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Formation of methane hydrate from water sorbed by anthracite: An investgation by low-field NMR relaxation
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Formation of methane hydrate from water sorbed by anthracite: An investgation by low-field NMR relaxation ⁎
A.H. Turakhanova, M.Y. Shumskaytea, A.V. Ildyakovb, A.Y. Manakovb,c, , V.G. Smirnovd, V.N. Glinskikha,c, A.D. Duchkova a
Trofimuk Institute of Petroleum Geology and Geophysics SB RAS, Ac. Koptyug Ave., 3, Novosibirsk 630090, Russian Federation Nikolaev Institute of Inorganic Chemistry SB RAS, Ac. Lavrentiev Ave., 3, Novosibirsk 630090, Russian Federation c Novosibirsk State University, Pirogova Str., 2, Novosibirsk 630090, Russian Federation d Kuzbass State Technical University, Vesennaya Str., 28, Kemerovo 650026, Russian Federation b
G R A P H I C A L A B S T R A C T
Typical T2 spectra and their changes with time for the wet coal sample in which methane hydrate was obtained.
A R T I C LE I N FO
A B S T R A C T
Keywords: Gas hydrate Methane Coal Water adsorption NMR Spin-spin relaxation
Transverse relaxation time spectra T2 were studied for the samples (a) of wet coal in which methane hydrate was obtained, (b) wet coal exposed under the pressure of methane without the formation of the hydrate, and (c) initial wet coal. In all these cases, the samples of wet coal contained only water adsorbed in the fine pores of the coal substance. It was demonstrated that hydrate formation and the competitive sorption of methane at the positions previously occupied by water causes replacement of a part of water from coal mesopores into larger pores. The data obtained confirm a previously proposed multistage model (Smirnov et al., 2018) of hydrate formation in natural coal, which includes the stages of replacement of a part of sorbed water from coal pores by gas, the formation of microscopic water fragments in larger coal pores, and the formation of hydrate from this water.
⁎
Corresponding author at: Head of Laboratory, Nikolaev Institute of Inorganic Chemistry SB RAS, Ac. Lavrentiev Ave. 3, Novosibirsk 630090, Russian Federation E-mail addresses: [email protected] (A.Y. Manakov), [email protected] (V.G. Smirnov).
https://doi.org/10.1016/j.fuel.2019.116656 Received 4 November 2018; Received in revised form 8 November 2019; Accepted 12 November 2019 0016-2361/ © 2019 Elsevier Ltd. All rights reserved.
Please cite this article as: A.H. Turakhanov, et al., Fuel, https://doi.org/10.1016/j.fuel.2019.116656
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1. Introduction
humidity of gas less than 100% in equilibrium with coal, water is adsorbed in micropores and small mesopores of coal [21]. This water is strongly bound with the functional groups on pore walls, and its properties differ from those of bulk water. In particular, it does not form ice during cooling; instead, it forms glass-like phase. It was demonstrated in [13,16] that a part of water adsorbed in coal matter is able to form a hydrate for which the equilibrium conditions are practically the same as those for the bulk hydrate. This may be possible only in the case if the formed hydrate particles are outside of the pore space of coal. It was assumed in [16] that the formation of hydrate in coal proceeds in two stages: (a) displacement of a part of sorbed water from the pores by gas, and the formation of macroscopic water fragments in large mesoand macropores of coal, and (b) the formation of hydrate from this water. This mechanism is in good agreement with the data on the displacement of water adsorbed in small pores of coal by carbon dioxide [7]. The authors of the latter work demonstrated high information value of the low-field NMR relaxation for the studies of the distribution of adsorbed species over the pores of different types in coal. In the present work, we applied low-field NMR relaxation to the studies of water redistribution in anthracite samples saturated with water under the action of methane hydrate formation in these samples. The major goal of these studies was to confirm or reject the above-mentioned mechanism of hydrate formation in coal.
At present, coal is one of the major sources of energy, and the products of coal processing find wide application in metallurgy and in chemical industry [1,2]. Coal deposits were formed as a result of metamorphism of peat beds buried under the sedimentary layers, and their age varies from 50 to 300 million years [2,3]. Coal itself is a solid, mainly hydrocarbon polymer with extremely complicated chemical composition and internal structure. According to modern notions, the central part of coal macromolecules (a framework) consists mainly of condensed aromatic groups, while the peripheral part consists of alkyl, carboxyl, carbonyl and other functional groups attached to the framework. The size of the aromatic central part of coal macromolecules can vary from small in brown coals to very large in anthracite which is almost fully aromatic. These molecules are packed in larger associates; the space between them is a complicated system of pores with a broad size range, from micro- to macropores. It is accepted at present that the inner surface of micro- and mesopores is mainly formed by the functional groups of the peripheral parts of coal macromolecules. The degree of coal metamorphism increases in the sequence from brown coal (lignite) to various kinds of bituminous coal and then to anthracite. As a rule, with an increase in the degree of metamorphism, water-absorbing capacity of coal and the loss of volatiles during heating decrease. The Ushape dependence of mesopore specific surface area (SSA) vs. coal rank is observed, demonstrating that the number of mesopores within the lower rank coals decreases with increasing coal rank, and coalification mainly affects the mesopore structure. For the higher rank coals the mesopore size diminishes and the number of micropores ascends. Smaller mesopores and micropores gradually become dominant. This phenomenon is due to the effect of intensive compaction within bulk coal [4]. In this situation, the average characteristic size of mesopores decreases in the metamorphism sequence from 18 nm for brown coal to 3.5 nm for anthracite. Total pore volume increases with an increase in the degree of coal maturation [4]. In an undisturbed coal bed, pores and cracks are occupied by water, methane, CO2 and other gases, which may be present in the sorbed and free states [5]. It has been demonstrated that the sorption of gases and water by coal involves competitive processes, in particular, gases adsorbed in micropores may be partially displaced by water molecules and vice versa [6,7]. Gas hydrates are inclusion compounds in which the molecules of gases or some volatile liquids are arranged inside the cavities of a framework composed of hydrogen bonded water molecules [8]. Hydrateforming gases may be lower hydrocarbons, inert gases etc.; one volume of the hydrate may hold up to 170 volumes of gas (STP). The hydrates of hydrocarbon gases are widespread in nature [9] and are considered as a promising source of hydrocarbons [10]. The accumulations of natural hydrates are most frequently localized in the upper layers of marine and oceanic deposits (in the case of the sufficient depth of water reservoir) and in permafrost [8,9]. The possibility for gas hydrate inclusions to exist in coal beds at present or in the past is under consideration during the recent years [11–15]. Thermobaric conditions favorable for the existence of hydrates are most probable in coal deposits situated in the permafrost zone. It was demonstrated experimentally in [13,15,16] that the equilibrium conditions for methane and carbon dioxide hydrates in a coal matrix do not differ from those for bulk hydrate; only a part of water present in coal may take part in hydrate formation. The structure of the formed hydrate does not differ from that of bulk hydrate [15]. The data reported in [13,15,16] show that hydrate formation from water sorbed by coal has some qualitative differences from hydrate formation from water sorbed in other porous media, for example silica gel, porous glass etc. It is well known that the particles inside small limiting pores melt at lower temperature than the same substance in the bulk phase does [17]. Thus, a decrease for hydrate particles in silica gel pores about 30 nm in size may be several degrees, while it will be more than ten degrees in the pores 3–5 nm in size [18–20]. For the relative
2. Material and methods The experiments were carried out with the sample of anthracite coal drilled from Sibenergougol mine situated in Kemerovo region of the Kuznetsk Basin (Russian Federation). Some properties of the coal are summarized in Table 1, detailed information concerning this coal including specific surface area, total pore volume and water adsorption isotherm one may found in [16]. Fraction 0.5–2.0 mm were picked for the experiments. Prior to the experiment the samples were heated for 4 h in a vacuum oven at 110 °C and pressure below 100 Pa. Coal samples with 4.1 wt% of adsorbed water were prepared by one week exposing of the dried coal to water vapor over pure water. Water saturation was performed at room temperature (20 ± 2 °C); water adsorption was controlled by weighting of the samples. Methane with purity 99.99% was used. Five coal samples differing from each other by the method of their preliminary treatment were prepared for experiments. Preparation methods for each sample are described below. (1) Coal sample directly after drying (see above) was taken for the reference experiment. As expected, signal intensity was practically zero in this case. The observed low-intensity signal may be due to the presence of water that is not removed by drying, for example in closed pores. The value of this signal was practically unnoticeable in comparison with the signal from wet coal; this sample will not be discussed below. (2) Wet coal sample taken immediately after the removal from the desiccator in which sample saturation with water was carried out. (3) Dry coal sample similar to that described for method (1), placed in the autoclave and saturated with methane for a week at room temperature. (4) Wet coal sample as that from method (2), placed in the autoclave and saturated with methane for a week at room temperature. Methane hydrate could not be Table 1 Some information on the coal used in the experiments. Ash content Adaf*, 1, wt. % Volatile matter Vdaf*, wt. % Cdaf* Content, wt. % on organic Hdaf* ∑(O + N + S)daf* matter of coal Specific surface area SBET, m2/g Total pore volume V∑, cm3/g
2
eC]O eCOOH eOH eC]O eCOOH
0.109 0.010 0.046 0.87 0.32
13.0 6.5 93.10 3.69 3.21
Functional composition, mg-equ/g (daf*) Oxygen in group, mg/g (daf*)
2.1343 0.0051
eOH 0.74 * daf – dry-ash-free basis 1 – ash content of the drilled coal
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solution was found by means of the least squares with the condition of nonnegative variables [24]. The amplitudes of all 40 given times of transverse relaxation were calculated, and then a quasi-continuous T2 time spectrum was plotted. It is known that T2 value is affected by the bulk, surface, and diffuse relaxation, and the contribution of the latter may be neglected under the conditions of homogeneous internal field gradient [24,25]. This condition is fulfilled in the experimental set-up used by us, so, T2 may be described by the following equation [7]:
formed under these conditions. (5) The same wet coal sample as that in method (2), placed in the autoclave and saturated with methane for a week at a temperature of + 1 °C. Methane hydrate was surely formed under these conditions [13]. Before charging coal samples into the autoclave, it was blown with methane, then the working pressure of methane was set at 11 MPa; due to gas absorption by coal and temperature-related compression, the gas was pumped into the autoclave several times. The autoclaves were depressurized and opened directly before NMR experiments. Coal was loaded into tightly closed teflon containers, and measurements were carried out. The time from pressure relief to the start of measurement was not longer than 2–3 min. Then the samples were stored in the teflon containers, and measurements were repeated periodically for several hours. NMR measurements were carried out with a MCT-05 relaxometer with the working frequency 2.2 MHz and magnetic field induction 55 mT at a temperature of 25 °C. The system of the permanent magnets of the relaxometer was made on the basis of samarium-cobalt alloy with the working temperature range from –60 °C to + 350 °C. The major characteristics and parameters of experiments are: magnetic field nonuniformity does not exceed 10−3, fixed gradient: 0.3%, the minimal distance between spin echo signals: 0.18 ms, dead time: 90 μs, maximal polarization time: 10 s, the measurable transverse relaxation time range (T2): 600 μc − 10 s, the radio-frequency field is uniform over the sample volume. A result of NMR measurement is a relaxation curve; its initial amplitude corresponds to total water content of the sample. Magnetization of the sample, or polarization of the magnetic momenta of hydrogen nuclei, proceeded while the sample was placed in the static magnetic field, which caused macroscopic magnetization directed along the axis of the applied field. The vector of macroscopic magnetization was turned into the transverse plane under the action of the 90° pulse of the CPMG sequence (Carr-Purcell-Meiboom-Gill). Subsequent series of 180° pulses causes the formation of spin echo signals; their envelope curve is the relaxation curve to be investigated. Its initial amplitude corresponds to the electromotive force in the receiver coil after the first 90° pulse and is proportional to the number of hydrogen nuclei in the sample under study, which is then re-calculated into total water content. Before measuring, the sample was placed in a cylindrical teflone container 25 mm in diameter and 40 mm high. The container was tightly closed, in the lid there was a small opening to release gases emitted from the coal. The sample filled the container completely. The entire sample was in the coil, the magnetic field was uniform over the sample volume. The duration of a 90° pulse was 27 μs. Information concerning the relaxometer and signal processing can be found in [34]. The absolute error of T2 does not exceed 50 ms for fluids and 5 ms for porous samples, water content – not more than 2% (0.02 rel. units); the relative error of T2 determination is not more than 5%, water content – not more than 10%. The treatment and interpretation of the relaxation curve included the calculation of the distribution of transverse relaxation time values, which characterizes the size distribution of pores filled with water or methane [23]. This transformation is reduced to solving the integral Fredholm equation of the 1st type [22]
∫T
T2 max
2 min
e
− t T2 z (T2 ) dT2
1 1 S = +ρ T2 T2B 2 V where T2 is relaxation time, T2B is bulk relaxation time, S and V are the surface and volume of the pores filled with water, ρ2 is surface relaxivity representing the transverse relaxation strength. Here the first term depicts the contribution from bulk relaxation, and the second term is for the surface relaxation. Here T2B depends only on the nature of the liquid (in our case it is water), S and V depend on the size and shape of pore space, ρ2 depends on the properties of pore surface. It is known (for example, see [7,26–31]) that the major contribution into T2 for water adsorbed in the pore space (in particular, in natural coal) is made by surface relaxation; there are substantial differences in the behavior of water in micro-, meso- and macropores, and bulk water. This makes NMR relaxometry in the weak field a very efficient tool to study the state of water adsorbed in porous matter. 3. Results and discussion Typical spectra of transverse relaxation time T2 and their changes with time are shown in Fig. 1. The zero time here corresponds to the moment when the first spectrum starts to be recorded. As shown above, for the samples kept in the autoclave under methane pressure, this happened not later than within 2–3 min after pressure relief. According to the data of [7,26,29,30,31,32], relaxation time less than 5–10 ms in coal saturated with water corresponds to water in mesopores less than 100 nm in size. Relaxation time longer than ∼ 100 ms corresponds to water in large cracks and (more than 1000 ms) to bulk water. Intermediate relaxation time corresponds to water in macropores several hundred nanometers in size. Water filling micropores is characterized by relaxation time less than 0.1 ms; as a rule, it is not recorded by instruments. To analyze the results of the present work, we chose the boundary values 7 and 100 ms. Below we will consider the types of water present in the samples under investigation. The spectra of humid coal samples in which methane hydrate was formed (referred to as Sample 1 below) embrace a broad T2 time range (Fig. 1a). The shape of the spectra provides evidence of the presence of all the three types of water considered above (water in meso- and macropores, and in large cracks). The spectrum changes with time, showing a trend to increase the intensity in the region of small relaxation time and a decrease in intensity in the region of long relaxation time. Similar trends are demonstrated by the spectrum of the wet coal sample kept under methane pressure at room temperature (referred to as Sample 2), however, unlike for Sample 1, the intensity of the spectrum at T2 longer than 100 ms is almost zero, which means that water is in meso- and macropores (Fig. 1b). Appearance and disappearance of the peaks in the spectra presented in Fig. 1a and 1b may be explained by local features of water diffusion (and/or overflow) within the coal pores. This process is rather stochastic, and we can not interpret it quantitatively. Each spectrum demonstrates the features of water distribution within the sample at given moment of time. Finally, the spectrum of coal saturated with water but not exposed to methane pressure (Sample 3) has a single mode with the maximum at 0.5 – 0.8 ms (Fig. 1c). This spectrum corresponds to the situation when water is present almost completely in the mesopores less than 100 nm in size. This spectrum exhibits no changes with time. For comparison, the spectrum of dry coal kept under methane pressure was also recorded
= f (t ),
Here T2, T2min and T2max are the time of transverse relaxation, its minimal and maximal values, t is the time of the action of static magnetic field, z(T2) is the function of the differential distribution of signal amplitudes over transverse relaxation times, f(t) is the NMR signal. The solution of the inverse problem is traditionally done by means of Tikhonov regularization. Expansion limits T2min and T2max were introduced to carry out the expansion of the initial signal over the transverse relaxation times; a linear version was applied on a fixed net of 40 transverse relaxation time values; the amplitudes corresponding to each time value from the range from T2min to T2max were found. The 3
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Fig. 1. Typical spectra of transverse relaxation time T2 and their changes with time. (a) Wet coal sample in which methane hydrate was obtained (Sample 1). (b) Wet coal sample exposed under methane pressure (Sample 2). (c) Initial wet coal sample (Sample 3). (d) Dry coal sample saturated with methane. The time that passes from the start of experiments is indicated near the plots. Dashed vertical lines show the boundaries between relaxation times for water present in the pores of different types. For further explanations, see the text.
within the initial 5–10 min of the experiment. It is impossible to take this contribution into account. It is known that the intensity of T2 spectra under consideration in any point is proportional to the amount of protons relaxing with time constant T2. The sum of all calculated intensities is proportional to the total amount of water in the sample (excluding water present in micropores), and this value may be used to calculate the total amount of water if calibration is made. These values are shown in Fig. 2. One can see that the total amount of water in Samples 1–3 remains approximately constant during experiments, which means that water is not evaporated and sorbed from air. In the experiment with dry coal saturated with methane, the total area beneath the signal decreases with time, quite as expected, due to methane desorption from the sample. Substantial scattering of the experimental values is most likely due to a
(Fig. 1d). One can see that initially the spectrum of methane protons is bimodal with the maximum at 0.6 ms and a shoulder at 9 ms. Due to the release of adsorbed methane from the sample, the intensity of the spectrum decreases rapidly with time, and the line shape changes for a broad band with a center at about 5 ms. After that, the rate of methane loss by the sample decreases, and the intensity of the observed broad band decreases relatively slowly. It should be noted that the amount of methane in humid Samples (1–3) is surely lower than that in the dry sample at the same moments of time after the start of measurements, because some positions in coal accessible for methane sorption are occupied by water in humid samples. So, for this reason, and relying on the experimental spectra of dry coal saturated with methane, we suppose that any noticeable contribution from the protons of adsorbed methane into the total spectrum of Samples 1 – 3 may take place only 4
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Sample 3 provide reliable evidence of mesopores filled with sorbed water. The situation corresponds to the equilibrium, so no redistribution of spectrum intensity occurs with time (Fig. 3c). In the case of Sample 2, the spectra show that about 40% of water in the initial sample is present in macropores. While the sample is stored, redistribution of water from macropores into mesopores occurs (Fig. 3b). This shows that the initial distribution of water over the pores is not equilibrium The observed phenomena fully correspond to the model of the competitive sorption of water and methane in coal pores, implying partial replacement of sorbed water from the pore space into larger pores under the pressure of methane (or another gas) in coal sample saturated with water [7]. After depressurization, gas desorption occurs. Sorption positions occupied by gas get free, and gradual transfer of previously displaced water to the initial positions takes place. Water transfer from larger pores into smaller ones was previously observed for the wetted coal samples as described in [29,30]. A more complicated situation was observed for Sample 1. In this sample, water was detected to be present in all pore types meso- and macropores, as well as cracks. The fraction of water in macropores and cracks increases within the first ten minutes of the experiment, while the fraction of water in mesopores decreases. Then the situation changes for the opposite, with great scattering of experimental values observed for the fraction of water in meso- and macropores. So, substantial redistribution of water over the pore space is observed during hydrate formation in wet coal samples; a part of water is brought into large pores. An increase in water content in macropores and cracks, which is observed at the first stage, may be related to the continuing decomposition of the hydrate in the sample. It is known that the heat conductivity of natural coal is not high [33]. Hydrate decomposition is accompanied with heat absorption [8], therefore, the hydrate is cooled during its decomposition. The indicated factors lead to intense cooling of decomposing hydrate particles and decelerated hydrate decomposition in the sample. Most probably, it is this process that is observed within the first ten minutes of the experiment with Sample 1. The total amount of water and its fraction in macropores and cracks increase due to the formation of liquid water from the hydrate. A reason of a decrease in water fraction in mesopores may be related with two factors. First of all, intense gas flows passing in the pore space of the sample as a result of hydrate decomposition and coal degassing may cause mechanical removal of some amount of water from mesopores. Second, an increase in the total amount of water in macropores and cracks itself leads to a decrease in the fraction of water in mesopores. After the
Fig. 2. Dependence of the total water content in the samples on time. Solid squares – wet coal sample in which methane hydrate was obtained (Sample 1); open triangles – wet coal sample exposed under methane pressure (Sample 2); open squares – initial wet coal sample (Sample 3). Solid triangles correspond to the dry coal sample saturated with methane. In the latter case, sample magnetization is related not with the protons of water but with the protons of methane, so an apparent water content of the sample is presented.
small total amount of water in the sample and hence to a small measurable total magnetization of the sample. Correspondingly, the observed scattering is due to small experimental errors of measurements, imperceptible at the background of intense signals. In order to determine the fractions of water in the pores of different types, summing of the spectra of Samples 1–3 was carried out within the ranges 0.1–7, 7–100 ms, and more than 100 ms. The results are shown in Fig. 3. One can see that water is present in Sample 3 almost entirely in mesopores. Samples 1, 2 and 3 were prepared from the same parcel of coal saturated with water, so it is evident that the initial distribution of water in Samples 1 and 2 corresponds to that for Sample 3. This distribution of water in the initial sample is reasonably corresponding to the expected one for the samples obtained by the saturation of dry coal with water vapor through the gas phase. Indeed, water sorption in this case proceeds only in thin pores. The aqueous phase is limited by water menisci above which the equilibrium pressure is less than that above pure water [17]. Water vapor pressure above the filled macropores only slightly differs from the pressure above bulk water, so only micro- and mesopores are filled during sorption. The spectra of
Fig. 3. Dependence of the fraction of water present in the pores of different types on the time that passes from the start of the experiment. (a) Wet coal sample in which methane hydrate is obtained (Sample 1); (b) wt coal sample exposed under methane pressure (Sample 2); (c) initial wet coal sample (Sample 3). Open squares – water in cracks, solid squares – water in macropores, open triangles – water in mesopores. Pore types are discussed in the text. The lines are drawn schematically. Estimated error of each experimental point is ± 3%. 5
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completion of hydrate decomposition, the back transfer of water from large pores into smaller ones starts, as considered above for Sample 2. Relatively large scattering of experimental points in the experiments with Sample 1 may be explained by redistribution of water within the sample. Phenomenological model provided a satisfactory explanation of all observed features of hydrate formation in coals was suggested in Ref. [16]. The basic assumptions of this model are as follows. (1) Initially, the entire water sorbed by coal is present in micro- and mesopores. Free water is absent in the sample. (2) After pressurising of the sample with gas, a competition for sorption positions arises between the gas and sorbed water (see e.g. [35]). A part of water is displaced from the pores by sorbed gas and form water drops in macropores of coal. (3) Water drops react with the gas to form the hydrate. The process is over when the available water is consumed. Decomposition of thus formed hydrate proceeds under the conditions close to the equilibrium. Redistribution of water in the pore space which was observed in this work completely corresponds to the proposed model and can serve as its confirmation.
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4. Conclusions Thus, experiments show that the formation of methane hydrate from water sorbed by natural coal causes redistribution of water in the pore space. A part of water, initially present in mesopores, is replaced into larger pores, which causes characteristic changes in the spectra of transverse relaxation time T2. So, these results provide a confirmation of a two-stage model of hydrate formation in natural coal, proposed in [16], which includes the stage at which a part of water sorbed in coal pores is replaced by the gas, the formation of macroscopic water fragments in the large pores of coal, and hydrate formation from this water. In addition, the obtained data confirm the model of the competitive sorption of water and methane in the pores of natural coal. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements The studies were carried out within the framework of the project “Study of the physicochemical properties of hydrate-containing rocks for the development of remote sensing methods and the characteristics of natural accumulations of gas hydrates” of the integrated program of basic research of the SB RAS “Interdisciplinary integration studies”. References [1] Ghosh TK, Prelas MA. Energy Resources and Systems: Volume 1: Fundamentals and Non-Renewable Resources. Springer Science + Business Media B.V. 2009. [2] Van Krevelen DW. Coal: typology, physics, chemistry, constitution. Amsterdam, London, New-York, Tokyo: Elsevier; 1993. [3] Greb SF. Coal more than a resource: Critical data for understanding a variety of earth-science concepts. Int J Coal Geol 2013;118:15–32. [4] Baisheng N, Xianfeng L, Longlong Y, Junqing M, Xiangchun L. Pore structure characterization of different rank coals using gas adsorption and scanning electron microscopy. Fuel 2015;158:908–17.
6
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Full Length Article
Formation of methane hydrate from water sorbed by anthracite: An investgation by low-field NMR relaxation ⁎
A.H. Turakhanova, M.Y. Shumskaytea, A.V. Ildyakovb, A.Y. Manakovb,c, , V.G. Smirnovd, V.N. Glinskikha,c, A.D. Duchkova a
Trofimuk Institute of Petroleum Geology and Geophysics SB RAS, Ac. Koptyug Ave., 3, Novosibirsk 630090, Russian Federation Nikolaev Institute of Inorganic Chemistry SB RAS, Ac. Lavrentiev Ave., 3, Novosibirsk 630090, Russian Federation c Novosibirsk State University, Pirogova Str., 2, Novosibirsk 630090, Russian Federation d Kuzbass State Technical University, Vesennaya Str., 28, Kemerovo 650026, Russian Federation b
G R A P H I C A L A B S T R A C T
Typical T2 spectra and their changes with time for the wet coal sample in which methane hydrate was obtained.
A R T I C LE I N FO
A B S T R A C T
Keywords: Gas hydrate Methane Coal Water adsorption NMR Spin-spin relaxation
Transverse relaxation time spectra T2 were studied for the samples (a) of wet coal in which methane hydrate was obtained, (b) wet coal exposed under the pressure of methane without the formation of the hydrate, and (c) initial wet coal. In all these cases, the samples of wet coal contained only water adsorbed in the fine pores of the coal substance. It was demonstrated that hydrate formation and the competitive sorption of methane at the positions previously occupied by water causes replacement of a part of water from coal mesopores into larger pores. The data obtained confirm a previously proposed multistage model (Smirnov et al., 2018) of hydrate formation in natural coal, which includes the stages of replacement of a part of sorbed water from coal pores by gas, the formation of microscopic water fragments in larger coal pores, and the formation of hydrate from this water.
⁎
Corresponding author at: Head of Laboratory, Nikolaev Institute of Inorganic Chemistry SB RAS, Ac. Lavrentiev Ave. 3, Novosibirsk 630090, Russian Federation E-mail addresses: [email protected] (A.Y. Manakov), [email protected] (V.G. Smirnov).
https://doi.org/10.1016/j.fuel.2019.116656 Received 4 November 2018; Received in revised form 8 November 2019; Accepted 12 November 2019 0016-2361/ © 2019 Elsevier Ltd. All rights reserved.
Please cite this article as: A.H. Turakhanov, et al., Fuel, https://doi.org/10.1016/j.fuel.2019.116656
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1. Introduction
humidity of gas less than 100% in equilibrium with coal, water is adsorbed in micropores and small mesopores of coal [21]. This water is strongly bound with the functional groups on pore walls, and its properties differ from those of bulk water. In particular, it does not form ice during cooling; instead, it forms glass-like phase. It was demonstrated in [13,16] that a part of water adsorbed in coal matter is able to form a hydrate for which the equilibrium conditions are practically the same as those for the bulk hydrate. This may be possible only in the case if the formed hydrate particles are outside of the pore space of coal. It was assumed in [16] that the formation of hydrate in coal proceeds in two stages: (a) displacement of a part of sorbed water from the pores by gas, and the formation of macroscopic water fragments in large mesoand macropores of coal, and (b) the formation of hydrate from this water. This mechanism is in good agreement with the data on the displacement of water adsorbed in small pores of coal by carbon dioxide [7]. The authors of the latter work demonstrated high information value of the low-field NMR relaxation for the studies of the distribution of adsorbed species over the pores of different types in coal. In the present work, we applied low-field NMR relaxation to the studies of water redistribution in anthracite samples saturated with water under the action of methane hydrate formation in these samples. The major goal of these studies was to confirm or reject the above-mentioned mechanism of hydrate formation in coal.
At present, coal is one of the major sources of energy, and the products of coal processing find wide application in metallurgy and in chemical industry [1,2]. Coal deposits were formed as a result of metamorphism of peat beds buried under the sedimentary layers, and their age varies from 50 to 300 million years [2,3]. Coal itself is a solid, mainly hydrocarbon polymer with extremely complicated chemical composition and internal structure. According to modern notions, the central part of coal macromolecules (a framework) consists mainly of condensed aromatic groups, while the peripheral part consists of alkyl, carboxyl, carbonyl and other functional groups attached to the framework. The size of the aromatic central part of coal macromolecules can vary from small in brown coals to very large in anthracite which is almost fully aromatic. These molecules are packed in larger associates; the space between them is a complicated system of pores with a broad size range, from micro- to macropores. It is accepted at present that the inner surface of micro- and mesopores is mainly formed by the functional groups of the peripheral parts of coal macromolecules. The degree of coal metamorphism increases in the sequence from brown coal (lignite) to various kinds of bituminous coal and then to anthracite. As a rule, with an increase in the degree of metamorphism, water-absorbing capacity of coal and the loss of volatiles during heating decrease. The Ushape dependence of mesopore specific surface area (SSA) vs. coal rank is observed, demonstrating that the number of mesopores within the lower rank coals decreases with increasing coal rank, and coalification mainly affects the mesopore structure. For the higher rank coals the mesopore size diminishes and the number of micropores ascends. Smaller mesopores and micropores gradually become dominant. This phenomenon is due to the effect of intensive compaction within bulk coal [4]. In this situation, the average characteristic size of mesopores decreases in the metamorphism sequence from 18 nm for brown coal to 3.5 nm for anthracite. Total pore volume increases with an increase in the degree of coal maturation [4]. In an undisturbed coal bed, pores and cracks are occupied by water, methane, CO2 and other gases, which may be present in the sorbed and free states [5]. It has been demonstrated that the sorption of gases and water by coal involves competitive processes, in particular, gases adsorbed in micropores may be partially displaced by water molecules and vice versa [6,7]. Gas hydrates are inclusion compounds in which the molecules of gases or some volatile liquids are arranged inside the cavities of a framework composed of hydrogen bonded water molecules [8]. Hydrateforming gases may be lower hydrocarbons, inert gases etc.; one volume of the hydrate may hold up to 170 volumes of gas (STP). The hydrates of hydrocarbon gases are widespread in nature [9] and are considered as a promising source of hydrocarbons [10]. The accumulations of natural hydrates are most frequently localized in the upper layers of marine and oceanic deposits (in the case of the sufficient depth of water reservoir) and in permafrost [8,9]. The possibility for gas hydrate inclusions to exist in coal beds at present or in the past is under consideration during the recent years [11–15]. Thermobaric conditions favorable for the existence of hydrates are most probable in coal deposits situated in the permafrost zone. It was demonstrated experimentally in [13,15,16] that the equilibrium conditions for methane and carbon dioxide hydrates in a coal matrix do not differ from those for bulk hydrate; only a part of water present in coal may take part in hydrate formation. The structure of the formed hydrate does not differ from that of bulk hydrate [15]. The data reported in [13,15,16] show that hydrate formation from water sorbed by coal has some qualitative differences from hydrate formation from water sorbed in other porous media, for example silica gel, porous glass etc. It is well known that the particles inside small limiting pores melt at lower temperature than the same substance in the bulk phase does [17]. Thus, a decrease for hydrate particles in silica gel pores about 30 nm in size may be several degrees, while it will be more than ten degrees in the pores 3–5 nm in size [18–20]. For the relative
2. Material and methods The experiments were carried out with the sample of anthracite coal drilled from Sibenergougol mine situated in Kemerovo region of the Kuznetsk Basin (Russian Federation). Some properties of the coal are summarized in Table 1, detailed information concerning this coal including specific surface area, total pore volume and water adsorption isotherm one may found in [16]. Fraction 0.5–2.0 mm were picked for the experiments. Prior to the experiment the samples were heated for 4 h in a vacuum oven at 110 °C and pressure below 100 Pa. Coal samples with 4.1 wt% of adsorbed water were prepared by one week exposing of the dried coal to water vapor over pure water. Water saturation was performed at room temperature (20 ± 2 °C); water adsorption was controlled by weighting of the samples. Methane with purity 99.99% was used. Five coal samples differing from each other by the method of their preliminary treatment were prepared for experiments. Preparation methods for each sample are described below. (1) Coal sample directly after drying (see above) was taken for the reference experiment. As expected, signal intensity was practically zero in this case. The observed low-intensity signal may be due to the presence of water that is not removed by drying, for example in closed pores. The value of this signal was practically unnoticeable in comparison with the signal from wet coal; this sample will not be discussed below. (2) Wet coal sample taken immediately after the removal from the desiccator in which sample saturation with water was carried out. (3) Dry coal sample similar to that described for method (1), placed in the autoclave and saturated with methane for a week at room temperature. (4) Wet coal sample as that from method (2), placed in the autoclave and saturated with methane for a week at room temperature. Methane hydrate could not be Table 1 Some information on the coal used in the experiments. Ash content Adaf*, 1, wt. % Volatile matter Vdaf*, wt. % Cdaf* Content, wt. % on organic Hdaf* ∑(O + N + S)daf* matter of coal Specific surface area SBET, m2/g Total pore volume V∑, cm3/g
2
eC]O eCOOH eOH eC]O eCOOH
0.109 0.010 0.046 0.87 0.32
13.0 6.5 93.10 3.69 3.21
Functional composition, mg-equ/g (daf*) Oxygen in group, mg/g (daf*)
2.1343 0.0051
eOH 0.74 * daf – dry-ash-free basis 1 – ash content of the drilled coal
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solution was found by means of the least squares with the condition of nonnegative variables [24]. The amplitudes of all 40 given times of transverse relaxation were calculated, and then a quasi-continuous T2 time spectrum was plotted. It is known that T2 value is affected by the bulk, surface, and diffuse relaxation, and the contribution of the latter may be neglected under the conditions of homogeneous internal field gradient [24,25]. This condition is fulfilled in the experimental set-up used by us, so, T2 may be described by the following equation [7]:
formed under these conditions. (5) The same wet coal sample as that in method (2), placed in the autoclave and saturated with methane for a week at a temperature of + 1 °C. Methane hydrate was surely formed under these conditions [13]. Before charging coal samples into the autoclave, it was blown with methane, then the working pressure of methane was set at 11 MPa; due to gas absorption by coal and temperature-related compression, the gas was pumped into the autoclave several times. The autoclaves were depressurized and opened directly before NMR experiments. Coal was loaded into tightly closed teflon containers, and measurements were carried out. The time from pressure relief to the start of measurement was not longer than 2–3 min. Then the samples were stored in the teflon containers, and measurements were repeated periodically for several hours. NMR measurements were carried out with a MCT-05 relaxometer with the working frequency 2.2 MHz and magnetic field induction 55 mT at a temperature of 25 °C. The system of the permanent magnets of the relaxometer was made on the basis of samarium-cobalt alloy with the working temperature range from –60 °C to + 350 °C. The major characteristics and parameters of experiments are: magnetic field nonuniformity does not exceed 10−3, fixed gradient: 0.3%, the minimal distance between spin echo signals: 0.18 ms, dead time: 90 μs, maximal polarization time: 10 s, the measurable transverse relaxation time range (T2): 600 μc − 10 s, the radio-frequency field is uniform over the sample volume. A result of NMR measurement is a relaxation curve; its initial amplitude corresponds to total water content of the sample. Magnetization of the sample, or polarization of the magnetic momenta of hydrogen nuclei, proceeded while the sample was placed in the static magnetic field, which caused macroscopic magnetization directed along the axis of the applied field. The vector of macroscopic magnetization was turned into the transverse plane under the action of the 90° pulse of the CPMG sequence (Carr-Purcell-Meiboom-Gill). Subsequent series of 180° pulses causes the formation of spin echo signals; their envelope curve is the relaxation curve to be investigated. Its initial amplitude corresponds to the electromotive force in the receiver coil after the first 90° pulse and is proportional to the number of hydrogen nuclei in the sample under study, which is then re-calculated into total water content. Before measuring, the sample was placed in a cylindrical teflone container 25 mm in diameter and 40 mm high. The container was tightly closed, in the lid there was a small opening to release gases emitted from the coal. The sample filled the container completely. The entire sample was in the coil, the magnetic field was uniform over the sample volume. The duration of a 90° pulse was 27 μs. Information concerning the relaxometer and signal processing can be found in [34]. The absolute error of T2 does not exceed 50 ms for fluids and 5 ms for porous samples, water content – not more than 2% (0.02 rel. units); the relative error of T2 determination is not more than 5%, water content – not more than 10%. The treatment and interpretation of the relaxation curve included the calculation of the distribution of transverse relaxation time values, which characterizes the size distribution of pores filled with water or methane [23]. This transformation is reduced to solving the integral Fredholm equation of the 1st type [22]
∫T
T2 max
2 min
e
− t T2 z (T2 ) dT2
1 1 S = +ρ T2 T2B 2 V where T2 is relaxation time, T2B is bulk relaxation time, S and V are the surface and volume of the pores filled with water, ρ2 is surface relaxivity representing the transverse relaxation strength. Here the first term depicts the contribution from bulk relaxation, and the second term is for the surface relaxation. Here T2B depends only on the nature of the liquid (in our case it is water), S and V depend on the size and shape of pore space, ρ2 depends on the properties of pore surface. It is known (for example, see [7,26–31]) that the major contribution into T2 for water adsorbed in the pore space (in particular, in natural coal) is made by surface relaxation; there are substantial differences in the behavior of water in micro-, meso- and macropores, and bulk water. This makes NMR relaxometry in the weak field a very efficient tool to study the state of water adsorbed in porous matter. 3. Results and discussion Typical spectra of transverse relaxation time T2 and their changes with time are shown in Fig. 1. The zero time here corresponds to the moment when the first spectrum starts to be recorded. As shown above, for the samples kept in the autoclave under methane pressure, this happened not later than within 2–3 min after pressure relief. According to the data of [7,26,29,30,31,32], relaxation time less than 5–10 ms in coal saturated with water corresponds to water in mesopores less than 100 nm in size. Relaxation time longer than ∼ 100 ms corresponds to water in large cracks and (more than 1000 ms) to bulk water. Intermediate relaxation time corresponds to water in macropores several hundred nanometers in size. Water filling micropores is characterized by relaxation time less than 0.1 ms; as a rule, it is not recorded by instruments. To analyze the results of the present work, we chose the boundary values 7 and 100 ms. Below we will consider the types of water present in the samples under investigation. The spectra of humid coal samples in which methane hydrate was formed (referred to as Sample 1 below) embrace a broad T2 time range (Fig. 1a). The shape of the spectra provides evidence of the presence of all the three types of water considered above (water in meso- and macropores, and in large cracks). The spectrum changes with time, showing a trend to increase the intensity in the region of small relaxation time and a decrease in intensity in the region of long relaxation time. Similar trends are demonstrated by the spectrum of the wet coal sample kept under methane pressure at room temperature (referred to as Sample 2), however, unlike for Sample 1, the intensity of the spectrum at T2 longer than 100 ms is almost zero, which means that water is in meso- and macropores (Fig. 1b). Appearance and disappearance of the peaks in the spectra presented in Fig. 1a and 1b may be explained by local features of water diffusion (and/or overflow) within the coal pores. This process is rather stochastic, and we can not interpret it quantitatively. Each spectrum demonstrates the features of water distribution within the sample at given moment of time. Finally, the spectrum of coal saturated with water but not exposed to methane pressure (Sample 3) has a single mode with the maximum at 0.5 – 0.8 ms (Fig. 1c). This spectrum corresponds to the situation when water is present almost completely in the mesopores less than 100 nm in size. This spectrum exhibits no changes with time. For comparison, the spectrum of dry coal kept under methane pressure was also recorded
= f (t ),
Here T2, T2min and T2max are the time of transverse relaxation, its minimal and maximal values, t is the time of the action of static magnetic field, z(T2) is the function of the differential distribution of signal amplitudes over transverse relaxation times, f(t) is the NMR signal. The solution of the inverse problem is traditionally done by means of Tikhonov regularization. Expansion limits T2min and T2max were introduced to carry out the expansion of the initial signal over the transverse relaxation times; a linear version was applied on a fixed net of 40 transverse relaxation time values; the amplitudes corresponding to each time value from the range from T2min to T2max were found. The 3
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Fig. 1. Typical spectra of transverse relaxation time T2 and their changes with time. (a) Wet coal sample in which methane hydrate was obtained (Sample 1). (b) Wet coal sample exposed under methane pressure (Sample 2). (c) Initial wet coal sample (Sample 3). (d) Dry coal sample saturated with methane. The time that passes from the start of experiments is indicated near the plots. Dashed vertical lines show the boundaries between relaxation times for water present in the pores of different types. For further explanations, see the text.
within the initial 5–10 min of the experiment. It is impossible to take this contribution into account. It is known that the intensity of T2 spectra under consideration in any point is proportional to the amount of protons relaxing with time constant T2. The sum of all calculated intensities is proportional to the total amount of water in the sample (excluding water present in micropores), and this value may be used to calculate the total amount of water if calibration is made. These values are shown in Fig. 2. One can see that the total amount of water in Samples 1–3 remains approximately constant during experiments, which means that water is not evaporated and sorbed from air. In the experiment with dry coal saturated with methane, the total area beneath the signal decreases with time, quite as expected, due to methane desorption from the sample. Substantial scattering of the experimental values is most likely due to a
(Fig. 1d). One can see that initially the spectrum of methane protons is bimodal with the maximum at 0.6 ms and a shoulder at 9 ms. Due to the release of adsorbed methane from the sample, the intensity of the spectrum decreases rapidly with time, and the line shape changes for a broad band with a center at about 5 ms. After that, the rate of methane loss by the sample decreases, and the intensity of the observed broad band decreases relatively slowly. It should be noted that the amount of methane in humid Samples (1–3) is surely lower than that in the dry sample at the same moments of time after the start of measurements, because some positions in coal accessible for methane sorption are occupied by water in humid samples. So, for this reason, and relying on the experimental spectra of dry coal saturated with methane, we suppose that any noticeable contribution from the protons of adsorbed methane into the total spectrum of Samples 1 – 3 may take place only 4
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Sample 3 provide reliable evidence of mesopores filled with sorbed water. The situation corresponds to the equilibrium, so no redistribution of spectrum intensity occurs with time (Fig. 3c). In the case of Sample 2, the spectra show that about 40% of water in the initial sample is present in macropores. While the sample is stored, redistribution of water from macropores into mesopores occurs (Fig. 3b). This shows that the initial distribution of water over the pores is not equilibrium The observed phenomena fully correspond to the model of the competitive sorption of water and methane in coal pores, implying partial replacement of sorbed water from the pore space into larger pores under the pressure of methane (or another gas) in coal sample saturated with water [7]. After depressurization, gas desorption occurs. Sorption positions occupied by gas get free, and gradual transfer of previously displaced water to the initial positions takes place. Water transfer from larger pores into smaller ones was previously observed for the wetted coal samples as described in [29,30]. A more complicated situation was observed for Sample 1. In this sample, water was detected to be present in all pore types meso- and macropores, as well as cracks. The fraction of water in macropores and cracks increases within the first ten minutes of the experiment, while the fraction of water in mesopores decreases. Then the situation changes for the opposite, with great scattering of experimental values observed for the fraction of water in meso- and macropores. So, substantial redistribution of water over the pore space is observed during hydrate formation in wet coal samples; a part of water is brought into large pores. An increase in water content in macropores and cracks, which is observed at the first stage, may be related to the continuing decomposition of the hydrate in the sample. It is known that the heat conductivity of natural coal is not high [33]. Hydrate decomposition is accompanied with heat absorption [8], therefore, the hydrate is cooled during its decomposition. The indicated factors lead to intense cooling of decomposing hydrate particles and decelerated hydrate decomposition in the sample. Most probably, it is this process that is observed within the first ten minutes of the experiment with Sample 1. The total amount of water and its fraction in macropores and cracks increase due to the formation of liquid water from the hydrate. A reason of a decrease in water fraction in mesopores may be related with two factors. First of all, intense gas flows passing in the pore space of the sample as a result of hydrate decomposition and coal degassing may cause mechanical removal of some amount of water from mesopores. Second, an increase in the total amount of water in macropores and cracks itself leads to a decrease in the fraction of water in mesopores. After the
Fig. 2. Dependence of the total water content in the samples on time. Solid squares – wet coal sample in which methane hydrate was obtained (Sample 1); open triangles – wet coal sample exposed under methane pressure (Sample 2); open squares – initial wet coal sample (Sample 3). Solid triangles correspond to the dry coal sample saturated with methane. In the latter case, sample magnetization is related not with the protons of water but with the protons of methane, so an apparent water content of the sample is presented.
small total amount of water in the sample and hence to a small measurable total magnetization of the sample. Correspondingly, the observed scattering is due to small experimental errors of measurements, imperceptible at the background of intense signals. In order to determine the fractions of water in the pores of different types, summing of the spectra of Samples 1–3 was carried out within the ranges 0.1–7, 7–100 ms, and more than 100 ms. The results are shown in Fig. 3. One can see that water is present in Sample 3 almost entirely in mesopores. Samples 1, 2 and 3 were prepared from the same parcel of coal saturated with water, so it is evident that the initial distribution of water in Samples 1 and 2 corresponds to that for Sample 3. This distribution of water in the initial sample is reasonably corresponding to the expected one for the samples obtained by the saturation of dry coal with water vapor through the gas phase. Indeed, water sorption in this case proceeds only in thin pores. The aqueous phase is limited by water menisci above which the equilibrium pressure is less than that above pure water [17]. Water vapor pressure above the filled macropores only slightly differs from the pressure above bulk water, so only micro- and mesopores are filled during sorption. The spectra of
Fig. 3. Dependence of the fraction of water present in the pores of different types on the time that passes from the start of the experiment. (a) Wet coal sample in which methane hydrate is obtained (Sample 1); (b) wt coal sample exposed under methane pressure (Sample 2); (c) initial wet coal sample (Sample 3). Open squares – water in cracks, solid squares – water in macropores, open triangles – water in mesopores. Pore types are discussed in the text. The lines are drawn schematically. Estimated error of each experimental point is ± 3%. 5
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completion of hydrate decomposition, the back transfer of water from large pores into smaller ones starts, as considered above for Sample 2. Relatively large scattering of experimental points in the experiments with Sample 1 may be explained by redistribution of water within the sample. Phenomenological model provided a satisfactory explanation of all observed features of hydrate formation in coals was suggested in Ref. [16]. The basic assumptions of this model are as follows. (1) Initially, the entire water sorbed by coal is present in micro- and mesopores. Free water is absent in the sample. (2) After pressurising of the sample with gas, a competition for sorption positions arises between the gas and sorbed water (see e.g. [35]). A part of water is displaced from the pores by sorbed gas and form water drops in macropores of coal. (3) Water drops react with the gas to form the hydrate. The process is over when the available water is consumed. Decomposition of thus formed hydrate proceeds under the conditions close to the equilibrium. Redistribution of water in the pore space which was observed in this work completely corresponds to the proposed model and can serve as its confirmation.
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4. Conclusions Thus, experiments show that the formation of methane hydrate from water sorbed by natural coal causes redistribution of water in the pore space. A part of water, initially present in mesopores, is replaced into larger pores, which causes characteristic changes in the spectra of transverse relaxation time T2. So, these results provide a confirmation of a two-stage model of hydrate formation in natural coal, proposed in [16], which includes the stage at which a part of water sorbed in coal pores is replaced by the gas, the formation of macroscopic water fragments in the large pores of coal, and hydrate formation from this water. In addition, the obtained data confirm the model of the competitive sorption of water and methane in the pores of natural coal. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements The studies were carried out within the framework of the project “Study of the physicochemical properties of hydrate-containing rocks for the development of remote sensing methods and the characteristics of natural accumulations of gas hydrates” of the integrated program of basic research of the SB RAS “Interdisciplinary integration studies”. References [1] Ghosh TK, Prelas MA. Energy Resources and Systems: Volume 1: Fundamentals and Non-Renewable Resources. Springer Science + Business Media B.V. 2009. [2] Van Krevelen DW. Coal: typology, physics, chemistry, constitution. Amsterdam, London, New-York, Tokyo: Elsevier; 1993. [3] Greb SF. Coal more than a resource: Critical data for understanding a variety of earth-science concepts. Int J Coal Geol 2013;118:15–32. [4] Baisheng N, Xianfeng L, Longlong Y, Junqing M, Xiangchun L. Pore structure characterization of different rank coals using gas adsorption and scanning electron microscopy. Fuel 2015;158:908–17.
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