Adsorption properties of South African bituminous coals relevant to carbon dioxide storage

Abstract

An investigation was undertaken to determine the sorption properties of South African bituminous coals relevant to carbon dioxide storage in unmineable coal seams. Four bituminous coal samples from underground coal seams of Witbank, Highveld, and Tshipise-Pafuri coalfields were selected for this study. Detailed sample characterisation was conducted on the coal samples using both standard and advanced techniques. The coal samples were further subjected to high pressure CO2, CH4, and N2 sorption experiments at 55 °C and up to 16 MPa pressure to simulate in-situ coal seam conditions, using a high pressure gravimetric sorption system. Petrographic data of the samples revealed that three of the samples (coals FOZ, DEN, and OGS) are iso-rank (bituminous medium rank C) coals (0.63 - 0.68 Rr.%), while coal TKD is classified as bituminous medium rank B (1.20 Rr.%). All three samples were found to be relatively high in ash yields (16.8 - 25.9 wt.%, adb). Using different techniques, it was found that the surface areas and porosity properties of the samples obtained from SAXS analysis were comparatively greater than similar data acquired from the more widely used techniques (CO2- and N2- LPGA, and MIP), and these results were observed to be significantly rank dependent. Characterisation results from solid state 13C NMR, WAXRD-CFA, and ATR-FTIR show that the lower iso-rank coal samples contained more aliphatic moieties; while the higher rank sample (coal TKD) contained higher fractions of polyaromatic moieties and saturated long chain hydrocarbons, hence, higher aromaticity. Furthermore, HRTEM data revealed that the lower iso-rank coals exhibited higher frequency of lower molecular weight fringes; while the higher rank coal TKD possessed more of the higher molecular weight fringes, and tend to be more preferentially aligned with more ordered carbon crystallites. Results from the high pressure sorption experiments show that the coal samples can store up to 4.1 - 8.7% of CO2, 1.0 - 1.8% of CH4, and 1.0 - 2.2 % of N2 relative to its weight at the experimental conditions. A new model, based on a hybrid DR and Henry law approach (DRHH), provided better fits to the experimental isotherm data than the previously used modified DR (M-DR) model. Physisorption was found to be the dominating sorption mechanism, with neat heat of sorption generally ≤ 12.8 kJ/mol. The sorption capacities of the samples were found to be rank dependent, while the micropore properties of the samples significantly impacted the sorption properties of the samples, more than both the mesopore and macropore properties. The sorption capacities of the samples were also found to be relatively influenced by the intermediate maceral abundance, suggesting that lithotype bandings enhances either the fluid transport processes or the micropore properties of the coal matrix.