Combustion Efficiency

Combustion Efficiency

Knowing the correct air to fuel ratio that achieves complete combustion, and the expected ratio of the products of combustion, allows boiler operators to calculate how efficiently combustion has occurred.

Combustion efficiency tables contain the information required to calculate combustion efficiency. Information provided in a combustion efficiency table typically includes:

• The % of excess air.
• The % of oxygen in the gases of combustion.
• The % of CO2 in the gases of combustion.
• The temperature of the air supplied for combustion.
• The temperature of the gases of combustion when measured from the stack.

Notes

The gases of combustion are also referred to as exhaust gases or flue gases. The term stack is used interchangeably with the word chimney.

A typical combustion efficiency table is shown below.

Natural Gas Combustion Efficiency Table (assuming complete combustion)

Rather than explaining the intricacies of the table, two examples are provided.

Example 1

A boiler has a stack temperature of 500°F. The boiler room temperature is 100°F. The air for combustion in the boiler is taken directly from the space surrounding the boiler, so we know the temperature of air supplied for combustion is the same as the boiler room temperature (100°F). The % of carbon dioxide (CO2) in the gases of combustion is measured at the stack and has a value of 10.6%.

• Net stack temperature is 500°F – 100°F = 400°F.
• % CO2 is 10.6%.

Checking the results with the combustion efficiency table indicates that the boiler is operating at 80.8% efficiency.

Example 2

A boiler has a stack temperature of 420°F and a boiler room temperature of 80°F. The % of oxygen in the gases of combustion is 7.2%.

• Net stack temperature is 420°F – 80°F = 340°F.
• % Oxygen is 7.2%.

Checking the results with the combustion efficiency table indicates that the boiler is operating at approximately 81.2% efficiency.

If we round the net stack temperature to 350°F, we can estimate the efficiency value using the table. The actual efficiency is likely to be between 81.2% and 78.2% because these are the two values given between 300°F and 400°F. 

For example:

1. Combustion efficiency value at 300°F and 7% oxygen is 81.2%
2. Combustion efficiency value at 400°F and 7% oxygen is 78.2%
3. 81.2% - 78.2% = 3%
4. So, there is a 3% reduction in efficiency if the net stack temperature changes from 300°F to 400°F.
5. Knowing there is a 3% reduction in combustion efficiency for a 100°F change on the net stack temperature, we also know that a 50°F change will correspond to a 1.5% reduction in combustion efficiency (because 50°F is half of 100°F and correspondingly 1.5% is half of 3%).
6. The final combustion efficiency is thus calculated as 82.2% - 1.5% = 79.7%.

Notice that combustion efficiency reduces as the net stack temperature increases. A high net stack temperature indicates that less of the heat generated by combustion was transferred to the water, whereas a low net stack temperature indicates more heat was transferred to the water. High exhaust gas temperatures represent a waste of energy and money.

Example 3

An exhaust gas temperature of 600°F with a combustion air temperature of 100°F, gives a net stack temperature of 500°F.

• 600°F -100°F = 500°F

An exhaust gas temperature of 400°F with a combustion air temperature of 100°F, gives a net stack temperature of 300°F.

• 400°F -100°F = 300°F

Notice in our example that all combustion efficiency values relating to a low net stack temperature, are higher than those possible for a high net stack temperature.

Additional Resources

https://en.wikipedia.org/wiki/Engine_efficiency

https://www.trutechtools.com/Understanding-Combustion-Efficiency_c_261.html

https://www.nrel.gov/docs/fy02osti/31496.pdf

https://www.sciencedirect.com/topics/earth-and-planetary-sciences/combustion-efficiency