Study On Optimization And Catalytic Performance Of Urea-SCR System For Diesel Engines

Commercial vehicle is the main type of vehicle used in the logistics and transportation industry.It is generally believed that,the high-power diesel engine,as the power source of commercial vehicle,can hardly be replaced by power battery in quite a long period of time.Diesel engine has the advantages of low fuel consumption under comprehensive working conditions and high torque at low speed,but it may produce relatively high NO_Xand particulate emissions during the combustion process,resulting in atmospheric and environmental pollution,and endangering the health of human being.Meanwhile,with the increasingly stringent emission regulations,the requirements for diesel engine emission post-treatment are higher and higher.To reduce the NO_Xemission of diesel engines,the Selective Catalytic Reduction（SCR）technology is widely used in China.Among them,the urea selective catalytic reduction system is the mainstream.In this SCR system,the urea spray characteristics,urea crystallization and catalyst performance may all significantly affect the NO_Xconversion efficiency.This paper aims at finding methods to improve the uniformity of urea distribution in the exhaust pipe of diesel engine,reduce the risk of urea crystallization and improve the low-temperature activity of catalyst,so as to provide a theoretical basis and implementation method for improving the NO_Xconversion efficiency of urea selective catalytic reduction system.The main research work and conclusions of this paper are as follows:（1）Based on the self-developed spray characteristic testing platform,the spray characteristics of two different nozzles,swirl nozzle and pressure nozzle,were tested with shadow imaging technology.The source program for spray image processing and the interface processing software were developed to achieve the efficient processing of nozzle spray test images.The results showed that the spray cone angles and penetration distances of both swirl and pressure nozzles increased with the rising of injection pressure.Under the same injection pressure,the spray cone angle of swirl nozzle was greater,while that of the pressure nozzle is relatively stable,with smaller fluctuation with time.The penetration distance of swirl nozzle was relatively small,but greatly influenced by pressure changes.The Sauter mean diameters of both nozzles decreased with the increase of the injection pressure,axial distance and radial distance.（2）The direct coupling numerical calculation of the internal flow and the near-field jet flow in urea nozzle orifice was realized based on the Eulerian multiphase flow simulation.The internal flow field information of the nozzle was loaded into the initial spray model,and the initial droplet size and the direction of movement were automatically corrected.Finally,a high-precision spray model coupled with the internal flow of the urea nozzle was constructed.Three urea nozzles of different structures（A,B and C）with 6 orifices and the diameter of 0.19mm were designed.The cavitation phenomenon,jet velocity,spray cone angle and spray characteristics of the three nozzles were simulated and compared.The results showed that the urea nozzle A with the minimum slit thickness and orifice height had the best atomization effect.（3）Aiming at the problem of urea crystallization in the exhaust pipe of the diesel engine,the optimization of the layout scheme of urea nozzle in the exhaust pipe was carried out.Based on the accurate calculation model of the internal flow and spray of urea nozzle,a numerical simulation study on urea selective catalytic reduction system of a diesel engine was carried out.On the basis of the analysis of the prototype SCR system,the offset angle between the axis of nozzle head and the central axis of the diesel engine exhaust pipe,and the offset angle between the orifice axis and the nozzle axis were redesigned.The orifice diameter was adjusted from 0.5 mm to 0.19 mm and four different orifice numbers were designed.The simulation results showed that with the improvement of the nozzle structure and installation position,the risk of urea crystallization in the exhaust pipe was effectively controlled.The NO_Xconversion efficiency of nozzles corresponding to four orifice numbers was improved to varying degrees.Among them,the 8-hole nozzle has the best comprehensive performance,with the uniformity index and the NO_Xconversion efficiency being 0.906 and 67.22%respectively.The simulation study on the influencing factors of 8-hole nozzle Urea-SCR system showed that in the improved system,it is necessary to keep the exhaust temperature,the space velocity in exhaust pipeline and the outlet velocity of nozzle within a reasonable range,so as to avoid the formation of urea crystallization in SCR system and maintain a high NO_Xconversion efficiency at the same time.（4）In order to improve the low-temperature activity of SCR system catalyst,Br-doped Ti O₂was proposed as the carrier of system catalyst and the modified V₂O₅/Ti O₂catalyst samples were prepared,based on the idea of doping modification of SCR system catalyst.The results of the activity test to samples doped with different amounts of Br showed that when the molar ratio of[Br]/[Ti]was 2,the catalyst VTi Br2.0 has the best activity.In this article,further experimental studies were carried out on the effects of carrier calcination temperature,SO₂and H₂O on the performance of catalyst VTi Br2.0.In addition,studies on the structural parameters,phase composition,reducibility and surface acidity of Br modified V₂O₅/Ti O₂catalyst were carried out with XRD,BET and other characterization technologies.The mechanism of improving the low-temperature activity of the catalyst developed in this paper was revealed,which is valuable for improving the NO_Xconversion efficiency of SCR system in engineering applications.