The Influence of Ammonia Slip Catalysts on Ammonia, N2O and NOX Emissions for Diesel Engines 2007-01-1572
The use of urea-based selective catalytic reduction (SCR) is a promising method for achieving U.S. Tier 2 diesel emission standards for NOx. To meet the Tier 2 standards for Particulate Matter (PM), a catalyzed diesel particulate filter (CDPF) will likely be present and any ammonia (NH3) that is not consumed over an SCR catalyst would pass over the CDPF to make nitrous oxide (N2O) emissions and/or oxides of nitrogen (NOx), or exit the exhaust system as NH3. N2O is undesirable due to its high greenhouse gas potential, while NOx production from the slipped NH3 would reduce overall system NOx conversion efficiency. This paper reviews certain conditions where NH3 slip past an SCR system may be a concern, looks at what would happen to this slipped NH3 over a CDPF, and evaluates the performance of various supplier NH3 slip catalysts under varied space velocities, temperatures and concentrations of NH3 and NOx. Current SCR technology shows that the greatest concern over NH3 slip comes at low temperatures during cold starts, and at moderate exhaust temperatures (250 to 350%C) with both moderate and very high space velocities. Slipped NH3 that passes over a CDPF is shown to form N2O at temperatures below around 350%C, and to remake form NOx at temperatures above approximately 350%C. A critical issue is size for an NH3 slip catalyst, since diesels equipped with SCR will likely also have a diesel oxidation catalyst and a CDPF (thus making addition of an NH3 slip catalyst difficult for both system backpressure and packaging space). Current state of the art NH3 slip catalysts are shown to be capable of removing nearly all NH3 at temperatures above 225%C at low (80,000 h-1) space velocities (SV) with good selectivity to N2. The “light-off” for high NH3 oxidation shifts to higher temperatures as the SV is increased for NH3 slip catalysts. At high space velocities and temperatures below about 250%C, NH3 slip catalysts remove some but not all of the slipped NH3. Thus, work should be done to minimize NH3 slip under these conditions. Use of an NH3 slip catalyst can selectively remove most slipped NH3 at medium and high temperatures, thus minimizing undesired NH3 reactions over a CDPF.