Simulation And Experimental Research On SCR System Control Strategy Of Medium Speed Marine Diesel Engine

The marine diesel engine nitrogen oxides (NOx) emission reduction has received much attention in recent years, and the Tier III standard published by the International Maritime Organization (IMO) will be forced to implement in 2016. In many NOx emission reduction technology, selective catalytic reduction (SCR) has been regarded as the most promising technology for marine diesel engine NOx emissions due to its high efficiency. SCR system control technology has become an important research topic in the field of marine diesel engine NOx emissions. In this paper, the SCR catalytic reaction model and the urea injection control test based on this model are studied. The whole paper is organized as follows:In view of the establishment of the SCR catalytic reaction model, the reaction principle of the chemical reactions in the catalyst of SCR system was analyzed comprehensively. The main chemical reactions in SCR vanadium based catalyst are analysed from aspect of temperature and catalyst type. The SCR catalytic reaction model is established based on the model hypothesis. Through the SCR system experimental bench test, it is proved that the SCR catalytic reaction model can accurately predict the NOx concentration and NH3 concentration of SCR reactor outlet.In order to identify those unknown parameters in SCR dynamic analysis, both the nonlinear least-square method and the trust region algorithm are applied. Experimental verifications of identified parameters are carried out and results of which show that the nonlinear least square method can accurately identify the unknown parameters of SCR dynamics,and improve the accuracy of SCR catalytic reaction model.Experimental analysis on the cross sensitivity of NOx sensor is conducted by using Calman filter method. The influence of exhaust temperature, ammonia nitrogen ratio and other factors on the cross sensitivity of NOx sensor was analyzed. The results show that the EKF method can effectively remove the NOx sensor reading error from ammonia cross sensitivity.The experimental bench of SCR system is built on medium speed marine diesel engine,and theoretical analysis of various urea injection control strategies formulated by S7-300 PLC controller are conducted. Experimental study on the open loop control strategy and the closed-loop PID control strategy are carried out. Comparisons of two control strategies are made and results of which verify that the two control strategies can make the NOx emission of marine diesel engine reach the Tier ? standard, but the control effect of the closed-loop control strategy is more excellent.