A reactive molecular dynamics simulation on the mechanism of the transformer oil pyrolysis at the high temperature

We have carried out the reactive force field simulations on the pyrolysis process of the transformer insulating oil at different temperatures from 2900 to 3500 K. The selected principal components of the oil molecules are alkanes (C₁₅H₃₂), cycloalkanes (C₁₅H₂₈), and aromatics (C₁₅H₁₆). The effects of formic acid (HCOOH) and acetic acid (CH₃COOH) on the pyrolysis mechanism of insulating oils are also studied. The calculated results suggest that with the increasing temperatures, the cracking process of three types of oil molecules accelerates and their speeds of cracking decrease in this order: C₁₅H₃₂ > C₁₅H₂₈ > C₁₅H₁₆. The main pyrolysis products are C₂H₄, CH₄, C₂H₂, ·CH₃, ·C₂H₃, etc. For the mixture of three types of hydrocarbons, the C₂H₄ is initially the most abundant product but its concentration decreases gradually and finally the C₂H₂ and CH₄ become the major products. The variation trend of the number of C‒C bonds indicates that the C‒C bonds of oil molecules (especially C₁₅H₃₂ molecules) are broken rapidly at the early stage of pyrolysis process to produce small molecular fragments and then these small fragments are recombined into new pyrolysis products. The evolution of the different types of carbon atoms (sp³-C, sp²-C, sp-C) shows that the sp³-C atoms are initially converted to sp²-C atoms. After the early stage of reactions, many sp²-C atoms are finally turned into sp-C atoms. The addition of formic acid and acetic acid in the insulating oils leads to the formation of the CO and H₂O molecules. Both the acetic acid and formic acid could accelerate the pyrolysis process of oil molecules and could further increase the degree of unsaturation of products, which is consistent with the experimental observations. Graphical abstract: [Figure not available: see fulltext.]