Research On SCR System Closed-loop Control Strategy Of Marine Diesel Engine

The International Maritime Organization(IMO)strictly enforced the Tier ? emission standards in 2016.According to the new standards,the NOx emission of marine diesel engine should be reduced by 75%in reference to the Tier ? standards released in 2011.Current marine diesel engine cannot meet the new requirements of the Tier ? emission standards without external purification devices.At present,selective catalytic reduction technology(SCR)is considered as the most promising technology for marine diesel engine to reduce NOx emission effectively,which is reported to decrease 80%-90%NO,emission.In addition,the technology has the advantage of a wide application field and less influence on the safety of engine and low long-term operating costs on the marine diesel engine,therefore,it is favored by many engine manufacturers and shipyards domestic and abroad.In this paper,a SCR system dynamic model is established,simulated with closed-loop control strategy and experimentally tested.The research content and results are presented as follows:Based on principle of SCR catalytic reaction,a dynamic system is modeled.The system parameters are identified using least square method and input into the simulation.The theoretic model is validated through experimental data under stable operating condition.The result is consistent with the experimental data with a small margin,which suggested that the obtained parameters were accurate.The influence of exhaust temperature,NH3 concentration and NOx concentration on the NOx sensor cross sensitivity was researched.Under each affecting factor,the fitted formula was put forward in terms of experimental data.Aiming at the effects of exhaust gas temperature,it showed that cross sensitive factor gradually rose along with the rise of temperature and the rising rate was increasingly fast within the range of research temperature.For the influence of the concentration of NH3,NH3 concentration did not affect the cross sensitivity factor.Finally aiming at the effects of NOx concentration,It found that cross sensitive factor gradually decreased with the rise of NOx concentration and The rate of cross sensitive factor reduction decreased with the NO,concentration increasing.This paper introduced the Extended Kalman Filtering(EKF)and Unscented Kalman Filter(UKF)methods,The algorithms were implemented in MATLAB to estimate true value of the NOx concentration.EKF and UKF exhibited strong robustness against the instrumental error in the experiments.Based on the closed-loop PID control strategy simulation studies were conducted,it showed that the control effect was very obvious under the stable conditions and cyclic conditions.Experimental research on the closed-loop control strategy based on SCR system test bench was carried out.With the closed-loop PID feedback and feed-forward control strategy of urea injection of PLC S7-300 controller,the control effect of the strategy was idea under the stable condition and cyclic conditions,and closed-loop emitted dose of urea was comparatively accurate.This control strategy had strong robustness for the SCR system.