The decomposition of sulfates present within bottom ashes produced during operation of a fluidized bed combustor is not complete under continuously reducing conditions (2% CO) at 950 degrees C. The product CaS accumulates within the particles. Under alternatively reducing and oxidizing conditions, a complete sulfur release can be obtained. The reduction product CaS is oxidized to SO2 during the oxidizing step. This prevents sulfate accumulation within the particles during the reducing step. Under these cycling conditions, the rate of sulfate decomposition is increased by using a high CO concentration during the reduction phase. The duration of the reduction phase must be optimized to avoid the formation of too much CaS within the particle. (C) 2007 Published by Elsevier Ltd.