1 Copyright © 2011 by ASME Proceedings of the ASME 2011 Internal Combustion Engine Division Fall Technical Conference ICEF2011 October 2-5, 2011, Morgantown, West Virginia, USA ICEF2011-60233 MODELING OF SCR NH 3 STORAGE IN THE PRESENCE OF H 2 O Michael A. Smith University of Michigan Ann Arbor, MI, USA Christopher D. Depcik University of Kansas Lawrence, KS, USA John W. Hoard University of Michigan Ann Arbor, MI, USA Stanislav V. Bohac University of Michigan Ann Arbor, MI, USA Dionissios N. Assanis University of Michigan Ann Arbor, MI, USA ABSTRACT Diesel engines offer excellent fuel economy, but this comes at the expense of higher emissions of nitrogen oxides (NO x ) and Particulate Matter (PM). To meet current emissions standards, diesel engines require aftertreatment devices. Concepts using combinations of catalysts are becoming more common in aftertreatment systems to reduce the cost and size of these aftertreatment systems. One combination is an LNT- SCR system where the LNT releases NH 3 during a regeneration to be used by the SCR catalyst for further NO x reduction. This involves rich-lean cycling of the exhaust stream, which alters species concentrations in the exhaust. Most notably H 2 O and CO 2 levels can vary from 4% - 14% during lean-rich cycling. An investigation was performed using multiple Temperature Programmed Desorption (TPD) experiments to determine how H 2 O and CO 2 affect NH 3 storage capacity of an Fe-based zeolite SCR catalyst. It was determined that H 2 O and CO 2 inhibit NH 3 storage capacity of the SCR catalyst. This inhibition has shown a linear dependence on H 2 O and CO 2 concentration at constant temperature. It was also determined that H 2 O is a much stronger inhibitor of NH 3 storage capacity then CO 2 . Additional Temperature Programmed Desorption (TPD) experiments, were run where H 2 O and CO 2 concentration (0%, 6%, and 10%) and the initial storage temperature (200°C, 250°C, 300°C, 350°C) were varied. Results suggest the addition of a reaction that creates competition for active sites on the catalyst between H 2 O and NH 3 . The additional reaction allows H 2 O and NH 3 to be stored on open catalytic sites and has improved model accuracy by accounting for large changes in H 2 O, CO 2 , and temperature. INTRODUCTION Future emission regulations target dramatic reductions in the levels of carbon monoxide (CO), hydrocarbons (HC), nitrogen oxides (NO x ) and particulate matter (PM) produced by internal combustion power plants. For the traditional spark- ignition engine, these problematic species are eliminated using Close Coupled Catalysts (CCC) near the exhaust manifold and underbody Three-Way Catalysts (TWC) in a system that approaches 100% efficiency [1, 2]. However, for lean burn engines, the choice is not as clear. The lean combustion characteristics of these engines often force the OEM to utilize multiple devices to convert all problematic species [3-5]. This is because a singular device does not exist for diesel engines for simultaneous conversion of CO, HC and NO x similar to a CCC or TWC in spark-ignition engines. Current efforts at lowering diesel NO x emission levels use retarded fuel injection and large amounts of cooled Exhaust Gas Recirculation (EGR) to reduce the in-cylinder combustion temperature [6, 7]. However, in- cylinder techniques alone cannot reduce emission levels to below regulatory standards [8]. Copper (Cu) and iron (Fe) are the two major types of zeolite cations for SCR catalysts. Fe-zeolites have the largest temperature operating range, while Cu-zeolites have lower sensitivity to the NO:NO 2 ratio in the exhaust stream. Cu- zeolites are also less susceptible to sulfur poisoning and have a higher NH 3 storage capacity than Fe-zeolites [9, 10]. While zeolites do have promise, a major issue with respect to SCR applications is the availability of NH 3 for conversion of NO x . Instead of using an on-board Diesel Exhaust Fluid (DEF) tank, it is possible to combine Lean NO x Trap (LNT) and SCR catalysts, similar to the Eaton Aftertreatment System (EAS) shown in Figure 1 [11, 12]. The LNT-SCR system uses the