Calculation of Excess Air Combustion Process

Calculation of Exces

The amount of air required to completely burn a certain amount of fuel can be calculated theoretically using the basic principles of stoichiometry In other words, if every molecule of fuel proper contact with the chemical reaction of oxygen in the air, then surely the overall fuel burn, and it will not be a definite amount of excess air ( excess ) were wasted. However, the reality is that this is not the case. The fuel molecules are not able to 100% meet directly with the oxygen it needs to burn. It takes a certain amount of excess air to ensure that all the fuel molecules can burn completely. 

What is the right amount of excess air for a combustion process?

Determining the amount of excess air combustion reaction in the boiler depends on several main factors such as the type of fuel, boiler design, burner design, and boiler load. Generally, coal-fired boilers use 15% to 30% excess air . For boilers with gas or petroleum fuel, they require a smaller amount of excess air . Gas-fired boilers require 5% to 10% excess air , while petroleum-fired boilers require 3% to 15% excess air . This condition indicates that gaseous and liquid phase fuels are easier to mix and react with oxygen, compared to solid phase fuels.

How big the boiler load has a big impact on the need for excess air . The diagonal design size of the boiler combustion chamber must be able to bear the gas flow discharge when the boiler is at full load. The opposite condition occurs when the boiler load is lower, where the gas flow rate decreases so that the mixing of fuel with air becomes more difficult. Therefore, when the boiler load is under full load, the amount of excess air needed becomes more to ensure a complete combustion process occurs. In a coal boiler, for example, at 50% load, twice as much excess air is required than when the load is 100%.

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Although excess air is important to ensure complete combustion, some excess air has a negative impact on boiler efficiency. The higher the amount of excess air, the more heat energy from combustion is wasted following the exhaust gases. Therefore, from an efficiency perspective , the amount of excess air must be kept as low as possible.

To keep excess air at its optimum, modern boilers are equipped with sensors for the amount of oxygen and carbon monoxide on the exhaust side of the boiler. These two parameters can be the basis for maintaining the amount of excess air to remain at the optimum level throughout the boiler operational time.

Excess air calculation

Where the stoichiometric combustion reaction equation is as follows:

CH The 0.74 o 0.061 N 0,018 S 0026 + 1,211 (o 2 + 3,762N 2 ) → CO 2 + 0,37H 2 o + 0,026SO 2 + 4,565N 2

Furthermore, if it is determined that the boiler uses an excess of 15% air, then the chemical reaction of combustion becomes as follows:

CH The 0.74 o 0.061 N 0,018 S 0026 + 1,393 (o 2 + 3,762N 2 ) → CO 2 + 0,37H 2 o + 0,026SO 2 + 5,24N 2 + 0,212O 2

From the chemical equation above, we can calculate the percentage of excess oxygen in the boiler exhaust gas:

O2 excess = \dfrac {0.212}{1+0.37+0.26+5,24+0.212}\times 100\%

O2 excess = 3,096%

Meanwhile, the air-fuel ratio was corrected to:

AFR = \dfrac {1,393\left( 32+3,762\times 28\right)}{12+1\times 0,74+16\times 0,061+14\times 0,018+32\times 0,026}

AFR = 12,926

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