Combustion

Combustion is the reaction engineers balance most: furnaces, boilers and engines all burn fuel in air. Air is taken as 21% oxygen and 79% nitrogen by moles, so every mole of O₂ brings 79 ÷ 21 = 3.76 mol of N₂ along with it. The nitrogen doesn't burn, but it has to be heated and goes out the stack.

Theoretical oxygen is what complete combustion needs: CH₄ + 2O₂ → CO₂ + 2H₂O, so 2 mol of O₂ per mol of methane. Theoretical air is that divided by 0.21. Real burners use excess air (often 10 to 20% for gas) so that every bit of fuel meets oxygen; too little and you get carbon monoxide and soot, too much and you waste heat warming air.

Burn 100 mol/h of methane with 20% excess air. Oxygen needed: 200 mol/h; fed: 240. Air: 240 ÷ 0.21 = 1143 mol/h, carrying 903 mol/h of N₂. Out of the stack: 100 CO₂, 200 H₂O, 40 unused O₂ and 903 N₂.

Flue gas analyzers usually report a dry basis, leaving out the water (it condenses before the gas is measured). Dry flue gas here is 100 + 40 + 903 = 1043 mol/h, so CO₂ is 100 ÷ 1043 = 9.6% and O₂ is 3.8%. On a wet basis, counting water, the CO₂ would be 100 ÷ 1243 = 8.0%.

Operators work backwards from those numbers: oxygen in the flue gas tells you how much excess air you're using, so burners are tuned by watching it.