Soot and Ash Layer Characteristics in Ceramic Diesel Particulate Filters

Detailed investigation of the morphology, internal structure and oxidative reactivity of PM (ash and soot) trapped in a loaded, uncoated, ca. 30cm long DPF made of SiC, used on a small truck engine operating on a test bench for ca. 250 hours without fuel borne additive revealed the following: ash PM accumulates at the outflow part of the DPF after repeated regenerations, filling up ca. 15% of the effective filter volume. The ash shows a tendency to accumulate more at central over periphery segments, probably following the corresponding differences in the flow rate pattern of the exhaust stream. The amount of ash diminishes significantly towards inflow and becomes minor at ca. 10cm from filter inlet. Soot occurs as aggregates forming a coherent ‘cake’ deposited only in inflow channels. Outlet channels are completely devoid of any deposited material. HRTEM studies show that soot forms aggregates consisting of individual, nearly spherical particles with an average size of 24nm in diameter. The internal particle structure consists of an outer part with nearly concentric alternating lamellae and an amorphous core. Raman spectra of the soot reveal a certain degree of crystallinity and are in line with spectra reported for carbon blacks produced at high temperatures exhibiting low surface area but high degree of ordered structure.