The Study On Non-uniform Spraying Technology Of SCR Denitrification System

Nowadays,Selective Catalytic Reduction(SCR)technology is the most widely used denitrification technology for coal-fired power plants in our country.However,with the increasingly stringent NOx emission limits of coal-fired power plants,especially in the implementation of ultra-low emissions,there are a lot of problems in the SCR denitrification projects such as ammonia escalation,air preheater corrosion blockage,catalyst wear and fan Load and so on.The main reason is that the conventional SCR denitrification technology uses uniform spray ammonia,and the distribution of flue gas velocity at the spray zone and the NOx concentration distribution in the flue gas are not uniform,in order to achieve high denitrification efficiency,it is necessary to inject excess ammonia which result in a large difference in the molar ratio of ammonia to ammonia in different areas,thus producing the above problems.To solve these problems,Southeast University proposed non-uniform spray ammonia technology.In this paper,the SCR system of 135MW coal-fired boiler and 2×130t/h coal-fired boiler were taken with the help of numerical simulation method and engineering survey to research the effect of Ammonia Injection Grid(AIG)on ammonia-nitrogen matchment under the non-uniform ammonia injection controlment.According to this,a new AIG which could be used for non-uniform spray ammonia adjustment was designed and some suggestions were proposed to achieve non-uniform ammonia injection.Firstly,the SCR system of 135 MW coal-fired boiler was taken as the research object.The validity of the numerical simulation was verified by comparing the cold test data and the numerical simulation results.The k-a model was used to simulate the flue gas flow and diffusion of the whole SCR system under non-uniform inlet conditions to analysis the influence of spray injection strategy,spout density N,opening rate ? and spout angle a on the ammonia-nitrogen matchment in SCR system.The results show that when non-uniform spray ammonia method was used and N=15.34per m2??=0.0024?a=90°,the NH3-NO matchment in the reactor is the best.As the LES model could describe the turbulence structure near the nozzles more finely,it was used to simulate the development of the reduce agent jet from AIG spouts.The interaction of the reducing agent jets and with the mainstream flue gas in sprayed ammonia region was studied to analysis the influence of spouts structure on the ammonia-nitrogen mixing effect.The results show that when y/(coupling coefficient of AIG)reaches 4 and y(nozzle angle of AIG)reaches 30°,it could effectively strengthen the NH3-NO mixing effect in the reactor.On this basis,a new AIG was designed for the 2×130t/h coal-fired boiler SCR system,it could be used for non-uniform spray ammonia adjustment.As the flue of AIG has a narrow structure,a single-dimensional non-uniform spray ammonia control method was adopted.It worked like this:in the flue length direction,the amount of ammonia were determined by the flue gas velocity and NOx concentration to achieve non-uniform spray ammonia;in the flue width direction,uniform spray ammonia was achieved by the spray ammonia grinder in the flow and heat transfer characteristics of the throttle components.In the meantime,the position of the AIG in the SCR system was also determined by numerical simulation.Finally,the flow field distribution and NH3/NO ratio distribution of the SCR system were obtained by field measurement to analyse the actual performance and existing problems of the designed AIG.It was found that the denitration efficiency of the SCR system could reach 96.3%,but there was a large area where NH3/NO ratio>1 in the upstream region of the SCR system,which would cause a lot of ammonia escapes and it was necessary to carry out non-uniform ammonia injection regulation.Then,the measurement data were used as the inlet condition of numerical simulation to determine the distribution of flue gas flow field in the cross section of the sprayed ammonia system.Based on this,a non-uniform ammonia injection scheme was established for the 2×130t/h coal-fired boiler SCR system.