A fluidized bed combustion chamber adaptable for use with relatively large gas turbines and the like, wherein a substantially spherically-shaped outer casing encloses an annular fluidized bed combustion chamber which is mounted on a double bottom air distribution box. A cleaning assembly is positioned within the fluidized bed and the entire combustion chamber is supported on a base plate which extends through the casing and into contact with the box. Compressed air enters the spherical casing, is directed through a pair of cooling gaps, is caused to enter the air distribution box and then passes through the fluidized bed.