The amount of air needed to completely burn a certain amount of fuel can be calculated theoretically using the basic principles of stoichiometry (see following article). In other words, if every fuel molecule precisely contact with oxygen in the air and reacted, then the entire fuel must be burned, and there will not be a certain amount of excess air at the exhaust. But in reality, it is impossible. Fuel molecules can not 100% met directly with the oxygen. It takes some excess air to make sure all the fuel molecules can burn completely. This is what we know as excess air (read the following article).
What is the right amount of excess air for a burning process?
Determining the amount of excess air in the boiler burning process depends on several main factors such as fuel type, boiler design, burner design, and boiler load. Generally, coal fired boilers use excess air as much as 15% to 30%. For boilers with gas or petroleum as its fuel, requires less excess amount of water. Gas-fired boilers require excess air of 5% to 10%, while petroleum-fired boilers require excess air of 3% to 15%. This condition indicates that the gas and liquid phase fuels more easily mix and react with oxygen, compared to solid phase fuel.
How much the boiler load has a great impact on excess air. The design of the diagonal cross-section of the boiler’s combustion chamber should be able to bear the flow of gas flow when the boiler is in full load. The opposite condition occurs when the boiler load is lower, where the flow of gas decreases so that mixing the fuel with air becomes more difficult. Therefore, when the boiler is below full load, the amount of excess air required increases to ensure a complete combustion. In coal-fired boilers for example, at 50% load, takes two times excess air than when the load is 100%.
Although the excess air is important to ensure complete combustion, excess air has a negative impact on boiler efficiency. The higher amount of excess air will make the heat energy wasted following the exhaust gas. Therefore, in terms of efficiency, the amount of excess air should be kept as low as possible.
To keep the excess air stay at the optimum value, modern boilers equipped with sensors of amount of oxygen and carbon monoxide on the boiler exhaust side. Both of these parameters can be use to keep the excess air amount stay at the optimum level throughout the boiler operating time.
Excess Air Calculations
Let’s use the example of coal data at the previous article as follows:
Which the stoichiometric combustion reaction equation is as follows:
CH0,74O0,061N0,018S0,026 + 1,211(O2 + 3,762N2) → CO2 + 0,37H2O + 0,026SO2 + 4,565N2
Furthermore, if determined boiler using excess air of 15%, then the chemical reaction of combustion becomes as follows:
CH0,74O0,061N0,018S0,026 + 1,393(O2 + 3,762N2) → CO2 + 0,37H2O + 0,026SO2 + 5,24N2 + 0,212O2
From the above reaction we can calculate the percentage of excess oxygen in boiler flue gas:
O2 excess =
O2 excess = 3,096%
While the air-fuel ratio corrected to:
AFR = 12,926