Question 209Steam Plants - 1st Asst/Chief

The correct answer is D) incomplete combustion. High carbon monoxide (CO) content in the flue gases of a boiler indicates incomplete combustion. This is because incomplete combustion results in inefficient burning of the fuel, leading to the release of CO instead of the complete conversion to carbon dioxide (CO2). Proper combustion with sufficient air supply would result in lower CO levels and more complete conversion of the fuel. The other options are incorrect - A) complete combustion would result in low CO levels, B) a high carbon content fuel may contribute to CO formation but does not directly indicate it, and C) too much excess air would typically result in lower CO levels by providing more oxygen for complete combustion.