Development and Validation of a Cu-Zeolite SCR Catalyst Model 2011-01-1299

A one-dimensional numerical model for a Cu-zeolite SCR catalyst has been developed. The model is based on kinetics developed from laboratory microreactor data for the various NH₃-NO_X reactions, as well as for NH₃ oxidation. The kinetic scheme used is discussed and evidence for it presented. The model is capable of predicting the conversion of NO and NO₂, NH₃ slip and the formation of N₂O, as well as effects associated with NH₃ storage and desorption. To obtain a good prediction of catalyst temperature during cold start tests, it was found necessary to include storage and desorption of H₂O in the model; storage of H₂O is associated with a sizable exotherm and the subsequent desorption of this water produces a correspondingly large endotherm. Extensive validation of the model has been carried out against engine/vehicle data for both HDD (light-off, heavy-duty FTP transient cycle, NRTC) and LDD (NEDC) test cycles for a range of catalyst volumes, urea injection levels and upstream DOC volume and loading. In particular, data measured with an oversized DOC was used to test the models ability to predict N₂O formation. In general, good agreement between model prediction and the experimental data was achieved.

Examples of application of the model to aftertreatment design are given. The model can be used to predict the effect of changing system parameters (such as catalyst size, NH₃:NO_X ratio (or urea injection strategy) and NO₂/NO_X ratio) and the effect of modifying engine calibration (catalyst inlet temperature, etc.).