Coal Swelling Research Articles

Laboratory Experiments on Environmental Friendly Means to Improve Coalbed Methane Production by Carbon Dioxide/Flue Gas Injection

Scaled in situ laboratory core flooding experiments with CO2, N2 and flue gas were carried out on coal in an experimental high P,T device. These experiments will be able to give an insight into the design of the injection system, management, control of the operations and the efficiency of an ECBM project. Although the experience gained by the oil industry represents a valuable starting point, several problems are still to be studied and solved before CO2 improved deep coalbed methane production may be operationally feasible. These are all related to the heterogeneous nature of the pore structure of coal, and in particular to the presence of fractures. More specifically, a number of questions need to be addressed, e.g. what are the conditions under which the fluid in the micro pores of the coal is displaced by the CO2 in the presence of competitive adsorption; what is the role of compositional heterogeneity and fracture anisotropy of coal for the injection design and the efficiency of the sequestration in relation to the swelling and shrinkage characteristics of coal; how does the mobile and the immobile water in the coal affect the exchange process. These questions can be answered by means of downscaled laboratory experiments that are capable of accurately describing the coupled process of multiphase flow, competitive adsorption and geo-mechanics. The laboratory conditions have been simulated to match pressure and temperature at depths of 800 to 1,000 m. Under those conditions the injected CO2 remains supercritical. Upto now, the results show that dewatering will be an essential step for successful ECBM combined with a CO2 sequestration process.

Competitive adsorption equilibria of CO2 and CH4 on a dry coal

Gases like CO2 and CH4 are able to adsorb on the coal surface, but also to dissolve into its structure causing the coal to swell. In this work, the binary adsorption of CO2 and CH4 on a dry coal (Sulcis Coal Province, Italy) and its swelling behavior are investigated. The competitive adsorption measurements are performed at 45 °C and up to 190 bar for pure CO2, CH4 and four mixtures of molar feed compositions of 20.0, 40.0, 60.0 and 80.0% CO2 using a gravimetric-chromatographic technique. The results show that carbon dioxide adsorbs more favorably than methane leading to an enrichment of the fluid phase in CH4. Coal swelling is determined using a high-pressure view cell, by exposing a coal disc to CO2, CH4 and He at 45 and 60 °C and up to 140 bar. For CO2 and CH4 a maximum swelling of about 4 and 2% is found, whereas He shows negligible swelling. The presented adsorption and swelling data are then discussed in terms of fundamental, thermodynamic aspects of adsorption and properties which are crucial for an ECBM operation, i.e. the CO2 storage capacity and the dynamics of the replacement of CH4 by CO2.

Kinetic study on solvent swelling of coal particles

AbstractSolvent swelling experiments of coal particles of Taiheiyo and Witbank coals in pyridine at 15–50°C were conducted by using an orthogonal microscope coupled with an image analysis system and the newly prepared temperature-controlled cell. Two kinetic parameters, n (a number that crudely indicates the nature of the solvent diffusion) and k (a rate constant), were calculated based on time profile of volumetric swelling ratio. The authors examined the correlation between these parameters and volumetric swelling ratios, to find that there was linear correlation between n and ln(1/k). By using the proportional constant, the authors calculated activation energy of the swelling of both coals. The energy changed with respect to each particle. Average values were 71kJ/mol for Taiheiyo coal and 55kJ/mol for Witbank coal, these values agree well with the values reported in the literature. The authors also examined the effects of pretreatments of both coals. O-Acetylation and pyridine pre-extraction lead to decrease the energies drastically.

Modeling of CO2 sorption on coal

This paper discusses moderate pressure CO2 sorption behavior of Illinois coals. The results fit the Langmuir and Dubinin–Astakhov (D–A) sorption models satisfactorily although the fit is better for D–A equation. Since factors like swelling of coal with CO2 sorption and CO2 dissolution in coal matrix contribute to uncertainties in estimating the void volume in and around the sample, an attempt was made to account for these by modifying the conventional adsorption equation. Re-fitting the experimental data using the modified equation results in improved fit for both models. The adsorption capacities of coals tested, as predicted by the equations, also reduce by 7% to 32%. The effect of volumetric uncertainty is more in lower rank coals than the higher rank ones. Furthermore, it explains the excess sorption behavior observed by others when extrapolated beyond the experimental pressure range.

Impacts of volumetric strain on CO2 sequestration in coals and enhanced CH4 recovery

Sequestration of CO2 into deep, unminable coal seams is an attractive option to reduce atmospheric emissions. However, coal seams commonly have low initial permeability, and CO2 adsorption causes the coal matrix to swell, which further reduces the permeability and may result in inefficient injection. We investigate numerically the impacts of coal swelling on coal permeability and, thus, CO2 injection efficiency with constraints determined by experimental adsorption-associated volumetric strain measurements on three western Canadian coals. Our results show that injecting pure CO2 markedly reduces permeability through time to the extent that it is not a feasible sequestration technology for almost all coals. However, injection of a gas mixture of N2 and CO2 (flue gas) markedly improved CO2 injection efficiency while mildly reducing CO2 sequestration capacity. The study also suggests that different geological settings and mechanical properties of specific coal seams strongly control coal seam permeability during gas injection and, thus, viability of CO2 sequestration.

Differential swelling and permeability change of coal in response to CO2 injection for ECBM

The matrix volume of coal swells when CO2/CH4 adsorb on the coal structure. In coalbed gas reservoirs, matrix swelling could cause the fracture aperture width to decrease, causing a considerable reduction in permeability. On a unit concentration basis, CO2 causes greater degree of coal matrix swelling compared to CH4. Much of this difference is attributable to the differing sorption capacity that coal has towards carbon dioxide and methane. This condition in a coal reservoir would lead to differential swelling. Differential swelling will have consequences in terms of porosity/permeability loss, with serious implication for the performance and implementation of carbon sequestration projects. Coal can be understood as a macromolecular cross-linked polymeric structure. An experimental effort has been made to measure the differential swelling effect of CO2/CH4 on this macromolecular structure and to theoretically translate that effect in terms of porosity and permeability. A unique feature of this work is that, real time permeability measurements were done to see the true effect of differential strain from CH4 saturated coal core flooding experiments.

Swelling of Australian coals in supercritical CO2

Abstract An apparatus capable of directly observing, by an optical method, swelling of coal in CO 2 and other gases is described. The system was used to measure CO 2 -induced swelling in three Australian bituminous coals at temperatures between 25 and 55 °C and pressures up to 15 MPa. All three coals showed similar behaviour, with swelling increasing as a function of pressure up to about 8 to 10 MPa, after which no further increase in swelling was observed. The maximum volumetric swelling was between about 1.7 and 1.9%, even in liquid CO 2 . Significant anisotropy was observed; swelling in the plane perpendicular to the bedding plane was always substantially higher than in the parallel plane. The swelling in these coals was completely reversible with each sample returning to its original dimensions after the gas was removed, even after multiple exposures. Temperature did not directly affect the maximum amount of swelling, however, the swelling tended to occur at lower pressures with decreasing temperature. Expressing the swelling as a function of gas density rather than gas pressure showed that swelling was independent of temperature; in all cases the swelling curves were identical. Swelling could be well represented by a modified Dubinin–Radushkevich model which has been used previously to describe gas adsorption on coal. Volumetric swelling data for two of the coals were combined with gas adsorption results previously measured for these coals. Swelling was roughly proportional to the amount of CO 2 adsorbed up to intermediate pressures, but at high pressures, the relationship was no longer linear; adsorption continued to increase but swelling did not. The swelling data were used to correct the adsorption isotherm measured for these coals, assuming that the volume occupied by the coal was inaccessible to gas and increased in proportion to volumetric swelling. At pressures up to about 5 MPa, the effect of correction for swelling on the adsorption value was negligible but at higher pressures, the volume changes due to the swelling of the coal resulted in an increased measured excess adsorption. At 15 MPa the corrected excess sorption was about 30% greater than the uncorrected value. Correction for swelling had little effect on the calculated sorption capacity of the coal or the heat of sorption (using the modified Dubinin–Radushkevich equation) but had strong effects on the term representing dissolution of carbon dioxide into the coal.

Adsorption‐induced coal swelling and stress: Implications for methane production and acid gas sequestration into coal seams

Sequestration of CO2 and H2S into deep unminable coal seams is an attractive option to reduce their emission into atmosphere and at the same time displace preadsorbed CH4 which is a clean energy resource. High coal seam permeability is required for efficient and practical sequestration of CO2 and H2S and recovery of CH4. However, adsorption of CO2 and H2S into coals induces strong swelling of the coal matrix (volumetric strain) and thus reduces significantly coal permeability by narrowing and even closing fracture apertures. Our experimental data on three western Canadian coals show that the adsorption‐induced volumetric strain is approximately linearly proportional to the volume of adsorbed gas, and for the same gas, different coals have very similar volumetric strain coefficient. Impacts of adsorption‐induced swelling on stress and permeability around wellbores were analytically investigated using our developed stress and permeability models. Our model results indicate that adsorption‐induced volumetric strain has significant controls on stress and permeability of producing and sequestrating coal seams and consequently the potential of acid gas sequestration. Coal seams may undergo >10 times enhancement of permeability around CH4‐producing wellbores due to a reduction in effective stress as a result of coal shrinking caused by methane desorption accompanying a reduction in reservoir pressure. Injection of H2S and CO2 on the other hand results in strong sorption‐induced swelling and a marked increase in effective stress which in turn leads to a reduction of coal seam permeability of up to several orders of magnitude. Injection of mixtures of N2 and CO2 such as found in flue gas results in weaker swelling, the amount of which varies with gas composition, and provides the greatest opportunity of sequestering CO2 and secondary recovery of CH4 for most coals. Because of the marked swelling of coal in the presence of H2S, even minor amounts of H2S result in a marked reduction in permeability, and hence sequestration of H2S in deep coals will be likely impractical. Furthermore, high stresses resulting from sorption of acid gases will potentially cause the coal to yield, fracture or slip, and produce fine particles, which further affect permeability and thus methane production and acid gas sequestration.

Effects of Matrix Shrinkage and Swelling on the Economics of Enhanced-Coalbed-Methane Production and CO2 Sequestration in Coal

Summary Increases in carbon dioxide (CO2) levels in the atmosphere and their contributions to global climate change are a major concern. CO2 sequestration in unmineable coals may be a very attractive option, for economic as well as environmental reasons, if a combination of enhanced-coalbed-methane (ECBM) production and tax incentives becomes sufficiently favorable compared to the costs of capture, transport, and injection of CO2. Darcy flow through cleats is an important transport mechanism in coal. Cleat compression and permeability changes caused by gas sorption/desorption, changes of effective stress, and matrix swelling and shrinkage introduce a high level of complexity into the feasibility of a coal sequestration project. The economic effects of CO2-induced swelling on permeabilities and injectivities has received little (if any) detailed attention. CO2 and methane (CH4) have different swelling effects on coal. In this work, the Palmer-Mansoori model for coal shrinkage and permeability increases during primary methane production was rewritten to also account for coal swelling caused by CO2 sorption. The generalized model was added to a compositional, dual-porosity coalbed-methane reservoir simulator for primary (CBM) and ECBM production. A standard five-spot of vertical wells and representative coal properties for Appalachian coals was used (Rogers 1994). Simulations and sensitivity analyses were performed with the modified simulator for nine different parameters, including coal seam and operational parameters and economic criteria. The coal properties and operating parameters that were varied included Young's modulus, Poisson's ratio, cleat porosity, and injection pressure. The economic variables included CH4 price, CO2 cost, CO2 credit, water disposal cost, and interest rate. Net-present-value (NPV) analyses of the simulation results included profits resulting from CH4 production and potential incentives for sequestered CO2. This work shows that for some coal seams, the combination of compressibility, cleat porosity, and shrinkage/swelling of the coal may have a significant impact on project economics.

Influence of Coal Shrinkage and Swelling on Coalbed-Methane Production and CO2 Sequestration

This article, written by Assistant Technology Editor Karen Bybee, contains highlights of paper SPE 102767, "Influence of Shrinkage and Swelling of Coal on Production of Coalbed Methane and Sequestration of Carbon Dioxide," by H.J. Siriwardane and D.H. Smith, Natl. Energy Technology Laboratory/DOE; F. Gorucu, SPE, Advanced Resources International; and T. Ertekin, SPE, Pennsylvania State University, prepared for the 2006 SPE Annual Technical Conference and Exhibition, San Antonio, Texas, 24–27 September. The potential for enhanced methane (CH4) production and geologic sequestration of carbon dioxide (CO2) in unmineable coal seams needs to be evaluated before large-scale sequestration projects are undertaken. Coal is known to competitively adsorb CO2 and CH4 gases. Laboratory experiments indicate that coal swells when a gas is adsorbed and shrinks when a gas is desorbed. The swelling and shrinkage may change the coal-cleat porosity and permeability. A 3D swelling and shrinkage model (SSM) was developed that is based on equations that account for coupled fluid-pressure/-deformation behavior of a porous medium that undergoes swelling and shrinkage. Introduction Greenhouse-gas emissions are increasing. Anthropogenic CO2 emissions can contribute to global warming, and attempts are being made to identify methods to reduce them. Geologic sequestration has been identified as one of the methods for CO2 storage. One of the promising methods is to store CO2 in unmineable coal seams. Coal seams can hold large amounts of CO2 compared to the amounts of CH4 gas that they contain. However, many technical and safety issues need to be investigated before such efforts can be undertaken. Several previous studies have addressed important aspects of CO2 sequestration in coal seams. One of the potential problems of CO2 injection into a coal seam is swelling of the coal. While more laboratory-research work is needed to resolve the exact amount of coal swelling, it is suspected that coal swelling will cause a reduction in permeability, which in turn may reduce injection volumes during large-scale injection operations. As part of a research and field-demonstration project of CO2 sequestration in deep unmineable coalbeds, a field study involving the injection of CO2 for enhanced coalbed-CH4 recovery was performed in the San Juan basin. The influence of coal swelling and shrinkage on reservoir performance of the reported field project was investigated by use of a new SSM. The full-length paper contains results of several reservoir simulations of the reported field study. Results from the present study are compared with previous studies published in the literature. Methodology A theoretical model for stress-dependent permeability caused by matrix shrinkage under uniaxial-strain conditions has been presented previously in the literature. It is anticipated that when CO2 is injected into a coal seam while CH4 is produced from the same layer, the seam will undergo simultaneous swelling and shrinkage at different locations, depending on the gas composition and pressure at each location. There have been numerous research investigations on reservoir modeling of gas flow in coal seams. In many earlier investigations, the influence of swelling and shrinkage of the reservoir matrix on reservoir permeability was not considered.

Coal Chemistry for Mechanical Engineers: From Macromolecular Thermodynamics to Reservoir Simulation

In pilot trials and commercial scale field demonstrations of CO2 storage in coal seams, quite often unexpected problems with coal swelling around injector and reducing injection efficiency (e.g., Allison unit in the San Juan Basin, RECOPOL in Poland, Hokkaido project in Japan, etc.) can stall or even terminate the site development. To avoid the costly mistakes with the prospective site evaluation, the state of the art in reservoir modeling needs to be improved by taking into account coal properties at the macromolecular level. The current models are based on the rock mechanics, which ignores decades of experimental and theoretical studies of interaction between coal and injected fluids. A pseudopolymer approach is introduced to the modelers as a viable alternative, especially, at medium to high fluid pressures. Further, it is discussed how the thermodynamics of CO2 dissolution in the macromolecular network of the coal matrix can be incorporated into geomechanical models.

Mass transfer in coal seams for CO2 sequestration

AbstractCO2 sequestration in coal seams is a relatively new technique to simultaneously achieve enhanced coal bed methane production and reduced CO2 emission. In this article, we integrate understandings in individual research fields to provide improved insight into the nature of this complex process. Our current overall model constructed from a number of sub‐models consists of mass transfer in four pore types, namely, fractures, micro‐, meso‐, and macro‐pores, all having pore size dependent characteristics. Key parameters are estimated using well established methods from the general literature. Three mechanisms of coal swelling leading to permeability variations during adsorption are proposed based on molecular simulations. The macroscopic level model is validated using a true tri‐axial stress coal permeameter, which provides previously unpublished, accurate dynamic measurements of systems properties in three orthogonal directions including changes to the coal matrix volume. The integrated model provides a more complete and flexible representation for this complex system. © 2007 American Institute of Chemical Engineers AIChE J, 2007

Preparation of coal slurry with organic solvents

In this study, various organic solvents were used to prepare coal slurries and the rheological and thermal properties of coal-organic solvent slurries were examined. Solvents with molecules containing unpaired electrons (high basicity) show high extraction power and cause swelling of coal. Therefore, coal-organic solvent slurries usually showed higher viscosities compared to coal-water slurry. In addition, coal slurries prepared by alcohols and cyclohexanone demonstrated lower settling rates but a high specific sedimentation volume presumably because these solvents swelled coal particles well and led to the formation of weak gel structures in the bulk. In addition, ethanol and cyclohexanone are capable of breaking a considerable amount of hydrogen bonds in coal and subsequently opening up the structures. Thus, more surface area is available for combustion and the combustion rate of coal slurries was increased.

Swelling-induced volumetric strains internal to a stressed coal associated with CO2 sorption

It is generally accepted that typical coalbed gases (methane and carbon dioxide) are sorbed (both adsorbed and absorbed) in the coal matrix causing it to swell and resulting in local stress and strain variations in a coalbed confined under overburden pressure. The swelling, interactions of gases within the coal matrix and the resultant changes in the permeability, sorption, gas flow mechanics in the reservoir, and stress state of the coal can impact a number of reservoir-related factors. These include effective production of coalbed methane, degasification of future mining areas by drilling horizontal and vertical degasification wells, injection of CO2 as an enhanced coalbed methane recovery technique, and concurrent CO2 sequestration. Such information can also provide an understanding of the mechanisms behind gas outbursts in underground coal mines. The spatio-temporal volumetric strains in a consolidated Pittsburgh seam coal sample were evaluated while both confining pressure and carbon dioxide (CO2) pore pressure were increased to keep a constant positive effective stress on the sample. The changes internal to the sample were evaluated by maps of density and atomic number determined by dual-energy X-ray computed tomography (X-ray CT). Early-time images, as soon as CO2 was introduced, were also used to calculate the macroporosity in the coal sample. Scanning electron microscopy (SEM) and photographic images of the polished section of the coal sample at X-ray CT image location were used to identify the microlithotypes and microstructures. The CO2 sorption-associated swelling and volumetric strains in consolidated coal under constant effective stress are heterogeneous processes depending on the lithotypes present. In the time scale of the experiment, vitrite showed the highest degree of swelling due to dissolution of CO2, while the clay (kaolinite) and inertite region was compressed in response. The volumetric strains associated with swelling and compression were between ± 15% depending on the location. Although the effective stress on the sample was constant, it varied within the sample as a result of the internal stresses created by gas sorption-related structural changes. SEM images and porosity calculations revealed that the kaolinite and inertite bearing layer was highly porous, which enabled the fastest CO2 uptake and the highest degree of compression.

アジア大での環境負荷軽減と石炭需給の安定化 ＣＯ２炭層固定化に関連した液体ＣＯ２に対する吸着，浸透および膨潤特性

A national project on a carbon dioxide (CO2) sequestration has been carried out from 2002 by injecting CO2 into one of the Ishikari coal seams located at the Minami Ishikari Coal Field in Hokkaido, Japan. The coal seam lies at 890 m under the surface with in-situ conditions of 29°C in temperature and 11MPa in static stress. Even if super critical CO2 over 70°C and 8 MPa is supplied at the surface, it becomes liquid CO2 with 29°C and 15MPa at the bottom of injector due to heat loss to low temperature strata and creates a problem of coal seam permeability reduction around the injector. Thus, coal characteristics for liquid CO2 injection are very important to carry out the project. However, measurements of permeability and coal swelling factor for liquid CO2 has not been presented and visualized clearly.In this study, the permeability and swelling of a core sample drilled from the coal seam has been investigated. Furthermore CO2 adsorption/desorption properties of the coal have been measured using with a volumetric method with comparisons of coal samples mined from the Kushiro Coal Mine. As a result, the swelling of the coal matrix has been observed clearly by visualization pictures of crushed coal sample and a hole drilled in the coal core sample sunk in CO2 liquid. The ratio of its area reduction by the swelling in liquid CO2 was 7% during 78hours.

Pyrolytic Mercury Removal from Coal and Its Adverse Effect on Coal Swelling

Coal combustion is one of the largest anthropogenic contributors of mercury emissions to the environment. The high level of mercury toxicity and its potential to bioaccumulate have prompted strict regulations for mercury emission control in many countries. While end-of-pipe technologies for mercury control are currently undergoing increased international development, precombustion pyrolytic mercury removal has also been proposed as a possible option. Mercury is highly volatile at low temperatures, and its removal from coal can be controlled by low-temperature pyrolysis. This work investigated the effect of temperature on mercury release while monitoring the adverse effects of the thermal pretreatment process on coal using coal swelling as an indicative parameter for combustion quality and char burnout rate. Three coal samples were pyrolyzed for 20 min to selected temperatures ranging between 200 and 600 °C at a heating rate of 50 °C/min. Mercury content in the coal chars was determined by a cold vapor atomic fluorescence method. The total mercury in coal was found to decrease between 5 and 25% when the coal samples were pyrolyzed at 200 °C, while at 300 °C, the amount of mercury removed from coal ranged between 55 and 90%. Representative coal char samples were also observed by long-distance microscopy in a single-particle reactor in which they were pyrolyzed at a heating rate of 200 °C/s. The results showed a significant decrease in the maximum transient swelling behavior when the char samples were thermally treated at temperatures above 400 °C for one coal and 500 °C for the remaining two coal samples. The results suggested that mercury removal from coal by prior thermal upgrading to around a maximum temperature of 300 °C may provide an option for partial mercury control for some coals. However, a better understanding and quantification of the strongly bonded fraction of mercury in the coal is required to model a feasible mercury retention process using the option of thermal upgrading.

Swelling of Coal in Response to CO2 Sequestration for ECBM and Its Effect on Fracture Permeability

Summary The "swelling" of coal by a penetrant refers to an increase in the volume occupied by the coal as a result of the viscoelastic relaxation of its highly crosslinked macromolecular structure. Projects relating to CO2 sequestration in coal seams suffer a serious setback in terms of injectivity loss resulting from the swelling of coal. Volumetric swelling associated with CO2 sorption on coal has a significant influence on the fracture porosity and permeability of the coal. Two coal samples differing in rank were used for volumetric strain measurements. With CO2, the high-rank Selar Cornish coal showed a maximum volumetric strain of 1.48% corresponding to an average pore pressure of 13 MPa. A matrix swelling coefficient (Cm) of 1.77×10−4 MPa−1 was calculated for this Selar Cornish coal. The low-rank Warndt Luisenthal coal exhibited higher strain of 1.6%, and a matrix swelling coefficient (Cm) of 8.98×10−5 MPa−1 was calculated. The rank dependence of swelling holds true in this set of experiments. Repeat volumetric strain measurement on the same Warndt Luisenthal coal core shows higher volumetric strain values for all pressure steps. A volumetric strain of 1.9% corresponding to a mean pore pressure of 14 MPa was measured. This confirms the process of sequential swelling. A unique feature of this work is that real-time permeability measurements were done under unconstrained conditions. Permeabilities were measured, reducing the pore pressure from 16 to 1 MPa at constant flow rate. Although measured permeability increased with increasing pore pressure under unconstrained swelling, in-situ permeability will actually decrease because of fracture closure in a constrained coal. To validate the permeability swelling relationship, both permeability measurements under unconstrained conditions and volumetric strain measurements were used.

06/02076 Interaction mechanism of flocculants with coal waste slurry: Sabah, E. and Erkan, Z. E. Fuel, 2006, 85, (3), 350–359.

This study explored the feasibility of using waste organic solvents as substitutes for water to prepare coal slurries. The rheological properties of coal–2-propanol slurries were examined and compared with that of coal–water slurry (CWS). The good compatibility between coal particles and 2-propanol resulted in stable particle suspension in slurry which usually exhibit Newtonian behaviour. However, coal–2-propanol slurries usually shown higher viscosities comparing to CWSs at a fixed solid loading due to swelling of coal by 2-propanol. In addition, coal–2-propanol slurries demonstrated lower settling rates (higher stability) compared to CWSs presumably due to good compatibility between coal particles and 2-propanol. Finally, coal–2-propanol slurries formed bulky sediment with loose structure even coal particles suspended in 2-propanol were more stable than coal particles in CWSs.

Swelling and shrinkage behavior of raw and processed coals during pyrolysis

Direct observation of transient swelling and shrinkage behavior of raw coals and processed coals was conducted by using a digital camera with a long focus lens, and quantitative description of swelling and shrinkage was resulted from subsequent image processing. The results showed that the raw and processed coals in pyrolysis behaved differently. The shrinkage of the processed coals was quite different, the maximum volumetric shrinkage ratio at 1000 °C as in the range of 32–38% with swelling ratios less than 5%, and the volumetric swelling ratio increased and the shrinkage ratio decreased with the increase of coal sample density. The maximum volumetric swelling ratio of the raw coals was more prominent than those of the processed coals. The raw coal A 2 showed a maximum swelling ratio of 20–85% and the raw coal B 2 showed a maximum swelling ratio of 25–45%. The volumetric swelling ratios decreased and the shrinkage increased with the decrease of the sample size. It is considered that the different pyrolysis behaviors were mainly due to the variation in their macroscopic structures. However, the similar swelling and shrinkage curves of coal samples were observed for the same raw or processed coals. In addition, the effect of volatile matter on the volumetric shrinkage ratios would be weakened if the raw coal was crushed into powder and then pressed into processed coal samples. Scanning electron microscope analysis showed that the structure of coke after pyrolysis was also different.

Measurement and interpretation of supercritical CO2 sorption on various coals

Abstract While the amount of CO 2 sorption data on various natural coals has increased in recent years, only few measurements have been reported under the experimental condition of supercritical CO 2 (scCO 2 ) at very high pressure (> 5 MPa). The estimation of realistic CO 2 -sorption capacities for different coals is crucial for the understanding of the processes associated with CO 2 storage and enhanced coalbed methane (ECBM) production. In this study CO 2 -sorption experiments up to 20 MPa at 45 °C have been performed on dry and water-containing coals from various coal basins. The coal samples cover a broad spectrum in rank, ranging from 0.52 to 2.41% vitrinite reflectance (VRr). As shown in various studies on activated carbon, scCO 2 surface excess sorption isotherms do not increase continuously with pressure up to complete surface coverage of the sample. This is mainly caused by the static interpretation of excess sorption isotherms, not taking into consideration changes in the sample volume such as the volume of the sorbed phase, coal swelling, etc. This leads to difficulties in the interpretation of the experimental data in terms of adsorption isotherms, as they require models for all volumetric effects. In this context, this study provides an approach to account for the volumetric effects and, hence for the estimation of total sorption capacities. For the fitting procedure, one generalized correction factor for the volume increase was calculated for each coal sample and applied to the whole experimental pressure range. Generally, it was observed that coals containing water show a smaller volume increase than their corresponding dry samples and no specific trend with coal rank was observed. Contrary, a trend for the dry samples was observed: The sample volume increase follows a U-shaped trend, i.e., decreasing from 0.5 to 1.1% VRr and increasing again from 1.1 to 1.7% VRr.