Dual-regime molecular restructuring in vacuum hot-pressed briquette coal: Closing the fidelity gap for gas outburst simulations

Briquette coal (BC) is used extensively as a coal analog in the physical modeling of coal and gas outbursts. However, the intrinsically weak mechanical structure and elevated permeability of BC relative to natural coal introduce systematic biases. To close this fidelity gap, we report an integrated vacuum hot-pressing thermal conditioning protocol (150–450 °C) coupled with triaxial rock mechanics and seepage characterization. Correlative X-ray diffraction, Fourier-transform infrared spectroscopy, and X-ray photoelectron spectroscopy analyses are performed to decode the molecular engineering levers that govern BC evolution. Two distinct thermal regimes are identified. Below 300 °C, the selective scission of aliphatic C–C/C–H and oxygen-bearing side chains (C–O, C–OH, and COOH) generates plasticizing fragments that drive matrix densification and strength accrual, concurrent with intensified aromatic layer ordering (decreased d 002 , increased L c and L a ). Above 300 °C, accelerated polycondensation consumes these plasticizing moieties, triggering embrittlement and permeability rebound. The 300 °C optimum delivers the closest macroscopic parametric convergence in terms of the compressive strength and permeability. A uniaxial compressive strength of 7.79 MPa and permeability of 1.72 mD are achieved, which are comparable to typical ranges for natural coal specimens. This optimum is attributed to balanced depolymerization–condensation chemistry and optimized microcrystalline packing. These insights establish thermal processing as a quantitative molecular tuning strategy for coal analog fidelity, providing a mechanistic foundation to enhance the predictive reliability of gas outburst simulations.