An experimental investigation of soot morphology and nanostructure in high-pressure co-flow laminar methane diffusion flames

Pressure is a significant factor affecting soot formation and oxidation. However, there are only scarce studies in the literature that probe soot samples in high-pressure laminar diffusion flames and then analyze the soot nanostructure quantitatively. In this study, a novel pneumatic probe sampling method was proposed to obtain soot samples in methane co-flow laminar diffusion flames with pressures ranging from 0.2 to 0.8 MPa. Compared with thermophoretic sampling, this method was more suitable for long-time sampling to obtain adequate soot samples in flames with low soot concentration. The soot morphology and nanostructure were analyzed quantitatively by high-resolution transmission electron microscopy (HRTEM). The results show that as the pressure increases, the soot samples at 10, 20, 30, and 40 mm heights above burner (HAB) exhibit larger soot particle size, more ordered nanostructure, longer crystallite fringe, the larger size of poly-aromatic hydrocarbons (PAHs), lower fringe tortuosity, and smaller fringe spacing. In particular, the soot samples at 10 mm HAB showed the largest differences in morphology and nanostructure as the pressure increased. The quantitative analysis results suggested that elevated pressure increased graphitization, maturity, and oxidation resistance of soot, which was most possibly attributed to enhanced soot nucleation and surface growth by pressure.