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Engineering ToolBox > Combustion Efficiency and Excess Air
To ensure complete combustion of the fuel used, combustion chambers are supplied with excess air. Excess air increase the amount of oxygen and the probability of combustion of all fuel.
The combustion efficiency will increase with increased excess air, until the heat loss in the excess air is larger than than the heat provided by more efficient combustion.
Typical excess air to achieve highest efficiency for different fuels are
Carbon dioxide - CO2 - is a product of the combustion and the content in the flue gas is an important indication of the combustion efficiency.
An optimal content of carbon dioxide - CO2 - after combustion is approximately 10% for natural gas, and approximately 13% for light oils.
Normal combustion efficiencies for natural gas at different amounts of excess air and flue gas temperatures are indicated below:
| Combustion Efficiency (%) | ||||||
| Excess % | Flue Gas Temperature (oF) | |||||
| Air | Oxygen | 200 | 300 | 400 | 500 | 600 |
| 9.5 | 2.0 | 85.4 | 83.1 | 80.8 | 78.4 | 76.0 |
| 15 | 3.0 | 85.2 | 82.8 | 80.4 | 77.9 | 75.4 |
| 28.1 | 5.0 | 84.7 | 82.1 | 79.5 | 76.7 | 74.0 |
| 44.9 | 7.0 | 84.1 | 81.2 | 78.2 | 75.2 | 72.1 |
| 81.6 | 10.0 | 82.8 | 79.3 | 75.6 | 71.9 | 68.2 |
The relationship between temperature difference flue gas and supply air, CO2 concentration in the flue gas, and the efficiency loss in the flue gas combustion oil, is expressed in the diagram below.
