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Abstract

The occurrence of coal and gas outbursts is closely linked to the presence of tectonic coal. To study the pore structure characteristics and adsorption characteristics of different destruction types of coal, nondestructive coal, destructive coal, strongly destructive coal, pulverized coal, and fully pulverized coal are selected based on the coal and gas outburst mine identification specifications. The experimental methods used are liquid nitrogen adsorption, mercury intrusion porosimetry and CH₄ isothermal adsorption. The results show that the pore volume obtained by the Barrett–Joyner–Halenda method and the specific surface area increase with increasing destruction type. For all tested coal samples, the N₂ adsorption/desorption hysteresis loop is not closed when the relative pressure is low, indicating the existence of ink-bottle pores, an elastic structure of the coal and nitrogen affinity in the coal. With increasing tectonic stress, it becomes more advantageous to produce micropores. The pore volume obtained by the mercury intrusion porosimetry experiment increases with increasing destruction types except for the case of fully pulverized coal. High-pressure mercury causes pore deformation and collapse. When the f value is <0.5, the compression effect of the pores is obvious. The smaller the value of f is, the wider is the pore range affected by the high-pressure mercury. The degree of destruction is positively correlated with the porosity, specific surface area, and Langmuir volume. However, the degree of destruction is negatively correlated with the f value and mercury extrusion efficiency.

Keywords

Pore structure liquid nitrogen adsorption mercury intrusion porosimetry different destruction types pore deformation

Introduction

Coal and gas outburst, which is a complex dynamic phenomenon accompanied by coal, rock, and gas in underground coal mines, is one of the most serious natural disasters in the process of coal seam mining (Cao et al., 2001; Jiang et al., 2015; Li et al., 2017; Wang et al., 2014, 2017; Yue et al., 2019). It is generally believed that coal and gas outbursts are the result of the combined action of gas pressure, in-situ stress and the mechanical properties of coal. Gas pressure and in-situ stress are positively correlated with the coal and outburst intensity. However, the mechanical properties of coals are negatively correlated with the coal and gas outburst intensity (Chen, 2011; Guan et al., 2009; Wang et al., 2013, 2017; Xue et al., 2014). Coals with different destruction types are formed by tectonic movement and are also called tectonic coals. Tectonic coal is not only closely linked to coal and gas outbursts but also a necessary condition for these outbursts (Hao and Yuan, 1999). Many studies have indicated that tectonic coal is very abundant in coal and gas outburst occurrence sites and the nearby areas. The range of the dangerous areas can be determined by investigating the distribution area of tectonic coal (Beamish and Crosdale, 1998; Frodsham and Gayer, 1999; Yan et al., 2013). Tectonic coal can be divided into five categories: nondestructive coal (I), destructive coal (II), strongly destructive coal (III), pulverized coal (IV), and fully pulverized coal (V) (Jiang and Ju, 2004). Coal and gas outbursts are unlikely to occur where nondestructive coal (I) and destructive coal (II) exist. However, the likelihood is much higher for strongly destructive coal, pulverized coal, and fully pulverized coal. In addition, the pore structure and adsorption properties of coal with different destruction types are different. Therefore, investigating the pore structure characteristics and adsorption characteristics of tectonic coals is important to prevent and control coal and gas outburst accidents and enhance coal bed methane (CBM) recovery.

The pores of coal samples can be divided into four categories based on Hotot’s pore classification: micropore (less than 10 nm), transition pore (10–100 nm), mesopore (100–1000 nm), and macropore (>1000 nm) (Hodot, 1996). The gas is mainly adsorbed and stored in the micropores (<10 nm) and transition pores (10–100 nm), and the micropores (<10 nm) and transition pores (10–100 nm) are usually called adsorption pores. In contrast, the mesopores (100–1000 nm) and macropores (>1000 nm) are usually called seepage pores, and the seepage pores are the main locations of methane and water (Chen et al., 2018; Peng et al., 2017; Liu et al., 2015; Meng et al., 2016). Many researchers have performed relevant investigations of pore structures in coals. Ju et al. (2005) revealed the evolution rule of nanoscale pore (<100 nm) structure by LNA experiments and scanning electron microscopy (SEM) experiments. Xu et al. (2019) investigated the differences in pore structures between disturbed coal and nondisturbed coal. Compared to the properties of primary coal, the specific surface area and volume of nanoscale pores (<100 nm) of the disturbed coal were much larger, indicating that the gas adsorption and reservoir capacity of the disturbed coal are stronger. Using the LNA experiment, CO₂ adsorption experiment and high-pressure mercury intrusion, Song et al. (2017) concluded that the contribution of micropores (<2 nm) to the specific surface areas is the largest in low- to medium-rank disturbed coals. Zhong et al. (2002) found that the adsorption methane capacity of coal was positively correlated with the total pore volume, total pore specific surface area, and micropore-specific surface area by the LNA experiment and CH₄ isotherm adsorption experiment. Qi (2013) studied the pore structure characteristics of nine coal samples from coal and outburst sites in underground coal mines and found that the proportion of micropores is positively correlated with the coal rank. Therefore, the specific surface areas of the nine coal samples were relatively large. Gregory et al. (2015) discussed the surface area and porosity of coal samples from different coal mines by different techniques. The pore structure in coals has been reported extensively by different test methods and classification standards from the above research. However, the pore structure characteristics and adsorption characteristics of coals with different destruction types remain to be reported.

To probe the pore structure and adsorption characteristics of coal with different destruction types, coal samples were obtained from the Jiaozuo coalfield. The coal samples are typical tectonic coals. The Jiaozuo coalfield is located in Henan Province, China. The Jiaozuo coalfield, which contains complex geological structures and abundant tectonic coal with a high gas content, high gas pressure, and low permeability, is one of the most notable coalfields in terms of coal and gas outburst disasters in China. The Jiaozuo coalfield has 15 coal mines, 10 of which are coal and gas outburst coal mines. In the coal and gas outburst mines, the outburst dangers of the Jiulishan coal mine and the Zhongmacun coal mine are great. Based on this status, this paper chooses the coal samples of the Jiulishan coal mine and Zhongmacun coal mine with severe outbursts as research objects. This paper analyzed the pore structure characteristics and adsorption characteristics of coals with different destruction types by the LNA experiment, high-pressure mercury intrusion experiment, and CH₄ isotherm adsorption experiment. Such research broadly explores the significance of preventing and controlling of coal and gas outburst accidents and enhancing CBM recovery.

Materials and experimental methods

Sample preparation

The anthracite samples with different destruction types used in the experiment were obtained from the Jiulishan coal mine and Zhongmacun coal mine in Jiaozuo, China. The location of the investigated coal mines and the structural outline of the Jiaozuo mining area are shown in Figure 1. The Jiulishan coal mine is a monoclinic structure that runs toward N40°E and tends to the SouthEast (SE). The coal mine field structure is dominated by faults, and the folds are not developed. There are two groups of large- and medium-sized faults in the minefield, all of which are high-angle normal faults. The overall structural form of the Zhongmacun coal mine is a gently inclined monoclinic structure. The strike of the stratum is 40°–100°, and the tendency is 130°–190°. The fault is relatively developed and is dominated by fault structures. The collection of coal samples with different destruction types was based on the specification for the identification of coal and gas outburst mines. The definitions and features of coal with different destruction types are shown in Table 1. Based on Table 1, coal samples with different destruction types can be easily identified.

Figure 1.

The location of the investigated coal mines and the structural outline of the Jiaozuo mining area.

Table 1.

The definitions and features of coal with different destruction types.

Destruction types	Luster	Structural feature	Fracture properties	Strength
Nondestructive coal	Bright and semibright	Layered structure; block structure; the strip is clearly visible	Stagger step; wavy shape	Hard and not easily broken
Destructive coal	Bright and semibright	Squeeze feature; distorted and displaced	Stagger	Medium hardness;it is very easy to separate into small pieces by hand.
Strongly destructive coal	Bright and semibright	Small-piece structure; small fragments	Stagger; granular	It can be ground into powder by hand and is low in hardness
Pulverized coal	Dull	Granular or small granular	Granular	It can be ground into powder by hand. It is occasionally hard
Fully pulverized coal	Dull	Soil-like structure	Soil-like	Loose

The coal samples of the Jiulishan coal mine used in this study were destructive coal (II), strongly destructive coal (III), pulverized coal (IV), fully pulverized coal (V), and the experimental coal samples with different destruction types were referred to as J2, J3, J4, and J5, respectively. The coal samples of Zhongmacun coal mine used in this study were nondestructive coal (I), strongly destructive coal (III), pulverized coal (IV), fully pulverized coal (V), and the experimental coal samples with different destruction types were referred to as Z1, Z3, Z4, and Z5, respectively. However, the nondestructive coal (I) in the Jiulishan coal mine and the destructive coal (II) in the Zhongmacun coal mine cannot be found and collected in all work faces. Therefore, the nondestructive coal (I) in the Jiulishan coal mine and the destructive coal (II) in the Zhongmacun coal mine have not been studied. Coal samples from the Jiulishan coal mine and Zhongmacun coal mine are shown in Figure 2.

Figure 2.

The experimental coal samples.

First, the anthracite coal samples with different destruction types were crushed with a jaw crusher. Coal samples with diameters between 180 µm and 250 µm (60 meshes and 80 meshes) and between 3 mm and 6 mm were screened out. Second, water could affect the experimental effects, and the coal samples should be dried first. The muffle furnace was used to dry the experimental coal samples, and the temperature of the muffle furnace was set as 105°C. Third, when the weight of the experimental coal samples became constant, the coal samples were considered to be completely dry.

The physical parameters of coal samples with different destruction types were tested. The testing procedures were in accordance with Chinese National Standards (GB212-200, GB/T23561.12–2010, GB/T217-2008). The physical parameters of coal samples are shown in Table 2, including A_ad (ash content), V_ad (volatile matter), M_ad (moisture), ARD (apparent relative density), TRD (true relative density), f value, and porosity ( $ϕ$ ). The f value is called the Protodyakonov coefficient and was tested by the drop hammer method. The Protodyakonov coefficient is not only used to distinguish hard coal from soft coal but also often used as a single index for identifying outburst coal seams (Paithankar and Misra, 1976). The f value decreased with increasing destruction type, which indicated that the uniaxial compressive strength decreased with increasing destruction type.

Table 2.

The physical parameters of the coal samples.

Coalsamples	M_ad (%)	A_ad (%)	V_ad (%)	TRD (g/cm³)	ARD (g/cm³)	Porosity (%)	f value
J2	2.01	11.85	7.56	1.56	1.47	5.95	1.2
J3	2.39	11.63	7.41	1.49	1.39	6.52	0.73
J4	3.00	12.99	7.13	1.59	1.45	6.95	0.61
J5	1.86	15.65	7.20	1.53	1.42	8.58	0.36
Z1	1.70	23.57	6.33	1.69	1.61	4.90	1.43
Z3	2.47	16.89	6.35	1.62	1.51	6.73	0.86
Z4	2.97	10.99	5.62	1.57	1.46	7.04	0.65
Z5	2.72	11.07	5.72	1.58	1.43	9.49	0.41

ARD: apparent relative density; TRD: true relative density.

Liquid nitrogen adsorption–desorption experiment

The liquid nitrogen adsorption (LNA)/desorption isotherms were characterized by using a Micrometrics ASAP2020 analyzer. The test temperature was 77 K, and the test method was based on the manometric method (GB/T19587-2004) (Qi et al., 2017). First, a certain quality of coal samples with diameters between 180 µm and 250 µm (60 meshes and 80 meshes) were weighed. Second, to ensure that the liquid nitrogen molecules could be effectively adsorbed on the surface of the experimental coal samples or fill in the pores, the prepared coal samples were degassed. The temperature and time of vacuum evacuation were 473.15 K and 5 h, respectively. Third, degassed experimental coal samples were exposed to low-temperature liquid nitrogen. The relative pressure (p/p₀) of the experiment ranged from 0.0010 to 0.9950; p₀ was the saturated vapor pressure, and p was the adsorption equilibrium pressure. Finally, the Micrometrics ASAP2020 analyzer was used to construct low-temperature LNA–desorption isotherms, which could be analyzed by the Brunauer–Emmett–Teller (BET) model and Barrett–Joyner–Halenda (BJH) model. Therefore, the specific surface area, pore volume, and pore size could be obtained. The pores of the experimental coal samples could be divided into four categories based on Hotot’s pore classification, including micropores (<10 nm), transition pores (10–100 nm), mesopores (100–1000 nm), and macropores (>1000 nm) (Hodot, 1996).

The BET model is typically used to calculate the BET surface area, as shown in equations (1) and (2). The BET model is applicable for relative pressures (p/p₀) ranging from 0.05 to 0.35. If the relative pressure is <0.05, the adsorption equilibrium of multimolecular layers cannot be established. If the relative pressure is >0.35, the capillary condensation becomes substantial, and the adsorption equilibrium is disrupted. $\frac{p / p_{0}}{V (1 - p / p_{0})} = \frac{1}{V_{m} C} + \frac{C - 1}{V_{m} C} \frac{p}{p_{0}}$ (1) $S_{g} = \frac{V_{m} N_{a} A_{m}}{2240 m}$ (2)where V_m is the monolayer amount of gas adsorbed, cm³/g; V is the total volume of adsorbed nitrogen, cm³/g; C is a constant related to adsorption; S_g is the specific surface area, m²/g; N_a is the Avogadro constant, 6.02 × 10²³; A_m is the nitrogen molecule surface area, m²; and m is the weight of the coal sample, g.

The BJH model assumes that the pore was a cylindrical model. The BJH model was proposed on the basis of the Kelvin equation, which is shown in equation (3). The relationship between the pore radius and relative pressure is given by the Kelvin equation under the condition of capillary condensation. The pore radius increases with increasing pressure at which capillary condensation occurs. $r_{k} = - \frac{γ v^{*}}{RT \ln \frac{p}{p_{0}}}$ (3)where r_k is Kelvin radius, m; v* is molar volume, L/mol; and $γ$ is the surface tension, N/m.

Mercury intrusion and extrusion experiment

Using an AutoPore 9505 Micrometrics Instrument, the mercury intrusion porosimetry (MIP) experiment of coal samples was performed. First, a certain quality of coal samples with diameters between 3 mm and 6 mm were weighed; second, the prepared coal samples were placed in the AutoPore 9505 Micrometrics Instrument and tested under low-voltage station and high-voltage station; third, the pore structure characteristics of experimental coal samples were obtained by analyzing the MIP curves. The pore diameter can be calculated by the Washburn equation, which is shown in equation (4). $P_{m} = \frac{4 σ \cos θ}{d}$ (4)where $θ$ is the contact angle, °; d is the pore diameter, m; P_m is the external pressure, N; and $σ$ is the surface tension, N/m.

CH₄ isothermal adsorption experiment

Using the CH₄ isothermal adsorption experimental device, the isothermal adsorption curves can be measured. A schematic map of the CH₄ isothermal adsorption experimental device is shown in Figure 3. The self-designed experimental device has six parts: a temperature control system, a vacuum degassing system, a gas injection system, a desorption system, an adsorption equilibrium system, and an automatic data acquisition system. The temperature control system can accurately control the experimental temperature. The accuracy of the temperature control is ±0.01°C. The vacuum degassing unit includes a resistance composite vacuum meter, some valves, and a direct-coupled rotary vane vacuum pump. The limit vacuum of the vacuum pump is 6.0 × 10⁻²Pa. The gas injection system includes a methane cylinder with 13.00 MPa pressure and 99.990% purity, a high-pressure helium cylinder with 13.00 MPa pressure and 99.990% purity, a reference tank, and a high-precision pressure sensor. The adsorption equilibrium system includes a high-precision pressure sensor, a coal sample tank, and some valves. The general experimental procedures are as follows.

The connections of the experimental device were double-checked to ensure that all valves, the reference tank and pipelines were tight enough.

Turning on the low-temperature control system. The thermostat was adjusted to the predefined temperature. The temperature of the thermostat should be not changed in the course of the experiment.

A certain quality of coal samples with diameters between 60 mesh and 80 mesh were weighed, after which all the experimental coal samples were loaded in the coal sample tank.

Measuring the free space volume. Helium with 99.99% purity was charged into the coal sample tank (8) by reference tank (3) to measure the free space volume, which excluded the skeleton volume of the coal.

Vacuum degassing. All the valves (a–g) were closed in the CH₄ isothermal adsorption experimental device. Valve (e) and the vacuum pump (5) were opened. The vacuum degree of the system can be observed by a resistance composite vacuum meter (4). If the indicator of the resistance composite vacuum meter (4) is <10.00 Pa, the vacuum degassing of the test system is completed. Valve (e) was closed.

Adsorption equilibrium. Valve (a) was opened, and methane with 99.99% purity was charged into the reference tank by the methane cylinder (2). If the value of the high-precision pressure sensor (h) was stable, valve (a) could be closed, and the number of pressure sensors (h) was recorded. Then, valve (d) was opened, and methane with 99.99% purity was charged into the coal sample tank by the reference tank (3). When the inflation process was completed, valve (d) was closed, and the readings of the pressure sensors (h) were recorded. To achieve the predefined adsorption equilibrium pressure, it was often necessary to perform inflation several times. It took at least 10–15 hours to reach adsorption equilibrium (Yue et al., 2017).

Calculation of the adsorption quantity. Based on equation (5), the quantity of adsorbed methane in the experimental coal samples under different adsorption equilibrium pressures could be calculated. $\begin{array}{l} Q = (Q_{1} - Q_{2} - Q_{3}) / m = \frac{273.15 * V}{(273.15 + T) * 0.101325 * m} (\frac{P_{1}}{Z_{1}} - \frac{P_{2}}{Z_{2}}) \\ - \frac{273.15 V_{1} P}{(273.15 + T) * Z * 0.101325 * m} \end{array}$ (5)

Figure 3.

Schematic map of the CH₄ isothermal adsorption experimental device. Note: 1. Helium cylinder; 2.High-pressure methane cylinder; 3. Reference tank; 4. Resistance composite vacuum meter; 5. Direct-coupled rotary vane vacuum pump; 6. Data acquisition panel; 7. PC; 8. Coal sample tank; 9. Temperature control system; 10. Desorption instrument; a–g—valves; h–i—high-precision pressure sensors; A–E—three way connectors.

where Q is the quantity of adsorbed methane, cm³/g; Q₁ is the volume of gas in the reference tank before inflation, cm³. Q₂ is the volume of gas in the reference tank before inflation, cm³. Q₃ is the volume of free methane in the coal sample tank and pipeline after adsorption equilibrium, cm³; V is the volume of reference tank and pipeline. The temperatures (T) are 30°C and 40°C; P₁ is the pressure of gas in the reference tank before inflation, MPa. P₂ is the pressure of gas in the reference tank after inflation, MPa; Z₁ and Z₂ are the compression factors for the methane pressures P₁ and P₂, respectively; P is the adsorption equilibrium pressure of the experimental coal samples, MPa; V₁ is the volume of free methane in the experimental coal sample tank and pipeline after adsorption equilibrium, cm³. Z is the compression factor for methane pressures P₁ and P₂, respectively. m is the quality of the coal sample, g.

8. The experimental process of the other adsorption equilibrium pressure tests is the same as the above steps.

Results and discussion

LNA isotherm analysis

According to the procedures of the LNA experiment, the LNA isotherms of coals with different destruction types could be obtained, which are shown in Figure 4. As shown in Figure 4, whether for the Jiulishan coal sample or the Zhongmacun coal sample, the adsorption capacity of fully pulverized coal (V) is the greatest. The adsorption capacity of the Jiulishan samples increases by the following order: J2, J3, J4, and J5. The adsorption capacity of the Zhongmacun samples increases by the following order: Z1, Z3, Z4, and Z5. The adsorption capacity increases with increasing destruction type.

Figure 4.

Liquid nitrogen adsorption–desorption isotherms of coals with different destruction types. (a) Jiulishan coal sample. (b) Zhongmacun coal sample.

The physical adsorption isotherms were classified into six types (Figure 5) based on International Union of Pure and Applied Chemistry (IUPAC) guidelines. Type I and type II are the most common types. The isotherm curves of low-temperature liquid nitrogen for the coal samples J2, J3, J4, J5, Z1, and Z3 are type II. However, the isotherm curves of low-temperature liquid nitrogen for coal samples Z4 and Z5 are type I-B, which shows that the adsorption potential of the coal samples micropores is very large. All coal samples contain relatively continuous complete pore systems. The pore models are shown in Figure 6. There are two pore types, namely, semiclosed pores and opened pores. The semiclosed pores include cylinder pores (Figure 6(a)), parallel plane pores (Figure 6(b)), wedge-shaped pores (Figure 6(c)), tapered pores (Figure 6(d)), and ink-bottle pores (Figure 6(e)), all of which are closed at one end. The opened pores include cylinder pores (Figure 6(f)) and slit-like pores (Figure 6(g)), both of which are opened at two ends. The desorption curve shows a downward inflection point in the range of relative pressure between 0.4 and 0.6, which indicates that a large amount of adsorbed gas has desorbed into the free state in an instant due to the existence of a special pore structure similar to the ink-bottle type.

Figure 5.

IUPAC classification adsorption isotherms.

Figure 6.

Pore structures and models.

All the LNA–desorption isotherms (Figure 4) exhibit hysteresis, which is generated by completely different processes. The adsorption curve is closely relevant to condensation. The desorption curve is relevant to the evaporation of the condensed liquid. However, the hysteresis loops are usually related to capillary condensation in transition pores. In general, the sorption environment and adsorbent determine the type of hysteresis loop (Machin, 1994). The hysteresis loops of the coal samples from the Jiulishan or Zhongmacun coal mine become more prominent with increasing destruction type. According to the IUPAC, adsorption loops have four types, namely, H₁, H₂, H₃, and H₄, which are shown in Figure 7.

Figure 7.

IUPAC classification of adsorption hysteresis loops.

The hysteresis loops of the coal samples J2, J3, J4, J5, Z1, and Z3 belong to type H₃. However, the hysteresis loops of the experimental coal samples Z4 and Z5 belong to type H₄, which shows that some narrow slit-like pores exist in the experimental coal samples (Z4 and Z5). In addition, the hysteresis loops are not closed when the pressure is low. There are several reasons for this phenomenon. First, the pore deformation of the coal body is caused by LNA or filling. Second, some liquid nitrogen molecules cannot be released. Third, the nitrogen molecules are trapped by the adsorption potential. Fourth, the existence of ink-bottle pores in the experimental coal sample may result in this situation (Cai et al., 2013; Larsen, 2004; Neimark et al., 2000; Ravikovitch et al., 1995).

The specific surface areas of coal samples are measured by the BET method. The testing results are shown in Figure 8. As shown in Figure 8, whether for the Jiulishan coal samples or the Zhongmacun coal samples, the specific surface areas of the experimental coal samples increase with increasing destruction type. The reason for this phenomenon is as follows. The specific surface area is closely related to the micropore structure of the coal body. The structural response of coal with different destruction types is different under the action of periodic and anisotropic tectonic stress. The coal sample with a low degree of destruction can increase the contact area by displacement, rotation, and rearrangement of the coal skeleton particles. The internal stress of the coal skeleton particles is again balanced. With the increase of destruction degree, the development of multigroup fissures gradually disappears in macroscopic aspects, and a large number of nanopores are generated in microscopic aspects. The tectonic stress produces strong crushing and pulverization, which has a relatively strong impact on the micropores. The specific surface areas of coal samples Z4 and Z5 are larger than those of the other samples, which may imply that coal samples Z4 and Z5 have a relatively high adsorption capacity.

Figure 8.

BET specific surface areas of the coal samples.

The percentage of the stage pore volume of coal samples with different destruction types measured by the BJH method is shown in Figure 9. The total pore volume increases with increasing destruction type. The pore volume in different stages increases correspondingly with increasing destruction type. Thus, the methane adsorption space also increases with increasing destruction type. This phenomenon shows that the proportions of micropores, transition pores, and mesopores were changed by tectonic deformation. Some macropores or fractures were transformed into micropores, transition pores, and mesopores under the action of tectonic stress. The percentage of the sum of the transition pore volume and mesopore pore volume decreases with increasing destruction type. However, the percentage of micropore pore volume increases with increasing destruction type, which implies that this effect is more conducive to the generation of micropore with increasing the tectonic action. For all the experimental samples, there is a peak value at approximately 50 nm on the x-axis, which indicates that the pore has a large contribution to the transition pore volume in the transition pore stage. In the range of approximately 50–100 nm, the pore volume of all experimental coal samples decreases with increasing pore diameter. Subsequently, the pore volume increases sharply in the range of 100–300 nm and then slowly at approximately 150 nm, which indicates that the pore makes a large contribution to the mesopore pore volume in the mesopore stage.

Figure 9.

The percentage of the stage pore volume and the pore volume in different stages. (a) The percentage of the stage pore volume. (b) The pore volume in different stages.

Yao et al. (2014) tested the specific surface area and pore volume of tectonic coals, including cataclastic coal, fractionation coal, scaly coal, and mylonite coal; this test was different from the classification method used in this paper. The results are shown in Table 3. The specific surface area and pore volume increase with increasing destruction type, similar to the results of this study. Xu et al. (2019) used Zhaozhuang coal mine area samples to test the specific surface area and pore volume of primary structure coal and tectonic coal based on the LNA method. The results, also shown in Table 3, indicate that the specific surface area and pore volume of tectonic coal are larger than those of primary structure coal. This phenomenon illustrates that tectonic action increases the specific surface area and pore volume of coal.

Table 3.

Research results of specific surface area and pore volume of coal samples in the literature.

Cited source	Coal sample	Specific surface area (m²/g)	Pore volume (cm³/g)
Yao et al. (2014)	Cataclastic coal	0.062	0.000694
	Fractionation coal	0.177	0.00163
	Scaly coal	0.791	0.00409
	Mylonite coal	6.584	0.013
Xu et al. (2019)	Primary structure coal	0.0958	0.000366
Xu et al. (2019)	Tectonic coal	0.2298	0.0013

MIP experiment analysis

The MIP curves of the coal samples with different destruction types could be obtained based on the procedures of the MIP experiment, which are shown in Figure 10. As shown in Figure 10, the curve of mercury intrusion of coal sample Z1 was essentially coincided with the curve of mercury extrusion. However, the MIP curves of other experimental coal samples all have hysteresis loops. The hysteresis loops of the strongly destructive coal, pulverized coal, and fully pulverized coal are obvious, showing that some ink bottle pores and open pores exist in the experimental coal samples. The mercury intrusion experiment of strongly destructive coal, pulverized coal, and fully pulverized coal shows that mercury intake increases almost linearly at the beginning and slowly at the end. There is no part similar to the horizontal stage, which indicates that the transition pore volume and micropore pore volume of strongly destructive coal, pulverized coal, and fully pulverized coal are large.

Figure 10.

The MIP curves for coals with different destruction types. (a) Jiulishan coal samples. (b) Zhongmacun coal samples.

The total pore volumes of the coal samples with different destruction types are shown in Figure 11. The total pore volumes of the Jiulishan and Zhongmacun coal samples with different destruction types increase by the following order: J2, J3, J5, J4 and Z1, Z3, Z5, and Z4. The total pore volumes of pulverized coals J4 and Z4 are the highest. The destructive coal and strongly destructive coal are less affected by tectonic stress that the other types. Therefore, the pore volumes of destructive coal and strongly destructive coal are small. However, the pore volume of fully pulverized coal is not the highest. Under the destruction action of tectonic stress, the coal body becomes soft, and the number of nanoscale pores further increases. In practical research and application, the high pressure produced by a micrometric instrument causes pore collapse, which indicates that the coal matrix is compressed. Only for the fully pulverized coal is the Protodyakonov coefficient is <0.5. It also shows that when the protodyakonov coefficient is <0.5, the compression effect of the coal matrix is obvious. In the previous literature on the study of the coal pore fractal dimension, the compression effect of the coal matrix also has been proven to exist based on the MIP experiment (Li et al., 1999; Zhang et al., 2018). Therefore, some pore volumes cannot be measured, and the measured results deviate from the theoretical values. The mercury extrusion efficiency can be used to characterize the connectivity of coal samples. The mercury extrusion efficiency can be calculated by equation (6). $η = \frac{S_{max} - S_{R}}{S_{\max}} \times 100 %$ (6)where $η$ is the mercury extrusion efficiency, %. S_m is the maximum saturation for mercury intrusion, %; and S_R is the residual saturation of mercury, %.

Figure 11.

The total pore volume of the experimental coal samples with different destruction types.

The mercury extrusion efficiencies of the Jiulishan coal samples J2, J3, J4, and J5 are as follows: 43.46%, 40.97%, 35.70%, and 32.82%, respectively. The mercury extrusion efficiencies of the Zhongmacun coal samples Z1, Z3, Z4, and Z5 are as follows: 52.37%, 43.44%, 34.10%, and 32.45%, respectively. The mercury extrusion efficiency decreases with the increasing destruction type, which indicates that the pore structure becomes more complicated and the coal connectivity becomes worse. The mercury extrusion efficiency of the J5 and Z5 coal samples is the lowest, which shows that the ink-bottle pores account for a large proportion of the pore system of J5 and Z5 coal samples. This phenomenon can easily lead to coal and gas outbursts. The distribution of the pore volume includes two types: incremental distribution and accumulative pore volume distribution (Huang et al., 2017; Kuila et al., 2012). The incremental pore volume distributions of coals with different destruction types are shown in Figure 12. Whether for the Jiulishan coal samples or the Zhongmacun coal samples, there is a peak value of the incremental pore volume at 10 nm on the x-axis. The incremental pore volume corresponds to the pore diameter, which contributes greatly to the total pore volume of the coal sample. The pore volumes of the nondestructive coal and destructive coal are lower, between 100 nm and 10,000 nm, and the curve trend is gentle, which indicates that the distributions of the pore size are relatively uniform during this stage. The pore volumes of strongly destructive coal (type III), pulverized coal (type IV), and fully pulverized coal (type V) are larger than the pore volumes of nondestructive coal and destructive coal between 100 nm and 100,000 nm. The fluctuation of the curve is obvious, which also indicates that the distribution of pore sizes is uneven during this stage. For strongly destructive coal (III), pulverized coal (IV) and fully pulverized coal (V), the pore volume increases with increasing destruction type between 100 nm and 10,000 nm. For the Jiulishan coal samples, the pore volume of the micropores and transition pores is highest, and the pore volume of the mesopores and macropores is between that of the strongly destructive coal (III) and pulverized coal (IV). As previously mentioned, the volume of the mesopores is maximal based on the LPA experiment. This result shows that some mesopores are deformed or transformed into micropores under the action of high-pressure mercury. For the Zhongmacun coal samples, the volumes of the micropores, transition pores, and mesopores of the fully pulverized coal (V) are smaller than those of pulverized coal (IV). However, compared to pulverized coal (IV), the volume of the micropores, transition pores, and mesopores of the fully pulverized coal (V) are maximal based on the LPA experiment, which indicates that the high-pressure mercury causes some pores to deform and even collapse. Compared to the characteristics of the other coal samples, the f value of the coal sample Z5 is the smallest, and the high-pressure mercury intrusion has the greatest influence on the pores of that coal sample. Therefore, a comprehensive analysis of the Jiulishan coal samples and the Zhongmacun coal samples revealed that high-pressure mercury has a great influence on the mesopores, namely, deformation or collapse. The smaller the f value is, the wider is the pore range affected by high-pressure mercury.

Figure 12.

The incremental pore volume distribution. (a) Jiulishan coal samples. (b) Zhongmacun coal samples.

CH₄ adsorption capacity of coal with different destruction types

CBM has two kinds of states: adsorbed-state methane and free-state methane. The adsorbed state usually accounts for more than 80% (Naveen et al., 2017; Wang and Tang, 2018). The methane adsorption capacity is closely related to the CBM content in coal seams. However, the pore structure affects the methane adsorption performance. Therefore, it is essential to measure the characteristics of gas adsorption for coals with different destruction types.

The adsorbed methane capacity of coal can be measured at different pressures by using the CH₄ isothermal adsorption experimental device (Figure 3), and the results are shown in Figure 13. The experimental temperatures are set as 303.15 K and 313.15 K, which are the temperatures of coal with different destruction-type reservoirs at sampling points. As shown in Figure 13, the methane adsorption capacity increases with increasing adsorption equilibrium pressure; however, the rate of increase is decreasing. The adsorption capacity increases with increasing destruction type at the same temperature. That is, the stronger the tectonic action is, the greater the coal adsorption capacity is. The adsorption isotherms of coal with different destruction types are in good agreement with the Langmuir equation, and the law is similar. The Langmuir equation is as follows (Guan et al., 2018; Meng et al., 2018; Wang et al., 2017). Zhao et al. (2019) and Li (2015) obtained the adsorption capacity of the primary structure coal and tectonic coal by a WY-98A gas adsorption constant analyzer at a temperature of 303 K, as shown in Figure 14. As shown in Figure 14, the adsorption quantity increases with increasing adsorption equilibrium pressure. Compared to the characteristics of primary structure coal, tectonic coal has a greater adsorption capacity, which agrees with the results of this study in terms of tectonic action. $V = \frac{abp}{1 + bp} = \frac{V_{L} p}{P_{L} + p}$ (7)where V is the adsorbed gas volume, cm³/g; V_L is the Langmuir volume, mL/g, which is the limit adsorption amount; $P_{L} = 1 / b$ is the Langmuir pressure, MPa; and P is adsorption equilibrium pressure, MPa.

Figure 13.

Adsorption isotherms of coals of different destruction types. (a) Jiulishan coal samples of different destruction types. (b) Zhonmacun coal samples of different destruction types.

Figure 14.

Adsorption isotherms of primary structure coal and tectonic coal.

The Langmuir volume (V_L) of coal with different destruction types increases with increasing destruction types. For coals with different destruction types, the laws of adsorption isotherm evolution are consistent with the pore volume and the specific surface area evolution, which are obtained by the BJH method and BET method, respectively. The adsorption properties of methane on coal with different destruction types are closely related to the pore structure. This finding indicates that the proportion of micropores increases with increasing destruction type. Therefore, the adsorption space and adsorption sites of coal for methane increase correspondingly, and the amount of methane that can be adsorbed naturally increases gradually.

Analysis of the control effect of coals with different destruction types on coal and gas outbursts

The relationships between the porosity, f value, specific surface area, mercury extrusion, efficiency and Langmuir volume of coals with different destruction types are shown in Figure 15.

Figure 15.

The relationships between the porosity, f value, specific surface area, mercury extrusion efficiency, and Langmuir volume of coal. (a) Jiulishan coal samples. (b) Zhongmacun coal samples.

As shown in Figure 15, the porosity value increases with increasing destruction type. The greater the porosity, the better suited it is for storing free gas. Furthermore, the mercury extrusion efficiency decreases with increasing destruction type, the pore structure becomes more complicated, and the coal connectivity becomes worse. That is, the permeability decreases with increasing destruction type, which is consistent with previous literature studies. The specific surface area and Langmuir volume increase with increasing destruction type. Therefore, generally speaking, the gas pressures in coal seams with more extreme types are greater. The three aspects contribute to the dynamic conditions required for coal and gas outburst, corresponding to the internal causes of coal and gas outbursts.

As shown in Figure 15, the f value decreases with increasing destruction type. Therefore, the ability to resist external forces decreases as the destruction type increases, which constitutes an external condition that facilitates the occurrence of coal and gas outbursts, namely, external causes of coal and gas outbursts.

Coals with different destruction types have different deformation ranges. After the mining activities, due to the effect of stress concentrations, the coal with a high destruction type has undergone large compression deformation. When the pressure relief condition is formed, the tension deformation degree with high destruction degree is large, which creates the conditions for rapid desorption and rapid flow of methane. Therefore, this state is more conducive to coal and gas outbursts.

Conclusions

In this paper, according to the coal and gas outburst mine identification specifications, coals with different destruction types are divided into nondestructive coal, destructive coal, strongly destructive coal, pulverized coal, and fully pulverized coal. The pore structure characteristics and adsorption characteristics of coal samples with different destruction types were studied by LNA, MIP, and CH₄ isothermal adsorption experiments. The results show the following:

The specific surface area of the experimental coal sample increases with increasing destruction type. The N₂ adsorption/desorption isotherms of the Jiulishan coal samples (II, III, IV, and V) and the Zhongmacun coal samples (I and III) are of type II. However, the N₂ adsorption/desorption isotherms of Zhongmacun coal samples (IV and V) are of type I-B, ascribed to the distinctive type and amount of pores. The N₂ adsorption/desorption hysteresis loop is not closed when the relative pressure is low in all tested coal samples, ascribed to the existence of ink-bottle pores, the elastic structure of coal and nitrogen affinity in coal. The pore volume obtained by the BJH method in different stages increases correspondingly with increasing destruction type, which is attributed to part of the macropores or fractures being transformed into micropores, transition pores, and mesopores under the action of tectonic stress. With increasing tectonic stress, it is more advantageous to produce micropores.

The pore volume obtained by the MIP experiment increases with increasing destruction type except fully pulverized coal. High-pressure mercury causes pore deformation and collapse. When the Protodyakonov coefficient is <0.5, the compression effect of the pores is obvious. The smaller the f value is, the wider the pore range affected by high-pressure mercury is. The MIP experiment shows that the hysteresis loops are more obvious with increasing destruction types implying that more open pores and ink bottle pores are existed in the coal samples with higher destructive type. The mercury extrusion efficiency decreases with increasing destruction type, which shows that the proportion of ink bottle configurations in the pore system increases with increasing destruction type.

The degree of destruction is positively correlated with the porosity, specific surface area, and Langmuir volume. However, the degree of destruction is negatively correlated with the f value and mercury extrusion efficiency.

Footnotes

Acknowledgments

The authors appreciate the editor and the anonymous reviewers for their careful reviews of this paper.

Declaration of Conflicting Interests

The author(s) declared no potential conflicts of interest with respect to the research,authorship,and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research,authorship,and/or publication of this article: The authors are grateful for financial support from the National Natural Science Foundation of China (No. 51504804;No. 51874122),the support from the key scientific research projects of Henan higher education institutions (No.19B440002),the support from program for innovative research team of Henan Polytechnic University and the support from key scientific research projects of Henan higher education institutions (No. 17IRTSTHN030).

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Investigation of pore structure characteristics and adsorption characteristics of coals with different destruction types

Abstract

Keywords

Introduction

Materials and experimental methods

Sample preparation

Liquid nitrogen adsorption–desorption experiment

Mercury intrusion and extrusion experiment

CH4 isothermal adsorption experiment

Results and discussion

LNA isotherm analysis

MIP experiment analysis

CH4 adsorption capacity of coal with different destruction types

Analysis of the control effect of coals with different destruction types on coal and gas outbursts

Conclusions

Footnotes

Acknowledgments

Declaration of Conflicting Interests

Funding

References

CH₄ isothermal adsorption experiment

CH₄ adsorption capacity of coal with different destruction types