PURPOSE: To remove economically NH3 from the fly ash removed from waste gas by transporting and preheating the fly ash with part of the waste gas prior to feeding of NH3 then decomposing the adsorbed NH3 compd. by heating.

CONSTITUTION: Gaseous NH3 is fed to the waste gas from a coal firing boiler 1, whereafter the gas is conducted to a dry denitration device 2. The denitrated waste gas is subjected to a heat exchange with the combustion air of the boiler with an air preheater 3, then the fly ash in the gas are captured with a dust collector 4. The captured fly ash are transported by the waste gas of the boiler prior to feeding of NH3 and is preheated simultaneously. The preheated gas is fed to gas-solid separator 5 to separate the fly ash. The separated gas is returned to the upstream of the device 2 and the separated fly ash is fed to a thermal decomposition furnace 6. The NH3 compd. adsorbed to the fly ash is decomposed by heating by the steam from the boiler 1 in the furnace 6 and the fly ash returned in a gas-solid separator 7 is fed to a cooler 8, from which the air contg. NH3 and SOx is returned to the upstream of the device 2.