Analysis And Prediction Of The Urea Crystallization Risk Based On Chemical Reaction Kinetics

Selective Catalytic Reduction(SCR)is the standard method for reducing NOx in diesel exhaust.However,there are serious problems of urea crystallization in SCR technology.Urea crystallization leads to reduced catalyst life,clogging of the exhaust line,and increased exhaust back pressure,which reduces engine economy and even affects engine durability.The SCR mixer structure is one of the important factors affecting the crystallization of urea.Therefore,a CFD method for accurately predicting the risk of urea crystallization is studied as a technical means to guide the structural design and optimization of the mixer,so as to avoid urea crystallization is the key to solve the above problems.In this paper,based on the "evaporation of urea with water" proposed by relevant scholars to simulate the evaporation process of pure water,the method of analyzing the liquid film thickness distribution of the wall of SCR mixer to predict the risk and location of urea crystallization,the establishment of urea based on chemical reaction kinetics CFD model of crystallization risk,thereby improving the accuracy of urea crystallization risk prediction.Main tasks as follows:Based on the CFD model proposed by related scholars to replace urea with pure water to simulate the risk of urea crystallization,a multi-component evaporation model of urea aqueous solution droplets was established.The accuracy of the model was verified by engine bench crystallization test.The pure water simulation results were compared and analyzed to verify the accuracy of the model.Based on the study of the decomposition of urea and the detailed chemical reaction process of solid by-products(biuret,cyanuric acid,etc.),a detailed decomposition model of urea was established,and the accuracy of the model was determined by engine bench crystallization test.Verification.The combination of the above two models is a more accurate and accurate CFD model for predicting the risk of urea crystallization.It is used as a technical means to guide the structural optimization design of the traditional SCR mixer,and the two optimization schemes proposed by the model are applied.Perform simulation analysis to determine the best optimization solution.The SCR catalyst and the traditional SCR catalyst are optimized for the steady-state performance test,transient ETC emission test and 30 h engine bench urea crystallization verification test.The test results show that compared with the traditional structure,the NOX conversion rate of the optimal optimization scheme is obviously improved,theNOX emission is obviously reduced,and the urea crystallization does not appear on the wall of the mixer,that is,the problem of urea crystallization is solved from the perspective of optimal design of the mixer structure..The SCR catalyst of the optimization scheme 2 and the conventional SCR catalyst were subjected to a steady-state point performance verification test,a transient ETC emission test,and a 30-hour engine bench urea crystallization verification test.The test results show that compared with the traditional structure,the NOx conversion rate of the structure of Scheme 2 is significantly improved,the NOx emission is significantly reduced,and urea crystals are not present on the mixer wall.