Experimental And Simulation Study On Ammonia Storage Characteristics Of Vehicle SCR

Due to the aggravation of air pollution,human life and production activities are seriously threatened.Exhaust aftertreatment technology has become a necessary means to reduce the exhaust pollutions of internal combustion engines.Selective catalytic reduction（SCR）is one of the most effective technologies to reduce NOx emission of diesel engines.It also brings ammonia leakage and N₂O and other harmful gases.With the increasingly strict limitation of ammonia leakage in emission regulations,more and more attention has been paid to the research on ammonia storage characteristics of SCR catalyst.In this paper,SCR small sample reaction test-bed was built to study the ammonia storage characteristics of copper based and vanadium based catalyst samples provided by Weichai Power Co.,Ltd.Through NH₃-TPD test,the whole process of NH₃ adsorption and desorption in catalyst are analyzed.The effects of temperature,space velocity,ammonia nitrogen ratio and other factors on ammonia storage capacity and ammonia leakage time of catalyst are studied.In addition,the relationship between ammonia storage and no conversion efficiency and N₂O production is also studied.Finally,the model of NH₃-TPD is built by AVL Boost software.After parameter optimization,the prediction of NH₃ concentration change in the process of adsorption and desorption is in good agreement with the experiment.It is found that the ammonia storage capacity of copper based and vanadium based catalysts decreases with the increase of temperature in the range of 150～550℃.The most obvious decrease is in the range of 250～450℃.The ammonia storage capacity of copper based catalysts is better than that of vanadium based catalysts,and the ammonia storage capacity of vanadium based catalysts is more affected by temperature.In the range of 10000～50000h^-1,the saturated ammonia storage capacity of copper based and vanadium based catalysts decreased with the increase of space velocity,but the reduction trend becomes slower with the increase of space velocity.The influence of space velocity on the saturated ammonia storage capacity of vanadium based catalysts was greater than that of copper based catalysts,and the influence of temperature on the ammonia storage capacity of the two catalysts is greater than that of space velocity.At 20000h^-1,the ammonia leakage time of copper-based and vanadium-based catalysts increased first and then reduces with the increase of temperature,because the increases of catalyst activity in the low temperature range plays a leading role,and the reduction of catalyst ammonia storage capacity in the high temperature range plays a leading role.In addition,the ammonia oxidation capacity of copper-based catalysts at high temperature consumes part of NH₃,making the ammonia leakage time decreased and released slow;the increase of air speed may lead to the advance of ammonia leakage,and its effect is not as good as that of temperature;the increase of ammonia nitrogen ratio may lead to the advance of ammonia leakage time and the steady increase of ammonia leakage concentration.The above three factors affect the ammonia leakage time from large to small:ammonia nitrogen ratio,temperature and air speed.The low temperature activity of Cu based catalyst is higher than that of V-based catalyst.The90%NO conversion efficiency window is 250～500℃,and the V-based catalyst is 300～450℃.At low temperature,the copper based catalyst still has good performance,and can achieve high NO conversion efficiency at low ammonia storage,while the ammonia storage restricts the increase of no conversion efficiency of vanadium based catalyst.At high temperature,the ammonia storage has little effect on no conversion efficiency.When the ammonia storage is low,it restricts the formation of N₂O.When the ammonia storage is high,the N₂O production of copper based catalyst is less affected by it.