Modeling and analysis of dual-layer NOx storage and reduction and selective catalytic reduction monolithic catalyst

Abstract

A simulation study of dual-layer NOx storage/reduction (NSR) and selective catalytic reduction (SCR) monolithic catalyst is carried out using (1+1)-D model of catalytic monolith with individually-calibrated global kinetic models. The model elucidates the complex spatiotemporal processes occurring within the washcoat and along the reactor length. Specifically, the simulations address: (i) general performance features of dual-layer NSR+SCR configuration, (ii) effects of temperature, (iii) effects of washcoat loading of individual components, and (iv) impact of catalyst architecture. In the dual-layer configuration, NH3 generated in the underlying NSR layer is stored in the outer SCR layer during the rich phase which then reacts with the NOx during the subsequent lean phase. The simulation results show that multiple combinations of catalyst (washcoat) loading can attain a given NOx conversion and N2 selectivity, and that there exists a loading of SCR washcoat for a given NSR catalyst for which the NOx conversion is maximum. For higher SCR washcoat loading, the amount of NH3 generated in the NSR catalyst is not sufficient to fully utilize the adsorption sites in the SCR. As a result, only a fraction of the SCR closer to the NSR layer is utilized while the rest acts as an inert layer, creating an undesired diffusional barrier which lowers the NOx conversion. Simulations of the dual-brick monoliths are also performed to analyze the architectural effects on performance of the combined system. Under identical conditions, the simulations show that the dual-layer configuration outperforms the dual-brick, largely because the NH3 generated in the LNT layer is better utilized in the SCR layer. At higher temperatures, the functionality of the SCR component is greatly reduced because of the higher rate of NH3 consumption in the NSR layer that lowers the NH3 yield. Under these conditions, comparable performances are obtained for both the configurations.