Conventional diesel aftertreatment systems use a discrete NOx control unit followed by a diesel particulate filter (DPF) for PM control. Putting the NOx and PM functions on the same device would reduce the overall number of catalyst devices, enabling packaging optimization that can potentially lower overall system back-pressure.
In addition to potential reductions in system cost, such a combination could allow for more thermally efficient engine operation and better support advanced engine concepts such as exhaust recovery by minimizing exhaust heat loss between the engine and the catalyst system downstream. Several catalyst providers and automakers are at work on such systems.
Umicore’s Automotive Catalysts group is developing a combined SCR (selective catalytic reduction) DPF (diesel particulate filter) unit—the SDPF—to deliver a single catalytic converter capable of controlling both NOx and PM emissions. Bukky Oladipo from Umicore presented the state of the SDPF work at the 2008 Diesel Engine-Efficiency and Emissions Reduction (DEER) conference in Dearborn, Michigan (4-7 August).
Umicore is evaluating Fe-based SCR coated on a 10.5" x 12" (about a 2.4 to 2.6 volumetric ratio to engine displacement) DPF. Samples, after oven-aging, were tested on a 2004-class 6.6L Duramax with a Pt DOC located upstream to generate NO2.
Tests were conducted with both NH3 and urea as reductants to separate out any impact of urea hydrolysis on the results. To minimize backpressure due to the SDPF, Umicore reduced the SCR catalyst washcoat loading in steps down to 52% of the standard level used in flow-through SCR.
Umicore achieved up to 80% NOx conversion with the reduced washcoat loading from steady state operation, compared to 90% in the flow-through SCR system. Further results show that it is possible to optimize the level of washcoat loading to meet target backpressure while maintaining NOx conversion.
An evaluation of the effect of extended soot loading on NOx reduction showed a drop of about 12 percentage points in conversion from start to end.
In the initial work, Umicore used its baseline SCR technology. Next steps include the use of more advanced SCR technology that it expects will improve its results.
In another presentation at DEER, Douglas Dobson from Ford described their work on designing an emissions system that would achieve Tier 2 Bin 5 for a light-duty truck. Ford developed and tested a “three-brick” system consisting of DOC, SCR and cordierite DPF units as well as a “two-brick” system featuring the DOC upstream of a combined SCR/DPF unit (which Ford calls the SCRF). The combined unit consisted of an SCR washcoat applied to the DPF.
Ford tested two versions of each configuration using material from different suppliers. Control of NMHC and CO by both types of systems were satisfactory. For NOx, the initial conversion efficiencies of the DOC-SCRF units were lower than expected, and degraded more. Ford determined that only about 60% of the washcoat was ending up on the filter walls, and expects that it can bring NOx conversion up to the target Tier 2 Bin 5 level.
Where the Ford system had the most problems was with PM—each of the systems failed at 50,000 miles. Ford suggests that the failures were due to aggressive filter regen, and some loss in filtration efficiency. Nonetheless, the NOx conversion for the SCRF remained high.
Ford has concluded, Dobson said, that the DOC-SCR-DPF systems were Bin 5 capable with little deterioration, given the development of a more robust PM capability. The SCRF system will require additional development to reach Tier 2 Bin 5 emissions. Dobson also noted that Ford thought it could leverage the work on this project to reach Bin 2 emission standards.
Jong Hwan Lee, Michael Paratore, David Brown (2008) Evaluation of Cu-based SCR/DPF Technology for Diesel Exhaust Emission Control (SAE 2008-01-0072)