Air–Fuel Ratio and Excess Air in Boiler Combustion
Every kilogram of fuel needs a definite amount of air to burn completely. Real burners and grates cannot mix fuel and air perfectly, so boilers run with some extra — excess air. Too little and fuel goes up the stack unburnt; too much and the extra air carries heat away. The right amount is the most powerful efficiency setting an operator controls.
You'll learn
- Calculate the theoretical air for a fuel
- Explain why excess air is needed and what it costs
- Estimate excess air from flue gas oxygen
- Know typical excess air by fuel and firing method
Before this: Coal and lignite, Oil and gas fuels
Burning needs the right amount of air
Combustion joins the carbon, hydrogen and sulfur in a fuel with oxygen from the air. There is an exact amount of air that supplies just enough oxygen to burn everything: the theoretical, or stoichiometric, air. In practice the fuel and air never mix perfectly, so some extra air is supplied to make sure every particle of fuel finds the oxygen it needs. That extra is the excess air.
Air is mostly nitrogen, which takes no part in combustion but is heated by it and carried up the chimney. Every bit of air beyond what is needed adds to that wasted heat. The goal is the least excess air that still burns the fuel out completely.
Theoretical air and excess air
| Symbol | Meaning | Unit |
|---|---|---|
| Carbon, hydrogen, oxygen, sulfur in the fuel | % by mass |
- Carbon 84%, hydrogen 12%, oxygen 1%, sulfur 3%.
- Theoretical air:
- The flue gas shows 4% oxygen. Excess air:
- Actual air: 13.89 × (1 + 0.235) = 17.15 kg per kg of oil.
Finding the right amount
Below the right excess air, combustion is incomplete: carbon monoxide appears in the flue gas, smoke forms, and unburnt carbon rises in the ash. Above it, CO stays near zero but the dry flue gas loss grows with every extra kilogram of air heated to stack temperature. The best setting is just above the point where CO starts to rise, and it differs by fuel and firing method: gas burners can run lean, oil a little higher, and solid fuels on grates need considerably more because mixing on a grate is poor.
| O₂ in flue gas, % | Excess air, % (DOE) | Simple formula, % |
|---|---|---|
| 2 | 9.5 | 10.5 |
| 3 | 15.0 | 16.7 |
| 5 | 28.1 | 31.3 |
| 7 | 44.9 | 50.0 |
| 10 | 81.6 | 90.9 |
US DOE Steam Tip Sheet #4, natural gas. The simple formula overstates excess air slightly because it ignores the water vapor and CO₂ the fuel forms; it is close enough for routine trimming.
Air leaks distort the reading. Air drawn in through gaps in the casing, inspection doors or a leaking air preheater dilutes the flue gas after combustion, so the oxygen probe reads high while the furnace itself is short of air. Compare oxygen at the furnace outlet with oxygen at the stack; a large difference means leaks, not a burner setting.
Key takeaways
- Theoretical air: kg per kg of fuel.
- Excess air ensures complete combustion; beyond that it only carries heat away.
- Estimate excess air from flue gas O₂: .
- Set excess air just above where CO starts to rise; check for air leaks before trimming.