Soot formation at elevated pressures and carbon concentrations in hydrocarbon pyrolysis

Abstract

For the formation of soot in mixtures of ethylene, n -hexane, and benzene in argon behind reflected shock-wave induction periods, soot growth rate constants and soot yields were measured at various temperatures, pressures, and C-atom concentrations using light extinction techniques. As expected, the three substances investigated behave differently regarding soot formation, the strongestdifference being between aromatic and nonaromatic fuels. Induction times and soot growth rate constants do not show any significant pressure effect within the accuracy of the experiment under the conditions employed. The data for ethylene and n -hexane agree quantitatively fairly well, whereas benzene shows induction times that are shorter and rate constants that are larger by about an order of magnitude than those of the nonaromatics for equal C-atom concentrations. The previously observed general shape and behaviour of the soot yield curves could be confirmed, withmaximum soot yields, at temperatures between 1800 and 1950 K. For similar experimental conditions of temperature, pressure, and carbon concentration, the sooting propensity increases from n -hexane to ethylene to benzene. The large pressure range covered in this study revealed different pressure dependencies of soot formation for the three hydrocarbons investigated. For n -hexane, the influence of pressure on soot yield in pyrolysis is very small. For ethylene, soot yield scales with pressure, whereas for benzene, a different behaviour was found, resulting in lower soot yields at higher pressures at otherwise equal conditions. Soot yields are also depending on C-atom concentration. Particle diameters of soot from pyrolysis determined by electron microscopy can be described by a narrow log-normal size distribution, with σ g ≈0.2. The average particle diameters are 20–30 nm for all conditions of temperature, pressure, and C-atom concentration employed in this study. Assuming spherical particles, this leads to final number densities of 10 11 to 10 13 /cm 3 .