Phase transitions in the coal-water-methane system

Sudden coal and gas outbursts as well as explosions of methane and air mixtures may be compared to permanent natural disasters taking lives of miners and inflicting tremendous losses. All the aspects of mining safety and methane combustive utilization are therefore very urgent. Coal is a porous substance. The pressure of overlying rocks and the action of metamorphic gases produce a very specific dispersed porous and fractured structure of fossil coals with multi-level size hierarchy ranging over seven orders of magnitude, from components comparable with molecular sizes of methane or water to cracks with openings up to several millimeters. Coal formation in virgin condition is a complicated natural system existing in the thermodynamic equilibrium and consisting of coal mass, gases (predominantly methane in Donets Basin deposits), and water. Methane and water are here not merely inert components of the system but active agents in a coal seam exerting substantial or even crucial effect on its behavior. One of the main difficulties in studying this system consists in its being a multiphase system constitutive of a solid, a liquid, and a gas. Thermodynamic equilibrium in such systems is quite poorly investigated. Most attention was focused on solid-water systems [1–7] due to unique properties of water among other liquids (in particular, polymeric nature of molecular structure, cooperative network of hydrogen bonds, etc.). One of the key issues studied was the effect of the solid surface on the behavior of the boundary phase. A great contribution to understanding the behavior of adsorbed molecules was made by E.K. Rideal [8], J. De Boer [9], and his followers [10]. J. De Boer [9] considered two extreme cases of monomolecular adsorbed layer, local and non-local adsorption. In the case of local adsorption the adsorbed molecules are strongly bonded to adsorption sites, they have no translational degrees of freedom and cannot oscillate either perpendicular or parallel to the surface. Local adsorption does not imply, however, a complete immobility of adsorbed molecules. The mere existence of adsorption equilibrium between the gas and the adsorbed layer indicates the molecular movements in both perpendicular and parallel directions to the solid surface. Migration of adsorbed molecules along the surface occurs by jumps from one adsorption site to another. A site residence time τc is related to the jump movement activation energy V0 using Frenkel equation [11] τc = τ0e V0/RT , (1)

Sudden coal and gas outbursts as well as explosions of methane and air mixtures may be compared to permanent natural disasters taking lives of miners and inflicting tremendous losses.All the aspects of mining safety and methane combustive utilization are therefore very urgent.
Coal is a porous substance.The pressure of overlying rocks and the action of metamorphic gases produce a very specific dispersed porous and fractured structure of fossil coals with multi-level size hierarchy ranging over seven orders of magnitude, from components comparable with molecular sizes of methane or water to cracks with openings up to several millimeters.Coal formation in virgin condition is a complicated natural system existing in the thermodynamic equilibrium and consisting of coal mass, gases (predominantly methane in Donets Basin deposits), and water.Methane and water are here not merely inert components of the system but active agents in a coal seam exerting substantial or even crucial effect on its behavior.One of the main difficulties in studying this system consists in its being a multiphase system constitutive of a solid, a liquid, and a gas.Thermodynamic equilibrium in such systems is quite poorly investigated.
Most attention was focused on solid-water systems [1][2][3][4][5][6][7] due to unique properties of water among other liquids (in particular, polymeric nature of molecular structure, cooperative network of hydrogen bonds, etc.).One of the key issues studied was the effect of the solid surface on the behavior of the boundary phase.
A great contribution to understanding the behavior of adsorbed molecules was made by E.K. Rideal [8], J. De Boer [9], and his followers [10].J. De Boer [9] considered two extreme cases of monomolecular adsorbed layer, local and non-local adsorption.In the case of local adsorption the adsorbed molecules are strongly bonded to adsorption sites, they have no translational degrees of freedom and cannot oscillate either perpendicular or parallel to the surface.Local adsorption does not imply, however, a complete immobility of adsorbed molecules.The mere existence of adsorption equilibrium between the gas and the adsorbed layer indicates the molecular movements in both perpendicular and parallel directions to the solid surface.
Migration of adsorbed molecules along the surface occurs by jumps from one adsorption site to another.A site residence time τ c is related to the jump movement activation energy V 0 using Frenkel equation [11] τ c = τ 0 e V0/RT , where τ 0 is a constant proportional to the oscillation period of the adsorbed molecule, V 0 is a difference between adsorption heat values at the active surface site and between two sites.Consequently, V 0 is much less than adsorption heat Q.At physical adsorption on hydrophilic (polar) surfaces, the V 0 value is usually from one third to half of Q value which is about 40 kJ/mol or higher [12].These values are much higher than the energy of thermal motion RT ∼ 2.4 kJ/mol at T = 290 to 295 K.As a result, the residence time τ c is higher than the oscillation period τ 0 ∼ 10 −13 s but certainly less than the adsorption time τ .At a physical adsorption on polar adsorbents, the τ /τ c ratio is equal to about 5,000 [9,12].When V 0 becomes less than energy of thermal motion, the adsorbed molecules begin to move freely along the surface in both directions.Adsorption layers consisting of such molecules show the behavior similar to a two-dimensional gas.In this case, adsorption is described by non-local model.According to De Boer, the adsorbed molecules retain all the rotational and two translational degrees of freedom at the perfect non-local adsorption.
Fossil coals are hydrophobic substances.Almost all hydrophobic adsorbents have hydrophilic adsorption sites (as a rule, energetically different) on their surfaces [13,14].However, definite data on the adsorbed water molecules on hydrophobic adsorbents with various concentrations and polarity of hydrophilic sites are very rare in the literature.This is due to experimental quantitative determination difficulties of hydrophilic species on the surfaces of hydrophobic adsorbents.
In coals, adsorption sites can be formed by various π-bonds, single electron pairs, small cations, oxygen-containing groups, radicals such as CH, CH 2 , CH 3 , etc. Just these sites primarily adsorb water molecules.
As for methane, its sorption in coal can hardly be interpreted as simply condensation from gas phase onto the pore surface.Firstly, pores in coals are bulk filled and hence the pores volume rather than the pore surface is the sorption parameter.Secondly, methane was incorporated into the coal mass due to a joint action of gas pressure and mechanical loads at coalification during metamorphism without forming any chemical bonds [15,16].Since such incorporated methane molecules are contained in the cavities of the same size, the concept of solid-gas interface, as well as the concept of adsorption layer, are unimportant.Such an incorporated methane was referred to as "methane in solid solution" [17,18].It accounts for about 80% of all the absorbed methane.
The use of classical models (developed for getters) to estimate the methane amount and behavior in coals is a common error by many researchers.In references [19][20][21] the authors estimate the methane behavior in coals based on the assumption that the methane sorption occurs on the pore surface.However, a conventional determination of methane amount and condition using sorption isotherms introduces a significant uncertainty into the results obtained.In particular, the applied approaches fail to explain why coking ("K") and fat ("J") coals are capable of containing as much methane as up to 35 m 3 /ton while possessing smallest (as compared to other coal types) specific porosity.Over a long time, coals were studied using laboratory techniques, which had caused coal destruction, with subsequent extraction of soluble products.The integrity of coal structure was thereby broken at all levels, from micro to macro ones.Such an approach precluded the study of the coal interaction with fluids.A substantial advancement in solving the global coal problems became possible only after the implementation of nondestructive physical techniques.
NMR techniques are among those being most successfully applied to the study of fluids in various heterogeneous systems [22][23][24].NMR wide line technique is especially useful in studying the effects of fluids in coals.
We have used 1 H NMR wide line spectrometer with the autodyne circuit in order to enhance the sensitivity and measurement accuracy.Raw experimental NMR absorption spectra in the coalwater, coal-methane, or coal-water-methane systems, see figure 1(a), were numerically differentiated into final derivative spectra, see figure 1(b), consisting of two superimposed lines, namely, wide and narrow lines [25].The wide line originates from the hydrogen incorporated into the coal structure while the narrow line is caused by the hydrogen of water and methane.Linewidth values ∆H are determined by 1 H nuclei mobility in the system.Wide and narrow line widths ∆H 1 and ∆H 2 in NMR spectra differ by about an order of magnitude [26,27].In particular, the resonance line width of the adsorbed water and methane ∆H 1 varies from 0.079 to 119.4 A/m depending on nuclei mobility while the linewidth ∆H 2 of coal organics varies from 437.7 to 517.3 A/m depending on the coal metamorphism extent.
NMR spectrum line intensities I 1 and I 2 depend on both numbers of resonating nuclei and their mobility.The numbers of resonating nuclei being equal, the intensity values I will be different if the mobility values are different [28].
NMR wide-line low-temperature technique made it possible to estimate the effects of porosity and surface active centers on the activation energy V 0 and correlation time τ c much simpler than, for instance, the measurements of entropy [29], frequency dependence of dielectric losses, or nuclear quadrupole resonance frequency (NQR) [30] in a wide temperature range.
The present studies were carried out, taking into consideration a complicated nature of the coal-water-methane three-phase system, in three steps.First, artificially prepared moistened coals, then methane saturated coals, and finally the complete coal-water-methane system were studied.Experimental data on low temperature NMR line spreading for adsorbed water were processed using approximate theory of Blombergen, Purcell, and Pound [31] to obtain correlation time values τ c .Correlation time is a characteristic of the mobility of water molecules.According to J. Uo and E. Fedin [32], correlation time τ c is related to the linewidth ∆H at given temperature and to the linewidth ∆H 0 at the temperature of rigid lattice by the equation where α = 8 ln 2; γ = 4258 s −1 Oe −1 , and ω c = 2π/τ c is a reorientation frequency.
The correlation time τ c can be derived from equation (2) as: Low temperature behavior of NMR line widths and second-order moments for several coal types is presented in figures 2 and 3.  Low temperature NMR spectra of coals have a distinctive feature of the liquid to crystal-like phase transition temperature in adsorbed water, see figure 4. The transition appears extended in a small temperature range rather than abrupt jump, since molecules more loosely bonded to solid surface freeze first.On the contrary, two or three monolayers closest to the surface remain mobile even at very low temperatures where rotational freedom becomes frozen, as it was shown by A. Bloch [33].This transition begins at 183 K in anthracites, 158 K in coals with V G = 26%, or 233 K in coals with V G = 39%.
In accordance with Uo-Fedin equation [32], the magnitude of the potential barrier V 0 hindering the molecular motion can be evaluated using a simple relation where T is the phase transition temperature.The highest measured value V 0 was 40.74 kJ/mole for coals with highest volatile content V G = 39%.Respectively, the lowest measured value V 0 was 28.43 kJ/mole for anthracites with V G = 5%.It can be concluded that water molecules are most strongly bonded to adsorption centers in coals of intermediate metamorphism degree.It can be derived from equation (3) that the correlation time τ c of water molecules' in coals with V G = 39% is equal to about 28 µs at the temperature 233 K.The same correlation time in coals with V G = 26% can be attained at the temperature 253 K while in anthracites with V G = 5%, correlation time of 31 µs was reached only at the temperature 173 K.
Water and methane in moistened plus methane-saturated coals are irresoluble in NMR spectra at room temperature because they both contribute into a narrow line.Therefore, the intensity and linewidth of the narrow NMR line in the coal-water-methane system give only a total information on the amount and mobility of constituent water and methane species.However, these two fluids would show different behavior at a lowering temperature due to different adsorption mechanisms.
Experimental measurements on methane-saturated coals have proved that the constituent methane does not undergo the phase transitions down to the lowest reached temperatures about 90 K, see figure 5.This fact enables us to separate water and methane contributions using low temperature measurements in water-methane saturated coals.
For example, figure 5 demonstrates the temperature dependence of NMR narrow line intensity for anthracite saturated with water-methane mixture.It can be seen that the constituent water of water/methane mixture converges into crystal-like phase state at the same temperature as the water in the moistened sample.The constituent methane stays thereat in a mobile state (narrow line intensity is equal to 110).Potential barrier hindering molecular motion is about 28.43 kJ/mole for the water adsorbed in anthracite.The same barrier value for constituent methane is as low as 14.1 kJ/mole or lower.It means that the phase transition of the absorbed water in constituent water/methane mixture will take place much earlier during cooling than the transition of constituent methane.This phenomenon makes the separation of water and methane contained in coal possible.
Following to J. De Boer, the adsorption in coals of early or intermediate metamorphism stage should be classified as localized adsorption.As it was shown above, methane adsorption in coals is a rather nonlocalized one.Based on this finding, individual amounts of water and methane in coal can be estimated separately using the low temperature NMR technique.

Figure 1 .
Figure 1.NMR absorption line of a solid-fluid system (a) and derivative of this line (b): I1 and I2 are intensities; ∆H1 and ∆H2 are linewidths for narrow and wide components of the spectrum line, respectively.

Figure 4 .
Figure 4. Temperature dependence of adsorbed water liquid-to-solid phase transition for various coal types (volatile content from 5 to 40).

Figure 5 .
Figure 5. Temperature dependencies of 1 H NMR lines for constituent water, methane, and their mixture.
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