The flue gas system consists of two air heaters. The combustion gas transfers its own heat to the water wall, the superheaters, the reheater and the economizer, then leaves the boiler to the flue gas ducts. The flue gas goes to the stack through the AH and is exhausted to the atmosphere.   Air pre-heater transfer heat from flue gas to air by means of a rotary matrix in which heat is absorbed by the heating elements passing through the hot gas stream and transferred to the combustion air stream.