Study On Deposit Mechanism And Modeling Of SCR Urea Injection Process In Diesel Engines

In order to meet increasingly stringent emission regulations,diesel SCR systems are widely used as exhaust aftertreatment technologies for efficient NOx elimination.Currently,diesel SCR systems commonly use urea water solution as the reductant additive.Due to the inherent characteristics of urea,its incomplete decomposition in the pipeline can lead to deposition.With the upgrading of emission regulations,the proportion of low-speed and low-load working condition emissions of diesel engines has increased,and the trend of low-temperature exhaust makes the urea deposit problem more prominent.Since the simulation tools for the urea deposit process are not perfect,it is common to optimize the SCR system to avoid urea deposit through a large number of experiments.Therefore,it is important to clarify the generation mechanism of urea deposit in diesel SCR system and establish an effective simulation model for the design and optimization of SCR system.In this dissertation,the urea deposit problem in SCR system of diesel engine was studied by visualizing the wall film and deposit characteristics of SCR urea injection process,and the mechanism of urea deposit was clarified by combining with the experimental study of thermal decomposition of urea,and the whole process model of SCR reductant addition coupled with urea decomposition mechanism was established accordingly to realize the simulation of urea deposit process,which provides a reliable simulation tool for diesel SCR urea injection optimization.The main work and conclusions of the dissertation are as follows:(1)Experimental study of urea deposit process: Based on the visualization test,this dissertation made a direct observation of the urea deposit process in SCR system and summarized two different modes of urea deposit generation: liquid wall film is produced on the wall surface of the pipe when urea is injected,the evaporation of water in the wall film leads to the precipitation of urea and other substances in over concentration and finally solidifies to form deposit,which can be quickly decomposed and eliminated when the exhaust gas temperature is maintained;some of the deposit formed will block the upstream wall film movement,and the wall film will accumulate there,causing further depostion to dam up and grow,and it is difficult to eliminate these crystals when the exhaust gas temperature is maintained.The results show that HNCO is an important intermediate product of urea decomposition byproducts,which will be dissolved in the molten reactants when generated.The accumulation of wall film during the growth of deposit dams leads to the aggregation of HNCO,so the resulting deposit are more difficult to decompose.(2)Study of urea wall film and deposit model: Based on the experimental study,a reaction kinetic model of urea pyrolysis was constructed,and the reaction kinetic parameters were optimized based on particle swarm algorithm.Moreover,the reaction was improved by considering the effect of reaction area on the decomposition process of cyanuric acid.The reaction kinetic model was coupled to a quasi-3D wall film model,which was able to calculate the component changes of the reactants within the wall film,and the liquid-solid physical transformation process of the wall film components was considered,and the deposit damming growth process was reflected by the viscosity change of the wall film.(3)Study of SCR reductant addition process model: The SCR system reductant addition process model was constructed by integrating the gas flow,spray motion,droplet evaporation and pyrolysis,spray-wall interaction,wall film formation and deposit process in the exhaust pipe.Among them,the differences in the pyrolysis process of urea in spray droplets and wall film were considered,and the kinetic model of droplet pyrolysis reaction was further optimized,and the model parameters were identified by the gas phase products.The model was comprehensively validated in terms of gas phase product distribution,wall film area,deposit location,deposit composition and quality,etc.The results show that the model has good accuracy and can realize a comprehensive simulation of the urea spray to deposit process,which can be used as a simulation tool to guide the design and optimization of the actual SCR system.(4)Simulation study of urea deposit process: Based on the established model,simulation study of factors affecting urea deposit generation in SCR system was carried out.The results show that temperature has the most direct influence on deposition,and increasing the exhaust temperature can significantly reduce the wall film and deposit quality,but the proportion of cyanuric acid and ammelide will also increase;pipe insulation can reduce the amount of wall film and deposit generation,but the effect of insulation is not obvious under continuous injection;pipe heating can significantly avoid the generation of wall film and deposit,and the effect is best when the exhaust temperature is relatively low;reducing the droplet size can increase the evaporative pyrolysis rate of droplets and effectively avoid deposit;increasing the spray cone angle and reducing the bending of the pipe can expand the range of wall film,reduce the thickness of the wall film,and reduce the proportion of hard-to-decompose substances in deposit.