Structural Optimization Of Selective Catalytic Reduction And Study On NH₃ Distribution Characteristics In Diesel Engine

Diesel engines are widely used in agricultural machinery,engineering machinery and automobile industries due to their high torque and high thermal efficiency.In recent years,internal combustion engine technology has been continuously developed,and the gap between domestic and foreign internal combustion engine technology is getting smaller and smaller.The application of new diesel engine technology in various fields in China is Continue to expand.However,the increase in the total number of diesel engines also brings about pollution problems.Nitrogen oxides?NO_x?and particulate matter?PM?are the main pollutants emitted by diesel engines.At present,in the air pollution,the pollution caused by automobile exhaust emissions accounts for 60⁷0%,in which NO_x emissions have a great destructive effect on the human body and the atmospheric environment.In the face of increasing environmental pollution problems,the national emission regulations are becoming more stringent,and Selective catalytic reduction?SCR?technology is one of the most effective technical means to deal with NO_x emissions.And the study of catalytic reaction characteristics has important theoretical guidance and practical application significance.This paper takes the D30 diesel engine as the research object,and builds the test rig of the DOC+CDPF+SCR system after installing the engine.It mainly surrounds the air-assisted injection as the urea injection system,and the SCR system at different speeds and torques.The working characteristics of working conditions,2 minutes and 5 minutes of stable speed variable working conditions test,and different torque 100N.m constant torque working conditions test.The research shows that:?1?The urea/NO_x equivalent ratio has a great influence on the conversion efficiency of SCR under low temperature conditions.When the urea/NO_x=1,the NO_x conversion efficiency is only 63%when the exhaust gas temperature is 180°C,and the exhaust gas temperature rises.The NO_x conversion efficiency is as high as nearly 90%at200°C.As the rotational speed increases,the pressure loss before and after the SCR increases,and at a rotational speed of 2880rpm,the pressure loss value reaches a maximum of about 4⁵kPa.?2?In the comparative test study of the steady-state variable speed of 2 minutes and 5minutes,the overall difference between the two tests is about 1.5%.The catalytic reduction reaction of urea injection system and the catalyst is in transient and steady state conditions.Both have good adaptability and stability.?3?In the SCR test study of constant torque working conditions with different speeds,during the process of increasing the speed from 1100rpm to2880rpm,the SCR inlet temperature is below 200°C at 1100rpm to 1300rpm,and the NO_x conversion efficiency is only 3%;When the temperature of 2880rpm drops to 1100rpm,the NO_x conversion efficiency remains at about 90%due to the SCR inlet temperature above220°C,indicating that the SCR inlet temperature has a great influence on the NO_x conversion efficiency under variable operating conditions.The SCR reactor was modeled based on the SCR bench test combined with the 3D simulation software AVL FIRE.The accuracy of the calculation model is determined by comparing the test values of the seven operating points with the simulated values.The study carried out two overall structural optimizations for the SCR catalyst,and designed a comparison of various different types of mixers and various rectifier schemes to analyze the pressure loss,the turbulent flow energy,the uniformity of NH₃ distribution and the NO_x conversion efficiency of different design schemes.The research indicates that:?1?For the optimization of the overall model,the removal of the rear-end rectifying plate of the catalyst has little effect on the distribution of NO_x in the carrier and the NO_x conversion efficiency,but has a great influence on the distribution of NO_x at the outlet of the catalyst.In practical applications,it will affect the detection of NO_x by NO_x sensors,but the calculation has little effect on NO_x conversion efficiency.?2?Pressure loss and turbulent flow energy comparison analysis of various SCR mixers.Strong turbulent flow energy can promote the hydrolysis and oxidation of urea,which is beneficial to the uniformity of NH₃ distribution and mixing with air,but it also causes Large pressure loss reduces engine combustion performance and power.Regarding the distribution characteristics of NH₃ in the catalyst,the distribution and slip of NH₃ of different mixer schemes are similar,and the difference between different schemes is small.In terms of NO_x conversion efficiency,the NO_x conversion efficiency of the original mixer is significantly improved at a low temperature operating temperature of 168°C.At the exhaust gas temperature of 195°C,the NO_x conversion efficiency of the original mixer is still higher than the other average of 4%.When the exhaust gas temperature is 260°C,the difference in NO_x conversion efficiency between the original mixer scheme and scheme B is small.In the scheme with better optimization results,there are lower pressure loss,better NH₃ distribution uniformity and higher NO_x conversion efficiency.?3?Comparing the three blade angle analysis of 20°and 30°and40°,the pressure loss and turbulent flow energy increase with the increase of the angle.In terms of NH₃ distribution,the original mixer has a higher turbulent flow energy,so the amount of NH₃ produced is higher.In terms of the NH₃ distribution of the inlet section of the carrier,the angle of the mixer blade increases and the uniformity of the NH₃ of the carrier inlet section increases significantly.In terms of NO_x conversion efficiency,the mixer blade angle has little effect on NO_x conversion efficiency,and the difference between conversion efficiencies is only2%.?4?Compared with different rectifier designs,the overall pressure loss of the rectifier is small,the maximum is less than 50Pa,and the pressure loss of the entire SCR system is negligible.In terms of NH₃ distribution characteristics,the uniformity of NH₃ of the inlet cross section of the carrier is quite different.The uniformity of NH₃ of the inlet of the carrier is the highest when there is no rectifier,and the uniformity of NH₃ is up to 98%under different working conditions,so the NO_x conversion in the absence of rectifier The most efficient.Since the NH₃ uniformity of the SCR is higher when there is no rectifier,the pressure loss,NH₃ distribution characteristics and NO_x of the carrier with different meshes,wall thickness and coating thickness under different working conditions are obtained based on the structurally optimized rectifierless model.Comparative analysis of conversion efficiency,and comparative analysis of urea wall membranes by SCR schemes optimized for different structures and different pore number carriers.The research shows that:?1?Compared with different carrier parameter schemes,due to the increase of mesh size and wall thickness,the cross-sectional area of the carrier channel increases and the flow resistance decreases.However,when the mesh number is 600 mesh,the increase of the carrier frictional resistance is greater than The amount of decrease in flow resistance,therefore,the magnitude of the pressure loss decreases first and then increases as the number of meshes of the carrier increases.?2?For the three kinds of carriers other than the original carrier,with the increase of the number of holes and the decrease of the wall thickness,the NO_x conversion efficiency is increasing.When the carrier is 600 mesh,when the discharge temperature is only 168°C.The NO_x conversion efficiency is nearly 15%higher than that of the original carrier.When the temperature exceeds 200°C,the NO_x conversion efficiency exceeds 90%.?3?In the comparison of urea wall membrane distribution in different structural optimization schemes of SCR,the original model NH₃ distribution uniformity and NO_x conversion efficiency are high,and the carrier exit region has strong turbulent flow energy,turbulent airflow is enhanced,and the wall membrane area is reduced..?4?In the comparison of urea wall film distribution of different carrier schemes,due to the increase of the number of pores in the carrier and the decrease of wall thickness,the contact area between urea and NH₃ and the catalyst through the pore channel increases,the NO_x conversion efficiency increases,and the NH₃ slip decreases.And the contact area of the NH₃oxidant coated on the carrier is increased,and urea and NH₃ are more oxidized,resulting in less urea membrane area at the shrink tube position.