Application of NMPC on optimization of ammonia coverage ratio references in two-can diesel SCR systems

This paper presents a cycle-based ammonia coverage ratio optimization for Diesel engine two-can selective catalyst reduction (SCR) systems. For a high NO_x conversion, it is intuitively required that the ammonia coverage ratio for the first catalyst can is as high as possible. However, a high ammonia coverage ratio would increase the ammonia slip downstream. In order to constrain the ammonia slip downstream, it is expected that the ammonia coverage ratio for the second catalyst can is as low as possible. The low ammonia coverage ratio whereas may not help for the NO_x reduction in the second can. Due to the conflict in the ammonia coverage ratios for these two catalyst cans, it is quite interesting and meaningful to study the optimization problem for the ammonia coverage ratios. With the developed system models, we propose a nonlinear model predictive control (NMPC) method to address this optimization problem. Various cases are studies to investigate the effects of the initial values on the ammonia coverage ratios.