Gas Fundamentals and Combustion
This domain tests how well you can predict what a fuel gas does when it is mixed with air and ignited: stoichiometry, flammable limits, ignition behavior, flame stability, and the meaning of combustion-analysis readings.
Core rules
- Combustion stoichiometry is mole-based. Methane reacts as CH4 + 2O2 -> CO2 + 2H2O. Each mole of O2 brings 3.76 mol N2, so 1 mol CH4 needs 2 × 4.76 = 9.52 mol air.
- Excess air is the air supplied above stoichiometric. In dry flue gas with no CO, % excess air ≈ O2 / (20.9 - O2) × 100.
- LFL and UFL bound flammability; LEL is the same concept expressed for detectors. Methane LFL is about 5% vol, UFL about 15% vol.
- Autoignition temperature is not the same as flash point. A fuel can flash below its autoignition temperature; ignition still needs an energy source.
- Wobbe index compares heat input through a fixed orifice and matters when gases are interchanged.
Worked example
A burner fires methane at 3.0% O2 dry in the flue gas. Estimate excess air. %EA = 3.0/(20.9 - 3.0) × 100 = 16.8%. If the same burner were set to 20% excess air, the O2 reading would rise to roughly O2 = 20.9 × EA/(100 + EA) = 20.9 × 20/120 = 3.48%. The direction matters: more excess air means more O2 and more stack loss.
For stoichiometric air mass: 1 kmol CH4 needs 9.52 kmol air; air at 28.97 kg/kmol gives 275.8 kg air per 16.04 kg CH4, or 17.2 kg air/kg fuel.
Traps
Using 21.0% rather than 20.9% is a small error; using wet O2 when the analyzer reports dry O2 is larger. Do not call 100% LEL pure gas: 100% LEL is the lower flammable limit, not 100% volume. Also check whether the analyzer corrected for CO and whether the burner is condensing, because water vapor changes readings.