Lecture 10 Material balance in combustion Contents Objectives How composition of POC is determined? Orsat analysis Reporting of Orsat analysis Orst analysis and material balance Exercise Keywords: Material balance, combustion, furnaces Objectives: •
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To calculate amount of air required for a given composition and amount of fuel To calculate amount of POC produced and its composition. To calculate excess air when composition of POC is known. To determine air leakage. One of the important requirements of material balance is to know composition of products of combustion (also called flue gas; in the following flue gas and POC are used interchangeably). How composition of POC is determined? Orsat analysis Composition of POC is determined by “Orsat Analysis”. The Orsat apparatus consists essentially of a calibrated water‐jacketed gas burette connected by glass capillary tubing to two or three absorption pipettes containing chemical solutions that absorbs the gases. The absorbents are potassium hydroxide for CO , alkaline pyrogallol for O and ammoniacal cuprous chloride for CO measurement. By means of rubber tubing arrangement, the gas to be analyzed is drawn into the burette and flushed through several times. Typically, 100ml is withdrawn for ease of calculation. Using the stopcocks that isolate the absorption burettes, the level of gas in the leveling bottle and the burette is adjusted to the zero point of the burette. The gas is then passed into the caustic potash burette, left to stand for about two minutes and then withdrawn, isolating the remaining gas via the stopcock arrangements. The process is repeated to ensure full absorption. After leveling the liquid in the bottle and burette, the remaining volume of gas in the burette indicates the percentage of carbon dioxide absorbed. The same technique is repeated for oxygen, using the pyrogallol solution, and carbon monoxide using the ammoniacal cuprous chloride solution. Important to note that Orsat % give actual analysis on a dry basis. The common Orsal gives % O % CO and % CO . N is found by % N
100 – % O
% CO
% CO . Since KOH solution absorbs both CO & SO % CO contains the amount of SO as well. Reporting of Orsat analysis Consider combustion of coal with dry air. The ultimate analysis of coal % dry basis is C 7 6, H 5, O 6, N 2, S 2 and Ash 9. Assume complete combustion. Determine a) Stoichiometric amount of air, b) Amount of POC. C) % POC analysis (wet basis), d) % POC analysis (dry basis) and e)% POC analysis (Orsat). Basis 100 Kg coal. In the following the balance on the elements is done in kg mole. Amount of stoichiometric air O required strichiometric
6.33
7.6425 kg moles. O from air
O for combustion
7.6425
1.25
O present in coal 0.1675
7.475 kg mole
Amount of air
35.581 kg moles. Amount and % composition of POC: Kg moles POC CO2 H2O N2 SO2 0.0625 6.33 1.25 28.18 0.0625 % analysis (wet basis) 17.67 3.49 78.67 0.7 % analysis *(dry basis) 18.31 ‐ 81.51 0.18 Orsat analysis 18.49$ ‐ 81.51 ‐ 35.82 100.00 100.00 100 Total analysis dry basis
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% POC wet $ Orsat analysis does not report SO separately, therefore % CO contains % SO2 as well CO
18.49 Note: Orsat analysis would have resulted into POC analysis as given in column 5 of the above table. Orsat analysis of the POC will report CO and N only. Orsat analysis is also on dry basis. This aspect must be understood while solving material balance problems on combustion Now suppose 20% excess air is used Amount of air
POC 35.581 1.2
Kg moles 42.70kg moles Analysis of POC in % Wet basis 14.74 2.91 78.73 0.15 3.47 100.00 % analysis (dry basis) Orsat analysis CO2 6.33 15.18 15.34 4.25 ‐‐‐‐‐‐ ‐‐‐‐‐‐ H2O 81.09 33.80 81.09 N2 0.0625 0.16 ‐‐‐‐‐‐‐ SO2 O2 1.49 3.57 3.57 Total 42.93 100.00 100.00 Orsat analysis and material balance For this purpose coal of the same composition as given above is combusted with stoichiometric 18.49% and N
air. The orsat analysis of POC is CO
81.51% . Find amount of POC. Now orsat analysis is on dry basis. Basis of selection of coal should also be dry basis. Basis 1 kg coal dry Let x kg mole dry POC. Performing carbon balance: C in CO of POC
S from coal kg moles C from coal
0.1849 x
6.33 0.0625 34.57 kg moles dry POC X
POC will contains H O also but Orsat analysis does not include H O Amount of POC
Amount pf POC
dry POC amount
water due to reaction between H& O 35.82 kg moles. This answer is same as calculated above. Now consider that the coal contains 4 % moisture when fired, & combusted with stoichiometric air, find amount of POC. Amount of POC
34.57
dry POC amount
1.25
water due to reaction between H and O
moisture of coal 36.04 kg moles Exercise Orsat analysis of POC is CO
15.34, % N
81.09 % and O
3.57 % obtained on combustion of coal %, dry basis C 76, H 5, O 6, N2, S2 and ash 9. Find amount of POC and % excess air. Ans: 42.93 kg moles and 2%. Assignment: 15.34, % N
81.09 % and O
3.57 % obtained on 1) Orsat analysis of POC is CO
combustion of coal %, dry basis C 76, H 5, O 6, N2, S2 and ash 9. Find amount of POC and % excess air. 2) Explain the procedure to determine percent composition of POC obtained on complete combustion of a fuel comprising of C and hydrogen with air 3) The ultimate analysis of coal % dry basis is C 80, H 5, O 7, N 2, S2 and Ash 4. Assume complete combustion. Determine a) Stoichiometric amount of air, b) Amount of POC. C) % POC analysis (wet basis), d) % POC analysis (dry basis) and e) % POC analysis (Orsat).