Coal Structure Research Articles

Micropores manipulation of medium-rank coal through aggregate structure evolution during coalification: A study based on HRTEM analysis

Micropores in coal, which can be manipulated by aggregate structure, have been widely considered to be a key factor affecting the gas adsorption of coal. In this study, to explore the aggregate structure evolution and its influence on micropores during coalification, a well characterized sample set of medium-rank coals from Shanxi province, covering a vitrinite reflectance (Ro) interval from 0.68% to 1.98%, was selected. High resolution transmission electron microscopy (HRTEM) and low-pressure CO2 adsorption experiments (LPCO2) were employed to quantify the evolution of aggregate structure and micropores in coal during coalification. The results show that the two methods give similar results for the micropore distribution. With the increase of coalification, four obvious stages are found for fringe length and distribution of micropores, with turning points appear near Ro = 1.20% and 1.50%, respectively, indicating that the coalification process are combination of continuity and jumping and related to the coalification jumps. The dominant fringes are short fringes (0.3 ∼ 0.54 nm) and intermediate fringes (0.55 ∼ 1.14 nm) in all coals, which contribute most to the curvature of c = 1.0 ∼ 1.04 and c greater than 1.04, separately, suggesting the existence of non-six-membered rings. The curvature of c = 1.0 ∼ 1.04 decreases while the curvature of c = 1.04 ∼ 1.15 increases with increasing Ro, indicating that the fringes mainly tend to bend in medium-rank coals. For micropores, most pores in coal were found to be in the range of 0.5 ∼ 0.8 nm based on the result of HRTEM images, which mostly appeared in the disorder regions featured with intermediate fringes. However, the pores of 0.8 ∼ 1.2 nm and 0.4 ∼ 0.5 nm are always found in the regions without the basic structural units (BSUs) in lower-rank coals and the edge regions stacked with long fringes in the higher-rank coals, respectively. This work provides a strategy to manipulate micropores of medium-rank coal through aggregate structure evolution during coalification, which is expected to provide theoretical basis for further research on gas adsorption of coal.

Simulation Study on Molecular Adsorption of Coal in Chicheng Coal Mine.

To study the importance of the adsorption mechanism of methane (CH4) and carbon dioxide (CO2) in coal for coalbed methane development, we aimed to reveal the influence mechanism of adsorption pressure, temperature, gas properties, water content, and other factors on gas molecular adsorption behavior from the molecular level. In this study, we selected the nonsticky coal in Chicheng Coal Mine as the research object. Based on the coal macromolecular model, we used the molecular dynamics (MD) and Monte Carlo (GCMC) methods to simulate and analyze the conditions of different pressure, temperature, and water content. The change rule and microscopic mechanism of the adsorption amount, equal adsorption heat, and interaction energy of CO2 and CH4 gas molecules in the coal macromolecular structure model establish a theoretical foundation for revealing the adsorption characteristics of coalbed methane in coal and provide technical support for further improving coalbed methane extraction.

Experimental Study on Catalytic Action of Intrinsic Metals in Coal Spontaneous Combustion.

In order to study the effect of inherent metals in coal on spontaneous combustion, Hongmiao lignite and Hongqingliang long-flame coal were demineralized by hydrochloric acid, the raw coal and demineralized coal were characterized by Fourier transform infrared spectrometry, X-ray diffraction, and synchronous thermal analysis experiments, and the corresponding ash content was detected by inductively coupled plasma mass spectrometry. The results show that the effect of demineralization on the volatile matter of low-rank coal is small, and the change of crystallite structure is not significant. The removed parts are mainly water-soluble salts and soluble minerals, such as carbonates and metal ions, that are not tightly bound to the organic matter of coal structure. The removed metal elements are mainly alkali metals Na and K, alkaline earth metals Ca, Mg, Sr, and Ba, and transition metals Fe, Mn, Ti, and so forth. The temperatures corresponding to the end of weight loss, ignition, and maximum weight loss rates were elevated on the thermogravimetric curves of the demineralized coal samples. The heat absorbed by evaporation of water in coal and the heat released by oxidation and combustion of coal are decreased to different degrees, indicating that the spontaneous combustion tendency of coal after demineralization is reduced, and alkali metal, alkaline earth metals, and transition metals in coal have a catalytic effect on spontaneous combustion of coal. After adding the metal chelating agent ethylenediaminetetraacetic acid (EDTA), the apparent activation energy decreased by 33.08 and 2.42%, respectively. EDTA and the alkali metal, alkaline earth metal, or transition-metal ions formed a stable chelate in coal. The catalytic activity of metals is weakened or even lost, thereby inhibiting spontaneous combustion of coal, and verifying the catalytic effect of internal metals in coal on the spontaneous combustion of coal.

Hydromechanical Impacts of CO2 Storage in Coal Seams of the Upper Silesian Coal Basin (Poland)

Deep un-mineable coal deposits are viable reservoirs for permanent and safe storage of carbon dioxide (CO2) due to their ability to adsorb large amounts of CO2 in the microporous coal structure. A reduced amount of CO2 released into the atmosphere contributes in turn to the mitigation of climate change. However, there are a number of geomechanical risks associated with the commercial-scale storage of CO2, such as potential fault or fracture reactivation, microseismic events, cap rock integrity or ground surface uplift. The present study assesses potential site-specific hydromechanical impacts for a coal deposit of the Upper Silesian Coal Basin by means of numerical simulations. For that purpose, a near-field model is developed to simulate the injection and migration of CO2, as well as the coal-CO2 interactions in the vicinity of horizontal wells along with the corresponding changes in permeability and stresses. The resulting effective stress changes are then integrated as boundary condition into a far-field numerical model to study the geomechanical response at site-scale. An extensive scenario analysis is carried out, consisting of 52 simulation runs, whereby the impacts of injection pressures, well arrangement within two target coal seams as well as the effect of different geological uncertainties (e.g., regional stress regime and rock properties) is examined for operational and post-operational scenarios. The injection-induced vertical displacements amount in maximum to 3.59 cm and 1.07 cm directly above the coal seam and at the ground surface, respectively. The results further demonstrate that neither fault slip nor dilation, as a potential consequence of slip, are to be expected during the investigated scenarios. Nevertheless, even if fault integrity is not compromised, dilation tendencies indicate that faults may be hydraulically conductive and could represent local pathways for upward fluid migration. Therefore, the site-specific stress regime has to be determined as accurately as possible by in-situ stress measurements, and also fault properties need to be accounted for an extensive risk assessment. The present study obtained a quantitative understanding of the geomechanical processes taking place at the operational and post-operational states, supporting the assessment and mitigation of environmental risks associated with CO2 storage in coal seams.

Effect of water immersion on pore structure of bituminous coal with different metamorphic degrees

It is important to understand the disaster-causing mechanism of dried water-immersed coal by investigating the influence of water immersion on the coal pore structure. In this study, low-temperature nitrogen adsorption experiments of eight bituminous coals with different metamorphic degrees and their dried water-immersed coals were carried out to analyze the adsorption/desorption mechanism and investigate the effects of water immersion on the pore shape, pore-structure parameters, and fractal dimension of coal. The results showed that adsorption of nitrogen on coal mainly includes micropore filling, monolayer adsorption, multilayer adsorption, and capillary condensation. The pores of coal were a continuous system from nanometer pores to relatively unlimited size pores. Water immersion did not significantly affect the pore shape of the coal, decreased the specific surface area and adsorption capacity, and increased the total pore volume and average pore diameter. It had the greatest effect on the average pore size of moderately metamorphosed coals, which may accelerate the oxidation process of coal and aggravate gas emission. The fractal results showed that water immersion reduced the surface fractal dimension of higher and lower metamorphic coal, while it increased that of moderately metamorphic coal, and it had the opposite effect on the structural fractal dimension.

Chemical structure and hydrocarbon generation characteristics of tectonic coal with different metamorphic degrees: Implications for gas adsorption capacity

Chemical structure and hydrocarbon generation characteristics of tectonic coal with different metamorphic degrees: Implications for gas adsorption capacity

Editorial: Advances in geochemistry and macromolecular structure of coal

EDITORIAL article Front. Earth Sci., 21 March 2023Sec. Geochemistry Volume 11 - 2023 | https://doi.org/10.3389/feart.2023.1177271

Quantitative characterization of pore-fracture structure of medium and high-rank coal based on micro-CT technology

ABSTRACT Coal is a natural and complex porous medium, which contains pore-fracture structures of different scales. The pore-fracture structures of coal directly affect the characteristics of gas storage and migration. In this paper, high-resolution X-ray CT scanning technology is used to quantitatively characterize the pore-fracture structure of medium and high-rank coals. First, the porosity inversion method is used to determine the optimal threshold segmentation of pores and fracture in medium and high-rank coals. Then, the pore and fracture model of representative element volume (REV) was extracted and established to characterize the area porosity, pore surface area, and shape factor of the two coals, and the relationship between pore surface area and shape factor was investigated. Finally, an equivalent pore network model (PNM) of connected pore topology was established, and pore-throat parameters are statistically analyzed, including pore volume, pore radius, throat radius, throat length, and coordination number. The results show that the average pore diameter and average surface area of macropores in medium-rank coals are larger than those of high-rank coals, and high-rank coals have fewer isolated pores. With the increase of coal rank, the length and radius of the throat gradually decrease, which indicates that the throat of the medium-rank coal has a higher degree of development, and the gas seepage ability in the throat is stronger. The outcome of this study is of great significance for comprehensively understanding the pore-fracture structure of coal in micro-scale.

Investigation on selective separation of oxygen-containing compounds in lignite and sub-bitumite

To realize the high value-added utilization of low-rank coal and elucidate the selectivity of solvent structure and oxygen-containing compounds in low-rank coal, Zhaotong (ZT) lignite and Naomaohu (NMH) sub-bitumite were subjected to thermal dissolution in three types of oxygen-containing solvents with different properties, namely ethanol (ET), tetrahydrofuran (THF) and ethyl acetate (EA). The soluble portions (SPs) obtained were separated by column chromatography into three types of fractions: hydrocarbon (HY), non-hydrocarbon (NHY) and asphaltene (ASP). The results indicated that the yields of SPs in all three solvents for each coal was ET > THF > EA, and the ratios of column chromatographic fractions of SPs of the two coals were significantly different, with lower asphaltene content in the SPs of NMH. The thermal dissolution process of the three solvents significantly reduced the oxygen content of low-rank coal, and the deoxygenation capacity was in the order of ET > THF > EA. More medium or low polarity compounds could be obtained from NMH than from ZT through thermal dissolution process. ET showed excellent selectivity for the generation of both phenols and esters, where the phenols were mainly alkylphenols. THF could release highly condensed polycyclic compounds from coal thus more naphthalenols containing substituent groups were detected in phenols. And it was more selective to COO bonds in NMH than ZT from the results of XPS. More esters were detected in the SPs of NMH compared with ZT. EA had a strong selectivity for CO in low-rank coals, and the thermal dissolution process released a large number of esters, which could be divided into alkanoates and aromatic acid esters. This study can provide a theoretical basis for extracting high value-added chemicals from low-rank coal under mild conditions.

Mechanism of supercritical CO2 on the chemical structure and composition of high-rank coals with different damage degrees

The interaction between carbon dioxide (CO2) and coal plays a vital role in CO2 geological storage enhanced coalbed methane recovery (CO2-ECBM). To investigate the mechanism of supercritical carbon dioxide (SC-CO2) extraction in technically deformed coals under deep coal seam conditions, the chemical composition and structure of technically deformed coals before and after SC-CO2 treatment were analyzed by Fourier transform infrared spectroscopy (FTIR), mass spectrometry (GC–MS) and X-ray diffraction (XRD). The results show that, after SC-CO2 extraction, the absorption peak intensities of weakly polar and non-polar compounds in coal are weakened. With the increase in tectonic deformation degree, the dissolution effect of aromatic compounds increases, while aliphatic compounds show a complementary evolution trend. The dissolution effect of SC-CO2 on carbonate minerals is the most significant, with the highest dissolution amount of 100 %, followed by clay minerals, with the dissolution amount between 1.3 % and 30.5 %, while oxide minerals are relatively stable and hardly react. With the increasing degree of tectonic deformation, the solubility of various minerals tends to increase. In summary, SC-CO2 has a dissolution effect on the organic functional groups and minerals of technically deformed coal, and the effect tends to be significant with the increase of coal tectonic deformation degrees. In view of the important role of mineral and organic composition on the porosity and adsorption of coal, when CO2 is injected into deep coal seams, the response characteristics of porosity, permeability and adsorption caused by the change in mineral and organic composition should also be considered.

Experimental study on the influence of acid fracturing fluid on coal wettability

With the continuous improvement of mechanized mining intensity, dust has seriously threatened the safety of mine production and the health of underground workers. Coal seam water injection is one of the effective means to moisten the primary coal dust in the coal body. Through testing by the surface tension tester, contact angle meter, FTIR, low-temperature liquid nitrogen adsorption, and SEM, the impact of acid fracturing fluid which can be used for coal seam water injection on the wettability and pore structure of coal was studyed. The results showed that HCl and CTAB had good synergistic acidification effect, which can effectively reduce the surface tension of CTAB by 10 ∼ 30 mN/m. Also the contact angle between the solution and the coal sample is significantly affected by the acidity of the solution. However, the impact of the acid fracturing fluid varies for coal with differing levels of metamorphism and mineral content. For low rank bituminous coal, the contact angle decreased by 39.55° after contact with the sample surface for 1 s, and the solution was completely immersed in the coal body after 5 s. The absorbance of hydrophilic functional groups of coal becomes stronger after treatment with acid fracturing fluid, which is conducive to upgrading the wettability of coal. Meanwhile, the acid fracturing fluid increases coal’s connectivity and average pore diameter, which in turn influences the wettability of coal. Exploring the influence of acid-based fracturing fluid on coal can provide guidance for improving the wettability of coal, reducing dust concentration and optimizing the underground working environment.

Experimental Study on the Properties of Gas Diffusion in Various Rank Coals under Positive Pressure.

A kind of closed mining and nonventilation working face is proposed, which provides a possibility for eliminating coal mine accidents and extracting high-purity gas. During mining, a confined space above normal pressure is formed in the face. Geological conditions cause significant differences in the physicochemical properties of coal and affect the occurrence and migration of gas in coal seams. The pore structures of five coal samples were obtained by mercury injection and low-temperature nitrogen adsorption. The self-made positive pressure desorption experimental device was used to conduct isothermal desorption experiments under different environmental pressures. An extended Langmuir model was proposed to carry out regression analysis on the curve of positive pressure desorption with time. The effect of coal pore structure on gas diffusion properties was discussed. The results indicate that the development degree of micropores in coal determines the amount of gas adsorption. With the increase of coal rank, both the ultimate desorption quantity and desorption rate first decrease and then increase. The positive pressure enhances the concentration of methane outside the coal and inhibits methane diffusion. With an increase of positive pressure, the desorption capacity of the high-rank ZG was significantly inhibited, and the desorption limit and initial desorption rate decreased by 15.80-44.54 and 16.92-47.93%, respectively. The diffusion coefficient of the middle-rank XS has the greatest decrease rate of 1.56-18.05%. The pore structure of coal is the essential reason that affects methane diffusion.

Study on the Variation Laws and Fractal Characteristics of Acoustic Emission during Coal Spontaneous Combustion

Acoustic emission (AE) technology has the advantage of online localization to study the change laws of AE in the process of coal spontaneous combustion and to reveal the generation mechanisms of AE signal during the process of heating and rupture of coal body from a microscopic perspective. This paper first constructs a large-scale coal spontaneous combustion AE test system and conducts experimental tests on the AE signal in the process of coal spontaneous combustion. The results show that with the increase of temperature in the process of coal spontaneous combustion, the AE signal shows a trend of increasing fluctuations. Low-temperature nitrogen adsorption experiments studied the pore structure of coal spontaneous combustion, and the results showed a correspondence between the development of pores and the temperature of coal spontaneous combustion. Further, through the analysis of the evolution of the pore structure of coal through Fourier transform and fractal theory, it is found that the high-frequency main frequency AE signal and average frequency are continuously enhanced with the increase of temperature. The fractal dimension of the pore structure and the fractal dimension of the AE count of the coal body first rise and then decline. The mechanism of coal spontaneous combustion AE of coal is revealed, and the pore development caused by thermal stress when coal heats up is the main source of AE signal generation. The research in this paper is of great significance for applying AE technology to detect the position of coal spontaneous combustion.

Experimental study on extraction of organic matter from coal by liquid CO2

Liquid CO2 injection into coal seam for CH4 displacement is one of the important means to strengthen gas extraction and carbon sequestration in recent years. Liquid CO2 is a non-polar solvent, based on the principle of “similar phase solubility”, part of non-polar or weakly polar organic matter can be dissolved in liquid CO2. By controlling the key parameters of liquid CO2 in the extraction process, small molecular organic matter can be dissolved and migrated from the pore structure of coal as far as possible. In order to clarify the main factors affecting the process of liquid CO2 extraction of coal body, the experimental platform of liquid CO2 extraction of organic matter in coal body was built by ourselves, the single factor experimental design method was used to analyze the evolution law of organic matter components in the process of liquid CO2 extraction of coal body. The experimental results showed that: the macromolecular structure of coal was not significantly damaged by liquid CO2 extraction; after liquid CO2 extraction, the relative contents of aliphatic hydrocarbon and aromatic hydrocarbon are positively correlated with pressure, the dissolution of ketones, alcohols, ethers and esters has the characteristics of stage, time sequence and explosion; at the initial stage of extraction, the dissolved compounds are free in the structure of the macromolecular network. With the increase of extraction pressure and time, the small molecular compounds of hydrocarbons and non-hydrocarbons in the embedded state are dissolved.

Influence of coalification on pore structure evolution in middle-ranked coals

The influence of coalification on coal structure evolution in middle ranked coals is significant for physical assessment of coalbed methane (CBM) reservoirs, which provides insights on the intrinsic connection between coalification jump and pore heterogeneity. A total of 26 middle-ranked coals were samples covering Liupanshui Coalfield in Guizhou Province, Anhe Coalfield in Henan Province, Huaibei Coalfield in Anhui Province, Sanjiang Basin in Heilongjiang Province, Ordos Basin in Shaanxi Province and Qinshui Basin in Shanxi Province. Based on a series of experiments including vitrinite reflectance, coal maceral identification, nitrogen adsorption and the pore fractal method, the inner link between physical property parameters of coal reservoirs and coal rank was revealed. The results show that the coal maceral in middle rank coals is dominated by vitrinite and inertinite and two types of adsorption pores are divided according to the nitrogen adsorption/desorption curves along with pore size distribution. The specific surface area is positively correlated with total pore volume, micropore volume and negatively correlated with averaged pore size and transitional pore volume. The coal samples with low average pore sizes have relatively high total pore volume, specific surface area and micropore volume per unit nm. With the increase of coal rank, the fluctuating points of micropore and transitional pore volume correspond to 1.16%–1.19%, 1.41%–1.43% and 1.86%–1.91% of Ro, max, respectively. The boundary of Ro, maxcorresponding to the second coalification jump can be more specifically defined as 1.16%–1.19% from the established nominal range of 1.1%–1.3%. The pore fractal dimension DNA1and DNA2increase with increasing specific surface area. Furthermore, the DNA2has a negative correlation with micropore volume and averaged pore size, indicating that the coal with smaller average pore diameter and lower micropore content has a more complex pore structure.

Investigation on the Effect of Calcium on the Properties of Geopolymer Prepared from Uncalcined Coal Gangue

In this paper, the influence of calcium on coal gangue and fly ash geopolymer is explored, and the problem of low utilization of unburned coal gangue is analyzed and solved. The experiment took uncalcined coal gangue and fly ash as raw materials, and a regression model was developed with the response surface methodology. The independent variables were the CG content, alkali activator concentration, and Ca(OH)2 to NaOH ratio (CH/SH). The response target value was the coal gangue and fly-ash geopolymer compressive strength. The compressive strength tests and the regression model obtained by the response surface methodology showed that the coal gangue and fly ash geopolymer prepared with the content of uncalcined coal gangue is 30%, alkali activator content of 15%, and the value of CH/SH is 1.727 had a dense structure and better performance. The microscopic results demonstrated that the uncalcined coal gangue structure is destroyed under an alkali activator's action, and a dense microstructure is formed based on C(N)-A-S-H and C-S-H gel, which provides a reasonable basis for the preparation of geopolymers from the uncalcined coal gangue.

Method for determining the ultimate sorption capacity of coal matter by EPR-spectroscopy

Purpose. Improving the method for determining the ultimate sorption capacity of coal matter using EPR-spectroscopy (electron paramagnetic resonance) by adjusting the proportionality coefficient between the ultimate sorption capacity of coal and the concentration of paramagnetic centers and the conjugation coefficient in accordance with the degree of coalification. Methodology. The ultimate sorption capacity of the matter was estimated by EPR-spectroscopy, based on the content of paramagnetic centers (PMC) in coal, which are able to come into physical (sorption) interaction with molecules of paramagnetic gas (O2) when the pressure increases. Processing of the research results was carried out by methods of mathematical statistics. Findings. Analysis of long-term results for determining the ultimate sorption capacity of coal matter by EPR-spectroscopy was carried out. The analysis testified about the need to adjust the proportionality coefficient  between the ultimate sorption capacity of coal and the concentration of paramagnetic centers Na and the conjugation coefficient Ksc, depending on the coal rank metamorphism. The values of the proportionality coefficient  by hard coal ranks for the yield of volatile components Vdaf and the reflectivity of vitrinite R° were calculated. Appropriate changes were made to the express-method for estimating the ultimate sorption capacity of coal by the EPR method. Originality. It is proved that the proportionality coefficient β between the ultimate sorption capacity of coal and the concentration of paramagnetic centers Na and the conjugation coefficient Ksc is not a constant value, but changes (decreases) with the degree of metamorphism. It is established that this relationship is satisfactorily characterized by the sigmoid model, whose inflection (on the graph) is confined to the gas and fat ranks of coals (volatile-matter yield is 29 %) and is caused by the second main jump of coalification during a cardinal change in the molecular structure of coal, associated with the completion of the intensive decomposition of the polymer-lipoidin component in the coal matter. Practical values. The express-method was improved for estimating the ultimate sorption capacity of coal by the EPR-method, which differs by specified proportionality coefficients according to ranks in the series of coalification.

Effect of acidification on microscopic properties and pore structure of coal

Acidification is a common method for removing minerals from coal. In this study, four groups of acidification samples were evaluated: raw coal (Raw), hydrofluoric acid (HF), hydrochloric acid (HCl), and mixed HF–HCl acid (Mix) groups. The concentrations of ash and various elements in the sample groups were measured by industrial and elemental analyses. The Mix solution most effectively removed the minerals from coal, with an ash removal fraction of 87.87%. The microscopic characteristics of the four groups were studied using Fourier-transform infrared (FT-IR) and carbon nuclear magnetic resonance spectroscopy (13C NMR). The oxygen-containing functional groups of the coal samples increased after acidification by 76.07%, 14.49%, and 51.06% for the HF, HCl, and Mix groups, respectively. In addition, the content of aliphatic structures decreased during acidification. The adiposity of the Mix, HCl, and HF groups decreased by 16.36%, 8.50%, and 12.73 %, respectively. The results of CO2 and liquid nitrogen adsorption showed that the cumulative pore volume of the HF and Mix groups increased by 24.90% and 21.39%, respectively, while that of the HCl group decreased by 3.23% compared to the Raw group. The changes in the coal microstructures were observed for both micropores and mesopores. The aliphatic content was inversely proportional to the pore volume, whereas some oxygen-containing functional groups enhanced the pore volume. These findings contribute to the body of knowledge related to coal-seam acidification.

Revealing reactive mechanism and nitrogen transformation of HSW coal combustions at molecule and particle scales

Understanding reactive mechanism of coal thermochemical conversion was the scientific fundamental for coal high-efficient utilization. However, the evolution of coal macromolecular structure, reactants and products remained unclear, in particularly at particle and molecular scales. The complex reaction networks for transformation of pollution elements were also not established. This work uncovered reactive mechanism and nitrogen transformation of HSW coal combustion at microscopic scales via reactive force field molecular dynamics method. The effects of chemical equivalent and combustion temperature were investigated to explore coal structural evolution, combustion reactants and products during reactive process. It was found that the fracture change of coal structure was observable with the continuous reaction. Based on the analysis of nitrogen-containing molecules in coal combustion, it was confirmed that HCN was an important nitrogen-containing intermediate. Further, the transformation networks of organic nitrogen in combustions were established, and the pathway for formation of HCN, NO and NO2 was obtained.

The Characteristic Development of Micropores in Deep Coal and Its Relationship with Adsorption Capacity on the Eastern Margin of the Ordos Basin, China

The accurate description of micro-/nanopores in deep coal reservoirs plays an important role in evaluating the reservoir properties and gas production capacity of coalbed methane (CBM). This work studies nine continuous samples of high–rank coal from the Daning–Jixian area of the Ordos Basin. Maceral analysis, proximate analysis, field emission scanning electron microscopy (FE-SEM), low-pressure CO2 adsorption (LPA), low-temperature N2 adsorption (LTA) and high-pressure methane adsorption (HPMA) experiments were conducted for each sample. The fractal dimension (D) of the LPA data was calculated by using the micropore fractal model. The characteristics of the deep coal reservoir pore structure, proximate analysis, relationship between maceral and fractal dimensions, and gas adsorption capacity of the micropores are discussed. The results showed that the combination of LPA with nonlocalized density functional theory (NLDFT) models and LTA with NLDFT models can more accurately determine the pore size distribution of the micropores. The pore volume (PV) and specific surface area (SSA) of the coals were distributed in the ranges of 0.059~0.086 cm3/g and 204.38~282.42 m2/g, respectively. Although the degree of micropore development varies greatly among different coal samples, the pore distribution characteristics are basically the same, and the PV and SSA are the most developed in the pore size range of 0.4–0.7 nm. Ash content (Ad) and mineral composition are two major factors affecting micropore structure, but they have different impacts on the fractal dimension. The higher the vitrinite content, moisture content (Mad) and Ad are, the larger the micropore fractal dimension (D) and the stronger the heterogeneity of the pore structure. Micropores account for 99% of the total SSA in coal, and most methane can be adsorbed in micropores. The fractal dimension of micropores can be used to evaluate the pore structure characteristics. The larger the fractal dimension, the smaller the micro-SSA and micro-PV of the coal sample. Fractal analysis is helpful to better understand the pore structure and adsorption capacity of CBM reservoirs.