Experiments and Analyses on Stability/Mid-Channel Collapse of Ash-Deposit Wall Layers and Pre-Mature Clogging of Diesel Particulate Filters 2019-01-0972
The conventional concept of soot and ash wall deposits (cake-layers) gradually building up along the channels of a ceramic honeycomb and then periodically or continuously being swept downstream toward the “end-plugs” of the channels may not always occur in practice. When deposits irregularly form on or detach from the walls (mid-channel collapse), causing premature clogging usually around the mid-sections of the channels, the particulate filter could experience a much more rapid rate of back pressure rise, resulting in the need of premature repair or replacement. Experiments were performed, accompanied by analysis and simulation, to investigate the factors that contribute to the pattern of wall deposits, particularly of ash, and the stability of the deposit layers.
Experiments to attempt to duplicate conditions where irregular deposits occur were carried out in a combination of actual engine and diesel-burner exhaust to achieve a variety of exhaust conditions, such as soot and ash concentrations and their relative ratios, flow rates (space velocities), and exhaust temperatures. Initial tests focused on the soot/ash concentration ratios, to be followed closely by temperatures and flow rates, although it is understood that the chemical composition of the exhaust such as oxides of nitrogen, sulfur, or steam, could also have effects on both the properties and stability (including stickiness) of the wall layers, but those parameters would be studied in future tests. The particulate filter was loaded up to pre-set levels, then regenerated; loading and regeneration continued until a preset level of loading was reached (nominally 10 grams per liter of nominal filter volume).
In addition to the routine experimental conditions of temperatures, flows, and concentrations measured, data included continuous pressure-drop measurements, and extensive detailed microscopic images. X-Ray CT (Computed Tomography) images were taken to record the spatial distributions of the ash deposits, radially and axially along the channels for the various conditions. FIB/ESEM (Focused Ion Beam/Scanning Electron Microscopy) images were recorded to study the detailed morphology of the ash deposits and their interactions with the substrate.
Thousands of images were statistically analyzed. Computer simulations were also conducted, in conjunction with the image analyses, to correlate the results of the physical-plugging and deposit distribution information, and to decipher the plausible connections to the exhaust or soot/ash deposit conditions. The ultimate objective is to isolate the causes and be able to control the stability and progression of the deposits.
Experiments have only covered a small subset of the entire test matrix covering all variables for now, and yet interesting data have begun to emerge. Results so far do indicate that exhaust and soot/ash accumulation parameters that affect the local temperatures (peak, duration, frequency) do appear to affect the properties and stability of the cake layer as well.
Yuesen Wang, Yosuke Obuchi, Jun Zhang, Ian Tracy, Victor Wong
Massachusetts Institute of Technology, Industrial Power Alliance, Ltd.