An explicit reduced-order model of Cu-Zeolite SCR catalyst for embedding in ECM

Abstract

A piecewise analytical solution is developed as part of an explicit reduced-order model for a state-of-the-art Cu-SSZ-13 SCR catalyst in the degreened state (550 °C-4 h). Simplifying the species, coverage and energy balance equations by treating the monolithic catalyst as an array of CSTRs in series, along with additional approximations based on intrinsic catalyst behavior allow for the decoupling of the conservation equations. This yields analytical solutions for all relevant catalyst states and outputs, which can be integrated using a simple fixed step explicit Euler scheme. The reduced-order model utilizes the same reaction kinetics as the high-fidelity 1D plug flow reactor model, developed using NH3-temperature programmed desorption (TPD) and 4-step [40] reactor data on the degreened catalyst. SCR kinetics are modeled here using a global single-site approach, accounting for ammonia (NH3) and ammonium nitrate (AN) dynamics. Washcoat diffusion is considered using the overall mass transfer approach [39], and in the case of SCR catalysts, we demonstrate the applicability of the asymptotic internal Sherwood number for realistic reaction rates and washcoat diffusivities. Model results show the ability of the piecewise analytical solution to produce nearly identical results as the high-fidelity model, which in turn is validated with experimental reactor data. The reduced-order model is also translated to a full-size catalyst under transient engine drive cycle conditions. SCR outlet temperature, NH3 and AN coverage, along with outlet NOx and N2O slip predictions are in line with the high-fidelity model and experimental measurements. The simplified model presented here is generally applicable to all single-layer monolithic catalysts utilizing linear reaction rate expressions for solved species. This model allows for the tracking of catalyst states (such as NH3 coverage) and outputs (such as NOx) onboard the engine control module (ECM), enabling the development of model-based estimators and control strategies necessary to meet the upcoming emission and durability requirements.