CH4 Adsorption Capacity Research Articles

Changes in Physicochemical Properties of Coal and Their Mechanism Due to Supercritical CO2–H2O Treatment

The dissolution of supercritical carbon dioxide (ScCO2) in water forms a ScCO2–H2O system, which exerts a transformative influence on the physicochemical characteristics of coal and significantly impacts the CO2-driven enhanced coalbed methane (CO2-ECBM) recovery process. Herein, the effect of ScCO2–H2O treatment on the physicochemical properties of coal was simulated in a high-pressure reactor. The migration of major elements, change in the pore structure, and change in the CH4 adsorption capacity of coal after the ScCO2–H2O treatment were detected using plasma emission spectroscopy, the low-temperature liquid nitrogen adsorption method, and the CH4 adsorption method, respectively. The results show that (1) the ScCO2–H2O treatment led to mineral reactions causing a significant migration of constant elements in the coal. The migration of Ca ions was the most significant, with an increase in their concentration in treated water from 0 to 16–970 mg·L−1, followed by Na, Mg, and K. Al migrated the least, from 0 to 0.004–2.555 mg·L−1. (2) The ScCO2–H2O treatment increased the pore volume and pore-specific surface area (SSA) of the coal via the dissolution and precipitation of minerals in the coal pores. The total pore volume increased from 0.000795–0.011543 to 0.001274–0.014644 cm3·g−1, and the total pore SSA increased from 0.084–3.332 to 0.400–6.061 m2·g−1. (3) Changes in the CH4 adsorption capacity were affected by the combined effects of a mineral reaction and pore structure change. The dissolved precipitates of the minerals in the coal pores after the ScCO2–H2O treatment caused elemental migration, which not only decreased the mineral content in the coal pores but also increased the total pore volume and total pore SSA, thus improving the CH4 adsorption capacity of the coal. This study provides theoretical support for CO2 sequestration and ECBM recovery.

Molecular simulation of adsorption behaviors of methane and carbon dioxide on typical clay minerals

Knowledge of the interaction mechanisms between shale and CH4/CO2 is crucial for the implementation of CO2 sequestration with enhanced CH4 recovery (CS-EGR) in shale reservoir. As one of the main constituents of shale, clay minerals can profoundly affect the storage capacity of gases in nanopores. In this paper, the adsorption behaviors of both CO2 and CH4 on montmorillonite, illite as well as kaolinite under dry condition are investigated by Grand Canonical Monte Carlo (GCMC) simulation. The results exhibit that the maximum adsorption capacity of single-component CH4 and CO2 is associated with the types of clay crystals. Specifically, the montmorillonite has the strongest adsorption capacity for CO2, followed by illite and kaolinite, while the sequence in maximum adsorption capacity of CH4 is predicted in the order of kaolinite > montmorillonite > illite. These discrepancies are closely related to the characteristics of adsorbate molecules as well as the different structures of clay crystals. Meanwhile, the maximum adsorption capacity of CH4 in studied clay minerals gradually decreases as pore size increases, while nanopores with 2-nm basal spacing demonstrate the highest adsorption capacity for CO2. In addition, it is observed that the studied clay minerals tend to preferentially adsorb CO2 rather than CH4 during binary gas mixtures simulation. The selectivity of CH4/CO2 mixtures in montmorillonite and kaolinite exhibits various performances as the adsorption pressure increases, with the selectivity in montmorillonite being the largest, especially at low pressure. The cation exchange significantly enhances the electrostatic interaction with CO2 molecules, leading to a higher loading of CO2 as well as larger value of selectivity. These findings can provide basis and guidance for the CS-EGR project in shale reservoirs.

Competitive adsorption law of multi-component gases during CO2 displacement of CH4 in coal seams

The CO2 enhanced coal bed methane recovery technology provides an excellent way to mitigate the greenhouse effect and energy crisis. The features and mechanism of CO2/CH4 competitive adsorption in the coal rock matrix have a critical impact on the production of CBM. For CO2/CH4 competitive adsorption process components and pressure changes, a multi-component gases competitive adsorption experiment was carried out with CO2 and CH4 binary gases as the study objects, and a multi-component adsorption model was developed by the expanded Langmuir equation. Based on the principles of molecular dynamics and thermodynamics, important parameters such as the average free range of gas molecules and adsorption potential are introduced to explain the competitive adsorption behavior from multiple perspectives and explore the competitive adsorption law of CH4 and CO2 under multiple component conditions, so as to provide some theoretical basis and field guidance for improving the extraction effect of CH4 in coal seams. The results show that: Under two critical conditions (100%CO2 +0%CH4 and 0%CO2 +100%CH4), the Langmuir volumes of CO2 for HN 1/3 coking coal and HL weakly caking coal respectively are 2.21 and 3.01 times higher than those of CH4; the overall adsorption capacity of binary gas is in between the adsorption capacities of both critical conditions and increases with increasing CO2 concentration in the gas source ratio; using E-L equation for binary gas component partitioning, the CH4 partition curves were all below CO2, the concentration of free-phase CH4 was always higher than that in the adsorbed phase, and coal samples had stronger adsorption ability for CO2 than CH4. CO2 has a stronger adsorption potential at the surface for HL weakly caking coal. CH4 has a slightly stronger adsorption potential at the surface for HN 1/3 coking coal. The higher coalification degree of the coal sample, the stronger the adsorption ability for CH4 and the weaker the adsorption ability for CO2. The slope of the overall adsorption curve of multi-component gases is analogous to the slope of the component with a high ratio of gas or strong adsorption capacity; the capacity of one-component adsorption for dual gas is closely related to the partial pressure of the free-phase gas and the separation factor α21. When the concentration ratio of CH4/CO2 in free phase is y2/y1 =α21, the concentration of both gases in adsorption phase is 50%, and α21 correlation is weakened, then the free phase gas partial pressure is dominant and there is a threshold value makes CH4 and CO2 adsorption capacity equal; when y2/y1 <α21, the coal preferentially adsorbs CO2 in the binary gas; when y2/y1 >α21, the coal preferentially adsorbs CH4 in the binary gas.

Influence of rock properties and prediction on the methane storage capacity in marine-continental transitional shale and coal from northern China

Systematic understanding of geological controls on methane storage capacity in marine-continental transitional shale can help to predict the potential gas resources in such shale reservoirs. In the present study, seven transitional shale samples were first collected from three different basins (Qinshui Basin, Ningwu Basin, and eastern Ordos Basin) in Northern China and characterized for their composition, thermal maturity, pore structure and CH4 adsorption capacity. Their total CH4 storage capacities (including adsorbed and free gas) at pressures up to 10 MPa were determined using an ingenious method from previous studies. Based on this, we investigated the properties controlling gas adsorption and total gas storage in these shales. Meanwhile, several coal samples from the same strata unit were analyzed through analogous processes and treated as the naturally occurring extreme total organic carbon (TOC) cases of transitional shale during discussion. Our findings indicate that the transitional shale’s composition (typically represented by the TOC and total clay content) directly influences the pore development and then controls the CH4 adsorption, which is consistent with the previous findings for marine shale. Though the positive correlations between CH4 adsorption capacity and both TOC and total clay content were observed for the studied shale samples, the observations demonstrate the dominance of organic components in rock for gas adsorption when the transitional shale and coal samples are considered together. In addition, the positive correlation between clay minerals and CH4 adsorption may be overshadowed, even reversed. Based on the method used, the total gas-in-place (GIP) storage in these transitional shales shows a stronger positive correlation with total clay content than TOC content. This is mainly attributed to the meso- and macroporosity of clay minerals. For the studied shale samples, the relationship between thermal maturity and CH4 storage capacity can be represented by a quadratic polynomial function, which is likely due to considerable variation in rock composition. Furthermore, it is emphasized that predicting the total GIP in shale and coal reservoirs using these correlations should be approached with caution, regardless of the method (direct/indirect) used to determine the GIP.

Study on Molecular Model Construction and Gas Adsorption Simulation of Anthracite in the Qinshui Basin.

Uncovering gas adsorption characteristics of coal at the molecular scale is of great theoretical significance for the study of gas occurrence, coalbed methane exploitation, and carbon dioxide sequestration. In this study, based on proximate analysis, ultimate analysis, 13C nuclear magnetic resonance, and Fourier-transform infrared spectroscopy experiments, the existence forms and relative contents of elements of anthracite in the Qinshui Basin were tested and analyzed, and a macromolecular structure model was constructed. Besides, three types of acidic oxygen-containing functional groups, namely, carboxyl groups, phenolic hydroxyl groups, and lactone groups, were added to the molecular model. Furthermore, CH4 adsorption simulation was conducted on the original molecular model of anthracite and models with three types of acidic functional groups added. The following research results were obtained. The molecular formula of the constructed macromolecular model of anthracite in the Qinshui Basin is C193H138N2O7. The molecular structure of coal becomes more compact and curved after structural optimization and annealing optimization. For the four models, the CH4 adsorption characteristics of coal molecules all conform to the Langmuir equation under the same simulation conditions. Among them, the original model has the largest CH4 adsorption capacity, while the addition of oxygen-containing functional groups reduces the CH4 adsorption capacity to varying extents. The reduction of CH4 adsorption capacity follows the order: adding carboxyl groups > adding phenolic hydroxyl groups > adding lactone groups, which is mainly attributed to the different adsorption heats and adsorptive potential wells triggered by the addition of acidic functional groups in molecules.

Carboxymethyl cellulose/polyvinyl alcohol composite aerogel supported beta molecular sieve for CH4 adsorption and storage

Biomass aerogel is attractive in various applications due to their renewable, biodegradable and eco-friendly advantages. Herein, a novel beta molecular sieve/carboxymethyl cellulose/polyvinyl alcohol composite aerogel (beta/CP) is prepared by direct mixing and directional freeze-drying as an efficient gas adsorbent with hierarchical porosity. The beta molecular sieve is uniformly dispersed in the three-dimensional skeleton of the aerogel. By adjusting the loading mass of the beta molecular sieve to constitute a reasonable porosity and pore size distribution, the synergistic effect between pore structures of different scales improves the adsorption performance. The experiment results of beta/CP-4 show that the CH4 adsorption capacity can reach 60.33 cm3/g at 298 K and 100 bar, which is almost the same as that of the pure beta molecular sieve (62.09 cm3/g). The strong interaction between the aerogel and it prevents the molecular sieve agglomeration, improves the pore utilization, and also reduces the cost of using molecular sieve adsorbent. The above results indicate that the composite has good potential for application in the field of CH4 storage.

Investigating the selective adsorption of CO2 by MIL-101(Cr)-NH2 and modeling the equilibrium data using a new three-parameter isotherm

Compared to MIL-101(Fe)-NH2, MIL-101(Cr)-NH2 has considerably higher adsorption of CO2. On the other hand, the adsorption of CH4 and N2 by MIL-101(Cr)-NH2 is lower than the adsorption of these gases by MIL-101(Fe)-NH2 leading to a higher selectivity of CO2 to CH4 and N2 for MIL-101(Cr)-NH2. The chemisorption mechanism is more influential in higher temperatures and pressure, which can be attributed to the performance of amines which can adsorb more CO2 at higher temperatures and pressure. The isosteric heat of CO2 adsorption on MIL-101(Cr)-NH2 was estimated equal to − 31 kJ mol-1.MIL-101(Cr)-NH2 was synthesized and characterized by the BET, XRD, FESEM, TGA, and DTA methods. The BET surface area of the synthesized MOF was achieved equal to 1768 m2 g-1. The adsorption capacity of pure CO2, CH4, and N2 was estimated using the volumetric method. A new three-parameter equation was presented to model the experimental adsorption data including both types I and II isotherm shapes. Using the Ideal Adsorbed Solution Theory (IAST), the selectivity of CO2 to CH4 at high pressure, and CO2 to N2 at lower pressures was estimated equal to 4.1 and 4.7, respectively. In addition to 25 °C, the adsorption tests were performed at 18 and 30 °C to estimate the adsorption heat of CO2 on MIL-101(Cr)-NH2 achieved equal to − 31 kJ mol-1. The adsorption isotherm of CO2 at 25 and 30 °C have been almost identical showing the role of the chemisorption mechanism of CO2 by the amine functional group, especially at higher temperatures and pressures. Furthermore, the resistance of synthesized MOF to water was investigated by putting it in contact with water for 24 h. The result showed that MIL-101(Cr)-NH2 can be classified as thermodynamically stable to water.

Constructing Randomly Lamellar HKUST–1@Clinoptilolite through Polyethylene Glycol—Assisted Hydrothermal Method and Coordinated Complexation for Enhanced Adsorptive Separation for CO2 and CH4

Clinoptilolite (CP) was successfully synthesized via a hydrothermal route in the presence of polyethylene glycol (PEG), and it was then delaminated by washing using Zn2+ containing acid. HKUST-1, as one kind of the Cu-based MOFs, showed a high CO2 adsorption capacity owing to its large pore volume and specific surface area. In the present work, we selected one of the most efficient ways for preparing the HKUST-1@CP compounds via coordination between exchanged Cu2+ and ligand (trimesic acid). Their structural and textural properties were characterized by XRD, SAXS, N2 sorption isotherms, SEM, and TG-DSC profiles. Particularly, the effect of the additive PEG (average molecular weight of 600) on the induction (nucleation) periods and growth behaviors were detailed and investigated in the hydrothermal crystallization procedures of synthetic CPs. The corresponding activation energies of induction (En) and growth (Eg) periods during crystallization intervals were calculated. Meanwhile, the pore size of the inter-particles of HKUST-1@CP was 14.16 nm, and the BET specific area and pore volume were 55.2 m2/g and 0.20 cm3/g, respectively. Their CO2 and CH4 adsorption capacities and selectivity were preliminarily explored, showing 0.93 mmol/g for HKUST-1@CP at 298 K with the highest selective factor of 5.87 for CO2/CH4, and the dynamic separation performance was evaluated in column breakthrough experiments. These results suggested an efficient way of preparing zeolites and MOFs composites that is conducive to being a promising adsorbent for applications in gas separation.

Facile preparation of nitrogen-doped microporous carbon from potassium citrate/urea for effective CH4 separation and uptake

N-doped microporous carbons have attracted much attention for micromolecule gas separation and accumulation owning to their superior adsorption potential and surface polarizability. However, to prepare N-doped microporous carbon usually acquire excess corrosive KOH as activator, which limits its application in industry. Here, we propose a non-corrosion method for the manufacture of narrow N-doped microporous carbons using potassium citrate acts as activating agent, and urea acts as both a nitrogen doping agent and coactivator without any solvent. The BET surface area, micropore volume, pore size distribution and N content can be easily adjusted by changing the potassium citrate/urea ratio and activating temperature. The influence of porous structure and N content on CH4 uptake and separation was investigated, and ultra-micropore volume (<1nm) plays a dominate role in CH4 selectivity adsorption. ACK2N1 had the largest CH4 adsorption capacity of 3.00 mmol/g and CH4/N2 selectivity of 7.11 at 273.15 K and 100 KPa due to its largest ultra-micropore volume and N content. The adsorption breakthrough, regeneration experiments and adsorption thermodynamics showed that the prepared well-developed N-doped microporous carbons have very good potential for CH4 separation and enrichment from low coal bed methane. Furthermore, the pore formation mechanism of the well microporous carbon materials is proposed.

Polarization enhanced CH4/N2 separation in bromine functionalized ZIF-62 based mixed-matrix membranes

Owing to the similarities in polarity, kinetic size, and boiling point, CH4 and N2 separation remains difficult and energy-consuming. However, the difference in polarizability offers an opportunity for boosting the separation by differentiating their interactions with the metal-organic framework (MOF). Herein, bromine (Br) functional group is rationally introduced into ZIF-62 to strengthen CH4/N2 separation through the polarization effect. The anisotropic charge distribution on Br atom can induce electron cloud deformation of gas molecules, especially CH4 molecules with higher polarizability, thus generating differentiated interactions. As a result, ZIF-62-Br exhibits a 69.7% increase in CH4 adsorption capacity and obviously improved selectivity compared with the ZIF-62, and embedding ZIF-62-Br in the polyethyleneimine (PEI) matrix endows the obtained mixed-matrix membranes (MMMs) with good CH4/N2 separation performance (CH4 permeance of 3157 GPU and CH4/N2 ideal selectivity of 7.6 for single gas permeations, and CH4 permeance of 2235 GPU and CH4/N2 selectivity of 4.2 for mixed gas tests). The good separation performance and stability suggest that MMMs fabricated with ZIF-62-Br in this work can be a promising candidate for facilitating the exploration of CH4/N2 membrane separation.

N-doped sponge-like biochar: A promising CO2 sorbent for CO₂/CH₄ and CO2/N₂ gas separation

Sponge-like biochar sorbents were prepared from the dissolution of chitosan followed by freeze-drying methodology and pyrolysis at three different temperatures (400, 600, and 800 °C) to produce sustainable N-enriched carbon materials with enhanced CO2 uptake from CO2/CH4 and CO2/N2 gas mixtures. The pyrolysis process was reproduced by operando TGA-IR to study the gas evolved from the pyrolysis process. It was found that the pyrolysis temperature highly influences the textural properties of the chitosan sponge-like biochar materials, impacting mainly the amount and type of the N-species on the sample but also at the microporosity. XPS revealed the transformation of the amino groups from chitosan into pyridinic-N, pyrrolic-N, graphitic center-N, and graphitic valley-N or pyridine-N oxide species during the pyrolysis process. Increasing the pyrolysis temperature enhanced the quantity of the latter two N-type species. All sponge-like biochars adsorbed higher amounts of CO2 compared with CH4 and N2 gases, with maximum CO2 uptake (∼1.6 mmol⋅g−1) at 100 kPa and 25 °C for the sample pyrolyzed at 600 °C (named CTO_P600). Biochar produced at 800 °C showed no longer adsorption capacity for CH4 and N2, having the highest selectivity value for CO2/N2 separation under continuous flux conditions among all prepared biochar sorbents. Isobaric CO2 adsorption measurements on the CTO_P600 sorbent revealed that physisorption phenomena predominantly governed the CO2 adsorption process, which was confirmed by its consistent adsorption capacity after 10 consecutive adsorption–desorption cycles. Moreover, the biochar exhibited tolerance to water vapor adsorption, indicating its suitability to work under moisture-rich conditions.

Enhancing Size-Selective Adsorption of CO2/CH4 on ETS-4 via Ion-Exchange Coupled with Thermal Treatment

ETS-4 shows great potential to achieve effective separation of challenging mixtures due to its promising tunability of pores. However, the regulation of pore size is largely limited by its low thermal stability. Here, Na-ETS-4 was synthesized by the hydrothermal method and additionally modified via ion exchange coupled with thermal treatment. Structures of all synthesized samples were examined using a variety of characterization techniques. Both adsorption equilibrium and kinetics of CO2 and CH4 on these adsorbents were evaluated on custom-made adsorption units. In addition, the mechanism of CO2 adsorption on different structural models of ETS-4 was revealed via combining with the molecular dynamics (MD) simulations. Ion exchange largely improves the thermal stability of ETS-4 and its adsorption capacities for CO2 and CH4. Ba-ETS-4-350 exhibits the largest equilibrium selectivity of 12.42 and the greatest Henry’s selectivity of 2178.51. The radial distribution function (RDF) results reveal the main adsorption sites of CO2 in different ETS-4 structures.

Long-term supercritical CO2 exposure dependence of physicochemical properties and CH4 adsorbability of moisture-equilibrated coal matrices

ABSTRACT Further understanding into the impacts of supercritical CO2 fluid exposure toward CO2 storage and coalbed methane (CH4) recovery is significant to evaluate validity, stability, and safety of CO2-ECBM. For purpose of addressing the potential impacts, the long-term static interactions between supercritical CO2 fluid and moisture-equilibrated coal matrices with various degrees of coalification were conducted at 318.15 K and 12.00 MPa for 240 days in this study. Furthermore, the changes of the physicochemical properties dominating adsorbability of moisture-equilibrated coals due to the exposure were revealed. Ultimately, the impacts of the exposure toward CH4 adsorbability of moisture-equilibrated coals were addressed. Results indicate that the long-term supercritical CO2 exposure induces complex effects typically including organic matter extraction, mineral dissolution and formation, and matrix swelling to the coals, thereby further reducing the micropores within pore diameter ranges of 0.38–0.71 nm and 0.74–0.90 nm of the low-rank moisture-equilibrated coals, and slightly increasing those of 0.40–0.70 nm and 0.75–0.90 nm of the high-rank moisture-equilibrated coal. Additionally, the complex effects reduce the mesopores with pore diameter less than 8.00 nm for all the coal samples. Moreover, the exposure alters the surface chemistry of the coals. Particularly, the exposure increases the oxygen-containing functional groups typically comprising C-O, C=O, and -COOH of the low-rank moisture-equilibrated coals by 25.32–27.16%, and weakens the graphitization of the high-rank coal by 34.00%. Moreover, the induced synergistic effects decrease the number of pores with diameter less than approximately 8.00 nm of all the moisture-equilibrated coals. Furthermore, the CO2 exposure significantly reduces CH4 adsorption capacity of the moisture-equilibrated coal matrices indicated by the decreasing amplitude in absolute adsorption amount of 6.20–18.00%. Overall, the long-term supercritical CO2 exposure could favor CH4 production during CO2-ECBM. Future study should focus on in-situ characterization and accurate quantitative analysis on extraction, mineral dissolution and formation, and matrix swelling due to CO2 exposure, and distinguishment in the contribution of these effects to pores and functional groups of coals.

Isothermal adsorption characteristics of various phases of CO2 and CH4 in different rank coals

ABSTRACT Using the remaining coal seams of closed coal mines for CO2 geological storage can effectively slow down the greenhouse effect and realize the redevelopment and utilization of closed coal mines. The burial depth of coal seams in closed coal mines in China covers an extensive range, and CO2 and CH4 could exist in various phases in the coal. The existing experimental objects for CO2 adsorption in coal are mainly associated with the supercritical and gaseous phases, and a research gap comparison between the CO2 adsorption characteristics in the liquid phase and those of other phases is very obvious. Herein, the isotherm adsorption experiments of CO2 and CH4 in different phases on different rank coal samples are methodically conducted in the pressure range of 0-12MPa. The experimental results indicate that the excess adsorption capacity of gaseous CO2 and CH4 (gas pressure <4MPa) rapidly raises with the growth of the adsorption pressure; however, a significant difference for the case of the medium- and high-pressure stage (>4MPa) is detectable. With the growth of the adsorption pressure, the excess adsorption capacity of the supercritical CH4 gradually reaches the maximum value and then remains stable, where the maximum experimental value is 1.009 mmol/g. While the excess adsorption capacity of CO2 near the critical pressure of liquid and supercritical state rapidly reduces and then tends to be stable. The maximum adsorption capacity is capable of touching 2.096 mmol/g, which is stable around 0.5 mmol/g. The analysis indicates that the change in the excess adsorption amount of CO2 near the critical pressure corresponds to the change in its density. The absolute adsorption capacity of CO2 and CH4 monotonically grows in the pressure range of 0-12MPa, and the absolute adsorption capacity of the same coal sample follows the following order: liquid CO2 > supercritical CO2 > CH4. The maximum adsorption capacity values for these substances in order are 2.493, 2.345, and 1.296 mmol/g. With the growth of the coal rank, the Gibbs free energy, specific surface area, and adsorption capacity of the understudied coal samples exhibit a U-shape relationship as a function of their coal ranks.

Synthesis of nanosized IM-5 zeolite and its CH4/N2 adsorption and separation

Reduction of zeolite crystal size from the micron scale to the nanoscale leads to a significant increase in specific surface area, favoring mass-transfer pathways. Here, we have prepared nanoscale (ca. 30–80 nm) IM-5@100 zeolite (IMF type) by a facile strategy of regulating the amount of Al source in the starting gel, and have investigated its CH4/N2 adsorption and separation properties. We found that on decreasing the amount of Al source, the crystal size of the product changed significantly, decreasing from strips of length about 1 μm to irregular nanocrystals. Compared with micron-sized IM-5@50, the mesopore volume of IM-5@100 was increased fourfold from 0.12 cm3/g to 0.49 cm3/g, its external surface area was increased from 38 m2/g to 118 m2/g, and its DFT pore size distribution based on N2 adsorption/desorption showed a mesopore distribution in the range 5–10 nm. Adsorption kinetics tests showed that nano-IM-5@100 can achieve adsorption equilibrium more swiftly and exhibits better mass-transfer performance. Moreover, IM-5@100 showed good adsorption capacity for CH4 (17.8 cm3/g) and high IAST selectivity (4.7) for CH4/N2 (50:50, v/v). Breakthrough experiments have indicated that IM-5@100 permits efficient practical separation of CH4/N2.

Characterization of kerogen molecular structure and its effect on methane adsorption behavior: A comparative study on outcrop and core samples from Longmaxi shale

It has been known that adsorbed gas, mainly CH4, serves as the dominant source of shale gas yield in the middle and later stages of reservoir exploitation. Therefore, it is of great significance to study the microstructure of kerogen (organic matter rich in shale) and its effect on CH4 adsorption characteristics. In this paper, for the first time, a comparative study was conducted to experimentally characterize the microstructure of both outcrop and core kerogen samples from Longmaxi Shale, Sichuan Basin, SW China, using 13C NMR, FTIR, and XPS. Great differences in the chemical structure such as carbon skeleton (e.g., aromaticity, aliphatic branch) and heteroatoms (e.g., distribution and proportion of various types of functional groups) between the two kerogen samples have been identified. Molecular models of the outcrop and core kerogen samples were constructed based on the experimental data, that is, C237H219O21N5S4 for the former and C239H175O27N5S4 for the latter. CH4 adsorption isotherms obtained from gravimetric adsorption test and molecular simulation using proposed kerogen molecular models are in reasonable agreement with each other, especially at low-pressure range. Interestingly, the adsorption capacity of CH4 in the core sample is found higher than that in the outcrop sample under the same condition, indicating the effect of kerogen microstructure on CH4 adsorption behavior. As a result, to precisely evaluate the adsorbed gas content in shale gas reservoirs, it is strongly recommended to use a core sample rather than an outcrop sample.

New insight into efficient CH4/N2 separation based on Ni-decorated porous carbon functional composites

Separation of CH4/N2 mixture in coal-mine gas with low CH4 concentration is still challenging due to similar physical properties between CH4 and N2. Herein, we construct quantitative relationships between the pore distribution and CH4 adsorption capacity of porous carbon, exploring the specific role of micropores and meso-/macropores in the accommodation of CH4. Then, nickel (Ni) species selected on account of density functional theory (DFT) were employed for accurately regulating the size and surface properties of the pore structure, targeted to enhance the separation of CH4 against N2. The results indicate that 10 %Ni/C adsorbent achieved the highest CH4 adsorption (28.3 cm3/g) as well as CH4/N2 selectivity (5.29), compared to 20.0 cm3/g and 3.67 of the porous carbon. Additionally, a deep understanding of the synergistic effect between porous sizes and adsorption potentials provided by Ni species is triggered by the obtained results, which indicated Ni-decorated porous carbon as promising adsorbents and provided new insight for the improvement of CH4/N2 separation.

Thermodynamic‐kinetic synergistic separation of CH4/N2 on a robust aluminum‐based metal‐organic framework

AbstractA robust aluminum‐based metal–organic framework (Al‐MOF) MIL‐120Al with 1D rhombic ultra‐microporous was reported. The nonpolar porous walls composed of para‐benzene rings with a comparable pore size to the kinetic diameter of methane allow it to exhibit a novel thermodynamic‐kinetic synergistic separation of CH4/N2 mixtures. The CH4 adsorption capacity was as high as 33.7 cm3/g (298 K, 1 bar), which is the highest uptake value among the Al‐MOFs reported to date. The diffusion rates of CH4 were faster than N2 in this structure as confirmed by time‐dependent kinetic adsorption profiles. Breakthrough experiments confirm that this MOF can completely separate the CH4/N2 mixture and the separation performance is not affected in the presence of H2O. Theoretical calculations reveal that pore centers with more energetically‐favorable binding sites for CH4 than N2. The results of pressure swing adsorption (PSA) simulations indicate that MIL‐120Al is a potential candidate for selective capture coal‐mine methane.

Adsorption Equilibrium and Diffusion of CH4, CO2, and N2 in Coal-Based Activated Carbon.

Coal-based activated carbon is an ideal adsorbent for concentrating CH4 from coalbed methane and recovering CO2 from industrial waste gas. In order to upgrade the environmentally protective preparation technology of coal-based activated carbons and clarify the adsorption equilibrium and diffusion rules of CH4, CO2, and N2 in these materials, we prepared granular activated carbon (GAC) via air oxidation, carbonization, and physical activation using anthracite as the raw material. Also, we measured the adsorption isotherms and adsorption kinetic data of GAC by the gravimetric method and characterized its surface chemical properties. According to the results, GAC had abundant micropore structures with a pore size mainly in the range of 5.0-10.0 Å, and its surface was covered with plentiful oxygen-containing functional groups. The specific pore structure and surface chemical properties could effectively improve the separation and purification effects of GAC on CH4 and CO2. In the temperature range of 278-318 K, the equilibrium separation of CH4/N2 by GAC with a coefficient between 3 and 4 could be achieved. Also, the CO2/CH4 separation coefficient decreased with the increase in temperature but remained around 3. The bivariate Langmuir equation could describe the adsorption behaviors of GAC on CH4/N2, CO2/N2, and CH4/CO2. With the increase in the concentrations of CH4 and CO2 in the gas phase, the difference between the adsorption capacity of CH4 or CO2 and that of N2 became greater. The change of the gas ratio did not affect the characteristics of preferential adsorption of CH4 and CO2. At different temperatures (278, 298, and 318 K), the diffusion coefficients of CH4, N2, and CO2 at various pressure points showed predominately a small variation without an obvious trend. These results demonstrated that the separation of CH4/N2, CO2/N2, and CH4/CO2 by the activated carbon could only rely on the equilibrium separation effect rather than the kinetic effect.

Effect of binder on CO2, CH4, and N2 adsorption behavior, structural properties, and diffusion coefficients on extruded zeolite 13X

The effect of inorganic binder-based extrusion (Silica sol, Bentonite, Attapulgite, and SB1) in the selective adsorption of CO2, CH4, and N2 on zeolite 13X in the context of flue gas carbon capture and natural gas purification has been studied to reduce CO2 emissions. The effect of extrusion with binders was examined by adding 20 wt% of the mentioned binders to pristine zeolite and the results were analyzed by four approaches; (i) the effect on structural properties was analyzed by XRD patterns followed by Williamson–Hall (W–H) plot, FESEM images, and BET surface area. In addition, the mechanical strength of the shaped zeolites was measured by crush resistance testing; (ii) the effect on the adsorption capacity for CO2, CH4, and N2 were measured by volumetric apparatus up to 100 kPa; (iii) the impact on binary separation (CO2/CH4 and CO2/N2) were investigated; (iv) the influence on diffusion coefficients were estimated by micropore and macropore kinetic model. The results showed that the presence of a binder can cause reductions in BET surface area and pore volume, indicating partial pore blockage. It was found that the Sips model had the best adaptability to the experimental isotherms data. The trend of CO2 adsorption was 13X > pseudo-boehmite > bentonite > attapulgite > silica, in which the adsorption capacity reached 6.02, 5.60, 5.24, 5.00, and 4.71 mmol/g, respectively. Among all samples, silica was found the most suitable binder for CO2 capture in terms of selectivity, mechanical stability, and diffusion coefficients.