Ultimate Analysis y4r54

Ultimate Analysis of Fuels Fuels and Combustion

By- 1.) Meet Oza-14BME065 2.) Alay Patel-14BME073 3.) Prakash Parghi-14BME153D Batch- Mechanical, 2014 Guided By- Mr. Abhinaya Srinivas

Introduction

The composition of fuels vary widely and hence it is necessary to analyse and interpret the results for: ● Commercial classification ● Price fixation ● Proper industrial utilization The quality of fuels is ascertained by the following two types of analysis: ● Proximate Analysis and Ultimate Analysis

Ultimate Analysis ●

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Ultimate analysis refers to the determination of weight percentage of carbon, hydrogen, nitrogen, oxygen and sulphur. This analysis gives the elementary, ultimate constituents of the fuel. This analysis is essential for calculating heat balances in any process for which the fuel is employed. It is useful to the deg of fuel burning equipments and auxiliaries.

Carbon and Hydrogen: ●

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A known amount of coal is burnt in presence of oxygen thereby converting carbon and hydrogen into1. CO2 (C +O2 = CO2) 2. H2O (H2 + ½ O2 = H2O) The products of combustion CO2 and H2O are ing over weighed tubes of anhydrous CaCl2 and KOH which absorb H2O and CO2 respectively. The increase in the weight of CaCl2 tube represents the weight of water formed while the increase in the weight of KOH tube represents the weight of CO2 formed.

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In the ASTM standard method, a 0.2 gram sample is normally used. A combustion tube contains a section of heated copper oxide in wire form to complete the combustion. Lead chromate or silver are specified for removing oxides of sulfur, and silver for removing chlorine. A temperature of 850° to 900° C is specified for burning the sample. For the high-temperature combustion method, a coal sample of approximately 0.5 gram (accurately weighed) is used, and combustion is accomplished, without copper oxide, with a rapid flow of oxygen at a temperature of from 1250° to 1350° C.

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The percentage of carbon and hydrogen in coal can be calculated in the following way-

The weight of coal sample taken = x g The increase in the weight of KOH tube = y g The increase in the weight of CaCl2 tube = z g a.) Carbon: Consider the following reaction, C (12) + O2 = CO2 (44) 44 g of CO2 contains 12 g of carbon Therefore y g of CO2 contains = (y * 12/44) g of carbon. And, x g of coal contains = 12*y/44 g carbon % of carbon in coal = (12*y/44*x) * 100

b.) Hydrogen: Consider the following reaction, H2 (2) + ½ O2 = H2O(18) 18 g of water contains 2 g of hydrogen Therefore, z g of water contains = 2 z/18 g of hydrogen And, X g of coal contains = 2 z/18 g of hydrogen % of hydrogen in coal = (2*z) / (18*x) * 100

Significance of Total Carbon: ● It is the sum of fixed carbon and the carbon present in the volatile matters like CO, CO2, hydrocarbons. ● Thus, total carbon is always more than fixed carbon in any coal. ● High total carbon containing coal will have higher calorific value. Significance of Hydrogen: ● It increases the calorific value of the coal. ● It is associated with the volatile matter of the coal. ● When the coal containing more of hydrogen is heated, it combines with nitrogen present in coal forming ammonia. Ammonia is usually recovered as (NH4)2SO4, a valuable fertilizer.

Uses of Carbon and Hydrogen Data: ● Carbon and hydrogen values are used as parameters in certain proposed classification systems. ● They are also used in the calculation of heat values for checking determined calorific values and for calculating heat balances in boiler efficiency tests. ● They are probably most often used, however, in research investigations. Limitations: ● Incomplete combustion. ● Inclusion of the hydrogen of moisture and water of hydration. ● The temperature of the furnace section covering the lead chromate and/or the silver must be maintained carefully and must not be too high.

Determination of Nitrogen: ● ●

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This is done by Kjeldhal’s method. A known amount of powdered coal is heated with concentrated sulphuric acid in the presence of K2SO4 and CuSO4 in a long necked Kjeldhal’s flask. This converts nitrogen of coal to ammonium sulphate. When the clear solution is obtained (ie., the whole of nitrogen is converted into ammonium sulphate), it is heated with 50 % NaOH solution and the following reaction occurs:

(NH4)2SO4 + 2 NaOH

Na2SO4 + 2 NH3 + H2O

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The ammonia thus formed is distilled over and is absorbed in a known quantity of standard 0.1 N H2SO4 solution. The volume of unused 0.1 N H2SO4 is then determined by titrating against standard NaOH solution. Thus, the amount of acid neutralized by liberated ammonia from coal is determined using the formula:

% Nitrogen in coal = 1.4*volume of acid used*normality/x ●

The current ASTM standard method specifies mercury as the catalyst and is known as the Kjeldahl-Gunning method.

Significance: ● Presence of nitrogen decreases the calorific value of the coal. ● However, when coal is carbonized, its N2 and H2 combine and form NH3. Ammonia is recovered as (NH4)2SO4, a valuable fertilizer. Uses of Nitrogen data: ● Nitrogen data are used in comparing coals and in research. ● When the oxygen content of coal is estimated by difference, it is necessary to make a nitrogen determination. ● Nitrogen data probably have no other important uses.

Limitations: ● Certain combinations of nitrogen do not respond to the Kjeldahl methods. For example, nitro-nitrogen is not included unless provision is made to reduce the nitro group to ammonia. ● Incomplete conversion of the nitrogen to ammonium sulfate will lead to low results. ● In the distillation step, a too-rapid distillation rate may result in carry-over of the alkaline digestion mixture, which will ruin the determination. ● Pyridine carboxylic acids may be formed that are resistant to decomposition.

Determination of Sulphur ● ● ● ●

A known amount of coal is burnt completely in Bomb calorimeter in presence of oxygen. Ash thus obtained contains sulphur of coal as sulphate which is extracted with dil. HCl. The acid extract is then treated with BaCl2 solution to precipitate sulphate as BaSO4. The precipitate is filtered, washed, dried and weighed.

From the weight of BaSO4, the percentage of sulphur in coal is calculated in the following way: The weight of coal sample taken = x g The weight of BaSO4 precipitate = y g Consider the following equations,

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S (32) + 2O2 = SO4 (+ BaCl2) = BaSO4 (233) 233 g of BaSO4 contains 32 g of sulphur Therefore, y g of BaSO4 contains = 32*y/233 g sulphur. Therefore x g of coal contains = 32*y/233 g sulphur. % of sulphur in the coal = (32*y/233*x)*100.

Significance: ● It increases the calorific value of the coal, yet it has the following undesirable effect- The oxidation products of sulphur (SO2, SO3) especially in presence of moisture forms sulphuric acid which corrodes the equipment and pollutes the atmosphere. Uses of Total Sulfur Data: ● Purpose of cleaning coal is to reduce the total sulfur content. Sulfur values, therefore, must be available for use as a guide to successful commercial processing of coal. ● Sulfur is converted to oxides that may contribute to corrosion in the equipment, to slagging of the combustion or boiler equipment, and to atmospheric pollution. Sulfur data are therefore necessary for evaluation of coals to be used for combustion purposes.

Limitations: ●

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Due to the non uniform distribution of the pyritic portion of total sulfur in coal, adequate sampling is complicated. Failure to completely convert all the sulfur to sulfate will result in erratic values. As sulfur normally is first oxidized to sulfur dioxide during analysis, some of it may be lost if precautions are not taken.

Determination of oxygen ●

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According to the ASTM standards, the oxygen value has been obtained by subtracting the sum of the percentages of carbon, hydrogen, nitrogen, chlorine, total sulfur, and ash from 100 percent. It is calculated indirectly in the following way% of oxygen in coal = 100 - % (C + H + N + S + ash)

Significance: ● The less the oxygen content, the better is the coal. As the oxygen content increases, its moisture holding capacity also increases.

Uses of Oxygen Data: ● ●

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In calculating heat balances for boiler efficiency studies. In carbonization, a coal of high oxygen content is judged to be less suitable for making good metallurgical coke than one with a lower oxygen content. In gasification and liquefaction of coal, higher oxygen (lower rank) coals may be more reactive and thus easier to gasify or liquefy. In research, oxygen data, as a part of the ultimate analysis, help to characterize coal reserves and are useful in various technical investigations.

Numericals for fun… 1.

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A coal sample was subjected to ultimate analysis, 0.6 g of coal on combustion in a bomb calorimeter produces 0.05 g of BaSO4. Calculate the percentage of S in the sample. 1.56 g of coal was kjeldahlized and NH3 gas thus evolved was absorbed in 50 ml of 0.1 N H2SO4. After absorption, the excess acid required 6.25 ml of 0.1 N NaOH for exact neutralization. 2.6 g of coal sample in a quantitative analysis gave 0.1755 g of BaSO4. Calculate the % of N and S in the sample.

References ●

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Chemistry, Uses and Limitations of coal analyses by O.W. Rees, Illinois State Geological Survey, Department of Registration and Education, State of Illinois. Fundamentals for analysis and calculation of energy and environmental performance, Applied industrial energy and environmental management by Z. K. Morvay, D. D. Gvozdenac. Jain and Jain, Engineering Chemistry, Dhanpat Rai Publication James G. Speight, Fuel Science and Technology Hand Book, Marcel Dekker, New York.

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