Combustion, Fuels, and Burner Systems
This domain tests how fuel and air are mixed, ignited, and controlled safely. Expect stoichiometry, excess air, draft, gas and oil fuel trains, burner turndown, purge and light-off sequence, and flame safeguarding.
Combustion fundamentals
- Theoretical air is the air needed for complete combustion. Excess air ensures complete burning but lowers efficiency by carrying heat up the stack.
- Oxygen in flue gas indicates excess air. Carbon monoxide indicates incomplete combustion, poor mixing, or insufficient air.
- Draft moves products through the boiler; too little draft can cause poor combustion, and too much increases infiltration and stack loss.
- Stack temperature above the acid dew point avoids condensation and corrosion, especially with sulfur-bearing oil.
Burner systems
Gas trains include regulators, shutoff valves, proof-of-closure switches, vents, and pressure switches. Oil burners require atomization by pressure, steam, or air, plus correct oil temperature and pressure. A purge removes combustible gases before light-off. Burners normally light off at low fire and prove flame before the main fuel valve opens.
Worked example: Flue-gas oxygen reads 3.0% dry. Excess air equals O2 divided by (20.9 - O2), times 100: 3.0 / (20.9 - 3.0) = 3.0 / 17.9 = 0.168, or about 16.8% excess air. If CO is also rising, do not simply add air; check burner adjustment, fuel pressure, and air distribution.
Traps: purging with a fuel valve open; skipping proof-of-closure; light-off without proven purge; ignoring CO while chasing low O2; water or condensate in gas lines; wrong oil atomization.