Gas Diffusion Process Research Articles

A sectioning method for the kinetics study on anthracite pulverized coal combustion

The combustion process of pulverized coal is widely used in different industries as a supplier of energy. Taking Gaoping coal as an example, combustion reaction kinetic of anthracite was studied by thermal analysis curves at different heating rates. Based on the physical and chemical characteristics of combustion process, the characteristic temperatures of combustion process are obtained. Then, anthracite combustion process is divided into two ranges from ignition to burnout temperatures at the temperature where the mass loss rate reaches the maximum. At first range, the combustion reaction temperature is relatively low and the ash layer (solid reaction product) is thin. The primary limiting element is the chemical reaction stage. With the temperature rising and the reaction going on, the chemical reaction rate is increased. Simultaneously, as the solid product is accumulated, the gaseous diffusion process is impeded. The rate-controlling step is turned into the diffusion process. Therefore, interfacial chemical reaction model and internal diffusion model are developed to describe the two ranges of combustion process. The results illustrate that the experimental data are fitted well and correlation coefficients (r 2) exceed 0.90. Finally, the corresponding kinetic parameters are obtained for the utilization of coal resource.

Analytical analysis of gas diffusion into non-circular pores of shale organic matter

The total of the gas in shale gas reservoirs is sourced from a combination of free, adsorbed and dissolved/diffused gas. The mechanisms of production of free and adsorbed gas have been studied by numerous researchers. In contrast, the evolution of the dissolved gas and its contribution to the total gas production is not well understood. In this study we model the effect of pore micro-structure in organic matter (OM) on the rate of gas production in shale reservoirs. In this regard, first, we solve the gas-in-solid diffusion equation over a general doubly connected spatial domain with external Neumann and internal Dirichlet boundary conditions. The obtained solution is applied systematically to multi-pore porous OM domains and then the effect of pore morphology on the rate of gas production is studied. Our model results show that pore geometry has a slight effect on the gas diffusion process, while total organic carbon, and OM porosity, pore size distribution and specific surface area, are dominant parameters. An abundance of very small pores in OM tremendously increases the diffuse gas contribution in the total gas reserve and production.

High activity oxide Pr0.3Sr0.7Ti0.3Fe0.7O3−δ as cathode of SOEC for direct high-temperature steam electrolysis

A high activity ferrite Pr0.3Sr0.7Ti0.3Fe0.7O3−δ (PSTF) has been synthesized and examined as a cathode of solid oxide electrolysis cell (SOEC) for direct high-temperature steam electrolysis. The SOEC with a configuration of PSTF|YSZ|LSM-YSZ was operated under H2O concentrations ranging from 20%H2O/Ar to 60%H2O/Ar and exhibited excellent electrochemical performances. Polarization resistance of the electrolyzer was as small as 0.43 Ω cm2 in 60%H2O/Ar at 1.85 V at 800 °C. According to AC impendence spectra analyzing, gas diffusion process was the rate-determine-step (RDS) under smaller current density, while under larger current density, transport properties in the electrodes and the interfaces of electrode/electrolyte was RDS. The electrochemical properties of PSTF cathodes were systematically investigated and compared when they were exposed to gas atmosphere with and without safe gas (H2). The obtained results demonstrated that PSTF electrode could conceivably avoid any hydrogen feeding for steam electrolysis.

Effective gas diffusion coefficient in fibrous materials by mesoscopic modeling

This paper presents a method to establish a relationship between internal microstructure and the effective gas diffusion coefficient in fibrous materials via a mesoscopic modeling approach and, when possible and based on the analysis, to propose user-friendly formulas as functions of structural parameters for practical engineering applications. The entire numerical framework includes two main parts: a random generation-growth method to reconstruct the digital microstructures of fibrous materials based on experimental statistical information of the actual structure, and then a high-efficiency lattice Boltzmann algorithm for modeling the gas diffusion process through porous structures. The predictions are then validated by existing experimental data for both dry and saturated fibrous materials. Owing to the unique robustness of the developed modeling approaches, we are then able to conduct a parametric analysis, more detailed than ever, of the influences on the system effective diffusion coefficient in fibrous materials by such important parameters as structural anisotropy, system water content, microstructure morphology and the layering space in a laminated fibrous system. These results may improve our understanding of gas diffusion in fibrous materials, and this method may serve as a tool for easy estimation of effective diffusivity, leading to the optimal design of fibrous materials.

Time-Dependent Methane Diffusion Behavior in Coal: Measurement and Modeling

In-depth understanding of how methane diffuses in porous media like coal is critical for enhanced coalbed methane recovery and coal seam methane drainage. However, the classic unipore gas diffusion model can only describe methane diffusion behavior in coal at the early time diffusion stage. Describing the whole methane diffusion behavior in coal has not been reasonably addressed in the coalbed methane field. Considering the large surface-to-volume ratio of coal, the authors proposed a time-dependent gas diffusion model to describe the whole diffusion process. This work first proposed a power–law relationship between diffusion coefficient and time according to previous studies of the time-dependent gas diffusion process in porous media. Then, both the analytical solution and its approximation solution for the time-dependent gas diffusion model were derived. Six methane diffusion tests in crushed coal were conducted to validate the proposed model under different pressures (0.5, 1.5, 2.5 MPa) and temperatures (20 and \(30\,{^{\circ }}\hbox {C}\)). Modeling results show that the time-dependent gas diffusion model is superior to the classic unipore gas diffusion model for describing the whole methane diffusion process in coal. Increasing temperature always accelerates methane diffusion in coal; the higher the temperature the larger the diffusion coefficient.

Effects of gas diffusion from fractures to coal matrix on the evolution of coal strains: Experimental observations

In this study, we investigate coal matrix swelling during the process of gas diffusion from cleats and its impact on the evolution of strain. We inject helium into both jacketed and unjacketed coal samples within a pressurized core holder. In the unjacketed experiment, the coal sample was exposed to hydrostatic gas pressure. In the jacketed experiment, the coal sample was constrained by fixed displacement at the two ends and by constant confining pressure laterally. In both experiments, strains were measured continuously both parallel/perpendicular to the bedding plane of the coal as internal pressure of helium is increased. For the unjacketed test the initial rapid compactive deformation of the sample rebounds to dilative at long term — attributed to the gas diffusion-induced swelling of the coal matrix. For the jacketed test both axial and circumferential coal strains are first dilative due to the decrease in effective stress with subsequent changes of coal strain induced by the gas diffusion closely associated with the specific boundary conditions. For a constant stress boundary, the circumferential strain first remains unchanged for some time and then gradually increases to steady state. Such a phenomenon suggests that when gas diffuses from the cleat to the matrix, the coal matrix-cleat system probably undergoes a transition from local swelling to macroswelling. In contrast, for a fixed displacement boundary, the axial strain first decreases and then reaches a steady state. It also indicates that the swelling of the matrix not only decreases the aperture of the fracture, but also expands the circumferential profile of the coal sample due to the Poisson effect. In addition, helium diffusion in the matrix is estimated by using the bidisperse diffusion model. The calculated effective diffusivity increases with pore pressure. This suggested that the gas diffusion coefficient is a pore pressure-dependent parameter. These results demonstrate that gas diffusion from the cleat to the matrix can cause the swelling of the coal matrix, change the aperture of the cleat, and in turn impact the evolution of the coal permeability.

Gas content derivative data versus diffusion coefficient

The study of the gas diffusion process has a main role in both coalbed methane (CBM) production and CO2 injection in geological sequestration projects. The accurate determination of gas diffusion coefficients in unconventional reservoirs such as coal seams requires a consistent mathematical approach. The study of the gas diffusion process in coal seams was carried out using sorption isotherms. The Langmuir model for individual gases and the extended Langmuir model for multicomponent gas mixtures were applied to fit sorption isotherm data. “Gas content derivative data” and “gas content changes” emerged as crucial mathematical parameters to accurately study the gas diffusion process. The main goal of this paper is to define the degree of interaction between the gas content derivative data and the gas diffusion process. Experiments were performed on three samples selected from two different coals, which were submitted to three different gas compositions, viz 99.999% CH4; 99.999% CO2; and a gas mixture containing 74.99% CH4 + 19.99% CO2 + 5.02% N2, at 35℃, and at pressures ranging from 0 up to 50 bar. Experimental results obtained from the three samples indicate that during adsorption/desorption processes, the diffusion coefficients increase and the gas content changes decrease when the pressure decreases, due to the sample saturation degrees and to the kinetic mechanisms increase. Additionally, the “gas content derivative data” scattering is slightly lower during the desorption process than during the adsorption process. These behaviours are clearly identified when using methane, but are even more evident when using CO2 and the gas mixture, due to the CO2 interaction with coal porous structure, which induces a considerable resistance to CO2 release. The results show that sample B (CH4 + CO2 + N2) displays higher diffusion coefficient values (this behaviour is mainly related to the presence of N2) than sample C (CH4) and than sample A (CO2).

Three-dimensional modeling and experimental validation of thermomechanical response of FRP composites exposed to one-sided heat flux

The heat transfer, gas diffusion process and thermomechanical deformation are generally coupled and associated with the chemical decomposition for fiber reinforced polymer composites at elevated temperatures. The three-dimensional (3-D) governing differential equations for the coupled temperature-diffusion-deformation problem of porous elastomers were developed. The thermomechanical behavior of a silica/phenolic composite material was predicted using the mathematical model. The spatially dependent temperature and pore pressure, displacement, and stress contours of silica/phenolic composites exposed to one-sided radiant heat flux were investigated. Based on the digital image correlation technique, a non-contact high temperature deformation measurement test was conducted. The temperature profiles were measured by the thermocouples embedded in different depths of the specimen, while the full-field displacements and strains were provided by correlating the two digital images of the specimen surface in the un-deformed and deformed states, respectively. The accuracy of the proposed model was assessed by comparing the predicted temperatures and displacements with experimental values for the same boundary and initial conditions.

Quantitative visualization study on diffusion of oxygen using UV-LED induced phosphorescence

The aims of this study are to visualize gas diffusion process between gas–liquid interface and measure a diffusion coefficient of dissolved oxygen in water by using a microscale UV-LED-induced phosphorescence method. The mass transfer between air and water was visualized qualitatively in a Y-shaped microchannel and the diffusion coefficient D L was measured quantitatively. Oxygen-sensitive phosphorescence dye (tris (2, 2′-bipyridine) ruthenium (II) chloride hexahydrate) and a 450 nm UV-LED were used for the optical measurement of a dissolved oxygen concentration (DOC) field. In-situ pixel-by-pixel calibration was carried out to obtain Stern–Volmer constant and phosphorescent intensity was converted to DOC field with a spatial resolution of 0.625 μm/pixel. Instantaneous dissolved oxygen concentration field and diffusion coefficient D L were successfully measured and compared to the theoretical prediction based on the one-dimensional Fick’s law. It was confirmed that the experimental results were in good agreement with the numerical prediction.

Laser spectroscopic studies of gas diffusion in alumina ceramics.

Experiments for measuring gas diffusion through porous alumina ceramics are described. With the gas in scattering media absorption spectroscopy (GASMAS) technique, gaseous oxygen signals are measured continuously during the gas diffusion process. It is experimentally demonstrated that the time-dependence of the transient oxygen signal is described by an exponential curve. Moreover, the effect on gas diffusion of material porosity and whether water is present or not is also experimentally investigated.

The Influence of Gas Diffusion on the Dynamics of A Spherical Layer of Viscous Incompressible Liquid and Heat and Mass Transfer in it

The formation of spherical microballoons in the case of short-term weightlessness is investigated numerically. The algorithm of the numerical solution of the problem is described and the results of numerical studies of the formation of liquid glass microballoons, saturated with carbon dioxide, are presented. The results of calculations for the problem in the full statement (mathematical model includes the influence of inertial, thermal and diffusive factors) and simplified statement, when the process of gas diffusion is not taken into account, are compared.

Hydrate-based hydrogen purification from simulated syngas with synergic additives

Hydrated-based separation process has recently raised as a novel technology for hydrogen (H2) purification from mixture gases containing carbon dioxide (CO2). However, the practical application requires suitable hydrate formation condition, hydrate formation rate, gas selectivity, gas storage capacity and so on. This work presents the effect of synergic additives on the hydrate-based syngas purification process based on the thermodynamic, kinetic, purification characteristic studies as well as microcosmic structure analysis. The synergic additives comprise traditional hydrate promoter (tetra-n-butyl ammonium bromide, TBAB) and gas solvent (dimethyl sulfoxide, DMSO). 40.2 mol% CO2/H2 binary mixtures were selected as the simulated syngas. The results show that the synergic additives (TBAB-DMSO) can reduce the equilibrium hydrate formation pressure of the simulated syngas by 80%. Compared to single TBAB for the hydrate-based purification process, the synergic additives could improve the gas consumption rate of the unit system, gas storage capability and gas selectivity by about 32%, 105% and 51% respectively, and decrease the loss ratio of H2 by 1.45%. The Raman analysis reveals that the simulated syngas forms semiclathrate framework with TBAB-DMSO. DMSO only performs as a gas solvent during the gas dissolution and diffusion process, and not participates in the hydrate framework formation.

Analysis of fission gas release-to-birth ratio data from the AGR irradiations

A series of advanced gas reactor (AGR) irradiation tests is being conducted in the advanced test reactor (ATR) at Idaho National Laboratory (INL) in support of development and qualification of tristructural isotropic (TRISO) fuel used in the High temperature gas-cooled reactor (HTGR). Each AGR test consists of multiple independent capsules containing fuel compacts placed in a graphite cylinder shrouded by a steel shell. These capsules are instrumented with thermocouples (TC) embedded in the graphite enabling temperature control. For AGR-1, the first US irradiation of modern TRISO fuel completed in 2009, there were no particle failures detected. For AGR-2, a few exposed kernels existed in the fuel compacts based upon quality control data. For the AGR-3/4 experiment, particle failures in all capsules were expected because of the use of designed-to-fail (DTF) fuel particles whose kernels are identical to the driver fuel kernels and whose coatings are designed to fail under irradiation. The release-rate-to-birth-rate ratio (R/B) for each of krypton and xenon isotopes is calculated from release rates measured by the germanium detectors used in the AGR fission product monitoring (FPM) system installed downstream from each irradiated capsule. Birth rates are calculated based on the fission power in the experiment and fission product generation models. Thus, this R/B is a measure of the ability of fuel particle coating layers and compact matrix to retain fission gas atoms preventing their release into the sweep gas flow. The major factors that govern gaseous diffusion and release processes are found to be fuel material diffusion coefficient, temperature, and isotopic decay constant. To compare the release behavior among the AGR capsules and historic experiments, the R/B per failed particle is used. HTGR designers use this parameter in their fission product behavior models. For the U.S. TRISO fuel, a regression analysis is performed to establish functional relationships between R/B per failed particle of selected noble gas isotopes and temperature. The effect of isotopic half-life is also obtained from the data. To reduce measurement uncertainty of the release rate, the krypton and xenon isotopes selected for regression analysis have a short enough half-life to achieve equilibrium in the capsule, but are also long enough to provide a measurable signal in the FPM detector. The impact of uncertainty of the estimated number of failed particles in each capsule is also examined. This study found that R/B values for AGR test fuel are comparable to R/B obtained in historic tests. The correlation with isotopic half-life is very stable with time indicating no strong influence of burnup on release. The correlation can be used by reactor designers to estimate fission gas release from postulated failed fuel in HTGR cores.

A critical comparison of constant and pulsed flow systems exploiting gas diffusion

Considering the beneficial aspects arising from the implementation of pulsed flows in flow analysis, and the relevance of in-line gas diffusion as an analyte separation/concentration step, influence of flow pattern in flow systems with in-line gas diffusion was critically investigated. To this end, constant or pulsed flows delivered by syringe or solenoid pumps were exploited. For each flow pattern, two variants involving different interaction times of the donor with the acceptor streams were studied. In the first one, both the acceptor and donor streams were continuously flowing, whereas in the second one, the acceptor was stopped during the gas diffusion step. Four different volatile species (ammonia, ethanol, carbon dioxide and hydrogen sulfide) were selected as models. For the flow patterns and variants studied, the efficiencies of mass transport in the gas diffusion process were compared, and sensitivity, repeatability, sampling frequency and recorded peak shape were evaluated. Analysis of the results revealed that sensitivity is strongly dependent on the implemented variant, and that flow pattern is an important feature in flow systems with in-line gas diffusion.

Modeling photosynthesis of discontinuous plant canopies by linking the Geometric Optical Radiative Transfer model with biochemical processes

Abstract. Modeling vegetation photosynthesis is essential for understanding carbon exchanges between terrestrial ecosystems and the atmosphere. The radiative transfer process within plant canopies is one of the key drivers that regulate canopy photosynthesis. Most vegetation cover consists of discrete plant crowns, of which the physical observation departs from the underlying assumption of a homogenous and uniform medium in classic radiative transfer theory. Here we advance the Geometric Optical Radiative Transfer (GORT) model to simulate photosynthesis activities for discontinuous plant canopies. We separate radiation absorption into two components that are absorbed by sunlit and shaded leaves, and derive analytical solutions by integrating over the canopy layer. To model leaf-level and canopy-level photosynthesis, leaf light absorption is then linked to the biochemical process of gas diffusion through leaf stomata. The canopy gap probability derived from GORT differs from classic radiative transfer theory, especially when the leaf area index is high, due to leaf clumping effects. Tree characteristics such as tree density, crown shape, and canopy length affect leaf clumping and regulate radiation interception. Modeled gross primary production (GPP) for two deciduous forest stands could explain more than 80% of the variance of flux tower measurements at both near hourly and daily timescales. We demonstrate that ambient CO2 concentrations influence daytime vegetation photosynthesis, which needs to be considered in biogeochemical models. The proposed model is complementary to classic radiative transfer theory and shows promise in modeling the radiative transfer process and photosynthetic activities over discontinuous forest canopies.

Preparation of Janus Pd/SiO₂ nanocomposite particles in inverse miniemulsions.

Janus Pd/SiO2 nanocomposite particles (NCPs) were successfully synthesized through a combination of the sol-gel process of tetramethoxysilane in inverse miniemulsions and in situ reduction of Pd salts via a gas diffusion process of hydrazine. The formation of Pd nanoparticles (NPs) was verified by X-ray diffraction. The Janus morphology of the Pd/SiO2 NCPs was confirmed by microscopic observation. The Pd/SiO2 NCPs displayed a mesoporous structure. The content of Pd NPs in the NCPs could be conveniently adjusted by the K2PdCl4 loading. A formation mechanism of the Janus Pd/SiO2 NCPs was proposed. The mesoporous Janus Pd/SiO2 NCPs show good catalytic activity toward the reduction of p-nitrophenol with NaBH4.

Hydrate-based CO2 capture and CH4 purification from simulated biogas with synergic additives based on gas solvent

Treatment and subsequent use of biogas are garnering huge interest for both energy recovery and mitigation of environmental impact. However, separation process is pivotal for increasing its calorific value and removing CO2. This work presents the kinetic and separation efficiency study as well as microcosmic structure analysis for purifying simulated biogas (45.0mol% CO2/CH4 binary mixture) through hydrate crystallization approach. Particularly, synergic additives comprise gas solvent (dimethyl sulfoxide (DMSO)) and traditional hydrate promoter (tetrahydrofuran (THF) or tetra-n-butyl ammonium bromide (TBAB)) were proposed to enhance the hydrate-based separation process. The promotion mechanism was explored through in-situ Raman spectroscopy. The residual gas phase and the decomposition gas phase from the hydrate slurry were sampled and analyzed. Based on the experimental data, the gas storage capacity, unit system gas consumed rate, gas selectivity and separation efficiency were calculated for evaluating the separation process. It was found that, the synergic additives could promote the mixture hydrate formation process due to DMSO (acid gas solvent) could improve both rate and selectivity of CO2 during the dissolution and diffusion processes. In addition, the Raman analysis reveals that the simulated biogas forms structure II hydrate and semiclathrate framework with THF–DMSO and TBAB–DMSO respectively, and CH4 molecules are only found in the smaller (512) cages of the mixture hydrates. It is inferred that DMSO just performs as an acid gas solvent during the gas dissolution and diffusion processes but not participate in the hydrate framework formation. It will be of practical interest in relation to resolving the bottleneck of hydrate-based biogas purification technology and of potential importance for the industry application of gas hydrate.

Durability Evolution of RC Bridge under Coupling Action of Chloride Corrosion and Carbonization Based on DLA Model

Chloride attack and carbonization are the main factors which affect the durability of concrete structures, and the respective theoretical models are systematically established. However, the quantitative analysis and models about the coupling effect of chloride attack and carbonization are less, so the precision and level of durability analysis of reinforced concrete are restricted. Diffusion-limited aggregation (DLA) model can finely simulate the process of gas diffusion and condensation with randomness and fractal characteristics, which is suitable for revealing the durability evolution process of the chloride attack, carbonization, and the coupling action in concrete. Based on the principle of DLA, considering the factors such as diffusion depth, concrete properties, and exposure conditions which influence the characteristics of chloride diffusion and carbonization, as well as the coupling effect, an integrated DLA model is established. The concentration of carbon dioxide and chloride at any time and any location can be obtained and dynamically displayed based on the DLA model. The performance predict method for concrete and steel bars considering fatigue effect is presented based on DLA, according to the demand for bridge durability analysis. Numerical examples show that the method can dynamically and intensively simulate the durability evolution process of reinforced concrete bridge.

Mass-transport-controlled, large-area, uniform deposition of carbon nanofibers and their application in gas diffusion layers of fuel cells.

The effect of mass transport on the growth characteristics of large-area vapor-grown carbon nanofibers (CNFs) was investigated by adjusting the substrate deposition angle (α). The catalyst precursor solution was coated onto one side of a 2D porous carbon paper substrate via a decal printing method. The results showed that the CNFs were grown on only one side of the substrate and α was found to significantly affect the growth uniformity. At α = 0°, the growth thickness, the density, the microstructure and the yield of the CNF film were uniform across the substrate surface, whereas the growth uniformity decreased with increasing α, suggesting that the large-area CNF deposition processes were mass-transport-controlled. Computational fluid dynamics simulations of the gas diffusion processes revealed the homogeneous distributions of the carbon-source-gas concentration, pressure, and velocity near the substrate surface at α = 0°, which were the important factors in achieving the mass-transport-limited uniform CNF growth. The homogeneity of the field distributions decreased with increasing α, in accordance with the variation in the growth uniformity with α. When used as a micro-porous layer, the uniform CNF film enabled higher proton exchange membrane fuel cell performance in comparison with commercial carbon black by virtue of its improved electronic and mass-transport properties confirmed by the electrochemical impedance spectroscopy results.

Measurement of gas diffusion coefficient in liquid-saturated porous media using magnetic resonance imaging

In this study, the dual-chamber pressure decay method and magnetic resonance imaging (MRI) were used to dynamically visualize the gas diffusion process in liquid-saturated porous media, and the relationship of concentration-distance for gas diffusing into liquid-saturated porous media at different times were obtained by MR images quantitative analysis. A non-iterative finite volume method was successfully applied to calculate the local gas diffusion coefficient in liquid-saturated porous media. The results agreed very well with the conventional pressure decay method, thus it demonstrates that the method was feasible of determining the local diffusion coefficient of gas in liquid-saturated porous media at different times during diffusion process.