Abstract: In a fluidized boiler system, the rate of fuel flow and bed depth are simultaneously controlled in response to variations in a load demand signal representing the need for steam output from the system. When the load demand changes, the rate of fuel flow is varied accordingly to provide a change in the bed temperature to thus provide a rapid response to the change in the demand signal. At the same time, the system changes the rate of flow of limestone to the bed and the rate of removal of spent particulate material of the bed to change the bed depth to respond more slowly to the change in the demand signal. As the depth of the fluidized bed approaches a value corresponding to the demand signal, the temperature of the bed will change back toward a median value.