Influences of Evolution of Pore Structures in Tectonic Coal under Acidization on Methane Desorption

Abstract

Experiments on corrosion reactions of pulverized coal with monomeric and polymeric (mixed) acid solutions reveal that monomeric acids are listed in a descending order as HF, HCl, and CH₃COOH according to their corrosion effects on tectonic coal collected in Faer Coal Mine (Liupanshui City, Guizhou Province, China). In addition, the optimal mixing ratio of mixed acids is 6% HCl + 6% HF + 3% CH₃COOH + 2% KCl. The mineral grains filled in pores in coal samples treated with mixed acid solutions are dissolved, so the porosity increases. The volumes of transition pores and mesopores are obviously affected by acidization, and some transition pores are transformed into mesopores and macropores to form dissolved pores. At the same time, inkbottle-shaped pores reduce, while slit pores or open pores increase. The coal samples after acidization show a higher aromatization degree and an increased relative content of oxygen-containing functional groups, with a generally lower hydroxyl content, so the methane (CH₄) adsorption capacity of coal declines, which promotes CH₄ desorption. The control effect of pore structures after acidization reactions on CH₄ desorption was revealed from perspectives of the diffusion coefficient (Kn), adsorption volume (ω), average pore–throat ratio (P_T), and average sinuosity (τ_av). That is, CH₄ molecules in tectonic coal after acidization turn from Knudsen diffusion to transitional diffusion, the adsorption volume of CH₄ molecules shrinks, the average pore–throat ratio decreases, and the average sinuosity reduces, which promotes CH₄ desorption from tectonic coal.

1. Introduction

Coalbed methane (CBM), as an unconventional gas (accounting for about 20% of natural gas resources), is a clean energy, and its extraction is a key link in the development of low-carbon energies.¹⁻⁴ There are abundant CBM resources in China; however, tectonic coal is widely distributed in China, in which the complex pore structures, molecular structures, and low permeability limit the mining activities of CBM. Therefore, studying the structural characteristics and increasing the gas permeability of coal are key steps in vigorous CBM mining.^5,6 To improve the CBM yield, acid solutions have been injected in coal seams to reform material properties of coal reservoirs, physically fracture coal seams, chemically dissolve minerals that block gas migration channels, and change migration paths of CBM in pores and fractures. This can reach the goal of increasing permeability of coal reservoirs and thus realize efficient CBM extraction.⁷⁻¹⁰

Generally, the microscopic physicochemical structures of coal include physical (pore) and chemical (molecule) structures. Pore structures in coal play an important role in the adsorption and desorption mechanisms of methane (CH₄).^11,12 Pores in coal include micropores, transition pores, mesopores, and macropores, the size and distribution of which significantly influence the adsorption and desorption process of CBM.¹³ At present, three types of methods are available for exploring pore structures in coal: fluid injection, nonfluid injection, and image analysis methods. The high-pressure mercury injection porosimetry (HP-MIP), low-temperature liquid nitrogen gas adsorption (LTN₂-GA), low-temperature CO₂ gas adsorption (LTCO₂-GA), and nuclear magnetic resonance (NMR) all belong to fluid injection methods. They can quantitatively characterize the pore volume, pore size, and pore size distribution in tectonic coal and have advantages including wide measurement ranges and high precision.¹⁴⁻¹⁷ However, limited by the measurement principles, different tests are applicable to different pore size ranges, so it generally requires to combine multiple tests to comprehensively quantitatively characterize pore and fracture structures.¹⁸ Nonfluid injection methods include focused ion beam scanning electron microscopy (FIB-SEM), micron-resolution computed tomography (μ-CT), small-angle X-ray scattering (SAXS), and small-angle neutron scattering (SANS), which are suitable for characterizing the porosity, pore size distribution, and surface area of coal.¹⁹⁻²³ Image analysis methods include scanning electron microscopy (SEM), transmission electron microscopy (TEM), and atomic force microscopy (AFM) that can directly observe geometry, mineral filling, and connectivity of pore structures in coal and rocks, while they cannot achieve quantitative analysis.^24,25

Moreover, the chemical structures of coal also affect the adsorption and desorption of CBM. For example, hydroxy, phenolic, and carboxyl groups in coal are all conducive to adsorption of CBM, while alkyl groups, alkyl side chains, and aromatics promote desorption of CBM. Existing research on chemical structures of coal mainly involves X-ray diffraction (XRD) to analyze mineral contents and Fourier transform infrared spectroscopy (FTIR) and Raman spectroscopy to study chemical properties of coal samples.^26,27 For acid solvents, many existing studies focus on their influences on minerals and then influences on changes in pore structures.²⁸ In fact, acid solvents also affect chemical structures of coal. For example, hydrochloric acid (HCl) can not only remove particles on coal surfaces but also can influence carboxyl- and oxygen-containing structures; besides, they also exert significant influences on aromatic structures and oxygen-containing groups.²⁹⁻³¹

However, research into influences of acid solutions on the properties of coal, such as wettability, adsorptivity, and gas permeability is still in its infancy.³²⁻³⁴ In previous research, HCl and hydrofluoric acid (HF) are commonly the first choices of acid solutions. Meanwhile, influences of evolution of physicochemical structures of tectonic coal under combination of acetic acid (CH₃COOH) with HCl and HF on CH₄ adsorption and desorption are seldom reported. Changes in the surface morphologies, pore size distribution, and functional groups of chemical structures in tectonic coal samples before and after acidization were analyzed through SEM, HP-MIP, LTN₂-GA, and FTIR. By carrying out CH₄ adsorption and desorption experiments, influences of evolution of physicochemical structures of tectonic coal under acidization on CH₄ adsorption and desorption were revealed. The research can provide a theoretical reference for coalbed methane mining in similar mining areas.

2. Samples and Experimental Methods

2.1. Samples and Material Basis

Coal samples in the experiments were tectonic coal collected from a heading face in a coal mine, namely, No. 5 coal seam in Faer Coal Mine in Liupanshui Coal Field in Guizhou Province, China (specific geographic location of the coal mine is shown in Figure 1a,b). The tectonic coal samples collected in Faer Coal Mine were seme-bright and showed rough textures, disordered beddings, angular fractures, and developed endogenous fissures. The coal samples were easily broken when twisted by hands. The sample is shown in Figure 1c.

Figure 1.

Sampling geographical location. (a) Geographical location of sampling coal mine. (b) Faer coal mine. (c) Tectonic coal.

To study conventional properties of the tectonic coal, the primary coal samples taken from the same mine lot were used for comparative analysis. Two proximate analysis (PA) and ultimate analysis (UA) were separately performed for the primary and tectonic coal samples and their mean values were obtained, as shown in Table 1.

Table 1. Routine Analysis of Coal Samples.

	proximate analysis (%)				ultimate analysis (%)
samples	Mad	Aad	Vdaf	FCad	C	H	N	S
primary coal	1.08	12.22	16.03	70.67	74.46	3.00	1.15	0.63
tectonic coal	1.45	21.91	13.69	62.95	57.95	2.56	1.02	2.23

The primary coal in the research area contains a lower water content with a higher volatile content than the tectonic coal, indicating that the tectonic coal has a larger internal surface area than the primary coal, along with loose internal structures, developed pores, and abundant inorganic minerals. Moreover, software MDI Jade 6 was adopted to analyze XRD data to obtain minerals and their relative contents in primary and tectonic coal samples, as listed in Table 2.

Table 2. Mineral Content of Coal Samples Based on XRD.

	mineral relative content (%)
samples	quartz	kaolinite	pyrite	illite	calcite	dolomite	chlorite
primary coal	56.4	35.7	0.8	1.9	4.4	0.7	0.1
tectonic coal	52.4	42.3	1.6	2.1	0.9	0.4	0.3

Three minerals are mainly present in primary and tectonic coal sampled in Faer Coal Mine, namely, clay minerals (kaolinite, chlorite, quartz, and illite), carbonate minerals (calcite and dolomite), and sulfide mineral (pyrite). This suggests that tectonic deformation hardly changes the mineral compositions in coal, while it may cause changes in the relative contents of minerals. Minerals in tectonic coal are mainly clay minerals, which lay a basis for material reactions in the subsequent acidization reaction experiments.

2.2. Experiments and Methods

2.2.1. Experimental Principles and Schemes

2.2.1.1. Acidization Corrosion Mechanism

According to analysis and experimental results of minerals in coal samples collected from Faer Coal Mine, HF can be used to treat all inorganic minerals and HCl can be utilized to treat carbonatites and some oxides in coal.²⁹ In the meantime, despite its lower corrosivity than HF and HCl, CH₃COOH, as an organic acid, can react with carbonatites, some oxides, and ester ether compounds in coal. In addition, it can also realize the goal of slowing down the corrosion rate of acid solutions and therefore enlarge the reaction area between acid solutions and coal samples.^35,36

2.2.1.2. Experimental Schemes

At first, UA, PA, and XRD were performed to analyze the material basis of coal samples, as shown in Figure 2a. Then, the following three steps were conducted: (1) the optimal mixing ratio of acid solutions was determined, as displayed in Figure 2b; according to data in the corrosion experiments using monomeric acid solutions, the mixing schemes for corrosion experiments using polymeric acid solutions were provided. By carrying out corrosion experiments using polymeric acid solutions, the optimal mixing ratio of acid solutions was selected to conduct acidization reactions between acid solutions and coal samples for 12 h. Coal samples after acidization were used in subsequent experiments. (2) In Figure 2c–g, characteristics of pore and fracture structures in tectonic coal samples before and after acidization were analyzed by combining SEM, HP-MIP, and LTN₂-GA. Meanwhile, UA and FTIR were adopted to analyze changes in elements and chemical structural parameters of coal samples. (3) As shown in Figure 2h, CH₄ adsorption and desorption experiments were performed on tectonic coal samples before and after acidization to explore the influences of evolution of pore and fracture structures before and after acidization on CH₄ desorption.

Figure 2.

Experimental process. (a) Analysis on mineral basis of coal: Vario EL cube ultimate analyzer, WS-G818 Automatic proximate analyzer, and Rigaku Smartlab X-ray diffractometer. (b) Acidization reaction of coal. (c–g) Experimental analysis of chemical and pore structure of coal samples before and after acidization: Vario EL cube ultimate analyzer, Nicolet 6700 Fourier transform infrared spectrometer, Zeiss Sigma 300 scanning electron microscope, Micromeritics AutoPore V 9620 high-performance automatic mercury porosimeter, and Autosorb IQ3 automatic surface area and porosity analyzer, respectively. (h) Experimental analysis of methane adsorption and desorption of coal before and after acidification: BSD-PH automatic high temperature and high pressure gas adsorption instrument.

2.2.2. Optimization of the Mixing Ratio of Acid Solutions

The mixing ratio of acid solutions for acidizing coal samples in the research was selected in the corrosion experiments. As shown in Figure 2b, the steps include:

① Drying. Pulverized coal of 60–80 mesh was selected and dried to a constant weight at 100 °C.

② Preparation of acid solutions. Distilled water was used to separately dilute 37% (mass fraction) HCl, 40% HF, and 100% CH₃COOH to prepare acid solutions at concentrations of 3, 6, 9, 12, and 15%, which were poured in 250 mL volumetric flasks for later use.

③ The needed volumes V of the above original acid solutions were calculated using eqs 1–3:

1

2

3

where V_HCl, V_HF, and V_CH3COOH are the needed volumes of HCl, HF, and CH₃COOH solutions; ρ_HCl, ρ_HF, and ρ_CH3COOH are the mass fractions of the original acid solutions, respectively.

④ Acidization of pulverized coal. Several portions of prepared dry pulverized coal (exact 3.0000 g for each) were weighed to sufficiently contact and react with HCl, HF, and CH₃COOH solutions prepared in step ② at a ratio of 1 g:10 mL for 1, 3, 6, 12, and 24 h.

⑤ Filtration and weighing. After reaction duration between acid solutions and pulverized coal reached the pre-set experimental conditions, filter paper was used to filter acid solution to neutral. Afterward, the filtered pulverized coal was placed in a drying oven at 60 °C to be dried to constant weight. After being cooled, the residual pulverized coal was weighed.

⑥ Analysis of corrosion rate. The corrosion rate is an important index for selecting the optimal mixing ratio of acid solutions and its value reflects the reaction degree of acid solutions and minerals in coal. According to the data of the corrosion rate, the optimal reaction concentration and duration of monomeric acid solutions were determined. The corrosion rate can be calculated using eq 4

4

where Y is the corrosion rate of coal samples (%); m₁ is the initial mass of pulverized coal (g); m₂ is the mass of filter paper (g); m₃ is the mass of filter paper (containing pulverized coal) after acidization (g). Therein, calculation of the corrosion rate in the case of using polymeric acid solutions follows the similar steps above. Figure 3 illustrates the specific steps of acidization reactions of coal samples.

Figure 3.

Acidization experimental process.

2.2.3. Experiments on Microscopic Pore Structures in Coal Samples

SEM was performed using a Zeiss Sigma 300 field emission scanning electron microscope equipped with an Oxford Xplore 50 energy-dispersive spectrometer (EDS), which could test low-voltage microstructures and elemental contents in nonconducting samples with different sizes and in different shapes.

HP-MIP was conducted using a Micromeritics AutoPore V 9620 high-performance automatic mercury porosimeter. After preheating the device, the mercury solution was added in a mercury pool 1–3 cm above the observation window. Then, after weighing and placing coal samples in the expansimeter, the tests began.

LTN₂-GA experiments were carried out using an Autosorb IQ3 automatic surface area and porosity analyzer (Quantachrome Instruments, USA). At first, the solid samples to be tested were placed in the degassing station at 105 °C to be pretreated through vacuum desorption for 12 h. Then, parameters including the adsorption pressure points were set according to the designed analysis method and the solid samples were coated with an insulation cover and put and tested in the workstation of the instrument.

2.2.4. Experiments on Chemical Structures of Coal Samples

A Vario EL cube ultimate analyzer (Elementar, Germany) was utilized to test the contents of elements C, H, N, and S. A Thermo Fisher Nicolet 6700 Fourier transform infrared spectrometer (USA) was used for experiments, and the obtained infrared spectra were subjected to smoothing, normalization, and peak fitting using OMNIC and Peakfit V4.12, thus quantitatively analyzing changes in parameters of functional groups and chemical structures of tectonic coal before and after acidization.

2.2.5. CH₄ Adsorption and Desorption Experiments of Coal Samples

A BSD-PH automatic high-temperature, high-pressure gas adsorption instrument produced by Bei Shi De was used to study the CH₄ adsorption and desorption properties before and after acidization. Pulverized coal (20 g) was weighed and placed in a clean sample cell, which was continuously shaken to compact the coal sample therein and thus reduce the experimental error. After checking the airtightness of the instrument, the sample cell was placed under a vacuum condition at 120 °C to be degassed for 180 min. The gas expansion method was adopted to measure the dead volume of the sample cell followed by adsorption and desorption tests.

3. Results and Discussion

3.1. Analysis of Acidization Corrosion Experimental Results

3.1.1. Corrosion Experiments with Monomeric Acid Solutions

The above research on mineral compositions of coal reveals that good acidization effects were achieved using HCl, CH₃COOH, and HF. Pulverized coal was allowed to react with monomeric acid solutions for different durations. Afterward, the corrosion rates of coal were obtained using eq 4, as shown in Figure 4.

Figure 4.

Coal powder corrosion rate under single component acid solution with different concentrations. (a) Corrosion rate of coal in different concentrations of hydrochloric acid. (b) Corrosion rate of coal in different concentrations of acetic acid. (c) Corrosion rate of pulverized coal in hydrofluoric acid with different concentrations.

It can be seen from Figure 4a that the corrosion rate of pulverized coal is positively correlated with the concentration of HCl solution, and the longer the acidization reaction duration is, the higher the corrosion rate. Within 1–12 h, the corrosion rate is high, which belongs to the rapid reaction stage; within 12–24 h, changes of the corrosion rate tend to stabilize. In the corrosion process with HCl, HCl tends to have a corrosion rate generally lower than 10% for coal samples with the increasing concentration and reaction duration of HCl. This is mainly because there are low contents of carbonate minerals in coal samples collected from Faer Coal Mine and the sample used in the corrosion experiments only weighed 3 g so that there are limited soluble minerals. According to Figure 4b, the higher the concentration of CH₃COOH solution is, the higher the corrosion rate of pulverized coal. At the same concentration of the acid solution, the corrosion rates reach the peaks after reaction with 3% CH₃COOH and 6% CH₃COOH for 12 h. At other concentrations, the reaction periods of 1–3 h and 3–6 h separately belong to the rapid reaction stage and steady reaction stage; the corrosion rate reduces slightly in 12–24 h, and it tends to stabilize in 12–24 h. As displayed in Figure 4c, generally the higher the concentration of acid solutions is, the higher the corrosion rate of pulverized coal. Meanwhile, the corrosion rate of coal samples treated with 9% HF is the highest (25.64%), and the corrosion rates of those treated with 12% HF and 15% HF are lower than that treated with 9% HF. This is because the higher the concentration of HF solution is, the easier the reactions of HF with minerals including quartz, kaolinite, and calcite to generate SiF₆^2– ions, which undergo secondary reactions to produce precipitates. In the process, the periods of 1–3 h, 3–6 h, and 6–9 h separately belong to the rapid reaction stage, steady corrosion stage, and stage with a decreased corrosion rate, respectively. This is because HF may react with calcite to generate CaF₂ precipitates. The period of 12–24 h corresponds to the stable equilibrium stage, in which the corrosion rate tends to stabilize.

The data analysis of the above corrosion experiments with monomeric acid solutions reveals that the three acids are listed in a descending order as HF, HCl, and CH₃COOH according to the corrosion rates of coal samples collected from Faer Coal Mine in the three acid solutions.

3.1.2. Corrosion Experiments with Polymeric Acid Solutions

According to results of corrosion experiments with monomeric acid solutions and considering the influences of acid concentrations and reaction durations in practical production on field construction, underground equipment, and other factors, the mixed acid with the overall concentration of 15% was prepared to carry out corrosion experiments with polymeric acid solutions (mixing ratios of mixed acid are listed in Table 3; the procedure is similar to the one-component acid solution procedure in Section 2.2.2). Coal samples were allowed to react with acid solutions with different mixing ratios for different durations, and the corrosion rates of pulverized coal were calculated (Table 3 and Figure 5). In this way, the optimal mixing ratio of acid solutions for acidization reactions of tectonic coal collected from Faer Coal Mine was determined.

Table 3. Coal Powder Corrosion Rate at Different Reaction Times under Different Mixed Acids.

	mixed acid ratio concentration (%)				corrosion rate at different reaction times (%)
number	HCl	HF	CH3COOH		1 h	3 h	6 h	12 h
A	9	3	3		6.38	7.59	7.20	11.63
B	6	6	3		5.34	7.75	9.90	11.87
C	4.5	6	4.5		3.70	5.72	9.24	10.38
D	3	6	6		2.45	6.82	8.32	11.90
E	3	9	3		6.28	11.36	14.83	14.90
F	3	3	9		1.98	3.39	4.21	5.87

Figure 5.

Coal powder corrosion rate at different reaction times in multicomponent acid solution. A: 9% HCl + 3% HF + 3% CH₃COOH. B: 6% HCl + 6% HF + 3% CH₃COOH. C: 4.5% HCl + 6% HF + 4.5% CH₃COOH. D: 3% HCl + 6% HF + 6% CH₃COOH. E: 3% HCl + 9% HF + 3% CH₃COOH. F: 3% HCl + 3% HF + 9% CH₃COOH.

It can be seen from Table 3 and Figure 5 that as the reaction duration prolongs, the corrosion rate of tectonic coal tends to rise on the whole, with fast reactions in the first 6 h and slow ascent of the corrosion rate in 6–12 h. Within the same reaction duration, the higher the concentration of CH₃COOH solution is, the lower the corrosion rate of tectonic coal samples, which is mainly because of the retarding effect of CH₃COOH in acidization reactions. For tectonic coal samples taken from Faer Coal Mine, the corrosion schemes are listed in a descending order as E, B, D, A, C, and F according to corrosion rates of coal samples after being soaked in acid solutions at different mixing ratios.

In accordance with the corrosion effect of acid solutions with different mixing schemes and the influences of the corrosion rate on acidization areas, the optimal mixing ratio of the mixed acid solution for tectonic coal in Faer Coal Mine is 6% HCl + 6% HF + 3% CH₃COOH + 2% KCl (shorted as scheme B). In the scheme, 2 % KCl was added to relieve expansion during acidization.

3.2. Characteristics of Pore Structures in Acidized Coal Samples

3.2.1. SEM-EDS Analysis

To study changes in surface characteristics of tectonic coal samples before and after acidization, the SEM-EDS was adopted to observe surface morphologies of coal samples before and after acidization, as shown in Figures 6 and 7. For the convenience of drawing charts, the tectonic coal samples before and after acidization were separately recorded as PT coal and AT coal, respectively.

Figure 6.

(a–d) SEM test results of PT and AT coal.

Figure 7.

(a) White precipitate and (b) EDS analysis.

According to Figure 6, the surface roughness increases, pore and fracture profiles become clear, and lots of dissolved pores appear after acidization reactions of coal samples. Moreover, white crystals are found on local surfaces of AT coal samples in the test process. According to compositional analysis of the white crystals using the EDS, the white crystals are dominated by F and Si, so it is inferred that the white crystals are mainly K₂SiF₆ and CaF₂ (Figure 7). This is because in the acidization process, HF solution reacts with quartz to form SiF₆^2–, which generates white precipitate potassium fluosilicate after reactions with the anti-swelling agent KCl solution. Meanwhile, white precipitate CaF₂ is generated when HF dissolves calcite, which seriously influences the acidization reaction effect. Hence, during acidization reactions of coal seams with minerals including calcite and dolomite that contain Ca, Fe, and Mg, the HF concentration should be strictly controlled. In addition, it is suggested to use a pre-acid to treat these minerals to avoid influences on pore connectivity in coal reservoirs due to generation of precipitates.

3.2.2. Analysis of HP-MIP Experiments

At the room temperature, the coal samples were soaked in the mixed acid solution prepared according to the mixing ratio in scheme B to experience acidization reactions for 12 h. The specific methods of acidization experiments are described in Section 2.2.2. Coal samples before and after acidization were separately used to carry out HP-MIP experiments. Test results are displayed in Table 4 and Figures 8 and 9.

Table 4. Parameters of Pore Structures of PT and AT Coal.

samples	total porosity (%)	specific surface area (m2/g)	average pore diameter(nm)	pore volume (mL/g)
PT coal	8.01	4.1812	7.27	0.1457
AT coal	10.51	5.320	7.88	0.1801

Figure 8.

Percentage of different aperture ratio surface areas and pore volumes in PT and AT coal. (a) Percentage of surface area. (b) Percentage of pore volume.

Figure 9.

Experiment curve of mercury injection and distribution diagram of mercury intake–pore size in stages of PT and AT coal. (a) Mercury injection and mercury ejection curves. (b) Stage mercury intake–pore size curve.

It can be seen from Table 4 that compared with the primary tectonic coal, the porosity of AT coal samples increases by 31.21% from the original 8.01 to 10.51%, and the total specific surface area, average pore size, and total pore volume all enlarge.

Figure 8a,b shows that the volume of micropores is well developed in PT coal samples and micropores account for the largest proportion in the total pore volume, reaching 50.48%, followed by transition pores, which account for 29.42%. After acidization, the proportions of specific surface area and volume of macropores separately increase by 1.22 and 14.39%; the proportion of specific surface area of mesopores reduces by 0.98%, while the volume proportion of mesopores enlarges by 1.89%; the proportions of specific surface area and volume of transition pores separately decline by 5.85 and 8.21%; the proportion of specific surface area of micropores grows by 5.61%, while the proportion of their volume decreases by 8.06%. These results indicate that some transition pores in the tectonic coal are transformed into mesopores and macropores in the acidization process and tiny dissolved pores appear, which increase the porosity of coal samples. Besides, the volume proportion of transition pores after acidization reactions is 21.21% (lower than 29.42% before reactions) and that of macropores is 26.32% (higher than 11.93% before reactions). This implies that acidization reactions cause transformation of some transition pores into macropores and enhance pore connectivity of tectonic coal.

According to Figure 9a, mercury injection and mercury ejection curves of PT coal are open with a small angle. As the pressure drops, the difference in the volumes of injected and ejected mercury gradually enlarges while at an unobvious amplitude, suggesting a low proportion of open pores in tectonic coal. Figure 9a reveals that the angle between mercury injection and mercury ejection curves of AT coal is larger than that of PT coal. As the pressure drops, the increase amplitude of the difference in the volumes of injected and ejected mercury is slightly greater than that of AT coal. It can be seen from the pore size distribution figure of PT coal that the staged mercury injection increases at first and then decreases with the enlarged pore size and reaches the lowest value at transition pores. The peak 1000 is the most prominent, where the pore size is mainly 10–100 nm, so pore structures of PT coal belong to the type of inverted-S shape (Figure 9b). In terms of pore size distribution, the staged mercury injection is stable in transition pores and mesopores and also reaches the lowest value at transition pores, while the peak 1000 is not prominent (Figure 9b). The pore sizes mainly cover transition pores and mesopores, so pore structures of AT coal belong to the M type.

3.2.3. LTN₂-GA Analysis

To more accurately analyze changes in transition pores and micropores of PT and AT coal, LTN₂-GA experiments were performed to study evolution of micropores and transition pores (10–100 nm). The LTN₂-GA results of PT and AT coal are illustrated in Figure 10.

Figure 10.

Low-temperature liquid nitrogen adsorption/desorption curves of PT and AT coal.

As shown in Figure 10, the adsorption–desorption curves of PT coal samples are not closed, accompanied by adsorption hysteresis. This is mainly because there are inkbottle-shaped pores or slit pores in tectonic coal, which are narrow in the front part while wide in the rear part, and after adsorbing nitrogen, the diameter of the pore mouth shrinks so that nitrogen fails to be completely desorbed in the desorption process. The adsorption–desorption curves of AT coal samples belong to type III in the IUPAC classification scheme, and the curves are closed. Meanwhile, under P/P₀ < 0.5, the adsorption–desorption curves are almost overlapped, suggesting complexity of pore structures in the pore size range. Moreover, under P/P₀ > 0.5, H₃ hysteresis loops appear, indicating the presence of parallel-plate-shaped pores or slit pores in AT coal and good overall connectivity of pores in coal samples. The nitrogen adsorption capacity of AT coal enlarges from the original 3.79 to 10.58 mL/g, the adsorption/desorption curves are closed, and the area of hysteresis loops widens, which indicate that the number of pores in AT coal increases.

Many scholars have used LTN₂-GA experiments to detect pore parameters such as the volume and specific surface area of pores to characterize changes in pore structures of coal.⁹ The test results are shown in Table 5 and Figure 11.

Table 5. Pore Volume and Specific Surface Area Based on a Low-Temperature Liquid Nitrogen Adsorption Test^a.

	pore volume (×10–2 cm3/g)			specific surface area (m2/g)			percentage (%)
samples	V1	V2	Vt	S1	S2	St	V1/Vt	V2/Vt	S1/St	S2/St
PT coal	0.200	0.500	0.700	2.6960	4.3180	7.0140	28.57	71.43	38.44	61.56
AT coal	0.002	1.638	1.640	0.1606	2.8785	3.0391	0.12	99.88	5.28	94.72

^aNote: V₁ is the volume of micropores; V₂ is the volume of transition pores; V_t is the total pore volume; S₁ is the specific surface area of micropores; S₂ is the specific surface area of transition pores; S_t is the total specific surface area.

Figure 11.

(a) Pore volume–pore size and (b) specific surface area–pore size distribution.

It can be seen from Table 5 that AT and PT coal samples show differences in both the total pore volume and total specific surface area. The total pore volume in PT coal is 0.7 × 10^–2 cm³/g, while it enlarges by 134.3% to 1.64 × 10^–2 cm³/g in AT coal. The total specific surface area of PT coal is 7.014 m²/g, while it reduces by 56.67% to 3.0391 m²/g after acidization. This indicates that acidization dissolves minerals in pores and fractures, expands volumes of pores with the size in the range of 10–100 nm, and narrows the specific surface area so that the gas adsorption site shrinks and pore connectivity is improved, which are favorable for gas desorption.

As shown in Figure 11, the pore volume and specific surface area of tectonic coal collected from Faer Coal Mine are greatly influenced by acidization. Before acidizing coal samples, the volume of micropores is mainly contributed by pores with the size of 5 nm around, and that of transition pores is mainly contributed by pores with the size of 25 nm (Figure 11a-1). It can be seen from Figure 11b-1 that, before acidizing coal samples, the specific surface area of micropores is mainly contributed by pores with the size of 4 nm, while that of transition pores is mainly composed of pores with the size of 26 nm. As displayed in Figure 11a-2, after acidizing coal samples, the volumes of micropores and transition pores are separately mainly contributed by pores with sizes of 3 and 10 nm. After acidizing coal samples, the specific surface areas of micropores and transition pores are mainly separately composed of pores with sizes of 3 and 12 nm (Figure 11b-2).

3.3. Analysis of Changes in Chemical Structures of PT and AT Coal

UA is commonly adopted to analyze changes in element contents in AT coal samples. Table 6 lists results of two tests of PT and AT coal.

Table 6. Ultimate Analysis of PT and AT Coal Samples.

samples	C (%)	H (%)	N (%)	S (%)	H/C (%)
PT coal-1	57.941	2.561	1.02	2.251	0.53
PT coal-2	57.954	2.549	1.014	2.21	0.52
AT coal-1	63.096	2.933	1.027	4.152	0.55
AT coal-2	63.348	3.015	1.042	4.169	0.57

The results in Table 6 show that the element contents in PT and AT samples change obviously. In the tests, the relative contents of elements tested including C, H, N, and S all change. In Figure 12, the characteristic absorption peaks of PT and AT coal basically appear at the same positions on the infrared spectra, while the difference lies in the intensity of peaks. According to the positions of the characteristic absorption peaks, the infrared spectra are divided into eight main areas.³⁷

Figure 12.

FTIR spectra before and after acidification.

3.3.1. Quantitative Analysis of Functional Groups in PT and AT Coal

Hydroxy groups, aliphatic hydrocarbons, oxygen-containing functional groups, and aromatics in coal are mainly distributed in ranges of 3000–3750, 2800–3000, 1000–1800, and 700–900 cm^–1, respectively. Through peak fitting of the above four ranges, the results are shown in Figure 13 and Tables 7–10.

Figure 13.

(a–d) FTIR spectra curve of PT and AT coal based on Peakfit V4.12 software fitting.

Table 7. Fitting Results of Hydroxyl Functional Groups for PT and AT Coal.

	PT coal			AT coal
number	peak position (cm–1)	area ratio (%)	functional group	peak position (cm–1)	area ratio (%)	functional group
1	3173	5.89	ring hydrogen bond	3061	2.54	free hydroxyl
2	3266	12.76	alcoholic hydroxyl	3154	4.96	ring hydrogen bond
3	3361	20.91	O–H or N–H	3223	9.13	alcoholic hydroxyl
4	3439	27.42	OH-OH	3290	11.52	alcoholic hydroxyl
5	3514	20.97	OH-OH	3355	14.28	alcoholic hydroxyl
6	3580	7.54	O-OH	3413	16.29	OH-OH
7	3608	4.51	free hydroxyl	3473	16.40	OH-OH
8				3537	13.00	hydroxyl-π bond
9				3605	8.06	free hydroxyl
10				3665	3.82	free hydroxyl

Table 10. Fitting Results of Aromatic Structures for PT and AT Coal.

	PT coal			AT coal
number	peak position (cm–1)	area ratio (%)	functional groupa	peak position (cm–1)	area ratio (%)	functional groupa
1	742	9.35	Di	731	6.96	Di
2	758	11.92	Di	745	21.67	Di
3	774	12.20	Tr	761	17.15	Tr
4	790	16.91	Tr	776	20.86	Tr
5	805	12.16	Tr	796	8.40	Tr
6	859	5.48	Te	797	13.15	Tr
7	872	11.44	Pe	869	6.85	Te
8	887	11.64	Pe	895	4.96	Pe
9	899	8.90	Pe

^aNote: D_i is benzene ring disubstitution, T_r is trisubstituted benzene ring, T_e is tetra-substituted benzene ring, and P_e is pentasubstituted benzene ring.

3.3.1.1. Hydroxy Groups in PT and AT Coal

Hydroxy groups in coal are hydrophilic and acidophilic, and they can form hydrogen bonds and van der Waals’ force with CH₄ molecules. The hydroxy content is obviously affected by CH₄ adsorption and desorption.¹⁰ In Figure 13a and Table 7, hydroxy groups in PT coal are mainly dominated by hydrogen bonds of hydroxy-hydroxy and N-hydroxy groups, while in AT coal, they are dominated by hydrogen bonds of alcoholic hydroxyl groups and hydroxy-hydroxy groups followed by free hydroxyl groups. A new peak appears at 3537 cm^–1, which is attributed to a hydroxy-π bond; after acidization, cyclic hydrogen bonds decrease slightly from 5.89 to 4.96%, suggesting slight influences of acidization on cyclic hydrogen bonds. After acidization, the crest area in the range of 3000–3750 cm^–1 narrows as much as 41.06%, which indicates that acidization reactions cause overall reduction of the hydroxyl content.

3.3.1.2. Aliphatic Hydrocarbons in PT and AT Coal

According to Figure 13b and Table 8, aliphatic hydrocarbons in PT coal are mainly dominated by the asymmetric stretching vibration of CH₂, while those in AT coal are dominated by asymmetric CH₃ and asymmetric CH₂. The analysis of contents of aliphatic hydrocarbons in PT and AT coal unveils that symmetric CH₂ declines from 28.98 to 9.62% after acidization, implying the weakened symmetric stretching vibration of CH₂ after acidization, and asymmetric CH₂ decreases from 39.93 to 20.24% after acidization, which means that the asymmetric stretching vibration of CH₂ weakens after acidization; after acidization, asymmetric CH₃ grows from 14.32 to 19.6%, indicating the enhanced asymmetric stretching vibration of CH₃ after acidization. After acidization, a new peak appears at 2861 cm^–1, which is ascribed to the symmetric stretching vibration of CH₃. After being soaked and acidized, the stretching vibration of CH₂ in the tectonic coal turns to that of CH₃ and the aliphatic side chain length reduces.

Table 8. Fitting Results of the Aliphatic Hydrocarbon Structure for PT and AT Coal.

	PT coal			AT coal
number	peak position (cm–1)	area ratio (%)	functional group	peak position (cm–1)	area ratio (%)	functional group
1	2918	39.93	–CH2(as)	2817	3.35	–CH2(s)
2	2949	14.32	–CH3(as)	2839	6.27	–CH2(s)
3	2853	24.78	–CH2(s)	2861	12.63	–CH3(s)
4	2831	4.20	–CH2(s)	2891	11.39	–CH
5	2889	16.77	–CH(as)	2914	25.75	–CH(v)
6				2932	20.24	–CH2(as)
7				2955	19.60	–CH3(as)
8				3000	0.75

3.3.1.3. Oxygen-Containing Functional Groups in PT and AT Coal

Oxygen-containing functional groups in coal influence the hydrophily and lipophilicity of coal. Coal with high hydrophily strongly interacts with water, which decreases the adsorption capacity of CH₄ in coal. The fitting results in Figure 13c and Table 9 show that the oxygen-containing functional groups in PT coal are shown as the stretching vibration of C-O-C bonds, conjugated C=O vibration, and vibration of aryl ether C–O bonds, accounting for 57.44%. In AT coal, oxygen-containing functional groups are mainly dominated by C=C in aromatic hydrocarbons and conjugated C=O, accounting for 67.8%. Therein, it is the stretching vibration of C-O-C bonds that is mainly in the range of 1027–1033 cm^–1, which accounts for 22.63% before acidization that decreases to 7.89% after acidization. It is C–O bonds of ether that are in the range of 1081–1109 cm^–1, which account for 17.54% before acidization that reduces to 4.02% after acidization. The stretching vibration of phenolic and hydroxyl groups is mainly observed at 1151–1238 cm^–1, which accounts for 13.32% before acidization that declines to 5.66% after acidization. The C=C double-bond vibration of aromatic hydrocarbons is mainly observed in the range of 1550–1606 cm^–1, and its proportion is 14.79% before acidization, which rises to 37.45% after acidization. It is conjugated C=O that is mainly found at 1628–1677 cm^–1, which accounts for 24.7% before acidization and increases to 30.35% after acidization. In summary, the oxygen-containing functional groups in AT coal grow by 51.6%.

Table 9. Fitting Results of Oxygen-Containing Functional Groups for PT and AT Coal.

	PT coal			AT coal
number	peak position (cm–1)	area ratio (%)	functional group	peak position (cm–1)	area ratio (%)	functional group
1	1027	7.81	C-O-C(v)	1033	7.89	C-O-C(v)
2	1028	14.82	C-O-C(v)	1083	4.02	ether C–O
3	1109	10.11	ether C–O	1113	4.34	phenolic hydroxyl
4	1189	4.79	phenolic hydroxyl	1165	1.32	phenolic hydroxyl
5	1238	2.36	phenolic hydroxyl	1351	8.79	–CH2(s)
6	1576	5.1	aromatics C=C	1390	3.39	–CH3(s)
7	1606	9.69	aromatics C=C	1440	2.45	–CH2(a)
8	1630	14.99	conjugate C=O	1550	3.94	aromatics C=C
9	1661	9.71	conjugate C=O	1593	33.51	aromatics C=C
10	1725	1.0	carboxylic C=O	1628	23.40	conjugate C=O
11	1081	7.43	ether C–O	1677	6.95	conjugate C=O
12	1151	6.17	phenolic hydroxyl
13	1444	4.45	–CH2(as)
14	1387	1.49	–CH3(s)

3.3.1.4. Aromatic Structures in PT and AT Coal

The wave bands of aromatic structures in PT and AT coal were subjected to peak fitting, and the results are illustrated in Figure 13d and Table 10. The proportions of the areas of characteristic absorption peaks in 731–758, 761–805, and 859–869 cm^–1 enlarge from 21.27 to 28.63%, 41.27 to 59.56%, and 5.48 to 6.85%, while the proportion of the area of characteristic absorption peaks in 872–899 cm^–1 decreases from 31.98 to 4.96%. Except that the relative content of pentasubstituted benzene ring decreased, the other aromatic structures increased obviously. This means that aromatic structures increase obviously after acidization, indicating that acidization promotes increment of the content of aromatic structures.

3.3.2. Evolution of Structural Parameters of PT and AT Coal

To quantitatively analyze the evolution of chemical structures in PT and AT coal, parameters including the aromaticity f_a and the aliphatic side chain length R were adopted to perform semiquantitative analyses based on previous research. They can be calculated using the following equations:³⁸

5

6

7

where A_{700 – 900}, A_{2800 – 3000}, A_{2900 – 2940}, and A_{2940 – 3000} separately correspond to the areas of spectral absorption bands in the wave number ranges of 700–900, 2800–3000, 2900–2940, and 2940–3000 cm^–1, respectively. H/C can be attained based on the EA results. H_al/C_al is the hydrogen-to-carbon ratio in aliphatic structures and is valued to be 1.8.³⁹ In addition, the relative abundance I of H in aromatics and aliphatic hydrocarbons and the structural parameter ‘C’ of oxygen-containing functional groups can be calculated using eqs 8 and 9.^40,41 ‘C’ can also be used to represent the maturity of coal

8

9

where A_{1630 – 1730} and A₁₆₀₀ separately correspond to the integral areas of spectral absorption bands in the wave number ranges of 1630–1730 and 1600 cm^–1, respectively. Table 11 lists the structural parameters of PT and AT coal, which are calculated using eqs 5–9.

Table 11. Structural Parameters of PT and AT Coal.

samples	fa	R	I	‘C’
PT coal	0.876	2.789	1.349	0.7268
AT coal	0.952	1.314	5.539	0.7710

According to Table 11, the aromatization degree of AT coal rises. Meanwhile, the relative content of H in aromatics also increases while the aliphatic side chain length is shortened, which indicates that acidization exerts slight influences on aromatics in tectonic coal while it decreases the aliphatic side chain length. Moreover, C=O is transformed into C=C, which implies that the maturity of coal also changes relatively.

3.4. Changes in CH₄ Adsorption and Desorption in PT and AT Coal Samples

The experimental temperature of CH₄ adsorption and desorption experiments on AT and PT coal was 298.15 K. The CH₄ isothermal adsorption and desorption test results are illustrated in Figure 14.

Figure 14.

Methane adsorption and desorption curves in PT and AT coal.

As the pressure rises, the CH₄ adsorption rate tends to decline gradually. The adsorption and desorption curves of PT and AT coal are different, that is, there is a phenomenon of adsorption hysteresis. However, the situation is improved under high pressures (P > 4 MPa). Compared with PT coal, the adsorption and desorption curves of AT coal begin to be overlapped under low pressures (2 < P < 3 MPa) and they are overlapped again under 8 < P < 10 MPa.

It can be seen from Figure 14 that the adsorption and desorption curves of the tectonic coal follow the Langmuir equation. To explore the CH₄ adsorption and desorption properties of PT coal, eq 10 is adopted to fit the CH₄ adsorption and desorption data:⁴²

10

where V is the CH₄ adsorption capacity at adsorption equilibrium (cm³/g); a is the saturated adsorption capacity of CH₄ (mL/g); b is the reciprocal of pressure when reaching the half of saturated adsorption capacity (MPa^–1); and P is the pressure at CH₄ adsorption equilibrium (MPa).

It can be seen from Figure 15 that the Langmuir equation performs well in fitting CH₄ adsorption and desorption data before and after acidization (R² > 0.99), and other parameters are displayed in Table 12. However, the correlation coefficient for fitting desorption after acidization is lower than that for fitting adsorption, which is because of the residual adsorption capacity in coal samples.

Figure 15.

(a, b) Fitting methane adsorption and desorption curve based on the Langmuir equation.

Table 12. Fitting Parameters Based on Langmuir Equation.

	adsorption			desorption
samples	a	b	R2	a	b	R2
PT coal	21.8714	0.7878	0.9965	21.5310	0.8834	0.9967
AT coal	17.4645	0.7403	0.9970	17.3551	0.7880	0.9919

3.5. Influences of Pore Structures before and after Acidization on CH₄ Adsorption and Desorption

Figure 16 shows the CH₄ adsorption and desorption of tectonic coal before and after acidization at the adsorption temperature of 298.15 K under adsorption pressures of 0–10 MPa. As shown in Figure 16a, with the rising pressure, the CH₄ adsorption capacities of tectonic coal before and after acidization both grow steadily and the adsorption curves conform to the Langmuir equation. The CH₄ adsorption capacity reduces to some extent by 20.74% after acidization. As displayed in Figure 16b, the residual gas content after acidization is 8.0338 mL/g as the pressure drops, which is lower than the residual gas content 10.3509 mL/g before acidization, and the desorption rate grows from 45.6283 to 46.7361%.

Figure 16.

Comparison of methane adsorption and desorption curves in PT and AT coal. (a) Adsorption of methane and Langmuir fitting curve. (b) Desorption of methane and Langmuir fitting curve.

Causes (in terms of pore structures) for the reduction of CH₄ adsorption capacity in the AT coal sample in Figure 16 include the following: ① acidization renders the pore and fracture system more complex in tectonic coal and pore size distribution in coal also changes. The proportions of the volumes and specific surface areas of micropores and transition pores, as the main contributors to the specific surface area and pore volume in coal, both decrease after acidization, while those of macropores increase. As a result, the CH₄ adsorption space in coal shrinks and the adsorption performance reduces. ② The numbers of inkbottle-shaped pores in AT coal decrease apparently. Instead, certain amounts of slit pores and open pores appear, which increase the porosity of tectonic coal samples.

As for the chemical structures of coal, ① acidization causes transfer of −CH₂ and −CH₃ side chains in coal, shortened aliphatic side chain length, higher branching degree, and weakened CH₄ adsorption capacity of coal. ② Carboxyl and hydroxy groups, as methanophilic molecular groups, can improve the CH₄ adsorption capacity of coal. The hydroxyl content in AT coal decreases on the whole, so the interaction force between coal and CH₄ molecules is weakened, which promotes CH₄ desorption. ③ The oxygen-containing functional groups increase by 51.6% after acidization so that the hydrophily of coal is enhanced and CH₄ adsorption sites decrease, which are unfavorable for CH₄ adsorption.

3.5.1. Influences of Pore and Fracture Structures on CH₄ Adsorption

To further study the influences of pore and fracture structures before and after acidization on CH₄ adsorption, the theory of adsorption potential was introduced. The adsorption potential refers to the work done by transformation of per unit mass of CH₄ from the free state to the adsorbed state, and it is expressed as follows:^43,44

11

where ε is the adsorption potential of CH₄ (J/mol); P_i is the ideal pressure at gas adsorption equilibrium (MPa); R is a gas constant and generally valued to be 8.3144 J/(mol·K); T is the absolute temperature (K); P is the gas adsorption pressure; P₀ is the saturated vapor pressure of gas (MPa). Therein, P₀ can be calculated using the Amankwah empirical formula (eq 12)⁴⁵

12

where P_c is the critical pressure of CH₄ and generally valued to be 4.59 MPa; T_c is the critical temperature of CH₄ and commonly valued to be 190.55 K; k is the correlation coefficient of CH₄ adsorption and generally valued to be 2.7. Meanwhile, the adsorption space theory was introduced to characterize the adsorption performance of PT and AT coal, as expressed by eqs 13 and 14:^43,46

13

14

where ω is the CH₄ adsorption space (volume) (cm³/g); M is the molar mass of CH₄ (g/mol); V_ad is the actual adsorption capacity (mol/g); ρ_ad is the relative adsorption density of CH₄ (g/cm³).

According to the CH₄ adsorption and desorption data and eqs 11–14, the adsorption potentials and adsorption spaces of CH₄ in PT and AT coal are calculated. The results show that under the condition that the adsorption potential is 1.066 KJ/mol and the relative adsorption density is 0.3708 g/cm³, the CH₄ adsorption capacity of AT coal declines by 20.74% from 19.0373 to 15.083 mL/g, and the CH₄ adsorption volume decreases by 20.71% from 0.0367 to 0.0291 cm³/g. This indicates that acidization exerts significant influence on the pore and fracture structures and CH₄ adsorption and desorption of tectonic coal.

3.5.2. Influences of Pore and Fracture Structures on CH₄ Desorption

CH₄ desorption from coal is an inverse process of CH₄ adsorption. To further study the influences of changes in pore and fracture structures before and after acidization on CH₄ desorption, the Knudsen number that can represent the pore diameter and the average free path of gas molecular motion was introduced to characterize the CH₄ desorption capacity of PT and AT tectonic coal. They are calculated using eqs 15 and 16:⁴⁷

15

16

where Kn is the Knudsen number; d is the average pore diameter in tectonic coal (nm); λ is the average free path of gas in pores (nm); K is the Boltzmann constant and generally valued to be 1.38 × 10^–23 (J/K); T is the absolute temperature (K); P is the gas pressure (MPa); dσ is the effective diameter of molecules (nm).

Based on data in LTN₂-GA experiments, the average pore diameters of PT and AT coal are separately 3.414 and 21.5472 nm, respectively. The Kn values of PT and AT coal are calculated using eqs 15 and 16, thus obtaining that Kn increases apparently from 0.06429 to 0.4058 after acidization. This indicates that CH₄ molecules before acidization are dominated by Knudsen diffusion, while those after acidization mainly experience transitional diffusion, which is conducive to desorption of CH₄ molecules.

3.6. Parameters of Pore and Fracture Structures for Effective CH₄ Desorption

Most CH₄ molecules in coal are present in the adsorbed state in pores. CH₄ molecules in micropores, as the diffusion source, constantly transfer toward diffusion pores, in which CH₄ molecules seep in fractures. To study microscopic pore and fracture conditions for effective CH₄ desorption from tectonic coal reservoirs, two parameters for pore structures including the average pore–throat ratio and the average sinuosity were introduced to evaluate the CH₄ desorption capacity of the coal reservoir. The average pore–throat ratio generally refers to the ratio of pore diameter to throat diameter and can also indicate the ratio of pore volume to throat volume. Their calculation formulas are expressed as eqs 17 and 18:^48,49

17

18

where P_T is the average pore–throat ratio; r₁ is the average radius of transition pores (nm); r₂ is the average radius of mesopores (nm); V₁ is the volume of transition pores (cc/g); V₂ is the volume of mesopores (cc/g); S₁ is the specific surface area of transition pores (m²/g); S₂ is the specific surface area of mesopores (m²/g).

The average sinuosity of coal is a ratio of the actual path to the linear characteristic path for gas migration in coal.⁵⁰ If the average sinuosity is small, it means that the gas migration path is short, gas can be rapidly desorbed, and gas can easily migrate and vice versa.⁵¹ According to eq 19(48)

19

where τ_av is the average sinuosity; m is the mass of coal particles and valued to be 0.000035 g; S_BJH is the total specific surface area (m²/g); V_BJH is the total pore volume (cc/g); l₀ is the linear characteristic path and valued to be the average particle size (nm).

The average pore–throat ratio was calculated based on data of the pore volume and specific surface area obtained in LTN₂-GA experiments, and the average sinuosity was calculated according to data of the total pore volume and total specific surface area. The calculation results are displayed in Table 13.

Table 13. Average Pore–Throat Ratio and Average Tortuosity.

	average pore radius (nm)
samples	micropores	mesopores	PT	τav (×106)
PT coal	2.1161	11.1303	9.62	7.8338
AT coal	2.8324	18.2681	6.51	6.2775

It can be seen from Table 13 that acidization enlarges the average radii of micropores and mesopores in tectonic coal, while the average pore–throat ratio decreases obviously. Compared with PT coal, the average pore–throat ratio of AT coal reduces by 32.33%, which implies that the difference in the pore and throat diameters in tectonic coal narrows after acidization, which decreases the resistance to gas migration. The average sinuosity declines by 19.87% in AT coal, suggesting that minerals that block gas migration channels are dissolved after acidization so that pores in the tectonic coal become straight, which shortens the gas migration path and is conducive to gas desorption and diffusion. This agrees with the increased CH₄ desorption rate after acidization.

In summary, the average pore–throat ratio and average sinuosity of AT coal both reduce. This indicates that acidization decreases the difference between the throat and pore diameters, shortens the migration path for gas desorption and diffusion, facilitates CH₄ desorption from tectonic coal, and improves the CH₄ desorption rate.

4. Conclusions

• (1)Corrosion experiments with monomeric acid solutions reveal that as the concentration and reaction duration of acid solutions increase, the acidization reactions can be roughly divided into three stages: the rapid reaction stage, steady corrosion stage, and stage with a decreased corrosion rate. Monomeric acid solutions are listed in a descending order as HF, HCl, and CH₃COOH according to the corrosion rates of coal. Corrosion experiments with polymeric acid solutions show that CH₃COOH solution plays a certain role in retarding the acidization reactions and the optimal mixing ratio of mixed acid solutions for tectonic coal collected from Faer Coal Mine is 6% HCl + 6% HF + 3% CH₃COOH + 2% KCl.
• (2)The activation degree of AT coal rises, which is shown as the increased overall porosity, specific surface area, and average pore diameter. Because acid solutions dissolve minerals in structures of coal, pore and fracture structural characteristics change. The volumes of transition pores and mesopores in coal are affected obviously by acidization so that some transition pores are transformed into mesopores and macropores and form dissolved pores, and inkbottle-shaped pores reduce, while slit pores or open pores increase.
• (3)Changes in functional groups in AT coal affect CH₄ adsorption and desorption. In AT coal, the −CH₂ and −CH₃ side chains are transferred, aliphatic side chain length shortens, and the branching degree rises. Moreover, the aromatization degree of AT coal rises, and oxygen-containing functional groups increase, while hydroxyl content declines generally, which reduces the CH₄ adsorption capacity of coal and promotes CH₄ desorption.
• (4)The adsorption volume of AT coal shrinks and CH₄ adsorption sites decrease. The Kn number was introduced to study CH₄ desorption performance of PT and AT coal, which reveals that CH₄ molecules after acidization change from being dominated by Knudsen diffusion to be dominated by transitional diffusion. The average pore–throat ratio and average sinuosity of AT coal both decline, the CH₄ diffusion path shortens, and the diffusion rate rises, which promotes CH₄ desorption.

The authors declare no competing financial interest.