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. [ABSTRACT FROM AUTHOR]