After the coal is fed to the hearth, it will be quickly mixed with the fluidized scorching hot materials. Firstly water is evaporated, volatile matter is separated and is burnt, at last the carbon is burnt, therefore the burning time of the coal with higher carbon content will be longer. The coal of large particle size will be concentrated in the dense-phase zone of the combustion chamber and will be discharged from the bottom of the boiler after it becomes slag. The coal of fine particle size will rise and burn and will be taken away from the hearth and will enter the cyclone separator to be collected by the returns feeder and will be sent to the hearth for secondary combustion. The continuous process above mentioned forms fluidizing and circulating burning, thus making gas, dust and steam full of the whole hearth and accomplishing convection and the radiation transferring to the heating surface.