SC-CO2 Treatment Research Articles

Improving UV Resistance of Aramid Fibers by Simultaneously Synthesizing TiO2 on Their Surfaces and in the Interfaces Between Fibrils/Microfibrils Using Supercritical Carbon Dioxide.

Aramid fibers with low density and high strength, modulus, and thermal resistance are widely used in applications such as bulletproof vests and cables. However, owing to their chemical structure, they are sensitive to ultraviolet light, which degrades the fibers’ useful mechanical properties. In this study, titanium dioxide (TiO2) nanoparticles were synthesized both on the aramid III fiber surface and in the interfacial space between the fibrils/microfibrils in supercritical carbon dioxide (scCO2) to improve the UV resistance of aramid fibers. The effects of scCO2 treatment pressure on the TiO2 structure, morphology, surface composition, thermal stability, photostability, and mechanical properties were investigated using Fourier transform infrared spectroscopy, X-ray diffraction, scanning electron microscopy, X-ray photoelectron spectroscopy, thermogravimetric analysis, ultraviolet–visible spectroscopy, and single-fiber test. The results show that amorphous TiO2 formed on the fiber surface and the interface between fibrils/microfibrils, and decreased the photodegradation rate of the aramid III fiber. Moreover, this modification can also improve the tensile strength via treatment at low temperature and without the use of a solvent. The simple synthesis process in scCO2, which is scalable, is used for mild modifications with a green solvent, providing a promising technique for synthesizing metal dioxide on polymers.

Can supercritical carbon dioxide be suitable for the green pretreatment of plant fibres dedicated to composite applications?

This work explores the use of supercritical carbon dioxide (sc-CO2) conditions as an innovative and environmentally friendly treatment of plant fibres to optimize their performance for integration into composite materials. This study evaluates, in particular, the influence of this treatment on the mechanical, thermal, hygroscopic properties and biochemical features of industrial hemp bast fibres. Two distinct settings were tested by tuning time, temperature and pressure parameters to assess the influence of the severity of the treatment on the fibre quality. Results show that sc-CO2 treatment induces an increase in the fibre fineness and a decrease in their moisture sensitivity while maintaining their initial resistance to temperature. These changes are consistent with the measured decrease in the relative content of hemicelluloses. A significant decrease in the tensile rigidity and strength is also observed as a function of the severity of sc-CO2 treatment, counterbalancing a little bit the benefits retained on the other properties.

Research Note: Microbial inactivation of raw chicken meat by supercritical carbon dioxide treatment alone and in combination with fresh culinary herbs

The objective of the present study was to assess the potential synergistic effect between supercritical carbon dioxide (SC-CO2) and fresh culinary herbs (Coriandrum sativum and Rosmarinus officinalis) on the microbial inactivation of raw chicken meat. The microbiological inactivation was performed on Escherichia coli and natural flora (total mesophilic bacteria, yeasts, and molds). High pressure treatments were carried out at 40°C, 80 or 140 bar from 15 to 45 min. Microbial inactivation had a strong dependence on treatment time, achieving 1.4 log CFU/g reduction of E. coli after 15 min, and up to 5 log after 45 min, while a pressure increase from 80 up to 140 bar was not significant on the microbial inactivation. Mesophilic microorganisms were strongly reduced (>2.6 log CFU/g) after 45 min, and yeasts and molds were below the detection limits of the technique (<100 CFU/g) in most cases. The combination of fresh herbs together with SC-CO2 treatment did not significantly increase the inactivation of either E. coli or natural flora, which was similar to the SC-CO2 alone. The synergistic effect was obtained on the inactivation of E. coli using a proper concentration of coriander essential oil (EO) (0.5% v/w), while rosemary EO did not show a significant effect. Color analysis after the treatment showed an increment of lightness (L*), and a decrease of redness (a*) on the surface of the sample, making the product visually similar to cooked meat. Texture analysis demonstrated the modification of the texture parameters as a function of the process pressure making the meat more similar to the cooked one.

Application of nanoindentation technology for characterizing the mechanical properties of shale before and after supercritical CO2 fluid treatment

To better implement super critical carbon dioxide (SC-CO2) fracturing on shale, it is important to investigate effects of SC-CO2 on rock mechanical properties. Because micro-scale mechanical properties determine the overall mechanical response, the determination of micro-scale mechanical properties is necessary. In this paper, continuous stiffness measurement (CSM) and constant loading rate (CLR) mode nanoindentation was performed on shale samples before and after SC-CO2 treatment. With the aid of scanning electron microscope and energy dispersive spectra mapping technique, mechanical strength difference on clay-rich and quartz –rich areas in shale sample can be identified. The results demonstrated that samples treated by SC-CO2 present lower Hardness, Young’s modulus and fracture toughness. The average decrease rates of hardness and Young’s modulus under CSM mode for three sample are 32.01 % and 11.75 %. The hardness, Young’s modulus and fracture toughness under CLR mode for the three samples have decreased at average rates of 29.5 %, 11.0 %, and 11.3 %, respectively. Moreover, the mechanical strength substantially decreases in the clay-rich area because CO2 adsorption and mineral dissolution happen after the SC-CO2 treatment. The change in mechanical properties in quartz-rich area is much smaller because there is little effect on quartz. The close examination of indentation fracture geometry indicate more micro fractures have been generated in the post SC-CO2 treated shale, thus we can expect the capability of SC-CO2 for rock weaken. This study can provide more insights for fracture mechanism in target areas with different mineral distribution, which is beneficial for SC-CO2 engineering applications.

Effects of Supercritical CO2 Treatment Temperature on Functional Groups and Pore Structure of Coals

The buried depth of a coal seam determines the temperature at which CO2 and coal interact. To better understand CO2 sequestration, the pore structure and organic functional groups of coal treated with different ScCO2 temperatures were studied. In this study, three different rank coals were treated with ScCO2 at different temperatures under 8 MPa for 96 h in a geochemical reactor. The changes in pore structure and chemical structure of coal after ScCO2 treatment were analyzed using mercury intrusion porosimetry, attenuated total reflection Fourier transform infra-red spectroscopy, fractal theory, and curve fitting. The results show that the enhancement effect of ScCO2 on pore structure of coal becomes less significant as the increase of buried depth. In most of the treated coal samples, the variation proportion of mesopores decreased and the variation proportion of macropores increased. In the relatively higher rank coals, the degree of condensation (DOC) of aromatic rings decreased after treatment with ScCO2. The DOC values showed a U-shape relationship with temperature, and the aromaticity showed a downward trend with increasing temperature. The chemical structural changes in the relatively lower rank coal sample were complex. These findings will provide an understanding of mechanisms relevant to CO2 sequestration with enhanced coalbed methane recovery under different geothermal gradients and for different ranks of coal.

Immobilized Laccase for Sustainable Technological Processes

Enzyme-catalyzed processes are currently a powerful tool in the chemical industry due to the reduction of process time, specificity, eco-friendly characteristics, intake of low energy input, lower cost and nontoxic properties. Laccases are versatile enzymes, which catalyze oxidation reactions coupled to four-electron reduction of molecular oxygen to water. Laccases are used for decolorization and detoxification of industrial effluents and are widely used in wastewater treatment. For improving enzyme utilization in biotechnological processes, the process cost has to be reduced, the enzyme stability during industrial processes should be enhanced and the recycle and reuse step should be favorable. The immobilization of the enzyme is an important step for enhancing enzyme catalytic properties and operational stability. In our study, two different types of immobilization were used for laccase: cross-linked enzyme aggregates (CLEA) and magnetic cross-linked enzyme aggregates (mCLEA). The enzyme activity of synthesized CLEA was 0.237 U/mg laccase and mCLEA 0.637 U/mg laccase, respectively. In the next step, immobilized laccase in the form of CLEA and mCLEA was treated under supercritical conditions. Supercritical fluids (SCFs) comprise a unique class of nonaqueous media for biocatalysis and bioseparation. It is very difficult (almost impossible) to predict the stability and activity of an enzyme in any SCF at a certain pressure. Therefore, the stability of both laccase immobilized forms in supercritical carbon dioxide (SC CO2) at different pressures was studied. The highest residual activity for both laccase immobilized forms were obtained after exposure to SC CO2 at 20 MPa, 35 °C for 5 h. The obtained residual activities were 145 ± 3 % for mCLEA and 128 ± 3 % for CLEA. Improved stability of immobilized laccase in the form of CLEA and mCLEA after SC CO2 treatment indicates the possibility of using laccase in supercritical media. Further application of CLEA laccase and mCLEA laccase could be reflected in the field of hospital wastewater treatment for removal of drugs.

Time-Dependence of Mechanical Property Alterations on Anthracite Coals Treated by Supercritical Carbon Dioxide

The injection of supercritical carbon dioxide (Sc-CO2) into coal seams remarkably changes the physical-chemical structures of coal mass and thereby improves coal mechanical properties. In this study, a series of unconfined and triaxial compressive strength tests were performed on anthracite coal samples under the pressure of 8 MPa at the temperature of 35°C for 24 h and 48 h, respectively. Besides, experiments with longer Sc-CO2 treatment time were carried out on coal mass without stress constraint to observe the damage mode of coal. The results show that Sc-CO2 treatment obviously alters time-dependent mechanical properties of anthracite coal. The coal samples treated with Sc-CO2 for 240 h and 960 h show different damage modes from limited fractures to a complicated fracture network. The time-dependence of the mechanical weakening model on Sc-CO2-treated coal was proposed to explain the link between CO2 flow and mechanical weakening effect by means of physical-chemical effects on time scales.

Influence of supercritical carbon dioxide treatment on the physicochemical properties of cellulose extracted from cassava pulp waste

This study aims at investigating the influence of supercritical carbon dioxide (ScCO2) treatment on the physicochemical properties of cassava–based cellulose. The ScCO2 treatment was performed in a high-pressure vessel by exposing the cellulose to ScCO2 without a co-solvent at variable pressure (P) ranging from 8 MPa to 20 MPa for 60 min. The system temperature (T) was set to 40 °C, 60 °C and 80 °C. Subsequently, a field emission scanning electron microscopy analysis showed that the average fibril diameter of the cellulose was approximately two times less than that of the untreated cellulose. Differential scanning calorimetry and thermogravimetric analyses showed higher temperature shifting of the decomposition peak, which indicated improved thermal stability of the ScCO2-treated celluloses. Additionally, a correlation test revealed that thermal stability of cellulose increased with the pressure of ScCO2.

Low temperature formation of CH3NH3PbI3 perovskite films in supercritical carbon dioxide

A new method for the production of high-quality CH3NH3PbI3 perovskite films at low temperature is reported. It is shown that CO2 above the critical point (31 ℃, 7.38 MPa (1071 psi)) acts as an anti-solvent to perovskite precursors and significantly enhances the solid-state film crystallization rate at low temperatures and also assists in the removal of residual solvent from within the film. Post deposition treatment in supercritical carbon dioxide (scCO2) resulted in complete conversion of perovskite compared to incomplete conversion at the same temperatures using thermal annealing alone. Complete crystallization and an average crystal grain size of 408 nm at 45 ℃ was observed using scCO2 treatment compared to incomplete crystal formation with average grain size of 175 nm at 45 ℃ with thermal annealing alone.

Experimental Investigation of the Rheological Properties of a Typical Waxy Crude Oil Treated with Supercritical CO2 and the Stability Change in Its Emulsion

The application of supercritical CO₂ (scCO₂)-flooding technology is increasing worldwide. After a waxy crude oil is treated with scCO₂, its composition and rheological properties are changed and the stability of its emulsion is accordingly altered as well, thus affecting the dewatering process in gathering stations. To probe the specific changes occurring in waxy crude oil and its emulsion after scCO₂ flooding, a scCO₂ treatment device was first designed to simulate the reservoir conditions. Next, the composition changes in the waxy crude oil caused by the scCO₂ treatment were studied, and the changes in the rheological properties, such as the pour point, viscosity, yield stress, and wax precipitation characteristics, caused by the composition variation were analyzed. Then, the stability of the degassed crude oil emulsion and the corresponding interfacial properties were examined. Finally, the demulsification characteristics of the emulsion under the action of a demulsifier were tested. The results reveal a content increase in the heavy components, such as asphaltenes, resins, and high-carbon-number hydrocarbons, in the waxy crude oil as a result of the scCO₂ extraction of the light components. This results in increases in the pour point, wax appearance temperature, and abnormal point as well as the growth of the apparent viscosity and yield stress. As a result of the structural enhancement of the interface, as reflected by the dilational modulus increase in the interface, the stability of the scCO₂-treated waxy crude oil emulsion is strengthened and its demulsification efficiency is reduced after dosing with the same demulsifier. All of these changes could unfavorably influence the safe transportation of the produced fluid in scCO₂-flooding oil fields and the subsequent dewatering process.

Thiol concentration, structural characteristics and gelling properties of bovine heart protein concentrates

Consumer demand for proteins from animal sources has been increasing with economic growth of the populations. Freeze-dried ground bovine heart was treated using supercritical-CO2 (SC–CO2) at different pressures (20, 30 and 40 MPa) and hexane to produce a protein concentrate. Thiol concentration, structural and gelling properties of the control bovine heart and bovine heart protein concentrates (BHPCs) were investigated. SC-CO2 treatment at 20 MPa led to an increased (p < 0.05) thiol (–SH) content (74.08 nmol/mg protein) than those for the control and other BHPCs. Fourier–transform infrared (FT–IR) spectroscopy showed increased secondary structure of α–helix and parallel β–sheet content for SC-CO2–treated bovine heart protein when compared to the control and hexane–treated bovine heart protein. Integrated light scattering in protein suspension showed SC-CO2–treated samples had higher cumulative values (Q3) and density (q3) for the particle size distribution. SC-CO2–treated BHPC had better physicochemical properties without structural deterioration of proteins resulting in increased functional properties such as solubility (31.08 g/100 g bovine heart), viscosity and gelling capacity (least gelation concentration 10 g/100 mL water). Bovine heart proteins may serve as a good source of quality proteins for different food applications.

Effect of supercritical CO2 exposure on the high-pressure CO2 adsorption performance of shales

The interactions between shales and supercritical carbon dioxide (ScCO2) as well as the influence of these interactions on the pore structure and the surface functional groups play a key role in long-term CO2 geological sequestration. To investigate the effect of ScCO2 treatment on the performance of high-pressure CO2 adsorption, three shale samples selected from different formations were treated with ScCO2 in a geochemical reactor to simulate the in situ geological sequestration process. The high-pressure adsorption behaviors of CO2 on untreated and ScCO2-exposed shale samples were measured by gravimetric method at 45 °C and pressures up to 18 MPa. Characterization methods included scanning electron microscope (SEM), thermogravimetric analysis (TGA), X-ray diffraction (XRD), low-pressure gases adsorption (LP-GA), element analysis (EA) and X-ray photoelectron spectroscopy (XPS). The experimental results indicated that ScCO2 fluid could significantly alter the structural and physicochemical properties of shales, leading to the variations in high-pressure CO2 adsorption capacity of shales. The moisture and oxygen contents in the ScCO2-exposed samples were lower than those in the untreated samples. XPS analysis showed that the relative content of the organic oxygen-containing functional groups (CO, CO, OCO and COO−) in the raw shale samples decreased after the ScCO2 treatment. After long-term ScCO2 immersion, the maximum CO2 adsorption capacity of the shale samples exhibited a decreasing trend when compared to the raw samples, which was linked to the changes in the specific surface area of the micro- and mesopores as well as the oxygen-containing functional groups. The results obtained in this paper provide a basis for further evaluation of CO2 storage mechanisms.

Investigating the effect of sterilisation methods on the physical properties and cytocompatibility of methyl cellulose used in combination with alginate for 3D-bioplotting of chondrocytes

For both the incorporation of cells and future therapeutic applications the sterility of a biomaterial must be ensured. However, common sterilisation techniques are intense and often negatively impact on material physicochemical attributes, which can affect its suitability for tissue engineering and 3D printing. In the present study four sterilisation methods, autoclave, supercritical CO2 (scCO2) treatment, UV- and gamma (γ) irradiation were evaluated regarding their impact on material properties and cellular responses. The investigations were performed on methyl cellulose (MC) as a component of an alginate/methyl cellulose (alg/MC) bioink, used for bioprinting embedded bovine primary chondrocytes (BPCs). In contrast to the autoclave, scCO2 and UV-treatments, the γ-irradiated MC resulted in a strong reduction in alg/MC viscosity and stability after extrusion which made this method unsuitable for precise bioprinting. Gel permeation chromatography analysis revealed a significant reduction in MC molecular mass only after γ-irradiation, which influenced MC chain mobility in the Ca2+-crosslinked alginate network as well as gel composition and microstructure. With regard to cell survival and proteoglycan matrix production, the results determined UV-irradiation and autoclaving as the best candidates for sterilisation. The scCO2-treatment of MC resulted in an unfavourable cell response indicating that this method needs careful optimisation prior to application for cell encapsulation. As proven by consistent FT-IR spectra, chemical alterations could be excluded as a cause for the differences seen between MC treatments on alg/MC behaviour. This investigation provides knowledge for the development of a clinically appropriate 3D-printing-based fabrication process to produce bioengineered tissue for cartilage regeneration.

Comparative effect of supercritical carbon dioxide and high pressure processing on structural changes and activity loss of oxidoreductive enzymes

Due to the CO2 specific characteristics, it has been used as supercritical (Sc) fluid for several applications, including enzyme inactivation. The influence of Sc-CO2 (10–65 MPa/10–30 min/35–65 °C) on mushroom polyphenol oxidase (PPO) and horseradish peroxidase (POD) was evaluated and the results were compared with those found using high pressure processing (HPP) (200–900 MPa/5–45 °C/1–15 min). The free ion concentration was also studied to compare the enzymatic activity and changes in electrical conductivity. Additionally both enzymes, untreated or treated using either Sc-CO2 or HPP, were used as additives in the CuCl2 crystallization method. The resulting additive-specific CuCl2 patterns were characterized based on different structural features. Sc-CO2 was found to have a significant influence on PPO and POD activities, and an increased reduction in the residual activity of both enzymes was observed when the Sc-CO2 pressure was increased. However, PPO was more resistant to temperature and pressure than POD. The D-value calculated for POD was in the range of 38.3 and 592.0 min, and 60.6 and 291.5 min, for HPP and Sc-CO2 treatments, respectively; whereas for PPO varied from 103.1 to 284.3 min under HPP and from 83.4 to 303.0 min using the Sc-CO2 treatment. It can be concluded that the application of Sc-CO2 could be an effective tool for inactivating PPO and POD enzymes. Moreover, both enzymes and treatments could be differentiated significantly based on the resulting CuCl2 crystallization patterns.

Evaluating the impact of supercritical-CO2 pressure on the recovery and quality of oil from “horchata” by-products: Fatty acid profile, α-tocopherol, phenolic compounds, and lipid oxidation parameters

The effect of supercritical carbon dioxide (SC-CO2) (10–40 MPa) and conventional extraction (CE) to recover oil from by-products obtained during “horchata” production was assessed. To evaluate both extraction techniques, the fatty acid composition, polyphenols, α-tocopherol, antioxidant capacity and lipid oxidation parameters of the extracts obtained were analysed. A linear relationship between extraction pressure and oil yield was observed. However, the highest oil yield was obtained under conventional extraction. The by-products from “horchata” presented a profile where monounsaturated fatty acids (MUFA) were the predominant, representing ≈ 70% of total fatty acids. The amount of saturated fatty acids (SFA) and polyunsaturated fatty acids (PUFA) was higher and MUFA lower at 10 MPa samples compared to the oils extracted using SC-CO2 at 20, 30 or 40 MPa, where no differences were detected. The content of α-tocopherol was significantly higher after SC-CO2 treatments compared to conventional extraction, independently of the applied treatment. On the other hand, the values of phenolic compounds and total antioxidant activity (TAC) increased as the pressure conditions of the SC-CO2 extraction increased, presenting a linear adjustment of the data. Regarding lipid oxidation, the lower oxidation indexes were obtained when the SC-CO2 pressure increased. Finally, our results confirmed that the application of SC-CO2 could be a potential alternative to conventional extraction in order to obtain oils from “horchata” by-products rich in high-added value compounds without the use of organic solvents which can be toxic.

Improvement of adhesion of kevlar fabrics to epoxy by surface modification with acetic anhydride in supercritical carbon dioxide

The effect of supercritical carbon dioxide (ScCO2) treatment with acetic anhydride (AA) for Kevlar fabric on the interfacial adhesion and mechanical properties of Kevlar fabric/epoxy composites was investigated. ScCO2 treatment with AA remarkably affects the interfacial adhesion, thus improving mechanical properties. The effects of treating time on the surface modification and mechanical properties of composites were studied. Results demonstrated that the optimal condition is 100°C at 10 MPa for 60 min, as shown by the tensile strength and shear strength of the ScCO2‐treated composites being 21.3 and 47.9% higher than those of the untreated ones, respectively. The mechanical properties demonstrated that ScCO2 treatment provides an efficient and environmentally safe method for surface modification to improve interfacial interaction between the treated fiber and matrix with ScCO2 treatment. POLYM. COMPOS., 40:E920–E927, 2019. © 2018 Society of Plastics Engineers

Polymeric membrane gas separation performance improvements through supercritical CO2 treatment

Supercritical carbon dioxide (sc-CO2) will plasticize and partially solubilise polymeric membranes, resulting in alteration to the polymer morphology, impacting the gas separation properties. Here, cellulose triacetate (CTA) and polyimides, Matrimid and 6FDA-TMPDA membranes were exposed to supercritical CO2 for 2 and 8 h, followed by two depressurization protocols; a rapid depressurization of 12 MPa/min and a slow depressurization of 0.17 MPa/min. The resulting impact on He, N2, CH4 and CO2 permeability as well as the corresponding selectivities were then quantified. Matrimid membranes undergo substantial plasticization in the presence of sc-CO2 resulting in significant increases in permeability and loss of selectivity, irrespective of the sc-CO2 exposure protocol. CTA and 6FDA-TMPDA membranes experience competing phenomenon under supercritical conditions, both demonstrate limited CO2 plasticization which is offset by sc-CO2 partly solubilising the polymers, enabling rearrangement to a more compact morphology. The depressurization protocol strongly impacted the underlying morphology for these two membranes. Rapid depressurization resulted in a higher fractional free volume and greater gas permeability, which is attributed to the sudden expansion of CO2 sorbed in the polymer opening up the morphology. Slower depressurization resulted in a lower fractional free volume and decreased gas permeabilities, because the CO2 gradually desorbs, leaving behind a more dense morphology. The resulting decrease in permeability also corresponded with a significant increase in selectivity. For both CTA and 6FDA-TMPDA membranes the sc-CO2 treatment improved the gas permselectivity relative to the original state. Therefore, exposure to sc-CO2 presents an advantageous process to improve the performance of common polymeric membranes for gas separation.

Synthesis of ZnO nanowires with supercritical carbon dioxide and post heat treatment

Zinc oxide (ZnO) nanowires are used in applications such as gas sensors and solar cells. This work presents a novel synthesis route for ZnO nanowires using supercritical carbon dioxide (scCO2) and post heat treatment. The method used scCO2 and a precursor solution as reactants to form nanowires on a galvanized surface. After the scCO2 treatment, the substrate was heat-treated. The surfaces were characterized with SEM, TEM, EDS, FTIR, XRD and optical spectroscopy. The FTIR results showed that the surface structure had changed from zinc hydroxycarbonate to ZnO during the heat treatment. The nanowires were slightly bent due to the heat treatment according to the SEM images. The presence of ZnO was further confirmed with XRD. The bandgap of the structure was determined by reflectance measurements and showed a value of 3.23 eV. The synthesis method presented in this study offers a unique approach into the formation of ZnO nanowires in a facile, rapid and environmentally friendly process.

Effect of supercritical carbon dioxide processing on Vibrio parahaemolyticus in nutrient broth and in oysters (Crassostrea gigas).

This study aimed to evaluate the technical feasibility of supercritical carbon dioxide (sc-CO2) treatment for Vibrio parahaemolyticus inactivation in oysters (Crassostrea gigas) and in nutrient broth. For this purpose, a variable-volume reactor was used as experimental system and a 23 factorial design was adopted considering the mass ratio between carbon dioxide and the product, pressurization and depressurization rate and pressurization cycles. Through statistical analysis of the experimental data, the mass ratio of 1:0.8 (product:carbon dioxide), depressurization rate of 10.0MPa/min and one cycle of pressurization was determined as the best process condition to eliminate V. parahaemolyticus, and this was the condition used for the inactivation kinetic analysis. Comparison between the inactivation kinetics of V. parahaemolyticus showed that the behavior of this microorganism inactivation depends on the environment in which it operates and its initial count. The results confirm that the supercritical carbon dioxide is effective in inactivating microorganisms in oysters, including pathogenic V. parahaemolyticus, demonstrating the potential of this technology in the food industry.

The effects of supercritical CO2 on mesopore and macropore structure in bituminous and anthracite coal

The effects of supercritical CO2 (scCO2) on coal pores play a critical role in CO2 geological storage-enhanced coalbed methane recovery (CO2-ECBM). To investigate the effects of scCO2 on the pore structure and connectivity in coals of different ranks, CO2 sequestration processes were replicated using a scCO2 geochemical reactor. Four coal samples with different ranks were exposed to scCO2 and water for 240 h under 80 °C and 20 MPa. Mercury intrusion porosimetry, nitrogen adsorption tests, and scanning electron microscopy analysis were used to identify the pore volume, pore size distribution, pore connectivity, and pore morphology of the coal samples before and after scCO2-H2O treatment. The increase in the macropore volume caused by the scCO2 is mainly controlled by the pores >30 μm, resulted from the dissolution of carbonate minerals with large grain size in organic matter. The coal rank has a positive influence on the volume and specific surface area of the pores >30 μm, whereas coal rank is more significant for macropore specific surface area in the high-volatile bituminous coal and macropore volume in the low-volatile bituminous, semi-anthracite, and anthracite coal. With an increase in the coal rank, the volume and specific surface area of the mesopores increase. The changes in the mesopore volume in the high-volatile bituminous coal are caused by changes in the pores 20–50 nm in diameter, while these changes in the low-volatile bituminous coal, semi-anthracite, and anthracite are caused by changes in the pores <20 nm, due to the formation of dissolution-created pores in minerals; additionally, many pores <3 nm are generated by the scCO2. At the macropore scale, the scCO2 increases the volume of the non-effectively interconnected pores more than that of the effectively interconnected pores, indicating that the scCO2 has a weak effect on the increase in macropore connectivity. The volume of the effectively interconnected pores increases more at the mesopore scale than at the macropore scale, and the scCO2 treatment increases the connectivity of the mesopores.