Molecular modeling of tar-rich coals from northern Shaanxi and their molecular dynamic characteristics in the process of pyrolysis for tar extraction

Objective The pyrolysis process and mechanism of tar-rich coals remain unclear due to their complex structures. Therefore, it is necessary to investigate the structure-activity relationship between their structure and the distributions of their pyrolytic products and determine the mechanism behind their pyrolysis for tar extraction.

Methods Focusing on typical tar-rich coals from northern Shaanxi Province, this study calculated and constructed the molecular model of the coals using characterization methods such as carbon-13 nuclear magnetic resonance (¹³C NMR), Fourier transform infrared spectroscopy (FTIR), and X-ray photoelectron spectroscopy (XPS), determining the molecular formula at C₄₄₄H₃₈₀O₇₄N₈S. Based on the molecular model, this study examined the pyrolytic product distributions, pyrolysis characteristics, and tar production mechanism of the tar-rich coals using the reactive force field molecular dynamics (ReaxFF MD).

Results and Conclusions  The results reveal that the tar-rich coals from northern Shaanxi Province exhibit substantial hydrogen-rich structures, such as aliphatic side chains and bridge bonds, tending to decompose into volatile constituents like tar and gas when heated. The product distributions during the pyrolysis of tar-rich coals are significantly influenced by the temperature and heating rate. At a simulated pyrolysis temperature of 1500 K, the tar-rich coals underwent partial reactions, yielding volatile constituents with a mass fraction of 7.8%. As the simulated pyrolysis temperature rose to 2 000 to 3000 K, molecular and radical fragments moved more intensely, accompanied by more pronounced secondary reactions. With an increase in the heating rate, gas products decreased, whereas tar products increased first and then decreased. At heating rates between 100 and 200 K/ps, the tar-rich coals were partially pyrolyzed, leading to mass fractions of volatile constituents below 36%. As inferred from the distributions of pyrolytic products, the mechanism behind the pyrolysis of tar-rich coals from northern Shaanxi Province is that the hydrogen-rich structures in tar-rich coal macromolecules break under heating, forming radical fragments, which are further pyrolyzed to produce light tar, heavy tar, and gas.