Numerical Study On High Temperature Preheating Fuel Combustion Coupling With Reduction Of NO From Kiln Gas In A Cement Precalciner

With characteristics of large output,high energy consumption and high pollutant emissions,cement industry has become the third largest NOx emission source after the power and transportation industries in China.Low NOx emission control technologies for cement industry is an important prerequisite to ensure its sustainable development.However,with the increasing pressure of environmental protection,the existing technologies can hardly meet the increasingly stringent NOx emission standards.In view of the bottleneck problems faced by the existing technologies,Fuel In-Situ Reduction(FISR)denitrification technology,suitable for coal-fired cement kilns,was proposed by the Circulating Fluidized Bed Laboratory(CFBL)team from the Institute of Engineering Thermophysics,Chinese Academy of Sciences(IET,CAS).In FISR method,high-temperature preheating fuel(HTPF)containing syngas and semichar is firstly produced by means of a pretreatment process of pulverized coal(PC),and then fed into the precalciner for combustion and reduction of NO from kiln gas under the condition of staged air.The feasibility and effectiveness of FISR method have been verified by a pilot-scale experiment.Nevertheless,the relevant studies mainly focus on the influence of the front-end pretreatment process on NOx emission,while such investigations as combustion characteristics of HTPF in the back-end precalciner and influences of various factors on the reduction characteristics of NO from kiln gas are insufficient.Therefore,with the precalciner in the pilot-scale experiment as object,a series of numerical studies were conducted to investigate the process of HTPF combustion coupling with NO reduction using CFD method in this dissertation,in order to reveal the influence laws of relevant factors and lay a theoretical foundation for the engineering application and optimization design of FISR denitrification technology.In this paper,the RNG k-ε model was used to calculate the gas turbulence and the DPM model was employed to calculate the trajectory of discrete particles in the precalciner.The finite rate-eddy dissipation model was used to calculate the combustion process of fuel and the decomposition process of cement raw meal(CRM),and the P-1 model was used to calculate the radiation heat transfer.The two-competing-rates model and the multiple-surface-reactions model were employed to calculate the processes of volatilization,combustion and burnout of char,respectively.After the combustion calculation was completed,the generation and reduction of NOx were modeled using a post-processing approach.Compared with the experimental data for temperature distribution along the precalciner and final NOx emission,the reliability and accuracy of the used mathematical model were verified.On this basis,both similarities and differences of the combustion process between PC and HTPF in the precalciner were firstly analyzed.Due to the special structure of the precalciner,the flow field in the separate combustion process for two fuels presents the flow structure of large velocity and weak swirl trend in the inner core area and small velocity and strong swirl trend in the annular area near the wall.Compared with the situation of PC combustion,the swirl trend in the lower section of the precalciner is more obvious,and the particle trajectories of semichar are more concentrated and closer to the wall when HTPF burns in the precalciner.Besides,the range of high-temperature area is larger in the lower section of the precalciner,and O2 concentration is also lower and distributed in a smaller local area,while CO concentration is higher and distributed a larger local area near the wall.In comparison to the situation of PC combustion,the concentration of NOx produced in the combustion process decreases by 83.6%and the reduction rate of NO increases by 22%for HTPF combustion in the precalciner.Then,without CRM,the effects of NO initial concentration and O2 content in kiln gas,air-staged schemes,location and excess coefficient of tertiary air-I on the whole process were investigated.As it turns out,the NOx emission concentration increases linearly with increasing the initial NO concentration in kiln gas,while the reduction rate of NO remains unchanged.When the O2 content exceeds 4%,the inhibition of NO reduction is enhanced,while the promotion of NOx generation is enhanced when the O2 content exceeds 5%.Only when the reduction zone established by air-staged scheme is large enough,the the NOx emission can be significantly cut.In case of consistent size and location of the reduction zone,the use of air-staged technology in the oxidation zone can also inhibit the generation of NOx and promote the reduction of NO.For four-stage air-staged scheme,it is beneficial to reduce NOx formation and promote NO reduction when the inlet of tertiary air-Iis located above that of HTPF.When the dimensionless position of tertiary air-I inlet(Y*TA-I)is 0.6,the NOx emission concentration is the lowest and the NO reduction rate is highest,and the corresponding values are 31.19 mg/m3 and 97.14%,respectively.Under this condition,increasing the excess coefficient of tertiary air-I can further promote the reduction of NO,owning to the transformations of flow field and components concentration distribution caused by the velocity change of tertiary air-I.Further,with the presence of CRM,the effects of decomposition and feeding port of CRM on the processes of HTPF combustion and NO reduction were explored.The results show that,when the position of CRM inlet is located at YCRM=0 m,the addition of CRM can reduce the NOx generated in the combustion of HTPF by 26.91%,and increase the NO reduction rate by 4.0%.As the CRM inlet moves up in the range of Y=-0.6 m-0.6 m,both emission concentration of NOx and decomposition rate of CRM decrease,while the NO reduction rate increases gradually.Both low emission and high decomposition rate can be guaranteed simultaneously when the dimensionless position of CRM inlet(Y*CRM)is 1.6,and the corresponding values are 31.18 mg/m3 and 92.63%,respectively.Finally,regarding an actual precalciner with a scale of 2500 t/d as numerical example,this dissertation analyzed the changes of NOx emission,decomposition rate of CRM and burnout rate of fuel before and after using FISR method and two-stage air-staged scheme.The results indicate that,without ammonia,the NOx emission can decrease by 66.59%when applying the FISR method.Under this condition,the further use of two-stage air-staged scheme,can make the NOx emission basically reaches the ultra-low level required by cement industry.Nevertheless,both the decomposition rate of CRM and the burnout rate of fuel drop to some extent.