Some Results of Determining the Gas Emission Rate Index of Coal from a Seam Prone to Sudden Outbursts

G. I. Gritsko

(Presented by the scientific seminar of the departments of bedded-deposit mining, ore-deposit mining, mine construction, safety engineering, and mine ventilation)

According to present-day concepts of the nature and mechanism of sudden outbursts of coal and gas, the most important factors determining the presence of favorable conditions for the occurrence of sudden outbursts are the physico-mechanical properties of coal, its micro- and macrostructure, and also the structure of the coal seam. In this connection, the structure of the coal seam is a factor that determines the potential possibility of outbursts. The specificity of the structure of seams subject to sudden outbursts consists in the fact that, as a rule, they have intensely crushed bands of coal of disturbed structure, possessing reduced mechanical strength and an increased rate of gas emission.

The fact of an increased rate of absorption and emission of gas by coal from hazardous seams was noted by a number of foreign and Soviet investigators more than 30 years ago.

Investigations carried out in Belgium on coal samples of 60 different grades showed [1] that certain zones of coal seams are hazardous from the point of view of sudden gas outbursts not as a result of their abnormally high absorptive capacity with respect to gas, but because of a higher rate of its emission. At the same time it was noted that neither oxidation of coal samples nor their crushing affects the change in the absorptive capacity of coal.

In 1936 the French investigator Daval, on the basis of numerous experiments, also came to the conclusion that “coals prone to sudden emissions differ from other coals only in their ability to give up the gas contained in them extremely rapidly, which in turn may be regarded as a function of the physical structure of the coal” [2]. This same feature of coals from hazardous seams was noted by Bubnoff [3] and O. Ruff [10]. The latter also indicated that the quantity of gas absorbed by coal does not depend on the grain size, which determines only the rate of absorption and emission of gas by coal [4].

The work of I. L. Ettinger at the IGD, Academy of Sciences of the USSR, established that coals from hazardous seams, while not differing from coals from nonhazardous seams in the quantity of gas absorbed by them, at the same time differ greatly...

differ from the latter in the rate of release or absorption of gas by coal. At the same time, I. L. Ettinger suggested that this difference is caused by differences in the ultrafine structure of the coals, which does not change when coal samples are separated from the massif and is indistinguishable under the microscope [5]. On the basis of this difference, I. L. Ettinger and E. S. Zhupakhina proposed a method for determining zones dangerous in terms of sudden outbursts in coal seams [6]. According to this method, the degree of danger of a seam is established by determining a conditional index of the initial rate of gas emission, which is numerically equal to the increase in pressure of the gas released from the coal during the experiment. The gas-emission index was conventionally denoted as $\Delta P$. Later this same index was denoted by Fertelmeister as $F$ [7].

As a result of investigations carried out in the Donets Basin by workers of the Institute of Mining of the Academy of Sciences of the USSR, it was established that all seams subjected to sudden outbursts of coal and gas have disturbed coal benches or interbeds of coaly argillite with a high value of the $\Delta P$ index. Criteria were proposed for assessing the degree of danger of seams, namely: at $\Delta P$ up to 5 the seam was considered nonhazardous, at $\Delta P$ from 5 to 15—threatened, and at $\Delta P$ greater than 15—hazardous. The method in question proved so accurate that in several cases an increase in the value of $\Delta P$ to 25–30 in a certain part of a seam coincided with a sudden outburst at that same place [8]. At the “Yunkom” mine, continuous monitoring of the outburst danger of seams and of individual benches within them was established, giving practically reliable results [9].

On the basis of this same principle, we carried out a study of the properties of coal from the Kharakterny seam, dangerous in terms of sudden outbursts, at the Koksovaya-2 mine of the Stalinskugol trust in the Prokopyevsk district of Kuzbass.

For sampling, a section along the Kharakterny seam at the elevation of $+125$ m of the Koksovaya-2 mine was selected. In this section the seam is worked by a system of long pillars along the strike, with roof caving. The level is divided into three sublevels; at present, work is being carried out only in the two upper sublevels, while the lower sublevel has been left as a pillar above the main drift, which is used as a group drift. According to measurements made in 1955, the gas abundance of the section was $4.62\ \mathrm{m}^3$ of $\mathrm{CH}_4$ per ton of daily output.

At the sampling site, the Kharakterny seam is characterized by the following data: seam thickness 1.7–1.9 m, dip angle $56^\circ$. The roof of the seam is a stable conglomerate, and the floor is coaly argillite. The seam consists of two coal benches. The upper bench, 0.9 m thick, consists of semilustrous hard coal with inclusions of thin bands of more friable lustrous coal. The lower bench of the seam is represented by coal that is strongly crushed, has a disturbed structure, and is very weak, silky to the touch, with a lustrous fracture surface. The structure of the Kharakterny seam at the sampling points is shown in Fig. 1. It should be noted that the thickness of the strong coal bench lying near the roof decreases down the dip of the seam and, at a distance of 75 m from the ventilation drift, is only 0.4 m, while the thickness of the disturbed bench at this same place increases to 1.2 m.

Coal samples were taken in longwall No. 1, two meters from the ventilation drift, and in longwall No. 2, two meters from the parallel drift, at distances from the ground surface vertically correspond—

at 95 and 149 m. From each bench two samples were taken; the locations of the sampling points are also indicated in Fig. 1.

Determination of the conventional index of the initial gas-emission rate \(\Delta P\) was carried out in the laboratory of electrovacuum engineering of the Tomsk Polytechnic Institute. For this purpose an apparatus was assembled,

Labels in Fig. 1: sandstone with carbonaceous material; coal, semi-bright, undisturbed; coal, banded, disturbed; clayey argillite.

Fig. 1. Structure of the Characteristic seam at the level of +125 m of the Koksovaya-2 mine at the sampling points

the main part of which consists of glass ampoules 1, connected by means of valves to a vacuum pump 2 and a vacuum gauge 3, and also to a gasometer 4 filled with methane (Fig. 2).

Fig. 2. Diagram of the apparatus for determining the index of gas-emission rate \(\Delta P\)

The procedure for carrying out the experiments is as follows. Coal samples of the size fraction 0.5–0.25 mm, weighing 5 g, are loaded into the ampoules. The valves of the ampoules are opened toward the vacuum pump and the coal is degassed for one and a half hours. After this, by turning the same valves, the ampoules are connected to the gas-

meter filled with methane at a pressure close to atmospheric. The coal is also saturated with gas for one and a half hours, after which the ampoule is connected to the vacuum system and gas is released from the coal, with the rates of increase in gas pressure measured by the vacuum gauge. The ampoule valve is opened for 1 sec and the gas pressure is measured; after this the ampoule is closed for 50 sec, then opened again for 10 sec and the pressure of the released gas is measured a second time. As a result, two readings are obtained, the first of which expresses the pressure established after the pressure of the free gas located in the ampoule above the coal sample has been reduced to 10–12 mm Hg and after gas has been released from the coal during the first second of the experiment; the second corresponds to the amount of gas released from the coal over the entire duration of the experiment. The difference between these two readings, characterizing the increase in pressure of the gas released from the coal over one minute, is taken as the conventional index of the initial gas-emission rate, \(\Delta P\).

The changes that we introduced into the method of I. L. Ettinger, concerning the time of gas release from the coal during the first period of the experiment (this time was reduced from 10 to 1 sec while keeping the same total duration of the experiment, one minute), as well as the weight of the coal sample, which was taken as 5 g instead of 3.5 g, are due to the design and dimensions of the instrument for determining \(\Delta P\).

The value of the \(\Delta P\) index was checked by obtaining a large number of identical results. In all, about 170 experiments were carried out on 9 coal samples from the Kharakternyi seam. In addition to determining the index of the initial gas-emission rate for different bands of the seam, the change in this index as a function of the size of the coal fraction was also studied, and observations were made of the decrease in the rate of gas emission from coal over time. As a result of the investigations carried out, the following data were obtained.

1. The conventional index of the initial gas-emission rate, \(\Delta P\), determined in this section of the Kharakternyi seam, is 2–2.5 times higher for disturbed coal from the crumpled band of the seam than for coal from the undisturbed band, and is respectively 11–13 for the first of the indicated bands and 5–7.4 for the second. The increased gas-emission rate of the disturbed coal from the lower band of the seam agrees with the fact that was observed during the sudden outburst of coal and gas in the main drift of the Kharakternyi seam on April 10, 1953. In this case it was precisely the lower disturbed band of the seam that was affected by the outburst, distinguished by a higher index of gas-emission rate \(\Delta P\), whereas the upper band of the seam, for which the \(\Delta P\) index is considerably lower, remained untouched in the face and in the roof (Fig. 3).

2. The conventional index of the initial gas-emission rate \(\Delta P\) changes depending on the degree of crushing of the coal, and this change manifests itself differently for coal from different bands (Table 1).

Fig. 3. Sudden outburst of coal and gas in the main drift of the Kharakternyi seam at the +125 m horizon of the “Koksovaya-2” mine on April 10, 1953.

Table 1

Change in the index of the initial rate of gas emission \(\Delta P\) as a function of the size of the coal fraction

As follows from the table, coal from the strong band of the seam gives a considerable increase in \(\Delta P\) when it is crushed, especially in going from the 0.5–0.25 mm fraction to the fraction less than 0.25 mm. This confirms the fact that, when such coal is crushed, an additional surface is created in it, promoting more rapid absorption and release of gas by the coal. Coal from the disturbed band of the seam, however, gives a smaller increase in the rate of gas evolution when it is crushed, since in crushing coal of this band the additional exposed surface is produced in significantly smaller amounts than in the case of crushing strong coal. Such a situation may arise when the active surface of the coal was already sufficiently large, which, in turn, becomes possible only with a more highly developed network of microscopic pores and cracks in disturbed coals.

1. Gas evolution from coal proceeds most intensively during the first minute from the start of the experiment. Over the course of 10 minutes the rate of gas evolution decreases by a factor of 10–12 (when measurements of the gas-release-rate index \(\Delta P\) are repeated in the usual way at intervals of 1.5–2 min). Figure 4 shows the decrease in the rate of gas evolution for coal samples from different bands of the seam. From a comparison of curves 1 and 2 it is evident that the greatest difference in the indices of the gas-release rate \(\Delta P\) for disturbed and undisturbed coals is observed during the first minute from the start of the experiment; that is, an essential distinguishing feature of disturbed coals is that they are capable of releasing methane considerably more rapidly immediately after the pressure of free gas is reduced. This property of coal is one of the important factors in the mechanism of development of sudden outbursts of coal and gas. Subsequently the indicated difference becomes less significant, since the rate of gas evolution from disturbed coal decreases with time much faster than the rate of gas evolution from coal whose structure is undisturbed. The latter may also be explained by the considerably more developed character of the porosity and fissuring possessed by the disturbed bands of the seam, which represent the greatest danger with respect to the occurrence of sudden outbursts.

Conclusion

The investigations carried out make it possible to form an idea of certain properties of coal from the seam liable to sudden outbursts characteristic of the “Koksovaya-2” mine. These properties consist in the fact that disturbed coal from the crumpled band, occurring at the footwall...

of the seam, is distinguished by a higher initial rate of gas emission in comparison with coal from the undisturbed band. Being due to a more developed character of microscopic porosity and fissuring, this property plays an important role as one of the conditions for the occurrence of sudden outbursts. Confirmation is provided by the sudden outburst of coal and gas that occurred on April 10, 1953, in the main drift of the Characteristic seam at the Koksovaya-2 Mine and affected only the lower disturbed band of the seam.

The method of determining bands and zones in coal seams that are dangerous with respect to sudden outbursts, on the basis of establishing the index of the initial gas-emission rate \(\Delta P\), may be recommended for forecasting sudden outbursts of coal and gas in the Kuznetsk Basin. The criteria for assigning a seam to one category or another in accordance with the value of the index \(\Delta P\) are to be established separately for each seam.

Fig. 4. Change in the gas-emission-rate index \(\Delta P\) as a function of time: 1—undisturbed coal; 2—disturbed coal

For comparison of results obtained in various research institutes and in production, the instrument for determining the index \(\Delta P\) and the experimental procedure should be standardized.

LITERATURE

1. Adsorption capacity of Belgian hard coals. Abstract from Annales des Mines de Belgique, vol. XXXV, issue 1.

2. Sudden emissions of methane. Abstract of an article by Daval in Revue de l’Industrie Minérale, No. 372, 1936.

3. Bubnoff. Geologische Verhältnisse der durch Kohlensäureausbrüche heimgesuchten Gruben, Zeitschrift für das Berg-Hütten und Salinenwesen, H. 75, 1927.

4. Ruff. O. Die Ursachen von Gasausbrüchen in Steinkohlengruben, Zeit. Ang. Chem., 1930.

5. Ettinger I. L. Index of the tendency of coal toward sudden outbursts of coal and gas, Ugol’, No. 10, 1952.

6. Ettinger I. L., Zhupakhina E. S. Initial rate of gas emission from coal in seams dangerous with respect to sudden outbursts of coal and gas, Proceedings of the Institute of Mining, Academy of Sciences of the USSR, vol. 1, 1954.

7. Fertelmeister Ya. N. On the results of studying certain physicochemical factors in the investigation of coal seams dangerous with respect to outbursts. Proceedings of MakNII, vol. VIII, 1955.

8. Lidin G. D., Ettinger I. L., Zhupakhina E. S., Sazonov L. Ya. Determination of the gas-emission rate as a method for detecting zones in coal seams that are dangerous with respect to outbursts, Ugol’, No. 12, 1954.

9. Sazonov L. Ya. Determination of the gas-emission rate in the laboratory of the Yunkom Mine as a method for detecting outburst-hazardous zones in seams subject to sudden outbursts of coal and gas. Abstracts of papers of the scientific-technical conference on combating sudden outbursts of coal and gas in coal mines, Stalino, 1955.

10. Ruff O. Die chemischen und physikalischen Vorgänge bei Kohlensäureausbrüchen, Zeitschrift für das Berg-Hütten und Salinenwesen, H. 4, 1927.