Research On Urea Deposits Characteristics And Inhibition Technology Of Diesel Engine Integrated After-treatment Device

Diesel engine exhaust emission has become one of the main factors of air pollution in China.NOx and PM emitted by diesel engine are important causes of photochemical smog and PM 2.5.Urea-SCR technology is a key technology that can effectively reduce NOx emission of diesel engine,but this system will be affected by diesel engine working conditions,ambient temperature and properties of urea water solution.The urea water solution cannot be completely evaporated and pyrolyzed,then will be attached to the wall of the device to form deposits.Once urea deposits are formed,it will affect the working efficiency of the whole device,resulting in low SCR conversion efficiency,ammonia leakage and other problems.In serious cases,it will even block the exhaust pipe,increase the exhaust back pressure,so as to increase the fuel consumption of diesel engine.In addition,there are few studies on the thermal regeneration of urea deposits in the integrated after-treatment device,and there is still a lack of numerical simulation model establishment and verification methods for the whole device.Therefore,this study aims to inhibit urea deposits in an integrated after-treatment device.The theoretical analysis,numerical simulation and experimental test are comprehensively used to carry out the research on the related problems of urea deposits in the device.Firstly,the urea deposits characteristics are studied by the combination of experiments and numerical simulation,and the influence law of urea deposits under different working conditions are explored.Secondly,the formation of urea deposits is inhibited by optimizing the injection parameters and mixer structure;Finally,the regeneration law of urea deposits in the overall device is studied by experiments.The specific research contents include the following aspects:（1）Study on the establishment of integrated after-treatment device model.A highly effective numerical simulation model of integrated after-treatment device is established by the computational fluid dynamics analysis software.The influence of exhaust temperature and flow rate on the back pressure of each catalyst under different operating conditions is studied,the porous medium model of each catalyst is established,and the permeability coefficients of each catalyst are determined;In the same way,the functional relationship between the wall temperature during urea injection and the exhaust temperature and the urea injection mass is found out.It provides boundary conditions for establishing the numerical model of integrated after-treatment device.（2）Study on the deposits characteristics of urea and its influence law.The influence of diesel engine working conditions and urea injection mass on urea deposits and its variation law are studied by the combination of experiment and numerical simulation.The effects of different exhaust temperature,exhaust flow rate and urea injection mass on the mass of urea deposits,the mass of liquid film,the liquid film components,the main by-products,the urea decomposition rate and the uniformity coefficients are studied to reveal the formation process of liquid film.（3）Study on urea deposits inhibition technology.Based on the numerical model of the integrated after-treatment device,the performance of the mixer is evaluated by the parameters such as the mass of liquid film（urea decomposition rate）,the uniformity of NH₃,velocity and temperature in the front-end section of SCR catalyst,and the back pressure of the mixer.With the aim of the maximum NH₃uniformity,minimum urea deposition rate and minimum mixer back pressure,the effects of different injection parameters and mixer structure on the formation and distribution of liquid film and NH₃are studied,and the optimal scheme is put forward.（4）Study on urea deposits thermal regeneration.Based on the same mass of urea deposits generated under the same working condition,the experiments of the different exhaust temperature,different exhaust flow rate and different thermal regeneration time are carried out.The main factors affecting the thermal regeneration process of urea deposits are found out and establish the functional relationship between urea deposits regeneration rate and boundary conditions.It lays a theoretical foundation for the thermal regeneration and thermal management strategy of urea deposits in the integrated after-treatment device.