Abstract: Fluidised bed combustor comprising a housing (10) and an air diffuser bed support arrangement (12, 14, 15, 16) disposed in the housing (10) to support and fluidise a bed of material (11) in the housing (10) in such a manner that there is formed in the bed (11), a combustion zone (17) in which material is burned. A feed zone (19) into which material of fuel to be burned may be fed and mixed with the material of the bed (11), an ash segregation zone (20) in which ash resulting from combustion may be at least partially separated from the bed material, and in which the diffuser (13, 14, 15, 16) is arranged to cause the bed material to circulate in operation from the combustion zone (17), through the feed zone (19), through the ash segregation zone (20) and back into the combustion zone (17). In a second mode a circulation takes place through the combustion zone (17), a drive zone (18) over and onto the surface (26) of the bed (11), partly into the feed zone (19) and partly back into the combustion zone (17).