Abstract: A grate assembly for a fluidized bed boiler comprises a number of parallel sparge pipes or the like extending side-by-side in a substantially horizontal plane and provided with elements for supplying fluidizing air from within the sparge pipes or the like into a combustion chamber located above the grate assembly. The discharge of some of the fluidized bed materials is effected through an aperture system formed between the adjacent sparge pipes into a receiver unit fitted below the grate assembly. The sparge pipes are provided with a cooling medium circulation by means of a duct assembly.