CH4 Adsorption Capacity Research Articles

Thermodynamic characteristics of methane adsorption on coals from China with selected metamorphism degrees: Considering the influence of temperature, moisture content, and in situ modification

A better understanding of the thermodynamic characteristics of methane (CH4) adsorption on coal surfaces is needed for the high-efficiency coalbed methane (CBM) preextraction or prevention of coal and gas outbursts. In this study, atomistic representations of selected coal samples with different degrees of metamorphism, such as long flame coal, coking coal, and anthracite coal obtained from China were created; CH4 adsorption isotherms considering the temperature, moisture content, and microwave in situ modification for CBM preextraction were simulated and analysed. The coal molecular structural results showed that as the degree of coal metamorphism increased from long flame coal to coking coal and anthracite coal, the functional groups in the molecular structure of the coal reduced, while the degree of graphite increased. In situ modification of coal facilitated this process. The saturation adsorption capacity of CH4 decreased and then increased with increasing coal metamorphism degree and decreased linearly with increasing temperature. When water molecules were present, the adsorbed CH4 showed different degrees of decreasing trends with increasing moisture content. The isosteric heat of CH4 adsorption followed the order of coking coal > anthracite coal > long flame coal. The isosteric heat gradually decreased with increasing methane adsorption, and the presence of water molecules had a limited effect on the isosteric heat. In situ modification disrupted the self-conjugated hydrogen bonds, reduced the oxygen-containing functional groups, and decomposed some associated minerals in the coal samples. After in situ modification, the amount of methane adsorption decreased due to a greater contribution from the functional group change than that from the pore volume change, and the modification effect was more evident for the low-rank coal samples than for the middle and high metamorphism degree coal samples. In situ modification applied in the coal seams could contributed to designs for CBM exploration, and the results of this experiment and simulation could provide a novel theoretical basis for in situ modification technology.

Synthesis of grape-seed derived carbon with high specific surface area for CO2 selective adsorption

Nitrogen-doped porous carbons with BET surface area of 1068.2–3314.5 m2/g and nitrogen contents of 3.2–6.5% were prepared with solid waste grape-seed as raw material, NaNH2 as activator and nitrogen source at low activation temperature. Super The activation mechanism of NaNH2 on hydrothermal carbon precursors was first explored by thermodynamic analysis and TG-IR, which provided theoretical support for pore forming of carbon materials. Maximum CO2 and CH4 adsorption capacity at 273 K and pressure of 101 kPa was 5.42 and 1.76 mmol g−1 respectively, which are higher than those of majority of carbon derived from most solid wastes reported in literature. IAST selectivities of GS-3-450 with the largest BET surface area for CO2/CH4 (40v/60v), CO2/N2 (15v/85v), CH4/N2 (50v/50v) were found to be 20.3, 71.4, 6.0 under 101 kPa and 298 K respectively. The competitive adsorption of GS-3-450 for CO2/CH4 (40v/60v), CO2/N2 (15v/85v), CH4/N2 (50v/50v) gases mixture were examined through breakthrough experiments, and the results showed that the breakthrough time of CO2 was longer than that of CH4 and N2, which was beneficial to the separation of CO2 from gases mixture. Eight cycles of CH4 adsorption–desorption studies revealed that the material exhibited excellent recycling stability. Low temperature preparation method, excellent BET specific surface area and total pore volume, as well as excellent adsorption ability of CO2 and CH4 make it have a very great potential for the capture of CO2 and CH4.

Fractal characteristics and theirs influence on methane adsorption in high-rank coals with NMR

To further understand the pore structure characteristics and their effect on CH4 adsorption capacity for high-rank coals. Based on 11 fresh coal samples from the Zhina coalfield of South China. We analyzed the pore structure characteristics of coal samples by low-temperature liquid-nitrogen adsorption (LP-N2A) measurements. On the basis of nuclear magnetic resonance (NMR), we obtained the fractal dimensions of different types of pores by the new model, studied the relationship between the fractal dimensions, and the characteristic parameters of coals (composition and pore characteristics) and discussed the influence of the fractal dimensions on CH4 adsorption. The results show that according to LP-N2A isotherms, all coals can be classified into three types. The micropores provide the largest proportion of the specific surface area (SSA) of coals. Two fractal dimensions, Da (adsorption pore) and Ds (seepage pore), ranged from 2.471 to 2.805 and from 2.812 to 2.976, which were acquired in the saturated water condition by NMR. Furthermore, Da and Ds have different correlations with ash yield, carbon contents, moisture, SSA and irreducible fluid porosity. The coal composition and pore parameters have much greater control over fractal dimensions. Moreover, the different fractal dimensions have different influences on methane adsorption. With the increase of Da, the methane adsorption capacity is enhanced, but it is weakened with the increase of Ds. The high-rank coals have more SSA with higher Da and provide more adsorption sites for CH4. Langmuir pressure PL has different correlations with fractal dimensions. Da decreases with the increase of PL. The adsorption velocity is faster with higher Da. Thus, the fractal dimensions are the comprehensive reflection of differences among the physical properties of coal and are able to show the effect of coal properties on methane adsorption fully.

Computer-aided design of high-connectivity covalent organic frameworks as CH4/H2 adsorption and separation media

Six novel borophosphonate cube (–B4P4O12–) based covalent organic frameworks (BP-COFs) with high-connectivity have been computationally designed and proposed as CH4/H2 adsorption and separation media. The structural characterization reflects that six BP-COFs own high porosity, low density, applicable pore size, large pore volume and accessible surface area which are beneficial to gas adsorption. The adsorption isotherms for H2 at 77 K and 298 K and for CH4 at 298 K were obtained with grand canonical Monte Carlo (GCMC) simulations. The results reveal that BP-COF-10, -11 and −12 possess the higher CH4 and H2 adsorption capacity versus BP-COF-7, -8 and -9. The CH4/H2 adsorption separation simulation indicated that BP-COF-7, -8 and -9 owns the better CH4/H2 selectivity than BP-COF-10, -11 and −12 at 298 K. It is excited that both CH4/H2 adsorption capacity and selectivity of six BP-COFs are comparable to porous materials owing excellent gas adsorption and separation capacity. We expect this study may motivate researchers’ efforts to develop new high-performance gas adsorption/separation material.

Adsorption of methane and carbon dioxide by water-saturated clay minerals and clay rocks

Understanding the effects of water on gas adsorption in geological media is of high importance in order to efficiently control numerous subsurface engineering process operating at gas/rock interfaces. Due to preferential interaction with clay surfaces, water fills their porous body, greatly reducing CH4 and CO2 adsorption capacity. In order to quantitatively describe CH4 and CO2 adsorption by hydrated clay minerals, this work proposes to rely on the mechanism of gas uptake by dissolution in pre-adsorbed pore water. This approach was employed to characterise water-saturated porous media of increasing complexity: mesoporous silica SBA-15 and silica gel with different pore sizes and geometry, isolated illite and montmorillonite and natural clay-rich rock (the Callovo-Oxfordian formation – COx, France) in powdered and crushed states. It was found that the solubility in water can reliably explain the CO2 uptakes by hydrated pore systems regardless of their nature as well as the CH4 uptakes, but only for solids with large mesopores and montmorillonite mineral. A so called “adsorption enhanced gas uptake in pore water” for CH4, exceeding its solubility in bulk water by a factor of 5–8, was observed for the systems with narrow pore sizes, highlighting the impact of surface energy on gas uptake and the occurrence of the interaction of weakly-soluble methane with the surface, promoting its uptake in comparison to pure dissolution.

Molecular Simulation on Adsorption Difference of Water and Methane on Long-Flame Coal

The adsorption difference of water and methane by long-flame coal is compared and analyzed by molecular simulation. The results show that when the single-component isothermal adsorption pressure is less than 15 MPa, the adsorption capacity of CH4 and H2O increases gradually with the increase of the adsorption pressure, and the average adsorption amount of H2O is much higher than that of CH4; after more than 15 MPa, the average adsorption amount of CH4 and H2O is saturated. When the two-component isothermal adsorption pressure is less than 0.7595 MPa, the long-flame coal molecule can adsorb both CH4 and H2O; after more than 0.7595 MPa, the average adsorption amount of H2O is much larger than that of CH4 indicating that H2O is in a dominant position in the competitive adsorption with CH4.

Efficient separation of CO2 from CH4 and N2 in an ultra-stable microporous metal-organic framework.

The efficient separation of CO2 from CH4 and N2 is essential for the upgrading of biogas and reducing carbon emissions in flue gas, but is challenging in the energy industry. Developing ultra-stable adsorbents with high CO2 adsorption performance in adsorption separation technology is deemed an effective solution for the separation of CO2/CH4 and CO2/N2. Herein, we report an ultra-stable yttrium-based microporous metal-organic framework (Y-bptc) used for the efficient separation of CO2/CH4 and CO2/N2. The single-component equilibrium adsorption capacity of CO2 reached 55.1 cm3 g-1 at 1 bar and 298 K, while the adsorption capacity of CH4 and N2 was almost negligible and thus resulted in a high adsorption ratio for CO2/CH4 (45.5) and CO2/N2 (18.1). GCMC simulations revealed that the μ3-OH- functional groups distributed in the pore cage of Y-bptc provide stronger adsorption sites for CO2via hydrogen-bonding interactions. The relatively lower heat of adsorption of CO2 (24 kJ mol-1) further reduces the desorption regeneration energy consumption. Dynamic breakthrough experiments using Y-bptc for the separation of CO2/CH4 (1/1) and CO2/N2 (1/4) mixtures could obtain high purity (>99%) CH4 and N2, and the CO2 dynamic adsorption capacities reached 52 and 31 cm3 g-1, respectively. More importantly, the structure of Y-bptc remained intact under hydrothermal conditions. The high adsorption ratio, low heat of adsorption, great dynamic separation performance, and ultra-stable structure of Y-bptc make it one of the candidate adsorbents suitable for the separation of CO2/CH4 and CO2/N2 in real-world applications.

Occurrence of moisture in deep gas-bearing shale matrix and its impacts on methane adsorption/desorption capability under favorable reservoir conditions.

To comprehend impacts of moisture on exploring and producing shale gas, the rules of pseudo-in situ moisture occurrence in deep shales were revealed through low-pressure N2 adsorption and desorption, and CO2 adsorption measurements. The influences of pseudo-in situ moisture on CH4 adsorption/desorption in the shales were explored at 353.15 K and pressures up to 30 MPa by using the volumetric method. Results showed that the pseudo-in situ moisture content of the shales ranges between 0.57% and 0.94%, which positively correlates with clay mineral content but negatively correlates with organic matter and quartz. The clay minerals contribute more to moisture occurrence mainly via adsorption effect. The pores with the diameters of 1.10-4.10 nm of the shales serve as dominant space for accommodating moisture. Moreover, the pseudo-in situ moisture reduces saturated adsorption capacity and isosteric adsorption heat of CH4 on the shales, suggesting the weakened adsorption affinity toward CH4-shale system. Typically, the minor pseudo-in situ moisture could significantly weaken CH4 adsorption capability of the shales with low clay mineral content through blocking pore throats of organic matter-hosted pores. However, the abundant pseudo-in situ moisture only slightly reduces CH4 adsorption capability of the shales with high clay mineral content due to continuous distribution of organic matter-hosted pores. The aforementioned different roles are dominated by the difference in occurrence characteristics of organic matter-hosted pores and clay mineral-hosted pores between the shales with low clay mineral content and the shales with high clay mineral content. Furthermore, the pseudo-in situ moisture strengthens CH4 adsorption/desorption hysteresis on the shales associated with moisture uptake-induced clay mineral swelling, thereby raising difficulty for CH4 desorption from the shales.

The effect of organic matter fraction extracted on micropores development degree and CH4 adsorption capacity of coal

The percentage of micropores in the pore structure of coal plays a significant role in methane (CH4) adsorption, whereas the fraction of organic matter in coal has a positive effect on CH4 adsorption. Ultrasonic waves were used to excite mixtures of organic solvents and coals with different degrees of deterioration. This process facilitates the induction of surface organic matter in coal and the shedding of non-closed-ring side chains in the main structure of coal and the opening of closed rings. The samples extracted by organic solvents were subjected to electron microscopy detection, elemental analysis, and functional group analysis. The porosity of the samples showed distinct changes, and the sample with the best organic solvent extraction effect showed a sharp decrease in organic main carbon and a change in the distribution of partial functional group peaks. Through the experimental results of CH4, N2, and CO2 adsorption, the physical adsorption theory and density flooding theory with micropore filling theory were combined. It was found that the CH4 adsorption decreased, macro-and mesopore channels developed, and the percentage of micropores decreased. Overall, the filling effect caused by microchemistry had a significant influence on the macroscopic physical adsorption. Our research results provide theoretical guidance and a reference for the efficient exploitation of coalbed methane and mine gas control.

Potential effect of carbon dioxide injection on the functional groups of medium volatile bituminous coals analysed using in-situ diffuse reflectance Fourier-transform infrared spectroscopy

CO2 sequestration with enhanced coalbed methane recovery (CO2–ECBM) is a promising technology for mitigating CO2 emissions. To implement CO2–ECBM, complex interactions must be elucidated between CO2 and coal during CO2 sequestration, but the mechanism through which coal and CO2 interact remains controversial mainly because coal contains a complex macromolecular structure. Therefore, we used in-situ high-pressure diffuse reflectance Fourier-transform infrared spectroscopy to investigate the potential impact of the CO2 injection on the functional groups of medium volatile bituminous (MVB) coals. The results of this study showed that CO2 physisorption could be found near the MVB coal macromolecular CH bonds, and the interactions between the MVB coal macromolecular CH bonds and CO2 kept strengthening with increasing CO2 injection pressure. Injecting CO2 into the MVB coal disrupted the weak interactions between aromatic layers, and supercritical CO2 (ScCO2) could disrupt the physical association between small molecular compounds and the coal macromolecular structure, which extracted hydrocarbon and oxygen-containing compounds, and thus reduced the content of oxygen-containing functional groups and -CH2- in the MVB coal. The coal macromolecular CO bonds were not conducive to methane adsorption on the coal surfaces, injecting high-pressure CO2 (P = 7 MPa, 8 MPa) into the MVB coal matrix irreversibly damaged the coal macromolecular CO bonds, which enhanced the CH4 adsorption capacity and was not conducive to CH4 desorption. The results of this study showed that injecting ScCO2 into the MVB coal could change the coal macromolecular structure and reduce the content of oxygen-containing functional groups and -CH2-, thus affecting the properties of MVB coal reservoir, such as porosity, permeability, and mechanical characteristics, which could advance knowledge of the theoretical usefulness for CO2-ECBM recovery and sequestration in the MVB coal reservoirs.

Preparation of molded biomass carbon from coffee grounds and its CH4/N2 separation performance

Coffee grounds are promising precursors for excellent porous carbon adsorbents. During the preparation of the porous carbons, sodium silicate was used as a binder and pore-forming agent, and extrusion molding technology was used to prepare them in a columnar form. After carbonization, steam activation and silica removal by alkaline washing, high-strength columnar porous carbon adsorbents (CGCs) were obtained. Their CH4/N2 separation performance was studied by multicomponent breakthrough experiments. The Brunauer-Emmett-Teller (BET) surface area of CGC-1.5 (where 1.5 is the mass ratio of a 9 wt% sodium silicate aqueous solution to the coffee grounds) is 527 m2·g-1. Both the N2 and CO2 adsorption isotherms show that the CGCs are rich in micropores and mesopores, with the micropores size mainly centered at about 0.48 nm. FT-IR results show that CGC-1.5 has abundant oxygen-containing functional groups. At 298 K and 0.1 MPa, its equilibrium adsorption capacity for CH4 is 0.87 mmol·g-1, and the separation selectivity for a CH4/N2 mixture (3/7, vol/vol) is 10.3, which is better than most biomass-based porous carbon adsorbents and crystalline materials. Dynamic breakthrough tests show that CGC-1.5 has an excellent CH4/N2 separation performance at both high and atmospheric pressures. The dynamic selectivities at 298 K, 0.11 and 0.5 MPa are 10.4 and 17.9, respectively. The adsorption capacity is unchanged after 10 adsorption-desorption cycles. The mechanical strength of CGC-1.5 is as high as 123 N·cm-1, which meets the criteria of industrial applications.

Effect of dynamic fragmentation on microscopic pore structure in coal: New insights into CH4 adsorption characteristics

Tectonic coal is a necessary condition for coal and gas outburst, and its pore structure is dramatically altered. The pore structure of tectonic coal has unique characteristics in terms of mechanics and gas flow, which plays a critical role in eliminating outburst risk. Therefore, it is important to study the effects of dynamic fragmentation on microscopic pore structure and CH4 adsorption characteristics of coal body. In this study, coal samples from Xintian (XT) and Pingdingshan No.6 (P6) coal mines were crushed and sieved into three particle size fractions of 0.5–1 mm, 0.2–0.25 mm, 0.074–0.1 mm, respectively, simulating dynamic fragmentation by reducing the particle size. A novel index that evaluates the average single-particle equivalent pore length was proposed. This index, Lp, provides a more comprehensive quantitative characterization of the effects of dynamic fragmentation on microscopic pore structure by considering the dynamic particle size reduction process. The LP values of large-sized samples are 23.8 to 24.6 times that of medium-sized samples; the LP values of medium-sized samples are 12.8 to 17.1 times that of small-sized samples. The experimental results show that dynamic fragmentation significantly alters microscopic pore structure, as demonstrated by the positive correlation between the Lp and particle size. With the decrease in particle size, the amount of CH4 molecules adsorbed in XT samples increases from 105.870 × 1019 p/g to 106.934 × 1019 p/g, increasing by 1.0 %; that in P6 samples increases from 39.660 × 1019 p/g to 48.037 × 1019 p/g, rising by 21.1 %. Dynamic fragmentation has a relatively insignificant effect on CH4 adsorption capacity. The total adsorption equilibrium time for XT samples decreases from 242 h to 32 h, and that for P6 samples decreases from 1146 h to 100 h as the particle size decreases. Dynamic fragmentation significantly reduces CH4 adsorption equilibrium time. The results of this study contribute to the understanding of microscopic pore structure under the effects of dynamic fragmentation and new insights into CH4 adsorption characteristics, which are of great guiding significance to the occurrence mechanism of outburst prediction and risk.

Experimental study on the influence of gas pressure on CH4 adsorption-desorption-seepage and deformation characteristics of coal in the whole process under triaxial stress

CH4 adsorption-desorption-seepage characteristics of coal is an important indicator to measure the gas extraction capacity of coal seam. The deformation of coal caused by CH4 adsorption-desorption-seepage is an important factor affecting the extraction ability of CH4. Synchronous test experiments of CH4 adsorption-desorption-seepage and whole-process coal deformation under different gas pressures were carried out by using the self-developed comprehensive test experimental system of gas-solid coupling for coal measure gas. The timeliness characteristics of CH4 adsorption-desorption-seepage and coal deformation were analyzed. The influence of gas pressure on CH4 adsorption-desorption-seepage and coal deformation was clarified. The quantitative relationship between CH4 adsorption-desorption-seepage and coal deformation was discussed. A whole-process strain model of coal for CH4 adsorption-desorption-seepage was established. The results showed that increasing gas pressure could promote CH4 adsorption-desorption-seepage of coal. The higher the gas pressure was, the greater the capacity of CH4 adsorption-desorption-seepage in coal was. Increasing gas pressure could promote the expansion/contraction deformation caused by CH4 adsorption-desorption-seepage in coal. The higher the gas pressure was, the stronger the deformation ability of CH4 adsorption-desorption-seepage in coal was. The expansion or contraction deformation caused by CH4 adsorption-desorption-seepage in coal was simultaneously affected by coal bedding structure and external stress. The axial/circumferential strain caused by CH4 adsorption-desorption-seepage in coal showed exponential function, quadratic function, and exponential function growth relationship with adsorption capacity, desorption capacity, and seepage capacity of CH4, respectively. The stress in the process of CH4 adsorption-desorption-seepage in coal covered the expansion stress of adsorbed CH4 to coal matrix, the compression stress of free CH4 to coal matrix, and the expansion stress of free CH4 to coal microporous fracture. Considering three kinds of deformation effects by CH4 in two states, the whole process strain model of CH4 adsorption-desorption-seepage in coal was established, which could effectively reflect the deformation timeliness characteristics of the whole process of CH4 adsorption-desorption-seepage in coal. This research provides some theoretical support for the efficient extraction of coal seam gas.

Synthesis of carbon energized materials with directed regulation of specific surface and pore structure as potential adsorbent for methane mitigation

In this study, nanoporous carbon material (NCM) specimens were synthesized from polymer raw materials (a mixture of furfural, hydroquinone, and urotropine). The influence of the conditions and parameters of the synthesis method (temperature, weight ratio of the activating agent to the precursor and contact time) on the NCM structure was examined. The effect of the weight ratio of the initial components on the properties of the resulting NCM was investigated, and the optimal composition of the ternary system of furfural, hydroquinone and urotropine was found. The results of nitrogen adsorption-desorption showed that NCM specimens obtained by varying the activation temperature were microporous but with different micropore volumes and the same pore size distribution. Varying the weight ratio of the activating agent (KOH) to the precursor made it possible to obtain microporous, micromesoporous, and mesoporous NCMs. NCM specimens with different pore hierarchy and different specific surface area were also obtained, including microporous NCMs (SBET: 1546–3030 m3/g; Vmic/Vt: 76.0–100%), micro- mesoporous NCMs (SBET: 3204–3516 m3/g; Vmic/Vt: 41.2–57.4% Vmes/Vt: 42.6–58.8%) and mesoporous NCMs (SBET: 3641–3649 m3/g; Vmes/Vt: 74.0–83.8%). Further, a mechanism for the development of pores during KOH activation of polymeric carbon precursor was proposed. The outcomes of this study signify that the NCM5/750/1 has a high adsorption capacity for CH4 and has good prospects for monitoring CH4 emissions and reducing greenhouse gas emissions.

Investigation into the pore structures and CH4 adsorption capacities of clay minerals in coal reservoirs in the Yangquan Mining District, North China

Abstract The rising energy demands worldwide and difficulty in developing novel clean energy sources have greatly stimulated the exploitation of coalbed methane. Clay minerals are common fractions of coal; thus, understanding their CH4 adsorption capacities and pore structures is vital. In this study, coal, parting, roof, and floor samples were collected from the Yangquan Mining District. The mineral compositions, CH4 adsorption capacities, and pore structures of the samples were analyzed using X-ray diffraction, the CH4 isothermal adsorption method, and the low-temperature N2 adsorption method, respectively. The results indicated that organic matter had a much higher CH4 adsorption capacity (33.80 cm3/g, 35°C) than that of clay minerals. The CH4 adsorption capacities of various clay minerals are significantly different, with smectite (18.01 cm3/g), kaolinite (5.81 cm3/g), mixed-layer illite-smectite (4.47 cm3/g), and illite (2.08 cm3/g) present in decreasing order. The pore sizes of the samples consisted of sizes <6 nm, and six pore size groups (Groups 1–6) were identified in the PSD patterns. These pore size groups were associated with different clay minerals. We propose that the CH4 adsorption capacities of clay minerals are mainly influenced by their pore structures, which are in turn associated with their species and formation processes. Furthermore, the conversion of kaolinite to illite, and the illitization of mixed-layer smectite-illite, exerted a negative effect on their CH4 adsorption capacities.

The impact of humic acid on hydrogen adsorptive capacity of eagle ford shale: Implications for underground hydrogen storage

Hydrogen is a clean fuel that is anticipated to play a key role to phase-out fossil fuels and consequently decline the global warming challenges. Large-scale Underground Hydrogen Storage (UHS) is a critical link in hydrogen economy, but it is yet to be successfully achieved at industrial scale, and this is presently a main drawback in the hydrogen economy. Organic-rich shale pores have been suggested as promising geo-storage media for underground storage of hydrogen through the adsorption trapping mechanism. The hydrogen (H2) could be stored as an adsorbed phase on the kerogen surface or clay minerals, thus replacing methane (CH4) on adsorption sites during competitive adsorption. Moreover, methane has been suggested as a promising cushion gas for hydrogen. Experimental measurement of the H2 and CH4 adsorptive capacity of shale is essential for precise description of competitive adsorption behavior between CH4 and H2 in shale, as well as in designing of UHS in shale. Thus, adsorption capacities of methane and hydrogen were measured using a PCTpro adsorption analyzer in this study. Since organic acids are inherently present in geo-storage media, H2 and CH4 adsorption capacities in the fresh shale and shale aged with humic acid (Shale-HA) were compared to evaluate the impacts of adsorbed organic acids on their gas adsorption affinities. Our results showed that at 303 K, the adsorption of hydrogen increased by almost 170 % at 2.5 MPa and 350 % at 4.28 MPa in Shale-HA. The H2 adsorption trends on both shale samples are type IV isotherm adsorption curves that increased steeply with pressure but levelled beyond 3.5 MPa. The low-pressure N2 adsorption-desorption isotherms showed a type II adsorption isotherm. The CH4 adsorption capacity in raw shale was uniformly increased more than that of hydrogen, but there is no significant difference in CH4 adsorption capacity in shale-HA. This work provides fundamental data for hydrogen storage in shale reservoirs, thus facilitates full-scale implementation of hydrogen economy.

Binderless zeolite LTA beads with hierarchical porosity for selective CO2 adsorption in biogas upgrading

In the context of CO2 removal from biogas, a series of binderless zeolite LTA adsorbents with a macroscopic bead format (0.5–1.0 mm) and with hierarchical porosity (i.e. with the zeolitic micropores being accessible through meso- and macropores mainly in the 10–100 nm range) was synthesized with a variety of Si/Al ratios (1.2–3.9) using Amberlite IRA-900 anion-exchange resin beads as a hard template. The CO2 and CH4 adsorption capacity of the beads in Na-form with different Si/Al ratios were measured, reaching higher CO2/CH4 selectivity and similar, yet slightly higher CO2 adsorption compared to commercial zeolite LTA pellets containing a binder. Subsequently, one the zeolitic beads was subjected to different degrees of ion-exchange (0–96%) with KCl and then tested in the adsorption of CO2 and CH4. The best performance among all the ion-exchanged beads was achieved with Na58K42-LTA beads, which gave very high CO2/CH4 selectivity (1540). Although essentially no CH4 was adsorbed on these beads, the CO2 adsorption capacity was still substantial (1.9 mmol g−1 at 0.4 bar CO2, i.e. the partial pressure of CO2 in biogas).

Effect of Porous Carbons’ Intrinsic Parameters on the Pressure Swing Adsorption of CO2 from Biogas

The current study investigates the use of six commercially available activated carbons and one carbon aerogel for the removal of CO2 from biogas during biogas upgrading using pressure swing adsorption. Pure component CH4 isotherms were determined gravimetrically at temperatures of 10, 30, 50, 70, and 90 °C and pressures up to 6.5 atm. This data was used to investigate the effect of intrinsic parameters of the carbons such as pore size, surface area, %oxidation, and ash content. In the high-pressure region, pore size in the range of <1 nm, <2 nm, and <3 nm significantly correlates to the CH4 adsorption capacities. In the low-pressure region, the heterogeneous nature of the carbons dominates adsorption. Comparing CO2 data to CH4 data, carbons having a smaller pore size distribution, a higher %oxidation, and a higher %ash content are all beneficial for the separation of CO2 from biogas. In this study, NZ-AC, M-30, and ACC have these favorable intrinsic properties along with large estimated working capacities, indicating them as prospective porous carbons for separating CO2 from biogas.

Effects of pore structure changes on the CH4 adsorption capacity of coal during CO2-ECBM

The effects of supercritical CO2 (scCO2) on pores in coal play a critical role in determining the CH4 adsorption and desorption characteristics and CO2 capacity of coal seams during CO2 geological storage-enhanced coalbed methane recovery (CO2-ECBM). To investigate the effects of pore structure changes caused by scCO2 on the CH4 adsorption capacity of coals, six coal samples with different ranks are exposed to scCO2 and water for 240 h at 62.5 °C and 15 MPa using a scCO2 geochemical reactor. Then, gravimetric isothermal adsorption tests, mercury intrusion porosimetry, N2 adsorption tests, and CO2 adsorption tests are performed to determine the CH4 adsorption capacity, pore volume, and pore specific surface area (SSA) of the coal samples with and without scCO2-H2O treatment. Finally, the influences of scCO2 on the pore structure and the effects of pore structure changes on the CH4 adsorption capacity of the coal samples are analyzed. The results show that the CH4 adsorption capacity of coal and its changes after scCO2-H2O treatment are strongly affected by density effects and pore selection effects. The CH4 adsorption capacity of coal is determined by the micropore volume and SSA; however, their changes only affect the CH4 adsorption capacity in the micropore filling stage when the pressure is <5 MPa with weak effects on the total CH4 adsorption capacity, which is related to their weak changes after scCO2-H2O treatment compared with macropores and mesopores. The changes in CH4 adsorption capacity mainly depend on the changes in macropore and mesopore SSAs after scCO2-H2O treatment; scCO2 mainly changes the CH4 adsorption capacity in the transitional stage, when the pressure is 5–10 MPa, and the monolayer and multilayer adsorption stage, when the pressure is >10 MPa. Meanwhile, the effect of pore structure changes on the CH4 adsorption capacity of coal with different ranks is complex. The CH4 adsorption capacity of subbituminous coal and medium-volatile bituminous coal increases after scCO2-H2O treatment due to the increases in macropore SSA, that of high-volatile bituminous coal slightly decreases, and that of semianthracite coal and anthracite coal slightly increases because of the changes in mesopore SSA after scCO2-H2O treatment. However, the CH4 adsorption capacity of low-volatile bituminous coal increases, which is the result of changes in the SSA of the mesopores and macropores due to slight changes in the micropore volume and SSA.

Coupled adsorption-hydro-thermo-mechanical-chemical modeling for CO2 sequestration and well production during CO2-ECBM

Injecting CO2 into the methane-bearing coal seams can not only store CO2 but improve coalbed methane recovery. In this paper, a fully coupled adsorption-hydro-thermo-mechanical-chemical (AHTMC) model for CO2 enhanced coalbed methane (CO2-ECBM) is constructed, including the multiple fields with the coupling relationships of competitive adsorption of CO2 and CH4 in coal matrix (A), gas-water two-phase flow and gas diffusion in formation water (H), heat transfer and conversion (T), coal deformation (M), geochemical reactions of various anions and cations and the mineral dissolution in formation water (C). The model is verified by cored coal sample experiments in the laboratory and applied to simulate a CO2-ECBM pilot project in Qinshui Basin with different CO2 injection rates. The simulation results show that CH4 adsorption capacity decreases and CH4 production rate increases significantly with CO2 injection. When CO2 breaks through production well, the CO2 storage rate decreases and the pH in drainage decreases significantly. The CO2 injection rate has a significant influence on CO2-ECBM. Importantly, 25 t/d of CO2 injection is available for the pilot project with a CH4 recovery growth rate of 14.89% and a CO2 storage ratio exceeding 96% in a CO2 injection duration of 8000d.