Quantification of pore modification in coals due to pulverization using synchrotron small angle X-ray scattering

Small angle X-ray scattering (SAXS) has been widely employed to characterize the pore structure of coal and other geomaterials. However, in the many previous experimental investigations by SAXS, the probed samples were not standardized in terms of shape and particle size, even for the same geomaterial. The influence of pulverization on the pore structure of coal is generally ignored by the researchers using SAXS for structure characterization. In this study, SAXS is used to investigate the effect of pulverizing on pore modification of virgin coal. Two kinds of coal with different ranks were selected and pulverized for SAXS measurements. Both powder samples (five particle size fractions) obtained by grinding and slice samples were investigated. The results show that the nano-pore structure of coal has been significantly modified due to pulverization. The sample pore volume per unit mass increases with the decrease of sample particle size. Pulverizing the slice sample into the minimum particle sample particle size of 0.075-0.125 mm leads to an increase in the pore volume per unit mass of coal by 39.53% and 18.29% for tested medium volatile bituminous coal and anthracite, respectively. Pulverization can reduce the volume of smaller pores (20 nm) and increase the volume of larger pores (20 nm). After grinding the original sample into the minimum particle size of 0.075-0.125 mm, the fractal dimension of the tested medium volatile bituminous coal and anthracite decreased by 0.266 and 0.179, respectively. Surface fractal dimension of pore decreases with the reduction in coal particle size. In the case of the powder coal sample, the increase in pore volume (especially for pore greater than 20 nm) and the reduction of fractal dimension are beneficial to the diffusion of gas in coal and promoting the desorption of residual gas. This study may provide a quantitative insight into the effect of pulverization on pore structure of coal sample. Moreover, it is of great significance for understanding the mechanism of coalbed methane storage and migration and is helpful for the selection of the sample size in SAXS experiments.