Nanopores In Coal Research Articles

Response of Coal Nanopores and Their Complexity to Igneous Intrusion

Response of Coal Nanopores and Their Complexity to Igneous Intrusion

Fractal character of coal nanopore and effect of deviation corrected, coal rank, and gas adsorption

Pore structure in coal is fundamental to determining the flowability properties and mechanical functions. Various insights into pore structure, including pore size distribution, specific area, and pore volume, have been obtained in experiments with many methods. Systematic research is lacking on how to characterize pore structure with more detailed information using fractal theory. Here, using the small-angle X-ray scattering (SAXS) method, we studied the fractal characters of four coal samples and showed how the deviation corrected, coal rank, and gas adsorption affected the fractal dimension of the pore structure. The average pore diameters of four samples obtained by the gyration radius using Guinier curves were 50.25–52.95 nm. The surface fractal of coal samples was the dominant feature, and the fractal features altered with the q value. Scattering information of simple bodies, such as pores and fractures, could superimpose the non-integer nature of the fractal dimension. The Porod and Debye laws guided the deviation correction process in the high q region. Pore fractals might be changed into surface fractals through the fractal dimension after the deviation correction, and the SAXS data obtained by Debye law correction maintained the originality and validity better than Porod law correction. The irregular shape of M was observed on the curve of fractal dimension as the maximum reflectance of vitrinite increases, resulting in three stages. More adsorption sites per unit projection area could be found in a larger fractal dimension of the pore surface, which results in an increase in Langmuir adsorption volume. The increase in the adsorption pressure reduced the fractal dimension. Our findings enhance the experimental support for comprehending the structure of coal nanopores.

Effects of CS2 Solvent Extraction on Nanopores in Coal

Effects of CS₂ Solvent Extraction on Nanopores in Coal

Confinement effect on transport diffusivity of adsorbed CO2–CH4 mixture in coal nanopores for CO2 sequestration and enhanced CH4 recovery

The transport of CO2–CH4 mixture in coal nanopores is crucial to CO2-enhanced coalbed (CO2-ECBM) recovery, but is highly challenging on adsorption-diffusion dominated nanoscale due to the gas-surface and intermolecular interactions. Using Monte Carlo and molecular dynamics simulation, the confinement effect of nanopores with two slit widths (2 and 6 nm) on the CO2–CH4 binary diffusion in the adsorption layer are investigated. A modified computational method for multi-component transport diffusivity in the adsorption layer is proposed by incorporating the survival probability function. The results show that the transport diffusivity in the semi-confined adsorption layer shows a general decreasing tendency with the increase of pressure, while it slightly decreases when the pressure <6.4 MPa in the confined adsorption layer, following an inverse parabola trend with the maximum value at around 12 MPa. Three mechanisms are classified regarding this pressure-dependent evolution. Firstly, the collision between molecules of weaker attraction and surface dominates the transport capacity, and this collisional frequency reduces at the low-pressure region. Accompanying the pressure exceeds the critical threshold, surface diffusion becomes the primary contribution. Lastly, significant intermolecular interactions appear after the adsorption saturation. During the CO2-ECBM process, pressure drawdown is the primary method to release the adsorbed CH4, and the largest recovery ratio can reach 25.9% in 2 nm micropore and 30.1% in 6 nm mesopore. After the early CO2 injection, the transport diffusion of CH4 exceeds that of CO2, and the selectivity of CH4 over CO2 increases in the subsequent CO2 huff-n-puff cycles. The CO2–CH4 mixture under greater confinement effect manifests higher diffusion selectivity, which benefits the separation of the CO2–CH4 mixture and further contributes to the CH4 recovery.

A lattice Boltzmann model for simulating gas transport in coal nanopores considering surface adsorption and diffusion effects

The gas flow in coal is extremely complicated as a combined results of multiple flow mechanisms and complex pore structures. Comprehensive understanding of gas flow behaviors in coal is of great importance for the prediction of coalbed methane (CBM) production and the elimination of gas-induced disasters in coal mines. Based on the lattice Boltzmann method (LBM), this study constructs a LB model considering surface adsorption and diffusion effects, which involves multiple mechanisms of gas flow, including viscous flow, enhanced gas slippage (gas slippage is enhanced by surface diffusion) and surface diffusion. The results show that in micropores (<2 nm) and mesopores (2–50 nm), under the effect of adsorbed gas, bulk gas flow velocity increases at a low pressure but decreases at a high pressure. Adsorbed gas surface diffusion permeability, enhanced gas slippage permeability and viscous flow permeability decrease with the rise of pore pressure. In nanopores in coal reservoirs, surface diffusion more significantly enhances gas slippage at a low pressure and in smaller pores. Besides, in the flow calculation, gas slippage without the considering effect of adsorbed gas and Knudsen diffusion can replace each other. According to the results from the LB model, the apparent diffusion coefficients of gas in coal particles decreases with the increase of pore pressure in smaller pores(<10 nm) but increases in larger pores(>10 nm). The study further investigates flow mechanisms controlling the CBM extraction yield in different extraction stages. In the early stage of extraction, the CBM extraction yield is controlled by Darcy seepage in fractures; in the middle stage, it is controlled by viscous flow in larger matrix pores; in the late stage, it is controlled by gas diffusion induced by the superposition of surface diffusion and enhanced gas slippage in matrix micropores and mesopores.

Experiment, Model, and Mechanism of Multiscale Dynamic Diffusion-Permeability in Coal under Different Fluids.

The permeability of coal exhibits multiscale characteristics in space and time, which is caused by the presence of micro and nanopores in coal. Water, free gas, and adsorbed gas are common engineering fluids in coal seams during gas extraction. Thus, it is of significance to study the multiscale characteristics and mechanisms of seepage-diffusion of different fluids in coal for gas extraction engineering. Experiments of seepage-diffusion for He, CH4, and water are carried out using ϕ50 × 100 mm cylindrical coal. It is found that the apparent diffusion coefficient for He, CH4, and water are not a constant but a variable that decays dynamically with time. The phenomenon is independent of fluid properties and determined only by the multiscale characteristics of pores in coal, and then a novel model of multiscale dynamic apparent diffusion that can accurately describe the full-time process of flow in various fluids is proposed. The mechanism of dynamic seepage-diffusion for different fluids in coal is elucidated based on a new proposed geometrical model of multiscale pores. At the early stage of flow, fluids first flow in or out of the largest pores outside coal, and at the later stage of flow, fluids flow in the micro and nanopores inside the coal matrix. The pore sizes through which the fluid flows decrease with time, which leads to a dynamic attenuation of the apparent diffusion-permeability with time. The initial apparent permeability K 0 for He/CH4 shows a "U"-shaped pattern of decreasing and then increasing with the decrease of gas pressure. When the gas pressure is lower than the turning point, the slip effect dominates, making the initial apparent permeability K 0 decrease with the increase of gas pressure. When the gas pressure is higher than the turning point, the effect of the pressure difference of the gas dominates, and K 0 increases with the increase of gas pressure. When water flows in multiscale pores, it occupies the space of large pores, making the homogeneity of pores increase, and the decay coefficient of apparent permeability becomes smaller. The multiscale permeability can explain the reason for the rapid attenuation of coalbed methane production in the late stage. It is of great significance for coalbed methane productivity prediction.

Experimental study on the law of transitional gas flow in porous media

There are many nanoscale pores in deep low permeability coal seams. The flow of coalbed methane (gas) in nanoscale pores belongs to the gas flow in porous media with medium and high Knudsen numbers. Its flow mechanism is one of the key unsolved scientific problems. In order to explore the gas transport law in nanopores of coal, a gas transport model based on microscopic boundary restriction was adopted to describe the gas flow law, and its rationality was verified by experimental data. The Field Emission Scanning Electron Microscope was used to scan the nanopores of customized anodized aluminum membrane. Then, the Maximally Stable Extremal Regions (MSER) algorithm of MATLAB and binarization algorithm were employed to quantify the pore structure parameters (equivalent pore size and porosity) of the membrane nanoscale pores. Finally, PMI micro-flow permeability tester was used to carry out different rarefied degree gas penetration experiments through anodized aluminum membranes, and the adopted gas transport model was verified. The results show that the binarization method is more accurate to characterize sample whose theoretical pore sizes are 20–35 nm, while MSER characterizes samples whose theoretical pore sizes are 110–150 nm and 200–300 nm more accurately. In other words, binarization method is more accurate for characterizing mesopores, while MSER algorithm is more accurate for macropores. The results have important reference value for more accurate extraction of nanopore parameters of porous media. Compared with traditional gas transport model, the adopted model considering the microscopic boundary restriction in this paper is closer to the experimental results. Moreover, it is suitable for describing the gas flow law in multiscale nanopores. The study provided important guiding significance for ascertaining the gas migration law in low permeability coal seam, improving the prediction accuracy of gas extraction and taking effective measures to increase production. The research results can further enrich the theoretical system of gas transport in coal, which is conducive to the efficient gas extraction, and is of great significance to promote the realization of carbon peak and carbon neutrality.

In situ SAXS study on the evolution of coal nanopore structures with uniaxial compressive stress

In this study, an in situ synchrotron radiation small-angle X-ray scattering (SAXS) experiment is used to characterize the evolution of coal nanopore structures under uniaxial compression. The variation in the coal nanopores is measured by analyzing the obtained scattering data. From the results, the coal scattering data show a positive Porod deviation in which the deviation slope decreases with increasing stress. Smaller nanopores are more sensitive to uniaxial compressive stress. There is a positive correlation between scattering intensity I and stress at less than 30 nm. The scattering intensity I of pores larger than 30 nm is negatively correlated with uniaxial compressive stress. The structure of smaller pores is more complex. The surface fractal dimension DS and pore fractal dimension DP increases and decreases with increasing uniaxial compressive stress, respectively. The specific surface area is positively correlated with DS. In the measuring range of 3–80 nm, the coal nanopores show a bimodal distribution. At the stage of below 0.6σp, the average diameter decreases by 1.98%, the porosity and specific surface area increases by 6.21% and 31.5%, respectively; At the stage of above 0.6σp, the average diameter decreases by 1.04%, the porosity and specific surface area increases by 1.18% and 5.57%, respectively. These results suggest that the variation of coal nanopores with stress have phased characteristics, and the evolution of the coal nanopores under uniaxial stress can be divided into two stages. In the nanopore fracture stage, the nanopores are deform, fracture and create new smaller pores with increasing stress, and the roughness of pore surface increase. In the nanopore closure stage, high stress intensifies the degree of pore fracture, and nanopores begin to close and disappear. This study reveals the evolution characteristics of the pore structures of coal under uniaxial compression at the nanoscale.

Applicability Analysis of Determination Models for Nanopores in Coal Using Low-Pressure CO₂ and N₂ Adsorption Methods.

The development characteristics of nanopores (with pore sizes <200 nm) in coal are a key factor affecting the accumulation and migration of coalbed methane (CBM). Thus, an appropriate determination method and calculation model are essential for accurate nanopore representation. Based on the experiments of low-pressure CO₂ adsorption (LP-CO₂GA) at 273 K and low-pressure N₂ adsorption (LP-N₂GA) at 77 K on four coals with different ranks, the abilities of different models (e.g., Langmuir, Dubinin-Radushkevich (D-R), Dubinin-Astakhov (D-A), Brunauer-Emmett-Teller (BET) and nonlocal density functional theory (NLDFT)) to accurately predict the pore parameters were analyzed. The results showed that (1) for LP-N₂GA, the Langmuir model is only suitable for gas adsorptions at low relative pressure conditions (P/P0 < 0.01), and its error value increased with the relative adsorption pressure. The fitting results of the D-R model showed good agreement with the D-A model under low relative pressure of LP-CO₂GA (P/P0 < 0.01), and the D-A model had more accurate fitting results. The BET model is more accurate than the other models (φ = -1.2733%) only in the interval of LP-N₂GA with 0.05 < P/P0 < 0.35. The data also showed that the NLDFT model can maintain a higher fitting accuracy for LPCO₂/N₂GA processes at relative adsorption pressures from 0.001-0.9996. (2) Using LP-CO₂GA with the Langmuir, D-R, D-A, and NLDFT models, the micropore specific surface area (SSA; 66.9570-248.6736 m²/g) and pore volume (0.0201- 0.0997 cm³/g) were obtained, while the values of meso-/macropore SSA (0.0007-2.3398 m²/g) and pore volume (0.0036-0.04 cm³/g) were calculated by LP-N₂GA with the BET and NLDFT models. The results showed that the fitting accuracy in descending order was the D-R, D-A, Langmuir and NLDFT models. (3) In combination with the applicable model range, LP-CO₂GA with the NLDFT model was recommended for micropore analysis of the coal pore sizes from 0.36-1.1 nm, while LP-N₂GA combined with the NLDFT model was recommended for nanopore analysis of pore sizes from 1.1-200 nm. (4) The characteristics of pore development in the Beiloutian coal were analyzed using LP-CO₂/N₂GA combined with the NLDFT model. It was found that a pore volume and SSA less than 1.0 nm accounted for 88.82% of the total pore volume and 98.05% of the total SSA, indicating that micropores in coal are the main space for CBM storage and are key physical factors for the occurrence and migration of coalbed methane. The conclusions of this article will provide a basis for the accurate calculation of nanopores in coal.

Small Angle X-ray Scattering Test of Hami Coal Sample Nanostructure Parameters with Gas Adsorbed Under Different Pressures.

The complexity and multiscale structure of coal pores significantly influence the gas diffusion and seepage characteristics of coal. To apply small angle X-ray scattering (SAXS) to study the coal pore structure parameters within the scale of 1-100 nm in the methane adsorption process, the X-ray window was optimized and a gas adsorption chamber was designed to interface with the small angle X-ray scattering platform. The fractal dimension and porosity of Hami coal samples under different methane pressures were studied using the small angle X-ray scattering platform and adsorption chamber. The surface and nanopore fractal information of the nanopores in coal were distinguished. The variation trends of the pores and surface fractal dimension with time under the same methane pressure were compared. The results indicate that the surface dimension changes from 2.56 to 2.75, and the extremum point may indicate that the primary nanopore structure is crushed by the adsorbed gas after approximately 15 minutes. This work clarifies that the fractal dimension can characterize the changes in nanopores in the process of gas adsorption by using SAXS. According to the fractal characteristics, the adsorption of gas in coal nanopores is summarized as four steps: expansion from adsorbance, deformation, crushing and recombination. The minimum porosity is 0.95% and the extreme value point is 1.47%. This work also shows that decrease in surface energy affect the porosity changes in nano-size pores. This work is of some significance to coalbed methane permeability improvement and gas extraction.

Experimental Study on the Physisorption Characteristics of O2 in Coal Powder are Effected by Coal Nanopore Structure

Coal is a porous medium. Oxygen molecules in the air penetrate through the pores of coal and are adsorbed on the coal surface. Low-temperature oxidation of coal then occurs, by which coal spontaneous combustion is promoted. Given this process, the authors analysed the physisorption characteristics of O2 in pulverized coal from the perspective of nanopore structure. In this study, five different kinds of coal samples (two lignites, one bituminous coal, and two anthracites) were selected, and the surface morphology, pore structure parameters and oxygen physisorption capacity of the pulverized coals were determined by scanning electron microscopy (SEM), mercury intrusion porosimetry (MIP) and oxygen adsorption with chromatography (OAC), respectively. The experimental results of SEM and MIP show that with the development of coal, the surface folds increase, and the pores increase in number and shrink, which leads to the nanopores of anthracite and bituminous coal being smaller and more complex than those of lignite. The experimental results of OAC show that adsorbed oxygen is physisorbed by pulverized coal in the order lignite > bituminous coal > anthracite. Analysis of the oxygen desorption curves shows that the oxygen desorption rates of the anthracites and bituminous coal are slower than those of the lignites. The results show that the amount of oxygen physisorbed by pulverized coal is proportional to the fractal dimension of the coal pores, proportional to the pore volume of the nanoscale pores, and inversely proportional to the number of closed pores in the coal. Based on the results of the analyses mentioned above, it is important to analyse the process of coal-oxygen chemisorption and the mechanism for low-temperature oxidation of coal to prevent coal spontaneous combustion.

Three‐dimensional characterization of open and closed coal nanopores based on a multi‐scale analysis including CO2 adsorption, mercury intrusion, low‐temperature nitrogen adsorption, and small‐angle X‐ray scattering

AbstractCoal seams in China have poor permeability and a complex pore structure, leading to a low gas extraction efficiency. Studying the pore structure of different metamorphic degrees of coal by using modern technological methods and employing appropriate pumping measures for different pore structures can improve the gas extraction efficiency. In this study, the pore sizes of three coal samples with different metamorphic grades were measured at different pore scales through CO2 adsorption, nitrogen adsorption, small‐angle X‐ray scattering, and mercury pressure methods. The pore structure was three‐dimensionally reconstructed using the lattice Boltzmann method‐based quartet structure generation set algorithm. The optimum pore size measurements performed using the CO2 adsorption, nitrogen adsorption, small‐angle X‐ray scattering, and mercury pressure methods were in the ranges of 0.3‐2, 1.7‐300, 1‐50, and 7.5‐11 000 nm, respectively; this demonstrates that the structure of a full‐scale coal model cannot be accurately tested using a single technique. The pore structure varies depending on the metamorphic degree of the coal; therefore, coal metamorphism should be considered for effective gas extraction.

Impact of coal composition and pore structure on gas adsorption: a study based on a synchrotron radiation facility

AbstractA better knowledge about the impact of coal composition and pore structure on gas adsorption is of great significance for coalbed methane exploration and uneconomic coalbed CO2 storage. Coal is a porous medium with complex organic components, and most pores in coal are nanopores that have a complex geometrical morphology. As a result, it is significantly necessary to investigate the impact of coal composition and pore structure on gas adsorption based on coal 3D nanopore structure. Synchrotron radiation nano‐computed tomography (CT) was applied to acquire 3D coal nanopore structure and synchrotron radiation small‐angle X‐ray scattering (SAXS) was used to obtain the pore surface fractal dimension. The coal with higher pore surface fractal dimension presents higher Langmuir volume. Based on the 3D nanopore structure acquired by synchrotron radiation nano‐CT, gas adsorption capacity was characterized by the adsorbed gas amount on unit pore surface area. It was found that gas adsorption capacity depends on the coal composition. There is a negative correlation between gas adsorption capacity and ash content and oxygen‐containing groups, and a positive correlation between gas adsorption capacity and vitrinite contents. Hysteresis between desorption and adsorption isotherms is dependent upon the ratio of throat number to pore number, and the hysteresis becomes more significant when the ratio is larger. © 2019 Society of Chemical Industry and John Wiley &amp; Sons, Ltd.

Methodology of three-dimensional visualization and quantitative characterization of nanopores in coal by using FIB-SEM and its application with anthracite in Qinshui basin

Analysis of the three-dimensional (3D) visualization and reconstruction methods of nanopores, and its morphology and connectivity are the key to explore the storage and migration mechanism of coalbed methane (CBM). The Bofang (BF) sample collected from the Qinshui basin was scanned by the focused ion beam scanning electron microscopy (FIB-SEM), which can simultaneously image the nanopores through 2D slice image and 3D reconstruction image. Firstly, the 2D morphology and development characteristics of nanopores were analyzed. Secondly, the 3D visualization and reconstruction of nanopores were carried out. Then, the pore connectivity, permeability, and geometric characteristics were discussed. Finally, the effect of pore structure on the storage and migration of CBM was emphatically analyzed. The results show that the organic pores, inorganic pores, shrinkage-induced pores and micro-fractures were developed in coal, and their morphology and connectivity were significant differences. The pores and throats can be extracted from the digital cores with sub-volume of 4.9 × 4.9 × 4.5 μm3, the pore number and throat length (avg.) of BF sample are 21951 and 18.2789 nm, respectively, the porosity and permeability of BF sample is 2.004% and 4.405 × 10−3 mD, respectively, which all indicates the BF reservoir has good storage and connectivity capacity. For pores <50 nm, the number, volume and area indicate that the BF sample has good storage capacity. For pores >200 nm, the pores are all interconnected, which indicates the BF sample can provide effective channels for CBM migration. The distribution of pores and throats and their geometric and topological structures indicate that the BF sample has good migration ability. This study on the methodology of 3D visualization and quantitative characterization of nanopores using FIB-SEM can broaden the research method of pores, and lay good foundations for studying the storage and migration of CBM.

Nanopore structure of deep-burial coals explored by AFM

With the increase of mining depth, the complex burial conditions enhance the chemical and physical heterogeneity of deep-burial coal. The accurate characterization of the nanopore within deep-burial coals is still a challenge. In this work, atomic force microscopy (AFM) measurement was conducted on 12 deep-burial coal samples with different ranks varying from bituminous C to anthracite, attempting to gain clearer insights into the nanopore characteristics and surface roughness. Based on the 3-D surface topographies by AFM, nanopore parameters were determined through image processing and analysis. Quantitative study has been carried out on coal surface roughness. Besides, the validation and limitation of AFM experiment were also analyzed, and the correlations between these nanopore parameters and coal rank (indicated by vitrinite reflectance, Ro) were established. The results show that the mean pore size of studied coal samples ranges from 6.83 nm to 40.16 nm with the areal porosity of 0.65%–9.76%. AFM images indicate that the macromolecular of lower rank coals (bituminous C) is arranged loosely with abundant larger pores, but the anthracite molecular is more ordered with regular network structures. A U-shape curve relationship with the bottom at Ro approximately 1.25% has been found between both the nanopore number and areal porosity with increasing Ro, demonstrating two evolution stages. The values of surface roughness parameters Ra and Rq are 6.43–30.27 nm and 7.51–35.27 nm, respectively. The exponential decline of surface roughness with increasing coal rank suggests that coalification enables coal surface to become smooth. Comparison of nanopore size distribution explored by AFM and low-pressure nitrogen gas adsorption (LP-N2GA) has also been made, indicating that AFM is an effective tool for coal nanopore characterization.

Quantitative analysis of coal nanopore characteristics using atomic force microscopy

Accurate characterization of nanopore structure within coal is of great significance for understanding the mechanism of coalbed methane transportation and accumulation. In order to quantitatively analyze the nanopore characteristics and surface roughness of coal, atomic force microscopy (AFM) experiments were conducted on nine coal samples with different ranks from the Qinshui Basin in Shanxi Province, North Chia in this study. The visible surface three-dimensional topographies of coal were obtained. The nanopore parameters including pore number, mean pore size, areal porosity and form factor were acquired through AFM image processing on the scanning area of 1 μm × 1 μm. Comparison of nanopore distribution determined by AFM and low-pressure nitrogen gas adsorption (LP-N2GA) has also been made to validate the reliability of AFM. Based on the experimental data, relationships between these nanopore parameters and the degree of coalification (indicated by vitrinite reflectance, Ro = 1.19%–3.15%) were studied. The results show that the pore number and areal porosity of studied samples are 1716–6885 and 0.91%–10.47%, respectively, with the mean pore size of 8.75–24.76 nm. The linearly positive correlation between the form factor and Ro suggests the enhancement of coal maturity can make the nanopore shape within coal tend to be more regular. As a response, the values of Ra and Rq demonstrate a similar nonlinear decreasing trend with the increase of Ro. Besides, dramatic changes of nanopore distribution have been observed during coalification jump, resulting in the shift to smaller nanopores. Comparison of nanopore distribution between AFM and LP-N2GA was made, indicating that AFM result is closer to the true value than LP-N2GA. This study enables us to better understand the surface characteristic of coal at nanoscale.

Influence of water invasion on methane adsorption behavior in coal

Fluid displacement is the fundamental process for subsurface fossil fuels extraction. Water invasion in coal seams is one of the routinely used stimulation approaches for coal seam methane extraction in underground coal mines. However, how the invading bulk water interacts with adsorbed/gaseous methane in coal is rarely considered even though it is known that moisture presence in coal decreases methane uptake by occupying adsorption sites. Here we study how the invading water interacts with adsorbed/gaseous methane in molded coal under elevated pressures using a custom-designed instrument; the test procedure mimics the real water invasion process in engineering applications. Experimental results demonstrate that invasion water displaces adsorbed methane in nanopores of coal and thus enhances the free gas content. The displacement mechanism can be attributed to capillary effect and preferential flow in a coating mode. It was found that Philip's sorptivity model can simulate the relationship between displaced methane content and time, and the obtained sorptivity increases with increasing water invasion content and is independent of gas pressure. It was observed that the higher the initial adsorption equilibrium pressure, the larger the displaced methane content, and this can be attributed to the pressure-dependent feature of adsorbed methane density. The higher the invasion water content, the higher the displaced methane content. These experimental results are also applied for optimizing gas drainage borehole arrangement to efficiently drain coal seam gas in underground coal mines. These findings provide a new perspective to understand the interactions between bulk water and methane in coals and pave the way for developing new technologies for methane recovery in coal seams.

Characterization of coal porosity for naturally tectonically stressed coals in Huaibei coal field, China.

The enrichment of coalbed methane (CBM) and the outburst of gas in a coal mine are closely related to the nanopore structure of coal. The evolutionary characteristics of 12 coal nanopore structures under different natural deformational mechanisms (brittle and ductile deformation) are studied using a scanning electron microscope (SEM) and low-temperature nitrogen adsorption. The results indicate that there are mainly submicropores (2~5 nm) and supermicropores (<2 nm) in ductile deformed coal and mesopores (10~100 nm) and micropores (5~10 nm) in brittle deformed coal. The cumulative pore volume (V) and surface area (S) in brittle deformed coal are smaller than those in ductile deformed coal which indicates more adsorption space for gas. The coal with the smaller pores exhibits a large surface area, and coal with the larger pores exhibits a large volume for a given pore volume. We also found that the relationship between S and V turns from a positive correlation to a negative correlation when S > 4 m2/g, with pore sizes <5 nm in ductile deformed coal. The nanopore structure (<100 nm) and its distribution could be affected by macromolecular structure in two ways. Interconversion will occur among the different size nanopores especially in ductile deformed coal.

An atomic force microscopy study of coal nanopore structure

Coal nanopore structure is an important factor in understanding the storage and migration of absorbed gas in coal. A new method for studying coal nanopore structures is proposed. This idea is based on the nano-level resolution of atomic force microscopy, which can be employed to observe the structural features of coal nanopores clearly, conduct quantitative three-dimensional measurements and obtain structural parameters. Analysis results show that coal nanopores are mainly metamorphic pores and intermolecular pores. The metamorphic pores are commonly rounded and elliptical, increasing quantitatively with the coalification degree. The forms of intermolecular pores change markedly. The average pore size of low-rank coal is bigger than high-rank coal, and the number of intermolecular pores decreases as the coal rank increases. Section analysis effectively characterizes the coal pore microstructure, bearing analysis is a vital approach to measure microporosity, and grain analysis can be employed to study the pore size distribution. Atomic force microscopy is a tool for the in-depth research of coal pore microstructure and the coal-bed methane adsorption mechanism.