Influence of coal rank, ash, mineral content, and maceral composition on CO2 adsorption in South African coals

Abstract

This study investigated the influence of coal rank, ash content, mineral matter, and maceral composition on the CO₂ adsorption capacity of ten distinct South African coal samples. A high-pressure volumetric adsorption system (HPVAS) was utilized to assess CO₂ sorption characteristics under supercritical conditions at 35 °C and pressures up to 85 bar. Comprehensive characterization, including proximate and ultimate analysis, petrographic analysis, and density determination, was conducted to understand how these factors influence CO₂ adsorption. The findings reveal that higher-rank coals (HRC), particularly those with vitrinite reflectance above 1.2%, demonstrated superior CO₂ adsorption capacities, reaching up to 2.17 mmol/g. Medium-rank coals (MRC) with higher inertinite content showed lower adsorption capacities, with the lowest recorded at 0.78 mmol/g, except for the IN coal sample. CO₂ adsorption increased with vitrinite reflectance, particularly within the 0.51% to 0.81% range for medium-rank coals. Linear increase in CO₂ adsorption capacity was noted as carbon content increased from MRC towards HRC particularly in SM and AN coals. An increase in volatile matter content corresponded with a significant decline in CO₂ sorption capacity. Additionally, a negative correlation between ash content, mineral matter, liptinite, inertinite, and CO₂ adsorption capacity was evident, likely due to pore obstruction and reduced surface area. Liptinite-rich coals, such as BL, GN, EM, and WG, exhibited decreased adsorption capacity, with BL showing the highest liptinite content at 5.5%. The analysis indicates that while ash content influences sorption capacity, the organic matter, especially vitrinite, serve as the primary sites for gas adsorption. The findings of this study will enhance understanding of the CO₂ adsorption behaviour of South African coals supporting the funding from highly intensive CO₂ emitting industries to enable further research of carbon capture and storage (CCS) pilot projects tailored to regional coal properties.