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УДК 622.272.6
РОЛЬ ТЕМПЕРАТУРЫ И МИКРОСТРУКТУРЫ УГОЛЬНОГО ВЕЩЕСТВА В ЭНЕРГОМАССООБМЕННЫХ ПРОЦЕССАХ
Татьяна Анатольевна Киряева
Институт горного дела им. Н. А. Чинакала СО РАН, 630091, Россия, г. Новосибирск, Красный пр., 54, кандидат технических наук, тел. (923)170-32-11, е-mail: coalmetan@mail.ru
Чжоу Аитао
Школа ресурсов и техники безопасности, Китайский университет горного дела и технологий, 100083, Китай, г. Пекин, тел. 86 010 62339036. е-mail: safety226@126.com
Представлены экспериментальные данные лабораторных и натурных исследований по особенностям взаимодействия между геомеханическими и физико-химическими процессами в угольных пластах Кузбасса различной стадии метаморфизма с учетом влияния температурного фактора. Изучается влияние температуры и микроструктуры угольного вещества в энергомассообменных процессах (изменение массы, выхода летучих, удельной поверхности частиц угля, внутренней энергии релаксации метаноносности).
Ключевые слова: внезапные выбросы угля и газа, углеметановые геоматериалы, выход летучих, удельная площадь поверхности, разрушение, структура горных пород.
ROLE OF TEMPERATURE AND MICROSTRUCTURE OF COAL IN ENERGY AND MASS EXCHANGE PROCESSES
Tatiana A. Kiryaeva
Chinakal Institute of Mining SB RAS, 630091, Russia, Novosibirsk, 54 Krasny Prospect, Ph. D., tel. (923)170-32-11, е-mail: coalmetan@mail.ru
Zhou Aitao
School of Resource and Safety Engineering, China University of Mining and Technology, 100083, China, Beijing, tel. 86 010 62339036, e-mail: safety226@126.com
The paper presents laboratory and in situ research data on interaction of geomechanical and physicochemical processes in Kuzbass coal layers of various metamorphism ranks, considering temperature effect. The authors study the role of temperature and microstructure of coal in energy and mass exchange processes (variation of mass, volatile yield, specific surface, internal energy of methane adsorption capacity decrease).
Key words: coal and gas outbursts, coal and methane geomaterials, volatile yield, specific surface area, destruction, rock structure.
Coal and gas outbursts are a kind of extremely complex dynamic phenomenon [1, 2], during an outburst, the coals and rocks around the coal mining face are quickly broken and ejected, large amounts of gas release from the pulverized coal. The pulverized coal and gas flow induced by an outburst have higher energy [3], which can lead to fatalities and destroy underground equipment. In recent years, many coal and gas outburst accidents occurred in China. For example, October 20, 2004, a serious outburst occurred in Daping coal mine of Zhengzhou Coal Group in Henan province.
In this accident, the outburst coal and rock was about 1894 t as well as about 250 thousand m3 outburst gases. Due to a great overpressure of outburst gas flow, some unground ventilation facilities were destroyed, a large amount of gases reversed, which led the gas concentration in intake roadway to exceed the explosion limit, 148 people were killed and 32 people injured.
Hence, it is necessary to investigate the factors that influence on the propagation characteristic of outburst gas flow.
Researchers understand the fundamental significance of the structural aspect in interpreting wide-range geomechanical, geophysical, and complex mass exchange processes. The structure of rocks governs their physicochemical, physico-mechanical and process properties. The knowledge of the structure of rocks provides an insight on occurrence of methane in coal and allows specifying physicochemical principles of gas-dynamic control in mines.
In recent decades Russian scientists dynamically expand concept on coal and methane seam as a solid coal and gas solution [4]. The process of coal and methane decomposition proceeds with release of elastic energy resulting in dynamic fracture of coal and pushing out its considerable volumes into excavation area by emitted gas flows.
Important characteristic of Kuzbass coal is an internal energy relaxation in methane, which characterizes coal and methane system condition quantitatively when translating from one metastable state to another under methane content changed 2 times.
Coal and methane are connected physicochemically and form a coal-and-methane solution. Methane emission is only possible under alteration of thermody-namic state and decomposition of the solution. The solution decomposition is accompanied by the release of energy that goes to destruction of coal with formation of additional inner surface. With the higher gas content of coal, the gas-dynamic destruction is more intensive, up to total self-destruction in the form of coal and gas outbursts. With equal gas content, the mid-metamorphism coal is a stronger prone to self-destruction. With the higher decomposition energy, the broken coal is more ground. Conditions for micro-fracturing appear due to elastic energy of gas. So, destruction of the solid component of the coal-and-methane solution is associated with the formation of a new inner specific surface.
The studies of coal specimens from the five beds in Kuzbass, with volatile content from 18 to 43 %, yielded an empirical relation of the internal specific surface in coal with different Protodyakonov's hardness f outburst force F (fig. 1) and coal-and-methane internal energy E.
The revealed connection of the coal-and-methane internal energy and the coal and gas outburst force was confirmed by statistical data on 197 instances of outbursts collected in mines in Karaganda, Kuznetsk and Pechora Basins gathered from 1943 to the present day. Due to weak gas-dynamic events the outbursts with the force lower than 20 tons were excluded from the data sampling. The remaining 114 outbursts were grouped by coal-and-methane internal energy per 10 kJ/kg. The average outburst force was estimated in each group (the points in fig. 1).
0 50 100 150 200 250
£. kj/kg
Fig. 1. The curves of the specific internal surface Ssp, outburst force F and internal energy Е of coal-and-methane:
1 - experiment; 2 - averaged statistics
Fig. 2. Percentage of TG (Thermo-gravimetry) versus temperature for the tested coal specimens. The figures are the numbers of the specimens
Thus, the change in the specific internal surface of coal particles and the self-destruction of coal have allied physical mechanism connected with the elastic energy of the gas component of the coal-and-methane material. The prompt decision of gas safety problems in mines requires express-methods. One of such methods can be the advanced estimation of the specific internal surface of coal sampled in deposits and producing faces.
An important characteristic of coal gas-dynamic activity is the porous structure of coal beds with the stored internal energy of relaxation, Е, on transition between the
stages of metastability. Such ideas on properties of mineral coal are based on the variety of processes of the natural origination (metamorphic grades), with formation of carbonic systems with a certain porous structure and changeable thermomechanical and physicochemical properties [5].
The porous structure was studied by the low-temperature (77 K) nitrogen adsorption on DigiSorb 26 00 plant. The specimens were pre-aged in vacuum 10-4 mm hg, at the temperature of 150 °C for 5 h. The real density p was measured by the volumetric method by helium on AutoPycnometer-1320 device.
TG was performed for all tested coal specimens in order to specify residual mass of a specimen under heating (the initial mass was assumed 100 %). As shown in Fig. 2, all specimens lose weight under heating, with different intensity within prominent intervals of 40-50 °C, 400-500 °C and higher. So, the experimental data split into three characteristic test series.
By studies, the mass loss Am (percentage of original mass) in coal specimens mostly takes place in two temperature intervals: Am1 at 40-50 °C (T1) and Am2 at 440-510 °C (T). The data of TG and DTG (Differential Thermal Analysis) are compiled in Table 1. According to Table 1, the behavior of Am2 at T2 is more intricate than the behavior of Am1 at T1; Am2 reaches 15 % on average.
The analysis of the pore structure parameters (size and volume of pores) by nitrogen adsorption technique at the temperature of 77 K shows the prevailing size is the size of macropores (d > 100 nm) and therefore coal cannot be examined reliably with the capillary condensation methods.
Table 1
The change in the specimen mass, Am, versus T by DTG data
Specimen T1, ° C Am1, % T2, ° C Am2, % Specimen T1, ° C Am1, % T2, ° C Am2, %
1 42 4.4 513 8.2 16 47 6.0 493 13.3
2 38 4.9 506 10.6 30 49 0.7 450 31.1
3 47 6.0 506 8.7 31 39 4.6 436 28.8
1n 38 4.0 510 8.9 32 47 4.8 487 18.2
3n 38 4.8 508 7.8 34 39 3.9 488 19.3
6n 39 4.8 466 21.6 35 48 3.5 476 21.5
7n 47 4.7 469 24.5 36 40 3.9 500 10.3
11 45 3.4 509 8.2 37 43 4.5 493 16
12 40 3.9 509 8.0 38 41 3.5 505 9.9
15 41 4.3 494 14.4 39 43 3.7 507 9.1
In future it is planned to use the mercury injection method. Table 2 describes the real density of Kuzbass coal: p1 - density of sample after storage; p2 - density of "fresh" sample. The value of p2 corresponds to the typical density of coal of various ranks (1.3-1.4 g/cm3).
Table 2
Real density of Kuzbass coal
Specimen P, g/cm3 Specimen p, g/cm3
Pi P P1 P
1 1.447 1.370 30 2.066 1.308
1 ground 1.334 — 31 2.779 1.366
3 1.418 1.364 31 ground 1.996 —
11 1.457 1.395 32 — 1.381
12 1.544 1.380 34 — 1.369
15 1.456 1.306 35 — 1.397
15 ground 1.420 — 36 2.081 1.489
16 ground 1.355 1.348 37 — 1.373
12 1.389 — 39 2.106 1.471
It was experimentally proved that with the time of storage of the excavated coal samples, the bond of coal and methane weakens. In particular, the change in density and mass of coal after storage was analyzed (Figs. 3 and 4). After 12 years of storage, coal specimens, heated to 500 °C, lost 3 times the mass of the specimens stored for 1 year. This confirms the dependence of the internal energy E of methane release on the coal mass loss. Figure 5 shows the correlation dependence of E and coal mass loss at T2 (refer to Table 1). It is seen that the presence of difficult-to-remove (tightly bound) volatile components (samples nos. 1, 3, 38, 39) results in the higher energy of methane release, and vice versa, the specimens with easy-to-remove volatile matter (specimens nos. 30, 31) posse lower E [6, 7].
11 12 15 16 30 31 36 After sampling ■ After storage
Fig. 3. The real density change after storage
35
30
cx 25
<N 20
K < 15
10
•
•
•
• • R2= 0.6102
• •
0
10
15
Storage time, years Fig. 4. Mass loss at 500 0C versus coal storage time
By now it is only presumable that this is conditioned by methane ability to fuse in the "fluid-like" coal components, which requires further investigation. On the other hand, the maximum internal energy of coal-and-methane is typical of middle-rank coal. Specimens with tight bonding have volatile content from 18 to 25% and refer to middle rank that is the most outburst-hazardous.
250
Fig. 5. Methane energy release versus mass of Kuzbass coal specimens
REFERENCES
1. Yuan L. Control of coal and gas outbursts in Huainan mines in China: A review // Journal of Rock Mechanics and Geotechnical Engineering. - 2016. - 8 (4). - P. 559-567.
2. Zhou H. et al. Methane drainage and utilization in coal mines with strong coal and gas outburst dangers: A case study in Luling mine, China // Journal of Natural Gas Science and Engineering. - 2014. - 20. - P. 357-365.
3. Guo H. et al. Pulverization characteristics of coal from a strong outburst-prone coal seam and their impact on gas desorption and diffusion properties // Journal of Natural Gas Science and Engineering. - 2016. - 33. - P. 867-878.
4. Alekseev A. D., Airuni A. T., Zverev I. V. et al. Property of an Organic Matter in Coal to Form Meta-Stable Single-Phase Systems with Gas by the Type of Solid Solutions // Dipl. Nauch. Utkryt. - 1994. - No. 9.
5. Interaction of Geomechanical and Physicochemical Processes in Kuzbass Coal / V. N. Oparin, T. A. Kiryaeva, V. Yu. Gavrilov, R. A. Shutilov, A. P. Kovchavtsev, A. S. Tanaino, V. P. Efimov, I. E. Astrakhantsev, I. V. Grenev // J. Min. Sci. - 2014. - Vol. 50, no. 2. - P. 191-214.
6. Kiryaeva T. A., Mel'gunov M. S. Preliminary Data on State-of-the-Art Investigation into Coal Structure, GIAB, Special issue no. 7, Kuzbass-1, 2009. - P. 155-160.
7. Nonlinear Deformation-Wave Processes in Various Rank Coal Specimens Loaded to Failure under Varied Temperature / V. N. Oparin, T. A. Kiryaeva, O. M. Usol'tseva, P. A. Tsoi, V. N. Semenov // J. Min. Sci. - 2015. - Vol. 51, no. 4. - P. 641-658.
© T. A. Kupneea, H. Aumao, 2017
