Abstract
Supercritical carbon dioxide is known to change the pore structure of coals and thus affect their carbon dioxide sequestration capacity. In this study, supercritical carbon dioxide dependence of pore morphology of coals was reviewed. Results indicated that the micropore surface area and volume of dry coals varied between –20% and 20% after exposure to supercritical carbon dioxide. Changes in the micropore size distribution of dry coals after supercritical carbon dioxide exposure were not found to be significant; however, the change in meso- and macropores with diameter of 2–8 nm was observed to be significant. Supercritical carbon dioxide and H2O exposure mainly influenced pores with diameters of 0.4–0.7, 0.7–0.9 and 2–8 nm. The variation in the pore fractal dimensions of the coals ranged from –0.5% to 0.5% after supercritical carbon dioxide exposure. Furthermore, the dependence of supercritical carbon dioxide on the pore structure of coals relies on the coal rank. The change in the pore structure of the coals after supercritical carbon dioxide exposure was observed to be related to the following aspects. First, supercritical carbon dioxide induced swelling in coal matrix, thus reducing the pore surface area and volume of the coal matrix and compressing the cleat system. Next, the extraction of supercritical carbon dioxide mobilised the small organic molecules dispersed in the coal matrix; this increased the pore volume, particularly of micropores. Finally, the mineral dissolution/precipitation also changed the pore structure of the coals. To further examine supercritical carbon dioxide dependence of coal pore morphology, the following studies should be performed. The characterisation of the chemical and pore structure of coals should be combined with existing coal structure models to account for the mechanism of supercritical carbon dioxide changing the pore structure of coals. Combination of physical experiments and numerical simulations is recommended to predict the changes in porosity and permeability of coals due to long-term carbon dioxide sequestration.
Highlights
Carbon dioxide (CO2) capture and sequestration (CCS) has great potential for mitigating anthropogenic greenhouse CO2 emissions
Various studies have suggested that the effects of ScCO2 on the pore structure of coals are associated with the physicochemical property of coals, including coal rank (Ro,max), volatile matter and minerals, and coal reservoir conditions, such as temperature, pressure and moisture content (Arami-Niya et al, 2016; Radlinski et al, 2004)
The stored CO2 is known as a supercritical fluid (ScCO2), with great potential to change the pore structure of coals
Summary
Carbon dioxide (CO2) capture and sequestration (CCS) has great potential for mitigating anthropogenic greenhouse CO2 emissions. Changes in the micropores of coals are known to mainly stem from the coal matrix swelling, extraction and mineral dissolution and precipitation because of the exposure to ScCO2. The effects of ScCO2 on meso- and macropore structure of coals have been associated with ScCO2-induced matrix swelling and small organic compounds extraction (Du et al, 2018; Gathitu et al, 2009; Li et al, 2017; Liu et al, 2018; Zhang et al, 2013a).
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