Pore morphology changes and quantitative analysis and evaluation of high‐rank coal samples after impact

The influencing mechanism of impact loads on the pore morphology of high‐rank coal has been extensively investigated since mining safety is greatly determined by the ensuing state of gas occurrence. In this work, we propose a combined use of a scanning electron microscope and Image Pro Plus software to examine the impact‐induced changes in pore characteristics. Impact loads of 0, 28.46, 51.8, and 58.7 MPa were applied to the coal samples in various directions (parallel/vertical/45° oblique). The results show that the pore connectivity of coal samples is enhanced following the impact load, and the number and diameter of pores are significantly increased on the whole. Surface porosity and roundness of the coal body first rise and then fall as the impact load increases. Following the impact load, surface porosity and roundness of the coal body significantly increase in the vertical and oblique bedding directions, while decreasing in the horizontal bedding direction.


| INTRODUCTION
Scanning electron microscope (SEM) facilitates visualization of an object's microstructure and surface morphology through magnification technologies.Furthermore, it is capable of quantitatively analyzing the microstructure characteristics of the object by combining high-performance computers and the corresponding image-processing software. 1,2SEM has been widely applied in the fields of materials, geology, metallurgy, mining, and so forth.For instance, Wang et al. 3 used SEM to study the influence of the microstructure of soil on its liquefaction characteristics and identified the mechanism for different liquefaction potentials between loess in China and USA.Similarly, the mesoscopic creep deformation mechanism of fractured rock in the weak layer under water pressure was investigated through SEM imaging. 4Also, Qi et al. 5 observed the internal morphology of the granite samples in the landslide zone along the Sichuan-Tibet railway under frequent freeze-thaw environmental conditions by analyzing SEM results.It was concluded that the freeze-thaw times had a great influence on the microscopic pores of the granite and its mechanical characteristics.On the basis of the fracture structure of the coal mass obtained by the SEM, Cao et al. 6 applied COMSOL software to study the impact of fracture opening on coal seepage and modified the gas-filled fissure flow model.Moreover, SEM was combined with the pore-fracture system and gas absorption to quantitatively characterize the pore structure of the coal.A new joint characterization mode for the coal nanopore structure was subsequently established. 7omestic and foreign scholars have studied extensively micropore structure changes of coal under different conditions.Liu et al. 8 analyzed the influence of coalification on the micropore structure of coal using SEM and Fourier Transform Infrared Spectroscopy.The results showed that the degree of coalification could greatly affect the microsurface functional groups of coal and thus the gas adsorption effect.The Hopkinson impact test system was also employed to simulate the shock and stress waves to study the effect of impact load on the micropore structures of anthracite samples.The changes in mercury injection and low-temperature liquid nitrogen of the samples were evaluated based on the fractal theory. 9Liang et al. 10 carried out blast and impact tests on high-rank anthracite from the II1 coal seam of Shanxi Formation, Zhaogu Coal Mine, Henan Province.They studied the X-ray diffraction data of the coal samples subject to different impact loads and analyzed how these loads influenced coal's microcrystalline structure.Impact experiments on lignite, bituminous, and anthracite coals may also be conducted by an split hopkinson pressure bar test system.According to lowtemperature liquid nitrogen and Raman spectra data of coal samples of different ranks before and after impact, it was found that the impact load could accelerate the effective desorption of gas and lead to the destruction of the macromolecule structure. 11 summary, SEM has been a powerful tool to study the physical and mechanical properties of rock and coal masses.However, SEM has rarely been used to examine the change in microscopic pores of coal mass following impact loading, which is a popular research topic.In this paper, such change in high-rank coals will be observed through SEM imaging, and qualitative and quantitative methods are jointly used to further evaluate how impact load influences the microstructure of high-rank coal.With this research, we hope to gain a better knowledge of the behavior of pores after an impact and improve safety protocols for gas extraction and coalbed methane mining.

| EXPERIMENTAL PROCEDURE
The coal samples were anthracites from No. 2 Zhaogu and Chengzhuang Coal Mines with clear beddings.The coal cores were drilled along the directions of vertical bedding, parallel bedding, and 45°oblique bedding.Subsequently, the cylindrical coal specimens of 50 mm × 50 mm were prepared by cutting and grinding.The thorough coal sample preparation procedure was provided by Wang et al. 12 Impact loading on coal pillars in different coal mine orientations was performed by the Hopkinson impact test system of Henan Technical College of Construction.The surface morphology of coal samples was examined using the SEM before and after impact.In the meantime, the change in coal sample characteristics was quantitatively analyzed by Image Pro Plus software.The particles close to the central axis of the coal were sampled to enhance the contrast of SEM images taken before and after the impact, as shown in Figure 1.

| Selection of coal
Considering that a limited number of coal samples would be used for SEM testing, the following requirements should be satisfied when selecting coal bodies so that the statistical significance of experimental data and the reliability of comparison before and after impact could be ensured.
(1) Coal blocks should be collected from the same coal seam and location to make sure that they belong to a single coal body.Also, the collection location should be marked on the original coal blocks.(2) Coal samples subject to the same impact load in the same direction should be drilled from the same coal block.If the quantity of the coal sample was insufficient, drilling should be performed on the adjacent coal blocks according to the number marked during the coal collection process.(3) The longitudinal wave velocity of a cylinder of 50 mm in diameter and 50 mm in length should be measured using the HS-YS301C rock acoustic wave tester.This ensured that the acoustic wave velocity of coal samples in the same impact direction was similar.(4) The test samples before and after the impact were taken from coal particles near the central axis of the cylinder for experimentation.

| Coal sample preparation
The coal samples used in this study were prepared according to the following procedure: (1) Cylindrical specimens with a diameter of 50 mm and a length of 50 mm were made using the rock core drilling machine (Figure 2A), the cutting machine (Figure 2B), and the double-end face polishing machine (Figure 2C).Selecting a specimen with a length of 50 mm for the Hopkinson impact test was for the purposes of satisfying the one-dimensional and uniform stress assumptions, properly reflecting the dynamic mechanical properties of the material, and facilitating pore determination using SEM with a test block weighing about 150 g. ( 2) As shown in Figure 3, coal samples were drilled from the coal mine in the X-, Y-, and Zorientations.The X-direction was parallel to the coal bedding; the Y-direction was perpendicular to it; and the Z-direction was 45°oblique to the bedding.(3) A double-end face grinder (Figure 2C) was used to polish the specimen in accordance with the "Engineering Rock Mass Test Method Standard" (GB/T50266-2013) and "Methods for Determining the Physical and Mechanical Properties of Coal and Rock" (GB/T23561.7-2009).This ensured that the nonparallelism of the two ends of the coal sample was no greater than 0.02 mm, and the unevenness of the two ends was no more than 0.05 mm.| 2521 (4) A dial gauge was used to measure the parallelism of the two end surfaces while the specimen was moving on a horizontal testing platform.The deviation between the maximum and minimum surface flatnesses should be less than 0.02 mm.Turn the specimen upside down and repeat the above operation.The operation method is shown in Figure 4. rod is used to precisely hit the incident rod, and the sample placed between the incident rod and the transmission rod is axially loaded.The electrical signal is transmitted to the super dynamic test analysis system through the highprecision resistance strain gauge to test the dynamic mechanical properties of materials or components.The schematic diagram is shown in Figure 5.

| Comparison of SEM images of high-rank coal before and after impact
Figures 6-8 compare the SEM images of coal samples before and after various impact loads in different directions.These images suggest that: (1) Before the impact, coal pores are neatly aligned mostly along the same direction.Their shapes are relatively simple and uniform.For example, the pores are distributed in bands in Figures 6A and 7A, and are oriented linearly in Figure 8A.After the impact, the pores are arranged disorderly with diverse shapes, such as crescent, triangle, oval, gourd, irregular polygon, and so forth.Moreover, their number increases significantly.(2) Before the impact, the majority of the pores' diameters are at the nanometer scale, with few being micron-sized.After the impact, there is an obvious surge in their diameter.As the impact load increases, micron-sized pores become more and more common, until finally all pores are micron-sized.(3) The impact induces an obvious increase in the pore depth.The morphology around the pores changes from a sharp and banded shape into a circular arc.Most of the pores are intragranular before the impact, while there are many solution pores with obvious external damage following the impact.
(4) Before the impact, most pores show obvious static compression (Figure 6A) or sedimentation (Figures 7A and 8A).After the impact, obvious impact cracking is observed, as shown in Figures 6G and 7E.( 5) The nanoscale pores are isolated before the impact.
Many of them are connected and form microcracks following the impact, resulting in an obvious increase in their depth and width, as shown in Figures 6B and 8C,E.

| QUANTITATIVE EVALUATION OF PORE STRUCTURE CHARACTERISTICS OF HIGH-RANK COAL BEFORE AND AFTER IMPACT
SEM has been extensively employed for qualitative analyses of surface morphology; however, the data extracted has rarely been used for quantitative evaluations.In this paper, the Image Pro Plus software is applied to binarize the SEM images based on statistical principles.Quantitative information on the SEM images will be obtained, such as plane porosity, pore area, circle of pores, long axis, short axis, and so forth.The influencing mechanism of the impact load on the micro-nano-pore structures of high-rank coal is discussed.

| Threshold determination and pore parameters extraction
The SEM images are gray-scale, with the gray level proportional to density.Given that pore density is the smallest among all coal microstructures, a proper segmentation algorithm can be used for quantitative data extraction and pore identification.Such an algorithm usually adopts the watershed segmentation method or the edge and threshold segmentation method, with the latter being the most commonly applied.The challenge and focus of the threshold segmentation method is the reasonable determination of the threshold.In this view, the iterative method, the two-peak method, and the Otsu method 13,14 are proposed to determine the binary threshold.The iterative algorithm is a global binarization method based on an approximation algorithm.The calculation procedure is as follows: ① the initial value of the global threshold m is determined; ② image segmentation is conducted by assigning A 1 if the pixel exceeds m or A 2 if the pixel is less than m.③ The average gray values n 1 and n 2 are calculated for A 1 and A 2 , respectively; ④ a new threshold value t is calculated as (n 1 + n 2 )/2; ⑤ steps ② and ③ are repeated until m in the continuous iteration reaches the ideal value.
The two-peak method is mainly used for gray-level histograms with two obvious peaks, where the gray level corresponding to the valley between these peaks is chosen as the threshold. 15The Otsu method is based on the clustering principle, where the image pixels are divided into two groups according to the gray level.The optimal threshold is one that may maximize the ratio of the class square error to the within-class variance of the two pixel groups.The Otsu method is credited for simplicity and strong self-adaptability.
(1) Determination of optimal thresholds In this paper, the MATLAB program was employed to calculate the thresholds of six coal samples of No. 2 Zhaogu Coal Mine before and after the impact from three directions.The optimal threshold of each coal sample is shown in Table 1.
It can be seen that the optimal thresholds obtained by the iterative method and the Otsu method are equivalent, while the thresholds predicted by the two-peak method are significantly larger.Therefore, the iterative method and the Otsu method are suitable for further investigation of the SEM images.However, the iterative method involves a complex calculation process and consumes a lot of memory space and time.In contrast, the Otsu method is simple and efficient by making direct use of the inbuilt Matlab function "graythresh."In this view, the Otsu method is used in this study to determine the optimal thresholds.
(2) Binary processing procedure The Image Pro Plus software, used for binary processing on the SEM images, carries out the procedure in the following steps.① Images are imported, ② pores in the SEM images are recognized, and ③ the quantitative information of the pores is extracted.Taking the coal sample from No. 2 Zhaogu Coal Mine under the impact load of 58.70 MPa in the oblique bedding direction as an example, their images are imported into the software (Figure 9A), and then the SEM image thresholds are adjusted to identify the pores (Figure 9B).Subsequently, binary SEM images are generated (Figure 9C), with white indicating pores and black representing the coal mass.More detailed results are shown in Figures 6-8.Finally, original values of pore area, the long axis, the short axis, and diameter are exported to calculate plane porosity and roundness, as indicated in Table 2. Plane porosity shown in Table 3, refers to the ratio of the total pore area to the total area of the SEM image. 16Roundness, as shown in Table 4, is defined as the ratio of the short axis to the long axis of the pore. 17To reduce the influence of the discreteness of coal itself, these calculated variables are the average values of seven samples under the same impact magnitude and direction.

| Plane porosity
According to Table 3 the vertical bedding direction, averaging 4.83%.The maximum plane porosity of 10.30% is reached when the impact load is 41.43 MPa, representing only a 0.20% increase when the load is 51.80 MPa.In the parallel bedding direction, the plane porosity ranges from 1.04% to 8.22%, with an average of 4.44%.The maximum porosity of 8.22% corresponded to an impact load of 41.43 MPa.In the oblique bedding direction, the porosity is between 1.20% and 12.82%, with an average of 7.13%.At an impact load of 41.43 MPa, the maximum porosity is 12.82%; this is only 1.72% higher than at an impact load of 32.59 MPa.The porosities are at their lowest before impact for all three directions.In the case of Chengzhuang Mine, the porosity is between 6.51% and 34.09% in the vertical bedding direction, with an average of 20.34%.The maximum porosity is 34.09% when the impact load is 41.43 MPa.In the parallel bedding direction, the porosity is between 8.68% and 24.21%, with an average of 16.39%.The highest value is 24.21% when the impact load is 32.59 MPa.In the oblique bedding direction, the porosity ranges between 8.19% and 30.81%, with an average of 17.48%.The maximum value is 30.81% with an impact load of 28.46 MPa.Similar to the No. 2 Zhaogu Coal Mine, the porosities are the lowest before impact for all three directions.This indicated that the impact load could increase the porosity and the number of pores, as well as create more migration and diffusion channels.
Figure 10 shows that the plane porosity of the coal first increased and then decreased with the increase of impact load in different directions.There was a maximum plane porosity and the corresponding optimal impact load in each direction, and the optimal impact load size was different in different directions.

| Roundness
Roundness refers to the ratio of the minimum axial length to the maximum axial length of a pore.Greater roundness implies an axial length varying in a smaller range, a pore shape closer to a circle, a smoother pore surface, a lower pore wall friction, and subsequently more favorable conditions for gas diffusion and migration.Table 4 shows that the roundness of coal from No. 2 Zhaogu Coal Mine is between 0.31 and 0.49 in the vertical bedding direction, with an average of 0.39.The maximum value is 0.49 when the impact load is 51.80 MPa.The minimum value before impact is 0.31.The pore roundness averages 0.34 in the parallel bedding direction, ranging from 0.23 to 0.46.The highest and lowest results represent impact loads of 41.43 and 58.70 MPa, respectively.In the oblique bedding direction, the roundness ranges from 0.32 to 0.41, with an average of 0.37.The maximum value is obtained when the impact is 32.59 MPa.For coal samples from the Chengzhuang Coal Mine, the pore roundness is between 0.41 and 0.56 in the vertical bedding direction, with an average of 0.45.The maximum roundness is associated with an impact load of 41.43 MPa.The smallest roundness is 0.41 before the impact.In the parallel bedding direction, roundness ranges from 0.39 to 0.48, with an average of 0.42.The impact load of 32.59 MPa results in the maximum value, while impact loads of 41.43 and 58.70 MPa result in the smallest value.In the oblique bedding direction, the roundness is between 0.36 and 0.46, with an average value of 0.42.The impact load that corresponds to the maximum value of 0.46 is 28.46 MPa.The roundness is the smallest before impact for samples taken from both mines.Figure 11 indicates that for the coal samples taken from both No. 2 Zhaogu Coal Mine and Chengzhuang Coal Mine, pore roundness increases with the impact load gradually at first, then rapidly, and eventually decreases in the directions of vertical bedding and 45°oblique bedding.The pore roundness generally becomes greater after the impact.In the parallel bedding direction, pore roundness undergoes a gradual increase with the impact load, followed by a quick and then gradual decline.Contrary to the other impact directions, the pore roundness in the parallel bedding direction becomes lower after the impact.There are two potential causes.First, When the impact loads occur on the coal in the parallel bedding direction, the impact and stress waves act directly along the fractures in the coal body.Without sufficient attenuation, the impact energy will crush the coal, thus increasing the irregularity and complexity of the pores and finally reducing the pore roundness after the impact.Second, coal is a complex porous medium with strong anisotropy and heterogeneity.T A B L E 3 Calculated plane porosities K before and after impact (%).

Coal mine
Impact direction Impact load (MPa) | 2531 Impact tests on coal samples from No. 2 Zhaogu Coal Mine and Chengzhuang Coal Mine in vertical, parallel, and 45°oblique bedding directions show that impact loads can alter the apparent morphology and pore characteristics of coal.This can be explained by the following factors.
(1) The energy carried by the impact load can encourage the transformation of micropores into mesopores and subsequently macropores.The concentrated stress accumulated at the pore tip changes the pores into a smooth and rounded shape, increasing porosity and roundness.Due to the nonuniformity of pore size, 18 when subjected to impact loads, local stress concentration will be formed at the tip of the smaller pore size, which will cause local microdamage to the pore tip. 19This local microdamage mainly manifests as local modification of the pores, thereby promoting the roundness and smoothness of the tip pores.(2) A larger impact load does not always equate to higher porosity.The reason could be that excessive impact load breaks down the coal matrix inside the coal body, causing small molecule compounds in the coal matrix to block the large pores, thereby reducing porosity. 20,21There are a large number of pores and cracks in the coal body, 22 and there are pore throats and throats of different sizes. 23,24When the coal matrix is broken by the impact load, it will cause small molecule compounds to block the throats at the connection between the medium and large pores, thus making the large pores become micropores and reducing the plane porosity of the coal body.(3) Energy embodied by excessive impact loads tends to damage rather than smooth the pores.In this instance, the original, regular, and round pores will become rough and irregular because the impact force will crack and destroy them, decreasing their roundness. 21The coal skeleton supports the pores of the coal body, 25,26 and has a certain strength 27 , so the integrity of the coal skeleton will not be affected under the action of small loads.However, when excessive impact loads affect the coal body, a large amount of energy acts on the coal skeleton, resulting in the deformation or fracture of the coal skeleton.When the deformation exceeds the fracture toughness of the coal body, the fracture of the coal skeleton will be caused.As a result, the skeleton is broken, the pores are broken, and the regular and round pores become rough and irregular.

| CONCLUSION
The micro/nanopore morphology, plane porosity, and roundness of coal samples from No. 2 Zhaogu Coal Mine and Chengzhuang Coal Mine were investigated by SEM imaging and the Image Pro Plus software.The main conclusions are as follows: (1) Impact loading causes the pore morphology of coal to become more diversified and complicated, with a more chaotic pore organization and increased pore number, diameter, depth as well as connectivity.
(2) The plane porosity and roundness of coal undergoes an increase and a subsequent decrease with the increase of impact loads.In the directions of vertical and oblique bedding, the plane porosity and roundness become greater following the impact, while the roundness was reduced in the parallel bedding, indicating the strong impact anisotropy of the micro-nano-pore structure of high-rank coal.(3) The impact load changes the pore morphology and size distribution of the coal body, improves the face ratio and roundness of the coal body, increases the pore diameter and depth, and stimulates the pore to become rounded and smooth.][25][26][27]

F I G U R E 1
Position of SEM sample extraction.(A) Before the impact and (B) after the impact.SEM, scanning electron microscope.

F I G U R E 2
Equipment for making coal samples.(A) Rock core drilling machine, (B) rock cutting machine, and (C) double-end face polishing machine.WANG ET AL.

2. 3 |
Electromagnetic-driven Hopkinson experimental system Electromagnetic-driven Hopkinson experimental system uses electromagnetic driving technology to replace traditional pneumatic loading.It is a new dynamic loading method developed by combining electromagnetic technology and Hopkinson rod experimental technology.The impact F I G U R E 3 Schematic of coal sample drilling directions and pictures of coal samples.F I G U R E 4 Test method for nonparallelism.(A) The schematic diagram and (B) The object pictures.

F I G U R E 5
Schematic of the electromagnetic-driven Hopkinson impact test system.F I G U R E 6 Comparison of SEM images of coal samples from No. 2 Zhaogu Coal Mine before and after impact in vertical bedding direction.(A) The SEM image (impact load: 0 MPa), (B) the binarized image (impact load: 0 MPa), (C) the SEM image (impact load: 28.46 MPa), (D) the binarized image (impact load: 28.46 MPa), (E) the SEM image (impact load: 41.43 MPa), (F) the binarized image (impact load: 41.43 MPa), (G) the SEM image (impact load: 58.7 MPa), and (H) the binarized image (impact load: 58.7 MPa).SEM, scanning electron microscope.F I G U R E 7 Comparison of SEM images of coal samples from Chengzhuang Coal Mine before and after impact in parallel bedding direction.(A) The SEM image (impact load: 0 MPa), (B) the binarized image (impact load: 0 MPa), (C) the SEM image (impact load: 28.46 MPa), (D) the binarized image (impact load: 28.46 MPa), (E) the SEM image (impact load: 41.43 MPa), (F) the binarized image (impact load: 41.43 MPa), (G) the SEM image (impact load: 58.7 MPa), and (H) the binarized image (impact load: 58.7 MPa).SEM, scanning electron microscope.F I G U R E 8 Comparison of SEM images of coal samples from No. 2 Zhaogu Coal Mine before and after impact in oblique bedding direction.(A) The SEM image (impact load: 0 MPa), (B) the binarized image (impact load: 0 MPa), (C) the SEM image (impact load: 28.46 MPa), (D) the binarized image (impact load: 28.46 MPa), (E) the SEM image (impact load: 51.8 MPa), (F) the binarized image (impact load: 51.8 MPa), (G) the SEM image (impact load: 58.7 MPa), and (H) the binarized image (impact load: 58.7 MPa).SEM, scanning electron microscope.

F I G U R E 9
SEM images processed by the Image Pro Plus software.(A) SEM image imported into Image Pro Plus, (B) pores in the SEM image, and (C) SEM image in the binary form.SEM, scanning electron microscope.

F I G U R E 10
Relationship between plane porosity and impact load.(A) No. 2 Zhaogu Coal Mine and (B) Chengzhuang Coal Mine.F I G U R E 11 Relationship between roundness and impact load.(A) No. 2 Zhaogu Coal Mine and (B) Chengzhuang Coal Mine.
, the plane porosity of the coal from No. 2 Zhaogu Coal Mine is between 1.10% and 10.30% in T A B L E 1 Optimal thresholds are obtained by different methods.
T A B L E 2 Data extracted and calculated from the scanning electron microscope result.
K 1 , K 2 , K 3 , K 4 , K 5 , K 6 , and K 7 are the plane porosities of seven samples, and K ¯represents the average.Calculated roundness o before and after impact.