Study on the Microscopic Mechanism of CH₄ Formation During Pyrolysis of High-Voltage Cable PVC Outer Sheath from a Dual-Pathway Perspective

Polyvinyl chloride (PVC), owing to its excellent insulation and flame-retardant properties, is widely used as the outer sheath in high-voltage cables. Although PVC undergoes pyrolysis under thermal fault conditions and releases combustible gases such as methane (CH₄), this process also provides a feasible basis for the application of gas signals in early fault warning of cables. Among the pyrolysis products, CH₄ has been widely observed experimentally; however, its specific microscopic formation pathways remain insufficiently explored. To address this, this study employs (DFT) density functional theory to construct and compare two potential CH₄ formation pathways during PVC chain scission: (1) concerted cleavage at the terminal β-position and (2) methyl radical recombination following in-chain cleavage. Computational results show that the β-position cleavage pathway, despite its higher energy barrier, exhibits stronger spontaneity at elevated temperatures. In contrast, the in-chain pathway has a slightly lower activation energy but incurs greater overall thermal cost, contributing significantly to CH₄ yield only under high-temperature or strongly excited conditions. This study systematically elucidates the dual-pathway mechanism of CH₄ formation during PVC pyrolysis, providing microscopic theoretical support for thermal fault diagnosis strategies based on methane monitoring.