Prediction Of Crystallization Risk Of Urea Deposits In Diesel SCR System

The implementation of China VI standard emission regulations has prompted how to reduce the NO_X emission level of diesel engines become a research hotspot in the field of internal combustion engines.However,after the urea water solution?UWS,67.5%deionized water and 32.5%urea?was injected into the exhaust pipe by the urea injector,the poor quality of spray and the low temperature of exhaust gas cause urea decomposed partially into solid deposits such as Biuret and cyanuric acid?CYA?.These deposits will not only affect the efficiency of SCR system ammonia conversion,but also may block the exhaust pipe and even cause shut down.In this paper,the process of thermal decomposition of urea and the formation of deposits of the Urea-SCR system are studied in detail.The kinetic model of urea thermal decomposition chemical reaction is established.The evaporation,pyrolysis,wall collision,crystallization and other processes of urea water solution were simulated,a crystallization risk assessment of the mixer surface and exhaust pipe wall was established finally.The gas chromatograph and thermal gravimetric analyzer was used to qualitatively and quantitatively analyze the deposit components,it is clear that the main components of the deposit are Biuret,CYA and CYA homologues,and also contain a small amount of urea and High-temperature resistant products such as heptazines.The pyrolysis characteristic curve of each deposit component was obtained by thermogravimetric test,and a kinetic mechanism model of urea decomposition including 11-step elementary reaction was constructed.Pearson correlation analysis showed that the model had a high correlation with the thermogravimetric test.The kinetic mechanism model shows that the formation temperatures of Biuret,CYA,Ammelide,and Ammeline are 430K,480K,500K and 640 K;The initial decomposition temperature and complete decomposition temperature of Urea,Biuret,CYA and Ammelide are 420K—540K,430K—540K,500K—670K,580K—720K.The processes of UWS injection,droplet collision,liquid film formation and deposit crystallization in the exhaust pipe were modeled in detail,and the state change of the liquid film on a long-term scale was considered,finally a three-dimensional gas-liquid phase flow CFD mathematical model was established.Based on the CFD model,the deposit crystallization risk prediction model is constructed with the liquid film temperature,liquid film thickness,liquid film urea concentration and gaseous isocyanate concentration around the liquid film as influencing factors.The prediction model results show that the highest risk of deposit crystallization in the exhaust pipe is the mixer surface and the exhaust pipe wall on both sides of the mixer.The main component of the deposit on the surface of mixer column is urea,and the main composition of the deposit on the surface of mixer blade is urea by-product.The effect of exhaust temperature,exhaust flow rate,thermal conductivity of mixer and exhaust pipe materials,thickness of mixer and exhaust pipe,UWS injection rate and UWS temperature on Urea-NH₃ conversion efficiency,deposit formation rate and crystal area ratio of deposit were analyzed by single factor analysis.The sensitivity analysis of each parameter shows that:in the range of variable setting,the changes of exhaust temperature,exhaust flow rate and UWS injection rate have a significant effect on urea decomposition.Increasing exhaust temperature and exhaust flow rate can effectively improve the Urea-NH₃conversion efficiency and reduce the formation of deposits,however,if the injection rate of UWS is too high,the effect of urea decomposition will be reduced;The effect of changing the material thermal conductivity,material thickness and UWS temperature on urea decomposition is not obvious,but using the low thermal conductivity exhaust pipe can reduce the deposit crystallization area on exhaust pipe wall more effectively than increasing the exhaust flow.