Gas Transport Processes Research Articles

Methane Diffusion and Adsorption in Shale Rocks: A Numerical Study Using the Dusty Gas Model in TOUGH2/EOS7C-ECBM

Gas production from shale gas reservoirs plays a significant role in satisfying increasing energy demands. Compared with conventional sandstone and carbonate reservoirs, shale gas reservoirs are characterized by extremely low porosity, ultra-low permeability and high clay content. Slip flow, diffusion, adsorption and desorption are the primary gas transport processes in shale matrix, while Darcy flow is restricted to fractures. Understanding methane diffusion and adsorption, and gas flow and equilibrium in the low-permeability matrix of shale is crucial for shale formation evaluation and for predicting gas production. Modeling of diffusion in low-permeability shale rocks requires use of the Dusty gas model (DGM) rather than Fick’s law. The DGM is incorporated in the TOUGH2 module EOS7C-ECBM, a modified version of EOS7C that simulates multicomponent gas mixture transport in porous media. Also included in EOS7C-ECBM is the extended Langmuir model for adsorption and desorption of gases. In this study, a column shale model was constructed to simulate methane diffusion and adsorption through shale rocks. The process of binary $$\hbox {CH}_{4}{-}\hbox {N}_{2}$$ diffusion and adsorption was analyzed. A sensitivity study was performed to investigate the effects of pressure, temperature and permeability on diffusion and adsorption in shale rocks. The results show that methane gas diffusion and adsorption in shale is a slow process of dynamic equilibrium, which can be illustrated by the slope of a curve in $$\hbox {CH}_{4}$$ mass variation. The amount of adsorption increases with the pressure increase at the low pressure, and the mass change by gas diffusion will decrease due to the decrease in the compressibility factor of the gas. With the elevated temperature, the gas molecules move faster and then the greater gas diffusion rates make the process duration shorter. The gas diffusion rate decreases with the permeability decrease, and there is a limit of gas diffusion if the permeability is less than $$1.0\,\times \,10^{-15}\, \hbox { m}^{2}$$ . The results can provide insights for a better understanding of methane diffusion and adsorption in the shale rocks so as to optimize gas production performance of shale gas reservoirs.

The Root Zone: Soil Physics and Beyond

Core Ideas This special section has 15 contributions focused on physical and biochemical processes in the root zone. Contributions stem from presentations at the 2016 Kirkham conference in Sede Boqer Campus, Israel. The root system plays a key role in water, nutrient, and gas transport processes in the vadose zone. Novel measurement and modeling tools are still needed to understand and manage root zone processes. This special section of Vadose Zone Journal (VZJ) contains 15 contributions focusing on the physical, biological, and chemical aspects of water and solute transport into and through the soil root zone, including root water and solute uptake. The papers stem from presentations at the 2016 Kirkham Conference, “The Root Zone: Soil Physics and Beyond,” which took place at the Sede Boqer campus of Ben‐Gurion University of the Negev, Israel, during 10 to 14 April (https://www.soils.org/membership/divisions/soil‐physics‐and‐hydrology/kirkham‐conferences). Much consensus existed at the conference for a special VZJ section to reflect the importance of the soil root zone from many disciplinary perspectives (soil, environmental, agricultural, hydrologic, and atmospheric) and the complexity of the basic processes involved. The contributions deal with root zone processes at different scales and from different disciplinary viewpoints, while covering a broad range of topics from the very theoretical to important practical applications. Special emphasis is on novel measurement and modeling tools at various scales and the need for interdisciplinary research.

Characterization of Grain‐Size Distribution, Thermal Conductivity, and Gas Diffusivity in Variably Saturated Binary Sand Mixtures

Core Ideas Parameterized models are relevant to applications where different porous media mixtures are used. Models include a grain‐size distribution function to describe bimodal behavior for binary mixtures. Improved model describes observed thermal conductivity–saturation relations for binary mixtures. Combined Buckingham–Penman model used to describe observed gas diffusivity–air content relations. This work is relevant for proper simulation of mixed porous media. Characterization of differently textured porous materials, as well as different volumetric porous media mixtures, in relation to mass and heat transport is vital for many engineering and research applications. Functional relations describing physical (e.g., grain‐size distribution, total porosity), thermal, and gas diffusion properties of porous media and mixtures are necessary to optimize the design of porous systems that involve heat and gas transport processes. However, only a limited number of studies provide characterization of soil physical, thermal, and gas diffusion properties and the functional relationships of these properties under varying soil water contents, especially for soil mixtures, complicating optimization efforts. To better understand how mixing controls the physical, thermal, and gas diffusion properties of porous media, a set of laboratory experiments was performed using five volumetric mixtures of coarse‐ and fine‐grained sand particles. For each mixture, the grain‐size distribution (GSD), thermal conductivity, and gas diffusivity were obtained and parameterized using existing and suggested parametric models. Results show that the extended, two‐region Rosin–Rammler particle‐size distribution model proposed in this study could successfully describe the bimodal behavior of the GSD of binary mixtures. Further, the modified Côté and Konrad thermal conductivity model adequately described the thermal conductivity–water saturation relations observed in different mixtures. The proposed simple soil‐gas diffusivity descriptive model parameterized the upper limit, average, and lower limit behavior in gas diffusivity–air content relations in apparently texture‐invariant gas diffusivity data. Results further show a close analogy between gas diffusivity and thermal conductivity and their variation with saturation across different binary mixtures. Overall, the results of the study provide useful numerical insight into the physical, thermal, and gas transport characteristics of binary mixtures, with wide implications for future engineering and research applications that involve multicomponent porous systems.

Effect of microscale compressibility on apparent porosity and permeability in shale gas reservoirs

The pore network in shale reservoirs comprise of nanoporous organic matter (OM) and micron-size pores in inorganic material (iOM). Accurate gas transport models in shale must include gas slippage, Knudsen diffusion, surface diffusion, and sorption. The change in pore size due to the applied stress could consequently affect gas transport processes. In this study we a compression coefficient to characterize the influence of stress sensitivity on key parameters for gas transport. We consider separate stress response in nanoporous organic matter and iOM because of their different mechanical properties. The effects of compressibility on apparent permeability of OM and iOM are analyzed at different pore sizes, pore pressures and for different gas compositions. Our results show that compressibility has a greater influence on the apparent permeability of iOM than on OM when pore sizes are smaller than 10 nm, whereas compression has similar impact on apparent permeability of both media when pore sizes are larger than 10 nm. With the same effective stress, lower pore pressure results in greater impair in permeability. We conducted a reservoir simulation study using conventional dual-continua model with our developed pressure dependent porosity and permeability to showcase field implication of this study. This work is an important and timely investigation of the development of shale-reservoir-flow simulators.

Morphological Studies of Mixed Methane Tetrahydrofuran Hydrates in Saline Water for Energy Storage Application

Solidified natural gas (SNG) in the form of clathrate hydrates has been considered one of the most promising technologies for natural gas storage and transportation because it is safe and economically feasible compared to liquefied natural gas (LNG) and compressed natural gas (CNG). In this direction, we recently observed that mixed methane tetrahydrofuran hydrates had the ability to store a significant amount of methane gas close to ambient temperature conditions. To further improve the economic feasibility of methane gas storage and transportation process, utilization of saline water was recommended. In the present work, we investigate the morphology of mixed CH4/THF hydrate formation and dissociation in presence of NaCl for the first time. Morphology changes observed during the nucleation, hydrate growth and dissociation of mixed hydrates at two different operating conditions are presented. All the experiments were performed with a stoichiometric concentration of THF (5.56 mol% THF). More over, the gas uptake was also measured and compared with non-salt experiments.

Stochastic algorithm for simulating gas transport coefficients

The aim of this paper is to create a molecular algorithm for modeling the transport processes in gases that will be more efficient than molecular dynamics method. To this end, the dynamics of molecules are modeled stochastically. In a rarefied gas, it is sufficient to consider the evolution of molecules only in the velocity space, whereas for a dense gas it is necessary to model the dynamics of molecules also in the physical space. Adequate integral characteristics of the studied system are obtained by averaging over a sufficiently large number of independent phase trajectories. The efficiency of the proposed algorithm was demonstrated by modeling the coefficients of self-diffusion and the viscosity of several gases. It was shown that the accuracy comparable to the experimental one can be obtained on a relatively small number of molecules. The modeling accuracy increases with the growth of used number of molecules and phase trajectories.

Why shale permeability changes under variable effective stresses: New insights

Experimental measurements of shale permeability are normally conducted under conditions of constant effective stresses. Based on the theory of poroelasticity, shale permeability in non-sorbing media is determined by the effective stress alone and remains unchanged if the effective stress does not change. This theoretical conclusion contradicts some experimental observations in sorbing media. These enigmatic phenomena are analyzed through a model accommodating gas slippage. This approach can explain why shale permeability is significant at low gas pressure but does not explain the contradiction with poromechanics. In this work, we develop and apply an alternative approach to resolve this dilemma. The model comprises flow within a nanotube capillary embedded within the shale matrix (discrete approach) and allows the evolution of shale permeability to be followed during the processes of shale gas flow. In the formulation, we define four strains: global strain of the shale, fracture-local strain, matrix-global strain, and pore-local strain. Shale permeability is defined as a function of these strains that are, in turn, a consequence of effective stress transfer between the matrix and the fracture systems. This behavior is regulated by the differential compliance of the various components and by gas diffusion from the fracture system to the matrix. We use the strain evolution to define how shale permeability changes with time or gas pressure in the matrix system. We apply the new model to generate a series of shale permeability profiles. These profiles are consistent with experimental observations reported in the literature. Through this study, we demonstrate that the experimental observations can indeed be explained through the inclusion of explicit interactions between shale microstructures and gas transport processes.

Experimental investigation of gas mass transport and diffusion coefficients in porous media with nanopores

Understanding gas mass transport and determining diffusion coefficients are essential for investigating the gas flow mechanisms and evaluating porous media with nanopores. Multiple gas transport mechanisms coexist in porous media with complex pore size distribution, including viscous flow, Knudsen diffusion and surface diffusion. During pressure depletion of a reservoir, the adsorbed gas desorbs into pore space asadditional ‘free gas’, and meanwhile, diffuses along the surface of nanopores in porous media. The surface diffusion itself increases the total gas transport capacity in pores and its effect cannot be neglected. The bulk gas transport (non-surface diffusion) data was excluded experimentally to intensively investigate the surface diffusion during gas mass transport based on the gas storage and flow mechanisms. Accordingly, a mathematical model is developed by incorporating the surface diffusion. The results show that the equilibrium time for gas transport process decreases quickly with temperature. Higher saturation pressure could accelerate the process and increase the amount of produced gas. Besides, the two-stage process of the gas mass transport can be identified by recording the decay of gas pressure, which implies that the surface diffusion dominates the late stage of the gas mass transport. The surface diffusion coefficient for shale is between 10−18 and 10−16m2/s. This study provides a straightforward method to describe the gas mass transport in shale, simple but information–rich for the assessment of shale gas targets.

Second-order accurate genuine BGK schemes for the ultra-relativistic flow simulations

This paper presents second-order accurate genuine BGK schemes in the framework of finite volume method for the ultra-relativistic flows. Different from the existing kinetic flux-vector splitting (KFVS) or BGK-type schemes for the ultra-relativistic Euler equations, the present schemes are derived from the analytical solution of the Anderson–Witting model, which is given for the first time and includes the “genuine” particle collisions in the gas transport process. The proposed schemes for the ultra-relativistic viscous flows are also developed and two examples of ultra-relativistic viscous flow are designed. Several 1D and 2D numerical experiments are conducted to demonstrate that the proposed schemes not only are accurate and stable in simulating ultra-relativistic inviscid and viscous flows, but also have higher resolution at the contact discontinuity than the KFVS or BGK-type schemes.

Soil Greenhouse Gases: Relations to Soil Attributes in a Sugarcane Production Area

Core Ideas The production of the main soil greenhouse gases (CO2, CH4 and N2O) is influenced by agricultural practices. The soil bulk density and micropores showed negative correlation with soil CO2 emission. The factor analysis showed the formation of two independent processes that explained almost 72% of the total variance observed in the data. The soil moisture is a controlling factor of soil CO2 emission. The production of the main soil greenhouse gases (GHG: carbon dioxide [CO2], methane [CH4,] and nitrous oxide [N2O]) is influenced by agricultural practices that cause changes in soil physical, chemical, and biological attributes, directly affecting their emission to the atmosphere. The aim of this study was to investigate the infield soil carbon dioxide emissions (FCO2) and soil CO2, methane, and nitrous oxide production potentials (PCO2, PCH4, and PN2O, respectively) under laboratory conditions and their relationship to soil attributes in a mechanically harvested sugarcane area. Soil carbon dioxide emissions presented an infield average emission value of 1.19 µmol CO2 m‐2 s‐1, while GHG production in the laboratory was 2.34 µg C–CO2 g‐1 soil d‐1 and 0.20 ng N–N2O g‐1 soil d‐1 for PCO2 and PN2O, respectively. No significant production or oxidation was observed for CH4. Factor analysis showed the formation of two independent processes that explained almost 72% of the total variance observed in the data. The first process was related to FCO2 transport and its relation to soil physical attributes such as microporosity, macroporosity, the C/N ratio, soil moisture, and soil bulk density, showing the dependence between FCO2 and soil porosity. The second process was related to soil CO2 and N2O production potentials under laboratory conditions and their relation to soil chemical attributes such as the sum of bases, pH, and available phosphorus, which affect microbial activity and contribute to GHG production. Although presented as independent, these processes are coupled and occur simultaneously in the soil, in addition to providing information about their variability and showing if the infield emissions are due to gas transport processes or soil carbon levels and their quality.

Analytical model of gas transport in heterogeneous hydraulically-fractured organic-rich shale media

Multiple gas transport mechanisms and geological parameters influence total gas production in hydraulically-fractured organic-rich shale reservoirs. The absolute and relative contributions of these processes and parameters vary significantly with the scale of the medium, from pore scale to field scale. The objective of this study is to investigate the impact of these geological parameters and transport processes in organic-rich shale production at field scale.We develop a multi-porosity analytical model which honors pressure- and space-dependent geological properties and gas transport processes in a multi-stage, hydraulically-fractured horizontal well of shale reservoir. The model premises distinct pores in kerogen, inorganic clay, induced fracture, and hydraulic fracture. Multiple flow zones are modeled: unstimulated reservoir volume (USRV), stimulated reservoir volume (SRV), and hydraulic fractures (HF). Compared to the earlier analytical models, the new model accounts for more comprehensive transport mechanisms and medium properties: gas diffusion and desorption in kerogen matrix and organopores, slip flow in inorganic matrix pores, transient Darcy flow from matrix to induced fractures, Darcy flow in both induced and hydraulic fractures, tortuosity of induced fractures, and pressure-dependent permeabilities. These enhanced features introduce important complexities to the analytical solution challenges that are discussed and resolved in detail.Extensive lab and field data for Eagle Ford shale are presented in the new model. Sensitivity analyses are conducted to study the flow contribution from organic and inorganic media. The results show that first, the impacts of gas diffusion, TOC, and organoporosity at pore and core scale are largely eclipsed at field scale by slip flow in inorganic matrix and anomalous diffusion within complex induced-fracture network. Second, fracture tortuosity and the Klinkenberg effect are dominant factors during early- and intermediate-time production. Third, gas diffusion in kerogen matrix is negligible compared to gas desorption and diffusion in organopores. Fourth, permeability variations with pore pressure have a significant impact on late-time production from fractured low-permeability reservoirs.The derived analytical solutions in Laplace space serve as simple but robust tools for studying gas transport in fractured organic shale formation. The field application is validated by matching results for an Eagle Ford shale well. Finally, there is a discussion of the present model’s limitations and potential future extensions.

Internal Reforming of Methane Fuel within Proton-Conducting Ceramic Anode Supports

The objective of this work is to advance our understanding of hydrocarbon internal-reforming processes within the anode supports of proton-conducting ceramic fuel cells. This work is centered on an exciting new class of protonic-ceramic materials that are emerging from the laboratory to play important roles in the commercial sector. While proton-conducting ceramics have been studied since the early 1980s, the unique properties of these materials are only now being harnessed to address societal challenges in electricity generation, energy storage, and chemicals synthesis. Protonic-ceramic fuel cells (PCFCs) have demonstrated remarkable power density and stable operation under hydrocarbon fuels at operating temperatures that are well below that of the more-well established oxygen-ion conducting ceramics. Performance of recent unit-cell PCFC stacks based on barium cerate-zirconates developed at the Colorado Fuel Cell Center exceeds 300 mW / cm2 at 0.78 V during operation under direct methane-steam fuel at 550 ºC. Such performance brings confidence that protonic ceramics can play a meaningful role in efficient electricity generation. These protonic-ceramic fuel cells have demonstrated remarkable stability under methane-steam fuel mixtures. Under these conditions, no upstream fuel processor is used to convert the methane fuel into synthesis gas (H2, CO, CO2, H2O, etc.). Rather, the methane-steam fuel feed is reformed within the PCFC anode support. This internal reforming brings many benefits, including higher efficiency, improved thermal uniformity, reduced thermal stress, and simpler, lower-cost balance-of-plant components. Such internal reforming is common is oxygen-ion conducting ceramics like yttria-stabilized zirconia (YSZ). However, YSZ-based devices operate at significantly higher temperatures (~ 800 ºC), where the chemical kinetics of methane-steam reforming are more favorable to high conversion. In contrast, PCFCs operate at temperatures at or below 550 ºC, where reaction kinetics are substantially slower. This lower-temperature operation under direct hydrocarbon fuels brings questions about the effectiveness of the internal reforming process. Inadequate reforming can lead to inefficient utilization of the parent hydrocarbon fuel, lower cell performance, and deleterious solid-carbon deposit formation. To explore these questions in more depth, we have developed unique experimentation that decouples the thermochemical phenomena of heterogeneous methane-steam reforming from the electrochemical processes underway during fuel-cell operation and electricity generation. The “Separated Anode Experiment” has been developed to study heterogeneous chemistry and gas-transport processes in solid-oxide fuel cell anodes. This experiment decouples gas-transport and hydrocarbon internal-reforming processes from electrochemistry by simulating operational conditions within an anode support in the absence of a dense electrolyte. The planar anode support is compression-sealed within two alumina manifolds into which flow channels have been machined. Feed gases cross-diffuse and heterogeneously react within the tortuous morphology of the anode. Exhaust-gas compositions are quantified to understand what gases reach the triple-phase boundaries at the anode-electrolyte interfaces, bringing insight to the conditions at which charge transfer occurs. A detailed computational model aids in interpretation of the experimental results. In this presentation, the performance of protonic-ceramic anode supports will be presented from the perspective of internal-reforming effectiveness. Cell morphology and operating conditions that promote efficient internal reforming will be presented. Figure 1

Multiscale pore structure and its effect on gas transport in organic‐rich shale

AbstractA systematic investigation of multiscale pore structure in organic‐rich shale by means of the combination of various imaging techniques is presented, including the state‐of‐the‐art Helium‐Ion‐Microscope (HIM). The study achieves insight into the major features at each scale and suggests the affordable techniques for specific objectives from the aspects of resolution, dimension, and cost. The pores, which appear to be isolated, are connected by smaller pores resolved by higher‐resolution imaging. This observation provides valuable information, from the microscopic perspective of pore structure, for understanding how gas accumulates and transports from where it is generated. A comprehensive workflow is proposed based on the characteristics acquired from the multiscale pore structure analysis to simulate the gas transport process. The simulations are completed with three levels: the microscopic mechanisms should be taken into consideration at level I; the spatial distribution features of organic matter, inorganic matter, and macropores constitute the major issue at level II; and the microfracture orientation and topological structure are dominant factors at level III. The results of apparent permeability from simulations agree well with the values acquired from experiments. By means of the workflow, the impact of various gas transport mechanisms at different scales can be investigated more individually and precisely than conventional experiments.

Solid particle spreading in gas-dispersed confined swirling flow. Eulerian and Lagrangian approaches

Dynamics of a disperse phase in a swirling two-phase flow behind a sudden tube expansion is simulated with the aid of Eulerian and full Lagrangian descriptions. The carrier phase is described by three-dimensional Reynolds averaged Navier–Stokes equations with consideration of inverse influence of particles on the transport processes in gas. The velocity profiles calculated using these two approaches are practically the same. It is shown that the main difference between the Eulerian and Lagrangian approaches is presented by the concentration profile of the dispersed phase. The Eulerian approach underpredicts the value of particle concentration as compared with the Lagrangian approach (the difference reaches 15−20 %). The dispersed phase concentration predicted by the Lagrangian approach agrees with the measurement data somewhat better than the data obtained through the Eulerian approach.

An in situ method for real‐time measurement of gas transport in soil

SummarySoil aeration is central to the biogeochemistry of soil. Gas transport in soil is governed mainly by molecular diffusion and depends on the soil gas diffusion coefficient (DS). Several methods exist to determine this property based on field and laboratory measurements. These methods, however, are unsuitable for continuous monitoring of DS over time. Moreover, non‐diffusive processes can affect gas transport in soil during certain situations that last several hours only and discontinuous measurements might fail to identify such processes. We developed a novel in situ method for the real‐time monitoring of gas transport in soil. Helium (He) was injected continuously into the soil and the resulting steady‐state gas profile was monitored. We used a gas sampling probe with very permeable membranes, which enables passive sampling of the soil gas at different depths. The DS profile was modelled by inverse finite‐element modelling (FEM) of an exact geometrical model of the sampler and soil. Molecular diffusion was assumed to be the only process of gas transport. The method was tested in the laboratory with different granular materials and in two field studies. The DS values obtained with this new method agreed well with reference measurements from soil cores and diffusion models. Soil gas diffusivity was monitored over a few days that included a major rain event. During this period the effect of increasing soil moisture on gas transport in soil could be observed in real time. Our novel method is suitable for monitoring gas transport in soil over several days. This enables the monitoring and investigation of situations including non‐diffusive transport processes that result from barometric pressure changes or strong wind events.Highlights We present a novel method for in situ monitoring of gas transport in soil. Helium was continuously injected into the soil and its distribution in the soil modelled. Soil gas diffusivity profiles determined in situ agreed well with laboratory measurements of DS. Our novel method is ideal for the investigation of specific situations in gas transport in soil.

Modeling of local gas transport in catalyst layers of PEM fuel cells

A mathematical model focusing on local gas transport in a single carbon primary particle was developed by combining the classical agglomerate model with local gas transport in the vicinity of a Pt particle. Two local gas transport processes—gas transport through an ionomer thin film at Pt particles deposited on the surface of the carbon primary particle and gas transport through water at Pt particles deposited inside the carbon primary particle—were newly introduced. The model showed qualitative agreement with the experimentally measured gas transport resistance of catalyst layers with different effective Pt surface areas, which had not been captured by the classical agglomerate approach. The experimental data were fitted by significantly reducing the gas permeability for the local gas transport processes in the vicinity of the Pt surface, which was 1/4–1/20 of the permeability measured in the bulk materials. The corrections of gas permeability were all related to the local gas transport in the vicinity of the Pt surface, which implied the presence of a high energy barrier for reactant gas transport at ionomer/Pt and water/Pt interfaces. The influence of structural parameters of catalyst layers on the gas transport resistance was presented to provide a guideline toward the reduction of the gas transport resistance at low Pt loading.

Significance of oxygen transport through aquaporins

Aquaporins are membrane integral proteins responsible for the transmembrane transport of water and other small neutral molecules. Despite their well-acknowledged importance in water transport, their significance in gas transport processes remains unclear. Growing evidence points to the involvement of plant aquaporins in CO2 delivery for photosynthesis. The role of these channel proteins in the transport of O2 and other gases may also be more important than previously envisioned. In this study, we examined O2 permeability of various human, plant, and fungal aquaporins by co-expressing heterologous aquaporin and myoglobin in yeast. Two of the most promising O2-transporters (Homo sapiens AQP1 and Nicotiana tabacum PIP1;3) were confirmed to facilitate O2 transport in the spectrophotometric assay using yeast protoplasts. The over-expression of NtPIP1;3 in yeasts significantly increased their O2 uptake rates in suspension culture. In N. tabacum roots subjected to hypoxic hydroponic conditions, the transcript levels of the O2-transporting aquaporin NtPIP1;3 significantly increased after the seven-day hypoxia treatment, which was accompanied by the increase of ATP levels in the apical root segments. Our results suggest that the functional significance of aquaporin-mediated O2 transport and the possibility of controlling the rate of transmembrane O2 transport should be further explored.

Separation of Hydrogen from Carbon Dioxide through Porous Ceramics.

The gas permeability of α-alumina, yttria-stabilized zirconia (YSZ), and silicon carbide porous ceramics toward H2, CO2, and H2–CO2 mixtures were investigated at room temperature. The permeation of H2 and CO2 single gases occurred above a critical pressure gradient, which was smaller for H2 gas than for CO2 gas. When the Knudsen number (λ/r ratio, λ: molecular mean free path, r: pore radius) of a single gas was larger than unity, Knudsen flow became the dominant gas transportation process. The H2 fraction for the mixed gas of (20%–80%) H2–(80%–20%) CO2 through porous Al2O3, YSZ, and SiC approached unity with decreasing pressure gradient. The high fraction of H2 gas was closely related to the difference in the critical pressure gradient values of H2 and CO2 single gas, the inlet mixed gas composition, and the gas flow mechanism of the mixed gas. Moisture in the atmosphere adsorbed easily on the porous ceramics and affected the critical pressure gradient, leading to the increased selectivity of H2 gas.

Gas Transportation and Enhanced Coalbed Methane Recovery Processes in Deep Coal Seams: A Review

Coalbed methane (CBM) is a potential green energy supply for addressing the worldwide energy crisis. However, the recovery of economically viable amounts of methane requires the application of production-enhancement techniques. The greater effectiveness of enhanced coalbed methane (ECBM) recovery compared to traditional pressure depletion and hydraulic stimulation techniques has been identified in terms of higher CBM recovery with minimal pollution risk and the ability to contribute to CO2 sequestration. Gas transport behavior in a coal seam is the governing factor for ECBM recovery, which includes sorption/desorption and diffusion in the matrix and advective flux in cleats. The interactions among sorption, diffusion, and flow indicate the complexity and abstruseness of gas transport in coal. Therefore, the purpose of this paper is to provide comprehensive knowledge of the gas transport process in deep coal seams, particularly in relation to the ECBM process. According to the review, the dual-porosity sys...

Synthesis and structure of molybdenum diselenide nanosheets produced from MoO3 and Se powders

In this paper, we report on the synthesis of molybdenum diselenide (MoSe2) nanosheets produced by hot filament chemical vapor deposition using the admixture of molybdenum trioxide (MoO3) and selenium (Se) powders. The results of field emission electron scanning microscope, transmission electron microscope, micro-Raman spectroscope and X-ray photoelectron spectroscope indicate that the MoSe2 nanosheets are well synthesized when the mixing mass ratios of Se to MoO3 powders are 5 and 10, which are lower than the ratios of Se to MoO3 powders reported elsewhere. However, other nanomaterials made of Se or molybdenum oxide are formed if the mixing ratios of 20 and 2. Based on the theory related to gas transport process, the formation of different nanomaterials in different mixing ratios is studied. The formation of MoSe2 nanosheets under relatively low ratios results from the increase of collision probability of MoO3 molecules with Se atoms due to the prolonged residence time of MoO3 molecules in gas environment. Furthermore, the photoluminescence (PL) properties of the synthesized nanomaterials appear to be different from the reported results. The excited PL bands of the MoSe2 specimens have a blue shift of ∼19 nm, which is due to the bandgap expansion caused by the residual stress in the MoSe2 nanosheets. These outcomes are environment-benign, enable structural control of MoSe2 nanosheets and facilitate the development of optoelectronic devices incorporating MoSe2-based materials.