Methane Transport Research Articles

Methane Transport through Simulated Landfill Covers with a PE Geofilm or Ethylene-Vinyl Alcohol, LLDPE, or PVC Geomembrane

AbstractMethane transport was evaluated using column tests for simulated interim cover profiles composed of soil only or soil over a geomembrane (GM). Tests were conducted with a co-extruded GM of ...

Molecular-level insight of the differences in the diffusion and solubility of penetrants in polypropylene, poly(propylmethylsiloxane) and poly(4-methyl-2-pentyne)

In order to design optimal polymer membranes it is important to have a molecular level understanding of penetrant transport in such membranes. As such, molecular dynamics simulation was employed to study the diffusion and solubility of gas molecules in polypropylene (PP), poly(propylmethylsiloxane) (PPMS) and poly (4-methyl-2-pentyne) (PMP). After the structures of PP, PPMS and PMP were established and relaxed, the average amplitudes of the oscillation of the main and branch chains of PP, PPMS and PMP were evaluated with a proposed analysis method; in addition the cavity size distributions of PP, PPMS and PMP were determined. PPMS has the largest average cavity size and accessible cavity fraction for penetrants, followed by PMP and PP, which has the smallest. The logarithm plot of mean-squared displacements (MSDs) versus time for the transport of methane in the three polymers revealed three regimes, namely the ballistic, the subdiffusive and the Fickian diffusive regimes. The ballistic regime of PMP is longer than that of PP and shorter than that of PPMS, because the average cavity size and accessible cavity fraction of PMP are larger than that of PP and smaller than that of PPMS and the penetrants have less space in PP and more space in PPMS to move freely than they do in PMP before they hit any matrix units. Statistical tests showed that the random walk on a fractal (RWF) model was the most appropriate model to explain the subdiffusion phenomena. The subdiffusive regime is induced by the trap of penetrants in cavities before they get the chances to jump from cavities to cavities nearby. Next the diffusivities of methane and n-butane were estimated from the plot of MSDs versus time for the penetrant transport in the Fickian diffusive regime. The diffusivities of penetrants are much smaller in PP than they are in PPMS and PMP and these values are larger in PPMS than they are in PMP. With both unbiased and biased Widom insertion methods, the solubility coefficients K of gas molecules in the polymers were calculated, though biased Widom insertion method has significantly faster calculation speed. PPMS has the largest solubilities of methane and n-butane, followed by PMP and PP, which has the smallest. Though the permeabilities of both methane and n-butane in PPMS are larger than the corresponding values in PMP, the selectivity of n-butane over methane in PPMS is lower than that in PMP. The research results may be employed in membrane design.

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.

GROWTH OF LNG GAS MARKET AS ALTERNATIVE FUEL IN TRANSPORT

The aim of the article is to present the possibility of reducing the negative impact of vehicles on the human environment. The article presents the directions of the development of methane storage and transport technology as a fuel for the supply of vehicles of different branches of transport.

Exploration of gas molecules transport in composite poly (4-methyl-2-pentyne) and carbon nanotubes employing molecular dynamics simulation

In order to design optimal membrane composites it is important to have an in-depth understanding of penetrants transport in such membranes. In that spirit, molecular dynamics was employed to study the transport of gas molecules in a composite membrane consisting of poly (4-methyl-2-pentyne) (PMP) and close-ended carbon nanotubes (PMPCNTC) or open-ended carbon nanotubes (PMPCNTO). The logarithm plot of mean-squared displacement (MSD) versus time for the transport of methane in PMPCNTC and PMPCNTO was drawn. It is observed that there are three regimes: the ballistic regime, the subdiffusive regime and the Fickian diffusive regime. The ballistic regime in PMPCNTC is shorter than it is in PMPCNTO, because the average cavity size of PMPCNTC is smaller than that of PMPCNTO and the penetrants have more space to move freely in PMPCNTO than they do in PMPCNTC before they hit any matrix units. Then the diffusivities of penetrants were achieved according to the plot of MSD versus time for the transport of penetrants in the Fickian diffusive regime. The solubility coefficients and the permeability of gas molecules in the composites were also calculated. In PMPCNTC, the 25 locations that contribute most to solubilities are outside the nanotubes while in PMPCNTO, they are inside the nanotubes. In PMPCNTO it is the nanotube that solvates the majority of the gas molecules. The huge channels inside the nanotube cause the high solubility of these penetrants. Finally, the selectivity of n-C4H10 over CH4 in PMPCNTC is observed to be higher than it is in both PMPCNTO and pure PMP. The research results may be employed in membrane design.

Non-linear gas desorption and transport behavior in coal matrix: Experiments and numerical modeling

Gas desorption and transport in coal matrix plays pivot roles to estimate in situ gas content, forecast gas production from coalbed methane (CBM) wellbores, classify the gas/coal outburst proneness of coal seams and estimate gas emission rate for active mine ventilation planning. Only using Fick’s law to depict methane transport in coal matrix may result in an erroneous prediction because it uses only adsorbed phase gas to calculate methane concentration gradient. In this study, a series of coal-methane ad/desorption experiments were carried out under different pressure boundary conditions. Following this, an effort is made to propose a semi-empirical desorption model describing the entire methane diffusion process and discuss its superiority and applicability by comparing to various commonly used models. The proposed approach includes two different theoretical models (Fick diffusion model, assuming concentration-difference transports gas; and Density model, assuming density-difference transports gas), to model methane diffusion corresponding to the experimental sections conducted in this study. Afterward a series of comparisons between the experimental desorption data and two sets of simulated desorption data obtained by numerically calculating the two theoretical models were conducted, and it shows that Density model exhibited a higher accuracy over Fick model. The proposed Density model is more effective in describing the non-linear gas diffusion behavior in coal matrix for the experimentally studied coals. Essentially, the Density model covers and promotes the Fick diffusion model, and is competent in mathematically modeling both adsorbing gas and non-adsorbing gas transport behavior in porous media. Moreover, the Density model can be directly incorporated to the existing dual-porosity model to model methane migration in coal matrix in coal seam.

Premixed combustion of low-concentration coal mine methane with water vapor addition in a two-section porous media burner

During the transport of low-concentration coal mine methane (CMM), water mist spraying into the pipeline is used to eliminate the risk of explosion, but it leaves abundant vapor in the methane. This study was aimed to explore the effects of water vapor addition on the low-concentration CMM combustion in porous media. Thereby, a 2D numerical model based on a two-section ceramic foam burner setup with high flame stability was established and multi-step kinetics mechanisms were imported to the model. In this paper, the effects of vapor concentrations on the temperature distribution, flame stability limit, and chemical reaction during low-concentration CMM combustion in ceramic foam were investigated. Results indicate that with the increase of vapor mole fraction in the inlet methane, the overall temperatures in the downstream section of the burner gradually decreased, while the vapor mole fractions were linearly and negatively correlated with the peak temperatures in the burner. A small amount of vapor was involved in the chemical reactions of combustion, and with the increase of vapor mole fraction, more vapor took part in the reactions when the vapor addition into the inlet methane was unchanged. As the vapor mole fraction in the low-concentration CMM increased, the velocity range of flame stability limit was gradually narrowed down. In addition, the lower limit of velocity changed very slightly and maintained at 0.13–0.20 m/s, while the upper limit dropped obviously. The key elementary reactions underlying the effect of vapor on combustion reactions were determined by defining the changing rate of peak reaction rate. Addition of vapor into methane affected the peak rate of each elementary reaction, and altered the area of axial region where elementary reactions occurred.

A simulation model for methane emissions from landfills with interaction of vegetation and cover soil

Global climate change and ecological problems brought about by greenhouse gas effect have become a severe threat to humanity in the 21st century. Vegetation plays an important role in methane (CH4) transport, oxidation and emissions from municipal solid waste (MSW) landfills as it modifies the physical and chemical properties of the cover soil, and transports CH4 to the atmosphere directly via their conduits, which are mainly aerenchymatous structures. In this study, a novel 2-D simulation CH4 emission model was established, based on an interactive mechanism of cover soil and vegetation, to model CH4 transport, oxidation and emissions in landfill cover soil. Results of the simulation model showed that the distribution of CH4 concentration and emission fluxes displayed a significant difference between vegetated and non-vegetated areas. CH4 emission flux was 1–2 orders of magnitude higher than bare areas in simulation conditions. Vegetation play a negative role in CH4 emissions from landfill cover soil due to the strong CH4 transport capacity even though vegetation also promotes CH4 oxidation via changing properties of cover soil and emitting O2 via root system. The model will be proposed to allow decision makers to reconsider the actual CH4 emission from vegetated and non-vegetated covered landfills.

Methane hydrate formation in thick sand reservoirs: 1. Short-range methane diffusion

We show with a comprehensive hydrate formation model that diffusion of dissolved methane along the local concentration gradient alone cannot transport enough methane to form the concentrated methane hydrates in thick (>= several m) marine sand reservoirs. In this model, methane is generated locally by biodegradation of organic matter in mudstones and diffused into the bounding sandstone where the methane hydrate solubility decreases abruptly and hydrate precipitates from the extra methane in water. Hydrate formed by local diffusion has peak concentrations at the top and base of the sandstone, and it decreases sharply toward the center; salinity is slightly elevated relative to seawater, and its gradient is negligible across the sandstone and adjacent mudstones; dissolved methane concentration decreases from the mudstones to the sandstone. Overpressure is generated in the sediment due to reduction of sediment permeability and volume expansion when hydrate replaces pore water. There is a shallow zone where pore pressure converges toward the lithostatic pressure and fracturing is possible. This study provides further insights into hydrate formation by local diffusion of dissolved methane. In addition, the numerical model we developed in this study fully couples the hydrate-liquid capillary effect, dynamic pore size change, and transient pore pressure-dependent sediment compaction with the multiphase (gas, liquid and hydrate) flow and multicomponent (methane, water and salt) transport. Our model not only predicts pore pressure, but also incorporates the effect of hydrate formation on overpressure generation and subsequent dissipation. These modeling approaches are now at a level of sophistication that we can start to distinguish what physical processes, of the several proposed, are viable explanations for observed hydrate deposits.

A kinetic Monte Carlo approach to study fluid transport in pore networks.

The mechanism of fluid migration in porous networks continues to attract great interest. Darcy's law (phenomenological continuum theory), which is often used to describe macroscopically fluid flow through a porous material, is thought to fail in nano-channels. Transport through heterogeneous and anisotropic systems, characterized by a broad distribution of pores, occurs via a contribution of different transport mechanisms, all of which need to be accounted for. The situation is likely more complicated when immiscible fluid mixtures are present. To generalize the study of fluid transport through a porous network, we developed a stochastic kinetic Monte Carlo (KMC) model. In our lattice model, the pore network is represented as a set of connected finite volumes (voxels), and transport is simulated as a random walk of molecules, which "hop" from voxel to voxel. We simulated fluid transport along an effectively 1D pore and we compared the results to those expected by solving analytically the diffusion equation. The KMC model was then implemented to quantify the transport of methane through hydrated micropores, in which case atomistic molecular dynamic simulation results were reproduced. The model was then used to study flow through pore networks, where it was able to quantify the effect of the pore length and the effect of the network's connectivity. The results are consistent with experiments but also provide additional physical insights. Extension of the model will be useful to better understand fluid transport in shale rocks.

Sulfate geoengineering impact on methane transport and lifetime: results from the Geoengineering Model Intercomparison Project (GeoMIP)

Abstract. Sulfate geoengineering (SG), made by sustained injection of SO2 in the tropical lower stratosphere, may impact the CH4 abundance through several photochemical mechanisms affecting tropospheric OH and hence the methane lifetime. (a) The reflection of incoming solar radiation increases the planetary albedo and cools the surface, with a tropospheric H2O decrease. (b) The tropospheric UV budget is upset by the additional aerosol scattering and stratospheric ozone changes: the net effect is meridionally not uniform, with a net decrease in the tropics, thus producing less tropospheric O(1D). (c) The extratropical downwelling motion from the lower stratosphere tends to increase the sulfate aerosol surface area density available for heterogeneous chemical reactions in the mid-to-upper troposphere, thus reducing the amount of NOx and O3 production. (d) The tropical lower stratosphere is warmed by solar and planetary radiation absorption by the aerosols. The heating rate perturbation is highly latitude dependent, producing a stronger meridional component of the Brewer–Dobson circulation. The net effect on tropospheric OH due to the enhanced stratosphere–troposphere exchange may be positive or negative depending on the net result of different superimposed species perturbations (CH4, NOy, O3, SO4) in the extratropical upper troposphere and lower stratosphere (UTLS). In addition, the atmospheric stabilization resulting from the tropospheric cooling and lower stratospheric warming favors an additional decrease of the UTLS extratropical CH4 by lowering the horizontal eddy mixing. Two climate–chemistry coupled models are used to explore the above radiative, chemical and dynamical mechanisms affecting CH4 transport and lifetime (ULAQ-CCM and GEOSCCM). The CH4 lifetime may become significantly longer (by approximately 16 %) with a sustained injection of 8 Tg-SO2 yr−1 starting in the year 2020, which implies an increase of tropospheric CH4 (200 ppbv) and a positive indirect radiative forcing of sulfate geoengineering due to CH4 changes (+0.10 W m−2 in the 2040–2049 decade and +0.15 W m−2 in the 2060–2069 decade).

Identification of conditions for increased methane storage capacity in sII and sH clathrate hydrates from Monte Carlo simulations

Abstract The current study utilizes Grand Canonical Monte Carlo simulations in order to calculate the amount of methane gas that can be stored inside hydrate structures sII and sH. The temperature and pressure conditions examined include those of interest to the industrial methane transportation, and the storage of methane in industrial settings or naturally-occurring hydrate deposits. To this purpose, the amount of methane stored in the different type of cages that form the hydrate structures is presented as Langmuir-type “absorption isotherms”, which are functions of pressure, temperature and hydrate structure. The obtained expressions are subsequently used in order to identify conditions (i.e., pressure temperature, hydrate promoter molecular weight, and cage fraction occupied by the hydrate promoter) where the considered hydrate structures can have storage capacities that are higher than the storage capacity of the more common sI methane hydrate. The results are also discussed in the context of storage capacities of natural hydrates that are under conditions of existing hydrate-bearing sediments.

The vertical transport of methane from different potential emission types on Mars

AbstractThe contrasting evolutionary behavior of the vertical profile of methane from three potential release scenarios is analyzed using a global circulation model with assimilated temperature profiles. Understanding the evolving methane distribution is essential for interpretation of future retrievals of the methane vertical profile taken by instruments on the ExoMars Trace Gas Orbiter spacecraft. We show that at methane release rates constrained by previous observations and modeling studies, discriminating whether the methane source is a sustained or instantaneous surface emission requires at least 10 sols of tracking the emission. A methane source must also be observed within 5 to 10 sols of the initial emission to distinguish whether the emission occurs directly at the surface or within the atmosphere via destabilization of metastable clathrates. Assimilation of thermal data is shown to be critical for the most accurate backtracking of an observed methane plume to its origin.

Production, consumption, and migration of methane in accretionary prism of southwestern Taiwan

AbstractTo systematically quantify the production, consumption, and migration of methane, 210 sediment cores were collected from offshore southwestern Taiwan and analyzed for their gas and aqueous geochemistry. These data, combined with published results, were used to calculate the diffusive methane fluxes across different geochemical transitions and to develop scenarios of mass balance and constrain deep microbial and thermogenic methane production rates within the accretionary prism. The results showed that methane diffusive fluxes ranged from 2.71 × 10−3 to 2.78 × 10−1 and from –1.88 × 10−1 to 3.97 mmol m−2 d−1 at the sulfate‐methane‐transition‐zone (SMTZ) and sediment‐seawater interfaces, respectively. High methane fluxes tend to be associated with structural features, suggesting a strong structural control on the methane transport. A significant portion of ascending methane (>50%) is consumed by anaerobic oxidation of methane at the SMTZ at most sites, indicating effective biological filtration. Gas compositions and isotopes revealed a transition from the predominance of microbial methane in the passive margin to thermogenic methane at the upper slope of the active margin and onshore mud volcanoes. Methane production and consumption at shallow depths were nearly offset with a small fraction of residual methane discharged into seawater. The flux imbalance arose primarily due to the larger production of methane through deep microbial and thermogenic processes at a magnitude of 1512–43,096 Tg Myr−1 and could be likely accounted for by the sequestration of methane into hydrate forms, and clay absorption.

Assessing marine gas emission activity and contribution to the atmospheric methane inventory: A multidisciplinary approach from the Dutch Dogger Bank seep area (North Sea)

AbstractWe present a comprehensive study showing new results from a shallow gas seep area in ∼40 m water depth located in the North Sea, Netherlands sector B13 that we call “Dutch Dogger Bank seep area.” It has been postulated that methane presumably originating from a gas reservoir in ∼600 m depth below the seafloor is naturally leaking to the seafloor. Our ship‐based subbottom echosounder data indicate that the migrating gas is trapped in numerous gas pockets in the shallow sediments. The gas pockets are located at the boundary between the top of the Late Pliocene section and overlying fine‐grained sediments, which were deposited during the early Holocene marine transgression after the last glaciation. We mapped gas emissions during three R/V Heincke cruises in 2014, 2015, and 2016 and repeatedly observed up to 850 flares in the study area. Most of them (∼80%) were concentrated at five flare clusters. Our repeated analysis revealed spatial similarities of seep clusters, but also heterogeneities in emission intensities. A first calculation of the methane released from these clusters into the water column revealed a flow rate of 277 L/min (SD = 140), with two clusters emitting 132 and 142 L/min representing the most significant seepage sites. Above these two flare clusters, elevated methane concentrations were recorded in atmospheric measurements. Our results illustrate the effective transport of methane via gas bubbles through a ∼40 m water column, and furthermore provide an estimate of the emission rate needed to allow for a contribution to the atmospheric methane concentration.

Inhibitory Effect of Adsorbed Water on the Transport of Methane in Carbon Nanotubes.

We investigate the transport diffusion of methane at 300 K and pressures of up to 15 bar in dry and wetted carbon nanotubes (CNTs) having diameters ranging from 0.95 to 2.034 nm using nonequilibrium molecular dynamics (NEMD) simulation. Because of their strong hydrogen bonding, preadsorbed water molecules transport in the form of clusters and block the diffusion of methane, reducing the Onsager coefficient of methane dramatically compared to that in dry CNTs. The reduction in the methane Onsager coefficient is greater in narrower CNTs or at higher water densities. Because the diameter of the water clusters is almost invariant with water density, the Onsager coefficient of water in the (10, 10) CNT increases linearly with water density. It is further found that whereas decreasing the CNT diameter from 2.034 to 0.95 nm enhances the Onsager coefficient of pure methane by about 1 order of magnitude, the Onsager coefficient of water is almost independent of the CNT diameter at a water density of 0.05 g/cm3. We propose a theoretical model for the strong dependency of methane diffusion in wetted CNTs on the Onsager coefficient of water, the preadsorbed water density, and the CNT diameter. The model predicts the Onsager coefficients of the methane/water mixture from the Onsager coefficients of the pure components. Our study provides a basic understanding of the coupled diffusion of immiscible components in nanochannels and will facilitate progress in gas storage and carbon capture as well as nanofiltration and biomedical and biotechnological applications.

Modeling and Application of Gas Pressure Measurement in Water-Saturated Coal Seam Based on Methane Solubility

A method based on methane solubility was proposed to determine the gas pressure of a coal seam after a borehole was invaded by formation water. First, methane transport equations in a water-saturated coal seam and a water-filled borehole were established, which considered the effect of stress, density-driven convection and solute diffusion. Numerical simulation software was used to investigate the characteristics of the methane dissolution and transport. Second, we established a set of measuring devices to test the methane concentration in the borehole water. Using the No. 10 coal seam of the Taoyuan Coal Mine as the focus area, we tested the dissolved methane concentration over time, and the results were well correlated with the numerical simulation. The coal seam gas pressure was calculated to be 0.35 MPa by testing Henry’s constant of the field water. Finally, the applicability of this method was analyzed. The results showed that the effects of stress and convection had little impact on methane transport, indicating that this method was less affected by external influences. This paper provides a new idea for gas pressure measurement in water-rich coal strata.

Transport Mechanism of Guest Methane in Water-Filled Nanopores

We computed the transport of methane through 1 nm wide slit-shaped pores carved out of selected solid substrates using classical molecular dynamics simulations. The transport mechanism was elucidated via the implementation of the well-tempered metadynamics algorithm, which allowed for the quantification and visualization of the free energy landscape sampled by the guest molecule. Models for silica, magnesium oxide, alumina, muscovite, and calcite were used as solid substrates. Slit-shaped pores of width 1 nm were carved out of these materials and filled with liquid water. Methane was then inserted at low concentration. The results show that the diffusion of methane through the hydrated pores is strongly dependent on the solid substrate. While methane molecules diffuse isotropically along the directions parallel to the pore surfaces in most of the pores considered, anisotropic diffusion was observed in the hydrated calcite pore. The differences observed in the various pores are due to local molecular prope...

Combined application of biochar and slow-release fertilizer reduces methane emission but enhances rice yield by different mechanisms

There has been an increased interest in and wide application of biochar and slow-release fertilizer (SRF) to agricultural soils in recent years because they can reduce greenhouse gas emissions, but increase rice productivity. However, the studies considering combined effects of biochar and SRF are rare. This study examined the combined effects of biochar and SRF on the biogeochemistry, rice productivity, methane emission, and microbial abundances in rice paddy. The study sites included six different treatment combinations: urea (NU), SRF (NS), straw+urea (SU), straw+SRF (SS), biochar+urea (BU), and biochar+SRF (BS). Both the biochar and SRF reduced the methane emission, and the BS paddy soil had the lowest methane emission, while it had the highest rice yield. The biochar inhibited methanogenesis by increasing the soil aeration and oxygen availability. The SRF decreased the plant biomass, thus they may decrease plant-mediated methane transport and carbon substrate from plant debris and root exudates. Increasing in the abundance of methane-oxidizing bacteria was assumed to have critical impact on the reduction in methane emission by biochar. In conclusion, combined application of biochar and SRF highly recommended in rice cultivation, because they can minimize the methane emission but maximize rice yield.

Semiclathrate hydrates of methane + tetraalkylammonium hydroxides

Clathrate hydrate crystallisation is a promising method for the capture, storage and transport of methane, but one of its major technical barriers is the high pressure required for hydrate formation. In search of suitable and cost-effective promoters for hydrate-based gas processing and handling, this paper reports the pressures required for the formation of clathrate hydrates of methane in the presence of one of the following three quaternary ammonium salts with the same anion (OH−) but different carbon chain lengths: tetrapropylammonium hydroxide (TPrAOH), tetrabutylammonium hydroxide (TBAOH), and tetrapentylammonium hydroxide (TPeAOH). The phase boundary between hydrate–liquid–vapor (H-L-V) phases and liquid–vapor (L-V) phases in the temperature range of 281.81–301.91K and pressure range of 4.46–16.05MPa was determined using an isochoric equilibrium step-heating pressure search method. The results show that TBAOH is a strong thermodynamic promoter for methane hydrate formation whereas TPrAOH and TPeAOH are weak hydrate inhibitors. We then compared the phase equilibrium conditions and gas storage capacities of TBAOH and a widely studied hydrate promoter, tetrabutylammoium bromide (TBAB). It was found that at a given dosage, TBAOH substantially outperformed TBAB in thermodynamically promoting methane hydrate formation and with an appropriate driving force TBAOH gave only slightly lower methane storage capacity than TBAB. These features make TBAOH a prospective promoter for hydrate-based methane storage and transport.