Model-Based Optimization of Ammonia Dosing in NH₃-SCR of NO _x for Transient Driving Cycle: Model Development and Simulation

The ability to catalytically reduce NO _x efficiently with ammonia (NH₃) has made the selective catalytic reduction (SCR) technology indispensable in diesel after-treatment systems. Ammonia is dosed periodically in the form of urea which decomposes to ammonia and reacts with NO _x . During the application of real-world driving cycles, owing to fast transients of the emissions, it is not efficient to dose constant amount of ammonia. An efficient optimized dosing strategy is quite essential to improve the efficiency and contain the NH₃ excursions within the permissible limits. Development of such an optimized dosing solution requires the usage of simple yet powerful mathematical models capable of simulating the process with reasonable accuracy and robust optimization schemes, which can ensure optimal solutions in spite of having high number of parameters to be optimized. Several efficient model reduction techniques presented in the literature have been used to obtain a grey-box model which can effectively simulate the process. Direct collocation method based on Orthogonal Collocation over Finite Elements (OCFE) is used to transform the Differential Algebraic Equation-constrained optimization problem into a nonlinear program. The developed model is applied to the World Harmonic Transient Driving cycle to optimize NH₃ dosing for each second of the driving cycle. The obtained optimal dosing solution not only improved the efficiency of the process but also maintained the NH₃ excursions below 10 ppm over the entire driving cycle. The developed methodology is applicable to other systems to obtain such large-scale optimal solutions efficiently.