Modeling of SCR NH3 Storage in the Presence of H2O

Abstract

Diesel engines offer excellent fuel economy, but this comes at the expense of higher emissions of nitrogen oxides (NOx) and Particulate Matter (PM). To meet current emissions standards, diesel engines require aftertreatment devices. Concepts using combinations of catalysts are becoming more common in aftertreatment systems to reduce the cost and size of these aftertreatment systems. One combination is an LNT-SCR system where the LNT releases NH3 during a regeneration to be used by the SCR catalyst for further NOx reduction. This involves rich-lean cycling of the exhaust stream, which alters species concentrations in the exhaust. Most notably H2O and CO2 levels can vary from 4%–14% during lean-rich cycling. An investigation was performed using multiple Temperature Programmed Desorption (TPD) experiments to determine how H2O and CO2 affect NH3 storage capacity of an Fe-based zeolite SCR catalyst. It was determined that H2O and CO2 inhibit NH3 storage capacity of the SCR catalyst. This inhibition has shown a linear dependence on H2O and CO2 concentration at constant temperature. It was also determined that H2O is a much stronger inhibitor of NH3 storage capacity then CO2. Additional Temperature Programmed Desorption (TPD) experiments, were run where H2O and CO2 concentration (0%, 6%, and 10%) and the initial storage temperature (200°C, 250°C, 300°C, 350°C) were varied. Results suggest the addition of a reaction that creates competition for active sites on the catalyst between H2O and NH3. The additional reaction allows H2O and NH3 to be stored on open catalytic sites and has improved model accuracy by accounting for large changes in H2O, CO2, and temperature.