A theoretical investigation on the transformer oil pyrolysis mechanism and the effect of the small molecule acid in oils

The reactive force field (ReaxFF) simulations have been performed to investigate the pyrolysis process of the transformer insulating oil from 2900 to 3500 K. We selected C17H36, C17H32, and C17H20 as the principal components of the oil molecules and studied the effects of formic acid (HCOOH) and acetic acid (CH3COOH) on the pyrolysis mechanism of insulating oils. The calculated results show that the cracking process are accelerated at high temperatures and the speeds of cracking are in this order: C17H36 > C17H32 > C17H20. The predicted main pyrolysis products are H2, C2H4, CH4, C2H2, CH3, C2H3, etc. Except for H2 molecules, the C2H4 is initially the most abundant product but its number decreases gradually and finally the C2H2 and CH4 are the major products. The evolution of the number of C–C bonds suggests that the C–C bonds of oil molecules are recombined from the early stage of pyrolysis process to the final equilibration state. The pyrolysis mechanisms of transformer oil have been proposed. The typical elementary steps are H2C = CH2 → H-H + H-C≡C-H, CH4 → H-H + CH2, 2(H2C = CH2) → H-H + 2(H2C = CH), and 2(H2C = CH) → H-H + 2(H-C≡C-H). Formic acid (HCOOH) could be decomposed into CO, H2O, and CO2 by the following reactions: HCOOH → COH + O-H, COH → CO + H, H + O-H → H2O, and HCOOH → CO2 + H-H. The similar steps are found for acetic acid (CH3COOH). Both the acetic acid and formic acid could increase the degree of unsaturation of products, which is consistent with the experimental results.