Detailed Urea Decomposition Reaction Kinetics For Diesel Engine SCR System

National VI emission regulations have been implemented in some cities.High-efficiency and low-emission diesel engines have become the focus of future research.Urea-SCR technology has a wide range of catalytic temperatures and high temperature when combined with diesel exhaust gas treatment.Restore safety and good fuel economy advantages.Urea is used as the storage source of ammonia(NH3)as a reducing agent.In the actual diesel engine SCR system,urea should be converted into ammonia in a short time.Otherwise,incomplete urea decomposition easily leads to deposit formation,affecting the conversion efficiency of NH3 and reducing the catalyst actively.The detailed decomposition model of urea is constructed,and the formation process of deposits in SCR system is discussed from the perspective of chemical reaction.The structure of urea decomposition tube is constructed by simulation method.The operating efficiency of urea is set to monitor the efficiency of urea transformation and the uniformity of NH3 at the outlet of decomposition tube.The distribution of the amount of deposit formation changed,and the internal structure of the pipeline was modified to optimize the distribution of NH3.From the structural aspect,the solution to reduce deposit formation was proposed.The formation and decomposition model of biuret,cyanuric acid and cyanuric acid homologues,the main by-products of urea deposits,were established.The transition state theory of quantum chemical method was used to calculate the formation and decomposition of cyanuric acid homologues.Activation energy and pre-exponential factor,thermogravimetric analysis of each component of the deposit to study the specific decomposition temperature of each component,and the test data obtained from the thermogravimetric curve to calculate the dynamics of the reaction of each component at different temperature stages Learn the parameters,combined with the sub-model to verify the possible reactions of each component in the decomposition process and compare the results of the two methods.The 13-step detailed urea decomposition reaction mechanism was transferred to the STAR-CCM+ fluid simulation software.The kinetic parameters of the quantum chemical method and experimental data were verified by comparing the molar mass fraction of NH3 at the outlet of different exhaust temperatures with the total package reaction.Feasibility,and for further verification,established three evaluation indexes of urea transformation rate,NH3 uniformity coefficient at the exit and deposit content change,and observed two kinds of exhaust gas temperature,urea injection amount and spray cone angle.The resulting trend of the method and the design of a new mixer to improve the uneven distribution of NH3 and the large amount of deposit formation.The results show that the three evaluation indicators show that the results of the two methods are consistent,and the results of the experimental kinetic calculation method are superior to the quantum chemistry method.From the urea transformation effect analysis,the exhaust gas temperature promotes the urea transformation rate,but the increase of the urea injection amount inhibits the transformation efficiency,indicating that the urea injection amount should be strictly controlled in the specific operation process,and the different spray cone angle pairs The effect of ammonia conversion efficiency is small,and as long as it is within a reasonable range,the effective conversion of the urea decomposition process can be guaranteed.Considering the change in the amount of deposit formation,the active region generated by by-products from 1750C to 225°C should be avoided in this temperature range for a long time,and the increase of exhaust gas temperature is beneficial to the decomposition of various components of the deposit.Gradually increasing the urea injection amount and the spray cone angle at a temperature of 250°C promotes the formation of by-products.The urea injection amount should not exceed 2kg.h-1,and the spray cone angle should not exceed 70°.The uniformity of NH3 at the outlet shows that increasing the exhaust gas temperature and the spray cone angle are favorable for the uniform distribution of ammonia gas,and the increase of urea injection amount makes the decomposition of urea not timely and increases the formation of by-products.The new vane mixer can increase the NH3 uniformity coefficient at the outlet by 27%,and the deposit has a greater influence on melamine,which can effectively reduce the amount of melamine produced,but the new design grid mixer has NH3 uniformity and sediment at the outlet.The effect of changes in content is limited.