The minimum amount of air which supplies the requisite amount of oxygen for complete combustion of fuel is called theoretical or stoichiometric air.
The reaction is, however, not likely to proceed to completion if the fuel is supplied just with stoichiometric air. The objective of complete combustion is achieved by supplying excess air; excess air means the amount of air supplied in excess to that theoretically needed to burn fuel completely. The quantity of excess air depends on the type of fuel and the method of combustion used and may vary from 25 to 100 percent.
Excess air insures better combustion but at the same time cools the furnace and increases heat loss in the exhaust gases.
When more air is supplied than required by the fuel for its complete combustion, the mixture is said to be weak or lean.
However, when insufficient air is supplied, we call the air-fuel mixture rich. When the air supplied is just enough to burn the fuel completely, the mixture is called stoichiometric or chemically correct.
To burn certain amount of fuel, some theoretical air is required.
It is given by
4.35[(8/3 C + 8H2 + S) – O2] kg
This is the theoretical air required. But, in practice, some more air or extra air is supplied to ensure complete combustion of the fuel.
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If more air is supplied, then there will be a cooling effect and the efficiency will decrease. Also, if less air is supplied, then complete combustion will not take place. Combustible substances will escape from the stack. Again, there will be a loss in efficiency. So, it is required to adjust the air supply in such a way that complete combustion will take place without much extra or excess air.
It is to be kept in mind that excess air and excess oxygen are not same. Air has roughly 21% oxygen by volume. So, 100% excess air is approximately equal to 10.5% oxygen remaining at the boiler exhaust stack.
Excess air percentage = [oxygen % in flue gas/(21 - oxygen %)] x 100
Actual air quantity = theoretical air x [1 + (excess air %/100)]
Problem (Gujarat BOE 2016, 2023, 5 marks)
Following parameters are noted from the ultimate analysis of coal sample:
Carbon: 40%
Sulphur: 2%
Hydrogen: 4%
Calculate the theoretical quantity of air required in kilograms. If the boiler is operated at 4% excess oxygen, then calculate the actual air quantity in kilograms.
Solution
Quantity of theoretical air required (by weight) per kilogramme of fuel is given by
4.35[(8/3 C + 8H2 + S) – O2] kg
= 4.35[(8/3) x 0.4 + 8 x 0.04 + 0.02)] - 0 = 6.12 kg
Here O2 is taken 0 as its value in the coal is not given.
Excess air percentage = [oxygen % in flue gas/(21 - oxygen %)] x 100
= [4/(21 - 4)] x 100
= 23.5%
Actual air quantity = theoretical air x [1 + (excess air %/100)]
= 6.12 x [1 + (23.5/100)]
= 7.55 kg/kg of fuel
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