Numerical Simulation And Structure Optimization Of SCR Reactor For Marine Diesel Engine

The shipping industry has been developing effective and feasible exhaust gas aftertreatment measures to meet the increasing rigorous regulations revealed by the International Maritime Organization on NOx emissions generated from marine diesel engines.Among the NOx mitigation technologies,selective catalytic reduction(SCR),regarded as one of the effective measures for reducing NOx emissions,has good application prospects.However,the oversize marine SCR system would occupy more limited ship space and reduce the ship operation economy,which restricts its application in shipping industry to a certain extent.Therefore,it is necessary to optimize the structure of the marine SCR system to make it compact and being downsized.In this paper,based on the simulation software,the SCR reactor model is established,and the corresponding simulation analysis,structure optimization and matching analysis are carried out on the basis of the established model.Firstly,the structural sizes of the SCR reactor are preliminarily designed according to the marine engine exhaust testing data and the design concept of the SCR reactor.Secondly,the matching analysis is conducted based on the established marine diesel engine model in BOOST software.The simulation results show that the maxium impact on the engine power output and fuel economy of the marine diesel engine is about 3%after the configuration of the SCR reactor,indicating that the SCR reactor shows acceptable match ability with the marine engine.The comparison of the simulation results of different configuration scheme of the SCR reactor shows that,when the SCR reactor is configurated in front of the turbocharger rather than the rear of the turbocharger,it has less impact on the performance of the marine engine.When the reactor is installed in pre-turbo position,the power output of the engine is increased by about 1%and the fuel consumption could reduce by 0.66%at full engine load compared to post-turbo SCR configuration.The denitrification performance of the pre-turbo SCR configuration scheme is overall higher than the post-turbo SCR configuration scheme.Then,the SCR reactor model established in BOOST software is calibrated by single step calibration method,so that the established SCR reactor model could simulate the actual denitrification reaction process.Then the calibrated model is used to analyze the influence of single structural parameters on the denitrification efficiency and pressure drop of the SCR reactor,and the structural parameters of the SCR reactor are optimized by the response surface method.The volume of SCR reactor,pressure drop generated by SCR reactor and NOx conversion efficiency are regarded as the objective of the optimization.The optimization results are showed as followed:the CPSI of SCR reactor is 510,the coating thickness is 0.02 mm,the substrate wall thickness is 0.1 mm,the catalyst length is 0.5 m and the reactor cross-sectional area is 0.566 m~2.The optimized SCR reactor volume is reduced by 23.82%,the pressure drop generated by SCR reactor is reduced by 10.38%,and the denitrification performance is merely decreased by 0.51%at full engine load.Finally,the FIRE software is used to optimize the shape of the reactor,the cone angle of the expansion tube and the layer numbers of the SCR reactor.The simulation result shows that the velocity uniformity at the inlet section of the cylindrical reactor is higher than that of the square reactor,resulting in a higher NOx conversion rate than that of the square reactor.The velocity uniformity in the inlet section is highest when the cone angle of the expansion tube is 90o,so that the NOx conversion rate of the 90°cone angle reactor is slightly higher than that of the 60oand 120ocone angle SCR reactor,but the difference among them is not evident.Compared with the single layer catalyst,the NOx conversion rate of double layers arrangement of catalyst is reduced by 0.4%,and when the catalyst is arranged in three layers,the value is reduced by about 1%.With the increase of catalyst layers,the pressure drop generated by the SCR reactor is reduced.When the number of arrangement layers of the catalyst is 2,the SCR reactor shows preferable overall performance.