Study On Deposit In Urea-SCR System Of Diesel Engine Based On The Detailed Decomposition Mechanism Of Urea

Urea-Selective Catalytic Reduction（urea-SCR）technology has been widely used in diesel engine exhaust aftertreatment system to control NO_x emissions as emission regulations are increasingly stricter on harmful emission limits of engines.Urea deposition risk exists in the process of urea-SCR technology use,and the efficiency of urea decomposition reduce.In order to solve this problem,it is necessary to further explore the formation mechanism and influencing factors of urea deposits.In this paper,the detailed decomposition mechanism of urea coupled with CFD method is used to simulate the evaporation,decomposition and deposit（contains solid urea and urea decomposition byproducts）formation process of urea under various working conditions.The effects of exhaust temperature,exhaust flow rate,mixer structure and injection parameters on the deposit are studied.Firstly,the deposition risk and the decomposition efficiency of urea aqueous solution under different exhaust temperatures and flow rates were analyzed by establishing a urea-injection model under an approximate exhaust environment.The results show that the deposition rate of urea decreases with the increase of exhaust temperature,and film is even not formed at higher exhaust temperature;the composition of by-products is closely related to temperature;when the temperature is 473K,the inactive decomposition of urea leads to the decrease of decomposition rate,and when the temperature is higher than 673K,the decomposition rate of urea is also low,but the decomposition efficiency of urea can be improved by increasing the residence time of droplets.Considering the urea deposition rate and urea decomposition rate,the optimal exhaust temperature is 573K.The exhaust flow rate mainly affects the decomposition rate of urea,and then affects the time scale of urea decomposition and NH₃ response time.The higher the exhaust flow,the faster the change of solid urea quality,the faster the solid urea was completely decompose,and the earlier the mass of biuret and cyanuric acid reached the peak value,and the lower the peak value.Secondly,a numerical model of sprayed urea-affecting a metal plate is established.Based on this model,four heat transfer methods（conduction,convection,boiling and evaporation）and complex chemical reactions in the film were calculated.The results show that the main heat transfer methods between the film and the plate are conduction and evaporation.The mass of the film with heat flux can be divided into four stages.In the first stage,the intense heat exchange occurs.The main components of the film are solid-liquid two-phase urea and water.The main reason for the mass loss of the film is the evaporation of water.In the second stage,the film increases slowly and the decomposition mass of urea increases.The main component of the film is solid urea.In the third stage,the mass of the film remains unchanged,but the mass of solid urea begins to decrease.In the fourth stage,film mass decreases,and a large amount of urea is decomposed.The main byproduct of urea decomposition is biuret in the first three stages,and CYA（cyanuric acid）in the fourth stage.The solid urea and byproducts are mainly distributed in regions with low heat flux.With an increase in the initial plate temperature,the heat flux increases,the masses of the film and solid urea decrease,the masses of byproducts also decrease.Finally,a three-dimensional numerical model of a straight cylinder and S-type exhaust after-treatment systems were established to to simulate the deposit formation process under different mixer structures and injection parameters.The results indicated that the quality of deposits can be reduced by using the mixer with higher turbulence intensity,reducing injection angle or injection pressure.In the S-type exhaust post-treatment system,the film only appears in the mixer and nozzle areas.It is found that the production of biuret,CYA,and ammelide,which are by-products of urea decomposition,is mainly affected by the NCO^-concentration,and film temperature and height.The thicker the liquid film and the smaller the rate of evaporation,the smaller the mass fraction of urea solid and the larger the mass fraction of urea decomposition by-products.According to the analysis results,the optimization scheme of the mixer is proposed,and the simulation analysis is performed.The analysis results show that the NH₃concentration uniformity at the SCR inlet increases,and the pressure drop in the mixer decreases.The deposits,such as urea solids,biuret,CYA,and ammelide（cyanuramide melanuric acid）decrease.Consequently,the performance of the exhaust post-treatment system is improved.