Abstract: A method of controlling the particle size distribution in a fluidized bed reactor in which a primary bed of particulate material having a varying size and including fuel is established. Air is introduced into the bed to fluidize the material and promote the combustion of the fuel. A portion of the particulate material is discharged from the bed and into a secondary bed. Air is introduced to the secondary bed at a velocity sufficient to fluidize the particulate material and entrain some of the finer portions of the particulate material. The air and the entrained finer material from the secondary bed are passed back to the primary bed and the velocity of the air introduced to the secondary bed is varied to vary the amount of the finer material that is entrained and thus control the size of the material in the primary bed.