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Influence of soluble organic matter on mechanical properties of coal and occurrence of coal and gas outburst
Powder Technology 332 (2018) 8–17
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Influence of soluble organic matter on mechanical properties of coal and occurrence of coal and gas outburst Yongliang Yang ⁎, Jiaji Sun, Zenghua Li, Jinhu Li, Xiaoyan Zhang, Liwei Liu, Daocheng Yan, Yinbo Zhou Key Laboratory of Coal Methane and Fire Control, Ministry of Education, China University of Mining and Technology, Xuzhou 221008, China School of Safety Engineering, China University of Mining and Technology, Xuzhou 221116, China
a r t i c l e
i n f o
Article history: Received 5 July 2017 Received in revised form 11 January 2018 Accepted 19 March 2018 Available online 21 March 2018 Keywords: Soluble organic matters Coal pore structure Adsorption characteristics Mechanical parameters Coal and gas outburst
a b s t r a c t Coal is a complex organic rock. A large number of soluble organic matters (SOMs) existing in pores of the coal matrix and even in the macromolecular network structure will inevitably have some impact on the coal seam gas occurrence, migration and outburst risk. From the perspective of structure and composition of coal, a series of experiments were carried out to reveal the mechanism of SOMs on the gas outbursts. The results show that SOMs can improve the coal adsorption ability due to the strongly adsorbent groups. Compared with the raw coal, the SOMs-extracted residual coal undergoes a decrease in specific surface area and an increase in total pore volume, average pore diameter and porosity. Furthermore, compressive strength, tensile strength and elastic modulus of the residual coal are all higher than that of the raw coal, while gas content, gas emission initial velocity, drill cuttings desorption indexes (K1 and Δh2) of the residual coal are all lower than that of the raw coal. In summary, the SOMs in coal can promote coal and gas outbursts. Research results of the project provide a new way of promoting gas extraction and outburst prediction. © 2017 Published by Elsevier B.V.
1. Introduction Coal and gas outbursts are the most serious mine gas disaster. To reduce and prevent coal and gas outburst disasters, many scholars hold that an outburst results from the combination of three factors, namely, ground stress, gas pressure and coal structure properties. They have also conducted numerous researches on outburst sensitive indexes and outburst prediction models, concluding gas pressure, gas content, initial velocity of gas emission, consistency coefficient of coal, failure mode of coal, cuttings index, etc. [1–2]. However, in recent years, many outburst accidents took place before the outburst indexes had reached critical values [3–4], so the outburst mechanism and its influencing factors still need further study. In the 1980s, some scholars found that, in a similar occurrence conditions, the higher the content of heavy hydrocarbons C2-C8 (i.e. C2-C8 hydrocarbons), the greater the outburst danger [5–6]. Hydrocarbons are widely distributed from C1 to C30, and SOMs which are composed of aliphatic hydrocarbons, aromatic hydrocarbons and heteroatom compounds containing heavy hydrocarbons C2-C8. However, the influence of SOMs on outburst is rarely reported. In addition to a macromolecular solid skeleton, coal, a kind of complex organic rock, contains numerous small organic compounds. Existing in pores of the coal matrix and even in the macromolecular ⁎ Corresponding author. E-mail address: [email protected] (Y. Yang).
https://doi.org/10.1016/j.powtec.2018.03.053 0032-5910/© 2017 Published by Elsevier B.V.
network structure, these small soluble organic compounds account for about 10%–23% of organic matters in coal and even for as high as 30% in some coal [7–9]. Therefore, it is evident that SOMs in coal is a component that cannot be ignored, so it is bound to have a certain influence on coal seam gas storage, transportation and gas outburst hazards. SOMs are free or embedded in the coal polymer network structure with hydrogen bonds and Van der Waals forces. In the past, studies on SOMs in coal were mainly concentrated on industries such as coal chemical industry and coal coking industry and focused on the occurrence forms, compositions and coal chemical structures of SOMs in coal [10–12]. Only few scholars made preliminary discussions and qualitative analyses on the effect of SOMs in coal on outburst hazards. For example, Zhang [13] conducted a SOMs geochemical study on Nantong coal samples and found that the higher the SOMs content in the coal seam, the greater the outburst risk. Zhang et al. [14] determined that a large amount of soluble low-molecular compounds were formed and preserved during the formation of tectonic coal, and they pointed out that the chloroform extraction rate of coal could be used as an index to predict coal and gas outburst. In a study of the mechanism of sudden methane release, Zhang et al. [15] noted that the porous structure of coal colloid contains molecule pores that are very small in diameter and able to prevent the escape of methane, which results in an uneven gas flow within the coal seam and thus conduces to the emergence of coal and gas outburst. From the perspective of structure and material composition of coal, this paper tested the mechanical strength of raw coal and residual
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coal, the desorption index of cutting gas and the characteristics of gas adsorption and emission. On this basis, it analysed the influence of SOMs on outburst indexes and further revealed the role that SOMs play in the mechanism of coal and gas outburst risk by combining theoretical analysis with experimental simulation and experimental test. 2. Experimental process 2.1. Coal sample preparation Three coal samples of different metamorphic grades, namely, Qingdong coal (QD) from Qingdong No. 8 seam, Zhangji coal (ZJ) from Zhangji No. 9 seam and Tongting coal (TT) from Tongting No. 10 seam, were selected as experimental coal samples. The proximate analysis of each coal sample (Proximate (air-dry-basis)) is listed in Table 1. Of the three, ZJ No. 9 seam and QD No. 8 seam belong to outburst coal seam, while TT No. 10 seam belongs to non-outburst coal seam. In addition, the coal sample before THF treatment is called “raw coal”, while the coal after THF treatment is called “residual coal”. Three coal samples with particle ranging from 60 to 80 meshes (0.180–0.250 mm) were selected. Each of them was divided into two parts, of which one was taken as the raw coal and the other was extracted by tetrahydrofuran (THF). The extraction experiments were conducted via a CW-2008 multi-microwave reaction/extraction device produced by Zeming Tech. Co., Ltd [20]. The 50 g coal sample and 300 ml THF solvent (analytic reagent) were mixed and then put into an extraction vessel. Next, after the connections were checked, the extraction experiment was started at 30 °C and atmospheric pressure. After 4 h, the mixtures were separated by a vacuum filter device. Finally, the separated coal samples were further dried at 80 °C for 12 h in vacuum. In this paper, the coal sample treated by THF was called “residual coal”. Further, the composition of liquid was analysed via an Agilent 6890/5973 GC/MS in the School of Chemical Engineering of Nanjing University. 2.2. Experimental methods Parameters such as coal strength, the drill cuttings desorption indexes and gas content are all important indexes for the evaluation of coal and gas outburst risk. In order to study the effect of SOMs in coal seam on outburst risks, this paper tested the mechanical parameters, drill cuttings desorption (K1 and Δh2) and gas contents (desorbed gas content and residual gas content) of the coal before and after the extraction respectively.
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2.2.2. Test of the drill cuttings desorption indexes The drill cuttings desorption indexes (K1 and Δh2) are commonly used as outburst prediction indexes in China. Their value can reflect the outburst danger. K1 is the total amount of gas desorption for exposure within 1 min per gram of coal sample, which can reflect the gas amount and the attention of desorption rate, thus its unit is mL · (g·min0.5)−1. Δh2 is expressed as the total gas desorption amount at 4th and 5th minutes of the coal sample (10 g) exposed to the air, and the unit is Pa [16]. The index K1 was measured using a WTC-type analyser produced by the Coal Research Institute of Chongqing, and Δh2 was measured using the MD-2-type gas desorption instrument produced by the Coal Research Institute of Fushun. During the simulation of K1 and Δh2 in the laboratory, the samples were loaded into a coal sample tank. After vacuum degassing, the gas was injected into the tank until it reached the preset pressure. Then, loading machine was started at the preset uniaxial force. After the coal samples had adsorbed gas for 4 h, the axial force was unloaded and the tank was opened. Then, the coal samples were rapidly drilled with a small drill to test the drill cuttings desorption indexes. 2.2.3. Test of gas content parameters Pulverized coal with particle diameters ranging from 0.18 to 0.35 mm (40–80 mesh) were selected as experimental samples and then dried and degassed in vacuum at 80 °C for 10 h. 50 g of raw coal and residual coal were respectively put into the coal sample tank and vacuum degassed for 8 h. After the equilibrium pressure was reached, methane was injected into the tank at 30 °C for 8 h. Next, the valve was opened and the gas desorption was measured by a gas desorption device. The measured total desorption amount divided by the coal weight is the desorbed gas content. The measured coal samples were placed in air for 3 h, and the samples before and after pulverization were then vacuum degassed respectively using a vacuum pump. The measured degassing volume divided by the coal weight is the residual gas content. Considering the fact that the SOMs-solubility of methane will inevitably affect the content of coal seam gas, this paper chose the SOMs model compound for the methane dissolution experiment. In this study, the amounts of methane dissolved in model materials under different pressures and temperatures were determined using the gasliquid equilibrium static method. The pressure values before and after the gas filling and the pressure changes before and after the balance in the reactor were measured respectively. The change in volume was calculated according to the gas state equation, and the amounts of methane dissolved under different pressures and temperatures were obtained finally [17]. 3. Results and discussion
2.2.1. Test of mechanical parameters To test the mechanical parameters of raw coal and residual coal, the powder samples were placed in the mould, respectively. Then, the standard briquettes (Φ50 × 100 mm) were suppressed by a loading machine under 300 MPa for 2 h. Further, the briquette samples were used to carry out the uniaxial compression and tensile tests by using a YAW4306 servo loading machine at loading rates of 10 N/s and 3 N/s, respectively.
Table 1 Analysis of basic features of coal samples. Sample number
ZJ No. 9 QD No. 8 TT No. 10
Proximate analysis/% Mad
Ad
Vdaf
1.67 0.56 0.96
22.31 13.85 15.57
33.42 24.19 25.56
Rv, max (%)
Extraction yield (wt%, daf)
0.93 1.37 0.78
2.521 2.130 3.149
Mad = air-dried moisture; Ad = dry ash; daf = dry ash free; Rv, max = maximum vitrinite reflectance; wt = weight.
3.1. Soluble organic matter composition and composition characteristics Fig. 1 shows the chromatograms of THF extracts from various coal samples. As the main components of SOMs (coal extract) in coal, aromatic hydrocarbons are mainly composed of benzene compounds of single or multiple benzene rings [18], and they include naphthalene, phenanthrene, anthracene and alkyl-containing polycyclic aromatic hydrocarbons of even more benzene rings. Aliphatic hydrocarbons are mainly composed of n-alkanes which are mostly C14 to C28 in carbon chain lengths and have odd carbon dominance, and they also include a small number of branched alkanes and cycloalkanes. Heteroatomcontaining compounds are predominantly oxygen-containing compounds, and only a silicon-containing compound was detected in the n-hexane extract of QD coal. The contents of SOMs in the outburst and non-outburst coal were comparatively analysed. It is found that SOMs of the outburst coal contain high contents of methyl groups and methylene groups such as 2,3,6trimethylnaphthalene, 1,4,5,8-tetramethylnaphthalene, heptadecane,
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Fig. 1. Total ion chromatogram (TIC) of THF extract.
octacosane and benzofluoranthene, while SOMs of the non-outburst coal contain more butyl groups and hydroxyl groups such as 2,6-di-tertbutyl-4-(hydroxymethyl)phenol and 2,4-di-t-butylphenol. Some scholars used infrared spectroscopy to test the change in coal body function before and after the extraction [19–20]. By comparing the results of outburst coal and non-outburst coal, it was found that, after extraction, the relative contents of free OH bonds, OH relative self-association hydrogen bonds and CH2 anti-symmetrical stretching vibration and other
characteristic peaks of non-outburst coal dropped significantly, and the relative contents of characteristic peaks such as CH2 symmetrical stretching vibration and CH3 deformation vibration of outburst coal decreased obviously. This is in good agreement with the mass spectrometry test results. Jiang [21] revealed that the effect of different side chain groups on the methane adsorption of the coal structure, results showed that: methylene N methyl N hydroxyl N butyl. Among them, the methane
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Table 2 Results of pore size distribution for raw coal and residual coal. Coal sample
ZJ No. 9 QD No. 8 TT No. 10
Raw coal Residual coal Raw coal Residual coal Raw coal Residual coal
Average pore diameter (nm)
Total intrusion volume (mL/g)
Total pore area (m2/g)
6.81 12.73 9.32 10.19 14.90 16.54
0.0371 0.0587 0.0351 0.0389 0.0430 0.0481
18.679 17.390 15.534 14.338 23.403 21.095
adsorption capacity of methylene groups and methyl groups in coal was higher than that of hydroxyl groups and butyl groups. In this study, SOMs of the outburst coal were found to contain more methylene groups and methyl groups, indicating that SOMs have a certain influence on the outburst risk of coal, Therefore, under the same conditions, the outburst coal shows a larger gas adsorption capacity. 3.2. Pore structure of raw coal and residual coal From the mercury penetration experiments, the calculated pore structure parameters of raw coal and residual coal (total pore volume, specific surface area, average pore diameter and pore size distribution) are shown in Table 2. As can be seen from Table 2, after extraction, the pore structure parameters show a significant change. The SOMs-extracted residual coal undergoes a decrease in specific surface area and an increase in total pore volume, average pore diameter and porosity; and the increase and decrease ranges are consistent with the extraction rate. Although the solvent extraction can produce a good pore increase and expansion effect, it reduces the specific surface area of the coal sample. The higher the extraction rate, the more obvious the reduction of specific surface
The area of the different types of pores/proportion (ml/g)/% Micropores
Small pores
Mesopore
Macropore
16.619/88.97 15.357/88.31 13.743/88.48 13.487/87.09 20.515/87.66 18.072/85.67
2.03/10.87 1.94/11.16 1.76/11.30 1.79/12.48 2.84/12.15 2.94/13.94
0.026/0.14 0.074/0.4 0.03/0.19 0.052/0.36 0.037/0.16 0.065/0.31
0.001/0.01 0.011/0.06 0.004/0.03 0.007/0.05 0.007/0.03 0.017/0.08
area. The results indicate that although the extraction can increase the pore size of coal, it reduces the number of micro pores, resulting in the decrease of specific surface area. It can be seen from the pore size distribution characteristics, both raw coal and residual coal show a similar distribution characteristics: the micropores (b10 nm) take up the highest proportion, followed by small pores (10–100 nm) and then mesopores (100–1000 nm), with macropores (N1000 nm) being the least. However, the solvent extraction can reduce the proportion of micropore and small-pore volumes and raise the proportion of mesopore and macropore volumes. Meanwhile, the higher the extraction rate, the greater the increase ranges of mesopores and macropores. This is because that, after SOMs are dissolved, some micropores can be connected and expanded into larger pores, causing the reduction of micropores.
3.3. Effect of SOMs on the mechanical strength of coal The stress-strain curves of coal samples under uniaxial compression are shown in Fig. 2, and the curves of the time loading tests are shown in Fig. 3.
Fig. 2. Stress-strain curves of raw coal and residual coal under uniaxial compression.
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Fig. 3. Tensile loading curves of raw coal and residual coal.
As can be seen from Fig. 2, the raw coal and residual coal exhibit a similar deformation laws in the whole stress-strain process. The deformation processes of coal samples can be generally divided into 5 phases: initial compression phase, elastic phase, yield phase, peak intensity phase and strain-softening phase [22]. From the initial compression phase to the peak intensity phase, axial stress of the residual coal is higher than that of the raw coal, and peak intensity of the raw coal is smaller than that of the residual coal (see Table 3). This law is reflected more obviously by the outburst coal samples ZJ and QD. Moreover, the strains of outburst raw coal samples at peak intensities are all greater than those of outburst residual coal samples, while the strains of nonoutburst raw coal samples are smaller than those of non-outburst residual coal samples. According to the tensile test results on the coal samples, the time loading curves of the raw coal and residual coal are similar. In addition, the tensile strengths of residual coal samples of different metamorphic grades are all greater than those of raw coal samples. The mechanical parameters are summarized in Table 4. It can be seen from Figs. 2 and 3 and Table 4 that whether for the outburst or for the non-outburst coal, comparing with the raw, the residual coal exhibits the higher compressive strength, tensile strength and elastic modulus. In addition, the strength of outburst residual coal is notably higher than that of outburst raw coal, while the strength of nonoutburst residual coal is slightly higher than that of non-outburst raw coal. The reason is that the extraction of SOMs in coal weakens the Table 3 Typical stress-strain briquettes. Parameters
Peak intensity/MPa Axial strains/%
ZJ
QD
TT
Raw coal
Residual coal
Raw coal
Residual coal
Raw coal
Residual coal
1.77 1.85
3.14 1.46
1.77 1.84
2.61 1.74
1.25 1.82
1.47 1.89
creep ability of coal, so that the pores within the coal matrix tend to be homogeneous and simplified [23–25]. Besides, the surface of extracted residual coal particles is relatively rough due to the dissolution of SOMs, causing that the coal particles in briquette combine more closely, so that the strength of residual coal is generally higher than that of raw coal. According to the results, the elastic limit and yield stress of the raw coal are smaller than those of the residual coal. The smaller yield stress means that ground stress and gas can combine more easily to jointly cause coal and gas outburst [26], which shows that the presence of SOMs is conducive to gas outburst. 3.4. The drill cuttings desorption indexes of raw coal and residual coal As the most commonly used outburst test indexes for Chinese coal mine, the drill cuttings desorption indexes K1 and Δh2 can well reflect the outburst danger of coal seams [2,16]. According to the field test procedures, the simulation tests for raw coal and residual coal are carried out in the laboratory. The test results of K1 and Δh2 are shown in Figs. 4 and 5. From Figs. 4 and 5, K1 and Δh2 of residual coal are less than those of raw coal, indicating that the outburst risk of residual coal is lower than that of raw coal. This also shows that SOMs in coal have a promoting effect on coal and gas outburst, because SOMs in coal can not only occupy Table 4 Comparison of mechanical parameters of raw coal and residual coal. Coal sample
Compressive strength/MPa
Tensile strength/MPa
Elastic modulus/GPa
Raw coal (ZJ) Residual coal (ZJ) Raw coal (TT) Residual coal (TT) Raw coal (QD) Residual coal (QD)
1.75 3.11 1.29 1.44 1.82 2.51
0.108 0.475 0.095 0.345 0.207 0.403
10.57 32.88 8.16 15.01 9.79 16.81
Y. Yang et al. / Powder Technology 332 (2018) 8–17
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Fig. 4. Test results of K1 values of raw coal and residual coal.
part of the pores and affect pore structure characteristics of the coal but also dissolve some methane. After THF extraction, pores in the coal matrix tend to be homogeneous and simplified due to the dissolution of SOMs, leading to the increase of the number of mesopores and macropores in the residual coal grows, while the reduction of the number of micropores drops. Therefore, the gas adsorption capacity is decreased.
3.5. SOMs dissolved gas and its effect on the gas content in coal seams Gas content is one of the significant indexes for evaluating the outburst danger of coal seams. To study the effect of SOMs on gas content, this paper analysed the extraction product composition by taking 0# diesel oil, lubricating oil and coal tar as the model materials to carry out a methane dissolution experiment. The results are listed in Table 5.
Fig. 5. Test results of △h2.
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Table 5 Methane dissolution amounts of model materials under different equilibrium pressures. Experimental sample
Inflation pressure/Mpa
Equilibrium pressure/MPa
Unit mass dissolved amount/ml·g−1
Diesel oil
0.95 2.20 2.97 4.10 5.07 1.06 2.04 3.15 4.25 5.10 1.05 2.10 3.05 4.20 5.00
0.88 1.99 2.75 3.71 4.60 1.03 1.91 2.98 4.01 4.76 0.98 1.97 2.86 3.92 4.56
1.13 2.89 3.34 6.03 7.50 0.47 1.71 2.44 3.80 5.24 0.45 0.82 1.25 1.90 3.12
Lubricating oil
Coal tar
The results show that the amounts of methane dissolved in different model materials were different. Among them, diesel oil accounts for the largest dissolved amount, followed by lubricating oil, with coal tar being the least. It is obtained through analysis that this is related to physical and chemical properties of the model materials. Diesel oil is primarily composed of normal alkane hydrocarbon organic matters whose properties are more similar to those of methane. Thus, according to the principle that the similar substance is more likely to be dissolved by each other, the solubility of methane will be greater than those of coal tar and lubricating oil. A large number of oxygen-containing compounds such as benzene, hydroxyl groups and phenols contained in coal tar contribute to the polarity of coal tar and will inevitably affect the amount of methane dissolved in coal tar. Even though the amount of methane dissolved in coal tar is the least, it is also significantly increased with
the rise of pressure. For example, when the equilibrium pressure is 4.56 MPa, the unit mass dissolved amount is 3.12 cm3/g. The gas adsorption amount, desorbed gas content and residual gas content are important parameters characterizing the coal seam gas content and emission characteristics. In this study, experimental results of desorbed gas contents of raw coal and residual coal under different equilibrium gas pressure conditions are shown in Fig. 6. As can be seen from Fig. 6, the desorbed gas content of raw coal is always higher than that of residual coal under different equilibrium pressures, suggesting that the adsorption capacity of residual coal is smaller than that of raw coal. This is possibly because the pore structure of coal has been changed during the extraction process. As SOMs are dissolved, some of the pores in the raw coal are expanded. Further, some plugging pores and internally connected pores in the coal may even be opened and integrated. This results in the increase of macropores and mesopores as well as the reduction of micropores and the specific surface area of coal, which reduces the gas adsorption capacity [27]. In summary, the SOMs can improve the methane adsorption capacity of coal to a certain extent, which is conducive to the occurrence of gas in coal seam and thus will promote the gas emission during mining. To verify this, isothermal adsorption experiments were carried out on the coal samples before and after extraction. The results are shown in Table 6. The gas adsorption experiment shows that the adsorption capacities of three kinds of raw coal samples are all larger than those of residual coal samples, and the a value of residual coal is smaller than that of raw coal, indicating that the methane adsorption capacity is decreased after small SOMs in coal are dissolved in the THF solution. The b value of residual coal is larger than that of raw coal, showing that the corresponding adsorption pressure of residual coal is lower than that of raw coal when the same adsorption saturation is reached, but the gas adsorption rate of residual coal is higher than that of raw coal under low-pressure conditions.
Fig. 6. Test curves of desorptable gas contents of raw and residual coal.
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4.1. Coal pore structure and gas emission characteristics
Table 6 Adsorption data fitted results for raw coal and residual coal. Parameters
Residual gas content (ml·g−1) Gas-absorbent constant a (cm3/g) Gas-absorbent constant b (MPa−1)
ZJ
QD
TT
Raw coal
Residual coal
Raw coal
Residual coal
Raw coal
Residual coal
2.86
2.45
2.54
2.26
2.29
2.10
24.3689 20.0814
18.2739 16.0210
20.7692 18.6381
0.6341
0.5533
0.5628
0.7083
15
0.5857
0.6532
Table 7 ΔP values of raw coal and residual coal. Coal sample
ZJ
QD
TT
ΔP of raw coal/mm Hg ΔP of residual coal/mm Hg
10.8 8.2
9.1 7.9
5.2 4.1
3.6. Initial speed of gas emission of raw coal and residual coal The initial speeds of gas emission of raw coal and residual coal were tested by a detector. The results are shown in Table 7. The results show that the initial velocity of gas emission of residual coal is lower than that of raw coal. Previous studies have pointed out that the gas adsorption amount of residual coal is decreased. Although the porosity of residual coal is more developed, the drop of adsorbed gas content leads to the decrease in the gas source and the initial velocity of gas emission. In addition, gas can be dissolved in SOMs in coal, resulting in a drop of gas desorption amount of residual coal under high pressure, which also leads to a decrease in the initial velocity of gas emission. ΔP is decreased after the extraction, indicating that the solid solution gas dissolved in the SOMs will be released more quickly when the coal seam is damaged by mining and will thus increase the risk of coal seam outburst.
Jang et al. [28] found that undeveloped coal pores will lead to poor connectivity between fractures and easy accumulation of gas, thus promoting coal and gas outburst accidents. However, it was discovered in this study that the SOMs-extracted residual coal undergoes a decrease in specific surface area and an increase in total pore volume, average pore diameter and porosity. The raw coal micropores that contain a large number of SOMs are more developed and take up a larger specific surface area [29–30], which constitutes the main space and creates favorable conditions for gas occurrence. Wang et al. [31] held that connectivity between fractures was enhanced after the dissolution of SOMs free or embedded in the coal polymer network structure. It can thus be seen that the presence of SOMs leads to poor connectivity between natural exogenous fractures and endogenous fractures, which is not conducive to the migration of coal seam gas. As a result, compared with the residual coal, it is easier for gas to accumulate in the raw coal, which creates a more favorable condition for coal and gas outburst. The gas emissions characteristics are controlled by the pore structure and adsorption ability. For the pore structure, some studies showed that [32], the methane diffusion coefficients of mesopores/macropores are obviously (about 105 times) larger than that of micropores, and the methane diffusion rate in coal is mainly determined by the ratio of mesopores/macropores in coal. For the adsorption ability, some studies showed that, the SOMs can improve coal adsorption ability because the gas can dissolved in the SOMs [24,25]. As we all know, the stronger adsorption ability, the greater emissions ability. In this paper, after THF treatment, the mesopores/macropores is increased, while the adsorption ability is reduced. Therefore, after THF treatment, there is a competitive relationship between pore structure and adsorption ability. Further, as shown in Table 7, the initial velocity of gas emission is decreased after THF treatment, indicating that the initial emission ability is dominated by the adsorption ability. As is known, the stronger the gas emission ability, the greater the risk of coal seam outburst. Therefore, from the viewpoint of gas emission, SOMs can improve the gas emission capacity of coal and increase the risk of outburst.
4. The influencing mechanism of SOMs on coal and gas outburst Pore characteristics, gas energy and strength of coal all directly affect coal and gas outburst danger. Through a large number of experiments and theoretical analyses in the previous section, this study analysed the influence of SOMs on the pore characteristics, gas energy and strength of coal, so as to obtain the influencing mechanism of SOMs on coal and gas outburst, as shown in Fig. 7.
4.2. Gas energy Wang et al. [33] put forward that as gas energy was the main energy source for an outburst, gas content can reflect coal internal energy most directly, and that under the same conditions, the higher the gas content, the greater the risk of outburst. From the aforesaid test results of coal
Fig. 7. Effects of SOMs on gas storage and outburst danger.
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pore structure (see Table 2) and SOMs dissolved gas (see Table 5), it can be obtained that when the SOMs are reduced, the number of gas adsorption groups and the amount of solid solution gas are both reduced, and the gas adsorption capacity of coal is weakened, which is reflected by the decrease in the gas limit adsorption, the rise of the low-pressure gas adsorption growth rate, the drop of the initial amount of adsorption heat, etc. Finally, the reduction of SOMs will lower original coal gas content, desorptable gas content and residual gas content. Hence, it is clear that the presence of SOMs enables coal to adsorb and save more gas within it and thus compresses its storage potential. Meanwhile, high desorptable gas content of the raw coal decides that its gas desorption speed is relatively high, so higher gas pressure is formed in a short time. Correspondingly, the initial velocity of gas emission rises, and thereby coal and gas outburst accidents are more likely to occur [34].
5. Conclusions The mechanical properties of raw coal and residual coal were studied. The results show that the compressive strength, tensile strength and elastic modulus of residual coal obtained through THF extraction are all higher than those of raw coal. In addition, the strength of outburst residual coal is notably higher than that of outburst raw coal, while the strength of non-outburst residual coal is slightly higher than that of non-outburst raw coal. A variety of outburst parameters of raw and residual coal were tested. After the extraction of SOMs, K1 and Δh2 are decreased and the initial velocity of gas emission is reduced. SOMs could dissolve the quantitative methane and finally revealed that SOMs can improve the gas adsorption capacity of coal and promote coal and gas outburst. These results provide a new means for the development of gas drainage and outburst prediction.
4.3. Coal strength The point that the lower the coal strength, the greater the outburst risk has been commonly acknowledged [35–39]. The previous experimental results show that mechanical parameters such as compressive strength and tensile strength of extracted residual coal are all larger than those of the raw coal, indicating that the mechanical strength of coal has been enhanced after the extraction. In order to further analyse the influence of SOMs on the creep law of coal, the uniaxial compression creep strain duration curves of raw coal and residual coal under the same conditions were plotted, as shown in Fig. 8. For reaching the same deformation degree, the residual coal takes more time than the raw coal, and the maximum strain of residual coal is smaller than that of raw coal, suggesting that the residual coal obtained through THF extraction has weakened axial creep ability and thus is more unlikely to fail under the same axial strain. On the contrary, when exposed, the coal seam containing more organic matters may fail under minor deformation. The experimental data measured show that the less the SOMs in coal, the smaller the parameters such as K1 value and Δh2 value, and the lower the initial velocity of gas emission. Combining change trend of gas content, it can be concluded that the presence of SOMs in coal has a certain promoting effect on coal and gas outburst, which is also verified by the difference of test results of outburst coal and non-outburst coal. From the above results, SOMs in coal can promote coal and gas outburst. In a mining area rich in SOMs, SOMs can be reduced by injecting supercritical CO2 into the coal seam. On the one hand, this can reduce the coal seam outburst danger, and on the other, it can increase permeability of the coal seam and improve the effect of gas drainage to ensure the safety of mine production.
Fig. 8. Uniaxial compression creep strain duration curves.
Acknowledgements This research was supported by the National Natural Science Foundation of China (No. 51304189), ‘the Fundamental Research Funds for the Central Universities’ (2017CXNL02), and the State Scholarship Fund of China Scholarship Council (Grant number 201606425011) for visiting scholars to pursue studies in Australia. References [1] J.P. Wei, F.J. Zhao, M.J. Liu, Q.B. Mou, Research on prediction index and critical value for coal and gas outburst area in Yi'an mine, Coal Sci. Technol. 36 (2008) 39–41. [2] L.W. Chen, Prediction analysis on seam outburst danger in D group seam of No.1 mine of Pingdingshan coal mining group, Coal Sci. Technol. 35 (2007) 40–43. [3] J.C. Chen, Z.F. Wang, H.M. Yang, H.H. Xia, L.W. Chen, The cause analysis and solutions of low-index outburst phenomenon during K1 value forecast, Safety in Coal Mines, vol. 43, 2012, pp. 145–147. [4] B.S. Nie, X.Q. He, E.Y. Wang, Z.T. Liu, Z.Y. Sa, Present situation and progress trend of prediction technology of coal and gas outburst, China Saf. Sci. J. 13 (2003) 40–43. [5] S.H. Yu, Phase behavior of the natural gas in coal beds, J. China Coal Soc. 2 (1981) 1–5. [6] C.L. Jiang, Z.H. Li, Y. Han, Distribution of heavy hydrocarbon in coal seams and its use in predicting outburst of coal, J. China Univ. Min. Technol. 13 (2003) 29–34. [7] A. Marzec, Towards an understanding of the coal structure: a review, Fuel Process. Technol. 77 (2002) 25–32. [8] J.P. Mathews, A.L. Chaffee, The molecular representations of coal - a review, Fuel 96 (2002) (2012) 1–14. [9] N.D. Rus'Ianova, N.E. Maksimova, V.S. Jdanov, V.L. Butakova, Structure and reactivity of coals, Fuel 69 (1990) 1448–1453. [10] T. Ishizuka, T. Takanohashi, O. Ito, M. Lino, Effects of additives and oxygen on extraction yield with CS2-NMP mixed solvent for argonne premium coal samples, Fuel 72 (1993) 579–580. [11] F. Söğüt, F.S. Image, Dissolution of lignites in tetralin at ambient temperature: effects of ultraviolet irradiation, Fuel Process. Technol. 55 (1998) 107–115. [12] P.H. Fang, R. Wong, Evidence for fullerene in a coal of Yunnan, Southwestern China, Mater. Res. Innov. 1 (1997) 130–132. [13] H.W. Zhang, Application of geo-dynamic division method in prediction of coal and gas outburst region, Chin. J. Rock Mech. Eng. 22 (2003) 621–624. [14] Y.G. Zhang, Z.M. Zhang, Y.X. Cao, Deformed-coal structure and control to coal-gas outburst, J. China Coal Soc. 32 (2007) 281–284. [15] H. Zhang, Y. Li, S. Ha, H. Li, M. Sun, Effects of included minerals in pulverized coals on the pore structure of chars, J. China Univ. Min. Technol. 38 (2009) 810–814. [16] L.B. Cheng, L. Wang, Y.P. Cheng, K. Jin, W. Zhao, L.S. Sun, Gas desorption index of drill cuttings affected by magmatic sills for predicting outbursts in coal seams, Arab. J. Geosci. 9 (2016) 61–75. [17] X.H. Fu, Y. Qin, Y.G. Yang, J.N. Peng, Experimental study of the solubility of methane in coalbed water, Nat. Gas Geosci. 15 (2004) 345–348. [18] M. Radke, R.G. Schaefer, D. Leythaeuser, M. Teichmüller, Composition of soluble organic matter in coals: relation to rank and liptinite fluorescence, Geochim. Cosmochim. Acta 44 (1980) 1787–1800. [19] Y.L. Yang, Z.H. Li, L.L. Si, F.J. Gu, Y.B. Zhou, Q.Q. Qi, X.M. Sun, Study governing the impact of long-term water immersion on coal spontaneous ignition, Arab. J. Sci. Eng. 42 (2017) 1359–1369. [20] Y.L. Yang, Z.H. Li, Y.B. Tang, F.J. Gu, H.J. Ji, Z. Liu, Effects of low molecular weight compounds in coal on the characteristics of its spontaneous combustion, Can. J. Chem. Eng. 93 (2015) 648–657. [21] W.P. Jiang, Microscopic mechanism study on the influence of coal rank on adsorption capacity, China Coalbed Methane 6 (2009) 19–22. [22] G.Z. Yin, C.B. Jiang, X.Q. Li, W.Z. Wang, B. Cai, An experimental study of gas permeabilities of outburst and nonoutburst coals under complete stress-strain process, Rock Soil Mech. 32 (2011) 1613–1619.
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Influence of soluble organic matter on mechanical properties of coal and occurrence of coal and gas outburst Yongliang Yang ⁎, Jiaji Sun, Zenghua Li, Jinhu Li, Xiaoyan Zhang, Liwei Liu, Daocheng Yan, Yinbo Zhou Key Laboratory of Coal Methane and Fire Control, Ministry of Education, China University of Mining and Technology, Xuzhou 221008, China School of Safety Engineering, China University of Mining and Technology, Xuzhou 221116, China
a r t i c l e
i n f o
Article history: Received 5 July 2017 Received in revised form 11 January 2018 Accepted 19 March 2018 Available online 21 March 2018 Keywords: Soluble organic matters Coal pore structure Adsorption characteristics Mechanical parameters Coal and gas outburst
a b s t r a c t Coal is a complex organic rock. A large number of soluble organic matters (SOMs) existing in pores of the coal matrix and even in the macromolecular network structure will inevitably have some impact on the coal seam gas occurrence, migration and outburst risk. From the perspective of structure and composition of coal, a series of experiments were carried out to reveal the mechanism of SOMs on the gas outbursts. The results show that SOMs can improve the coal adsorption ability due to the strongly adsorbent groups. Compared with the raw coal, the SOMs-extracted residual coal undergoes a decrease in specific surface area and an increase in total pore volume, average pore diameter and porosity. Furthermore, compressive strength, tensile strength and elastic modulus of the residual coal are all higher than that of the raw coal, while gas content, gas emission initial velocity, drill cuttings desorption indexes (K1 and Δh2) of the residual coal are all lower than that of the raw coal. In summary, the SOMs in coal can promote coal and gas outbursts. Research results of the project provide a new way of promoting gas extraction and outburst prediction. © 2017 Published by Elsevier B.V.
1. Introduction Coal and gas outbursts are the most serious mine gas disaster. To reduce and prevent coal and gas outburst disasters, many scholars hold that an outburst results from the combination of three factors, namely, ground stress, gas pressure and coal structure properties. They have also conducted numerous researches on outburst sensitive indexes and outburst prediction models, concluding gas pressure, gas content, initial velocity of gas emission, consistency coefficient of coal, failure mode of coal, cuttings index, etc. [1–2]. However, in recent years, many outburst accidents took place before the outburst indexes had reached critical values [3–4], so the outburst mechanism and its influencing factors still need further study. In the 1980s, some scholars found that, in a similar occurrence conditions, the higher the content of heavy hydrocarbons C2-C8 (i.e. C2-C8 hydrocarbons), the greater the outburst danger [5–6]. Hydrocarbons are widely distributed from C1 to C30, and SOMs which are composed of aliphatic hydrocarbons, aromatic hydrocarbons and heteroatom compounds containing heavy hydrocarbons C2-C8. However, the influence of SOMs on outburst is rarely reported. In addition to a macromolecular solid skeleton, coal, a kind of complex organic rock, contains numerous small organic compounds. Existing in pores of the coal matrix and even in the macromolecular ⁎ Corresponding author. E-mail address: [email protected] (Y. Yang).
https://doi.org/10.1016/j.powtec.2018.03.053 0032-5910/© 2017 Published by Elsevier B.V.
network structure, these small soluble organic compounds account for about 10%–23% of organic matters in coal and even for as high as 30% in some coal [7–9]. Therefore, it is evident that SOMs in coal is a component that cannot be ignored, so it is bound to have a certain influence on coal seam gas storage, transportation and gas outburst hazards. SOMs are free or embedded in the coal polymer network structure with hydrogen bonds and Van der Waals forces. In the past, studies on SOMs in coal were mainly concentrated on industries such as coal chemical industry and coal coking industry and focused on the occurrence forms, compositions and coal chemical structures of SOMs in coal [10–12]. Only few scholars made preliminary discussions and qualitative analyses on the effect of SOMs in coal on outburst hazards. For example, Zhang [13] conducted a SOMs geochemical study on Nantong coal samples and found that the higher the SOMs content in the coal seam, the greater the outburst risk. Zhang et al. [14] determined that a large amount of soluble low-molecular compounds were formed and preserved during the formation of tectonic coal, and they pointed out that the chloroform extraction rate of coal could be used as an index to predict coal and gas outburst. In a study of the mechanism of sudden methane release, Zhang et al. [15] noted that the porous structure of coal colloid contains molecule pores that are very small in diameter and able to prevent the escape of methane, which results in an uneven gas flow within the coal seam and thus conduces to the emergence of coal and gas outburst. From the perspective of structure and material composition of coal, this paper tested the mechanical strength of raw coal and residual
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coal, the desorption index of cutting gas and the characteristics of gas adsorption and emission. On this basis, it analysed the influence of SOMs on outburst indexes and further revealed the role that SOMs play in the mechanism of coal and gas outburst risk by combining theoretical analysis with experimental simulation and experimental test. 2. Experimental process 2.1. Coal sample preparation Three coal samples of different metamorphic grades, namely, Qingdong coal (QD) from Qingdong No. 8 seam, Zhangji coal (ZJ) from Zhangji No. 9 seam and Tongting coal (TT) from Tongting No. 10 seam, were selected as experimental coal samples. The proximate analysis of each coal sample (Proximate (air-dry-basis)) is listed in Table 1. Of the three, ZJ No. 9 seam and QD No. 8 seam belong to outburst coal seam, while TT No. 10 seam belongs to non-outburst coal seam. In addition, the coal sample before THF treatment is called “raw coal”, while the coal after THF treatment is called “residual coal”. Three coal samples with particle ranging from 60 to 80 meshes (0.180–0.250 mm) were selected. Each of them was divided into two parts, of which one was taken as the raw coal and the other was extracted by tetrahydrofuran (THF). The extraction experiments were conducted via a CW-2008 multi-microwave reaction/extraction device produced by Zeming Tech. Co., Ltd [20]. The 50 g coal sample and 300 ml THF solvent (analytic reagent) were mixed and then put into an extraction vessel. Next, after the connections were checked, the extraction experiment was started at 30 °C and atmospheric pressure. After 4 h, the mixtures were separated by a vacuum filter device. Finally, the separated coal samples were further dried at 80 °C for 12 h in vacuum. In this paper, the coal sample treated by THF was called “residual coal”. Further, the composition of liquid was analysed via an Agilent 6890/5973 GC/MS in the School of Chemical Engineering of Nanjing University. 2.2. Experimental methods Parameters such as coal strength, the drill cuttings desorption indexes and gas content are all important indexes for the evaluation of coal and gas outburst risk. In order to study the effect of SOMs in coal seam on outburst risks, this paper tested the mechanical parameters, drill cuttings desorption (K1 and Δh2) and gas contents (desorbed gas content and residual gas content) of the coal before and after the extraction respectively.
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2.2.2. Test of the drill cuttings desorption indexes The drill cuttings desorption indexes (K1 and Δh2) are commonly used as outburst prediction indexes in China. Their value can reflect the outburst danger. K1 is the total amount of gas desorption for exposure within 1 min per gram of coal sample, which can reflect the gas amount and the attention of desorption rate, thus its unit is mL · (g·min0.5)−1. Δh2 is expressed as the total gas desorption amount at 4th and 5th minutes of the coal sample (10 g) exposed to the air, and the unit is Pa [16]. The index K1 was measured using a WTC-type analyser produced by the Coal Research Institute of Chongqing, and Δh2 was measured using the MD-2-type gas desorption instrument produced by the Coal Research Institute of Fushun. During the simulation of K1 and Δh2 in the laboratory, the samples were loaded into a coal sample tank. After vacuum degassing, the gas was injected into the tank until it reached the preset pressure. Then, loading machine was started at the preset uniaxial force. After the coal samples had adsorbed gas for 4 h, the axial force was unloaded and the tank was opened. Then, the coal samples were rapidly drilled with a small drill to test the drill cuttings desorption indexes. 2.2.3. Test of gas content parameters Pulverized coal with particle diameters ranging from 0.18 to 0.35 mm (40–80 mesh) were selected as experimental samples and then dried and degassed in vacuum at 80 °C for 10 h. 50 g of raw coal and residual coal were respectively put into the coal sample tank and vacuum degassed for 8 h. After the equilibrium pressure was reached, methane was injected into the tank at 30 °C for 8 h. Next, the valve was opened and the gas desorption was measured by a gas desorption device. The measured total desorption amount divided by the coal weight is the desorbed gas content. The measured coal samples were placed in air for 3 h, and the samples before and after pulverization were then vacuum degassed respectively using a vacuum pump. The measured degassing volume divided by the coal weight is the residual gas content. Considering the fact that the SOMs-solubility of methane will inevitably affect the content of coal seam gas, this paper chose the SOMs model compound for the methane dissolution experiment. In this study, the amounts of methane dissolved in model materials under different pressures and temperatures were determined using the gasliquid equilibrium static method. The pressure values before and after the gas filling and the pressure changes before and after the balance in the reactor were measured respectively. The change in volume was calculated according to the gas state equation, and the amounts of methane dissolved under different pressures and temperatures were obtained finally [17]. 3. Results and discussion
2.2.1. Test of mechanical parameters To test the mechanical parameters of raw coal and residual coal, the powder samples were placed in the mould, respectively. Then, the standard briquettes (Φ50 × 100 mm) were suppressed by a loading machine under 300 MPa for 2 h. Further, the briquette samples were used to carry out the uniaxial compression and tensile tests by using a YAW4306 servo loading machine at loading rates of 10 N/s and 3 N/s, respectively.
Table 1 Analysis of basic features of coal samples. Sample number
ZJ No. 9 QD No. 8 TT No. 10
Proximate analysis/% Mad
Ad
Vdaf
1.67 0.56 0.96
22.31 13.85 15.57
33.42 24.19 25.56
Rv, max (%)
Extraction yield (wt%, daf)
0.93 1.37 0.78
2.521 2.130 3.149
Mad = air-dried moisture; Ad = dry ash; daf = dry ash free; Rv, max = maximum vitrinite reflectance; wt = weight.
3.1. Soluble organic matter composition and composition characteristics Fig. 1 shows the chromatograms of THF extracts from various coal samples. As the main components of SOMs (coal extract) in coal, aromatic hydrocarbons are mainly composed of benzene compounds of single or multiple benzene rings [18], and they include naphthalene, phenanthrene, anthracene and alkyl-containing polycyclic aromatic hydrocarbons of even more benzene rings. Aliphatic hydrocarbons are mainly composed of n-alkanes which are mostly C14 to C28 in carbon chain lengths and have odd carbon dominance, and they also include a small number of branched alkanes and cycloalkanes. Heteroatomcontaining compounds are predominantly oxygen-containing compounds, and only a silicon-containing compound was detected in the n-hexane extract of QD coal. The contents of SOMs in the outburst and non-outburst coal were comparatively analysed. It is found that SOMs of the outburst coal contain high contents of methyl groups and methylene groups such as 2,3,6trimethylnaphthalene, 1,4,5,8-tetramethylnaphthalene, heptadecane,
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Fig. 1. Total ion chromatogram (TIC) of THF extract.
octacosane and benzofluoranthene, while SOMs of the non-outburst coal contain more butyl groups and hydroxyl groups such as 2,6-di-tertbutyl-4-(hydroxymethyl)phenol and 2,4-di-t-butylphenol. Some scholars used infrared spectroscopy to test the change in coal body function before and after the extraction [19–20]. By comparing the results of outburst coal and non-outburst coal, it was found that, after extraction, the relative contents of free OH bonds, OH relative self-association hydrogen bonds and CH2 anti-symmetrical stretching vibration and other
characteristic peaks of non-outburst coal dropped significantly, and the relative contents of characteristic peaks such as CH2 symmetrical stretching vibration and CH3 deformation vibration of outburst coal decreased obviously. This is in good agreement with the mass spectrometry test results. Jiang [21] revealed that the effect of different side chain groups on the methane adsorption of the coal structure, results showed that: methylene N methyl N hydroxyl N butyl. Among them, the methane
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Table 2 Results of pore size distribution for raw coal and residual coal. Coal sample
ZJ No. 9 QD No. 8 TT No. 10
Raw coal Residual coal Raw coal Residual coal Raw coal Residual coal
Average pore diameter (nm)
Total intrusion volume (mL/g)
Total pore area (m2/g)
6.81 12.73 9.32 10.19 14.90 16.54
0.0371 0.0587 0.0351 0.0389 0.0430 0.0481
18.679 17.390 15.534 14.338 23.403 21.095
adsorption capacity of methylene groups and methyl groups in coal was higher than that of hydroxyl groups and butyl groups. In this study, SOMs of the outburst coal were found to contain more methylene groups and methyl groups, indicating that SOMs have a certain influence on the outburst risk of coal, Therefore, under the same conditions, the outburst coal shows a larger gas adsorption capacity. 3.2. Pore structure of raw coal and residual coal From the mercury penetration experiments, the calculated pore structure parameters of raw coal and residual coal (total pore volume, specific surface area, average pore diameter and pore size distribution) are shown in Table 2. As can be seen from Table 2, after extraction, the pore structure parameters show a significant change. The SOMs-extracted residual coal undergoes a decrease in specific surface area and an increase in total pore volume, average pore diameter and porosity; and the increase and decrease ranges are consistent with the extraction rate. Although the solvent extraction can produce a good pore increase and expansion effect, it reduces the specific surface area of the coal sample. The higher the extraction rate, the more obvious the reduction of specific surface
The area of the different types of pores/proportion (ml/g)/% Micropores
Small pores
Mesopore
Macropore
16.619/88.97 15.357/88.31 13.743/88.48 13.487/87.09 20.515/87.66 18.072/85.67
2.03/10.87 1.94/11.16 1.76/11.30 1.79/12.48 2.84/12.15 2.94/13.94
0.026/0.14 0.074/0.4 0.03/0.19 0.052/0.36 0.037/0.16 0.065/0.31
0.001/0.01 0.011/0.06 0.004/0.03 0.007/0.05 0.007/0.03 0.017/0.08
area. The results indicate that although the extraction can increase the pore size of coal, it reduces the number of micro pores, resulting in the decrease of specific surface area. It can be seen from the pore size distribution characteristics, both raw coal and residual coal show a similar distribution characteristics: the micropores (b10 nm) take up the highest proportion, followed by small pores (10–100 nm) and then mesopores (100–1000 nm), with macropores (N1000 nm) being the least. However, the solvent extraction can reduce the proportion of micropore and small-pore volumes and raise the proportion of mesopore and macropore volumes. Meanwhile, the higher the extraction rate, the greater the increase ranges of mesopores and macropores. This is because that, after SOMs are dissolved, some micropores can be connected and expanded into larger pores, causing the reduction of micropores.
3.3. Effect of SOMs on the mechanical strength of coal The stress-strain curves of coal samples under uniaxial compression are shown in Fig. 2, and the curves of the time loading tests are shown in Fig. 3.
Fig. 2. Stress-strain curves of raw coal and residual coal under uniaxial compression.
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Fig. 3. Tensile loading curves of raw coal and residual coal.
As can be seen from Fig. 2, the raw coal and residual coal exhibit a similar deformation laws in the whole stress-strain process. The deformation processes of coal samples can be generally divided into 5 phases: initial compression phase, elastic phase, yield phase, peak intensity phase and strain-softening phase [22]. From the initial compression phase to the peak intensity phase, axial stress of the residual coal is higher than that of the raw coal, and peak intensity of the raw coal is smaller than that of the residual coal (see Table 3). This law is reflected more obviously by the outburst coal samples ZJ and QD. Moreover, the strains of outburst raw coal samples at peak intensities are all greater than those of outburst residual coal samples, while the strains of nonoutburst raw coal samples are smaller than those of non-outburst residual coal samples. According to the tensile test results on the coal samples, the time loading curves of the raw coal and residual coal are similar. In addition, the tensile strengths of residual coal samples of different metamorphic grades are all greater than those of raw coal samples. The mechanical parameters are summarized in Table 4. It can be seen from Figs. 2 and 3 and Table 4 that whether for the outburst or for the non-outburst coal, comparing with the raw, the residual coal exhibits the higher compressive strength, tensile strength and elastic modulus. In addition, the strength of outburst residual coal is notably higher than that of outburst raw coal, while the strength of nonoutburst residual coal is slightly higher than that of non-outburst raw coal. The reason is that the extraction of SOMs in coal weakens the Table 3 Typical stress-strain briquettes. Parameters
Peak intensity/MPa Axial strains/%
ZJ
QD
TT
Raw coal
Residual coal
Raw coal
Residual coal
Raw coal
Residual coal
1.77 1.85
3.14 1.46
1.77 1.84
2.61 1.74
1.25 1.82
1.47 1.89
creep ability of coal, so that the pores within the coal matrix tend to be homogeneous and simplified [23–25]. Besides, the surface of extracted residual coal particles is relatively rough due to the dissolution of SOMs, causing that the coal particles in briquette combine more closely, so that the strength of residual coal is generally higher than that of raw coal. According to the results, the elastic limit and yield stress of the raw coal are smaller than those of the residual coal. The smaller yield stress means that ground stress and gas can combine more easily to jointly cause coal and gas outburst [26], which shows that the presence of SOMs is conducive to gas outburst. 3.4. The drill cuttings desorption indexes of raw coal and residual coal As the most commonly used outburst test indexes for Chinese coal mine, the drill cuttings desorption indexes K1 and Δh2 can well reflect the outburst danger of coal seams [2,16]. According to the field test procedures, the simulation tests for raw coal and residual coal are carried out in the laboratory. The test results of K1 and Δh2 are shown in Figs. 4 and 5. From Figs. 4 and 5, K1 and Δh2 of residual coal are less than those of raw coal, indicating that the outburst risk of residual coal is lower than that of raw coal. This also shows that SOMs in coal have a promoting effect on coal and gas outburst, because SOMs in coal can not only occupy Table 4 Comparison of mechanical parameters of raw coal and residual coal. Coal sample
Compressive strength/MPa
Tensile strength/MPa
Elastic modulus/GPa
Raw coal (ZJ) Residual coal (ZJ) Raw coal (TT) Residual coal (TT) Raw coal (QD) Residual coal (QD)
1.75 3.11 1.29 1.44 1.82 2.51
0.108 0.475 0.095 0.345 0.207 0.403
10.57 32.88 8.16 15.01 9.79 16.81
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Fig. 4. Test results of K1 values of raw coal and residual coal.
part of the pores and affect pore structure characteristics of the coal but also dissolve some methane. After THF extraction, pores in the coal matrix tend to be homogeneous and simplified due to the dissolution of SOMs, leading to the increase of the number of mesopores and macropores in the residual coal grows, while the reduction of the number of micropores drops. Therefore, the gas adsorption capacity is decreased.
3.5. SOMs dissolved gas and its effect on the gas content in coal seams Gas content is one of the significant indexes for evaluating the outburst danger of coal seams. To study the effect of SOMs on gas content, this paper analysed the extraction product composition by taking 0# diesel oil, lubricating oil and coal tar as the model materials to carry out a methane dissolution experiment. The results are listed in Table 5.
Fig. 5. Test results of △h2.
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Table 5 Methane dissolution amounts of model materials under different equilibrium pressures. Experimental sample
Inflation pressure/Mpa
Equilibrium pressure/MPa
Unit mass dissolved amount/ml·g−1
Diesel oil
0.95 2.20 2.97 4.10 5.07 1.06 2.04 3.15 4.25 5.10 1.05 2.10 3.05 4.20 5.00
0.88 1.99 2.75 3.71 4.60 1.03 1.91 2.98 4.01 4.76 0.98 1.97 2.86 3.92 4.56
1.13 2.89 3.34 6.03 7.50 0.47 1.71 2.44 3.80 5.24 0.45 0.82 1.25 1.90 3.12
Lubricating oil
Coal tar
The results show that the amounts of methane dissolved in different model materials were different. Among them, diesel oil accounts for the largest dissolved amount, followed by lubricating oil, with coal tar being the least. It is obtained through analysis that this is related to physical and chemical properties of the model materials. Diesel oil is primarily composed of normal alkane hydrocarbon organic matters whose properties are more similar to those of methane. Thus, according to the principle that the similar substance is more likely to be dissolved by each other, the solubility of methane will be greater than those of coal tar and lubricating oil. A large number of oxygen-containing compounds such as benzene, hydroxyl groups and phenols contained in coal tar contribute to the polarity of coal tar and will inevitably affect the amount of methane dissolved in coal tar. Even though the amount of methane dissolved in coal tar is the least, it is also significantly increased with
the rise of pressure. For example, when the equilibrium pressure is 4.56 MPa, the unit mass dissolved amount is 3.12 cm3/g. The gas adsorption amount, desorbed gas content and residual gas content are important parameters characterizing the coal seam gas content and emission characteristics. In this study, experimental results of desorbed gas contents of raw coal and residual coal under different equilibrium gas pressure conditions are shown in Fig. 6. As can be seen from Fig. 6, the desorbed gas content of raw coal is always higher than that of residual coal under different equilibrium pressures, suggesting that the adsorption capacity of residual coal is smaller than that of raw coal. This is possibly because the pore structure of coal has been changed during the extraction process. As SOMs are dissolved, some of the pores in the raw coal are expanded. Further, some plugging pores and internally connected pores in the coal may even be opened and integrated. This results in the increase of macropores and mesopores as well as the reduction of micropores and the specific surface area of coal, which reduces the gas adsorption capacity [27]. In summary, the SOMs can improve the methane adsorption capacity of coal to a certain extent, which is conducive to the occurrence of gas in coal seam and thus will promote the gas emission during mining. To verify this, isothermal adsorption experiments were carried out on the coal samples before and after extraction. The results are shown in Table 6. The gas adsorption experiment shows that the adsorption capacities of three kinds of raw coal samples are all larger than those of residual coal samples, and the a value of residual coal is smaller than that of raw coal, indicating that the methane adsorption capacity is decreased after small SOMs in coal are dissolved in the THF solution. The b value of residual coal is larger than that of raw coal, showing that the corresponding adsorption pressure of residual coal is lower than that of raw coal when the same adsorption saturation is reached, but the gas adsorption rate of residual coal is higher than that of raw coal under low-pressure conditions.
Fig. 6. Test curves of desorptable gas contents of raw and residual coal.
Y. Yang et al. / Powder Technology 332 (2018) 8–17
4.1. Coal pore structure and gas emission characteristics
Table 6 Adsorption data fitted results for raw coal and residual coal. Parameters
Residual gas content (ml·g−1) Gas-absorbent constant a (cm3/g) Gas-absorbent constant b (MPa−1)
ZJ
QD
TT
Raw coal
Residual coal
Raw coal
Residual coal
Raw coal
Residual coal
2.86
2.45
2.54
2.26
2.29
2.10
24.3689 20.0814
18.2739 16.0210
20.7692 18.6381
0.6341
0.5533
0.5628
0.7083
15
0.5857
0.6532
Table 7 ΔP values of raw coal and residual coal. Coal sample
ZJ
QD
TT
ΔP of raw coal/mm Hg ΔP of residual coal/mm Hg
10.8 8.2
9.1 7.9
5.2 4.1
3.6. Initial speed of gas emission of raw coal and residual coal The initial speeds of gas emission of raw coal and residual coal were tested by a detector. The results are shown in Table 7. The results show that the initial velocity of gas emission of residual coal is lower than that of raw coal. Previous studies have pointed out that the gas adsorption amount of residual coal is decreased. Although the porosity of residual coal is more developed, the drop of adsorbed gas content leads to the decrease in the gas source and the initial velocity of gas emission. In addition, gas can be dissolved in SOMs in coal, resulting in a drop of gas desorption amount of residual coal under high pressure, which also leads to a decrease in the initial velocity of gas emission. ΔP is decreased after the extraction, indicating that the solid solution gas dissolved in the SOMs will be released more quickly when the coal seam is damaged by mining and will thus increase the risk of coal seam outburst.
Jang et al. [28] found that undeveloped coal pores will lead to poor connectivity between fractures and easy accumulation of gas, thus promoting coal and gas outburst accidents. However, it was discovered in this study that the SOMs-extracted residual coal undergoes a decrease in specific surface area and an increase in total pore volume, average pore diameter and porosity. The raw coal micropores that contain a large number of SOMs are more developed and take up a larger specific surface area [29–30], which constitutes the main space and creates favorable conditions for gas occurrence. Wang et al. [31] held that connectivity between fractures was enhanced after the dissolution of SOMs free or embedded in the coal polymer network structure. It can thus be seen that the presence of SOMs leads to poor connectivity between natural exogenous fractures and endogenous fractures, which is not conducive to the migration of coal seam gas. As a result, compared with the residual coal, it is easier for gas to accumulate in the raw coal, which creates a more favorable condition for coal and gas outburst. The gas emissions characteristics are controlled by the pore structure and adsorption ability. For the pore structure, some studies showed that [32], the methane diffusion coefficients of mesopores/macropores are obviously (about 105 times) larger than that of micropores, and the methane diffusion rate in coal is mainly determined by the ratio of mesopores/macropores in coal. For the adsorption ability, some studies showed that, the SOMs can improve coal adsorption ability because the gas can dissolved in the SOMs [24,25]. As we all know, the stronger adsorption ability, the greater emissions ability. In this paper, after THF treatment, the mesopores/macropores is increased, while the adsorption ability is reduced. Therefore, after THF treatment, there is a competitive relationship between pore structure and adsorption ability. Further, as shown in Table 7, the initial velocity of gas emission is decreased after THF treatment, indicating that the initial emission ability is dominated by the adsorption ability. As is known, the stronger the gas emission ability, the greater the risk of coal seam outburst. Therefore, from the viewpoint of gas emission, SOMs can improve the gas emission capacity of coal and increase the risk of outburst.
4. The influencing mechanism of SOMs on coal and gas outburst Pore characteristics, gas energy and strength of coal all directly affect coal and gas outburst danger. Through a large number of experiments and theoretical analyses in the previous section, this study analysed the influence of SOMs on the pore characteristics, gas energy and strength of coal, so as to obtain the influencing mechanism of SOMs on coal and gas outburst, as shown in Fig. 7.
4.2. Gas energy Wang et al. [33] put forward that as gas energy was the main energy source for an outburst, gas content can reflect coal internal energy most directly, and that under the same conditions, the higher the gas content, the greater the risk of outburst. From the aforesaid test results of coal
Fig. 7. Effects of SOMs on gas storage and outburst danger.
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Y. Yang et al. / Powder Technology 332 (2018) 8–17
pore structure (see Table 2) and SOMs dissolved gas (see Table 5), it can be obtained that when the SOMs are reduced, the number of gas adsorption groups and the amount of solid solution gas are both reduced, and the gas adsorption capacity of coal is weakened, which is reflected by the decrease in the gas limit adsorption, the rise of the low-pressure gas adsorption growth rate, the drop of the initial amount of adsorption heat, etc. Finally, the reduction of SOMs will lower original coal gas content, desorptable gas content and residual gas content. Hence, it is clear that the presence of SOMs enables coal to adsorb and save more gas within it and thus compresses its storage potential. Meanwhile, high desorptable gas content of the raw coal decides that its gas desorption speed is relatively high, so higher gas pressure is formed in a short time. Correspondingly, the initial velocity of gas emission rises, and thereby coal and gas outburst accidents are more likely to occur [34].
5. Conclusions The mechanical properties of raw coal and residual coal were studied. The results show that the compressive strength, tensile strength and elastic modulus of residual coal obtained through THF extraction are all higher than those of raw coal. In addition, the strength of outburst residual coal is notably higher than that of outburst raw coal, while the strength of non-outburst residual coal is slightly higher than that of non-outburst raw coal. A variety of outburst parameters of raw and residual coal were tested. After the extraction of SOMs, K1 and Δh2 are decreased and the initial velocity of gas emission is reduced. SOMs could dissolve the quantitative methane and finally revealed that SOMs can improve the gas adsorption capacity of coal and promote coal and gas outburst. These results provide a new means for the development of gas drainage and outburst prediction.
4.3. Coal strength The point that the lower the coal strength, the greater the outburst risk has been commonly acknowledged [35–39]. The previous experimental results show that mechanical parameters such as compressive strength and tensile strength of extracted residual coal are all larger than those of the raw coal, indicating that the mechanical strength of coal has been enhanced after the extraction. In order to further analyse the influence of SOMs on the creep law of coal, the uniaxial compression creep strain duration curves of raw coal and residual coal under the same conditions were plotted, as shown in Fig. 8. For reaching the same deformation degree, the residual coal takes more time than the raw coal, and the maximum strain of residual coal is smaller than that of raw coal, suggesting that the residual coal obtained through THF extraction has weakened axial creep ability and thus is more unlikely to fail under the same axial strain. On the contrary, when exposed, the coal seam containing more organic matters may fail under minor deformation. The experimental data measured show that the less the SOMs in coal, the smaller the parameters such as K1 value and Δh2 value, and the lower the initial velocity of gas emission. Combining change trend of gas content, it can be concluded that the presence of SOMs in coal has a certain promoting effect on coal and gas outburst, which is also verified by the difference of test results of outburst coal and non-outburst coal. From the above results, SOMs in coal can promote coal and gas outburst. In a mining area rich in SOMs, SOMs can be reduced by injecting supercritical CO2 into the coal seam. On the one hand, this can reduce the coal seam outburst danger, and on the other, it can increase permeability of the coal seam and improve the effect of gas drainage to ensure the safety of mine production.
Fig. 8. Uniaxial compression creep strain duration curves.
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