How to find theoretical combustion air
Theoretical air is the minimum air a fuel needs for complete combustion. It sets burner turndown, forced draft fan capacity and the flue gas volume that drives stack and heat recovery sizing, so it is usually the first number worked out on a combustion job.
The calculation is pure stoichiometry from an ultimate analysis. One pound of carbon needs 2.667 lb of oxygen, hydrogen needs 8 lb and sulfur needs 1 lb, giving theoretical oxygen = 2.667C + 8H + S − O. Because air is 23.2% oxygen by weight, theoretical air = oxygen ÷ 0.232, and volume follows by dividing by 0.0765 lb/ft³, the density of standard air at 60°F and 14.7 psia.
A residual oil at 86% C, 12% H, 1% S and 0.5% O works out to roughly 14.0 lb of air per lb of fuel, about 184 SCF/lb. Add 20% excess air and the burner actually needs around 220 SCF/lb. Natural gas, being mostly methane, lands near 17.2 lb/lb, which is why gas trains move far more air per pound of fuel.
Too much excess air throws heat out of the stack; too little produces smoke and carbon monoxide. Field practice is to trim against a measured flue gas oxygen reading, and to confirm the final burner and fan selection against the manufacturer's combustion data.
Frequently asked questions
Oxygen already bound in the fuel takes part in combustion, so that much less has to arrive with the combustion air. Subtracting it is the classic available hydrogen correction.
It depends on fuel and burner. Gas firing typically runs 10 to 20%, distillate oil 15 to 25%, residual oil 20 to 30% and pulverized coal 20 to 40%. More excess air means more heat up the stack, so trim it against a measured flue gas oxygen reading.