Development of Model Predictive Control Strategy of SCR System for Heavy-Duty Diesel Engines with a One-State Control-Oriented SCR Model

摘要

Urea-based selective catalytic reduction (SCR) of nitric oxides (NO_x) is a key technology for heavy-duty diesel engines to achieve the increasingly stringent NO_x emission standards. The aqueous urea injection control is critical for urea-SCR systems in order to achieve high NO_x conversion efficiency while restricting the tailpipe ammonia (NH_3) slip. For Euro VI emission regulation, an advanced control strategy is essential for SCR systems since its NO_x emission limits are tighter and test procedure are more stringent compared to Euro IV and Euro V. The complex chemical kinetics of the SCR process has motivated model-based control design approaches. However, the model is too complex to allow real-time implementation. Therefore, it is very important to have a reduced order model for SCR control system. Based on a continuous stirred tank reactor (CSTR) and mass conservation law, a one-State control-oriented SCR model was developed and validated to estimate the downstream NO_x emission, NH_3 slip, and NH_3 storage of the SCR catalyst. A model predictive control (MPC) coupled to the control-oriented SCR model was established to control the urea injection rate and maintain the NH_3 storage at a required level. The control-oriented SCR model can capture the main dynamics of the steady state condition as well as the transient condition very well, and the NO_x emissions, NH_3 storage, and NH_3 slip can be well predicted. With the MPC, the simulation results showed that the NH_3 storage in the SCR catalyst could be well controlled, the NO_x emission in the world harmonized transient cycle (WHTC) after the SCR system of a heavy-duty diesel engine with raw NO_x emission at 8.66 g/kW·h was reduced to about 0.37 g/kW·h, and the averaged NO_x conversion efficiency was up to 96%, while the averaged NH_3 slip in the cycle was kept below 10 ppm, which met the requirements of the Euro VI emission regulation.