Study On The Urea Dosing Strategy Of SCR System For Marine Diesel Engine

Shipping industry has been seeking for applicable technologies for NOx emission reduction to meet the IMO Tier III standard, which sets strict NOx emission limits for marine diesel engine. Among various NOx reduction techniques, Selective Catalytic Reduction (SCR) is one of the most effective measures recognized by the industry. Appropriate urea dosing strategy has a significant importance for the fully exertion of catalytic reduction performance of SCR system. In this case, urea dosing strategy of SCR system is studied in this paper.Firstly, targeted on the SCR system for 6105AZLD marine diesel engine, the experiment of the SCR catalyst perfomance is carried out, and the experimental data is collected. Then, SCR catalyst model is established with AVL BOOST and its boundary conditions are set based on experimental data. To narrow the gap of catalyst activity between the model and the actual catalyst, genetic algorithm is used to identify the kinetic parameters. Identification result shows that the catalyst in the model and the actual catalyst have similar activity.Secondly, the steady state urea dosing strategy is developed for the target engine SCR system. Based on the SCR catalyst model, the optimum NSR valve and optimum urea dosing rate are determined for each steady state of the target engine SCR system. D2 test cycle is conducted on the target engine SCR system, and the test results show that the steady state urea dosing strategy is reasonable, meanwhile the simulation results of the SCR catalyst model are identical with the test results, which further validates the validity of the SCR catalyst model.Finally, based on the SCR catalyst model, the ammonia storage characteristics of the SCR catalyst and its effect on SCR system performnce were studied in different temperatures, space velocities and NSR. The simulation results show that under the same space velocity the ammonia storage capacity of the catalyst decreases with the increase of temperature, which results in the decrease of ammonia storage amount. Under the same catalyst temperature, the increase of space velocity causes earlier appearance of ammonia slip, then the ammonia storage amount decrease consequently. When in the low temperature the NOx conversion efficiency increases as the ammonia storage amount rises, with increasing temperature the effect of ammonia storage amount on the NOx conversion efficiency gets weaker. Since ammonia storage amount has significant effect on NH3 slip, in transient response the temperature and space velocity of SCR catalyst rise rapidly which will cause NH3 slip. Considering the effects of ammonia storage amount on NOx conversion efficiency and NH3 slip rate, therefore a urea dosing strategy with amendment of ammonia storage amount is presented, which can maximize the NOx conversion efficiency in rapid change of operating condition while keeping the NH3 slip rate as low as possible.