Formation mechanism of polycyclic aromatic hydrocarbons during mineral oil pyrolysis: A ReaxFF molecular dynamics study

Recycling used mineral oil (UMO) through pyrolysis is a flexible and efficient way. However, polycyclic aromatic hydrocarbons (PAHs) characterized by high toxicity can be produced during this process. This work attempts to reveal the evolution process of mineral oil and the formation mechanism of PAHs during mineral oil pyrolysis through the ReaxFF molecular dynamics (MD) simulation. Besides, the principles of heating rate (10 K/ps, 100 K/ps, and ∼ 1000 K/ps), temperature (2200 ∼ 3200 K), mineral oil components (naphthenes and aromatics content), and atmosphere (CO2) influencing PAHs formation were also explored. A two-staged pyrolysis evolution of mineral oil with the first decomposition and the followed polymerization reaction was observed. High temperature can make a quick conversion of the thermal decomposition to the polymerization reaction stage. By tracking the evolvement of crucial intermediates/products and aromatic structures, hydrogen abstraction-vinyl radical addition (HAVA) reaction was found to dominate PAHs formation during mineral oil pyrolysis. Besides, the polycondensation-cyclization of branched-chain groups attached to carbon clusters made considerable contribution to the formation of large PAHs. Vinyl radical addition and polycondensation-dehydrogenation were the two emblematic reactions of PAHs formation during mineral oil pyrolysis. The contribution of aromatic component in mineral oil to PAHs formation was found to be about 6.5 times that of naphthene component. CO2 can reduce PAHs formation through the oxidation of intermediates/product and also the inhibition of dehydrogenation with the followed vinyl radical/acetylene addition reactions.