Molecular structure model construction and pyrolysis mechanism analysis of lignite by experiments and ReaxFF-MD

• The single molecule and particle of Dayan lignite is thoroughly characterized. • The coal particle is predominantly assembled through non-covalent interactions. • The rationality and accuracy of single molecule and particle is verified. • The isothermal pyrolysis mechanism of Dayan lignite was clarified using ReaxFF-MD. To investigate the intrinsic chemical reactions in the process of clean and efficient coal utilization, a thorough understanding of coal’s molecular structure is essential. In this work, integrated experimental and theoretical approaches were applied to characterize the molecular configuration of Dayan lignite from Inner Mongolia, China. Reactive force field (ReaxFF) simulations were further conducted to explore the pyrolysis mechanism of Dayan lignite. Results indicate that the dominant framework consists mainly of aromatic clusters. The ratio of aromatic bridge carbon to aromatic peripheral carbon (XBP) is 0.05, indicating that the structure mainly consists of benzene and naphthalene rings. Oxygen exists primarily in functional groups including ether (C–O), carbonyl (C=O), and carboxyl (–COO) groups. Nitrogen occurs in forms such as pyrrole, pyridine, and quaternary nitrogen functionalities. Among aliphatic constituents, methylene groups are predominant. Accordingly, an average macromolecular model with the molecular formula C 174 H 143 O 41 N 4 was established. Validation of the proposed structural model was achieved by aligning 13 C NMR spectra with corresponding experimental measurements. Moreover, agreement between thermogravimetric analysis profiles and ReaxFF-derived curves confirms the reliability of the model in predicting pyrolysis predicting pyrolysis behavior and thermal stability characteristics. Ultimately, based on the developed molecular representation, mechanistic insights into the pyrolysis behavior of Dayan lignite over the temperature range of 1200–3000 K was examined. It was observed that primary decomposition prevails at relatively low temperatures, whereas secondary cracking of tar becomes pronounced above 2200 K, resulting in elevated char yields. Additionally, underlying mechanisms driving secondary reactions of tars have also been elucidated.