1D Kinetic Modeling of NH3-SCR with Two-Sites of NH3-Storage and Its application in LD Vehicle Emissions Control
Zhiming Gao, Oak Ridge National Laboratory
1D Kinetic Modeling of NH3-SCR with Two-Sites of NH3-Storage and Its application in LD Vehicle Emissions Control
Zhiming Gao, Josh A. Pihl, James E. Parks II, C. Stuart Daw
Oak Ridge National Laboratory
We present a NH3-SCR model which was structured for estimating the global reaction rate between the ammonia and nitrogen oxides reaching the SCR catalyst through using 1-D differential transient balances. The model accounts for 19 key global kinetic reactions for NH3 adsorption and desorption and multiple reactions between the adsorbed NH3 and NOx over two-sites of NH3 storage on the catalyst. The considered two NH3 storage sites include zeolite Bronsted acid sties and Lewis acid sites. For NH3 adsorption, the impact of H2O competitive adsorption on NH3 storage sites is further deliberated in the model. To understand the chemistry of fast SCR reactions for the optimal equimolar NO to NO2 feed ratio, as well the selectivity to NH4NO3, we adopted a sequential scheme (Nova et.al., Catalysis Today, 114(1), 3-12.), whereby the fast SCR reaction results from the formation of NH4NO3 reaction and its subsequent reduction by NO. Since Ammonium nitrate may decompose to HNO3 and NH3 and the subsequent decomposition of HNO3 to NO2, O2 and H2O (Sjövall, Industrial & Engineering Chemistry Research, 49(1), 39-52.), we thus added two reactions to describe Ammonium nitrate decomposition. Finally, the reactions for N2O formation and dissociation are also considered appropriately.
The SCR model has been well fitted with experimental measurements from laboratory measurements of Cu-SSZ-13, a commercial SCR formulation, according to the CLEERS transient SCR characterization protocol. Furthermore we have integrated the SCR model with our previous physically-based DOC models to study their impact on drive cycle performance of light-duty vehicles. The integrated aftertreatment-train simulation provides insights of controlling vehicle tailpipe emissions and optimizing NH3 dosing over city and highway drive cycles. The developed model is capable of implementation into Autonomie and other Commercial software in a Matlab/Simulink format.
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