Influences of supercritical carbon dioxide fluid on pore morphology of various rank coals: A review

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.

Coal Pores: Methods, Types, and Characteristics

“…The calculation of the fractal dimension of the mercury intrusion method can be solved jointly by Washburn's equation and scaling law, 74 or can be based on the Menger model, thermodynamic model, and Sierpinski model. 66,75 66,67 According to Lu et al, 66 the Sierpinski model is able to describe the heterogeneity of seepage pores in tectonically deformed coal and can be used in comparison with the thermodynamic model. The FHH model can characterize the heterogeneity of adsorption pores, while the Sierpinski model can be used as a supplement.…”

“…In view of the complex pore structure complexities, conventional pore characterization methods are difficult to quantitatively characterize the heterogeneity and complexity of pores . It has been shown that coal pore structure and surface morphology are self-similar in a certain range, and fractal theory has been widely recognized as an effective tool to describe the complexity of coal pore structure. − …”

“…Fractal dimension, as one of the most important fundamental parameters in fractal geometry, is used to describe the heterogeneity and complexity of pore structure. , Acquisition of the fractal dimension relies on the fundamental pore parameters (pore volume, pore size distribution, etc.) obtained by other pore measurement methods.…”

“…It is found that gas adsorption, SAXS, mercury intrusion, image analysis, and SAXS can be used to calculate the fractal dimension. − Mercury intrusion and N 2 adsorption methods are simple and are often used to calculate the fractal dimension. The calculation of the fractal dimension of the mercury intrusion method can be solved jointly by Washburn’s equation and scaling law, or can be based on the Menger model, thermodynamic model, and Sierpinski model. , N 2 adsorption data transformation models include the Frenkel–Halsey–Hill model (FHH), Avnir model, Neimark model, Sierpinski model, and Menger model, but different models have different scopes of application. , According to Lu et al, the Sierpinski model is able to describe the heterogeneity of seepage pores in tectonically deformed coal and can be used in comparison with the thermodynamic model. The FHH model can characterize the heterogeneity of adsorption pores, while the Sierpinski model can be used as a supplement.…”

Analysis of the Change Feature and Action Mechanism of Coal Pore Structure under the Action of Supercritical CO2

“…Yang et al [11] investigated the seepage law of coal under the action of ScCO 2 and discovered that ScCO 2 triggered the appearance of honeycomb-shaped pores and improved the permeability of the coal body. The coal sample treated with ScCO 2 -H 2 O mainly experienced changes in micropores, while the low-rank coal sample mainly experienced changes in macropores and mesopores [12]. In the experimental study on fracture initiation and extension of ScCO 2 -H 2 O fractured coal, it was observed that ScCO 2 -H 2 O led to the formation of numerous complex laminar bifurcation fractures in the coal body [13].…”