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The Effects of Sulfur Poisoning and Desulfation Temperature on the NOx Conversion of LNT+SCR Systems for Diesel Applications
Abstract:
A laboratory study was performed to assess the effects of sulfur poisoning and desulfation temperature on the NO
conversion of a LNT+(Cu/SCR) in-situ system. Four LNT+(Cu/SCR) systems were aged for 4.5 hours without sulfur at 600, 700, 750, and 800°C using A/F ratio modulations to represent 23K miles of desulfations at different temperatures. NO
conversion tests were performed on the LNT alone and on the LNT+SCR system using a 60 s lean/5 s rich cycle. The catalysts were then sulfur-poisoned at 400°C and desulfated four times and re-evaluated on the 60/5 tests. This test sequence was repeated 3 more times to represent 100K miles of desulfations. After simulating 23K miles of desulfations, the Cu-based SCR catalysts improved the NO
conversion of the LNT at low temperatures (e.g., 300°C), although the benefit decreased as the desulfation temperature increased from 600°C to 800°C. During the poisonings and desulfations, the SCR catalyst was effective at converting H₂S and COS produced by the LNT back into SO₂. After simulating 47K miles of desulfations, the benefit of the SCR catalyst for NO
conversion on the 60/5 test was minimal, regardless of the aging temperature. However, the rich purge time was increased to 6 seconds after simulating 73K and 100K miles, and the benefit of the SCR catalyst for low temperature NO
conversion was observed again for all four desulfation temperatures. These results suggested that, as the LNT+SCR system ages on a vehicle, the rich purge time may need to be increased to maintain the benefit of the SCR catalyst for low temperature NO
conversion. This can be attributed to a decrease in the purge kinetics of the LNT with thermal aging. The optimal desulfation temperature for this LNT+SCR system was 750°C, as lower temperatures resulted in incomplete desulfations and consequently lower NO
conversions at high temperatures, while 800°C desulfations resulted in thermal degradation.