Influence of particle size on pore structure and multifractal characteristics in coal using low-pressure gas adsorption

Abstract

Understanding pore heterogeneity is essential to obtain insights into the gas storage, transport, and recovery from coal. The pore structure in analytical sample particles of different sizes varies significantly, leading to inaccurate pore characterization. In this study, we discuss the effect of particle size on the pore structures and multifractal characteristics of two samples with six particle sizes using low-pressure gas adsorption. The results show that, with decreasing particle size, the mesopore and macropore volumes and specific surface areas (SSAs) of the two samples fluctuate. The variations of micropores that contribute to most of the total pore volume and SSA (>90%) are different. Multifractal analysis shows that the subsamples with the smallest particle size have the most homogeneous pore structures and the highest pore connectivity. We reveal a negative power function relationship among particle size and pore volume, and SSA, indicating that crushing opens up more pores that were initially closed. The differences between the two coal samples may be caused by the combined effects of crushing and associated impacts (e.g., pore collapse), coalification, incomplete equilibration of N2 adsorption, and mineralogical inhomogeneity. Our results demonstrate that in the range of test particles, the finer the particle size used, the more accurate the evaluation of highly mature coal adsorption properties.