Investigation of soot precursor molecules during inception by acetylene pyrolysis using reactive molecular dynamics

Abstract

Soot inception by acetylene pyrolysis at 1350-1800 K is investigated using reactive molecular dynamics. The composition and chemical structure of soot precursor molecules formed during inception are elucidated. During soot inception, increasing the process temperature leads to faster depletion of C₂H₂ molecules and faster formation of C₂H₃, C₂H₄, C₂H₆, CH₄, and C₂ with the concurrent appearance of H₂ molecules. Small molecules consisting of 1-5 C atoms (C₁-C₅) are formed due to reactive collisions and grow further to larger hydrocarbon compounds consisting of 6-10 C atoms. At initial stages of inception, prior to the formation of incipient soot, three-member rings are formed, which are associated with the formation of compounds with fewer than 10 C atoms. Once incipient soot is formed, the number of C₁-C₁₀ compounds and the number of three-member rings drop, while the number of five- and six-member rings increases, indicating that the formation of larger rings is associated with the growth of soot clusters. The chemical structure of soot precursor molecules obtained by bond order analysis reveals that molecules with up to 10 C atoms are either linear or branched aliphatic compounds or may contain three-member rings fused with aliphatic components. Molecules with more than 10 C atoms often exhibit structures composed of five- or six-member C rings, decorated by aliphatic components. The identification of molecular precursors contributing to soot inception provides crucial insights into soot formation mechanisms, pinpointing potential pathways of soot formation during combustion.