Study on the difference of pore structure of anthracite under different particle sizes using low-temperature nitrogen adsorption method

Abstract

The low-temperature nitrogen adsorption test was used to study anthracite from Jiulishan coal mine with different particle size ranges of 60-80 mesh, 150-200 mesh, and > 200 mesh. The adsorption isotherm, adsorption capacity, pore volume, pore specific surface area, and average pore diameter of coal samples were analyzed by BET and DFT models in order to study the influence of particle size on the pore structure of anthracite and determine the optimal range of particle size for low-temperature nitrogen adsorption test. The results indicate that the particle size plays a significant effect on the pore structure of anthracite and the adsorption capacity of soft coal is less affected by particle size, while hard coal is substantially affected by particle size. The adsorption capacity of hard coal with particle size of > 200 mesh is increased by 7 times when compared with the particle size of 60-80 mesh, indicating that the gas molecular mobility hindrance decline and pore connectivity improves with the decrease of particle size. The average pore diameter of hard coal decreases continuously from 3.1424 to 2.854 nm, while that of soft coal expands from 2.8947 to 3.2515 nm and then to 3.0362 nm with the decrease of particle size. The effects of particle size on the pore surface area of soft and hard coal are concentrated within the < 10 nm pore aperture. Effect of particle size on hard coal pore volume is mainly focused in the pore size < 10 nm, whereas that of soft coal is primarily concentrated in the pore with aperture ranges of 2-100 nm. When the particle sizes varies from 60-80 mesh to 150-200 mesh, the collapse of large pore of hard coal appears better than that of closed pore. When the particle size of hard coal reaches > 200 mesh, the collapse of closed pores and the damage to small pores are stronger than the collapse of large pores. The fractal dimensions with relative pressure of 0-0.20 and 0.20-0.995 are defined as D₁ and D₂, respectively, and when the fractal dimension D₁ increases, the surface roughness and structural complexity of coal samples increase with the decrease of anthracite particle size, while the fractal dimension D₂ shows the opposite trend, which indicates that anthracite of smaller particle size possess higher adsorption capacity. Therefore, 150-200 mesh is recommended as the preferred anthracite particle size in low-temperature nitrogen adsorption test.

Keywords: Adsorption isotherm; Anthracite; Different particle sizes; Fractal dimension; Low-temperature nitrogen; Pore structure.