For coal charring, a Circochar plant uses a variation of the circulating fluidized bed technology. This Outotec technology replaces the traditional nozzle grate arrangement with a central nozzle surrounded by an annular fluidized bed.
Heat required to char the coal is provided by the partial combustion of the pre-dried coal with combustion air injected into the CFB reactor via the central nozzle. The central nozzle allows the contacting of coal in the CFB reactor with the combustion air, thus making the use of a nozzle grate unnecessary. About 25 percent of the total fluidizing gas required for the CFB reactor is supplied from the fluidizing air blower. This gas is supplied to an annular fluidized bed using traditional fluidizing nozzles which circumvent the central nozzle. The main combustion air, along with the fluidizing air from the annular fluidized bed, serves as a transport gas to move the solids in the CFB reactor to the recycle cyclone.
Hot gases and solids leaving the fluidized bed furnace enter the recycle cyclone where solids in the CFB reactor off-gas are separated and collected at the base of the recycle cyclone and returned to the CFB reactor via the CFB seal pot. This recirculation of solids helps to maintain the circulating fluidized bed system, which ensures stable and controlled charring of the coal feed. The hot off-gas from the cyclone is then delivered to a waste heat boiler for steam generation by heat recovery. The off-gas, which contains significant amounts of char fines and combustible gases, can be delivered for burning in a coal-fired boiler. Another option involves the separation of solids in a multi-cyclone which are then cleaned in scrubber in preparation for an additional gas treatment.
