The bed depth in a power plant burning sulfur-containing fuel in a fluidized bed of material which constitutes a sulfur absorbent in a combustion chamber is controlled on the basis of plant power output. A plurality of sensors connected to the combustion chamber determine the existing bed depth. A bed material mill grinds material removed from the bed prior to returning it to the bed, the rate at which the mill operates being controlled by the extent to which the actual bed depth differs from the desired bed depth. The mill grinds some bed material to small enough size for it to be carried from the combustion chamber with the combustion gases.