Effects of carbon dioxide storage in coal on the physical and chemical properties of coal
Date
2013-07-15
Authors
Maphala, Tshifhiwa
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Abstract
Unlike other carbon dioxide (CO2) storage options, CO2 storage in coal seams is still in its infancy, and one of the many questions that 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 due to CO2 adsorption have been done over the period of adsorption (hours or 2-3 days), and mostly at low pressures which are not representative of actual storage conditions. The aim of this study is to determine chemical and physical changes that might occur due to long term interaction of coal with CO2 under pressurised conditions.
Air dried coal samples of different maceral groups (vitrinite and inertinite) were exposed to CO2 in reactors under subcritical and supercritical conditions for different period of time (up to 6 months). The samples were well characterised pre- and post-CO2 treatment. A newly-built high-pressure volumetric system was used for adsorption and desorption isotherms measurements of the coals before CO2 sorption. The characterisation techniques used were BET, XRD, FTIR and 13C NMR.
Surface area analysis of untreated and treated coal particles showed that vitrinite-rich coal samples have a greater surface area and pore size distribution change in pore structure following CO2 treatment than the inertinite-rich coal particles. Analysis of the crystalline part of the CO2 untreated and treated inertinite-rich particles showed that there was slight increase in the average crystallite height following CO2 treatment, although no changes were observed for the d002 aromatic interplanar spacing.
In comparing the effects of subcritical and supercritical treatment on CO2 sorption behaviour over time, the study determined that the long term effects of CO2 storage in coal were found to be dependent on the maceral composition of the coal, with the vitrinite-rich coal showing a more pronounced structural and properties change after CO2 storage. This change did, however, differ under subcritical and supercritical conditions. The adsorption capacity of the supercritical treated coal was found to be lower than the subcritical treated coal over the same period of 6 months for the vitrinite-rich coal. Inertinite-rich coal was found to be less prone to changes under CO2 storage. A major implication of this finding is the inclusion of maceral composition as major criteria for determining CO2 storage in coal capability.
The results in this study contribute significantly to the understanding of coal-CO2 interactions and the implications for coal structure and properties changes; the results could be used by decision-makers on the effects of storing CO2 in coal.