Soot formation in shock-tube pyrolysis of acetylene, allene, and 1,3-butadiene

Soot formation in argon-diluted mixtures of acetylene, allene, and 1,3-butadiene was studied behind reflected shock waves by monitoring attenuation of a laser beam in both the visible (632.8 nm) and the infrared (3.39 μm) regions of the spectrum. Experiments were conducted for temperatures in the range 1500–3100K, reflected shock pressures in the range 0.3–7.0 bar, and total carbon atom concentrations in the range (2–20) × 1017 atoms/cm3. During the pyrolysis of individual hydrocarbons, a bell-shaped dependence of soot yield on temperature, similar to that previously reported for toluene, was observed for all three compounds. For acetylene, a decrease in total pressure shifted the soot bell to higher temperatures with a significant increase in the maximum soot yield. The analysis of a computer simulation for acetylene pyrolysis indicated that the reactions involving C2H3, C4H3, and C4H4 may be those which lead to the formation of aromatic structures. The experimental results also show that soot is formed much faster and in much larger quantities from allene than from 1,3-butadiene. A conceptual model which explains the observed phenomena is suggested.