In-situ temperature-depth profile evaluation of South African unmineable coal seams to determine CO2 sequestration potential

This study aimed to investigate the sorption behaviour of South African coal seams with relation to the effect of temperature during CO₂ sequestration. The excess adsorption isotherms of CO₂ adsorption were undertaken using a high-pressure volumetric system for four coals of different coal rank (denoted by Somkele [SK], anthracite KZN [AN], Tshikondeni [TD], and Syferfontein [SF]). The volumetric pressure step method was conducted at increments of system temperature of 35, 45, 55, and 65°C for pure CO₂ adsorption at incremental pressures up to 93 bar. The results showed that high temperatures have a very significant negative effect on the amount of CO₂ adsorbed on the coal samples. The high-rank coal samples (SK and AN) demonstrated elevated CO₂ adsorption capacity across all tested temperatures due to their high vitrinite content. The medium-rank coals (TD and SF) exhibited comparatively lower CO₂ adsorption capacity, attributed to the presence of adsorption hindrances such as higher ash content and volatile and mineral matter. The isosteric heat of adsorption revealed an increasing trend with coverage for all coal samples, with higher rank coals displaying greater slopes. The determined range of the isosteric heat of adsorption, spanning from 10 to 59 kJ/mol, indicated that the adsorption process is primarily of a physisorption nature. Three theoretical models (Langmuir, Freundlich, and Temkin) were evaluated and fitted to the sorption experimental data. The Temkin model exhibited superior fitting compared to the Langmuir and Freundlich isotherms. The Temkin isotherm parameters suggest that the adsorption of CO₂ onto coal is a physisorption process.