Combustion, Fuels, and Burner Systems
Combustion, Fuels, and Burner Systems
This domain tests combustion chemistry, fuel properties, burner sequence, draft, and efficiency. You should be able to relate excess air, stack temperature, CO, and smoke to burner adjustment and safe operation.
Fuel and air basics
Stoichiometric combustion uses exactly enough oxygen; practical firing uses excess air to complete combustion. Natural gas is mostly methane, burns cleanly, and is lighter than air; No. 2 oil is a distillate with higher Btu per gallon; heavier oils need heating and atomization. Burners must atomize oil or meter gas, mix fuel with air, and stabilize flame. Draft overcomes flow resistance; too little draft causes incomplete combustion, too much raises stack loss.
Burner sequence
A typical sequence is prepurge, pilot ignition, pilot proving, main flame at low fire, modulation to demand, and shutdown with postpurge. Flame safeguards must prove pilot and main flame; a flame failure must close fuel valves within the safety shutoff time.
Worked example
A boiler fires 500 gal/hr of No. 2 oil at about 140,000 Btu/gal. Input is 500 × 140,000 = 70,000,000 Btu/hr (70 MMBtu/hr). If stack O2 is 3% and CO is 50 ppm with visible smoke, the mixture is not simply efficient: smoke and CO show incomplete combustion, so check atomization, air/fuel ratio, draft, and fouled heat transfer before claiming savings from low excess air.
Traps
Do not read low stack temperature as efficiency if the boiler is sooting or condensing acid. Do not adjust fuel pressure without checking flame shape and CO. Do not purge with fuel present, and do not treat a flame scanner as a substitute for a proper purge.