Molecular insights into bituminous coals pyrolysis: a combined study using spectroscopic techniques, thermogravimetric-mass spectrometry and ReaxFF molecular dynamics simulations

Understanding the coal molecular structure and its evolution in the pyrolysis process is essential for coal's clean and efficient utilization to achieve a climate-neutral society. Advanced physical characterization techniques, including solid-state 13C nuclear magnetic resonance spectroscopy (13C NMR), X-ray photoelectron spectroscopy (XPS), and Fourier transform infrared spectroscopy (FTIR), were employed to characterize the structures of six typical bituminous coals in atomistic scale. The observed structural information, including aromatic carbon ring skeleton, aromatic substituents, oxygen-containing functional groups, and aliphatic carbon, was used to build the two-dimensional coal molecular structures. The simulated 13C NMR spectrum of the coal molecular structure is consistent with the experimental 13C NMR spectrum, confirming the reliability of the coal molecular structure. Then, pyrolysis of coal samples was carried out through thermogravimetric-mass spectrometry (TG-MS) experiments and reactive force field molecular dynamics (ReaxFF MD) simulations. The TG experiments and ReaxFF MD simulations show that the weight loss curve during coal pyrolysis follows the same trend as the temperature change, validating the ReaxFF MD simulations. The MS and simulation results showed that H2 and CH4 increased with increasing temperature. Furthermore, the structural evolution, especially the quantity of C-C and C-H bonds during ReaxFF MD simulations, was investigated, indicating the pyrolysis mechanism.