Structural Optimization And Simulation Research Of SCR Flue Gas Denitration System For A 660MW Thermal Power Unit

With the rapid development of social economy,the social electricity consumption is increasing,and the main way of power generation is still coal-fired power generation.Coal-fired power generation produces many air pollutants,including nitrogen oxides(NOx).According to the air pollutant emission standard for thermal power plants(GB13223-2011)implemented by the state in 2012,the emission limit of NOx from newly built or existing thermal power plants is 100mg/m~3.According to the standard,each fire power unit must adopt the flue gas denitrification technology and install the flue gas denitrification equipments to reach the NOx emission standard stipulated by the state.Selective catalytic reduction(SCR)technology is widely used in flue gas denitrification of thermal power units at present.This paper introduces NOx control technology,research status of SCR by scholars at home and abroad,reaction principle of SCR and structure of SCR denitrification system.Taking the SCR denitrification system of a 660MW thermal power unit as the research object,STAR-CCM+software is used to carry out geometric modeling,grid division,numerical simulation and optimization of the SCR denitrification system.In the BMCR working condition of coal-fired boilers,the influence of the change of the value of L on the SCR denitrification system was simulated and analyzed by changing the distance L between the bottom of the second deflector plate above the rectifier grid of SCR denitrification system and the rectifier grid.When L increase gradually,the first layer of catalyst velocity distribution coefficient of the standard deviation of the entrance to the horizontal plane and the second layer on the surface of the catalyst export levels NO concentration standard deviation coefficient increase with the decrease of the first,and when L is 250mm,the minimum is 7.52%and 8.15%,and the denitration efficiency increased to 87.37%from 83.16%which is the original design scheme,250mm is relative optimal optimization scheme for SCR denitration system entrance guide plate.By keeping the angle between the two rows of static mixer unchanged,change the angle A between the upper row of static mixer and the horizontal plane,which ranges from19.2°to 31.2°.When A is 28.2 degrees,compared with the original design scheme,the first layer of catalyst entry level surface velocity distribution coefficient of the standard deviation decreases from 12.35%to 9.03%,the second layer on the surface of the catalyst export levels NO concentration decreases from 10.59%to 9.21%,the standard deviation coefficient denitration efficiency increased from 83.16%to 85.78%,28.2 degrees is the relatively optimal optimization scheme of static mixer.The operation of SCR denitration system under different load was simulated and analyzed.When the load reduces from BMCR to 50%THA,the denitrification efficiency reduces from 83.16%to 78.95%.When the load is 50%THA,the denitrification efficiency is78.95%,which does not meet the design standard.When the load is low,the denitrification efficiency should be improved through smoke temperature improvement or ammonia spraying optimization and other technical means.In the BMCR working condition,the molar ratio of ammonia and nitrogen is changed to explore the influence of changes of the molar ratio of ammonia and nitrogen on SCR denitrification system.The molar ratio of ammonia and nitrogen changs from 0.85 to 1.0,the escape rate of ammonia is 2.8 ppm,and the denitrification efficiency is 87.81%,the denitrification effect is relatively ideal.If the molar ratio of ammonia nitrogen continues to increase to 1.0,the escape rate of ammonia is 3.3 ppm,which exceeds the design standard.Therefore,0.95 is the relatively optimal molar ratio of ammonia and nitrogen.When the molar ratio of ammonia and nitrogen is 0.95,by changing the SCR denitration system of the ammonia injection grid up and down two rows of nozzles spraying ammonia ratio,the paper simulations and analysises the influence of different spraying ammonia proportional to the velocity field and concentration field,the spray nozzle on the amount of ammonia and the original ammonia injection volume ratio of the value range of 0.4 to 1.4.When the spray nozzle on the amount of ammonia and the original value of the ratio of the amount of ammonia injection of 0.6,the first layer of the velocity distribution of the entrance to the horizontal plane and ammonia catalyst concentration is more uniform,the second layer on the surface of the catalyst export levels NO concentration 8.28%.Therefore,the ratio of ammonia spraying amount of the upper nozzle to the original ammonia spraying amount is 0.6,which is the relatively optimal plan of ammonia spraying ratio.When the molar ratio of ammonia and nitrogen is 0.95,the paper keeps the SCR denitration system up and down two rows of nozzle angle relatively constant,changes the angle of the nozzle with horizontal plane angle on the C,analysises the influence of different angles on velocity field and concentration field.When the angle is 12 degrees,the first layer of catalyst velocity distribution coefficient of the standard deviation of the entrance to the horizontal plane and ammonia concentration reaches the minimum values,which are 8.23%and 8.18%,respectively,and maximum denitration efficiency reached 88.51%.Therefore,12degrees is the relatively optimal scheme of ammonia spraying angle.The numerical simulation and optimization of SCR denitration system of a660MW thermal power unit provide theoretical guidance for the actual operation of the thermal power unit.