Experimental Study of Gas Desorption Law of Deformed Coal

Experimental Study of Gas Desorption Law of Deformed Coal

Available online at www.sciencedirect.com Procedia Engineering ProcediaProcedia Engineering 00 (2011) 000–000 Engineering 26 (2011) 1083 – 1088 www...

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Procedia Engineering

ProcediaProcedia Engineering 00 (2011) 000–000 Engineering 26 (2011) 1083 – 1088 www.elsevier.com/locate/procedia

First International Symposium on Mine Safety Science and Engineering

Experimental Study of Gas Desorption Law of Deformed Coal Zhihui Wen* , Jianping Wei , Dengke Wang , Chao Wang School of Safety Science and Engineering, Henan Polytechnic University, Jiaozuo, Henan 454003, China

Abstract Experimental system for investigating gas-desorption characteristics of deformed coal was established. The gas desorption law of deformed coal with different damage extent was investigated. The desorption characteristics of deformed coal having different size under different equilibrium pressures were determined. This provides a theoretical basis for the prediction of coal and gas outburst, the measurement of pressure and content of coal seam gas, and the estimation of gas emission of coal mining fell.

© 2011 Published by Elsevier Ltd. Selection and/or peer-review under responsibility of China Academy of Safety Science and Technology, China University of Mining and Technology(Beijing), McGill University and University of Wollongong. Keywords: deformed coal; gas desorption law; simulation experiment

1. Introduction When coal seem was subject to compressive and shear force, uniform and clear banded structure of coal seem was damaged, thus the deformed coal formed [1]. Study results show that soft deformed-coal seam with certain thickness is the necessary condition for coal and gas outburst [2]. This is because the deformed coal with high porosity and the low permeability has high gas pressure and gas content, and stores potential for outburst. Furthermore, the deformed coal has a lower strength. It is difficult to resist strong external force for deformed coal. The deformed coal is, therefore, easily damaged and thrown. It is urgent to investigate the gas desorption law of deformed coal with different damage extent. This has important significance for the prediction of coal and gas outburst, the measurement of pressure and content of coal seam gas, and the estimation of gas emission of coal mining fell. Nomenclature

* Corresponding author. Tel.: +86 391 3987430; fax: +86 391 3987881. E-mail address: [email protected] (Z. Wen).

1877-7058 © 2011 Published by Elsevier Ltd. doi:10.1016/j.proeng.2011.11.2277

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hw

height of water column

Patm

the atmospheric pressure

Ps

saturation vapor pressure

Qt'

standard volume of desorbed gas

Qt''

measured gas volume of desorbed gas

tw

temperature of water

2. Experimental 2.1. Experimental system The self-made experimental system of deformed-coal methane adsorption is shown in Figure 1. The system was established based on the measurement method of coal methane adsorption (MT/T752-1997). It consists of a vacuum degassing unit, gas adsorption-desorption unit, desorption apparatus, high pressure methane cylinders, and volume correction device.

Connected with vaccum bag

2 1

5 3

4

Fig.1. Schematic diagram of experimental system of gas desorption of coal: 1-High pressure methane gas cylinder; 2-gas buffer; 3Vacuum pump; 4-Coal sample tank; 5-Desorption apparatus

2.2. Experimental method Vacuum degassing of the pre-treated coal samples were performed. The pure methane was adsorbed by the samples for a long time to achieve adsorption equilibrium. The pressure was immediately discharged before gas desorption. The gas desorption process of the coal sample exposed to air was measured using desorption apparatus under an air temperature of 30±1 ℃ and a gas outlet pressure of near 0.1MPa ( the effect of water column height on the desorption apparatus was ignored). It is believed that the gas desorption was carried out under isothermal–isobaric condition. (1) Pre-treatment of coal samples. The 600-1200g coal samples from Yian mine, Jiulishan mine, and Zhaogu mine were removed moisture using a drier for 1.5 hours. The coal tank was fulfilled with one of

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the coal sample. The sample was covered using absorbent cotton and copper net with a 80 mesh aperture. The coal tank was then sealed. (2) Vacuum degassing of coal sample. The vacuum degassing of coal sample was carried out in a water bath with a (60±1)℃ temperature, until the pressure of 0.1Mpa lasted for 2h. (3) Gas adsorption equilibrium. The temperature of water bath was adjusted to 30±1℃. The valves of high pressure gas cylinder and gas buffer were respectively opened. When the pressure of gas buffer is double the pressure of ideal gas adsorption equilibrium, the valve of the gas cylinder was closed. The valve, which connected the gas cylinder and the gas buffer, was kept open for more than 1.2 hours to obtain absorption equilibrium. (4) Measurement of gas desorption of coal sample. The vacuum bag was connected with the desorption apparatus. The valve connecting the gas cylinder and the gas buffer was then closed. The valve of the vacuum bag was opened to collect the free gas in the coal tank. When the gauge pressure of the coal tank was zero the valve of the vacuum bag was closed. The valve of the desorption apparatus was opened. The value of desorbed gas volume in the desorption apparatus was recorded at a certain time frequency. (5) Data processing. To compare the gas desorption characteristics of different coal samples, the standard volume of the desorbed gas should be calculated by:

Qt' =

273.2 ( Patm − 9.81hw − PS )Qt'' 101325(273.2 + t w )

(1)

3. Results 3.1. Effect of damage extent on gas desorption Two types of coals (i.e. anthracite and meager lean coal) were used to investigate the effect of damage extent on the rate of gas desorption. The rate curves of five 3~1mm coal samples under an absorption equilibrium pressure of 2.5MPa and a desorption temperature of 30±1℃ were shown in Figure 2. Q(ml/g.r/min)

6 Yian soft coal

5

Yian hard coal 4

Jiulishan soft coal

3

Jiulishan hard coal

2

Zhaogu coal with primary structure

1 0 0

2

4

6

8

10

12 t(min)

Fig.2. Curves of gas desorption rate for coal samples with different damage extent

It can be seen from Figure 2 that the initial gas-desorption rate of deformed coal is several times higher than that of coal with primary structure. The gas-desorption rate of deformed coal was related to its damage extent. Additionally, for soft coal and hard coal from the same coal seam, the initial rate of the

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former was much higher than that of the later. The initial rates of the two were both higher than that of the coal with primary structure. 3.2. Effect of particle size on gas desorption

Qt(ml/g.r)

Illustrated in Figure 3 is the variation of desorbed gas volumes of 0.2~0.5mm, 0.5~1mm and 1~3mm coals from Yian mine at a 1.0MPa absorption equilibrium pressure. As shown there, the desorbed gas volumes of the three coals increase gradually with time. The upper limit of the volume is equal to the value of absorbed gas volume of per gram coal. Furthermore, a decrease in coal particle size leads to an increase in desorbed gas volume. 10 8 6 4

0.2~0.5mm 0.5~1mm

2

1~3mm

0 0

20

40

60

80

100

120

140 t(min)

Fig.3. Desorbed gas volume versus time under different sizes

3.3. Effect of adsorption equilibrium pressure on gas desorption

Qt(ml/g.r)

The variation of adsorbed gas volume of 1~3mm deformed coal from Yian mine under a 30±1℃ water temperature and different adsorption equilibrium pressures was illustrated in Figure 4. Obviously, the desorbed gas volumes under different pressures increase gradually with time. The upper limit of the volume is equal to desorbed gas volume of per gram coal exposed to air. Additionally, the desorbed gas volume increases with increasing the adsorption equilibrium pressure. 16 14 12 10 8 6 4

0.5MPa 2.5MPa

2

1.0MPa 4.0MPa

0 0

20

40

60

80

100

120

Fig.4. Desorbed gas volume versus time under different adsorption equilibrium pressures

4. Discussion

140 t(min)

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The gas desorption of deformed coal is influenced by many factors. These factors can be categorized into two types. One is the physical-chemical properties of deformed coal, such as metamorphic grade, adsorption capacity, damage type, internal moisture, and particle size, etc. The other is external factors, which include gas adsorption equilibrium pressure and environmental temperature, etc [3]. The influences of damage type, particle size, and adsorption equilibrium pressure were investigated in the present work. (1) Initial rate of gas desorption was very high, and then decreased sharply. The volume of desorbed gas within the first one minute was high. The rate curve tended to be mild three minutes later. Therefore, the indexes, such as K1 value and ⊿h2 value, which characterize the gas desorption of coal mining fell within the first several minutes, are extensively used. The rate increased with an increase in damage extent. Compared to the coal with primary structure and the coal having small damage extent, the deformed coal has more cracks and pores, and thus larger specific surface area. This leads to higher absorption capability. Obviously, the rate of gas desorption is influenced by the specific surface area. The strength of deformed coal is relatively low. Both the gasdesorption rate and the desorbed-gas volume were, therefore, far higher than that of the coal with primary structure. (2) The displacement of desorbed gas and hindrance force acting on the gas were influenced by coal particle size. Smaller particle size leads to shorter gas displacement and higher gas desorption rate [4]. The initial rate of gas desorption decreases with an increase in particle size. Extreme particle size was proposed by Yang [5]. Extreme particle size is related to physical-chemical properties of coal, but irrelevant to damage extent. When coal particle size is larger than extreme particle size, the coal is divided into lots of fine particles by the cracks and large pores. The hindrance force of pores within the coal having extreme particle size is regarded to be constant. Moreover, the hindrance of the cracks and large pores is far lower than that of all of the coals with extreme particle size. Hence, when the coal size was up to a certain value (extreme particle size), the initial rate of gas desorption did not decrease anymore. (3) The gas-absorption equilibrium pressure characterizes the gas content of coal. The higher pressure indicates higher coal gas content. The gas is desorbed because of the pressure. The amount of absorbed gas varies with the pressure. When the pressure was relatively low the amount was directly proportional to the pressure. When the pressure was relatively high the amount hardly increased. Because of the reversibility of the desorption process, the pressure is one of key parameters which influence the amount of desorbed gas. 5. Conclusions The gas desorption law of deformed coal was experimentally investigated. The following conclusions can be made:  The damage type of deformed coals effects the gas desorption speed. For soft and hard stratified coal samples collected from the same seam, the gas desorption early speed of soft coal is great higher than that of hard coal under the same temperature, grain size and balance pressure. The greater the damage of deformed coals, the higher the gas desorption early speed is. When discharging the pressure, desorption speed decreased quickly.  The factors which effect the gas desorption of deformed coal were analyzed, especially the damage type, particle size and absorption equilibrium pressure.  To improve the prediction accuracy of the outburst dangerous, the gas desorption characteristics of deformed coal and coal with primary structure should be further investigated.

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Acknowledgements This work was financially supported by National Natural Science Foundation of China (Nos. 51074067, 50904024), Foundation and Advanced Research Program of Henan Province (No. 102300413220), Natural Science Research Program of Education Department of Henan Province (No. 2011B440004) and Youth Foundation of Henan Polytechnic University (No. Q2010-47a). References [1] Jiaozuo Mining College. Introduction to gas geology. Beijing: China Coal Industry Publishing House; 1990. (in Chinese) [2] Hao JS. The applying of fuzzy network techniques in prediction of coal and gas outbursts. Journal of China Coal Society 1999; 24: 624–27. (in Chinese) [3] Wen ZH. Experimental study on gas desorption laws of tectonically coal (master's degree dissertation). Jiaozuo: Henan Polytechnic University; 2008. (in Chinese) [4] Li XY. Characteristics and effecting factors of adsorption time of coalbed gas reservoir. Natural Gas Geoscience 2003; 14: 502–5. (in Chinese) [5] Yang QL. Experimental study of gas desorption law of coal. Safety in Coal Mines 1987; 18: 9–16. (in Chinese)