Fluidized bed combustion system having a heat exchanger in the upper furnace

A fluidized bed combustion system including a furnace for receiving combustible material and comprising a floor and four upright walls extending perpendicular to said floor and formed at least in part by a plurality of tubes, means for introducing air in the lower portion of said furnace to establish a fluidized bed of said material including fuel in said lower portion of said furnace, a separator disposed adjacent one wall of said furnace, a furnace interior duct in the upper portion of said furnace interior and adjacent said one wall for receiving a mixture of flue gases from the combustion of said fuel and entrained solids from said fluidized bed, an exterior duct exterior to said furnace and means connecting said furnace interior duct and said separating means for passing said mixture of flue gases and entrained solids to said separating means for separating said solids from said flue gases, a plurality of heat exchange tubes disposed in said upper portion of said furnace interior and adjacent said furnace interior duct for removing heat from a portion of said mixture of flue gases and entrained solids, and means for passing water through said tubes to heat said water, said mixture passing from the lower portion of said furnace upwardly in said furnace, and into said furnace interior duct for passage to said separator.