Investigation of coal pore structure evolution under temperature-time coupling using low-field nuclear magnetic resonance.

To investigate the coupled effects of temperature and time on the evolution of coal pore structure, this study employed low-field nuclear magnetic resonance (LF-NMR) combined with oil bath heat treatment experiments to systematically characterize variations in T 2 relaxation spectra, pore size distribution, throat structure, and porosity in coal samples. The experimental findings demonstrate that the T 2 spectra of all coal samples display a bimodal distribution, wherein micropores represent the predominant pore type, whereas macropores and fractures remain predominantly undeveloped. After heat treatment, the micropore peak intensity decreased and shifted to a lower value, indicating pore contraction or collapse. At 35 °C, the pore-throat volume and porosity exhibit characteristics of a dynamic equilibrium, showing partial recovery after an initial short-term contraction. In contrast, at 65 °C, the thermal stress induced by differential thermal expansion exceeds the strength of the coal matrix, resulting in the irreversible collapse of micropores and a monotonic decrease in porosity. Submicron-sized pore throats (<0.1 µm) exhibited higher sensitivity to the coupled effects of temperature and time, whereas macropore throats remained largely unaffected. Furthermore, this study reveals that the coupled temperature-time effect primarily governs the irreversible damage processes in the micropore-throat system through a thermal stress mechanism. This, in turn, modulates the coal's oxidative activity and spontaneous combustion (SC) propensity, thereby providing a critical theoretical foundation for the early prediction and warning of coal spontaneous combustion (CSC), as well as for the development of targeted inhibition and control strategies.