Expansion of Hard Coal Accompanying the Sorption of Methane and

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Expansion of Hard Coal Accompanying the Sorption of Methane and Carbon Dioxide in Isothermal and Non-isothermal Processes Paweł Baran,*,† Katarzyna Zarębska,† and Mirosława Bukowska‡ †

AGH University of Science and Technology, Faculty of Energy and Fuels, Aleja Mickiewicza 30, 30-059 Cracow, Poland Central Mining Institute, Plac Gwarków 1, 40-166 Katowice, Poland

‡

ABSTRACT: Coal swelling because of methane and carbon dioxide was measured by the volumetric method in isothermal and non-isothermal conditions. These investigations are of key importance in the context of potential CO2 sequestration in deep unmined coalbeds. Changes of the temperature underground may disturb the adsorption balance, leading to volumetric processes in the coal strata (swelling or shrinking), which can give rise to leaks and gas desorption toward the ground surface. The isothermal results show that the strain exhibited by a coal sample during CO2 sorption is about twice that of CH4. The liner strain kinetics also show that the swelling of the sample when exposed to both gases is anisotropic and greater in the direction perpendicular to the bedding plane than parallel to it. In the case of the non-isothermal process, the pattern of dilatometric processes seems to be different. The temperature increase gives rise to the sample swelling when exposed to methane, yet the presence of CO2 leads to sample contraction, which can be attributed to the different mechanisms involved in CO2 deposition. CO2 accumulated in pores undergoes a rapid phase transition as a result of capillary condensation, leading to rapid desorption and, in consequence, shrinking of the coal sample.

1. INTRODUCTION Coal swelling has been extensively studied for years.1,2 On the basis of the research data, it is reasonable to conclude that coal is a biporous system involving transport and sorption processes, in which, under elevated pressures of gases, the micropore regions become compressed and macropores expand, while swelling of the microporous coal substance accompanying the sorption processes causes the contraction of transport pores and reduction of the system permeability in situ.3,4 Research data summarized in the literature on the subject indicate the linear relationship between coal swelling and the scale of sorption that has induced the process.5−7 Some authors, however, suggest that the sorption processes and the extent of involved strains are not linearly related.8,9 The original approach involving the concurrent sorption and dilatometry measurement was suggested by the research team of Majewska and Ceglarska-Stefańska.10 Apart from simultaneous measurements of the sorption kinetics and expansion rates in the system, measurements were also taken of the acoustic emission levels. Measurements were taken for cuboid coal samples, and the sorbates used were methane, carbon dioxide, and their mixtures. The coal expansion was found to be nonlinearly related to the amounts of absorbed/adsorbed gas, and the swelling process was found to be irreversible. The preferential sorption of methane was demonstrated, and the affinity of CH4 and CO2 was found to be comparable at elevated pressures. That indicates that coalbeds sharing these properties are useless in the context of potential CO2 sequestration, which is broadly described in the literature as CO2 storage in a deep saline aquifer. 11−13 However, in the case of coalbeds, most publications on the subject of CO2 sequestration (in the area of experimental testing) are focused on sorption tests and establishing the influence of relevant parameters (coal rank, maceral content, porosity, and moisture content) on sorption © XXXX American Chemical Society

capacity. In most cases, sorption tests are performed on grainy samples or on samples in the dust form.14−22 This approach makes the experimental procedure less time-consuming (the sorption equilibrium is promptly established),23 although a question arises whether the sample crushing should not destroy the porous structure, and hence, the experimental results may not be directly related to the conditions prevailing in situ. In the case of coal expansion tests, the samples to be tested must not be crushed. There are no references in the literature showing the effects of the temperature on volumetric strains in hard coal that accompany the sorption of mine gases. Temperature fluctuations may disturb the sorption equilibrium of gases present in the coalbed (either endogenous or injected and stored CO2). Disturbing the sorption equilibrium will produce the volumetric strains in the coal mass, which may lead to stresses in the rock strata and, in consequence, to uncontrolled leaks in the gas reservoir (in the case of CO2 sequestration) or rock and gas outbursts in the case of mining exploitation of the coal deposit.24 Hence, the investigation of the effects of the temperature on the sorption and dilatometry phenomena will be of paramount importance.

2. EXPERIMENTAL SECTION 2.1. Test Material. Testing was performed on a hard coal sample from a Polish colliery (KWK Pniówek) localized in the Higher Silesia Basin. The coal seam 360/1 belongs to one of the sedimentary cyclothems of coal in the Upper Silesian Coal Basin. Similar to all other cyclothems, the cyclothem forming a mudstone series consists of sedimentary rocks. These are clastic rocks (sandstones made of grains differing in size and mudstones) and clay rocks (siltstones and coal Received: October 14, 2014 Revised: February 3, 2015

A

DOI: 10.1021/ef502312p Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels Table 1. Characteristic of the Coal Materiala colliery/coal seam

Cdaf

Sdaf

Hdaf

Ndaf

Odaf

Wa

Aa

Vdaf

Pniówek p. 360/1

84.24

0.39

4.58

1.52

4.58

1.75

3.01

27.12

a

The elemental analysis was performed in the Central Mining Institute in Katowice. The moisture content was determined in accordance with the procedure set forth in the standard PN-80/G-04511, and the ash content was established in accordance with PN-80/G-04512. The oxygen content was computed as the remainder of 100%, taking into account the moisture and ash content.

shales). In the mudstone series, coal seams marked with numbers 301−406 are surrounded by gangue. The seam 360/1 belongs to Orzesze beds. Selected results of elemental and technological analyses of the coal sample are summarized in Table 1. The petrography of the tested coal sample is summarized in Table 2. Microscopy testing was performed using a polarizing microscope Polmi and the Olympus device (supplementary measurements, used in diagnostics of minute objects).

volume becomes decompressed and moves from the reference cell to the sample cell 1 containing the sorbent. Knowing the dead volume of the apparatus and the volume of the dosing unit, the amounts of absorbed/adsorbed gas can be computed on the basis of gas laws. In the case of both gases, the deviations from the ideal gas properties have to be taken into account. In the pressure and temperature range investigated here, the difference between an ideal and real gas can be determined by the equation of state for CO225 and CH4.26 The volume of the dosing unit is computed by measuring the nitrogen outflow rate with a flow meter EL-FLOW F-111B (Bronkhorst). The dead volume of the sample cell was leveled with glass balls, and its value was derived with the use of helium. Coal sample deformations were measured with a strain gauge engineered at the Strata Mechanics Research Institute of the Polish Academy of Sciences. Linear strains are measured with an electric resistant wire strain gauge incorporating a resistance (bridge) transducer (Figure 2). Measurement data can be shown via an alphanumeric display, stored in internal memory, or further transmitted via a serial port. The strain-measuring system can be programmed by entering the parameters of in-connected strain gauges and transducers, setting the current time and defining the criteria for measurement data recording. The strain gauge incorporates a resistant element fixed to the sample with a special adhesive. Terminals used to connect the strain gauge with cables leading to the measuring apparatus are welded to the resistance element. The operating principle of the strain gauge meter uses the physical changes produced by the resistance of the conductor under strain. Minor resistance changes (of the order of several percentages) are typically measured by the Wheatstone bridge systems, which can be well-applied in measurements of resistance induced by static or dynamic strain. 2.3. Test Procedure. Diversity of physicochemical properties that control the sorption effects makes the interpretation of results difficult. Use of a relatively large one-piece sample weighing about 20 g should

Table 2. Petrography of the Tested Coal Sample maceral (%) coal

vitrinite

liptinite

inertinite

mineral matter

reflectance

P

73

7

20

1

0.92

2.2. Test Facilities. The experimental program will use a purposebuilt apparatus, enabling the concurrent measurements of sorption and dilatometric parameters of cuboid-shaped coal samples (Figure 1). The design of the apparatus is such that two samples can be handled simultaneously. Measurements of sorption capacity are taken by the manometric method. Pressure measurements use the pressure transducers (P1, P2, and P3). In the dosing section of the apparatus, there is a transducer S-10 WIKA (P3), operated in the pressure range of 0−100 bar, with the accuracy level of 0.25% best fit straight line (BFSL). Pressure measurements in sample cells 1 and 2 (P1 and P2) are taken with UT-10 transducers, manufactured by the WIKA Instrument Corporation, enabling pressure measurements up to 100 bar and providing the regulated measurement range in the scale of 1:20. The accuracy level equals 0.1% of the measurement range. The device is placed inside a water thermostat, maintaining a constant temperature with an accuracy of 0.1 K. Gas of known pressure and

Figure 1. Layout of the measuring apparatus. B


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Figure 2. Cuboidal coal sample with the location of strain gauges. render the results more representative in relation to the data reported in the literature. For the purpose of measurements, a cuboid sample (18 × 18 × 40 mm) was cut from the coal fragment. On its opposite walls, resistance strain gauges were attached, arranged alongside and transverse to the bedding plane. The sample was placed inside a test ampule and connected to the strain gauge. Prior to the main experiment, the sample was degassed, the dead volume of the sample cell 1 was determined using helium, and the system was degassed again to achieve the static vacuum of the order of 10−2 Pa. With all valves being locked and the strain gauge being set to zero, the prepared system was thus ready for the main part of the experiment. Carbon dioxide was then admitted to the reference cell, and when the constant pressure value was established, the gas was admitted to sample cell 1. The moment that gas was introduced, the recording began of the kinetics of the pressure drop (P1) in the measurement system. The data on kinetics of linear strains of the coal sample were stored in the internal memory of the strain gauge. The kinetics of sorption and volumetric changes taking place at the temperature of 298 K were being monitored for 50 h. The end of the isothermal part of the measurement was the starting point for the second phase of the test program, involving the registering of gas pressure and linear strains in the system, which occurred when the temperature of the system went up from 298 to 323 K. The temperature increased at the rate of 1 K per 5 min. The temperature of 323 K was reached after nearly 3 h, and for the next 7 h, the registered processes were isothermal. After the experiment was concluded using carbon dioxide, the system was thoroughly degassed and another measurement using methane was performed, in accordance with the outlined procedure. On the basis of the obtained kinetics of coal-induced linear strains in the perpendicular (ε⊥) and parallel (ε∥) directions, the kinetics of volumetric strains (εV) were determined in accordance with the relation

more easily penetrated by a molecule of CO2, having the kinetic diameter of 0.33 nm, than by CH4 molecules, whose kinetic diameter is 0.38 nm.27 Better similarity of carbon dioxide to the porous structure is associated with not only its physicochemical properties but also the electric properties (the quadrupole moment) and the presence of functional groups in coal. Another reason may be the presence of reactive oxygen groups present in the coal copolymer structure, which restricts the access of tetrahedral CH4 molecules to the pores, while in the case of CO2, their interactions with oxygen groups may have a different nature.20,28 The preferential sorption of CO2 is also associated with differences between the boiling temperatures for CO2 and CH4. The boiling temperature of carbon dioxide is higher (194.5 K) than of that of methane (111.4 K), and that is why sorption affinity for this gas is higher than that for methane. Reduced sorption capacity of coals with respect to methane (when compared to carbon dioxide) indicates that some portion of pores are inaccessible to CH4 molecules because the molecules of this sorbate need to expend considerable energy to expand the pore walls to penetrate them. As the time of the coal sample−methane interaction becomes prolonged and the adsorption space becomes saturated, vibrations of the coal copolymer network and the presence of an elastic phase allow for the spherical methane molecule to penetrate those regions, which did not play any role in coal−gas interactions required to expand the pore walls. It is likely that CH4 molecules cause the stress relaxation within molecular elements of the structure and creeping. When the high-pressure measurement data are analyzed, one should consider the specificity of sorption processes in coal. In the initial stage, gas molecules are adsorbed relatively fast on the coal surface, and then the adsorbed molecules tend to diffuse into the internal structure of the coal, blurring the distinction between pure adsorption and absorption, prompting the co-occurrence of the two processes, jointly referred to as sorption. After a considerable period of time, the coal structure becomes rearranged. Because this transformation involves the large-scale motion of molecules, the process is typically slow. When exposed to vapors of dissolved substances, coal tends to behave like a deformed glassy polymer.29−31 Under such conditions, carbon dioxide dissolves in coal, giving rise to coal swelling, and in such a case, coal should not be treated like a rigid solid because this may lead to incorrect interpretation of measurement data.

εV = ε⊥ + 2ε

3. RESULTS AND DISCUSSION The higher sorption capacity for CO2 than CH4 is associated with the energy (adsorption affinity) and the kinetic diameter of a gas molecule because the favored sorption process is that involving particles whose adsorption energy is higher and kinetic diameter is smaller. Carbon dioxide satisfies these two requirements because it has a smaller kinetic diameter and higher adsorption energy than methane; hence, a CO2 molecule would diffuse into a microporous matrix much easier than methane. That is why a coal matrix is able to sorb larger amounts of carbon dioxide than methane. Hard coal contains a heterogeneous network of pores, narrowed to the ultramicropore size. It is reasonable to suppose that it will be C


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Figure 3. (a) Kinetics of linear strains (in the directions parallel and perpendicular to the bedding plane) of the coal sample when exposed to methane and carbon dioxide at 298 K. (b) Kinetics of CO2 and CH4 sorption and volume strains at 298 K.

It is readily seen in Figure 3 that the linear expansion of the coal sample tends to increase when the two gases are admitted; coal tends to expand more in the direction perpendicular to the bedding plane. It is consistent with results reported by other authors.8,10,32,33 Even though the initial pressure of the gas being dosed was identical, the sample would expand at the faster rate and nearly twice as much when exposed to carbon dioxide, when compared to methane. The shape of kinetic plots in Figure 3b is nearly identical for sorption−dilatometric (volumetric) phenomena, which seems to confirm the hypothesis that deformations of the coal sample are attributable to gas deposition. During the mining operations, the temperature in the coalbed may change by 10 K or more, which will disturb the equilibrium of the coal−gas system. The gas pressure in the coalbed and, hence, the amount of gas absorbed by coal will change as well. These phenomena should be taken into account in the descriptions of the behavior of the system. That is why the concluded measurement at 298 K became the starting point for further stages of the experiment, which involved the recording of pressure and volumetric strain variations that occurred when the temperature of the coal−gas system was increased. Results are summarized in Figure 4, revealing the pressure increase with increased temperatures (by about 2 bar for CO2 and 1.5 bar for CH4). It is worthwhile to mention that volumetric changes follow a different pattern for the two investigated gases. In the case of CO2, the initial temperature change causes only slight swelling of the coal sample. After 25 min, the coal sample shrinks rapidly (its lateral and longitudinal dimensions change by about 1%), which is followed by gradual contraction and shrinking. The shrinking of the coal substance can give rise to expansion of the existing cracks and fissures in the coalbed or cause new cracks to be formed. Because of the shrinkage of the coal substance, the lateral stresses, which maintain the coal in the compact form, tend to disappear. A rapid shrinking is registered near the critical temperatures of CO2 (Figure 5). In the near critical regions of particular interest are the surface tension, density of liquid and vapor under the equilibrium conditions, pressure of saturated vapor, and variations of these parameters with the temperature and pressure. Sorption in the near critical region shows the feature of mechanism transition. The transition will take a more or less continuous way of the isotherms on both sides of the critical temperature, belonging to the same type;

Figure 4. Kinetics of the coal sample strain during the temperature increase.

Figure 5. Volumetric strains during the temperature increase.

D


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interactions required to expand the pore walls. It appears that methane adsorbed in sub-micropores will desorb because of the temperature increase, causing the expansion of transport pores and, hence, the observed increase of the outside dimensions of the sample. A longer time is required for the trapped molecule to leave the coal substance. These rheological effects are responsible for coal expansion during the temperature increase and the resulting gas desorption. During the temperature change, additional stress is generated between macromolecular elements, which can be interpreted as a contribution of mechanical interactions. Changes of the state of stress in the coal rock and in neighboring rocks ahead of the face front cause the rock strata to break, which leads to a nonelastic relative increase of the rock volume. These non-elastic volumetric strains give rise to an increase of gas pressure and coalbed permeability and, in consequence, lead to enhanced gas release. At some distance from the face front, these factors will trigger the phase transition of methane, from the adsorbed phase into the gaseous phase, which is a major cause of the rock−gas outbursts. It appears that, in the coal−CH4 systems, the sorption and volumetric (dilatometric) processes are not parallel.

however, a discontinuous transition could be observed on the isotherms in this region if there is a transformation of isotherm types.34 The observed changes can be explained by a changed mechanism of the CO2 molecule deposition. CO2 accumulated in pores undergoes a rapid phase transition as a result of capillary condensation, leading to rapid desorption and, in consequence, shrinking of the coal sample. The analysis of strain kinetics reveals that the two plots are shifted within a time interval by a similar value, which indicates the absence of the preferential influence of the temperature on the measured linear strains of the coal sample. It is reasonable to suppose that the influence of the temperature on each parameter being measured is constant. In the case of methane, temperature changes give rise to sample swelling, and this process is actually linear. The behavior of the entire system is rather unexpected. It is a well-established fact that the desorption processes give rise to shrinkage of the coal rock;32,33 therefore, the temperature increase in the system must lead to methane desorption. At the same time, the pressure increase causes the equilibrium point to be shifted toward the adsorption range. As a result, the amount of methane absorbed/adsorbed at the final temperature of 323 K is slightly lower than at 298 K (Vsorb of 15.4 → 14.6 dm3/kg; Figure 3), even though the dimensions of the coal sample will increase. It is reasonable to conclude, therefore, that the temperature increase is in this case a major determinant of volumetric strains experienced by the coal sample. In light of the copolymer model of the coal substance, the studies of the properties and behavior of the sorption systems should take into account its porosity because the gas deposition mechanism is dependent upon the pore size. In middle-rank coals, whose structural elements vary in size (as in the investigated coal sample P), pores having the size like the sorbate occupy the smallest volume, and hence, the proportion of the absorbing elastic phase is lower than in other coals. In the context of the adopted model, the sorption process involves the interactions of gas molecules with the absorbing phase in coal (sub-micropores and the molecular phase) and with the elastic phase and in the molecular phase. Sorbate molecules are deposited between surface groups and molecular bonds in coal, straightening or expanding them, and thus increasing the distance between molecular structures. It is a necessary condition that the distance between the groups should be equal to or smaller than the diameter of sorbate molecules. This is an adsorption/absorption phenomenon because sorbate molecules accumulate on the surface of the coal substance, at the same time retaining contact with aliphatic structures on the sides. A volumetric change is involved, and deformation of the coal substance is like deformation of an elastic macromolecular network, in which the sorbate plays the role of a “lubricant” between its structural elements. It is reasonable to expect that, because of its kinetic diameter (0.38 nm) and lower adsorption energy than CO2, a stable and spherical molecule of methane at the onset of the sorption process permeates those pores in the coal substance in which it is physically adsorbed, while a portion of pores will remain inaccessible because sorbate molecules require considerable energy to expand the pore walls. As the time of the coal sample−methane interaction becomes prolonged and the adsorption space becomes saturated, vibrations of the coal copolymer network and the presence of an elastic phase allow for the spherical methane molecule to penetrate those regions, which did not play any role in coal−gas

4. SUMMARY AND CONCLUSION Coal swelling because of methane and carbon dioxide was measured by the volumetric method in isothermal and nonisothermal conditions. These investigations are of key importance in the context of potential CO2 sequestration in deep unmined coalbeds. Changes of temperature underground may disturb the adsorption balance, leading to volumetric processes in the coal strata (swelling or shrinking), which can give rise to leaks and gas desorption toward the ground surface. The results show that the strain exhibited by a coal sample during CO2 sorption is about twice that of CH4. The liner strain kinetics also show that the swelling of the sample when exposed to both gases is anisotropic and greater in the direction perpendicular to the bedding plane than parallel to it. Even though the sorption capacity of coal for carbon dioxide is twice as large as for methane, the deformation of the coal sample in the presence of methane is nearly twice as low as that registered for CO2 in the same pressure range. In the case of the two sorbates, the contraction of the sorbent proceeds more slowly than its expansion. In the case of the non-isothermal process, the pattern of dilatometric processes seems to be different. The temperature increase gives rise to the sample swelling when exposed to methane, yet the presence of CO2 leads to the sample contraction, which can be attributed to the different mechanism involved in CO2 deposition. CO2 accumulated in pores undergoes a rapid phase transition as a result of capillary condensation, leading to rapid desorption and, in consequence, shrinking of the coal sample. Of particular interest is the behavior of the system at 323 K. The measurement procedure was performed for the two gases, at identical pressure. The sorbates were admitted in similar amounts. At 323 K, the volumetric changes of coal samples exposed to methane and carbon dioxide follow a different pattern. At 298 K, coal expansion when exposed to CO2 was greater, while at 323 K, it is not so, even though the amount of absorbed CO2 is larger than that of methane. It appears that of particular importance is the free gas pressure (higher for methane) accumulated in the coal sample. It leads us to the E


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(26) Setzmann, U.; Wagner, W. J. Phys. Chem. Ref. Data 1991, 20 (6), 1061−1151. (27) Li, J. L.; Ma, Y.; McCarthy, M. C.; Sculley, J.; Yu, J.; Jeong, H. K.; Balbuena, P. B.; Zhou, H. C. Coord. Chem. Rev. 2011, 255 (15−16), 1791−1823. (28) Gensterblum, Y.; Merkel, A.; Busch, A.; Krooss, B. M. Int. J. Coal Geol. 2013, 118, 45−57. (29) Milewska-Duda, J.; Duda, J.; Nodzeński, A.; Lakatos, J. Langmuir 2000, 16 (12), 5458−5466. (30) Larsen, J. W. Int. J. Coal Geol. 2004, 57, 63−70. (31) Goodman, A. L.; Favors, R. N.; Larsen, J. W. Energy Fuels. 2006, 20 (6), 2537−2543. (32) Majewska, Z.; Majewski, St.; Ziętek, J. Int. J. Coal Geol. 2010, 83, 475−483. (33) Czerw, K. Int. J. Coal Geol. 2011, 85, 72−77. (34) Zhou, L. Adsorption isotherms for the supercritical region. In Adsorption: Theory, Modeling and Analysis; Toth, J., Ed.; Marcel Dekker, Inc.: New York, 2002; pp 211250.

conclusion that the process of gas release from coal is largely determined by textural factors and the role of the temperature is far from minor. Those factors lead to changes of the coalbed permeability in situ because of the competing sorption and desorption processes and, hence, may be responsible for disturbing the equilibrium condition within the rock strata.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study is supported as a part of the AGH University of Science and Technology Research Project 11.11.210.244.

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REFERENCES

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Expansion of Hard Coal Accompanying the Sorption of Methane and

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