Numerical Simulation Of Multiple Adsorption Sites Model For Diesel SCR Catalyst

Diesel vehicles have an irreplaceable position in production and life,but the exhaust pollutants produced in the process of its use have brought some harm to people’s living environment.With the publication and implementation of China’sⅥstage emission regulations for heavy-duty diesel vehicles,engine manufacturers have higher and higher requirements for the aftertreatment technology to reduce NO_x emissions from diesel vehicles.Selective Catalytic Reduction（Urea-SCR）is one of the most commonly used aftertreatment technologies to reduce NO_x emissions from diesel vehicle exhaust,which has attracted extensive attention from researchers because of its greatest potential for development.At present,researchers focus on the ammonia storage performance of the catalyst,NO_x conversion efficiency and hydrothermal aging stability.In this paper,single,double and three adsorption sites NH₃adsorption models were established to study the NH₃ storage capacity for the new generation of Cu-based SCR catalysts（Cu/SSZ-13）.Comparing the three NH₃adsorption models,it was found that the three-adsorption site NH₃adsorption model can quantitatively predict the NH₃desorption peak and peak temperature.Based on the three-adsorption site NH₃adsorption model with the best prediction trend,the global reactions of different adsorption sites in NH₃ oxidation,NO oxidation,standard SCR,fast SCR and NO₂-SCR reaction processes was studied.By optimizing the kinetic parameters of each reaction,the model could better predict the reaction processes of catalyst.Based on the three-adsorption site reaction kinetics model,the NH₃storage process,NH₃ oxidation and NO oxidation process of the catalyst after hydrothermal aging were verified,and the migration mechanism of the active site during the hydrothermal aging process was discussed.The NH₃ storage capacity of the catalyst fundamentally determines the NO_x conversion efficiency of the catalyst.The main factors affecting NH₃ storage include temperature,space velocity and NH₃ concentration at the reactor inlet.Based on the NH₃ adsorption model,the influence of the above factors on the NH₃ storage process was ananlyzed.It was found that the higher the isothermal adsorption temperature and the higher the space velocity,the weaker the NH₃ storage capacity of the catalyst.The change of NH₃ cconcentration at the reactor inlet only changes the NH₃ storage rate,but does not change the NH₃ storage capacity of the catclyst.Because the NH₃ storage capacity of the catalyst changes with the temperature and space velocity,the ammonia supply quantity should be reasonably adjusted during the operation of the urea injector to avoid NH₃ leakage.In addition,hydrothermal aging can change the distribution of active sites,thus affecting the NH₃ storage performance of the catalyst.Considering that it is difficult to simultaneously measure the concentration of NH₃ and NO_x in different axial positions of SCR catalyst in the small sample test by multiple points and the cost of the test is high,this paper analyzed the axial reactivity of catalysts in different reaction processes based on the kinetic model of three-adsorption site,aiming to reduce the development cost of catalysts.The results showed that the oxidation activity of NH₃ and NO increased with the increase of the axial position of the catalyst.The utilization efficiency of the first half of the catalyst was higher in the reaction process of standard SCR and fast SCR.Therefore,when optimizing the catalyst size,it is necessary to improve the catalyst formula,so as to reduce the side reaction activity of NH₃ oxidation,improve the conversion of NO_x at high remperature and broaden the temperature window of catalyst activity.