Research Of SCR System Urea Dosing Bench Calibration For Marine Diesel Engine

With the implementation of the Tier Ⅲ standard in Annex VI of the MAPPOL Convention,the purification technology of marine diesel engine has difficulty in meeting the requirements of Tier Ⅲ for NO_X emission.SCR(selective catalytic reduction)technology has become one of the most promising technologies for NO_X emission control of marine diesel engines.In order to achieve the optimal control of urea dosing in SCR system of marine diesel engines,the research of SCR system urea dosing bench calibration has been required.The main research contents are as follows:Firstly,the emission characteristic test of the target engine was carried out according to the experimental design scheme,and the exhaust flow was calculated by the carbon balance method,then the sub-cubic Hermite interpolation polynomial method was used to obtain the NO_X concentration map,exhaust flow map,and exhaust temperature map.The experimental results showed that the original emission characteristics by the proposed method that were more accurate and could be used to calculate the basic dosing amount of urea in SCR system.Secondly,the urea dosing bench calibration method of engine SCR system was established,and the closed-loop control strategy of urea dosing based on the emission characteristic model was proposed.Based on the basic urea dosing amount that calculated by the engine emission characteristic MAP,the urea dosing amount was modified through the measures of ammonia storage,feedward and feedback.The NO_X demand conversion rate was determined by the weight coefficient method was proposed,the basic urea dosing map was obtained.Thirdly,based on the experimental data of SCR system,a one-dimensional SCR catalyst model was established by BOOST software,and the chemical reaction kinetic parameters were optimized by genetic algorithm.The simulation value of NO_X concentration in the optimized model was in good agreement with the experimental value,and the mean square difference was decreased by nearly 82%.Through the optimized simulation model,the saturated ammonia storage of SCR catalyst was obtained,and the ammonia storage correction was carried on for the dosing flow of the SCR system.Finally,the calibration test of the urea dosing control of SCR system was carried out,and the results showed that the closed-loop control strategy proposed in this paper has better control effect and higher control precision than the open-loop control strategy.Based on the closed-loopcontrol strategy that proposed in this paper,the parameters of urea dosing control were nominal optimized.Then the bench test results of E3,D2 test cycle showed that the single load NO_X rate emission could satisfy the requirement of emission limit(3.12g/kWh),and the weighted NO_X rate emission was 1.72 g/kWh,1.94 g/kWh respectively,which could meet the requirement of the rate emission limit(2.08 g/kWh)and the Tier Ⅲ emission limit standards.