Performance Simulation And Optimization Design Of Passive SCR System Based On Methane Reforming

Selective catalytic reduction technology is widely used in the field of marine as a aftertreatment technology that can meet the more stringent emission regulations in the future.At present,a major problem in the selective catalytic reduction of NOx with urea（urea-SCR）is the formation of urea decomposition byproducts,which adhere to the exhaust pipe in the form of crystallization,and even affect the engine operation in serious cases.The passive SCR system produces hydrogen through methane reforming,H₂ reduces NO to generate NH₃ and stores it,and then uses stored NH₃ to reduce NOx.The urea crystallization problem of urea SCR system can be avoided.In this paper,the performance simulation and optimization design of the passive SCR system based on methane reforming are carried out,which can provide guidance for the formulation of engine control strategy and the co-simulation of the coupled passive SCR system.The main work and conclusions of this paper are as follows:（1）Based on the detailed mechanism,the characteristics of methane reforming over palladium catalyst were carried out.The Chemkin software was used to study the characteristics of methane reforming over palladium catalyst.The effects of gas components,temperature and space velocity on methane reforming were investigated.The results showed that the hydrogen production was the highest in the form of methane reforming with the oxygen to carbon ratio at 0.4,and the methane conversion gradually increased with the increase of oxygen to carbon ratio.The increase of water vapor content would reduce the hydrogen production,and the increase of temperature could promote the generation of hydrogen and increase the methane conversion rate.Due to the high catalytic activity of palladium,increasing space velocity had little effect on the hydrogen production.The key element reaction in methane reforming process is analyzed by sensitivity analysis method and the reaction path diagram is drawn.The results show that methane reforming follows an indirect path,that is,total oxidation occurs first,then steam reforming and water gas shift to produce hydrogen.（2）The NH₃ generation characteristic and ammonia storage performance of passive SCR system were carried out.By simplifying the methane reforming mechanism and extending the NH₃ formation mechanism,a kinetic model of methane reforming assisted reduction of NOx was obtained.Based on the software GT-Power,the formation characteristics of NH₃ and N₂O under different gas components were studied.The results show that reducing the oxygen concentration and increasing the hydrogen concentration could promote the formation of NH₃and reduce the formation of N₂O.A kinetic model of SCR reaction with vanadium-based catalyst was established,and the effects of temperature,space velocity and inlet NH₃concentration on the ammonia storage characteristics of SCR reactor were studied.The results show that the increase of temperature and space velocity will reduce the saturated ammonia storage capacity and shorten the time to reach the saturated ammonia storage capacity.The increase of inlet NH₃ concentration will shorten the time to reach the saturated ammonia storage capacity and increase the catalyst saturated ammonia storage capacity.The results can provide guidance for the performance optimization of the passive SCR system,that is,the reduction of oxygen content in the system inlet and the enhancement of methane reforming are conducive to the generation of NH₃ and further increase the storage of ammonia in the SCR reactor.（3）The work performance of passive SCR system was carried out.The performance of passive SCR system was simulated,and it was found that temperature would affect the ammonia storage capacity of SCR system and further affect the overall denitration efficiency of the system,resulting in the increase of ammonia leakage.The lean/rich cycle time has a great influence on the performance of the passive SCR system.If the cycle time is too long,the ammonia storage capacity of the SCR will reach saturation,and the risk of ammonia leakage will be increased.Under the same cycle time,the higher the ratio of rich time,the higher the denitration efficiency of the system.The performance of the system can be improved by enlarging the size of the SCR reactor and using a multistage reactor.（4）The performance and emission characteristics of natural gas engine were analyzed under the condition of hydrogen-doped,and the passive SCR system integrated with it was optimized.A natural gas engine model was built and the influence of different hydrogen-blend ratios on engine performance and emissions was studied.The results show that hydrogen-blend could improve engine thermal efficiency and fuel economy and reduce methane emissions,but it would increase NOx emissions.The passive SCR system is designed and verified under the condition of hydrogen-blend combustion.The results show that the denitrification efficiency of the system can reach more than 80%without ammonia leakage.Finally,the cost analysis of conventional urea SCR system and passive SCR system is carried out.Passive SCR system uses palladium as catalyst to generate ammonia,so the cost is slightly higher than urea SCR system,but the cost of passive SCR system may be lower due to the fluctuations of precious metal price.