Numerical Simulation And Experimental Evaluation On Aging Of Urea-SCR Catalytic Converter

The limit of NOx emission is getting increasingly stringent withthedevelopmentof emission regulations in China.SCR emission control technology is recognized as one ofthe effective after-treatmenttechnologiesfor reducing NOx emission,whichis either implemented independently or combined with other emission control technologies.The durability of an SCR catalystisstipulatedtobe no less than 160 thousand kilometersby regulations,leading to amore attention being paidtothe durability study for SCR catalysts.In this dissertation,a numerical simulation of the thermal aging of a commercially-available SCR catalyst was conducted.This involved the testing and performance evaluationofthe hydrothermal aging sample of the catalyst.Based on the research on the models and experiments,the reaction kinetics model of the SCR catalyst was established in combination with the basic theory of chemical reaction kinetics,in the form of Eley-Rideal reaction mechanisms,chemical reaction rate model,basic control model of gas-solid reaction and the storage model of NH3.FLUENT coupling with CHEMKIN software code was utilized for simulating the reaction kinetics model,which was compared with results from test bench for the purpose of verifying the accuracy of the model.Subsequently,the influenceof average particle diameter and catalytic reaction rate during the thermal aging process of the SCR catalyst to the relative activity of catalyst were analyzed.This was furtherusedto establish the chemical reaction kinetics model for the thermal aging bycorrecting the reaction kinetics model.Numerical simulation of this model was implemented according to SCR bench rapid aging test cycle step by step,which concluded that during the aging process,both the catalyst relative activity and the conversion efficiency of NO decreased,with the temperature window shrinking,the light-off temperature increasing and the light-off characteristic deteriorating.Through calculation,it was concluded that the average diameter of catalyst particle increased with the aging process,with the increase appearing particularly in the area of entrance and center of the carrier.Catalyst samples with different aging levels were produced by simulating thehydrothermal aging process,and the performance and microstructure characteristics of evaluated and tested.A higher calcination temperature and longer calcination time worsened the aging deactivation,decreased the conversion efficiency,shrunk the temperature window,increased the light-off temperature and worsened the light-off characteristic,as revealed by the numerical simulation results.The SEM graph indicated that the surfactant accumulated with the aging process.The XRD analysis showed thatthe anatase-phasedeposited on the surface of catalyst is reduced and converted into rutile-phase.Dynamic parameters were calculated using performance data resulting from evaluation tests,showing that the activation energy(E)increased gradually while pre-exponential factor(A)decreased gradually with the deteriorating hydrothermal aging level of the catalyst.In this thesis,the study of SCR catalyst performance during the aging process provided a reference for the design,improvement of a commercially-available SCR catalytic converteras well as the diagnostics,control and performance matching of OBD.