On-Engine Investigation of SCR on Filters (SCRoF) for HDD Passive Applications 2013-01-1066

A 4-way Heavy-Duty Diesel (HDD) emissions control aftertreatment system typically consists of diesel oxidation catalyst (DOC), catalyzed soot filter (CSF), urea-based selective catalytic NOx reduction (SCR) and NH₃ slip control catalyst (AMOX). Incorporating the SCR functionality into the soot filter (SCRoF) has great potential to reduce system costs and package volume/weight.

In this paper, we discuss some of the recent Cu-Zeolite-based SCR on filter (SCRoF) developments targeting Passive filter regeneration applications. The on-engine investigation of complete DOC+SCRoF+AMOX system focused on three major areas: 1) SCR performance of NOx conversion efficiency and NH₃ slip under both steady state and transient testing conditions; 2) SCRoF system response to sulfur exposure and subsequent sulfur removal for activity recovery; and 3) Characteristics of filter soot load, pressure drop, and passive soot oxidation in SCRoF.

Overall, the SCRoF system showed comparable NOx conversion efficiency and sulfur response as the conventional DOC+CSF+SCR+AMOX system. A 92+% cycle cumulative NOx conversion efficiency was demonstrated in a transient emission testing cycle while the peak tailpipe NH₃ slip was well controlled below 5 ppm. With the tested system, soot load does not adversely affect NOx reduction. After sulfur exposure, system performance was fully recovered by desulfation at 500°C - often the threshold temperature for passive systems. It is also important to point out that passive soot oxidation (i.e., C+NO₂) in a SCRoF will be considerably lower than in a catalyzed soot filter (CSF). Passive regeneration capability is inhibited due to SCR reactions competing for NO₂. In this work, intrinsic mechanisms and possible approaches to further enhance passive soot oxidation are reviewed and discussed.