Research On Urea Injection Control Method Of Ship Exhaust Denitrification Unit Based On SCR

In response to the air pollution caused by nitrogen oxides in marine diesel exhaust,the International Maritime Organization has developed and adopted a series of regulations to impose strict limits on the NO_X content of marine diesel exhaust.Selective Catalytic Reduction（SCR）technology is recognized as the most effective NO_X emission reduction technology in the shipping industry.The key to achieve the NO_X emission reduction target of SCR system lies in the control of urea injection quantity.In this paper,the control model of SCR system and the prediction model of NO_X concentration in diesel exhaust are established,and then the control method of urea injection in SCR system is studied.The mechanism and reaction process of chemical reaction in SCR catalyst were analyzed and determined.The standard and rapid reaction of nitrogen oxides and ammonia,the adsorption and desorption reaction of ammonia and the oxidation reaction of adsorbed ammonia were determined.The mathematical model of SCR system was established with the change of concentration of each component as the state variable.On this basis,the validity of the model is verified.The simulation results show that the simulation model which divides the SCR system into two micro-elements fully conforms to the real SCR system characteristics.In view of the high demand of SCR system of diesel engine on exhaust flow,exhaust composition and exhaust temperature accuracy,Least Squares Support Vector Machine（LSSVM）was used to establish the NO_X concentration prediction model of diesel engine exhaust,and the improved Particle Swarm Optimization（PSO）was used to find the optimal LSSVM kernel function parameters and penalty factors.According to the rotational speed and load of diesel engine during normal navigation,the NO_X concentration,exhaust temperature and mass flow in exhaust were predicted in real time,so as to calculate the basic injection amount of urea.Simulation results show that the NO_X concentration prediction model can accurately predict the NO_X concentration,exhaust mass flow and temperature in diesel engine exhaust,and the maximum relative errors are 1.51%,1.24%and 1.05%,respectively.The gaseous NO_X and gaseous NH₃ of the SCR system were regarded as static elements,and the nonlinear model predictive controller was designed for the SCR system of Marine diesel engine.The concentration of NO_X at the outlet of the catalytic converter of the SCR system was taken as the control output,and ammonia escape was taken as the constrained output of the system to control the injection amount of urea.Simulation tests were carried out under stable and cyclic conditions respectively to verify its control performance.Simulation results showed that the highest NO_X conversion efficiency of SCR system under stable conditions was 93.30%,and ammonia escape could be guaranteed below 9.545 ppm.Under cyclic conditions,the NOX emission is 1.095g/（kW?h）,which meets the Tier Ⅲ emission standard.