Numerical Simulation And Experimental Study On NO_x Generation And Control Of Large CFB Boiler

Circulating fluidized bed（CFB）boilers have been vigorously developed in China due to their higher combustion efficiency,lower original emissions,and combustible inferior coal types.In order to achieve strict environmental protection requirements,it is necessary to improve the economics of combustion in the furnace and reduce the pollution emissions of the boiler,so as to achieve the requirements of ultra-low emissions.Therefore,it is necessary to further improve the uniformity of combustion in the furnace and the denitration efficiency.Due to the extremely complicated combustion process in the CFB boiler furnace,it is very difficult to test the actual furnace,but it is urgent to understand the gas-solid flow,combustion and denitration reaction processes in the furnace,and find further optimized technical solutions.Therefore,the topic of the thesis is to carry out a series of numerical simulation and actual furnace measurement for the typical 300MW subcritical CFB boiler..In this paper,the Euler-Lagrangian method is used to numerically study the gas-solid two-phase flow of a 300MW subcritical CFB boiler with different return deviations.The components were sampled and analyzed,and then the flue gas distribution obtained from the real furnace test was taken as the boundary condition.The Selective Non-Catalytic Reduction（SNCR）denitration process in the cyclone separator under different conditions was numerically simulated.The simulation results of the effect of the return deviation on the gas-solid flow uniformity in the furnace show that the influence of the return deviation on the particle concentration distribution along the height,width and depth of the furnace is mainly concentrated in the dense phase region.It has little effect on the particle concentration and distribution in the area above the upper secondary air outlet.The return deviation of the external circulation loop is not the main factor causing uneven gas-solid flow and uneven combustion in the furnace.Even if the deviation of the circulating ash in the furnace reaches 30%,most of this deviation will disappear in the dense phase region.The actual furnace flue gas test results show that there is indeed combustion inhomogeneity in the furnace,which leads to uneven distribution of flue gas in the horizontal section of the furnace’s dilute phase zone.In the depth direction of the boiler,the amount of coal in the front wall is higher than that in the rear wall;in the width direction of the boiler,more coal is fed to the walls on both sides and less coal in the middle area,which makes the vertical distribution of NO_x at the inlet of different separators uneven.The NO_x in the horizontal flue of#1 and#3 separators increased first and then decreased,the highest point was at a depth of 5m,while the vertical distribution of NO_x in the#2 separator basically showed a gradual increasing trend.By comparing the test results of the flue gas in the dilute phase zone and the inlet of the separator,it is found that the distribution of flue gas components in the dilute phase zone of the furnace will be mostly transmitted to the inlet of the cyclone separator.The flue gas component distribution at the entrance of the separator is the result of the combined effect of flue gas component distribution and flue gas component diffusion in the furnace.The flue gas component concentration in the top area of the horizontal flue mainly comes from the flue gas in the front wall,while the bottom area mainly affected by the flue gas from the rear wall,the central area is affected by the flue gas from the central area of the furnace.Based on the actual furnace flue gas test,a numerical simulation study was carried out on the SNCR denitration process in the cyclone separator under different inlet flue gas distribution and different urea flow distribution.The results show that the NO concentration in the vertebral segment of the separator is lower than that in the horizontal flue section;the NO concentration and NH₃ concentration near the wall of the separator are higher than those in the center area;the reduction reaction of NH₃ and NO mainly occurs in the middle and lower part of the horizontal flue and the vertebral body segment.The influence of different inlet flue gas distributions on the denitration reaction is mainly reflected in the inlet horizontal flue.After adjusting the urea flow rate of the spray guns at different positions according to the measured NO_x distribution,the denitration efficiency can be improved to a certain extent,indicating that the denitration plan is feasible,and the urea flow rate can be more accurately distributed to obtain the best denitration optimization plan.