Structural evolution and spontaneous combustion and activation mechanisms of coals treated under inert atmospheres in a coal mine goaf

Objective When used for the treatment of fire zones in deep coal mine goafs, low-oxygen and inertization techniques suffer from a scientifically blind spot, i.e., the fire prevention and control bottleneck caused by a high susceptibility to re-ignition of coals after being treated with high-temperature, low-oxygen (or inert) gas. To overcome this limitation, this study aims to systematically determine the mechanisms by which the evolution of the physical and chemical structures of coals after heat treatment under different atmospheres influences their spontaneous combustion and activation behavior. The purpose is to provide theoretical support for the treatment and safe opening of fire zones in a coal mine goaf.

Methods This study investigated gas-bearing coals in the Zhujidong Coal Mine, Anhui Province. Using a series of techniques, such as the Fourier transform infrared spectroscopy (FTIR), specific surface area and pore size analyses, thermal conductivity tests, and thermogravimetric analysis, this study systematically investigated the evolution of the chemical functional groups and changes in the pore structures in coal samples after heat treatment under atmospheres of 13% O₂, N₂, and CO₂. In combination with low-temperature oxidation experiments, this study monitored the release patterns of indicator gases such as CO and C₂H₄, oxygen consumption rates, and exothermic intensities of these coal samples. Additionally, variations in the activation energy for coal spontaneous combustion were quantitatively assessed through kinetic parameter-based inversion.

Results and Conclusions  FTIR analysis revealed that coal samples after heat treatment under different atmospheres exhibited significantly decreased ―OH content and noticeably increased contents of C―O and C=O. Among these, samples treated under a CO₂ atmosphere showed the most prominent effect of functional group reconstruction. Pore structure analysis suggests that coal samples after heat treatment exhibited significantly increased pore volumes and micropore specific surface areas, as well as improved pore connectivity. Most especially, samples treated under a CO₂ atmosphere exhibited the most developed pores with pore sizes ranging from 2 nm to 50 nm, with a micropore volume increasing by 0.3 μL/g and micropore specific surface area reaching up to 1.56 m²/g. These samples were followed by those treated under a N₂ atmosphere and then those treated under a 13% O₂ atmosphere in this regard. The test results of thermophysical properties and low-temperature oxidation characteristics revealed a generally enhanced thermal conductivity and low-temperature oxidation activity of coal samples after heat treatment, as manifested by significantly increased volume fractions of CO and C₂H₄, elevated oxygen consumption rates, and enhanced exothermic intensities. Kinetic analysis demonstrated that the apparent activation energy of the coal samples treated under 13% O₂, N₂, and CO₂ atmospheres decreased by 8.3%, 17.6%, and 21.6%, respectively. Thermogravimetric analysis (TGA) further confirmed that the active temperature t₃ of coal samples after heat treatment dropped by 2‒11 ℃. Furthermore, the mass loss rates in stages from t₃ to t₅ of the coal samples decreased in the order of those treated under CO₂, N₂, and 13% O₂ atmospheres. These findings suggest that although heat treatment under inert atmospheres (especially CO₂) can temporarily suppress oxygen, it could increase the tendency of coals to spontaneous combustion. In engineering, it is advisable to adopt N₂ injection or N₂-CO₂ co-injection, supplemented by measures including temperature control, heat insulation, and fire suppression agents, to establish a multi-element fire prevention and re-ignition control system. This will help enhance the safety and long-term stability of the fire zones of coal mine goafs.