Combustion, Fuels, and Burner Systems
The three T's and the air/fuel ratio
Combustion needs time, temperature, and turbulence, plus the right air-to-fuel ratio. The theoretical (stoichiometric) air is the exact amount needed for complete burning; anything above that is excess air. Too little air produces CO and smoke; too much air wastes heat up the stack. Flue-gas analysis - O2, CO2, CO, and stack temperature - is how you judge the balance. Natural gas needs roughly 10 volumes of air per volume of gas; No. 2 oil roughly 14 lb air per lb oil, and heavier oils need preheating for atomization.
Hardware
Know atmospheric and power (gun) burners, rotary-cup burners, stokers, and pulverized-coal systems; the gas train (regulator, low- and high-pressure switches, safety shutoff valves, proof-of-closure, leak test); oil trains (pump, heater, strainer, atomizing air or steam); draft systems (natural, forced, induced, balanced); and the flame safeguard with its scanner and pilot. Turndown ratio describes the range between high and low fire. Every light-off starts with a timed purge to clear the furnace.
Traps
Candidates wrongly believe "more excess air is always safer." Excess air above what is needed lowers efficiency; below it you get CO and soot. Also distinguish draft (pressure difference) from capacity, and remember that a high stack temperature is a maintenance or heat-transfer problem, not a combustion-tuning one. Never adjust a burner to chase a CO2 number while ignoring CO.
Worked example
A burner fires 1,000 ft³/hr of natural gas. Theoretical air is 10,000 ft³/hr; at 10% excess air the requirement is 11,000 ft³/hr. If the combustion-air duct measures 10 ft × 10 ft, average velocity is 11,000 ÷ (100 × 60) ≈ 1.8 ft/min - a reminder that airflow numbers must be converted to actual conditions before comparing to a fan curve.