What is the effect of excess air on boiler efficiency and emissions?

• A. Excess air increases stack losses and fuel consumption, lowering efficiency; it can also dilute flame and alter NOx and CO formation, potentially increasing emissions if not controlled.
• B. Excess air always improves efficiency.
• C. Excess air has no impact on emissions.
• D. Excess air reduces stack losses.

Answer: (D)

Excess air mainly lowers boiler efficiency by increasing stack losses. When more air than needed flows through the furnace, a larger portion of the heat from combustion ends up in the flue gas that leaves the stack rather than heating the water/steam. That extra hot exhaust carries energy away, so the useful energy transfer to the boiler decreases.

The flame is also diluted by the extra air, which lowers the flame temperature and alters combustion conditions. That change in temperature and mixing can shift the formation of pollutants: NOx and CO can be affected in different ways depending on how the combustion zone is controlled. If the air is in excess and not properly balanced, emissions can rise or become harder to control.

In short, excessive air tends to reduce efficiency due to higher heat losses in the stack and can complicate emissions control, especially if combustion is not carefully managed.