Effects of Fuel Type on Dual SCR Aftertreatment for Lean NOx Reduction

Global demand for alternative fuels to combat rising energy costs has sparked a renewed interest in catalysts that can effectively remediate NOx emissions resulting from combustion of a range of HC based fuels. Because many of these new engine technologies rely on lean operating environments to produce efficient power, the resulting emissions are also present in a lean atmosphere. While HCs are easily controlled in such environments, achieving high NOx conversion to N₂ has continued to elude fully satisfactory solution. Until recently, most approaches have relied on catalysts with precious metals to either store NOx and subsequently release it as N₂ under rich conditions, or use NH₃ SCR catalysts with urea injection to reduce NOx under lean conditions. However, new improvements in Ag based technologies also look very promising for NOx reduction in lean environments. Early in 2009 we proposed “Dual SCR,” a further enhancement to Ag HC-SCR, where an NH₃-SCR catalyst follows the HC-SCR catalyst. The second catalyst improves overall efficiency using ammonia formed on the Ag HC-SCR catalyst to further reduce NOx. This lean NOx aftertreatment approach, with a catalyst cost at a fraction of precious metal systems, has shown NOx reduction activity exceeding 90% under a variety of temperature and feed conditions in our bench reactor studies. Here we extend our efforts to an engine test cell and use a range of fossil and biofuels as the reductants for the Ag HC-SCR catalysts and we continue to find excellent NOx reduction efficiencies. Although relatively low concentrations of H₂ (near 0.1%) are useful for enhancing NOx activity at low temperatures (below 250 °C), the benefit decreases for exhaust temperatures near and above 400 °C. This technology could be a key enabler for low cost and efficient NOx reduction to help auto manufacturers meet Tier2/Bin5 and Tier2/Bin2 emissions standards.