Development and validation of a single site kinetic model for severely aged Cu-SSZ-13 SCR: Inclusion of AN chemistry and high temperature NH3 oxidation by NO2

Chintan  Desai, Isuzu Technical Center of America

• SCR model for severely aged condition
• AN chemistry
• High temp NH3 oxidation by NO2 to NO

In this study we present the development and validation of an one-dimensional numerical SCR model over Cu-SSZ-13 catalyst. The kinetic model was developed for severely aged catalyst based on the micro reactor data collected using ORNL SCR protocol. The operating condition for this data collection and model development were one aging condition, two space velocities and temperature range of 150-550°C. The protocol used for this work had argon as balance gas giving us important information about N2 generation and also assist in model calibration. Overall, the model consists of NH3 adsorption/desorption, NH3 oxidation by O2 generating N2,N2O and NO, NH3 oxidation by NO2 generating NO, SCR reactions generating N2 and N2O, AN chemistry.

The developed model is a semi – single site model. Only NH3 adsorption and desorption is developed using 2 sites, while all the SCR reactions are modeled using 1 site to adhere to the predominantly single site behavior after severe aging and also to keep the model simple. At low temperatures, effects of AN (ammonium nitrate) were observed and were modeled accordingly, 4 key reactions contributed to the AN chemistry such as AN generation, AN consumption by NO, AN decomposition to N2O and AN decomposition to HNO3 and NH3. It was found that AN decomposition to HNO3 and NH3 was crucial in predicting the experimental behavior. This modeling efforts gave us some more insights for AN which will be important in the future for the low load operations where temperatures are around 170°C. At higher temperatures, NH3 oxidation to NO was observed, its effect was pronounced and sharp at 500°C and 550°C, this behavior was captured by modeling NH3 oxidation by O2 generating NO and also NH3 oxidation by NO2 generating NO, the later reaction was important in predicting the experimental data in the presence of NO2. This SCR model predicts the important trends from 150-550°C for severely aged catalyst. The developed model is validated with additional micro reactor data at different ANRs (Ammonia to NOx ratio) and engine dynamometer testing. The final developed model has numerous applications such as future hardware selection, DEF (diesel exhaust fluid) dosing calibration, model based controls etc. Future works include the modeling of different aging conditions and poisoning effects such as sulfur.