Research On Adsorption Characteristics And Desorption Characteristics Of Ammonia Storage Materials And Application In Solid SCR System

In recent years,with the deterioration of atmospheric environment and the increasingly strict requirements of automobile emission regulations,the research on automobile exhaust emission control technology has attracted more and more attention.For NO_x,which is the most difficult to deal with,selective catalytic reduction（SCR）technology is basically adopted to remove it at present,and its conversion efficiency can reach over 90%.The reducing agent required for SCR comes from urea solution.Traditional SCR technology has a good effect in reducing NO_x emission of diesel engine,but it also has many problems that are difficult to solve:low density of ammonia storage,freezing of urea solution below-11℃,low efficiency at low exhaust temperature.In recent years,the solid SCR technology（SSCR）developed to effectively solve these problems which provides NH₃ by decomposition of solid ammonia storage material.SSCR technology has many advantages such as high density of ammonia storage,good safety,and high NO_x removal efficiency at low temperature.As one of the core factors of SSCR system,the performance of solid ammonia storage material affects the NO_x removal efficiency of SSCR system directly.Therefore,it is very important and urgent to study the characteristics of ammonia storage materials in SSCR technology.In this study,adsorption and desorption characteristics of various kinds of ammonia storage materials were studied on the solid SCR system.Besides,the reasons for the decrease in adsorption capacity were analyzed by TGA,BET,SEM and a method to solve the problem of adsorption capacity reduction was proposed.The results show that Sr（NH₃）₈Cl₂/expanded graphite is the best ammonia storage material.The main contents and conclusions of this paper are as follows:Through thermogravimetric analysis and theoretical calculation,the ammonia storage characteristics of three types of ammonia storage materials were studied.The results show that the amine complex has the highest ammonia storage density,which can reach 3 times that of urea aqueous solution.Sr（NH₃）₈Cl₂ requires the lowest temperature and minimum energy for desorbing ammonia.It only needs 34℃to release ammonia.For each mole of ammonia mass desorbed,the energy required is only 40 k J which less than half of urea aqueous solution.The equilibrium vapor pressure of the amine complex is below 1 bar,and the price is low.The adsorption characteristics of SrCl₂ and desorption characteristics of Sr（NH₃）₈Cl₂ were studied on the solid SCR system.In terms of adsorption characteristics,it takes only 15 minutes to reach more than 90%of the saturated adsorption capacity by increasing the adsorption pressure to 4 bar.A phenomenon was found during the adsorption cycle,which is caused by the change of the material structure and agglomeration.In terms of ammonia desorption,the set injection pressure can be achieved in less than 5 minutes by increasing the heating temperature to 250℃.Material modification was studied in order to solve the problem of adsorption capacity reduction,and the composite adsorbent of SrCl₂/activated carbon and SrCl₂/expanded graphite were prepared.The show that the performance of SrCl₂/expanded graphite is better.When the mass of expanded graphite is one-sixth of the mass of strontium chloride and the expansion times of graphite is 148,the cyclic adsorption capacity is stable,reaching 0.79 g/g.By suppressing Sr（NH₃）₈Cl₂,the volume density of ammonia storage is increased,and the time for solid SCR system to reach the set injection pressure reduced to 3 minutes.The application of Sr（NH₃）₈Cl₂/expanded graphite in solid SCR system has been studied.The results show that the NO_x removal efficiency is still81%in the low exhaust temperature section of 180℃.When the space velocity increases to 90,000 h^-1,the NO_x removal efficiency is still 61%,and the performance is significantly better than the traditional Adblue-SCR system.By setting the start-up unit in the solid SCR system,the time required to start ammonia injection can be shortened to 40 s.