Study On Optimizaton For SCR System In Diesel Based On A Detailed Mechanism Of Urea Deposit Formation

In response to meet more strict nox emissions in the present China IV and the imminent China V,and the future China IVemission regulatios,the external purification measure,the selective catalytic reduction has become a indispensable key technology.However,this system also brings new problem—urea deposit risk.The existence of urea deposit will affectthe uniformity of NH3 mole concentration on outlet,which affects NOx conversion efficiency.In this paper,CFD simulation method was used toestablish a detailed mechanism model of deposit formation,studying the deposit formation rule,to seek the influence factors for deposit formation.Basing on these factors,multiple parameter structure optimization to the SCR system was implemented by a advanced hybrid algorithm.The purpose is to reduce or even avoid deposit formation in SCR system,which can bring down the back pressure and improve the NOx conversion efficiency.The paper presented a detailed chemical kinetic model of urea decomposit formation.This model consists of 15 species and 11 reactions,which including mechanisms of biuret,cyanuric acid,and ammelide formation.This model made up for the deficiency of film model.At the same time,by using group contribution method,the relation between the ammlide’s specific heat and the temperature was constructed,to obtain the thermodynamic parameters of the mechanism model is needed.On the premise which we cannot obtain the activation energy and pre-exponential factor of the reaction C3H3N3O3+NH3→C3H4N4O2+H20 through the present literatures and experiments,the paper basing on the quantum chemical calculation method used the transition state to solve chemical reaction rate constant.And the activation energy and pre-exponential factor will be calculated according to chemical reaction rate constant by the Arrhenius equation.A study was carried out to find the impact relation between the geometric structure、injection parameters and urea deposit.NH3 uniformity.Research shows that,increasing the pipe length and pipe radius will extend the residence time of urea droplets in the exhaust line,which can improve NH3 uniformity on outlet.Athough pipe length to the NH3 uniformity influence is lower than the pipeline radius,the impact on the urea decomposition efficiency will be significantly than that of the pipe radius.Amounting mixer can increase the intensity of turbulence and vortex in the pipeline,which can greatly enhance NH3 uniformity and reducing the generation of urea deposits.The change of the spray cone angle and the spray angle to NH3 uniformity and urea deposit influence is not obvious.In this paper eight design variables-the pipe length,radius of pipe,mixer installation location,blade number of mixer,blade length,blade angle,the spray cone angle and injection angle were set up to build optimization search space.Then advanced SHERPA algorithm was used to optimize the SCR model.Results show that the best NH3 uniformity is 0.816,which has a 39%improvement.The pipeline length is 1.2 m,diameter of the pipeline is 150 mm,mixer installation location is 0.6 m,blade number is 3,blade length is 0.02 m,the blade angle is 30 degrees,the injection angle is 45 degrees,and the spray cone angle is 80 degrees.