What is Theoretical air?
Theoretical air, also known as stoichiometric air, is the minimum quantity of air required for complete combustion of a given fuel. At this condition, all the fuel is burned completely with no unburnt fuel and no excess oxygen in flue gas.
On what factors, Theoretical air requirement depends?
Theoretical air requirement depends on;
Percentage of Carbon in fuel
Percentage of Hydrogen in fuel
Percentage of Sulphur in fuel
Moisture percentage in fuel
Upon complete combustion carbon forms to CO2
Hydrogen forms to H2O
Sulphur forms to SO2
Formula used for the calculation of Excess air;
Theoretical air Thair = ((11.6 X C% + (34.8 X (H2-O2/8)) + (4.35 X %S))/100
Where C = % of carbon in fuel
H2 = % of Hydrogen present in fuel
S = % of sulphur present in fuel
Excess air is the amount of air supplied over and above the theoretical air to ensure complete combustion.
supplying exactly theoretical air is not practical due to:
- Imperfect mixing of air and fuel
- Variations in fuel quality
- Burner design limitations
- Operational fluctuations
Excess air requirement for combustion
%EA = O2% / (21-O2%)
Where O2 = % of oxygen present in flue gas
Mass of actual air supplied
Actual air supplied is calculated by the below formula.
AAS = (1 + EA / 100) X Theoretical air
Calculate the Theoretical air & Excess air required to burn 20 MT of coal having carbon (C) 50%, Hydrogen (H2) 4%, Oxygen (O2) 9% & Sulphur (S) 0.5% in it. For complete combustion operator is maintaining 4.5% of O2 in flue gas.
We have Theoretical air, Th = (11.6 X %C + 34.8 X (H2-O2/8) + 4.35 X S) / 100
Th = (11.6 X 50 + 34.8 X (4-9/8) + 4.35 X 0.5) / 100
Th = 6.82 kg/kg of fuel
We have excess air EA = O2 X 100 / (21-O2)
= 4.5 X 100 /(21-4.5) = 27.27%
Total air = (1 + EA/100) X Theoretical air
Total air = (1 + 27.27/100) X 6.82 = 8.68 kg of air per kg of fuel burnt
A Boiler’s combustion system requires 6 kg of air for burning 1 kg of fuel, then calculate the total air required for complete combustion if its flue gas has 4.7% of O2
We have,
Total air = (1 + EA/100) X Theoretical air
EA = O2 X 100 / (21-O2)
EA = 4.7 X 100 / (21-4.7) = 28.83%
Therefore, Total air = (1 + 28.83/100) X 6 = 7.72 kg of air per kg of fuel burnt
What will happen to theoretical air requirement if carbon percentage in fuel decreases?
Reduction of 5% carbon in fuel reduces the Theoretical air requirement for complete combustion by 8 to 9%.
What will happen to theoretical air requirement if Hydrogen percentage in fuel decreases?
Reduction of 1% Hydrogen in fuel reduces the Theoretical air requirement for complete combustion by 5%.
What will happen to theoretical air requirement if Oxygen percentage in fuel decreases?
Reduction of 5% carbon in fuel reduces the Theoretical air requirement for complete combustion by 8 to 9% upon.
