Impact of sulfur functional groups on physicochemical properties and oxidation reactivity of diesel soot particles

Soot particles with sulfur functional groups (SFGs) are one of the main concerns from shipping emissions, especially when they are produced from fuel combustion with high fuel sulfur content (FSC). The present study investigates the physicochemical characteristics of soot particles generated from different FSC fuels on a marine auxiliary diesel engine. A comprehensive set of experimental techniques including Fourier transform infrared (FT-IR), inductively coupled plasma optical emission spectroscopy (ICP-OES), transmission electron microscopy (TEM) and thermogravimetric analysis (TGA) are employed to characterize SFGs, chemical elements, morphology, nanostructure, volatile organic fraction (VOF) content and oxidation behavior, respectively. Results reveal that, FSC has negligible influences on combustion performance, yet its high content increases the concentrations of SFGs (-SH, C-S-S, C-S, SO42-) and sulfur element VOF content and primary particle size (d(p)) of soot particles. Linear correlations between soot fringe length (La), oxidation activation energy (Ea) and relative concentrations of -SH and C-S are observed, respectively. Higher relative concentration of C-S bond corresponds to shorter La. The relative amounts of -SH and C-S could improve soot oxidation reactivity, associated with lower Ea. In addition, metal-sulfates are formed from the combination of SO42- with metal elements contained in fuel or lube oils, among which CaSO4 is shown an enhancing factor for soot oxidation.