| The implementation and promulgation of the China V and VI emission regulations have significantly reduced the emission levels of NOx(nitrogen oxides)from diesel engines.Urea-SCR(Urea-Selective Catalytic Reduction)technology has become the most ideal NOx aftertreatment technology in diesel engine for its superior fuel economy,high NOx conversion efficiency,safe supply of reducing agent,and wide range of catalyst temperature adaptation.However,with the injection of urea as the supply source for the reducing agent NH3(ammonia),it is required that the decomposition process of urea-ammonia conversion can be completed quickly and efficiently.The incomplete decomposition of urea directly affects the conversion efficiency of NH3,which increases the occurance probability of side reactions and the risk of urea deposit formation in the SCR system.In this paper,the process of urea decomposition was studied in detail,and the detailed reaction network framework for urea decomposition was perfected and proposed.From the perspective of microscopic chemical reaction mechanism,the reason for deposit formation was explored,and the theoretical method for reducing deposit formation was sought.A detailed reaction mechanism model for urea decomposition and deposit formation was constructed.The urea decomposition section of the SCR system exhaust pipe was physically modeled in combination with the CFD method to realize the simulation analysis of the urea decomposition of the Urea-SCR system.Three evaluation indexes were established to quantify the urea decomposition effect,and the effect of operating parameters on the urea decomposition effect was studied to provide a theoretical reference for optimizing the matching of Urea-SCR system operating parameters.Through the methods of literature review and chemical structure analysis,a detailed reaction network for urea decomposition was constructed containing seven sub-models,which not only explained the formation process of common products such as NH3 and HNCO but also explained the reaction sources and formation pathways of by-products such as biuret,triuret,cyanuric acid,and triazines.The process of formation of deposit components such as melon,melam,and melem,which had been mentioned in many literatures but are not thoroughly described,has been studied and improved.According to the detailed reaction framework of urea decomposition,a chemical kinetic model of urea decomposition which contained 18 components,15-step reactions were constructed based on the original 11-step reaction mechanism,including new components such as triuret,ammeline,and melamine,etc.The new kinetic model further extends the prediction range of the mechanism.Quantum chemistry and transition state theory were used to estimate the chemical kinetic parameters of ammeline and melamine generation reactions.The group contribution method was used to estimate the thermodynamic parameters of the ammeline,melamine,triuret,which solved the problem that the reaction kinetic parameters and physical parameters could not be obtained through experiments and literature.The urea decomposition process was simulated using the STAR CCM+software in a 15-step detailed mechanism.The reliability of the detailed mechanism was preliminarily verified by comparing the molar fraction curves of NH3 and mole fraction curve of deposit like biuret,ammeline with temperature changes on outlet section obtained by the simulations using total package reaction and detailed reaction mechanism.The temperature distribution,the distribution of components along the section of the pipe length and diameter,the distribution of the outlet section NH3 in urea decomposition section of SCR system exhaust pipe were simulated and analyzed.Control variable method was adopted to study the effect of operating parameters such as exhaust temperature,exhaust flow,urea injection flow,spray cone angle,and spray temperature on urea-ammonia conversion efficiency,the NH3 surface uniformity on outlet section,and deposit production,making a comprehensive evaluation of their role in urea decomposition process.The study shows that,within the variable setting range,the exhaust temperature,exhaust flow,urea solution injection rate has the most obvious effect on urea decomposition result;improving the exhaust temperature and appropriately increasing exhaust flow gas are beneficial to the urea decomposition process;excessive injection amount of urea solution causes a rapid deterioration of urea decomposition effect;the spray cone angle,and spray temperature have a weak effect on urea decomposition.The change of urea-ammonia conversion efficiency with spray cone angle,and spray temperature does not exceed 5%,and is basically stable at around 52%.The ammonia uniformity coefficient of the outlet is almost unaffected by variations in the spray cone angle,and spray temperature.The spray cone angle,spray temperature have a certain influence on the change of deposit production,but the change range is limited.And different types of by-products may show different patterns of change. |
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