Fluidised beds

A fluidised bed combustor comprising a housing and an air diffuser bed support arrangement disposed in the housing to support and fluidise a bed of material in the housing, there being formed in the bed a combustion zone in which material to be burned is burned, a feed zone into which material or fuel to be burned may be fed and mixed with the material of the bed, an ash segregation zone in which ash resulting from combustion may be at least partially separated from the bed material, and in which a diffuser is arranged to cause the bed material to circulate in operation around a path extending from the combustion zone, through the feed zone, through the ash segregation zone, and back into the combustion zone, in which an ash collecting trough is provided below said diffuser to lie adjacent said path between said ash segregation zone and said combustion zone, in which a conduit leads from the bottom of said ash trough to the bottom of a generally vertical conduit extending outside the housing to above the surface of the bed, means being provided in the bottom of the vertical conduit to fluidise bed material and ash therein, in which a weir is provided in the vertical column over which ash material and any entrained bed material may be ejected, and in which in operation the height of the surface of the bed is controlled by the height of said weir and the material in the vertical conduit is fluidised to a sufficient extent to achieve flow of excess material over the weir.