PURPOSE: To reduce necessary quantity of heat to achieve the miniaturization of an installation and the lowering of temp., by decomposing an ammonia compound adsorbed by fly ash under heating in a heating fluidized layer using steam from a boiler as a heating source.

CONSTITUTION: NH3 gas 11 is injected into the exhaust gas of a coal fired boiler 1 to be guided to a denitration apparatus 2. Fly ash contg. leaked NH3 adsorbed from the exhaust gas after denitration is recovered by an electric dust collector 4. The collected fly ash is transported by utilizing the exhaust boiler gas prior to NH3 injection to be sent to a gas-solid separator 3. The separated fly ash is sent to a heat decomposition furnace 6 to decompose an ammonia compound under heating. The exhaust gas 18 from a fluidized layer is returned to the upstream of the denitration apparatus 2 through a gas-solid separator 7 and the fly ash recovered from the gas-solid separator is sent to a cooler 8 along with the ash 21 discharged from the fluidized layer to be discharged after cooling.