Bituminous Coal Samples Research Articles

Experimental simulation of biogenic coalbed gas generation from lignite and high-volatile bituminous coals

To deeply understand the formation of biogenic gas and develop evaluation methods of coalbed methane resources, it is important to clarify the influence mechanism of coal on the formation of biogenic coalbed methane. This paper selected lignite samples from Longkou coalfield and high-volatile bituminous coal samples from the Huainan coalfield, China as substrates of biogenic gas generation in simulation experiment, and aimed to investigate the characteristics of biogas formation from two different coal ranks. The experiments lasted for 90 days, and the results showed that both low rank coals (lignite) and higher rank coals (bituminous coal) can be utilized by methanogenic bacteria to produce biogas. Lignite, which has a greater abundance of hydrogen and oxygen atoms, had a greater potential for biogas production than bituminous coal. However, the maximum generation rate of bituminous coal groups was higher than that of lignite groups in the initial stage. The whole process of biogas generation can be divided into the following three stages: 1) a rapid growth stage; 2) slow descent stage; and 3) inhibition stage, which was consistent with the fluctuant change of the CH4 and CO2 contents in headspace gas, indicating that CO2-utilizing methanogens are present in all experimental groups throughout the process, especially in the first stage in the lignite groups. The gas components from the bituminous coal groups consisted of CH4, CO2, and small amounts of heavy hydrocarbons (but H2 not ruled out), suggesting the presence of acetoclastic methanogenesis, which was also confirmed by more positive δ13C values of CH4 and CO2 (−39.2‰ to −30.2‰ and −20.6‰ to −19.2‰, respectively) and more negative δD-CH4 values (−334‰ to −325‰, except for lignite groups on the 20th day of the experiment). However, more positive δD-CH4 values for the lignite groups (−244‰ and −239‰) demonstrated the occurrence of hydrogenotrophic methanogenesis, which coincided with a high CO2 content and low CH4 content on the 20th day of the experiment.

Structural Evolution Characteristics of Middle–High Rank Coal Samples Subjected to High-Voltage Electrical Pulse

High-voltage electrical pulse (HVEP) technology has been proposed to increase the gas production of low-permeability coal reservoirs in recent years. In this study, we investigated the variation characteristics of the pore structure of coal samples by combining scanning electrical microscopy with mercury intrusion porosimetry analysis, to better understand the structural evolution characteristics of middle–high rank coal subjected to HVEP. Furthermore, changes in the chemical structure of the coal samples before and after HVEP treatment were investigated by Fourier transform infrared spectroscopy analysis. The results show that, under the action of HVEP, both anthracite and bituminous coal samples can be crushed into many small pieces. Because the conductivity of anthracite coal samples is better than that of bituminous coal samples, the average breakdown voltage of anthracite coal samples is lower than that of bituminous coal samples. It was found that the greater the breakdown voltage, the more the numb...

Application of thermally stimulated acoustic emission method to assess the thermal resistance and related properties of coals

Traditional methods of coal thermal resistance characterization are informative but considerably time-consuming and require utilization of a complex and expensive equipment. This limits the effectiveness of their application. In this paper, authors experimentally investigated potential application of thermally stimulated acoustic emission method for developing of relatively simple and rapid coals thermal resistance assessment method. Features of thermally stimulated acoustic emission of anthracite, lignites and bituminous coal samples subject to cyclic thermal loading have been experimentally investigated. For the first time, it has been shown that there exists a relationship of such patterns with structural parameters and properties of the coal samples, as well as their thermal resistance. The results indicate the possibility of applying the method of thermally stimulated acoustic emission to control processes of cryogenic disintegration and thermal resistance of fossil coals. The description of the equipment and methodological support needed for the implementation of this method have been provided.

Nanometre-sized pores in coal: Variations between coal basins and coal origin

We have used small angle neutron scattering (SANS) to investigate the differences in methane and hexane penetration in pores in bituminous coal samples from the U.S., Canada, South Africa, and China, and maceral concentrates from Australian coals. This work is an extension of previous work that showed consistent differences between the extent of penetration by methane into 10–20nm size pores in inertinite in bituminous coals from Australia, North America and Poland.In this study we have confirmed that there are differences in the response of inertinite to methane and hexane penetration in coals sourced from different coal basins. Inertinite in Permian Australian coals generally has relatively high numbers of pores in the 2.5–250nm size range and the pores are highly penetrable by methane and hexane; coals sourced from Western Canada had similar penetrability to these Australian coals. However, the penetrability of methane and hexane into inertinite from the Australian Illawarra Coal Measures (also Permian) is substantially less than that of the other Australian coals; there are about 80% fewer 12nm pores in Illawarra inertinite compared to the other Australian coals examined. The inertinite in coals sourced from South Africa and China had accessibility intermediate between the Illawarra coals and the other Australian coals.The extent of hexane penetration was 10–20% less than CD4 penetration into the same coal and this difference was most pronounced in the 5–50nm pore size range. Hexane and methane penetrability into the coals showed similar trends with inertinite content.The observed variations in inertinite porosity between coals from different coal regions and coal basins may explain why previous studies differ in their observations of the relationships between gas sorption behavior, permeability, porosity, and maceral composition. These variations are not simply a demarcation between Northern and Southern Hemisphere coals.

Study on coal char ignition by radiant heat flux.

The study on coal char ignition by CO2-continuous laser was carried out. The coal char samples of T-grade bituminous coal and 2B-grade lignite were studied via CO2-laser ignition setup. Ignition delay times were determined at ambient condition in heat flux density range 90–200 W/cm2. The average ignition delay time value for lignite samples were 2 times lower while this difference is larger in high heat flux region and lower in low heat flux region. The kinetic constants for overall oxidation reaction were determined using analytic solution of simplified one-dimensional heat transfer equation with radiant heat transfer boundary condition. The activation energy for lignite char was found to be less than it is for bituminous coal char by approximately 20 %.

Biogenic methane generation from Pakistani bituminous coal samples

ABSTRACTThree bituminous coal samples, from Salt Range, Makarwal, and Khost coalfields of Pakistan, were subjected to a bioassay, based on WBC-2, for determining the potential of biogenic methane generation from higher-rank coals. Under strict anerobic conditions, maximum methane yield was observed from Makarwal coal sample, that is, 0.275 µmoles per gram of coal. The results supported the idea of inhibited susceptibility of higher-rank coal toward biogenic methane generation. Low-rank coal still appears as a favorable target for stimulation or regeneration of methane, though, before achieving commercial-scale methane generation, there are a number of crucial factors, which need to be addressed.

Identification and nanoporosity of macerals in coal by scanning electron microscopy

Recent applications of scanning electron microscopy (SEM) to shale resource plays at magnifications of <500–>80,000× have reported nanoporosity in organic matter with limited interpretations of organic matter type. Macerals, inclusive of kerogen and solid bitumen, are recognized and distinguished in reflected white and epifluorescent light in coal and shale samples at magnifications of 200–750×. The objectives of this study are to identify macerals by SEM and evaluate which macerals contain primary and secondary nanoporosity. Since coals are organic rich with a better chance of identifying adjacent maceral types than when dispersed in shales, broad ion beam milled samples of humic and sapropelic (boghead and cannel) coals ranging in rank from peat to semianthracite were examined in backscattered electron (BSE) mode at low magnification (≤2,500×) to identify maceral type. Once identified, macerals were examined at higher magnifications of 1200–75,000× to assess maceral nanoporosity.Manipulation of the accelerating voltage to 10kV in BSE mode of a high volatile bituminous humic coal durain lithotype sample revealed a contrast between maceral groups (vitrinite, inertinite, liptinite), with limited identification of individual maceral types. Vitrinite maceral subgroups telovitrinite and detrovitrinite are distinguished based on their relative gray scale appearance compared to other macerals and occurrence as bands or groundmass, respectively. The liptinite macerals alginite, sporinite and cutinite are distinguished based on dark relative gray level and their shape. The liptinite maceral bituminite/amorphinite was recognized by dark relative gray level and occurrence as groundmass in a boghead coal. The inertinite macerals fusinite and semifusinite are recognized by light gray level appearance compared to other macerals and bogen structure but are not distinguishable separately. Macerals dispersed in shale, lacking the subtle contrast of adjacent macerals, are much more difficult to identify.Even though porosity is revealed at high magnification in BSE mode, too high of a magnification (>15,000×) prohibits identification of maceral types. The best approach is to examine samples at a lower magnification (e.g., 650×) at 10kV accelerating voltage in BSE mode to identify the maceral type and then go to a higher magnification at 1–2kV accelerating voltage to observe nanoporosity.Primary nanoporosity is observed within coal macerals at low rank (peat and subbituminous), but decreases in amount with increasing rank. Primary microporosity occurs as woody cell lumens in semifusinite and fusinite macerals. Secondary nanoporosity develops in post-oil solid bitumen in shale beginning below the peak of the oil window with a lack of nanoporosity at lower thermal maturity. Compared to the abundant nanoporosity of post-oil solid bitumen in shale, only trace amounts of nanoporosity is observed in other macerals in coals of high volatile bituminous rank and higher under the SEM.The emphasis of this study was the identification and nanoporosity of macerals in coal by SEM. The same results may extend to the same macerals in shale. Knowledge of organic matter porosity distribution by maceral type and development by thermal maturity provides insight for coalbed methane, shale gas and tight oil production potential.

Structural investigations of Eocene coals from foreland basin of central Nepal Himalaya

Structural investigations of Eocene coal from foreland basin of central Nepal Himalaya were subjected to attenuated total reflection Fourier transform infrared spectroscopy, X-ray diffraction, high-resolution transmission electron microscopy and scanning electron microscopy to study their chemical and structural characteristics. The attenuated total reflection Fourier transform infrared spectroscopy data show that the less matured coal sample (sub-bituminous) has more aliphatic moieties, whereas the relatively more matured (bituminous) coal samples contain higher fractions of poly-aromatic moieties and saturated long-chain hydrocarbons. Fourier transform infrared spectroscopy spectra reveals the presence of stretching vibrations of aliphatic –CH, –CH2 and –CH3 absorptions, –OH bonds, C=C and –CH of aromatic structures and C=O stretching vibrations of carbonyl groups. X-ray diffraction indicates the presence of crystalline carbon having turbostratic structure along with some highly disordered amorphous car...

Comparison of bituminous coal apparent activation energy in different heating rates and oxygen concentrations based on thermo gravimetric analysis

Simultaneous thermal analysis (STA) experiments are carried on to study the relationship between the coal apparent activation energy and the heating rate as well as the oxygen concentration, respectively. Bituminous coal samples are analyzed using the simultaneous thermal analyzer. Experimental condition involves two aspects: different heating rates (2, 5, 10, 20 and 30 °C min−1) and different oxygen concentrations (5, 10, 15, 21 and 30%). A mixture of oxygen and nitrogen flows through the coal sample. Characteristic points at different temperatures are obtained; Coats–Redfern integral method is utilized to calculate the apparent activation energy in different experimental conditions. The results indicate that the variation of apparent activation energy in different heating rates follows the exponential function mathematically, which falls in order as the heating rate increases. The apparent activation energy of coal increases in order as the oxygen concentration increases, and the variation of apparent activation energy in different oxygen concentrations follows the logarithmic function mathematically.

Physicochemical structure and gasification reactivity of co-pyrolysis char from two kinds of coal blended with lignocellulosic biomass: Effects of the carboxymethylcellulose sodium

To investigate the influencing mechanism of alkalis on char structure evolution and gasification reactivity during co-pyrolysis of coal and lignocellulosic biomass, carboxymethylcellulose sodium (CMC) was selected as a typical organic sodium salt and introduced to the pyrolysis of bituminous coal (BC) and anthracite coal (AC) respectively. Physicochemical characteristics of the char samples were examined by N2 adsorption/desorption measurement, scanning electron microscopy (SEM) and Raman spectra under different CMC mass ratio. Fractal theory and deconvolution method were applied to quantitatively analyze the surface morphology, pore property and microcrystalline structure of the char. Thermogravimetric analyzer and non-isothermal kinetics method were used to determine the gasification reactivity and kinetic parameters of the char. The results indicated that the addition of CMC promoted the development of pore structure from both BC and AC char samples. The fractal dimension can quantitatively describe the complexity and heterogeneity of pore structure and surface morphology of char sample. The fractal dimension obtained from SEM images of co-pyrolysis char was in range 1.41–1.68 and higher than that of the coal char, which meant CMC promoted the heterogeneity of co-pyrolysis char. Peak fitting analysis on the Raman spectra illustrated that the value of AD/AAll and AD/AG increased with the mass ratio of CMC, indicating that addition of CMC reduced the ordering of co-pyrolysis char structure. Synergistic effect was observed during the gasification process of co-pyrolysis char. The evolution of physicochemical structure and organic sodium lead to higher reactivity and lower activation energy during gasification of co-pyrolysis char than the raw coal char. This paper provides insight on the effects of organic sodium salt on products evolution during co-pyrolysis of coal and biomass.

Insight into effects of pore diffusion on smoldering kinetics of coal using a 4-step chemical reaction model

Pore diffusion plays a crucial role in coal smoldering fires, however, its effects have been inadequately investigated. This paper aims to study influences of pore diffusion of oxygen (O2) on chemical reaction and apparent reaction rate of coal smoldering. Non-isothermal thermogravimetric (TG) tests for two bituminous coal samples at three heating rates (2, 5 and 10 K/min) were conducted, and a 4-step decomposition scheme was constructed. The decomposition schemes contained two competing paths: Coal oxidation → β-Char oxidation → Ash (Path One), and Pyrolysis → α-Char oxidation → Ash (Path Two). A number of kinetic triplets (reaction order, pre-exponential factor and activation energy) and stoichiometric coefficients of the developed decomposition schemes were optimized by a Genetic Algorithm (GA) tool. Emphasis was put on influences of pore diffusion on chemical reactions (e.g. pyrolysis, coal matrix's and chars' oxidation) and on competing paths. Research results show that Knudsen diffusion plays a dominant role in pore diffusion that retards coal oxidation and its successive char oxidation on the Path One, but basically enhances char oxidation on the Path Two. Pore diffusion may exert ignorable influence on pyrolysis as it takes place without O2. The investigation is beneficial for understanding mechanism of smoldering of coal piles and coal dust layers.

Results of comparative sorption studies of the coal-methane system carried out by means of an original volumetric device and a reference gravimetric instrument

The present paper discusses the idea of an innovative device for sorption measurements, working under isobaric conditions and based on a precise piston pressure regulator. As part of the research, the author compared the results of sorption measurements obtained with the constructed piston sorptomat with the results obtained by means of a reference device. The sorption capacities of 15 granular high-volatile bituminous coal samples, determined on the basis of methane sorption courses registered with both instruments were compared. The author also defined the criterion of compatibility of the obtained results, which stated that the compared results are compatible if their values fall within the range of measurement uncertainties of the instruments with which they had been established. After comparing the results obtained with the original instrument with the results obtained with the reference gravimetric device, it was observed that 30 out of 33 measurements of sorption capacities were classified as compatible (based on the defined measurement compatibility criterion). The mean difference in the readings provided by both instruments was 1.64%, for sorption capacity determined in 33 measurement points and the methane pressure range of 1–10 bar.

3D characterization and quantitative evaluation of pore-fracture networks of two Chinese coals using FIB-SEM tomography

To more finely describe the physical basis of coalbed methane (CBM) accumulation and flow mechanisms, two different rank coals (Ro,m of 0.4% and 0.85%) from the southern Junggar Basin of northwest China were systematically investigated to characterize pore-fracture spaces and their nanoscale connectivity in three dimensions (3D) using focused ion beam scanning electron microscopy (FIB-SEM) tomography. We reconstructed the 3D pore morphology of two different rank coals, quantify the pore size/volumes distribution and developed a novel pore network extraction (PNE) model. Samples of subbituminous coal (SC) and high-volatile bituminous coal (HBC) presented significant pore spaces for CBM adsorption, with total voxel numbers of 28,944 and 83,866, respectively; both samples predominantly consisted of closed-pore diameters of 10–50nm. The numbers, volumes, and areas of pores (<100nm) were less in SC than in HBC, revealing that the SC adsorption capability was weaker than HBC. The porosities acquired by FIB-SEM were 26.06 and 42.85% for the samples SC and HBC, respectively, which could indicate the storage capability of the SC reservoir was lower than that of the HBC reservoir. The SC and HBC samples revealed a significant connectivity for CBM seepage, with a connected pores proportion of ~100% in pores >300nm. The SC sample allowed pore space connections throughout, whereas the HBC sample partially displayed pore space connections in a 3D pore network. It was found that a throat size of 400–500nm could possibly dominate coal permeability in the SC sample; the numbers of throats (fractures) decreased with rising throat size. These results show that the gas accumulation capacity of the SC reservoir are lower than that of the HBC, whereas the fluid flow capability of the SC reservoir is higher than that of the HBC reservoir. These findings may be scientifically significant for lucubrating nanoscale pores to affect CBM storage and seepage. This paper comments on the reduction of the gas sorption capability and mass transport rate in CBM development, and we provide recommendations for relevant future work.

Characterisation of creep in coal and its impact on permeability: An experimental study

Creep is a time-dependent deformation that affects coal permeability and should be considered in the prediction of Coalbed Methane (CBM) production. This study experimentally characterises and quantifies the impact of creep on coal permeability. The experiments were conducted on a bituminous coal sample, excavated from Bowen Basin, Australia, using a triaxial gas rig equipped with strain and displacement transducers. Two different types of gases (helium and methane) were injected into the sample under various stress and pore pressure conditions. It was found that for the experiments with helium, creep caused permanent partial closure of cleats and pathways under constant effective stress, and hence a reduction in permeability. Under hydrostatic stress only, a Residual Deformation Ratio (RDR) of 14.1% and a Permeability Loss Ratio (PLR) of 71% were found following the removal of the axial load. This can be due to the damage to coal microstructure along with closure of cleats. For the experiments with methane, coal experienced an instantaneous elastic deformation, at the onset of pore pressure depletion, followed by consolidation and matrix shrinkage. Then, creep occurred when gas desorption ceased. A total permeability loss of 26% was achieved due to an increase of 1.91MPa in effective stress caused by gas desorption. In addition, the model previously developed by authors was validated against the experimental permeability data. A good agreement was found between the model-predicted permeability data and the experimental permeability data, particularly for higher pore pressure ranges.

Coupling of swelling, internal stress evolution, and diffusion in coal matrix material during exposure to methane

AbstractWe construct theoretical models for time‐dependent swelling of coal matrix material upon adsorption of a single gas, taking into account a coupling between stress, strain, chemical potential, and diffusion. Two models are developed. The first (model A) corresponds to diffusion and hence swelling rates being controlled by the jump frequency of adsorbed molecules between closely spaced adsorption sites and the second (model B) to transport controlled by diffusion of unadsorbed molecules through diffusion paths linking distant adsorption sites. To test these models, we performed axial swelling experiments on a single 4 mm sized cylindrical sample of medium volatile bituminous coal, exposed to CH4 at pressures up to 40 MPa, at 40°C, using 1‐D, high‐pressure dilatometry. The models were calibrated to the experimental data by adjustment of a single‐valued diffusion coefficient, independent of gas pressure and adsorbed concentration. The results show that the data can be accurately explained only by model B. The implication is that the gas transport, the associated adsorption, and hence time‐dependent swelling are controlled by the diffusion of unadsorbed molecules and not by molecules jumping between the adjacent adsorption sites. Our model describes a full coupling between stress, strain, sorption, and diffusion in coal matrix material in terms of parameters that have clear physical meaning and are easily obtained from sorption and swelling experiments on coal of any rank exposed to any gas. It therefore offers an important tool for modeling permeability evolution with time during (enhanced) coalbed methane operations.

Liquid chromatography-atmospheric pressure laser ionization-mass spectrometry (LC-APLI-MS) analysis of polycyclic aromatic hydrocarbons with 6-8 rings in the environment.

A method has been developed for the sensitive and rapid analysis of polycyclic aromatic hydrocarbons (PAHs) in environmental samples using liquid chromatography time-of-flight mass spectrometry as well as the selective atmospheric pressure laser ionization (APLI) process (LC-APLI-MS). Upon analyzing 34 PAHs, the limits of detection of this method were found to range from 0.008 to 1.824pg (0.024pg for benzo[a]pyrene). The method therefore provides 30-fold to 5,400-fold increased sensitivity compared with the established GC-MS technique. This LC-APLI-MS method was optimized for higher molecular weight PAHs (C24-C30 PAHs with 6-8 rings), which are difficult to detect or cannot be detected by GC-MS. Using the LC-APLI-MS method, various 6- to 8-ring PAHs were detected in environmental samples for the first time. After developing the method, it was successfully validated in ruggedness tests. The concentrations determined by the LC-APLI-MS method were in good accord with the certified concentrations in three certified reference materials (contaminated soils and sediments). Upon applying the method to environmental samples, it was found that (1) the presence of dibenzo[a,i]pyrene and dibenzo[a,h]pyrene in urban soil samples could only be detected using LC-APLI-MS (i.e., not GC-MS) due to its high sensitivity, (2) a bituminous coal sample yielded 211 tentative peaks from aromatic compounds in the C24-C30 range, and (3) eleven of those compounds occurred in different environmental samples in similar patterns. Hence, 6- to 8-ring PAHs occur in solid environmental samples in which other 6-ring PAHs such as indeno[1,2,3-cd]pyrene or benzo[ghi]perylene may also be present. Some of these numerous higher molecular weight PAH compounds could have very high carcinogenic potential, which will need to be elucidated to ensure the reliability of PAH risk assessments.

Non-isothermal kinetic study of bituminous coal and lignite conversion in air and in argon/air mixtures

The exact characteristics of the pyrolysis and combustion kinetics for Siberian bituminous coals and lignites were studied for better understanding of the fuel conversion processes. The bituminous coal samples of Kuznetskiy deposit and lignite samples of Kansko-Achinsk deposit were investigated in argon and in argon/air gas mixtures. The pyrolysis and oxidation experiments were executed at four heating rates (5, 10, 20 and 30°C/min) under TG/DSC analyses coupled with mass-spectrometry. The activation energy for bituminous coal and lignite samples was decreased with conversion degree during oxidation. The combustion products of highest oxidation degree (CO2 and H2O) were dominated in gases released in oxidizing atmosphere. Contrariwise, activation energy was increased during pyrolysis with high content of CO, CO2, H2O, CH4, and H2 in released gases.The two isoconversional models were applied to determine the activation energy dependence on fuel samples conversion: Starink model and Ozawa iterative procedure. The mean arithmetic values of the resulted activation energy were 60kJ/mole and 400kJ/mole for oxidation and pyrolysis processes, respectively. These values are in good agreement with the results, presented previously for the other coals. The Starink model showed higher accuracy and lower activation energy values. The heating rate by non-isothermal oxidation and pyrolysis had the significant influence on the reaction rate because of evolution processes of the reactive surface and pore structure of the coal samples.

Effectiveness of thin spray-on liners as a gas management tool in underground coal mines

A thin spray-on liner (TSL) is defined as a chemical-based layer or coating (2–5mm) that is sprayed onto the rock surface to support the mining excavations. Besides ground support, TSLs can potentially be used as a gas management tool for underground coal mining. However, the permeability characteristics and effectiveness of TSLs at in-situ gas pressure are the key considerations in application of TSLs for gas management.This paper presents an experimental study to investigate the potential use of TSLs for gas management in underground coal mines. The coal samples were collected from an underground coal mine in New South Wales, Australia. The selected bituminous coal samples were initially characterised using petrographic and uniaxial compression strength (UCS) tests. Single phase gas permeability tests were then conducted on the coal samples. Methane (CH4) and carbon dioxide (CO2) were used for the tests as they are the gases of main concern in underground coal mines, and Helium (He) was used as a control gas to investigate the effect of gas adsorption on the coal permeability. Permeability tests were first conducted on coal samples before the TSL application. The coal samples were then coated with two types of TSL materials with a thickness of 5mm and the permeability tests were repeated. The experimental observations show that both TSLs are very effective in reducing the coal permeability, with the permeability values in the range of nanodarcies.High injection pressure gas flow tests were also conducted to investigate the amount of pressure that the TSLs can resist. The injection pressure was increased until the TSLs failed. The failure of TSLs was demonstrated by a sudden increase in the permeability. The test results show that the failure pressure of the TSLs is directly related to their adhesion strength on coal.

Estimation of Pressure-Dependent Diffusive Permeability of Coal Using Methane Diffusion Coefficient: Laboratory Measurements and Modeling

Gas diffusion in coal is critical for the prediction of coalbed methane (CBM) production, especially for the late-stage CBM reservoir when both gas pressure and permeability are relative low. Using only Darcy permeability to evaluate the quality of gas transport might not be effective. Diffusive flow can be the dominant flow at low reservoir pressures. In this work, the methane diffusion coefficient was measured for pulverized San Juan sub-bituminous and Pittsburgh bituminous coal samples using the classic unipore model and particle method. The diffusion coefficient results showed a negative correlation with pressure, as reported previously. The significance of diffusion flow is strongly related to the rate of the methane ad-/desorption process, more severely in the low pressure range [<2 MPa (280 psi)]. The measured diffusion coefficient can be converted to the equivalent permeability. This equivalent permeability, termed diffusive permeability, can be considered as the contribution of diffusion flow, in...

Anisotropic swelling behaviour of coal matrix cubes exposed to water vapour: Effects of relative humidity and sample size

Understanding the swelling behaviour of coal matrix material exposed to water vapour is of direct relevance to optimising (E)CBM recovery. However, accurate measurement of coal swelling due to water sorption presents a substantial challenge, because while measurement accuracy increases with sample size, so does equilibration time hence experiment duration. This paper reports dilatometry experiments conducted on 1 and 4mm sized cubic samples of Brzeszcze high volatile bituminous coal. These were performed using a purpose-build, 3-dimensional (3D) dilatometer, consisting of sensitive eddy-current gap sensors. The aim was to accurately and continuously measure the volumetric response of coal matrix material during exposure to water vapour at relative humidities varied in the range of 0.1 to 95%, at a temperature of 40°C. Our results show that the swelling strains attained at apparent equilibrium tend to be a factor of up to 1.45 higher perpendicular to bedding than in the bedding plane. In addition, the sample size strongly influences the swelling kinetics, but does not influence the equilibrium swelling strains. Moreover, the volumetric swelling strains attained at equilibrium show a near-linear dependence on relative humidity, reaching 1.37–1.43% at around 95% relative humidity. In an attempt to explain the observed behaviour, three models for swelling of unconfined coal matrix material due to water sorption were developed. These correspond to mono-layer adsorption, multiple-layer sorption, and combined mono plus multiple-layer sorption. The experimental data are equally well fitted by all three models, so that the mechanism responsible for swelling could not be uniquely identified. However, our findings do demonstrate that decreasing in-situ water activity causes significant anisotropic shrinkage of coal matrix material, pointing to injection and recirculation of dry nitrogen as a promising strategy for stimulating coal seams for CBM production and as a pre-treatment for later CO2 injection and storage.