A heat exchanger is provided in an air duct between an air heater and a windbox of a combustion means such as a boiler. Dilution air is carried to the heat exchanger from a dilution air fan in a separate duct. The heat in the air from the air heater is transferred to the dilution air via the heat exchanger and the heated dilution air is sent to vaporize ammonia before the ammonia is introduced into an exhaust duct with a selective catalytic reduction (SCR) chamber. A bypass valve is provided around the heat exchanger for blending heated and unheated air to control the temperature of the dilution air.