Optimization of Combustion Processes - Comparison of Excess Air-Fuel

Optimization of Combustion Processes - Comparison of Excess Air-Fuel
DUNIAPEMBANGKITLISTRIK.COM - To find out the efficient operation of the boiler, the operator must understand what the combustion process is like.

Good combustion will always require the right combination of fuel and oxygen to produce products in the form of heat energy, carbon dioxide, water vapor, nitrogen and other gases (other than oxygen).

In theory there is a specific combustion process setting to determine the ratio between oxygen and fuel so that a perfect combustion process occurs, but actually it will never happen ideally.

Therefore, in order for the fuel to burn completely, a higher amount of air is needed compared to air in ideal conditions (theoretical air), and the amount of air that is known as excess air.

Excess water can be calculated through the following equation:

Excess Air = 100 x (20.9%) / (20.9% – O2%) – 100%

Where O2 is the element of Oxygen measured at the boiler outlet (flue gas outlet).

Normally the percentage of excess water in a boiler operation with coal fuel ranges from 15% to 20%, oil-fired boilers 10% - 20%, natural gas 5% - 10%, while turbine gas requires very high excess water that is up to 300%.

Here is a comparison table of excess water needed in boilers with different types of fuel.

Jenis Bahan Bakar
Excess of Air (%)
Coke oven gas
Natural Gas
Coal, pulverized
Coal, stoker
Oil (No. 2 and No. 6)
10 to 20
Semi anthracite, hand firing
70 to 20
Semi anthracite, with stoker
40 to 70
Semi anthracite, with travelling grate
30 to 60

To determine how much excess water will be needed in the combustion process, it can be done by calculating the stoichiometric ratio between air and fuel, or better known as stoichiometric combustion.

During stoichiometry combustion there is a process of mixing chemical elements between air and fuel that produces H, SOx, NOx, CO2, etc., so that by paying attention to this it can be determined how much chemical elements are produced in the combustion process and can be determined whether the ratio of air to fuel is suitable or not.

If there is a mismatch between the amount of air supplied with the fuel transferred into the burner, an unburned fuel, soot, smoke and carbon monoxide will come out through the stack.

Resulting in heat transfer in the boiler will be hampered, the occurrence of pollution, combustion efficiency will be low, fire patterns become unstable and cause the potential for explosion. To avoid inefficient processes and unsafe conditions, the boiler is normally operated at a certain level of excess water.

This level of excess air will also be a protection from the combustion oxygen element that is too low due to differences in the composition of the fuel and also the poor operation of the air and fuel control systems.

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