The Mechanism Of Urea Deposits For Diesel SCR After-treatment

In this paper, we study the urea deposits which appeared in Urea-SCR system, explorethe reaction mechanism in Urea-SCR system from the perspective of a chemical reaction.Study on the problems of chemical and heart transfer in urea-ammonia chemicalconversion process which by the meams of Chemical reaction kinetics. Search aqueoussolution of urea Laws of the mass transfer process of gas-solid multiphase flow and heattransfer by the theory of multiphase fluid, and seeking factors of which affecting ureadeposit formation. At last, find the solutions to urea deposits by using these factors.Specific studies are divided into the following three parts:First part The conversion mechanism of urea-ammoniaAt first, explore the effects of different catalysts on the pyrolysis of pure ureahydrolysis by the means of sample test, the product of urea hydrolysis and pyrolysis isHNCO and H2O. Explore the Hydrolysis Mechanism of HNCO, and analyze the effect ofdifferent catalysts on HNCO hydrolysis. Secondly, in the sample test, explore the impact ofthermal hydrolysis of urea with temperature, oxygen concentration and the initialconcentration of urea, then, we know the importance of temperature on urea-ammoniaconversion process.Second part The multiphase flow model of the aqueous solution of ureaFor the physical and chemical processes of aqueous solution of urea in Urea-SCRsystem, we use multiple models to simulate this changing process. In the first place，selectthe model of discrete droplets to simulate the process of spraying an aqueous solution ofurea. Next, With Dukowicz model to describe the droplet evaporation process.Third，Select Huh-Gosman model to describe the urea droplets broken. Finally, using multiphaseflow NS (Navier-Stokes) equations coupled entire model.Using AVL Fire for the entire simulation of Multiphase flow model. Only by this way can we get further explore ofmicroscopic processes in urea-ammonia conversion and heterogeneous quality, andprovide a theoretical basis for the study of the formation of urea deposits and prevention.Third part Study the impact of mixer on uera depositsExplore the influence of mixer on exhaust back pressure, mixing of exhaust gas, NOxconversion efficiency and urea deposits, and compare the impact of different points of themixer (second-order mixers, butterfly blades, grid-like) on exhaust back pressure, mixingof exhaust gas, NOx conversion efficiency and urea deposits.To validate the factors which affecting the formation of urea deposits by usingartificial crystallization experiments, and find ways to to reduce the generation of ureadeposits by improving the urea injection system, adding intelligent control system whichcontrol exhaust gas temperature and suitable mixer.