Fractal characteristics of coal surface structure during low-temperature oxidation and its effect on oxidizability

Pore structure and surface morphology are the key factors affecting coal spontaneous combustion. However, there are few studies that focus on quantitative descriptions of them. This paper presents an investigation into the evolution of the surface physical structure of different metamorphic coal during low-temperature oxidation. Specific surface area analyzer and atomic force microscopy were used to quantify the physical structure. The results indicate that the average pore size of coal decreases as the temperature increase, with low-metamorphic coal demonstrating a more pronounced reduction. Conversely, the specific surface area exhibits a gradual increase with temperature, particularly for the lignite. The evolution of surface morphology gradually changes from the fluctuation of low temperature to a flat surface with fewer high peaks. Meanwhile, the fractal dimension increasing with temperature also explain that the pore structure and surface morphology tend to become more complex during coal oxidation, especially the lignite. That is, the slopes of the fitting lines of fractal dimension vs temperature are higher for XLT coal than for YL coal and SG coal. The effect of coal surface structure on oxidizability is interpreted by the pore connectivity and oxygen adsorption on the surface during the coal oxygen compound reaction.