Aviation Fuels 4u2b4z

Aviation Fuels

Editor

Bhupendra Khandelwal

Department of Mechanical Engineering, The University of Alabama, Tuscaloosa, AL, United States

Table of Contents

Cover image

Title page

Copyright

Dedication

Contributors

Preface

Introduction

Chapter 1. Feedstock and pathways for alternative aviation fuels

1. Introduction

2. Alternative jet fuel research drivers

3. Life cycle assessments

4. Feedstock

5. Pathways

6. Conclusion

Chapter 2. General compositions and alternative aviation fuel approval process

1. Introduction

2. Fuel standards

3. Approval of conventional fuels

4. Approval of alternative fuels

5. Summary

Chapter 3. Ignition and relight, and impact of alternative fuels

1. Why ignition and relight are important

2. Gas turbine igniters

3. Parameters affecting the minimum ignition energy

4. Parameters affecting the ignition time delay

5. Alternative fuel impact on the ignition performance

6. Conclusion

Chapter 4. Impact of alternative fuels on gaseous emissions

1. Introduction

2. NOx formation mechanisms

3. Gaseous emissions and alternative fuels

4. Conclusion

Chapter 5. Impact of alternative fuels and fuel properties on PM emissions

1. What are particulate matter emissions?

2. Impact of particulate matter emissions

3. Measurement of particulate matter emissions

4. Ways to reduce particulate material emissions

5. Impact of alternative fuels on particulate matter emissions

6. Summary

Chapter 6. Impact of alternative fuels and properties on elastomer compatibility

1. Introduction

2. Phenomena in seals

3. Elastomer material

4. Testing methods

5. Recent advancements in understanding elastomer fuel compatibility

6. Summary

Chapter 7. Impact of fuel properties on lean blow off limit

1. Introduction

2. LBO mechanism

3. LBO detection method

4. Hardware and fuel impact on LBO

5. LBO prediction

6. LBO summary

Chapter 8. Thermal stability and impact of alternative fuels

1. Introduction

2. Thermal stability

3. Fuel composition and thermal stability

4. Blending

5. Trace species

6. Generation of chemical kinetic mechanism

7. Experimental apparatus

8. Incidents involving fuel thermal stability

9. Outlook

Chapter 9. Impact of alternative fuel properties on combustion instabilities, noise, and vibrations

1. Introduction to combustion instabilities

2. Categorizing instabilities

3. Combustion noise

4. Gas turbine instability, noise, and vibrations and previous work

5. Alternative fuels and their impact on noise and vibrations

6. Conclusions

Chapter 10. Hydrogen fuel for aviation

1. Introduction

2. Hydrogen fuel

3. Historical aerospace applications of hydrogen

4. Hydrogen combustors

Chapter 11. Electric-powered aircraft

1. Introduction

2. History of development of electric and hybrid aircraft

3. The future, conclusions

Index

Copyright

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Notices

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Dedication

This book is dedicated to Late Prof. Chris Wilson and Prof. Riti Singh for their remarkable guidance and work on fuels and combustion.

—Bhupendra Khandelwal

Contributors

Bandar Awadh Almohammadi, Department of Mechanical Engineering, College of Engineering, Taibah University, Yanbu, Saudi Arabia

Matthew Dwyer, Department of Mechanical Engineering, The University of Sheffield, Sheffield, United Kingdom

Pierre Ghali, Department of Mechanical Engineering, The University of Sheffield, Sheffield, United Kingdom

Maciej Piotr Gierulski, Department of Mechanical Engineering, The University of Sheffield, Sheffield, United Kingdom

Jerry Hamilton, Department of Mechanical Engineering, The University of Alabama, Tuscaloosa, AL, United States

Bhupendra Khandelwal, Department of Mechanical Engineering, The University of Alabama, Tuscaloosa, AL, United States

Huanrong Lei, Department of Mechanical Engineering, The University of Sheffield, Sheffield, United Kingdom

Chenxing Ling, Department of Mechanical Engineering, The University of Sheffield, Sheffield, United Kingdom

Yousef Sadat, Department of Mechanical Engineering, The University of Sheffield, Sheffield, United Kingdom

Vamsi Krishna Undavalli, Moscow Aviation Institute (National Research University), Moscow, Russian Federation

Charith J. Wijesinghe, Department of Mechanical Engineering, The University of Sheffield, Sheffield, United Kingdom

L. Zheng, Nanjing Institute of Technology, Nanjing, Jiangsu, People's Republic of China

Preface

Aviation fuels have long been considered as a closed box from an outsider’s point of view. As I have worked in the area of alternative aviation fuels for 10 years, it has always been an inspiration for me to simplify this closed box for the world. Much work has been done and progress has been made in this field over the past decade. This book summarizes the work done in the field of alternative and sustainable aviation fuels. It has been written by several authors, including myself, associated with the University of Sheffield and the University of Alabama.

Another inspiration to write this book is to provide a one-stop solution for gathering basic knowledge about all the aspects of aviation fuels. This book covers the important areas related to aviation fuels, including feedstocks, production pathways, general composition, fuel approval, impact of fuels on gaseous emissions, PM emissions, vibrations, noise, fuel thermal stability, and elastomer compatibility. In addition to these areas, two future options, hydrogenand electric-powered systems, are also discussed in different chapters of this book.

I would like to acknowledge all those who have helped and encouraged the writing and compilation of this book. The team at Low Carbon Combustion Centre (LCCC) has been tremendous in assisting with this work, in addition to the team at Engines and Combustion Laboratory (ECL). I must thank my wife, Trupti, for putting up with my lack of house maintenance and the many evenings spent concentrating on this work.

Bhupendra Khandelwal

Introduction

This book presents readers with the latest in-depth details on alternative aviation fuels and their performance in aircraft. Topics from feedstocks, to fuel systems performance, to emissions and overall aircraft impact are covered in the book. A comprehensive understanding of aviation fuel's science, combustion performance, and environmental impacts can point in a positive direction for further alternative fuel development, and is crucial for the beneficial development of future fuels and engines.

Chapter 1 of this book provides details of different feedstocks used and production pathways for aviation fuels. As this area is constantly evolving, the areas covered in this chapter are never fully complete. However, this chapter provides a good grounding for anyone new to this area.

Chapter 2 discusses the normative standards in the certification process of conventional and alternative fuels used by the aviation industry. The normative standards are listed by various regulatory organizations, such as ASTM International, UK Defence Standardization, and other institutions. These standards are discussed on the basis of fuel performance specifications, also referred to as fit-to-purpose in the normative documents. As the fuel is one of the vital components, its performance is of the utmost importance in the airworthiness of the aircraft and engine, the safety of the engers, and also the effects on environment pollution. The aviation fuel is a complex mixture of hydrocarbon chains, and it undergoes various processes from the initial stage of production to the final stage of burning in the combustor. There are several external factors that impact the fuel during this process, yet we require the fuel to have the desired performance. These performance parameters dictate the fuel specifications listed by various organizations throughout the world as standards,

and the roles of the individuals involved are clearly discussed. The author also elaborates on the streamlined approval process involved with future alternative fuels in the selection of new candidate fuels or new additives, and establishes the fact that a new standardization and certification process with the evolution of fuels is required.

Chapter 3 of this book discusses the issue of ignition performance and ignitors in detail. Moreover, the impact of different alternative fuels and their properties on ignition time delay from an aviation point of view is also discussed.

Chapters 4 and 5 discuss the impact of different alternative fuels on gaseous emissions and PM emissions. When considering alternative fuels, it is important to measure and evaluate the emissions produced when burning them in gas turbines, so that an informed comparison can be made with conventionally derived fuels such as Jet-A1 and JP-8. A detailed assessment of current PM measurement methods is presented in this chapter, in addition to PM formation.

Chapter 6 discusses the fuel interaction with aircraft materials, especially with regard to elastomers, which is one of the crucial components for airworthiness and efficient operation throughout the machine's lifetime. This chapter also discusses the basics of seal working mechanisms and their interaction with fuels. A brief discussion is also presented on the recent developments in seal materials and their interaction with alternative fuels that lack aromatic components, the role of aromatic content in fuels, the need for additives for enhancement swelling, the testing procedures involved with understanding the compatibility of seals, and their swelling behavior in new alternative fuels.

Chapter 7 discusses the work done in the area of assessment of alternative fuels and their properties with regard to lean blow-off performance. Owing to the fact that different fuels can have different properties, alternative fuels can have a significant impact on lean blowout and flame stability of the combustor.

Chapter 8 discusses the impact of different fuel compositions and their impact on thermal stability of aviation fuels. The fact that turbine engines are running at much higher temperatures and compression ratios drives the need to use fuel, prior to combustion, as a heat removal source, which confirms the need for a clear understanding of fuel thermal stability. All fuels contain numerous compounds and additives that have an effect on fuel thermal stability. It is important to review fuel composition and the reactions that occur when exposed to oxygen, when heated, and also when blended with other fuels. Currently, the issue of thermal stability in fossil jet fuels and alternative jet fuels is a barrier to using blends of greater than 50% for commercial use. Different pathways and schemes to carbonaceous deposition are discussed, along with results of current laboratory testing and historical research. Methods of testing and analysis of the thermal stability of different fuels and blends are also presented in this chapter. The goal of carbon net-zero fuels can be accomplished with increased research and attention to these topics.

Chapter 9 aims to describe how the fuel used in a gas turbine can impact the stability of a flame in the combustion chamber, with a special focus on the use of alternative fuels in gas turbines, a pressing concern given the climate imperative. Combustion instability is an event where small disturbances in a flame, caused by fuel flow rate fluctuations, heat release rate fluctuations, combustor designs in which poor mixing occurs, and other factors, grow in an uncontrolled manner and then affect the flame in a detrimental manner, such as excessive noise, vibration increased flame turbulence, and, in dire situations, flame-outs. This chapter discusses the impact of different fuels and their properties on combustion instabilities, noise, and vibrations. This area of research is fairly new in aviation, and very little literature is available. Overall, this chapter has shown that different fuels and their properties can impact noise, vibrations, and combustion instability at both combustor and engine levels.

Chapter 10 explores hydrogen as an aviation fuel. In aviation fuel development, hydrogen has an important role to play. Compared with the other fuels, hydrogen has the correct chemical properties such as energy density and provides a

reduction in harmful emissions. In history, hydrogen has been actively explored for use as an aircraft fuel since the 17th century. The exploration of hydrogen is wide ranging, from hydrogen combustion to hydrogen fuel cell application. For modern aircraft, the hydrogen combustors such as the micromix combustor and lean-direct injection combustor are being studied. These combustors are different from conventional combustors and can burn pure hydrogen without the risks of hydrogen combustion.

Chapter 11 aims to answer the question as to whether electric aircraft are a viable alternative to internal combustion engine aircraft. It conducts a quick review of electric aerospace propulsion technologies available in early 2020, including means of energy storage and electric motors. It also talks about the development of electric aircraft since their beginning in the 19th century until the present day. A market review is also conducted to show which electric airplanes are commercially available or are under development. Finally, the most significant barriers to the technology, as well as potential opportunities, are described. The conclusion is that electric airplanes can be successfully flown with today's technology and are able to perform adequately in some roles; however, it will be years before they become an everyday choice in aviation.

Chapter 1: Feedstock and pathways for alternative aviation fuels

Chenxing Ling ¹ , Jerry Hamilton ² , and Bhupendra Khandelwal ² ¹ Department of Mechanical Engineering, The University of Sheffield, Sheffield, United Kingdom ² Department of Mechanical Engineering, The University of Alabama, Tuscaloosa, AL, United States

Abstract

With the advent of alternative fuels for aviation, and constantly increasing demands, it is vitally important that feedstocks for these fuels are selected in the right way. Replacing food producing crops to fuel producing crops may not always be a good idea. This chapter discusses different feedstocks used and production pathways for aviation fuels. As this area is constantly evolving, the areas covered in this chapter are never fully complete. However, this chapter will give a good head start for anyone new to this area.

Keywords

Alternative aviation fuels; ATJ; Biomass; Feedstocks; Fischer–Tropsch; HEFA;

Life cycle; Pathways

1. Introduction

The subject of feedstock and pathways may not be the most glamorous of topics in aviation, but it may arguably be the most important with regard to sustainability, economics, and especially environmental impacts. As the global reliance on aviation transportation increases, the supply network for the industry must be uncompromisable. There are problems looming on the horizon that threaten aviation operations. These problems include the ever-decreasing fossil fuel resources and the increasing of greenhouse gases (GHGs). Luckily all is not doom and gloom nor is this a lost cause, and the attention to these problems has been on the rise over the past few decades. Researchers across the globe have put increasing emphasis into new feedstocks and processes that can reduce dependence on fossil fuels. Research into GHG life cycles and carbon neutral alternative fuels has already had an impact on decreasing the GHG footprint from aviation operations. While the goal is to lessen the impact that aviation operations have on the environment as well as the global economy, researchers must have clear understanding of the fuel life cycle. The impact of aviation fuels does not begin at combustion. The actual life cycle begins at the location of the raw products and follows through to the extraction and transportation to a processing plant. The life cycle is not complete until the fuel is converted to energy and gas has been released to the atmosphere. All the energy expended during the life cycle, not just at combustion, counts toward the overall impact and cost of the resource. The analysis of fuel life cycles has been made easier through software such as the Greenhouse gases, Regulated Emissions, and Energy use in Transportation (GREET) program by the Argonne National Laboratory.

2. Alternative jet fuel research drivers

The use of jet fuel has had an environmental impact as well as an economic impact. These impacts are negative and can affect future generations by releasing sequestered CO2 with other harmful compounds into the atmosphere. For the past few decades, the research on sustainable alternative fuels (SAFs) has increased dramatically. Broadly speaking, there are four main drivers for the development of SAF. These are economic and environmental sustainability, diversity in energy supplies, and the competition of different sectors for these resources/feedstocks. The cost of energy fluctuates overtime, and jet fuel cost is no exception. The stability of the market may be up for debate, but the actual numbers speak for themselves. According to the indexmundi.com Website [1], the price of jet fuel has increased from $0.54 per gallon in 1990 to $3.89 in 2008. However, the price remained between $0.30 and $0.70 per gallon from 1991 until 2004. The market has had three major events in the past 30 years that has significantly dropped the price. From 2008 to 2009 and 2014 to 2016, the price dropped by 33%. In December 2019, the price fell from $1.89 to $0.61 by April 2020. As of this writing, the price has risen to $0.98 per gallon, as shown in Fig. 1. Although there have been significant drops, the overall trend is upward, and each time, there is a large increase in price, there is a resurgence in attention given to SAF research and development. Until the aviation industry can deploy a viable alternative fuel solution, the production of engines and aircraft will be based on conventional jet fuels. This is a challenge to researchers, and the concept of “drop-in” fuels may be the bridge that leads to a departure from fossil fuels. These drop-in fuels must be not only renewable but also sustainable while adhering to the standards placed upon conventional jet fuels [2]. The advantage to using drop-in fuels does not require the current infrastructure to go through a tremendous amount of change.

Figure 1 Jet fuel prices from 1990 to 2020 [1].

According to the Air Transport Action Group (ATAG), aviation operations for 12% of CO2 emissions from all sources of transportation. CO2 is just one of the other harmful emissions, which include NOx, CO, UHC, SOx, and other nonvolatile particulate matter (PM). Most of the compounds released by aviation combustion processes are designated hazardous air pollutants to the environment as well as people. As important research continues in alternate jet fuels and new technologies, these hazardous emissions could be dramatically reduced. The greatest effect comes with both new combustion technologies and fuel compositions. It is important to address nonaviation advancements that may also impact the future of conventional jet fuel production. The ground transportation sector has been shifting to more eco-friendly vehicles. Our roads are seeing a yearly increase in hybrid, electric, and other fuel cell type vehicles. What does this mean for aviation? According to the US Energy Information istration (EIA), gasoline comprised 49% of the US petroleum refineries total production, and kerosene-type jet fuel ed for 9% [3]. The total amount of petroleum products was produced from 7.55 billion barrels of crude oil, as shown in Fig. 2. If the demand for gasoline diminishes, the jet fuel prices could rise significantly.

Figure 2 US refiner and blender net production of petroleum products, 2018 [3].

3. Life cycle assessments

Effective life cycle assessments (LCAs) of aviation fuel pathways help guide the industry in making critical decisions that will help reduce the annual 3% increase of GHG emissions by the aviation sector [4]. An LCA can also provide extensive data on the environmental effects and impact that each portion of the production contributes to the pathway. A great amount of effort has been spent over the past few decades developing standards and benchmarking to compare the different feedstock and pathways. Within an assessment, there are foundations in the methodology that include unit definitions, data resource information, and the scope of the system boundaries. The overall boundary for alternative aviation fuel pathways is referred to as well-to-wake (WtWa). Fig. 3 shows a visualization of the WtWa cycle. Other system boundaries are contained in WtWa; these are well-to-gate (WtG), well-to-pump (WtP), and gate-to-gate (GtG). Each of these system boundaries contains many other subprocesses required for the pathway. Some of these are the feedstock extraction, processing, refinement, and storage along with the transportation system in between. Each process requires energy and resources for completion. It also goes without saying that the outputs for these processes are GHG emissions as well as coproducts. These coproducts may be an addition or waste products. A proper LCA will take all the aforementioned considerations and more into to assess the viability and sustainability of the feedstock and pathway proposed. Differences between fossil-based and sustainable aviation fuel life cycles are illustrated in Fig. 4. In the past, research has only focused on the carbon emission at the combustion stage due to the availability of fossil oil in the wild. However, in case of the alternative fuel, most of the feedstocks are not typically located in mass quantities by nature, and extra processes are required to produce the desired jet fuels. Therefore, an overall calculation of carbon emissions should be considered during pathway evaluation as the life cycle tends to generate much higher carbon than during the combustion phase. On example is the coal-to-liquid (CTL) product. If consideration is only placed on the carbon emission during the

combustion phase, CTL will be lower than conventional fuel. However, additional carbon will be produced during the production of CTL, and the total carbon emission from WtWa will be much higher than that of convention fuel. Another example is jet fuel produced by biomass. For a long time, biomass has been considered as the raw material for renewable jet fuel (RJF), which has zero carbon emission or a carbon neutrality. This concept is based upon the carbon balance between the growth phase of the plant (absorption of carbon from atmosphere) and the combustion phase (release carbon to atmosphere). During the process, the total amount of absorbed carbon by the feedstock and the released carbon during WtWa is canceled, which therefore would not introduce extra carbon to the environment. However, recent studies have indicated the opposite results for some pathways. Also, other studies have shown that the feedstock that requires more fertilization has carbon released during the growth phase. In addition, with large scale of cultivation, deforestation, land exploitation, and land reclamation operations, more carbon will be released to the environment from the sequestered carbon from inside the soil. This is of particular interest to researchers of land use change (LUC). Complex life cycle calculations for selected feedstocks and pathways are shown in Fig. 5.

Figure 3 Alternative aviation fuel well-to-wake pathway [5].

Figure 4 Fossil and SAF life cycles [6].

Figure 5 Comparison of the life cycle GHG emissions [5]. GHG, greenhouse gas.

Carbon emission from fuel recovery, feed stock transportation, land use, fuel processing, and combustion are all considered throughout the calculation. According to the Group on International Aviation and Climate Change (GIACC), the highest whole carbon emission is the hydroprocessed renewable fuel produced by palm oil and soy oil. Note that a large portion of the emission is produced by LUC. The large emission from LUC is reasonable because cultivation of these energy plants involves deforestation and reclamation. In the case of palm oil production, tropical and peat land rainforests would have to be converted to palm plantation fields. In general, the production of plant oil could result in large quantities of carbon emissions during the land use phase. As compared with biofeedstock, fossil feedstock such as coal and natural gas has the emission advantage on the recovery stage. However, the major carbon emission is during the processing stage, which is more than half of the total carbon emission in the process. With the aid of carbon capture and sequestration (CCS) technology, the amount of carbon emission during the converting process has been dramatically reduced. In general, compared with fossil feedstock, biobased feedstock has a major advantage because of the reduction in total carbon emission throughout the growth phase. Still, further investigated should still be conducted as large-scale demand will be required in the future especially in of food and fuel feedstock balance.

4. Feedstock

Conventional jet fuel is derived from crude oil, and about 5.8% of the total oil produced globally is consumed by aviation. Demand for more SAFs from sustainable feedstocks continues to grow. Research has predicted a shortage of fossil-based fuel in coming times. Hence, there is a need to develop new feedstock and pathways for alternative aviation fuel production [7]. Alternative fuels can be categorized broadly as fossil-based and bio-based. The fossil-based examples in the figure are gases and coal. The bio-based examples are solid waste, biomass, and crops. As shown in Fig. 6, the fossil-based and biobased feedstocks (labeled as “Resource”) are not relocated to one specific path each. In fact, fossil-based and bio-based feedstocks can occupy the same processes.

4.1. Bio-based feedstock

Compared with fossil-based fuels, biofuels are considered to be renewable fuels such as renewable jet fuel (RJF), which is a substitute for Jet A-1 [8]. Biomass feedstocks are generally classified into three groups: first, second, and third generations, as shown in Fig. 7. First-generation feedstocks are generally food crops that are used to produce bioethanol and biodiesel. These feedstocks have been shown to have lower GHG emissions but are contrasted by very high production costs. Some mitigations to high costs are government subsidies and using well-established infrastructures [2]. Second-generation feedstocks have an advantage in that competition with food crops is virtually nonexistent. Research into these feedstocks has shown that there is sustainability and carbon neutrality potential. Unlike first-generation feedstocks with established infrastructures, producers must design the production process for the selected feedstock. Of course, engineers may discover new more efficient processes that might not have

been possible with older infrastructure models. The final feedstock is composed entirely of algae, and cutting-edge research is ongoing.

Figure 6 Feedstock and pathways for alternative fuels, fossil-based and biobased [5].

Figure 7 Breakdown of the total cost of a typical pelleted biomass supply chain [2].

Another way to view biofuel feedstocks is to divide them into two groups: edible and inedible. Edible types such as crop oil, corn, and sugarcane are considered in the first-generation feedstock [9]. Until recently, most of the production of biofuel was made from edible bio-based feedstocks. In general, 350 different species of crops could be used as the feedstock of biofuels [10]. However, the main requirements of the biofuel feedstock are low price and a high production rate. Some researchers have indicated that almost 75% of the total cost is the feedstock itself [11]. Therefore, finding an inexpensive feedstock is necessary. Other factors under consideration are the place of origin and climate, which will affect the production of biomass since most species are sensitive to geography. Species that provides a large crop yield and has a high energy content is extreme valuable. The emission of the biofuel must be lower than that of conventional fuel counterpart, if it is to be considered for production. However, when the total carbon life cycle is calculated, the results could be opposite. Carbon stored within the soil can be released to the atmosphere during cultivation. For a carbon neutral feedstock, the carbon released during the life cycle will be consumed during the growth of the plants.

4.2. Fossil feedstock

Until renewable feedstock processes are commonplace, fossil feedstock could also be used in the production of alternative jet fuel. Coal, natural gas, and oil shale are available to convert into jet fuel. Fossil feedstock has a longer production history and well understood conversion technology more so than biobased feedstock. The largest portion of alternative jet fuel is produced by coal and natural gas through the Fischer–Tropsch (FT) process (biomass feedstock can also produce a liquid fuel through the FT process). The advantage of fossil feedstock is the fully developed production pathway and less input during the recovery phase. As all fossil feedstock is generated from geologically

sequestered carbon sources, there is no additional process required, whereas biobased feedstock requires fertilization and water to become the desired fuel feedstock. However, due to the absence of the recovery process, the carbon dioxide generated through the combustion process will interrupt the carbon balance in the atmosphere which is the largest drawback of fossil feedstock. Moreover, as the large-scale production increases, there may be an energy competition between jet fuel feedstock and daily energy usage (electricity, heating). Coal is another source that can also converted into liquid fuel. The largest coal direct liquefaction project is in Erdos China [12]. The annual production of CTL was 2.54 million tons in china at 2015, and according to the announcement from the National Energy istration of China, by 2020, the production of CTL will reach upward to 13 million tons per year [13]. As a coal-rich country, China has a wide variety of coal types, which could be used as feedstock of CTL processing, which is shown in Table 1. The performance, as well as the fuel production, of a CTL processing plant is different for each type of coal. Table 2 indicates the amount of fuel production by different types of coal. Research has shown that compared with conventional fuel, the production of carbon dioxide is 2.4% less with FT fuels [12]. And as the lack of aromatic content in the FT fuels, particulate matter production is much lower, which could be up to 90% less than convention jet fuel. In addition, sulfur is not existent in FT fuel. These attributes demonstrate a clear superiority over conventional jet fuels. However, even with the mentioned advantages the production cost is significantly higher. The total carbon emission could also be twice as high when the carbon life cycle is factored into the calculation. Therefore, applying carbon capture and sequestration (CCS) is necessary to reduce the total carbon emission. However, the result of combined CCS with CTL is highly capital-intensive [15].

Table 1

Ash

Mass fraction (%)

Mass fraction (%)

C

H

N

S

O

Water

5.20

90.93

1.13

0.30

0.25

1.27

3.21

9.81

85.13

3.74

1.23

1.34

2.49

1.57

15.63

84.16

4.30

1.46

0.43

3.60

1.47

20.57

80.65

4.66

1.23

0.17

2.82

0.93

9.45

76.40

5.27

0.99

0.27

11.10

4.12

Table 2

Tops (kg/s)

Gasoil (kg/s)

Keo (kg/s)

FT oil (kg/s)

0.044

0.215

0.102

0.361

Anthracite coal (760524)

0.045

0.219

0.105

0.369

Meager coal (760527)

0.042

0.0204

0.097

0.344

Lean coal (73–1)

0.041

0.0199

0.095

0.334

Coking coal (73–6)

0.044

0.0211

0.101

0.356

Gas coal (73–7)

4.3. Biomass

Biomass is any kind of organic feedstock that can be replenished or renewed in a natural way. Fig. 8 illustrates a biomass flowchart; not all biomass feedstocks are present in the figure. As shown in the figure, the main components to the overall biomass feedstock are lignocellulosic and food biomass. Lignocellulosic feedstocks are wood, high yield/energy crops, agricultural, waste (industrial and municipal), food waste, and more. Food biomass are organic material that can be used for human consumption as well as alternate fuel sources. Oils are a great example of food biomass. Research into biomass for the development for an alternative jet fuel has only recently been investigated compared with how long fossil-based feedstocks have been used. The greatest advantage of using biomass as feedstock is the low price of the raw materials in relation to fossil fuels. Moreover, biomass is also considered as environmentally friendly as it absorbs carbon during the recovery phase. It is predictable that the portion of FT fuels produced by biomass will increase in the near future.

Figure 8 Biomass classifications [16].

4.4. Oils

Oils are a biological material that can be converted to alternative aviation fuel using hydroprocessing [17]. This type of feedstock includes (but not limited to) palm oil, soybean oil, sunflower oil, and jatropha oil. Fig. 9 indicates that soybeans and palm have a large quantity of annual production. However, as most of the production is used as food, only small amount of feedstock can be used for biofuel. It should be pointed out that almost 84% of edible biodiesel product is contributed by rapeseed [18]. According to Fig. 9, it is obvious that algae have the huge advantage of oil yield due to their ease of cultivation. Palm and coconut also show an ability to provide a sustainable oil supply.

Figure 9 Oil yield and cost [2].

4.4.1. Camelina

Camelina, within the Brassicaceae plant family, is an annual flowering plant. Various oils produced by rapeseed, canola, and mustard are included in this plant family. The advantage of camelina is the high 35% oil content. In addition, compared with conventional oil crops, camelina shows its high tolerance of dry environments during production. This is a huge advantage as large swaths of land that is not suitable for other biofuel plants are able to grow camelina. Furthermore, camelina requires low quantities of fertilizer and water. Although some research has indicated that camelina is full of omega-3 fatty acids, which could be used as high-quality food oil, it has not been included in the main food supply feedstock thus eliminating any competition.

4.4.2. Jatropha

Jatropha is a toxic flowering plant that grows in Africa, Asia, and American. Compared with conventional food crops, jatropha can survive in unproductive land and is insensitive to extreme weather. Moreover, the water necessary to produce jatropha is extremely low. The properties of jatropha result in a high potential for production, and it does not need to compete with food crops. The oil content in jatropha could reach up to 35% of the original weight [19]. Traditionally, jatropha family provides oil feedstock for the medicinal soap industry. Seedcake of jatropha could also be used as fertilizer or animal food. The oil derived from jatropha has been selected as a biofuel feedstock in the past decade and labeled as miracle crop as it provides high-quality biofuel with low input requirements. Recently, however, jatropha has lost its position because the

total yield is far lower the predicted previously. Farmers are hesitant to produce jatropha because of uncertainty in the market because it has not been completely domesticated.

4.4.3. Palm

With 2600 species [20], oil extract from palm is considered as a major candidate for renewable jet fuel feedstock. As one of the most extensively cultivated plants in the world, palm family has their position in the human history. The uses are so widespread that in some places, palm oil is used as food flavor. Most of the content in the palm oil is saturated fat, and at room temperature, it will become semisolid. One of the advantages of using palm as fuel feedstock is that the waste material during oil extraction process such as shells and fruit bunches could also be treated as wasted material to be used as energy production. The desired cultivation environment is tropical and subtropical climates, Indonesia, Malaysia, and Nigeria, which has a long history producing palm.

4.4.4. Algae

Algae are one of the oldest life forms and has evolved into thousands of different species spanning the globe. Recently labeled as the third generation of biofuel feedstock due to the high oil content, high growth rate, and easiness of cultivation [8]. Like the oil plants, algae consumes water, carbon dioxide, and sunlight, but with far more efficiency than convention plants. Cultivation of microalgae is much easier and faster than oil plants with yields almost 30 times higher than convention plants per acre [21]. In the wild, algae species could be found in every habitat on the Earth including extreme salt water or even glaciers. The quantity of carbon dioxide absorption during the microalgae lifetime shows a strong advantage over conventional biomass producers. Moreover, considering their extremely high growth rate, microalgae seem to be the one of the important

feedstock that has the potential to satisfy the world’s liquid fuel demand. With these advantages and more the use algae as a sustainable jet fuel will no doubt increase.

4.5. Waste oil

Waste oil is considered as part of the second-generation biomass feedstock [4]. Waste oil has a unique advantage in that the price can be free (from abandonment, for example) or very low. Moreover, using waste oil is considered to be environmentally friendly. However, the major problem when using waste oil is that the production is not stable, and the collect points are disposal locations. Therefore, waste oil does not have enough ability to be the long-term feedstock supplement to supply the large demand of the market. Moreover, waste oils and animal fat usually have a large content of saturated fatty acids, which increase the production prices.

4.6. Natural gas

Fossil fuel–based natural gas is an unrenewable energy. As a sort of hydrocarbon gas mixture, the major component is methane. The impurities of natural gas usually include other types of alkanes. The process that converts natural gas to liquefied fuel is referred to as gas-to-liquid (GTL) [14]. Speight (2014) adds that GTL is a refinery process that can be direct or using the FT process to convert natural gas into longer-chain hydrocarbons [22]. Synthesis gas (syngas), which is a mix of carbon monoxide and hydrogen, is produced through gasification of carbonaceous feedstocks and could be produced with natural gas [23]. FT process is then applied to convert synthesis gas into liquid hydrocarbons [11]. The US Energy Information istration estimated that by 2017, the average production of GTL was 230,000 barrels per day, which is 0.2% of global liquids production [24].

Fig. 10 show the global production of liquid hydrocarbons though the GTL process. The production of GTL has dramatically increased in the past decade as seen before. GTL plant constructions is projected to rise dramatically as a reaction to the ever-increasing demand.

5. Pathways

The term “pathway” is defined as the process by which a raw material is turned into fuel, as discussed earlier. The kerosene-type jet fuel pathway from crude oil feedstock is considered as the conventional pathway. This pathway has had very little competition until recent developments in SAFs.

Figure 10 Global GTL plant production [25]. GTL, gas-to-liquid.

The pathway processes of converting feedstock to aviation alternative fuels vary considerably depending on the feedstocks selected. The FT process is the most developed and mature technology, which is widely used and fully commercialized. Other pathways have been developed recently due to the feedstock crisis of conventional jet fuel. For example, hydrotreatment ester and fatty acid (HEFA) technology has provided an alternative method to producing aviation jet fuel by bio-oil type feedstocks.

5.1. Conventional pathways

The system for producing fuel as we know it today has its origins in 1859 with Edwin Drake's first oil well in Pennsylvania. Of course, the product was for completely different purposes, and much of the refined products potential, like gasoline, was not fully realized. With the invention of the automobile and later the aircraft, the production of gasoline increased dramatically. The methods and infrastructure that are used to produce and deliver these conventional fuels, also known as fossil fuels, have had a 150-year head start to new innovative fuel pathway technology. It must be noted that prior to the invention of the modern car, the internal combustion engines were designed for use with biofuels, ethanol, and turpentine.

5.2. Fischer–Tropsch

The FT process was developed more than a century ago by Germans Franz Fischer and Hans Tropsch. Hydrocarbons are produced from low boiling gases

to high-boiling wax [22]. Studies have shown that the FT pathways can have a GHG emission reduction as high as 86%–104% as compared with the fossil fuel pathway counterparts [8]. The feedstocks of FT pathways are dominated by coal, natural gas, and biomass. In general, synthetic paraffinic kerosene (SPK) is generated through gasification and catalytic cracking process. The resultant SPK fuel could be used directly in the jet engine, and no significant problems have been reported in the past few years. At present, FT fuels derived from coal and natural gas have been used in the commercial flights with 50% blends of conventional jet fuel [26]. Depending on the producer, the basic feedstock for FT process varies (coal for Sasol, natural gas for Shell). However, all processes required to produce synthesis gas must first go through gasification. FT jet fuel produced by Sasol is the first approved Alternative Jet Fuel(s) (AJF) to be used with the blending of Commercial Jet Fuel(s) (CJF). The resultant FT fuel produced by Shell is a narrow-cut kerosene compared with JP8. As compared with coal and natural gas, biomass has a much shorter history of the development regarding FT processes. The main drawback of using biomass is the cleaning of the syngas and finding sustainable resources. The cost of the biomass also significantly affects the process of large-scale production. It should be noted that approximately 75% of the price of biomass FT fuel is used on the purchase of the feedstock itself. Some scale-up testing has been done, and since 2009, up to 50% blending with petroleum has been used commercially. In addition, some recent research has indicated the possibility of using coal and biomass as feedstock, sometimes referred to as CBTL. Liquid fuel is generated though the newer and more efficient single-step hydrolysis process. The byproduct is a high-purity carbon dioxide, which could be reused and repurposed. The detailed information of this novel pathway is limited as it still needs further development.

5.3. Fuel Readiness Level

Standardization of alternate jet fuel development has been codified by the

Commercial Aviation Alternative Fuels Initiative (CAAFI) and is called the Fuel Readiness Level. There are varying levels that help developmental stage verifications. The classifications help to track the progress of a fuel from initial research and development, certification, to demonstration. Each stage in the process must meet certain milestones to progress to the next level [12].

5.4. Hydrotreated esters and fatty acids Fuel Readiness Level 7

In general, HEFA technology transfers raw feedstock to paraffins through hydrotreatment (Fig. 11) and isomerization process. (As of this writing, HEFA is one of seven authorized pathways for sustainable aviation fuel.) For all RJF with biomass feedstock, HEFA is the most developed method. Most of the feedstocks used in HEFA are vegetable oil, animal fat, and waste oil, which are firstgeneration feedstock. Second-generation resources include nonedible crops such as camelina and jatropha. Oil derived from algae, the third-generation feedstock, is also being rapidly developed. Both Honeywell and Neste use HEFA technologies to produce alternative fuel. However, due to the different processes used, the final output is different. Honeywell uses hydrotreatment followed by selective cracking with the combination of isomerization. Honeywell's process maximizes the RJF output. Neste's HEFA process targets diesel production. It is still possible to produce AJF, but further adjustment is required. One of the complications of HEFA is the production feedstock. Overall, the world supply of vegetable oil is 178 million tons in 2015. Recently, however, the aviation industry has increased the research and development of different oil feedstocks to convert into AJF. Due to the concern of competition with food production and environmental impact, waste oils have been investigated as alternative HEFA feedstock. Used cooking oil (UCO) is the most widely used feedstock, but the production is much smaller when compared with vegetable oil production. It has been estimated that 29 million tons per year of UCO is produced globally [27]. The actual useable waste oil feedstock must compete with the increased demand of biodiesel production for vehicles. Biodiesel used for land transportation has already been well developed, and AJF production will no doubt be using the experience of the biodiesel production.

Figure 11 General flow diagram for hydroprocessing [16].

One of the major drawbacks of using the edible feedstock is that when the demand and production both increase dramatically, it may affect the food security. In addition, the price of vegetable oil has increased more than 10 times during the past 10 years and global production has increased 600% from 1972 to 2015 [28]. Therefore, more development of nonedible feedstock is reasonable. Compared with edible plants, nonedible plants are not competing with the demand of food supply, and most of these plants could survive in land, which is not suitable for growing edible oil plants. Therefore, the price is significantly lower than edible plants.

5.5. Alcohol-to-jet Fuel Readiness Level 4–6

Ethanol is a clear, colorless alcohol produced from biomass such as corn and sugarcane. The world's leading ethanol producers are the United States and Brazil. Corn is the major feedstock of ethanol in the United States because of its rich cultivation and massive reserve quantity. Only a few pathways to jet fuel have been approved, and alcohol-to-jet synthetic paraffinic kerosene (ATJ-SPK) was approved in 2016 with a blending limit of 30% [26]. This method transfers alcohol intermediates to jet fuel. Through dehydration, oligomerization, and hydrogeneration, alcohol can be converted to hydrocarbon fuel. The pathway for converting biomass feedstock to alcohol intermediates has been well developed for decades, which provides strong technology advanced for this type of AJF. In general, two steps are required to produce this type of AJF. First, ethanol should be derived from sugar or starch through processes such as fermentation. This process is a fully mature technology especially in American and Brazil. However, for the feedstock such as lignocellulose resources, the converting process is still in development. The advantage of ATJ is the side range of

feedstock and methods for alcohol intermediate production.

5.5.1. Corn

As one of the most important crops, maize also known as corn is widely cultivated in every continent. According to the statistics from the Food and Agriculture Organization of the United Nations (FAO), the global production of corn in 2020 is forecast to be 1.207 billion metric tons, and the United States is set to contribute the largest percentage of the total number. In 2019, maize production in the United States reached 347 million metric tons and is expected to reach 406 million metric tons [29]. Corn is the primary feedstock of ethanol in the United States, and large corn harvests have contributed to increased ethanol production in recent years. Increased corn production and relatively stable corn prices have helped make increased ethanol production more profitable and less susceptible to corn price shocks that had affected ethanol profitability and output in the past.

5.5.2. Sugarcane

Originated in Asia, sugarcane is one of the largest feedstocks of ethanol production. Brazil is the leading country of sugarcane cultivation in global market and has the largest sugarcane production which is almost 41% of global production. Compared with the corn ethanol in the United States, Brazil uses sugarcane as the feedstock for ethanol. Since the United States and Brazil are the world's leaders in ethanol production, there is a tremendous amount of competition between the two nations [7].

5.6. Lignocellulose resources

Inedible biomass such as trees, grass, and agricultural waste could also be the feedstock of ATJ. Fibers in the plants are extracted first, and then after cellulose breaking, cellulose ethanol can be produced. Compared with traditional ethanol crops, inedible feedstock has a large potential. The water and fertilizer required during the cultivation is far less than conventional crops, and inedible plants can grow in the land that is not suitable for edible crops.

5.7. Direct sugar-to-hydrocarbon Fuel Readiness Level 5–7

The feedstock of direct sugar-to-hydrocarbon process is similar to ATJ fuel. The difference is that this method converts sugar directly to alkane-type fuels through anaerobic fermentation. Therefore, there is no need to produce alcohol as intermediate to convert into jet fuel [21]. Considering the complexity of fermentation, simple hexose sugars generated from sugar cane are used as the primary feedstock. However, this technology has the potential to expand to more complex structures of sugar such as the sugar extract from lignocellulosic biomass feedstock.

5.8. Coprocessing and catalytic hydrothermolysis Fuel Readiness Level 6

This pathway transfers vegetable oil-to-jet fuel with the middle distillation in the refineries. As this pathway is comparable with the existing hydrotreated technology, the investment cost is low. The catalytic hydrothermolysis uses water to produce aromatic and paraffinic compounds, which has a lower consumption of hydrogen.

6. Conclusion

Due to growing concerns of the impact that burning fossil fuels have on the environment, the interest for more environmentally friendly methods for air travel has increased. The problems and complexity to developing sustainable, economical, and environmentally safer fuels are daunting, especially when the safety standards of the fuel must not be compromised. Advancements in aviation technologies have made the industry much more efficient in the past half-decade, and as air travel demand continues to grow, the next major step is SAFs. Fossil-based (e.g., oil, natural gas, and coal) feedstock has historically been the main feedstock for jet fuel, which is the product of biological organisms, pressure, heat, and time. These organisms required CO2 to survive, and then the CO2 is released back into the atmosphere when used as fuel. Along with CO2, NOx and other compounds from the combustion process are described as GHGs. Generally, bio-based feedstocks also release the same compounds to the atmosphere. The striking difference is that biomass fuels absorb GHG compounds during the growth stage of the feedstock. Using life cycle analysis, the amount of gases going into the growth and through to the combustion is balanced. As alluded to, feedstocks are the key to unlocking the potential benefits to sustainable alternative fuel. Different types of feedstocks provide their own unique challenges. For example, feedstocks such as corn and sugar must compete with the food sector, and nonfood feedstocks may require new costly infrastructure. The other side of the spectrum is developing the feedstock into fuel. This is part of the pathway process and must be specially certified to be considered as a “drop-in” fuel. The term “drop-in” refers to alternate fuels that can be either blended with conventional fuel or used 100% as the jet fuel. Sustainable alternative fuels that can completely replace fossil-based are the goal; some flights have already been performed using 100% SAF. As mentioned earlier, fuel safety standards must be kept, and organizations such as the CAAFI have created tools to help with pathway approval. Currently, there are six sustainable aviation fuel pathways approved by the American Society for Testing and Materials

(ASTM) with many more on the way. As more feedstocks and pathways are approved, the dependency on fossil fuels will decrease, creating a more stable economy and cleaner air for generations to come.

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10. Sani Y.M, Daud W.M.A.W, Abdul Aziz A.R. Biodiesel feedstock and production technologies: successes, challenges and prospects, biodiesel feedstocks, production and applications. In: Biodiesel - feedstocks, production and applications . IntechOpen; 2012. 11. Atabani A.E, et al. A comprehensive review on biodiesel as an alternative energy resource and its characteristics. Renewable and Sustainable Energy Reviews . 2012;16(4):2070–2093. 12. Zhang C, et al. Recent development in studies of alternative jet fuel combustion: progress, challenges, and opportunities. Renewable and Sustainable Energy Reviews . 2016;54:120–138. 13. The outlook for energy: a view to 2040 . Exxon Mobil Corporation; 2015. 14. Zhou L, Duan M, Yu Y. Exergy and economic analyses of indirect coal-toliquid technology coupling carbon capture and storage. Journal of Cleaner Production . 2018;174:87–95. 15. Lane J. Beta renewables: biofuels digest's 2015 5-minute guide. [Online]. Biofuels Digest . 2015. https://www.biofuelsdigest.com/bdigest/2015/01/15/betarenewables-biofuels-digests-2015-5-minute-guide/. 16. Patel M, Kumar A. Production of renewable diesel through the hydroprocessing of lignocellulosic biomass-derived bio-oil: a review. Renewable and Sustainable Energy Reviews . 2016;58:1293–1307. 17. Tao L, et al. Techno-economic and resource analysis of hydroprocessed renewable jet fuel. Biotechnology for Biofuels . 2017;10(1):261. 18. Bart J.C.J, Palmeri N, Cavallaro S. Feedstocks for biodiesel production. In: Bart J.C.J, Palmeri N, Cavallaro S, eds. Biodiesel science and technology . Woodhead Publishing; 2010:130–225. 19. Nithiyanantham S, Siddhuraju P, Francis G. Potential of jatropha curcas as a biofuel, animal feed and health products. Journal of the American Oil Chemists' Society . 2012;89. 20. Tobi A.R, et al. Comparative analysis of physiochemical properties of physically activated carbon from palm bio-waste. Journal of Materials Research

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Chapter 2: General compositions and alternative aviation fuel approval process

Vamsi Krishna Undavalli ¹ , and Bhupendra Khandelwal ² ¹ Moscow Aviation Institute (National Research University), Moscow, Russian Federation ² Department of Mechanical Engineering, The University of Alabama, Tuscaloosa, AL, United States

Abstract

This chapter discusses the normative standards in certification process of conventional and alternative fuels used by the aviation industry aviation fuels. The normative standards are listed by various regulatory organizations such as ASTM International, Defence Standards of UK, and other institutions. These standards are discussed on the basis of fuel performance specifications or also referred as fit-to-purpose in the normative documents. As the fuel is one of the vital components, its performance is of utmost importance in airworthiness of aircraft and engine, safety of the engers, and also its effects on environment pollution. The aviation fuel is a complex mixture of hydrocarbon chains, and it undergoes through various processes from the initial stage of its production to its final stage of burning in the combustor. There are several external factors that impact the fuel during this process and yet we require the fuel to have the desired performance. These performance parameters dictate the fuel specifications listed by various organizations throughout the world as standards, with a clear discussion on the roles of the individuals. The author also elaborates on the streamlined approval process involved with future alternative fuels in

selection of new candidate fuels or new additives and establishes the fact that the new standardization and certification process with evolution of fuels are required.

Keywords

Alternative aviation fuels; Aviation fuel approval; Aviation fuel standards; Sustainable aviation fuels

1. Introduction

Aviation fuels usage varies according to the type of engines and their operational applicability. These fuels can be broadly classified as shown in Fig. 1. At present, the internal piston engines are powered by AVGAS (aviation gasoline) and turbine engines on aviation turbine fuels (ATFs). Although all ATFs are based on petroleum, their composition varies depending on production techniques, source of crude oil, and their availability. Due to this fact, the aviation fuels certification and standards are drafted on the basis of their performance. The performance of the fuel is determined by the properties such as calorific value, combustion quality, fuel stability, volatility, fluidity, lubricity, noncorrosive, cleanliness, safety properties, and emissions. The most common aviation turbine fuel is the wide-cut Jet-A1 fuel, and whereas Jet-A is only used domestically in the United States, while Jet-B is still being used in the colder regions of the world. The aviation fuels are also required to have high energy and high heat sink capability, low–high temperature deposition characteristics, good thermal stability for heat exchange, fuel operability, storage, and material compatibility.

Figure 1 Classification of aviation fuels.

With time, aviation fuels simultaneously evolved with aircraft and engine technology. Aviation fuels are basically derived from crude oil, which are either petroleum-based or kerosene-based or blend of both often referred to as wide-cut fuels. Aviation fuels have quite similar chemical structure, and it is very hard to define the exact composition of the fuel, as different producers adopt different production technology. Ultimately, the performance of fuel only matters, so the standard fuel specifications (fuel specs) are derived on the basis of fuel performance. Also, the fuel chemical properties are enhanced over a period of time to meet the increasing demands of engine performance. The early experience on piston engine fuels have provided a baseline in the development of current jet fuel specs for the industry and military applications. This leads to commonly accepted set of fuel properties, which are issued as fuel specs in standards. Some of the widely used fuels around the world are mentioned in Table 1.

2. Fuel standards

Standards are set of fuel specs for controlling aviation fuel quality and performance. These specifications are issued by individual national organizations, which will eventually be the focal point in evaluation of aviation fuels. The early focus was on improving the engine performance and operation capabilities such as overcoming the detonation problems and increasing the altitude of operation. An early experience on piston engine fuels has provided a baseline in the development of current fuel standard specifications with a refinement and revision over a period of time to accommodate any necessities that are required by the civil aviation and military aviation. The aviation industry employs these requirements to regulate the output performance and fuel properties in such activities as R&D of airframes, power plants, their operational deployment, production of fuel, and transport and storage of fuel.

Table 1



Fuels

Jet-A1, Jet-A

International Ai

Jet- A1, Jet-A

United States

Navy JP-4 [F-40] a , JP-5 [F-44] Air-force JP-7, JP-8 [F-34/35]

US military

AVCAT high-flash kerosene [F-44], AVTAG wide-cut kerosene [F-40], AVTUR Jet-A1 [F-35]

United Kingdom

T-1, TS-1, TS-1 , T-1S special kerosene, T-3 wide-cut kerosene, RT kerosene

Russia

Jet-A, Jet-A1 [F-35], AVTUR kerosene [F-34], AVCAT high-flash kerosene [F-44]

Canada

a NATO CODE is represented in [ ].

Airworthiness authorities approve the aircraft and aircraft engines but not the fuels. So they accept the standard specifications as operating limits upon specification by original equipment manufacturers (OEM). First alternative fuels were introduced with approval of coal-to-liquid (CTL) semisynthetic jet fuel through Sasol's Fischer–Tropsch (FT) process in 1999, and since then, they have evolved. This led to the basis in the development of alternative jet fuels and their approval criteria by the ASTM International Aviation Fuel Subcommittee J, which is accepted by other corresponding organizations. As alternative fuels possess identical fuel properties as crude oil–derived jet fuel, these replace the conventional jet fuels and are often referred to as “drop-in” fuels. Drop-in fuels offer similar performance as conventional jet fuels; the Federal Aviation istration (FAA) and European Aviation Safety Agency (EASA) adopted the concept of “drop-in” to avoid the recertification of existing jet engines and aircrafts. The Commercial Aviation Alternative Fuels Initiative (CAAFI) is actively working with aviation fuel community and other partners including the aviation OEMs in partnership with US military to build the qualification and certification processes for alternative fuel's production and their deployment. Initially in 2009, a comprehensive standard specification is issued for the FT-based fuel with 50% blend, and as of 2020, we have specifications for seven types of alternative fuels.

3. Approval of conventional fuels

Conventional fuels are derived from crude petroleum through refining process. These fuels may be based on either petroleum or kerosene or even blend of both and often referred to as “wide-cut” fuels. Wide-cut fuels are hydrocarbon mixture extended over boiling ranges of both gasoline and kerosene. Due to this fact, these are available in greater volume. The development of conventional fuels predates back to early 20th century, where the human's quest for flying was in amateur stages. With human's desire to fly higher and faster, there has been a necessity to improve fuel characteristics and control the fuel properties to meet the operational requirements of aircraft. In the mid-1930s–40s, series of attempts were made to resolve the piston engine combustion instabilities due to detonation in a keyway to improve engine performance. Later on, understanding the intricate relation between combustion and fuel composition led to compositional change in the AVGAS, which eventually led to development of detonation resistance gasoline. A new additive gasoline is developed by adding branched paraffin and tetraethyl, which leads to modern AVGAS. Over several decades, military organizations were developing the fuel specs to improve the fuel performance. These specifications introduced fuel testing methods and fuel qualification criteria and also controlled the fuel requirements to match the aviation designers's necessities. These early encounters with AVGAS in piston engines helped us to shape a foundation for the development of jet fuel. In his initial prototype experiments, German inventor Dr. Hans Von Ohain opted to use gaseous hydrogen and later turned to aviation gasoline in the first ever in-flight jet engine that powered aircraft Henkel-178. This was due to the large supply of AVGAS and no complexities with fuel storage and transportation. On other hand Sir Frank Whittle in England used illuminating kerosene due to its low freezing point. As in the 1930s–40s, it was a hit-and-trial process involved with hunting a suitable jet fuel. An iterative trial-and-error process over years, with initial accomplishments by scientists and engineers, through investigated trade-offs between fuel properties and aircraft operational

requirements. For example, the jet aircraft high-altitude operations require high volatility and freezing points. Aircraft carriers in view of safety concern on fuel handling limited the flash point. So choosing the fuel purely based on operational requirements is not desirable, as it limits the fuel availability due to refining costs. With the operational experience gained over years, there has been a continuous refinement in the early fuel specifications, which led to commonly agreed set of properties for the present jet fuel description. These specifications evolved with advancements in fuels, aircraft, and engines. Currently, the rate of change in specifications is very little, as over years we have very stable and relative specification. The fuel produced from crude oil has many properties, but only fitto-purpose (FTP) properties such as (1) thermal coefficient of expansion, (2) surface tension, (3) specific heat, (4) thermal conductivity, (5) enthalpy of evaporation, (6) dielectric constant, (7) electrical conductivity, (8) flammability limits with altitude, (9) minimum spark ignition energy, (10) spontaneous ignition, (11) bulk modulus solubility of gases and water, and (12) thermal oxidation stability [1] are evaluated and controlled due to their effect on fuel performance; remaining properties have very limited variations due to the fact of consistent refinery processing techniques. Petroleum and Lubricants Committee, D02 under ASTM International and Aviation Fuels Committee under Ministry of Defence, UK (DEF STAN) are the major contributors in establishing current fuel standards. These organizations work closely with major aviation industry stake holders including civil and military to coordinate specification's criteria for jet fuel. Many of the individual nations do issue jet fuel specs, yet most are developed from one of the aforementioned committees. ASTM D1655 “Standard Specification for Aviation Turbine Fuels” [2] is the most common standard used by the aviation industry standard along with DEF STAN 91-91. These both standards specify the specifications for the aviation turbine fuels Jet-A1 and Jet-A. The US military fuel JP-8 specifications are mentioned in MIL DTL 83133. The Commonwealth of Independent States (CIS) uses the Russian standards GOST 10227 specification [3] for TS-1 and RT fuels and for Jet-A1 kerosene GOST R 52050. Canada specifies Jet-A1 under CGSB 3.24, whereas China specifies its No. 3 jet fuels under GB 6537. Although in all of these jet fuel specs the fuel requirements, fuel properties and fuel limitations are quite close but not equivalent, some of the fuels specs are tabulated in Table 2. The AVGAS is specified in a separate standard under ASTM D910 for four grades 80L, 90L,

100L, and 100LL and in DEF STAN 91-90 for three grades 80L, 100L, and 100LL. Airworthiness authorities such as Federal Aviation startion (FAA) and European Union Aviation Safety Agency (EASA) indirectly oversee the aviation fuel through an end such as airliners unlike the direct oversight of airframes, aircraft power plants, and their components. Regardless of the fact, the fuel is subjected to several outer influences that result modifications in the fuel quality and properties across the fuel supply chain. It is nearly impossible to tag every molecule “approved” like an aircraft or engine part once manufactured. In addition to this, each batch of fuel composition keeps on varying due to the source of fossil oil and manufacturing techniques utilized by the fuel producer. So eventually the airworthiness regulations evolved to rely on regulating fuel performance rather than fuel composition as mentioned earlier. It allows any stakeholder in the fuel supply chain to validate the fuel performance that is appropriate at every stage. In a simplified manner, the airworthiness authority oversees through airliners, who are obliged to use the fuel specified by OEMs in operating manuals. The OEMs identify the fuels and their specs that are to be used in their products through product testing and certification in accordance with US CFR Title 14, Part 33.7, and EASA CS-E 40 (d) or EASA AMC 40 (d) (3) (c) [4]. So once fuel in compliance with airworthiness regulations is demonstrated for the aircraft or engine, the fuel specs become part of airframe or power plant operation limitations, which are specified in operating manuals and certification documents [5]. Historically the fuel specs used as operating limitations have provided enough control of the fuel composition and properties to meet the aircraft of engine airworthiness certification.

Table 2

Fuel ASTM D1655

Jet-A DEF STAN 91-91

Jet-A1 GOST 10227

TS-1 CGSB-3.22

Jet-B Specification standards

0.100

0.015

0.700

0.100

Acidity (mg KOH/g)

25

25

22

25

Aromatics (% vol.), max

0.300

0.300

0.250

0.400

Sulfur (mass%), max.

0.003

0.003

0.005

0.003

Sulfur, mercaptan (mass Distillation (°C)

205

205

165

Report

10%

Report

Report

165

Report

50%

300

300

250

270

100% or end point

–

–

–

21

Vapor pressure (kPa), m

38

38

28

–

Flash point (°C), min.

775–840

775–840

Min 774 at 20°C

750–801

Density at 15°C (kg/m³)

−40

−47

−50

−51

Freezing point (°C), max

8

8

8 at −40°C

–

Viscosity at –20°C (mm²/

42.8

42.8

42.9

42.8

Calorific value (MJ/kg),

18

19

25

20

Smoke point (mm), min.

3

3

–

3

Naphthalenes (vol. %), m

No. 1

No. 1

(3 h at 100°C)

No. 1

Copper corrosion for 2 Thermal stability

25

25

–

25

Filter pressure drop (mm

<3

<3

–

<3

Visual tube rating, max.

–

–

18

–

Static test for 4 h at 150

7

7

5

–

Existent gum (mg/100 m

Based on the specifications of ASTM D1655 and DEF STAN 91-91, t Inspection Group (JIG), which was founded by a consortium of oil companies, issues the most restrictive fuel requirements by combining both standards into a single document as “Aviation Fuel Quality Requirements for tly Operated Systems.” Often, this document is also referred as “t Checklist” and used by manufacturers as a checklist to meet the specifications of fuel standards [6].

4. Approval of alternative fuels

Alternative fuels include synthetic fuels and renewable fuels, which are often referred to as sustainable aviation fuels (SAFs). Synthetic fuels are produced from coal, natural gas, or other hydrocarbon feedstocks such as biomass using FT process. The feed stocks are gasified to create a mixture of carbon monoxide and hydrogen further recombined to form the hydrocarbon fuel [7]. Renewable fuels are produced from biological feedstock sources such as plant lipids, fats, oils, and greases. The lipids are processed for transesterification or hydrotreating to obtain jet fuels. The alternative fuels are referred to as “drop-In” fuels, as these fuels are having similar composition of conventional fuels and have achieved required operational performance with blending of conventional and alternative fuels. The drop-in is something more off to simplify or negate the recertification of aircraft equipment, as these are used to supplement or replace the conventional fuels. The initial attempts for producing alternative fuels date back to the early 1950s by Sasol Limited in South Africa. Since 1955, Sasol has been producing coalderived synthetic blend fuels using the FT CTL process. With the increase in demand for jet fuels and depletion of petroleum sources, Sasol began investigating CTL semisynthetic jet fuel (SSJF) in mid-1990s. By 1999, Sasol had obtained DEF STAN 91-91 Issue 3 approval of semisynthetic Jet-A1 aviation kerosene, which is based on FT process with blends of coal and jet fuel from petroleum. In 2008, Sasol had obtained DEF STAN 91-91 Issue 6 approval for a fully synthetic jet fuel (FSJF) produced from blending of five synthetic distillates streams of coal [8]. Furthermore, it also contains the procedure for acceptance testing of the components such as fuel nozzle, combustor, and even full-scale engine test if required. The United States Air Force (USAF) established a testing and evaluation procedure for alternate jet fuels depending on the aforementioned two approvals [2]. Later on, ASTM International has also approved both semi-synthetic jet fuel (SSJF) and Fully Synthetic Jet Fuel (FSJF) and has categorized them as “dropin” fuels. With similar fuel properties, Sasol fuels are incorporated as Jet-A and

Jet-A1 fuels into the existing aviation fuel specs. FAA and EASA have accepted the concept of “drop-in,” so further certification of engines or aircraft is not required anymore for usage of drop-in fuels. With these approvals, Sasol has established a pathway for universal approval alternative fuels for other producers, which are being adopted by the industry currently. With growing interests in alternative fuels, Commercial Aviation Alternative Fuels Initiative (CAAFI) was formed in May 2006. It is now actively coordinating the development and commercialization of “drop-in” fuels by streamlining the certification process [9]. The USAF and CAAFI in partnership work with aviation fuel community with more than 100 industry stakeholders including fuel producers, OEMs, airliners, and military organizations. CAAFI chairs a member of Aviation Fuel Subcommittee J of ASTM International, which comes under the Petroleum, Liquid Fuels and Lubricants Committee (Committee D02). It functions in determining the assessment procedure and fuel specs requirements for the emerging renewable fuels, with certain subcommittees. Although alternative fuels (drop-in fuels) are identical to petroleum-based fuels, the subcommittee decided to establish a stringent criteria for “drop-in” fuels due to lack of understanding with renewable fuels and modern manufacturing technologies in producing them where the feedstock had to be transformed from a broad range of resources such as coal, biomass, and plant oils. So in September 2009, a standard ASTM D7566 “Standard Specification for Aviation Turbine Fuel Containing Synthesized Hydrocarbons” was issued, as shown in Fig. 2 and in addition, Subcommittee J issued a standard ASTM D4054 “Standard Practice for Qualification and Approval of New Aviation Turbine Fuels and Fuel Additives” for defining the candidate fuel evaluation criteria [4] as shown in Fig. 3.

Figure 2 Structure of D7566 its alignment with D1655.

As absence of some specific hydrocarbons in the alternative fuels, they are required to be blended with regular jet fuel to achieve improved structure and composition yet they possess similar performance to regular jet fuel. So ASTM D7566 is composed of annexes for each synthetic fuel by defining their properties and compositional requirements. The main document of ASTM D7566 consists of table of specifications, which includes the permissible volume of fuel in percentage that can be combined with regular jet fuel in each annex for every new type of fuel listed. Once the new alternative fuels are listed in D7566 as “drop-in” fuels, they can be incorporated as conventional jet fuels in accordance with ASTM D1655 “Standard Specification for Aviation Turbine Fuels” [2]. This provides an ease with recertification or practically no further certification is required for the existing fuel supply chain infrastructure, aircraft, and engines. This allows the fuels listed in D7566 to be readily used, but it controls the fuel properties with more stringency and at the same time incorporating into existing fuel supply chain infrastructure with no additional certification.

Figure 3 ASTM D4054 candidate fuel qualification process, adapted from CAAFI [9].

Prior to successfully listing a new alternative fuel in ASTM D7566, the fuel must be a successful qualified candidate fuel. So the new candidate fuel must undergo a three-phase evaluation process in accordance with ASTM D4054 “Standard Practice for Qualification and Approval of New Aviation Turbine Fuels and Fuel Additives” [10] as shown in Fig. 3. During the four-tier evaluation process in three phases, candidate fuel producer will test the fuel samples to measure the compositions and performance through an iterative process. As shown in Fig. 4 intially a new fuel or additive is proposed in tier 1 by the producer to Subcommittee J of ASTM Committee D02. This proposal must be endorsed by the airliners and military to initiate the candidate qualification process. On endorsement, the fuel undergoes tier 2 test process where the fuel undergoes technical evaluation. The technical evaluation comprehensively evaluates the FTP properties, which include determination of chemical composition of hydrocarbon structure and trace materials. The performance properties and bulk physical properties are measured and cross-checked to limits of the ASTM D7566. Major tests include thermal stability to the adequate resistance to oxidative breakdown at operating temperatures [7]. Additive compatibility to check whether the fuel or additive is compatible with existing fuels and material compatibility to see whether lubricating oil seals do not change their elastomer behavior and the fuel supply systems do not corrode. Electrical safety and ground handling are also checked to see the commercial deployment of the fuel. On obtaining the data from tier 1 and tier 2, a research report is sent to OEMs, and a review is requested. Generally, it is referred as phase I research report. With the from OEMs, the qualification process is moved on to phase II where tier 3 and tier 4 are evaluated. Tier 3 mainly involves the engine hot section testing such as turbine and combustor to identify the behavior of combustion products with turbine blades and metallurgical coatings. In addition to that, component testing is conducted for the systems, which are in with fuel, such as fuel pumps, nozzles, fuel pipes, and APU altitude starting capability. On obtaining the performance and

operational components test data, a full-engine test is determined. Upon requirement of full-scale test, the engine testing is conducted to gather data on engine emissions, durability, and relight performance, which follows a Subcommittee J review through a research report of phase II where the inputs are taken from OEM manufactures as OEM reviews and approves. Once approved by OEMs, the process is moved on to phase III, and a review by airworthiness regulatory is taken into consideration. So the ASTM task force prepares the specifications and presents it for final voting process through ballot process. Upon successful voting, ASTM establishes the specification as a standard by listing them in Annex A of ASTM D7566.

Figure 4 ASTM D4054 candidate fuel phase-wise qualification process.

Currently, seven ways of producing alternative fuel for turbine engines which are approved and in practice as tabulated in Table 3, and listed as Annexes A1–A7 in D7566 [11]. As AVGAS contains lead that effects the environment negatively and is toxic in nature, it is necessary to avoid the knocking and detonation problems and to meet the operational safety of the piston engine. So an alternative additive to lead is being investigated by the FAA, Environmental Protection Agency (EPA), and CAAFI at William J. Hughes Technical Center in Atlantic City. In 2019, three shell fuels are evaluated preliminary but require additional refinements [12]. In recent times, there are a large number of efforts to reduce the cost and time required for alternative fuels approval. One example is creation of “clearing house” in the United States.

Table 3

Type

Year of approval, annex of D4054

Blend % value

2009 Annex A1

50.00% max.

Synthesis of CO and H2 gas mixture of urban solid waste

2011 Annex A2

50.00% max

Fatty acids, fatty acid esters, and lipids from plants and an

2014 Annex A3

10.00% max.

Sugars

2015 Annex A4

50.00% max.

Synthesis of CO and H2 gas mixture of urban solid waste

2016 Annex A5

50.00% max.

Ethanol and isobutanol from - Fermented starches of suga

2020 Annex A6

50.00% max.

Fatty acids, fatty acid esters, and lipids from plants and an

2020 Annex A7

10.00% max

Bio-derived hydrocarbons, fatty acid esters, and free fatty

5. Summary

Depletion of fossil fuel resources and their negative effects on environment has increased the attention towards the replacement of fossil fuels with alternative fuel sources. But there is only limited availability of renewable sources: still the conventional fuels are prime movers of today's aviation industry. With growing environmental concerns and to allow the fuels to meet technological design challenges, there is a necessity for fuel modification. This has been a proven fact with aviation gasoline by adding the lead additive to overcome the detonation combustion instabilities. The same is being achieved with convention jet fuel by allowing them to bend with additives. With the concept of carbon neutrality, use of the carbon depositions as fuels resulted in major changes in the standardization and certification process of aviation fuels. By streamlining the approval process and bringing the concept of “drop-in” through aligning the ASTM D7566 with ASTM D1655, it not only eases the process of new fuel deployment in accordance with ASTM D1655, but at the same time it also allows to maintain a stringent policy on new fuels production that allows us to control the fuel specs to meet the operational requirements. New procedures and process are being developed to streamline the process of aviation fuels approval, which will help in reducing the cost and time required to get the fuels approved.

References

1. Wilson G.R, Edwards T, Corporan E. Certification of alternative aviation fuels and blend components. Energy & Fuels . 2013;27(2):962–966. 2. ASTM D1655-20b. Standard specification for aviation turbine fuels . West Conshohocken, PA: ASTM International; 2020. 3. GOST 10227-2013. Jet fuels specifications . Federal Agency on Technical

Regulating and Metrology (ROSSTANDART); 2013. 4. Rumizen M. Aviation biofuel standards and airworthiness approval. In: Kaltschmitt M, Neuling U, eds. Bio-kerosene status and prospects . Berlin, Heidelberg: Springer; 2018:639–664. 5. Edwards T, Moses C, Drye F. Evaluation of combustion performance of alternative aviation fuels. In: 46th AIAA/ASME/SAE/ASEE t propulsion conference & exhibit . July 2010. 6. Aviation fuel quality requirements for tly operated systems. t Inspection Group . 2019(31). 7. Daggett D, Hadaller O, Maurice L, Rumizen M, Brown N, Altman R, Aylesworth H.The commercial aviation alternative fuels initiative. SAE Transactions . 2007;116:953–965. 8. DEF STAN 91-91,Issue-11. Turbine fuel, kerosene type, jet A1; NATO code: F-35; t service designation: AVTUR . UK: Ministry of Defense; 2019. 9. . Approved alternative fuels. www.caafi.org. [Accessed 5 August 2020]. 10. ASTM D4054-20a. Standard practice for evaluation of new aviation turbine fuels and fuel additives . West Conshohocken, PA: ASTM International; 2020. 11. ASTM D7566-20a. Standard specification for aviation turbine fuel containing synthesized hydrocarbons . West Conshohocken, PA: ASTM International; 2020. 12. Piston Aviation Fuels Initiative (PAFI). www.faa.gov. [Accessed 5 August 2020].

Chapter 3: Ignition and relight, and impact of alternative fuels

Bandar Awadh Almohammadi ¹ , Vamsi Krishna Undavalli ² , and Bhupendra Khandelwal ³ ¹ Department of Mechanical Engineering, College of Engineering, Taibah University, Yanbu, Saudi Arabia ² Moscow Aviation Institute (National Research University), Moscow, Russian Federation ³ Department of Mechanical Engineering, The University of Alabama, Tuscaloosa, AL, United States

Abstract

Study of igniter operability in gas turbines is important for both high-altitude relight performance and ground starting at cold conditions. Ignition performance is impacted by cetane number and fuel composition such as concentrations of aromatics and paraffins in a fuel. Also, ignition behaviour is impacted by the type of igniter and operating conditions. All these impactive parameters have been explored and discussed in this chapter. Among different types of igniters, the laser igniter has many advantages much like the ability to regulate ignition timing, minimum ignition energy, and ignition location. Moreover, the impact of different alternative fuels and their properties on ignition time delay from an aviation point of view have been discussed in this chapter.

Keywords

Igniter and alternative fuels; Ignition; Ignition delay; Performance; Relight

Nomenclature

AJFs alternative jet fuels

ATF aviation turbine fuel

BHD benzene–hexane–decane surrogate fuel

CID cetane ignition delay

DCN derived cetane number

MIE minimum ignition energy

P pressure

sps spark per second—spark frequency

T temperature

Φ equivalence ratio

τ ignition time delay

1. Why ignition and relight are important

For aviation gas turbines, the volume and operability of the combustor are determined by the ability to ignite and relight. Thus, it is an implicative factor for the engine emissions and engine dry weight. Predicting the combustibility of a gas turbine combustor will favor the design of the combustor and fuels development, but it is currently limited by the complexity of the phenomenon [1]. The spray flame ignition can be concluded in three phases [2]:

1. Flame formation and propagation 2. Sparking and initial kernel growth 3. Stabilization of combustor-scale flame through the recirculation zones [3]

2. Gas turbine igniters

Spark and torch igniters are not only igniters that can be used in gas turbines. In this section, different types of gas turbine igniters have been discussed. However, some of the ignition sources, such as glow plugs, chemical ignition, and hot-surface ignition, are currently outdated. Since glow plugs are normally intended for preheating of fuels to an instantaneous combustion temperature requiring a continuous power source, the chemical ignition source needs a mixture holding area and very large quantities of heat transfer are needed for hot surface ignition [3].

2.1. Spark ignition system

The spark igniter is important as it initiates combustion process and then sustains engine operation. Furthermore, the spark igniter needs to provide a rapid reignition when flame is completely out at high altitude [4]. Electrodes are the main spark plug component, through which sparking is achieved. The characteristic of spark plug is delivering the very high ignition energy to fuel–air mixture in a very small time periods, i.e., in 0.1 s and to a confined area in millimeters making the mixture ionize rapidly. This results in creating very-highenergy spark channel with temperature in the order of 6e⁴ K, thus heating the neighboring fuel mixture instantly. Spark ignition system is easy to use due to its ability to control the spark frequency, spark time span, and ionization energy [3].

2.2. Torch ignition system

The torch igniter comprises of an additional fuel/air stream and a spark plug in a separate housing, in which the initial ignition is created through fuel mixture and spark plug, thus using the flame as ignition source in combustor by creating a torch. Then, the formed torch ignites the main fuel. There are many parameters that can affect torch igniter performance, such as volatile nature of fuel, air temperature, and air-to-fuel ratio, but the performance is not affected by its position. The major disadvantages of torch igniters in gas turbines are gumming and cracking of fuels during the igniter idle operations. To overcome gumming and fuel cracking, clean purging air and solenoid valves could be incorporated in the ignition systems for starting the gas turbine. The ideal positioning of torch igniters is at 195 and 345 degrees in the bottom quadrants of the combustor [5].

2.3. Laser ignition system

Compared with other ignition systems, laser ignition system is most promising due to its ability to regulate ignition timing, location, and energy. With the prior said features and potential of laser igniters' wide applications, these are believed to be dominantly pervaded in the near future [6]. On the other hand, a lot of research is required to investigate the igniter operability in various operating conditions, performance of ignition in high-speed streams, and beam quality prior to mass production and application in jet engines. An example of laser ignition system has been shown in Fig. 1 [7].

2.4. Plasma jet ignition system

Plasma jet ignition system ignites a plasma medium created by virtue of electric discharge in an auxiliary combustion chamber although it is analogous spark ignition system unlike it uses a plasma medium resulting high temperature. The electric discharge creates a plasma medium by generating extremely high temperatures and pressures. This then lets the plasma flow through the port,

which works more like a torch igniter, but at supersonic velocities. The temperature and velocity of the plasma medium can be changed by varying plasma ports, gas feed, and chamber size and discharge, as a result of altering the distance it can penetrate [3]. Although it is advantageous in operating at highstream-speed flames and high-speed ignition, but it does pose a greater disadvantage. The requirement of high-power source to create high energy discharges, thus making system vulnerable to high energy consumption compared with other systems and prone to have shorter life span.

3. Parameters affecting the minimum ignition energy

Minimum ignition energy (MIE) is a vital parameter for ignition system; thus, the least MIE results in efficient ignition source, resulting the system to operate on low energy power supply. There are a wide number of factors that affect the ignition energy, and this section presents a detail review in the following sections.

3.1. Hardware and operation condition

3.1.1. Airflow

With air flow, the MIE increases. This is largely due to the improved airflow over the igniter, which cools the igniter and dilutes the concentrated energy. With increasing air flow [8], the range of equivalence ratios is decreased at which we can ignite the mixture. The MIE has approximately linear relationship with the velocity, but it is a function of several variables such as spacing of electrodes and time duration of spark. However, pressure does not affect the rate of MIE increment with velocity increment [9]. Turbulence is another factor that influences the MIE. With increasing turbulence, spark kernel loses high heat fluxes and also increases burning velocity resulting in the marginally increasing MIE. The effect of turbulence scale on the MIE is significant at higher intensities of turbulence [10]. According to the study, Shy et al. [11] suggested that an MIE transition occurs at a specific intensity. The transition in case of lean mixtures occurs at a normalized turbulence intensity of around 25, while in case of rich mixture and

stoichiometric mixture, it occurs about intensity 15. Before transition, MIE is in linear proportion to the intensity; once transition happens, MIE increases to exponent 4 of intensity.

3.1.2. Temperature

In addition to air flow, the temperature of air/fuel mixtures influences the MIE. The MIE reduces about 50%, increasing the fuel mixture temperature from 300 to 400 K [12]. The relationship between the MIE and temperature varies exponentially [13]. So, with preheating fuel mixtures, the equivalence ratio Φ is slightly richer for a least MIE than fuel mixtures that are not preheated. As well, to achieve ignition at higher temperatures, Φ must be small [12].

3.1.3. Pressure

MIE is proportional to pressure (P) with negative exponent of 2, which is experimentally agreed when the ignition process depends mainly on chemical rates [3]. So, increase in pressure leads to decrease in MIE. This is experimentally demonstrated true for pressures of small and higher magnitudes. But with pressures above 30 kPa, there is a slight increased reduction in MIE compared with exponent −2 of P. However, the pressure exponent is also influenced by equivalence ratio Φ and the air flow. When quiescent mixtures are at an equivalence ratio that is lightly higher than one, the exponent is at its lowest (around −2) i.e., with rich mixtures with increased pressure leading to decreased MIE. However, for air flow around 10 m/s, the exponent grows to −1.25, and then it remains constant with increasing air flow. Similarly, the power rises even more with changes in Φ, and it is around −0.5 for an Φ = 1.5 [8]. At increased mass flow rates, changes in pressure do affect the MIE, and at lower

values of Φ, increase in pressure leads to greater requirement of MIE compared with the same pressure at higher values Φ.

3.1.4. Spark plug location

Spark plug positioning should be correctly identified to increase the possibility of ignition. Ideal positioning should be in the regions of equivalence ratio equal to 1 or slightly greater than 1. However, if the spark plug location is moved toward the leaner regions around 5–10 mm away from the rich or stochiometric regions, the probability of the ignition may reduce significantly from 100% to 0% [14]. These results are also agreed by Ahmed [3]. Also Marchione et al. [3] conducted the study to find the optimal spark plug location and found that the recirculation zone is the best location as it densely contains fuel particles. In addition, multiple sparks have been used in this experiment: in contrast to single-spark igniter, multiple-spark igniter can position only alongside the combustor liner. Multiple-spark igniter position may be around the burner diameter in an axial direction to align and be influence of recirculation zone. In addition, it has been observed that shifting the position of the sparks 5–10 mm away from the above optimal distance, the probability of ignition has reduced to about 60%. In the case of a single spark, it is opposed to ignite near to the wall, as for multiple sparks, it is a default [3].

3.1.5. Spark duration

The spark duration or time period of spark is an important factor that directly affects energy consumption. High energy is consumed as the spark discharge happens quickly. However, if the discharge happens slowly, the loss of energy is distributed over too much of the fuel mixture, which leads the mixture to burn at a too low temperature. Airflow effects the duration of the spark, with increased

air flow reducing optimum duration of the spark. When Φ = 1, the optimal duration of the spark is the longest in time compared with equivalence ratio deviating far from Φ = 1. When equivalence ratio deviating far from Φ = 1, the duration of spark becomes shorter and shorter in time. Increased pressure reduces the optimal spark duration. While intensity of turbulence does not affect the optimum duration of the spark [15], the duration of the spark should always be lower than the amount of the ignition time delay [16].

3.1.6. Spark frequency (spark rate)

On using a spark plug, spark frequency or the spark rate (spark per second-sps) impacts the ignition. The chance of ignition can be increased with increasing frequency although utilizing identical electric source. On the other hand, a higher spark frequency gives a larger ignition unit that contributes to an optimum spark rate that requires to be detected based mostly on the application [3]. Foster [17] observed the same results in his experimental research, in which the spark frequency was changed from 150 to 3 sps and observed that greater chance of ignition is at the highest spark frequency (sps). Three explanations were given by him explaining the results [17]:

• Increasing the spark probability can happen when the instant flammable fuel mixture es through the spark. • Fuel and air are not completely mixed in small, localized regions between the electrodes. However, a higher spark frequency (sps) raises the chance of smaller fuel mixture packets moving through the electrodes and in return, which increases the chance of ignition. • Raising the spark electrodes temperature that in return serves the purpose of preheating fuel mixture makes it much easier for the mixture to ignite.

With experiments, it has been observed that multiple-spark igniter improved ignition reliability compared with single-spark igniter with frequency of 100 sps. It is because multiple sparks improved reliability as identified by Foster [3]. In contrast, single-spark igniter uses three times the energy used by multiplespark igniter, making multiple-spark igniter energy efficient and reliable.

3.1.7. Electrode gap and materials

The nature electrodes and gap between electrodes (electrode gap) are significant in deg the spark plug. In fact, optimal electrode gap can refer as the distance between electrodes with least MIE for successful ignition, which is critical factor for ignition. The purpose for an optimal electrode gap is that other than optimum gap, other electrode gaps result in higher MIE for both bigger and smaller electrode gaps. If electrode gap is too small, it results in quenching effect because a more number fuel mixture packets through electrodes, resulting in requirement of a higher MIE. On the other hand, if electrode gap is big, it also results in higher MIE due to the fact that energy is dissipated in producing a bigger spark kernel, which is more than required for ignition [3]. That being said so far, a very little is known from existing research about the parameters that affect the optimum spark width. Studies [10] carried out by Ballal et al. in various gas mediums have demonstrated highest optimum spark width, which is in the order of carbon dioxide, helium, nitrogen, and argon, respectively. The MIE decreases with smaller cross-sectional area electrodes. Smaller crosssectional electrode erodes sooner resulting nonuniform energy discharges. So, MIE is also influenced by the electrode material. MIE varies with exponent 0.25 of material boiling point; a material with low boiling point is more preferable. But it is also of great concern when selecting a material that is wear resistant and economical [15].

3.2. Fuels

Volatility is the most critical consideration when it comes to MIE and ignition for liquid fuels. Fuels with high volatility have a lower MIE compared with lowvolatile fuels. Heptane (n-heptane) with extreme volatility in nature has a roughly five times less MIE compared with a low-volatile fuel methanol [18].

3.2.1. Equivalence ratio (Φ)

In the case of liquid fuels, a least feasible MIE can be attained only at very fuelrich conditions where Φ > 2. However, equivalence ratio does not depend on volatility of fuel. That being said, fuels in the form of prevapor or gases has a very little significance on equivalence ratio as shown in Fig. 2. It is because, at condition Φ > 2, the fuel content in the form of gas keeps on growing in zone of combustion irrespective of either the fuel is liquid or gas with increasing equivalence ratio [18].

Figure 1 Laser ignition system of hydrogen-enriched natural gas developed by Prasad et al. [7].

Figure 2 Effect of equivalence ratio and droplet size on minimum ignition energy of methanol [18].

Similar observation is made by Lee et al. [19] that optimal Φ for liquid fuels is high compared with gaseous fuels. Experimentation with higher equivalence ratios carried out by Danis et al. proved that the optimal Φ for liquid fuels is about 4 [19].

3.2.2. Droplet size

In process of ignition, a better atomization is desired for better mixing of fuel and air. With atomization, the fuel molecules are broken down to minute droplets. But MIE would usually vary with droplet size (diameter). The smaller the droplet size, the lower will be MIE as in Fig. 2. Consequently, it is observed that with diameters <10–15 μm, the MIE tends to increase. So, it is evident that there is a necessity for optimal droplet diameter [18]. The droplets in smaller sizes would be carried away from zone of combustion or ignition due to Brownian motion of gas molecules. Thus, the droplets, which acquired heat, are not sufficiently heated to temperatures of ignition. The explanation for higher MIE for bigger size droplets for known equivalence ratio is due to the fact the surface area of single bigger droplet is smaller compared with total surface area of smaller droplets combined. So, with a larger droplet, there is a very little surface area of fuel droplet exposed for heating to ignition temperatures. The other reason would be flame kernel distribution, and bigger droplets do not migrate swiftly as compared with smaller droplets causing them to remain inside the kernel. This makes almost impossible for bigger droplets in flame kernel to interact with neighboring droplets out of flame kernel [18].

3.2.3. Prevaporization

The MIE decreases for prevaporized liquid fuels, and also the optimum equivalence ratio does decrease as shown in Fig. 2, to slightly leaner conditions from Φ = 2 to Φ = 1.5. Besides this, prevaporization only has a slight impact on extremely volatile fuels, while it has a serious effect on low-volatile fuels [18]. The disparity among low-volatile fuels of prevaporized and nonprevaporized studied and observed that MIEs of prevaporized fuels have approximately 20 times lesser compared with nonprevaporized fuels. Temperature difference among prevaporized and nonprevaporized fuels is about 25 degrees; evidently, temperature could hardly effect.

4. Parameters affecting the ignition time delay

4.1. Operation conditions

4.1.1. Temperature

Temperature of the flame is a significant factor that influences the ignition time delay. Ignition time delay (τ) is very low approximately 1 μs at high temperatures of 1800 K at 20 bars [20]. That being said, this analysis preheated the fuel to evaporation temperatures before the fuel enters into the combustion chamber [20]. Jet-A fuel has been used in the study. It is a bit more volatile compared with diesel fuel, but both fuels have the same ignition temperature. A research by Zhang et al. [21] found that ignition time delay was rapidly reduced as flame temperature increased. Also, it has been observed that ignition time delay (τ) decreases the flame temperature exponentially. The temperature of the inlet air is of high significance when a torch igniter is used. With decrease in standard temperature to 220 K for 90% flame to propagate through the combustor, it would be taken many times longer compared with standard temperature [5]. However, the time for complete flame propagation is comparatively short. Another study conducted [22] with kerosene fuel, at two position ignitions at180 degrees, inlet temperature 273.15 K, and combustors with diameter 35 cm, takes approximately 0.5 s for complete flame propagation.

4.1.2. Pressure

Ignition time delay (τ) is greatly influenced by pressure. The ignition time delay was found to be proportional to the (Pressure) −¹. From experimental studies, it is noticed that (Pressure) − . ⁸ is fitted best, but given the uncertainty of the experiment, (Pressure) −¹ can be considered appropriate [20]. However, Spadaccini and TeVelde [23] noticed that with five different liquid fuels, the ignition time delay (τ) is proportional to (Pressure) −² as suited for the data obtained. Many other studies mentioned by Spadaccini et al. suggested an exponent value to be in −1 to −2. Similar results have been identified by Zhang et al. [21] for gaseous fuels, but it has also been noticed that type of fuel used also has significant influence on pressure, and thus, no definitive exponent has been established.

4.2. Fuels

4.2.1. Fuel and droplet size

With the increment in droplet sizes (diameter), the ignition time delay (τ) does increase for low-volatile fuels. It is due to the fact that ignition takes place when the mixture is in the form of vapor or in vapor-controlled zone, but with increment in droplet size makes it longer to attain boiling temperatures. On contrary, τ with high-volatile fuels does decrease or remains unchanged, as the droplet size increases. It is due to combustion in high-volatile fuels happening in reaction-controlled zone. With droplets in smaller size, τ is roughly identical in both high- and low-volatile fuels; however, high-volatile fuels achieve speedier τ, as the droplet size increases [24].

4.2.2. Equivalence ratio Φ

Equivalence ratio Φ is lower which is in the range of Φ = 0.5 to 1 for a highvolatile fuel, which will eventually result in a shorter ignition time delay τ. It is due to the fact that a higher equivalence ratio would introduce additional fuel particles, and hence, maximizing the evaporative cooling results in a local quenching [25]. In the case of low-volatile fuels, the optimal equivalence ratio is very high Φ > 2. When combining fuels (combined fuels) with both natures, high and low volatility, the proportion of fuel with high volatility is critical. If the combined fuels contain 50% high-volatile fuel, the optimal Φ is as small as when a single component of high-volatile fuel is present in combined fuel. However, if the combined has 10% of fuel with high volatility, the optimal Φ would be marginally >2, which is substantially less compared with singlecomponent low-volatile fuel [26].

4.2.3. Temperature

The volatility of the fuel is of high significance for nonpreheated fuels. As the torch flame temperature rises from 1100 to 1700 K, for decane (low-volatile) mixture droplet size in 100 μm diameter, the ignition time delay τ will decrease from 0.1 to 0.03 s, while τ will decrease from 0.1 to 0.015 s with 10% hexane added to mixture, which is a more volatile fuel [27].

4.2.4. Velocity

Velocity affects the ignition time delay τ when the non-prevapored fuels, because of its immediate effect on heating and vaporization time of fuel. Increment in velocity reduces vaporization time. With increment in velocity to 10 m/s from 1 m/s, vaporization time decreases around 30% and first vaporization time decreases by about 1/2, where the studies used diesel fuels [28].

5. Alternative fuel impact on the ignition performance

There are many studies that have dealt with the impact of alternative fuels on ignition delay, as this is one of the most critical combustion characteristics. Zhu et al. conducted experiments on the ignition time delay (τ) for two conventional fuels, nine alternative fuels, and six blended fuels with of 50% alternative and 50% conventional fuels [29]. The ignition time delay (τ) was measured over a temperature of 1047–1520 K in two distinct pressure scenarios and different mixture regions using shock tube and laser absorption methods. In both pressure ranges, the overall results provided insignificant differences in τ among conventional fuels and alternative fuels. Consequentially, type of fuel does not impact the ignition delay. Another study conducted by Flora et al. investigated the ignition performance of JP 8 fuel and alternative jet fuels (Fischer–Tropsch), four biojet fuels, direct hydrocarbon sugar (DSHC), jet alcohol (ATJ), and biodiesel fuel using a singlepulse shock tube working in lean fuel mixture at high pressure conditions [30]. For all fuels, the results revealed that the ignition time delay was well within ±25% as shown in Fig. 3. These minor differences can be due to variations in the composition of the fuel, such as aromatic content. Dean at el. dealt with the autoignition of mixtures of hexane, benzene, and decane and Jet-A with the goal of finding appropriate surrogate fuels for kerosene [31]. The experiment was carried out using a heated shock tube of temperatures between 1000 and 1700 K at a mean pressure of 8.5 ± 1 atm. It was found that mixtures of hexane–decane (20%–80%), benzene–decane (20%–80%), and benzene–hexane–decane (18.2%–9.1%–72.7%) perform approximately similar ignition time delay compared with Jet-A for a range of temperature 1450–1750 K. In particular, the mixture of n-decane, n-hexane, and benzene led to a small increase in ignition time delay at low temperature as shown in Fig. 4. Conversely, this did not impact the critical temperature required for the initiation of direct detonation.

Figure 3 Comparison of ignition time delay τ for JP-8 and alternative jet fuels [30].

The influence of biojet fuel kerosene on ignition delay was studied by Han et al. [32]. The blend properties were similar to those of a kerosene jet fuel (Jet-A1) such as density, hydrocarbon ratio (H/C), and the lower heating value. The ignition time delay was recorded using a shock tube, and delay is compared with Jet-A and Jet-A1 in different temperatures ranging from 700 to 1200 K at 20 atm pressure. The ignition time delay of the blend was shorter at low temperature <900 K. The reason for this was associated with short ignition delay of biojet fuel components in the blend at low temperature. Autoignition characteristics of Jet-A, JP-8, and nonpetroleum fuel (S-8) were investigated by Kumar and Sung [33]. S-8 jet fuel was consisted of a hydrocarbon mixture rich in branched and linear alkanes (C7–C18). The ignition time delay was measured in the range of temperatures of 615–933 K, pressure of 7–30 bar, and equivalence ratio of 0.43–2.29. The ignition time delay recorded for S-8 is shortest compared with Jet-A and JP-8 as shown in Fig. 5. The difference in ignition probability was impacted by fuel properties such as cetane number.

Figure 4 Ignition time delay τ versus reciprocal temperature in stoichiometric benzene–hexane–decane (BHD) surrogate and Jet-A/air mixtures at equivalent conditions [31].

Mosbach et al. [34] dealt with the effect of five different alternative fuels such as GTL fuel composition on ignition and combustion performance of aviation gas turbine. The study has used a lean-burn aviation combustor at temperatures, subatmospheric air pressures, and a simulated flight altitude between 25,000 and 30,000 ft. Jet-A1 was used as a reference fuel. The differences in ignition delay times among test fuels were insignificant.

Figure 5 Ignition time delay τ as function of (A) pressure and (B) temperature [33].

Hui et al. [35] studied the effect of six alternative fuels, which include three hydrotreated renewable jet (HRJ) fuels and three Fischer–Tropsch (synthetic paraffinic kerosene) fuels on combustion characteristics. It was observed that ignition delay was affected by fuel composition, especially n-paraffins content. Test fuel with high n-paraffins content had shortest ignition delay compared with that of low n-paraffins content. For example, HRJ-camelina containing 10.2% by weight had short ignition delay compared with 8.8% n-paraffin content in HRJtallow fuel. The performance of ignition delay of distillate fuels was studied by Davidson et al. [36]. The study was conducted at different pressures, temperatures, and mixtures by using a shock tube. The distillate fuels were rocket propellants (RP2), jet fuels (JP-5, JP-8, and Jet-A), gasoline, and diesel fuels (F-76 and DF-2). The effect of variation in fuel components on ignition delay was insignificant. The activation energy of ignition time delay depended on oxygen concentration over a set of equivalence ratios near unity. Low oxygen concentrations led to high activation energies. For example, Fig. 6 depicts a contrast of ignition time delay for A2 jet fuel with different oxygen concentrations for equivalence ratio range near unity. As the variation in oxygen concentrations depends on temperature, the activation energy is increased with lower oxygen concentrations.

Figure 6 Ignition time delay τ of A2 jet fuel for φ = 1 for different amounts of N2 and O2 in argon [36].

Valco et al. [37] investigated the autoignition performance of two military grade aviation fuels (JP-5 and JP-8), Fischer–Tropsch fuels (Sasol and Shell), proposed camelina-derived hydroprocessed renewable fuels (HRJ-5 and HRJ-8), three Sasol isoparaffinic solvents, and three Sasol isoparaffinic solvents. Experiments were performed at various temperatures from 625 to 1000 K in shock tube at compressed pressures of 20 bar using rapid compression machine in lean and stoichiometric conditions. The results indicated that ignition time delay τ was reduced with increasing branch in chemical structure of fuel components. Furthermore, ignition reactivity was affected by cycloparaffin content at low and intermediate temperatures. The autoignition response results from Hui et al. [5] explained that alternative jet fuels ignite faster than Jet-A, and autoignition behavior can be quite susceptible to fuel structure and composition. The study shows that adding cetane additive to Jet-A will linearly increase the DCN and reduce the ignition time delay (τ). The overall ignition time delay of all seven alternative jet fuels seems to be much shorter than Jet-A, while Shell GTL is the fastest one with least ignition delay. Wang and Oehlschlaeger [38] documented autoignition studies of conventional and Fischer–Tropsch jet fuels with a total of five different fuel compositions. The results indicate that the fuel has a common ignition time delay τ if T > 1000 K as illustrated in Fig. 7. During low temperatures, the impact of composition of fuel is quite distinct, and GTL shows that shortest period of ignition time delay is in good correlation with Burger's observation [6]. He also demonstrated that GTL kerosene had the best performance in both evaporationlimited and reaction-limited efficiencies between the eight types of fuel tested. However, Mosbach et al. [4] tested five different GTL-like fuels with Jet-A1 as a standard fuel in a typical lean-burn aero combustor at subatmospheric air pressures and temperatures and noticed that ignition and combustion performance was weakly dependent on the fuels under investigation. As Lecourt's experimental work [7], he pointed out that the aromatic content in the fuel with the range of 0%–20% has positive effect on ignition performance.

But the pure aromatic is difficult to be ignited. According to Colket's modeling results, for all the tested aromatics species, higher ambient temperature can significantly reduce the ignition time delay τ [37]. The isooctane and toluene show longer ignition time delay compared with Jet-A, while atomization pressure has positive effect for ignition process.

Figure 7 Comparison between ignition time delay τ of alternative fuels at 20 atm [38].

Another one such recent study conducted by Kang et al. [39] investigated the autoignition properties of synthetic fuels compared with convention fuels. The studies used Jet A, JP-5, and JP-8 conventional fuels in comparison with synthetic fuels Syntroleum S-8 (S8), Shell synthetic paraffinic kerosene (Shell SPK), Sasol isoparaffinic kerosene (Sasol IPK), hydroprocessed renewable jet (HRJ8), and alcohol-to-jet (ATJ) fuels. The alternative fuels in the study can be grouped as high-reactive fuels (S8, Shell SPK and HRJ8) and low-reactive fuels (Sasol IPK and ATJ). Figs. 8 and 9 shows the ignition delays of physical ignition delay (a), chemical ignition delay (b), and total ignition delay in cetane ignition delay ignition (CID) by maintaining the fuel mixture at isobaric condition of 20 bar with varying temperatures of 20 K between 813 and 913 K. The time delays are measured by cetane ignition delay instrument, and the respective results are depicted in Figs. 8 and 9: 50% pure synthetic fuels and 50% blend of synthetic and conventional fuels, respectively. This study in particular demonstrated that these physical, chemical, and total time delays are not only function of physical, chemical, and thermodynamical properties of fuels but also interdependent on each other. Arrhenius plots of synthetic fuels from Fig. 8 show that τphy varies linearly with mixture temperature. With increase in mixture temperature, τphy decreases, as the mixture temperature directly affects the process of fuel spray vaporization. Alternative fuels have less τphy compared with JP-8 due to quick atomization of fuel, as they are of comparatively lower density and viscosity with the exception of Sasol SPK fuel. This is clearly showing the τphy is dependent and effected with τchem due to its low reactivity with low oxidation temperatures, leading to delay in total ignition process.

Figure 8 Ignition time delay comparison between synthetic and a conventional (A) physical ignition delay, (B) chemical ignition delay, and (C) total ignition delay measured in cetane ignition delay [39].

Figure 9 Ignition time delay comparison between 50% blend and a conventional (A) physical ignition delay, (B) chemical ignition delay, and (C) total ignition delay measured in cetane ignition delay [39].

Arrhenius plots of 50% blend fuels from Fig. 9, which clear shows that τphy varies linearly with mixture temperature and also depicts the effect on addition of JP-8 by delaying or advancing ignition process of blend fuels compared with alone synthetic fuels as in Fig. 8: a Sasol IPK blend and ATJ blend have same τphy although their τtot does vary significantly. It is also observed that highreactive alternative fuels with blends exhibit as delay in spray process due to the changes in fuel properties on JP-8 addition, whereas τchem of blend fuels shifts more toward the synthetic fuels although the values are expected to be in between JP-8 and synthetic fuels. It is also clearly evident the ignition process is greatly affected by blending process and having perfect ratio of blend showed 90% improved ignition reactivity in ATJ fuel; the same is also observed by Perez et al. Synergistic and antagonistic behaviors of fuel blends have greater influence on ignition reactivity, as it depicts the ratio of reactive and unreactive fuel components. Apparently at lower mixture temperatures, τchem has severe differences, with these becoming marginal with increase in mixture temperature for fuels with similar derived cetane number [40].

6. Conclusion

The ignition performance plays a role in the operability of airplane and combustor volume, which is an important factor for cost, weight, and exhaust emissions of aviation gas turbine. MIE and ignition delay time are affected by operating conditions and fuels used. Many igniter types can be used in gas turbine such as spark ignition, torch, laser, and plasma jet igniters. Laser igniters are likely to be more promising due to their number of benefits as compared with the conventional ignition systems due to their ability to regulate position of ignition, ignition time delay τ, and ignition energy. In of alternative fuels for aviation, ignition time delay τ is affected by the concentration of fuel components such as aromatics, paraffins, and cetane number. Increasing aromatic content in fuel increases ignition delay time. According to the literature reviewed in this chapter, the use of biojet fuel contributes to reduced ignition delay at low temperature compared with conventional fuel.

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Chapter 4: Impact of alternative fuels on gaseous emissions

Charith J. Wijesinghe ¹ , and Bhupendra Khandelwal ² ¹ Department of Mechanical Engineering, The University of Sheffield, Sheffield, United Kingdom ² Department of Mechanical Engineering, The University of Alabama, Tuscaloosa, AL, United States

Abstract

When considering alternative fuels, it is important to measure and evaluate the emissions produced when burning them in gas turbines so that an informed comparison can be made with conventionally derived fuels such as Jet-A1 and JP-8. To this end, this chapter focuses upon the current state of the art in relation to emissions measurement from the combustion of alternative fuels in aviation gas turbines.

Keywords

Alternative fuels; CO; CO2; Gaseous emissions; NOx

1. Introduction

When considering alternative fuels, it is important to measure and evaluate the emissions produced when burning them in gas turbines so that an informed comparison can be made with conventionally derived fuels such as Jet-A1 and JP-8. To this end, this chapter focuses upon the current state of the art in relation to emissions measurement from the combustion of alternative fuels in aviation gas turbines. Gas turbine emissions are generally separated into two categories namely, gaseous and particulates. Particulates, as the name suggests, are physical particles emitted from the combustion process. These are generally solids but may also include liquids. Gaseous emissions are, as the name suggests, formed of gases such as CO2, NO, and NO2. The major gaseous emissions generated during combustion in an aviation gas turbine can be classified as shown in Tables 1 and 2 [1]. The clear majority of the mass fraction for gaseous emissions from gas turbines is from CO2, which is considered a greenhouse gas (GHG). This is corroborated by a study conducted by Turgut et al. [2] whose team conducted a large-scale commercial turbine engine gaseous emissions study on conventional fuels using Turkish airlines CFM-56 engines. This study clearly shows the behavior of CO, NOx, and CO2 emissions with respect to fuel flow rate and engine throttle position.

Table 1

Major emissions species

Typical concentrations (v/v)

Source

66%–72%

Inlet air

Nitrogen (N2)

12%–18%

Inlet air

Oxygen(O2)

1%–5%

Oxidation of fuel carbon

Carbon dioxide (CO2)

1%–5%

Oxidation of fuel hydrogen

Water vapor (H2O)

Adapted from Pavri R, Moore GD. Gas turbine emissions and control. Atlanta,GA; n.d.

Table 2

Minor emissions species

Typical concentrations (ppmv)

Source

20–220

Oxidation of atmospheric nitrogen, fuel bound nitrogen

Nitric oxide (NO)

2–20

Oxidation of atmospheric nitrogen, fuel bound nitrogen

Nitrogen dioxide (NO2)

5–330

Incomplete combustion of fuel carbon

Carbon monoxide (CO)

Trace-100

Oxidation of fuel sulfur

Sulfur dioxide (SO2)

Trace-4

Oxidation of fuel sulfur

Sulfur trioxide (SO3)

Adapted from Pavri R, Moore GD. Gas turbine emissions and control. Atlanta,GA; n.d.

Fig. 1 denotes the CO and NOx emissions observed in this study as a function of both fuel flow rate and N1C which the corrected fan speed of the engine that equates to the thrust produced by the engine. As can be seen, the amount of CO is very high at low equivalence ratios and exponentially decreases to almost negligible amounts as the fuel flow rates are increased. The NOx, on the other hand, shows a reversed trend and peaks as the fuel flow and fan speed peaks. Both these phenomena are well described in literature, and this study serves as a good demonstration of how emissions are formed in the gas turbine cycle. CO is a result of incomplete combustion [3], and therefore, it is logical for there to be higher amounts of CO present at low equivalence ratios and low fan speeds, as there is not enough airflow to completely combust the fuel. This problem is mitigated with increasing fuel and air flow through the combustion chamber. NOx meanwhile is produced in high-temperature zones of the combustion chamber where the extreme temperature causes atmospheric nitrogen in the airflow to get oxidized to form either nitrogen monoxide or dioxide, both of which are harmful to both humans and the environment. Therefore, from Fig. 1, it is logical that the NOx rate increases as the fuel flow and fan speed increases, as this corresponds to a general increase in combustion temperature.

Figure 1 CO and NOx emissions for a CFM-56 engine running on Jet-A1 as a function of fuel flow rate and N1C the corrected fan speed [2].

Turgut et al. [2] further go on to describe the variation in CO2 emissions during gas turbine operation cycles as described in Fig. 2. CO2 is the product of complete combustion, and it is expected that the amount of CO2 would increase with the increase of the fuel flow rate and the fan speed in question. This holds true when compared with the reaction equation for stoichiometric combustion of hydrocarbons in the presence of air.

[1]

From the aforementioned equation, it is clear that the formation of CO2 is set in stone when combustion is involved and can only be reduced by reducing the carbon input to the combustion process, which is not very practical. And CO is produced when the hydrocarbon fuel is burned in a low oxygen environment where not enough oxygen molecules are available for complete combustion to occur.

Figure 2 CO2 versus fuel flow rate for CFM-56 gas turbine running on standard Jet-A1 [2].

SOx (oxides of sulfur) in gas turbine emissions are trace in amounts and, in general, are solely derived from sulfur present in the fuel. Oxides of sulfur were a major concern in the past, as these can combine with water vapor to form sulfuric acids, which is a very harmful substance. SOx is also a contributing factor to acid rain, which is harmful to the environment. Therefore, control of sulfur has always been controlled by limiting the amount of sulfur allowed in the fuel specification. Current specifications for jet A1 in the United Kingdom specify sulfur content to be limited at maximum 0.3% fraction of mass [4].

2. NOx formation mechanisms

Much like the process for CO and CO2, NOx can also be produced using fuelborne nitrogen. However, unlike them, the vast majority of NOx is produced from the reaction of combustion gases with diatomic nitrogen from atmospheric air. This process is known as thermal NOx formation. The final mechanism for the formation of NOx is termed prompt NOx, which is the mechanism in which the least amount of NOx is produced and involves the reaction of atmospheric nitrogen with hydrocarbon radicals (i.e., CH2, CH, etc.) to form various unstable species such as NH (nitrogen monohydride), which can then easily oxidize into NOx. Thermal NOx, in general, produced at very high temperatures when the atmospheric nitrogen reacts with atmospheric oxygen. This phenomenon starts becoming a significant issue at temperatures above 1500°C [6]. Therefore, it is of utmost importance to design engines that combust below this temperature if ultralow emission is an objective. The main reactions of thermal NOx production are known as the Zeldovich mechanism portrayed in the following [6]:

The former two equations were first brought to the fore by Zeldovich, and the final reaction was added by Lavoie et al. [7,8]. Fenimore conducted the preliminary work on another type of combustion NOx named prompt NOx [9]. They go on to explain the relationship between the equivalence ratio of the flame (richness) and the formation of prompt NOx. It is also of note that prompt NOx s for around 27% of total NOx formed during under normal operating conditions of gas turbines. The reaction mechanism for prompt NOx is as follows:

Lefebvre then goes on to explain that NOx formed by the thermal mechanism and the HCN from the prompt process oxidize in lean conditions to form nitrogen monoxide as follows:

However, it should be noted that in aviation and stationary gas turbine, thermal NOx is the driving force behind the total amount of NOx produced.

3. Gaseous emissions and alternative fuels

A study conducted by Corporan et al. [10] analyzed, among other aspects, the gaseous emissions produced by several commercial alternative jet fuels while running on an Allison T63-A-700 Turboshaft engine. The fuels in question were derived using Fischer–Tropsch (FT) processes from various feedstocks, i.e., fuels form Shell and Rentech were derived from natural gas, whereas the fuel from Sasol was derived from coal. The other types of fuel used in this investigation are hydroprocessed renewable jet fuels. The fuels had several common characteristics in that they had very low or no aromatic content, higher hydrogen content, and lower density and negligible sulfur content with respect to the baseline reference fuel JP-8. The gaseous emissions that were measured in this investigation were collected using FTIR (Fourier transform infrared spectroscopy)-type analyzers. The authors have concluded that their results showed negligible impact on CO2, NOx, and formaldehyde. The significant finding of this work has been a reduction in the amounts of CO and UHC (unburned hydrocarbon). The results of which are portrayed in Fig. 3. As is visible from Fig. 3, all the fuels performed significantly better in of UHC and CO at idle engine condition with respect to JP-8. The lowest emissions were from the Shell FT fuel. This may be attributed to the fact that it had the highest hydrogen content of all the tested fuels.

Figure 3 CO and UHC emissions of alternative fuels normalized to JP-8 [10]. UHC, unburned hydrocarbon.

A study conducted by Cain et al. [11] has observed the gaseous emissions of a turboshaft-type engine running on several fuels, both conventional and alternative fuels. The main conventional fuel tested by them was JP-8 (kerosene for military applications with several minor additives with respect to standard Jet A). Among the alternatives tested was a fully synthetic paraffinic kerosene (SPK) as well as several component mixtures such as m-xylene (an aromatic compound) and methylcyclohexane (MCH) and finally some aliphatic hydrocarbons such as heptane and octane. The emission indices they obtained for CO and CO2 are depicted in Fig. 4. As can be seen from Fig. 4, the alternative fuel SPK performs better than JP8 at all engine conditions; however, the difference increases as the engine power increases to the benefit of SPK. Furthermore, the worst performer in of CO and CO2 was m-xylene, which may be because of its aromaticity. The best performer was octane, which is an aliphatic hydrocarbon and therefore the easiest to burn cleanly as per Eq. [1]. This is in effect because the aliphatic compounds have a higher H/C (hydrogen to carbon ratio) generally due to the compounds in question being saturated compounds (no double bonds). Moreover, it should be noted that as engine power increases, the amount of CO2 increases dramatically; however, the amount of CO decreases almost proportionately, which can be attributed to the instances of incomplete combustion decreasing with increasing engine power. This is explained by the fact that the engine is designed to run most efficiently at higher power ratings. Hence, at low engine power ratings, it is inefficient and is not combusting the fuel–air mixture adequately.

Figure 4 Emission indices for CO2 on the left and CO on the right with respect to engine power for six fuels [11].

Another study has been conducted by Lobo et al. [5] where two FT process– derived fuels were tested in an auxiliary power unit against reference Jet-A1. The two fuels have been termed GTL (gas-to-liquid) and CTL (coal-to-liquid). The emissions and UHC data for the tests conducted are depicted in Tables 3 and 4. As can be seen, the NOx emissions between Jet A1, GTL, and the 50:50 blend show no significant differences at both idle and full power; however, the CTL fuel does show a 5% reduction in NOx with respect to Jet-A1. This difference has been attributed to chemical differences between the alternative and conventional fuels in the sense fuel-borne nitrogen is lower if not nonexistent in the alternative fuel [12,13]. Furthermore, the CTL fuel has presented a 7% increase in the UHC, whereas the GTL fuel has presented a 40% reduction with respect to Jet-A1.

Table 3

Fuel

NOx (g/kg fuel)

CO (g/kg fuel)

UHC (g/kg fuel)

Smoke number

2.26 ± 0.09

70.3 ± 0.3

39.8 ± 2.8

10.3 ± 0.6

Jet-A1

2.13 ± 0.09

67.0 ± 0.3

42.7 ± 2.7

2.6 ± 1.5

CTL

2.30 ± 0.09

63.4 ± 0.2

25.6 ± 3.1

0.7 ± 0.6

GTL

2.31 ± 0.09

65.2 ± 0.2

28.5 ± 3.0

2.0 ± 1.0

50:50 GTL Jet-A1

CTL, coal-to-liquid; GTL, gas-to-liquid; UHC, unburned hydrocarbon.

Table 4

Fuel

NOx (g/kg fuel)

CO (g/kg fuel)

UHC (g/kg fuel)

Smoke number

4.10 ± 0.05

28.3 ± 0.5

8.5 ± 3.4

27.3 ± 0.6

Jet-A1

3.87 ± 0.05

27.3 ± 0.5

7.0 ± 3.3

10.3 ± 1.2

CTL

4.00 ± 0.05

28.3 ± 0.5

5.2 ± 3.5

0.7 ± 0.6

GTL

4.01 ± 0.05

28.4 ± 0.5

6.0 ± 3.4

6.3 ± 0.6

50:50 GTL Jet-A1

CTL, coal-to-liquid; GTL, gas-to-liquid; PM, particulate matter; UHC, unburned hydrocarbon.

Furthermore, a separate study has been conducted by Chiariello et al. to study the emissions effects of a microgas turbine (MGT) running on several blends of vegetable oil [14]. The vegetable oils in question (rapeseed and sunflower) had been blended with Jet-A1 in two ratios by volume: 10% and 20%. This generates a fuel matrix of five fuels, namely, fuel K (pure Jet-A1), fuel A (90% Jet-A1 and 10% rapeseed oil), fuel B (80% Jet-A1 and 20% rapeseed oil), fuel C (90% JetA1 and 10% sunflower oil), and fuel D (80% Jet-A1 and 20% sunflower oil). A graph of the NOx and CO emissions from the five fuels for two power conditions of the MGT (15 and 25 kW) is portrayed in Fig. 5. Fig. 5 indicates that the addition of the two vegetable oils only has a marginal impact on the gaseous emissions of the MGT with respect to Jet-A1. This can be attributed to the small concentration of the SVO in the blends. However, it should be noted that this figure illustrates the formation of thermal NOx very well; at low power settings, the NOx emissions are a quarter of that at the higher power settings. This is due to the lower combustion temperatures at the lower power settings.

Figure 5 NOx and CO data from the MGT [14]. MGT, microgas turbine.

Figure 6 NOx emissions index for three types of t7xx engines according to their power settings [13].

A further study has been conducted by Corporan and Cheng investigating the performance of three helicopter engines running on FT-derived fuel and Jet-A1 [13]. Fig. 6 indicates the three engines that have been tested; the T700 running on Jet-A1 and the two T701C engines one running on Jet-A1 and the other running on FT-derived fuel. As is corroborated by the other sources in this chapter, the NOx is only marginally affected by the fuel composition [13] (Fig. 6). Furthermore, Corporan and Cheng go on to point out that the CO emissions for the engine running on the FT fuel have a slight reduction in the order of around 5%–10% as is shown in Fig. 7.

4. Conclusion

In conclusion, it can be noted that fuel properties have a limited impact upon the gaseous emissions characteristics of gas turbine engines running on liquid alternative fuels. This is because the rates of production for CO and CO2 are bound by combustion chemistry and can only be reduced by removing the carbon from the fuel, in which case it would become a hydrogen-based fuel, which brings upon its own challenges, such as increased NOx production due to the higher flame temperatures associated with hydrogen combustion. NOx production also is not affected by fuel properties alone when fuel-borne nitrogen is disregarded (alternatives contain no fuel-bound nitrogen) since it is atmospheric nitrogen that contributes toward NOx emissions in gas turbines. However, it should be noted that NOx can be avoided by using new technology such as oxy-fuel combustion (the process of burning the fuel in a pure oxygen environment instead of atmospheric air, but the practicality of such techniques for aviation gas turbines is questionable). To this end, it must be concluded that the gaseous emissions characteristics of alternative fuels are similar to conventional fuel in use today according to the sources reviewed in this chapter [3,5,13–16].

Figure 7 CO emissions index from Corporan and Cheng [13].

References

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Technology . 1970;1:313–326. doi: 10.1080/00102206908952211. 9. Fenimore C.P. Formation of nitric oxide in premixed hydrocarbon flames. Symposium (International) on Combustion . 1971;13:373– 380. doi: 10.1016/S0082-0784(71)80040-1.

10. Corporan E, Edwards T, Shafer L, Dewitt M.J, Klingshirn C, Zabarnick S, West Z, Striebich R thermal stability, seal swell, and emissions studies of alternative jet fuels. Energy & Fuels . 2011;25:955–966. doi: 10.1021/ef101520v.

11. Cain J, Dewitt M.J, Blunck D, Corporan E, Striebich R, Anneken D, Klingshirn C, Roquemor Wal R. Characterisation of gaseous and particulate emissions from a turbo-shaft engine burning conventional, alternative, and surrogate fuels. Energy & Fuels . 2013;27:2290–2302. doi: 10.1021/ef400009c.

12. Timko M.T, Yu Z, Onasch T.B, Wong H.W, MiakeLye R.C, Beyersdorf A.J, Anderson B.E, Thornhill K.L, Winstead E.L, Corporan E, Dewitt M emissions of gas turbine engine combustion of a fischer-tropsch synthetic fuel. Energy & Fuels . 2010;24:5883–5896. doi: 10.1021/ef100727t. 13. Corporan E, DeWitt M.J, Klingshirn C.D, Striebich R, Cheng M.D. Emissions characteristics of military helicopter engines with JP-8 and Fischer-Tropsch fuels. Journal of Propulsion and Power . 2010;26:317– 324. doi: 10.2514/1.43928. 14. Chiariello F, Allouis C, Reale F, Massoli P. Gaseous and particulate emissions of a micro gas turbine fuelled by straight vegetable oil–kerosene blends. Experimental Thermal and Fluid Science . 2014;56:16– 22. doi: 10.1016/j.expthermflusci.2013.11.013. 15. Khandelwal B, Cronly J, Ahmed I.S, Wijesinghe C.J, Lewis C. The effect of alternative fuels on gaseous and particulate matter (PM) emission performance in an auxiliary power unit (APU). Aeronautical Journal . 2019;123:617– 634. doi: 10.1017/aer.2019.16. 16. Corporan E, DeWitt M.J, Belovich V, Pawlik R, Lynch A.C, Gord J.R, Meyer T.R.Emissions

characteristics of a turbine engine and research combustor burning a FischerTropsch jet fuel. Energy & Fuels . 2007;21:2615–2626. doi: 10.1021/ef070015j.

Chapter 5: Impact of alternative fuels and fuel properties on PM emissions

Vamsi Krishna Undavalli ¹ , and Bhupendra Khandelwal ² ¹ Moscow Aviation Institute (National Research University), Moscow, Russian Federation ² Department of Mechanical Engineering, The University of Alabama, Tuscaloosa, AL, United States

Abstract

This chapter aims to provide the readers with material required to understand the particulate matter (PM) emissions and its adverse effect on human health, environment, ecosystem and what has been done from an fuels and engines prospective to reduce PM emissions. It also describes the factors that are responsible for PM emissions in aviation industry, such as from airports and aircraft powerplants which inlcudes primary engines and auxiliary power units. With growing concerns on environemntal pollution, International Civil Aviation Organisation (ICAO) imposed restrictions to regulate the PM emissions with Committee on Aviation Environmental Protection -11 regualtory norms (CAEP11). There is urgency to meet these regulations at all stages of aviation operational cycles. Conventional hydrocarbon based fuels produce more PM emissions than alternative fuels due to presence of high aromatic content in conventional fuels. In particular, the aromatic content in the fuels serve as the major soot precursors; however, a certain minimum aromatic content is required to meet compatibility requirements of aircraft systems such as elastomers, sealing of systems, and so on. To reduce this harmful emission, it is essential to

reduce the aromatic content in the fuels as suggested by different research studies. So this chapter in particular addresses the recent developments in a systematic manner from fuel selections, fuel properties, aromatic content, and aromatic species to PM emission reduction.

Keywords

Alternative fuels; Aviation; Impact of fuel properties on PM emissions; PM emissions; Sustainable fuels

Nomenclature

ACARE Advisory Council for Aeronautics Research in Europe

ASTM American Society for Testing and Materials

ATFs aviation turbine fuels

ATJ alcohol to jet

CAEP Committee on Aviation Environmental Protection

M condensable particulate matter

CTL coal to liquid

EMA electric mobility analyzer

EPA Environmental Protection Agency

FAME fatty acid methyl esters

FPM filterable particulate matter

FT Fischer–Tropsch

GHG greenhouse gases

GTL gas to liquid

H/C hydrogen-to-carbon ratio

ICAO International Civil Aviation Organization

LBO lean blowout

LII laser-induced incandescence

nvPM nonvolatile particulate matter (mass and number)

PAHs polyaromatic hydrocarbons

PM particulate matter

PM0.1 particulate matter less than 0.1-micron meter

PM10 particulate matter less than 10-micron meter but greater than 2.5micron meter

PM2.5 particulate matter less than 2.5-micron meter but greater than 0.1micron meter

SAFs sustainable aviation fuels

SPK synthetic paraffinic kerosene

vPM volatile particulate matter

1. What are particulate matter emissions?

Particulate matter (PM) emissions are often termed as particle emissions referring to presence of small solid or liquid particles in the exhaust gases. PM emissions are result of formation of soot or black carbon, which are frequently used by general public, and these are usually referring to fine or ultrafine particles in the environment. Airborne PM is nearly ubiquitous due to a vast number of available sources (e.g., gas phase reactions occurring within the fuel's combustion, abrasion, particle resuspension, etc.). However, a significant portion of very-small-size PM results from the combustion process. Both solid and liquid particles are included in this category, the composition being diverse with carbonaceous particles, nitrates, and sulfates besides including other compounds. The PM emissions can be categorized into volatile particulate matter (vPM) and nonvolatile particulate matter (nvPM). The PM emitted especially through gas turbines is a complex pollutant composing particles of solid in nature and also varies in size, shape, and chemical composition (Fig. 1).

1.1. Volatile particulate matter

Volatile particulate matter (vPM) is formed due to nucleation of gaseous precursors mainly containing sulfuric acid and other such organic compounds, that are generally formed in the colder regions of downstream combustion wake [1,4,22]. vPMs are prone to change their state from gaseous to solid when they are existing into atmosphere depending upon the local temperature, pressure, and relative humidity as the exhaust gases are relatively at high temperatures to atmospheric conditions [8]. In some cases, PM is condensed to liquid, and it is coated to the solid particles when cooled down in the exhaust, as shown in Fig. 2. These multiphase particles are also called condensable PM (M). M transforms from gaseous state to solid/liquid immediately after leaving engine exhaust. These gaseous PMs will either condense on existing fine particles or

form a new core for aggregation (nucleus).

Figure 1 PM particle size and variation from EPA [14]. EPA, Environmental Protection Agency; PM, particulate matter.

Figure 2 Volatile and nonvolatile PM formation [35]. PM, particulate matter.

1.2. Nonvolatile particulate matter

Nonvolatile particulate matter (nvPM) remains solid, after leaving engine exhaust. These particles will not change their physical state due to external atomposheric conditions such as pressure and temperature of the surrounding environment. But the smaller nvPM particles can accumulate to form bigger particles as shown in Fig. 2. The nvPM emissions mainly consist of carbonaceous materials, which are also termed as soot or smoke emissions. There also exists a term called filterable PM (FPM) which is usually correlated to nvPM terminology. FPM describes about the content of PM particles that can be captured through a filter on its release from the fuel burning during the engine testing period. In addition to vPM and nvPM, other such as primary and secondary PM are also used. This differentiation is also important when assessing the environmental impact of PM emissions and also the state of particle emissions. Primary particles are described as particulate emissions that have been emitted into the atmosphere (including nucleation which is dealt in latter section of the chapter) and are readily available to undergo physical and chemical changes simultaneously through dispersion or transport phenomenon. On the other hand, the secondary PMs refer to those that are formed due to transformation of precursor gases and primary PM; however, they are not directly emitted from the engine. The formation of secondary PMs is influenced by various parameters such as atmospheric conditions including pressure and humidity, availability of other compounds or atoms, and their concentrations. The summary of primary and secondary PM formation paths is depicted in Fig. 3 according to Chang et al. [10]. This primary and secondary fine PMs to the total fine PM in the local environment, thus effecting environment by degrading the visibility.

Figure 3 Typical primary and secondary particulate matter formation pathways.

1.3. Particle size

PM, soot, and black carbon particles are a complex fractal structures of primary particles. Depending on parameters such as temperature and pressure during the development phase, the size of aggregate will grow. Each aggregate or individual usually has some irregularity in shape and geometrical complexity, which causes difficulty in determining the size. Therefore, the term “equivalent diameter” is introduced to describe size of aggregate. Equivalent diameter refers to a perfect sphere particle, which has the same behavior of electrical mobility, or the aerodynamic equivalent diameter compared with a single PM aggregate or particles. Depending on the specific value of the equivalent diameter, praticles are classifed on basis of various size modes and usually divided into three categories-coarse, fine, and ultrafine as shown in Fig. 4. Course particles feature diameters in the order of 2.5 –10 μm (PM10), fine particles have diameters less than 2.5 μm (PM2.5), and ultrafine particles have diameters less than 0.1 μm (PM0.1). Based on the particle formation, Kittelson [10] described three modes. “Nuclei mode” refers to the particles that have little probability to grow by condensation or through coagulation. A course particle typically consists of volatile organic and sulfur compounds and also solid carbon and metal compounds. The “accumulation mode” refers to the particles in growth phase through either condensation or coagulation. This mode consists of agglomerated nvPM and volatile material condensed on the nvPM where soot and adsorbed material exist. The “coarse mode” consists of restrained accumulation mode particles and crankcase fumes.

Figure 4 PM particle size, particle number, and particle mass size distribution [11]. PM, particulate matter.

1.4. Particulate matter formation mechanism

The formation of PM in aviation and industrial gas turbines is a result of interactions between inorganic and organic compounds of fuel and the working fluid in the engine, in most cases its air. In addition to that, there can be catalysts that trigger PM formation such as steam (generally used in industrial gas turbines) fed to limit NOx by forming solid precipitation and also erosion of metals in the flow tract. Due to the high energy density in the combustor, the inorganic compounds added through the fuel–oxidizer mixture would undergo physical and chemical changes. These changes are dependent on the compound properties and operational conditions of combustor, but nevertheless, at engine exhasut, the PMs emitted are composed of nonvolatile inorganic compounds. These changes may also result in particle size reduction and degradation of oxidation, thus leading to pyrolysis. In general, this is due to the presence of inorganic compounds predominantly in heavy liquid fuels compared to gaseous fuels. Additionally, the presence of organometallics in fuels, in particular, as additive leads to formation of inorganic metal oxides on flame oxidation thus forming PM. The mechanism of PM formation from fuel molecules during combustion is been a major research subject for decades. But through numerous flame studies [15,19,34], a commonly held mechanism is established as shown in Fig. 5. This mechanism of hydrocarbon fuel molecule evolution from liquid or vapor to solid PM can be described in a five-phase process, that is, pyrolysis, nucleation, coalescence/surface growth, agglomeration, and oxidation, which will be discussed in detail in Section 5.3. All the aforementioned processes occur almost simultaneously but start with the precursor formation, i.e., polyaromatic hydrocarbons (PAHs), which are highly influenced by the aromatic content in the fuel. The availability of aromatic content in fuel results in enhanced and quicker formation of PAHs when compared to the fuels with least aromatic content

[1,19]. In addition to fuel properties, there are other factors related to combustion that greatly contribute to nvPM formation, which will be later discussed in the chapter. One of such vital factors in alternative fuels is kinetic time delay for pyrolysis when compared with conventional fossil fuels. Due to the lack of aromatic content in alternative fuels, they need to intially form sufficient quantity of PAHs to eventually form PM matter; thus, kinetic time delay imposes a restriction on alternative fuels compared to the conventional fuels. The modern aero combustors possess a high turbulent flow kinetics, which provides a better dilution and low local equivalence ratios resulting a further delayed PAH formation. With alternative fuels that are either pure or blended with conventional fuels, their complete fuel–air mixture atomization time period is well within the kinetic time delays; thus, these blended fuels form least PM compared with pure conventional fuels [34,40]. These blended fuels also result in decreased particle size as the primary particle's sizes are influenced by the incomplete pyrolysis and insufficient PAH precursors.

Figure 5 nvPM formation in Jet-A compared with alternative fuels with paraffinic molecules. nvPM, nonvolatile particulate matter.

2. Impact of particulate matter emissions

PM emissions lead to the deterioration of air quality, thereby causing a significant risk to local air quality, health, and the climate. At present, many countries and the related international regulatory organizations have air quality requirements specifying the maximum permissible levels of PM, besides other emissions in the atmosphere for a period of time to minimize the harmful effect of the pollutant. So most of the countries try to address the impact of PM emissions with limitations of PM 2.5 and PM 10 emission for either 24 h or annual period of cycle. PM limits proposed by some of the countries and the WHO are depicted in Fig. 6; though these limits have been only instructive, in real-time measurements, the emissions are far higher than the proposed or adopted limits. This is a grave situation, and many regulatory organizations are working hand in hand with all spectrums of the industry to limit the emissions.

Figure 6 PM2.5 annual regulatory limit for various countries [45]. PM, particulate matter.

2.1. Environmental impact

High concentrations of PM emissions, in particular position of atmosphere, could reduce visibility significantly, and at present, many countries such as China, India, United States, and others are greatly affected with the bad air quality. Low visibility may increase the number of car accidents and negative impacts on public lives through lung disease, asthma etc. Normally, PM particles contain sulfur, which could pollute water, river, and lake, and the nutrition balance within soil and water will also be contaminated. In addition, since it could exist in the atmosphere and propagate through wind, PM could contribute to the formation of acid rain. Moreover, materials in the open air such as building and statues could also be damaged by PM emissions. Most importantly, they greatly affect the whole ecosystem. PM deposition on snow lowers the albedo and thus causes the glaciers to absorb more heat resulting in accelerated melting. Glaciers in the Himalayas are melting at unprecedented rates as a result of global warming. It is estimated that by the year 2100, up to 5500 glaciers in the Hindu-Kush region would be melted by 70%–90% [33] which further affects the potential of hydropower. Also, PM deposition on plants is easily visible in the regions of high-content PM such as construction sites; these depositions affect not only the physical structure of plant life but also chemical structure depending on the chemical composition of deposited particles. Studies also found that toxic effects of PM in plants are due to the acidity, metal content, nutrient, surface properties, and salinity [13].

2.2. Health impact

A large number of studies have emphasized the relation between PM pollution and human health problems. Health problems due to PM can be of short-term and long-term. Small and fine particles such as PM 2.5, especially those smaller than 10 nm, could easily go deep into human lungs and even inside blood vessels. These may prone to deadly diseases such as heart attack asthma and lung damage on prolonged exposure to PM environment [14]. Statistics shows that there is a proportional relationship between morbidity of respiratory disease and PM exhaust. Since PM will be accumulated inside lungs and difficult to treat, the damage of respiratory system is both short and long term. It is therefore widely acknowledged in the medical community that PM emissions have adverse health effects, including several-from deteriorating lung conditions to cardiopulmonary diseases [13].

3. Measurement of particulate matter emissions

The characterization of PM emissions is vital to standardize the maximum permissible pollution levels. Measurement of PM emissions in gas turbines and jet engines is one of the complex procedures, and still, it is within the phases of procedure evolution. Currently, the mass and number concentration are the regulatory measures for the aviation according to ICAO—Committee on Aviation Environmental Protection (CAEP) 11 [16]. Majority of PM measuring instruments are measuring either the PM mass/number emission index or size distribution. Though a brief summary is presented on nvPM measurements in this section, we encourage readers to refer public domain materials [20,27,29,43]. The most common and simple method to measure PM emissions from an engine exhaust is using weighing method such as gravimetric method. In this method, a filter is used to collect PM from the exhaust. Filter will be weighing before and after the experiment to determine the emitted particles mass. Since PM and filter are extremely light in weight and are highly affected by environmental conditions (e.g., air humidity), extreme accurate measurement is required. Instead of weighting filter, other techniques could also be used to measure the PM such as optical method. Optical methods basically measures the reflectivity of PM and “smoke number” for aviation engines. Some of these techniques will be discussed in detail in the following section. But the measurement techniques are not only limited to these, but there are also a wide variety of instruments used across the industry. A brief view of such methods is tabulated in Table 1, and basically, these measuring instruments can be classified in three categories such as mass concentration on filter-based, particle size, and chemical speciation. The selection of a particular measuring instrument is affected by its correlation to gravimetric measurements, instrument response time, lower detection limit, minimum upper range, particle size, and traceable measurements.

3.1. Gravimetric method/filter-based method

Gravimetric method is the most widely used technique to measure the PM suspended in air. This method determines PM concentration based on weight difference of filters pre- and postsampling. So always an additional analytical method is required to analyze the sample when they are collected under gravimetric sampling. The procedure is such that two preweighted filters are placed one on top of the other in single filter holder. The first filter toward the collector is sample filter and the other is the control filter. PM is collected on the sample filter. Both the filters are weighed after sampling, and any change in control filter is adapted as correctional factor for weighing sampling filter. The precision of the gravimetric analysis relies on the minimization of electrostatic effects, regulation of moisture or other volatiles absorption, stable temperature, and utmost caution in the handling of samples. Gravimetric methods require less expertise, apparatus, and resources compared with other methods. Once it is calibrated to the specifications that are defined by weight, the gravimetric approach provides a convenient, economical, and highly reproducible measurement. This method is recommended by American Society for Testing and Materials (ASTM) International standards D2274 for Petroleum Products and for D2276 for Aviation Fuels in measuring PM emissions.

Table 1

Instrument

Property measured

Principle

Mass concentration on filter based Mass

Beta attenuation

60 s

Mass

Oscillatory inertial microbalance

2 s

Mass

Resonance frequency

60 s

Particle size and number Size, number

Electric mobility, electrical detection

200 s

Size, number

Light scattering

secs

Size, number

Electric mobility, condensation particle counting

16 s

Size, number, density

Electric mobility, impaction, electrical detection

2 s

Size, number

Impactor principle, electrical detection

1 s

Size, number

Time of flight

secs

Soot

Photoacoustic absorption

1 s

Black carbon

Absorption photometry

2 min

Chemical speciation

Table Continued

Instrument

Property measured

Refractory carbon

Laser-induced incandescence

Precursor gas speciation

Absorption spectra

Organic and elemental Carbon of filterable particulate matter

In situ thermal/optical carbon analyser

Soot

Light extinction and opacimeter

3.2. Laser-induced incandescence

Laser-induced incandescence (LII) is a laser optical measurement technique to obtain the results in real time, and it also provides measurements in volume fraction. Under LII technique, the sample exhaust will be heated by a pulsed high-energy laser beam. Principle of LII is to energize the sample by heating the sample using pulsed laser systems to temperatures ranging from 2500 to 4500 K depending on the laser system equipped [39]. The basic theory is that particles present in exhaust will absorb laser energy and increase the light emission, which is captured by the sensor and further detects the particles incandescent. Intensity of emission is attributed to the volume fraction. On instrument calibration, quantitative soot volume fraction measurements can be performed readily with high temporal and spatial resolution [43]. Since this is a real-time method, LII provides a relative convenient path to monitor PM emissions continually. However, there are some drawbacks. The temperature of PM is extremely high, but it is required not to exceed sublimation point. However, still there will be some mass loss during the heating. Some portion of particles such as volatile PMs are sensitive to high temperature and will vaporize during the heating process. Therefore, LII can only detect nvPM. Apart from LII, there are other novel optical methods, which are investigated and validated recently. Optical method uses color intensity of combustor flame to determine the level of PM production. The basic theory of optical method is a little similar compared with LII. The instruments based on optical methods are generally used to measure the suspended particle size and particle count based on the principle of light scattering [43]. Often instruments based on optical methods cannot for some of the physical properties of particle such as refractive index, density, shape, and absorption. Due to this, instruments provide equivalent particle diameter corresponding to scattered light but not the real-time geometrical diameter of particle.

3.3. Electrical mobility analyzer

Conventional weighting methods could not provide the detailed information about number concertation and size distribution for a control volume of exhaust. Since PMs are extremely small, this particular method was developed to close the gap of traditional measuring methods. The electrical mobility analyzers (EMAs) are classifiers in accordance with particle size used in any particle detector or particle counters. As shown in Fig. 7, an EMA consists of two concentric cylinders where the particles enter into the concentric region through filtered air from top [43]. Through a charge neutralizer, particles with a charge of known magnitude and polarity are obtained. Particles entered into classifier through the outer cylinder wall slot are deflected towards the inner cylinder wall upon imposed electric field. The trajectory of the particles is dependent on voltage applied, and thus, segregation of particle sizes is possible by applying the attractive electric field on the collector rod. Particles of a finite mobility range are collected by the annular inlet in the center cylinder and counted by condensation particle counter [43].

Figure 7 Working scheme of differential electrical mobility analyzer. Adopted from Roger WW, Chapter 4 - Size analysis and identification of particles. In: Mittal, Kohli R, Kash, editor. Developments in surface contamination and cleaning. Wiiliam Andrew; 2012. p. 180–212.

4. Ways to reduce particulate material emissions

4.1. Combustor designs

In general, a conventional combustor could be divided into three zones: primary zone, intermediate zone, and dilution zone. Usually, primary zone is right after the fuel injector, which means fuel spray will significantly increase the local fuel–air ratio. With zones of high equivalence ratios especially primary zone, start to produce PM with influence of other factors such fuel air mixing, the fuel breakup and the evaporation, spray droplets diameter, injector nozzle design, and other various factors. According to the goals of ACARE Flightpath 2050, the industry is aimed to reduce 75% in CO2, 90% in NOx, and 65% in noise compared with 2000 [2]. Also, with technological advancements in materials and additive manufacturing at present, the modern combustors are expected to perform their operations at pressure ratio greater than 45 and combustor exit temperatures of 2100 K. With these operational requirements, the rate of NOx formation would increase drastically in modern combustors. The major players in the aero engines have redesigned the conventional combustors to meet modern demands, which adopted the schemes such as TAPS (twin annular premixed) by General Electric, lean premixed prevaporized phase 5 tiled by Rolls-Royce, double annular combustor by CFM International (consortium of General Electric and Safran), and TALON by Pratt and Whitney [41]. Most of these showed promising results in decreasing the emissions and meeting current industry demands, but still require technological improvements to meet the future needs. With the understanding of PM formation mechanism, and experimental investigations, it is found that PM initially forms in the fuel rich region generally in the primary zone of combustion. In conventional gas turbine design, fuel injection zone will cause high fuel/air ratio and high temperature, which enhances the generation of the PM. Air will be introduced into the secondary and dilution zones to finish the combustion. Some of the PM will be consumed through these zones. In short, primary zone determines the rate of PM formation, and dilution zone determines the rate of PM consumption [6]. In

general, the combustion in whole is affected by several factors that include equivalence ratio temperature, pressure, velocity, fuel droplet size, atomization, evaporation, minimum ignition energy, ignition system type and location, and ignition time delays, which are already dealt in detail in Chapter 3. Of all equivalence ratio is a vital parameter deciding the content of fuel availability to combust air, and thus, all the modern aeroengine manufactures adopting the lean combustor designs as stated earlier.

4.2. Airport emission control

Airport emissions are of two types: one is by airport itself and other is by aircrafts. The aircraft emissions in vicinity of airport are mapped out through its landing–takeoff (LTO) cycle. LTO cycle comprises of an operating mode of the aircraft and the approximate duration in the respective mode as shown in Table 2. The PM emissions are related to the rate of fuel consumption and incomplete combustion. High thrust operations will require high amount of fuel feeding and high-level thrust will be only required for some specific flying stages for an aircraft such as takeoff. Therefore, other phases of flight such as taxing will not require engine to operate at maximum thrust levels. So, these modes are prone to incomplete combustion, as the engine operates at off-design points during these modes of flight. Some of the airlines such as Virgin Atlantic has suggested to shutoff one or two engines during taxing to reduce emission and save fuel at same time [16]. Moreover, reducing takeoff thrust could also decrease PM emissions significantly. Thomson Airway suggested that reducing up to 33% takeoff thrust among their 737–800 fleet which depends on the takeoff condition including length of runway and weight of aircraft [16]. Suitable takeoff management systems could also be introduced to avoid unnecessary stop during the taxi to reduce fuel consumption and PM emission.

Table 2

Operating mode

Thrust setting (%)

Operation time (min)

100

0.7

Takeoff

85

2.2

Climb

30

4

Approach

7

26

Taxi

ICAO, International Civil Aviation Organization; LTO, landing–takeoff.

4.3. Jet fuel composition

Depending on the analysis of the PM emissions test campaign using different jet fuels, there is a conclusion that high sulfur and aromatic contents in the fuel play a vital role in PM formation. Aromatics provide fuel energy density and enhance seal performance in fuel system. Therefore, it is not trivial for fuel industry to develop aromatic-free jet fuels to reduce PM emissions without design changes in gas turbine engine by the respective stakeholder to counter the problem of incompatibility. The modern fuels usually referred as alternative jet fuels are produced entirely from different feedstocks such as plant oil, biomass, and so on through Fischer–Tropsch (FT) process compared with conventional jet fuel. If the fuels or their blends fulfill the respective ASTM specifications, they are referred to as “drop-in fuels.” The engine tests indicated that these alternative fuels could burn without any aromatic content, and tests observed the fuel leakage problem with these less aromatic content jet fuels [8,23]. Therefore, the jet fuel ASTM standard D1655 specifically indicates that aromatic content in Jet A-1 should be less than 25% to reduce PM emissions, but higher than 8% to prevent fuel leaking problem according to ASTM D1319.

5. Impact of alternative fuels on particulate matter emissions

The main motive for the development of alternative jet fuels is the reduction of the carbon dioxide emission of aviation. However, the reduction of PM emission is a positive side effect. So, the PM emissions are also expected to reduce with alternative fuels compared with conventional fuels. In practice, this is not the fact as most pollutants are dependent on one or many fuel properties that may have physical or chemical properties. For example, CO2 emissions are dependent on the H/C ratio of fuel, and with increase in H/C ratio, there would be a significant decrease in CO2 emissions. But this would be at the expense of decreased energy density. Many studies concluded that PM emissions would decrease with decreased aromatic content in fuel. So, with the usage of alternative fuels, the PM emissions would reduce due to lack of aromatic content in the alternative fuels. It raises other problems such as elastomer compatibility issues, which has been discussed in detail in Chapter 6. The PM emissions are also due to the fact of incomplete combustion, which is directly related to the combustor inefficiency of atomization, vaporization, and complete chemical reaction due to surface tension, density, and chemical composition. This casts questions such as which type of alternative fuels is better and which fuel properties needed to be focused in order to reduce PM emissions. So, in this section, we address the questions such as the impacts of different alternative fuels, fuel properties, aromatic content, and aromatic species on PM emissions.

5.1. Impact of different alternative fuels on particulate matter emissions

The field of alternative fuels is recently emerged and is still in the phase of evolutions where new drop-in fuels are being certified. Also, no concrete

knowledge base is available on performance of alternative fuels. Very few comprehensive research studies are conducted on the evaluation of alternative fuel emissions, and this research is still at the phase of genesis. It is even more difficult to anticipate combustion behavior and predict emissions with a diverse fuel composition, feedstock, and production pathways of alternative fuels. It becomes more cumbersome to predict the emissions by blending aviation turbine fuel (ATF) with conventional fuels. Considering the chemical composition of fuels and its effect on soot formation as shown in Fig. 8 and according to Zheng et al. [21] and other researchers, the order of soot formation is given as follows: naphthalene > alkylbenzenes > alkynes > cycloparaffins ≈ isoparaffins > n-paraffins. This order shows that the greater influence is felt by the presence of aromatics like naphthalene and alkylbenzenes in fuel. It is further ed by Won et al. [34] as the soot propensity decreases in the order of aromatics > cyclic alkenes > branched alkenes > linear alkanes. With the absence of aromatics in alternative jet fuels, it raises questions such as how the soot formation is affected by the type of ATF; it is advised to consider the paraffinic content and molecular size for understanding the soot formation among ATFs, which is especially ed by the case study of Xue et al. [44] where alcohol-to-jet (ATJ) fuels, which have low aromatic content, showed higher soot formation compared with FT-based synthetic paraffinic kerosene (SPK) fuels. Lobo et al. [30] conducted a comparative study of alternative fuels on a commercial jet engine CFM-56. In their study different types of alternative fuels based on FT conversion and biomass derived, compared to crude oil derived JetA1 fuel. Different blends are investigated such as 50% FT with Jet-A1, 100% FT fuel, Blend of Jet-A1 with fatty acid methyl esters (FAME) in different proportions 20%, 40% [29]. The jet engine operated on different thrust levels to obtain the data of complete LTO cycle for fuel blend that is under investigation. The results of the investigations clearly indicate that adoption of alternative fuels in blend or pure form decreases the PM emissions as tabulated in Table 3, with the greatest reduction in case of pure alternative fuel of 100% FT fuel blend. In practice, this is not a suitable fuel as its aromatic content is way below the suggested aromatic content by few researchers of minimum 8% [17,23] for elastomer compatibility. The reductions in PM emissions can be related to fuels' low aromatic content, increased H/C ratio, and also presence of oxygen. Oxygen content in methyl esters (FAME fuels) reduces soot emissions and therefore PM emissions [3]. FAME fuels, on the other hand, have a propensity to transport

metal pollutants from raw feedstocks, causing compatibility issues with fuel pipelines. However, the FAME fuels have tendency to carry over the metal contaminants from raw feedstocks, leading to compatibility issue with fuel pipelines. This is because the FAME producing methods and processes are not yet standardized to satisfy aviation specifications.

Figure 8 Tendency of soot to hydrocarbons [15].

Other alternative fuels derived from coal to liquid (CTL) and gas to liquid (GTL) show similar behavior of reduced PM emissions. This is concluded by the study of Williams et al. [28] under which they investigated fuels based on GTL, CTL, blends of GTL with Jet-A1, and biodiesel and diesel with Jet-A1 as reference fuel. Williams et al. explained the behavior of reduced vPM emission by measurement of organic matter concentrations [28]. They noticed that Jet-A1 organic mass emissions are greater than the emissions of CTL and GTL blends used at various thrust levels. In addition to that, they also suggested that vPMs are sensible to location of measurements as the vPMs condense to nvPM downstream the exhaust as temperature drops as shown in Fig. 9. Another one such study conducted by Christie et al. [32] investigates the effect of different alternative fuels and their effect on especially PM formed in the exhaust of gas turbine engine, where authors evaluated Sasol's fully synthetic jet fuel (FSFJ), Shell's GTL, kerosene, and Jet-A1/GTL 50:50 blended kerosene. From the results, it has been affirmed that all the alternative fuels formed less PAH compared to Jet-A1; thus, strengthening the statement of low aromatic content in alternative fuels leads to low nvPM emissions. Of the fuels investigated, GTL has shown the least PAH formation, while FSFJ forms the highest. Thus, indicating the use of GTL kerosene as blend or replacement to JetA1 would lead to improve local air quality by reducing the emissions. However, it has been said that the first-generation fuels such as GTL and CTL are FT fuels and have remarkable greater life cycle of greenhouse gases (GHGs) than petroleum-derived jet fuel. Although the short emissions are less compared with conventional fuel, in long term the carbon footprint and carbon life are of major concern in use of the GTL fuels.

Figure 9 vPM as function of probe location for GTL and CTL blends with Jet-A1 [28]. CTL, coal to liquid; GTL, gas to liquid; vPM, volatile particulate matter.

Table 3

Fuel blend

Specific gravity @15°C

Kinematic viscosity @20°C (mm²/s)

Heat content (kJ/kg)

0.797

4.27

43,300

1.92

0.808

4.74

42,000

1.94

0.825

5.62

40,300

1.94

0.776

4.4

43,600

2.04

0.755

4.65

44,100

2.17

Kumal et al. investigations on two biofuel blends of camelina in 30% and 70% of volume, which showed that with decreasing aromatic concentration, PM emissions also decreases [19]. The presence of biofuels would increase the paraffinic content, thus decreasing the aggregate size of PM. The behavior of PM aggregate size as a function of engine thrust with variation in aromatic content is shown in Fig. 10, thus showing Jet-A with highest particle size irrespective of any thrust levels. The particle size for blends did increase with engine rating but was comparatively low with Jet-A. This is attributed to cumulative effect of fuel properties at higher temperatures such as increased H/C ratio of biofuels with respect to Jet-A1. Within the framework of the Emission and Climate Impact of Alternative Jet Fuels (ECLIF), a project of the German Aerospace Centre, two campaigns were performed to investigate the emission properties of several alternative jet fuel blends [38]. The measurements were performed on ground and inflight. The ground measurements provide insight into the emission over the different power settings of the engine, while the flight measurements reveal the atmospheric impact. The experiment was performed with an A320 with IAE V2527 engines as source aircraft. Six different fuels were investigated in the 2015 campaign. The fuels were selected to feature different aromatic contents but with no strict correlation to the fuel hydrogen content. The experiment demonstrated that the hydrogen content of the fuel is a better predictor for PM emission than the aromatic content. Overall reduction of 70% in PM emission could be achieved with fuel blends that fulfills the respective ASTM requirements. Also, BraunUnkhoff et al. concluded that less soot particles emissions were reported in mass and in number concentration for GTL, HEFA, and farnesane fuel blends in comparison with Jet-A fuel [18,25].

Figure 10 PM aggregate particle size variation with engine thrust and aromatic content [19]. PM, particulate matter.

Under NASA Alternative Aviation Fuel Experiment (AAFEX) program, FTbased alternative fuels are systematically evaluated in comparison with conventional jet fuel JP-8. The alternative fuels are compared with JP-8 as blend of 50/50 and pure FT fuel. Experiments conducted on CFM56-2C1 engine configured to NASA's DC-8 aircraft parked on the ground and simulated all the LTO operating conditions through engine thrust settings as described in Table 2. The results shows that at all the operating conditions, PM emissions as a function of particle number density are in the following order: pure FT fuels < 50/50 blend of JP-8 and FT fuel < JP-8 [9]. Simultaneously with growing interests in alternative fuels in recent years, there has been a significant increase of end fuel testing programs by the airline industry and OEM manufactures. These programs include the Continuous Lower Energy, Emissions, and Noise (CLEEN) program project JETSCREEN by European Union, Alternative Jet Fuels and Environment(ASCENT) by Federal Aviation istration, National Jet Fuels Combustion Program (NJF) led by the Federal Aviation istration, the US Air Force Research Laboratory, and the NASA. All these studies concluded that sustainable aviation fuels are future and would help in achieving the emission reduction goals.

5.2. Impact of fuel properties on particulate matter emissions

A large group of fuel properties influence PM emissions such as density, viscosity, H/C ratio, cetane number, kinematic viscosity, boiling point, smoke point, surface tension, ignition delays, aromatic species and content, and also aromatic molecular weight. Some of these fuel properties directly influence the combustion process of atomization, vaporization, and oxidations while some indirectly, but in total, all these properties influence the PM formations by

effecting combustion performance. Although the trend varies with equivalence ratio, increasing average droplet diameter causes increases in not only PM emissions but also other gaseous emissions such as nitrogen oxide (NO), carbon monoxide (CO), and unburned hydrocarbon (UHC). PM emissions formation is significantly influenced by the particle size, particle size growth, and particle number. According to Braun-Unkhoff et al., other than fuel properties, adiabatic flame temperature, pressure, equivalence ratio, and kinetic/ignition time delay do influence PM formation through the effect of fuel pyrolysis [24,25,36,46]. Also, the engine thrust rating, which greatly affects the primary particle size, aggregates particle size, as with increase in engine rating, there is an availability of high temperatures locally, which in turn enhances the particles sizes. In regard to adiabatic flame temperatures and equivalence ratios, a study investigated by Riebl et al. [36] measured the PM emissions for four different FT fuels in comparison with two Jet-A1 and Jet-A fuels. The study measured that benzene and acetylene content at flame temperatures of 1800 and 2200K for equivalence rations ranging from 0.25 to 1.8, as these are primarily responsible in PAH formation. The study concluded that with increasing flame temperatures and equivalence ratios, benzene and acetylene contents increase irrespective of fuels. But in comparison with Jet-A1 and Jet-A, the GTL and CTL fuels show higher acetylene content at lower flame temperature of 1800K, whereas the JetA1 and Jet-A show higher acetylene content at 2200K. On the other hand, benzene content of Jet-A1 and Jet-A fuels is higher than that of CTL and GTL fuels for both flame temperatures [36]. So, it is very much evident that aromatic species is also a significant influencer in PM formation, which will be discussed in the later sections of this chapter. Fuels with lower boiling points provide a quick vaporization and less ignition delays, and thus the fuels do not participate in nucleation so the PM emissions would decrease with lowering boiling points. The longer ignition delays also mean more homogenous fuel mixture and thus maximum heat releases, thus promoting higher temperatures in combustion zones with limited oxygen availability in combustor primary zones resulting pyrolysis, thus promoting PM formation by restricting complete oxidation [26], which is explained in later section of PM formation. That being said, fuels with high boiling points contributed to high particle numbers according to studies of Roquemore et al. [42]. An equation for threshold soot index (TSI) as a function of smoke point and molecular weight of the fuel was given by Calcote et al. [15], whereas Qian et al. [31] argued that TSI is proportional to smoke point and molecular weight.

This has been further strengthened by experimental studies of Christie et al. [32] where a claim made that nvPM formation is linearly proportional to smoke number. One of the fuel properties that greatly influences PM emissions is aromatic content. A considerable number of studies already concluded that increasing in aromatic content leads to increase in PM emissions. Results of the study of Dewitt at al. substantiate that PM emissions are proportional to fuel aromatic content [23]. The data obtained in Dewitt et al. study show that FT-derived fuels emitted smaller particles compared with JP-8 fuel emissions. Another one such study conducted by Brem et al. [7] uses an in-production jet engine of high-by turbofan. Under this study, two petroleum-derived aromatic solvents were mixed to vary the aromatic content in the fuel. One aromatic solvent composed of 6% (v/v) naphthalene, and another was based on depleted naphthalene Solvesso 150 ND. Fig. 11 shows that increasing aromatic concentration leads to increased PM emission indices. Further in contrast to industry belief that soot formation is a result of incomplete combustion, Brem et al. strongly put forward a notion as most modern jet engines combustion efficiency is in the order of >99%. Another such experiment conducted by Zheng et al. [47] concludes that increasing aromatic content leads to increase in PM emissions as shown in Fig. 12 presenting an understanding of soot formation and consumption with aromatic content. With increasing of aromatic content of indene aromatic spices from 8% to 18% at 0.04 s (−4/100) before lean blowout (LBO),the soot formation increases very drastically as shown in extreme left column of Fig. 12. On moving toward lean blowout conditions at 0 s (1/100), i.e., at extreme lean combustion state also, the fame is not uniform. This clearly depicts that even at lean combustion state, the flame exhibits different levels of soot and the intensity of flame is dependent on the aromatic content and also aromatic species, which we will discuss in detail in later sections of this chapter.

Figure 11 Emission indices of nvPM number with aromatic concentration. nvPM, nonvolatile particulate matter.

Figure 12 Formation and consumption of soot 0.05 s prior to LBO with variation of aromatic content for indene fuel blends [47]. LBO, lean blowout.

Zheng et al. evaluated effect of fuel properties and composition on PM emissions using a Rolls-Royce Tay gas combustor with 13 different alternative fuels and 3 reference fuels as tabulated in Table 4 [21]. The three reference fuels used are JP-8, Jet-A1, and JP-5 and designated as F1, F2, and F3 samples (Fig. 13). The 13 different fuels are designated from F4 to F16 (Fig. 13). According to Zheng et al. [21], there are diverse physical and chemical properties that influence emissions, and they presented the influence in a compressive manner as shown in Fig. 14 for PM and soot emissions by analyzing the combustion performance of the fuels mentioned in Table 4. The aromatic content has always influenced the emissions, and it has positive effect, thus increasing the emissions with increase in aromatic content. Increasing hydrogen content will increase H/C ratio, thus decreasing the emissions levels with a negative effect on emissions. The result shows that the aromatic content and hydrogen content have the highest levels of impact on soot tendency. Increasing the aromatic content of the fuel will promote soot emission. In contrast, higher hydrogen content will help the fuel resist soot formation. In addition, beside aromatic content, a higher density and surface tension may also have a soot promotion effect to some extent, whereas fuels with a higher net heat combustion, smoke point, and isoparaffin content intend to burn cleaner [21].

Figure 13 PM number with size distribution for various fuels [21]. PM, particulate matter.

Figure 14 Influence of fuel properties on emissions [21].

According to Kumal et al. [19], aggregate particle size is function of engine thrust rating and fuel aromatic content. Aggregate particle size increases with increase of engine thrust irrespective of fuel; on the other hand, it decreases with decreasing fuel aromatic content. This can be explained by lower aromatic content enforcing the necessity of fuel pyrolysis to form PAHs to a sufficient level for particle inception and growth, so as the primary particle concentrations lower with low aromatic content. This results in overall smaller concentrations of particle number and particle size. Thus, in a cyclic behavior, a smaller primary particle results in smaller aggregates in size. As shown in Fig. 15 with increase in aggregate particle size, the primary particle size also increases. According to Kumal et al., this trend of primary particle size is observed as a function of aggregate size, fuel composition, and local equivalence ratio. Other studies conducted on commercial engines have shown that the primary particle sizes are in the order of 13, 20, and 24 nm for static thrust ratings of 7%, 65%, and 100% on a CFM56 Turbofan engine [5]. An increase of particle size from 14.2 to 16.1 nm for thrust levels varying from 7% to 100% on a SaM146 commercial turbofan engine was also observed by Delhaye et al. [12].

Figure 15 Typical variation of primary particle size and aggregate size [19].

Table 4

Fuel Code

Fuel

JP-8

C10.8 H21.8

Jet-A1

C11.4 H22.1

JP-5

C11.9 H22.6

Isobutanol-derived fuel (over 99% of isoparaffins)

C12.6 H27.2

Blend of 64% vol JP-5 and 36% vol farnesane

C12.8 H25.3

Blend of 60% vol coal-to-jet fuel and 40% vol alcohol-to-jet fuel

C11.4 H24.8

Blend of 73% vol narrow boiling fuel C10 isoparaffins and 27% vol trimethyl benzene

C9.7 H18.7

75% vol rocket Propellant-2 (RP-2) blending with 23% vol A3

C12.1 H23.9

Table Continued

Fuel Code

Fuel

Chemical formula

Jet-A1 blending with aromatic (jet reference fluid)

C11.6 H21.4

823

80%/20% blend of hydroprocessed esters and fatty acids and n-dodecane

C12.6 H23.2

759

Jet-A1 with cetane additives to have DCN = 31

C11.7 H23.6

777

Jet-A1 with cetane additives to have DCN = 45

C11.4 H23

786

Jet-A1 with cetane additives to have DCN = 55

C11.7 H24.1

786

Biofuel-1

Confidential

791

Biofuel-2

Confidential

782.5

Biofuel-3

Confidential

804.4

Regarding the size and distribution of nvPM, it was noticed from the study of Liati et al. [20] that the particles in greater distribution and larger size were found at maximum thrust rating of the engine, with decreasing the thrust rating to lower thrust levels the size and distribution of nvPM particles also decreased. It has been said that the smaller size particles are prone to be more oxidative and reactive in comparison with large particles. So, there are a wide variety of possible ways to decrease the PM emissions with low emission combustor designs that limit equivalence ratio in the combustion zone and also reduce the residence time in high-temperature zones of combustor [20].

5.3. Impact of different aromatic species on particulate matter emissions

Before dwelling into the impact of aromatic species, we need to answer the following basic questions: What are aromatics? and How do aromatics promote PM emissions? Aromatic compounds by definition are chemical compounds composed of one or more planar rings (like benzene) of atoms bonded through different kinds of covalent bonds. Examples are benzene and toluene. Aromatic compounds are, by nature, unsaturated hydrocarbons and second largest in mass of crude oil composition. From the earlier sections, we already discussed that the effect of increased content of aromatic would increase the PM emissions; in this section, we will understand the role of aromatic species. The aromatic species can be grouped as monocyclic and polycyclic aromatic hydrocarbons based on their structural composition [37]. As in alternative fuels, the aromatic content is almost negligible, and to meet industry standards as mentioned in chapters of approval process and elastomer compatibility, the alternative fuels are blended with conventional fuel, thus reducing the overall aromatic content in the fuel. So, the fuels with aromatic blends attain significant changes in their properties such as density, viscosity, surface tension, H/C ratio, and cetane number compared

with conventional fuels [37]. How do aromatics promote PM emissions? By nature, aromatics are highly unsaturated, they form intermediary PAH compounds. As explained in PM formation sections, PAHs are primarily precursors for PM formation as shown in Fig. 16. The PAHs are formed on pyrosynthesis (combustion) of aromatic compounds and further incomplete oxidation (pyrolysis) resulting in acetylene or phenyl radical or first ring, which is a prerequisite for benzene formation. These PAHs further grow as carbonization of first ring due to nucleation, thus resulting in coagulation. Coagulation is a process through which the macro- or nanomolecules collide to form bigger structures; thus, surface growth of molecule happens. With agglomeration, long chain–like or clusters of agglomerates are formed, which is PM. Often, this is referred as primary PM, and furthermore, oxidation results to form secondary PM. So, it is very much mandatory to completely oxidize the PAHs to have no PM formation, but in practice, this is impossible to achieve. So there exists a trade-off for low PM formation, resulting in choice of aromatic species and aromatic content.

Figure 16 Stages of PM formation [40]. PM, particulate matter.

As described, the aromatic compounds vary in physical and chemical properties, so as their tendency to form PM emissions does vary with those properties. So, aromatics can be grouped as monocyclic and polycyclic aromatic hydrocarbons based on their structural composition as shown in Fig. 17. In 2020, a study conducted by Sharma et al. [37] on various aromatic species and their effects on emission formulation inspected 13 types of aromatic species and 1 diaromatic species. The experiments are carried out in diesel engine and studied the results in respective diesel as a reference fuel. The study reveals that not only the aromatic content but also nature of aromatic compound does affect PM emissions.

Figure 17 Different aromatic species and their structures [37].

Figure 18 PM emissions of various aromatic species at high and low power operations [37]. PM, particulate matter.

PAHs show increased PM emissions compared with monoaromatic hydrocarbons at any aromatic content level and at any load condition as shown in Fig. 18. This is because higher boiling points of PAHs result in delayed fuel vaporization rate compared with monoaromatics that in turn affects the process of combustion, for example, PAHs such as tetralin—207°C and indene—185°C and monoaromatics such as ethylbenzene—135°C and toluene—107°C. Also, in general, fuels with aromatic contents exhibit longer ignition delays, but in specific, PAHs have longer ignition delays compared with monocyclic aromatics, thus promoting pyrolysis in primary zone of combustors and thus restricting complete oxidation [26]. Qian et al. [31] explained in his study on diesel-blended monoaromatics and polyaromatics that in PM emissions, a maximum number of particles are observed in the region of the small particle diameter. A maximum number of smaller particles are formed for monoaromatics compared with PAHs; this can be explained by the phenomenon of nucleation in PAHs. This is also ed by the experiments of Dewitt et al. [23] where they conveyed that mean diameter is increased as tabulated in Table 5 with aromatic content and further went on saying that mean particle diameter increases linearly to the aromatic content. Another study conducted by DeWitt et al. [23] in 2008 investigated three aromatic solvents of different molecular weight in conjunction of FT fuels and compared with standard fuel of JP-8 fuel. During the study, the emissions were measured on the turboshaft engine at Wright–Patterson Air force base to understand FT fuels behavior with emissions and elastomer compatibility. The experiment concluded that PM emissions increased with increasing molecular weight. The solvents inspected were tabulated in Table 5 while the aromatic blend is mixture of solvent 100, 150, and 200 in volumetric proportions of 25%, 53%, and 22%, respectively, to imitate the behavior of JP-8 fuel. The experiments were conducted using these solvents as blends to FT fuel in volumetric concentrations of 5%, 10%, 15%, 20%, and 25%, respectively.

Table 5

Molecular weight Aromatic 100

Aromatic 150

Aromatic 200

Aromatic blend (25:53:22)

121

135

151

135

138

–

6

82

21

9.3

Mean particle diameters (nm)

Aromatic concentration

Aromatic 100

Aromatic 150

Aromatic 200

Idle

Cruise

Idle

Cruise

Idle

13

21

15

21

13

21

14

23

15

24

16

26

16

26

16

26

20

30

17

28

18

29

23

34

19

32

20

33

26

38

21

34

22

35

29

41

The average dilution-corrected PM for pure FT was 3.1e06 particles/cm³, while JP-8 fuel has 7.1e07 particles/cm³. The results are normalized to pure FT fuel as depicted in Fig. 19 being that JP-8 emits 23 times more than pure alternative FT fuel, and it is evident that with increased molecular weight, the particle number increases and the highest particle number is observed in 25 %vol aromatic solvent 200 in FT fuel for both operating conditions. At cruise condition, it is of 8:1, while at idle condition, it is of 71:1. This substantial ratio increase at idle condition is due to inefficient combustion and low operating conditions such as pressure and temperature, and the effect shores up as molecular weight increases [23]. The particle mass emissions are consistent with particle number emissions, but particle mass ratios are considerably lower in absolute value as particle mass emissions are related to particle size, and the larger particles correspond to total particle mass emissions [23].

Figure 19 Ratio of particle number of aromatic solvents to neat FT fuel at (A) idle condition and (B) cruise condition in comparison with JP-8 [23].

According to Zeng et al. [21], all the aromatic species do not influence the emissions same way, which is evident from fuel F7, which has the highest aromatic content (30.68%) of all the fuels inspected but still has relatively low PM number as shown in Fig. 20. The aromatic used in the study can be grouped as alkyl benzenes, cycloaromatics, and diaromatics. Based on the results, they concluded that alkyl benzenes have less effect compared with other types of aromatics. This can be explained as the diaromatics and cycloaromatics have a relatively stable chemical structure, which cannot be easily decomposed and thus becomes precursors of soot formation. So, with this, it is really impossible to comprehend the effect of all existing aromatic species, and their combinations of multiple species would require additional studies to comprehend their effect on emissions in a holistic approach.

Figure 20 Effect of aromatic species on PM number and soot concertation [21]. PM, particulate matter.

Figure 21 Soot radiation power of aromatics species at LBO condition [47]. LBO, lean blowout.

So, on investigating further the effect of each aromatic species, another study, conducted by Zeng et al. [47], investigates 16 different aromatic spices. These 16 aromatic species can be grouped into three aromatic species groups as alkylbenzenes, diaromatics, and cycloaromatics and studied in three proportional blends with fuel in 8%, 13%, and 18%. As shown in Fig. 21 for all aromatic species, the proportional increment in aromatic content promoted soot radiation strength. Amplitude of soot radiation power of methylnaphthalene belonging to diaromatics group is recognizably higher compared with other two groups of aromatics species such as alkyl benzenes and cycloaromatics. It is also evident that the highest soot formation especially at higher content of aromatics is in the order of diaromatics > cycloaromatics > alkyl benzenes. In contrast, αmethyl styrene and pseudocumene of alkyl benzenes have slightly higher soot intensity compared with the tetrahydronaphthalene of cycloaromatics. This can attribute to increase H/C ratio, leading to have decreased RMS soot radiation power. Almost all alkyl benzene aromatic species produced comparatively low soot radiation, and there were no obvious disparities among the alkyl benzene species. With desired physical and chemical properties such as high enthalpy, low boiling point, and low autoignition temperatures of aromatic species pcymene, tert-butylbenzene, and 3-isoropylcumene has formed least PM emissions [47]. Results from both the aforementioned studies by Zheng et al. [21,47] conclude the importance of aromatic content and selection of aromatic species for reduced emissions. The experimental results of Zheng et al. [47] from Fig. 21 show that PM emission intensities do vary with aromatic content and aromatic species for aviation combustor. But the question arises: Does the combustion environment affect the PM emission and how do these variations happen in accordance with aromatic species? The general idea of the combustion environment is affected through chamber pressure, fuel injection systems, atomization, and fuel vaporization. Zheng et al. investigated same aromatic species in two different combustions: aviation Tay combustion and internal combustion (IC). The investigation revealed quite interesting facts that behavior of aromatic species does not change though amplitude of PM differed.

The results in Fig. 22 showed that IC engine forms higher concentration of PM. This is attributed to the presence of high pressure and intensity of aromatics in fuel blend of the cylinder. Both the test rigs, Tay combustor rig and IC engine rig show similar trend of PM formation for each aromatics species and the formation is in the order of diaromatics > cycloaromatics > alkyl benzenes. This can conclude that combustion environment can only affect the magnitude of PM emission but not the trend. This has a lot of other factors to be considered such as local equivalence ratios, as the mass flow rates of fuel and air do vary in both types of engines. But it is true that the behavior of aromatic species with respective PM formation is identical. This fact is very interesting as at present there is very little research available on aromatic species and their effect on PM emissions, also the aviation test rig operational costs are significantly expensive and it is practically impossible for researchers to investigate the effect of each species with expensive experimental test runs.

Figure 22 Effect of test environment and aromatics species on PM mass concentration[47]. PM, particulate matter.

6. Summary

In summary, the PM emissions have adverse effects on health, environment, and climate in whole. It is of utmost importance to reduce the emission from the sources. It is evident that the PM formation initiates with precursor of PAHs, which are predominant in aromatic compounds. Though PAHs can be synthesized in the absence of aromatics, the process generally fastens with the presence of aromatics. There are a wide variety of factors that are needed to be considered for PM reduction starting from combustor design philosophy, fuel compositions, fuel properties, aromatic quantity, and also selection of aromatic species. The fuel composition is one of the primary factors, as each chemical compound in fuel reacts in a unique way at constantly changing operational conditions. Researchers concluded that the order of soot formation is as follows: naphthalene > alkylbenzenes >alkynes >cycloparaffins ≈ isoparaffins > n-paraffins. When it comes to fuel properties such as density, viscosity, H/C ratio, cetane number, kinematic viscosity, boiling point, smoke point, surface tension, and ignition delays, these properties largely affect the physical process of fuel injection, atomization, and mixing, which in turn influence the PM formation. The greater influencers of PM formation are aromatic content and aromatic spices of highly branched aliphatic. With increase in aromatic content, it is evident that PM formation is increased irrespective of aromatic species. In particular to aromatic species, the PM formation is observed in the order of diaromatics > cycloaromatics > alkyl benzenes.

Acknowledgments

We are thankful to Marina Braun-Unkhoff and Tobias Schripp from the Institute of Combustion Technology, German Aerospace Centre (DLR), Stuttgart, , for helping us to improve the quality of the chapter with their valuable s.

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Further reading

[1] Grantz D.A, Garner J.H.B, Johnson D.W. Ecological effects of particulate matter. Environment International . 2003;29:213–239. [2] Maldonado M.M, Maricq and H. Directions for combustion engine aerosol measurement in the 21st century. Journal of the Air & Waste Management Association . 2010;60:1165–1176. [3] Manos H.F, Calcote and D.M. Effect of molecular structure on incipient soot formation. Combustion and Flame . 1983;49:289–304. [4] Christie S, Raper D, Lee D.S, Williams P.I, Rye L, Simon B. Polycyclic aromatic hydrocarbon emissions from the combustion of alternative fuels in a gas turbine engine. Environmental Science and Technology . 2012;46:6393– 6400. [5] Chong S.T, et al. Large eddy simulation of pressure and dilution-jet effects on soot formation in a model aircraft swirl combustor. Combustion and Flame . 2018:452–472.

Chapter 6: Impact of alternative fuels and properties on elastomer compatibility

Vamsi Krishna Undavalli ¹ , Chenxing Ling ² , and Bhupendra Khandelwal ³ ¹ Moscow Aviation Institute (National Research University), Moscow, Russian Federation ² Department of Mechanical Engineering, The University of Sheffield, Sheffield, United Kingdom ³ Department of Mechanical Engineering, The University of Alabama, Tuscaloosa, AL, United States

Abstract

This chapter discusses the fuel interaction with aircraft materials especially in regard to elastomers, which is one of the crucial components for making the machine airworthy and operate through its life. This chapter emphasizes on the basics of seal working mechanisms and their interaction with fuels. Recent advancements in the fuels and invention of modern production mechanisms of alternative fuels in view of environmental effects, instigates new challenges to design engineers, especially the fuel compatibility issues. In this chapter authors also discuss the recent developments in seal materials and their interaction with alternative fuels- which are lacking aromatic components, role of aromatic content in fuels, need of additives for swelling enhancement, testing procedures involved with understanding the compatibility of seals, and their swelling behavior in new alternative fuels.

Keywords

Aviation fuels; Elastomer compatibility; Future fuels; O-ring compatibility

Nomenclature

AJF Alternative jet fuels

ASTM American Society for Testing and Materials

ATF Aviation turbine fuel

FEPM Fluoro rubber of the polymethylene type

FFKM Perfluoro rubbers of the polymethylene type

FKM Fluoro rubber of the polymethylene type

HSP Hansen solubility parameter

ISO International Organization for Standardization

JIG t Inspection Group

PM Particle matter

SPK Synthetic paraffinic kerosene

1. Introduction

Elastomers are polymers of rubber-like materials, which possess the viscoelastic property, exhibiting both viscous and elastic nature under deformation. These elastomers are used as seals in the aircraft fuel supply systems, and throughout chapter, we may often refer elastomer as seal. By ISO 1382:1996, elastomers are defined as materials of high molar mass, which quickly regain to their original shape and size when the deformation load has been removed. But under fuel supply systems, these must act as stoppers to prevent fuel leakages by swelling when in with fuel. Since the invention of alternative jet fuels (AJF) and synthesized alternative jet fuels (SAFs), a significant reduction of the particle matter (PM) emissions are achieved by adequately controlling the aromatic content in AJFs. However, a study conducted by the NASA showed that low percentage of aromatic content in the fuel reduces the elastomer swelling and causes fuel leakages as depicted in Fig. 1 [4]. The elastomer fuel compatibility is one of the most significant drawbacks of the AJF, which started to draw industry's attention. For current solution, to prevent the risk of possible leaking during the engine operation, fuels are recommended to have a minimum 8% of aromatic content [5,9]. Most of the commercialized alternative fuels are required to be blended with a conventional fuel before using in aircrafts. Elastomers based on fluorocarbon, fluorosilicone, and nitrile are the most common used seals in the aircraft fuel systems.

Figure 1 Fuel leaking using AJFs [4]. AJFs, alternative jet fuels.

Recent research studies indicate that every compositional constituent of fuel does participate in the swelling process. The propensity of swell behavior depends on the strength of fuel interaction, fuel specific molar volume, chemical structure and molecular size of fuel components, fuel polarity, and the potential to swiftly act as hydrogen donor in the formation of hydrogen bonding with elastomer. In detail, the large molar volume fuels will inhibit the swell behavior as they are weak in diffusing into an elastomer. Also due to the lack of the ability to form the hydrogen bond due to the less or no aromatic content, the swell problem for alternative fuels depends majorly on their molar volume and chemical structure.

2. Phenomena in seals

In general, seals stop leakage of flow by acting as a medium between two surfaces by phenomenon of adsorption and absorption. Through adsorption, a hydrodynamic lubricant film is developed in between the seal and surface [18]. As the sealing phenomenon process shown in Fig. 2, height of the fluid film is dependent on the nature of (rotatory or fixed), nature of load, and lubricant properties such as viscosity, temperature, and seal wedge angles. The film between the seal lip and surface behaves as meniscus of sealed fluid, which exerts capillary force and prevents the leakage of flow. The seals undergo deformation under an applied mechanical load through surface over several cycles in its lifetime. Other than these mechanical loads, over a time period, the seals wore out of stresses and needed to be replaced due to a chemical and physical absorption. Due to absorption, the seals exhibit other phenomena, such as swell and shrink, which are utmost important in determining the fuel interaction with elastomers and a deciding factor for compatible alternative fuel as either of phenomenon results in deformation of seal effecting its performance and lifetime. Understanding of swell phenomenon varied among scientists: some have understood it in of chemical change, whereas others in the viewpoint of thermodynamics [19]. Fig. 3 shows the three phenomena of seal—swell, shrink, and degradation due to interaction between seal and oil over a time period in A, B, and C, respectively. During swelling, the fuel components are absorbed by the elastomer or seal by increasing the seal volume. Seal shrinkage is caused because seal components are dissolute into lubricant/fuel causing the seal to decrease in its volume and shrink. Seal degradation is due to the chemical reactivity between the elastomer and oils/fuels, which causes the seal hardening and loose seal flexibility and cracks the seal. Swelling behavior occurs in both natural and synthetic elastomers. The chemical content such as naphthalene, alkanes, and alkyl-benzenes in the fuel is absorbed by the elastomer when immersed into the fuel, which results in volumetric expansion of the elastomer as shown in Fig. 3A. Naphthalene is a very good hydrogen donor compared with alkyl-benzene and alkanes as explained by Dewitt et al. [9]. This is because the

nonpolar alkanes make it become a less efficient hydrogen donor while the alkyl-benzene polar varies making it a weak hydrogen donor. In addition, DeWitt et al. [9] explained that the separation of fuel constituents increases as in the order of alkanes < alkyl - benzenes < naphthalene [9]. The acceptable seal swell of the elastomer in the automotive industry is about 12%, while in the aviation industry, it is ranging from 18% to 30% [4,8]. Graham et al. [5] described swell in of thermodynamics, where the fuel and polymer intermolecular bonds break releasing energies. The energy released helps to form new polymer–fuel bonds till an equilibrium state attained, which is dependent on the fuel and polymer properties. In addition, Treloar [2] explained in a way that liquid molecules diffuse into polymers increasing the entropy when a polymer in with low molecular weight fluid and swelling occurs till equilibrium concentration is achieved.

Figure 2 Sealing process.

Figure 3 Phenomenon in seals.

From thermodynamics and chemical kinetics, the concept of equilibrium concentrations for chemical reaction and dilution can be well established. For a constant pressure system, the equilibrium could be represented by Gibbs free energy equation. The total free energy of dilution could be transferred into an equation of dilution and the entropy as

where H is enthalpy of mixing and can be expressed as , where U represents the internal energy, V and p are volume and pressure, respectively. In case of defining through single variable, the most appropriate variable is vapor pressure, so the free molar dilution energy could be represented as the following equation:

2.1. Hansen solubility parameter

Hansen solubility parameter (HSP) developed by Charles Hansen from 1967 is a widely utilized mechanism based on the total (Hildebrand) solubility parameters in determining the elastomer solubility. The fundamental priniciple of HSP is that the total vaporization energy E for a liquid consists of three individual parts that include dispersion cohesive energy E D , polar cohesive energy E P , and hydrogen bonding E H and can be expressed as . Hansen space δ is a threedimensional parameter that forms the coordination between the Hildebrand solubility parameters and total energy E. It is expressed as the ratio of energy of each component to the molar volume. So HSP can be expressed as

where is dispersion force, is dipole to dipole force, and is hydrogen bond. Each molecule of elastomer or fuel will possess a unique coordination. The closer the two molecules of fuel and elastomer in the Hansen space, the greater the possibility of dissolving into each other. The parameter of distance could be calculated with the equation as

whereas constant “4” indicates the solubility data as a sphere, which is a convenient way to display the HSP characteristics. To put the equation to more practical use, an interaction radius or the radius of the sphere R 0 is defined for two materials to determine whether their HSPs are well within range of dissolving. HSPs are located at the center of the sphere, and a new parameter RED (relative energy difference) number is defined. It gives a simple analogy of solubility in of relativeness, and it follows as RED = R a /R 0: if RED <1. the two materials will dissolve; if RED = 1, materials dissolve partially; and if RED >1, two materials do not dissolve. A detailed review is written by Smith et al. [7] explaining the use of solubility parameters for selection of materials.

3. Elastomer material

Elastomer materials such as O-rings and other very crucial components for aerospace and aviation industry are expected to perform without flaws during their operational life. So they are required to comply with criteria such as resistant to fuel and tolerant to high temperatures and heat. Various types of aviation elastomers can be broadly grouped based on material composition in three groups. Nitrile group majorly was utilized in older-generation aircraft, fluorocarbon group is currently utilized in present-generation aircraft broadly, and fluorosilicone group is being utilized in the modern- and future-generation aircraft. Nitrile elastomers are predominantly utilized elastomer groups in the seal industry due to their well balance of desirable properties such as high wear resistance and economical efficiency. But nitrile group is no longer suitable to aircraft industry as it alone cannot satisfy the needs of aviation industry. It is due to their significant constraints over poor weather resistance and moderate heat resistance. On the other hand, fluorine-based elastomers offer the needs of high temperature for aviation industry. The fluorine-based elastomers are categorized on the basis of their chemical compositions by ASTM international in the standard ASTM D1418. Of those categories, aviation industry predominantly uses elastomers from the family of FKM, FFKM, and FEPM categories. FKM is a fluoro rubber of the polymethylene type that utilizes vinylidene fluoride as a co-monomer and has substituent fluoro, alkyl, perfluoroalkyl, or perfluoroalkoxy groups in the polymer chain, with or without a cure site monomer [21]. FFKM is a perfluoro rubber of the polymethylene type having all substituent groups on the polymer chain either fluoro, perfluoroalkyl, or perfluoroalkoxy groups. FEPM is a fluoro rubber of the polymethylene type only containing one or more of the monomeric alkyl, perfluoroalkyl, and/or perfluoroalkoxy groups, with or without a cure site monomer (having a reactive pendant group) [21]. These are resistant to heat, non-flammable, inert to aliphatic, and chemically inert to aromatic hydrocarbons. But these are poor resistant to chemicals such as ether, ketones, esters, amines, hydraulic fluids, and phosphate-based hydraulic fluids. Elastomers based on fluorocarbon can sustain temperatures up to 204°C and are resistant to a greater range of chemicals compared with any other elastomer; however, they offer poor sealing performance under low temperatures, which is

not desirable. Fluorosilicone integrates the features of silicone, which provides excellent performance at extreme low temperatures upto −73°C and good resistant to petroleum oils and fuels, but it is poor in physical strength and abrasion resistance so it is not allowed to use as static seal. In general, three types of O-ring seals, fluorosilicone, nitrile, and fluorocarbon, are used in the aviation industry, and their physical properties are described as in Table 1.

Table 1

Specification or physical property

Fluorosilicone

Nitrile

Fluorocarbon

Blue

Black

Black

Color code

1650

1000

1850

Density (kg/m³)

250

300

350

Elongation (%)

9

18

12

Tensile strength (MPa)

5.6

4

7.8

Young's modulus (MPa)

55

70

80

Hardness (Shore)

0.3

0.25

0.5

Thermal conductivity (W/mK)

−56

−40

−25

Glass temperature (°C)

4. Testing methods

In general, seals are tested in static and dynamic scenarios. The static testing procedures are meant to observe the changes in physical properties of the elastomers. During the static testing, the seals are immersed in fuels for a certain time period and compared for the changes in properties. The static testing methods are used in the past, as their advantage restricts to an extent especially in assessing the elastomer volumetric changes. However, with the static testing, we cannot predict the real-time changes in seals. So there is a necessity for dynamic testing methods under which the seals can be subjected to real-time working situations. There could be several ways to do dynamic testing, under which the seals can be influenced through both pressure and temperature simultaneously. In addition to that, one needs to understand the elastomer and seal molecular interactions occurring through diffusion process to give certain answers for the seal compatibility with new fuels and percentage of aromatic content. Common testing methods adopted by current industry are listed in Table 2. Dynamically the physical change of the seal such as changes in volume, mass, and hardness can be studied through the methods prescribed in the first column of Table 2. The chemical changes such as chemical content extracted from elastomer by fuel and molecular interactions can be studied through the second column methods.

4.1. Stress relaxation

Stress relaxation is a technique through which we can simulate the real-time working conditions on seals by maintaining desired temperatures and pressures. In modern stress relaxation equipment, both tensile and compressive tests are performed by isothermal conditioning or temperature cycling. Understanding stress relaxation behavior of elastomers will enhance our understanding of seal compatibility with new fuels. It is one of the principle testing methods adopted

by industry in seal testing and used to predict the lifetime of elastomers.

Table 2

Physical changes

Chemical changes

Fourier transform infrared spectrometer

Stress relaxation (pressure and temperature)

Gas chromatography–mass spectrometry

Optical method (high-speed camera)

Thermogravimetric analysis

The principle of stress relaxation is based upon the deformation, when an elastomer deforms either through compression or tension; the elastomer develops counter-responsive strain. Due to this strain, a counterforce is developed in elastomer, and this counterforce decays over a time period rather than to remain applied constantly. So, with counterforce decay, the strain does not change, but the stress decreases, and the process is referred to as stress relaxation. In standard conditions, the stress relaxation process of elastomer occurs by physical and chemical changes or both simultaneously. Physical process dominates the stress relaxation at lower temperatures or during short intervals of time. Chemical processes dominate relaxation at higher temperatures or long intervals. During the physical relaxation, the physical changes occur such as realignment of the elastomer strands under strain, through detachment, reconfiguration of chain entanglements and chain end movement. It also involves with breakdown and rearrangement of bonds by virtue of secondary valence forces between the polymer and filler materials. Chemical changes are caused by thermal or thermal oxidation reactions and scission of polymer chains or thermal breakdown of cross-link. Chain scission reduces the density of network strands that generate new dangling ends with released stress. The release of stress is often triggered by the breakdown of cross-links, leading to longer strands and disorganization. These chemical changes cause the chemical relaxations in elastomers [13]. In elastomers, stress relaxation process is affected by various aspects such as type of solvents, materials of polymer, filler and their density, cross-link type and density, vulcanization process, strands network structure, and thermal oxidative processes. Instruments include a compression rig and aging oven; specimen is set inside the gap between two compression plate as shown in Fig. 4. Compression force is then applied to the fuel immersed sample to reach a certain deformation. To maintain the desired temperatures, the compression rig is placed in the aging oven. So that a specific temperature is maintained during the seal testing. The general stress relaxation behavior of nitrile elastomer is represented in Fig. 5. The elastomer undergoes three phases of interaction with fuels. During the first phase, the counterforce will decrease dramatically during the first 30–60 min of force application. This is because the rubber seal shows both elastic and viscous behaviors, so a damping force is developed to counter the applied force.

The relaxation ratio starts to increase after 2 hrs of seal exposition to fuel, which is referred to as the second phase, caused due to the physical interaction between fuel and seal. The aromatic content in the fuel starts to penetrate inside the seal, which results in the increase of seal volume. The third phase occurs after the peak of the counterforce, which is followed by a continuous decrease in relaxation ratio. However, the stresses continues to apply on the seal which causes the molecular movement. In general, the polymer structure changes and becomes hard.

Figure 4 Stress relaxation test rig [11].

4.2. Optical method

Optical method is one of the simplest methods to detect the change of volume swell through thickness measurement of the elastomer. Swell determination methods of elastomer are based on measurements of mass or volume increments in an elastomer due to solvent absorption, generally in the case of aviation fuels. The optical methods provide an advantage by sensitively detecting the changes in film thickness, which is in general in the order of 1–2 μ over the direct volumetric and gravimetric methods. The indirect methods that include mechanical, hydraulic or aerodynamic devices can record variations in some properties related to volumetric changes, but they are limited in measuring equilibrium swelling. For capturing the elastomer swelling in thin films, the methods and instruments involved in measuring should not affect the shape or consistency in advancing layers in the elastomer. So, to achieve this precision, optical methods based on interferometry or microscopy are suitable [10]. With current advancements in optical instruments, they provide much better resolution and better sensitivity to record any minute changes in the elastomer. In general, the optical measurement equipment includes LED , container, and digital camera. Sample is placed at the bottom of the container, which is full of fuel or solvent. Camera is set under neath the optical plate to record the shape change during the experiment with the assistance of the LED light. Modern day digital camera systems can perfrom each sampling in avaerage of 6 mins which include 2 min of fuel exposition and photographing cycle of 20 secs for a period of 3 mins. This process of sampling can be conducted over an extensive time period ranging to 40 h.

Figure 5 General stress relaxation behavior of a nitrile elastomer.

4.3. Gas Chromatography–Mass Spectroscopy

Gas chromatography–mass spectroscopy (GC-MS) is involved in determining composition of the fuel absorbed by the seal ring during the test. It is an analytical technique based on two fundamental techniques of gas chromatography and mass spectroscopy. The gas chromatography initially isolates the elements of a complex mixture in such a way that it can be further processed for qualitative purposes. Then mass spectrometer provides mass spectra information that helps in processing the structural identification of components in the sample mixture. The combination of the two techniques provides a greater advantage in compositional determination. The limitation of this technique is that it cannot be used to analyze the compounds that are volatile in nature and also, thermally unstable compounds [12]. The working nature of the GC-MS is formation of two sections of the GC and the mass spectrometer. GC section consists of inert gas injection with sample mixture into oven-regulated capillary tubes through which molecular separation is achieved over the retention time period. The retention period of individual molecule type depends on the capillary tube dimensions and film thickness. After separation of molecules, the molecules diffuse at different times, which is generally referred to as retention time. Then these molecules are processed further through MS by ionizing, accelerating, deflecting, and detecting by the principle of charge to mass ratio.

4.4. Thermogravimetric analysis

Thermogravimetric analysis (TGA) is another analytical technique used in the

polymer industry to determine an elastomer fuel absorption capacity, thermal stability, organic content, and volatile content. This technique is widely used in thermal analysis of elastomers by determining the chemical decomposition of an elastomer due to effect of sustained static-temperature exposures. TGA is conducted on an instrument referred to as a thermogravimetric analyzer. It continuously measures mass while the temperature of a sample is changed over time. Mass, temperature, and time are considered base measurements in TGA. The technique is based on the continuous measurements of mass change in a sample due to heating the sample at steady rate, which also involves in temperature change over a time period. The measured data provide information about physical phenomena, such as phase transitions, absorption, adsorption, and desorption. Also, chemical phenomena include chemisorption, thermal decomposition, and solid–gas interactions such as oxidation or reduction [14].

4.5. Fourier Transform infrared spectrometer

Fourier transform infrared spectrometer (FTIR) is one of the instruments based on infrared spectroscopy. It is the most modern type and preferred over the other dispersive spectrometers. It is because of its high precision, accuracy, speed, enhanced sensitivity, ease of operation, and sample nondestructiveness. The fundamental of infrared spectroscopic technology is on atomic vibrations of a molecule that only absorbs specific frequencies and energies of infrared radiation. The molecules could be detected and classified by FTIR because different molecules will have different infrared spectrum. A block diagram of FTIR working process is shown in Fig. 6. The FTIR spectrometer essentially uses an interferometer to measure the energy that is being transmitted to the sample. The infrared radiation emitted from the black body reaches the interferometer where the spectral encoding of signals happens. The resultant interferogram signal is transmitted through or bounces from the sample surface, where specific energy wavelengths are absorbed. The beam eventually es through the detector and further ed on to processing computer for Fourier transformation of energy signals.

5. Recent advancements in understanding elastomer fuel compatibility

Lack of aromatic content is the primary reason for fuel leaking, which has been found in research area. Jeyashekar et.al [16] figured that the possible reasons for the poor performance of elastomers with R-8 fuel could be a combination of lack of aromatics, thermal effects, and/or fuel lubricity. Alves et al. [8] indicated that weight change and mass loss for NBR increase with increasing the concentration of biodiesel. Little differences in oil raw composition and consequently biodiesel composition do not affect the swelling ability of elastomers. Corporan et al. [9] also found that significant swelling capability was observed in nitrile O-rings with the alternative fuels; however, it was substantially less than the aromatic containing JP-8 fuel, which may lead to fuel leaks in aircraft systems. Link [17] using computational modeling of the interaction energy between a nitrile rubber monomer and various chemical species has determined that species have the potential for dual interactions, one with the polar nitrile group and the other with the electrons in the butadiene system.

Figure 6 FTIR processing [15]. FTIR, Fourier transform infrared.

The fact that molar volume affect swelling behavior of O-ring has been widely investigated in laboratory test. Saleem et al. [3] claim the diffusivity generally decrease with increasing of molar volume, which was also shown by Mathai et al. [1] and Sombatsompop et al. [6] who found that the rate of swelling decreased with increase in molar volume. Dewitt et al. [9] indicated that the volume swell trends were attributed to decreasing molar volume and increased polarity and hydrogen-bonding potential. A general summary for these studies is that the swelling behaviors are caused by both chemical and physical processes. Chemical process corresponds to polarity, hydrogen bond, and dispersion, which are the three main properties that affect the degree of swelling, whereas physical process corresponds to molar weight and molar volume as illustrated in Table 3. The performance of fuel seal is based on its high polarity and hydrogen bond. Therefore, according to Hansen solubility parameter, chemical content, which has higher value of polarity and hydrogen bond, will increase solubility for fuel and elastomer and hence promote the swelling process. One of the major differences between conventional fuel and alternative fuels is the percentage of aromatic content. Due to the absence of aromatic content in the fuel, the chemical factors have low impact on the swell behavior; therefore, for alternative fuel, the process could be considered as pure physical, and the major impact fact is the molar volume shown in Table 4.

Table 3

Property

Impact on seal swelling

Chemical factors Promoting

Polarity

Promoting

Hydrogen bond

Promoting

Dispersion

Physical factors Obstructing

Molar weight

Obstructing

Molar volume

5.1. Swell behavior with different aromatic species

Swell behavior for a certain O-ring highly depends on the aromatic content in the fuel, and further study is necessary. As discussed previously, molar volume, polarity, and H-bonding are three main parameters that affect the solubility due to the HSP theory. Therefore, it is obvious that each aromatic species will behave differently in of swell promotion. A study carried out by Boeing Company under CLEEN project [20] evaluated different spices of alkanes, aromatic, and diaromatics for seal swell experiments. These aromatic species are selected to evaluate each individual “isolate solubility parameter”. Propyl benzene, butylbenzene, and pentyl benzene are used for molar volume evaluation. Similarly, 1,3,5 trimethylbenzene, 1,2,4, trimethylbenzene, and 1,2,3- trimethylbenzene are used for polarity, and tetrahydronaphthalene and naphthalene are used for hydrogen bonding. Two extra species, indane and methylidene, are selected, as they are common components in the normal jet fuel, which has similar structure with tetrahydronaphthalene. Most of the aromatic spieces are blended in volume of 8% with SPK except naphthalene and methylidene which are blended in 3% volume, as this is the maximum allowable percentage in jet fuel. The detailed fuel properties includes the value of HSP, and molar volume as shown in Table 5. According to Table 6, four species (1,2,3-trimethylbenzene, tetrahydronaphthalene, naphthalene, and methylidene) provide same or higher swell capability than Jet-A for nitrile rubber. In case of fluorosilicone, only 1,2,3-trimethylbenzene shows poor swell capability compared to Jet-A fuel. Similarly, only 1,2,4-trimethylbenzene provides lower specific volume swell. In general, the results show its accuracy with respect to HSP theory. With larger molar volume fuels, the specific volume swell of an elastomer is poor due to difficulty in penetration of fuel molecules into elastomer. So, the specific volume of swell increases with the choice of lower molar volume fuels, i.e., with the decrease of molar volume in fuels, the specific volume swell increases. The

specific volume swell of elastomer does depend on polarity and H-bonding, and it increases with polarity and H-bonding. This shows that polarity and Hbonding will enhance the seal swelling capability. Nitrile rubber has higher responsiveness toward the aromatic content in the fuel than other types of elastomers such as fluorocarbon and fluorosilicone, and the same can be identified as in Table 6 as nitrile rubber shows higher specific swell value compared with other elastomers in the same level of aromatic content fuel.

Table 4

Fuel types

Property

Polarity, hydrogen bond, dispersion, molar volume

Conventional fuel

Molar volume

Alternative fuel (lack of aromatic)

Table 5

Compound

Fuel component

Hansen solubility parameters

Dispersion

Polar

H-bond

Molar volume mL/mol

Nonane

15.7

0

0

15.7

0

0

196

16

0

0

229

16.3

0

0

294

Benzene

18.4

0

2

18

1.4

2

107

17.8

1

3.1

121

17.8

0.6

1.4

123

17.8

0.4

1

134

18

0

0.6

140

17.7

0.1

1

154

18

0

0.6

157

17.4

0.1

1.1

157

17.6

0.1

1

171

17.4

0.1

1

174

Naphthalene

19.2

2

5.9

20.6

0.8

4.7

139

Table 6

Characteristic

Aromatic

O-rings

N0602

N0602e

L1120

V0835

0.55

0.69

0.06

0.02

Average Jet-A

Propyl benzene

0.34

0.57

0.26

0.08

Molar volume

0.29

0.41

0.19

0.04

Butylbenzene

0.21

0.28

0.06

0.03

Pentyl benzene

1,3,5-Trimethylbenzene

0.33

0.53

0.08

0.07

0.45

0.66

0.04

0.1

1,2,4-Trimethylbenzene

0.57

0.82

0.16

0.1

1,2,3-Trimethylbenzene

Tetrahydronaphthalene

0.61

0.61

0.12

0.01

1.22

1.92

0.29

0.15

Naphthalene (3%)

Indane

0.55

0.66

0.06

0.09

0.81

1.07

0.61

0.18

Methylidene (3%)

Polarity

H-bonding

Other

With respect to the influence of the molecular structure of the fuel, volume swell was found to be increasing as the molar volume of the fuel components decreases and as their polarity and hydrogen bonding increases. Among these three factors (molar volume, polarity, and hydrogen bonding), molar volume had the least influence on the volume swell followed by polarity and hydrogen bonding. Furthermore, the influence of hydrogen bonding tended to be significantly higher than that of polarity. This suggests that the volume swell of jet fuel will increase as the boiling range shifts toward lower temperatures (lower molecular weight) and as the overall polarity and hydrogen bonding increase the specific volume swell, and vice versa. Noting that the bulk of Jet-A and all typical SPKs are paraffinic and therefore nonpolar, the polarity and hydrogen bonding character of the fuel will be dominated by the aromatics. However, emphasizing that the bulk of the fuel is paraffinic, particularly as the aromatic content is lowered, the influence of the molecular weight distribution of the fuel must be well considered. To understand in greater extent the effect of paraffin component in the fuel on the swelling performance, an experimental study was conducted by Liu and Wilson [22]. The study was carried out on three types of paraffin compounds such as cycloparaffin or decalin, normal paraffin or n-decalin, and isoparaffin. Like aromatics, the molar volume, molar structure, ring size, and chain length of a paraffin compound could also affect the penetration of the fuel content into the elastomer. As summarized in Table 7, the greater the content of decalin in fuel, the better the sealing performance, and vice versa, and it may not be true for all cycloparaffin compounds. But in case of n-decalin, it is quite opposite that the seal swelling performance degrades with greater content. However, the isoparaffins do not participate in the seal swelling. Although the three components participate in polymer interaction or material extraction, the ndecalin is relatively higher than other two compounds and then followed by isoparaffins and decalin. In general, the results indicated that the swelling behavior is high for cycloparaffin than normal paraffin and isoparaffin. This certainly concludes that the structure and molar volume are the primary factors that affect the swelling behavior.

Table 7

Paraffin compounds

Impact on seal swelling

Polymer interaction

Promote

Great

Decalin (cycloparaffin)

Obstruct

Greatest

N-decalin (normal paraffin)

Do not participate

Greater

Isoparaffins

As previously discussed, aromatic content in the fuel will do affect the swelling behavior for elastomer and other rubber materials in the fuel system. But it is not the same in the case of paraffin content. This emphasizes the importance of considering the composition of a fuel as a whole when evaluating how it interacts with nonmetallic materials rather than emphasizing on individual fuel components of the fuel. In an another study by Anuar et al., impact of different aromatic species and seals prepration method on seal swell has been presented [23]. It was observed that both different species of aromatics and how seals are prepared for the test has an impact on seal swell performance.

6. Summary

With the lack of aromatic components in new alternative fuels, there is less swelling behavior in seals, which results in degradation of seals performance. This restricts the use of new fuel's availability for the aircraft and aircraft engines, and the requirement of minimum aromatic content becomes a necessity. So a minimum of 8% aromatic content with maximum 50% SPK blends is recommended through studies conducted by Graham et al. [5] and Dewitt et al. [9].The same is adopted by industry through ASTM standard D7566-20b and by t Inspection Group (JIG) through a document “Aviation fuel Quality Requirements for tly Operated Systems.” The current studies do indicate that the swelling behavior does not solely depend upon aromatic content, so there needs a greater depth of understanding of the subject. The commercially available alternative fuels are required to be blended with conventional fuels before using. Elastomers based on fluorocarbon, fluorosilicone, and nitrile are most common used seals in the aircraft fuel systems.

References

1. Mathai A.E, Thomas S. Transport of aromatic hydrocarbons through crosslinked nitrile rubber membranes. Journal of Macromolecular Science . 1966;B35(2):229–253. 2. Treloar L.R.G. The elasticity and related properties of rubbers. Reports on Progress in Physics . July 1, 1973;36(7):755–826. 3. Saleem M, Asfour A.F.A, De Kee D, Harrison B. Diffusion of organic penetrants through low density polyethylene (LDPE) films: effect of size and shape of penetrant molecules. Journal of Applied Polymer Sciences . 1989;37(3):617–625.

4. Del Rosario R, Koudelka J, Wahls R, Madavan N, Bulzan D. Alternative aviation fuel experiment II (AAFEX II) overview NASA technical reports document id 20150010381. 2012. 5. Graham J.L, Striebich R.C, Myers K.J, Minus D.K, Harrison W.E. Swelling of nitrile rubber by selected aromatics blended in a synthetic jet fuel. Energy & Fuels . 2006;20(2):759–765. 6. Sombatsompop N, Lertkamolsin P. Effects of chemical blowing agents on swelling properties of expanded elastomers. Journal of Elastomers and Plastics . 2000;32(4):311–328. 7. Buckley Smith M. K., Fee C. J. The use of hansen solubility parameters for the selection of materials for organic/organic separations by pervaporation. Proceedings of the 9th APCChE congress and CHEMECA, September 29– October 3, 2002. 8. Alves S.M, Mello V.S, Medeiros J.S. Palm and soybean biodiesel compatibility with fuel system elastomers. Tribology International . 2013;65:74–80. 9. Dewitt M.J, Corporan E, Graham J, Minus D. Effects of aromatic type and concentration in Fischer−Tropsch fuel on emissions production and material compatibility. Energy Fuels . 2008(9):2411–2418. 10. Chiklis C.K, Grasshoff J.M. An optical microscope technique for measuring the swelling of thin films. Journal of Polymer Science Part A-2 . 1969(7):1619– 1621. 11. Yue L. Investigation on elastomer compatibility with alternative aviation fuels Doctoral thesis. University of Sheffield; 2013.

12. Gilbert J.R, Mc Caskill D, Fishman V.N, Brzak K, Markham D, Bartels M.J, Winniford B, Mohsin S.B, Godbe applications of high-resolution GC/MS. Comprehensive Analytical Chemistry . 2013;61:403–429. 13. Brown R.P, Butler T, Hawley S.W. Ageing of rubber: accelerated heat ageing test results . Rapra Technology Ltd; 2001.

14. Coats A.W, Redfern J.P. Thermogravimetric analysis. A review. The Analysts . 1963;88:906–924. 15. Mohamed M.A, Jaafar J, Ismail A.F, Othman M.H.D, Rahman M.A. Fourier transform infrared (FTIR) spectroscopy. Membrane characterization . 2017:3– 29. 16. Jeyashekar N, Frame E, Wilson G, Hutzler S, Brien S.O. Elastomer – fuel compatibility studies with alternative jet fuels . 2013:1–8 GT2013-95941. 17. . Link D.D, Gormley R.J, Baltrus J.P, Anderson R.R, Zandhuis P.H. Potential additives to promote seal swell in synthetic fuels and their effect on thermal stability. Energy & Fuels . 2008;22(2):1115–1120. 18. ERIKS Technology. ERIKS oil seals boucher. https://static.eriksgroup.com/en/oilseals/brochures/oil%20seals%20technical%20manual/eriks%20%20sealing%20technology%20technical%20manual.pdf. (Accessed 16 October 2020). 19. Exxon Mobil Corporation. Seal compatibility in hydraulic systems. https://www.mobil.ru/industrial/-/media/Files/global/us/industrial/techtopics/seal-compatibility-hydraulic-system.pdf. (Accessed 16 October 2020). 20. Rahmes T.F, Kay M.C, Belières J.P, Kinder J.D, Millett S.A, Ray J, Vannice W.L. Impact of alternative jet fuel and fuel blends on non-metallic materials used in commercial aircraft fuel systems . report by Boeing company and University of Dayton Research Institute; July 29, 2011. 21. ASTM international. ASTM D1418-17 standard practice for rubber and rubber latices – nomenclature . January 1, 2017. 22. Liu Y, Wilson C.W. Investigation into the impact of n-decane, decalin, and isoparaffinic solvent on elastomeric sealing materials. Advances in Mechanical Engineering . January 2012 doi: 10.1155/2012/127430. 23. Anuar A, Undavalli V.K, Khandelwal B, Blakey S. Effect of fuels, aromatics and preparation methods on seal swell. The Aeronautical Journal . 2021 doi: 10.1017/aer.2021.25.

Further reading

[1] Risher J, Bittner P.M, Rhodes S. Toxicological profile for jet fuels JP-5 & JP-8 . Atlanta, GA: Agency for Toxic Substances and Disease Registry, U.S. Department of Health and Human Services, Public Health Service; 1998. [2] Winchester N, McConnachie D, Wollersheim C, Waitz I.A. Economic and emissions impacts of renewable fuel goals for aviation in the US. Transport Research Part A . 2013;58:116–128. [3] Kass M.D, Janke C.J, Connatser R.M, Lewis S.A, Keiser J.R, Gaston K. Compatibility assessment of fuel system infrastructure plastics with bio-oil and diesel fuel. Energy & Fuels . 2018;32(1):542–553. [4] Arias Quintero S.A, Ricklick M, Kapat J. Synthetic jet fuels and their impact in aircraft performance and elastomer materials . AIAA, 3965; 2012. [5] Carrell J, Lewis R, Slatter T. Elastomer solubility and stress relaxation in biolubricants. Tribology International . 2020;141:105947. [6] Chen K, Liu H, Xia Z. The impacts of aromatic contents in aviation jet fuel on the volume swell of the aircraft fuel tank sealants. SAE International. Journal for Aerospace . 2013;6(1).

Chapter 7: Impact of fuel properties on lean blow off limit

Bhupendra Khandelwal ¹ , L. Zheng ² , and Charith J. Wijesinghe ³ ¹ Department of Mechanical Engineering, The University of Alabama, Tuscaloosa, AL, United States ² Nanjing Institute of Technology, Nanjing, Jiangsu, People's Republic of China ³ Department of Mechanical Engineering, The University of Sheffield, Sheffield, United Kingdom

Abstract

Owing to the fact that different fuels can have different properties, alternative fuels can have significant impact on lean blowout and flame stability of the combustor. This chapter discusses the work done in the area of assessment of alternative fuels and their properties on lean blow-off performance.

Keywords

Alternative aviation fuels; Fuel properties; Gas turbine combustion; Lean blowoff limit; Sustainable fuels

1. Introduction

Lean Blow Off (LBO) is a condition in combustion engine where flame in the engine gets extinguished due to operating at very high air-to-fuel ratio. With continuous improvements in technology for lower emissions, lean blow-off performance of engines has become significantly important, as modern engines are operating at significantly leaner conditions to keep emissions low. LBO performance of a combustion system in power generation and propulsion is an important parameter. LBO becomes a practical issue when it impacts the rate of descent, maneuverability, and other performance-based characteristics of military aircraft while also being an issue in land-based gas turbines, which must be reignited frequently if operating under LBO conditions, which is timeconsuming and therefore impacts profitability [1]. Both the fuel and combustor geometry play an important role in flame stability. With demand for cleaner and more efficient aviation engines with ever-increasing power requirements, engines are required to perform closer and closer to the lean fuel limit, which requires changes to combustor design to allow stable operation in and near the LBO limit. Even with the prodigious advancement in the gas turbine industry in the recent past, flame stability in lean operation has not been without its issues such as a global extinction events or even LBO, which has the undesirable effect of catastrophic loss of thrust in an aircraft, which is more likely at high altitude. Flame stabilization is a complex thermofluidic process as well as a chemical reaction, which is affected by reaction rates as well as fluid turbulence and diffusion of the reactants, which can cause localized ignition and extinctions [2]. This chapter explains different LBO mechanisms, detection methods, prediction methods, and impact of alternative fuels and their properties on LBO.

2. LBO mechanism

Flames, in general, are stabilized in adverse conditions especially in gas turbines in speed reactant streams. The blowout point is also known as “static stability” limit in a given combustion device. Recently, the drives for leaner combustion to combat emissions and increase efficiency have brought the LBO point into contention as a safety and performance concern. Esclapez et al. [3] summarized the three main theories for LBO:

• Along the flame sheet, the extinction of the recirculation region and large-scale wake disruption, which acts as a well-stirred reactor. • Inability to ignite the in-coming reactants in the shear layer of the recirculation domain. • Local flame loss by aerodynamic effects including wake cooling and shrinking.

Sigfrid et al. [4] explained the LBO from the flame lifted-off theory, which is the precursor to blowout [5].

• Local flame velocity is reduced with respect to the oncoming fluid [6,7]. • The extinction strain rate is higher than the local strain rate [8]. • The mixing time of the large-scale structures is less with respect to the reaction time; hence, the fresh mixture cannot be ignited by the hot mixture [9].

LBO can be mitigated by means of a pilot flame utilizing richer conditions with respect to the core flame. This approach has the drawback in that the higher combustion temperatures of the pilot flame can cause increased NOx emissions. Therefore, it would be prudent to use a pilot flame only as a last resort by achieving a compromise in understanding the true LBO limits of a given combustor for its operating conditions and avoiding it [10].

3. LBO detection method

Given that LBO by nature is a transient process understanding the LBO phenomenon is imperative. Many intricate analytic schemes have been established to deliver instantaneous data of these measurable dynamic quantities. Early LBO detection and prediction methods involved identifying pressure oscillations or drop in the burner environment as per Snyder et al. and Lucenko et al. [11,12]. Piezoelectric transducers were used to measure the heat release rate by Lee and Santavicca [13]. Li et al. measured the temperature oscillations by a variable diode laser temperature device [14]. The most widespread methods involve noise and optical methods of observing LBO [15–18]. Nair and Lieuwen showcased the viability of acoustic measurements using a high-speed monitoring approach to define the LBO characteristics [19]. Infrared-capable high-data-rate cameras have also been used to detect the LBO phenomenon [20]. Planar laserinduced fluorescence (PLIF) has been used to monitor equivalence ratio fluctuations [21]. The synchronized stereo-PIV (particle image velocimetry) and OH-PLIF approach have been applied by Stöhr et al. to detect the unsteady vortex and gas mixing at LBO conditions [22]. UV imaging implemented via OH chemiluminescence has also been used to develop control systems [23– 25]. Muruganandam and Seitzman [26] utilized high-speed OH chemiluminescence methods to examine the performance of a swirled premixed combustor near the LBO limit [26].

4. Hardware and fuel impact on LBO

Fuel properties, operating condition, and the design of the combustor itself have an impact on the LBO limit [27]. Equivalence ratio, inlet airflow velocity, bluff body geometry, stratification, and inlet temperature are also to be noted, as they have impact on LBO [28]. Lefebvre's theory dictates that the LBO equivalence ratio is impacted mainly by fuel properties, combustor design, and thermodynamic quantities as per Eq. (1) [29].

[1]

4.1. Hardware configuration impact

The flame blow-off behavior is mainly composed of autoignition with consecutive extinction and reignition events of the recirculation bubble. Therefore, the design of the primary combustion zone has a deep impact on the LBO limits. Shanbhogue et al. [8] reviewed the dynamic configurations of flame stabilization and described the phenomenology of the blow-off process. This investigation observed that the Damköhler number and LBO limits are well correlated. For modern aviation combustor, the swirl spray has been found to have a noticeable effect to enhance the mixing and improve the flame stability [26,30]. Ateshkadi et al. [30] were identified that the best hardware configuration for LBO is 45 degrees primary swirler and 55 degrees secondary swirler and counterswirl. Through the simultaneous stereo-PIV and chemiluminescence imaging study, Stöhr et al. [22] described the importance of the flame root. He suggested that the better LBO limit can be achieved with targeted modifications of the mixture fraction or the flow field in the flame root region. Later on, Xiao and Huang [31] tested three different types of swirl cups for their impact on LBO using a dome rectangular model combustor. It was claimed that the swirl intensity is a major factor, which impacts LBO limits. It was also observed that different types of swirlers have different impact on LBO limits. They also noticed that the impact of atomization on LBO limits is not a significant factor. Overall combustion pressure was found to be of little import by Strakey [32]. In his experimental work, increasing the pressures from 1 to 8 bar and increasing the nozzle velocity from 40 to 80 m/s, it only increased at blowout limited by roughly 0.05. Daniele et al. [33] also claimed that there is no significant dependence of the LBO limits on the inlet velocity and pressure. Moreover, spray characteristics were thought not be a major factor for LBO.

4.2. Fuel property and composition impact

Multiple investigations have been conducted to evaluate the impact of fuel properties on LBO limit in different combustion environment. Ateshkadi et al. [30] considered the flame steadiness in a spray combustor and claimed that the liquid evaporation rate is the dominant factor for the flame stabilization at low gas temperatures conditions. However, at higher temperatures, the oxidizer and fuel mixture are the controlling factor for stabilization process. Low flash point and low viscosity have beneficial effect on LBO performance [34]. Hydrogen content of the fuel has significant impact on the LBO limits. It was found that fuels with high hydrogen contents can help in extending the lean operational limits of the combustor without flashback or flame fluctuation [35,36]. Starkey et al. [32] experimentally studied the high hydrogen concentration influence on the combustion performance with a swirl-stabilized combustor. The presence of increased H2 impacted the presence of OH radicals and hence the reaction rates, which in turn causes increased flame speeds and reduced LBO limits. Burger et al. [37,38] presented results from a LBO study of 16 different fuel formulations in a pressure atomizer combustor. He advocated that the DCN (derived cetane number) have positive influence to extend the blow off limit. Burger also found that LBO behavior exhibited a strong inverse correlation with ignition delays. Colket et al. [39] also describe that the fuel with advanced DCN and increased C/H ratio can have better ability to resist blow off. Fig. 1 shows the impact of DCN on LBO equivalence ratio, while testing multicomponent fuels, which shows the DCN (according to ASTM 6890) that is a measure of a liquids' tendency for autoignition [40]. The investigation conducted by Won et al. is corroborated with this remark [41]. This puts forward the concept that DCN and LBO are related concepts. Ahmed et al. [42] reported the impact of DCN on LBO at different inlet mass flow conditions using a gas turbine combustor. It can be observed in Fig. 2 that with increasing DCN number, flame stability limits get better and equivalence ratio of LBO goes down. Investigations observed that the DCN is impacted when alternative fuels are blended together [44,45], which is due to the change in the chemical composition and the amount of high cetane compounds. Braun-Unkhoff stated

that the cetane numbers of n-paraffins were greater than those of isoparaffins. In several recent studies, as part of National Jet Fuel Combustion Program (NJF), impact of DCN and other fuel properties on lean blow-off limit has been experimentally and numerically evaluated [45–50]. One of the NJF's main criteria was testing the LBO, which was done experimentally with 10 different combustors and geometries as well as various operating conditions and fuel atomizers as well as varied fuels. The outcome of these tests was the identification of two distinct types of LBO limits, namely the chemical reactivity limit and the physical volatility limit. Also, LBO has been found to be first order dependent on the DCN of the fuel, which is a common property tested for in diesel fuels but not so far required for aviation jet fuels. It was observed that fuels with lower than 30 DCN led to worse LBO performance; 35–55 DCN range is typical for conventional jet fuels. Higher than 55 DCN led to improved lean blow-off performance but could have negative impact on high compression and LDI engines [46].

Figure 1 Impact of DCN on LBO equivalence ratio [43]. DCN, derived cetane number.

Figure 2 Distillation curves for NJF fuels [48]. NJF, national jet fuel combustion program.

The NJF studies were the first studies to have identified cetane number as impacting the LBO limits, in that fuels with high cetane number have extended LBO limits and fuels with low cetane numbers have lower LBO limit causing earlier blow off. There are several other fuel properties that can impact LBO performance of an engine. In one of the recent articles by Stachler et al. [47], impact of fuel H/C ratio, TSI, and average molecular weight have been discussed. The impact a particular combustion rig has toward specific fuel properties has been identified using statistical regressions and can be expressed as a bias that a given combustion rig leans toward certain thermodynamic conditions. For instance, the Honeywell APU rig has been shown to be biased toward the physical volatility limit of LBO with negligible dependency on DCN, which would imply that the evaporative timescale encoming the primary and secondary spray evaporation is greater than the mixing and chemical timescales. However, the well-stirred reactor, which is a prevaporized and premixed research reactor, was only impacted by the chemical fuel properties. Furthermore, volatility, which is represented by the distillation curve determined by ASTM standards D2887 and D86, has been correlated to lean blow off due to its impact on the time to evaporate. Essentially the greater the volatile of the fuel, the shorter the evaporation time and hence long LBO [48]. Alternative fuels, which have distillation curves that are similar to the conventional fuels tested (colored green in Fig. 2), have been observed to have performance characteristics similar to the conventional fuels. The fuels that had higher distillation temperatures were found to portray poorer LBO limits with respect to the conventional fuels (colored red in Fig. 2). Fuels that have higher distillate temperatures may require cold ignition and LBO tests. The yellow region in Fig. 2 indicates fuel with distillation temperatures below that of Jet-A1; these fuels may fail flashpoint requirements for the fuel [48]. In a recent study by Ahmed et al. [42], impact of ignition delay on LBO limits and combustion instability has been presented. It was observed that ignition delay also has a substantial influence on combustion volatility in a gas turbine combustor and on lean blow-off limits. In another study, lean blow-off

performance was measured on a Honeywell APU [49]. In Fig. 3, importance of different fuel properties and combustor properties has been presented for impact on LBO. In APU testing for LBO, among fuel properties, distillation curve has the highest impact on LBO, whereas reference velocity has the highest impact on LBO among combustor properties. In recent years, significant progress has been done toward computational prediction of LBO for alternative fuels. In a recent study by Hasti et al. [50], step-by-step model and its validation has been presented. Impact on LBO as a result of fuel property change on T63 turboshaft engine has been presented in a recent study [51]. It was observed on T63 engine that fuel vaporization and atomization processes had the most impact on LBO. 10% recovered distillation provided the highest correlation with LBO [51]. This was in-line with testing done by Honeywell [49]. In another work by Zheng et al. [52], high-speed imaging was used as a part of evaluating the impact of fuel on LBO process. This study was done on a Rolls-Royce Tay single can combustor and has given different insight of LBO process. Fig. 4 shows the images of different types of fuels just before the lean blow off happens.

Figure 3 Average feature importance on LBO [49].

In a recent study by Corporan et al. an Allison T63 turboshaft engine was used to evaluate the impact of fuel properties on lean out (LO) performance [51]. The LO has been indicated in the study as stalled compressor or blown-out flame. Fig. 5 shows the impact of different fuels on the LO process. It can be seen that 10% recovered distillation temperature, which is a fuel property, has a significant impact on LO process, where heat input rate at LO has changed significantly when the fuel is changed. The Strongest correlations of R² = 0.66 and R² = 0.85 were observed for fuel viscosity and 10% recovered distillation temperature respectively.

Figure 4 Images of different types of fuels during the last 0.1 s of LBO [52].

Figure 5 T63 engine heat input rate against recovered temperature curve [51].

5. LBO prediction

LBO predictions in gas turbine combustors can provide technical for the development of the combustors and fuels in the primary design stage. Lots of early studies focused on building semiempirical models between and the parameters related to operating conditions and combustor configuration. A widely used LBO prediction model was proposed by Lefebvre [29] based on the conception of well-stirred reactor, which describes the overall fuel/air ratio at LBO as a function of combustor configurations, operating conditions, fuel properties, and atomization qualities, shown in Eq. (2). But the discrepancy rate between the calculated result and actual value is within approximately 30%.

[2]

There is an argument on this model about the neglection of the upstream dilution influence. Therefore, an improved semiempirical LBO is introduced by Xie et al. [53] and written as Eq. (3), where α is the combustor air fraction and β is the dimensionless flame volume. The model involved the concept of flame volume observed in the experiment, and the effect of dome design and primary zone arrangement has been considered. Xiao and Huang [31] tested the model on 15 different burners with diverse flow regimes to validate the correlation between the LBO prediction and practical combustor. The uncertainty of this approach to measuring LBO limits is within ±20%. But the flame volume is obtained from experiment results, and it loses the guiding significance for initial combustor design.

[3]

Progresses in computational and kinetics solvers (CFD and chemical kinetics) provide researchers with new insight into the dynamic process of LBO. Hu et al. [54,55] developed a method of fuel iterative approximation to simulate flame volume and minimize the uncertainty to 10%–16%. Wang et al. [56] developed a new method based on Damköhler number and OH concentration reaction zone. The Reynolds-averaged Navier–Stokes method has been widely used in LBO prediction. But Angelberger et al. [57] argued that this approach is inapplicable to lean blow off because LBO is an unsteady phenomenon. The unsteady Reynolds-averaged Navier–Stokes and large eddy simulation (LES) were conducted by Black et al. on a low-emission gas turbine fuel injector to assess their ability to capture LBO [58]. Later on, the LES combined with other methods, including the Lagrangian, transport element method [59], eddy breakup model [60], laminar chemistry model, eddy dissipation concept mode [61], probability density function model [62,63], and feature–parameter–criterion [64] were employed to explore the transient dynamic characteristics of flow and flame at LBO. Casselberry et al. [43] developed a model to predict LBO using supercritical pyrolysis products. Fig. 6 shows the calculated LBO performance as compared with measured LBO performance for the model developed by Casselberry et al. Won et al. have stated that of various fuel substitutes, the chemical and physical properties are novel to the wider conventional fuel industry [41,65], which means that the performance of novel alternative fuels cannot be directly extrapolated from research conducted when fuel was a commodity. Currently, there is no model that defines the chemical and physical properties of jet fuel and their combustion performance [41]. Pilot-scale experimental investigation is imperative to broaden the knowledge regarding the impact of alternative fuels on the operating life, performance as well as the environmental impact with respect to engines and their components.

Figure 6 Calculated LBO versus measured LBO equivalence ratio [43].

6. LBO summary

Overall, it has been observed that fuel has significant impact on lean blow off and could eventually impact engine operability. Several studies have shown significant impact of DCN on LBO [42,48–51]: with increasing DCN number, LBO becomes better. There are also studies that show impact of distillation curve, viscosity, and other fuel properties on LBO [49]. Combustor properties also play a vital role in LBO. As a conclusion of this chapter, it can be said that for future fuels, impact on LBO could be considered.

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Chapter 8: Thermal stability and impact of alternative fuels

Jerry Hamilton ¹ , Yousef Sadat ² , Matthew Dwyer ² , Pierre Ghali ² , and Bhupendra Khandelwal ¹ ¹ Department of Mechanical Engineering, The University of Alabama, Tuscaloosa, AL, United States ² Department of Mechanical Engineering, The University of Sheffield, Sheffield, United Kingdom

Abstract

The fact that turbine engines are running at much higher temperatures and compression ratios drives the need to use fuel, prior to combustion, as a heat removal source confirms the need for a clear understanding of fuel thermal stability. As new alternative jet fuels are introduced into the commercial market, it is imperative that fuel thermal stability is well known and understood. All fuels contain numerous compounds and additives that have an effect on fuel thermal stability. It is important to review fuel composition and the reactions that occur when exposed to oxygen, when heated, and also when blended with other fuels. Currently, the issue of thermal stability in fossil jet fuels and alternative jet fuels is a barrier to having blends of greater than 50% for commercial use. Different pathways and schemes to carbonaceous deposition are discussed along with results of current laboratory testing and historical research. Methods of testing and analysis of the thermal stability of different fuels and blends are also presented in this chapter. The goal carbon net-zero fuels can be accomplished with increased research and attention to these topics.

Keywords

Alternative jet fuel; Oxidation; Thermal stability

1. Introduction

The jet (turbine) engine became an operational opponent to the conventional piston engine during the 1930s, but for jet engines to become the forerunner in aviation propulsion, a tremendous amount of effort was needed to make the engine much more efficient. As turbine engine technology advanced, the powerto-weight ratios increased through ingenuity and determination of engineers to meet the demands of the ever-growing aviation sector. The turbine engine had to compress air, combust fuel, and produce thrust more efficiently than the piston counterparts. Engineers have been able to increase the efficiencies due in part to higher compression ratios and fuel burn rates. Consequently, as more heat is generated during the production of work, excess heat energy must be removed from the system. As new technologies drove turbine engine operating at higher temperatures, the task of removing the excess heat without adding elaborate cooling systems was needed. One method for removing excess heat from turbine engine systems is using a “liquid to liquid” cooler. This system allows heat to be exchanged between the hot engine oil to the colder fuel as shown in Fig. 1. One of the major advances to jet engine performance is the increase of engine compressor pressure ratios [1]. This enhancement increased the efficiency of the engine compressor, which reduced the fuel burn and power output. However, the decrease in fuel burn is relative to the older engine model attempting to produce the same amount of work. With an increased fuel burn, the power output increases along with the operating temperature and excess thermal energy. The excess thermal energy is removed by various methods, but in general, engine oil systems absorb the majority of the heat and are then cooled by a combination of ambient air and heat transfer to the fuel.

Figure 1 Example of liquid-to-liquid heat exchanger. Hot oil is cooled through cold fuel bath while heating fuel.

Additionally, energy is the most important factor shaping the current society in the 21st century [2]. Since the start of the industrial revolution, carbon dioxide emissions have been growing rapidly due to excessive use of fossil fuels, resulting in an increase in overall temperature of the atmosphere [3]. Therefore, there is a need to consider replacement of conventional fossil fuels with alternative fuels; this switch requires a deep understanding of the current fuel systems on top of knowing the properties of these alternatives. Aviation as one of the most important sectors of transportation heavily depends on fossil fuels. It currently s for 2%–3% of annual CO2 emissions, and it is projected to increase to 11% by 2100 [4]. To mitigate the CO2 emissions and reach an optimum performance, it is essential to have a thorough understanding of aircraft systems: jet engine in particular. In comparison with a conventional piston engine, the jet engine is capable of containing 70 times more worth of air into the system and producing 20 times more energy. Gas turbine or a turbo engine consists of an open tube in both ends with a free flow of air. Prior to getting heated in the combustor and producing thrust, air is compressed and premixed with fuel. Basic principles of thermodynamics suggest that for a larger jet velocity, a hotter engine with higher temperature difference is desired [5,6]. Running the engine at such elevated temperatures is associated with some problems, which needs to be addressed. This chapter provides details of the fuel degradation process as heat is added prior to the combustion process. Before diving into thermal stability, an overview of different fuels that are used in the aviation sector to include methods of alternative fuel production is introduced for better clarity. Because thermal stability is occurring at the molecular level, attention has been given to describe components of fuels and the contributions or hindrances to the main topic of thermal stability. In addition, topics such as ongoing research, cutting-edge concepts, methods and test apparatus for data collection, and more are included for a better understanding of how and why fuel thermal stability is such a vital area worthy of intense study.

1.1. Jet fuels

During the early developments of the jet turbine engine, it was thought, and demonstrated in use, that aviation was insensitive to the fuel being used for combustion. As such, a wide cut of kerosene was used. However, due to the increased development and specialization of jet engines, it has become important to regulate the properties of the fuels in service and thus provide controls on its chemical composition. This is carried out by the fuel specification documents released by aviation regulation bodies around the world. The most followed fuel specifications are controlled by the American Society for Testing and Materials international (ASTM) and MIL-STD-3004D (US Department of Defense) in the United States and the Defense Standard (DEFSTAN) in the United Kingdom as well as European Union regulations. The fuel specifications set out the property requirements and methods of production for aviation fuel. Currently, jet fuel needs to meet a substantial amount of specifications such as vapor pressure, flash point, density, viscosity, freezing point, and density, to name a few, [7]. These requirements have an impact on the chemical compositions of the fuel that can be used, as straight chain alkanes will have freezing points to high for use in cold environments, while highly branched alkanes will lead to densities that are too low. As well as these implied controls on fuel composition, the specifications also provide explicit controls on the concentrations of specific species in the fuel. The volume of aromatics in aviation fuels is limited to 25%, and the volume of naphthalene is limited to 3%. The fuel specifications also limit the number of impurities, which can be found in the fuel, such as copper and sulfur species. Aviation fuel is a complex mixture of long-chain hydrocarbons C12–C14 with various compositions depending on where the crude comes from and what manufacturing process it goes through. Liu et al. [8] have characterized jet fuel into four main groups depending on which source and what route it has been processed through:

Figure 2 Kerosene production from conventional crude sources. Reprinted from Liu G, Yan B, Chen G. Technical review on jet fuel production. Renewable and Sustainable Energy Reviews 2013;25:59–70. https://doi.org/10.1016/j.rser.2013.03.025, Copyright 2013, with permissions from Elsevier.

1. Conventional process from crude oil 2. Unconventional oil sources, e.g., oil sands and oil shale 3. Fischer–Tropsch synthesis 4. Renewable sources

The majority of jet fuels come from crude oil sources, having processed through a method known as atmospheric and vacuum fractional distillation, which operates based on different boiling points of individual compounds present in the crude oil. The middle section of the distillation contains the raw materials required for production of jet fuel, mainly known as kerosene [8] (see Fig. 2). The raw materials will be later hydrogenated to create kerosene, aiming for a high ratio of hydrogen/carbon components and production of n-, cyclo-, and isoparaffins [9]. These constituents provide a suitable energy content and combustion properties due to their high hydrogen/carbon ratio and very low freezing point, which prevents the jet fuel from crystallization at high altitudes. In contrast to these paraffins, aromatics with lower hydrogen/carbon ratios have lower energy content and poor combustion properties, ultimately leaving carbon deposits on the tube walls resulting in engine damage or failure [10]. Conventional jet fuel contains about 20%–25% aromatics [11]; this problem poses a deposition risk to fuel and engine systems, which needs close attention. In contrast, having too little aromatics would negatively affect seals in the systems the fuel encounters. Since jet fuel could be originated from a variety of sources in different geographical areas, it could have different compositions

depending on the source (Fig. 3). This could make complications in predicting the fuel behavior in the engine systems.

Figure 3 Simplified view of the crude oil fractional distillation products.

As the aviation sector moves toward a more sustainable future by reducing CO2 emissions, there is a need to consider how jet fuel could be made from more environmentally friendly sources. Among the four categories mentioned earlier, Fischer–Tropsch (FT) process synthesis and renewable jet fuel could be promising. The FT process is particularly interesting as it could use the CO2 from carbon capture units and reduce the overall amount of CO2 in the atmosphere from a life cycle perspective. The FT process is widely known as the main technological process for conversion of syngas to liquid fuels. Firstly, a mixture of hydrogen and carbon monoxide is created; secondly, the undesired compounds and impurities such as CO2 are removed; thirdly, straight chains of hydrocarbons are formed through the FT process, and finally, the long-chain hydrocarbon goes under treatments such as cracking to create a desired fuel [12]. Main reactions involved in the FT process are shown in the following. Reactions 1 and 2 represent synthesis of paraffinic or olefinic chains. Reaction 3 known as the water–gas shift reaction produces CO2 and hydrogen. Side reactions 4–6 also happen to generate undesired methane and small amounts of alcohol [13].

[1]

[2]

[3]

[4]

[5]

[6]

FT-generated fuels have several properties that could make them suitable jet fuels; they offer increased energy performance and a reduced payload capacity due to their high specific energy leads [14]; they do not contain sulfuric compounds such as SO2 and H2SO4; therefore, having improved thermal stability [15] and lack of aromatics make them cleaner burning fuels. However, aromatic-free FT fuels fail to meet minimum density requirements for jet fuel, and this absence could cause leaks in fuel and engine systems sealing as explained earlier [8]. Nonetheless, this issue could be resolved by blending certain FT fuels such as synthetic paraffinic kerosene (SPK) with conventional aviation fuels. Fig. 4 illustrates a schematic example of FT synthetic fuel production through sewage sludge and CO2 capture. This study by Dimitriou et al. [16] has examined four routes for production of liquid transport fuels from carbon capture and utilization (CCU) processes; after performing techno economic and sensitivity analysis, it states that production of liquid fuels from CCU still has a significant economic disadvantage. Nevertheless, Wilson and Styring [17] believe that synthetic fuels will play a key role in future of energy systems due to their lower price of handling, transport, and storage in comparison with electrical batteries and other renewable energy sources.

Figure 4 An example of process schematic for production of synthetic transportation fuel from FT Process. FT, Fischer–Tropsch. Dimitriou I, García-Gutiérrez P, Elder RH, Cuéllar-Franca RM, Azapagic A, Allen RWK. Carbon dioxide utilisation for production of transport fuels: process and economic analysis. Energy & Environmental Science 2015;8(6):1775–89. https://doi.org/10.1039/C4EE04117H, with permissions from The Royal Society of Chemistry.

Vegetable oils and animal fats, which can be hydrotreated to create long-chain hydrocarbons (C9–C18), are another source of alternative aviation [18]. Similar to the FT process, SPK fuel (Fig. 5) could be created from biorenewable sources such as fatty acids and glycerides. One of the most popular routes for production of bioderived SPK is Synfining, which involves three main steps. Firstly, the impurities such as water and catalysts are removed; secondly, in a hydrotreater, fatty acid chains go through exothermic hydrogenation and deoxygenation reactions to produce n-paraffins; and lastly, long-chain paraffins get hydrocracked to produce the desired fuel [8]. Similarly, the SPK fuel from renewable sources requires blending to reach the optimum number of aromatics before being used. Although these types of renewable fuels illustrate a positive perspective in future of aviation fuel, they are yet to become economically viable, as previously stated. Most of them are associated with significant capital investment; therefore, further research is needed to look into reducing their production costs.

Figure 5 Illustration of typical syngas products.

The fuel types discussed all have different compositions and also demonstrate markedly different thermal stabilities during use. Straight-run kerosene, produced from fractional distillation, is the easiest fuel to manufacture as no postdistillation treatment occurs. Straight-run fuels are formed from the middle distillate fraction of the oil and have an average carbon chain length of 10–16. Because straight-run fuel is not treated to the extent of other fuels, the hydrocarbon and chemical composition is controlled by the oil source feedstock. This in turn may affect the properties of the fuel from that feedstock or distillery. Due to the lack of control that the distillation has in regard to the fuel, the product may have high isoalkane, alkene, and aromatic concentrations compared with fuels produced using other methods. Due to the broad ranges of fuel compositions, it is very difficult to predict how a straight-run fuel will behave at high temperatures. For instance, fuel from Brent crude or west Texas sources demonstrates better thermal stability that those from Venezuelan or Indonesian sources. It is not uncommon for petroleum products to vary in the proportionality and type of hydrocarbons from oil fields. The composition of petroleum products could be radically different from others in the same location [19]. To improve the control over the composition of the fuel, and thus provide improvements of its thermal stability, the straight-run kerosene can undergo postdistillation processing. This is done through hydrotreating the middle distillate using catalysts. Hydrogenation catalysts are used to chemically reduce the heteroatomic species found in the fuel, such as sulfur and phenolic species, into hydrocarbons. Studies have shown that changing the extent of hydrotreatment of a straight-run fuel impacts its thermal stability [20]. Mildly hydrotreating a fuel improved its breakpoint by 68–354°C, a significant increase, so clearly these polar species have a negative effect on the thermal stability of a fuel. Further hydrotreatment can occur if more specialized catalysts are used to further alter the fuel [21]. They hydrogenate the unsaturated hydrocarbons, such as alkenes and aromatics to alkanes (Fig. 6). These processes have the effect of producing a fuel, which is more chemically pure, and they demonstrate improved thermal stability. This more severe hydrotreatment increased the fuels

stability of the same fuel to >371°C. The polar species found in a fuel clearly reduce the breakpoint of the fuel, but the hydrocarbon composition of the fuel also plays an important part in the formation of oxidation products and insoluble gums even in the single fuel that was used in these tests.

Figure 6 Illustration of hydrocarbon alkanes.

Another class of jet fuel is manufactured from the heavy oil residues left over after the distillation process. The heavy waxes can be found in greater concentration in poorer-quality oil reserves that are harder to exploit, such as in Venezuela or Indonesia. However, due to the economic drive for oil, they are increasingly being exploited [8,22]. These heavy oils require greater processing for them to have the correct physical properties so that they can be used as jet fuel. They are hydrocracked to lower-molecular-weight produces, using metal catalysts. If untreated, these fuels have poor thermal stability, as they have higher metal contamination and polar species concentrations. This means that fuels produced using this method usually undergo postproduction treatment to remove the high concentration of impurities. These production methods to improve the fuels thermal stability, however, increase the cost of production and the fuel. These changes also have the result of negatively affecting other physical properties such as reducing the lubricity of the fuel. While the middle distillates of light oils from the Middle East and the North Sea are very good, in general, straight-run kerosene has lower thermal stability than fuels produced from other methods due to the high levels of polar species and unsaturated hydrocarbons, which are not removed from these fuels. Alternative fuels provide a solution to this issue, and they offer greater control over the chemical composition and provide energy security. They offer a number of benefits that cannot be currently achieved using conventional fuels. For this to become clear, the chemistry of the oxidation process must be introduced and understood.

1.2. “Drop-in” fuels

In recent years, significant research and testing have been carried out regarding alternative fuels. The ability to “drop-in” alternative fuels to replace Jet-A1 or JP-8 is a highly sought-after characteristic of the new fuels, as it would enable

implementation with little-to-no modification of current infrastructure and propulsion systems. Ideally, alternative fuels would have combustion characteristics similar to current jet fuels, and their aromatic content would allow for fuel system seal swell to prevent leaks. These fuels should also present improved emissions characteristics to comply with current and expected regulations. Laminar flame speed and ignition delay can be used to determine whether two different fuels behave similarly. Richter et al. (2018) compared those two properties for four “drop-in” fuels (compliant with ASTM D7566) with that of Jet-A1 [23]. The tested fuels were two variations of FT fuels, namely coal-toliquid and gas-to-liquid (CtL and GtL, respectively), farnesane, and alcohol-tojet (AtJ-SPK). Findings show that AtJ-SPK is less reactive than other fuels over the tested equivalence ratios and temperatures. The authors noted additionally that “in general, the differences between AtJ-SPK, farnesane, and Jet A-1 at Φ = 2.0 are small within the temperature range of this study. This is also true for the measurements performed at Φ = 0.5 for GtL, CtL, and Jet A-1” (Richter 2018). Effectively, the investigated fuels' flame speed and time delay variations from Jet-A1 are small enough to consider the combustion performance to be the same (Richter 2018). This work shows that alternative fuels can perform similarly to existing ones, which would facilitate their implementation as far as combustors are concerned. It is important to note that blending alternative fuels with current fuels is also a possible solution, which has been investigated previously. As mentioned, existing fuels contain aromatics that cause various seals in the fuel system to swell, thereby preventing leaks. Therefore, alternative fuels must have the same seal-swelling capabilities to replace current fuels without incurring costs due to required retrofitting of fuel systems. Link et al. (2008) investigated different additives for FT fuels to restore seal swelling characteristics, as well as their impact on the resulting thermal stability of the fuels. First, the authors determined that “species having an aromatic ring along with a polar substituent linked to the ring, such as phenols, naphthols, and benzyl alcohol” were capable of causing seal swell comparable with that of conventional jet fuel [24]. Another study by Corporan et al. focused on six potential jet fuel alternatives from different feedstocks, comparing them to JP-8 with respect to seal swell and emissions [25]. Using Shell, Sasol, and Rentech FT fuels, R-8, and camelina and tallow hydroprocessed renewable jets, the authors determined that the alternative fuels were able to swell nitrile O-rings

(7.0%–9.6%), though not at the same level as JP-8 (16.6%). Their findings are reproduced in Table 1.

Table 1

Fuel

Volume swell (%)

9.6

Shell FT

9.5

Sasol FT

7.8

Rentech FT

7.0

R-8

9.1

Camelina HRJ

8.6

Tallow HRJ

16.6

JP-8

8.1

Syntroleum S-8

FT, Fischer–Tropsch; HRJ, hydroprocessed renewable jet.

Reprinted with permission from Corporan E, Edwards T, Shafer L, DeWitt MJ, Klingshirn C, Zabarnick S, et al. Chemical, thermal stability, seal swell, and emissions studies of alternative jet fuels. Energy & Fuels 2011;25(3):955–66. https://doi.org/10.1021/ef101520v. Copyright 2011 American Chemical Society.

Alternative fuels do have some of the seal-swelling characteristics of current jet fuels, although they tend to underperform slightly. More research is required to identify aromatics or additives that can improve alternative fuel seal swell capabilities. Beyond new fuels' effects on the fuel system, it is important to consider the environmental impact associated with their combustion. Pure alternative fuels such as SPKs and fully formulated synthetic jet fuel emitted substantially less particulate matter (PM) when tested in an APU [26]. Bulzan et al. (2010) noted during their testing that when running a pure FT fuel with 0.6% aromatics, leakage from the fuel system was observed, which only gradually subsided once an FT/JP-8 blend was introduced in the fuel system [27]. This highlights the importance of seal swell characteristics, which must be considered alongside the emissions impact of including aromatics. Corporan et al. (2011) found in their study that the six alternative fuels also had significantly fewer particulate emissions at low engine power (where the largest differences were observed). The emissions index number was reduced by 90%– 98% and 60%–80% in idle and cruise conditions, respectively. Gaseous emissions testing showed negligible effects on carbon dioxide (CO2) and nitrogen oxides (NOx), though normalized carbon monoxide (CO) and unburned hydrocarbon (UHC) emissions reached reductions of 10%–25% and 20%–30%, respectively. However, using additives can increase soot production, as determined by DeWitt et al. (2008). They studied the seal-swelling abilities of three commercially available aromatic solvents mixed with an FT fuel and then

tested these in a T63 gas turbine to gather emissions data. The authors found that increasing the aromatic molecular weight and concentration increased swelling abilities, which is beneficial, but also increased PM emissions—a relationship shared by all the aromatic solvents they tested with the FT fuel (both independently and blended) [28]. An option for lessening the dependence on current fuels and reducing their environmental impact is to blend them with new fuels. This also helps offset the lack of aromatics in some alternative fuels. Corporan et al. tested various blend percentages of JP-8 and a FT synthetic jet fuel (synjet) in a T63 helicopter gas turbine as well as a research swirl combustor [29]. Under cruise and idle conditions, the particle number and mass decreased with an increase in percentage of synjet in the blended fuel. Using the synthetic fuel in the T63 had a reduced effect on gaseous emissions; only the sulfur oxides reduced.

Figure 7 Reduction in PM mass emissions from a T63 engine as a function of synjet fuel in blend, for idle and cruise conditions. PM, particulate matter. Reprinted with permissions from Corporan E, DeWitt MJ, Belovich V, Pawlik R, Lynch AC, Gord JR, Meyer TR. Emissions characteristics of a turbine engine and research combustor burning a Fischer−Tropsch jet fuel. Energy & Fuels 2007;21(5):2615–26. https://doi.org/10.1021/ef070015j. Copyright 2007 American Chemical Society.

For the swirl combustor, several blends of synjet and JP-8 were burned at various equivalence ratios ranging from Φ = 0.6 to Φ = 1.1. The chief results mirror the data from the T63 turbine; blends of synjet and JP-8 have decreasing S02 concentrations for increasing percentage of synjet in the blend. Though differences exist between the T63 turbine and the experimental combustor (combustion pressure and equivalence ratios), the results from both experiments suggest that blending synjet with JP-8 can offer reductions in PM (Fig. 7). Another study tested four alternative fuels on a DC-8's CFM56-2C1 engines [27]. These fuels were comprised of two pure FT fuels, and two 50/50 blends by volume of the FT fuels and JP-8. Strikingly, their findings show that the engine performed similarly from a combustion perspective across all fuels tested. Interestingly, one of the FT fuels lowered gaseous emissions in of NOx at higher power settings. For particulate emissions, the FT fuels and blends performed better than JP-8, with lower black carbon mass at all analyzed fuel flows. Additionally, experiments were also run on the aircraft's APU, one running pure FT fuel and comparing with emissions performance using JP-8. These results indicate slight improvement in NOx, CO, and UHC emissions indices, as well as significant reduction in black carbon mass across all temperatures of operation compared with JP-8. Another study involving blends of fatty methyl esters (FAMEs) and FT fuels with Jet-A1 shows that the alternative fuels they tested had lower mass-based and number-based emissions indices compared with the Jet-A1 baseline [30]. Emissions from alternative fuels show promising results, where these new fuels performed better especially in of PM emissions. Further research is necessary to identify the right aromatics that provide the proper seal-swelling characteristics without significantly increasing emissions.

The reduction in reliance on petroleum confirms not only an environmental advantage but potentially a strategic advantage as well. When effectively implemented, a nation with reduced dependency on the international oil market reaps the benefits in of logistics and energy supply. The US Air Force (USAF) conducted tests with synthetic fuel on a Boeing B-52H strategic bomber platform; in 2007, a FT blend was approved for use on the aircraft. The test campaign included operating the aircraft exclusively on the new blend, as well as cold weather testing to evaluate the fuel's thermal stability. The tests were successful, and based on these results, other aircraft in the USAF's fleet, such as the Boeing C-17 transport aircraft and Lockheed Martin F-22 stealth air superiority fighter, were slated to begin synthetic fuel operation testing in 2010 [25,31].

1.3. Alternative jet fuel pathways

According to a report by the US Department of Transportation (2020), alternative fuel pathways must rigorous examinations to be approved for commercial use. The standards that must be met are published under ASTM D4054, and approved pathways are located in ASTM D7566. As of 2019, there are 10 pathways that have been approved. Some of these, along with the date of approval, are listed in the following [32]:

1. FT—SPK (2009) 2. Hydroprocessed esters and fatty acids (HEFAs)—SPK (2011) 3. Hydroprocessed fermented sugar—synthetic isoparaffins (2014) 4. AtJ—SPK (2016) 5. Applied Research Associates' catalytic hydrothermolysis jet, or ARA CHJ (2020)

6. Synthesized paraffinic kerosene from hydroprocessed hydrocarbons, esters, and fatty acids (HC-HEFA) (2020)

These different pathways produce fuels that are used as drop-in additives. The percentage allowed for these fuels differ because of the different components that are produced through each process. Fuels that have high aromatic or noncyclic alkanes are restricted in very low concentrations for blending.

2. Thermal stability

Among all of the standardized specifications and requirements to be classified as a jet fuel, the thermal stability of fuels and potential fuels has attracted much interest in researchers due to the high level of importance. To understand thermal stability, researchers first must define the problem. According to Hazlett [7], the stability of a fuel could be determined by the fuels resistance to degradation, therefore undermining engine performance after degradation. Thermal stability, also known as thermal oxidative stability, is governed by the composition of the jet fuel to include the additives and trace components as well as the system operating conditions.

2.1. Autoxidation

As noted earlier, aviation fuel is used as a method of extracting heat generated by the turbine engine. As a result, the fuel is then thermally stressed. The hydrocarbon chains within the thermally stressed fuel react with dissolved oxygen to form oxidized products [34]. These reactions occur through a set of radical reaction pathways, in a process called autoxidation. Autoxidation pathways are known as bulk reactions and surface reactions, which occur separately but can influence each other. Fig. 8 illustrates the two pathways. The rate at which autoxidation occurs differs significantly during different engine operating regimes. At higher engine loads, the rate of autoxidation is very rapid as compared with the engine at ground idle due to the increased heat produced by the engine. The rate is also affected by the speed at which the fuel proceeds through the system. For instance, during descent from altitude, the fuel flow is drastically reduced giving the fuel more opportunity for oxidization reactions. As these reactions progress, the hydrocarbons become more oxidized and less soluble in the nonpolar fuel. These oxidized species will agglomerate together and react further, forming more extensive polar products. Eventually the

oxidation products become too polar to remain in solution and crash out, forming the sediments and deposits. Because the fuel system is designed to act as a heat sink to take away heat from different components of the aircraft, the absorbed heat causes the fuel to degrade as the autoxidation process initiates around 140°C. At this temperature, the hydrocarbons begin to react with the dissolved oxygen that is present in the fuel, around 70 ppm. The formation of carbonaceous deposits is a known product of this reaction. Many methods such as fuel deoxygenation, temperature minimization, fuel additives, and surface coating have been developed to mitigate this problem, but none have been able to propose a universal solution [35].

Figure 8 ASTM pathways for synthetic kerosene. Reprinted from Brooks KP, Snowden-Swan LJ, Jones SB, Butcher MG, Lee GSJ, Anderson DM, et al. Chapter 6-low-carbon aviation fuel through the alcohol to jet pathway. In: Chuck CJ, Editor. Biofuels for aviation. Academic Press; 2016, p. 109–50. https://doi.org/10.1016/B978-0-12-804568-8.00006-8, Copyright 2016, with permissions from Elsevier.

As illustrated in Fig. 9, autoxidation starts with a hydrocarbon radical (R˙) formation, which is ittedly not understood completely in the literature. The hydrocarbon radical reacts with the oxygen present in the fuel to form a peroxy radical (RO2 ). This peroxy radical can react with the hydrocarbon fuel (RH) to produce hydroperoxide (RO2H) and close the cycle by creation of another hydrocarbon radical (R ). Presence of antioxidant of phenolic nature (AH) and hydroperoxide decomposing compounds (SH), which include a subset of sulfur compounds such as sulfides and disulfides, can slow down the oxidation process [35]. According to West et al. [36] at temperatures below 120°C, hydroperoxides tend to be the main output of these chain reactions. However, as temperatures increase, the tendency to decompose and create radicals increases the rate of the oxidation process. It should be noted that dissolved metals and organic acids also play a catalytic role to further assist this decomposition. Hydroperoxides also have the ability to react with the reactive sulfur species inside the fuel to accelerate deposit formation, as Jones et al. explain it [20].

Figure 9 Formation of deposition via surface and bulk oxidation reactions.

Because jet fuel is a complex system made of many different components, even smallest amounts of trace materials could have a significant effect on autoxidation and thermal stability. Several studies have investigated the effects that trace species has on the autoxidation process. The autoxidation chain reaction is mainly responsible for fuel thermal stability, and deg experiments to look at the oxygen depletion rates is considerably important. To further understand the process of the autoxidation chain reaction, chemical kinetic models have been developed, which simulate the main chemical reactions during autoxidation. Zabarnick [37] introduced a model in the early 1990s, which illustrated that peroxy radicals are responsible for completing the chain when reacting with oxygen and antioxidants (Fig. 10). This process will create a decline in the oxidation rate in lower concentrations but will have an opposite effect in higher concentrations. The paper also stated how important the role hydroperoxides have as they get thermally decomposed in higher temperatures to increase the number of free radicals. In a much-improved kinetic model, Kuprowicz et al. [35] propose an 18-step reaction mechanism, in which the activation energy and preexponential factor have been represented (Table 2).

Figure 10 Depiction of antioxidants and free radicals.

As previously mentioned, jet fuel is a complex mixture that contains several chains of hydrocarbons and trace species such as sulfur (also written as sulfur), nitrogen, metal and oxygen compounds. Although most of the bulk properties of the fuel remain unchanged during storage or operation, some properties could degrade due to chemical reactions inside the fuel resulting into formation of undesired products such as hydroperoxides, which can form insoluble materials or gums [7]. Although these trace species are present at very low concentrations (ppb levels), they have a significant effect on thermal stability of the fuel. The process occurs as hydrocarbons react with the dissolved oxygen inside the fuel, creating a chain reaction known as autoxidation (Fig. 11), which will be discussed further in the next section. Formation of carbonaceous deposits does not only oppose a safety risk, but it also can be responsible for engine failure and increase in operational costs of aircrafts, which involves airlines to add the appropriate additives to prevent this from happening [38].

Table 2

#

Reaction

A (mol, L, s)

E a (kcal mol −¹)

0

1

0

2

12

3

0

4

5

5

12

6

0

7

0

8

39

9

10

10

15

11

10

12

0

13

10

14

18

15

19

16

0

17

15

18

Reprinted with permissions from Kuprowicz NJ, Zabarnick S, West ZJ, Ervin JS. Use of measured species class concentrations with chemical kinetic modeling for the prediction of autoxidation and deposition of jet fuels. Energy & Fuels 2007;21(2):530–44. https://doi.org/10.1021/ef060391o, Copyright 2007 American Chemical Society.

Figure 11 Diagram of the autoxidation reaction process showing the role of peroxy radical inhibiting (AH) and hydroperoxide decomposing (SH) species classes. Reprinted with permissions from Kuprowicz NJ, Zabarnick S, West ZJ, Ervin JS. Use of measured species class concentrations with chemical kinetic modeling for the prediction of autoxidation and deposition of jet fuels. Energy & Fuels 2007;21(2):530–44. https://doi.org/10.1021/ef060391o, Copyright 2007 American Chemical Society.

2.2. Deposit formation

Deposits have the ability reduce the efficiency of heat exchangers by preventing heat transfer to the fuel and can also create pressure drops in the fuel system. It is important to understand the underlying mechanisms behind the formation of deposits. According to Moses [39], heat transfer to the fuel and chemical reactions initiated by that results in the formation of insoluble gums, which later move toward the walls and form carbonaceous deposits. For decades, it has been suspected that polar aromatic compounds are the preliminary cause of formation of deposits, both during storage and elevated temperatures [40]. This could be explained by the polar nature of the end products of the autoxidation mechanism, in which there is high concentration of oxygen and sulfur [41]. The polarity of these oxidized end products leads them to collectively react together to create large molecules that are insoluble inside the fuel. Although efforts of Hardy and Wechter in 1990s [42] are mainly on diesel distillate fuel, they shine a light on the soluble macromolecular oxidatively reactive species (SMORS) mechanism, which could also explain deposition in jet fuel. For fuels older than 6 months, the amount of extractioninduced precipitate (EIP) is proportional to insoluble materials observed by stability tests, called SMORS. Later by Aksoy et al. [43], initiation of SMORS mechanism has been conceptualized in the following reaction. In this scheme, when a quinone is formed due to autoxidation of indigenous phenol inside fuel,

it reacts with electron-rich aromatics like the carbazole, ultimately resulting in a formation of lightweight SMORS, shown in Fig. 12.

Figure 12 Representation of Scheme 1 SMORS mechanism. SMORS, soluble macromolecular oxidatively reactive species. Reproduced with permissions from Aksoy P, Gül Ö, Cetiner R, Fonseca DA, Sobkowiak M, Falcone-Miller S, et al. Insight into the mechanisms of middle distillate fuel oxidative degradation. Part 2: on the relationship between jet fuel thermal oxidative deposit, soluble macromolecular oxidatively reactive species, and smoke point. Energy & Fuels 2009;23(4):2047–51. https://doi.org/10.1021/ef8007008. Copyright 2009 American Chemical Society.

Beaver et al. [44] suggest that the main step for formation of deposits is the coupling of products of autoxidation chain (electrophiles) with electron-rich aromatics resulting in the formation of SMORS. Further to that, SMORS with electrophilic and nucleophilic ability increase the number of deposits by further reactions. In the later research, Kabana et al. [41] propose a much more detailed mechanism. It basically follows the same principle as Fig. 12: as phenolic compounds get oxidized through a peroxyl radical chain mechanism resulting in the formation of resonance stabilized radicals. They later react with a second peroxyl radical yielding benzoquinone in step 3 of Fig. 13. During step 4, quinones as strong electrophiles have the ability to attack other molecules, resulting in an electrophilic aromatic substitution between quinones and electron-rich aromatics. Quinone dimers are produced in step 5. Ultimately, compounds (3) are formed in reaction 6 as a result of EAS and oxidization, which happens to accurately be aligned with previously studied deposits [41– 43]. A visual representation of the scheme is described in Fig. 13. Formation of deposits creates an insulation layer between the engine walls and jet fuel. This layer disturbs the heat transferred to the fuel, and consequently, the engine will run at a higher temperature to ensure fuel gets the sufficient amount of heat it needs. That decreases the engine' overall efficiency and increases emissions while also leaving a negative impact on the environment. Formation of deposits also increases operational and maintenance costs for airlines since periodic inspections are needed to remove filter blockages. Therefore, thermal stability should remain an active area of research to minimize deposit formation.

Figure 13 SMORS mechanism for formation of deposits in jet fuel. SMORS, soluble macromolecular oxidatively reactive species. Reprinted with permissions from Kabana CG, Botha S, Schmucker C, Woolard C, Beaver B. Oxidative stability of middle distillate fuels. Part 1: exploring the soluble macromolecular oxidatively reactive species (SMORS) mechanism with jet fuels. Energy & Fuels 2011;25(11):5145–57. https://doi.org/10.1021/ef200964z. Copyright 2011 American Chemical Society.

2.3. Oxidation chemistry

Fuels can meet specifications and also have a wide variety of chemical compositions and properties. As such, it is important to discuss what species are in fuel, how this affects its properties, and what consequences this has on its chemistry. Hydrocarbons are the major components of aviation fuel, and each hydrocarbon family or group effects fuel properties differently. The hydrocarbon species found in jet fuel include straight chain n-alkanes, branched isoalkanes, cyclic alkanes of varied sizes, aromatics, and potentially unsaturated alkenes (Fig. 14). It is clear that the aviation fuel is a complex mix of chemical components, which can react to form insoluble sediments and deposits, which can lead to in-service problems. This occurs through the hydrocarbon species reacting with the dissolved oxygen in the fuel. These reactions proceed through radical chemistry mechanisms to form peroxides and other hydrocarbon radicals. The oxidation reaction is initiated by the abstraction of a hydrogen from a fuel hydrocarbon. The hydrogen abstraction can be caused by a metal surface, dissolved metals, or peroxides found in the fuel. The hydrocarbon radical can now react with the dissolved oxygen to form the peroxide radical, which can propagate the oxidation reactions [36]. The reason that the hydrocarbon cannot react directly with oxygen present in the fuel is that they are closed-shell singlet species, while the oxygen is in a triplet spin state with two unpaired electrons. The reaction between two species in different spin states is slow, as reactions must obey the

law of the conservation of momentum, which includes the angular momentum of electrons. This also affects the rate of fission of hydrogen peroxide as the reaction involves the unpairing of two electrons.

Figure 14 Hydrocarbon classes in jet fuel. Reprinted from Brooks KP, Snowden-Swan LJ, Jones SB, Butcher MG, Lee GSJ, Anderson DM, et al. Chapter 6-low-carbon aviation fuel through the alcohol to jet pathway. In: Chuck CJ, Editor. Biofuels for aviation. Academic Press; 2016, p. 109–50. https://doi.org/10.1016/B978-0-12-804568-8.00006-8, Copyright 2016, with permissions from Elsevier.

The hydrocarbon radical and peroxide can react further to form the primary oxidation products, through hydrogen abstraction and radical termination reactions [37]. A number of mechanisms have been developed to describe the processes that form the primary oxidation products. Some of these mechanisms were developed by Zabarnick et al. and include the roles that metals and sulfur species play in the oxidation of fuel. The products of these reactions are alcohols, aldehydes, ketones (Fig. 15), and ethers. These species are more polar than the majority hydrocarbon fuel, and as such will agglomerate together to minimize the negative enthalpic interactions with the apolar hydrocarbons.

Figure 15 Ketone example, acetone.

This agglomeration brings the primary oxidation products close together. The oxygenated species then react with the heteroatomic species found in the fuel to form larger partially soluble species. These reactions proceed to form highermolecular-weight, more-polar species until they are no longer soluble in the jet fuel. These reactions are thought to progress through a mix of radical reaction mechanisms and electrophilic aromatic substitution reactions with the aromatic oxidation products and heteroatomic species; this is known as the SMORS mechanism and was proposed by Hardy et al. and developed by Beaver et al. [41,42], as discussed previously. These high-molecular-weight polar species have high heteroatomic and aromatic content. They agglomerate together, forming insoluble sediments at storage temperatures. At increased operating temperatures, these reactions not only proceed at a much higher rate but also cause the products to bind to the surface forming insoluble deposits on the fuel system working parts. This suggest that binding to the surface is not barrierless and requires energy to be put into the system to form the chemical bond to the surface, in this case by an increase in temperature and thereby the kinetic energy. As the literature indicates, the initial deposit on the surface has a high concentration of sulfur. Thus, at increased temperature, the sulfur in the insoluble sediments must be able to be chemisorbed to the surface and form strong covalent bonds. The initial deposit is then able to react through the same mechanisms as in the liquid phase increasing the weight of deposit over time. All of these processes lead to a full picture of a global oxidation mechanism, which describes the formation of sediments at storage temperatures, and the deposits found at higher temperature synonymous with thermal stability issues [45]. Early work on the effect that a hydrocarbons chemical structure has on its oxidation rate was carried out in the polymer industry, investigating their use as monomers (Fig. 16) as well as in relation to their thermal stability as fuel. The work by Mayo et al. and Taylor et al. investigated the rate of oxidation of a number of saturated and unsaturated hydrocarbons including aromatic species [46,47]. They found that olefins, particularly styrene, were very reactive to radical species and formed large-molecular-weight products. Under oxidation

condition, these products would also have high amounts of oxygen present in them. It is unclear if the styrene monomers are oxidized prior to polymerization or if the extended oligomer system, with high aromatic stabilization of oxidation products, are oxidized postpolymerization. Due to the fast rate of polymerization of the styrene monomers, once initiated, it would be suggested that the oligomer is oxidized after formation. The literature indicates that all alkenes have a very poor effect on thermal stability. The alkenes can form oligomers in the fuel, through reacting with the initiated radical and propagating the reaction. The alkenes are also easily oxidized, as the hydrogen next to an unsaturated double bond can be easily abstracted and the radical formed is stabilized by conjugation with the double bond. Alkenes are also labile toward addition reactions with the oxygenated products of the fuel, yielding extended oxygenated compounds. For this reason, alkenes are seen as undesirable in the fuel, as they do not provide any benefits to the other properties of the fuel and are excluded.

Figure 16 Depiction of monomer and polymer molecules.

Alkanes fall into two branches, cyclic and noncyclic, which can be branched or unbranched [48]. Previous work investigating the effect that branching has on the abstraction has shown that tertiary carbons react quicker, forming the radical products. This is caused by the adjacent carbons donating electron density to the electron deficient radical center, stabilizing the radical through inductive effects. Experimental work carried out at IFPEN found that increasing chain length caused a reduction in the induction period of the fuel. This continued up to a plateau reached at 12 carbons chains and longer. Mixtures of hydrocarbon by chain lengths showed a linear response behavior, indicating that it is to do with the ratio of terminal and chain carbons in the hydrocarbon. This s the theory that the rate of hydrogen abstraction is linked to stabilization of the radical through inductive effects. The roles that cyclic alkanes have on the thermal stability are open to debate. They have increased density compared with isoalkanes. This means that if they show improved thermal stability compared with aromatics, they could be used to improve the range of a fuel while not reducing its thermal stability. Previous work indicates that they do not increase the inductions period, so are not acting as an antioxidant in the fuel. Nor do they increase the deposit noticeably when compared with aromatic species. However, the literature shows that not all aromatics and cyclics behave the same, so the exact nature of these mechanisms needs to be clarified before development of the fuel can proceed.

3. Fuel composition and thermal stability

Fuels must be able to withstand thermal stresses due to the customary practice in aircraft design of using fuel as heat sinks or coolants; moreover, as the fuel approaches the engine and combustors through the fuel system, its temperature begins to rise, and a range of chemical reactions begin to take place. These reactions may give rise to particles in the fuel, which may or may not be soluble [7,49]. Particles such as these would have an obvious detrimental effect upon filters and valves in the fuel system, which may lead to blockages and disruptions during fuel flow [50]. Increased pressure ratios in turbines of the 21st century pose several issues in of thermal stability, in which the higher-pressure ratios result in higher thermal loads upon the lubrication system. This results in an increased heat sink demand [51]. The increased thermal efficiency of the turbine cycle caused by the increased pressure ratios also results in decreased fuel flow rates, which while ideal in when considering efficiency puts more strain upon the thermal characteristics of the fuel in that a lower volume of fuel must absorb more heat. This increase in engine heat also results in convection transfer into the mechanical components of the fuel system such as the swirlers and burner feed arms [51]. All these factors individually and combined result in the degradation of the fuel and may cause carbonaceous deposits to form in areas that come into with the fuel, much like atherosclerosis in human blood vessels. The causes for these deposits are multifactorial, and some of these causes are as follows [7]: fuel composition, fuel temperature, duration of thermal exposure, flow characteristics, surface roughness of the fuel wetted areas, etc. Consideration must be given to the effect of fuel composition on engine components as well as the adverse effects to the environment. Although the aviation industry contributes very little to emissions pollution as compared with all other sources (approximately 6% of global fossil fuel consumption [52]), there is a determination to produce fuels that will minimize the negative effects of the use of fossil fuels. The pathway to fully synthetic fuels has already begun with blending of fossil and synthesized fuels. This method attempts to utilize the

current equipment without radical or complete redesign [52,53]. Perhaps new engine systems will be developed for fully synthetic fuels in the future, but that is a massive undertaking. That being said, new fuels are being developed to be compatible with the current engine technologies in mind. This is where the composition of the fuel plays a vital role and understanding what the thermal stability will be when engineering new fuels. It is well known that crude oil composition varies from different locations across the globe, possible within the same oil field as well, and the time of year may also affect the composition as well. The crude oil is then refined to create many different products including kerosene. Kerosene is mainly composed of hydrocarbon chains with 6–20 carbon atoms per chain (Fig. 17). Conventional jet fuel, like Jet-A and Jet-A1, is produced by fractional distillation of petroleum processes and is a special branch of kerosene. This special category of kerosene typically contains hydrocarbon chains of 9–16. The hydrocarbon molecular chain length is a factor in the physical state of the fuel. Because of the length of the kerosene molecules, jet fuel is a liquid at room temperatures. Kerosene can also be produced by other methods. The process described before by distilling crude oil produces “straight-run” kerosene. Another way to extract the kerosene is from “cracking” heavier petroleum feedstock hydrocarbon chains [54] through thermal or catalytic processes.

Figure 17 Hydrocarbon groups in a three-dimensional illustration.

Because the feedstock of jet fuel varies so much, different processes are employed to remove unwanted components after the initial production of the jet fuel. Sulfur and nitrogen are just two components that when removed enrich the kerosene. Hydrodesulfurization and hydrotreating are two methods of removing these compounds. Even with all the variations of these fuels and processes for enriching, the basic hydrocarbon components, shown in Figs. 18 and 19, of jet fuel are as follows [52]:

1. n-alkanes—long-chain, unbranched 2. isoalkanes—long-chain, branched 3. cyclic alkanes—including naphthenes 4. aromatics

Figure 18 Depiction of jet fuel components, Braun and Riedel [52]. Reprinted with permissions from Springer Nature, Copyright 2014.

Figure 19 Common hydrocarbons found in jet fuel: (A) paraffins; (B) naphthenes; (C) olefins, and (D) aromatics. Reprinted from Baena-Zambrana S, Repetto SL, Lawson , Lam JKW. Behaviour of water in jet fuel—a literature review. Progress in Aerospace Sciences 2013;60:35–44. https://doi.org/10.1016/j.paerosci.2012.12.001, Copyright 2013, with permissions from Elsevier.

3.1. Alkanes

n-Alkanes form the majority of the hydrocarbon fuel with high hydrogen to carbon ratios. Due to n-alkanes uniform structure, they have high freeze points, as they can pack tighter, forming stronger van der Waals forces between the chains. They also have high viscosity at low temperatures, reducing the ability to pump the fuel to the engines. To combat these issues, isoalkanes can be used to reduce the freezing point of the fuel and improve their viscosity. However, this reduces the energy density and the maximum range of the fuel. With the drive for efficiency in aviation, energy density is vital for the engine to get the most out of the fuel. To improve the energy density of the fuel, aromatic hydrocarbons can be added. They are hydrogen deficient compared with saturated hydrocarbons, meaning they have very poor combustion properties, and form a lot of smoke and carbon particulates when burned. For this reason, aromatic concentrations by volume are limited to 25% in aviation fuel. It is clear that a number of combinations of these hydrocarbons would meet the fuel specifications and the ratio of them is determined by the feedstock [33]. Alkanes (generally CnH2n+2) are organic compounds with varying lengths of carbon chains, which are the main components of crude oil and thus jet fuel [55]. This group of organic compounds are known to be inert and are also known as paraffins (para—“little” and affine—“affinity”) as well as naphthenes [19]. The inertness of alkanes is due to the molecular structure being apolar. Although alkanes are inert, they are very susceptible to microbial degradation when oxygen is present. Degradation is prone during storage and transportation of the

fuels, and additives are used for prevention. Under the rules of the International Union of Pure and Applied Chemistry (IUPAC), the number of carbon atoms dictates the name. For example, the simplest of the alkanes is methane, which is a straight chain saturated—each atom is fully saturated with hydrogen—hydrocarbon that contains one carbon atom. The different molecule names have -ane in common with the suffix used to identify the number of carbon atoms. Eth, porp, but, pent, hex, hept, oct, non, dec, undec, and dodec are used when the carbon atoms range from 2 to 12, respectively. Generally, the length of the carbon chain dictates the property of being gaseous, liquid, or solid at room temperature. Chains of 1–4 are gaseous, 5–16 are liquid, and greater than 16 are solids [55]. The arrangement of alkane molecules is important to the properties of the fuel. Alkanes can be straight-chained (n-alkanes), branched (isoalkanes), or cyclic (cycloalkanes) as illustrated in Fig. 20.

3.2. Isoalkanes

Isoalkane molecules have properties that are very beneficial to combustion such as high specific energy and low freezing levels. Importantly, isoalkanes have good thermal stability properties. These molecules are special cases of alkanes that are known as isomers. Isomers are two or more molecules that contain the same number of constituent parts but differ in the connectivity [56]. Although the molecular formula is the same, the different compounds have different properties such as boiling point. There are two ways that the compounds can differ in structure. One is that the component connectiveness is different known as constitutional isomers or by the arrangement of the atoms in the Cartesian system (see example shown in Fig. 19A).

Figure 20 Hydrocarbon alkane formulas.

3.3. Cycloalkanes

Cyclic structures are the preferred compounds as compared with the linear alkane structures [28]. Fuels, for example, derived from coal or coal petroleum blends and contain cycloalkanes (Fig. 21) have been shown to have better thermal stability properties [57]. Naphthenes are specific kinds of alkane molecules that contain one or more cyclic carbon ring. Cycloalkanes add value to a fuel by increasing the density and seal-swelling capabilities of the fuel.

Figure 21 Four cycloalkane molecular structures and constituents.

3.4. Olefins

Olefins (generally, CnH2n) are hydrocarbon structures known for having at least one of the carbon pairs in the molecule double bounded. The naming convention of olefin compounds, as well as aromatics, differs from saturated hydrocarbons by the changing of the -ane at the end of the name to -ene. These compounds are in very little quantity in crude oil but become more prevalent in the refining process [58]. Approved fuels only allow trace quantities of olefins.

3.5. Aromatics

Aromatic hydrocarbons are allowed in aviation fuel, as they improve the energy density in of both volume and weight, which are controlled by the fuel specifications. Aromatics also have a positive property, which is the absorption of aromatic hydrocarbons into nitrile rubber O-rings. During this process, the Oring will swell in, which helps seal the system [59]. However, aromatic hydrocarbons have the negative effect on the combustion properties of the fuel; hence, limits on the concentration allowed are imposed. Their effect on the thermal stability of aviation fuel is complex and not completely understood. Most research studies into aromatics are focused on the effects of aromatics after combustion (see Almohammadi [60]). The literature indicates that the aromatic hydrocarbons can improve the stability of the fuel, by reducing the rate of the deposition, while increasing the amount of deposit formed overall. Work carried out at IFPEN indicated that the addition of aromatic increased the induction period of the oxidation of the fuel. This increased induction period was found to be particularly effective for larger aromatic hydrocarbons with benzylic carbons.

This s the earlier work carried out by Igarashi et al. and Taylor et al. That work showed that 1-methylnaphthalene (1-MN) had a dramatic effect in increasing the induction period of the fuel. It is suggested that this is caused by the stabilization of the radical formed on the benzylic carbon by conjugation through the two aromatic rings. The effect was seen by Taylor [61] in a number of aromatic hydrocarbons, with the increased aromaticity of the hydrocarbon used leading to decreasing rates of deposition. This reinforces the theory that the aromaticity of the hydrocarbon increases the number of resonance structures of the radical, stabilizing the radical through conjugation. This stability of the radicals of aromatic hydrocarbons means they can act as antioxidants in aviation. They behave as a sink for the radicals formed during oxidation by donating their hydrogen to the radicals of products from other hydrocarbons, similar to the mechanism that phenols exhibit. Despite increasing the induction period of oxidation in aviation fuels, aromatic hydrocarbons increase the total amount of deposits formed during the oxidation process. When the size of the aromatic species added to a test fuel is increased, the amount of deposit also increases, with 1-MN leading to a dramatic increase in deposit over single-ringed aromatics. This leads to a picture where aromatics are easier to oxidize and form more deposits but slows the rate of deposition. This is highlighted by Heneghan et al. where the addition of antioxidant to a fuel leads to slow deposition but more deposit overall. This would suggest that the product-forming reactions for the AO2 recombination are much slower than that for RO2 recombination, but the products must have at least a higher tendency to form deposit. Similar to the homolytic fission of peroxides, this is an interesting problem, as it is a radical reaction that involves a number of complex spin states and potential pathways to the products. The rate of recombination and product formation could be controlled by steric and electronic factors, which need further investigation. Like olefins, aromatics are composed of unsaturated hydrocarbons. These compounds are defined by the connectiveness of the carbon atoms. Fig. 22 shows the complexity of several different aromatic molecules. Depicted in Fig. 19D, the toluene molecule, which has double-bonded carbon atoms along with a ring structure, is a good illustration of the complexity in aromatic structures. Typically, aromatic compounds pose the most difficulty in the refining process. Because of the structure of the aromatics, they are molecularly

stable, which increases the difficulty of achieving complete combustion [62]. Aromatics are also responsible for higher particulate emissions and do not burn as cleanly as alkanes. Research conducted by Brem et al. [63] corroborates the view that soot formation is the result of aromatic content in the fuel as opposed to incomplete combustion, as modern-day turbines are highly efficient achieving 99.9% combustion efficiency [63]. Lastly, aromatics have a lower specific energy content than alkanes.

Figure 22 Structure of selected polycyclic aromatic hydrocarbons.

3.6. Energy content

The performance and value added of any jet fuel, conventional or alternative, is based on four major areas: specific energy, energy density, emissions, and thermal stability [32]. Specific energy is the amount of energy per unit mass (MJ/kg), and energy density is the amount of energy per unit volume (MJ/L). These two areas are vital to the performance of aviation operations. Specific energy and energy density will affect the amount of fuel required to achieve the enger/cargo/mission requirements. Kosir, Heyne, and Graham (2020) looked at O-ring swell prediction in relation to conventional and sustainable fuels. During the study, a neural network was used to test iso- and cycloalkane compounds for future high-performance jet fuels. Currently, jet fuels can only have a blend up to 50% due to material compatibility issues. To achieve results, the researchers required accurate specific energy and energy densities of the fuel components as well as the combinations of different components. Fig. 23 shows the results of optimized blends of 10 compounds [64].

3.7. Alternative jet fuel composition and thermal stability

Alternative or advanced fuels can be derived from a myriad of sources such as biomass, coal, and natural gas. Currently, there are commercial alternatives to traditional gasoline and diesel fuel [25], but a fully functional stand-alone alternative jet fuel has yet to be approved. The properties of the alternatives for

the gasoline and diesel fuels limit the use to nonaviation operations. These properties are either incompatible with jet engines, notwithstanding low temperatures, performance of combustion, or safety considerations in storage and/or transport. The majority of alternative jet fuels used today are only authorized as a fractional fuel and limited in percentage of total fuel of a maximum 50%. Each alternative fuel is individually scrutinized, and the properties help determine the maximum percentage authorized for that fuel. Alternative fuel chemical and physical composition is a very important process for determining the maximum percentage as well as for future 100% alternative aviation fuels. Development and testing are ongoing to understand how these alternative jet fuels will react under thermal stresses. Analysis of these fuels is completed with an array of different instruments, for example, gas chromatography/mass spectrometry (GC/MS), flame ionization detection, quartz crystal microbalance (QCM), emissions instruments, and more. Several methods of testing thermal stability are also covered later in this chapter. Zabarnick [65] used QCM to study deposition of fuels and heteroatomic species (Fig. 24). Use of these tools is expanding the testing possibilities for alternative jet fuels with complex mixtures, which will lead to a stand-alone alternative jet fuel.

Figure 23 SE plotted against ED with the high-performance fuels (HPFs) Pareto front representing blends of iso- and cycloalkanes meeting nitrile rubber volume swell requirements in addition to other operability limits. Composition and compound structure of solutions across the HPF. Kosir [64]. ED, energy density; SE, specific energy. Reprinted with permissions from Elsevier.

Quantification of oxygenated compounds can be determined by using GC/MS and can be used in thermal stability research. Understanding the differences in the different alternative jet fuel compositions is vital for this research. In a recent paper published in 2020, researchers used GC/MS to find the compounds of four different alternative fuels. The researchers, Mainali and Garcia-Perez [66], used (1) Amyris, (2) Gevo, (3) UOP, and (4) Syntroleum alternative fuels. The research identified the trace oxygenated compounds in these alternative jet fuels as well as quantifying the results. This research found that the main compounds in these fuels were isoparaffins (38 wt%), n-paraffins (28 wt%), naphthenes (15 wt%), aromatics (14 wt%), and olefins (1.2 wt%). Fig. 25 and Table 3 show the results of the analysis of the four types of alternate jet fuels.

Figure 24 QCM plots at 140 C of deposition and oxygen consumption. QCM, quartz crystal microbalance. Reprinted with permissions from Zabarnick S, West ZJ, Shafer LM, Mueller SS, Striebich RC, Wrzesinski PJ. Studies of the role of heteroatomic species in jet fuel thermal stability: model fuel mixtures and real fuels. Energy & Fuels 2019;33(9):8557–65. https://doi.org/10.1021/acs.energyfuels.9b02345. Copyright 2019 American Chemical Society.

In 2001, research into biodiesel blends was studied by Dunn [67]. This study revealed that although BioJet fuels are feasible, there are several thermal stability issues that must be resolved. The BioJet fuel studied was a blend of biodiesel and JP-8. The biodiesel (fatty acid monoalkyl esters) was developed through processing vegetable oil, animal fat, and used frying oil. The appeal to this renewable fuel is that it can be manufactured locally, easily stored, and relatively safe. Unfortunately, the fuel, when blended with JP-8, has many drawbacks including an increase in cloud point. This directly relates to low thermal stability resulting in the likelihood of increased deposition and the clogging of fuel lines. The use of this biodiesel also has many benefits to the environment, for instance, lowered emissions and negative carbon dioxide balance.

Figure 25 GC/MS chromatograph of alternative jet fuels: (A) Amyris, (B) Gevo, (C) UOP, and (D) Syntroleum. GC/MS, gas chromatography/mass spectrometry. Reprinted from Mainali K, Garcia-Perez M. Identification and quantification of trace oxygenated compounds in alternative jet fuels: fluorescence methods for fast detection of phenolic compounds in operational field conditions. Fuel 2020;271:117652. https://doi.org/10.1016/j.fuel.2020.117652, Copyright 2020, with permissions from Elsevier.

Ellipsometry is a method of determining the amount of deposition from a fuel. This process is different from the jet fuel thermal oxidation test (JFTOT). Ellipsometry is not limited to specific compositions and substrate material. Also, the results are objective meaning that the results are quantitative and repeatable. The JFTOT requires a color observation to be made, which could be different for each observer. Nash [68] has used this method on alternative fuels with different compositions to determine thermal stability and deposition thickness and volume. Other analysis techniques in determining the effects of the alternative jet fuel components with thermal stability are the use of PetroOxy and, as mentioned earlier, JFTOT. These two methods were used by Amara [48] determined that SPK derived from HEFA-SPK had a high thermal stability. The study focused on the addition of cyclic molecules to the fuel compounds. During the tests, the deposit formations from tetralin and decalin/HEFA-SPK blends were evaluated, which showed a high thermal stability (Fig. 26).

Table 3

Compound

MF

Classification

Octane

C8H18

Paraffin

1

Octane, 4-methyl-

C9H20

Paraffin

2

Octane, 2-methyl

C9H20

Paraffin

3

Heptane, 2,5-dimethyl-

C9H20

Paraffin

4

Cyclopentane, butyl-

C9H18

Naphthene

5

Nonane

C9H20

Paraffin

6

Nonane, 3-methyl-

C10H22

Paraffin

7

Heptane, 2,2,4,6,6-pentamethyl-

C12H26

Paraffin

8

Nonane, 2,3-dimethyl-

C11H24

Paraffin

9

Decane

C10H22

Paraffin

10

Heptane, 5-ethyl-2,2,3-trimethyl-

C12H26

Paraffin

11

Decane, 3-methyl-

C11H24

Paraffin

12

Undecane, 4-methyl-

C12H26

Paraffin

13

Undecane

C11H24

Paraffin

14

Undecane, 3-methyl-

C12H26

Paraffin

15

Z-1,6-tridecadiene

C13H24

Olefin

16

Dodecane

C12H26

Paraffin

17

Dodecane, 3-methyl-

C13H28

Paraffin

18

Naphthalene, 1,2,3,4-tetrahydro-2-methyl-

C11H14

Aromatic

19

Tridecane

C13H28

Paraffin

20

Nonane, 2,2,4,4,6,8,8-heptamethyl-

C16H34

Paraffin

21

Tridecane, 3-methyl-

C14H30

Paraffin

22

Dodecane, 2,6,10-trimethyl-

C15H32

Paraffin

23

Naphthalene, 1,2,3,4-tetrahydro-2,7-dimethyl-

C12H16

Aromatic

24

Tetradecane

C14H30

Paraffin

25

Pentadecane, 7-methyl-

C16H34

Paraffin

26

Tetradecane, 3-methyl-

C16H34

Paraffin

27

Pentadecane

C15H32

Paraffin

28

Hexadecane

C16H34

Paraffin

29

Reprinted from Mainali K, Garcia-Perez M. Identification and quantification of trace oxygenated compounds in alternative jet fuels: fluorescence methods for fast detection of phenolic compounds in operational field conditions. Fuel 2020;271:117652. https://doi.org/10.1016/j.fuel.2020.117652, Copyright 2020, with permissions from Elsevier.

One of the major processes for production of alternative jet fuel is the FT process. In essence, fuel produced from any feedstock that conforms to strict fuels standard and has been through appropriate fuels approval process could be used as a jet fuel. SASOL of South Africa was the first company to get its 50% synthetic blend approved as a jet fuel. It was also the first company to supply alternative jet fuel commercially. Significant steps have been taken by the alternative fuels industry to make alternative jet fuel a reality. Daggett et al. [70] studied various commercial aviation alternative fuels.

Figure 26 Evaluation of thermal stability by JFTOT of HEFA-SPK, HEFASPK/cyclic molecules blends at 5–25 vol%. (A) Tube deposit rating after JFTOT test. (B) Filter clogging measurement by pressure gradient variation after JFTOT test. HEFA, hydroprocessed esters and fatty acids; JFTOT, jet fuel thermal oxidation test; SPK, synthetic paraffinic kerosene. Reprinted from Bacha K, Ben-Amara A, Vannier A, Alves-Fortunato M, Nardin M. Oxidation stability of diesel/biodiesel fuels measured by a PetroOxy device and characterization of oxidation products. Energy & Fuels 2015;29(7):4345–55. https://doi.org/10.1021/acs.energyfuels.5b00450, Copyright 2015, with permissions from Elsevier.

In of alternative fuels, FT-derived fuels have been observed to perform well with respect to thermal stability while contributing far less PM emissions when compared with Jet-A1 [29,71]. The FT process uses CO, H2, and catalysts to initiate highly exothermic reactions that produce paraffins. The following equation illustrates the reaction:

When considering SPK fuels, which have been derived from syngas (CO and H2), thermal stability is considered insignificant because of the reduced number of impurities contained in syngas, which has already been processed. JFTOT is conventionally used to test the thermal stability of the fuels. Due to the JFTOT tubes being aluminum in construction, the breakpoint temperature cannot be measured. In a recent study by Moses [72], different blends of semisynthetic fuels were tested for their JFTOT breakpoint. It was observed that semisynthetic jet fuel blends under study were having very high breakpoint, which indicates very high thermal stability. It was also observed that as the tube temperature increases, the deposit depth at the end of the test increases. One of the suggestions of the study was that SPK could be used to upgrade fuels that are marginal on thermal stability. In another study by Corporan et al. [25], thermal stability of six different paraffinic fuels was tested and compared with JP8. This shows that all the paraffinic fuels tested in the study have higher resistance to carbon formation and could be used in elevated temperature environment as a coolant. Fig. 27 shows headspace oxygen profiles and mass accumulation for all the fuels tested in the study. It can be observed from the figure that each fuel shows a different deposition and oxidation characteristic. Oxidation profiles were also observed very high in variance.

Figure 27 Mass accumulation (solid lines, closed markers) and headspace oxygen (dashed lines, open markers) profiles of alternative fuels. Adapted with permissions from Corporan E, Edwards T, Shafer L, DeWitt MJ, Klingshirn C, Zabarnick S, et al. Chemical, thermal stability, seal swell, and emissions studies of alternative jet fuels. Energy & Fuels 2011;25(3):955–66. https://doi.org/10.1021/ef101520v. Copyright 2011 American Chemical Society.

4. Blending

The composition of alternate jet fuels can be very different when compared with other alternative jet fuels. Not only do these fuels differ among themselves but also with fossil fuels. Pires et al. found in a study of many different fuels that there were fuels containing only one component while others contained hundreds [73]. The composition of alternate fuels versus fossil fuels may also vary widely by the length of the hydrocarbon chains components. Alternative fuels may also lack critical components when compared with fossil fuels. The synthetic fuel– type gas-to-liquid (GtL), for example, does not contain aromatic compounds. This is also a driving force behind fuel blending. The blending of different fuels will also have very different compositions. Since there are a myriad of different combinations blending can produce, researchers are experimenting with different blends, including additives, to better understand what happens when different fuels are combined. To illustrate, Braun and Riedel (2014) experimented with synthetic surrogates of GtL and CtL fuels. Four mixtures were used with different molar percentages of species. Just in this one study, focus was only on two major combustion properties of these mixtures [52]. Fig. 28 shows the different amounts of the chemical families present in each of the fuel samples studied. One of the major goals of studying the blending of fuels, as previously mentioned, is to exploit features of one fuel's components with the features of a different fuel. For example, natural inhibitors within straight-run fuels help slow the process of autoxidation [20]. It should be noted that the blending of fuels could also be beneficial in that a lower quality of fuel can be mixed with a higher quality fuel. However, this process could also be detrimental to the thermal stability of the blended fuel. Grant et al. (1996) ran experiments on two commercial jet fuels, a straight-run Jet-A and hydrotreated Jet-A1. The study measured oxygen consumption, formation of hydroperoxides, and insoluble and deposition rates. They group accurately measured oxygen consumption and deposition rates of both types of fuels individually and then with different percentages of blending (Fig. 29). The experiments have shown that the thermal

stability of the blended fuel was benefited only if the lesser quality fuel was diluted with the higher-quality fuel 8–1, or more.

Figure 28 Fuel blends and composition used in Braun and Riedel [52]. Adapted with permissions from Springer Nature (2014).

The emergence of these new fuel technologies requires a better understanding of the effects of the introduction of various compounds on the fuel's thermal stability. Previous treatment of oxidation mechanisms for hydrocarbons has constrained these fuels as a single, broad class. However, with the increased focus on fuel composition, which is essential for alternative fuels, it is important to consider the role that the chemical composition of new fuels has on the oxidation process. In turn, this will allow for developed fuels to have greater thermal stability.

5. Trace species

The hydrocarbon component of the fuel controls its bulk properties, but a number of fuel specification properties (Table 4) are controlled by the trace species found in the fuel. These include the fuels lubricity and cleanliness. These minor components of the fuel include polar oxygenated species, sulfur- and nitrogen-containing compounds, and metal impurities. The exact composition and concentrations of these species are determined by a fuel's feedstock and the postproduction processing that it undergoes. Metal contaminants, especially copper, have a negative effect on the fuel's thermal stability and cleanliness. This is thought to be because they facilitate the fission of peroxides, either through catalysis or by reacting with the peroxides in a one-electron reaction. Nitrogenand oxygen-containing species, particularly heteroatomic aromatic species, have been suggested to have negative effects on the fuels thermal stability and cleanliness. They are thought to react with radical species to form the sediments and deposits in the fuel. Not all of these species have a negative effect on the fuel. Sulfur species improve the lubricity of the fuel by binding to the metal surfaces, improving the slip between the fuel and the surface. Phenolic species found in the fuel can improve the storage stability of the fuel, by acting as antioxidants donating a hydrogen atom to the radical species, slowing the oxidation process [48]. However, both have been linked in to reduce the thermal stability of a fuel, as sulfur and oxygen have been found in the deposits in high concentrations.

Figure 29 Histogram of quantity of insolubles formed in Jet-A and Jet-A1 blends: (A) total, surface, and bulk, (B) IS and IG. Reprinted with permissions from Balster LM, Corporan E, DeWitt MJ, Edwards JT, Ervin JS, Graham JL, et al. Development of an advanced, thermally stable, coal-based jet fuel. Fuel Processing Technology 2008;89(4):364–78. https://doi.org/10.1016/j.fuproc.2007.11.018. Copyright 1996 American Chemical Society.

Table 4

Class

High heat release per unit weight and burn cleanly; isoparaffins offer potential for low-temperature fluidity not offered b

Lower hydrogen-to-carbon ratio, lower heat release per unit weight, greater density, lower freeze point than the paraffins Good combustion characteristics, but reactive.

Burns with a smoky flame, releases more of its chemical energy as thermal radiation than other hydrocarbons (ASTM D Sulfur, nitrogen, oxygen.

Adapted from Brooks KP, Snowden-Swan LJ, Jones SB, Butcher MG, Lee GSJ, Anderson DM, et al. Chapter 6-low-carbon aviation fuel through the alcohol to jet pathway. In: Chuck CJ, Editor. Biofuels for aviation. Academic Press; 2016, p. 109–50. https://doi.org/10.1016/B978-0-12-804568-8.00006-8, Copyright 2016, with permissions from Elsevier.

Phenolic compounds are found naturally in the fuel but are also added to some fuels, to improve their storage stability, in the form of additive packages. The antioxidant used is butylated hydroxytoluene (BHT), a highly sterically hindered phenol [20]. The hindrance means that the radical formed from its donation of a hydrogen to a hydrocarbon radical is stable to further reactions by the steric hindrance of the radical site and the aromaticity of the phenol, slowing the rate of formation deposit. Other additive packages can be added to the fuel to improve its properties. Static dissipators are used to improve the conductivity of the fuel, stopping the accidental sparking and ignition of the fuel. Fuel system icing inhibitors are added to stop the formation of water ice in cold climates. For fuel with poor lubricity, lubricity improvers can be added, and metal deactivators can be used to chelate to copper and stop it from reacting with the fuel. As explained in the previous section, trace materials play a significant role in thermal stability of jet fuel. With continuous advancements of jet engine, fuels will be run at much more elevated temperatures; therefore, further research is required to fully understand the role of individual species to prevent the detrimental effect of carbonaceous deposits on the engine. In this section, a literature review is done to understand the role of key players in thermal oxidation of jet fuel. Main species explored in this section are antioxidants, sulfur compounds, dissolved metals, and hydroperoxides.

5.1. ROOH species (hydroperoxides)

Another important group of species present in jet fuel are ROOH or hydroperoxides, which have a key role in autoxidation chain present at concentrations of 3–30 μM [35]. According to Benson and Nangia [74], as the temperature in the system increases to above 100°C, hydroperoxides are decomposed through a radical induced reaction and become unstable to unimolecular bond fission above 150°C due to relatively weak RO-OH bond. That process produces further radicals, which accelerate the autoxidation reaction. These hydroperoxides species may interact with other species present in jet fuel. As discussed in the previous section, sulfur species play an active role in decomposition of hydroperoxides. Dissolved metals and organic acids (discussed in the next section) are also considered as catalyzers of hydroperoxide decomposition pathways. Although numerous studies have been done on decomposition of hydroperoxides, little of them kept the conditions relatively similar to chemical pathways in jet fuel. West et al. [36] state that homolytic decomposition of hydroperoxides is energetically barrier-less, having a near-zero activation energy (E a ). This is in contrast with basic definition of a catalyst, which acts by offering a lower activation energy pathway. The lower energy pathway could be from formation of different radicals such as a new metal compound. In addition, it has been found by Morris et al. that the trace copper species in jet fuel may be removed from jet fuel and precipitate in wall deposits, which also contradict the catalytic role of trace metals in hydroperoxide decomposition [75]. Rate of thermal decomposition of hydroperoxides increases in the presence of dissolved metals such as Cu, but it is not much affected by acids such as naphthenic acid (NA). However, the presence of NA and dissolved metals was found to synergistically assist decomposition if present together [36]. Since hydroperoxides are a key player in the autoxidation chain of jet fuel, it is important to be able to measure and monitor their concentration analytically. A quantification technique has been deployed using triphenylphosphine followed by chromatography analysis [76]. This technique offers a low detection limit (<0.02 mM), and it is capable of using small volumes (<1 mL). Unlike other species, several hydroperoxides present in the fuel have similar reactivity; therefore, they do not need to be distinguished individually.

5.2. AH species

The term AH refers to peroxy radical inhibitors or antioxidants, which can be naturally present in jet fuel or synthetically added in the range of 100–600 mg/L [35]. They are polar species by nature, and in case of jet fuel, phenol is the main polar species present in bulk of jet fuel [61]. Because they have an easily extractable hydrogen in their structure, they are able to inhibit further oxidation. Phenols react with oxygen to produce quinones, which are strong electrophilic compounds. As a result of that, electron-rich aromatics go under electrophilic aromatic substitution to form insoluble gums [40]. As previously described, antioxidants slow the autoxidation chain down by reacting with peroxy radicals and stopping reproduction of hydrocarbon radicals by producing phenoxy radicals and hydroperoxides during the following reaction:

In a molecular modeling by Zabarnick, it is deduced that addition of antioxidants at lower concentrations lowers the rate of autoxidation dramatically, whereas it acts opposite at higher concentrations [37]. In another study by Kuprowicz et al., it is strongly stated that naturally occurring phenols in jet fuel play an important role in slowing autoxidation and promoting surface deposition. Promotion of surface deposition could be simply explained by the chemistry behind the difference in solubility properties of polar and nonpolar compounds. As hydrocarbons are a nonpolar environment, the polar species attract each other to form insoluble gums, which become detrimental deposition to wall surfaces. When treating polar species in a pseudo-detailed mechanism, it should be noted that there are a few different types of naturally occurring phenols such as methyl-substituted, dimethyl-substituted, and larger phenols with different reactivity present in the fuel [35].

5.3. SH species

Compounds containing sulfur are also an important form of trace material present in jet fuel. According to Fathoni and Batts (1992), this has been a point of argument in the literature of fuel stability. Generally, sulfur species tend to act as antioxidants in fuel by decomposing hydroperoxides, as they react with fuel hydroperoxide via a nonradical producing pathway represented in the following [35]:

Their study identifies elemental sulfur, disulfides, and polysulfides as causes of instability, whereas aliphatic mercaptans and aliphatic sulfides do not show much reactivity within the same fuel [77]. As shown in the reactions earlier, sulfides (RSR) and disulfides (RSSR), also known as “reactive sulphur” in this case, tend to be the most important sulfur species inside jet fuel present from 1 to 1100 ppm(wt). Nevertheless, other compounds such as thiophenes, benzothiophenes, and dibenzothiophenes are present in parallel [35]. Although Zabarnick and Mick [78] believe that oxidation rate is unaffected by presence of sulfur species such as alkyl sulfides at 140°C, Kuprowicz et al. state that reactive sulfur components slow down the autoxidation chain by hindering thermal or catalytic hydroperoxide decomposition [35]. Due to their polar nature, they are also understood to contribute to deposit formation as well, but the mechanism is poorly understood in the literature and requires further research. These sulfurcontaining species in jet fuel could be measured by a hydroperoxide reaction pathway followed by high-resolution gas chromatography—atomic emission detection using group selective chemical treatment explained by Stumpf et al. [79].

5.4. Dissolved metals

Although metals are present at ppb levels in jet fuel, they have a significant effect on thermal stability. As previously mentioned, their main role is to offer a lower energy pathway for decomposition of hydroperoxides by forming new radicals and complex metals. West et al. initially selected five metals: Cu, Fe, Mn, Mg, and Zn; however, only Cu and Mn were investigated further, as they showed a much higher sensitivity to lowering activation energy for decomposition of hydroperoxides, whereas the rest of metals did not have much of an influence [36]. The concentration, at which these sensitivity analyses were run, is a point of argument since Hazlett [7] shows Fe and Zn both have an increasing effect on autoxidation rate at higher concentrations. Catalytic effects of metals can be mitigated in the presence of additives, known as metal deactivator additives (MDAs), which have the following chemical composition

(N, N-disalicylidene-1,2-propane diamine). West et al. [36] state that MDA inhibits both deposition levels and oxidation rates by lowering the reactivity of metals, particularly for copper. As not many studies have investigated the effect of MDA on other metals than copper, this could be a point of improvement to see whether it has a similar effect on other metals too. Low concentration of metals makes the measurements challenging. Nevertheless, techniques such as inductively coupled plasma followed by atomic emission spectroscopy or mass spectrometry could be used to quantitatively measure metal presence in jet fuel [35]. The effects of dissolved metals were researched as far back as 1976 by Exxon and the US Navy [50]. The main interest was to evaluate deoxygenated fuel for high-speed flight without the need for cryogenic or specialized fuels. The study analyzed varieties of different metals including copper, iron, and vanadium mixed into air saturated and deoxygenated Humble JP-5, also called High Stability JP-5. The researchers believed that metals were the likely cause of deposit formation by catalyzing autoxidation and found that the effects were lessened in deoxygenated fuel. For the case of dissolved copper, the data showed a 30% increase in deposits in deoxygenated fuel and an 80% increase in air saturated fuel as compared with a fuel with no dissolved copper. Dissolved iron was also tested, and the deposits were found to be significantly greater than that of the copper tests.

6. Generation of chemical kinetic mechanism

As previously mentioned, several methods have been tested to mitigate deposit formation or offer a systematic method to govern thermal stability and be able to predict autoxidation chain reactions, which ultimately leads to deposit formation. Constructing a chemical kinetic mechanism from a range of elementary reaction happening in the system with the desired conditions should be able to offer a good prediction of thermally stressed fuel. Fig. 30 from Carr's Modeling of Chemical Reactions book [80] schematically illustrates how a detailed chemical kinetic mechanism could be generated step by step: According to Swihart [80], all the preliminary reactions should be identified in the first step with the means of literature, chemical intuition, and analogy. Secondly, the reaction thermochemistry should be found from the literature, group additivity, analogy with other reactions, and quantum chemistry calculations. Once the thermochemistry data such as standard enthalpy of formation, standard entropy are known, the Gibbs free energy could be calculated. In the third step, rate parameters could be identified using the literature data and a combination of quantum chemistry calculations with rate theories. Step 4 is applying the conditions of interest in of temperature and pressure, and the final step includes sensitivity analysis, rate flux analysis, and experimental validation. The final step, sensitivity analysis in particular, helps us to iterate the process and reduce the mechanism by eliminating the unimportant reaction steps to achieve optimization.

Figure 30 Schematic representation of mechanism generation. Reprinted with permissions from Swihart MT. Chapter 5 constructing reaction mechanisms. In: Carr RW, editor. Comprehensive chemical kinetics. Elsevier; 2007. p. 185–242. https://doi.org/10.1016/S0069-8040(07)42005–2. Copyright 2007 Elsevier.

6.1. Rate of reaction

In understanding the thermal stability of jet fuel, rate of the reactions in which the oxygen is consumed plays a crucial role. Therefore, a thorough understanding of reaction rate is needed. According to Turanyi and Tomlin [81], in reaction stoichiometry, it is assumed that a reaction can be described with the means of a single stoichiometric equation. This equation determines the molar ratio of the reactants and the products. It can be shown as follows:

[7]

N s is the number of species, is the stoichiometric coefficient of the jth species, and A j represents the formula of the jth species in the overall reaction. Following Eq. (7), the rate of reaction can be determined from measuring the concentration of different species (Y) at different times:

[8]

Although reaction rate does not depend on index j, meaning the rate is the same

for all other species, it does depend on the stoichiometric coefficient. Finally, reaction rate can be estimated using the following equation where k is the rate coefficient, independent of concentration but dependent on temperature, pressure, and quantity of nonreactive solvent present:

[9]

The exponents are positive numbers or zero. It can be related to overall order of reaction in Eq. (10), where a is the overall order:

[10]

As previously explained, the rate coefficient K is dependent on temperature. This temperature dependency can be illustrated by the Arrhenius equation:

[11]

A is the preexponential factor, E is the activation energy, T is temperature, and R is the universal gas constant. Arrhenius equation is valuable, as it could help us obtain activation energy for different steps of the chemical mechanism by means of Arrhenius plots, in which ln K is the y axis, T −¹ is the x axis, and the slope is –E/R.

6.2. Sensitivity analysis

The final step of the reaction mechanism generation was to perform a sensitivity analysis to identify the insignificant reaction steps so that the reaction could be reduced and optimized. Sensitivity analysis shows to what extent a degree of change in one parameter has an effect on another parameter. According to Turanyi and Tomlin [81], the main reason for performing a sensitivity or uncertainty analysis is to improve the predictive ability of the model so that the design could be trusted. Assuming a homogenous dynamic system, the change of parameter in time could be calculated from the following formula:

[12]

The parameter x could be rate coefficients, Arrhenius parameters, or thermodynamic data. If we assume the solution of the differential Eq. (12) is solved between t = 0 and t = t 1, then parameter j is changed by Δxj, and the solution of the ordinary differential equation (ODE) gets continued until t 2. If Y i (t 2) is the original solution and the Y' i (t 2) is the modified solution, the mathematical expression can be written as

[13]

The is called the first-order local sensitivity coefficient (see Turanyi and Tomlin [81]). One of the simplest methods of performing sensitivity analysis is the Brute Force method. This procedure is one of the one at the time methods, meaning one parameter is changed at the time and the degree of its change is observed on the system. Although this method offers a simple way of performing sensitivity analysis, it is not always the most efficient and accurate way.

6.3. Ordinary differential equation solver— temperature dependency

A MATLAB code has been previously designed by Alborzi [51], which requires the following inputs: rate parameters including activation energy and preexponential factor, initial concentration at reaction onset for each species, and time intervals with a range of tolerances for accuracy of computation. This code has been integrated with the 18-step global reaction mechanism by Kuprowicz et al. [35] to model the oxygen depletion against time. A preliminary parametric study was performed to examine the effect of change of temperature on oxygen depletion rate. Changing the temperature by 5% showed a drastic change in the curve represented in Fig. 31. The code was originally at 458 K (185°C); the temperature was altered as shown in the figure. This drastic change shows the experimental rig, nearisothermal flowing test rig (NIFTR) that will be introduced in the next section, is potentially subjected to significant inaccuracies as further thermal analysis shows that there is a relatively long nonisothermal region at the entry (see Section 7.3), and isothermal region is not achieved for higher flow rates at all. Therefore, the existing mechanism could be improved on this basis, by deg a rig, which is able to offer a uniform temperature and avoid the nonisothermal entry length.

Figure 31 MATLAB graph showing the effect of temperature change on oxygen depletion rate.

7. Experimental apparatus

According to Sander et al. [82] and Moses [39], several experimental techniques have been designed in the past to examine the role of geometry and flow rate on thermal stability by the use of high Reynolds number rigs, computational fluid dynamics, and flight tests. However, examining the role of chemical composition on thermal stability requires smaller rigs with simpler test geometries. These devices are able to model oxygen depletion and deposition mechanism on the tube walls. The results obtained from these test rigs can be used to validate modeling data. Fig. 32 illustrates an overview of thermal stability tests available, which can link the research to effect of geometry, flow rate, and chemical composition on thermal stability.

7.1. Isothermal tube reactor (ITTR)

The current isothermal tube reactor (ITTR) device at the University of Sheffield Low Carbon Combustion Centre (LCCC) has the capability to measure the rate of oxygen depletion as a function of time. It provides chemical kinetic data to strengthen the understanding of fundamentals of liquid-phase oxidation of hydrocarbons, jet fuel in particular. These data are valuable as they can be used as a validation of modeling data generated by the ODE solver discussed earlier. The current ITTR rig consists of a fuel supply system, high-performance liquid chromatography pump, tubular heater, and water jacket followed by an online oxygen sensor with a tube length of around 3 m long; it is illustrated in Fig. 33 [83].

Figure 32 Different thermal stability tests shown as a function of fuel needed and scale.

Figure 33 Illustration of the isothermal tube reactor at LCCC lab. LCCC, Low Carbon Combustion Centre. Reprinted with permissions from Alborzi E, Gadsby P, Ismail MS, Sheikhansari A, Dwyer MR, Meijer AJHM, et al. Comparative study of the effect of fuel deoxygenation and polar species removal on jet fuel surface deposition. Energy & Fuels 2019;33(3):1825–36. https://doi.org/10.1021/acs.energyfuels.8b03468. Copyright 2019 American Chemical Society.

This current formation has a number of limitations; it is a large device that requires a high volume of fuel (around 20 L) and a greater power output, resulting in significant operational and maintenance costs. Also, in the following section, a thermal modeling has been conducted, which shows that there is a considerable region of nonisothermal flow at the entry region, and it gets smaller as flow rate is increased. Performing a sensitivity analysis on the effect of temperature on oxygen depletion in the integrated MATLAB code illustrated how oxygen depletion is dramatically affected by the smallest changes in temperature. Therefore, it is vital to maintain temperature alterations to a minimum and have a uniform temperature across the reactor. In addition, sampling small volumes of fuel for posttest analytical analysis is challenging due to large volume of fuel being used. Posttest analysis of fuel samples is a key step in determining the effects of chemical composition on thermal stability as it shines a light on the aftermath of autoxidation process.

7.2. Thermal modeling

Knowing the importance of obtaining an isothermal region within the rig, an isothermal region modeling has been done based on the Internal Forced Convection chapter of Cengel and Ghajar [84]. The following formula has been to find T x , which is the temperature at a particular position along the tube:

where

: position temperature along the tube;

: bulk temperature;

: inlet temperature;

: heat transfer coefficient;

: heat transfer section area;

: mass flow rate; and

: specific heat capacity

The following specifications from the ITTR rig at LCCC have been used in this model:

Note: CRC Handbook of Aviation Fuel Properties [85] has been used to find the exact values for density, viscosity, and specific heat capacity based on Jet-A1 fuel properties and formulations. It has also been assumed that Nusselt number equates to 1, since Reynolds numbers are in the range of 0.03, meaning that convective and conductive forces are equally involved in the flow regime. A comparison between Nu = 1 and Nu = 3.66 has also been done, in which the experimental data agree more with Nu = 1. The graphs will be provided later in this chapter. Another assumption is the negligible amount of heat loss, as the tube was insulated. Also, the fuel temperature is assumed to be the same as outer tube temperature, since thermocouples are placed on the outer wall and bulk temperature is assumed to agree with the set temperature, which is 200°C.

7.3. Comparison with Near-Isothermal Flowing Test Rig

Originally described by Jones and Balster [49,86], an has been used for experimental validations. Kuprowicz et al. [35] have also used a similar test rig to experimentally validate the global kinetic mechanism previously explained. It contains a single- heat exchanger with 0.125-in o.d. and 0085-in i.d. made of commercial stainless steel. Fuel is pumped into the device by a syringe, and temperature is controlled along the tube. At the end, an oxygen sensor is placed to measure oxygen concentration, and a pressure regulator is used to overcome thermal expansion. Fig. 34 provides a schematic diagram of NIFTR. Since NIFTR and ITTR are fairly similar devices, a comparative study has been done based on the thermal modeling explained earlier. The main differences are shown in Table 5.

Figure 34 Schematic diagram of NIFTR. NIFTR, near-isothermal flowing test rig. Reprinted with permission from Jones EG, Balster WJ. Surface fouling in aviation fuel: short- vs long-term isothermal tests. Energy & Fuels 1995;9(4):610–15. https://doi.org/10.1021/ef00052a006. Copyright 1995 American Chemical Society.

Table 5

Test rig

ITTR

NIFTR

306.5 cm

81.2 cm

Tube length

4.75 mm

2.16 mm

Inner diameter

25°C

25°C

Inlet temperature

200°C

185°C

Bulk temperature

Jet A1

Jet A1

Fuel type

NIFTR, near-isothermal flowing test rig.

7.4. Jet fuel thermal oxidation test

One of the popular thermal stability tests is known as JFTOT. The aim of this test is to investigate the deposits formed at a particular temperature, after aerating the fuel and ing through a metal surface. The fuel degradation is determined by two methods: one is the color change on the heated tube due to formation of deposits, and the other is by evaluating the pressure drop due to formation of particulates [87]. JFTOT uses around 500 mL fuel and runs the test for 2.5 h, examining the tendency of a fuel to form insoluble materials [82]. Although color charts have been previously used for disqualifying a fuel for further use, but more modern devices such as DR10 (ITR Tube Deposit Rater Instrument) [88] can be used to generate more detailed graphs, which will be presented later in this section.

7.5. High Reynolds Number Thermal Stability tester

In opposed to the JFTOT rig, which operates on laminar flow, the high-Reynolds number thermal stability (HiReTS) tester has been designed to perform the test in turbulent regime. It also aims to quantify the deposits formed and correct the tube metallurgy since it uses stainless steel rather than aluminum. HiReTS is advantageous as it provides an environment more realistic to real flight conditions [89].

7.6. PetroOxy

PetroOxy device is one of the small-scale thermal stability tests. It needs a small volume of sample to provide analysis in a short period of time. This is a great advantage as the results are reproducible, and they can be run at several temperatures [69]. The device operates based on pressure change; as it reaches maximum pressure, the pressure decrease is then converted into oxygen depletion as a function of time [90]. A visual depiction is provided in Fig. 35.

Figure 35 Oxidative degradation of fragrant aldehydes. Autoxidation by molecular oxygen. Reprinted from Marteau C, Ruyffelaere F, Aubry JM, Penverne C, Favier D, Nardello-Rataj V. Oxidative degradation of fragrant aldehydes. Autoxidation by molecular oxygen. Tetrahedron 2013;69(10):2268–75. https://doi.org/10.1016/j.tet.2013.01.034, Copyright 2013, with permissions from Elsevier.

7.7. Aviation Fuels Thermal Stability Unit

Alborzi et al. [1] investigated the effect of surface deposition on fuel injector feed arm, which was simulated for sudden contraction and expansion. The study was conducted using an Aviation Fuels Thermal Stability Unit (AFTSTU), which was capable of conducting a representative test at full-scale 1000 flight hours for surface deposition, to determine how long it takes for surface deposition to start occurring and its associated performance impact upon the turbine. This is a different type of rig for testing thermal stability of the fuels. It was observed that deposition in contraction/expansion tube is significantly higher than straight tube. Balster et al. [28] presented thermal stability data for a novel coal derived fuel. It was observed that surface deposition for novel fuel was significantly lower than other fuels tested in the study that includes JP8.

8. Incidents involving fuel thermal stability

Although increasing the temperature of the fuel (preheating) aids in the combustion process, incidents began to occur due to fuel, which had become thermally stressed. One of the first investigations related to thermal stability was in the 1950s when Pratt and Whitney J57 engines were experiencing turbine blade failures [62,91]. Fuel lines in these engines were heated from compressor exhaust, which led to carbon deposits. These deposits blocked fuel filters, which led to undesirable changes in heat generation at the turbine inlet. Thermally stressed fuel can form insoluble gums and deposits, carbon deposits with the J57 case, within fuel systems. These gums and deposits may cause blockages and degrade the performance of fuel and/or engine systems due to pressure drops across filters and other components [92]. These blockages can occur not only on fuel filters but also on pumps within the fuel systems or any other component that may have a decrease in wetted surface area. Blockages that occur at the fuel/oil heat exchanger (also called liquid-to-liquid cooler) can limit the heat transfer process and therefore increase the temperature of the engine oil and reduce the performance of the engine. In extreme cases, severe blockages can lead to fuel system failures and inadvertent shutdown of the engine. To stop these extreme cases from occurring, airlines inspect and service engine fuel systems on a regular basis. This is costly for the airlines both in servicing and in the time lost in aircraft operation. As airlines are often running low liquidity, prolonged grounding of aircraft can lead to increased profit losses and when political issues have led to widespread grounding, bankruptcy. Clearly the loss of flight hours due to the issue is detrimental to airlines, but the solutions also increase the operating costs. These solutions include not only cleaning and servicing of the components but also the use of additive packages to improve and control the thermal stability of the fuel. The integration of alternative fuels in the aerospace sector allows aircraft to operate in harsher environments, for instance, ones with lower outside air temperatures, or where the risk of contamination is greater. However, there have been instances where fuel composition—including additives—contributed to an

accident involving an aircraft. One notable example where the evidence strongly suggests that fuel thermal stability caused an accident is that of British Airways Flight 38, which crashed just short of the runway at London Heathrow Airport after both of its engines suddenly failed to deliver thrust while the Boeing 777 was on final approach. The investigation conducted by the Air Accidents Investigation Branch (AAIB) found that ice accretion at the face of the fuel–oil heat exchanger (FOHE) caused a fuel flow restriction during rollback of the engines while on approach; upon movement of the throttle up to maintain airspeed, the ice that had accreted restricted additional fuel from flowing to the engines, thereby preventing them from generating thrust as commanded by the throttle. The investigating body found that the ice was formed from water that was expected to be in the fuel system—both “dissolved and entrained water present in aviation fuel” [93]. Additionally, the water in the fuel was not found to be in abnormally high concentrations. The AAIB further notes the following points in the report (AAIB 2010):

1. Ice can form on the inside of fuel pipes when warm fuel at a temperature of +5°C flows through cold pipes. 2. There is a “sticky range” between −5°C and −20°C, when ice crystals in aviation fuel are most likely to adhere to their surroundings. 3. Ice does not appear to stick to the inside of the fuel pipes when the fuel temperature is at −35°C or below. 4. Ice that accumulated in the fuel system, during testing, was always soft and mobile. 5. Increasing the fuel flow can cause accreted ice to be released from the walls of the fuel pipes. 6. Ice released from within the fuel pipes could form a restriction at the face of the FOHE. Beyond determining the cause of the crash, the AAIB's accident report makes three recommendations that pertain directly to fuel additives. It is recommended

that the Federal Aviation istration (FAA) and the European Aviation Safety Agency (EASA) tly conduct the following:

1. “a study into the feasibility of expanding the use of antiice additives in aviation turbine fuel on civil aircraft” 2. “research into ice formation in aviation turbine fuels” 3. “research into ice accumulation and subsequent release mechanisms within aircraft and engine fuel systems” (AAIB 2010)

7. Therefore, further research into alternative fuels, including the addition of antiicing agents (for the present case) but also biocides or other additives, is necessary, as it may help prevent accidents such as British Airways Flight 38 by expanding the envelope in which the fuels are capable of operating without having adverse effect on propulsion systems' operation.

The COVID-19 pandemic of 2020 has had a tremendous effect globally. One mitigating approach was to ground aircraft in hopes of slowing the spread of the virus. Because of the grounding, aircraft became an unofficial storage facility for fuel. It was understood that the fuel would succumb to the effects discussed in this chapter. Operators applied techniques such as adding biocide additives to the aircraft tanks to prevent the growth of microbial contamination [94]. The growth of biomass in fuel tanks has the potential for catastrophic consequences. Aircraft that are located in areas of high temperatures and humidity are most vulnerable. Operators, airlines, engineers, and regulatory istrations are actively perusing methods that will ensure safe operations once travel bans are lifted. The most common technique is frequent testing of the fuel and mitigation of contamination. However, once unsafe levels are discovered, then a full defueling and cleaning process occurs, which is extremely costly and time-consuming.

9. Outlook

Beyond the development of alternative fuels, there have been studies conducted on the feasibility of their implementation into the aviation sector. Hileman and Stratton (2014) consider the implementation of alternative fuels from an economic and production impact on the environment standpoint. Indeed, the authors consider the impact of alternative fuels at every step of manufacture and implementation, allowing them to establish an estimate on the cost of manufacturing and consuming these alternative fuels over their life cycle [22] (Fig. 36). Their conclusions include that only petroleum and synthetic fuels are currently compatible with existing aircraft and airport infrastructure. Other fuels, such as liquefied natural gas, hydrogen, and alcohols (e.g., ethanol), are not capable of satisfying the stringent performance and safety requirements of aviation. However, they can be used as fuels for systems, which are less stringent with respect to the fuel used, thereby increasing the amount of petroleum products available to the aviation sector [22]. Another consideration is that of aircraft range performance using alternative fuels. Blakey et al. [25] followed a theoretical analysis method to establish the impact of alternative fuels on range and payload for the Gulfstream G550, Boeing 737, and Boeing 747 aircraft [95]. Using the Breguet range equation, paired with available data from the aircraft (total and allowable weights for payload and fuel, as well as fuel tank volume) and with data on various alternative fuels, the authors determined for each aircraft what the best fuel would be to maximize range for maximum payload, and for maximum allowable fuel volume. Using different sizes of aircraft allowed for representation of the different types of aircraft used today, from small regional jets to twin-aisle wide bodies. The study has several salient findings:

Figure 36 Life cycle greenhouse gas (GHG) emissions for various alternative fuels. Reproduced from Hileman JI, Stratton RW. Alternative jet fuel feasibility. Transport Policy 2014;34:52–62. https://doi.org/10.1016/j.tranpol.2014.02.018, Copyright 2014, with permission from Elsevier.

1. Less dense fuels, such as SPKs, yield better range for maximum aircraft payloads. 2. Denser fuels allow for greater range at less-than-maximum aircraft payloads. 3. Due to the fact that not all missions require the aircraft to carry its maximum payload or reach its maximum range, different payload mass and fuel type configurations may be considered. This point specifically allows for alternative fuels to be used more widely, as their use no longer imposes a limitation on the aircraft for a specific subset of mission types. 4. Though alternative fuels with smaller densities and higher caloric content have a smaller CO2 impact, complete life cycle analyses need to be carried out. 5. Though SPKs are lower in density and higher in caloric content than kerosene, they are not “drop-in” fuels. First, they need to be mixed with Jet-A1 at 50% to comply with existing ASTM D7566 standards; next, significant modification to plumbing systems, injectors, atomizers, and combustion devices would need to be carried out before SPKs could be safely and efficiently burned in existing gas turbines.

Though further analysis is required to more accurately evaluate the performance impact of alternative fuels on jet aircraft's range and payload capabilities, work of Blakey et al. demonstrates that such an implementation can generally provide the performance required of gas turbine-powered aircraft at various mission ranges and payload requirements.

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Chapter 9: Impact of alternative fuel properties on combustion instabilities, noise, and vibrations

Charith J. Wijesinghe ¹ , and Bhupendra Khandelwal ² ¹ Department of Mechanical Engineering, The University of Sheffield, Sheffield, United Kingdom ² Department of Mechanical Engineering, The University of Alabama, Tuscaloosa, AL, United States

Abstract

Combustion instability is an event where small disturbances in a flame caused by fuel flow rate fluctuations, heat release rate fluctuations combustor design where poor mixing occurs and other factors grow in an uncontrolled manner to then affect the flame in detrimental manner such as excessive noise, vibration increased flame turbulence and in dire situations flame outs. This chapter aims to describe how the fuel used in a gas turbine can impact the stability of a flame in the combustion chamber with a special focus trained upon the use of alternative fuels in gas turbines, a pressing concern given the climate imperative. This chapter discusses the impact of different fuels and their properties on combustion instabilities, noise, and vibrations. This area of research is fairly new in aviation, and very little literature is available about this topic. Overall, this chapter has shown that different fuels and their properties can impact noise, vibrations, and combustion instability at combustor and engine levels.

Keywords

Alternative fuels; Aviation fuel; Combustion instability; Flame stability; Gas turbines

1. Introduction to combustion instabilities

In the basic sense, combustion can be defined as a process where chemical energy is converted to heat energy in an oxidative manner [1]. In this process, the reactants undergo a process where the chemical bonds are broken and then new molecules are formed as the products of the reaction; for the reaction to be beneficial for combustion, the energy released from the reaction in of heat must be greater than the activation energy required to start the reaction. This localized increase in temperature in a given flame can be unsteady and give rise to pressure oscillations, which can then lead to noise and vibrations in the global system. This is the basis of combustion instability. Noise is defined as the unwanted oscillation of air particles, while vibration is the oscillation of solid material [2]. What differentiates combustion instabilities from normal combustion process is the unsteady nature of it. Pressure oscillation that appears due to unsteady heat release from combustion can start to self-propagate and grow causing, among other things, thrust fluctuations, vibrations, and thermal stresses. The circumstances that lead to combustion instabilities have been described by Lord Rayleigh in the so-named Rayleigh's criterion [3,4]. Fig. 1 denotes the loop describing combustion instability where the thermal oscillations feed the pressure and acoustic oscillations, which in turn affect the thermodynamic quantities of the fluid (flame of gas turbines occurs in a fluid domain). If ignored, combustion oscillations that cause pressure oscillations in the combustion chambers will therefore cause thrust oscillations that are detrimental. Moreover, there can be possible interference of the engine control systems, leading to malfunctions and premature wear of components due to cyclic fatigue [3]. In addition, if the vibrations caused match the natural frequencies of the components being vibrated, catastrophic failure could occur; however, most aviation-related components are tested for their natural frequencies, and safe ranges of frequencies are specified for each component [6,7]. Furthermore, if vibrations and instability are not handled correctly, then damage to the combustors can occur.

Figure 1 Combustion instability loop [5].

Rayleigh's criterion can be described by the following equation:

[1]

where and are the pressure fluctuations and heat release fluctuations, respectively, is the time interval, is the chamber volume, is the surface, is the velocity fluctuations, and finally is the normal to the surface boundary. In effect, when the left portion of the inequality is greater than the right portion, combustion instabilities can occur. It has been determined that there are several separate phenomena that are capable of inducing and then driving combustion instabilities; these are as follows:

• Interactions between the flow vortices and the flame inside the combustion chamber • Oscillations of the flame area as well as the reaction rate of combustion

Figure 2 Combustion interactions that can cause instability [ 8].

• Fluctuations of the AFR (air–fuel ratio) locally in the mixture as well as globally in the fuel system • Fluctuations of the fuel flow rate entering the combustion chamber • Inlet air flow rate oscillations • Unequal mixing and atomization of the fuel from the injector nozzles

The phenomenon of combustion instabilities is far more well documented for rocket engines where instabilities have caused catastrophic loss of spacecraft. An illustration of the interactions that can lead to combustion instability is depicted in Fig. 2.

2. Categorizing instabilities

In general, combustion instabilities can be categorized into two variants namely: linear and nonlinear instabilities. However, linear instabilities are caused by miniscule disturbances and therefore are not observable in nature according to Zinn et al. [9]. Zinn further goes on to give an intriguing example of a linear instability where a ball is balanced perfectly on the top of hill, where it can be dislodged by a minute disturbance. Nonlinear behavior, on the other hand, is dominant in combustion instability. According to Jacobs, there are several defining characteristics of instability such as limit cycle amplitudes, wave steepening, and mean pressure shift [10]. A simpler way to define a nonlinear system is to say that the inputs of the system and the change in the outputs are not linearly proportional to each other; therefore, it must be that in the system equations, the output variables are governed by at least one higher-order polynomial.

Figure 3 Combustion instability frequency ranges [13].

Combustion fluctuations typically occur in three frequency ranges: lowfrequency dynamics (LFDs) that occur <50 Hz and referred to as Helmholtz modes; intermediate-frequency dynamics (IFDs) that occur between 50 and 1000 Hz; and high-frequency dynamics (HFDs) that occur >1000 Hz [11]. These frequency ranges respond differently to changes in operating conditions, although their heat release oscillations and acoustic pressure fields might be coupled [11]. Fig. 3 shows the instabilities represented in amplitude spectrum occurring in different frequency ranges. LFDs, also known as cold tones, occur at lean AFRs, and their amplitude increases as the flame temperature decreases [12]. Excessive LFDs (cold tones) have detrimental impact on the turbine section and can be visible on combustor parts in the form of worn-out surfaces and fretting. HFDs or screech damages the hardware rapidly [12].

3. Combustion noise

Combustion noise is of great importance to the aviation sector in of the greatly increased noise pollution that surrounds busy hub airports such as London's Heathrow and New York's John F. Kennedy International Airport. The extremity of the issue is such that Heathrow has curtailed its operations from 2300 to 0700 and has only allowed 5800 flights per year during this time period [14]. This phenomenon started becoming an issue with the introduction of the conventional jet engine beginning in the 1950s onward. At this point, the jet engines were much louder than the piston engines that they replaced. These early engines mainly exhibited noise in the form of jet noise (high-speed jet exhaust that generated noise); according to studies conducted by M.J. Lighthill, this jet noise contributed the vast majority of the noise proportion [15]. According to Lighthill, jet noise produced by the engine scales by the eighth power of the exhaust speed. However, with time and increased efficiency, the speed of the jet exhaust has been reduced to increase the efficiency of the engines without compromising thrust by increasing the total mass flow by introducing the highby turbofans. Today, the noise reduction research has turned to other directions instead of jet speed, as this method is approaching its limits and innovative solutions such as chevrons have appeared, which helps the jet exhaust mix well and therefore dissipate the noise generated. However, these are compromise solutions, as the chevrons tend to reduce the total noise generated by the engine and thereby increase fuel consumption [16]. At first, these turbofan engines with greater by ratios reduced the jet noise rapidly. However, these engines came with their own issues of fan noise. And therefore, most research focused upon the reduction of noise generated by the turbomachinery of the jet engine. It was not until the advent of combustion chamber designed to produce less NOx that combustion noise became a focus of research. Engine employing techniques such as lean-premixed, dry-low emission, or rich-burn—quick quench—lean-burn combustors has higher noise fraction in of combustion noise. The issue with this type of lean-burning novel combustors is that they are prone

to increased combustion instabilities when the turbulent flame intermingles acoustic characteristics of the combustion chamber in a beneficial way driving the instabilities and increasing the acoustic energy, which results in high noise and perhaps even combustor damage. These heat release fluctuations generate two types of acoustic waves and entropy waves; these two waves are the precursors to the two types of combustion noise [16]. It is commonly accepted that there are two mechanisms for the generation of combustion noise: direct and indirect combustion noise (Fig. 4).

4. Gas turbine instability, noise, and vibrations and previous work

The extent to which vibration and noise manifest is partially dependent on the different properties of the fuel. Khandelwal et al. investigated the role of different fuel composition and its impact on combustion vibrations [18]. Testing was done on a Honeywell GT85 APU using four different fuels. Fuels 1 and 2 are Jet-A1 sourced from two different sources, whereas fuels 3 and 4 are FT process–produced fuel from different sources. It was observed that the FT process fuel, which has the lowest density from the fuels tested in this study, produced higher-frequency spectra of vibrations. However, the highest amplitude of the vibration was produced by Jet-A1 from fuel 1. It is to be noted that Jet-A1 sourced from two different sources has similar frequency but significantly different vibrations (Fig. 5).

Figure 4 Combustion noise mechanism [17].

Figure 5 Vibration amplitude and frequency with four different fuels [18].

Furthermore, the relationship between fuel density and vibration is more explicitly considered in the work done by Khandelwal et al. [18], where a higher-density, coconut oil–blend biodiesel was compared with conventional diesel [18]. In this study, the higher-density fuel blend displayed a significant reduction in vibrational acceleration. Overall, it is observed that there has been little research on the combustion instability, noise, and variations caused by combustion-induced vibration with alternative fuels. This pattern is reflected across all the major public domain journals under current situations. Stricter emission legislations demand the use of lean-premixed combustion, but combustion instability is more likely in these types of lean combustors compared with current rich-burn systems. It was observed that the instabilities may be due to oscillations in pressure, velocity, temperature, or equivalence ratio of fuel. Among these variables, Rayleigh's criterion was made to be one of the primary conditions to be met for a selfexcited combustion oscillation to occur. Equivalence ratio oscillations are a possible cause of combustion instabilities. The underlying reason for change in combustion instability could be due to change in fuel's boiling point, viscosity, vapor pressure, flame speed, stoichiometric equivalence ratio, cetane rating, density, energy density, and/or composition. So far, these instabilities are suppressed in existing engines by using Helmholtz resonators, active combustion control, and noise suppressors; controlling the droplet size; changing the flame speed and length; and varying the flame injector geometry and location. But with the arrival of new alternatives, the instabilities can be understood better and designed out by fuel selection and combustion design optimization. A study conducted by Leyko et al. [19] has linked high noise through gas turbine outlet nozzles to a high outlet-to-inlet Mach number ratio, which is caused by shock waves developing as flow goes past the nozzle in this case of supercritical design. These shockwaves that are caused by high-outlet Mach number and cause severe pressure oscillations downstream of the nozzle are also responsible for increased indirect noise emanating from the turbine as shown in Fig. 7.

Moreover, linked to this is the calculation of nozzle transfer functions which allow the computational simulation of the noise and pressure fluctuations generated by a particular nozzle design. The nozzle transfer function also plays a key role in aiding the design process of engine exit nozzles by allowing the simulation of their acoustic signature, reducing amount of prototypes required during the actual testing phase [20]. This is especially important for military applications in helicopters where turbine and blade noise is highly undesirable. Moreover, a study conducted by Tam et al. has investigated the methods by which the direct combustion noise generated from an APU can be isolated and therefore measured [21]. The process by which they have achieved this is rather interesting, in which they have developed a numerical model using the governing equations for entropy and acoustic waves to compare previous research with their own gathered experimental noise data from a Honeywell RE220 APU. This experimental data have been compared with the model, which simulated a nozzle similar to that which is present in the APU combustor to turbine transition area. Fig. 6 depicts a frequency spectrum from their work, which indicates the indirect combustion noise isolated. Table 1 summarises the advances made in the field of combustion instability from its inception to the present date, table has been adapted from the work of Khanna VK [22].

Figure 6 Frequency spectrum data from a Honeywell RE 220 APU [21].

5. Alternative fuels and their impact on noise and vibrations

Studies have been conducted by Wijesinghe et al. [46] where the noise and vibrations occurring form an APU were measured. The key aspect of this particular study was that an alternative fuel was used and compared against JetA1. The study compared the noise and vibrations characteristics of an HEFA (hydroprocessed esters and fatty acids)-derived alternative fuel in various blend ratios to the standard Jet-A1 fuel. It was found in this study by way of comparison of the power spectral density data from accelerometers and microphones attached to and placed in the near field of the APU. Fig. 7 depicts this.

Table 1

Year

Author (s)

Putnam AA [23]

Studied instabilities on simple combustors and theoretical investigation of Rayleigh

Barrere M, Williams FA [24]

Described different combustion instability mechanisms observed in various combus

Hadvig S [25]

Studied the problem of oscillating combustion in industrial oil–gas fired boiler syste

Goldschmidt VM, et al. [26]

Attempted to predict thermoacoustic instability by applying the systems levels appro

Whitelaw JH [27]

Studied thermoacoustic instability occurring in premixed turbulent flow environmen

Culick FEC, et al. [28]

Concentrated in understanding the nonlinear aspects involved in thermoacoustic ins

Bloxsidge GJ, et al. [29]

Studied causes of ϕ oscillations at low Mach number flows and their impact in therm

Vandsburger U, et al. [30]

Studied the effects of fuel composition on the occurrence of thermoacoustic instabil

Keller JO, et al. [31]

Studied various mechanisms responsible for thermoacoustic instability and the role

Candel SM, et al. [32]

Studied dynamics of flame and their interaction with vortices present in the flow. Al

Dowling AP, et al. [33]

Experimentally studied instability on the afterburner geometry. Developed models f

Yang V, et al. [34]

Studied the occurrence of thermoacoustic instability in rocket engines and develope

Janus M, et al. [35]

Experimentally studied instabilities and their control. Correlation of occurrence of in

Table Continued

Year

Author (s)

Zinn BT, et al. [36]

Investigated thermoacoustic instability in combustors using solid, liquid, and gaseous p

Mongia R, et al. [37]

Developed sensors to capture ϕ oscillations caused by coupling of heat release rate to th

Peracchio AA, et al. [38]

Developed models predicting thermoacoustic instability and compared with experimen

Paschereit, et al. [39]

Studied the interactions of the combustion process with fluid mechanical instabilities an

Paschereit CO, et al. [39]

Concentrated on the effects of coherent structures present in swirling turbulent flows on

Lieuwen T, et al. [40]

Discussed the effects of oscillation on the dynamic heat release especially under lean o

Flores R, et al. [41]

Studied effects of fuel compositions and unmixedness on the occurrence of thermoacou

Annaswamy A, et al. [42]

Described the state of the art of active control for combustion systems as of 2002.

Poinsot, et al. [43]

The difference in the chemical ignition delay of fuels causes phase change between the

Leyko M, et al. [19]

Carried out numerical and analytical work on how indirect noise is generated in nozzle

Choi O, et al. [44]

Investigated the combustion instability of synthetic natural gas on a gas turbine combus

Table Continued

Year

Author (s)

Li B, et al. [45]

Investigated combustion instability of methane in oxy-fuel tubular flame burners, includ

Wijesinghe CJ, et al. [46]

Investigated the noise and vibrations characteristics of a Honeywell APU running on H

HEFA, hydroprocessed esters and fatty acids.

Adapted from Khanna VK. A study of the dynamics of laminar and turbulent fully and partially premixed flames. Virginia Tech; 2001. http://hdl.handle.net/10919/28527.

It should be noted that the 100% Jet-A1 PSD (power spectral density) displays lower vibrational amplitudes with respect to the alternative fuel blends. This is indicative of the motivations behind the testing of noise and vibrations in gas turbines running on alternative fuels. Another study has been conducted by Zhuohui He et al. where the impact of fuel was investigated for emissions lean blowout and combustion dynamics [47]. This study was part of the NJF (National Jet Fuels Combustion Program). The fuels used by them were divided into two categories: “A” category fuels were conventional petroleum-based fuels, whereas “C” category fuels were alternative fuels. Further fuel details are given in Table 2. A key feature of this study was the use of a novel fuel injector with nine injection points injecting directly into the combustion chamber and hence the term lean-direct injection. The primary function of this combustor design is to burn cleaner in of NOx and particulate emissions (Fig. 8). Fig. 9 shows the pressure frequency data conducted in 2015 and 2016 by Zhuohui He et al. [47], which appears to show no high magnitude combustion instability in both figures (2015, left of figure). The tests have been carried out near the lean blowout limit of the combustor. Furthermore, the data appear to overlap indicating that the alternative fuels are as good as the conventional fuel in of combustion dynamics. Some key features of the test conditions are as follows: pressure at 1034 kPa, temperature at 644 K, and equivalence ratio of 0.35.

Moreover, they go on to explain that the two wide-spectrum noise peaks present at 900 and 1700 Hz are different between the two test years and are possibly caused due to variation in the pilot swirler rotation of their combustor. According to Zhuohui, the 2015 tests used a counterrotating pilot swirler, whereas the 2016 tests were carried out under corotating pilot swirlers, which has the effect of changing the interaction of shear layers between the pilot and the main fuel mixers. Finally, the paper goes onto show how the alternative fuels are significantly better than the conventional fuel in of NOx emissions.

Figure 7 Power spectral density for several blends of HEFA and pure Jet-A1 [46]. HEFA, hydroprocessed esters and fatty acids.

Figure 8 NASA Glenn CE-5 Stand 2 test rig used in the experiment [47].

Figure 9 Pressure–frequency spectrum data approaching LBO at equivalence 0.35 for several alternative fuels in nine-point LDI combustor [47]. LBO, lean blowout; LDI, lean-direct injection.

Table 2

Properties

A2

C1

Petroleum Jet-A (w/average properties)

Gevo ATJ; C12/C16 highly branched isoparaffins

4.5

4.9

48.3

16

244

228

230

195

205

182

A further study has been conducted by Simons and Soloiu who have investigated the noise and vibration characteristics of a miniature SR-30 gas turbine running on two alternative F-T type fuels and Jet-A as a reference fuel [48]. The experimental setup used by them involves a single triaxial accelerometer and microphone spaced 1m away from the turbine. The study further goes on to illustrate the impacts that sound-dampening material manufactured from melamine and soybean-based products have on the sound propagation from the gas turbine. Moreover, as can be seen from Figs. 10 and 11, the study shows the fast Fourier transform data for the three fuels that were tested in the frequency ranges of 0– 800, which is considered the range in which combustion noise and vibrations tend to occur according to Lefebvre et al. [49].

Figure 10 FFT accelerometer data for Jet-A (red), S8 (blue), and IPK (green) in the x-direction with amplitude truncation [48]. FFT, fast Fourier transform.

Figure 11 FFT accelerometer data for Jet-A (red), S8 (blue), and IPK (green) in the y-direction with amplitude truncation [48]. FFT, fast Fourier transform.

The fuel properties for the fuels used in the study are shown in Table 3.

Table 3

Property

Jet-A

S-8

IPK

Viscosity at 40°C (mm²/s)

1.40

1.44

1.16

CNa

45

63(20)

31.3

Density (g/cm³)

0.803

0.755

0.76

Lower heating value (MJ/kg)

43.1

41.5

42.8

Flash pointb (°C)

48

38–52

42

TA10 (°C)

84.4

82.0

71.8

TA50 (°C)

130.8

130.7

108.2

TA90 (°C)

164.8

158.9

131.3

Ignition delay (21) (ms)

3.47

2.57

5.11

aSource Oakridge National Laboratory

bFuel safety data sheet

6. Conclusions

In conclusion, it can be seen that alternative fuels do have an impact on the noise vibrations and instability characteristics of gas turbine engines in most cases to the detriment of those qualities. However, the increases are relatively minor, and the root cause of these must be further investigated. It can be hypothesized that the bulk properties of the fuel will have an impact on the fuel injection characteristics, which in turn will impact the instability of the combustion and in turn affect the noise and vibrations produced by the engines. From the aforementioned review, it can be seen that the alternative fuels are, in general, less dense than the conventional Jet-A and Jet-A1. And this may impact the fuel flow characteristics of the fuel and hence impact the spray pattern characteristics of engines designed to run on Jet-A1; fluctuations of equivalence ratio (by fuel flow variations) have been known to cause combustion instability in aircraft engines. It is also noted that further research is required into the effect that other properties such as viscosity, H/C ratio, flash points, and so on have on combustion instability in operational gas turbines.

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Chapter 10: Hydrogen fuel for aviation

Huanrong Lei ¹ , and Bhupendra Khandelwal ² ¹ Department of Mechanical Engineering, The University of Sheffield, Sheffield, United Kingdom ² Department of Mechanical Engineering, The University of Alabama, Tuscaloosa, AL, United States

Abstract

In the aviation fuel development, hydrogen has an important role to play. Compared with the other fuels, hydrogen has right chemical properties such as energy density and gives reduction in harmful emissions. In history, hydrogen has been actively explored to be used as an aircraft fuel since the 17th century. The exploration of hydrogen is wide ranging from hydrogen combustion to hydrogen fuel cell application. For modern aircraft, the hydrogen combustors such as micromix combustor and lean-direct injection combustor are being studied. These combustors are different as compared with conventional combustors and can burn pure hydrogen and without the risks of hydrogen combustion.

Keywords

Alternative fuel; Aviation fuel; Hydrogen combustion; Hydrogen fuel; Micromix combustor

1. Introduction

With the aviation industry developing, the air travels and transports became an indispensable part of human life. The frequent air flights increase the use of traditional fossil fuel, which has become a quite important environmental concern. Since the invention of airplane, the gasoline been used on the reciprocating engine on aircraft and jet fuel on turbo engine in the next few decades. The development of commercial aircraft benefited the public, and the popularization of these commercial aircraft massively increased the consumption of fossil fuels, which has a severe impact on the global environment. Nowadays, the most popular used aviation fuels are kerosene and aviation gasoline (AVGAS); the combustion of these fuels produces greenhouse gases emissions such as COx, NOx, and SOx. All these types of gas discharge into the atmosphere and cause climate changes. The statistics from the aircraft emissions at Turkish airports states that about 6% petrol products are used as aviation fuels, and the burning of these fuels discharges about 3% NOx and 2.6% CO2 of total anthropogenic emissions [1]. Not only the environmental concerns, the economic problem comes with the consumption of fossil fuels as well. The fossil fuel on our earth is a nonrenewable energy source; after entering the 20th century, the fuel price has risen rapidly, and the demand for the investigation of new clean alternative energy sources is necessary. In fact, aside from improving the existing combustion technologies and enhancing the combustors, the use of alternative clean energy is the most feasible solution. The reduction of emissions of existed combustion is limited, and solving the problem from the root is necessary. These considerations drove people to develop and investigate alternative fuels. Following initial investigations, several kinds of clean renewable energy are considered, including tidal power, solar power, hydrogen power, and wind power. However, at this stage, it is believed that only hydrogen power is feasible for the application of clean power in the area of the aviation industry. In contrast to conventional fuel energy, hydrogen only burns with a by-product of water and nitrogen oxides, while there is no emission of gases that include carbon or sulfur. Also, compared with the other fuels, hydrogen fuel has many advantages.

2. Hydrogen fuel

2.1. Hydrogen fuel properties

During the combustion, the hydrogen reacts only with air to produce water vapor and nitrogen oxides. Compared with the other fuels, the hydrogen fuel does not produce any harmful pollutants or particulate matter (PM). The only problem that needs to be solved associated with hydrogen combustion is the NOx formation. Due to the high combustion temperature of hydrogen, the production of NOx is higher than the conventional fuels. Appropriate method to reduce the NOx formation is controlling the combustion equivalence ratio or reducing the flame length [2]. As an alternative energy source, hydrogen fuel is not only a clean energy source but also has the highest energy density (120 MJ/kg excluding the radioactive element) [3]. Other aviation fuels used such as methane and JET-A only contain energy density about 50 MJ/kg [3,4]. Hydrogen fuel can provide high specific impulse; the burning 1 kg/s of hydrogen fuel could provide 450 kg force thrust. Because of this property, hydrogen has been used for the space exploration as the fuel of space shuttle and rocket by NASA since the 1970s [5]. It is worth mentioning that the hydrogen used in rocket propellant is in solid form. It is because the density of natural hydrogen gas is too small, and only solid hydrogen could satisfy the rocket propelling. As the previous studies show, hydrogen has the lowest weight of all molecules, which means that for the same weight of fuel, the storage tank must be much larger. Fig. 1 demonstrates that compared with conventional kerosene fuel, the volume of hydrogen is three times larger for the same energy content. This means that if hydrogen is to be used on the existing aircraft, although the weight of the fuel tank could be reduced, the space of fuel tank needs to be much larger as compared with conventional fueled aircraft. At this stage, to apply solid hydrogen fuel in the commercial aircraft is impractical and not economic. The

only possible consideration from an aircraft point of view is using hydrogen in liquid form. However, the storage of the liquid hydrogen fuel is a complex issue. For the use of liquid hydrogen, NASA stores the liquid hydrogen in a vacuum cryogenic tank and keeps the temperature lower than −423°F [6]. Although hydrogen fuel is the best option to solve environmental pollution and long-term development, the hydrogen fuel storage on commercial aircraft is still a difficult challenge, which needs to be overcome.

Figure 1 Comparison between hydrogen and kerosene.

2.2. Hydrogen production

To widely apply hydrogen fuels for commercial purpose, large amount of hydrogen is required, and the large-scale production is a problem that must be faced. Before the promotion of hydrogen powered aircraft, the production of hydrogen fuel on a large-scale industrial level needs to be solved. In fact, the International Energy Agency (IEA) has discussed the production of hydrogen [7]. Their study indicates that, at present, there are several feedstocks that could be used to produce hydrogen, such as fossil fuel, water, or biomass. At present, almost half amount of hydrogen production is from the steam reforming of natural gas, part comes from oil/naphtha and the gasification of coal, and only small partial of hydrogen is produced by water electrolysis [8]. Producing hydrogen fuel from fossil fuel is the main large-scale process with low costs and high efficiency. However, this process always has COX as the by-product, which negates the main reason for using hydrogen fuel. Hence, to use hydrogen through a total clean method, hydrogen is split from water. The basic method is electrolysis, which simply uses electric power to produce hydrogen, but it has high costs and low efficiency. In addition, the electricity energy used in the process comes mostly from fossil fuel. From the simple water electrolysis hydrogen production, more and more improved methods have been developed from it, such as high-temperature electrolysis, photolysis, and alkaline electrolysis. For all the aforementioned methods, high-temperature electrolysis method has been proved that it could be a promising technology for efficient large-scale production of hydrogen [9]. The photoelectrolysis method is also a commercial way to produce hydrogen gas in large scale; it has high hydrogen splitting efficiency at a relative low cost. In addition, the photoelectrolysis is a production process using solar energy, which makes the hydrogen to be directly produced without any carbon oxides emissions. If the photolysis industries be constructed at high solar energy potential area (such as Algeria), the large-scale hydrogen production with low

cost is possible [10]. The simple concept of hydrogen production could be described as shown in Fig. 2. In this process, the feedstock could be water or hydrocarbons. The feedstocks go through the extraction process, which could be photolysis, electrolysis, and thermochemical [11]. For the extraction of hydrogen molecules from the feedstock, the external energy is required to help molecules escape. This energy could be hydrocarbons, nuclear energy, and renewable. For example, the external energy used for high-temperature electrolysis could be nuclear energy; for photolysis process, it could be solar power [11]. It has been shown that the photolysis seems to be the most efficient and low-cost method to extract hydrogen by far [11]. Although the hydrogen production technology is not mature enough to produce hydrogen in large-scale as an aviation fuel, it has a potential to obtain good progress in the near future. The aim of this chapter is to review of application of hydrogen fuels, and the focus has been put on the using of hydrogen. Therefore, the hydrogen fuel supply has not been considered in detail. Previous studies have already shown that industrially produced hydrogen as an aviation fuel is possible, which means that the fuel supply for future hydrogen-powered aircraft is promising.

Figure 2 Hydrogen production process.

2.3. Hydrogen fuel storage

2.3.1. Tank configuration

At room temperature, hydrogen is in gas state; for the utilization of hydrogen as an aviation fuel, the hydrogen gaseous and hydride is impractical due to the large volume and excessive weight of the tank [12]. Storage of the hydrogen in solid or slush is feasible; however, keeping the hydrogen in solid state demands significant amount of energy to pressurize it and subcool the hydrogen. Compared with the liquid hydrogen, the density of solid hydrogen is only improved about 20% (from 70.8 to 86 kg/m³), but the energy demanded for the solidification is much higher. Hence, the storage of hydrogen in liquid state is the most suitable solution for the long endurance aircraft application. To store the hydrogen in liquid state, high pressure and low temperature are necessary, hydrogen must be cooled to −253°C (about 20 K) at 1 atm, or the pressure of hydrogen gas must be increased to 5000–10,000 psi at the room temperature [13]. Due to the low density of hydrogen, as shown in Fig. 1, in the same weight hydrogen fuel and kerosene, hydrogen has 2.8 times energy content than kerosene. However, this advantage was compensated by the weight of complex hydrogen fuel supply system to some extent. But it could be expected that the hydrogen could allow higher payload at the given takeoff weight. On the other hand, for same amount of hydrogen with respect to weight, the volume of liquid hydrogen is four times larger than aviation kerosene. Therefore, the hydrogen fuel requires a much larger fuel storage space than conventional fuels. The previous wing storage design would be too heavy for liquid hydrogen tanks; new tank layout design has to be implemented. To install the big hydrogen tank to the

commercial aircraft, the previous nonintegral fuel tank structure cannot be used. The wing structure cannot bear the weight of the hydrogen tank. In addition, the volume of hydrogen tank is much larger than kerosene tank; the high frontal and surface will cause higher drag force and affect the aerodynamic of the aircraft. Therefore, the integral tank design has been considered in many studies; for this unusual configuration, mounting the integral tank on the top of the fuselage is a feasible way for big commercial airplane [14]. To install the fuel tank in the aircraft fuselage, the tank shape and weight is a crucial challenge for commercial aircraft. Also, the storage of liquid requires the cryogenic fuel tank to keep the LH2 at 20 K; therefore, the insulation of the tank and cryogenic device are important for the tank design.

2.3.2. Tank shape

For the storage of the liquid of hydrogen, to apply the hydrogen as the aviation fuel, the integral tank structure should be considered. For the integral tank design, the small frontal and surface area is required for the minimal weight. Also, the minimization of the tank surface area could result the reduction of the amount of conductive heat entering the fuel tank. As mentioned before, the boiling point of liquid hydrogen is extremely low, which is −253°C; hence, the less heat entering the tank means less fuel loss during the storage [12]. For the nonintegral fuel, tanks only function as the fuel container and do not have to conform the shape of the aircraft; the design purpose is minimizing the drag for and reducing the fuel loss. The most common design shape of fuel tanks is spherical or cylindrical for the insulation requirements [15]. The spherical geometry is the simplest shape to design and manufacturing but has the least clearance with the existing airframe design [16]. In addition, the spherical geometry of the fuel tank has the minimum surface for same volume, which means that the ive heat conduct into the tank is minimum as well, so the loss of the fuel will be reduced [17]. These are some studies discussed about hydrogen fuel tank design of spherical shape. As Xu. et al. stated that the spherical liquid hydrogen fuel tank includes an inner tank and an outer tank, with eight point- insulating s [18].

In Figs. 3 and 4, it can be observed that between these two concentric spherical shells, there are insulation layers [18]. To reduce the heat leakage from the structures, a new design was studied, as shown in Fig. 4. This new structure contains several ceramic balls. It could reduce the radius; the point between the gasket, inner tank shell, and balls could reduce the heat leakage by more than 85% as compared with the normal pull rod structure.

Figure 3 Spherical hydrogen fuel tank design [18].

Figure 4 Low thermal structure design [18].

Except the spherical fuel tank, cylindrical fuel tanks have also been widely used. Compared with the spherical tanks, for same capacity, the cylindrical fuel tank has larger surface area than the spherical tank. It resulted in more heat being absorbed through the larger surface. Also, the pressure inside the cylindrical is not equally distributed, which means that the wall of cylindrical tanks needs to be thicker and larger than that of spherical tanks [12]. In addition, due to the length of the cylindrical tank, if this tank wants to be mounted into the commercial aircraft, the sloshing problem needs to be solved to keep the weight balance. Therefore, the weight of the cylindrical tank is greater than tank in sphere shape for same capacity. Based on the aforementioned information, the spherical fuel tank is better than cylindrical tank for the integral liquid hydrogen fuel tanks. In the LH2 tank design, the insulation layer is a significant design to store hydrogen and minimize the boil-off of liquid hydrogen. There are mainly two types of insulations, which are internal insulation and external insulation. In the usage of external insulation, the problem comes with tank expansion and contraction. Also this external insulation design is easily damaged [19]. However, compared with internal insulation, this design has less technical difficulties. For the internal insulation, there are different insulation methods such as multilayer insulation (MLI), vacuum insulation, and foam insulation. The MLI has a number of different material layers to prevent the heat leakage. Mostly, the materials used are metal foil and insulating material; these insulating layers pile up alternatively [20]. Because more heat shields the better function, the MLI product will be heavy. The vacuum insulation is an effective method to be used on LH2 tank. But to keep the insulation layer vacuum is very difficult; also the material tank wall needs to be strong enough against the pressure due to vacuum. In addition, the external equipment is required to exhaust air. This method requires a thicker tank wall and external equipment to maintain the pressure in the vacuum layer [21].

3. Historical aerospace applications of hydrogen

3.1. Hydrogen aircraft development

3.1.1. Balloon and Zeppelin—1783 and 1911

Hydrogen as a clean alternative fuel has been applied in aeronautics area from hundreds of years. Initially, the “lighter than air” concept was developed; the low density of hydrogen gas made the using of hydrogen as uplift power feasible. In 1783, Roberts brothers used the hydrogen gas in balloons for the inflation, under the direction of French physicist Jacques Charles [2]. At the end of the same year, a large manned balloon was built, which had ability to carry two engers. Charles and one of Roberts became the first persons to fly with hydrogen balloons. Compared with hot air balloon that uses heated air as the uplift force, hydrogen balloon does not need extra power such as flame heating. Another famous aeronautic application of uplift force of hydrogen gas is Zeppelin airship. Since the late 19th century, a rigid airship with direction and speed control was designed by Graf von Zepplin. The internal combustion engines were used for propulsion. Zeppelin began the commercial use of airship by German transportation company DELAG (Deutshce Luftschiffahrts-AktienGesellschaft) [22]. In next 5 years, more than 1500 flights were made and carried over 10,000 engers [23]. This is the first commercial use of hydrogen powered aircraft in the world. An accident in 1937 made people realized that hydrogen gas used in commercial airship is risky and dangerous [24]. On May 6, at the end of scheduled flight of Hindenburg, the airship burst into flames and 35 engers died in this accident. The Hindenburg accident signaled the end of using hydrogen gas in commercial flight of airship.

3.1.2. HeS 1-1937

Both the aforementioned examples just used the physical properties of hydrogen such as low density. In the history, people started using hydrogen as a chemical fuel from 1937. Von Ohain, who was considered the designer of the first turbojet engine [25], worked with Ernst Heinkel in 1936 and decided to produce an engine that runs on hydrogen gas. The simple Heinkel-Strahltriebwerk 1 (HeS 1) was completed in March 1937 and successfully ran with hydrogen gas and produced 551 lb of thrust [26]. Although this engine was never designed for flight purpose, it has still been shown that the basic concept of hydrogen power engine was workable.

3.1.3. Air Force B-57-1955

The first successful flight of hydrogen-powered aircraft was conducted in 1957, which was conducted by NASA. In the autumn of 1955, the National Advisory Committee for Aeronautics (NACA) of the United States started the investigation of aircraft with liquid hydrogen fuel. This project was known as Project Bee; the initial study was start on an Air Force B-57B bomber with two Curitss Wright J-65 turbo engines [27]. One engine of the airplane was modified to test liquid hydrogen fuel (LH2) in this test. In the plane, two types of fuels were used in the test: one engine uses regular kerosene JP-4 for taking off, and then after climbing the reach height 16,400 m, another engine with independent hydrogen fuel system started working. The configuration design of the hydrogen B-57 is shown in Fig. 5 [7]. The liquid hydrogen was stored in the tank at the tip of the wing, and on the opposite wing tip, a helium tank was mounted to pressurize the hydrogen tank and pump liquid hydrogen. In addition, the heat exchanger was fixed on the wing near the hydrogen tank; the function of this design is gasifying the liquid hydrogen to

make it burn easier in the turbojet engine. In the engine test, they found that for stable combustion, the hydrogen engine flight altitude was 7000 m than JP-4 engine. The thrust of hydrogen engine was 2–4% higher than JP-4, and there was 60–70% lower of specific fuel consumption [28]. There were three flight tests of this hydrogen power B-57: first two flight tests were unsuccessful because of the hydrogen vaporization and pressure rise problem. Although the airplane takeoff and climb were successful, during cruising, the venting of hydrogen tank was required frequently to keep the pressure within operation limitation. These two flights still proved that the hydrogen power aircraft is workable. The third test was made in February 1957, which was the first successful flight of hydrogen-powered aircraft in history, which had historical significance in hydrogen fuel application. The test flight took 17 min with altitude 50,000 ft, and the speed of this hydrogen power B57 was around 0.75 Mach [29]. The pilot reported that the engine runs well when using hydrogen alone, and the switch between hydrogen and JP-4 was quite smooth. In addition, in his book, G. Daniel Brewer recorded that “there were no operational safety problems encountered with the hydrogen fuel system” [2].

Figure 5 Configuration of B-57 hydrogen fuel aircraft [7].

Although the flight test of hydrogen aircraft was made successfully, there were some shortcomings revealed. This hydrogen engine only ran about 20 min and almost exhausted, which means that the flight endurance of hydrogen engine is a problem. Also the condensation cloud trail was left after hydrogen combustion, it may have impact on the climate [7].

3.1.3.1. Lockheed CL-400—1956

At the same time, the United States also started a secret project of hydrogen fuel aircraft called Suntan. During the late-1950s, Lockheed received a contract from the United States for design of two prototype reconnaissance aircraft, which has the speed of Mach 2.5 and 30,300 m altitude [30]. The CL-400 high-flying reconnaissance aircraft was selected with two engines designed by Pratt and Whitney. For the CL-400, the three views of CL-400 are shown in Fig. 6, which demonstrated that CL-400 had a 3 m diameter and 49 m length fuselage to carry 9707 kg of liquid hydrogen fuel (LH2). It had the similar appearance design as F-104, and two engines are mounted at the tip of the wings. For this airplane, Pratt and Whitney designed a 304 engine for the using of LH2: each engine weighed 2850 kg and could provide 27 kN at Mach 2.5 and 29,000 m altitude. For this project, a series of prestudies had been done including hydrogen tanks and fuel system and practicability of hydrogen fuel. Few tests were conducted about the storage of hydrogen, and they found that hydrogen was easily escaping without ignition.

Figure 6 Configuration of CL-400 hydrogen fuel aircraft [7].

Initially, Pratt and Whitney decided to analyze the different hydrogen engine designs and design a new one. However, with further analysis of various hydrogen engines, they realized that modifying an existing engine to LH2 was much convenient. They made an agreement with Air Force and modified the J57 engine for this contract. The engine was built and tested in 1957; after several tests and modifications, the 304-2 hydrogen engines were tested successfully at the end of 1958, and the engines were operated for about 25 h with hydrogen without problem. These prove that the hydrogen engine development satisfied the design required [7]. At the end of the 1960s, because of the differences in technical opinion, the Suntan project gradually demised. This was mainly because of the slow development of hydrogen aircraft and hydrogen fuel storage and transport problem. However, from the results of the project, it was shown that the hydrogen aircraft had higher flight altitude than traditional fuel-powered aircraft, which shows the potential of keep developing hydrogen aircraft.

3.1.4. TU 155-1988

In the 1970s, Soviet Union started the studying of alternative fuel: liquid hydrogen (LH2) and liquid natural gas (LNG) due to the Oil Crisis. On April 15, 1988, the world first liquid-powered aircraft made its flight test successfully [31]. The Soviet announcement stated that the building of hydrogen Tu-155 could show that the hydrogen could be a practical fuel [32]. A former Soviet Union plane manufacturer Tupolev redesigned the engine system of TU-154 to test liquid hydrogen fuel; one of the three NK-8 engines was converted to NK88, which was fitted at the starboard position. This engine was operated with hydrogen alone, and the other two engines were still fueled with conventional fuel; in addition, the rear part of the enger cabin was modified for the hydrogen fuel tank [33]. The engine fed with LH2 was tested at the altitudes

7000 m and with the maximum speed 900 km/h; the starts and failures of engine were simulated. However, although the successful flight was made, this LH2 program only lasted for five flights and had then been cut down. The reason was mainly because of the high cost of liquid hydrogen and the lack of LH2 facilities in airports.

3.1.5. A310-1991

After Soviet Union stated their alternative fuel projects, Daimler-Chrysler Aerospace Airbus cooperated with a German–Russian team and started the investigation of alternative fuel aircraft from 1990. Then in 1992, the investigation concluded that liquid hydrogen is safer than natural gas and more environmental friendlier. A study of liquid hydrogen airplane based on Airbus A310 was started, and the design of modified airbus A310 with cryogenic fuel tank above the full length of the cabin because of the four times volume than conventional kerosene was proposed. Long-range hydrogen aircraft was found to have 15% less of takeoff weight than normal aircraft. However, due to the complexity of hydrogen combustion operation, the final mean flight weight increased [34]. This study found the base of a system of hydrogen project CRYOPLANE project.

3.1.6. CRYOPLANE-2000

Due to the continuous increase of conventional aviation and the environmental concerns, in year 2000, a consortium of 35 partners from 11 countries came together and started a project called “CRYOPLANE,” which was ed by the European Commission with 4.5 million euros in a 2-year period [35]. This project aimed on a comprehensive study of applying liquid hydrogen fuel on commercial aircraft to replace the conventional fossil-based fuels such as kerosene. This study was a large-scale research of liquid hydrogen, which

covered all the aspects that may be related, such as technical, environmental, and economic consideration. In this project, the production of hydrogen, the configuration of the hydrogen-powered aircraft, the combustion system and components, safety and propulsion, environmental compatibility, and many other aspects were studied. For example, the project considered different configurations of different flightrange commercial aircraft and thought out different hydrogen tank carrier plans. In addition, the final report of this project also mentioned that the using hydrogen in conventional turbo engine was feasible and has even shown the better performance on specific fuel consumption and engine temperature [35]. As the result of this project, it proved that applying the liquid hydrogen on commercial aircraft is technically feasible but still requires much more research and development work on it. As the estimations from the project analysis, at the 2000s because of the missing of material and components, the hydrogen fuel cannot be furtherly researched. Also, from the practical point of view, the hydrogen fuel will not be implemented until the hydrogen storage and production had been fully developed. In addition, because the kerosene is cheaper and easy to produce, compared with kerosene, hydrogen is unattractive. Hence the project team estimated that the earliest implementation of the hydrogen engine technology at least needs 15–20 years [36]. In fact, as the scenario of the project assumed before, the first commercial operation of hydrogen aircraft would have happened in 2015. In fact, the largesize hydrogen aircraft have not been developed, but a hydrogen fuel cell– powered enger airplane HY4 was successfully tested before in 2016 by DLR [37].

3.1.7. Scramjet-2004

Liquid hydrogen was also used with scramjet. NASA′ Hyper-X team planned to develop a small-scale hypersonic vehicle, with liquid hydrogen fuel. Therefore, the experimental unmanned scramjet X-43 was developed for the hypersonic flight and LH2 fuel testing; the design of X-43 was followed the no-landing and no-recovered concept, which is for test only [38]. Due to the operation condition

of scramjet, hydrogen has huge advantage as fuel in hypersonic flights. This Hyper-X program was investigated in 1996, and first scramjet X-43A was developed, which was powered by a single hydrogen-fueled propulsion system. There were several flights attempted from 2001 to 2004. The second and the third flight tests successfully demonstrated that the hydrogen power air-breathing propulsion system is feasible. In the third flight test on November 18, 2004, the flight lasted for 11 s and covered the distance of 20 miles with top speed of 9.6 Mach [39].

3.1.8. Boeing Phantom Eye −2010

Hydrogen fuel on the commercial aircraft has met severe difficulties due to the hydrogen fuel's high volume. In the area of unmanned aerial vehicle (UAV), liquid hydrogen fuel was successfully employed. In 2010, Boeing announced that it started to build its first liquid hydrogen–powered, high-altitude longendurance (HALE) UAE [40]. Boeing stated that this demonstrator UAV could carry a 450 pounds payload and fly for 4 days. For the full-size Phantom Eye, it has 10 days endurance and 2000-pound payload carry ability. For the Phantom Eye, the liquid hydrogen was employed as fuel, and the feature of Phantom Eye had two 2.3 L turbocharged liquid hydrogen internal combustion engines, which were mounted on 150-foot distance from the fuselage on the wings. The engines used were designed for Ford cars. To ensure the engine could work normally at high altitude, the engines used a multiple turbocharge to compress low density air. From 2010 to 2014, Boeing demonstrator Phantom Eye has carried nine flight tests; the recent one in 2014 demonstrated that Phantom Eye could fly at 54,000 ft altitude for more than 8 h [41]. The success of Phantom Eye has shown the great potential of liquid hydrogen fuel in the area of HALE aircraft.

3.1.9. Hydrogen fuel cell aircraft

For the application of hydrogen fuel in aviation industry, except combustion with hydrogen fuel in engine, the utilization of the hydrogen fuel cell has great potential in future commercial flight industry. On September 29, 2016, the world first hydrogen fuel cell–powered four-seater enger aircraft did a successful flight test at Stuttgart Airport; it was ed by German National Innovation Programme for Hydrogen and Fuel Cell Technology (NIP) [42]. Compared with the previous tests using of liquid fuel as a combustion fuel, the hydrogen fuel cell was used to drive an electric motor on the aircraft. As the project described, this HY4 aircraft is a central single-propeller plane with two-seater fuselages on both wings. The electric motor was powered by a fuel battery, which was placed behind the propeller system, and the liquid hydrogen was stored in two carbon fiber tanks at high pressure of 4300–5800 psi, which are mounted in two fuselages [43]. The fuel cell could convert the liquid hydrogen fuel into electrical power and drive the motor [44]; in the flight, the electric motor could provide a maximum speed of 200 km/h and approximately 145 km/h during the cruise. With various speeds and loads, the range of HY4 is between 750 and 1500 km [37]. Not only DLR built hydrogen fuel cell successfully, but China also made a successful flight test in 2017 and became the third nation to test the hydrogen fuel cell airplane successfully. The plane named RX1E has a fuel cell system and lithium batteries; during the takeoff and climbing, the lithium works and then power system switches to hydrogen fuel at cruise, and it could fly for 60 min with the speed of 120 km/h [45]. In addition, during the flight test of hydrogen fuel cell aircraft, there was zero emissions, which only had water vapor exhausted. Theses hydrogen fuel cell aircraft make people see the potential of hydrogen fuel cell, as Josef Kallo, HY4 project leader at DLR, said that these small enger airplanes could be applied in regional air travel such as the air taxis in the future [46]. The history of the hydrogen powered aircraft is shown in Fig. 7.

Figure 7 Timeline of hydrogen aircraft history.

Figure 8 Tank arrangement of small regional and business aircraft [11].

3.2. Aircraft configuration

Due to the low density of hydrogen, the fuel storage space required is much large than that required for kerosene and other conventional fuels. In CRYOPLANE project, the LH2 tanks used were not in a specific shape. In the CRYOPLANE project, multiple commercial aircraft with different sizes and types were studied. The space for tanks in the fuselage was different; therefore the size requirements of the fuel tanks were various for different sizes of airplanes. In small shortrange aircraft, the fuel storage space is too small for conventional tank shape; in this situation, the sphere shape may be adapted. In the long-range enger airplane, there is bigger fuselage, which has more space for the fuel tank. For the details of the tank layout of the conventional aircraft configuration in CRYOPLANE project, there were several different tank layout plans for the integral tank design studied. Based on the seat number on the aircraft and flight range, from small regional turboprops to large long-range jet aircraft, total seven categories were classified for aircraft. In addition, three different tank configurations were proposed [36]. In the project, mainly three fuel tank arrangement designs were given related to four different aircraft types, respectively. The first tank layout design was applied to “small regional aircraft” and “business aircraft,” and the fuel tanks were placed at the rear part of the fuselage behind the rear pressure bulkhead. In the second tank arrangement design, the fuel tanks were arranged above the cabin and rear part of the fuselage, and the “regional aircraft up to 100 seats,” “short-range aircraft,” and “medium-range aircraft” were fitted with the design. The third design was suitable for large aircraft, which were named as “longrange aircraft” and “very large long-range aircraft” in the project. For these aircraft, the fuel tanks were mounted at the rear part of airplane aft of the pressure bulkhead and behind the cockpit, respectively. Aside from these conventional air configurations, there were some irregular designs as well. The

design detail and general weakness are concluded in the following table.

Layout Small regional and business aircraft (Fig. 8)

The fuel tank is mounted behind the pressure bulkhead at the rear part. Because of the consideration of the center of grav

Center of gravity consideration, and compensation methods are required such as larger tail. The trim drag will increase,

Figures of fuel tank arrangement (Figs. 8, 9, and 10).

Figure 9 Tank arrangement of short/medium-range aircraft [11].

Figure 10 Tank arrangement of long-range aircraft [11].

4. Hydrogen combustors

4.1. Hydrogen in civilian use

For the utilization of the hydrogen fuel, the conventional combustors cannot be used directly. Before the hydrogen fuel could be burned in existing combustors, there are several challenges that need to be overcome. Because the properties of the hydrogen are quite different from the conventional fuels such as kerosene, the high burning speed, large diffusivity, and the large range of flammability result in the high risk of autoignition. In the conventional combustor, the largescale center flame will be produced when burning fuels; for the hydrogen fuel, the flame may burn back long the fuel injectors [47]. The application of hydrogen in the conventional gas turbine combustion system may produce the flashback problem; this phenomenon happens during the ignition when burning fuels at high flame speed [48]. Even if the flashback problem is solved, the high production of NOX will be another challenge, which needs to be taken care of for the application of hydrogen fuel. The conventional fuel combustors have limited number of fuel injectors; this geometry makes the mixing of hydrogen and air inadequate. When the combustion starts in the combustor, a high-temperature stoichiometric layer is formed in the combustor chamber along with large flame. The high temperature concentrated in the combustor will result in the production of higher NOX emission [49,50]. As mentioned earlier, the only emission of hydrogen fuel is NOX; the formation of the NOX happens when the flame temperature reaches 1800 K and above [51]. The higher the temperature, the NOx formation rate would also be higher. Therefore, demand for new concept of combustor for hydrogen fuel came up; this concept needs to solve the autoignition risk and the high NOx emission problem. The combustor being developed should be able to reduce the flame size, and to achieve it, smaller and multiple-hole injector might be required. To

reduce the NOx emission, the flame temperature in the combustor chamber, as well as the residence time of reaction, needs to be decreased. For this purpose, the increasing of the fuel/air mixing intensity is a critical requirement in this design. There are two combustor concepts that satisfy these requirements. These are micromix combustion concept and lean-direct injection (LDJ); both concepts could avoid the risk of flashback and improve the mixing of fuel and air to reduce the combustion temperature in the chamber.

4.1.1. Micromix combustor

For the utilization of the hydrogen fuel in existing gas turbine aircraft system, two problems need to be considered. The first one is how to burn hydrogen in gas turbine engine safely without flashback, and the second question is how to change the engine power level rapidly and precisely with hydrogen fuel. Fachhochschule (FHA) Aachen university considered these questions and made a further movement since 1988. FHA converted a small shaft gas turbine engine KHD T216 to a gaseous hydrogen-powered engine in 1992 [52]. In a further step made in July 1994, FHA worked with DASA, BGT, ASA, and Sundstrand together on an auxiliary power unit (APU) GT 36-300 of A320 to convert conventional A320 APU to a hydrogen-powered APU [49]. The previous gas turbines burned with fossil fuels work well in premixing design; the physical properties of natural gas make the risk of flashback low. However, the hydrogen and air mixture were highly combustible; the high risk of premature burning needs to be looked at. In this research, a new combustion concept had been developed by FHA, compared with the conventional combustor. In this new micromix concept, the number of air/fuel mixing zones was rapidly increased. This change was done to minimize the scale of mixing and to maximize the intensity of the air/fuel mixing [49]. From Fig. 11, it could find that the conventional air inlet was replaced by a ringed , and a very large number of injection holes are arranged on this combustor main cross section. This design could make change the conventional large-scale flame into multiple miniaturized diffusion flames. This modification of the combustor helped the research achieve the design purpose. The thousands of the hydrogen/air mixture gas enter the chamber and burn into the miniaturized

diffusion flames; as a result, the risk of self-ignition was greatly reduced. For the number of the injection holes, in principle, the number would be unlimited even to molecular size, and a combustor inlet section could have even millions of air/fuel inlets. For more injectors, the scale of the mixing zones would be reduced as well, and then the mixture of air and fuel could be stirred perfectly.

Figure 11 Hydrogen micromix combustor schematic [53].

Compared with fossil fuel–based conventional combustor, the emission of NOx is much lower for hydrogen-based micromix combustor, and it could be further reduced. The NOx formation goes exponentially high above temperatures of 1800 K [51]. Therefore, to reduce the NOx emission of the hydrogen combustor, the flame temperature needs to be controlled. Also, the insufficient of air/fuel mixing impacts the combustion temperature, and the micromix concept combustor greatly avoids this problem. For the maximization of the mixing intensity, multiple-injector configuration can split the conventional large flames into miniaturized diffusion flames, which makes a large increase of the number of local mixing zones. This increases the total mixing intensity of hydrogen and air. In addition, the improved mixing of fuel and air increases the turbulence generation and eddy breakdown, which could rapidly reduce local stoichiometric combustion zones. As a result, the residence time of the flame is reduced, and the formation of NOx is decreased [54]. However, although increase in the number of the injector holes could improve the mixing intensity and reduce the NOx emission, the number of injectors cannot be increased unlimitedly. There are two factors that restrict the miniaturization of the flames: the first factor is the limitation of manufacturing technology. With the increase in amount of the injectors, the size of the injectors downs too small, and the existing manufacturing skills cannot produce the components in that size and integrity. Another critical challenge is the pressure drop, which limits the number of the injection holes. In the modern combustors, the pressure loss should not exceed 3%–4%, and the flow energy is used to increase the mixing of fuel and air; hence, the pressure drop impacts total mixing intensity. The research results of the micromix combustion concept by FHA were applied to a practical GT 36-300 combustor; the further study about the NOx reduction and the micromix improvements was planned. This APU was built by AlliedSignal and used for experiments [49]. In this research, the configuration of the combustor has been modified to a hydrogen-operated combustor. The schematic of the comparison between the conventional combustor and the micromix concept combustor of A320 APU is shown in Fig. 12. It can be

observed that the inlet section of the original fossil fuel combustor has been replaced with a multiple-hole inlet. Theoretically, these miniaturized injection holes at the combustor inlet could guide the air/fuel mixture burning into numerous miniaturized flames, and the mixing intensity of the reactants would be rapidly increased. In addition, the reduction of the flamelets length would also reduce the residence time in high-temperature regions [55].

Figure 12 The comparison between conventional and micromix combustor [49].

The practical modified LH2 APU GT 36-300 based on micromix concept was built and tested. In Fig. 13, it can be observed that annular combustor inlet was modified into a ring shape micromix combustor. This combustor included two parts: the hydrogen-segment ring, which has a large number of micro–hydrogen injection holes distributed uniformly on it, and the air guiding s (AGPs), which consist of multiple AGP gates corresponding to each hydrogen injector. The final modification makes this combustor burn with thermal loading power of 1.6 MW. Also, in the combustor chamber, there were total 1600 miniaturized diffusion flames, which have great positive impact on the mixing of the reactants [56]. The function of the AGPs is illustrated in Fig. 14. To improve the mixing ability of the injectors, the injection method was developed and modified, as shown in Fig. 14B. The hydrogen was injected into the air flow perpendicularly. This design makes the mixing of hydrogen and air faster and more intense, which eventually reduces the NOx emissions. In this AGP design, an important parameter needs to be considered, which is called as blockage ratio (BR). The BR would impact the grade of mixing and the fame shape. From Fig. 14B, it can observed that if the hydrogen injection depth is too small less than the critical injection depth , the hydrogen fuel cannot penetrate the shear layer of AGP vortex. In this case, the hydrogen fuel would not mix with air stream efficiently. It can result in the increase of NOx emission [56].

Figure 13 Hydrogen micromix injector design [57].

Figure 14 Hydrogen injection with AGP [57]. AGP, air guiding .

The AGP structure is an important component in micromix concept, and many previous investigations about this design have been carried by Funke et al. [55– 58]. The studies have proven that the micromix combustor concept is promising in the reduction of NOx emission. It has been shown that the splitting of largescale flame to micromix flames could lower the residence time of NOx and results in the reduction of NOx level during the combustion [55,59]. In their papers, AGP has been considered as a key factor of NOx emission control. Several studies have been carried out on the AGP structure micromix injector. It has also been said that the BR of AGP could influence the vortex formation in the combustor chamber, which could result in the flame shape and the temperature distribution. The numerical Computational Fluid Dynamics (CFD) study about AGP effect on the combustion performance has been conducted by Funke et al. [55–58]. In one of the CFD models shown in Fig. 15A, realizable model has been used as the viscous model, and y+ wall treatment has been used. These viscous models could be a good reference for the future studies. To ensure the accuracy of the numerical solution, a very fine mesh has been applied on this model; the total mesh cell number is about 900,000 [60]. In a numerical study on hydrogen combustors, the combustion performance is studied with different BRs and geometries [60]. It was found that the height of the AGP could impact the vortex formed in the combustor. As shown in Fig. 15B, there are two different types of vortex formed. The air flow through the air guiding forms the inner vortex, and the recirculation of hot gas downstream of the hydrogen segment results in the outer vortex. By increasing the height of AGP, the size of the vortex could be increased, which could increase the length of vortex shear layer and reduce the amount of NOx emissions. Another method is increasing the mixing distance in the chamber, which could reduce the NOx emission as well.

Figure 15 (A) CFD model of micromix combustor. (B) Gas recirculation in the chamber [57].

The studies by Funke et al. [57] have proved that the NOx emission could be effectively reduced about 80% by adjusting the flame profile, which can be achieved by adjusting the injector geometry. The modification of the burner geometry could positively influence the vortex shear layer length, which means that the combustion zone length and the residence time could be affected. Funke et al. [60] have shown a high-quality study on the AGP geometry effect on NOx emission; the main objective of this investigation is the gas recirculation in the combustor chamber. Different geometry impacts on the vortices formed in the combustor chamber were studied in detail. The basic geometry of the micromix combustor design investigated was based on the previous study carried out by Ying Chun Li et al. [5] at Cranfield University. Their study has shown the essential concept of the hydrogen micromix burner; different injection methods have been researched by them. The focus of their study was simulating the micromix combustion with varied injection directions. This combustor had a main air inlet and four perpendicular hydrogen injection holes. The original injection method involved injecting air from the main pipe in the combustion chamber, and hydrogen was getting injected by four cross-flow injectors. Several designs of hydrogen and air injection designs were studied in this research. The results show that the different injection method did had the impact on the combustion performance and emissions inside the chamber [61]. The temperature contour shown in the study reflects reduction in the high-temperature areas. The reduction of the hightemperature zone not only lowers the NOx emission level but also makes the reduction of chamber length possible. The injection method modification was effective and efficient. As the conclusion of this study, it is clear that the adjustment of the injection method could effectively influence the combustion performance; the varying of the flame shape in the chamber makes the chamber modification feasible, and also the material life span of the chamber could be increased.

4.1.2. LDI combustor

For the application of the hydrogen fuel to the conventional combustor, the key factor is the injector. In the micromix combustor concept, changes are done to the inlet of the current annular combustor inlet to produce miniaturized flame rather than large-scale flames to avoid the risk of self-ignition. Except the micromix concept that is based on the nonpremix combustion, the LDI studied by Marek modified a current LDI combustor injector to adapt the hydrogen fuel [62]. This design was based on the premixed combustion; this premixed system was highly susceptible to flashback. To avoid the risk of flashback, injector was modified to have multiple injection holes with quick mixing technology. The reaction rate of hydrogen is over seven times the conventional Jet-A fuel, and to reduce the high risk of flashback, the design concept makes the mixing time short and the mixing velocity high as possible. Although this LDI injector is a kind of premixed combustion concept design, it could be considered as a kind of nonpremix concept. In this design, the separated nonpremixed air/fuel flows were injected and then mixed in small premix zones before combustion [62–65]. The previous studies found that the NOx formation is highly dependent on the mixing of the fuel and air [66]. As we know, the design of fuel injector could greatly impact the mixing of air and fuel, which is the key factor in reducing NOx emission. Based on this, that NASA has initiated the Zero CO2 Engine Technology project, which had conducted several experiments on combustion emission [67]. The experimental results were also compared with the numerical predictions. To examine the function of the new LDI injector designs, an experiment was conducted in NASA Glenn Research Center [62]. For this test, the cell 23 of Research Combustion Laboratory (RCL-23) was modified to accommodate gaseous hydrogen fuel [68]. The overall view of the test rig has been shown in Fig. 16. This test rig contains a modified fuel injector, which employed the side injection design, and the air flow enters the combustor from the front of the injector and hydrogen could be injected with the perpendicular direction with air flow. The detailed design of the injector is shown in Fig. 17A and B, which could be considered as the basic concept of the LDI combustor. The injector contains multiple rings, air enters from the holes on the front side, and hydrogen is injected from the H2 orifice along the rings side, flows through the rings part, and mixes inside. Compared with the conventional combustors, the hot mixed gas stream has been divided into multiple small

streams to reduce the risk of flashback and autoignition. The hydrogen gas jets in through two opposing holes, and the cross-flow could improve the mixing intensity in the mixing tube.

Figure 16 Schematic of NASA LDI combustor test rig [62]. LDI, lean-direct injection.

Figure 17 (A) Tree-piece injector brazing assembly detail—25 air holes total. (B) NASA low-emissions LDI hydrogen combustor assembly [62]. LDI, lean-direct injection.

In this experiment, the combustor chamber was inserted with three gas sample probes. From Fig. 16, it can be observed that at the entrance of the combustor chamber, a specially designed hydrogen/air torch igniter was installed. At the outside of the combustor chamber, the water-cooling pipes were twinned on it for chamber cooling. Three gas probes were installed at the location 2.54, 7.937, and 13.652 cm from the injection surface, respectively. Only the first probe was inserted into the flow path, and the other two probes were inserted just slightly over the inner surface of the chamber [62]. To study the different injector design effects on the combustion performance, different injector configurations have been tested with this test rig. These total five injector designs are shown in Fig. 18. The injector design in Fig. 18A is the NASA GRC (N1) injector, which was generated at NASA Glenn. The other four injectors in Fig. 18B,C and D were developed from NASA N1 injector and manufactured by experienced major fuel injector manufacturers. These manufacturers have experience on testing configuration of rocket injectors and low-emission injectors. N1 is the basic injector configuration, which contains 25 injections holes at the air entrance; the hydrogen gas will be injected from side holes in opposing directions. The jet in cross-flow could improve the mixing and gas penetration [69,70]. The air/fuel could premix in a short distance and enter the combustion chamber. The diameter of air inlet is 0.25 and 0.02 in. for the hydrogen injection orifice.

Figure 18 Four injectors tested. (A) NASA N1 injector. (B) Configuration C1. (C) Configuration C2. (D) Configuration C3 and Mod C4 [62].

Fig. 18B shows the configuration of injector C1; from the picture, it could be observed that the simple air inlet has been replaced with a “snowflake” inlet. For each inlet, eight air injection holes arrayed with a hydrogen jet at the center. This design was based on the rocket injection technology. It could create a rich region for ignition and flame hold near the face by using four inner air injection holes. The air through the rest of air jet could create the quenching section. The other injector design C2 changed the jet hole shape from circle in N1 to triangle. It reduced the size but increased the number of injection elements. The configuration C3 just modified the existed gas turbine injector; it added amount of counterswirler to increase mixing. C4 has the same design as C3 but using four small radial hydrogen jets instead of center hydrogen holes. It was observed that different combustor injectors did show different NOx emission performances. The NOx emission of all the injector configurations is shown in Fig. 19. The experimental results of N1 injector show that at the centerline of the combustor, the NOx emission was lower than Jet-A injector, but at the wall area, the NOx value was much higher. With different liners, the values are greatly different. In 3.5 in. liner, the recirculation zones were larger than the 2.5 in. liner, and the residence times also get longer. Hence the NOx emission is higher than 2.5-in. liner. In the other injector designs, the C1 injectors have been tested with different inlet temperatures and pressures. It has been shown that the NOx emission with different positions changes; for example, at 1-in. position, the NOx is lower, but at 3.125 point, it grows to double of the Jet-A values. In addition, with higher inlet temperature, the NOx value increases. C2 design has the best performance at the initial. The overall conclusion from this study is that the NOx emission is related to the injector configurations. Different injectors show different emission performance, but the NOx levels were all lower than Jet-A. From the results, it could be said that the more injection points could provide a lower level of NOx, but the manufacturing techniques limit the design of the injectors. Also, the material and cooling could be a problem, and two injector designs have failed during the test [62].

References

Figure 19 NOx emission of different injector designs [62].

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Chapter 11: Electric-powered aircraft

Maciej Piotr Gierulski ¹ , and Bhupendra Khandelwal ² ¹ Department of Mechanical Engineering, The University of Sheffield, Sheffield, United Kingdom ² Department of Mechanical Engineering, The University of Alabama, Tuscaloosa, AL, United States

Abstract

This chapter aims to answer the question whether electric aircraft are a viable alternative to internal combustion engine aircraft. It conducts a quick review of electric aerospace propulsion technologies available by early 2020, including means of energy storage and electric motors. It also talks about the development of electric aircraft since its beginning in the 19th century until the present day. A market review is also conducted to show what electric airplanes are commercially available or are under development. Finally, most significant barriers to the technology, as well as potential chances, are described. The conclusion is that as long as electric airplanes can be successfully flied with today's technology and are able to perform adequately in some roles, it is years until they become an everyday choice in aviation.

Keywords

Aviation; Battery applications; Commercial aviation; Electric aircraft; Fossil fuels; enger aircraft; Sustainability

1. Introduction

1.1. Background

The second decade of the 21st century brought a sharp rise in interest in electric vehicles. After being discarded in the very beginning of automobiles' existence as inefficient, they have recently been brought back to life because of the political and economic situation, as well as an improvement in battery and electric motors technology. The argument mentioned most often to their case is that they contribute less to the production of toxic exhaust fumes or at least allow for producing them in more easily controlled conditions than it happens in standard combustion engines, away from densely inhabited areas—in power plants. Another important advantage is that they might set human civilization free from dependence on fossil fuels, the supply of which is limited and whose prices have been driven very high in recent decades. All this prompts the question: why shall we not go a step further and also build electrically propelled aircraft? In this chapter, this option is discussed.

1.2. Basic characteristics of aerial electric propulsion systems—energy storage

An electric aircraft is the one propelled by an electric motor. Power for it could be supplied in several ways, including:

• batteries,

• capacitors, • hydrogen fuel cells, • solar cells, • nuclear solutions, • a combustion engine with an electric generator (a hybrid), and • microwave or cable transmission from a stationary source section 2.1.3.

One of the most important factors in choosing the power source for an aircraft is energy density together with specific energy. It says how many watt-hours of energy the given substance contains in a unit of space and mass, as in Eqs. (1a and 1b).

(1a)

(1b)

where P is power, t is time, and V and m are volume and mass. These values can be derived from each other if the density of the medium is known. Fossil fuels such as kerosene feature one of the highest specific volumes among all known substances in both volume and mass. No commercially available batteries at the moment can match these values, e.g., for the batteries in currently (early 2020) developed Eviation Alice (see Section 2.4.1) it is 256 Wh/kg

compared with 11 968 Wh/kg (for calorific value of 43.15 MJ/kg [1]) for Jet A1 – over 46 times less. A fuel that sures kerosene in mass specific energy is hydrogen. However, its storage is problematic, which is more widely explained in Section 1.2.3. Other aspects include the ability to supply high power for extended periods of time, not obstructing the general structure of the aircraft and being safe for the plane and its engers; moreover, in this situation, it is desired that they have smaller environmental impact and are more affordable in the long run than fossil fuels. One advantage of electric propulsion is its good tolerance of altitude. Combustion engines need a minimum density and pressure of air to sustain combustion; electric motors work regardless of it. However, they still require a dense medium around the propeller for it to exert any force. They can fly, unlike jet aircraft, effortlessly in highly polluted air, for example during a volcanic explosion, like that of Eyjafjallajökull in 2010 when the majority of European air traffic had to be halted. This ability might be also a significant advantage in search and rescue missions. The next advantage often mentioned is smaller noise emission. A big problem is that electric aircraft do not lose mass during flight like combustion engine aircraft do because of burning their fuel. This feature of the latter allows for deg maximum takeoff weight (MTOW) higher than maximum landing weight, which makes flight more economical than if the mass were to stay constant. This is the case with electric aircraft—aeroplanes will have to be designed with a different approach (smaller MTOW), considering that they have to land with the same mass that they started with, so they will be able to take less engers or cargo. Below follows a review of currently available electric energy storage methods.

1.2.1. Batteries

Batteries are the most widely used source of power for electric propulsion. They often aid other means of storage, e.g., fuel cells (see Section 2.6.2), when extra power is needed.

Batteries are generally cells or sets of cells that convert chemical into electrical energy. They consist of two electrodes, cathode and anode, placed in an electrolyte. The flow of electrons from the cathode to the anode creates the electric current. Galvanic cells, the first example of the technology, were invented in the late 18th century by Alessandro Volta. Their basic form was a vessel with sulfuric acid (H2SO4) with two electrodes made from metals of different potentials: copper cathode and zinc anode. There are two main types of batteries: primary and secondary. Primary batteries cannot easily be recharged, and the secondary batteries can be recharged using a reversed current. The former are inapplicable for use in aerospace industry, even though they have higher specific energy, because they need to be disposed of and recycled after being discharged and do not perform well under high loads. They are used in low-power portable devices. Thus, the section will focus on secondary batteries. The main factors in choosing a battery to put in a plane are

• light weight, • high energy density, • mechanical safety (resistance to vibrations, acceleration), • compactness, • fire or explosion safety, • ability to supply high power for a considerable amount of time, • good tolerance of frequent recharging, • acceptable performance in all temperatures.

There is no single type that would fulfill all the desired characteristics; compromises have to be made.

First batteries to power a manned electric aircraft (see Section 2.2.2) were nickel–cium. This used to be one of the more popular types in multiple applications because of its reliability, durability, and long life, but it lost its market share to newer generations, superior in specific energy and not suffering from memory effect. Energy density is up to 110 Wh/kg [2]. The other species of batteries widely used in aerospace propulsion is lithium-ion (including lithium-polymer). In this type, the electrodes are lithium compounds with cobalt, magnesium, nickel, or porous carbon. The main advantages include good specific energy, reaching 250 Wh/kg (lithium is one of the lightest elements) [2], long cycle life, no memory effect, and satisfactory discharge characteristics. The biggest disadvantage is the risk of thermal runaway: when overcharged or damaged, the batteries swell and start burning violently. The reaction is very fast and practically unstoppable, which makes them risky to utilize in an aircraft [3]. Battery technology is under constant development, and improvements can be expected.

1.2.2. Capacitors

A capacitor is one of the basic electrical instruments, storing charge in an electric field created between two terminals (e.g., conductive plates) separated with a dielectric material. If connected one way, they accumulate electric charge; if connected reversely, they provide electric current. They have the simplicity advantage over batteries, meaning no chemical reactions taking place, which results in long cycle life. They can also be recharged much quicker than any batteries. Currently, they exhibit very low energy densities, up to 5 Wh/kg, but research has shown that they can be developed to compete with Li-ion batteries, reaching 286 Wh/kg [4]. It can be expected that further work will bring next advancements [5].

1.2.3. Hydrogen fuel cells

Hydrogen is the lightest of all, and a very reactive element; it is then obvious it has high mass specific energy. However, its density is very low, so it would take too much space to be stored on an aircraft. Storage of hydrogen is a complex problem with multiple research projects touching it. There are a number of solutions:

• Liquefied hydrogen —means keeping it under −252°C, requiring large and heavy cooling equipment, not applicable on an aircraft. • Compressed hydrogen —in pressure vessels made of metal or composites. The material needs to be chosen carefully—hydrogen dissolves in steel and embrittles it, which might result in a burst; moreover, as its particles are very small, they can permeate through layers of polymers. Usually, the materials of choice are grades of polyethylene and polyamide or aluminum [6]. Currently, the highest pressure obtained in a hydrogen container is 700bar, which gives an energy density of 767Wh/L. Knowing that density of hydrogen in these conditions at room temperature is around 0.038kg/L, it can be calculated that it results in mass specific energy of 20.2kWh/kg, which is already more than in fossil fuel [7]. • Chemically bonded hydrogen —for example, in the form of metal hydrides and borohydrides, metal–organic frameworks, carbon nanostructures, and liquid organic carriers. They are in relatively early stage of development and not yet suitable for mobile applications [8].

Electric energy is produced from hydrogen using fuel cells. The principle of operation is the following: hydrogen approaches the anode of the cell, and oxygen (air) is delivered to the cathode. At the latter, a platinum catalyst extracts one of the hydrogen's two electrons, creating a positively charged H+ ion and a stream of free electrons. The H+ is ed through an electrolyte layer (also known as proton exchange membrane)—this can be made from several

materials, e.g., polymers or ceramics—and the electrons travel to the terminal and then to the load circuit. On the other side, at the cathode, electrons exit the load circuit and combine with H+ ions and oxygen to create water and leave the cell. It is a clean and easily controlled process. The reason it did not become popular till late 20th century was high cost of platinum catalysts, but in the past 20 years new catalyst materials and electrolytes were invented that made cells affordable, for example to be used as power sources for cars. Also, new ways have been invented for hydrogen production, for example, by electrolysis using wind or solar power, which is a very good application for these power sources [9]. However, as long as hydrogen provides excellent energy density, the specific power of fuel cells is still far from sufficient for aircraft to take off. It goes up to 100 W/kg, when batteries', capacitors' and fossil fuels' capability is 10 times bigger [10]. For this reason, they have to be coupled with batteries so a power reserve is always available (see Section 2.6.2). In calculating mass specific energy of hydrogen, the masses of all equipment necessary for the system to function were not taken into consideration. In practice, the value is lower and, as claimed by Romeo et al. [11], is the main factor limiting the use of fuel cells in aeroplanes.

1.2.4. Solar cells

A solar cell is a device converting the energy of light into electricity using photovoltaic effect: the absorption of light excites electrons in a semiconductor, producing electric potential. Contrary to previously mentioned solutions, solar cells do not have to store energy; they produce it in real time. In this case, not specific energy, but specific power with respect to mass and area—will be applicable (see Eqs. 2a and 2b). These physical quantities also characterise an engine or an electric motor. Ideally, engines in an aircraft should have the highest specific power possible. It is especially important during take-off.

(2a)

(2b)

In early trials of solar powered aircraft (Project Sunrise II, 1975), solar arrays weighing 2.523 kg delivered 580 W [12], which equals 230 W/kg and 162 W/m². More recently (2000) developed available high-efficiency Si cells could have 676 W/kg and 169 W/m² [13]. Taking the example of ENFICA-FC (Section 2.6.2), where in steady flight, the motor drew 20 kW of power, and solar s necessary to produce it (assuming 100% efficiency in transmission) would, respectively, weigh 87 and 30 kg, which is less than hydrogen storage did — it can be concluded that weight is not a problem. However, the area to be covered with cells would be 124 and 118 m², which is more than 10 times the actual wing area of the plane. One brand of currently (beginning of 2020) commercially available solar cells, from a Norwegian company REC, can reach 217.14 W/m² [14]. This value allows for reducing the area to 69.5 m², but is still not a satisfying result, requiring a complete redesign of the aircraft. However, it gives some hope for the future and may allow solar cell power to be used in some applications (see Section 2.3). It has to be ed, though, that its performance is always off the theoretical value and never constant because of the changing angle of sunlight; they often cannot be used below the cloud altitude and are useless at night. This means that, just as in the case of hydrogen fuel cells, this system has to be coupled with batteries.

1.2.5. Nuclear solutions

There are a number of technologies for generating electricity using nuclear

energy. The first and foremost is a nuclear reactor, which could hypothetically produce current for electric motors. There have been attempts to propel a plane with one, however it would not produce electricity, but steam that powered turbine engines. The experimental projects Convair NB-36H and Tupolev Tu119 that employed this idea were cancelled before the novel propellant system was used due to safety concerns. The main fear about nuclear reactors onboard an aircraft is engers' safety and the risk of contamination after an accident, so if nuclear power is to be used, it should avoid utilizing the more dangerous fuels, such as uranium. Devices that do not require chain reactions are the alternative, for example:

• radioisotope generator—nuclear fuel, hot because of radiation, is placed in a container where one thermocouple terminal is installed; the other is connected to a radiator to keep it cool; the temperature drop creates potential difference; they have been used to power satellites or lighthouses—devices that need to operate for months without maintenance; • betavoltaic cell—principle similar to solar cells (see Section 1.2.4), but a radioactive material is the source of radiation; an example was presented in December 2018 by Rosatom, the Russian nuclear operator [15]—an atomic battery using enriched nickel-63 isotope; it is claimed to have a life of 50years and be safe for use even in consumer electronics [16].

These solutions are only for low power devices though and there have been no attempts to use them for propelling any aircraft systems.

1.2.6. Hybrids

A series hybrid consists of a combustion engine—piston or turbine—turning an electricity generator, which charges a battery or powers the propeller directly. A parallel hybrid means that both an electric motor and a combustion engine

provide power to the propeller(s) simultaneously. A hybrid plane may use a battery to keep the engine at lower power and draw more current from the battery whenever it is needed, or not use one, if the aim is only to change the layout of propelling the motors, e.g., with distributed propulsion. This idea solves the issue of not being able to build an energy storage system sufficient to match the kerosene characteristics, but does not solve the fossil fuel dependency problem. It might, though, quieten the noise and decrease fuel consumption, by allowing the engines to work in stable, static conditions, without being forced to produce high momentum at nonideal rotation speed. Examining the efficiency chains shows that a hybrid setup is the second best after a classic combustion engine [17].

1.3. Basic characteristics of aerial electric propulsion systems—electric motors

Electric motors are machines converting electrical into mechanical energy, exploiting force acting on a current-carrying wire placed in a magnetic field, perpendicularly to the direction of the current. As usually rotational energy is required, motors are designed to have two basic parts—one stationary and one rotational—a stator and a rotor. Depending on the type, they can have either coils or permanent magnets on either. The two basic types are alternating current (AC) and direct current (DC) motors. The DC motors divide into brushed and brushless. Brushes are parts that keep and conduct current between static and rotating sections of a motor. DC motors usually feature a commutator—a switch that reverses the direction of the current inside the rotor once it is parallel to the magnetic field to keep it rotating. For aerospace purposes, motors with highest power to weight ratio are needed, and the available electricity sources produce direct current. Thus, DC motors are a natural choice [18]. Brushless should also be used over the other, as brushes are parts that produce friction heat and wear off, and generally reduce motor performance [19]. The same choice was made in most planes reviewed in Section 2.

High-temperature superconducting motors seem attractive for future aerospace propulsion, but they require extensive cooling systems—too big and heavy to fit into an aeroplane. At the moment, there are no electric motors capable of reaching the jet turbine specific power.

2. History of development of electric and hybrid aircraft

Because of their characteristics, electric aircraft have not been suitable for general aviation, but there are some particular applications in which they could perform well:

• Surveillance—a solar-powered aircraft could theoretically stay airborne for long periods, which is perfect for flying over an enemy territory and photographing it or observing natural phenomena. • Pilot training—with rising prices of fuel, electricity has become a cheap alternative for relatively short training flights. • Motor gliders—an electric motor enables a glider to start without a towing airplane, and power is not necessary later on. • Remote-controlled hobby aeroplanes—electric motors are cheap and easy to maintain, convenient for amateur aeromodellers. • Aerobatics—acrobatic planes are small, light, and airborne for only a short time. • Recent developments also seem to allow for their use in regional enger transport.

Until now, most prototypes and technology demonstrators were successful; however, the majority of them were canceled and saw no direct follow-up, probably because of lack of economic sense. To facilitate understanding to what extent a technology is developed and how far

it is from being commercially successful and mass produced, the Technology Readiness Level scale as defined by NASA is used [20]:

1. basic principles observed and reported 2. technology concept and/or application formulated 3. analytical and experimental critical function and/or characteristic proof-ofconcept 4. component and/or breadboard validation in laboratory environment 5. component and/or breadboard validation in relevant environment 6. system/subsystem model or prototype demonstration in a relevant environment (ground or space) 7. system prototype demonstration in a space environment 8. actual system completed and “flight qualified” though test and demonstration (ground or space) 9. actual system “flight proven” through successful mission operations.

2.1. Early attempts

2.1.1. Gaston Tissandier

The first attempts to use electric propulsion to power an aerial vehicle were with blimps. A Frenchman Gaston Tissandier flew one in year 1883, two decades

before the Wright brothers' flight. A Siemens electric motor used was producing 1.1 kW at 180 rpm and was powered by 24 potassium bichromate cells [21]. Some other airships were also equipped with electric motors in later years.

2.1.2. Petróczy–Kármán–Žurovec

The next interesting development happened during World War I. A helicopter meant to be a tethered observation device harder to destroy and easier to operate than hydrogen balloons was built by Austro-Hungarian trio of Petróczy, Kármán, and Žurovec. An Austro-Daimler DC electric motor weighing 195 kg (total helicopter mass being 650 kg) and capable of producing 140 kW of power at 6000 rpm was used; the electricity for it was supplied with the tethering cable. The technology, however, was not mature enough—the wiring of the motor burnt after several trials [22].

2.1.3. William C. Brown

A further early example of efforts to build an electrically powered aircraft was another tethered observation device made by William C. Brown during his work in Raytheon. It was conceived as a way for testing microwave power transmission. The result was a small (2.3 kg, 1.8 m rotor diameter) helicopter flown at the altitude of 15 m over a generator and antenna (see Fig. 1). A rectifying antenna was installed on it to convert AC current induced by microwaves to DC current to power the motor—a 0.11 kW hand drill motor. The design was successfully tested for 10 h, and the author imagined it could be developed to soar at altitudes up to 1500 m, but it saw no follow-up—it was decided that rectennas capable of receiving large amounts of power from long distances would be too big to build and strong microwaves in open air can harm humans. But this technology has recently been used in a small scale for charging mobile phones [23].

Figure 1 W.C. Brown's microwave-powered helicopter [23].

2.2. First electric aeroplanes

2.2.1. Radio Queen

The first radio-controlled electric aircraft, a small model, was flown on June 30, 1957, by Colonel J. Taplin in the United Kingdom. “Radio Queen” featured a permanent magnet motor and silver–zinc battery. Project was not taken further but gave birth to the whole branch of electric hobby models [24].

2.2.2. Militky MB-E1

The first ever manned electric airplane took off in Austria on October 21, 1973. It was Militky MB-E1, a conversion from the propelled sailplane Brditschka HB-3. The aircraft climbed to the altitude of 300 m, made a few circles around the airfield, and landed after over 9 min, using half of the battery capacity during the flight. Its empty weight was 370 and MTOW was 440–60 kg more than the piston engine version. Its motor was a 10 kW (at 2400 propeller rpm, compared with 26 kW for piston engine) Bosch DC motor with Varta Ni-Cd battery. A few more test flights were done, and the machine performed flawlessly; however, the flying time was still very limited [25].

2.2.3. AstroFlight Sunrise

Next year brought the first unmanned solar-powered airplane, AstroFlight Sunrise, by Roland Boucher (Fig. 2). It was meant to be a technology demonstration that an aircraft could be made that would remain in the air constantly for months. During daylight, it would rise to the altitude of 30 km, and when the sun sets, it would descend to 2 km. The machine was designed according to the principles of a sailplane with the length of 4.36 m and the wingspan of 9.8 m. Empty weight would be 10 kg, MTOW—12 kg. Off-the-shelf solar s with 10% efficiency were used, and a newly purpose-built samarium-cobalt magnet 0.75 kW (at 2000 rpm) motor would rotate the propeller. All flight tests showed that the design was successful; however, during the 28th test the plane was destroyed by a turbulence at the altitude of 2.5 km [26].

Figure 2 AstroFlight Sunrise—“The Designer, Roland Boucher is checking the voltage and current under full power while Col Thacker looks on as Bob Boucher operates the throttle. The little black square about afoot aft of the Wing is the sun com which weighed less than one ounce. Circular solar cells covered a large part of the wing” [26]. Photo © Roland Boucher.

2.2.4. Mauro Solar Riser

Next, in 1979, came the first manned solar-powered plane, Mauro Solar Riser, made by Larry Mauro of Ultralight Flying Machines. It was a conversion from a standard biplane glider, with the addition of a 2.6-kW Bosch motor and a Ni-Cd battery. Empty weight—57 kg, MTOW—137 kg. Solar cells only took half of the top wing surface (see Fig. 3, [27]), so they could only provide up to 350 W of power. It was, however, enough to charge the batteries in 1.5 h, and then the plane could take off (the motor could run 3–5 min on batteries) and keep gliding until the batteries were full again to rise higher or perform a powered landing. The plane was successfully tested by flying at the height of 12 m and the distance of 0.8 km and then placed in a museum [27].

Figure 3 Mauro Solar Riser [27]. Photo © Experimental Aircraft Association.

2.3. Later notable examples

2.3.1. NASA: Pathfinder (Plus), Centurion, Helios

The idea of a solar powered airplane able to stay permanently airborne (highaltitude pseudo-satellite, or HAPS, see section 2.2.3) was not forgotten; however, its realisation was slowed down, probably by the development of satellites, which were initially proved easier to use for intelligence purposes. NASA used to develop a series of solar-powered unmanned aerial vehicles in the years 1983–2003. They were a continuation of a project conducted by the company AeroVironment from California. It was a high-aspect-ratio, 30-m-long flying wing with six to eight electric motors, covered with solar s (Fig. 4). It would fly extremely slowly, at 24–40 km/h, and was remotely controlled with small elevators in pitch and motor power in yaw; the top power achieved was 7.5 kW. It reached a record altitude for both propeller-driven and solar-powered planes: 21.8 km. Later, with improvements (reaching 35 kW of solar power and 75 m wingspan), as Pathfinder Plus [28], Centurion [29], and Helios [30], it rose to as high as 29.5 km. Mass: 252–1052 kg. For energy storage to be able to fly at night, it was equipped with lithium batteries and, later, with hydrogen cells.

Figure 4 NASA Helios in flight [31]. Photo by NASA (public domain).

The aeroplane was used for research for imaging of marine and terrestrial ecosystems and for transmitting radio and TV signal as an “atmospheric satellite.” Eventually, Helios was destroyed in a crash in 2003. Longest flight duration achieved was over 17 h.

2.3.2. Solar Impulse

Solar Impulse is a private initiative of two Swiss citizens in cooperation with several universities and companies. It is a manned airplane whose target was to fly around the world solely on solar energy, which was accomplished in 2015 by its second prototype. The circumnavigation took nearly 17 months, with a 9month-long stop for servicing damaged batteries. Longest nonstop flights were over the Pacific Ocean, 4 days and 7212 km long, and set records for longest solo flight and longest solar-powered flight [32]. The plane's wingspan is 71.9 m (as big as the one of Boeing 747), MTOW is 2300 kg, and cruising speed is 60–140 km/h depending on altitude; the four motors deliver 52 kW together. During the day, when sunshine is available, the plane climbs to 8.5 km and descends to 1.5 km at night [33].

2.3.3. QinetiQ/Airbus: Zephyr

A similar project was also conducted in the United Kingdom, starting from 2003, by QinetiQ and later Airbus. After development, in 2018, the unmanned machine reached a record in continuous unrefueled flight—almost 26 days. It stayed

above the air traffic and weather phenomena, at 15–22 km, finally reaching the goals of a satellite-replacing aeroplane. Data: MTOW—53 kg, wingspan—11.5 m, power—0.9 kW, speed—56 km/h [34].

2.3.4. Lange Antares

The first commercially available electric aircraft, a German-built motor glider, was first flown in 2003. 42-kW own-made brushless DC motor allows it to take off without towing. SAFT VL41M Li-ion battery allows for a climb to 3.5 km after recharging for 10 h. MTOW = 660 kg [35].

2.3.5. Pipistrel Taurus Electro

It is the first two-seat electric commercially available plane. This self-launching motor glider (Fig. 5) was introduced in 2007. The motor is capable of running at 30 kW, MTOW = 550 kg; Li-Po battery with 4.75 kWh capacity, cruise speed of 141 km/h [36].

Figure 5 Pipistrel Taurus Electro in flight. Photo © Pipistrel.

2.3.6. ElectraFlyer-ULS

It is another commercially available electric plane, built in the US in 2012. With a MTOW of 238 kg, 2 kWh × 3.3 kWh Li-Po batteries and a 15 kW motor, it is able to cruise for 2 h at 64 km/h [37].

2.4. Current state of the art

As of winter 2020, there are several ongoing projects concerning electric aircraft. A few of them were chosen to be described in this chapter.

2.4.1. Eviation Alice (TRL: 4)

This is meant to be the first commercial electric enger plane and was presented at Paris Airshow in June 2019. The Israeli company Eviation claims that it is capable of competing with regional turboprops: taking nine engers over 1000 km at the speed of 440 km/h at service ceiling of 3 km. It has a nonstandard layout: three pusher electric motors (780 kW together, provided by Siemens and magniX) are placed at the rear of the fuselage and on wingtips to reduce drag, the tail is V-shaped, and the fuselage is flattened at the bottom to

provide extra lift with wings being short and thin (Fig. 6). Battery is a 3600-kg 920-kWh lithium-ion pack. MTOW: 6350 kg. It is supposed to start serving customers in 2022, and the company has already received orders for multiple aircraft from regional airlines, lured by potential 60% reduction in maintenance and fuel costs compared with turboprops [38]. However, by the time this chapter was written, the aircraft had not yet flown; the first prototype burnt during trials on January 22, 2020 [39].

Figure 6 Eviation Alice at Paris Airshow [38]. Photo © Eviation.

2.4.2. Bye Aerospace eFlyer (TRL: 7)

This aircraft is planned as a cheap alternative for pilot trainers, which now run combustion engines and make training prohibitively expensive. Its prototype was successfully tested in April 2018. It features Siemens SP70D electric motor capable of delivering up to 90 kW and weighing 25.8 kg. Li-ion batteries let it stay airborne for 3.5 h and achieve top speed of 220 km/h. Wingspan: 11.58 m, MTOW: 862 kg. The producer claims that operating cost is $14 per hour, compared with $88 for Cessna 172 [40].

2.4.3. Pipistrel Alpha Electro (TRL: 9)

Meanwhile, a Slovenian company has been producing and selling the world's first FAA-certified electric aircraft since 2015 (again, a trainer). It is a conversion from an existing, commercially available piston engine airplane. More than 60 are already being used around the globe. The aircraft (Fig. 7) can stay airborne for 1.5 h (the petrol version—3.5 h), recharging the batteries takes 45 min, or they can be replaced in 5 min. Own-made motor produces 60 kW of power, which allows for reaching a cruising speed of 160 km/h at 3.9 km altitude. Wingspan: 10.5 m, MTOW: 550 kg [41].

Figure 7 Pipistrel Alpha Electro in flight. Photo © Pipistrel.

2.4.4. NASA X-57 Maxwell (TRL: 4)

This plane, a modified Tecnam P2006T, is supposed to demonstrate the advantages of distributed propulsion. This means mounting 14 electric motors along the leading edge in regular intervals (Fig. 8). Interestingly, the 12 smaller motors will only be activated for takeoff, when the strongest lift is needed, and then turned off for cruising. So far, this setup has been proved to work by ground tests, and so have been the main motors and batteries. The prototype is yet to be built. The designed characteristics are the following: cruise speed—277 km/h; maximum altitude—4.3 km; mass—1360 kg; batteries—Li ion, 390 kg, 69.1 kWh; two cruise motors 60 kW and 53 kg each, 12 “high-lift” motors 10.5 kW and 5 kg each [42].

Figure 8 A render of X-57 Maxwell [42]. Picture by NASA (public domain).

2.4.5. Airbus E-fan (TRL: 7)

One of the two leading aircraft producers, Airbus, also had its attempt at an electric airplane. It was an evolution of EADS' project to convert the plane Cricri to electric propulsion (finished in 2010). The plane in question was called Efan and made its maiden flight in early 2014. It was imagined as a trainer. It featured two ducted fans with two motors 64 kW in total. Interestingly, the main landing gear wheel was also powered by a 6-kW motor to assist with takeoff. MTOW: 600 kg, cruise speed: 160 km/h; 167 kg of Li-ion batteries allow it to keep airborne for 1 h [43]. Initially, the company planned to develop it into a larger plane [44], possibly with a range extending motor, but the project was terminated in April 2020.

2.4.6. Zunum Aero (TRL: 2)

The other aerospace giant, Boeing, had plans to build an electric plane with a company called Zunum, but they decided to divest before even a prototype was produced.

2.4.7. Harbour Air and magniX (TRL: 7)

Harbour Air is a sailplane airline operating between cities at the Strait of Georgia in British Columbia in Canada, which means covering distances below 300 km. Currently, they use piston and turboprop aircraft, mainly DeHavilland Canada. In 2019, they signed an agreement with magniX, an electric motor company, to convert their whole fleet of 40 aircraft to electric. This resulted in the 4-min test flight that took place on the December 11, 2019 (Fig. 9). The plane was a sixenger DHC-2 with a 560-kW (135 kg) motor. The lithium-ion battery allows it to stay airborne for 40 min; however, it has yet to complete all certifications to be used commercially, and specifications are expected to change, too [45].

Figure 9 Harbour Air and magniX ePlane in its maiden flight. Photo © Harbour Air Seaplanes.

2.4.8. Extra 330 LE (TRL: 8)

It is an experimental aerobatic electric plane. It holds two speed records: one of 342.86 km/h for an electric plane heavier than 1000 kg and the other of 337.5 km/h for one lighter than 1000 kg and utilizes a Siemens SP260D weighing 50 kg and delivering 260 kW. The battery is Li-ion with 18.6 kWh capacity. It is expected to perform 20-min long flights including a 5-min full throttle show [46].

2.4.9. Air taxis (TRL: 1-7)

Recently, a new idea has attracted the interest of aerospace companies: a small electric helicopter capable of carrying two engers and flying short distances at low speeds, meant for use as an “aerial taxi” or for commuting in cities where street traffic has become inefficient. Several companies are working on such an aircraft: Boeing PAV, Airbus A³ Vahana, CityAirbus, Beijing Yi-Hang Creation Science & Technology Ehang, Lilium Jet, Opener Black Fly, Bell Nexus, and Volocopter. They are in various stages of development, some prototypes have already flown, and none of them had been used commercially by the time this chapter was written.

2.5. Hybrid airplanes

2.5.1. Diamond Aircraft: DA36 E-star (TRL: 7)

The first hybrid aircraft was built very recently, only in 2011: DA36 E-star by Diamond Aircraft with cooperation with Siemens, Austro Engine, and EADS. It is a conversion from the Diamond HK36 motor glider. It was propelled by a 70kW electric motor with a 30-kW engine as the generator. The energy from the batteries was used for takeoff and climb and later, in level flight, they were charged [47].

2.5.2. Ampaire Electric EEL (TRL: 8)

An American company called Ampaire is developing a parallel hybrid plane on the basis of Cessna 337 Skymaster, originally a twin push–pull engine aircraft. The modification is about replacing the rear engine with a battery-powered electric motor. This configuration reportedly guarantees better flight characteristics including less fuel consumption and less emissions as well as serves as a reliable testbed because of its centreline thrust layout. A prototype was flown in June 2019, and the company has already won an order for 100 of them [48].

2.6. Hydrogen fuel cell aircraft

2.6.1. Boeing DA-20/Theator (TRL: 7)

Boeing, in cooperation with several other companies, converted an Austrian Diamond Aircraft DA-20 to fly on fuel cells, making it the first of its kind. It was flown three times in 2008, climbing to 1000 m and keeping cruise speed of 100 km/h for 20 min. The climb was ed by batteries, and level flight was performed with fuel cell power only [49].

2.6.2. ENFICA-FC (TRL: 6)

Another attempt was made by researchers from Turin Polytechnic. A two-seat Czech microlight Rapid200 by Skyleader was equipped with a 35-kW motor and fuel cell as well as Li-Po battery, both 20 kW. The battery, with its 18 min capacity, as in previous case, is only to be used during takeoff and in emergency. The complete aircraft mass is 554 kg, and a breakdown is shown in Fig. 10; the wing area is equal to 11.85 m². Hydrogen storage tanks weigh 52 kg and contain 1.2 kg (26 L at 350 bar) of H2. The plane made six test flights in May 2010, during which a velocity of 135 km/h (in level flight) and the endurance of 39 min were reached [11].

Figure 10 ENFICA-FC mass breakdown.

2.6.3. DLR HY4 (TRL: 6)

This plane was collaboratively made by the aforementioned Pipistrel, Hydrogenics (fuel cell producers), University of Ulm and DLR (German Aerospace Centre) on the basis of Pipistrel Taurus G4, a twin-fuselage electric motor glider. It is powered by an 80-kW electric motor, for which the energy is provided by a hydrogen fuel system. The H2 is stored in tanks at pressures of 300–400 bar, which is then reduced to 7 bar and fed to fuel cells. A battery was installed to provide extra power for takeoff and landing and as backup power in case of cells failure and can run the plane for 15 min. MTOW is 1500 kg and three engers can be taken aboard. Cruise speed is 145 km/h, and top speed is 200 km/h. Velocity depending range can reportedly be from 750 to as much as 1500 km. Its maiden flight took place on September 29, 2016 and lasted for 10 min. The project is under further development, and its target is to create 6–10 and, later, possibly, 40-enger aircraft that would reach 300–400 km/h on distances of 1000 km [50].

2.6.4. ZeroAvia (TRL: 6)

On June 23, 2020, another hybrid electric prototype was flown, one built by a British company called ZeroAvia. It is a conversion from the 260-kW pistonengine Piper Malibu, featuring a 300-kW battery. The ten test flights allowed to find the most efficient flight conditions at 167 km/h at sea level altitude—the energy consumption was 75 kW. The company representatives claim that the biggest challenge was the thermal management of the propulsion system. In future flights, they are aiming to test a hydrogen system [51].

3. The future, conclusions

As the quick market research shows, there is some future for electric airplanes; however, by early 2020, only one ultralight and three motor gliders have achieved commercial success. There have been ambitious plans for aircraft going electric, but most of these projects were cancelled in earlier or later stages of development. Certainly, in the segment of pilot trainers, because of the way planes are used, there is no reason why electric propulsion will not succeed; the air taxis also seem manageable. In long-distance enger or cargo transport, it is not that likely. A case study conducted by M. Hepperle of DLR [17] investigating a potential regional turboprop plane finds that if electric propulsion were fitted into a Dornier Do 328, it would be slowed down from 472 to 300 km/h and the range would go down from 2200 to 50 km, which makes no commercial sense. With significant modifications done to the fuselage and structure, the range could be extended to 339 km with speed as low as 255 km/h, but it is only a leap in battery technology that would enable it to match the turboprop performance. Another case study was done by B. Łukasik and W. Wiśniowski from Institute of Aviation in Poland for a business jet Gulfstream G150 [52]. Provided that there existed electric motors that would produce enough power for such a plane to take off, which was calculated to be over 3 MW, and be small and light enough to be installed on an aircraft, batteries that would contain enough energy for a 1800 km (1/3 of the original range) flight would weigh two to five times the MTOW assuming the current technology level. Unless a sudden breakthrough is performed, it is not earlier than in 15 years' time that sufficient batteries might become available. Hydrogen fuel cells do not seem to be a viable alternative due to problems with storing H2. Nuclear batteries might satisfy the energy density expectations but are unlikely to be used due to safety concerns. Considering all the aforementioned, the only currently viable solution for electric propulsion for long-distance aircraft would be a hybrid with an internal combustion engine powered generator—but it does not solve the fossil fuel dependency problem. Moreover, even switching to fully electric is also

problematic when it comes to sustainability—it requires mining significant quantities of elements such as nickel, lithium, and cobalt for batteries, which are, too, a limited resource. 60%–70% of global cobalt production comes from the Democratic Republic of Congo, which can be considered a high-risk country for business. The demand for these minerals will be growing quickly (tenfold by 2030), as there are plans for switching to electric power for cars and lorries, too. The today's production chains are not capable of delivering enough cobalt and nickel without an enormous investment ($140 billion [53]), and the amount needed to put batteries in over a billion vehicles exceeds the currently known cobalt resources of the Earth and almost completely drains the nickel resources [54]. In conclusion, without a breakthrough in battery technology, not only will it be impossible to build functioning long distance aircraft but also to even construct sufficient amounts of batteries for the short distance ones. However, some interesting technologies are in development that could aid electric propulsion. One would be catapulting aircraft instead of having them take off on their own. This method has been a standard on aircraft carriers since they were developed, now it is being considered for airliners as well [55]. It would possibly mean decreasing the required peak motor power. Also, less battery capacity would be needed—according to the data presented by Łukasik and Wiśniowski—by 6.5% on a 1800-km journey. Another intriguing development lies in the field of composites: electroactive polymers. These are used to make laminae that change their shape when voltage is applied to them. Such mechanism might allow for building an aircraft utilizing flapping wings that can move in a sophisticated, nature-inspired way, rather than propeller blades, to gain speed. The idea is already being researched, e.g., SmartBird by a German company named Festo [56]. Finally, one more nonstandard form of electric propulsion would be a plasma jet engine. In one of these, air would be inhaled, compressed, and excited using electromagnetic field and form plasma—the dense and hot ionized state of matter—that is then exhaled at high speed. Such solutions have been tested in laboratory conditions with the intention to be installed in satellites, but maybe, one day, they can also be used in atmospheric aircraft. At present, they are impossible to implement for the same reason as other mentioned technologies— lack of satisfactory batteries [57]. In conclusion, the world has yet to wait for electric aircraft to dominate the sky.

Solutions to aforementioned problems are investigated, but it might take decades before they are fully resolved. All the arguments stated above make one appreciate the paramount importance of fossil fuels to human economy.

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Index

‘Note: Page numbers followed by “f” indicate figures and “t” indicate tables.’

A

Aerial electric propulsion systems, basic characteristics of

electric motors, 277–278

energy storage, 271–277

Air Transport Action Group (ATAG), 3

Airbus E-fan, 287–288

Airport emission control, 85–86

Airworthiness, 25

Alcohol-to-jet fuels (ATJ fuels), 54

Alcohol-to-jet synthetic paraffinic kerosene (ATJ-SPK), 18–19

Algae, 14

Alkaline electrolysis, 240

Alkanes, 178–179

Alternating current motors (AC motors), 277–278

Alternative aviation fuels (AAF), 2–3

approval process

approval of alternative fuels, 29–34

approval of conventional fuels, 25–29

approved types of alternative fuels, 34t–36t

classification of aviation fuels, 23f

different aviation turbine fuels in use, 24t

fuel standards, 24–25

specifications values of various jet fuels, 28t

feedstock, 7–15

life cycle assessment, 4–6

Alternative fuels, 59 , 139 , 160

approval, 29–34

gas turbine igniters, 40–42

gaseous emissions and, 63–67

ignition and relight, 39

impact on ignition performance, 49–56

and impact on noise and vibrations, 226–232

impact on particulate matter emissions, 87–108

different alternative fuels impact on particulate matter emissions, 87–93

different aromatic species impact on particulate matter emissions, 100–108

fuel properties impact on particulate matter emissions, 93–100

parameters affecting ignition time delay, 47–49

operation conditions, 47–48

parameters affecting minimum ignition energy, 42–47

fuels, 45–47

Alternative jet fuels (AJFs), 113

composition and thermal stability, 184–190

pathways, 162–163

American Society for Testing and Materials international (ASTM), 151

International Aviation Fuel Subcommittee J, 25

Ampaire Electric EEL, 290

Animal fats, 155–156

Aromatic species impact on particulate matter emissions, 100–108

Aromatics, 182–184

AstroFlight Sunrise, 281

Atmospheric fractional distillation, 152

Autoxidation, 163–167

Auxiliary power unit (APU), 256

Aviation, 1

aviation-related components, 219–220

fuels, 23–24 , 151–152 , 237–238

gas turbines, 39

Aviation Fuels Thermal Stability Unit (AFTSTU), 208

Aviation gasoline (AVGAS), 23–24 , 26 , 237

Aviation turbine fuels (ATFs), 23–24 , 87–88

B

Batteries, 273–274

Bio-based feedstock, 7–9

Biological feedstock sources, 29

Biomass, 11–12

Blending, 191–192

Blockage ratio (BR), 259

Blow-off process, 135–136

Boeing Phantom Eye, 250–251

Butylated hydroxytoluene (BHT), 194

Butylbenzene, 127

Bye Aerospace eFlyer, 286

C

Camelina, 13

Capacitors, 274

Carbon capture and sequestration technology (CCS technology), 6 , 9–11

Carbon capture and utilization processes (CCU processes), 154–155

Carbon emission, 6

Catalytic hydrothermolysis, 20

CBTL, 17

Centurion, 282–284

Cetane ignition delay ignition (CID), 54

Chemical kinetic mechanism

generation of, 198–203

ordinary differential equation solver, 202–203

rate of reaction, 200–201

sensitivity analysis, 201–202

Civilian use

hydrogen in, 255–266

LDI combustor, 262–266

micromix combustor, 256–262

CO, 60–61

CO2 emissions, 150 , 153–154

Coal, 9

Coal-to-liquid (CTL), 4–5 , 25 , 65 , 89

Cold tones, See Low frequency dynamics (LFDs)

Combustion

engines, 272

instabilities, 219–221

alternative fuels and impact on noise and vibrations, 226–232

categorizing instabilities, 221–222

combustion noise, 222–223

loop, 220f

frequency ranges, 222f

gas turbine instability, noise, and vibrations, 223–226

noise, 222–223

technologies, 237–238

Combustor designs, 84–85

Commercial aircraft, 237

Commercial Aviation Alternative Fuels Initiative (CAAFI), 17 , 25 , 30

Committee on Aviation Environmental Protection (CAEP), 80

Commonwealth of Independent States (CIS), 27

Compression force, 121

Condensable PM (M), 72–73

Conventional fuels, 59–60

See also Alternative fuels

approval, 25–29

energy, 237–238

Conventional jet fuel, 7

Conventional pathways, 16

Coprocessing, 20

Corn, 18–19

CRYOPLANE project, 249–250

Cycloalkanes, 180–181

D

DA36 E-star, 289

Deposit formation, 168–170

Derived cetane number (DCN), 137

Diamond Aircraft, 289

Direct combustion noise, 223

Direct current motors (DC motors), 277–278

Direct hydrocarbon sugar (DSHC), 49

Direct sugar-to-hydrocarbon Fuel, 20

Dissolved metals, 197–198

DLR HY4, 290–291

Drop-in fuels, 2–3 , 25 , 29–30 , 86–87 , 158–162

Droplet size, 46

E

Edible feedstock types, 8–9

Elastomers, 113

fuel compatibility, 113–114

fuel leaking using AJFs, 114f

material, 118–119

phenomena in seals, 114–118 , 116f

elastomer material, 118–119

Hansen solubility parameter, 117–118

swell behavior with different aromatic species, 127–130

testing methods, 120–125

FTIR spectrometer, 125

GC-MS, 124

optical method, 122–123

stress relaxation, 120–122

thermogravimetric analysis, 124

ElectraFlyer-ULS, 285

Electric aircraft, 271

history of electric aircraft development, 278–291

Electric motors, 277–278

Electric propulsion, 272

Electric-powered aircraft

background, 271

basic characteristics of aerial electric propulsion systems, 271–277

history of electric and hybrid aircraft development, 278–291

current state of art, 285–289

early attempts, 279–280

ElectraFlyer-ULS, 285

first electric airplanes, 281–282

hybrid airplanes, 289–290

hydrogen fuel cell aircraft, 290–291

Lange Antares, 284

Pathfinder (Plus), Centurion, Helios, 282–284

Pipistrel Taurus Electro, 284

QinetiQ/Airbus, 284

Solar Impulse, 284

Electrical energy, 275

Electrical mobility analyzer, 83

Electrodes, 40

gap and materials, 45

Electrolysis, 239

Ellipsometry, 187

Emission instruments, 184–185

Energy, 150

content, 184

density, 271–272

storage, 271–277

batteries, 273–274

capacitors, 274

hybrids, 277

hydrogen fuel cells, 274–275

nuclear solutions, 276–277

solar cells, 275–276

Energy Information istration (EIA), 3

ENvironmentally Friendly Inter City Aircraft powered by Fuel Cells (ENFICAFC), 290

Environmental Protection Agency (EPA), 34

Equivalence ratio, 45–46 , 48

Ethanol, 18–19

European Aviation Safety Agency (EASA), 25 , 210

Eviation Alice, 285–286

Excessive LFDs, 222

Extraction-induced precipitate (EIP), 168–169

F

Fatty acid methyl esters (FAME), 88–89 , 161–162

Federal Aviation istration/Agency (FAA), 25 , 27–29 , 210

Feedstock, 7–15

bio-based, 7–9

biomass, 11–12

fossil, 9–11

natural gas, 15

oils, 12–14

pathways, 15–20

alcohol-to-jet fuel, 18–19

conventional pathways, 16

coprocessing and catalytic hydrothermolysis Fuel Readiness Level 6, 20

direct sugar-to-hydrocarbon Fuel, 20

Fischer–Tropsch, 16–17

fuel readiness level, 17

Hydrotreated esters and fatty acids, 17–18

lignocellulose resources, 20

waste oil, 14

First electric airplanes, 281–282

AstroFlight Sunrise, 281

Mauro Solar Riser, 281–282

Militky MB-E1, 281

Radio Queen, 281

Fischer–Tropsch process (FT process), 9 , 16–17 , 25 , 63 , 86–87 , 153–154

Flame

blow-off behavior, 135–136

ionization detection, 184–185

Food biomass, 11–12

Fossil feedstock, 9–11

Fossil fuels, 237 , 239 , 272

Fourier transform infrared spectroscopy (FTIR), 63 , 125

Fuel–oil heat exchanger (FOHE), 209–210

Fuels, 45–47

composition and thermal stability, 175–190

degradation process, 150–151

droplet size, 46

equivalence ratio, 45–46 , 48

fuel and droplet size, 48

prevaporization, 47

properties

impact on LBO, 136–140

impact on particulate matter emissions, 93–100

standards, 24–25

velocity, 49

Fuel's Fit-to-purpose properties (FTP properties), 26–27

Fully synthetic jet fuel (FSJF), 29–30

G

Gas chromatography–mass spectroscopy (GC-MS), 124 , 184–185

Gas turbine

emissions, 59

igniters, 40–42

laser ignition system, 41

plasma jet ignition system, 41–42

spark ignition system, 40

torch ignition system, 40

instability, noise, and vibrations, 223–226

Gas-to-liquid (GTL), 15 , 65 , 188–189

Gaseous emissions, 59

See also Particulate matter emissions (PM emissions)

gas turbine gaseous emissions

major species and sources, 59t

minor species and sources, 60t

NOx formation mechanisms, 62–63

Gate-to-gate (GtG), 4

Gibbs free energy equation, 117

Gravimetric method/filter-based method, 80–82

Greenhouse gases (GHGs), 1 , 59–60

emissions, 4 , 237

Greenhouse gases, Regulated Emissions, and Energy use in Transportation program (GREET program), 1

Ground transportation sector, 3

H

Hansen solubility parameter (HSP), 117–118

Helios, 282–284

High Reynolds number thermal stability tester, 207

High temperature electrolysis, 240

High-altitude long-endurance (HALE), 250–251

High-frequency dynamics (HFDs), 222

Hybrid aircraft development, history of, 278–291

Hybrid airplanes, 289–290

Ampaire Electric EEL, 290

Diamond Aircraft, 289

Hydrogen aircraft development

Balloon and Zeppelin, 244–245

Boeing Phantom Eye, 250–251

CRYOPLANE, 249–250

HeS 1, 245

hydrogen fuel cell aircraft, 251

Hydrogen fuel, 238–244

Boeing DA-20/Theator, 290

DLR HY4, 290–291

ENFICA-FC, 290

ZeroAvia, 291

historical aerospace applications of hydrogen, 244–254

aircraft configuration, 252–254

hydrogen aircraft development, 244–251

hydrogen combustors, 255–266

hydrogen in civilian use, 255–266

hydrogen production, 239–240

properties, 238–239

storage, 241–244

tank configuration, 241

tank shape, 242–244

Hydroperoxides, 195–196

Hydroprocessed esters and fatty acids (HEFA), 226–229

Hydrotreated renewable jet (HRJ), 52

HRJ8, 54

Hydrotreatment ester and fatty acid (HEFA), 16

I

Ignition, 39

parameters affecting ignition time delay, 47–49

Indirect combustion noise, 223

Inedible biomass, 20

Intermediate-frequency dynamics (IFDs), 222

Internal combustion (IC), 107

International Energy Agency (IEA), 239

Isoalkanes, 179–180

Isothermal tube reactor (ITTR), 203–204

Iterative trial-and-error process, 26

J

Jatropha, 13

Jet fuel, 2

composition, 86–87

Jet fuel thermal oxidation test (JFTOT), 187 , 207

JET-A, 238

Jet-A1, 59

t Inspection Group (JIG), 29

JP-8, 59

K

Kerosene, 152 , 237 , 249

kerosene-type jet fuel pathway, 15

L

Land use change (LUC), 4–5

Landing–takeoff cycle (LTO cycle), 85–86

Large eddy simulation (LES), 142

Laser ignition system, 41

Laser-induced incandescence (LII), 82–83

LDI combustor, 262–266

Lean blow off (LBO), 133–134

detection method, 134–135

hardware and fuel impact on, 135–140

fuel property and composition impact, 136–140

hardware configuration impact, 135–136

limits, 134

prediction, 141–142

Lean blowout (LBO), 94–95

Lean fuel limit, 133

Lean out (LO), 140

Lean-burning novel combustors, 223

Lean-direct injection (LDJ), 256

Life cycle assessments (LCAs), 4–6

Lignocellulosic feedstocks, 11–12

Linear instabilities, 221

Liquid hydrogen (LH2), 248

Liquid natural gas (LNG), 248

Lithium ion, 274

Lockheed CL-400 hydrogen fuel aircraft, 247–248

Low frequency dynamics (LFDs), 222

M

MagniX, 288–289

Metal deactivator additives (MDAs), 197–198

Methane, 238

Methylcyclohexane (MCH), 64

Methylidene, 127–129

Microgas turbine (MGT), 66

Micromix combustor, 256–262

Militky MB-E1, 281

Minimum ignition energy (MIE), 42

Multilayer insulation (MLI), 244

N

Naphthalene, 127–129

NASA X-57 Maxwell, 286–287

National Advisory Committee for Aeronautics (NACA), 245–246

National Jet Fuel Combustion Program (NJF), 137–138 , 229

Natural gas, 15

Near-isothermal flowing test rig (NIFTR), 202–203

Nitrile group of elastomers, 118–119

Noise, 219

Nonlinear instabilities, 221

Nonvolatile particulate matter (nvPM), 72–74

NOx formation mechanisms, 62–63

Nozzle transfer function, 225

O

O-ring, 118–119

physical properties, 119t

swelling behavior of, 126

OH-PLIF approach, 134–135

Oils, 11–14

algae, 14

camelina, 13

jatropha, 13

palm, 13–14

Olefins, 181

Omega-3 fatty acids, 13

Optical method, 122–123

Original equipment manufacturers (OEMs), 25

Oxidation chemistry, 170–175

P

Palm, 13–14

Particle image velocimetry (PIV), 134–135

Particle size, 75–76

Particulate matter emissions (PM emissions), 3 , 72–77 , 113 , 238

See also Gaseous emissions

impact of, 78–79

environmental impact, 79

health impact, 79

alternative fuels impact on, 87–108

measurement of, 80–83

electrical mobility analyzer, 83

gravimetric method/filter-based method, 80–82

LII, 82–83

nvPM, 73–74

particle size, 75–76

particulate matter formation mechanism, 76–77

vPM, 72–73

ways to reduce PM, 84–87

airport emission control, 85–86

combustor designs, 84–85

jet fuel composition, 86–87

enger aircraft, 290–291

Pathways, 15–20

alcohol-to-jet fuel Readiness Level 4–6, 18–19

conventional pathways, 16

coprocessing and catalytic hydrothermolysis Fuel Readiness Level 6, 20

direct sugar-to-hydrocarbon Fuel Readiness Level 5–7, 20

Fischer–Tropsch, 16–17

fuel readiness level, 17

Hydrotreated esters and fatty acids Fuel Readiness Level 7, 17–18

lignocellulose resources, 20

Pentyl benzene, 127

Perfect sphere particle, 75

PetroOxy, 207

Photoelectrolysis, 240

Planar laser-induced fluorescence (PLIF), 134–135

Plant lipids, 29

Plasma jet ignition system, 41–42

Polyaromatic hydrocarbons (PAHs), 76–77

Power spectrum density (PSD), 229

Pressure, 43 , 47–48

oscillation, 219

Prevaporization, 47

Primary batteries, 273

Propyl benzene, 127

Q

Quartz crystal microbalance (QCM), 184–185

R

Rayleigh's criterion, 219–221 , 225

Relight, 39

Renewable fuels, 29

Reusable jet fuel (RJF), 4–5

ROOH species, 195–196

S

Sasol isoparaffinic kerosene (Sasol IPK), 54

Seal degradation, 115–117

Seal shrinkage, 115–117

Sealing phenomenon, 114–118

Semisynthetic jet fuel (SSJF), 29–30

Sensitivity analysis, 201–202

Shell synthetic paraffinic kerosene (Shell SPK), 54

Small regional aircraft, 252–253

Solar Impulse, 284

Soluble macromolecular oxidatively reactive species (SMORS), 168–169

Spark duration, 44

Spark frequency, 44

Spark igniters, 40

Spark ignition system, 40

Stress relaxation, 120–122

Sugarcane, 19

Sulfur oxides (SOx), 62

Sustainable aviation fuels (SAFs), 2 , 7 , 15 , 29

Synthesis gas (syngas), 15

Synthetic paraffinic kerosene (SPK), 16 , 64 , 88 , 154–155

Syntroleum S-8 (S8), 54

T

Temperature

of air/fuel mixtures, 42–43

of flame, 47–49

Tetrahydronaphthalene, 127–129

Thermal modeling, 205–206

Thermal stability of alternative jet fuel, 150–151 , 163–175

alkanes, 178–179

alternative jet fuel

composition and thermal stability, 184–190

pathways, 162–163

aromatics, 182–184

autoxidation, 163–167

blending, 191–192

cycloalkanes, 180–181

deposit formation, 168–170

drop-in fuels, 158–162

energy content, 184

experimental apparatus, 203–208

AFTSTU, 208

ITTR, 203–204

jet fuel thermal oxidation test, 207

PetroOxy, 207

thermal modeling, 205–206

fuel composition and thermal stability, 175–190

generation of chemical kinetic mechanism, 198–203

incidents involving fuel thermal stability, 209–211

isoalkanes, 179–180

jet fuels, 151–158

liquid-to-liquid heat exchanger, 150f

olefins, 181

oxidation chemistry, 170–175

trace species, 192–198

AH species, 196

dissolved metals, 197–198

ROOH species, 195–196

SH species, 196–197

Thermogravimetric analysis (TGA), 124

Threshold soot index (TSI), 94

Torch igniters, 40

Torch ignition system, 40

Trace species, 192–198

Traditional fossil fuel, 237

1,2,3-trimethylbenzene, 127–129

U

United States Air Force (USAF), 29–30

Unmanned aerial vehicle (UAV), 250–251

Used cooking oil (UCO), 17–18

V

Vacuum fractional distillation, 152

Vegetable oils, 155–156

Vibration, 219

Volatile particulate matter (vPM), 72–73

Volatilitys, 45

W

Waste oil, 14

Well-to-gate (WtG), 4

Well-to-pump (WtP), 4

Well-to-wake (WtWa), 4

Wide-cut fuels, 25–26

Wide-cut Jet-A1 fuel, 23–24