Question 479Steam Plants - Assistant Engineer

The correct answer is D) excess air is increasing. As the percentage of CO2 in the stack gas decreases, it indicates that there is more excess air being introduced into the combustion process. This excess air dilutes the stack gas, reducing the concentration of CO2. With more excess air, the fuel-to-air ratio is decreasing, which is the opposite of option A. The increased excess air does not necessarily mean the fuel is being burned with greater economy (option B). And the reduced CO2 concentration is not an indication of approaching secondary combustion (option C).