Ash Accumulation and Impact on Sintered Metal Fiber Diesel Particulate Filters 2015-01-1012
While metal fiber filters have successfully shown a high degree of particle retention functionality for various sizes of diesel engines with a low pressure drop and a relatively high filtration efficiency, little is known about the effects of lubricant-derived ash on the fiber filter systems.
Sintered metal fiber filters (SMF-DPF), when used downstream from a diesel engine, effectively trap and oxidize diesel particulate matter via an electrically heated regeneration process where a specific voltage and current are applied to the sintered alloy fibers. In this manner the filter media essentially acts as a resistive heater to generate temperatures high enough to oxidize the carbonaceous particulate matter, which is typically in excess of 600°C. The regeneration process does not require additional fuel, such as in the case of active regeneration in conventional/ceramic wall-flow diesel particulate filters (DPF), since the energy required for regeneration typically comes from another source like an auxiliary battery. In addition, the SMF-DPF is not sensitive to fuel quality (especially fuel sulfur levels), engine load and exhaust temperature.
Just as in the case of the wall-flow DPF, lubricant derived ash (typically comprised of ionic crystalline solids including the following elements: Ca, Zn and Mg in the form of solid phosphates, sulfates and oxides) is observed to accumulate in the fiber filter systems over its lifetime.
This study examines the behavior of lubricant-derived ash accumulation in SMF filters which were used to remove particulate emissions from diesel engine exhaust in various field applications. High resolution visualization and quantification of both inherent ash properties and ash accumulation characteristics are presented. Specifically, the regeneration strategy (active vs. passive) utilized in each filter is shown to significantly affect the inherent ash properties, such as ash particle size and interaction with the metal fibers within the filter. A number of analytical tools have been used in this study including focused ion beam (FIB) milling, Ar+ ion milling for cross sectional analysis, environmental scanning electron microscopy (ESEM), energy dispersive X-ray spectroscopy (EDX) and X-ray Computed Tomography (CT). The primary objective of this study is to show, for the first time, some of the general characteristics of lubricant-derived ash within SMF particulate filters, and to provide some insight into the role ash plays over the useful lifetime of the fiber filters.