A furnace for carrying out combustion of refuse or for reclaiming particles contaminated with combustible material comprises a bed of particles which is fluidized by feeding air and gaseous fuel into the bed. The refuse or contaminated material is fed into the bed and it is heated by combustion of the fuel therein. Incombustible residue is discharged from the bed over a weir. The furnace includes a heat exchanger for extracting heat from a part of the bed which can be fluidized selectively.