Experimental study on the variation of coal pore structure under the coupling effect of oxidation temperature and duration

To elucidate the evolution of coal pore structure under coupled temperature-time effects, this study conducted programmed temperature oil-bath experiments to treat coal samples under varying thermal conditions. The pore characteristics, including pore volume, specific surface area, and pore size distribution, were systematically analyzed through low-temperature nitrogen adsorption (LTNA) measurements and BET theory. Furthermore, the FHH fractal theory was employed to quantitatively characterize the pore structure heterogeneity. The results indicate that the shape of the coal adsorption-desorption curves remained largely unchanged with increasing heating temperature and duration. In terms of pore volume distribution, the mesopore fraction exhibited a consistent decreasing trend, the micropore fraction initially declined before increasing, while the macropore fraction exhibited an initial increase followed by a subsequent reduction. Regarding the surface area distribution, the proportion of mesopores and macropores exhibited an initial decrease followed by an increase, whereas the micropore fraction demonstrated a converse trend, increasing first and subsequently decreasing. The fractal characteristics of pores are significant, with the fractal dimension <i>D</i> ₁ > <i>D</i> ₂ indicating that the surface pores exhibit rougher structures compared to the internal pore network. Under fixed heating duration, the elevation of temperature lead to concurrent decreases in both <i>D</i> ₁ and <i>D</i> ₂ values. When maintaining constant temperature conditions, the value of <i>D</i> ₁ demonstrated an initial decline followed by recovery, while that of <i>D</i> ₂ showed a trend of initial increase and subsequent reduction. This reveals the dynamic evolutionary process of pore structure.