PURPOSE: To perform a superior denitration treatment by a method wherein flyashes recirculated from a dust collector to a slag-tap boiler are ignited to remove arsenic contained in the flyashes.

CONSTITUTION: Both coal and air are supplied into a furnace 2 of a slat-tap boiler 1, they are heated to cause a primary combustion of the coal, thereafter they are guided into a furnace 3 to perform a complete combustion. Exhausted gas or discharged gas is denitrated with a denitration device 4, the flyashes are collected by an electrical dust collector 5 and the remaining discharged gas is discharged into surrounding atmosphere. In turn, the flyashes collected at the electrical dust collector 5 are forcibly fed within a pressure feeding pipe 6, an incinerating treatment is carried out in an incinerator 7, arsenic contained in the flyashes is gasified and separated from the flyashes. The flyashes from which arsenic is removed are forcibly fed in the pressure feeding pipe 6 and then recirculated into the furnace 2 of the slag-tap boiler 1.