The differential pressure is controlled by the slag discharged from the boiler bottom. During normal operation, the bed pressure shall be controlled between 8500Pa-9500Pa through the adjustment to the rotating speed of the cold slag machine. If the differential pressure of the bed of materials is too high, the fluidizing speed will be reduced and the output of boiler will be lowered. The bed temperature rise will cause coking in combustion chamber in severe case. If the differential pressure of the bed of materials is too low, the fluidizing will not even, and the local of the bed of materials will be empty by blowing and the bed temperature difference will be slightly bigger, in sever case, the fire of the boiler will go out.