Latent methane in fossil coals

Fuel 83 (2004) 1407–1411 www.fuelfirst.com

Latent methane in fossil coals A.D. Alexeev*, E.V. Ulyanova, G.P. Starikov, N.N. Kovriga Academy of Sciences of Ukraine, Institute for Physics of Mining Processes1, 72 R. Luxemburg Str., Donetsk 83114, Ukraine Received 4 November 2002; revised 4 July 2003; accepted 7 July 2003; available online 4 March 2004

Abstract It is established experimentally using 1H NMR wide line spectroscopy that methane can exist in coals not only in open or closed porosity and fracture systems but also in solid solutions in coal substance, in particular, under methane pressure 2 MPa or higher. Methane dissolved in coal minerals reversibly modifies their lattice parameters as determined from X-ray diffraction analysis. Co-existence of these methane forms in fossil coals causes multi-step desorption kinetics. It is shown experimentally that the long-term latent methane desorption is effected mainly by closed porosity, which in turn is determined by coal rank. q 2004 Published by Elsevier Ltd. Keywords: Coalbed methane; NMR; X-ray diffraction; Desorption

1. Introduction Coalbed methane is an important non-traditional energy resource. It has a number of industrial and consumer applications. At the same time, methane in coal seams induces various gas dynamic phenomena: explosions, outbursts, etc. [1 – 4]. Therefore, studying physical aspects of methane– coal interactions can provide, from one hand, better methane extraction from coal seams and, from other hand, safe underground mining. Methane in coal exists as a free gas, adsorbed gas on inner surfaces, and in solid solution [5 – 8]. Hence, mobility of methane molecules would be different depending on their bonding strength. The purpose of the present work was studying methane phase states in coal through respective molecular mobilities. Studies were conducted using wide line NMR spectrometer since analysis of wide line NMR spectra can provide information on the mobility of resonating nuclei and its dependence on various factors. Dissolved methane effects on the coal crystalline structure were investigated by means of X-ray structural analysis. * Corresponding author. Tel.: þ380-622-904-908; fax: þ 380-622-557626. E-mail address: [email protected] (A.D. Alexeev). 1 Former Division for Physical and Technical Problems of Mining. 0016-2361/$ - see front matter q 2004 Published by Elsevier Ltd. doi:10.1016/j.fuel.2003.07.001

2. Experimental methods and materials A stationary 1H wide line NMR radio spectrometer consisted of permanent magnet, NMR signal recording system and gas pressure system was used for NMR studies. Permanent magnetic field strength of the magnet was 4600 Oe with field homogeneity 2 £ 1026 Oe/cm; resonance frequency was 19.5 MHz. A modulating (autodyne) technique was applied to enhance spectrometer’s sensitivity and accuracy, consisting in superposition of alternating magnetic field Hm cos vt on the polarizing field H0 : Modulating amplitude value Hm was kept within the range 0:1DH # Hm # 0:5DH; where DH is the linewidth, in order for retaining spectra undistorted [9,10]. A linewidth DH is defined as the distance between two extremums of a spectrum line or its component. Recorded 1 H NMR signal is a resonance absorption line consisting of two components, wide and narrow ones, see Fig. 1(a). These components can be more conveniently analyzed in the firstorder derivative of the absorption line, see Fig. 1(b). A wide component is contributed by hydrogen incorporated in coal organic substance while the narrow component is originated from hydrogen of methane. The linewidth DH characterizes hydrogen nuclei mobility in the substance, the higher being mobility, the narrower being the NMR line. Linewidths DH1 and DH2 corresponding to NMR lines narrow and wide component, respectively, differ by nearly the order. Such difference is caused by very

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nuclei. At equal numbers but different mobility of nuclei line intensities are different [12]. X-ray diffraction studies of coals at larger angles (XDLA) were performed using X-ray diffractometer DRON-0.5 [13 –16]. XDLA curves were recorded continuously at the rate 1 deg/min. Cu Ka emission line was chosen. Heights and widths of diffraction peaks were measured after background subtraction. Gamma band was obtained by graphic subtraction of symmetrical reflex (002) from the experimental peak containing the band. Atomic plane spacing d002 and dg was calculated from angular positions of respective peaks ui ; ‘crystallite’ sizes La and Lc were estimated using formulae La ¼ 1:84l=B10 cos u10 ; Lc ¼ 0:9l=B002 cos u002 ; Fig. 1. Typical NMR absorption line (a) and its derivative for a solid containing water, (b) I1 and I2 are intensities; DH1 and DH2 are linewidths for narrow and wide components of the spectrum line, respectively. Numerical notations: (1) absorption line; (2) first-order derivative of the absorption line; (3) wide component of absorption line; (4) wide component of the first-order derivative of absorption line; (5) narrow component of absorption line; (6) narrow component of first-order derivative of the absorption line.

different molecular mobility in gases or liquids and in solids [11]. In particular, linewidth DH1 for coal –methane varies from 0.001 to 1 Oe depending on methane phase state whereas, linewidth DH2 for coal organics lies between 5.5 and 6.5 Oe depending on extent of coal metamorphism, cp. Fig. 1. It can be concluded from above that the modulating field amplitude Hm should be applied different for wide or narrow NMR spectrum components. Therefore, we had recorded at first a general coal – methane spectra at Hm ¼ 0:44 Oe optimal for wide component but distorting narrow one, and then only narrow component at Hm ¼ 0:05 Oe. In the last case, wide component intensity becomes negligible and the narrow component can be easily numerically divided onto subcomponents DH11 and DH12 ; if any, see Fig. 2. NMR spectrum components intensities I1 and I2 in Fig. 1 are determined by both number and mobility of resonating

Fig. 2. Narrow component expansion onto sub-components for NMR spectrum line of adsorbed methane in coal: DH3 is a linewidth for methane adsorbed in coal structure; DH4 is a linewidth for methane adsorbed in pores.

where B10 and B002 are respective bandwidths, l is the wavelength [17]. Carbon parcels and aliphatic layers alignment perfection was evaluated using parameter w ¼ h=l where h is height of the diffraction peak, l is its halfwidth. Experimental errors were estimated as mean square errors in series of three to five individual curves. Present studies were carried out using Donets Basin coals (Ukraine). Samples of the following coal types were investigated: (A) Bright low-volatile (4.8 wt%) anthracite from coal seam with natural methane content 11.0 m3/ton and vitrinite content ca. 90%; (F) Predominantly bright high-volatile (31.0 wt%) fat weak coal from coal seam with natural methane content about 24 m3/ton and vitrinite content up to 85%; and (C) Half-bright high-volatile (39.0 wt%) cannel coal from coal seam with natural methane content about 20 m3/ton and vitrinite content ca. 75%. For NMR studies, pre-dried and degassed powder coal samples with mean particle size 0.25 or 0.5 mm, or alternatively solid cylindrical coal specimens 20 mm in diameter and 8 mm in length were prepared. Reference coal samples or specimens were stored and tested in air while methane saturated ones were exposed in a special sorption installation with few chambers (a separate chamber was destined for each coal type) during 40 days under methane pressure 4.0 MPa and then tested in air. In order to simulate the natural conditions, another solid cylindrical coal specimens tightly fitted into NMR resonator cavity in order for minimizing effects of free methane were put down into high pressure container, and the container was mounted in magnet gap of NMR spectrometer. Container was pumped down with methane under pressure 2 or 10 MPa. Highpressure coal specimens were exposed to chosen methane pressure during 10 days for complete saturation and tested under that pressure. For X-ray studies, smooth rectangular bars with dimensions 25 £ 10 £ 4 mm3 were machined from coal lumps of

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Table 1 Time dependence of intensities ratio I1 =I2 during methane desorption from coal samples Coal type

NMR line intensities ratio I1 =I2 in time (h)

Particle size (mm)

0

0.5

1

1.5

2

3

4

5

6

7

8

9

10

– –

5.1 8.7

– –

4.0 6.2

2.8 4.8

2.2 4.2

2.0 3.1

1.8 3.0

1.5 2.9

1.45 2.5

1.4 2.3

1.3 2.2

A

0.2 0.5

10.0 12.0

F

0.2 0.5

2.3 2.6

1.35 –

1.2 2.0

0.95 –

– 1.7

– 1.5

– 1.4

– 1.1

– 1.0

– –

– –

– –

– –

C

0.2 0.5

2.1 2.2

1.4 –

1.0 1.8

– -

– 1.7

– 1.45

– 1.35

– 1.3

1.2

– 1.1

– –

– –

– -

types C or A. X-ray diffraction curves were recorded on both degassed and gas saturated specimens. Methane saturation was conducted in the same sorption installation and at the same conditions. Prior X-ray studies, all bar specimens were tested using NMR spectrometer. Closed porosity in test samples was estimated using method of methane adsorption at high gas pressures in combination with helium measurements of free volume [17].

3. Experimental results Fine structure of narrow NMR line components was revealed using computer analysis of NMR signals recorded at the modulating field amplitude 0.05 Oe under methane pressures 10.0 or 2.0 MPa as well as at atmospheric pressure. It was found that these narrow components generally have a complex nature. At atmospheric pressure only most narrow subcomponents were registered. One extremely narrow subcomponent with linewidth DH10 < 0:05 Oe corresponds to free methane with highest mobility in macropores and fractures. This feature is omitted in Fig. 2 since free volume in test specimens and NMR resonator was minimized, as mentioned above. Methane contained in micropores yields second subcomponent with DH11 < 0:2 Oe. At methane pressure 2.0 MPa a third subcomponent arises with linewidth DH12 < 0:7 Oe and at pressure 10.0 MPa its linewidth increases up to DH12 < 1:2 Oe. Quantitative estimation of these NMR line components and evaluation

of methane constituents in coal is beyond scope of present work. NMR data on the gas desorption kinetics from methane saturated powder coal samples in air are presented in Table 1. These data were recorded at modulating field amplitude 0.44 Oe that enables to trace only total methane line with intensity I1 ; cp. Fig. 1. Since, the wide NMR component (with linewidth DH2 and intensity I2 in Fig. 1) remains constant during desorption in any coal type, its intensity was adopted as reference value in each experiment. Intensities ratio I1 =I2 therefore indicates a share of residual methane in a sample or specimen. At I1 =I2 < 1:0; methane may be considered completely extracted. Total desorption time was found to be dependent on coal rank. Indeed, this time for coal particle sizes 0.25 vs. 0.5 mm was: 1 vs. 6 h for coal type D; 1.5 vs. 7 h for coal type F, and 10 vs. 50 h for coal type A. This phenomenon can be understood in terms of closed porosity in all coal types, far exceeding open porosity [18,19]. Namely, measured specific open and closed pores volumes in coals under study were: 0.061 and 0.103 cm3/g for type C; 0.037 and 0.199 cm3/g for type F; 0.102 and 0.669 cm3/g for type A. As for solid coal, it was established that narrower NMR subcomponents are completely absent in all degassed specimens in contrast to raw coal. At highest possible methane saturation, intensities ratio I1 =I2 was equal to 3.6 for coal type C or 20.1 for coal type A. In the course of methane desorption values I1 =I2 decrease similarly to powder samples. Results are presented in Table 2. Correlations between X-ray structure parameters and NMR line intensities ratio I1 =I2 for coal type C are shown in

Table 2 Dependence of X-ray structure parameters on sorbed methane amount (as indicated by NMR line intensities ratio I1 =I2 ) for different coals Coal type and condition

I1 =I2

˚) d002 (A

˚) Lc (A

˚) La (A

˚) d10 (A

w002

w10

˚) dg (A

˚) Lg (A

C (as-dried) C (saturated) C (saturated and desorbed) C (saturated and desorbed) A (as-dried) A (saturated)

– 3.6 2.4 1.7 – 20.1

3.75 3.75 3.675 3.67 3.57 3.63

13.5 16.0 15.5 13.5 22.5 19.5

16.5 13.0 13.5 14.0 – –

2.145 2.165 2.16 2.15 2.055 2.15

3.5 5.0 4.0 3.5 5.0 4.3

0.455 0.55 0.5 0.5 3.0 2.7

5.2 4.5 4.8 5.1 – –

15.0 13.0 15.0 15.0 – –

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contained in fractures and open porosity. Available methods or models of reservoir evaluation are mostly based on the methane resources calculated using typical porosity values and, as a result, underestimate real methane productivity [20]. Indeed, methane transportation from closed pores or solid solution can proceed only by means of solid-state diffusion. Estimation of methane solid-state diffusivity in coal gives about 1029 cm2/s as compared with about 1024 cm2/s for methane filtration along cleats or interconnecting pores [5,6,21]. However, present results on NMR-controlled desorption demonstrate that latent methane can effectively be extracted at long-term coalbed reservoir exploitation. Its reserves hence should be taken into account at the reservoir evaluation as well. Some preliminary results have shown that the dissolved methane’s content in coal may be at least equal to content of free methane in pores or even higher. Such estimations will be a subject of our future papers. Coal substance is known to be a very complex system consisting of graphite-like crystallites and polymeric strata. X-ray data in combination with NMR studies under methane pressure can be understood in assumption that the dissolved methane responsible for subcomponent with DH12 < 0:7 Oe is incorporated into aliphatic coal constituents while methane providing DH12 < 1:2 Oe is contained in crystallites.

5. Conclusions

Fig. 3. X-ray structure data as functions of NMR line intensities ratio I1 =I2 for coal type C.

Fig. 3. It is well seen that longitudinal ðLa Þ and transversal ðLc Þ carbon parcel sizes as well as atomic plane spacing d002 and dg in coal class C change at methane saturation as compared with raw material even when taking into consideration accuracy of X-ray measurements. Carbon structure perfection degree w reversibly increases at methane saturation and returns to initial value at desorption. Other X-ray structure parameters return to initial values as well indicating that sorbed methane modifies the coal crystalline structure.

4. Discussion ‘Latent’ coalbed methane is nothing else but methane ‘reserve’ contained in closed porosity and solid solutions in contrast to technically extractable methane ‘resource’

Methane resources in coal include both storage in void systems (open and closed porosity, fracture, cleat) and incorporation into coal substance in the form of solid solutions without chemical bonding. Most methane resource estimations discount or neglect methane in closed porosity and solid solution. Such estimations can be drastically improved or made more reliable when taking into account this ‘latent’ methane. Coal mining can be made more safe as well.

References [1] Alexeev FA, Voitov GI. The methane. Moscow: Nedra; 1978. [2] Ettinger IL. Physical chemistry of gas bearing coal seam. Moscow: Nauka; 1988. [3] Mobbat M, Weale KE. Fuel 1955;34:841. [4] Harpalani S, Chen G. Fuel 1995;74:1491. [5] Ettinger IL. Immense resources and abrupt catastrophes. Moscow: Nauka; 1988. [6] Alexeev AD, Vasilenko TA, Zaidenvarg VE, Sinolitsky VV. Solid State Chem (Russ) 1993;1:16. [7] Weishauptova´ Z, Medek J. Fuel 1998;77:71. [8] Clarkson CR, Bustin RM. Fuel 1999;78:1345. [9] Abraham A. Nuclear magnetism. Moscow: Inostr. Literatura; 1963. [10] Pool C. Techniques of EPR spectroscopy. Moscow: Mir; 1970.

A.D. Alexeev et al. / Fuel 83 (2004) 1407–1411 [11] Slonim IY, Lubimov AN. Nuclear magnetic resonance in polymers. Moscow: Khimiya; 1996. [12] Alexeev AD, Serebrova NN, Ulyanova EV. Rep Acad Sci Ukr SSR 1989;9:25. [13] Lazarov LK, Angelova GK. Structure and reactions of coals. Sophia: Bulg. AS Press; 1990. [14] Korolev YM. Chem Solid Fuel (Russ) 1989;6:11. [15] Lukovnikov AF, Korolev YM, Golovin GS, Gulmaliyevam AM, Gagarin SG, Rade VV. Chem Solid Fuel (Russ) 1996;5:3.

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[16] Golovin GS, Korolev YM, Lunin VV, Rode VV. Chem Solid Fuel (Russ) 1999;4:7. [17] Kitaigorodsky AI. X-ray structure analysis of fine-grained or amorphous bodies. Moscow: Gostechteorizdat; 1952. [18] Alexeev AD, Vasilenko TA, Ulyanova ED. Fuel 1999;78:635. [19] Alexeev AD, Feldman EP, Vasilenko TA. Fuel 2000;79:939. [20] Davidson RM, Sloss LL, Clarke LB. Coalbed methane extraction; January 1995. 67 pp, Ref IEACR/76, ISBN 92-9029-248-2. [21] Alexeev AD, Sinolitsky VV. Engng Tech J (Russ) 1985;49:648.