Spatial Distribution of N2O in Selective Catalyst Reduction Systems: Implications for Catalyst Zoning

Abstract

N₂O generation during the selective catalytic reduction (SCR) of NO_x is a significant environmental concern. Zoning design of SCR systems based on the spatial distribution characteristics of chemical species offers an effective strategy for optimization. However, conducting experimental tests in complex, high-temperature/pressure reactor systems presents significant challenges. Therefore, this study utilizes a three-dimensional computational fluid dynamics model to investigate the spatial distribution of N₂O within a diesel SCR catalyst under various typical operating conditions. The simulation results revealed significant nonuniformity in the N₂O spatial distribution. Temperature was found to significantly influence the uniformity of N₂O distribution; at lower temperatures, nonuniformity was pronounced and linked to mixer geometry and the temperature-dependent kinetics, whereas higher temperatures led to substantially more uniform profiles. The exhaust NO₂/NO_x ratio was identified as the most critical factor affecting both the axial distribution of N₂O and its overall selectivity, with high NO₂ ratios, particularly exceeding 0.56, significantly increasing N₂O selectivity. The detailed spatial distribution information obtained provides valuable theoretical guidance for zoned SCR catalyst design strategies aimed at suppressing N₂O formation in identified high-generation regions while maintaining high NO_x conversion.