Simulation Research On Zeolite Monolith Catalysts Used By Heavy-Duty Diesel Engine SCR System

Nitrogen oxides?NO_x?emitted by diesel engines are an important cause of air pollution.With the stricter emission regulations,ammonia-selective catalytic reduction technology?NH₃-SCR?has become the dominant technology for diesel vehicles to remove NO_x.The NH₃-SCR catalyst is the core of the selective catalytic reduction system and determines the removal efficiency of NO_x.Since the high N₂ selectivity,a wide temperature active window and high hydrothermal stability,Cu-SAPO-34 catalysts has been becoming a hot spot among the current researchers.Based on the Cu-SAPO-34 catalyst sample,a fixed denitrification test bed was built in this paper.And then this paper built a one-dimensional calculation model of the sample experiment using the GT-Suite software.As a result,the kinetic model of the catalyst chemical reaction was verified by comparing the experimental results and simulation results.Based on the validated model,this paper used AVL Fire software to simulate the effect of ammonia-to-nitrogen ratio,inlet NO₂ ratio,space velocity,and catalyst active site density on catalyst denitrification performance.The results shows that:?1?When NH₃/NO<1.2,the NO_x conversion rate and the active temperature window increases with the increase of ammonia-to-nitrogen ratio at the inlet;?2?The ratio of NO₂ at the inlet increases,the NO_x conversion rate increases slightly,but the number of by-products increases too,although with the maximum by-product at the temperature of 200?;?3?As the air speed increases,the contact time between the catalyst and NO_x becomes shorter and as a result the denitrification performance decreases,but this effect is not significant at high temperature.?4?When the reaction temperature is less than 250?,the NO_x conversion rate increases with the increase of the active site density.In the high temperature stage,the effect is reversed.According to the active temperature window,the optimum active site density of the Cu-SAPO-34 catalyst is 360 mol/m³.While running,the operating conditions of the diesel engine are complex,and the performance of the catalyst is the result of multiple factors.Based on the above single factor influence results,this paper uses the air speed,ammonia nitrogen ratio and temperature as experimental factors,and the NO_x conversion rate as the target variable.Through the data regression of the experimental factors using the secondary corresponding surface model,the influence of different factors and their combination on the denitrification performance was obtained.To solve the problem that NO_x cannot be reduced due to low exhaust temperature under low load conditions in diesel engines,Cu-SAPO-34 was used as a catalyst to simulate the denitrification of a heavy-duty diesel engine SCR system.The effect was analyzed and the cause of the low NO_x conversion rate of the system was analyzed,which leads to the optimization of the system.The optimization results showed that:?1?The addition of a mixer facilitates the uniform mixing of NH₃ and the NO_x conversion rate is improved by nearly 30%;?2?The increase of mixing section length is beneficial to the hydrolysis and pyrolysis of urea droplets.The NO_x conversion rates of the urea SCR model with 4 times,6 times,and 8 times the diameter of the mixing section are 64.6%,66.8%,and 68.8 respectively;?3?Finally,the influence of urea injection parameters on the performance of the SCR system was studied,including injection velocity,nozzle position,and injection angle.The results showed that the case which has best denitrification performance is a combination of the droplet injected 30m/s,the injection position located at point B and the injectied with the angle of 45°.For this case,the NO_x conversion rate reached 77.9%,satisfying the low load and low discharge temperature.Under the operating conditions,the National V Emission Requirements was achieved that provide the basis for optimizing the urea SCR system of heavy-duty diesel engines.