A Study of the Early History of Soot Formation in Various Hydrocarbon Diffusion Flames

Abstract

Diffusion flames of methane, ethane, ethene, acetylene, propane, allene 1-butene, 1,3-butadiene and benzene in coflowing normal atmospheric air were investigated by analyzing samples of malerials collected on fine probes inserted into the flames. To focus attention on early slages of soot formation, small flames (heights 2cm to 4cm) on a burner 1.6cm in diameter were studied, and the more heavily sooting fuels were diluted with nitrogen (up to a nitrogen mole fraction of 90% for acetylene). Radiation-corrected thermocouples were employed to measure temperatures in the regions of earliest soot deposits, and the distances from the point of earliest deposit to that of the first visible soot emissions of radiation were also recorded. Scanning electron micrography (SEM) and X-ray photoelectric spectroscopy (ESCA) were applied lo the samples to investigate changes in soot morphology with increasing axial distance. The results showed the earliest deposils to be highly fuel-specific but later deposits to be quite similar for all fuels. For methane and benzene the first deposils appeared to be smooth and light brown under an oplical microscope, unique and subjectively polymer-like by SEM, and essentially aliphatic by ESCA. Later deposits for these two fuels, and all deposits for the other fuels, were black and initially shiny bul becoming progressively duller and rougher with increasing height as seen under an optical microscope, mainly sool-like agglomerates of spheres by SEM, and virtually completely aromatic by ESCA. Contrary to results of many earlier studies, these observations suggest a large effect of the initial fuel on the early history of soot production in diffusion flames.