Properties and Combustion of Fuel Gases
Domain 2 tests fuel-gas properties, stoichiometry, and the combustion process. You need to predict air requirements, recognize incomplete combustion, and explain how gas characteristics affect orifice sizing, venting, and appliance performance.
Properties
Natural gas is mostly methane (CH4), with specific gravity about 0.60 (air = 1.0), so it rises and disperses. Propane (C3H8) has specific gravity about 1.52, so it pools in low areas. Heating values, Wobbe index, and flammable limits differ; an orifice drilled for natural gas will over-fire on propane unless converted. Odorant is added so leaks are detectable, but you never use smell as a test.
Combustion
Complete combustion of methane: CH4 + 2O2 -> CO2 + 2H2O. One volume of methane needs two volumes of oxygen; since air is about 21% O2, that is 2/0.21 = 9.52 volumes of air. Practical burners use excess air, often 15-50% for natural draft, to complete combustion. Too little air yields CO; too much lowers efficiency and can cool the flame.
Worked example
A burner consumes 1.0 m3/h of methane. Stoichiometric air = 1.0 x 9.52 = 9.52 m3/h. At 20% excess air, actual air = 9.52 x 1.20 = 11.42 m3/h. The flue gas then contains unused O2 and N2, which is why CO2 readings are diluted below the theoretical maximum.
Traps
- Confusing specific gravity with density; propane is heavier than air, natural gas is lighter.
- Using the wrong heating value (gross vs net) in efficiency calculations.
- Assuming a blue flame proves complete combustion; CO can still be present.
- Ignoring altitude and gas-quality derating.
- Converting an appliance without changing the orifice or spring.