Can a heat exchanger improve the cold-start performance of a PNA–SCR system for NOx abatement?

Petr  Koci, University of Chemistry and Technology, Prague, Czech Republic

Petr Kočí, Martin Leskovjan
University of Chemistry and Technology, Prague, Czech Republic
http://www.vscht.cz/monolith

Combined exhaust aftertreatment systems including a passive NOx adsorber (PNA) followed by a catalyst for selective catalytic reduction of NOx (SCR) will be required to meet the upcoming strict NOx emission limits for Diesel engines. The PNA should adsorb NOx during the cold start and release it when the downstream SCR (including the urea injection unit) reaches its operating temperature. However, the NOx desorption from PNA may occur earlier than the SCR unit is heated enough, resulting in an undesired NOx slip. The actual evolution of NOx desorption depends on the PNA type, size, composition and flow rate of the exhaust gas, and exhaust line heat-up dynamics.

In this contribution we present a simulation study exploring the cold-start performance of a PNA–SCR system newly extended with a counter-current heat exchanger (HEX). The heat recuperation accelerates the SCR heat-up while slowing down the increase of the PNA temperature, which effectively delays the NOx desorption peak from the PNA and increases the chances that the desorbed NOx are completely reduced in the SCR. Two different PNA formulations are considered: Pd-BEA zeolite (BEA-PNA) and PtPd/ceria-zirconia-alumina (CZA-PNA) [1]. A new 1D model is developed for the heat exchanger device.

The results reveal that a decrease of the cold-start NOx emissions with the PNA–SCR compared to the baseline SCR is not ensured under all conditions; a proper temperature management is necessary. It must be considered that the PNA (as well as any other device located upstream) slows down the SCR light-off. The use of a suitably sized heat exchanger can help to maintain more favorable temperature profiles in the PNA and SCR during the heat-up period. A considerable improvement of the NOx conversion in the presence of a heat exchanger is predicted for the BEA-PNA. On the other hand, the use of a heat exchanger seems irrelevant to the CZA-PNA because of its higher NOx desorption temperature.

References
[1] Kvasničková A., Kočí P., Ji Y., Crocker M. Effective Model of NOx Adsorption and Desorption on PtPd/CeO2-ZrO2 Passive NOx Adsorber. Catalysis Letters 150 (2020), 3223-3233. https://doi.org/10.1007/s10562-020-03186-z