Effect of gas pressure and particle size on dynamic diffusion-adsorption characteristics of CO2 in coal

Abstract

This study investigates the influence mechanism of particle size and gas pressure on the dynamic diffusion law of CO2 in coal.We used a self-developed high pressure adsorption and desorption experimental system for coal seam to analyze the dynamic adsorption-diffusion characteristics before the adsorption equilibrium of gas in coal based on the volumetric adsorption experiment, which was combined with the classical single hole diffusion model to calculate the CO2 diffusion coefficient at different times.The results show that the adsorption equilibrium time is positively correlated with the coal particle size, and decreases with the increase of adsorption equilibrium pressure.Larger coal particle size would result in more obvious decreasing trend.The effective diffusion coefficient is found to be more effective than the diffusion coefficient in explaining the relationship between particle size and equilibrium time.Smaller particle size would result in higher effective diffusion coefficient and shorter adsorption equilibrium time.The effective diffusion coefficient shows a power function with time.The initial effective diffusion coefficient (De0) and effective diffusion attenuation coefficient (β) both decrease with the increase of particle size.De0 first increases and then decreases with the increase of equilibrium pressure, and the β exhibits varying relationships with the increase of equilibrium pressure depending on different particle sizes of the coal.Smaller particle size would result in better connectivity between the pores of coal, leading to shorter diffusion paths of gas molecules and lower diffusion resistance, thus more facilitated to adsorption.Gas pressure poses a driving effect on CO2 adsorption and diffusion, but with the continuous adsorption of coal samples, the coal matrix expands and blocks the pores, leading to a narrowing of the diffusion channel and diminished driving effects.

Keywords

• particle size
• co2 adsorption
• equilibrium pressure
• equilibrium time
• diffusion characteristics