Pore and gas desorption characteristics of primary coal with different degrees of metamorphism

Pore difference characteristics and adsorption/desorption experiments of primary coal with different degrees were explored to study the characteristics of pore structure and gas desorption. The results show that the pore volume increases with the increase in the degree of metamorphism. In primary coal with different degrees of metamorphism, the distribution of micropores, small pores, and medium‐sized pores (by volume) is WY < PM < CY. The distribution of large pore volume and visible pore volume was WY > PM > CY. According to the pore volume distribution law of different metamorphic primary coals, the average pore diameters of WY, PM, and CY were 68.40, 45.60, and 30.50 nm, respectively. The porosity and specific surface area of primary coal with different degrees of metamorphism show a distribution pattern such that WY > PM > CY. For large pore and visible pore, the pore volumes of WY, PM, and CY coal were 0.0785, 0.0587, and 0.0300 mL/g, respectively, and the proportions were 81.86%, 74.49%, and 55.25%, respectively. The porosity of WY, PM, and CY were 11.57%, 9.03%, and 6.57%, respectively, and the specific surface areas were 6.917, 5.826, and 5.611 cm3/g. According to the desorption characteristics of coal bodies with different metamorphic degrees at different time nodes found that the gas desorption at different time nodes shows similar changes under the same adsorption equilibrium pressure. The higher the degree of coal metamorphism, the greater the amount of gas desorption, and the faster the desorption amplitude. The desorption intensity of WY coal is significantly higher than that of PM and CY coal with the increase of desorption time which also verifies the measurement results of pore structure and adsorption constant.

Technology Project, Grant/Award Number: HNKJ19-H10 the desorption amplitude. The desorption intensity of WY coal is significantly higher than that of PM and CY coal with the increase of desorption time which also verifies the measurement results of pore structure and adsorption constant.

K E Y W O R D S
degree of metamorphism, gas desorption, pore volume, primary coal, specific surface area

| INTRODUCTION
Coal is a natural porous medium and its internal pore structure is relatively complex which varies greatly with the degree of metamorphism. 1,2 The study and evaluation of the pore structure in coal is the basic premise for accurately mastering the adsorption and desorption characteristics of coal, evaluating the permeability of coal, preventing coal and gas outbursts, and realizing efficient gas extraction. 3,4 Scholars have often examined the pore structure of coal: Jiang et al. studied the pore characteristics of coal with different coal structures by low-temperature liquid nitrogen adsorption test. [5][6][7] Qi et al. found that the burst coal and block coal had similar properties in terms of pore size distribution and surface area by collecting the burst coal and block coal and using a low-temperature nitrogen adsorption test. With the increase of coal rank, the proportion of micropores and surface area increase. 8 Yue et al. studied the pore structure characteristics and adsorption characteristics of coal with different failure types by liquid nitrogen adsorption, mercury injection, and methane isothermal adsorption methods. 9 Chen et al. tested the pore structure of nine groups of coal samples with different degrees of metamorphism through mercury injection testing, and investigated the fractal characteristics of pore structure of coal samples with different degrees of metamorphism using the Menger sponge model. Combined with the adsorption constant of coal samples, the influence of fractal characteristics of pore structure on gas adsorption characteristics was studied. 10,11 Wang et al. studied the effects of pore structure and the gas diffusion properties on the lost gas from tectonic and intact coals were investigated by the mercury intrusion porosimetry method, N 2 and CO 2 adsorption methods, and gas adsorption equilibrium/ desorption tests. 12 Using a method combining scanning electron microscopy energy dispersive X-ray spectroscopy (SEM-EDS) and microcomputed tomography scan, mesostructural deformations of coal during three methane adsorption-desorption cycles were observed. 13 Zhang et al. investigated the difference of gas emission process between structural coal and primary coal by using the constant-temperature coal particle gas emission experiment method, and tested the pore structure of coal by mercury injection method and low-temperature nitrogen adsorption method. The distribution of mesopore and macropore was found to be the main factor leading to differences in the gas emission characteristics of structural coal and primary coal. 14, 15 Nie et al. studied the nonuniformity in form and distribution of nanopores of different coal ranks by means of SEM, liquid nitrogen adsorption, and small-angle X-ray analysis. 16,17 Xue et al. measured the low-temperature liquid nitrogen adsorption curve of coal samples with automatic nitrogen adsorption apparatus, and calculated the pore fractal dimension, capillary mean tortuosity fractal dimension, and permeability of coal samples according to fractal theory, a capillary mean tortuosity fractal model, and a permeability model. From the perspective of fractality, the relationship between the fractal characteristics of micropores and their adsorption properties and permeability was studied. 18 Sun et al. used the porosity and permeability in confining stress experiments to simulate the porosity and permeability variations in coal samples at different depths. 19 Jian et al. found that mercury intrusion porosimetry and isothermal adsorption experiments could be conducted on of coal samples with Ro max ranging between 0.24% and 0.65%. The results show that the pores with sizes ranging between 10 and 100 nm are the most important in lowrank coal. 20 To study the pore structure of low-rank coal and the characteristics of gas adsorption and diffusion, and its influence on accidents related to gas escape, N 2 / CO 2 adsorption, small-angle X-ray scattering, gas adsorption/desorption experiments and fractal theory were used to study six groups of coal samples. According to the relationship between coal sample pore structure parameters and gas adsorption/desorption characteristic parameters, the correlation between the micropore structure of low-rank coal and the macrogas adsorption and diffusion characteristics was obtained. 21 Wang et al. found experimental studies on the changes in pore structure and permeability during N 2 injection have been limited. In this study, N 2 injection experiments, mercury intrusion porosimetry, and permeability measurements were conducted to estimate the changes in pore structure and permeability caused by N 2 injection of semianthracite coal from the Lu'an mining area in the Qinshui basin, Shanxi Province, China. The results show that the total pore volume markedly increases during N 2 injection, with increases in transition pores, mesopores, and macropores of 8.0%, 50.0%, and 138.3%, respectively. 22 Bai et al. used the LF-NMR method to evaluate the transformation of adsorbed and free CH 4 during the CO 2 injection process, and the enhancement of adsorbed CH 4 recovery efficiency using CO 2 was quantified. 23,24 Relevant scholars studied improved model for the fractal characteristics of pore structure in high-rank coal seams and the effects of pore structure on methane adsorption behavior of ductile tectonically deformed coals and the effects of pore morphology and moisture on coalbed methane (CBM)-related sorption-induced coal deformation. [25][26][27] Liu et al. studied the role of sorption-induced coal matrix shrinkage on permeability and stress evolutions under replicated in situ conditions for CBM reservoirs The models for quantifying matrix shrinkage-related effects including horizontal stress loss, vertical strain variation, and permeability evolution were proposed. 28 Bodden III and Ehrlich studied the influence of coal permeability and desorption characteristics on CBM production. 29 Tang et al. isothermal desorption hysteretic model for deep CBM development. 30 Li et al. studied multiscale pore fractal characteristics of differently ranked coal and its impact on gas adsorption. 31 Yue et al. studied pore structure characteristics and adsorption characteristics of coals with different destruction types. 9 Zhu et al. studied the coal pore characteristics in different coal mine dynamic disasters and the pore characteristics of gas outburst coal and bursting liability coal are analyzed and compared. The pore structure, the pore volume, and the pore compressibility associated with different dynamic disasters were comprehensively analyzed. The experimental data of mercury intrusion were corrected by liquid nitrogen adsorption experiment data, and the distribution of pore volume was obtained. 32 Sun et al. studied pore size distributions and pore multifractal characteristics of medium and low-rank coals. 19 Jian et al. studied the characteristics of pores and methane adsorption of low-rank coal. 20 Previous studies are mainly focused on the highly metamorphic anthracite and the structural coal with a relatively complex structure, but the areas where the primary coal bodies occur prevail during mining. On the other hand, due to the highly metamorphic coal, structural coal is often weak, most of the research from the pore structure and coal seam gas adsorption and desorption, coal and gas outburst, and other aspects and gradually becomes the key link in experimental and field research, for primary coal is relatively weak. Therefore, it is more practical to experimentally study the pore structure of primary coal in different mining areas and different degrees of metamorphism to determine the difference characteristics of pore structure and the coal desorption characteristics derived therefrom.

| Sample preparation
According to the purpose and content of the experiment, three groups of primary coal samples with different degrees of metamorphism were collected, including the anthracite (WY) of Jiulishan Coal Mine in Henan Province, the poor coal (PM) of Xinyuan Coal Mine in Shanxi Province, China, and the long-flame coal (CY) of Gengcun Coal Mine in Henan Province, China. Samples were selected from freshly exposed coal, and the collected coal samples were labeled and placed into sealed bags to avoid contact with the air as much as possible. The samples were prepared immediately after being taken back to the laboratory and sealed for preservation. Information pertaining to coal samples is shown in Table 1, and industrial analysis and adsorption constants of coal samples are shown in Table 2.

| Mercury injection experiment
To study the pore characteristics of coal samples with different degrees of metamorphism at the microscale, the mercury injection method was used to measure the pore structure of coal samples. The pore strength of coal is supposed to be large enough (i.e., it will not break under high pressure), mercury injection methods can be used to T A B L E 1 Information pertaining to coal samples. | 3187 study and calculate the microscopic pore structure of coal as a porous medium. 10,33,34 When mercury is forced into coal pores under external pressure, the pore size is calculated according to Equation (1):

Sampling location
where D i is the diameter of mercury entry hole under pressure (nm), W denotes the Washburn constant of mercury (0.145038), γ is the mercury surface tension (485 dyn/cm), θ is the mercury contact angle (130°), and P i is the pressure on the mercury in time period i (psi).
In the experiment, the pressure on the mercury increases slowly, and the average pore diameter D mi from i − 1 to i can be calculated according to Equation (2): where D mi is the average diameter of the pores (nm), D i represents the pore diameter at time i (nm), and D i−1 is the pore diameter at time i − 1 (nm). The volume of pores invaded by mercury in coal is calculated according to Equation (3).
where I i is the volume of mercury intrusion per unit mass of coal at time i (cm 3 /g), V i is the volume of mercury intruded at time i (cm 3 ), and W s denotes the weight of the coal sample (g). Assuming the volume of mercury forced into the pore is equal to that of the pore, the volume I ii of mercury intruding at time i is calculated according to Equation (4): where I ii is the volume of mercury forced into the pore at time i (cm 3 /g), I i−1 represents the volume of mercury intrusion per unit mass of coal at time i − 1 (cm 3 /g).
The flooded pore area at time i is calculated according to Equation (5): where A ii is the flooded pore area at time i (m 2 /g). The cumulative surface area of the hole is calculated according to Equation (6): where A i is the cumulative surface area of the hole (m 2 /g).
An AUTOPORE9505 mercury injection machine was mainly used to estimate the size distribution, total pore volume, sample bulk/true density, fluid transport, and other physical properties of powder or bulk solids. The collected coal samples were crushed to 3-6 mm particle size and oven-dried at 150°C (or at a higher temperature) for 1 h before mercury injection porosimetry. 35

| Experimental equipment and procedure
Gas desorption experimental test device including a vacuum degassing unit, quantitative charging unit of high-pressure methane gas cylinder, adsorption balance unit, constant-temperature control unit, and gas desorption measurement unit was established according to the determination method of methane adsorption capacity of coal. Schematic diagram of the experimental device is shown in Figure 1

| Volume calibration
The vacuum pumping method is used for volume calibration of public pipelines and reference tanks. Valves 2-4 are opened to connect the charging system to the atmosphere when all valves are closed. Then the vacuuming device is started to depressurize the charging T A B L E 2 Industrial analysis and adsorption constants of coal samples. system. When the system vacuum reaches 20 Pa, Valve 4 is closed and then the vacuum system is closed. All valves must be closed, before opening Valves 2 and 4 to connect the charging system to the atmosphere; the vacuuming device is started to depressurize the charging system. When the system vacuum reaches 20 Pa, Valve 4 is closed and then the vacuuming system is closed. Filling a certain amount of N 2 into the measuring cylinder, the position of the liquid filling bottle is adjusted, to make the liquid level of the measuring cylinder and the liquid level of the liquid filling bottle flush. The volume of gas (V 1 ) in the measuring cylinder at that time is recorded. Valve 3 is slowly opened to allow the N 2 in the measuring cylinder to slowly enter the main pipeline and the reference tank. After the pressure is balanced, the position of the liquid bottle is adjusted again, so that the liquid level of the measuring cylinder and the liquid level of the liquid bottle are kept balanced, the volume of gas (V 2 ) in the measuring cylinder at this time is recorded, and the atmospheric pressure (P) and laboratory temperature (T) are also recorded. The same method was used to calibrate the adsorption tank volume. The difference is that the regulating valves are Valves 2-4 and the volume of gas in the measuring cylinder should be marked as (V 3 ) and (V 4 ). The standard volume (V p0 ) is calculated according to Equations (7) and (8). The aforementioned steps are repeated three times and the average value is taken. The unit volumes are shown in Table 3.

| Procedure for the adsorption and desorption experiment
The experimental process includes sample loading, airtight detection, vacuum, quantitative aeration, constanttemperature adsorption, quantitative air release, constant-temperature desorption, and experimental data recording.
The amount of methane gas charged into the adsorption tank can be calculated according to Equation (9).
where Q ci is the standard volume of methane filled into the adsorption tank (cm 3 ), P 1i and P 2i refer to the absolute pressure in the tank before and after inflating (MPa), K 1i and K 2i denote the compression of the methane (1/MPa), t 1 is the temperature of laboratory (°C), and V 0 denotes the standard volume of the tank and connecting pipe (cm 3 ). The amount of methane gas released by the adsorption tank can be calculated according to Equation (10).
where Q ci is the standard volume of methane released by the adsorption tank (cm 3 ), V L is the volume of gas read by the desorption apparatus (cm 3 ), P C is the atmospheric pressure of laboratory (kPa), 0.02t 2 is the pressure correction for the volumetric expansivity of mercury as the manometer reading varies with temperature (kPa), and W P is the saturated vapor pressure of salt water (kPa).

| Analysis of mercury characteristics
In the stage of mercury injection into primary coal with different degrees of metamorphism, with the increasing F I G U R E 2 Mercury intrusion curve of coal samples with different degrees of metamorphism.
F I G U R E 3 Mercury intake increment curve. pressure, mercury successively fills the visible pores and fissures, macropores, mesopores, and micropores in coal sample particles, until mercury completely intrudes into all pores of coal sample that can be reached under the applied experimental pressure. 17 Under the pressure provided by the experimental instrument, mercury saturation intruded into the pores of the coal sample and then began to depressurize. With the decrease of pressure, mercury withdrew successively from micropores, mesopores, macropores, and visible pores and cracks until no mercury withdrew. The mercury intake curve of primary coal with different degrees of metamorphism is always below the mercury withdrawal curve, and the limit point of mercury invasion is reached at higher pressure, and then mercury withdrawal begins. The mercury intake curve intersects the mercury withdrawal curve at the limit point of mercury invasion, but the straight lines of the two curves do not completely coincide, but produce a certain degree of "lag ring," indicating that there is a pore structure characteristic similar to an "ink bottle" in the coal. This phenomenon is closely related to the pore structure attributes of the coal itself. The mercury intrusion curve is shown in Figure 2.
For all tested coal samples, the pore volume obtained by the mercury intrusion hysteresis loop is not closed when the relative pressure is low, indicating the existence of ink-bottle pores and an elastic structure of the coal. However, the mercury flows in and out of PM coal samples show good reversibility.
The pore volume acquired by the mercury intrusion porosimetry experiment increases with increasing destruction types. The comparison of the total amount of mercury intrusion in nonstructural primary coal with different degrees of metamorphism shows that the total amount of mercury intrusion increases with the increase of degrees of metamorphism. The total amount of mercury intrusion in WY, PM, and CY coal is 0.0959, 0.0778, and 0.0543 mL/g, respectively. The difference in the total amount of mercury intrusion in coal samples with different degrees of metamorphism indicates that the pore volume within the pore size measured by the instrument increases with the increase in degrees of metamorphism. The differences of mercury advance and retreat of primary coals of the three degrees of metamorphism are shown to be CY > WY > PM. The difference values of mercury advance and retreat of primary coals of WY, PM, and CY are 0.0046, 0.0014, and 0.0107 mL/g, respectively.

| Mercury intake increment
Studies at home and abroad usually classify the pore structure in coal as follows. Micropores with diameters less than 10 nm constitute the adsorption zone of coal and are usually impossible to compress. Pores with diameters ranging between 10 and 100 nm form a capillary condensation and gas diffusion space, mesopores with diameters ranging between 100 and 1000 nm constitute the slow laminar infiltration zone of coal gas, large pores having diameters ranging between 1000 and 100,000 nm constitute a strong laminar flow penetration zone and can determine the failure surface of coal body with strong failure structure; visible holes and cracks with diameters larger than 0.10 mm from the mixed  According to the analysis of the mercury intrusion into primary coals with different degrees of metamorphism, the maximum mercury increment range of the primary coals with WY, PM, and CY coal was 0.10-10 psi (relative pressure), and it can be obviously found that WY > PM > CY. The maximum increments of Hg into WY, PM, and CY coal were 0.0142, 0.0072, and 0.0036 mL/g, respectively. With the increase in the degree of metamorphism, the maximum increment of mercury in the range of 0.10-10 psi increases gradually. The maximum mercury injection increment difference between WY and PM coal was 0.0070 mL/g, and that between PM and CY coal was 0.0036 mL/g, and that between WY and CY coal was 0.0106 mL/g. The minimum change of mercury intake increment in WY, PM, and CY primary coals was 10-1000 psi (relative pressure), and the mercury intake increments were small. As the pressure increases, the mercury intake increments showed a slow increase above 1000 psi. The mercury intake increment curve is illustrated in Figure 3.
When the relative pressure of WY increased from 0.60 to 1.19 psi, the increment of mercury intake showed a rapid growth; when the relative pressure increased from 1.19 to 7.47 psi, the increment of mercury intake showed a rapid decrease; when the relative pressure increased from 1.19 psi the increment of mercury intake reached a maximum of 0.0142 mL/g. When the relative pressure increased from 10 to 1000 psi, there was almost no increase in mercury intake as it remained at 0.0001-0.0002 mL/g; when the relative pressure exceeded 1000 psi, it slowly increased from 0.0002 to 0.0013 mL/g.
When the relative pressure of PM increased from 0.60 to 1.39 psi, the increment of mercury intake increased rapidly; when the relative pressure increased from 1.39 to 8.47 psi, the increment of mercury intake decreased rapidly; when the relative pressure increased from 1.39 to 8.47 psi, the increment of mercury intake reached the maximum value of 0.0072 mL/g. When the relative pressure increased from 10 to 1000 psi, there was almost no increase in mercury intake as it remained at 0.0001-0.0002 mL/g; when the relative pressure exceeded 1000 psi, it slowly increased from 0.0002 to 0.0021 mL/g.
When the relative pressure of CY increased from 0.60 to 1.19 psi, the increment of mercury intake showed a rapid increase; when the relative pressure increased from 1.19 to 9.47 psi, the increment of mercury intake showed a rapid decrease; when the relative pressure increased from 1.19 to 9.47 psi, the increment of mercury intake reached a maximum value of 0.0036 mL/g. When the relative pressure of CY increased from 10 to 1000 psi, it showed increased slightly, then decreased slightly with a peak inflection point which was different from tests on WY and PM specimens. The increment of mercury intake remained at 0.0001-0.0006 mL/g which increased compared with WY and PM; when the relative pressure exceeded 1000 psi it slowly increased from 0.0006 to 0.0021 mL/g.
Through comprehensive analysis of mercury intake increment curves of primary coal with different degrees of metamorphism, it can be found that the mercury intake increment of WY and PM is large in both the low and high-pressure stages, and there is a nonincremental curve in the process of relative pressure from low to high,

| Pore volume
According to the pore volume trend analysis of the three primary coals with different degrees of metamorphism, the maximum pore volume range of WY, PM, and CY was 10,000-1,000,000 nm and the pattern of WY > PM > CY can be clearly seen. The maximum pore volume of WY, PM, and CY in this region was 0.0142, 0.0072, and 0.0036 mL/g, respectively. The maximum pore volume in this region increases gradually with the increase of coal sample metamorphism. The maximum pore volume difference between WY and PM was 0.0070 mL, and that between PM and CY was 0.0036 mL, and that between WY and CY was 0.0106 mL. The pore volume of WY, PM, and CY primary coal body changes within the lowest range between 100 and 10,000 nm and the inner pore volume tended to be flat. The pore volume increased slowly in the range of 1-100 nm. The pore volume distribution curve is shown in Figure 5.
Through comprehensive analysis of pore volume curves of primary coal with different degrees of metamorphism, it can be found that the pore volume of WY and PM was large in the micropore, small hole and large hole interval, and relatively small in the middle hole interval. The pore volume and pore size presented a U-shaped curve. The pore volume of CY was larger in the micropore, small pore, and large pore regions, while the pore volume increased and decreased slightly in the middle and large pore regions. The pore volume and pore size showed obvious W-shaped curves with three peak values. The pore volume of coal with different degrees of metamorphism is shown in Figure 6.
According to the pore volume distribution law of different metamorphic primary coals, the average pore diameters of WY, PM, and CY were 68.40, 45.60, and 30.50 nm, respectively. The average diameter of the volume holes of WY relative to PM is increased by 22.80 nm, and that of PM relative to CY is increased by 15.10 nm, and that of WY relative to CY is increased by 37.90 nm. The average pore diameter increases gradually with the increase in the degree of metamorphism of primary coal. The average pore diameter of coal with different degrees of metamorphism is shown in Figure 7.

| Pore distribution characteristics
To facilitate the quantitative characterization of pore size and volume distribution characteristics of primary coal samples with different degrees of metamorphism, the coal samples were classified according to three grades which including micropore and small pore, medium pore, large pore, and visible pore. For micropore and small pore, the pore volumes of WY, PM, and CY coal were 0.0154, 0.0183, and 0.0197 mL/g, respectively, and the proportions were 16.06%, 23.22%, and 36.28%, respectively. For medium pore, the pore volumes of WY, PM, and CY coal were 0.0002, 0.0018, and 0.00467 mL/g, respectively, and the proportions were 2.09%, 2.28%, and 8.47%, respectively. For large pore and visible pore, the pore volumes of WY, PM, and CY coal were 0.0785, 0.0587, and 0.0300 mL/g, respectively, and the proportions were 81.86%, 74.49%, and 55.25%, respectively. For the primary coal with different degrees of metamorphism, the pore volume and proportion of micropore and small pore, medium pore showed a distribution pattern such that WY < PM < CY. The pore volume and proportion of large and visible pores showed a distribution such that WY > PM > CY. The distribution of pore size and volume of coal with different degrees of metamorphism is shown in Figure 8.
The porosity of WY, PM, and CY were 11.57%, 9.03%, and 6.57%, respectively, and the specific surface areas were 6.917, 5.826, and 5.611 cm 3 /g. The porosity and T A B L E 4 Pore structure and coal methane parameters with different metamorphic degrees.

| Internal relationship between coal pore and adsorption
Mercury injection characterizes the size and distribution of coal pore structure intuitively. Coal methane adsorption and desorption are mainly controlled by the internal pore structure. Porosity is the key factor in intuitively characterizing the coal internal pore structure. Distribution patterns and pore volume characteristics of pores are both key factors affecting coal methane adsorption and desorption. Through experimental testing could be found that porosity gradually increases as the degree of metamorphism increases. At the same time, the adsorption constant, average pore diameters, and specific surface areas gradually increase which indicating that porosity is the key factor affecting coal methane adsorption potential energy. Pore structure and coal methane parameters with different metamorphic degrees are shown in Table 4. The characterization of coal methane parameters by porosity is shown in Figure 10. Porosity, specific surface area, average pore diameters, and maximum pore volume show a gradually increasing trend as the degree of primary coal metamorphism increases. Meanwhile, the adsorption constant showed the same trend. Through experimental characterization found that anthracite has an obvious advantage in the characteristic parameters such as pore structure and adsorption constant which is also the essential characteristic that anthracite has obvious differences from other low metamorphic coals in gas content, gas pressure, adsorption potential energy, and desorption potential energy. Tree diagrams of coal pore structures are shown in Figure 11.
The corresponding relationships between adsorption constants a and b, as well as specific surface areas, average pore diameters, and maximum pore volume, were compared and analyzed based on experimental results. Specific surface area, average pore diameters, and maximum pore volume show a relatively consistent linear increase as the adsorption constant increases from different perspectives. Characterization of specific surface area, average pore diameters, and maximum by adsorption constants are shown in Figures 12-14, respectively. F I G U R E 11 Tree diagram of coal pore structure.

| Gas adsorption and desorption characteristics
To investigate the differential characteristics of gas desorption of native media with different degrees of metamorphism, coal samples with a diameter of 1-3 mm were prepared, and adsorption-desorption experiment was conducted at a constant temperature (30 ± 0.5°C) under the adsorption equilibrium pressure of 3 MPa. The initial cumulative desorption amount in 30 min and the initial desorption rate in 15 min of coal with different degrees of metamorphism are shown in Figure 15.   According to the analysis of cumulative desorption amount at different observation times, when the adsorption equilibrium pressure is 3 MPa and the particle size of coal samples is 1-3 mm, the cumulative desorption amount of coal samples with different degrees of metamorphism increases gradually over time within 30 min of cumulative desorption. Among them, the cumulative desorption capacity of WY, PM, and CY coals in 1 min is 186, 127, and 111 mL, respectively; at 5 min these are 379, 240, and 213 mL, respectively; at 10 min these are 498, 320, and 272 mL, respectively; at 15 min these are 576, 371, and 313 mL, respectively; at 20 min these are 635, 406, and 339.50 mL, respectively; at 25 min these are 678, 435, and 360.50 mL, respectively; at 30 min these are 716, 460.50, and 378.50 mL, respectively.
According to the analysis of gas desorption velocity in different observation times, when the adsorption equilibrium pressure is 3 MPa and the particle size of coal samples is 1-3 mm, the gas desorption velocity of coal samples with different degrees of metamorphism gradually decreases with the passage of time within 15 min of initial desorption. Among them, the gas desorption rates of WY, PM, and CY coals at 1 min are 1.5076, 1.1485, and 0.9332 mL/g·min, respectively; at 5 min these are 0.3891, 0.3158, and 0.1866 mL/g·min, respectively; at 10 min these are 0.2918, 0.1723, and 0.1333 mL/g·min, respectively; at 15 min these are 0.1459, 0.1149, and 0.1067 mL/ g·min, respectively. The more directly to reflect the gas desorption velocity of coal samples with different degrees of metamorphism, the gas desorption velocity F I G U R E 15 Desorption capacity and desorption rate of coal with different degrees of metamorphism.
Through the previous experiment could be found that porosity, specific surface area, average pore diameters, and maximum pore volume show a gradually increasing trend as the degree of primary coal metamorphism increases. Meanwhile, the adsorption constant showed the same trend. Through experimental characterization found that anthracite has an obvious advantage in the characteristic parameters such as pore structure and adsorption constant which is also the essential characteristic that anthracite has an obvious difference from other low metamorphic coals in gas content, gas pressure, adsorption potential energy, and desorption potential energy.
According to the desorption characteristics of coal bodies with different metamorphic degrees at different time nodes found that the gas desorption at different time nodes shows similar changes under the same adsorption equilibrium pressure. The higher the degree of coal metamorphism, the greater the amount of gas desorption, and the faster the desorption amplitude. The desorption intensity of WY coal is significantly higher than that of PM and CY coal with the increase of desorption time which also verifies the measurement results of pore structure and adsorption constant. Coal methane desorption with different metamorphic degrees at different time nodes is shown in Figure 16. Porosity is the key factor in intuitively characterizing the coal internal pore structure. Distribution patterns and pore volume characteristics of pores are both key factors affecting coal methane adsorption and desorption. Through experimental testing could be found that porosity gradually increases as the degree of metamorphism increases. At the same time, the adsorption constant, average pore diameters, and specific surface areas gradually increase which indicating that porosity is the key factor affecting coal methane adsorption potential energy.
According to the desorption characteristics of coal bodies with different metamorphic degrees at different time nodes found that the gas desorption at different time nodes shows similar changes under the same adsorption equilibrium pressure. The higher the degree of coal metamorphism, the greater the amount of gas desorption, and the faster the desorption amplitude. The desorption intensity of WY coal is significantly higher than that of PM and CY coal with the increase of desorption time which also verifies the measurement results of pore structure and adsorption constant.
Due to the limitations of experimental research samples and devices, further work needs to be done on experimental specimens of coal with different degrees of metamorphism in different coal measure strata, different coal-forming regions, and different mines, so as to more accurately grasp the pore structure and adsorption and desorption characteristics of primary coal with different degrees of metamorphism.