at least two fluidized bed sections positioned in series along a solids flow path, each section containing fluidizing means, the first section along the solids path being separated from a following section with a threshold, the first section containing thermocouple means for measuring a bed temperature T₁ in the vicinity of the fluidizing means and at a higher elevation T₂ within the fluidized bed, and means for removing oversized bed material from the first section when a temperature difference (T₂−T₁) between stagnant bed material and fluidized bed material above, indicative of an accumulation of bed material in a lower part of the first section, is detected by the thermocouple means.