PURPOSE: To permit efficient recombustion as well as the reduction of grain load, power loss and wearing of a device by providing a fly ash silo capable of recirculation and throwing of ash, a grain size of ash classifying device, a forced air sending blower for classification and a fine granular ash circulating pipe.

CONSTITUTION: Ash in the ash recirculation side hopper unit 13 of a fly ash silo 9 is supplied into a grain size of ash classifying device 20 and forms bubbling fluidized layer 23 by air forcibly sent from a blower 25 whereby fine granular ash is levitated and rough granular ash is descended thereby permitting the classification of both of them. Accordingly, only the levitated fine granular ash is returned into a furnace from a fine granular ash circulating pipe 24 by air transportation. On the other hand, the amount of air for fluidizing is regulated and air speed in a device is changed arbitrarily as desired whereby the grain size of ash to be returned into the furnace can be controlled. According to this method, only fine granular ash, containing much unburnt constituents, can be returned into the furnace to permit efficient recombustion while the grain load, power loss and abrasion of the device can be reduced.