Which gas components are commonly analyzed in a flue gas analysis to assess combustion?

• A. NOx alone.
• B. CO2 and O2 levels only.
• C. CO2, O2, CO, and NOx; used to judge excess air and emissions.
• D. Humidity and moisture content.

Answer: (C)

Measuring a set of gas components in the flue provides a complete picture of how the combustion is performing and what emissions it’s producing. CO2 and O2 are the foundational readings: CO2 indicates how completely the fuel has been burned, with higher CO2 generally pointing to better fuel utilization, while O2 shows how much excess air remains in the flue. The amount of excess air can be inferred from the O2 level—more O2 means more excess air, which affects efficiency and heat transfer.

CO reveals incomplete combustion. If CO appears in the flue, it signals that some fuel isn’t burning fully, often due to poor fuel-air mixing or insufficient residence time. NOx provides information about emissions, since these oxides form more readily at higher temperatures and are a key pollutant to monitor and control.

Humidity or moisture content isn’t a primary indicator of combustion performance or emissions, so it isn’t typically used for this analysis. By combining CO2, O2, CO, and NOx, you get a practical view of both how completely the fuel is burning and how clean the combustion is, which is why this set is the best choice.