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-NOX 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:NOX ratio (or urea
injection strategy) and NO₂/NOX ratio) and the effect of
modifying engine calibration (catalyst inlet temperature,
etc.).