A study on the abnormal thermal behaviors of barkinite by ReaxFF molecular dynamics simulation

Barkinite is termed as a maceral in the Chinese bituminous coal, but it has not been recognized by International Committee for Coal and Organic Petrology, which is related to the unclear chemical structure of barkinite. The abnormal thermal behaviors of barkinite/bark coal were reported in our previous works, but the reason is not fully understood. For discussing these issues, the chemical structural model of barkinite with a molecular formula of C128H166N2O11 was constructed by elemental analysis, C-13 nuclear magnetic resonance spectroscopy, and Fourier transform infrared spectroscopy. Besides, a heat up simulation of barkinite model was also performed by ReaxFF. The results showed that the chemical structural model of barkinite has long aliphatic chains and many hydroxyl and ether functional groups, which act as side chains and bridges. Naphthalene is main aromatic unit. When barkinite model system was heated from 300 to 2850 K, the change trends of pyrolysis products were described from four pyrolysis stages. The most noticeable changes of tar yields occurred in the simulated temperature range of 2100-2600 K, which caused the abnormal thermal behaviors of barkinite. According to the reaction pathways, the formation of tar products is related to the cleavage of aliphatic chains and the recombination of small molecular free radicals.