Modeling and Simulation of Coupled LNT – SCR Catalytic System under Anaerobic and Aerobic Operating Conditions
Kota, Arun S.
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An important goal is to minimize the required precious metal loading in the LNT while keeping the NOx emission below a specified level. We present a mathematical model of this system using hydrogen as the reductant. Simulations are used to determine the influence of the architecture of the LNT-SCR bricks, non-uniform precious metal loading in the LNT bricks, and the cycle time under aerobic and anaerobic operating conditions. Simulations reveal that low temperature reduction is the limiting step in determining the optimal precious metal loading. Architectural changes (sequential) in the LNT-SCR brick arrangement improved the NOx conversion and reached an asymptotic limit. Non-uniform precious metal loading in the LNT resulted only in a minor improvement in the NOx conversion, while cycle time affected the NOx conversion significantly. The importance of considering diffusional limitations in the models has been highlighted. The lack of which over predicts the deNOx efficiency of the catalyst. Aerobic operation with heat supplied by H2 oxidation improved the NOx conversion. The relationship between fuel penalty and precious metal loading in the coupled LNT-SCR system at low temperature is determined. There exists an optimum length of the catalyst during the adiabatic aerobic operation which results in improved performance compared to anaerobic operation. Impact of the substrate material when switched from ceramic to metal has been small but positive. Decreasing the cycle time and increasing the pulse duty resulted in overall performance improvement by reducing the NOx slip from the catalyst. A comprehensive kinetic model based on the mechanistic details is being developed, to closely match the realistic operation conditions of the coupled catalyst system.