The resultant expressions make it possible to predict the gas capacity of the coals in the Georgian deposits, while the proposed graphoanalytical method used to establish the methane capacity of the microcomponent groups of coals can be used in solving similar problems, even those involving other deposits. LITERATURE CITED 1.

2. 3. 4. 5. 6.

7.

Yu. D. Tsintsadze, T. M. Danelishvili, G. E. Surguladze, and M. A. Tatarishvili, Rock Mechanics and Mine Aerology [in Russianl, Metsniereba, Tbilisi (1972). I. L. Ettinger, Gas Capacity of Coals [in Russian], Nedra, Moscow (1966). I. L. Ettinger, I. V. Eremin, B. M. Zimakov, and A. P. Bakaldina, Dokl. Akad. Nauk SSSR, 155, No. 2 (1964). I. V. Eremin, B. M. Zimakov, I. L. Ettinger, and M. F. Yanovskaya, Tekh. Ekonom. Ugledob., No. 97 (i) (1965). B. K. Chichua, Yu. D. Tsintsadze, and G. E. Surguladze, Soobshch. Akad. Nauk Gruz. SSR, 63, No. 2 (1971). O. A. Radchenko, "Physical and technical properties of fossil coals," Proceedings of the Laboratory of Coal Geology [in Russian], No. 16, Izd. Akad. Nauk SSSR, Moscow (1962). Yu. A. Zhemchuzhnikov and A. I. Ginsburg, Fundamentals of Coal Petrology [in Russian], Izd. Akad. Nauk SSSR, Moscow (1960).

VARIATION OF THE MOLECULAR STRUCTURE OF COAL AND THE RISK OF BURSTS IN COAL SEAMS IN THE PROCESS OF METAMORPHISM E. A. Bineev, V. M. Zotov, G. I. Stepovoi, and A. V. Frolov

One of the primary tasks associated with the problem of controlling sudden bursts of coal and gas is the timely and reliable prediction of the burst risk of coal seams. The acquisition, increase, and loss of burst-risk properties are associated with specific stages of regional metamorphism. Sudden bursts of coal and gas occur in coal seams with a volatile content vg < 35% and anthracites with a specific electric-resistance indicator log p > 3.5 [i, 2], or, according to Petrosyan and Ivanov [3], in seams with a vg of from 36 to 3.5%. The probability of bursts increases initially according to an established rule with continuing metamorphism, reaching a maximum when vg ~ 15%, and then diminishes [2]. This is most likely governed by qualitative variations in the molecular and supramolecular structure of coal throughout the metamorphism sequence. Knowledge of the physical essence of the variations occurring in the moelcular structure of coal in the coal-formation process will make it possible to develop new and to perfect already-familiar indicators of the risk of burst in coal seams. Specimens of coals with vg ranging from 44 to 8% and anthracites with log 0 from 3.8 to 0.4 (a total of 124 specimens) were selected to investigate the structural features of coals in different stages of metamorphism and to determine the feasibility of using parameters of their molecular structure for the regional prediction of burst risk in shafts of the Donbass. In the laboratory, we conducted element and technical analyses and investigated the characteristic features of the molecular structure by the method of electron paramagnetic resonance (EPR) and x-ray structural analysis. The element and technical analyses were performed by methods approved by the GOST. The atomic fractions of the three basic elements found in an organic coal mass -- carbon, hydrogen, and o x y g e n - - a n d also the number B of

S. Ordzhonikidze Polytechnic Institute, Novocherkassk. Translated from Fiziko-Tekhnicheskie Problemy RazrabotkiPoleznykhIskopaemykh, No. 4, pp. 81-86, July-August, 1982. Original article submitted April 13, 1981.

0038-5581/82/1804-0323507.50

9 1983 Plenum Publishing Corporation

323

K

K 2O

40~'~,

"'

0 -21

"i ~ 0 - 20

'

.

b

0

;'6

s4

9z

'0

if

"0

!

Fig. i. Variation in atomic fractions of hydrogen i and carbon 3 throughout metamorphism sequence; 2, 4) curvature of Ha(Cg) and Ca(Cg) regression lines, respectively.

c g,%

~. fO-te Paramagnetic cenr.em/g 24-

f6

~,

~9

o 76

Fig. 2

aromatic rings in a statistical carbon layer and its diameter L a were computed from the results of the element analysis. The computational method used is described in [4]. The EPR spectra were produced on an RE-1301 x-ray spectrometer. Diphenylpicrylhydrazyl was used as a standard. Absorption curves were recorded in the form of the first derivative. The concentration N of paramagnetic centers was computed in accordance with [5]. The x-ray structural analysis was conducted using a Dron-i diffractometer with a Bsv-9 tube in filtered copper radiation. Analyzing the 002 diffraction maximum, we computed the following from the resultant radiographs: the interplane distance doo2; size of the region with ordered stacking of layers (thickness of the packet of parallel carbon layers] Lc, which was determined from the Selyakov equation [6]; and structural-ordering index h/~ (the ratio of the intensity of the line at the maximum (pulses/sec) to the angular half width of the line (radians), which was determined after Kitaigorodskii [7]. The method of least squares [8] was used to define empirical relationships between the atomic fractions of hydrogen (Ha) , carbon (Ca) , oxygen (Oa) , and the ratio Ha/C a and the carbon content cg. A polynomial of fifth degree in terms of the argument cg was selected as a nonlinear relationship between variables, since polynomials of higher order do not increase the correlation ratio, while polynomials of lower order significantly reduce it. The closeness of the relation was determined by the theoretical correlation coefficient of a random value relative to the regression line. The curvature (K) was used to find the characteristic features of the Ha(Cg), Ha/Ca(Cg), and Ca(Cg ) regression lines. The plotted data illustrate graphically the possibility of using curvature as a criterion exposing the various segments of the relationships under consideration. Basic laws governing the variation of N throughout the metamorphism sequence were judged from the average-interval values of N and cg. The width of the interval was 1% for cg. All calculations were performed on an "Mir-l" computer. Relationships showing the variation of H a (curve i) and Ca (curve 2) throughout the metamorphism sequence are presented in Fig. i. The correlation coefficients for these re-

324

18 J

.,.

h/~ ~/"

e

~

~o

.

fO00-

~000

J,G ]

.

""

0

$,4 I 78

1

. 80

84

88

Fig. 3

3~ C

Fig. 4

lationships were 0.96 and 0.98, respectively (curves 2 and 4). An increase in the degree of metamorphism is accompanied by loss of hydrogen; this would indicate a reduction in the atomic fraction of hydrogen and Ha/C a . The rates of H a loss are different for different segments of the metamorphism. Analysis of the curvature of the theoretical Ha(Cg) regression line (see Fig. i) made it possible to isolate the following stages in the metamorphism sequence: The first stage (with a carbon content of up to 83-84%) is characterized by a virtually constant value for the atomic fraction of hydrogen (41-44%); in the second, the H a decreases gradually (84% < cg < 90%). In the third (cg < 90%), the rates of hydrogen loss increase and go to the maximum. The increase in the atomic fraction of hydrogen occurs in the same way. As the degree of metamorphism increases, a sharp reduction occurs in the oxygen content (Oa); this reduction is most vigorous in the first and second stages (cg < 90%). The stepwise character of the variation in the atomic fraction of oxygen could not be clearly isolated. Analysis of median-interval values of the concentration of paramagnetic centers (Fig. 2) indicated that the nature of the variation of N depends on the stage of the metamorphism. In connection with the small number of points when cg < 81%, average values were not computed in this interval. In the 81 < C g < 85% interval, values of N remain virtually unchanged. An increase in the degree of metamorphism (the 85 < cg < 90% interval) will result in a sharp increase in values of the concentration of paramagnetic centers, which are stabilized when c g > 90%. The concentration drops on transition to anthracites (cg > 95%). The vigorous increase, stabilization, and drop in N values coincide with transition from one isolated stage of metamorphism to another. The stepwise nature of the variation in the atomic fractions of hydrogen and carbon and the concentration of paramagnetic centers throughout the metamorphism sequence is apparently governed by the fact that a particular molecular and supramolecular structure, which, all things considered, determines their physicochemical properties, and, among other things, the predisposition to sudden bursts, is characteristic for coals of each of these stages (cg < 84-85%; 84-85 < cg < 90%; 90 < cg < 95%). And, in effect, transition from the first to the second stage of metamorphism corresponds to the acquisition of burst-risk properties by the coals, and transition from the second to the third stage to the maximum probability of burst development according to the data contained in [1-3]. The acquisition, increase, and loss of burst risk by fossil coals are apparently associated with qualitative variations in their structure throughout the metamorphism sequence, which make themselves felt via structural parameters, e.g., via the paramagnetic characteristics. It should be noted that although increased values of the concentration of paramagnetic centers are observed for coals in dangerous shafted seams, laws governing the variation of N throughout the metamorphism are the same for coals in dangerous and safe shafted seams. This is most likely associated with the fact that sudden bursts are local in nature, developing only in certain zones of gas-bearing coal seams. Results of the x-ray structural analysis and the L a and B values of the aromatic rings in a statistical carbon layer, which were computed from data of the element analysis, are presented in Table i. The character of the variation of doo2, thickness Lc, and the index h/l throughout the metamorphism sequence (Figs. 3 and 4) is indicative of the stepwise nature of structural changes in fossil coals. A decrease in the internetwork distance, i.e., densification of the packets of aromatic carbon layers, occurs in the first stage (cg < 84%). At the same time, the thickness of the packet increases (Fig. 3). This suggests that the

325

carbon layers have the possibility of being stacked parallel to the existing packet. It is apparent from Fig. 4 that in the first stage these processes are accompanied by a reduction in the degree of structural ordering. The latter may occur due to deflections of networks of packets, i.e., due to an increase in the mobility of the structural elements. The second stage (84% < cg < 90%) is characterized by the constancy of the mean-statistical values of the structural parameters, and d0o= and L c vary slowly, maintaining virtually the same mean-statistical values in this stage (Fig. 3). The degree of structural ordering, having reached the minimum in the 84-86% cg region, begins to increase slowly, gradually increasing the ordering rate (see Fig. 4). In the third stage (cg > 90%), the packets of carbon layers continue to densify. This is manifested in a further reduction of doo=. A rapid increase in the thickness of the carbon-layer packet, which is accompanied by a sharp increase in the degree of structural ordering of the coal matter, begins in this stage. Comparison of the results of the x-ray structural examinations with data derived from the element analysis and with the concentration of paramagnetic centers permits a more profound understanding of the structural changes that take place in the organic matter of the coals. According to the results of the element analysis, carbonization of the coal will proceed in the first stage primarily due to loss of oxygen atoms. Detachment of rather large oxygen atoms located on the boundaries of the carbon networks (or between them) creates the potential for their convergence and a reduction in doo2. At the same time, detachment of the bridging oxygen creates the potential for the deflection of certain layers. As a result, several neighboring layers attach to existing packets, this leads to an increase in L c. At this stage (see Table i) La's dimensions are small and it contains several aromatic rings, owing to which the Van der Waals forces of the interaction between layers are relatively weak. Thermal oscillations, therefore, constantly disturb the parallelism of layer stacking in the packet in the mutual orientation of different packets. This leads to a reduction in the mean-statistical~values of the degree of structural ordering. The presence of connecting aliphatic chains between packets of layers in this stage apparently still ensures adequate rigidity and length of the macromolecules. The structure still remains too highly bonded for the acquisition of burst-risk properties. It becomes increasingly mobile and less ordered as the oxygen is detached and the extent of carbonization increases. Transition to the second stage is governed by the start of the dehydroxylization reaction [9]. Detachment of hydroxyl radicals, together with the loss of oxygen, markedly reduces the number of intermolecular bonds; this, in turn, sharply increases the mobility of the structure. The structure acquires burst-risk properties. In this stage, thermal oscillations apparently lead to the rupture of certain aliphatic chains bonding the packets, and to an increase in the mean-statistical number of ruptured bonds. The presence of these ruptures is confirmed by the jump in the concentration of paramagnetic centers in this stage. Since the networks are still surrounded by aliphatic chains, they do not become laced together, and the diameter of the layer remains permanently small and the interaction between networks weak. As a result of increasing mobility, thermal oscillations make it impossible to stack even neighboring networks parallel to the existing packets, and the interaction of the networks is smaller than the energy of the thermal oscillations. This reduces to the fact that the thickness of the packet of layers remains constant under equilibrium. Dissociation of peripheral aliphatics increases the mobility of the structure all the more. Even now, the increasing mobility is ~ t sufficient to separate a layer from the packet. The packets are transformed into closed, stable formations -- globules. The placement of layers in the globule is more parallel than is suggested by the small increase in the degree of structural ordering. Since the basic portion of the bonds at the globule boundaries is closed, and their dimensions remain small (and are determined by the thickness of the packet and the layer diameter), the globules possess extremely high mobility. The structure becomes rigid and exhibits extremely high mobility in this state. An increase in the probability of sudden bursts is therefore observed in this stage. In the third~ stage of metamorphism (cg > 90%), the demytilization reaction [9] results in a sharp decrease in the number of aliphatic CH x radicals, and this, in turn, results in the condensation of aromatic rings and a growth in network diameter (see Table i). As a result of the destruction of the peripheral segments, the globules apparently begin to decompose, and the carbon networks that have been liberated acquire the potential to be drawn together; this leads to a sharp increase in the layer diameter. The increased area of interaction gives rise to an increase in the forces of attraction between the layers and to their

326

TABLE I. gation

Structural Characteristics of Coals Under Investi-

Sample Seam index I'~o. i 2 3 4 5

k8

6

ls

7

Is

Is ls ks hs

8 9 t0 ii t2 t3 i4 i5 i6 17 18 t9 20 21 22 23 24 25 26 27 28 29 3O 3i 32 33 34 35

ls hs hs

Is 16 h~

le ls ls ls Is h~

ls ia ia k2 hs

ls i2 hs k2 ia

Vr

41,8 37,5 39,8 4t,9 39,9 36,5 4t ,2 40,0 39,3 39,8 35,2 27,9 38,4 36,7 35,2 35,2 32,5 34,2 28,5 23,6 23,2 23,7 21,0 23,9 23,i 24,7 lO,i i4,6 t7,3 i7,i t7,0 i5,7 tt ,7 2,8 i,6

I %

Cr

77,7 79,3 79,5 79,8 80,5 80,5 82,9 83,4 83,8 83,8 83,9 84 ,t 84,2 84,4 85,5 86,0 86,2 87,i 87,2 87,7 88,i 88,4 88,7 89,0 89,2 89,2 89,2 89,8 89,9 90,2 90,8 91,8 92,8 95,5 98,3

doo2

3,65 3,67 3,59 3,6t 3,58 3,52 3,54 3,56 3,59 3,58 3,64 3,56 3,59 3,57 3,58 3,57 3,55 3,54 3,58 3,55 3,54 3,59 3,56 3,58 3,56 3,52 3,53 3,~8 3,49 3,45 3,52 3,52 3,45 3,51 3,46

4

La

Lc

~uls~ }lec.radian 10,2

11,7 3,3

3,5 3,5 3,7 3,5 3,8 4,2 3,7 3,8 3,3 3,9 4,1 4,1 5,1

4,9 6,9 5,9 5,2 5,i 7,8

t2,5 t2,4 t5,8 t ! ,8 t4,2 i4,8 t4,3 14,0 t6,0 i4,5 15,3 i5,0 17,2 t6,0 i5,8 i6,2 i7,0 i7,0 i7,0 14,2 i5,8 i4,5 i7,t i7,0 i8,7 t9,5 i6,6 22,2 17,2 24,5 23,0 i4,5 17,4

592 918 240 645 580 993 240 320 240 500 550 280 260 390 38O 380 44O 44O 6OO 620 ii00 t417 it22 1962 il9i i290 78O 850 1667 9OO 2636 i680 770 2709 3096

i,7

t,8 i,9 2,0 1,8 2,2 2,6 2,i 2,t i,6 2,4 2,6 2,5 3,9

3,6 7,i 5,2 4,i 3,9 9,1

convergence; this manifests itself as a decrease in doo=. The forces of interaction also lead to a pronounced growth in packet thickness due to the joining of neighboring free layers, An increase in the dimensions of the packets and in the interaction forces results in a marked increase in the degree of structural ordering and a reduction in mobility. According to [1-3], the probability of sudden bursts diminishes gradually in this segment of metamorphism. The transition from the first to the second stage of metamorphism, which c o i n c i d e s w i t h the acquisition of burst-risk properties by coals, is associated with a qualitative change in the structure, which manifests itself through structural parameters. This makes it possible to use these parameters for the regional prediction of the risk of bursts in coals, which permits the isolation of seam shafts, during the excavation of which bursts of coal and gas are not expected, since the coal has still not acquired a structural predisposition to bursts in the metamorphism process. The structural parameters that we investigated (like the existing indicator vg) yield only average-statistical values of the transition from the first to the second stage, and do not permit sufficiently reliable isolation of seams during whose excavation there will be no coal and gas bursts. For this purpose, we need a complex structural indicator that would combine in itself both the degree of carbonization of the coal substance, which is associated with the degree of metamorphism, and also the degree of defectiveness of the structure of

327

the coal substance, which is associated with characteristic features of the molecular structure of the coals. Analysis of the structural parameters under investigation indicated that the product of the concentration of paramagnetic centers and the ratio of atomic fractions of hydrogen and carbon N(Ha/C a) is the most informative. The concentration of paramagnetic centers characterizes the defectiveness of the structure [i0], and the ratio Ha/C a characterizes the degree of carbonization of the substance. The reliability of the N(Ha/Ca) indicator was confirmed on specimens removed from 37 shafted seams, ii of which are referred to the no-risk category as regards sudden bursts. As analysis of the results for the sample under consideration indicated, shafted seams should be classed with the no-risk category for values of the indicator N(H
2. 3.

.

5.

8

7.

8. 9. i0.

328

V. I. Nikolin, B. A. Lysikov, and V. Ya. Tkach, Predicting the Burst Risk of Coal and Rock Seams [in Russian], Donetsk, Donbass (1972). V. I. Nikolin and L. N. Karagodin, "Burst-risk prediction from the discharge of volatile substances," Ugol', No. 4 (1973). A. E. Petrosyn and B. M. Ivan.v, "Causes of the development of sudden bursts of coal and gas," in: Fundamentals of the Theory of Sudden Bursts of Coal, Rock, and Gas [in Russian], Nedra, Moscow (1978). Ya. Yurkevich and S. Rosin'skii, Coal Chemistry [in Russian], Metallurgiya, Moscow (1973). # A. V. Artemov, E. A. Bineev, and V. N. Glukhodedov, "Investigation of physic.chemical means of an effect of an anthracite seam for the purpose of its gasification," Fiz.Tekh. Probl. Razrab. Polezn. Iskop., No. 4 (1976). L. I. Mirkin, Handbook on the X-Ray Structural Analysis of Polycrystals [in Russian], Fizmatgiz, Moscow (1961). A. I. Kitaigorodskii, X-Ray Structural Analysis of Finely Crystalline and Amorphous Solids [in Russian], Gosgortekhizdat, Moscow--Leningrad (1952). V. M. Ivanov et al., Mathematical Statistics [in Russian], Vysshaya Shkola, Moscow (1975). B. K. Mazumdar, "Hydrogen in coal," Fuel, 51 (1972). Yu. N. Nedoshivin and V. I. Kasatochkin, "On the nature of spin centers in highcarbon substances with a developed system of conjugate bonds~" in: X-Ray Spectroscopy of a Solid [in Russian], Atomizdat, Moscow (1967).