To remove smoke soot and effectively utilize it by a method wherein a dust collector is arranged at a stage followed by an air preheater, smoke soot is collected, and returned to a boiler and a carbon content contained therein is caused to effect recombustion.

Combustion exhaust gas generated in a boiler 1 contains smoke soot removed by a dust collector 3 and the smoke soot is returned to the boiler 1 for recombustion. An electric dust collector and a cyclone are used as the dust collector 3. Combustion exhaust gas flowing out from the dust collector 3 heats air supplied to the boiler 1 for combustion by a blower 6 and the combustion exhaust gas itself is cooled and fed to an exhaust gas treating device 5. The exhaust gas treating device is provided with a desulfurization device 5 and a chimney and smoke soot is discharged to the atmosphere. Since the carbon component of smoke soot returned to the boiler 1 is reburnt, combustion efficiency is improved. Further, NOx in exhaust gas is reduced and the generation of NOx is reduced. Further, when the generation of N0x is reduced by a denitration device, an amount of using ammonia is decreased.