Effect of H/C ratio of feedstock composition on particle size distribution of soot in C2 hydrocarbon pyrolysis

The effect of the H/C ratio of feedstock on soot formation was investigated in this study. Soot was continuously generated by pyrolyzing C2 hydrocarbons in an atmosphere without oxygenated species at a maximum temperature of 1683 K. H2 was added to the feedstock to vary the H/C ratio, and the particle size distribution of soot was determined using a scanning mobility particle sizer. Most of our results indicate that the ease of thermal dissociation of the feedstock is more important than the degree of unsaturation in the feedstock. With increasing H/C ratio of the feedstock, more radicals are generated, and polycyclic aromatic hydrocarbons (PAHs) are easily formed, leading to the formation of soot. Interestingly, many previous studies on combustion have reported the opposite behavior, indicating that the ease of radical formation is important in pure pyrolysis without oxygenated species, which generate radicals easily. In the case of C2H2 as a feedstock in this study, the addition of a small amount of H2 increased the amount of radicals and promoted soot formation. However, with increasing amounts of H2, soot formation was inhibited because the inhibitory effect of H2 on PAH formation increased. When H/C = 2, the H/C ratio of the feedstock was dominant for soot formation, while the degree of unsaturation of the chemical species in the feedstock was dominant when the H/C ratios were 3 and 4.