Simulating the advantages of a LNT+ passive SCR system with a bypass concept

Yang  Zheng, PNNL

A simulation study is conducted for a LNT + passive SCR system featuring the use of an exhaust bypass around the LNT and an out-line fuel reformer for effective regeneration. Specifically, during regeneration the main exhaust stream flows around the LNT via the bypass to the downstream SCR while the LNT is locally enriched by feeding the reformate gas from the fuel reformer at a low space velocity. Compared to conventional sequential layout, the bypass concept enables engine-independent LNT regeneration (i.e., no need to adjust engine lambda), higher NH3 yield and lower fuel penalty [1]. 1-D LNT and SCR monolithic models with individually calibrated global kinetics are adopted for system performance analysis. The simulation first focuses on the regeneration strategy for the bypass concept including LNT NOx load, cycling frequency, regeneration duration and space velocity, aiming to deliver high NH3 emission for the downstream SCR. Then key operating parameters are identified and assessed to provide a general guideline for optimal compromise among overall NOx conversion, fuel penalty and catalyst cost. Based on the modeling results, the combined system with a bypass concept exhibits great advantages to meet the upcoming stricter NOx and CO2 emission standards as compared to a conventional sequential LNT + SCR system.

[1] Wittka, T., Holderbaum, B., Maunula, T., Weissner, M., SAE Int. J. Engines 7(3):1269-1279, 2014

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