PAHs Behavior and Graphitization Degree of Soot in a Hexane Diffusion Flame

Pool combustion flame was analyzed using KrF Excimer laser optical system to characterize in-flame soot and its precursors such as polycyclic aromatic hydrocarbons (PAHs). Test fuel was hexane and it was a typical paraffin-type hydrocarbon fuel. Spatial distributions of PAHs and soot were measured using laser induced fluorescence (LIF) and laser induced incandescence (LII) diagnostics. Some of PAH concentrations were quantitatively obtained using gas chromatography (GC) system. As for the analysis of polymerization and graphitization process from PAHs to soot, Raman spectroscopy analysis was adopted. Peaks of G-band corresponding to the vibration mode of graphite lattice structure and D-band attributing to a disordered graphite lattice were detected. Further graphitization degree of soot was discussed using I(G)/I(D) ratio of Raman spectra and thermo-gravimetric analysis (TGA). Transition temperature from PAHs to soot was clearly obtained as the valley of PAH-LIF and soot-LII intensities. I(G)/I(D) ratio of PAHs in the later LIF stage was low, but it increased with a solidification progress from PAHs to soot. I(G)/I(D) ratio increased further when the soot inspection position approached downstream of the flame. Thermal decomposition temperature (TGA result) of sampled soot was elevated at the downstream position. Soot in high temperature region near the flame tip showed high I(G)/I(D) ratio and high decomposition/oxidation temperature, it means that graphitization of soot was well progressed near the flame tip.