Characterization of coal before and after supercritical CO2 exposure via feature relocation using field-emission scanning electron microscopy

The solvent and swelling effects of supercritical CO2 on coal structure and porosity were examined using high-resolution field-emission scanning electron microscopy (FE-SEM) and surface area techniques to investigate any irreversible CO2 induced alterations of the micro-, meso-, and macropores. Dry, 1 in. unconfined cores of Pittsburgh and Sewickly bituminous coals were exposed to supercritical CO2 at 15.3 MPa (2200 psig) and 328 K (55 °C) for 104 days. Prior to CO2 exposure, coal structure and porosity – specifically macropores (>50 nm) – were imaged using FE-SEM. After CO2 exposure, the imaged features were relocated, reimaged, and analyzed for structural changes. Brunauer–Emmett–Teller (BET) surface areas were evaluated from the adsorption isotherms of N2 at 77 K and P/P0 = 0.1–0.3. Micropore surface areas were determined from the low-pressure adsorption isotherms of CO2 at 293 K using the Dubinin–Polanyi equation. FE-SEM analysis indicated that there were no significant changes observed in the pore areas in all coal samples after CO2 exposure. Meso- and micropore characteristics were slightly affected by supercritical CO2 exposure.