Heterogeneous development of micropores in medium-high rank coal and its relationship with adsorption capacity

Abstract

In order to better understand the micropores (pore size less than 2 nm) in some medium-high rank coals, the development, distribution, and structure of micropores in five medium-high ranks coal samples (0.68% < Ro, max < 6.22%) were determined by low-pressure CO2/N2 adsorption (LP-CO2/N2 GA) experiments, and high-resolution transmission electron microscopy (HRTEM). The mathematical models and methods for accurately determining micropores were further discussed. Then, the high-pressure methane isotherm adsorption (HP-CH4 GA) experiment was carried out to analyze the gas adsorption capacity of the micropores. The results showed that LP-CO2/N2 GA in conjunction with non-localized and quenched-solid density functional theory models could more accurately determine the pore volume (PV), specific surface area (SSA), and pore size distribution (PSD) of the micropores. The PV and SSA of the coals were distributed between 0.046 and 0.089 cm3/g and 139.6–305.2 m2/g, respectively. Although the micropore development differed greatly between the samples, their PSD was very similar, with more than half of the micropore PV and SSA distributed in the size of the pore between 0.35 and 0.70 nm. The micropores could be mainly derived from pores in organic macromolecular structure and were evenly distributed in the coal matrix in complex form. The development and size of the micropores were controlled to some extent by the size and arrangement of the organic structure. Micropores accounted for 86.9%–98.5% of the total pore SSA in the coals, so most methane could be adsorbed in micropores. However, the SSA of micropore has not restricted the linear relationship with the methane adsorption capacity.