CH4 Diffusion Research Articles

Molecular simulation of CH4 adsorption characteristics in bituminous coal after different functional group fractures

To explore the effect of different functional group fractures in coal on the adsorption characteristics of CH4, molecular models of bituminous coal (Wiser) with fractured oxygen-containing functional groups (W–O5), sulfur-containing functional groups (W–S5), and nitrogen-containing functional groups (W–N5) were constructed. The microscopic mechanism by which the fracture of different functional groups in bituminous coal molecules affects CH4 adsorption was analyzed using Density Functional Theory (DFT), Molecular Dynamics (MD), and Grand Canonical Monte Carlo (GCMC) methods. The results show that functional groups facilitated the polarization of CH4 during the adsorption process, with the following influence relationship: oxygen-containing functional groups > sulfur-containing functional groups > nitrogen-containing functional groups; Fracturing functional groups in bituminous coal molecules weakened the adsorption capacity and reduced adsorption sites of CH4, the sequence of CH4 adsorption was Wiser > W–N5 > W–S5 > W–O5. High temperature reduced the effect of nitrogen-containing functional group fractures on CH4 adsorption, but had no obvious effect on sulfur-containing and oxygen-containing functional groups. The interaction between CH4 and the coal molecule decreased after functional group fractures, causing the enhanced diffusion of CH4 on bituminous coal molecules, and the order was W–O5 > W–S5 > W–N5 > Wiser. Additionally, the fracture of functional groups decreased the pore size of the bituminous coal molecules, which inhibited CH4 adsorption but facilitated its diffusion. The research provides theoretical support for enhancing CH4 desorption and improving the recovery rate of coalbed CH4.

Molecular insights on influence of CO2 on CH4 adsorption and diffusion behaviour in coal under ultrasonic excitation

Decoupling the molecular structure from the pore structure is challenging under experimental conditions; this hinders the independent investigation of the effect of the changes in molecular structure on methane (CH4) transport. In this study, the molecular model of coal was reconstructed based on the test results of X-ray photoelectron spectroscopy (XPS) and 13C nuclear magnetic resonance (13C NMR). A grand canonical Monte Carlo (GCMC) based molecular dynamics (MD) simulation method was used to independently investigate the effect of CO2 on CH4 adsorption and diffusion behaviour in coal under changes in the molecular structure of coal after ultrasonic excitation. The experimental results show that, under ultrasonic excitation, the proportion of oxygen content in the coal increases and the oxygen-containing functional groups (carboxyl groups, ether bonds/hydroxyl groups) increase compared to the original coal. The simulation results show that the amount of CH4 adsorbed, the isosteric heat and the proportion of low-energy CH4 in the coal sample decrease after ultrasonic excitation for the same pore size. Furthermore, for the same pore size and CO2 partial pressure ratio, adsorption selectivity factor increases after ultrasonic treatment. Additionally, for the same pore size and the same CO2 partial pressure ratio, the interaction energy between CH4 and coal in the CH4–CO2–coal mixed system decreases and the density of high-energy CH4 increases after ultrasonic excitation; this leads to an increase in CH4 self-diffusion and transport diffusion coefficient. The presence of oxygen-containing functional groups causes a significant increase in the electrostatic potential difference between CO2 and coal as compared to that between CH4 and coal, which is the fundamental reason for the change in the effect of CO2 on the adsorption-diffusion behaviours of CH4. Our findings revealed the micro-mechanism of ultrasonic-assisted coalbed methane extraction and demonstrated the effectiveness of ultrasonic-CO2 collaborative technology.

A theoretical study on the separation of CO2/CH4 through MFI zeolite

The separation of carbon dioxide (CO2) and methane (CH4) is important in addressing global warming and energy shortages. Porous materials play a large role, of which MFI zeolite has received much attention. To better analyse mechanisms or guide experiments, the adsorption and diffusion properties of CO2 and CH4 in MFI zeolite have been studied by Monte Carlo and molecular dynamics simulations. Due to stronger interactions, the loading of pure CO2 is significantly higher than that of CH4. For mixtures (biogas or natural gas), CO2 dominates in competitive adsorption. Lower temperatures and higher pressures correspond to higher adsorption selectivity. In addition, the self-diffusion coefficients of guest molecules in MFI zeolite were also calculated. The diffusion performance of pure CH4 is superior to CO2 under the same conditions. And their self-diffusion coefficient increases with increasing temperature and decreasing load. For mixed components, CO2 greatly limits the diffusion of CH4.

First Calibrated Methane Bubble Wintertime Observations in the Siberian Arctic Seas: Selected Results from the Fast Ice

This paper presents the results of an acoustic survey carried out from the fast ice in the shallow waters of the East Siberian Arctic Shelf (ESAS) using a single beam echosounder. The aim of this paper is to demonstrate an improved approach to study seafloor seepages in the Arctic coastal zone with an echosounder calibrated on site. During wintertime field observations of natural rising gas bubbles, we recorded three periods of their increased activity with a total of 63 short-term ejections of bubbles from the seabed. This study presents quantitative estimates of the methane (CH4) flux obtained in wintertime at two levels of the water column: (1) at the bottom/water interface and (2) at the water/sea ice interface. In winter, the flux of CH4 transported by rising bubbles to the bottom water in the shallow part of the ESAS was estimated at ~19 g·m−2 per day, while the flux reaching the water/sea ice interface was calculated as ~15 g·m−2 per day taking into account the diffusion of CH4 in the surrounding water and the enrichment of rising bubbles with nitrogen and oxygen. We suggest that this bubble-transported CH4 flux reaching the water /sea ice interface can be emitted into the atmosphere through numerous ice trenches, leads, and polynyas. This CH4 ebullition value detected at the water/sea ice interface is in the mid high range of CH4 ebullition value estimated for the entire ESAS, and two orders higher than the upper range of CH4 ebullition from the northern thermocarst lakes, which are considered as a significant source to the atmospheric methane budget.

Influence of uniaxial strain loading on the adsorption-diffusion properties of binary components of CH4/CO2 in micropores of bituminous coal by macromolecular simulation

Heterogeneous and anisotropic coal tends to deform under strong mining disturbances, affecting the microscopic adsorption and diffusion of binary CH4-CO2 mixtures in micropores. Therefore, the grand canonical Monte Carlo and molecular dynamics methods were used to study the influence of the uniaxial strain loading on the adsorption-diffusion properties of binary components of CH4/CO2 in coal micropores. The results show that compression strain has significant effects on adsorption selectivity and thermodynamic factor compared to tension strain, and CH4 displacement efficiency is greater when the strain is less than −0.12. Both compression and tension strains have dual effects on promotion and inhibition of volumetric swelling of the gas-containing coal matrix, micropore development and gas diffusion. Furthermore, the diffusivity of CH4 is significantly larger than that of CO2, and Einstein diffusion is dominated by low-velocity motion. Results provide an important basis for the optimal coalbed methane recovery design in deformed coal reservoirs.

Confinement effect on transport diffusivity of adsorbed CO2–CH4 mixture in coal nanopores for CO2 sequestration and enhanced CH4 recovery

The transport of CO2–CH4 mixture in coal nanopores is crucial to CO2-enhanced coalbed (CO2-ECBM) recovery, but is highly challenging on adsorption-diffusion dominated nanoscale due to the gas-surface and intermolecular interactions. Using Monte Carlo and molecular dynamics simulation, the confinement effect of nanopores with two slit widths (2 and 6 nm) on the CO2–CH4 binary diffusion in the adsorption layer are investigated. A modified computational method for multi-component transport diffusivity in the adsorption layer is proposed by incorporating the survival probability function. The results show that the transport diffusivity in the semi-confined adsorption layer shows a general decreasing tendency with the increase of pressure, while it slightly decreases when the pressure <6.4 MPa in the confined adsorption layer, following an inverse parabola trend with the maximum value at around 12 MPa. Three mechanisms are classified regarding this pressure-dependent evolution. Firstly, the collision between molecules of weaker attraction and surface dominates the transport capacity, and this collisional frequency reduces at the low-pressure region. Accompanying the pressure exceeds the critical threshold, surface diffusion becomes the primary contribution. Lastly, significant intermolecular interactions appear after the adsorption saturation. During the CO2-ECBM process, pressure drawdown is the primary method to release the adsorbed CH4, and the largest recovery ratio can reach 25.9% in 2 nm micropore and 30.1% in 6 nm mesopore. After the early CO2 injection, the transport diffusion of CH4 exceeds that of CO2, and the selectivity of CH4 over CO2 increases in the subsequent CO2 huff-n-puff cycles. The CO2–CH4 mixture under greater confinement effect manifests higher diffusion selectivity, which benefits the separation of the CO2–CH4 mixture and further contributes to the CH4 recovery.

Modes of multi-mechanistic gas diffusion in shale matrix at varied effective stresses: Observations and analysis

Gas diffusion in the shale matrix has a dominant effect on late-stage production from shale gas reservoirs. However, adequate research on the mechanisms and contributions of gas diffusion for varied pore size populations in shale matrix under recreated in situ stress is lacking. We report gas-diffusion measurements under constant in situ stress but variable gas pressures for contrasting non-adsorbent (helium (He)) and adsorbed (methane (CH4)) gases to investigate the impact of effective stress on the evolution of dominant mechanisms of diffusion. An intact sample replicates true pore-network topology and diffusion paths. An integrated diffusion model is proposed that combines the effects of slip flow, Knudsen flow, and surface diffusion to constrain the evolution of these flow regimes and their respective contributions to the observational data. Finally, a probability density function (PDF) is employed to separate the gas content distributions of macropores and micropores from the total gas content and to investigate gas contributions in various pores. The results reveal that the diffusion coefficients of both He and CH4 in macropores and micropores increase with gas pressure but decrease with increasing effective stress. The diffusion coefficients of He and CH4 are different in macropores but remain nearly the same in micropores. The diffusion coefficients of slip flow and surface diffusion increase with decreasing effective stress except for CH4 diffusion in the micropores, while the evolution of Knudsen diffusion shows the opposite trend. Slip flow plays a dominant role in He and CH4 diffusion within macropores (pore size 45 nm). Knudsen diffusion gradually becomes significant for He diffusion in the micropores (pore size 4 nm), conversely, for CH4 diffusion in the micropores, surface diffusion becomes significant. Related to gas production from reservoirs, the contributions of the micropores will increase gradually with the duration of gas recovery, indicating the significant role of gas diffusion in micropores to steady supply during late-stage production.

A Numerical Study of CO2 Decoupled Effects on Extinction Limit and Flame Microstructure in CH4/Air Counterflow Diffusion Flame with Various Pressures

Extinction limits and flame microstructures of CH4 versus air (N2/O2) counterflow diffusion flames with additional CO2 to oxidizer stream under various pressure conditions were numerically investigated by a proposed decoupled strategy. The chemical and thermal effects of CO2 addition were isolated from the synergistic effect on extinction limits and flame microstructures. The results show that the extinction limits were decreased due to the CO2 addition and extended caused by the enhanced pressure, respectively. The changes of flame microstructures from far away from extinction to near extinction mainly are reflected in a narrower combustion zone and thinner flame thickness, while there is no visible displacement of the flame front location. Quantitative analyses manifest the thermal effect of CO2 addition on extinction limit is greater than chemical effect, and elevated pressure can enhance both thermal and chemical effects. An interesting phenomenon occurred between the thermal and chemical effects resulting from increasing CO2 mole fraction in an oxidizer on extinction limit; the former monotonically increase along with CO2 mole fraction, while the latter represents a kind of approximate quadratic-parabola tend along with CO2 mole fraction. The distributions of the production and consumption rate of hydroxyl (OH) radicals were presented to clarify the contribution of the kinetic term in the asymptotic solution to the extinction limits with increasing CO2 molar fraction and various pressures.

Carbon nanofibers with gas selective layer containing rich and accessible ultramicropores for methane/nitrogen separation

The adsorption separation and purification of coalbed methane (CH4) has attracted global interest as an important technique for the utilization of unconventional natural gas. The success for this method relies on the development of adsorbent materials that can selectively enrich CH4. Toward this goal, we report a facile in-situ growth method to prepare carbon nanofibers with ultramicroporous gas selective layer as adsorbent for CH4/N2 separation. The thickness of gas selective layer can be precisely tuned from 90 nm to 525 nm. The optimal carbon nanofibers exhibited a high CH4/N2 selectivity of 6.8 with the CH4 uptake reaching 0.97 mmol g−1 as well as two orders of magnitude faster CH4 diffusion kinetics than the commercial activated carbon adsorbent. The high accessibility to the adsorption sites and short diffusion path of the nanofibers facilitate the rapid mass transfer and intensify the dynamic CH4/N2 separation. Pressure swing adsorption simulations indicated the recovery and productivity over the carbon nanofibers with 90 nm-thick gas selective layer increased by 40 % and 20 %, respectively, as compared to the 525 nm-thick counterparts. This work offers a new insight for designing high-performance CH4/N2 adsorbents through suitable combination between rich ultramicropores and short diffusion path.

Thermal-Hydraulic-Mechanical Coupling Simulation of CO2 Enhanced Coalbed Methane Recovery with Regards to Low-Rank but Relatively Shallow Coal Seams

CO2 injection technology into coal seams to enhance CH4 recovery (CO2-ECBM), therefore presenting the dual benefit of greenhouse gas emission reduction and clean fossil energy development. In order to gaze into the features of CO2 injection’s influence on reservoir pressure and permeability, the Thermal-Hydraulic-Mechanical coupling mechanism of CO2 injection into the coal seam is considered for investigation. The competitive adsorption, diffusion, and seepage flowing of CO2 and CH4 as well as the dynamic evolution of fracture porosity of coal seams are considered. Fluid physical parameters are obtained by the fitting equation using MATLAB to call EOS software Refprop. Based on the Canadian CO2-ECBM project CSEMP, the numerical simulation targeting shallow low-rank coal is carried out, and the finite element method is used in the software COMSOL Multiphysics. Firstly, the direct recovery (CBM) and CO2-ECBM are compared, and it is confirmed that the injection of CO2 has a significant improvement effect on methane production. Secondly, the influence of injection pressure and temperature is discussed. Increasing the injection pressure can increase the pressure difference in the reservoir in a short time, so as to improve the CH4 production and CO2 storage. However, the increase in gas injection pressure will also lead to the rapid attenuation of near-well reservoir permeability, resulting in the weakening of injection capacity. Also, when the injection temperature increases, the CO2 concentration is relatively reduced, and the replacement effect on CH4 is weakened, resulting in a slight decrease in CBM production and CO2 storage.

Recyclable and efficient hydrate-based CH4 storage strengthened by fabrics

Due to the high storage capacity and low cost, gas hydrates have great potential in CH4 storage. In this work, four different common fabric materials were used as the CH4 hydrate carriers to enhance the gas–liquid interface and the mass transfer during hydrate growth at 278.2 K and an initial pressure of 6.7 MPa. The strengthen mechanism was further analyzed using a kinetic model and SEM images. Results showed that the water conversions ratios in different fabric materials were 76–92 % which corresponded to the storage capacities ranged from 60 to 65 v/v and the decay in storage capacity was less than 6.3 % within 6 cycles of hydrate formation. The beach pants fabric was found to be the most active in the promotion of CH4 dissolution and hydrate growth where the water conversion ratios were over 90 % and the time required for 63 % water conversion is only about 0.6 h. Kinetic analysis showed that the CH4 diffusion resistance through the hydrate layer was the rate-limiting step although some fabric materials performed well in hydrate growth. SEM images revealed that beach pants induced the hydrate formation with foamy surface which facilitated the gas transportation through the hydrate layer. Based on the multiscale analysis, an ideal fabric material for hydrate formation should be thin, light and knitted where the fibers can induce particle like or foamy hydrates instead of dense and continuous hydrate layer.

Study on gas transport behavior in coal matrix based on free gas density gradient diffusion: Comparison with fick model

Understanding the mechanism of gas transport in coal matrix is crucial for coalbed methane production and gas disaster prevention. The conventional Fick's law may have certain theoretical flaws because its content gradient includes both adsorbed and free gases, ignoring the fact that free gas dominate gas transport. In this study, enclosed isothermal adsorption tests with adsorbable methane (CH4) and non-adsorbable helium (He) were done first. Then, combined with Fick's law and free gas density gradient diffusion (FGDGD), the corresponding mathematical models of gas diffusion-adsorption were numerically established and solved with finite difference method (FDM). Finally, the numerical simulation results of these two models were validated. The results show that: 1) whether CH4 or He, the simulation curves based on the FGDGD model are always consistent with the experimental data, verifying the correctness of the FGDGD model; 2) the Fick model fits the experimental data with a substantial error when simulating the CH4 diffusion and adsorption process, but fits well when modeling He, demonstrating that the model's failure can be attributed to the misconception that adsorbed gas is involved in transport; and 3) the microchannel diffusion coefficient of free gas (Dm) shows that Dm (He) is greater than Dm (CH4), which can be attributed to the pore structure and gas adsorption-desorption. Consequently, the FGDGD model is reasonable in assuming that gas migration is mainly driven by free gas density gradient, and it also offers a novel method for further research into gas flow in coal matrix.

Influence of Supercritical CO2 Fluid on CH4 and CO2 Diffusion in Vitrinite-Rich Coals and Inertinite-Rich Coals

Coal maceral composition has a great effect on gas adsorption and diffusion. The interaction between maceral composition and supercritical CO2 (SCCO2) fluid will affect gas diffusion behavior in coals. Thus, the diffusivity derived from adsorption kinetics of CH4 and CO2 in vitrinite- and inertinite-rich coals with low-violate bituminous rank collected from the Hancheng mine of the Weibei coalfield pre- and post-SCCO2 fluid exposure (SFE) were tested at the conditions of 45 °C and 0.9 MPa. In combination with pore distribution and functional group content, the possible mechanism of the alterations in gas diffusion characteristics in coals with various maceral compositions was addressed. The results show that for vitrinite-rich coals, SFE increases the macropore apparent diffusion coefficient of CH4, while this treatment decreases the micropore apparent diffusion coefficient of CH4. However, the reverse trend is found for CO2 diffusion–adsorption rate. For inertinite-rich coals post-SFE, CH4 diffusion–adsorption rate increases, while an increase and a decrease in diffusivity CO2 occur for macropore and micropore, respectively. Generally, SFE shows a stronger impact on CO2 adsorption rate than CH4 in coals. The results suggest that the diffusion of CH4 and CO2 in coals with different maceral compositions show selectivity to SCCO2 fluid. The possible reason can be attributed to the changes in pore structure and surface functional group content. SFE causes an increase in macro/mesopore volume of all samples. However, SFE induces a reduction in oxygen-containing species content and micropore volume of inertinite-rich coals, while the opposite trend occurs in vitrinite-rich coals. Thus, the changes in pore volume and surface functional group account for the difference in gas diffusivity of coals with different maceral compositions. With regard to the micropore diffusion–adsorption behavior of CH4 and CO2, the impact of oxygen-containing species is superior to pore volume. The oxygen-containing species favor CO2 diffusion–adsorption but go against CH4 transport. This effect accounts for the reduction in the micropore diffusion–adsorption rate of CH4 and the increase in micropore diffusivity of CO2 in vitrinite-rich coals, respectively. However, the aforementioned effect is the opposite for inertinite-rich coals. Overall, the changes in gas diffusion in coals with different maceral composition during the CO2-ECBM process requires further attention.

Modeled production, oxidation, and transport processes of wetland methane emissions in temperate, boreal, and Arctic regions.

Wetlands are the largest natural source of methane (CH4 ) to the atmosphere. The eddy covariance method provides robust measurements of net ecosystem exchange of CH4 , but interpreting its spatiotemporal variations is challenging due to the co-occurrence of CH4 production, oxidation, and transport dynamics. Here, we estimate these three processes using a data-model fusion approach across 25 wetlands in temperate, boreal, and Arctic regions. Our data-constrained model-iPEACE-reasonably reproduced CH4 emissions at 19 of the 25 sites with normalized root mean square error of 0.59, correlation coefficient of 0.82, and normalized standard deviation of 0.87. Among the three processes, CH4 production appeared to be the most important process, followed by oxidation in explaining inter-site variations in CH4 emissions. Based on a sensitivity analysis, CH4 emissions were generally more sensitive to decreased water table than to increased gross primary productivity or soil temperature. For periods with leaf area index (LAI) of ≥20% of its annual peak, plant-mediated transport appeared to be the major pathway for CH4 transport. Contributions from ebullition and diffusion were relatively high during low LAI (<20%) periods. The lag time between CH4 production and CH4 emissions tended to be short in fen sites (3 ± 2 days) and long in bog sites (13 ± 10 days). Based on a principal component analysis, we found that parameters for CH4 production, plant-mediated transport, and diffusion through water explained 77% of the variance in the parameters across the 19 sites, highlighting the importance of these parameters for predicting wetland CH4 emissions across biomes. These processes and associated parameters for CH4 emissions among and within the wetlands provide useful insights for interpreting observed net CH4 fluxes, estimating sensitivities to biophysical variables, and modeling global CH4 fluxes.

Insight into the effect of biaxial compression strain on adsorption structure of bituminous coal matrix as well as gas diffusion and permeability properties by macromolecule simulation

The concentrated stress induced by mining activities leads to deformation of the coal, especially the coal matrix, which affects the production of coalbed methane (CBM). Therefore, grand canonical Monte Carlo and molecular dynamics methods were used to study the effect of biaxial compression strain on the adsorption structure of the bituminous coal matrix as well as the CH4/CO2/N2 diffusion and permeability properties based on the constructed macromolecular coal model. The results show that the dual-mode equation fits the gas adsorption isotherm and the order of influence of adsorption concentration on the model structure is N2 > CO2 > CH4. For the diffusion models, the orders of swelling rate and specific volume are CH4 > N2 > CO2, while those of the fractional free volume and pore volume are opposite. Compression (negative) strain (0.00 ∼ −0.20) reduces the bulk modulus but has a negligible effect on the shear modulus and wave velocity. Also the Poisson’s ratio of the CH4 diffusion model is smaller compared to the CO2/N2 diffusion model. The cohesive energy density (CED) order is CO2 > N2 > CH4 and its van der Waals component accounts for the major part. The mean square displacement distribution of gases successively increases in the power function and linearity with time. The self-diffusion coefficient of CH4 is larger than that of CO2/N2, which is greatly affected by strain. The (negative) interaction energy order between gas and coal matrix is CH4 > N2 > CO2, and the order of intermolecular interaction distance of gases is N2 > CO2 > CH4. Most importantly, the solubility order of gases is CO2 > CH4 > N2, and the strain has the dual effect of promoting and inhibiting the permeability of the coal matrix. The research results provide molecular-level insights into CBM recoveries in coal reservoirs with different strain degrees, with some technical value and theoretical importance.

Diffusion and transformation of methane within the soil profile and surface uptake in dryland spring maize fields under different fertilizer application depths

It is important to assess the potential uptake of methane (CH4) in dryland farming and its contribution to global warming mitigation by elucidating the migration characteristics of CH4 in the soil profile. Therefore, before planting spring maize in the Loess Plateau region of China in 2019–2020, we applied fertilizer at soil depths of 5, 15, 25, and 35 cm (D5, D15, D25, and D35, respectively). The uptake of CH4 and diffusion flux were determined simultaneously in the soil surface and profile. We found that the dryland soil was a sink for CH4 and the cumulative CH4 uptake amount by soil without fertilization during the maize growth stage was 1.54 kg ha–1. Fertilization inhibited the uptake of CH4 by the soil, but deep fertilization promoted its uptake. The cumulative surface CH4 uptake amounts under D15, D25, and D35 were significantly higher than that under D5, i.e., by 42.4%, 105.6%, and 169.1%, respectively. The CH4 concentration and diffusion flux decreased as the soil depth increased, and CH4 diffused downward in the soil profile. The CH4 consumption rate was greater than the production rate in the 0–20 cm soil layer, whereas the opposite was found in the 20–30 cm layer. The surface CH4 uptake flux was significantly positively correlated with the diffusion flux in the 0–30 cm soil layer. Inorganic nitrogen (N) inhibited the diffusion of CH4 from the surface through the soil profile and its diffusion within the profile. The highest maize yield and biomass were obtained under D25. Therefore, increasing the fertilization depth can promote the uptake of CH4 by the soil in maize fields to alleviate climate change, and a fertilization depth of 25 cm is suitable for balancing the requirements for environmental protection and agricultural production. Our findings provide important insights into the production, consumption, diffusion, and uptake of CH4 in dryland farming areas.

Pressure Measurement of Coal Seam with Active Nitrogen Injection: Model and Experimental Analysis.

Coal seam gas pressure is the core index of coal mine gas disaster prevention and control. Research on active nitrogen injection pressure measurement can effectively shorten the pressure measurement time and is of great significance to improve the pressure measurement efficiency. The purpose of this paper is to study the variation in gas pressure in the process of active nitrogen injection and the influence of different gas injection pressures on pressure measurement. Through the design of active pressure test device, pressure measurement experiments with different nitrogen injection pressures were carried out. The multicomponent gas migration model of active pressure measurement was constructed, and the active pressure numerical solver was developed based on OpenFOAM and C++ language. The borehole gas pressure equation was verified by field pressure data. The results show that when the injected nitrogen pressure is closer to the original coal seam gas pressure, the borehole gas pressure balance time is shorter. The gas change rate in this process depends on the pressure difference and concentration difference between the borehole and the surrounding coal. When the nitrogen injection pressure is greater than the gas pressure around the borehole, N2 will percolate, diffuse, and adsorb into the coal. In contrast, it is dominated by the desorption, diffusion, and seepage of CH4 to the borehole. The correlation coefficient R 2 between the borehole gas pressure equation and the field pressure measurement data is more than 0.88. Therefore, the equation can verify the reliability of the measurement results of active gas injection pressure in the field. The research results provide a reference for further understanding the mechanism and application of pressure measurement in coal seam injected with active nitrogen.

Molecular insights into CH4/H2O transport and hydrate formation in hydrophobic metal-organic frameworks ZIF-8: Implication for CH4 storage by adsorption-hydration hybrid method

The method of adsorption-hydration hybrid has been considered to have great application potential for future CH4 storage. However, the mechanism of CH4 /H2O transport and hydrate formation in metal–organic frameworks (MOFs) systems is not clear. In this work, the hydrophobicity of ZIF-8 was demonstrated by measuring the equilibrium contact angle (115.8 ± 1.2°) of the substrate at molecular level. The interaction between H2O molecules promotes the hydrogen bonds network formation on ZIF-8 surface. Because of the strong physisorption of CH4 by ZIF-8 cavities, excess CH4 accelerates migration and forms larger flat nanobubbles on ZIF-8 surface to cause separation of gas and liquid. On the basis of CH4 /H2O transport results, the whole process of hydrate nucleation and growth in ZIF-8 system was first investigated to reveal the microscopic mechanism of promoting CH4 storage by adsorption-hydration hybrid method. For one thing, a preferential direction may exist during the hydrate nucleation. For another, the migration of intergranular CH4 and H2O is enhanced to promote hydrate nucleation at early stage. Finally, the link between transportation of CH4 /H2O and hydrate formation were discussed in three ZIF-8 systems, elucidating that the larger gas–liquid ratio can accelerate the transportation of intergranular CH4 and H2O at early stage to promote hydrate nucleation and growth. However, because of the competitive adsorption between ZIF-8 inner cavities and hydrate cages, the flat nanobubbles aggregates and the solubility of intergranular CH4 decreases, which accelerate the diffusion of CH4 and H2O to hinder the hydrate continues growth and nucleation, respectively. This study provides crucial microscopic insight into the transportation of CH4 /H2O promotes hydrate formation, which facilitates application for natural gas storage in MOFs by adsorption-hydration hybrid method.

Effects of supercritical CO2 exposure on diffusion and adsorption kinetics of CH4, CO2 and water vapor in various rank coals

The physico-chemical effects caused by supercritical CO2 (ScCO2) exposure is one of the leading problems for CO2 storage in deep coal seams as it will significantly alter the flow behaviors of gases. The main objective of this study was to investigate the effects of ScCO2 injection on diffusion and adsorption kinetics of CH4, CO2 and water vapor in various rank coals. The powdered coal samples were immersed in ScCO2 for 30 days using a high-pressure sealed reactor. Then, the diffusion and adsorption kinetics of CH4, CO2 and water vapor in the coals both before and after exposure were examined. Results indicate that the diffusivities of CH4 and CO2 are significantly increased due to the combined matrix swelling and solvent effect caused by ScCO2 exposure, which may induce secondary faults and remove some volatile matters that block the pore throats. On the other hand, the diffusivities of water vapor are reduced due to the elimination of surface functional groups with ScCO2 exposure. It is concluded that density of the surface function groups is the controlling factor for water vapor diffusion rather than the pore properties. The unipore model and pseudo-first-order equation can simulate the diffusion and adsorption kinetics of CH4 and CO2 very well, but the unipore model is not capable of well describing water vapor diffusion. The effective diffusivity (De), diffusion coefficient (D) and adsorption rates (k1) of CH4 and CO2 are significantly increased after ScCO2 exposure, while the values of water vapor are decreased notably. Thus, the injection of ScCO2 will efficiently improve the transport properties of CH4 and CO2 but hinder the movement of water molecules in coal seams.

A novel bidisperse diffusion model for coal particles considering micropores as the major sites of CH4 adsorption

Pores in coal have the characteristic of double peaks, and the bidisperse diffusion model is an effective method to describe the diffusion behavior in coal. At present, the traditional bidisperse diffusion models can be divided into “independent” mode and “dependent” mode, which have three main deficiencies as follows. Firstly, since the percentage of adsorbed CH4 in micropores or macropores cannot be accurately determined, the bidisperse diffusion model consists of two equations with three unknown parameters. It is impossible to get a unique solution only based on satisfying a high R2, which brings uncertainty to the solution of the model. Secondly, the ratio of macropore adsorption mass to the total adsorption mass β in some cases shows that CH4 is mainly stored in macropores, which is inconsistent with the physical phenomenon. Thirdly, the independent model is based on the assumption that the macropore diffusion and micropore diffusion are separate from each other, which is not consistent with the pore structure characteristics of coal. To solve the above problems, we put forward a novel two-stage diffusion mode via insightful analysis of the CH4 adsorption form, and the newly-established bidisperse diffusion model can better describe the CH4 diffusion behavior in coal. By using the novel model to analysis the CH4 diffusion behavior, we obtained the following results: (1) The CH4 adsorption form presents the characteristics of micropore filling and a monomolecular layer adsorption. Micropores are the main sites of CH4 adsorption. This adsorption form makes the desorption and diffusion resistance of CH4 vary in different pores, which leads to the CH4 source dynamic evolution during diffusion. (2) The novel model has been proved by experimental validation to have good applicability and can accurately describe the whole process of CH4 diffusion. Meanwhile, the obtained parameters by the new model not only with clear physical significance but also can explain the adsorption-diffusion mechanism of CH4 in multi-scale pores. (3) The traditional dependent model may degenerate into the unipore diffusion model under the condition of satisfying the CH4 occurrence characteristics and fitting accuracy, which results in losing the physical significance of the bidisperse diffusion model.