Study On Urea Deposit Risk Prediction And Assessment Of Diesel Engine SCR System

Under the current situation of emission regulation becoming increasingly stringent,selective catalytic reduction(SCR)technology plays an important role in the field of diesel engine NOx emission control.The close-coupled aftertreatment system and low exhaust temperature of efficient diesel engine contribute to the appearance of deposit issue.Deposit matters have inverse effect on working efficiency of aftertreatment system and fuel economy of diesel engine,therefore,it is of great necessity to conduct studies surrounding deposit risk of SCR system.In order to further study the deposit risk of diesel engine SCR system,establishment and validation of the ammonia addition process model was conducted primarily.Then,based on the simulation model of SCR system mixing section,the forming process of deposit precursor and the influence of engine-side and aftertreatment-side factors on deposit risk were studied.Finally,according to the influence law of exhaust temperature and urea water solution injection rate on deposit risk,the detailed analysis and accurate assessment of deposit risk of a China VI diesel engine SCR system were realized with bench test and simulation model.Main works and conclusions of this paper as follows:(1)Modeling and validation of ammonia addition process in SCR system:Taking relevant processes inside SCR system into consideration,including gas phase flow,heat transfer,spray atomization,component evaporation,spray-wall interaction,liquid film formation,and urea thermal decomposition reaction,the ammonia addition process model was established on the simulation software platform of Starccm+,and the model was validated comprehensively from four aspects,i.e.,urea water solution atomization,liquid film formation,ammonia conversion,and by-product generation of urea thermal decomposition.The results show that the model can well simulate the processes of urea water solution atomization,liquid film formation,ammonia conversion,and urea thermal decomposition reaction.The modeling is reasonable and can be used for follow-up study on deposit risk.(2)Study on the formation process of liquid film(precursor of urea deposit)and influencing factors of deposit risk in SCR system deposit precursor:Simulation model of SCR system mixing section was established,and evaluation index was defined.Based on the simulation model,the forming process of liquid film under cold and hot conditions,and the influence of engine-side and aftertreatment-side factors on deposit risk were studied.The results show that,(a)there exists a consistency between cold and hot conditions in both the evolution process of urea water solution droplets and the formation site of liquid film,but the formation process of liquid film is quite different.Meanwhile,there exists deposit risk under cold condition,whereas there doesn’t exists deposit risk under hot condition.(b)The increase of exhaust temperature can effectively reduce the deposit risk.(c)The exhaust flow rate has little effect on deposit risk.(d)The urea water solution injection rate has a significant effect on the deposit risk.(e)Increasing injection angle of single hole will increase deposit risk.(f)The number of orifices and the spray angle of single spray bundle have opposite effects on the maximum thickness and accumulation of liquid film,which make it difficult to identify the deterministic correlation between factors and deposit risk.(g)The angle of swirl plate has little effect on deposit risk,considering pressure loss and reductant uniformity,it can be concluded that the 40° layout is more reasonable than other.(3)Analysis and assessment study of deposit risk for a China VI diesel engine SCR system:The test system was built firstly.According to the influence law of exhaust temperature and urea water solution injection rate on deposit risk,representative operating conditions were selected within all operating conditions.A full-size three-dimensional simulation model of SCR system was established,and the deposit risk was analyzed and assessed based on the simulation results of the representative operating conditions.The deposit test was carried out to validate the accuracy of the representative operating conditions selection and assessment results.The study shows that the liquid film forms on the inner wall of perforated tube with a macroscopic morphology like spiral band under the rated condition and the typical condition.The accumulation of urea and liquid film,and the loss rate of urea increase with the extension of injection time,and both representative operating conditions are judged to have deposit risk.After diesel engine was operated for 8 hours under the WHTC cycle condition,urea deposit was found on the inner wall of perforated tube.The location and characteristics of deposit distribution are consistent with the simulation analysis results,which proves that the representative operating conditions selection and assessment results are accurate.