Dual SCR Aftertreatment for Lean NOx Reduction

Galen  Fisher, Delphi Powertrain Systems

Galen B. Fisher*, Craig L. DiMaggio, Ken M. Rahmoeller, Mark Sellnau
Delphi Powertrain Systems
3000 University Drive, Auburn Hills, MI 48326
(*Presently at the Dept. of Chemical Engineering, University of Michigan, gbfisher@umich.edu)

Low-cost lean NOx aftertreatment is one of the main challenges facing high efficiency gasoline and diesel engines operating with lean exhausts.  While there are many candidate technologies, they all offer tradeoffs.  We have investigated a multi-catalyst Dual SCR aftertreatment system that is capable of obtaining NOx reduction efficiencies of greater than 90% under lean conditions, without the use of platinum group metals (PGMs) or urea injection into the exhaust.  This Dual SCR approach uses an Ag/Al2O3 HC-SCR catalyst followed by an NH3-SCR catalyst.  In bench reactor studies from 150 °C to 500 °C with modest C/N ratios, NOx reacts over the first catalyst to predominantly form nitrogen.  In addition, it also forms ammonia in sufficient quantities to react on the second NH3-SCR catalyst to improve overall performance.  The operational window and the formation of NH3 are improved in the presence of small quantities of hydrogen (0.1-1.0%).  The response of the system to other factors such as exhaust oxygen content and space velocity has also been explored.  In addition, we have begun to develop a model in GT Power for the HC-SCR reaction to combine with a model of the NH3-SCR reaction to be able to cover the Dual SCR system.  This Dual SCR technology for lean NOx reduction is especially well matched for use with pre-mixed low-temperature diesel combustion.  It has the potential to be a low-cost, non-urea enabler for satisfying Tier2/Bin5 and Tier2/Bin2 emissions standards.