Structural Optimization And Numerical Simulation Of High Pressure SCR System For Low-Speed Diesel Engine

With the gradual deterioration of the natural environment,the International Maritime Organization establishes stricter emission standards on the NOx of marine diesel engines.SCR technology,the most developed and effective NOx after-treatment technology both at home and abroad,meets the IMO Tier Ⅲ emission regulations,and it has been widely used in the field of diesel ships..In the background of Tier Ⅲ emission regulations,a diesel engine was chosen as the research subject.In this paper,the CFD numerical simulation method was used to study the evaporation decomposition process and catalytic decomposition of urea solution in the SCR system.Also,the NH3 concentration distribution of the SCR system flow field and the catalyst windward side,and the system pressure drop were considered to optimize the SCR system structure,to meet the design requirements of the SCR systems and NOx emission regulations.At first,in this paper,the SCR evaporation-mixer was chosen as the research subject.Two kinds of static mixer were selected and the numerical simulation models were constructed.From the point of view of the velocity uniformity at the outlet,NH3 concentration uniformity and pressure drop,the effects of urea solution evaporation and decomposition process in different types of static mixer were studied.The simulation results show that the static mixer has less influence on the velocity uniformity at the outlet;the static mixer 2 makes the uniformity of NH3 concentration at the outlet increase by 8.2%;the static mixer makes the system pressure drop slightly higher.Secondly,based on the SCR evaporation mixer,the whole SCR system was taken as the research subject,and the flow field of the whole SCR system was optimized from the point of view of the velocity uniformity at the windward side of the catalyst and the pressure drop of the system.After the first numerical calculation of the SCR system,it is found that the flow distribution is bad in the elbow and flaring parts.A method of adding a diversion device was adopted to optimize flow,and the influences of the shape and size parameters of the diversion device were discussed.The simulation results show that based on the initial scheme,the optimization effect of scheme 11 is the best;the velocity uniformity at the windward side of the catalyst is increased by 16%;the system pressure drop is decreased by 419.5Pa.Finally,the scheme with best flow field was chosen as the research.From the point of view of mass concentration variation of NH3,NO and NO2 and the denitration efficiency under different load,the catalyst reduction process of SCR was studied.The simulation results show that,in the SCR reactor,the SCR catalytic reduction process is basically consistent with the actual situation;and according to the E3 cycle calculation,the NOx weighted emission value is 2.665g/kWh,which meets the requirements of Tier Ⅲ emission regulations.The research results of this paper can provide reference for the design and optimization of high speed SCR system of low speed marine diesel engine,and lay a foundation for further research on SCR system.