Effects of CO₂ storage in coal on coal properties

Unlike other storage options, CO₂ storage in coal seams is still in its infancy and requires advanced research to address the number of unknowns. One of the many questions that still need to be answered concerns the long term effects of storing CO₂ in coal, specifically on the structure and properties of the coal. Most studies on coal structural changes have been conducted over a short period of adsorption (hours or 2-3 days) and at low pressures (1-20 bar). Available literature has shown that the volumetric strains are a function of the local petrographic composition, and, in general, a coal's response to CO₂ sorption is based on the coal petrographic composition. The aim of this study was to determine any molecular structural and physical changes that may occur in coal following long term exposure (up to 6 months) to CO₂ under subcritical conditions (up to 42 bar). Inertinite-rich coal particles have a higher microporous surface area than vitrinite-rich particles of the same rank, as demonstrated by BET analysis. XRD results on the preand post-sorption coal samples demonstrated that CO ₂ sorption in coal causes some structural changes. The structural changes were found to be different for coals of different maceral composition, confirming the initial hypothesis. The coal samples that were exposed to CO ₂ over longer periods of time (6 months) displayed a more pronounced change in coal structure than those that were exposed for a shorter period of time (14 days). Volumetric sorption studies demonstrated that CO₂ treated vitrinite-rich particles have a greater change in sorption properties than inertinite-rich particles. The study provides an insight into the chemical and physical changes caused by sorption of CO₂ into coal, and could be used for modelling CO₂-coal interactions, and for assisting in CO₂-coal uptake calculations.