Which indicator best signals poor combustion efficiency in flue gas analysis?

• A. Low CO
• B. High CO2 with low O2
• C. Elevated CO levels or a combination of high CO with low CO2 or excessive excess air indicated by high O2 with low CO2
• D. No CO detected

Answer: (C)

When looking at flue gas, the clearest signal of poor combustion efficiency is what the combustion products reveal about how completely the fuel is burned. Carbon monoxide is a key indicator because it is formed when the flame isn’t hot enough or isn’t well mixed to fully oxidize carbon to carbon dioxide. So seeing elevated CO means combustion isn’t going to completion.

If CO is high and CO2 is low, that tells you most of the carbon isn’t being oxidized to CO2, which means inefficient combustion. It gets even more telling if there’s high O2 with low CO2: lots of excess air dilutes the flame and lowers the fraction of fuel carbon converted to CO2, again signaling poor efficiency. The combination of these patterns—high CO with low CO2, or high O2 with low CO2—best indicates that combustion efficiency is not being achieved.

In contrast, very low or no CO suggests little unburned carbon, and high CO2 with low O2 points to effective, efficient combustion.