Experimental investigation on variation of physical properties of coal samples subjected to microwave irradiation

    Experimental investigation on variation of physical properties of coal particles subjected to microwave irradiation Guozhong Hu, Nan Yang, Guang Xu, Jialin Xu PII: DOI: Reference:

S0926-9851(17)30345-2 doi:10.1016/j.jappgeo.2017.12.011 APPGEO 3391

To appear in:

Journal of Applied Geophysics

Received date: Revised date: Accepted date:

6 April 2017 23 November 2017 8 December 2017

Please cite this article as: Hu, Guozhong, Yang, Nan, Xu, Guang, Xu, Jialin, Experimental investigation on variation of physical properties of coal particles subjected to microwave irradiation, Journal of Applied Geophysics (2017), doi:10.1016/j.jappgeo.2017.12.011

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ACCEPTED MANUSCRIPT Experimental investigation on variation of physical properties of coal particles subjected to microwave irradiation

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Guozhong Hua, b，Nan Yanga，Guang Xuc，Jialin Xua, b, * School of Mines, Key Laboratory of Deep Coal Resource Mining, Ministry of Education of China, China University of

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Mining and Technology, Xuzhou, China

State Key Laboratory of Coal Resources and Safe Mining, China University of Mining and Technology, Xuzhou, China

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Western Australian School of Mines, Curtin university, Kalgoorlie, WA, Australia

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b

Abstract: The gas drainage rate of low-permeability coal seam is generally low. This leads to the gas disaster of coal mine, and largely restricts the extraction of coalbed methane (CBM), and increases the

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emission of greenhouse gases in the mining area. Consequently, enhancing the gas drainage rate for a coal seam is an urgent challenge to be solved from the viewpoint of the exploitation of CBM. To solve

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this problem, the new approach of using microwave irradiation (MWR) as a non-contact physical field excitation method to enhance gas drainage from a coal seam has been attempted. In order to evaluate the feasibility of this method, the methane adsorption, diffusion and penetrability of coal subjected to MWR was experimentally investigated in this study. Thus, the variation of methane adsorbed amount,

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methane diffusion speed and absorption loop for the coal sample before and after MWR were obtained.

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The findings show that the MWR can change the adsorption property of coal, and reduce the methane adsorption capacity of coal. Moreover, the irradiated coal samples and that original coal samples have

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the methane diffusion characteristic curves with a same trend. The irradiated coal samples have better methane diffusion ability than the original samples. As the adsorbed amount of methane for the coal samples decreased, the sample subjected to MWR had increased or equal methane diffusion speed. Furthermore, compared to the original coal samples, the area of absorption loop for those irradiated

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samples increases, especially for the micro-pore and medium-pore stage. This leads to the increase of proportion of open pores for coal sample, thus improving the gas penetrability of coal sample. From the above, this indicates the modification effect of the MWR on coal, causing pore structure variation in coal particles, thereby changing the methane adsorption, the methane diffusion and the gas penetrability properties of coal particles. Keywords: Microwave irradiation; coal particles; adsorption; diffusion; gas penetrability; coalbed methane

1 Introduction The gas in a coal seam is also called coalbed methane (CBM), and it is a clean and efficient energy source. It can be used as an alternative energy source for coal, petroleum, and natural gas (Karacan et al., 2011). However, the main component of coal seam gas is methane, disasters such as gas explosion and coal and gas outburst can be caused during coal mining and seriously affect the safety of coal mines. In addition, methane is also a potent greenhouse gas (Bibler et al., 1998; Saghafi *Corresponding author at: State Key Laboratory of Coal Resources and Safe Mining, China University of Mining and Technology, Xuzhou, Jiangsu 221116, China. Tel.: +86 516 8388 5581. E-mail address: [email protected] (G. Hu), [email protected] (J. Xu ).

ACCEPTED MANUSCRIPT et al., 1997) and is emitted into the atmosphere during coal mining, causing wastage of resources and pollution of the atmospheric environment. Thus, the basic approach to solve these problems is to develop and utilize the gas in coal seams.

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Pre-drainage of coal seam gas is one of the efficient ways to prevent gas disasters in coal mines, drain gas from coal seams, and reduce emission of greenhouse gas from mining area (Hu et al., 2015a;

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Hu et al., 2009; Hu et al., 2012; Peng, 2007; Hu, 2000; Palchik, 2002). The basic idea is to drain gas

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before coal mining for exploiting and utilizing gas as an energy source and guarantying safety in coal mining. This technology is suitable for application in the coal mine of a coal seam with high gas permeability (Palchik, 2002). However, in the coal mine of a low gas-permeability coal seam, the low drainage rate of gas (Peng, 2007) severely constrains the implementation of pre-drainage of gas, greatly

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restricts the exploitation of gas, and increases the emission of greenhouse gases. There are mainly two reasons for the low drainage rate of gas in a coal bed: first, low gas permeability in the coal seam (for example, 72% of coal seams in China are low-permeability coal seam (Hu, 2000), which makes pre-

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drainage of gas a challenge; second, coal contains pores and fissures, which make it a kind of porous fractured medium. Large amount of gas is absorbed and adhered to micropores in coal (80–90% of gas in original coal seams is absorbed on the surface of micropores and internality of micrograins (Palchik,

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2002), and it is difficult to desorb during gas drainage from a coal seam. It follows that to increase the

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drainage rate of gas in coal seam, we need to find out how to increase the gas permeability of coal seams, promote the desorption of absorbed gas in micropores in coal and accelerate the diffusion of

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free gas. This problem had become a universal challenge for scholars working in the field of the safety of coal mines. Hence, scholars worldwide conducted extensive research on improving the drainage rate of gas from coal seams, and developed many technologies (Lu et al., 2010; Kovaleva and Solov'eva, 2006; Baran et al., 2014; Busch and Gensterblum, 2011; Mazumder and Wolf, 2008; Huang et al.,

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2011; Gentzis and Bolen, 2008; Yi et al., 2005; Yuan et al., 2012; Jiang et al., 2010; Hou, 2007; Yan et al., 2015) such as presplitting blasting, excavation decompression, building holes and increasing fissures, hydraulic fracturing and hydraulic slotting, physicochemical process, replacing desorption (injecting gas to expel the original gas), heat injection into coal seams, approaches using bacteria (microbe), and physical field excitation technology. Owing to the different principles of these technologies and the occurrence conditions of coal seams in different mining area, these methods have different scopes of application, and some methods are not used widely. Accordingly, based on the physical characteristics of coal and MWRs (the electromagnetic field in the frequency range of 300 MHz to 300 GHz) in terms of the overall and selective heating characteristics of coal, the author proposed a non-contact physical field enhancing method using microwave irradiation (MWR) to improve the gas drainage rate of coal seam. Due to the natural coal that contains gas is a typical dielectric (Wang et al., 2004), the method of MWR is an approach of using the absorption of microwave radiation of a controllable source (artificially controlled field source) by coal to promoting gas desorption, diffusion and flowing. Hence, for evaluating the feasibility of this method, an experiment was conducted on the physical properties of

ACCEPTED MANUSCRIPT a coal sample subjected to MWR, thereby determining the influence of MWR on capacity of desorption, diffusion and penetrability of gas in coal.

2 Experimental

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2.1 Preparation of coal sample

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The coal samples for this experiment were made of meagre coal with high grade of metamorphism

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obtained from the Yangquan mining area in China and of lean coal from the Tianhong coal mine in Chongqing of China. The coal samples were prepared by peeling onsite fresh coal samples, and then grinding, sifting, and so on, to achieve a particle size of 60 to 80 meshes. The industrial analysis of the

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experimental coal samples was conducted as presented in Table 1.

Table 1 The industrial analysis of coal samples Mad/%

XY

1.38

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Coal sample number

TH

1.19

2.2 Experimental Method

Vad/%

6.17

10.4

16.92

13.12

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2.2.1 procedure

Aad/%

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To investigate the influence of the MWR on physical properties of the coal, an MWR experiment for coal was designed, as shown in Fig. 1. In this experiment, the industrial analysis, methane

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adsorption, methane diffusion and gas penetrability of coal were tested before and after MWR. For realizing the process of MWR to the coal samples, the container with as-prepared original coal sample was placed in the microwave oven at a set values, as shown in Table 2.

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Microwave oven

Container Original coal sample

Irradiated coal sample

Coal samples Physical properties tests

Methane adsorption and diffusion test

Revolving board

Microwave irradiation

Indirect gas penetrability test Fig.1. Procedure and apparatus of microwave irradiation

Physical properties tests

Methane adsorption and diffusion test

Indirect gas penetrability test

ACCEPTED MANUSCRIPT Table 2 The experimental parameters of microwave Coal sample

Output frequency / GHz

Irradiation time /s

Power /kW

XY

2.45

150, 250

0.7, 1.0

TH

2.45

200, 250

0.7, 1.0

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number

2.2.2 Methane adsorption and diffusion test

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In order to study these properties after irradiation, these coal samples irradiated in the MWR were placed in the absorption-diffusion experimental setup (as shown in Fig. 2), to derive the relation of gas adsorptive capacity and adsorption pressure for the original coal samples and those of the irradiated samples, and to derive the formula for the difference in gas diffusion speed and diffusion time for the

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original coal samples and those of the irradiated samples.

First, an isothermal adsorption experiment was conducted for the coal sample by adopting the

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volumetric method. After adsorption, the adsorption valve was closed. The gas desorption instrument and adsorption tank were connected, the adsorption valve was again opened to conduct the experiment on gas desorption and diffusion. Start measuring the quantity of methane diffusion from the coal

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sample in the adsorption tank when the pressure in adsorption tank reduced to zero.

Gasing tank

Pressure gauge Valve Valve Valve

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Valve

High pressure gas distributing system

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Gas container

Valve Graduated cylinder

Valve

Pressure gauge Vacuometer Adsorption tank

Vacuum pump

Thermostatic system

Fig.2. Experimental setup to study methane diffusion of the coal sample

2.2.3 Indirect gas penetrability test In general, the gas penetrability of coal depends on the development of permeable pores in the coal mass. However, natural coal seam fractures often remain closed owing to the impact of ground stress. Thus, adsorption pores in coal form the main space of the gas flow and the shapes of the adsorption pores dominate the gas flow in the adsorption pores. Consequently, the gas penetrability of coal seam is indirectly determined by the development of adsorption pores (Zhang et al, 1993). That is, the influence of MWR to the gas penetrability of coal can be revealed by analyzing the pore structure characteristics of coal samples before and after MWR. In this experiment, the pore structure features of coal samples were tested by the BELSORP-max pore tester.

ACCEPTED MANUSCRIPT 3 Results 3.1 The variation of adsorption property for coal Based on the experimental method and procedure of volumetric method, the adsorption isotherm

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Original sample-XY

Sample-XY subjected to MWR (Power: 700W)

25.5

Sample-XY subjected to MWR (Power: 1000W)

22.5

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19.5

16.5 13.5 10.5 7.5

4.5 1

2 3 Balanced pressure/MPa

4

5

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0

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Adsorbed amount of methane/cm3.g-1

28.5

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curves of coal samples XY and TH before and after MWR are obtained as shown in Fig.3.

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(a) WA coal sample

Sample-TH Sample-TH subjected to MWR (Power: 700W)

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Adsorbed amount of methane/cm3.g-1

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Sample-TH subjected to MWR (Power: 1000W)

14

12 10

8 6

4 2 0

1

2 3 Balanced pressure/MPa

4

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(b) TH coal sample Fig.3 Absorption isotherm curves of coal samples before and after MWR for 250 s

It can be seen from Fig. 3 that the methane adsorption isotherm curve of each irradiated sample shows the trend of growing rapidly at first, and then leveling off, which conforms to the Langmuir adsorption model. The correlation coefficients of adsorption model fitting are all above 0.98. Compared with the original coal samples, the adsorbed amount of methane for the irradiated coal sample are significantly reduced. Furthermore, the reduction of absorbed amount of methane in these irradiated coal samples is not obvious in the low adsorption pressure phase. In the high absorption pressure phase,

ACCEPTED MANUSCRIPT the absorbed amount of methane for these coal samples subjected to MWR decreases to a greater extent. In addition, compared with these original coal samples, with the increase in the power of the MWR, the absorbed amount of methane on these irradiated coal samples decreases gradually; but this decrease is

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greater in the case of coal sample XY than in the case of coal sample TH. Further, the coal sample XY is more sensitive to the power of the MWR than coal sample TH. Thus it can be seen, after these coal

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samples are subjected to MWR, the methane adsorption capacity of the coal samples decreases to some

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extent. 3.2 The variation of diffusion property for coal

To study the influence of MWR on the methane desorption and diffusion properties of coal, the

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irradiated coal sample was placed in the methane adsorption-desorption system to carry out the adsorption and diffusion tests. These tests were accomplished at an experimental temperature of 30 °C and with 1 MPa methane gas pressure. In addition, the coal samples were subjected to MWR at 700 W

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power for 250 s. The methane desorption and diffusion properties of irradiated coal samples were compared with those of original samples, as illustrated in Fig. 4 and 5, respectively.

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6.2

5.6

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5.8

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Desorption/cm3.g-1

6

5.4

Original sample-XY Sample-XY subjected to MWR

5.2

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5

0

1

2

3

4

5 6 Time/min

7

8

9

10

(a) desorption and time curve 0.3 Diffusion speed/cm3.(g.min)-1

Original sample-XY 0.25

Sample-XY subjected to MWR

0.2 0.15 0.1 0.05

0 0

1

2

3

4 5 Time/min

6

7

8

9

(b) diffusion speed and time curve Fig.4 Methane diffusion property curve of coal sample XY

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5.5

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5.1

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4.9 Original sample-TH

4.7

Sample-TH subjected to MWR 4.5 4.3

0

1

2

3

4

5 6 Time/min

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Desorption/cm3.g-1

5.3

7

8

9

10

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(a) desorption and time curve

Original sample-TH

0.35

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Sample-TH subjected to MWR

0.3 0.25 0.2

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0.15 0.1

0.05 0

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0

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Diffusion speed/cm3.(g.min)-1

0.4

1

2

3 Time/min

4

5

6

(b) diffusion speed and time curve

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Fig.5 Methane diffusion property curve of coal sample TH

The methane diffusion property curves of the irradiated coal samples are basically the same as those of the original coal samples. Under the same absorption pressure, the diffusion amount and adsorbed amount of methane for the irradiated coal sample decreased obviously, mainly because the absorption ability of coal samples was reduced after MWR. After diffusion for 1 min until desorption balance was attained, the methane diffusion speed of the irradiated coal sample was greater than or remained roughly the same as that of the original coal sample. Compared to the original coal sample XY, the methane diffusion speed of the irradiated coal sample XY increased slightly in the initial stage of desorption. Then, with the increase of the desorption time, the methane diffusion speed of the irradiated coal sample XY was maintained at the same level initially and subsequently increased as compared that of the original coal sample XY. In the case of the coal sample TH, the methane diffusion speed of the irradiated sample was clearly greater than that of the original sample in the initial stage of desorption. Then, with the increase in the desorption time, the methane diffusion speed of the irradiated sample gradually attained the same level as that of the original sample. Thus, the methane diffusion capacity of the irradiated coal sample is obviously greater than that of an original sample. Moreover,

ACCEPTED MANUSCRIPT the coal sample TH, having a higher increment of methane diffusion speed, is more sensitive to the MWR than coal sample XY. From the above mentioned, it is shown that the diffusion properties of irradiated coal samples are

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different from those of original samples. For the irradiated samples, the methane adsorption ability is lower, and the methane diffusion ability is enhanced, indicating that the MWR influenced the coal. This

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influence is mainly manifested as two aspects. First, the MWR changed the methane adsorption

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property of coal, decreasing the absorbed amount of methane on coal; the reduction in the absorbed amount of methane led to a reduction in the diffusion amount of methane in this coal sample. Second, although there is a decrease in adsorbed amount of methane, the diffusion speed of methane for the irradiated coal sample slightly increased or was maintained at the same level, indicating that the

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methane diffusion ability of the irradiated coal sample increased to some extent. 3.3 The variation of gas penetrability property for coal

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Low-temperature nitrogen adsorption is a commonly used method for studying the porosity characteristics of a coal mass. In this process, the adsorption and desorption branches of coal sorption isotherms overlap or become separated. In case the adsorption isotherm separates, the adsorption loop

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is formed (Chen et al, 2001). The pores in the coal mass can be divided into three types based on their shapes and whether an adsorption loop is formed. Type I is the impermeable pore with one end closed;

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this pore cannot generate an adsorption loop. The gas penetrability of coal with a large proportion of Type I pores is comparatively poor. Type II is the open permeable pore, which can generate an

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adsorption loop. The coal mass with a large proportion of Type II pores exhibits good gas penetrability. Type III is the flask-shaped pore, which can generate an adsorption loop; however, its desorption curve has a sharp decline point (Zhang, 2005). Much of research (Zhang et al, 1993) has shown that coal samples with poor gas penetrability

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cannot generate adsorption loops or their adsorption loops area are small; however, coal samples with good gas penetrability can generate adsorption loops with a greater adsorption loop area. Accordingly, the gas penetrability of the coal samples can be evaluated based on their adsorption loops. By analyzing the experimental results of low-temperature nitrogen adsorption, the adsorption isotherms of coal samples XY before and after MWR may be obtained, respectively, as shown in Fig. 6. From the Fig. 6, we can know that there exists a small-area adsorption loop in the medium relative pressure section of the adsorption isotherm for original coal sample XY. It shows that the pore structure of the original coal sample XY is mostly composed of closed pores, which cause greater resistance to gas flow in coal seam with poor gas penetrability. However, after MWR, the adsorption loop area of the coal sample XY increases, especially in small and medium pore-size sections. This is consistent with the pore-volume ratio change of each pore size of the coal samples (Hu et al, 2015). As a result, the proportion of the closed pores of the coal sample XY after MWR decreases moderately, while that of the open pores increases slightly. Thereby the gas flow resistance in the coal sample is reduced, and the gas penetrability of the coal sample is improved.

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2 ADS DES

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Va/cm3(STP) g-1

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1.5

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1

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0.5

0 0

0.2

0.4

0.6

0.8

1

p/p0

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(a) original coal sample 2

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D

DES

1

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Va/cm3(STP) g-1

1.5

ADS

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0.5

0

0

0.2

0.4

0.6

0.8

1

p/p0

(b) coal sample subjected to MWR at 1 kW for 150 s Fig. 6 The adsorption isotherm for coal samples XY

4 Discussion The MWR is the electromagnetic physical field formed by electromagnetic waves within the frequency range of 300 MHz to 300 GHz (Cui and Lin, 2005). Under the effect of this field, coal, which is a dielectric, absorbed part of the microwave energy, leading to its dielectric loss; that is, the microwave energy was converted to heat energy in coal. This increased the temperature of coal, leading to a series of changes in internal medium of coal. This resulted in the modification effects which are mainly characterized by the following two aspects: In the first place, the integral heating property of the MWR caused the interior and exterior media of coal to be heated simultaneously and caused the evaporation of water on the coal surface because of the high temperature; thus, a temperature gradient distribution with a hotter interior as compared to the

ACCEPTED MANUSCRIPT surface was formed. This resulted in the transfer and evaporation of moisture in coal and eliminated part of the pore water of coal (Fig. 7). This decreased the saturation of the pore water phase of coal, indirectly improving the gas permeation volume of coal pores, enhancing methane diffusion and gas

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penetrability in coal.

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1.5

Sample XY Sample TH

1.45

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1.35 1.3

1.25 1.2

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moisture content/%

1.4

1.15

1.1

1

0

50

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1.05

100 150 irradiation time/s

200

250

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Fig. 7 Moisture content of coal samples before and after MWR at 1 kW power

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There is one more point, the absorption of microwave energy in material is proportional to the electrical conductivity of the material. Because coal comprises a variety of minerals and elements, and

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the electrical conductivities of various minerals in coal are different (Cui and Lin, 2005), there are differences in the absorption abilities of microwave energy for the constituent minerals. This causes differences in the warming efficiency of various minerals in the internal coal medium, and thus produces thermal stress between the different minerals in coal. This thermal stress can lead to the

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deformation of the frame of coal, changing the pore structure of coal, as shown in Table 3. It is shown from Table 3 and Fig. 5, 6 that the specific surface area, pore size and pore proportion of the irradiated coal sample changed. Generally, this shows a decrease in the specific surface area decreases, an increase in the average pore diameter, an increase in the proportion of open pores. Further, the changes in the pore structure lead to the reduction in methane absorption ability, the enhancement in coal methane diffusion ability, and the improvement in the gas penetrability of coal.

Table 3 The change of pore structure of coal samples subjected to MWR at 1 kW Power for 250s (Hu et al, 2015) Coal sample

Average pore diameter (nm)

Specific surface area (m2.g-1)

Sample subjected to

Original

MWR

sample

7.7944

7.8407

3.9797

1.6696

7.9791

7.9904

1.6815

1.5883

number

Original sample

XY TH

Sample subjected to MWR

ACCEPTED MANUSCRIPT Consequently, the main reason on the variation of physical properties for irradiated coal samples is that the MWR causes multiple variations in the internal medium of coal, thus changing the pore structure of coal. This change results in increases of the pore connectivity of the coal, increasing the

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effective flow channel of the coal, as shown in Fig. 8.

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Before MWR

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after MWR

(a) coal samples XY

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Before MWR

after MWR

(b) coal samples TH Fig. 8 Surface structure of coal sample before and after MWR for 250 s

ACCEPTED MANUSCRIPT 5 Conclusion This paper elaborates the influences of MWR on the adsorption, diffusion and penetrability properties of coal. Adsorption, diffusion and penetrability tests were conducted on various coal samples

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before and after MWR. The variation law of the methane adsorbed amount, methane diffusion speed

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and gas penetrability for coal samples under different MWR conditions was obtained. The experimental results show that the variation trend of the isothermal adsorption curves of the irradiated coal sample

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has no obvious changes, and is in agreement with the Langmuir adsorption model. With the increase in MWR power, the reduction in adsorbed amount of methane for the irradiated coal sample increased, indicating that the MWR changed the adsorption property of coal. Moreover, the methane diffusion property curve of the irradiated coal sample showed no significant changes. When the adsorbed amount

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of methane in the irradiated coal sample decreased, the diffusion speed of methane in the irradiated coal samples increased or remained the same level, indicating that the methane diffusion capacity of

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irradiated coal samples increased to some extent. Furthermore, the proportion of open pores in the irradiated coal sample increased，indicating that the gas penetrability property of the irradiated coal samples was improved. Consequently, under the action of the MWR, the coal absorbs microwave

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energy, leading to dielectric loss, which heats the coal and cause temperature rise. Owing to the influence of integral heating and the selective heating property of the MWR, the irradiated coal

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undergoes variable changes, leading to lower specific surface area, higher average pore diameter and higher proportion of open pores for coal. This improves pore connectivity of coal, increases the

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effective flow channel of coal, and decreases the gas permeation resistance of coal, resulting in the reduction of the methane adsorption capacity and improvement of the methane diffusion ability and gas penetrability for coal.

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Acknowledgements

This work was supported by the Fundamental Research Funds for the Central Universities (Grant No. 2015XKZD04) and a Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (Grant No. SZBF2011-6-B35).

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ACCEPTED MANUSCRIPT Highlights: 1. The new approach of using microwave irradiation to enhance CBM drainage was proposed. 2. Variation of physical properties of coal subjected to microwave irradiation was investigated. 3. The microwave irradiation can reduce the methane adsorption capacity of coal.

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4. The microwave irradiation can improve the gas diffusion ability and penetrability of coal.

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CE P

TE

D

MA

NU

SC R

IP

5. The integral and selective heating of microwave irradiation led to the variation of coal.