Abstract: At a fluidbed furnace comprising a combustion shaft, a particle separator, a particle return passage and designed according to conventional design criteria for obtaining a good combustion at moderate temperature, a destruction of laughing gas (N.sub.2 O), and complete combustion of possible unburnt particles in the combustion gases is brought about in a reactor passage at the entrance of which a combustion means is located. The reactor passage is moderately cooled, so the increase of temperature in the combustion gases is maintained substantially constant unto the first convection heating surface.