Experimental Investigation On The Characteristics Of Heat And Volatile Release In Circulating Fluidized Bed Reactors

The characteristic of heat release is a key factor in the design and operation ofcirculating fluidized bed boilers;volatile release plays an important role in heatrelease which should be paid enough attention to.A bench scale circulating fluidized bed (CFB) combustor, with the diameter of150mm and 4.9m height, was reconstructed in the laboratory. A series of experimentswere carried out in it, which aimed to analyze the characteristics of heat release forcoal particles of three narrow size ranges between 500 and 6000μm. By the way, theconcentration profiles of SO2 and NOX were measured during the experiments. .The characteristic of volatile release was experimentally investigated in anelectrically heated fluidized bed reactor with the diameter of 45 mm. The bedtemperature was 600 to 900℃. The bed material was silica sand and the bed wasfluidized with nitrogen. The coal samples were lignite, bituminous and lean coal, andthe coal particle size range was 2000~6000μm in diameter.Based on the experiments, it was found that the heat release fraction in densezone of circulating fluidized bed was much less than that of bubbling bed and theevolution of CO was visible in dense zone even though the oxygen concentration washigh enough. These results show that the dense zone of circulating fluidized bed is inreducing condition, which agrees well with the previous conclusion. The heat releasedistribution was strongly affected by the coal particle diameter. For the larger particleof 3200~6000μm in diameter, most of the combustion occurred in dense zone, whilefor the smaller one of 500~600μm in diameter, the combustion in freeboard washigher than that for large particle, and the heat release distribution along the reactorheight was evener compared with the other two cases. The concentration of SO2 indense zone was negligible and increased gradually along the reactor height, whichsupports the theory of combustion in reducing atmosphere for CFB. However, theconcentration of NOX decreased gradually along the height.The evolution of volatile occurred markedly at the early stage after coal particleswere fed into the reactor, and then the rate of volatile release decreased gradually. It isconcluded that the rate of volatile release decays exponentially after the early stage,and a model was suggested. The release rate and yield of volatile were affected by thetemperature as well as coal type. A larger percentage of volatile was released forlignite and bituminous than that for lean coal at the early stage. For mm-size coalparticles, the larger the particle, the more slowly volatile was released;the effect ofparticle diameter was more significant for lean coal than that for lignite andbituminous.