A numerical study on oxygen adsorption in porous media of coal rock based on fractal geometry

In order to explore the factors affecting coal spontaneous combustion, the fractal characteristics of coal samples are tested, and a pore-scale model for oxygen adsorption in coal porous media is developed based on self-similar fractal model. The liquid nitrogen adsorption experiments show that the coal samples indicate evident fractal scaling laws at both low-pressure and high-pressure sections, and the fractal dimensions, respectively, represent surface morphology and pore structure of coal rock. The pore-scale model has been validated by comparing with available experimental data and numerical simulation. The present numerical results indicate that the oxygen adsorption depends on both the pore structures and temperature of coal rock. The oxygen adsorption increases with increased porosity, fractal dimension and ratio of minimum to maximum pore sizes. The edge effect can be clearly seen near the cavity/pore, where the oxygen concentration is low. The correlation between the oxygen adsorption and temperature is found to obey Langmuir adsorption theory, and a new formula for oxygen adsorption and porosity is proposed. This study may help understanding the mechanisms of oxygen adsorption and accordingly provide guidelines to lower the risk of spontaneous combustion of coal.