Gas Heat ESCO Practice Exam

If the exact amount of air is supplied and perfect combustion occurs, in a natural gas furnace, what percentage of CO2 content will be present in the flue gases?

1. 8.2% - 8.7%

2. 9.5% - 10.5%

3. 11.7% - 12.2%

4. 13.5% - 14.0%

The correct answer is: 11.7% - 12.2%

In a natural gas furnace, when perfect combustion occurs with the exact amount of air supplied, the primary products of combustion are carbon dioxide (CO2) and water vapor. The theoretical yield of CO2 from natural gas, which primarily consists of methane (CH4), is about 10% to 11% under optimal conditions. The presence of around 11.7% to 12.2% CO2 content in flue gases indicates a highly efficient combustion process where all the available carbon in the natural gas has been converted to CO2, and there is no unburned fuel or formation of carbon monoxide (CO). This efficiency is what leads to the correct range in this option. In contrast, the other ranges either fall short or exceed the expected CO2 concentration under ideal combustion conditions, suggesting a less complete combustion or variations in fuel composition and operational efficiency. Thus, the percentage of CO2 content in the flue gases directly correlates with the efficiency of combustion and the balance of air-fuel ratio, emphasizing the accuracy of the selected range.