Abstract: In a fast fluidized bed reactor having, in a lower region, downwardly and inwardly inclined walls, a change in the direction of the wall surface or a ledge or protrusion is provided to change the direction of the particles flowing downwardly adjacent the wall for directing the particles inwardly away from the wall and into the region acted on by the nozzles providing the fluidizing gas. The change of direction, abutment or projection is preferably provided along the refractory wall surface at an elevation 200-1100 mm above the grid plate. In this manner, clogging of the nozzles is prevented and more efficient and effective fluidization and combustion processes occur.