Pore structure characterization of different rank coals using gas adsorption and scanning electron microscopy

Eleven coal samples of different metamorphism are studied with regard to their pore structures. Both low-pressure nitrogen gas adsorption (LP-N2GA) and scanning electron microscopy (SEM) were performed. The use of these techniques allows us to gain clearer insight into the nature of the pore structure including the pore volume, specific area, pore size distribution (PSD) and pore shape. The LP-N2GA isotherms demonstrate strong differences in gas adsorption capacity between the coal samples studied, consistent with variability in specific surface area (SSA) of the samples. Pore geometry of coals with different metamorphism varies a lot, indicative of heterogeneity on coal surface, which was verified with SEM observation. Adsorption analysis revealed that mesopore size distributions are multi-modal, whereas, the micropore structure of the samples tested appears to be unimodal, with a major peak between 1.6 and 2.0 nm. The influence of coal rank on pore structure was also analyzed. The U-shape relationship between mesopore SSA and Vdaf is observed, demonstrating that the number of mesopores within the lower rank coals (Vdaf > 15%) decreases with increasing coal rank and the coalification mainly affects the mesopore structure. For the higher rank coals with Vdaf < 15%, as the coalification effect increases, the mesopore size diminishes and the number of micropores ascends. Smaller mesopores and micropores gradually become the dominant roles. This phenomenon is due to the effect of intensive compaction within the coal bulk. The combination of LP-N2GA and SEM techniques gives a better understanding of pore characteristics in coal. The research results will provide guidance for the gas control in coal mines.