Modeling Study On The Desulphurization,Denitrification And Heat Transfer Of Fujian Anthracite During Combustion In CFB Boiler

Coal is the dominant energy resource of FuJian province and 98.3% is anthracite. FuJian anthracite has tough structure,bed reaction,low DT temperature,strong thermal fragmentation and is difficult to be on fire and burnout, so there exist much difficulties in using it. The circulating fluidized bed boiler has so many advantages including fuel flexibility, high combustion efficiency, low pollutants emissions that it has been the first choice of combusting FuJian anthracite recently. Right now the main problem during combusting in CFB boiler is that many desulphurization, NO_x reduction instruments are out of operation. Besides, because of the high suspended solids concentration and very high furnace temperature(1050℃), the heat transfer characteristic in furnace is a little different from that of CFB boiler combusting other coals. Moreover, the heat transfer in top region of CFB boiler is a little important for total heat transfer. So, Modeling study of desulphurization,NO_x reduction and heat transfer has a great significance to popularize the clean combustion technology and optimize the design or operation of CFB boiler.Firstly, the principle,characteristics and international general situation of CFB boiler was introduced, the recent studies on clean combustion of FuJian anthracite was introduced, the modeling studies of desulphurization model,NO_x reduction and the heat transfer in CFB boiler was also introduced.Secondly, as a basis of thoroughly studying on desulphurization characteristics of FuJian anthracite during combustion in circulating fluidized bed boiler, one step reaction desulphurization model and two step reaction desulphurization model were established. The results calculated from these models agree well with that from industrial experiments. When Ca/S mole ratio is under 2.2, two step reaction model describes industry experiments better than one step reaction model. Numerical simulation indicates that, when Ca/s mole ratio is hold constant, desulphurization efficiency will reduce as the flue gas velocity increases and increase when the circulation ratio increases; with the rising of combustion temperature, desulphurization efficiency will also increase, but desulphurization efficiency stays almost constant with the change of load.Thirdly, after thoroughly studying on NO_x formation and reduction characteristics of FuJian anthracite during combustion in circulating fluidized bed boiler, a semi-experimental NO_x reduction model was established. This model was well validated against industrial experiments. Numerical simulation indicates that, when other operation parameters are hold constant, concentration of NO_x(NO) in furnace reduces as the secondary air ratio increases and reduces as the temperature increases; with the rising of flue gas velocity,char diameter and the Ca/S mole ratio, concentration of NO_x increases.Further, taking modeling studies on other coals' heat transfer during combustion in circulating fluidized bed boiler as references, the FuJian anthracite's heat transfer model during combustion in circulating fluidized bed boiler was revised. As a basis of studies on fluid dynamics and heat transfer characteristics in top region of circulating fluidized bed boiler, a top region heat transfer model of FuJian anthracite was established. Numerical simulation indicates that, abrupt exits enhance the heat transfer; however, the top region heat transfer coefficient increases little for circulating fluidized bed boiler combusting FuJian anthracite. It is not appropriate to add heat transfer surfaces in top region of circulating fluidized bed boiler combusting FuJian anthracite.Finally, some problems of these models were analyzed. The results of desulphurization efficiency with temperature calculated from desulphurization models were contradict to that from the industrial experiments. Effects of limestone addition on NO_x concentration in NO_x reduction model were also contradict to that from the industrial experiments. These problems indicate that, the desulphurization and NO_x reduction models of circulating fluidized bed boiler combusting FuJian anthracite need to be studied more thoroughly.