Physical and structural effects of carbon dioxide storage on vitrinite-rich coal particles under subcritical and supercritical conditions

• A vitrinite-rich medium rank C South African coal was sampled and characterised.
• The samples were kept in supercritical and subcritical CO2 in reactors for 6 months.
• The treated samples were characterised and changes in properties compared.
• Changes in structure and properties were observed in the treated samples.
• The changes observed differed at low (subcritical) and high (supercritical) pressures.

The effects of CO2 sequestration on the chemical and physical properties of coal have been shown to be influenced by the coal's maceral composition. Whilst inertinite-rich coal particles have insignificant changes upon exposure to subcritical CO2 conditions for extended time periods (6 months), vitrinite-rich coal particles had changes in the structure and physical properties. In the present paper, changes in the structural and physical characteristics of a South African vitrinite-rich dry coal were studied after exposure to subcritical (42 bar, 25 °C), and supercritical (125 bar, 35 °C) CO2 conditions for 6 months. The samples were characterised pre- and post-CO2 exposure. The characterisation techniques used were: CO2 adsorption capacity (using a custom built high pressure volumetric system), surface area and pore size distribution analysis using both N2 and CO2 gas and the Brunauer–Emmett–Teller method (BET) and Fourier-Transform Infrared Spectroscopy (FTIR). Structural and physical changes in the coal were observed; these changes differed for subcritical and supercritical conditions. It was found that the surface area and number of micro-pores decreased sharply for the supercritical treated coal sample when compared to the untreated and subcritical treated coal sample. The adsorption capacity of the supercritical treated coal was also reported to be lower than the subcritical treated coal over the same period.