PURPOSE: To obtain coal and combustion coal of high grade at a low cost, by a method wherein the collected fly ash is recirculated, and unburnt carbon contained is burnt and an ammonia compound is decomposed by the heat of a combustion device.

CONSTITUTION: Fly ash, collected by a dust collector 4, is supplied to an air flow convey line 14 through a rotary feeder 25 and a fly ash discharge line 13. Thereafter, the fly ash is conveyed to a cyclone separator 15 by means of boiler exhaust gas supplied through a line 20. The separated fly ash is covneyed through a fly ash line 17 to a hopper part 1a of a boiler 1. The fly ash, discharged in a furnace from a discharge part 30, flows down over a draft plate 36 by means of fluidized gas such as an air A', combustion exhaust gas. During such passage, a unburnt content is burnt and an ammonia compound is thermally decomposed by the heat in a furnace.