Study On The Generation And Distribution Uniformity Of Reducing Agent For Diesel Engine Urea-SCR

With the increasingly strict emission regulations,the technology of Selective Catalytic Reduction?SCR?has become an important technical means for emission control of medium and heavy diesel engines,Urea-SCR system is widely used in China.The uniformity of reducing agent distribution has a great impact on the performance of the SCR system.With the higher requirements of China VI emission standards on NOx conversion efficiency and NH₃ slip level,it is practical significance to study the uniformity of reducing agent distribution in the Urea-SCR system.The Urea-SCR system injects 32.5%of urea water solution?UWS?to exhaust pipe.After evaporation and thermolysis,UWS droplets generate reducing agent NH₃ and other gases.A three-dimension model of gas-liquid two-phase flow for reducing agent generation was developed on OpenFOAM,which used two-step evaporation method and kinetic reaction for thermolysis of urea and included the gas flow,UWS spray and wall heat transfer models.The model can describe the distribution of reducing agent with high accuracy,which provides reference to study the NH₃ uniformity of Urea-SCR system.The model was used to study the influence of different factors on the NH₃ distribution uniformity.The main work and conclusions of this paper are as follows:?1?On the basis of studying the specific decomposition process of urea water solution?UWS?,a three-dimension model of gas-liquid two-phase flow for reducing agent generation was developed on OpenFOAM.In terms of UWS evaporation model,the evaporation behavior of UWS droplets at different temperatures was considered and a two-step evaporation model was adopted.The complicated side reaction process was considered in the thermolysis process of urea.Since the distribution uniformity of gas phase reductants is mainly considered in this paper,the thermolysis reaction process was simplified into 7-step reaction.In addition,the gas flow,UWS spray and wall heat transfer models were considered.According to the gas-phase flow model,the flow test was carried out,and the results was used to validate the flow model.At the same time,the extra experimental data were used to validate the reducing agent addition model,and it was found that the model accurately predict the distribution and distribution uniformity of NH₃ and HNCO at the cross section,providing reference for the study of distribution characteristics of reducing agent.?2?The global genetic algorithm was used to identify the parameters related to the urea thermolysis process,and the objective function was the relative error of the simulation and test of the reducing agent concentration at 12 measurement points at the cross section.The result showed that the genetic algorithm effectively reduced the relative error between simulation and test results and improve the accuracy of the model.It can be seen that genetic algorithm is an effective identification and optimization algorithm for the process of reducing agent production in Urea-SCR system.?3?The model was used to study the influence of various factors on the uniformity of reducing agent distribution,which provides guidance to improve the uniformity of reducing agent distribution in engineering.The influence was studied from three aspects:injection,pipeline and environment parameters.In terms of injection parameters,the uniformity of NH₃ distribution can be improved by reducing the droplet size,expanding the droplet distribution range and selecting an appropriate injection angle.The spray quantity of UWS has little effect on the distribution uniformity of reducing agent.In terms of pipeline parameters:decreasing the pipeline diameter,increasing the length and adding a mixer lead the distribution uniformity getting better.However,there exists a threshold,when the length is longer than that,increasing the length has little effect on the distribution uniformity.In terms of environment parameters:under the normal working condition of SCR system,using higher exhaust temperature and smaller exhaust flow can improve the NH₃ distribution uniformity.The environment temperature has little effect on the NH₃ distribution uniformity in Urea-SCR system.