Pore structure evolution of Jharia coal for potential underground coal thermal treatment and associated CO2 sequestration

Abstract

Underground coal thermal treatment (UCTT) is an emerging technique for clean energy extraction from coal, which also creates a unique CO₂ sink environment in the form of pyrolytic char. In this study, a pathway for cleaner and efficient extraction of energy from coal is proposed. Early coalbed methane (CBM) extraction, application of UCTT followed by CO₂ sequestration in pyrolytic char formed during UCTT presents an opportunity to maximize the utility of coal in new energy scenarios. To characterize Jharia coal in terms of its pore size distribution (PSD), pore surface area, pore volume, thermal evolution, CO₂ adsorption attributes at low P/T (low-pressure and low-temperature), and surface morphology at different temperatures (30, 150, 300, 450, and 600 °C), a variety of analytical techniques such as low-pressure gas adsorption (LPGA), small angle X-ray scattering (SAXS), mercury intrusion porosimetry (MIP), thermogravimetric analysis (TGA), and scanning electron microscopy (SEM) were employed. The results show that the quantity of adsorbed CO₂ (at low P/T) increased by 138 % for coal subjected to the maximum pyrolysis temperature of 600 °C. The PSD showed significant variations at different pyrolytic temperatures. While the pores did not show large variations when coal was heated up to 300 °C, the micropores increased sharply, while the mesopores and small macropores reduced when heated further. The elevated pyrolytic temperatures resulted in the enlargement and merging of mesopores and small macropores, along with the formation of new pores due to thermal decomposition and release of volatiles. Consequently, this contributed to a significant increase in the volume of macropores, and overall porosity. The increase in the accessibility of pores under the UCTT environment could significantly boost the CO₂ storage capacity in coal.