Development of models for NH3 SCR storage and the impacts of hydrothermal aging
Austin Ladshaw, Oak Ridge National Laboratory
Under the anticipated regulatory changes to both NOx emissions limits and required emissions control system full useful life for heavy duty vehicles, urea selective catalytic reduction (SCR) systems will be required to achieve even higher performance over a much longer operating lifetime than what commercial systems currently deliver. Effective urea dosing strategies will be needed to meet the increasing performance requirements. Since urea dosing strategies are often developed through system simulations, the underlying SCR catalyst models must be able to accurately capture changes in ammonia storage capacity as a function of operating conditions and, especially, catalyst age. In this work, numerous capacity studies have been performed to assess the ammonia storage capacity of Cu-SSZ-13 as a function of ammonia concentration, water vapor content, temperature, and extent of hydrothermal aging. Isothermal capacities for ammonia storage on Cu-SSZ-13 are assessed using an extended heterogenous Langmuir function that identifies 3 different binding sites in the catalyst: (i) a Cu site bound at 1 framework Al, (ii) a Cu site bound at 2 framework Al, and (iii) a Brønsted acid site. Results show that this 3-site model can accurately capture ammonia storage across all temperatures, water contents, and aging conditions. The impacts for hydrothermal aging are considered via a set of 3 aging reactions that alter the site densities of each identified adsorption site. Consistent with literature, the model assessment in this work shows that during aging there is a significant loss in Brønsted acid sites with a simultaneous increase in available Cu sites. Those models for ammonia storage were then implemented into a simulation framework to demonstrate the validity of the approach for capturing the transient behavior of ammonia storage under the conditions of interest.