How do you perform a basic stoichiometric air calculation for a hydrocarbon fuel?

• A. Only compute O2 needed for carbon portion.
• B. Determine the fuel’s elemental composition, compute O2 needed for C to CO2 and H to H2O (and S to SO2), then divide by 0.21 to get theoretical air and add a margin for excess air.
• C. Ignore sulfur content.
• D. Guess approximate air based on experience.

Answer: (B)

Calculating the air needed for complete combustion starts with the total oxygen demand of all combustible elements in the fuel. You use the fuel’s elemental formula C_a H_b S_c and balance the amount of O2 required to form the final products: CO2 from carbon, H2O from hydrogen, and SO2 from sulfur. The oxygen needed is a for the carbon to CO2, plus b/4 for the hydrogen to H2O, plus c for the sulfur to SO2. So O2 needed per mole of fuel is a + b/4 + c. To turn that into theoretical air, divide by the oxygen fraction in air (about 0.21), giving theoretical air amount. Then add a margin for excess air to ensure complete combustion.

For example, a hydrocarbon like C8H18: O2 required = 8 + 18/4 = 12.5 moles of O2 per mole of fuel. Theoretical air ≈ 12.5 / 0.21 ≈ 59.5 moles of air per mole of fuel. If you want, say, 5% excess air, target about 62.5 moles of air. This approach accounts for all combustibles, not just carbon, and explains why sulfur (if present) also matters.