Abstract: A fluidized bed combustor for burning solid fuel or dirty liquid fuels comprised of a bubbling fluid bed with tubes in the bed, an isothermal, non-heat extractive freeboard/recycle cyclone flow path, a hot recycle cyclone, a connective heat exchanger and a particle filter. The bubbling fluid bed is operated at low superficial velocities of 0.5 to 7 ft/sec and is composed of fine particulates 45 to 2000 microns in diameter with up to 40% less than 200 microns. Fines elutriated from the bed are isothermally recycled back to the bed resulting in high combustion efficiency and good sulfur oxide suppression from sorbents contained in the bed. The material recycled in one hour is equivalent to twice the weight of the bed. Ammonia injected upstream of the recycle cyclone suppresses nitrogen oxides with high efficiency because of the excellent mixing in the cyclone. The heat transfer coefficient on the tubes in the bed is increased at least 2 to 4 times because of the fine particulate in the bed.