Combustion and Boiler Efficiency
Air–fuel ratio, excess air, flue gas losses, heating values, condensing boilers and boiler efficiency.
Combustion chemistry
Combustion is rapid oxidation. The carbon and hydrogen in a fuel combine with oxygen to give carbon dioxide and water and release heat. For methane, the main part of natural gas:
CH₄ + 2 O₂ → CO₂ + 2 H₂O
Worked example: the air needed to burn methane
- Molar masses: CH₄ 16.043, O₂ 31.998, CO₂ 44.009, H₂O 18.015 kg/kmol.
- Oxygen per kg of fuel = 2 × 31.998 ÷ 16.043 = 3.989 kg. Air is 23.14 percent oxygen by mass, so the stoichiometric air–fuel ratio is 17.24 kg air per kg fuel (about 9.5 m³ of air per m³ of gas).
- Products per kg: CO₂ 2.74, H₂O 2.25, N₂ 13.25 kg. Total 18.24 kg against 18.24 kg in: mass is conserved (difference 0.000).
- Burning 1 kg of methane produces 2.74 kg of CO₂; this ratio is how fuel use converts into emissions.
| Fuel | Stoichiometric air (kg/kg) | LHV (MJ/kg) | CO₂ per kg fuel (kg) |
|---|---|---|---|
| Methane (natural gas) | 17.2 | 50.0 | 2.75 |
| Propane | 15.7 | 46.4 | 3.00 |
| Petrol | 14.7 | 43.5 | 3.1 |
| Diesel | 14.5 | 42.8 | 3.17 |
| Coal (typical) | 10.5 | 25 to 30 | 2.4 to 2.8 |
| Wood (dry) | 6.0 | 16 to 18 | 1.8 (biogenic) |
| Hydrogen | 34.3 | 120 | 0 |
Air, excess air and oxygen in the flue
Burners use more air than the chemical minimum so that all the fuel burns. Excess air is the percentage above stoichiometric; equivalence ratio λ = actual ÷ stoichiometric air = 1 + excess. Too little air leaves carbon monoxide and soot; too much carries heat up the stack.
Flue gas analyzers read oxygen (and CO). For gas, the excess air follows from the oxygen reading: excess ≈ O₂ ÷ (21 − O₂), in percent of volume. For example, 4 percent O₂ in the dry flue means about 24 percent excess air. A well-set gas burner runs at 2 to 3 percent O₂ (10 to 15 percent excess air); oil burners need 3 to 5 percent.
Heating values: LHV and HHV
The higher heating value (HHV) counts the latent heat of the water vapor formed; the lower heating value (LHV) does not. For methane the LHV is about 50.0 MJ/kg and the HHV 55.5 MJ/kg, a difference of 5.5 MJ/kg, matching the 2.25 kg of water condensing at 2.44 MJ/kg = 5.5 MJ. North American utilities quote HHV; European engineers quote LHV. A boiler that is “109 percent efficient” is quoting LHV for a condensing unit; on an HHV basis nothing exceeds 100 percent.
Boiler losses and efficiency
Efficiency is useful heat to the water divided by the fuel energy. The direct method divides steam or hot-water output by fuel input; the indirect method subtracts the losses from 100 percent and is more accurate. The losses are:
- Dry flue gas loss: heat carried up the stack, ṁflue cp (Tflue − Tair). The largest loss, 5 to 15 percent.
- Latent heat of the water formed from the hydrogen in the fuel (about 11 percent of the HHV for gas; recovered only by condensing).
- Unburned fuel / CO, small if the burner is tuned.
- Radiation and convection from the shell, 0.5 to 2 percent (higher at low load).
- Blowdown (steam boilers): hot water thrown away to control dissolved solids, 1 to 5 percent.
The rule of thumb: each 20 K reduction in flue temperature saves about 1 percent of fuel, and each 10 percentage points of excess air costs about 0.5 percent. Lower the stack temperature with an economiser, which preheats the feed water with flue gas, but keep it above the acid dew point (about 120 to 150 °C for oil and coal, around 55 °C for gas) unless the boiler is built to condense.
Worked example: a 1 MW boiler
Worked example: a natural-gas hot-water boiler
A boiler delivers 1 000 kW of heat with 20 percent excess air, a flue temperature of 180 °C and an air temperature of 20 °C.
- Air per kg of fuel = 17.24 × 1.20 = 20.69 kg; flue gas = 21.69 kg per kg of fuel.
- Dry flue loss = 21.69 × 1.12 × (180 − 20) ÷ 50 000 = 7.8 percent; add 1 percent for radiation.
- Efficiency = 91.2 percent (LHV).
- Fuel input = 1 000 ÷ 0.912 = 1 096 kW, a gas flow of 78.9 kg/h, about 116 m³/h at 0.68 kg/m³.
- Run at stoichiometric air with the same flue: efficiency would be 92.5 percent; at 60 percent excess air it falls to 88.8 percent.
- Cooling the flue by 40 K (to 140 °C) with an economiser raises the efficiency from 91.2 to 93.2 percent, a fuel saving of 2.1 percent.
Condensing boilers
A condensing boiler cools the flue below its water dew point (about 55 °C for gas), so the 2.25 kg of water vapor per kg of fuel condenses and returns 5.5 MJ, 11 percent of the LHV. The return water must be below about 55 °C to condense: so condensing boilers shine with low-temperature heating (underfloor, oversized radiators) and less with high-temperature systems. At an 80 °C flue the sensible loss alone would be 2.9 percent; with condensation the LHV efficiency exceeds 100 percent (typically 105 to 109), or 95 to 98 percent on an HHV basis. The condensate is acidic (pH 3 to 4) and must drain through plastic pipe, often via a neutralizer.
Safety and emissions
- Carbon monoxide from incomplete combustion is invisible and lethal; fit alarms and keep flues clear.
- Fuel explosions: burners need purge, flame-failure and gas-valve safety systems to a recognized code (EN 746, NFPA 85, 86). Never override them.
- NOx forms at high flame temperature; low-NOx burners, flue-gas recirculation and staged air cut it. CO₂ follows the fuel (see the table).
- Boiler explosions from low water or blocked safety valves: test low-water cut-outs and safety valves on schedule and use a competent person.
Related
- Previous: Refrigeration and Heat Pumps. Next: Steam Systems, Traps and Troubleshooting.
- Steam Power Cycles: Rankine for the steam cycle that the boiler feeds.
- Heat Transfer for the heat exchange in the boiler and economiser.
- Thermodynamics and Steam: the whole guide.
FAQ
Why not burn with exactly the right amount of air?
Mixing is never perfect, so some excess air is needed to burn all the fuel. The tuned burner finds the balance between CO (too little air) and stack loss (too much).
How do I read the efficiency from a flue analyzer?
The analyzer measures O₂ (or CO₂), CO and flue temperature, and calculates the stack loss and efficiency using the fuel’s constants. It is the indirect method.
What is the difference between HHV and LHV efficiency?
The same boiler shows a number about 10 percent lower on an HHV basis for natural gas (multiply by 50.0 ÷ 55.5 = 0.901).
Where does the CO₂ go?
Up the stack: about 2.75 kg per kg of methane, or about 0.2 kg per kWh of gas energy.
References
- Babcock and Wilcox, Steam: Its Generation and Use, 42nd edition.
- ASME PTC 4, Fired Steam Generators; EN 12952-15 and EN 303-5 (boiler acceptance tests).
- Çengel, Y. A. and Boles, M. A., Thermodynamics: An Engineering Approach, McGraw-Hill (combustion).
- US DOE, Improving Steam System Performance: A Sourcebook for Industry.
Last reviewed: October 2026. Heating values and constants are typical for natural gas.