Coating Distribution in a Commercial SCR-Filter

Transcription

1 Coating Distribution in a Commercial SCR-Filter FENG GAO, YILIN WANG, MARK ENGELHARD, JARROD CRUM, MARK STEWART (PNNL) CARL JUSTIN KAMP (MIT, KYMANETICS) 2017 CLEERS Workshop October 10,

2 Volkswagen close-coupled aftertreatment module with SCR-filter One of the first commercially deployed SCR-filters in the US was used on the VW 2.0L TDI Clean Diesel EA288 engine (2015 Jetta, Passat) Part Number: 04L131606M Innovative compact system includes DOC, static mixer, filter with Cu-zeolite SCR coating Filter: 165mm diameter x 140mm Segmented silicon carbide ~300/12, asymmetric channels Additional SCR in under-floor monolith urea inj DOC static mixer SCR coated filter October 10,

3 Background and motivation Although relatively few of these systems found their way onto American roads due to the VW diesel emissions scandal, similar SCR-filter technology is available to other OEMs OEM partners had expressed interest in examining this system as a way of developing general characterization approaches for new multifunctional filter technologies It is also possible that future multi-functional filter technologies (e.g. TWC/GPF) may have some similar features goal is to develop general tool set Detailed, high-fidelity performance models may require: Catalyst loading Catalyst distribution (axial, radial, across filter wall) Local wall permeability (size of open pores, geometry of flow paths through the filter walls) October 10,

4 Sectioning The filter consists of 4 square segments in the center, 8 partial segments on edges, and 4 triangular segments in corners Segments were removed one at a time for various analyses Arrows indicating flow direction were transferred to each segment as they were removed October 10,

5 Overview of examinations and analyses Micro X-ray CT of small specimens from segment 1 was conducted by a third party laboratory (1.7 μm) Additional multi-resolution CT scans were performed by Dr. Justin Kamp (Kymanetics) with progressive sectioning of segments 1 and 2 (50 μm 0.7 μm) SEM images were taken of coated wall surfaces (segments 1 and 2) Several small samples from segment 6 were analyzed using mercury porosimetry Powder samples scraped from thick surface coatings in segments 0 and 1 were analyzed by XPS and XRD Crushed wall/catalyst samples from segment 6 were analyzed by XPS October 10,

6 Visual observations Flow direction 1 Pronounced striping on upstream end, lighter in inlet channels. 0 White regions present on both upstream and downstream sides of filter walls. Striping in some locations. Dark band between fronts from upstream and downstream only in triangular segment. Color more uniform on downstream end. Slightly lighter in outlet channels. Some lighter streaks that cross multiple inlet and outlet channels. Lighter color just in front of some plugs. Inlet channels. October 10,

7 Visual observations 6 Flow direction The same basic pattern was evident in every segment examined The size, shape, and location of the white, heavily coated regions varied between segments October 10,

8 Low resolution X-ray CT Segment 2 and a portion of neighboring segment 1 were scanned and further sectioned by Dr. Justin Kamp at Kymanetics Initial low resolution scan = 48.6 µm voxel size Thick coating regions with similar features in same locations in both segments (region 2) Intermediate coating at downstream end represents majority of the length (region 3) Portion of segment 1 Full segment 2 Flow direction Lighter coating at inlet end (region 1) Bright spots scattered throughout October 10,

9 Low resolution X-ray CT 2 October 10,

10 Low resolution X-ray CT 2 October 10,

11 Medium resolution X-ray CT 3.62 µm resolution Coating shown in false color Medium-resolution CT scan showing transitions to thick coating Coating thickness not uniform Coating may not completely cover surface pores throughout Region 2 October 10,

12 Mercury porosimetry 6 An attempt was made to retrieve small samples from central segment 6, representative of the three distinct regions Porosity (%) Median pore size (um) Region 1 (front) Region 2(heavy coating) Region 3 (back) Hypothetical bare substrate ~ 62 ~ 16 October 10,

13 Mercury porosimetry Incremental Hg Intrusion Vol (ml/mg) Hypothetical Bare Substrate Region 1 (Front) Region 2 (Heavy Coating) Region 3 (Back) Pore Diameter (micron) Cumulative Hg Intrusion Vol (ml/mg) A hypothetical pore size distribution was generated by extrapolating from the front region with the lightest coating, considering experience with other SCR-coated filter samples and assuming 62% porosity Pores with diameters greater than 50 µm are ignored Pore Diameter (micron) October 10,

14 Coating direction Gray/black striping apparent at certain locations indicating coating of plugged filters from one side Lighter color most apparent in inlet channels near inlet end Lighter color in outlet channels near outlet end, but difference more subtle Gradients through the filter walls are also apparent in some locations Radial gradient of lighter material in walls. Consistent with coating from upstream side in the larger inlet channels near the inlet end of the filter. October 10,

15 High resolution X-ray CT - Region 1 1.7μm resolution coarsened to 6.7μm Segmentation to match assumed substrate porosity and measured total Catalyst voxels indicated by false color Most of the catalyst observed is within the porous filter walls Catalyst within wall radiates out from large inlet channels Total porosity 42.4% Resolved porosity 35% October 10,

16 High resolution X-ray CT - Region 3 1.7μm resolution coarsened to 6.7μm Most of the catalyst observed is within the porous filter walls Coating within wall radiates out from small outlet channels Total porosity 37.3% Resolved porosity 29% October 10,

17 High resolution X-ray CT - Region 2 1.7μm resolution coarsened to 6.7μm Over 75% of the catalyst observed is within the porous filter walls Coating on both upstream and downstream wall surfaces - thicker at one end of this small sample Significant macro-porosity within walls and within catalyst deposits on wall surfaces Total porosity 35.1% Resolved porosity 26% October 10,

18 Electron microscopy of thick coating Inlet Outlet Region 2 (Kymanetics) 18

19 Electron microscopy Inlet Region 3 (Kymanetics) Outlet 19

20 Powder XRD 800 Support and/or binder Intensity (Counts) Catalyst + Support Catalyst from SCRF H/SSZ-13 Active phase is SSZ (Deg) October 10,

21 Wide Scan XPS No Fe, P so SCR catalyst is Cu/SSZ-13 Catalyst seems to have some zirconia October 10,

22 Narrow Scan XPS October 10,

23 Near surface concentration Element O 1s Al 2p Si 2p Cu 1s Zr 3d C 1s Fe 2p Atomic % <0.3 Near surface Cu weight percentage estimated to be ~3.0%. Low Si/Al ratio suggests that an alumina binder was used

24 Summary At least three distinct regions were observed: Region 1 Inlet end ~ 15-21% or less of effective length Relatively lightly coated, heavier on upstream side Region 2 Near inlet end ~ 14-20% or less of effective length Heavily coated with surface deposits visible on both sides Some empty pores in middle of wall Region 3 Outlet end ~ 65-70% of the effective length Intermediate loading, heavier on downstream side Regions will likely have different permeability, filtration behavior, and chemical activity, even if coating composition is identical October 10,

25 Ideas for future work Modeling Lattice-Boltzmann simulations to predict wall permeability at various locations Device scale SCR, filtration models Sensitivity studies possible impact of varying permeability, catalyst distribution Conduct permeability, filtration, SCR, filter regen experiments Small specimens from various discrete regions Full-length segments, with or without plugs Compare model predictions with experimental results, refine and iterate October 10,

26 Acknowledgements Our thanks to Ken Howden and Gurpreet Singh at the US Department of Energy Office of Vehicle Technologies for funding the CLEERS program Thanks to Dana Ruane of PNNL for helping us obtain the VW aftertreatment module Special thanks to Dr. Christine Lambert of Ford; and Dr. Timothy Johnson, Dr. Ameya Joshi, and Dr. Todd St. Clair of Corning for helpful discussions on coated filter microstructure October 10,