# Effect of Fragmentation on Pore Structures and Fractal Characteristics of Intact Coal and Deformed Coal Based on Experiments

Authors: Mingjie Li; Shouqing Lu; Chengfeng Wang; Jie Liu; Jiang Shi; Taibiao Bei
Journal: Geofluids
 (2022)

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## Abstract

Coal mining has to develop to deeper strata with the increasing demand for coal, which leads to frequent coal and gas outburst disasters. Especially, when there is a deformed coal in the coal seam, the outburst disasters are easier and more intense. Coal and gas outburst disasters can be promoted by the fragmentation. In order to clearly explain the reasons for the impact of fragmentation on deformed coal and gas outburst, mercury intrusion porosimetry, low-temperature liquid N2 adsorption test, and scanning electron microscope were carried out in this paper. The differences in pore characteristics of intact coal and deformed coal under different particle size conditions were analyzed from pore shapes, pore size distributions, and fractal dimensions. The damage paths of fragmentation on various kinds of pores in intact coal are as follows: macropores, mesopores, minipores, and micropores, and those on various kinds of pores in deformed coal are as follows: macropores, micropores, mesopores, and minipores. According to the fractal dimension theory and experimental results, the calculative results of fractal dimensions show that the fragmentation and tectonism reduce the surface roughness and structural complexity of macropores and mesopores and increase those of minipores and micropores in deformed coal. Finally, combined with the coupling effect of fragmentation and tectonism on pores, the reason of deformed coal with high crushing degree in fields of outburst was explained. The research provides the basis for the prevention of coal and gas outburst.


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## Body

## 1. Introduction

China is one of the most important coal production and consumption countries in the world [1–3]. The large increase in coal demand leads to the depletion of shallow coal resources and continues to extend to deep coal resources [4–6]. That leads coal and gas outburst disasters to become more and more serious, which greatly affects the safety of mine production [7–10]. A large number of research results found that there are deformed coal seams in the coal seams of gas outburst [11–14]. Deformed coal is formed by natural coal particles with minimal particle size under tectonic stress. Compared with the intact coal seam, there are characteristics of low permeability, low mechanical strength, high gas content, and weak resistance to disturbance of external mining stress in the deformed coal seam, which leads deformed coal to be prone to outburst [15, 16]. Therefore, it is very important to study the pore structure characteristics of intact coal and deformed coal.At present, the research on pore characteristics is from the aspects of pore shapes, pore size distributions, and fractal dimensions to analyze. The research test of pore shapes mainly include scanning electron microscope (SEM), mercury intrusion porosimetry (MIP), and low-temperature liquid N2 adsorption test (LPNAT). Among, SEM is directly observed to determine pore shapes. According to the results of SEM, Ren et al. [17] observed that closed pores transform into open pores in deformed coal. MIP and LPNAT are based on the results of the curve characteristics to determine pore shapes. According to the results of MIP, Chen et al. [18] and Guo et al. [19] found that the pores are open pores when there is a hysteresis loop in the curves; otherwise, they are closed pores. Deboer [20] and IUPAC [21] divided the hysteresis loops formed by low-temperature liquid N2 adsorption curves and the pore shapes into five categories and made a detailed introduction, respectively. Combining the five types of hysteresis loops which were defined by Deboer [20], Nie et al. [22] found that pore shapes are bottle-shaped pores and cylindrical pores. According to IUPAC, Wang et al. [23] found that hysteresis loops of intact coal and deformed coal samples are H4, which illustrates that the pore shapes are slit pores. Li et al. [24] found that hysteresis loops of coal samples had the comprehensive characteristics of H2 and H3, indicating that there are ink-bottle pores and slit pores.Pore size distributions are analyzed qualitatively and quantitatively by the results of MIP and LPNAT. Liang [25] believed that tectonism mainly caused the increase of macropores and mesopores in deformed coal and had a little effect on micropores. Guo et al. [26], Wang and Yang [27], Dong et al. [28], and Hou et al. [29] found that tectonism damages the original pore structures in coal samples and increases the degree of pulverization of coal, thus increasing the pore volumes and specific surface areas (SSAs). Li and Wu [30] studied on the pore characteristics of coal samples under different particle size conditions and found that the decrease of particle size reduces the connectivity. Wang et al. [31] believed that complex pores become simple pores with the damage of coal, which shortened the gas migration path. Liu [32] and Jin et al. [33] found that the pore volumes and SSAs can be promoted by fragmentation, thereby enhancing the gas adsorption capacity of coal samples.The surface roughness and structural complexity of pores in the coal are great, and the traditional geometric model is difficult to accurately describe them. Therefore, scholars often use fractal theory to describe the pore structures. Wang et al. [34] found that the surface roughness of pores gradually increases and the structural irregularity of pores gradually decreases after the coal sample was affected by tectonism. Song et al. [35] analyzed the fractal characteristics of deformed coal and found that the fractal dimensions are larger and the tectonism is more serious, which shows the larger pore volumes and SSAs. Guo et al. [19] found that the pores become more and more simple with the crushing process, which makes gas migration and storage more convenient. Jiang [36] studied the fractal dimension characteristics of coal samples under different particle size conditions and found that the decrease of particle size leads to the sharp increase in the adsorption/desorption rate of coal samples. Zhao et al. [37] analyzed the boundary point of fracture and matrix by fractal dimensions and proposed the size ranges of matrix.By summarizing the previous literatures, it is found that the current research results for pore structure characteristics mainly have the following problems. (1) For the analysis of pore shapes, most scholars have made qualitative analysis of pore shapes, but there are insufficient results on the change of pore shapes caused by fragmentation. (2) Most literatures only analyze the pore structure characteristics of single coal sample from the perspective of intact coal or deformed coal and lack the comparative study of two coal samples, which cannot fully explain the reason why deformed coal is prone to outburst. (3) The study of pore structure characteristics and fractal dimensions under the coupling effect of fragmentation and tectonism still needs to be further researched.In this paper, the commonly used pore classification method of Hodot et al. [38] was adopted, and the pore sizes of coal samples were divided into four categories: macropores (>1000 nm), mesopores (100~1000 nm), minipores (10~ 100 nm), and micropores (<10 nm). SEM, MIP, and LPNAT were carried out. Firstly, change of pore shapes caused by fragmentation and tectonism in intact coal and deformed coal was analyzed. Then, based on the results of MIP and LPNAT, pore size distributions were analyzed, and the effect of fragmentation on the damage paths of intact coal and deformed coal was proposed. Finally, the relationship between pore volumes and fractal dimensions was analyzed, and the critical particle size ranges of deformed coal were proposed. According to the evolution process of pore damage, the reasons why deformed coal is prone to outburst were comprehensively analyzed. The research results provide the research basis for the prevention and control of coal and gas outburst disasters.


## 2. Samples and Methods

### 2.1. Coal Samples

Intact coal and deformed coal, which come from No. 3 coal seam Daning Coal Mine, Shanxi Province, China, were selected. After repeated tectonism, the coal seam has the phenomenon of “soft and hard stratification.” The stratification phenomenon of coal seam and coal samples for four particle size conditions is shown in Figure1. According to the stratification phenomenon of coal seam in Figure 1, it can be found that the intact coal has clear joints, bedding, bright luster, and high strength. Joints of deformed coal are meaningless, glossy, low hardness, and easy to be kneaded into powder [39]. In order to study the coupling effect of fragmentation and tectonism on the pore structure characteristics, the collected intact coal and deformed coal were crushed in a completely clean hammer crusher, and then, the particle size required for the experiment was obtained by using the standard sieve (1-3 mm, 0.5-1 mm, 0.25-0.5 mm, and 0.074-0.25 mm).Figure 1
Sampling sites and coal samples.The proximate analysis of coal samples was measured by the 5E-MAG6600 automatic industrial analyzer which is produced by Changsha Kaiyuan Instrument Company. Langmuir parameter was measured by the HCA high-pressure capacity method gas adsorption device which is produced by Chongqing Coal Research Institute. Coal hardiness coefficient was measured; the drop hammer method was used for determination [40].The parameter results of coal samples are shown in Table1. The differences in moisture content and ash content between intact coal and deformed coal are not obvious. The ash content of intact coal is obviously lower than that of deformed coal, which shows that there are more impurities such as gangue in deformed coal. The VL and ΔP of deformed coal are larger than those of intact coal, indicating that the adsorption and diffusion ability of deformed coal are stronger. In addition, the coal hardiness coefficient of intact coal is 4.63 times that of deformed coal, which shows that deformed coal is more vulnerable to damage and leads to coal and gas outburst.Table 1
Basic physical parameters of coal samples.
SamplesProximate analysis (%)ΔP (mmHg)Coal hardiness coefficientLangmuir parameterMoisture contentAsh contentVolatile contentVL (m3/t)Vp (MPa-1)Intact coal2.6317.1910.2841.801.6223.981.03Deformed coal2.1924.3811.7345.600.3555.171.02Footnotes:ΔP = index of initial velocity of gas diffusion; VL = Langmuir volume; Vp = reciprocal of Langmuir pressure.


### 2.2. Scanning Electron Microscope (SEM)

SEM was carried out with The FEI Quanta TM250. Pore structures were measured by SEM as shown in Figure2. By observing the results, it can be seen that there are amygdaloidal pores, line pores, and round pores in intact coal. There are obvious slit pores in deformed coal, and there are a large number of broken grains at the slit pores, which indicates that the slit pores are easily destroyed.Figure 2
SEM results of coal samples.
(a)
Intact coal(b)
Intact coal(c)
Deformed coal(d)
Deformed coal


### 2.3. Mercury Intrusion Porosimetry (MIP)

In order to obtain pore shapes and pore size distributions of intact coal and deformed coal, pore structures were measured by MIP. The theoretical basis of MIP is Washburn’s equation. Pore structures were measured by POREMASTER-33 mercury porosimetry analyzer. Washburn’s equation is shown in(1)pm=−2πcosθr,wherepm is the mercury injection pressure (MPa), r is the pore radius (mm), and θ is the contact angle of mercury-coal surface (°).The pore structures of coal samples are compressed in the high pressure of MIP, which causes great errors for the results of minipores and micropores. Therefore, the results of MIP are used to analyze the pore structure characteristics of macropores and mesopores.


### 2.4. Low-Temperature Liquid N2 Adsorption Test (LPNAT)

The pore structure characteristics of minipores and micropores in intact coal and deformed coal were measured by AUTOSORB iQ2 gas adsorption analyzer manufactured by Quantachrome Instruments.


## 2.1. Coal Samples

Intact coal and deformed coal, which come from No. 3 coal seam Daning Coal Mine, Shanxi Province, China, were selected. After repeated tectonism, the coal seam has the phenomenon of “soft and hard stratification.” The stratification phenomenon of coal seam and coal samples for four particle size conditions is shown in Figure1. According to the stratification phenomenon of coal seam in Figure 1, it can be found that the intact coal has clear joints, bedding, bright luster, and high strength. Joints of deformed coal are meaningless, glossy, low hardness, and easy to be kneaded into powder [39]. In order to study the coupling effect of fragmentation and tectonism on the pore structure characteristics, the collected intact coal and deformed coal were crushed in a completely clean hammer crusher, and then, the particle size required for the experiment was obtained by using the standard sieve (1-3 mm, 0.5-1 mm, 0.25-0.5 mm, and 0.074-0.25 mm).Figure 1
Sampling sites and coal samples.The proximate analysis of coal samples was measured by the 5E-MAG6600 automatic industrial analyzer which is produced by Changsha Kaiyuan Instrument Company. Langmuir parameter was measured by the HCA high-pressure capacity method gas adsorption device which is produced by Chongqing Coal Research Institute. Coal hardiness coefficient was measured; the drop hammer method was used for determination [40].The parameter results of coal samples are shown in Table1. The differences in moisture content and ash content between intact coal and deformed coal are not obvious. The ash content of intact coal is obviously lower than that of deformed coal, which shows that there are more impurities such as gangue in deformed coal. The VL and ΔP of deformed coal are larger than those of intact coal, indicating that the adsorption and diffusion ability of deformed coal are stronger. In addition, the coal hardiness coefficient of intact coal is 4.63 times that of deformed coal, which shows that deformed coal is more vulnerable to damage and leads to coal and gas outburst.Table 1
Basic physical parameters of coal samples.
SamplesProximate analysis (%)ΔP (mmHg)Coal hardiness coefficientLangmuir parameterMoisture contentAsh contentVolatile contentVL (m3/t)Vp (MPa-1)Intact coal2.6317.1910.2841.801.6223.981.03Deformed coal2.1924.3811.7345.600.3555.171.02Footnotes:ΔP = index of initial velocity of gas diffusion; VL = Langmuir volume; Vp = reciprocal of Langmuir pressure.


## 2.2. Scanning Electron Microscope (SEM)

SEM was carried out with The FEI Quanta TM250. Pore structures were measured by SEM as shown in Figure2. By observing the results, it can be seen that there are amygdaloidal pores, line pores, and round pores in intact coal. There are obvious slit pores in deformed coal, and there are a large number of broken grains at the slit pores, which indicates that the slit pores are easily destroyed.Figure 2
SEM results of coal samples.
(a)
Intact coal(b)
Intact coal(c)
Deformed coal(d)
Deformed coal


## 2.3. Mercury Intrusion Porosimetry (MIP)

In order to obtain pore shapes and pore size distributions of intact coal and deformed coal, pore structures were measured by MIP. The theoretical basis of MIP is Washburn’s equation. Pore structures were measured by POREMASTER-33 mercury porosimetry analyzer. Washburn’s equation is shown in(1)pm=−2πcosθr,wherepm is the mercury injection pressure (MPa), r is the pore radius (mm), and θ is the contact angle of mercury-coal surface (°).The pore structures of coal samples are compressed in the high pressure of MIP, which causes great errors for the results of minipores and micropores. Therefore, the results of MIP are used to analyze the pore structure characteristics of macropores and mesopores.


## 2.4. Low-Temperature Liquid N2 Adsorption Test (LPNAT)

The pore structure characteristics of minipores and micropores in intact coal and deformed coal were measured by AUTOSORB iQ2 gas adsorption analyzer manufactured by Quantachrome Instruments.


## 3. Results and Analysis

### 3.1. Pore Shape Characteristics

#### 3.1.1. Mercury Injection/Ejection Curves

The mercury injection/ejection curves of coal samples under different particle size conditions are obtained from MIP. As shown in Figure3, the high-pressure segment (parts of small pore sizes) at the tails of mercury ejection curves of the 1-3 mm intact coal and deformed coal is obviously swelling. When the particle size decreases, the swelling of tail gradually flatten. When the particle size is 0.074-0.25 mm, the high-pressure segment of mercury ejection curves shows the horizontal shape. This is mainly due to the large amount of mercury injecting into the low-pressure segment (parts of large pore sizes). This shows that the particle sizes of intact coal and deformed coal are smaller, and the surges of pore volumes in the parts of large pore sizes are more obvious. In addition, it is found that the mercury injects of deformed coal are larger than those of intact coal when the particle size condition is same, indicating that both fragmentation and tectonism can promote developments of macropores and mesopores in deformed coal.Figure 3
Mercury injection and ejection curves.
(a)
Intact coal(b)
Intact coal(c)
Intact coal(d)
Intact coal(e)
Deformed coal(f)
Deformed coal(g)
Deformed coal(h)
Deformed coalAccording to literature [41], when there are hysteresis loops between the two curves, it indicates that there are mainly open pores. When there are no hysteresis loops between the two curves, it shows that there are mainly semiclosed pores. When the mercury injection curves show the upward concave trend and the mercury ejection curves show the downward trend, it shows that there are ink-bottle pores.As shown in Figures3(a)–3(d), there is no hysteresis loop between the mercury injection curve and mercury ejection curve of the 1-3 mm intact coal. As the particle size decreases, there are hysteresis loops in intact coal, and the mercury injection curve shows the obvious upward concave trend. It shows that the shapes of macropores and mesopores are mainly semienclosed pores in the 1-3 mm intact coal. As the particle size decreases, semienclosed pores gradually are opened, and open pores and ink-bottle pores begin to be generated. The shapes of macropores and mesopores are mainly ink-bottle pores in intact coal.As shown in Figures3(e)–3(h), there is the hysteresis loop between the mercury injection curve and mercury ejection curve of the 1-3 mm deformed coal, and the mercury injection curve shows the slightly upward concave trend. As the particle size decreases, the hysteresis loops and the upward concave trends of mercury injection curves are more and more obvious. It shows that the shapes of macropores and mesopores are ink-bottle pores and semiclosed pores in the 1-3 mm deformed coal, and the open pores and ink-bottle pores in deformed coal gradually increase. This also indicates that fragmentation can cause the changes of shapes of macropores and mesopores in intact coal and deformed coal. In addition, the hysteresis loops of the 0.25-0.5 mm and 0.074-0.25 mm deformed coals are similar. It indicates that the fragmentation has a little effect on the shapes of minipores and micropores under the two particle size conditions. When particle sizes are same, the hysteresis loops and upward concave degrees of mercury injection curves of deformed coal are more obvious than those of intact coal. This proves that tectonism promotes pore development and connections of pore structure in deformed coal.


#### 3.1.2. Low-Temperature Liquid N2 Adsorption/Desorption Curves

The LPNAT is usually used to analyze the pore characteristics of minipores and micropores in coal samples. The adsorption curves and desorption curves of intact coal and deformed coal under different particle size conditions are obtained, as shown in Figure4. According to the six types of adsorption isotherms in IUPAC, it can be determined that the adsorption curves are the combinations of type I and type II.Figure 4
Liquid N2 adsorption and desorption curves.
(a)
Intact coal(b)
Intact coal(c)
Intact coal(d)
Intact coal(e)
Deformed coal(f)
Deformed coal(g)
Deformed coal(h)
Deformed coalWhen the relative pressure is less than 0.02, the adsorption capacity increases sharply, and the change trends of curves are consistent with the characteristics of type I. In this process, liquid nitrogen molecules are mainly adsorbed as the form of micropore filling and monolayer adsorption, which shows that there are the strong interactions between coal particles and liquid nitrogen molecules. With the increase of relative pressure, there is the inflection point, which indicates that the monolayer adsorption process of liquid nitrogen molecules is completed, and the multilayer adsorption begins. When the relative pressure is close to 1, there is still no plateau curve, that is, the adsorption capacity is still not saturated, and this curve trend conforms to the characteristics of type II, which indicates the existence of macropore in coal samples. It is worth noting that when the particle size decreases and the relative pressure is close to 1, the phenomenon of unsaturation is more and more obvious. This phenomenon is more obvious in deformed coal, which also shows that fragmentation and tectonism can promote the development of macropores.According to the classification of hysteresis loops in IUPAC and corresponding pore shapes, as shown in Figure5, hysteresis loops of adsorption curves can be divided into five types: H1-H5, and the corresponding pore shapes are conical pores, cylindrical pores, ink-bottle pores, slit pores, and interstice pores, respectively. By Figures 4 and 5, it is found that the hysteresis loops of the 1-3 mm intact coal are H2 (b), and those of intact coal with other three particle sizes conditions are the combination of H2 and H4. This shows that the shapes of minipores and micropores in the 1-3 mm intact coal are ink-bottle pores, and the minipores and micropores transform from slit-like pores into slit pores which are generated by nonrigid particle as the particle size decreases. For deformed coal under different particle size conditions, the hysteresis loops show the combination of H2 and H4, which shows that the shapes of minipores and micropores are ink-bottle pores and slit pores produced by nonrigid particle. This shows that the fragmentation has the certain effect on the shape of minipores and mesopores in intact coal. The shapes of minipores and micropores in deformed coal are not obviously affected. In addition, there is a little difference in the types of hysteresis loops between intact coal and deformed coal under the same particle size condition, which shows that the shapes of minipores and micropores are not sensitive to the tectonism. It is worth noting that when the relative pressure of adsorption isotherm is greater than 0.5, the hysteresis loop width is larger, and open pores are more. According to Figure 4, when the relative pressure>0.5 and the particle size decreases, the hysteresis loop widths of intact coal and deformed coal increase, and the hysteresis loop width of deformed coal is larger than those of intact coal under the same particle size. It shows that fragmentation and tectonism can promote the development of open pores.Figure 5
Hysteresis loops and their corresponding pore shapes.Combined with the above analysis, the fragmentation makes the macropores and mesopores in intact coal gradually transform from the original semiclosed pores to open pores and ink-bottle pores. For the macropores and mesopores in deformed coal, the pore shapes gradually transform from the original ink-bottle pores and semiclosed pores to the ink-bottle pores and open pores. It shows that fragmentation can promote the development of minipores and micropores to open pores, but the effect is not obvious. Tectonism promotes the development of macropores, mesopores, minipores, and micropores in deformed coal to open pores, thereby enhancing the connectivity of deformed coal.


### 3.2. Pore Size Distributions

The pore size distributions (PSDs) and volumes of macropores, mesopores, minipores, and micropores under different particle size conditions are obtained from MIP and LPNAT, respectively, as shown in Figures6 and 7, as well as Tables 2 and 3. The pore size distributions of minipores and micropores are obtained by BJH (Barrett-Joyner-Halenda) theory [42].Figure 6
PSDs of macropores and mesopores.
(a)
Intact coal(b)
Intact coal(c)
Intact coal(d)
Intact coal(e)
Deformed coal(f)
Deformed coal(g)
Deformed coal(h)
Deformed coalFigure 7
PSDs of minipores and micropores.
(a)
Intact coal(b)
Intact coal(c)
Intact coal(d)
Intact coal(e)
Deformed coal(f)
Deformed coal(g)
Deformed coal(h)
Deformed coalTable 2
Pore structure characteristic parameters of macropores and mesopores.
Coal sampleParticle size (mm)Pore volume (×10-3 cm3/g)Total pore volumeMacroporesPercentage (%)MesoporesPercentage (%)Intact coal1-316.261.408.613.2019.680.5-116.551.509.063.6021.750.25-0.532.039.2828.974.6814.610.074-0.2553.7619.1035.5310.8020.09Deformed coal1-323.614.9020.765.7024.150.5-146.618.3017.8112.4026.600.25-0.561.8015.9025.7314.0022.650.074-0.25110.2638.8035.1920.7018.77Footnote: total pore volume is the sum of macropores, mesopores, minipores, and micropores.Table 3
Pore structure characteristic parameters of minipores and micropores.
Coal sampleParticle size (mm)Pore volume (×10-3 cm3/g)Total pore volumeMiniporesPercentage (%)MicroporesPercentage (%)Intact coal1-316.264.3926.997.2744.720.5-116.554.9029.596.5539.600.25-0.532.037.4723.3110.6133.120.074-0.2553.7610.1518.8913.7025.49Deformed coal1-323.615.6323.867.3731.240.5-146.619.9721.3915.9434.200.25-0.561.8011.7118.9520.1932.670.074-0.25110.2615.7614.3035.0031.74Footnote: total pore volume is the sum of macropores, mesopores, minipores, and micropores.


#### 3.2.1. Pore Size Distribution Characteristics of Macropores and Mesopores

According to Figure6, in the pore system with pore size greater than 100 nm, there are peaks of PSDs in the sections of mesopores and macropores in the 1-3 mm intact coal. As the particle size decreases, the peaks gradually transform to the section of macropores. When the particle size decreases to 0.25-0.5 mm, the peaks are completely located in the section of macropores. In addition, the peak values of intact coal gradually increase when the particle size decreases. For deformed coal under different particle size conditions, the pore size ranges corresponding to the peaks change slightly, but the pore size ranges of the peaks are always located in the section of macropores. The peak values do not change significantly as the particle size decreases.According to Table2, the macropore volumes for different particle size conditions are, respectively, 1.40×10−3 cm3/g, 1.50×10−3 cm3/g, 9.28×10−3 cm3/g, and 19.10×10−3 cm3/g in intact coal. The macropore volume in the 0.074-0.25 mm intact coal is 13.64 times larger than in the 1-3 mm intact coal. The mesopore volumes for four particle size conditions are 3.20×10−3 cm3/g, 3.60×10−3 cm3/g, 4.68×10−3 cm3/g, and 10.80×10−3 cm3/g, respectively. The mesopore volume of intact coal with minimum particle size is 3.38 times that with maximum particle size. Comparing the difference multiples between macropore volume and mesopore volume in the intact coal with the largest particle size and the smallest particle size, it is found that the difference multiple of macropore volume is 4.04 times that of mesopore volume. This shows that macropores are sensitive to fragmentation in the pore system of intact coal with pore sizes greater than 100 nm.In the deformed coal, the macropore volumes are4.90×10−3 cm3/g, 8.30×10−3 cm3/g, 15.90×10−3 cm3/g, and 38.80×10−3 cm3/g, respectively, and the mesopore volumes are 5.70×10−3 cm3/g, 12.40×10−3 cm3/g, 14.00×10−3 cm3/g, and 20.70×10−3 cm3/g, respectively. The macropore volume in the 0.074-0.25 mm deformed coal is 7.92 times that in the 1-3 mm deformed coal. The mesopore volume of the minimum particle size is 3.63 times that of maximum particle size. Comparing the difference multiples between macropore volume and mesopore volume in deformed coal with the maximum particle size and minimum particle size, it can be found that the difference multiple of macropore volume is 2.18 times that of mesopore volume. This shows that the effect of fragmentation on macropores is stronger than that on mesopores in the pore system of deformed coal with pore size greater than 100 nm.In addition, when the particle size condition is the same, the difference multiples of macropore volumes in intact coal and deformed coal are 3.50 times, 5.53 times, 1.71 times, and 2.03 times, respectively, and the difference multiples of mesopore volumes are 3.50 times, 5.53 times, 1.71 times, and 2.03 times, respectively. The difference multiples of macropore volumes and mesopore volumes have not exceeded the order of magnitude. This shows that the effect of tectonism on the macropore volumes and mesopore volumes is not significant.


#### 3.2.2. Pore Size Distribution Characteristics of Minipores and Micropores

As shown in Figure7, in the pore system with pore size less than 100 nm, the peaks of PSDs of intact coal under different particle size conditions are mainly located in the section of micropores, and the peaks are located in the pore size ranges of 2-4 nm. When the particle size decreases, the numbers of peaks gradually increase. In addition, the maximum peak values increase when the particle size decreases in intact coal, but there is a turning trend when the particle size is 0.25-0.5 mm. In the deformed coal, the peaks are mainly located in the microporous section with pore size of 2-3 nm. The numbers of peaks and the maximum peak values increase when the particle size decreases. This indicates that fragmentation can promote the development of micropores, thereby the complexities of pore system with pore size less than 100 nm are increased.As shown in Figure7, the minipore volumes of intact coal under different particle size conditions are 4.39×10−3 cm3/g, 4.90×10−3 cm3/g, 7.47×10−3 cm3/g, and 10.15×10−3 cm3/g, respectively. The minipore volume in the 0.074-0.25 mm intact coal is 2.31 times that in the 1-3 mm intact coal. The micropore volumes are 7.27×10−3 cm3/g, 6.55×10−3 cm3/g, 10.61×10−3 cm3/g, and 13.70×10−3 cm3/g, respectively. The micropore volume of intact coal with the minimum particle size is 3.63 times that of maximum particle size. The difference multiple of minipore volume between the maximum particle size and minimum particle size is 1.23 times that of micropores. This shows that the effect of fragmentation on minipores is more obvious than that of micropores in the intact coal with pore size less than 100 nm.When the particle size decreases, the minipore volumes of deformed coal for four particle size conditions are5.63×10−3 cm3/g, 9.97×10−3 cm3/g, 11.71×10−3 cm3/g, and 15.76×10−3 cm3/g. The micropore volumes are 7.37×10−3 cm3/g, 15.94×10−3 cm3/g, 20.19×10−3 cm3/g, and 35.00×10−3 cm3/g. The minipore volume of the minimum particle size is 2.80 times that of the maximum particle size, and the micropore volume of the minimum particle size is 4.75 times that of the maximum particle size, which shows that the effect of fragmentation on micropores is more obvious in deformed coal with pore size less than 100 nm.In addition, when the particle sizes of intact coal and deformed coal are the same, the difference multiples of the minipore volumes are 1.28 times, 2.03 times, 1.57 times, and 1.55 times, respectively, and the difference multiples of the micropore volumes are 1.01 times, 2.43 times, 1.90 times, and 2.55 times, respectively. The difference multiples of minipore volumes and micropore volumes are in the same order of magnitude, which indicates that the effect of tectonism on the micropore volumes and micropore volumes is not obvious.Based on the above analysis, fragmentation can promote the complexity of minipores and micropores in intact coal and deformed coal. According to the quantitative analysis results of pore volumes, the macropore volumes, mesopore volumes, minipore volumes, and micropore volumes in the 0.074-0.25 mm intact coal are 13.64 times, 3.38 times, 2.31 times, and 1.23 times than those in the 1-3 mm intact coal, respectively. The difference multiples of macropore volumes, mesopore volumes, minipore volumes, and micropore volumes are, respectively, 7.92 times, 3.63 times, 2.80 times, and 4.75 times in deformed coal with the minimum particle size and the corresponding maximum particle size. This shows that the effect order of fragmentation on pores in intact coal and deformed coal is different. That is, the effect of fragmentation on pores in intact coal is macropores, mesopores, minipores, and micropores, while that in deformed coal is in the order of macropores, micropores, mesopores, and minipores.


### 3.3. Pore Fractal Characteristics

The fractal dimensions of macropores and mesopores are calculated by using the experimental data of MIP and the calculation equation of fractal dimension (the experimental measurement equation of mercury injection) which is obtained by Menger’s sponge model. Based on the experimental data of low-temperature liquid N2 absorption test and FHH (Frenkel-Halsey-Hill) model, the fractal dimensions of minipores and micropores are calculated. The calculative fractal dimension values are between 2 and 3, and the larger the fractal dimensions are, the larger pores structural complexity and pores surface roughness are. The smaller the fractal dimensions are, the smaller the pores structural complexity and pores surface roughness are [43].The experimental measurement equation of mercury injection is shown in(2)−dVdpm∝r2−D.After combining Equations (1) and (2), Equation (3) can be obtained by logarithmic processing.
(3)lgdVdpm=C+D−4lgpm,whereV is the pore volumes (cm3/g), pm is the mercury injection pressure (MPa), r is the pores radius (mm), C is the constant, and D is the fractal dimensions.The calculative equation of FHH model is shown in(4)lnVaVm=C+D−3lnlnP0P,whereVa is the liquid nitrogen adsorbed volume (cm3/g), Vm is the monolayer adsorbed gas volume (cm3/g), P0 is the adsorbed gas saturated vapor pressure (MPa), and P is the liquid nitrogen adsorbed pressure (MPa).According to the slope of the curve obtained by Equation (3), the fractal dimensions of macropores and mesopores are calculated as D1 and D2, respectively. According to the slope of the curve obtained by Equation (4), the fractal dimensions of minipores and micropores are calculated as D3 and D4, respectively. The curves of fractal dimensions obtained by Equations (3) and (4) are shown in Figures 8 and 9, respectively. The fractal dimensions of macropores, mesopores, minipores, and micropores are recorded in Table 4.Figure 8
Fractal dimensions of macropores and mesopores.
(a)
Intact coal(b)
Intact coal(c)
Intact coal(d)
Intact coal(e)
Deformed coal(f)
Deformed coal(g)
Deformed coal(h)
Deformed coalFigure 9
Fractal dimensions of minipores and micropores.
(a)
Intact coal(b)
Intact coal(c)
Intact coal(d)
Intact coal(e)
Deformed coal(f)
Deformed coal(g)
Deformed coal(h)
Deformed coalTable 4
Fractal dimensions of coal samples.
Coal sampleParticle size (mm)Fractal dimensionsMacropores (D1)Mesopores (D2)Minipores (D3)Micropores (D4)Intact coal1-33.713352.984572.529512.096360.5-13.746142.959632.537002.134620.25-0.52.728902.339972.575642.097640.074-0.252.853312.538282.593012.01327Deformed coal1-32.968142.763782.541822.094630.5-12.995342.782442.589662.465870.25-0.52.993422.581462.477272.523810.074-0.252.800642.486792.433002.17702As shown in Figure8 and Table 4, under the particle size condition greater than 0.5 mm, the fractal dimensions of macropores are greater than 3, and the fitting coefficients are also less than 0.5 in intact coal. Because they do not conform to the fractal law, abandon them. Excluding fractal dimensions of these two particle sizes, the others are between 2 and 3, and the fitting coefficients are also greater than 0.9.In the intact coal, the fractal dimensions of macropores and mesopores are 2.72890-2.85331 and 2.33997-2.98457, respectively. When the particle size decreases, the fractal dimensions of macropores increase, and those of mesopores decrease. This shows that the fragmentation increases the surface roughness and structural complexity of macropores in intact coal and reduces those of mesopores. In the deformed coal, the fractal dimensions of macropores and mesopores are 2.80064-2.99534 and 2.48679-2.78244, respectively. The fractal dimensions of macropores and mesopores decrease with the decrease of particle size. This indicates that the fragmentation makes the surface roughness and structural complexity of macropores and mesopores develop towards simplification in deformed coal. In addition, it is found that the fractal dimensions of macropores and mesopores in deformed coal are less than those in intact coal, when the particle sizes of intact coal and deformed coal are same. This indicates that the tectonism simplifies the surface roughness and structural complexity of macropores and mesopores in deformed coal.As shown in Figure9 and Table 4, for intact coal under different particle size conditions, the fractal dimensions of minipores and micropores are 2.52951-2.59301 and 2.01327-2.13462, respectively. The fractal dimensions of minipores and micropores fluctuate as the particle size decreases, indicating that the fragmentation has no obvious influence on the surface roughness and structural complexity of minipores and micropores. For deformed coal under different particle size conditions, the fractal dimensions of minipores and micropores are 2.43300-2.58966 and 2.09463-2.52381, respectively. When the particle size decreases, the fractal dimensions of minipores decrease, and those of micropores increase. This shows that the fragmentation leads to the simplification of pore surface and pore structure of minipores and makes those of micropores more complicated. In addition, the fractal dimensions of minipores and micropores in deformed coal are larger than those of intact coal when the particle size condition is same, which indicates that tectonism leads to rougher pore surface and more complex pore structure.Based on the above analysis, for intact coal, the fragmentation increases the surface roughness and structural complexity of macropores and decreases those of mesopores but has no obvious effect on those of minipores and micropores. For deformed coal, the effect of fragmentation on macropores, mesopores, and minipores is completely different from that on micropores. Fragmentation simplifies the pore surface and pore structure of macropores, mesopores, and minipores and makes those of micropores more rough and complex. It is worth noting that the pore surface and pore structure of macropores and mesopores are simplified by tectonism, and those of minipores and micropores are complicated by tectonism.


## 3.1. Pore Shape Characteristics

### 3.1.1. Mercury Injection/Ejection Curves

The mercury injection/ejection curves of coal samples under different particle size conditions are obtained from MIP. As shown in Figure3, the high-pressure segment (parts of small pore sizes) at the tails of mercury ejection curves of the 1-3 mm intact coal and deformed coal is obviously swelling. When the particle size decreases, the swelling of tail gradually flatten. When the particle size is 0.074-0.25 mm, the high-pressure segment of mercury ejection curves shows the horizontal shape. This is mainly due to the large amount of mercury injecting into the low-pressure segment (parts of large pore sizes). This shows that the particle sizes of intact coal and deformed coal are smaller, and the surges of pore volumes in the parts of large pore sizes are more obvious. In addition, it is found that the mercury injects of deformed coal are larger than those of intact coal when the particle size condition is same, indicating that both fragmentation and tectonism can promote developments of macropores and mesopores in deformed coal.Figure 3
Mercury injection and ejection curves.
(a)
Intact coal(b)
Intact coal(c)
Intact coal(d)
Intact coal(e)
Deformed coal(f)
Deformed coal(g)
Deformed coal(h)
Deformed coalAccording to literature [41], when there are hysteresis loops between the two curves, it indicates that there are mainly open pores. When there are no hysteresis loops between the two curves, it shows that there are mainly semiclosed pores. When the mercury injection curves show the upward concave trend and the mercury ejection curves show the downward trend, it shows that there are ink-bottle pores.As shown in Figures3(a)–3(d), there is no hysteresis loop between the mercury injection curve and mercury ejection curve of the 1-3 mm intact coal. As the particle size decreases, there are hysteresis loops in intact coal, and the mercury injection curve shows the obvious upward concave trend. It shows that the shapes of macropores and mesopores are mainly semienclosed pores in the 1-3 mm intact coal. As the particle size decreases, semienclosed pores gradually are opened, and open pores and ink-bottle pores begin to be generated. The shapes of macropores and mesopores are mainly ink-bottle pores in intact coal.As shown in Figures3(e)–3(h), there is the hysteresis loop between the mercury injection curve and mercury ejection curve of the 1-3 mm deformed coal, and the mercury injection curve shows the slightly upward concave trend. As the particle size decreases, the hysteresis loops and the upward concave trends of mercury injection curves are more and more obvious. It shows that the shapes of macropores and mesopores are ink-bottle pores and semiclosed pores in the 1-3 mm deformed coal, and the open pores and ink-bottle pores in deformed coal gradually increase. This also indicates that fragmentation can cause the changes of shapes of macropores and mesopores in intact coal and deformed coal. In addition, the hysteresis loops of the 0.25-0.5 mm and 0.074-0.25 mm deformed coals are similar. It indicates that the fragmentation has a little effect on the shapes of minipores and micropores under the two particle size conditions. When particle sizes are same, the hysteresis loops and upward concave degrees of mercury injection curves of deformed coal are more obvious than those of intact coal. This proves that tectonism promotes pore development and connections of pore structure in deformed coal.


### 3.1.2. Low-Temperature Liquid N2 Adsorption/Desorption Curves

The LPNAT is usually used to analyze the pore characteristics of minipores and micropores in coal samples. The adsorption curves and desorption curves of intact coal and deformed coal under different particle size conditions are obtained, as shown in Figure4. According to the six types of adsorption isotherms in IUPAC, it can be determined that the adsorption curves are the combinations of type I and type II.Figure 4
Liquid N2 adsorption and desorption curves.
(a)
Intact coal(b)
Intact coal(c)
Intact coal(d)
Intact coal(e)
Deformed coal(f)
Deformed coal(g)
Deformed coal(h)
Deformed coalWhen the relative pressure is less than 0.02, the adsorption capacity increases sharply, and the change trends of curves are consistent with the characteristics of type I. In this process, liquid nitrogen molecules are mainly adsorbed as the form of micropore filling and monolayer adsorption, which shows that there are the strong interactions between coal particles and liquid nitrogen molecules. With the increase of relative pressure, there is the inflection point, which indicates that the monolayer adsorption process of liquid nitrogen molecules is completed, and the multilayer adsorption begins. When the relative pressure is close to 1, there is still no plateau curve, that is, the adsorption capacity is still not saturated, and this curve trend conforms to the characteristics of type II, which indicates the existence of macropore in coal samples. It is worth noting that when the particle size decreases and the relative pressure is close to 1, the phenomenon of unsaturation is more and more obvious. This phenomenon is more obvious in deformed coal, which also shows that fragmentation and tectonism can promote the development of macropores.According to the classification of hysteresis loops in IUPAC and corresponding pore shapes, as shown in Figure5, hysteresis loops of adsorption curves can be divided into five types: H1-H5, and the corresponding pore shapes are conical pores, cylindrical pores, ink-bottle pores, slit pores, and interstice pores, respectively. By Figures 4 and 5, it is found that the hysteresis loops of the 1-3 mm intact coal are H2 (b), and those of intact coal with other three particle sizes conditions are the combination of H2 and H4. This shows that the shapes of minipores and micropores in the 1-3 mm intact coal are ink-bottle pores, and the minipores and micropores transform from slit-like pores into slit pores which are generated by nonrigid particle as the particle size decreases. For deformed coal under different particle size conditions, the hysteresis loops show the combination of H2 and H4, which shows that the shapes of minipores and micropores are ink-bottle pores and slit pores produced by nonrigid particle. This shows that the fragmentation has the certain effect on the shape of minipores and mesopores in intact coal. The shapes of minipores and micropores in deformed coal are not obviously affected. In addition, there is a little difference in the types of hysteresis loops between intact coal and deformed coal under the same particle size condition, which shows that the shapes of minipores and micropores are not sensitive to the tectonism. It is worth noting that when the relative pressure of adsorption isotherm is greater than 0.5, the hysteresis loop width is larger, and open pores are more. According to Figure 4, when the relative pressure>0.5 and the particle size decreases, the hysteresis loop widths of intact coal and deformed coal increase, and the hysteresis loop width of deformed coal is larger than those of intact coal under the same particle size. It shows that fragmentation and tectonism can promote the development of open pores.Figure 5
Hysteresis loops and their corresponding pore shapes.Combined with the above analysis, the fragmentation makes the macropores and mesopores in intact coal gradually transform from the original semiclosed pores to open pores and ink-bottle pores. For the macropores and mesopores in deformed coal, the pore shapes gradually transform from the original ink-bottle pores and semiclosed pores to the ink-bottle pores and open pores. It shows that fragmentation can promote the development of minipores and micropores to open pores, but the effect is not obvious. Tectonism promotes the development of macropores, mesopores, minipores, and micropores in deformed coal to open pores, thereby enhancing the connectivity of deformed coal.


## 3.1.1. Mercury Injection/Ejection Curves

The mercury injection/ejection curves of coal samples under different particle size conditions are obtained from MIP. As shown in Figure3, the high-pressure segment (parts of small pore sizes) at the tails of mercury ejection curves of the 1-3 mm intact coal and deformed coal is obviously swelling. When the particle size decreases, the swelling of tail gradually flatten. When the particle size is 0.074-0.25 mm, the high-pressure segment of mercury ejection curves shows the horizontal shape. This is mainly due to the large amount of mercury injecting into the low-pressure segment (parts of large pore sizes). This shows that the particle sizes of intact coal and deformed coal are smaller, and the surges of pore volumes in the parts of large pore sizes are more obvious. In addition, it is found that the mercury injects of deformed coal are larger than those of intact coal when the particle size condition is same, indicating that both fragmentation and tectonism can promote developments of macropores and mesopores in deformed coal.Figure 3
Mercury injection and ejection curves.
(a)
Intact coal(b)
Intact coal(c)
Intact coal(d)
Intact coal(e)
Deformed coal(f)
Deformed coal(g)
Deformed coal(h)
Deformed coalAccording to literature [41], when there are hysteresis loops between the two curves, it indicates that there are mainly open pores. When there are no hysteresis loops between the two curves, it shows that there are mainly semiclosed pores. When the mercury injection curves show the upward concave trend and the mercury ejection curves show the downward trend, it shows that there are ink-bottle pores.As shown in Figures3(a)–3(d), there is no hysteresis loop between the mercury injection curve and mercury ejection curve of the 1-3 mm intact coal. As the particle size decreases, there are hysteresis loops in intact coal, and the mercury injection curve shows the obvious upward concave trend. It shows that the shapes of macropores and mesopores are mainly semienclosed pores in the 1-3 mm intact coal. As the particle size decreases, semienclosed pores gradually are opened, and open pores and ink-bottle pores begin to be generated. The shapes of macropores and mesopores are mainly ink-bottle pores in intact coal.As shown in Figures3(e)–3(h), there is the hysteresis loop between the mercury injection curve and mercury ejection curve of the 1-3 mm deformed coal, and the mercury injection curve shows the slightly upward concave trend. As the particle size decreases, the hysteresis loops and the upward concave trends of mercury injection curves are more and more obvious. It shows that the shapes of macropores and mesopores are ink-bottle pores and semiclosed pores in the 1-3 mm deformed coal, and the open pores and ink-bottle pores in deformed coal gradually increase. This also indicates that fragmentation can cause the changes of shapes of macropores and mesopores in intact coal and deformed coal. In addition, the hysteresis loops of the 0.25-0.5 mm and 0.074-0.25 mm deformed coals are similar. It indicates that the fragmentation has a little effect on the shapes of minipores and micropores under the two particle size conditions. When particle sizes are same, the hysteresis loops and upward concave degrees of mercury injection curves of deformed coal are more obvious than those of intact coal. This proves that tectonism promotes pore development and connections of pore structure in deformed coal.


## 3.1.2. Low-Temperature Liquid N2 Adsorption/Desorption Curves

The LPNAT is usually used to analyze the pore characteristics of minipores and micropores in coal samples. The adsorption curves and desorption curves of intact coal and deformed coal under different particle size conditions are obtained, as shown in Figure4. According to the six types of adsorption isotherms in IUPAC, it can be determined that the adsorption curves are the combinations of type I and type II.Figure 4
Liquid N2 adsorption and desorption curves.
(a)
Intact coal(b)
Intact coal(c)
Intact coal(d)
Intact coal(e)
Deformed coal(f)
Deformed coal(g)
Deformed coal(h)
Deformed coalWhen the relative pressure is less than 0.02, the adsorption capacity increases sharply, and the change trends of curves are consistent with the characteristics of type I. In this process, liquid nitrogen molecules are mainly adsorbed as the form of micropore filling and monolayer adsorption, which shows that there are the strong interactions between coal particles and liquid nitrogen molecules. With the increase of relative pressure, there is the inflection point, which indicates that the monolayer adsorption process of liquid nitrogen molecules is completed, and the multilayer adsorption begins. When the relative pressure is close to 1, there is still no plateau curve, that is, the adsorption capacity is still not saturated, and this curve trend conforms to the characteristics of type II, which indicates the existence of macropore in coal samples. It is worth noting that when the particle size decreases and the relative pressure is close to 1, the phenomenon of unsaturation is more and more obvious. This phenomenon is more obvious in deformed coal, which also shows that fragmentation and tectonism can promote the development of macropores.According to the classification of hysteresis loops in IUPAC and corresponding pore shapes, as shown in Figure5, hysteresis loops of adsorption curves can be divided into five types: H1-H5, and the corresponding pore shapes are conical pores, cylindrical pores, ink-bottle pores, slit pores, and interstice pores, respectively. By Figures 4 and 5, it is found that the hysteresis loops of the 1-3 mm intact coal are H2 (b), and those of intact coal with other three particle sizes conditions are the combination of H2 and H4. This shows that the shapes of minipores and micropores in the 1-3 mm intact coal are ink-bottle pores, and the minipores and micropores transform from slit-like pores into slit pores which are generated by nonrigid particle as the particle size decreases. For deformed coal under different particle size conditions, the hysteresis loops show the combination of H2 and H4, which shows that the shapes of minipores and micropores are ink-bottle pores and slit pores produced by nonrigid particle. This shows that the fragmentation has the certain effect on the shape of minipores and mesopores in intact coal. The shapes of minipores and micropores in deformed coal are not obviously affected. In addition, there is a little difference in the types of hysteresis loops between intact coal and deformed coal under the same particle size condition, which shows that the shapes of minipores and micropores are not sensitive to the tectonism. It is worth noting that when the relative pressure of adsorption isotherm is greater than 0.5, the hysteresis loop width is larger, and open pores are more. According to Figure 4, when the relative pressure>0.5 and the particle size decreases, the hysteresis loop widths of intact coal and deformed coal increase, and the hysteresis loop width of deformed coal is larger than those of intact coal under the same particle size. It shows that fragmentation and tectonism can promote the development of open pores.Figure 5
Hysteresis loops and their corresponding pore shapes.Combined with the above analysis, the fragmentation makes the macropores and mesopores in intact coal gradually transform from the original semiclosed pores to open pores and ink-bottle pores. For the macropores and mesopores in deformed coal, the pore shapes gradually transform from the original ink-bottle pores and semiclosed pores to the ink-bottle pores and open pores. It shows that fragmentation can promote the development of minipores and micropores to open pores, but the effect is not obvious. Tectonism promotes the development of macropores, mesopores, minipores, and micropores in deformed coal to open pores, thereby enhancing the connectivity of deformed coal.


## 3.2. Pore Size Distributions

The pore size distributions (PSDs) and volumes of macropores, mesopores, minipores, and micropores under different particle size conditions are obtained from MIP and LPNAT, respectively, as shown in Figures6 and 7, as well as Tables 2 and 3. The pore size distributions of minipores and micropores are obtained by BJH (Barrett-Joyner-Halenda) theory [42].Figure 6
PSDs of macropores and mesopores.
(a)
Intact coal(b)
Intact coal(c)
Intact coal(d)
Intact coal(e)
Deformed coal(f)
Deformed coal(g)
Deformed coal(h)
Deformed coalFigure 7
PSDs of minipores and micropores.
(a)
Intact coal(b)
Intact coal(c)
Intact coal(d)
Intact coal(e)
Deformed coal(f)
Deformed coal(g)
Deformed coal(h)
Deformed coalTable 2
Pore structure characteristic parameters of macropores and mesopores.
Coal sampleParticle size (mm)Pore volume (×10-3 cm3/g)Total pore volumeMacroporesPercentage (%)MesoporesPercentage (%)Intact coal1-316.261.408.613.2019.680.5-116.551.509.063.6021.750.25-0.532.039.2828.974.6814.610.074-0.2553.7619.1035.5310.8020.09Deformed coal1-323.614.9020.765.7024.150.5-146.618.3017.8112.4026.600.25-0.561.8015.9025.7314.0022.650.074-0.25110.2638.8035.1920.7018.77Footnote: total pore volume is the sum of macropores, mesopores, minipores, and micropores.Table 3
Pore structure characteristic parameters of minipores and micropores.
Coal sampleParticle size (mm)Pore volume (×10-3 cm3/g)Total pore volumeMiniporesPercentage (%)MicroporesPercentage (%)Intact coal1-316.264.3926.997.2744.720.5-116.554.9029.596.5539.600.25-0.532.037.4723.3110.6133.120.074-0.2553.7610.1518.8913.7025.49Deformed coal1-323.615.6323.867.3731.240.5-146.619.9721.3915.9434.200.25-0.561.8011.7118.9520.1932.670.074-0.25110.2615.7614.3035.0031.74Footnote: total pore volume is the sum of macropores, mesopores, minipores, and micropores.


### 3.2.1. Pore Size Distribution Characteristics of Macropores and Mesopores

According to Figure6, in the pore system with pore size greater than 100 nm, there are peaks of PSDs in the sections of mesopores and macropores in the 1-3 mm intact coal. As the particle size decreases, the peaks gradually transform to the section of macropores. When the particle size decreases to 0.25-0.5 mm, the peaks are completely located in the section of macropores. In addition, the peak values of intact coal gradually increase when the particle size decreases. For deformed coal under different particle size conditions, the pore size ranges corresponding to the peaks change slightly, but the pore size ranges of the peaks are always located in the section of macropores. The peak values do not change significantly as the particle size decreases.According to Table2, the macropore volumes for different particle size conditions are, respectively, 1.40×10−3 cm3/g, 1.50×10−3 cm3/g, 9.28×10−3 cm3/g, and 19.10×10−3 cm3/g in intact coal. The macropore volume in the 0.074-0.25 mm intact coal is 13.64 times larger than in the 1-3 mm intact coal. The mesopore volumes for four particle size conditions are 3.20×10−3 cm3/g, 3.60×10−3 cm3/g, 4.68×10−3 cm3/g, and 10.80×10−3 cm3/g, respectively. The mesopore volume of intact coal with minimum particle size is 3.38 times that with maximum particle size. Comparing the difference multiples between macropore volume and mesopore volume in the intact coal with the largest particle size and the smallest particle size, it is found that the difference multiple of macropore volume is 4.04 times that of mesopore volume. This shows that macropores are sensitive to fragmentation in the pore system of intact coal with pore sizes greater than 100 nm.In the deformed coal, the macropore volumes are4.90×10−3 cm3/g, 8.30×10−3 cm3/g, 15.90×10−3 cm3/g, and 38.80×10−3 cm3/g, respectively, and the mesopore volumes are 5.70×10−3 cm3/g, 12.40×10−3 cm3/g, 14.00×10−3 cm3/g, and 20.70×10−3 cm3/g, respectively. The macropore volume in the 0.074-0.25 mm deformed coal is 7.92 times that in the 1-3 mm deformed coal. The mesopore volume of the minimum particle size is 3.63 times that of maximum particle size. Comparing the difference multiples between macropore volume and mesopore volume in deformed coal with the maximum particle size and minimum particle size, it can be found that the difference multiple of macropore volume is 2.18 times that of mesopore volume. This shows that the effect of fragmentation on macropores is stronger than that on mesopores in the pore system of deformed coal with pore size greater than 100 nm.In addition, when the particle size condition is the same, the difference multiples of macropore volumes in intact coal and deformed coal are 3.50 times, 5.53 times, 1.71 times, and 2.03 times, respectively, and the difference multiples of mesopore volumes are 3.50 times, 5.53 times, 1.71 times, and 2.03 times, respectively. The difference multiples of macropore volumes and mesopore volumes have not exceeded the order of magnitude. This shows that the effect of tectonism on the macropore volumes and mesopore volumes is not significant.


### 3.2.2. Pore Size Distribution Characteristics of Minipores and Micropores

As shown in Figure7, in the pore system with pore size less than 100 nm, the peaks of PSDs of intact coal under different particle size conditions are mainly located in the section of micropores, and the peaks are located in the pore size ranges of 2-4 nm. When the particle size decreases, the numbers of peaks gradually increase. In addition, the maximum peak values increase when the particle size decreases in intact coal, but there is a turning trend when the particle size is 0.25-0.5 mm. In the deformed coal, the peaks are mainly located in the microporous section with pore size of 2-3 nm. The numbers of peaks and the maximum peak values increase when the particle size decreases. This indicates that fragmentation can promote the development of micropores, thereby the complexities of pore system with pore size less than 100 nm are increased.As shown in Figure7, the minipore volumes of intact coal under different particle size conditions are 4.39×10−3 cm3/g, 4.90×10−3 cm3/g, 7.47×10−3 cm3/g, and 10.15×10−3 cm3/g, respectively. The minipore volume in the 0.074-0.25 mm intact coal is 2.31 times that in the 1-3 mm intact coal. The micropore volumes are 7.27×10−3 cm3/g, 6.55×10−3 cm3/g, 10.61×10−3 cm3/g, and 13.70×10−3 cm3/g, respectively. The micropore volume of intact coal with the minimum particle size is 3.63 times that of maximum particle size. The difference multiple of minipore volume between the maximum particle size and minimum particle size is 1.23 times that of micropores. This shows that the effect of fragmentation on minipores is more obvious than that of micropores in the intact coal with pore size less than 100 nm.When the particle size decreases, the minipore volumes of deformed coal for four particle size conditions are5.63×10−3 cm3/g, 9.97×10−3 cm3/g, 11.71×10−3 cm3/g, and 15.76×10−3 cm3/g. The micropore volumes are 7.37×10−3 cm3/g, 15.94×10−3 cm3/g, 20.19×10−3 cm3/g, and 35.00×10−3 cm3/g. The minipore volume of the minimum particle size is 2.80 times that of the maximum particle size, and the micropore volume of the minimum particle size is 4.75 times that of the maximum particle size, which shows that the effect of fragmentation on micropores is more obvious in deformed coal with pore size less than 100 nm.In addition, when the particle sizes of intact coal and deformed coal are the same, the difference multiples of the minipore volumes are 1.28 times, 2.03 times, 1.57 times, and 1.55 times, respectively, and the difference multiples of the micropore volumes are 1.01 times, 2.43 times, 1.90 times, and 2.55 times, respectively. The difference multiples of minipore volumes and micropore volumes are in the same order of magnitude, which indicates that the effect of tectonism on the micropore volumes and micropore volumes is not obvious.Based on the above analysis, fragmentation can promote the complexity of minipores and micropores in intact coal and deformed coal. According to the quantitative analysis results of pore volumes, the macropore volumes, mesopore volumes, minipore volumes, and micropore volumes in the 0.074-0.25 mm intact coal are 13.64 times, 3.38 times, 2.31 times, and 1.23 times than those in the 1-3 mm intact coal, respectively. The difference multiples of macropore volumes, mesopore volumes, minipore volumes, and micropore volumes are, respectively, 7.92 times, 3.63 times, 2.80 times, and 4.75 times in deformed coal with the minimum particle size and the corresponding maximum particle size. This shows that the effect order of fragmentation on pores in intact coal and deformed coal is different. That is, the effect of fragmentation on pores in intact coal is macropores, mesopores, minipores, and micropores, while that in deformed coal is in the order of macropores, micropores, mesopores, and minipores.


## 3.2.1. Pore Size Distribution Characteristics of Macropores and Mesopores

According to Figure6, in the pore system with pore size greater than 100 nm, there are peaks of PSDs in the sections of mesopores and macropores in the 1-3 mm intact coal. As the particle size decreases, the peaks gradually transform to the section of macropores. When the particle size decreases to 0.25-0.5 mm, the peaks are completely located in the section of macropores. In addition, the peak values of intact coal gradually increase when the particle size decreases. For deformed coal under different particle size conditions, the pore size ranges corresponding to the peaks change slightly, but the pore size ranges of the peaks are always located in the section of macropores. The peak values do not change significantly as the particle size decreases.According to Table2, the macropore volumes for different particle size conditions are, respectively, 1.40×10−3 cm3/g, 1.50×10−3 cm3/g, 9.28×10−3 cm3/g, and 19.10×10−3 cm3/g in intact coal. The macropore volume in the 0.074-0.25 mm intact coal is 13.64 times larger than in the 1-3 mm intact coal. The mesopore volumes for four particle size conditions are 3.20×10−3 cm3/g, 3.60×10−3 cm3/g, 4.68×10−3 cm3/g, and 10.80×10−3 cm3/g, respectively. The mesopore volume of intact coal with minimum particle size is 3.38 times that with maximum particle size. Comparing the difference multiples between macropore volume and mesopore volume in the intact coal with the largest particle size and the smallest particle size, it is found that the difference multiple of macropore volume is 4.04 times that of mesopore volume. This shows that macropores are sensitive to fragmentation in the pore system of intact coal with pore sizes greater than 100 nm.In the deformed coal, the macropore volumes are4.90×10−3 cm3/g, 8.30×10−3 cm3/g, 15.90×10−3 cm3/g, and 38.80×10−3 cm3/g, respectively, and the mesopore volumes are 5.70×10−3 cm3/g, 12.40×10−3 cm3/g, 14.00×10−3 cm3/g, and 20.70×10−3 cm3/g, respectively. The macropore volume in the 0.074-0.25 mm deformed coal is 7.92 times that in the 1-3 mm deformed coal. The mesopore volume of the minimum particle size is 3.63 times that of maximum particle size. Comparing the difference multiples between macropore volume and mesopore volume in deformed coal with the maximum particle size and minimum particle size, it can be found that the difference multiple of macropore volume is 2.18 times that of mesopore volume. This shows that the effect of fragmentation on macropores is stronger than that on mesopores in the pore system of deformed coal with pore size greater than 100 nm.In addition, when the particle size condition is the same, the difference multiples of macropore volumes in intact coal and deformed coal are 3.50 times, 5.53 times, 1.71 times, and 2.03 times, respectively, and the difference multiples of mesopore volumes are 3.50 times, 5.53 times, 1.71 times, and 2.03 times, respectively. The difference multiples of macropore volumes and mesopore volumes have not exceeded the order of magnitude. This shows that the effect of tectonism on the macropore volumes and mesopore volumes is not significant.


## 3.2.2. Pore Size Distribution Characteristics of Minipores and Micropores

As shown in Figure7, in the pore system with pore size less than 100 nm, the peaks of PSDs of intact coal under different particle size conditions are mainly located in the section of micropores, and the peaks are located in the pore size ranges of 2-4 nm. When the particle size decreases, the numbers of peaks gradually increase. In addition, the maximum peak values increase when the particle size decreases in intact coal, but there is a turning trend when the particle size is 0.25-0.5 mm. In the deformed coal, the peaks are mainly located in the microporous section with pore size of 2-3 nm. The numbers of peaks and the maximum peak values increase when the particle size decreases. This indicates that fragmentation can promote the development of micropores, thereby the complexities of pore system with pore size less than 100 nm are increased.As shown in Figure7, the minipore volumes of intact coal under different particle size conditions are 4.39×10−3 cm3/g, 4.90×10−3 cm3/g, 7.47×10−3 cm3/g, and 10.15×10−3 cm3/g, respectively. The minipore volume in the 0.074-0.25 mm intact coal is 2.31 times that in the 1-3 mm intact coal. The micropore volumes are 7.27×10−3 cm3/g, 6.55×10−3 cm3/g, 10.61×10−3 cm3/g, and 13.70×10−3 cm3/g, respectively. The micropore volume of intact coal with the minimum particle size is 3.63 times that of maximum particle size. The difference multiple of minipore volume between the maximum particle size and minimum particle size is 1.23 times that of micropores. This shows that the effect of fragmentation on minipores is more obvious than that of micropores in the intact coal with pore size less than 100 nm.When the particle size decreases, the minipore volumes of deformed coal for four particle size conditions are5.63×10−3 cm3/g, 9.97×10−3 cm3/g, 11.71×10−3 cm3/g, and 15.76×10−3 cm3/g. The micropore volumes are 7.37×10−3 cm3/g, 15.94×10−3 cm3/g, 20.19×10−3 cm3/g, and 35.00×10−3 cm3/g. The minipore volume of the minimum particle size is 2.80 times that of the maximum particle size, and the micropore volume of the minimum particle size is 4.75 times that of the maximum particle size, which shows that the effect of fragmentation on micropores is more obvious in deformed coal with pore size less than 100 nm.In addition, when the particle sizes of intact coal and deformed coal are the same, the difference multiples of the minipore volumes are 1.28 times, 2.03 times, 1.57 times, and 1.55 times, respectively, and the difference multiples of the micropore volumes are 1.01 times, 2.43 times, 1.90 times, and 2.55 times, respectively. The difference multiples of minipore volumes and micropore volumes are in the same order of magnitude, which indicates that the effect of tectonism on the micropore volumes and micropore volumes is not obvious.Based on the above analysis, fragmentation can promote the complexity of minipores and micropores in intact coal and deformed coal. According to the quantitative analysis results of pore volumes, the macropore volumes, mesopore volumes, minipore volumes, and micropore volumes in the 0.074-0.25 mm intact coal are 13.64 times, 3.38 times, 2.31 times, and 1.23 times than those in the 1-3 mm intact coal, respectively. The difference multiples of macropore volumes, mesopore volumes, minipore volumes, and micropore volumes are, respectively, 7.92 times, 3.63 times, 2.80 times, and 4.75 times in deformed coal with the minimum particle size and the corresponding maximum particle size. This shows that the effect order of fragmentation on pores in intact coal and deformed coal is different. That is, the effect of fragmentation on pores in intact coal is macropores, mesopores, minipores, and micropores, while that in deformed coal is in the order of macropores, micropores, mesopores, and minipores.


## 3.3. Pore Fractal Characteristics

The fractal dimensions of macropores and mesopores are calculated by using the experimental data of MIP and the calculation equation of fractal dimension (the experimental measurement equation of mercury injection) which is obtained by Menger’s sponge model. Based on the experimental data of low-temperature liquid N2 absorption test and FHH (Frenkel-Halsey-Hill) model, the fractal dimensions of minipores and micropores are calculated. The calculative fractal dimension values are between 2 and 3, and the larger the fractal dimensions are, the larger pores structural complexity and pores surface roughness are. The smaller the fractal dimensions are, the smaller the pores structural complexity and pores surface roughness are [43].The experimental measurement equation of mercury injection is shown in(2)−dVdpm∝r2−D.After combining Equations (1) and (2), Equation (3) can be obtained by logarithmic processing.
(3)lgdVdpm=C+D−4lgpm,whereV is the pore volumes (cm3/g), pm is the mercury injection pressure (MPa), r is the pores radius (mm), C is the constant, and D is the fractal dimensions.The calculative equation of FHH model is shown in(4)lnVaVm=C+D−3lnlnP0P,whereVa is the liquid nitrogen adsorbed volume (cm3/g), Vm is the monolayer adsorbed gas volume (cm3/g), P0 is the adsorbed gas saturated vapor pressure (MPa), and P is the liquid nitrogen adsorbed pressure (MPa).According to the slope of the curve obtained by Equation (3), the fractal dimensions of macropores and mesopores are calculated as D1 and D2, respectively. According to the slope of the curve obtained by Equation (4), the fractal dimensions of minipores and micropores are calculated as D3 and D4, respectively. The curves of fractal dimensions obtained by Equations (3) and (4) are shown in Figures 8 and 9, respectively. The fractal dimensions of macropores, mesopores, minipores, and micropores are recorded in Table 4.Figure 8
Fractal dimensions of macropores and mesopores.
(a)
Intact coal(b)
Intact coal(c)
Intact coal(d)
Intact coal(e)
Deformed coal(f)
Deformed coal(g)
Deformed coal(h)
Deformed coalFigure 9
Fractal dimensions of minipores and micropores.
(a)
Intact coal(b)
Intact coal(c)
Intact coal(d)
Intact coal(e)
Deformed coal(f)
Deformed coal(g)
Deformed coal(h)
Deformed coalTable 4
Fractal dimensions of coal samples.
Coal sampleParticle size (mm)Fractal dimensionsMacropores (D1)Mesopores (D2)Minipores (D3)Micropores (D4)Intact coal1-33.713352.984572.529512.096360.5-13.746142.959632.537002.134620.25-0.52.728902.339972.575642.097640.074-0.252.853312.538282.593012.01327Deformed coal1-32.968142.763782.541822.094630.5-12.995342.782442.589662.465870.25-0.52.993422.581462.477272.523810.074-0.252.800642.486792.433002.17702As shown in Figure8 and Table 4, under the particle size condition greater than 0.5 mm, the fractal dimensions of macropores are greater than 3, and the fitting coefficients are also less than 0.5 in intact coal. Because they do not conform to the fractal law, abandon them. Excluding fractal dimensions of these two particle sizes, the others are between 2 and 3, and the fitting coefficients are also greater than 0.9.In the intact coal, the fractal dimensions of macropores and mesopores are 2.72890-2.85331 and 2.33997-2.98457, respectively. When the particle size decreases, the fractal dimensions of macropores increase, and those of mesopores decrease. This shows that the fragmentation increases the surface roughness and structural complexity of macropores in intact coal and reduces those of mesopores. In the deformed coal, the fractal dimensions of macropores and mesopores are 2.80064-2.99534 and 2.48679-2.78244, respectively. The fractal dimensions of macropores and mesopores decrease with the decrease of particle size. This indicates that the fragmentation makes the surface roughness and structural complexity of macropores and mesopores develop towards simplification in deformed coal. In addition, it is found that the fractal dimensions of macropores and mesopores in deformed coal are less than those in intact coal, when the particle sizes of intact coal and deformed coal are same. This indicates that the tectonism simplifies the surface roughness and structural complexity of macropores and mesopores in deformed coal.As shown in Figure9 and Table 4, for intact coal under different particle size conditions, the fractal dimensions of minipores and micropores are 2.52951-2.59301 and 2.01327-2.13462, respectively. The fractal dimensions of minipores and micropores fluctuate as the particle size decreases, indicating that the fragmentation has no obvious influence on the surface roughness and structural complexity of minipores and micropores. For deformed coal under different particle size conditions, the fractal dimensions of minipores and micropores are 2.43300-2.58966 and 2.09463-2.52381, respectively. When the particle size decreases, the fractal dimensions of minipores decrease, and those of micropores increase. This shows that the fragmentation leads to the simplification of pore surface and pore structure of minipores and makes those of micropores more complicated. In addition, the fractal dimensions of minipores and micropores in deformed coal are larger than those of intact coal when the particle size condition is same, which indicates that tectonism leads to rougher pore surface and more complex pore structure.Based on the above analysis, for intact coal, the fragmentation increases the surface roughness and structural complexity of macropores and decreases those of mesopores but has no obvious effect on those of minipores and micropores. For deformed coal, the effect of fragmentation on macropores, mesopores, and minipores is completely different from that on micropores. Fragmentation simplifies the pore surface and pore structure of macropores, mesopores, and minipores and makes those of micropores more rough and complex. It is worth noting that the pore surface and pore structure of macropores and mesopores are simplified by tectonism, and those of minipores and micropores are complicated by tectonism.


## 4. Discussion

According to the analysis results of PSDs and fractal dimensions, it was found that the fragmentation and tectonism affect the pore structure characteristics. In order to further analyze the effect of fragmentation and tectonism on the pore structure characteristics and explain the reasons why deformed coal is prone to outburst, this section analyzed the relation between different pore volumes and their corresponding fractal dimensions.


### 4.1. Relationship between Pore Volumes and Fractal Dimensions

The pore volumes and fractal dimensions are measured by MIP and LPNAT, which are recorded in Table5, and the relationship between pore volumes and fractal dimensions is shown in Figure 10.Table 5
Relationship between pore volume parameters and fractal dimensions.
Coal sampleParticle sizeD1VolumeD2VolumeD3VolumeD4VolumeIntact coal1-33.713351.402.984573.202.529514.392.096367.270.5-13.746141.502.959633.602.537004.902.134626.550.25-0.52.728909.282.339974.682.575647.472.0976410.610.074-0.252.8533119.102.5382810.802.5930110.152.0132713.70Deformed coal1-32.968144.902.763785.702.541825.632.094637.370.5-12.995348.302.7824412.402.589669.972.4658715.940.25-0.52.9934215.902.5814614.002.4772711.712.5238120.190.074-0.252.8006438.802.4867920.702.4330015.762.1770235.00Footnote: the unit of particle size is mm; the unit of pore volume is ×10-3 cm3/g.Figure 10
Relationship between pore volumes and fractal dimensions.
(a)
Macropores(b)
Mesopores(c)
Minipores(d)
MicroporesAs shown in Figure10, the fitting coefficient between the mesopore volumes and corresponding fractal dimensions of the 0.5-1 mm intact coal is 0.256; others are greater than 0.700. This indicates that the changes of macropore volumes, minipore volumes, and micropore volumes have the obvious effect on the surface roughness and structural complexity.According to Figure10, various kinds of pore volumes have the linear function relationship with their fractal dimensions in intact coal. The fractal dimensions of macropores and minipores increase with the increase of pore volumes, while the fractal dimensions of mesopores and micropores decrease with the increase of pore volumes. This indicates that the increase of pore volumes leads to more complex pore surface and pore structure of macropores and minipores and makes those of mesopores and micropores simpler. This also shows that the macropores as the main transport channels increase the resistance to gas migration, and the micropores mainly used for gas storage also decrease the adsorption sites.There is the quadratic function relationship between pore volumes and fractal dimensions in deformed coal. The fitting curves gradually present the obvious shape of inverted “U” with the decrease of pore sizes. The fractal dimensions of macropores and mesopores decrease with the increase of pore volumes in deformed coal, and there is the inflection point for the fractal dimensions of minipores and micropores with the change of pore volumes. The fractal dimensions increase with the increase of pore volumes before the inflection point. After the inflection point, the fractal dimensions decrease with the increase of pore volumes, and the decreasing trend becomes more and more obvious with the decrease of pore diameters. This shows that the surface roughness and structural complexity of macropores and mesopores decrease with the increase of pore volumes, and those of minipores and micropores increase first and then decrease with the increase of pore volumes. Meanwhile, this also shows that the gas migration channels in deformed coal are more convenient, and the adsorption sites increase before the inflection point of the relationship between pore volumes and fractal dimensions and decrease rapidly after the inflection point.The fitting curves of pore volumes and fractal dimensions of intact coal and deformed coal with the same particle size are significantly different, and the fitting curves of deformed coal have inflection points. It is worth noting that the particle size ranges of corresponding to the inflection points are 0.25-1 mm. Combined with the concept of critical particle size proposed by Airey [44] and research and analysis of critical particle size by scholars, Zhao et al. [37] measured the critical particle size ranges of coal samples which were 0.375-0.75 mm. Liu Y. and Liu M. [45] believed that the critical particle size was 0.75 mm. Li et al. [46] proposed that its size is 1 mm. The critical particle size ranges were measured as 0.45-2 mm by Liu et al. [47]. The critical particle size ranges of deformed coal in this paper have the cross interval with the previous ones, which can prove that those of deformed coal are 0.25-1 mm.


### 4.2. Evolution Process of Pore Damage and Outburst Causes of Deformed Coal

According to the measurement results of pore shapes, PSDs, and fractal dimensions, the relationships between pore volumes and fractal dimensions in Section4.1 are analyzed, as shown in Figure 11.Figure 11
Pulverization evolution processes of intact coal and deformed coal: (a) evolution of pore shapes, (b) evolution process of pore damage, and (c) damage paths.As shown in Figure11(a), the fragmentation makes the pores in intact coal and deformed coal develop towards open pores. The semienclosed pores in intact coal are gradually opened by effect of fragmentation, first developed into ink-bottle pores, and then developed towards opening pores. The fragmentation leads to transforming from the ink-bottle pores and semiclosed pores into open pores in deformed coal. According to Figure 11(b), the closed pore volumes in intact coal and deformed coal cannot be measured so that V0=0. With the increase of crushing degree, the closed pores are opened, and the original closed pore volumes are V1+V2, which explains the reason for the increase of various kinds of pore volumes.According to Tables2 and 3, the macropore volumes, mesopore volumes, minipore volumes, and micropore volumes of deformed coal are larger than those of intact coal. This is because tectonism leads to that some original semiclosed pores are gradually opened into ink-bottle pores or opening pores in deformed coal. At the same time, the original closed pores are also further opened in deformed coal. Finally, the various kinds of pores in deformed coal are more developed than those in intact coal.According to the analysis results of pore shapes, PSDs, and fractal dimensions caused by fragmentation and tectonism, the reasons why deformed coal is more prone to outburst than intact coal are as follows.(1)
The fragmentation makes the ink-bottle pores and semiclosed pores in deformed coal gradually transform into open pores, which enhances the connectivity of macropores and mesopores. Although the fragmentation has a little effect on the ink-bottle minipores and micropores, it also promotes the development of ink-bottle pores to open pores, which leads to the adsorption sites increase, resulting in enhanced adsorption capacity. In addition, macropores, mesopores, minipores, and micropores in deformed coal develop to open pores due to tectonism, which makes the gas migration channels of deformed coal more developing than those of intact coal(2)
According to the analysis results of PSDs and fractal dimensions, the effect of fragmentation on the damage paths of intact coal and deformed coal is different. As shown in Figure11(c), the damage path of intact coal is macropores, mesopores, minipores, and micropores, and that of deformed coal is macropores, micropores, mesopores, and micropores. The surface roughness and structural complexity of macropores and mesopores in intact coal are increased by the fragmentation. Those in deformed coal become simpler under the coupling effect of fragmentation and tectonism, and the surface and structure of micropores become more complex. This makes deformed coal have more adsorption sites and more convenient migration channels under the coupling effect than intact coal(3)
According to the relationship between pore volumes and fractal dimensions, there are obvious inflection points in the fractal dimensions of minipores and micropores in deformed coal with the change of pore volumes. The adsorption sites increase before the inflection points and decrease rapidly after the inflection points. This shows that when the particle size of deformed coal is broken to the critical particle size, the matrix is damaged, and the adsorption sites are rapidly reduced, resulting in a large number of desorption of adsorbed gas, which generates a large number of adsorbed gas expansion energy, but this phenomenon in intact coal did not appearBased on the above analysis results, it may be assumed that the same external force should be applied to intact coal and deformed coal, respectively. According to the damage path of intact coal, the macropores are first affected by fragmentation, followed by mesopores, minipores, and micropores. This may exist the condition, that is, when the energy generated by the fragmentation has been exhausted before acting on the micropores, or even if the micropores are affected, the amounts of adsorbed gas are small, and the gas migration paths are complex, which makes the gas expansion energy generated insufficient to supply the outburst. According to the damage paths of deformed coal, the external force first affects the macropores, resulting in the decrease of gas migration resistance. Then, the micropores are affected by the external force, resulting in the rapid desorption of adsorbed gas and generation of gas expansion energy. Meanwhile, with the effect of external force, the particle sizes of deformed coal reach the critical particle size, and the matrix is damaged. A large number of adsorption sites are rapidly reduced, and a large amount of adsorbed gas expansion energies are generated instantaneously. These adsorbed gas expansion energy and free gas expansion energy together aggravate the crushing of mesopores and minipores, which reduces the resistance of mesopores as the migration channels, and desorption of adsorbed gas in the micropores produces energy. The pore skeletons and stabilities of deformed coal are damaged by tectonism, which makes the required crushing energy decrease. When the gas expansion energy is greater than the crushing energy, the coal and gas outbursts are finally caused. This also explains the reason why many references [48–50] are proposed to find deformed coal with high crushing degree in many fields of outburst accidents.


## 4.1. Relationship between Pore Volumes and Fractal Dimensions

The pore volumes and fractal dimensions are measured by MIP and LPNAT, which are recorded in Table5, and the relationship between pore volumes and fractal dimensions is shown in Figure 10.Table 5
Relationship between pore volume parameters and fractal dimensions.
Coal sampleParticle sizeD1VolumeD2VolumeD3VolumeD4VolumeIntact coal1-33.713351.402.984573.202.529514.392.096367.270.5-13.746141.502.959633.602.537004.902.134626.550.25-0.52.728909.282.339974.682.575647.472.0976410.610.074-0.252.8533119.102.5382810.802.5930110.152.0132713.70Deformed coal1-32.968144.902.763785.702.541825.632.094637.370.5-12.995348.302.7824412.402.589669.972.4658715.940.25-0.52.9934215.902.5814614.002.4772711.712.5238120.190.074-0.252.8006438.802.4867920.702.4330015.762.1770235.00Footnote: the unit of particle size is mm; the unit of pore volume is ×10-3 cm3/g.Figure 10
Relationship between pore volumes and fractal dimensions.
(a)
Macropores(b)
Mesopores(c)
Minipores(d)
MicroporesAs shown in Figure10, the fitting coefficient between the mesopore volumes and corresponding fractal dimensions of the 0.5-1 mm intact coal is 0.256; others are greater than 0.700. This indicates that the changes of macropore volumes, minipore volumes, and micropore volumes have the obvious effect on the surface roughness and structural complexity.According to Figure10, various kinds of pore volumes have the linear function relationship with their fractal dimensions in intact coal. The fractal dimensions of macropores and minipores increase with the increase of pore volumes, while the fractal dimensions of mesopores and micropores decrease with the increase of pore volumes. This indicates that the increase of pore volumes leads to more complex pore surface and pore structure of macropores and minipores and makes those of mesopores and micropores simpler. This also shows that the macropores as the main transport channels increase the resistance to gas migration, and the micropores mainly used for gas storage also decrease the adsorption sites.There is the quadratic function relationship between pore volumes and fractal dimensions in deformed coal. The fitting curves gradually present the obvious shape of inverted “U” with the decrease of pore sizes. The fractal dimensions of macropores and mesopores decrease with the increase of pore volumes in deformed coal, and there is the inflection point for the fractal dimensions of minipores and micropores with the change of pore volumes. The fractal dimensions increase with the increase of pore volumes before the inflection point. After the inflection point, the fractal dimensions decrease with the increase of pore volumes, and the decreasing trend becomes more and more obvious with the decrease of pore diameters. This shows that the surface roughness and structural complexity of macropores and mesopores decrease with the increase of pore volumes, and those of minipores and micropores increase first and then decrease with the increase of pore volumes. Meanwhile, this also shows that the gas migration channels in deformed coal are more convenient, and the adsorption sites increase before the inflection point of the relationship between pore volumes and fractal dimensions and decrease rapidly after the inflection point.The fitting curves of pore volumes and fractal dimensions of intact coal and deformed coal with the same particle size are significantly different, and the fitting curves of deformed coal have inflection points. It is worth noting that the particle size ranges of corresponding to the inflection points are 0.25-1 mm. Combined with the concept of critical particle size proposed by Airey [44] and research and analysis of critical particle size by scholars, Zhao et al. [37] measured the critical particle size ranges of coal samples which were 0.375-0.75 mm. Liu Y. and Liu M. [45] believed that the critical particle size was 0.75 mm. Li et al. [46] proposed that its size is 1 mm. The critical particle size ranges were measured as 0.45-2 mm by Liu et al. [47]. The critical particle size ranges of deformed coal in this paper have the cross interval with the previous ones, which can prove that those of deformed coal are 0.25-1 mm.


## 4.2. Evolution Process of Pore Damage and Outburst Causes of Deformed Coal

According to the measurement results of pore shapes, PSDs, and fractal dimensions, the relationships between pore volumes and fractal dimensions in Section4.1 are analyzed, as shown in Figure 11.Figure 11
Pulverization evolution processes of intact coal and deformed coal: (a) evolution of pore shapes, (b) evolution process of pore damage, and (c) damage paths.As shown in Figure11(a), the fragmentation makes the pores in intact coal and deformed coal develop towards open pores. The semienclosed pores in intact coal are gradually opened by effect of fragmentation, first developed into ink-bottle pores, and then developed towards opening pores. The fragmentation leads to transforming from the ink-bottle pores and semiclosed pores into open pores in deformed coal. According to Figure 11(b), the closed pore volumes in intact coal and deformed coal cannot be measured so that V0=0. With the increase of crushing degree, the closed pores are opened, and the original closed pore volumes are V1+V2, which explains the reason for the increase of various kinds of pore volumes.According to Tables2 and 3, the macropore volumes, mesopore volumes, minipore volumes, and micropore volumes of deformed coal are larger than those of intact coal. This is because tectonism leads to that some original semiclosed pores are gradually opened into ink-bottle pores or opening pores in deformed coal. At the same time, the original closed pores are also further opened in deformed coal. Finally, the various kinds of pores in deformed coal are more developed than those in intact coal.According to the analysis results of pore shapes, PSDs, and fractal dimensions caused by fragmentation and tectonism, the reasons why deformed coal is more prone to outburst than intact coal are as follows.(1)
The fragmentation makes the ink-bottle pores and semiclosed pores in deformed coal gradually transform into open pores, which enhances the connectivity of macropores and mesopores. Although the fragmentation has a little effect on the ink-bottle minipores and micropores, it also promotes the development of ink-bottle pores to open pores, which leads to the adsorption sites increase, resulting in enhanced adsorption capacity. In addition, macropores, mesopores, minipores, and micropores in deformed coal develop to open pores due to tectonism, which makes the gas migration channels of deformed coal more developing than those of intact coal(2)
According to the analysis results of PSDs and fractal dimensions, the effect of fragmentation on the damage paths of intact coal and deformed coal is different. As shown in Figure11(c), the damage path of intact coal is macropores, mesopores, minipores, and micropores, and that of deformed coal is macropores, micropores, mesopores, and micropores. The surface roughness and structural complexity of macropores and mesopores in intact coal are increased by the fragmentation. Those in deformed coal become simpler under the coupling effect of fragmentation and tectonism, and the surface and structure of micropores become more complex. This makes deformed coal have more adsorption sites and more convenient migration channels under the coupling effect than intact coal(3)
According to the relationship between pore volumes and fractal dimensions, there are obvious inflection points in the fractal dimensions of minipores and micropores in deformed coal with the change of pore volumes. The adsorption sites increase before the inflection points and decrease rapidly after the inflection points. This shows that when the particle size of deformed coal is broken to the critical particle size, the matrix is damaged, and the adsorption sites are rapidly reduced, resulting in a large number of desorption of adsorbed gas, which generates a large number of adsorbed gas expansion energy, but this phenomenon in intact coal did not appearBased on the above analysis results, it may be assumed that the same external force should be applied to intact coal and deformed coal, respectively. According to the damage path of intact coal, the macropores are first affected by fragmentation, followed by mesopores, minipores, and micropores. This may exist the condition, that is, when the energy generated by the fragmentation has been exhausted before acting on the micropores, or even if the micropores are affected, the amounts of adsorbed gas are small, and the gas migration paths are complex, which makes the gas expansion energy generated insufficient to supply the outburst. According to the damage paths of deformed coal, the external force first affects the macropores, resulting in the decrease of gas migration resistance. Then, the micropores are affected by the external force, resulting in the rapid desorption of adsorbed gas and generation of gas expansion energy. Meanwhile, with the effect of external force, the particle sizes of deformed coal reach the critical particle size, and the matrix is damaged. A large number of adsorption sites are rapidly reduced, and a large amount of adsorbed gas expansion energies are generated instantaneously. These adsorbed gas expansion energy and free gas expansion energy together aggravate the crushing of mesopores and minipores, which reduces the resistance of mesopores as the migration channels, and desorption of adsorbed gas in the micropores produces energy. The pore skeletons and stabilities of deformed coal are damaged by tectonism, which makes the required crushing energy decrease. When the gas expansion energy is greater than the crushing energy, the coal and gas outbursts are finally caused. This also explains the reason why many references [48–50] are proposed to find deformed coal with high crushing degree in many fields of outburst accidents.


## 5. Conclusion

The methods of test and theoretical analysis are used to analyze the effect of fragmentation and tectonism on the pore structure characteristics of intact coal and deformed coal from pore shapes, PSDs, and fractal dimensions. The conclusions are as follows.(1)
According to the scanning electron microscope, mercury injection and ejection curves, and low-temperature liquid N2 adsorption and desorption curves, the change of pores shapes was analyzed. The results show that the shapes of macropores and mesopores are obviously affected by fragmentation. The original semiclosed pores are transformed into open pores and ink-bottle pores by the fragmentation in intact coal. The shapes of macropores and mesopores in deformed coal are gradually transformed from the original ink-bottle pores and semiclosed pores to the ink-bottle pores and open pores by the fragmentation. Comparing the differences of pore shapes between intact coal and deformed coal, it is found that tectonism promotes connectivity of pore in deformed coal(2)
Combined with the results of mercury intrusion porosimetry and low-temperature liquid N2 adsorption test, the pore size distribution characteristics of various kinds of pores were analyzed, and the effect of fragmentation on the damage paths of intact coal and deformed coal was proposed. The damage path of intact coal is macropores, mesopores, minipores, and micropores, and that of deformed coal is macropores, micropores, mesopores, and micropores(3)
According to the fractal dimension theory, the fractal dimensions of various kinds of pores were calculated. The calculative results show that the fragmentation and tectonism decrease the surface roughness and structural complexity of macropores and mesopores in deformed coal and increase those of minipores and micropores. According to the relationship between pore volumes and fractal dimensions, the critical particle size range of deformed coal is 0.25-1 mm(4)
Based on the analysis of effect of fragmentation and tectonism on the pore shapes, pore size distributions, and fractal dimensions, the reason for the existence of deformed coal with high crushing degree in the field of coal and gas outburst was explained. Under the coupling effect of fragmentation and tectonism, the matrix of deformed coal is prone to damage, resulting in rapid desorption of absorbed gas. At the same time, it also makes the gas migration resistance decrease, which leads to the phenomenon of deformed coal with high crushing degree in the fields of outburst accidents



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*Source: 8771617-2022-08-08.xml*