The evolution of soot precursor particles in a diffusion flame

The chemical evolution of soot precursor particles on the centerline of the laminar ethene diffusion flame has been analyzed using laser microprobe mass spectrometry (LMMS) as they undergo the transition to carbonaceous aggregates. LMMS is a reliable microanalytical technique for the detection of intermediate and heavy polycyclic aromatic hydrocarbons (PAHs) in particulate material. The analyses show that many of the masses present within the precursor particles coincide with those predicted by Stein and Fahr (1985) to be most thermodynamically stable (stabilomers). The stabilomer PAHs that consist solely of six-membered rings, the benzenoid PAHs, prove to be the most important members of the stabilomer grid. Pericondensed PAHs as large 472 amu, which is attributed to the molecule C38H16 with 12 hexagonal rings, are found to be constituents of the precursor particles. The PAH mass distribution diverges to the larger sizes with increasing height in the flame, and includes many of the species identified by others as gas-phase PAH constituents in hydrocarbon flames. Carbonization on the centerline of the flame occurs abruptly between 35 and 40 mm above the burner where the particle metamorphosis (from single precursor liquid-like particles to fused aggregates) and the decrease in hydrogen mole fraction (from 0.35 to 0.15) simultaneously occur. The presence of stabilomer PAHs reported by others in the particulate combustion product of a variety of fuels-aliphatic and aromatic gases, diesel fuel, elude oil, kerogen, carbon black feed stuck, cigarette tobacco, and biomass-suggests that the stabilomer grid represents the common path for the growth of PAHs which contribute to the formation of carbonaceous soot in these diverse instances. This observation can account for the previously noted invariance of the soot product of combustion from diverse fuels and devices. (C) 1998 by The Combustion Institute.