The Effect of Demineralization on Coal Functional Group Pyrolysis: Rank‐Dependent Dual Mechanisms of Structural Reorganization and Mineral Catalysis

Coal pyrolysis, as a key thermochemical conversion process, is significantly influenced by inherent minerals, yet their role across different coal ranks remains inadequately understood. This study investigates the effect of acid‐washing (demineralization) on the thermal decomposition of six functional groups (aliphatic C–H, aromatic C–H/C=C, C=O, C–O, and OH) in coals of varying ranks (lignite XDL, gas coal DJG, and coking coal QJC) using thermogravimetry (TG), X‐ray diffraction (XRD), and in situ transmission Fourier transform infrared spectroscopy (in situ FTIR). Sequential hydrochloric acid‐hydrofluoric acid (HCl‐HF) treatment removes most inorganic minerals in coal while inducing structural reorganization, particularly in the higher‐rank coal. For low‐rank XDL, the functional group evolution remains largely unaffected, whereas medium‐ and high‐rank DJG and QJC exhibit accelerated decomposition of aliphatic C–H, aromatic C=C, and C=O groups probably due to acid‐induced pore formation or macromolecular destabilization. Minerals in the raw coals stabilized aromatic frameworks and catalyzed the conversion of C–O to C=O. Consequently, their removal suppressed hydrogen transfer and the cleavage of substituents on aromatic rings. Notably, demineralization by sequential acid washing delays C–O decomposition across all ranks but enhances OH retention in lignite while promoting dehydroxylation in higher‐rank coals. These findings underscore a rank‐dependent mechanism: structural reorganization dominates in high‐rank coals, whereas mineral catalytic effects prevail in lower‐rank matrices. The study highlights the interplay between coal maturity, mineral composition, and structural integrity in governing pyrolysis dynamics.