Energy & Fuels 2003, 17, 159-163
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Mechano-activation as Initiation of Self-ignition of Coal J. Medek* and Z. Weishauptova´ Institute of Rock Structure and Mechanics, Academy of Sciences of the Czech Republic, 182 09 Prague 8, Czech Republic Received June 19, 2001
A new theory of the self-ignition of coal was developed, based on the principle of the spontaneous disintegration of coal resulting from the effect of potential energy, which is accumulated in the coal and causes mechanical stress. This leads to the formation of microcracks where a part of the potential energy is dissipated to heat, which may evoke, in a thermally insulated system, the initiation of microfires as primary centers of burning. The considered process is independent of the size of the coal body, humidity of the coal, and the degree of previous oxidation. A fundamental difference between this theory and other theories lies in the fact that a gradual increase in the coal temperature is not considered and that latent microfires occur directly.
Introduction A spontaneous increase in coal temperature resulting in coal self-ignition is a process of extraordinary importance, to which attention must be paid for two different reasons: (i) from the scientific point of view in order to elucidate the nature of the phenomenon called selfignition, to formulate the mechanism of the particular processes leading to the coal inflammation as the final stage of the whole process and to define the preconditions necessary for its inducing and realization, and (ii) from the practical point of view, as it is a dangerous natural phenomenon, especially with regard to the mining safety and in order to secure the subsequent protection of the extracted coal during further manipulation. Laboratory Investigation Since the second half of the 20th century when a number of scientific disciplines (geology, chemistry, physics, mining) together with their exact research methods started to deal with the problem of self-ignition, this phenomenon became the object of purposeful investigation. A number of theories have been evolved; however, none of them has been able to give a complex interpretation of the mechanism of the spontaneous temperature rise of coal up to its inflammation. Although in the laboratory under artificial conditions a great number of parameters participating in the temperature increase under real conditions are simulated, simultaneously the conditions for forced heat increase are formed artificially instead of letting the coal temperature rise spontaneously. Most frequently the following processes leading to coal self-ignition are considered: reactions of selective chemical groups or free radicals,1-6 chemical reactions or * Author to whom correspondence should be addressed. Fax: (+4202) 6880649, 6880105. E-mail:
[email protected]. (1) Davydova, Z. A.; Sukhov, V. A.; Lukovnikov, A. F. Khim. Tverd. Topl. 1983, No. 6, 38.
catalytic action of mineral admixtures,7-11 effect of pyrophoric compounds,12,13 heat developed by electrochemical processes,14-17 activity of bacteria,18,19 and the adsorption heat of water vapor.20-24 Among the published theories the preferred opinion is represented by the continuous oxidation of coal starting already at low temperatures, further progressing through the whole range of temperature increase, and finally ending by attaining the ignition point. Therefore, attention was paid to the study of partial oxidation reactions taking place in various temperature intervals, mostly within the low-temperature region.25-37 (2) Evans, J. C.; Rowlands, Ch. C.; Cross, R. M.; Rigbly, N. Fuel 1984, 63, 1471. (3) Dack, S.; Hobday, M. D.; Smith, T. D.; Pilbrow, J. R. Fuel 1984, 63, 39. (4) Khan, M. R. Energy Fuels 1987, 1, 366. (5) Neuman, H. J. Erdol, Kohle, Erdgas, Petrochem. 1987, 103, 131. (6) Wiese, R. G., Jr.; Powell, M. A.; Fyfe, W. S. Chem. Geol. 1987, 63, 29. (7) Cudmore, J. F.; Sanders, R. H. Australian Coal Ind. Res. Lab., ACIRL-PR.84-10, North Ryde, N.S.W., 1984. (8) Bardocz, V. 21st Conf. Safety Mines; Res. Inst. Sydney, 1985; p 475. (9) Norton, P. Proc. Int. Conf. Coal Sci., Sydney, 1985, p 467. (10) Ghosh, R. Fuel 1986, 65, 1042 . (11) Backes, C.; Pulford, I. D.; Duncan, H. J. J. Reclam. Revegetat. Res. 1987, 6, 279. (12) Mapstone, G. E. Chem. Ind. 1954, No. 23, 658. (13) Stadnikov, G. L. Spontaneous Combustion of Coals and Rocks; Ugletechizdat: Moscow, 1956. (14) Obukhov, N. K.; Burkov, P. A.; Alexandrov, I. V. Khim. Tverd. Topl. 1976, No. 4, 73. (15) Kossov, I. I.; Alexandrov, I. V.; Kamneva, A. I. Khim. Tverd. Topl. 1984, No. 4, 41. (16) Alexandrov, I. V. Khim. Tverd. Topl. 1987, No. 5, 29. (17) Fuerstenau, D. W.; Rosenbaum, J. M.; You, Y. S. Energy Fuels 1988, 2, 241. (18) Ichiro, I. J. Min. Metal Inst. Jpn. 1960, 76, 524. (19) Lifanov, J. V. Khim. Tverd. Topl. 1970, No. 3, 150. (20) Bhattacharyya, K. K. Fuel 1971, 50, 367. (21) Panaseiko, S. P. Khim. Tverd. Topl. 1974, No. 1, 26. (22) Saranchuk, V. I.; Gabishko, L. Y.; Pashchenko, L. V.; Lukyanenko, L. V. Khim. Tverd. Topl. 1978, No. 1, 9. (23) Nordon, P.; Bainbridge, N. W. Fuel 1983, 62, 619. (24) Singh, R. N.; Demirbilek, S. Min. Sci. Technol. 1987, 4, 155. (25) de Vries, H. A. W.; Bokhoven, C.; Dormnas, H. N. M. Brennst.Chemie 1969, 50, 289. (26) Fredericks, P. M.; Warbrooke, P.; Wilson, M. A. Org. Geochem. 1983, 5, 89.
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However, recently the authors published in the previous paper38 an opinion that the progressive oxidation of coal at various temperatures below its ignition point leads to the formation of a chemisorption complex that reduces significantly the oxyreactivity of the coal surface in the subsequent interaction with oxygen. This chemisorption proceeds very quickly, its rate increasing with the temperature. At temperatures in the vicinity of the ignition point a shock contact of the coal surface with oxygen resulted in an instantaneous inactivation of the surface so that no inflammation but, on the contrary, an elevation of the original ignition point took place. Despite numerous experiments, nobody has until now verified experimentally this simple mechanism of spontaneously progressing oxidation; many authors1-24 have come to the conclusion that an initiation factor is needed, which in the form of an exothermic side reaction would provide the system with necessary activation energy. The same concerns also the inducement of the starting low-temperature reaction, whichsin order to progress within the range of several tens of °Cscannot start spontaneously but must be initiated by some other exothermic reaction. New Conception of the Theory of Self-Ignition On the basis of a long-termed observation and analysis of self-ignition of coal both in situ and after its exploitation, we come to the opinion that the stimulus to coal ignition is caused by a very quick development of the necessary amount of heat generated in the interior of coal matter. A new theory has been suggested, respecting all phenomena accompanying the real process. This theory follows from the spontaneous disintegration of a coal body, which is a physical property common to all types of coal. This concept takes into account all significant circumstances connected with self-ignition, such as the origin of free radicals, the indeterminacy in the time and location, and the existence and course of spontaneous fire. The considered process is independent of the size of the coal body, the moisture, and the degree of previous oxidation. A fundamental difference between this theory and all other theories consists of the fact that a gradual increase in the coal temperature is not considered and that the latent microfires occur immediately. The aim of this study was to explain self-ignition by means of a new theory based on entirely different principles. (27) Beafore, F. J.; Cawiezel, K. F.; Montgomery, C. T. J. Coal Qual. 1984, 3, 17. (28) Kaji, R.; Hishimma, Y.; Makamura, Y. Fuel 1990, 64, 297. (29) Banerjee, B.; Bchattacharyya, N. C. Fuel Sci. Technol. 1988, 7, 11. (30) Taraba, B.; Dobal, V.; C ˇ a´p, K.; Harasˇta, N. Fuel 1988, 67, 758. (31) Clemens, A. H.; Matheson, T. W.; Rogers, D. E. Fuel 1990, 69, 255. (32) Kelemen, S. R.; Freund, H. Energy Fuels 1990, 4, 165. (33) Wang, H. H.; Dlugogorski, B. Z.; Kennedy, E. M. J. Loss Prevent. Proc. Ind. 1998, 11, 373. (34) Continillo, G.; Faraoni, V.; Maffetoni, P. L. Chem. Eng. Sci. 2000, 55, 303. (35) Moghtaderi, B.; Dlugogorski, B. Z.; Kennedy, E. M. Proc. Saf. Environ. Prot. 2000, 78, 445. (36) Akgun, F.; Essenhigh, R. H. Fuel 2001, 80, 409. (37) Nugroho, Y. S.; McIntosh, A. C.; Gobbs, B. M. Fuel 2000, 79, 1951. (38) Medek, J.; Weishauptova´, Z. Energy Fuels 1999, 13, 77.
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Theory of Microfires Spontaneous Disintegration of Coal. A common property of coal observed within a wide coalification range is the spontaneous disturbance of the inherent cohesive forces of the coal matter by crack formation. This phenomenon comes into effect through the latent potential energy accumulated in the coal mass as the sum of the work expended on its elastic deformation. The forces taking part in this deformation are either a mechanical stress in the form of rock pressure in the seam, or compacting pressure during briquetting, or a force derived from some physical process, mainly in the form of swelling pressure caused by the dissolution of molecules of water and coal-bed gases (methane, carbon dioxide) in the fine structure of the coal matter. After extinction of the deformation forces, coal matter tends to return to the original state to the detriment of accumulated potential energy. At the same time, a state of tensile or shearing stress is formed, which after attainment of the critical value in the places with the minimum of cohesion may result in the formation of plastic microdeformations (crazes or shear bands), from which the formation of a crack starts to develop. Energy of Microcracks. To substantiate theoretically the phenomenon considered above in the case of coal, it is suitable to introduce an analogy with the physical properties of amorphous polymers taking into account the near similarity of the structural arrangement of both substances. Should a crack be formed and propagate, the critical stress σc must exist, which by means of the modified Griffith’s energy criterion may be expressed by the formula
σc )
x
2efE lR
(1)
where E is Young’s modulus, ef the effective specific energy of the crack, l its length, and R is a constant. As the plastic zone in front of the crack tip is usually formed by crazes, ef consists of the following additive terms39
ef ) 2 γ0 + c + ve + d + dis
(2)
where γ0 is the specific surface energy of the crack, c the specific energy needed for the formation of the craze by plastic deformation, ve the specific energy connected with the viscoelastic deformation of the craze, d the specific energy connected with the formation of hollows in the craze, and dis is the specific energy dissipated in the form of heat. From these partial energy components, dis has the highest value corresponding to the heat affecting the surface area of the crack.39 The published values of the effective specific energy ef vary from 4 × 10-4 J cm-2 with inorganic glasses and 0.02 to 0.03 J cm-2 with polymer substances up to ef ) 0.25 J cm-2 with thermoplasts are given.40 For the coal, an analogical value of this quantity was not experimentally determined up to the present time. Therefore, taking into account the high structural similarity of coal (39) Meissner, B.; Zilvar, V. Physics of Polymers, SNTL/ALFA, Prague, 1987. (40) Pluharˇ, J. Physical Metallurgy and Critical States of Material; SNTL, Prague, 1987.
Mechano-activation as Initiation of Self-ignition of Coal
Figure 1. External surface of lump vitrinite from anthracite coal (Vdaf ) 13.1 wt %) disturbed by microcracks. Surface plane is parallel to bedding planes. Magnification 780×.
Figure 2. Internal surface of section of vitrinite from coke coal (Vdaf ) 19.8 wt %) disturbed by macro- and microcracks. Plane of the section is perpendicular to bedding planes. Magnification 490×.
with polymer substances, in the first approximation the lowest potential value of ef ) 0.01 J cm-2 for the organic coal matter may be suggested. From this value, the amount of specific energy dis ) 2.5 × 10-3 J cm-2 causing an effect in the form of dissipated heat Qdis, the unit surface area of crack may be estimated. Formation of Radicals. The initiation and propagation of microcracks and the formation of new surfaces are connected with the destabilization of the molecular coherence and with the disintegration of the solid phase. The mechanism of this process may be described according to the kinetic theory of Zhurkov41,42 and Bueche,43 supposing the interruption of covalent bonds in macromolecules with the formation of reactive macroradicals. As a single radical formed by the interruption of a covalent bond can initiate subsequently the interruption of a number of other covalent bonds, a chain repetition of this process during the formation of a microcrack takes place. In the calculation of the temperature increase of the coal surface it must be considered that the heat Qdis includes also the heat amount released by the formation of radicals. Temperature Increase in Crack Surface. Microcracks (Figures 1 and 2), the length of which may be accounted approximately in the range of 1 to 10 µm and the distance between opposite crack surfaces from 0.01 to 0.1 µm, are indicative of the amount of generated heat. These dimensions are sufficient for an effective insulation of heat and at the same time they enable the gases to move by diffusion within the space of the crack. (41) Zhurkov, S. N. Kinetic Concept of Strength of Solid Bodies; Bull. Acad. Sci. USSR, 1968; No. 3. (42) Kaush, H. H. Polymer Fracture; Springer: Berlin, 1978. (43) Bueche, F. Physical Properties of Polymers; Interscience Publishers, J. Wiley: New York, 1962.
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If the microcrack length is 10 µm and the rate of crack propagation is commensurable with the sound velocity (330 m s-1),44 then the formation time of a crack is τ ) 3 × 10-8 s. During the same time the heat Qdis ) 2.5 × 10-3 J cm-2 is released in the crack so that this process may be considered as adiabatic. To estimate the temperature rise in the limiting surface layer caused by the heat released in the front of the progressing crack, two expressions may be used, which contain the characteristic parameters of the coal. The first one is the equation expressing the heat conduction in the form
λτ(T - To) h
Qdis ) A
(3)
where Qdis [J cm-2] is the quantity of heat released per surface unit (A ) 1 cm2), λ [J cm-1 s-1 K-1] the heat conductivity, τ [s] time, T - To [°C] the temperature gradient, and h [cm] is the thickness of surface layer through which the heat passes. The time τ corresponding to crack formation is also the time interval, during which the heat Qdis released in the course of the progress of the crack front maintains its original value and enters the limiting surface layer, the thickness h of which must include a structural unit of the coal matter or its multiple. The second equation expresses the heating of the layer with the thickness h and the mass Ahd (A ) 1 cm2) by the supplied heat Qdis
T)
Qdis Ahdcp
(4)
where besides the symbols presented above also the coal matter density d [g cm-3] and the specific heat cp [J g-1 K-1] are used. The quantity T in eq 4 represents the temperature increase in the limiting layer related, only in this case, to the starting temperature To ) 0 °C. This assumption must be introduced in order to give the possibility of solution of both equations. The error in the calculated value of T obtained in this way is with regard to the relation T . To (0) practically negligible. If the layer thickness h is expressed by eq 4 and introduced into eq 3, the following quadratic equation is obtained:
T2 - ToT -
Q2dis )0 a
(5)
where a ) λτdcp. Only the positive root of this equation has a real meaning. When solving the equation, the value of To, which represents the coal temperature, is selected according to the actual conditions of coal. The thickness h of the layer is obtained by introducing T, for example, into eq 4. Consider a situation when the temperature To > 0 and the unit surface of the crack consists of partial surfaces, which correspond to the following individual components: the solid coal matter (CM), water (W) contained in interrupted pores, and mineral substances (A) dispersed within the coal matter. In the case where the portions of CM, W, and A expressed in wt % are (44) Neˇmec, J. Dynamics of Fracture; Academia: Prague, 1986.
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transformed into surface fractions, it will hold that CM ) 1 - W - A. As an example of temperature increase, a bituminous coal with 5 wt % of water (W ) 0.05) and 10 wt % of ash (A ) 0.1) may be used. The mean values of following characteristic parameters related to temperature of 30 °C and pressure of 1 bar can be introduced. For coal matter: real density dc ) 1.4 g cm-3, specific heat cp(c) ) 1.06 J g-1 K-1, and thermal conductivity λ(c) ) 2.43 × 10-3 J cm-1 s-1 K-1. For water W: cp(w) ) 4.18 J g-1 K-1 and λ(w) ) 6.21 × 10-3 J cm-1 s-1 K-1. For mineral component A: da ) 2.5 g cm-3 , cp(a) ) 0.69 J g-1 K-1, and λ(w) ) 2.65 × 10-3 J cm-1 s-1 K-1. The quantities λ, d, and cp in the eq 5 will consist of the respective proportions of these mentioned quantities for coal matter, water, and ash in the ratio 0.85:0.05:0.1. As the disconnection of the coal matter occurs in this case only in the area CM ) 0.85 cm2, it holds that Qdis(CM) ) 0.85Qdis ) 2.13 × 10-3 J cm-2. When selecting To ) 30 °C (which is the most frequent coal temperature under real conditions) and when applying the presented relations and values of the individual quantities, after the introduction into eq 5 we obtain T ) 192 °C. For the temperatures of To ) 40 and 50 °C, the quantity T attains the values of 198 and 203 °C, respectively. The corresponding thickness of the surface layer is h ) 7.46 × 10-6, 7.25 × 10-6, and 7.05 × 10-6 cm, respectively In the case where by crack propagation a water- and ash-free coal surface (CM ) 1) is formed, the temperature of the coal matter To ) 30 °C would be considered and the above-mentioned parameters λc, cp(c), and dc would be introduced; to the values of released heat amounts Qdis ) 2.0 × 10-3, 2.5 × 10-3, and 3.0 × 10-3 J cm-2 would correspond the increased temperatures T ) 192, 240, and 288 °C, respectively. Origin and Destiny of Microfires. The temperature increase in the crack surface by the addition of quantity T represents an already sufficiently high temperature lying in the vicinity of the ignition point of coal matter. For this reason, on the crack surface, especially in the presence of groups of reactive radicals, the ignition in very small centers with formation of microfires may take place. According to the arrangement and the chemical character of the crack surface these microfires, the size of which equals a molecular complex in the range of nanometers to micrometers, may either spread or integrate into larger units, or extinguish. For the propagation or, on the contrary, for the extinction of the microfires the following circumstances are decisive. The positive ones include the following: sufficiently high energy of primary microfire, a part of which may be transformed to the next center; the primary microfire spreads in a favorable direction into the neighborhood; further, the possibility of burning down to a deeper layer and in this way maintaining the local microfire in an active state and of preserving the conditions for the diffusion ensures the substitution of the gaseous combustion products by the air. The negative ones include the following: the center of microfire is energetically weak and collapses by itself; the unfavorable distribution of incombustible obstacles in the form of the cross-sections of the very small pores filled
Medek and Weishauptova´
with water and mineral agglomerates in the surface of the crack. In view of these factors, the probability of development of microfires as nuclei of coal ignition lies within the framework of the statistical uncertainty of their existence in both time and space, which is so typical for the occurrence of coal self-ignition. Discussion The microfires with extremely small dimensions need not manifest their existence outwardly, i.e., by the release of measurable amount of volatile products or by the increase in temperature of the external surface of coal. These effects occur only in case that the microfires change to macroscopic fire. However, such a development is not conditioned by unknown accidental factors but always by theoretically defined physical, chemical, and steric factors. The model of microfires agrees fully with the empirical knowledge concerning the heat insulation of fire centers and the presence of oxygen. It only presents a new explanation of the initiation of the self-ignition process, based on idea that this process can be started only by direct ignition of a microscopic, heatinsulated object of coal mass, not by its gradual oxidation. The presented model of the coal self-ignition gives the possibility of determining roughly the reaction heat of the process, which causes the local increase in coal temperature up to the ignition point. The calculation is based on the physical parameters of the coal, the values of which were obtained by direct measurements performed with coal of a defined character on one hand, and by a roughly quantitative estimation of the heat released during the formation of the crack, based on the similarity of the structural properties of the coals and plastics, on the other hand. The magnitudes of specific heat cp(c) and thermal conductivity λ(c) measured with lump coal are documented prevailingly with bituminous coals where only the values determined with the separated coal matter are of importance. According to the individual authors,49 these values differ only very slightly and within the coalification range of 10 to 45% Vdaf the cp(c) values vary in the average between 0.8 and 1.2 Jg-1K-1 with the mean value of 1.06 ( 0.1, and λ(c) varies from 2.0 × 10-3 to 2.85 × 10-3 with the mean value of 2.43 × 10-3 ( 0.36 × 10-3. Whereas the mean values were applied for the basic heat balance, when a combination of the extreme values within the range of both intervals was used under identical conditions (CM ) 0.85; W ) 0.05; A ) 0.1) at Q ) 2.13 × 10-3 J cm-2, the temperature T ) 196 ( 22 °C, and the layer thickness h ) 7.25 × 10-6 ( 0.23 × 10-6 cm were obtained. A change in the qualitative parameters of coal with a chosen heat Qdis does not affect substantially the values of T and h with the exception of extreme cases, which deviate from the type characteristics of the coal. For the estimation of the temperature T the heat Qdis (45) Pa´nek, P.; Taraba, B. Coal Interaction with Oxygen; Letters University Ostrava, No. 92, 1996. (46) Vydegzˇanin, V. N.; Vydegzˇanin, Vl. N. Dokl. Akad. Nauk USSR 1982, No. 5, 84. (47) Chrenkova, T. M.; Thubarova, M. A. Khim. Tverd. Topl. 1973, No. 1. (48) van Krevelen, D. W. COAL; Elsevier: New York, 1993. (49) Databank of IRSM, ASCR, Prague.
Mechano-activation as Initiation of Self-ignition of Coal
is of basic importance; however, there are no concrete experimental data concerning its real value. This heat, which is a component of the total energy accompanying the disintegration of the coal, could be at least approximately compared with the heat released when coal is crushed under strictly adiabatic conditions. However, the realization of such experiment with sufficiently high precision is connected with exceptionally difficult conditions of measurement considering that the change in the outer surface area of coal must be determined. Moreover, none of the other energetic members in expression (2) has been determined with coal, both with regard to both the specific properties of the coal matter and the lack of specific experimental procedures, which could be possibly used with coal. A certain similarity of selected physical parameters with coals48,49 in the range of 10-45 Vdaf and polymers48,50 (in parentheses) is evident. For example density d ) 1.39 ( 0.09 g cm-3 (1.27 ( 0.27), specific heat cp ) 1.06 ( 0.2 J g-1 K-1 (1.6 ( 0.55), thermal conductivity λ ) 2.0 × 10-3( 0.42 × 10-3 J cm-1 s-1 K-1 (2.22 × 10-3 ( 0.88 × 10- 3), Young’s modulus E ) 4.1 GPa (6.3 GPa), Vickers microhardness H ) 30 (10 kg mm-2 (25 ( 15), surface energy of solid γs ) 57 ( 5 mJm-2 (40 ( 5),48 electrical conductivity σ ) 10-12(3 S m1- (10-12(4), dielectric constant [in 106cycles s-1] ) 4.0 ( 0.5 (4.3 ( 1.7). Therefore, we can assume that an analogy exists also in the case of the effective specific energy ef and suppose that at least the order of the estimated value of Qdis corresponds to the real one and that it can be also numerically valid within a limited interval, which may include various types of coal. The method of the partial surfaces CM, W, and A gives the possibility of determining the effect of water content on the value of the surface temperature T. The temperature change under the same boundary conditions as in the preceding calculations may be given as an example. With Qdis ) 2.5 × 10-3 J cm-2, To ) 30 °C, and A ) 0.1 for the water fractions W ) 0, 0.05, 0.1, (50) Information materials of Manufacturers of Plastics.
Energy & Fuels, Vol. 17, No. 1, 2003 163
and 0.2, and CM ) 0.9, 0.85, 0.8, and 0.7 correspond to the surface temperatures T ) 229, 192, 166, 128 °C, respectively. It is evident in agreement with the wellknown practical experience that the probability of the coal self-ignition decreases with the increasing water content. For comparison, it is useful to mention the temperature rise during the chemisorption of oxygen on the coal surface. On the fresh surface of subbituminous coal the oxidation heat of 14.5 J g-1 was determined at 30 °C by pulse flow calorimetry.45 With its surface covered with oxygen molecules SO2 ) 6.5 m2 g-1 this corresponds to the heat QS ) 2.23 × 10-4 J cm-2. With the same values of cp, d, and h, from eq 4 it follows that the temperature increase is T ) 25 °C. With the prolonged time of exposure of coal to air oxygen the heat QS decreases substantially as the result of the gradual oxidation of the coal surface. Conclusions The contribution of the mechanical activation to the self-ignition of coal has already been theoretically considered previously46,47 but only as the first step in a series of several consecutive processes without a direct influence on the self-ignition. In this work, we have tried to perform a detailed analysis of the exothermic processes on the basis of the specific properties of the coal matter and a statistical analysis of the individual circumstances connected with the manifestation of the self-ignition. For the first time, the new theory was documented by the calculation of the possible thermal effect, the value of which within the scope of the applied suppositions has been shown to be sufficient for the immediate formation of miniature foci of coal matter ignition. Acknowledgment. This work was supported by the Grant Agency of the Academy of Sciences of the Czech Republic under Contract No. A2046101. EF010134D
