PURPOSE:To simplify combustion control mechanism for protecting metal of reheater tube at the start of boiler by dividing a fluidized bed into two beds, disposing one of said beds on top of another in a vertical pressure vessel to separately control combustion in each fluidized bed. CONSTITUTION:An upper fluidized bed furnace (evaporator furnace) 1 and a lower fluidized bed furnace (superheater/reheater furnace) 2 are disposed, on one another, separately in a vertical pressure vessel 3, each of them being supported by a supporter beam 4 and supporter frame 5. A main body of each fluidized bed consists of furnace wall tubes 6, furnace wall back stays 7, connecting pipes 8, fluidized bed supporter device 9, outlet duct 10, lower wind box 11, coal, lime and air feed device 12, etc. And ancillary devices of the fluidized bed such as cyclones 13, ash storage bin for controlling bed height 14, etc., are disposed in the vessel 3. These devices are disposed in a suitable and efficient manner in a space within the vessel 3 to minimize the diameter of the vessel. Since combustion control can be separately performed, the combustion control mechanism for protecting the metal of reheater tube at the start of boiler can be simplified.
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