Pore Structure Characterization of Coal by Synchrotron Small-Angle X-ray Scattering and Transmission Electron Microscopy

Coal is a porous medium with complex pore structures. The characteristics of the pore structure play an important role in various aspects of coal use, including extraction of methane from coal seams, CO2 sequestration in coal, and water purification by activated carbon. To describe comprehensively the pore structure of coal, we apply transmission electron microscopy (TEM) and synchrotron small-angle X-ray scattering (SAXS) measurements to six coal samples from medium to high rank. The positive deviation of SAXS data from Porod's law was observed. The positive deviation correction of SAXS data was carried out to quantitatively obtain the pore size distribution and specific surface area. We find that the pore size distribution is independent of the coal rank but varies with the vitrinite content; pores in vitrinite-rich coals are smaller than those in vitrinite-poor coals for the same rank. Channel-like and interconnected pores are observed for both high- and low-volatile bituminous coals. Among all coal samples, the low-volatile bituminous coal has the largest specific internal surface area, indicating the highest gas storage capacity and a favorable role as the best candidate for coalbed methane exploration and coal CO2 sequestration.