Numerical Simulation And Optimization Of SCR Denitrification System For Double Variable Section Flue Of 1000MW Unit

According to China’s current energy structure,the pattern dominated by thermal power will not change much in the future.NOx in the flue gas of coal-fired power stations is one of the main sources of artificial sources.Only by mastering its production mechanism and achieving effective control of NOx emissions can the national energy conservation and emission reduction policies be implemented.As the mainstream technology today,Selective Catalytic Reduction（SCR）has been widely used in domestic and international thermal power plant flue gas denitrification due to its high removal rate and high maturity.Based on CFD simulation analysis technology,the corresponding mathematical model of SCR denitrification system is established and numerical simulation is carried out,which can provide important basis and design parameters for the design and optimization of the denitrification reactor flue,internal guide plate and ammonia gas injection system.In this paper,the SCR denitrification system of a 1000 MW tower boiler in a power plant is taken as the research object.A three-dimensional model of the SCR denitrification system is established for the problem of large velocity field and concentration field deviation coefficient of the first layer catalyst.The influence of the arrangement of the deflector at the double-variable cross-section flue on the flow field was studied by numerical simulation.The simulation results show that the original SCR system is affected by the size of the double-section flue structure.By repeatedly optimizing the layout of the deflector,the boundary layer detachment phenomenon still exists when the flue gas flows through the area,and it cannot flow along the inclined direction of the left wall surface.On this basis,two kinds of flue structure reconstruction schemes are proposed,and the simulation results of the two schemes are compared qualitatively and quantitatively from the aspects of relative standard deviation,system pressure drop and fly ash wear.The comparison results show that after the modification of the scheme 1,the flow field uniformity optimization effect is more significant than that of the scheme 2 by splitting the double variable section flue of the original SCR system into two single variable section flue in the X direction and the Z direction.On the basis of optimizing the flow field of SCR denitrification system,it is an effective measure to improve the uniform mixing of flue gas and ammonia by reasonably designing the position and size of ammonia spray grille.By using the species transport model without considering chemical reaction to simulate the mixed diffusion of ammonia and flue gas,the influences of the distance between the nozzle in the direction of flue length and width to the wall（X direction and Z direction nozzle spacing）and the distance between the nozzle in the adjacent branch pipe on the ammonia concentration distribution of the first-layer catalyst were investigated respectively,and the optimal size of ammonia injection grating was determined.Then,the optimal turbulent mixing of reducing agent and NOx was achieved by adopting the optimization method of ammonia injection in different zones and adjusting the nozzle injection velocity in each zone through repeated simulation.The deviation coefficient of concentration distribution of NH₃ was reduced to 2.24%,which ensures the uniform denitrification reaction in all channels of the catalyst.The flue gas in the denitrification system often carries a large amount of fly ash particles,which will have a great impact on the wear of the deflector and catalyst layer.It is easy to cause the diversion failure in the long run,thus affecting the denitrification efficiency.In this paper,the DPM model is adopted to track the trajectory of fly ash particles,and the wear and deposition of the wall of each group of deflectors are monitored to predict the location of wear and the specific size of wear rate,which provides reference for anti-wear measures of deflector.