PURPOSE: To enhance the efficiency of desulfurization and to reduce the consumption amount of limestone, in carrying out in-furnace desulfurization by limestone, by a method wherein coal ash recovered from a dust collector succeeded to a boiler is again blown into a furnace while humidified to restore the activity of the recovered coal ash.

CONSTITUTION: Limistone is directly blown into a fluidized bed type coal combustion boiler 1 to carry out in-furnace desulfurization. The exhaust gas from the boiler is introduced into a multi-cyclone type dust collector 2 to recover coal ash consisting of a reaction product and unreacted lime and discharged out of a system through an air preheater as a clean gas. Among the coal ash collected in the dust collector, one from which ash to be disposed is removed is introduced into a humidifier 4 to convert the unreacted lime to slaked lime by humidication and the formed slaked lime is blown into the fluidized bed type coal combustion boiler 1 along with combustion air to carry out the in-furnace desulfurization again.