How do air preheaters improve boiler efficiency?

• A. By transferring heat from flue gas to incoming air, lowering fuel use and improving combustion efficiency.
• B. By removing sulfur from flue gas.
• C. By increasing boiler pressure.
• D. By cooling feedwater before it enters the boiler.

Answer: (A)

Air preheaters recover heat from hot flue gas and transfer it to the incoming combustion air. By warming the air before it enters the furnace, the furnace needs less energy to bring the air to the needed flame temperature, so less fuel is required for the same steam output. Warmer air also promotes more complete combustion, boosting combustion efficiency. In addition, cooling the flue gas reduces stack losses, which further increases overall boiler efficiency. The other options describe functions not performed by air preheaters: removing sulfur from flue gas is done by scrubbers, not air preheaters; increasing boiler pressure is achieved through the boiler system design, not preheating air; and cooling feedwater would waste energy—feedwater is typically heated by an economizer, not cooled.