Molecular structure model construction and pyrolysis mechanism study on low-rank coal by experiments and ReaxFF simulations

For an in-depth understanding of China's low-rank coal's organic structure and pyrolysis process, Meihuajing (MHJ) coal was typically studied. Firstly, the organic structures of MHJ coal were obtained, and its molecular model was constructed based on various physical characterizations. Then, the constructed molecular model was well verified, and the pyrolysis processes of this model under heat-up and isothermal conditions were explored using ReaxFF. The results indicated that the primary organic structure of MHJ coal was aromatic skeleton (76.03%). Naphthalene was more abundant in the aromatic skeleton than benzene and anthracene. Moreover, the chemical formula of the constructed molecular model was C388H259O89N5, which was well-validated regarding physical characterizations, thermogravimetric analysis, and ReaxFF simulations. As for pyrolysis simulation, increasing the heating rate primarily suppressed the secondary cracking reactions of tar while further raising the heating rate mainly slowed down the cracking reactions of coke. Besides, the pyrolysis process of MHJ coal was divided into three steps: the rapid cracking stage, the secondary reaction stage of tar, and the basic completion stage of pyrolysis.