Abstract: A bubbling fluid bed steam generator comprising: a reactor chamber having a lower combustion region and a freeboard region; heat exchange means for the circulation of a coolant disposed substantially throughout the freeboard region and along the walls of the reactor chamber; discharge conduit disposed near the top of the reactor chamber for the discharge of flue gas containing entrained solid particles therein; a particle separator connected to the discharge conduit for separating the solid particles from the discharged flue gas, the solid particles being returned to the lower combustion region of the reactor chamber via a recycle port; means for introducing a carbonaceous material to the lower combustion region of the reactor chamber; primary inlet means for introducing a fluidizing gas disposed at the bottom of the reactor chamber; and secondary inlet means for introducing a fluidizing gas disposed above the recycle port, wherein the improvement is characterized by: the lower combustion region of the reacto