Combustion, Fuels, and Burner Operation
The three T's and the air/fuel ratio
Combustion needs temperature, turbulence and time. Air is supplied by draft - natural, forced, induced or balanced. Methane's stoichiometric requirement is about 2 ft³ of oxygen, roughly 9.5–10 ft³ of air, per ft³ of gas. Real burners run with excess air: enough to finish combustion without wasting heat up the stack.
Reading flue gas
Oxygen is the practical handle. Excess air (percent) ≈ O2 ÷ (20.9 − O2) × 100, so 3% O2 is about 17% excess air - a typical gas target. Very low excess air raises CO and smoke; very high excess air lowers flame temperature and raises stack loss. Carbon dioxide rises as excess air falls, but 'highest CO2' is not best if CO or smoke appears.
Fuels and burners
Grades 2 through 6 fuel oil differ in viscosity, so heavier oils need preheating and fine atomization (pressure, air or steam). Flash point is a safety property, not an ignition temperature. Burners are on/off, staged or modulating; turndown describes how far they can throttle. Every start needs a proved purge, and a proved pilot before the main valve opens.
Traps
- Assuming more excess air always means cleaner, better combustion.
- Mixing up flash point with fire point or viscosity.
- Forgetting that a low-fire start protects the stack and the refractory.
Rule of thumb
Each point of flue O2 above the optimum costs roughly half a point of efficiency, while CO or smoke means you have gone too far the other way.