Determining pore structure changes through a filter system
Catalytic diesel engine exhaust after treatment components, especially the diesel particulate filter, are subject to degradation due to various reasons over normal component lifetimes. One form of degradation in the catalytic diesel particulate filter (cDPF) is the significant rise in pressure drop due to the accumulation of engine lubricant-derived ash (inorganic, incombustible ionic crystalline solids consisting of Ca, Mg, Zn, S, P and O) which coats the inlet channel walls effectively decreasing the permeability of the wall-flow component architecture. This form of catalyst degradation has been found to reduce vehicle fuel economy by as much as 5%.
MicroCT with a transmission X-ray source (voxel size ~600nm) has been used in combination with direct numerical simulation techniques to calculate the permeability and pore structure changes of the combined ash-catalyst substrate system to better understand the effects of ash accumulation on engine after treatment component functionality. The current CT resolution allows direct and accurate 3D visualization of the catalyst substrate structure, the individual ash particles (which have an average size of 1-2µm) and the ash which penetrates the substrate surface pores. This study discusses the sample preparations necessary for such high CT resolution, the combination of CT and direct numerical simulation (CT dataset segmentation and flow simulations), the comparison between calculated and experimentally measured permeability values, and the implications of the ability to calculate permeability in the combined ash-catalyst substrate system.
a) SEM image of an uncoated cordierite channel surface in an ash particulate filter, b) CT image of an uncoated surface, c) SEM image of a washcoated surface and d) CT image of a washcoated surface.