Interaction of NO x Reduction and Soot Oxidation in a DPF with Cu-Zeolite SCR Coating

Abstract

The combination of selective catalytic reduction (SCR) and diesel particulate filter (DPF) in one system (so called SDPF) enables a very compact design and has already found its way into commercial application for passenger cars. In the case of heavy-duty and non-road applications, the design and control of the SDPF heavily depends on the interaction between NO x reduction and soot oxidation reactions taking place in close vicinity. The governing phenomena are described and studied in the present work by means of multi-scale experimental testing and mathematical modeling. Detailed kinetic studies at a micro-scale laboratory test rig are used to acquire the intrinsic reaction rates of the NH3-SCR reactions over a Cu-based catalyst, as well as to obtain qualitative trends of the role of soot presence. The above reaction kinetics are then directly implemented in a physicochemical model of the transport and reaction processes in a wall-flow-catalyzed filter. This model is validated against medium-scale engine tests revealing the competitive effects of SCR and passive soot oxidation, by a simple superposition of SCR chemistry and soot oxidation chemistry. This indicates that proper modeling of the species diffusion processes in the channels, the soot layer, and the wall is sufficient to describe the inhibiting effect of NH3-SCR on soot oxidation. This allows for a straightforward decoupled model calibration process which can be used to develop efficient system design and dosing control methodologies. Finally, the model is applied to a much different SDPF scale relevant for large non-road engines. The necessary adaptations to the model parameters are discussed as well as a methodology to apply predictive simulation tools when the boundary conditions are changed.