Coal Micropores Research Articles

The occurrence of coalbed methane in the depocentre of the Upper Silesian Coal Basin in the light of the research from the Orzesze-1 deep exploratory well

The Orzesze-1 exploratory well with a depth of 3708 m (TVD) was drilled in 2019–2020 in the depocentre of the Upper Silesian Coal Basin (USCB). The methane content in the coal seams has been tested to a depth of 2840 m and the sorption capacity of the coal to a depth of 2576 m. These are the deepest measurements in the USCB so far. The vertical distribution of methane content in the borehole shows two depth zones of interest, the first at a depth 883 m to about 1300 m (maximum methane content about 12 m3/t coaldaf) and another in the range of 1500–2840 m, that is, to the maximum measurement depth, so the actual lower boundary depth of this zone is unknown. The maximum methane content here exceeds 18 m3/t coaldaf at a depth of >2800 m. Both zones are separated by an interval of reduced methane content of about 5 m3/t coaldaf at a depth of approximately 1400 m. The gas composition is dominated by methane (∼90%), and the content of carbon dioxide increases to approximately 15% at a depth of >2300 m. The methane-bearing zone at ∼900–1300 m corresponds to the zone of high- and medium-volatile bituminous coal (second coalification jump), while the highest methane content at a depth of >2800 m was determined in anthracite. The methane sorption capacity of the coal seams oscillates between 16 and 40 m3/t coaldaf with a maximum in anthracite at a depth of >2800 m, where the temperature of the rock approaches 100 °C and the deposit pressure exceeds 28 MPa. The highest sorption capacity in anthracite results from its inner structure characterised by the predominance of ordered aromatic lamellas and the dominance of vitrinite macerals (>70%), which contain coal micropores accumulating adsorbed methane. The comparison of the sorption capacity of the tested coal and the measured methane content displays undersaturation of 11–59%, however, due to significant gas content in the deep zone (depth > 1500 m), the drilling area can be considered as a prospect for further exploration and development of coalbed methane (CBM).

Investigation of methane adsorption mechanism in Low-Rank bituminous coal considering internal water Distribution: A molecular simulation and experimental study

The impact of high internal water in low-rank bituminous coal on methane adsorption properties has significant implications for the extraction and utilization of low-rank coalbed methane. Mercury-in-pressure (MIP), low-pressure N2/CO2 adsorption (LPNA/CA), low-field nuclear magnetic resonance (NMR), and isothermal adsorption of methane experiments were conducted on two types of low-rank bituminous coal from neighboring mines in the same region. Additionally, molecular dynamics simulations were performed by constructing a molecular model of the low-rank bituminous coal. Experimental and simulation results indicate that the unique internal water distribution characteristics of low-rank bituminous coal and the methane adsorption mechanism under its influence. The initial adsorption of water molecules occurs within the micropores of coal, forming a microporous filling. As relative humidity increases, water molecules migrate spontaneously to the macropores, but they do not seal the pores. This phenomenon is observed in low-rank bituminous coals. The internal water primarily affects the microporous filling of methane, yet it does not alter the absolute predominance of this form in adsorption, and the internal water spontaneously migrates to provide part of the filling site for the methane molecules with the increase in adsorption pressure. The results provide valuable insights into optimizing the extraction strategies for low-rank coalbed methane resources.

Targeted Stimulation of Micropores by CS2 Extraction on Molecular of Coal.

The targeted stimulation of micropores based on the transformation of coal's molecular structure is proposed due to the chemical properties and difficult-to-transform properties of micropores. Carbon disulfide (CS2) extraction is used as a targeted stimulation to reveal the internal evolution mechanism of micropore transformation. The variations of microcrystalline structures and micropores of bituminous coal and anthracite extracted by CS2 were analyzed with X-ray diffraction (XRD), low-temperature carbon dioxide (CO2) adsorption, and molecular simulation. The results show that CS2 extraction, with the broken chain effect, swelling effect, and aromatic ring rearrangement effect, can promote micropore generation of bituminous coal by transforming the microcrystalline structure. Furthermore, CS2 extraction on bituminous coal can decrease the average micropore size and increase the micropore volume and area. The aromatic layer fragmentation effect of CS2 extraction on anthracite, compared to the micropore generation effect of the broken chain effect and swelling effect, can enlarge micropores more remarkably, as it induces an enhancement in the average micropore size and a decline in the micropore volume and area. The research is expected to provide a theoretical basis for establishing reservoir stimulation technology based on CS2 extraction.

Pore structure response at different scales in coal to cyclical liquid nitrogen treatment and its impact on permeability and micromechanical properties

The methane in the coal seams of abandoned mines is a valuable natural gas resource. However, the ultra-low permeability of coal seams restricts the extraction of coalbed methane. The liquid nitrogen fracturing technology is a novel approach suitable for enhancing the permeability of coal seams in abandoned mines. The ultra-low temperature could potentially facilitate the growth and propagation of pores and fractures in coal seams. In this study, we observed inconsistent alterations in coal properties measured by multiple instruments at different scales, whether in dry or wet coal specimens. This suggests that the mechanisms influencing the pore structure due to LN2 treatment differ across various scales in dry and wet coal specimens. For dry specimens, heterogeneous thermal deformation and freezing shrinkage exhibited opposing effects during LN2 treatment. Thermal stress-induced micro-fractures might counteract the freezing contraction of micropores in coal matrices, preventing a significant decrease in coal macropores and fractures. In wet specimens, the effects of LN2 treatment on wet coal specimens were predominantly controlled by frost heaving. However, due to low water saturation, LN2 treatment had negligible effects on coal micropores, even in the presence of local frost heaving. In field applications, water migration from smaller to larger pores could further diminish the impact of LN2 treatment on micropores.

Prediction of the Effects of Pulsating Hydraulic Fracturing on the Porous Structure and Permeability of Coal Based on NMR and AE Spectra.

Pulsating hydraulic fracturing has been an environmentally friendly method to improve the permeability of rock formations to stimulate gas production and reduce hazard risks. It has the advantage of fracturing the reservoir with lower cracking pressure and less water volume, as the mechanical strength of rock materials has been reduced by the hydraulic pulse pressure. Many researchers have found significant changes in hard rocks after cyclic loading. However, the existing work still cannot clearly explain the mechanism of the rock damage by pulsating hydraulic fracturing within a short-time experiment. To solve the issue, an investigation of the effects of pulsating hydraulic fracturing on CBM production has been carried out in lab and field applications. Results indicate that the long-term hydraulic pulse pressure can cause a linear decline in cracking pressure directly measured in the lab. It plays an essential role in the permeability enhancement by generating more flow channels for CBM production. The low-field NMR quantitatively evaluates the increase in porosity, which reveals significant incremental ratios of over 20% in the porosity of macropores, mesopores, and micropores of coal caused by fatigue damage. It is first proven that hydraulic pulse pressure has a significant influence on the porosity components of macropores, mesopores, and micropores. To validate the effectiveness of the technique on the field scale, a field application of pulsating hydraulic fracturing has been carried out in a coal mine. It shows that gas production has been largely enhanced with a long and stable production stage and higher gas flux after the applied pulsating load. The gas concentration and gas flux of the fractured boreholes are about 2 times that of the nonfractured boreholes. This work provides an investigation of the effects of pulsating hydraulic fracturing on CBM production, which gives a better understanding of the mechanism for the engineers in the field.

Microbial Conversion of Sulfur-Containing Compounds Contributed the Natural Biodegradation of Bituminous Coals

Microorganisms can affect coal biodegradation by driving elemental biogeochemical cycling. A detailed description of the sulfur conversion process led by indigenous bacteria during coal degradation is important. This study obtained sulfur-containing compounds conversion bacteria through indoor enrichment and conducted anaerobic culture experiments using coal supplemented with enriched bacteria. The study showed that the addition of enriched bacteria changed the microbial composition, particularly the microbial groups within Pseudomonadota and Bacillota. In addition, the sulfur-containing compounds conversion bacteria increased the pore volume and specific surface area of micropores (<10 nm) and transition pores (10–100 nm), as well changed the content of sulfur-containing organic components including polypeptides, aromatic compounds, alkyl compounds, and thiazoles. The genera related to inorganic sulfur conversion were mainly belonging to Bacillota and Actinomycetota. Additionally, functional genera that can degrade sulfur-containing organic compounds of dibenzothiophenes through the ring-opening of benzene rings were found within Pseudomonadota, Bacillota and Actinomycetota. In summary, organic sulfur-related Pseudomonadota, Bacillota and Actinomycetota and inorganic sulfur-related Bacillota and Actinomycetota detected in this study could accelerate the sulfur-containing compounds conversion and ultimately increase micropores in coal, and this holds great significance for understanding the biogeochemical cycling process of coal.

Research on the process of coal’s pores and fissures splitting during liquid nitrogen freeze-thaw action

ABSTRACT A clean and effective unconventional fossil fuel is coalbed methane (CBM). The development of CBM is essential to reducing the lack of oil and gas resources, preventing coal mine gas mishaps, and safeguarding the atmosphere. After the coal samples prepared in this study were frozen and thawed by liquid nitrogen, the change rules of coal surface and internal pores were investigated by metallurgical microscopy and carbon dioxide adsorption. The results showed that: 1) The values of specific surface area, characteristic adsorption energy, and total SF microvolume of coal samples increased after freeze-thawing of liquid nitrogen, but the CO2 adsorption experiments showed that the pore width decreased, which indicated that the mechanism of freeze-thawing fracture effect coal by liquid nitrogen mainly increased the number of micropores in coal. 2) The water saturation of coal samples rose with the degree of surface cracking, but when the water content reached a specific level, the rise in surface porosity of coal reduced with the water saturation. 3) The increase in the number of cycles has a greater effect on the extent of expansion of pores on the coal surface than on the expansion of pores in the coal.

Examination of the Factors Inhibiting CO2 Adsorption on Coal: A Case Study from Shallow-Depth Low-Rank Coal Seams.

Understanding the inhibitory factors affecting the adsorption of CO2 on low-rank coal from shallow-depth coal seams is essential to identify potential coal seams for CO2 sequestration. The CO2 adsorption capacity of shallow-depth coals was measured at a low pressure on raw and dry coals. The samples were also prepared for organic analyses, scanning electron microscopy analyses, and low-temperature nitrogen adsorption analyses to evaluate the CO2 adsorption and identify the inhibitory factors. An investigation was conducted to determine how CO2 adsorption occurs on coal by fitting experimental data to adsorption isotherm models, followed by analyzing the results based on the statistical analysis. In addition, this study used Henry's law, surface potential, and Gibbs free energy to identify the adsorption inhibitor between CO2 and coal. The CO2 adsorption experiment was conducted on raw coal with a moisture content of 15.18-20.11% and dry coal with no moisture. The experimental data showed that the CO2 adsorption capacity in dry coal was 1.6-1.8 times greater than that in raw coal. A fitting graph between the adsorption data and the isotherm model indicated that CO2 adsorption on coal occurred on monolayers and multilayers under raw and dry conditions. Statistical evaluation of the adsorption isotherm models showed that the Langmuir and Freundlich models aligned more closely to the experimental data. According to this result, low-pressure adsorption of CO2 on coal occurred in monolayers and multilayers under raw and dry conditions. Coal containing a high huminite content had a higher potential for CO2 adsorption, and the drying increased the positive relationship. On the other hand, coal containing high inertinite content inhibited CO2 adsorption onto the coal, but the drying process did not adversely affect CO2 adsorption. Furthermore, coal with high moisture and inertinite content inhibited the affinity, accommodation, and spontaneous CO2 adsorption onto the coal. CO2 adsorption could lead to swelling, but moisture loss opened more sites and micropores, resulting in the swelling effect not closing all micropores in dry coal. Based on these results, coal seams with low moisture and inertinite content are the most promising for CO2 adsorption. Altogether, this study provides an understanding of the percentage of inhibitor factors that affects CO2 adsorption on low-rank coal from shallow depths, which may lead to different CO2 adsorption capacities.

Characterizing water vapor adsorption on coal by nuclear magnetic resonance: Influence of coal pore structure and surface properties

Steam injection is a novel and promising technology for enhancing coalbed methane (CBM) recovery. Accurate descriptions of the migration and adsorption of water vapor on coal is crucial for determining the occurrence state of the CBM and improving CBM production. This study quantitatively characterized the dynamic transport and adsorption processes of water vapor in coal pores and fractures using nuclear magnetic resonance (NMR). The results demonstrated that water vapor was preferentially adsorbed in the coal micropores, which was mainly related to capillary condensation, whereas liquid water filled a wide range of pores and fractures. Due to the differences in the microstructure and surface properties among different coals, the water vapor adsorption behaviors varied significantly. Among the coals used in this study, water vapor was most easily adsorbed on the bitumite with the biggest specific surface area, pore volume, pore area, the highest content of oxygen-containing functional groups, and the strongest wettability. Additionally, the coal pore structure had a greater impact on the water vapor adsorption capacity than the oxygen functional groups and the wettability. This research is of great significance for understanding the flow behaviors of water vapor in porous media and the interactions between coal and water.

Influence of uniaxial strain loading on the adsorption-diffusion properties of binary components of CH4/CO2 in micropores of bituminous coal by macromolecular simulation

Heterogeneous and anisotropic coal tends to deform under strong mining disturbances, affecting the microscopic adsorption and diffusion of binary CH4-CO2 mixtures in micropores. Therefore, the grand canonical Monte Carlo and molecular dynamics methods were used to study the influence of the uniaxial strain loading on the adsorption-diffusion properties of binary components of CH4/CO2 in coal micropores. The results show that compression strain has significant effects on adsorption selectivity and thermodynamic factor compared to tension strain, and CH4 displacement efficiency is greater when the strain is less than −0.12. Both compression and tension strains have dual effects on promotion and inhibition of volumetric swelling of the gas-containing coal matrix, micropore development and gas diffusion. Furthermore, the diffusivity of CH4 is significantly larger than that of CO2, and Einstein diffusion is dominated by low-velocity motion. Results provide an important basis for the optimal coalbed methane recovery design in deformed coal reservoirs.

Experimental study on the change of coal structure and microbial community structure during supercritical-CO2-H2O-microorganisms-coal interaction process

To study the effects on coal structure and microbial community diversity during CO2 storage in deep unminable water-bearing coal seams, the lignite in the mining area of Inner Mongolia was selected as the research object. The simulation experiments of supercritical-CO2 (ScCO2)-H2O-microbial-coal interaction under different pressure conditions performed by using an autoclave. Then the solid–liquid phases before and after the reaction were analysed by XPS, low-temperature liquid nitrogen adsorption, 16S rRNA and other testing methods. The results showed that CO2 injection caused the pH and OD600 values in the system to decrease, and the pH and OD600 values in the system decreased more obviously with the increase of ScCO2 pressure. Compared with raw coal, the relative content of carbon elements on the surface of the coal after the reaction increased, and the organic carbon (CC/CH) relative content decreased slightly. In contract, the specific surface area, total pore volume, and the pore size of micropores of coal increased, but the average pore size of the mesopores and macropores decreased. At the same time, with the ScCO2 injection under different pressure conditions, some bacterial cells in the system ruptured and inactivated, resulting in the decrease their relative abundance. In contract, some genera, such as Paraclostridium and Methanoculleus, may have the adaptability to the extreme environment and gradually evolve into the dominant genus. In conclusion, the results provide a theoretical basis for studying the changes of ScCO2 on coal structure and microbial community structure during CO2 storage in deep unminable water-bearing coal seams.

Model for Predicting CO2 Adsorption in Coal Left in Goaf Based on Backpropagation Neural Network

Injecting power plant flue gas into a goaf stores CO2 in the flue gas and effectively prevents the spontaneous combustion of the coal remaining in the goaf. Here, we investigated the adsorption behavior of three types of coal at normal temperature and pressure using a self-developed adsorption experimental device. We used a specific surface area and porosity analyzer to study the effects of pore structure, mineral content, and moisture content on CO2 adsorption in coal. Based on the experimental data, we designed a multifactor CO2 adsorption prediction model based on a backpropagation (BP) neural network. The results indicated that the pore size of most micropores in coal was in the range of 0.5–0.7 and 0.8–0.9 nm. The specific surface area and pore volume were positively correlated with the CO2-saturated adsorption capacity, whereas the mean pore diameter, mineral content, and moisture content were inversely associated with the CO2-saturated adsorption amount. The accuracy of the multifactor BP neural network prediction model was satisfactory: the determination coefficients (R2) of the training and test sets were both above 0.98, the root mean square error (RMSE) and mean absolute error (MAE) of the test set were both less than 0.1, and the prediction results satisfied the requirements. To optimize the prediction performance of the model, we used the random forest algorithm to calculate the importance of each factor. The sum of the importance weights of the specific surface area, moisture content, and pore volume was 91.6%, which was much higher than that of the other two factors. Therefore, we constructed an optimization model with specific surface area, moisture content, and pore volume as input variables. The R2 values of the training and test sets in the simplified model were improved compared with those of the multifactor model, the RMSE and MAE were reduced, and the fitting effect was ideal. The prediction model of CO2 adsorption in coal based on the BP neural network can predict the CO2 adsorption capacity of coal under different physical and chemical conditions, thereby providing theoretical support for the application of CO2 storage technology in goafs.

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.

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.

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.

Regularity of change in the volmer diffusion coefficient of methane adsorbed in the microsorption structure of the elastic zone of the coal seam bearing pressure

The Volmer diffusion coefficient of methane adsorbed in the micropores of coal in the elastic zone of the coal seam bearing pressure, which normally is under conditions of significant compressive stress, was calculated with taking into account the energy of the methane sorption connection with coal, the energy of Volmer diffusion activation in the porous space of coal, and the stressed state of the elastic zone with its influence on the change of Volmer porosity. During the calculations, such parameters as the diameter of Volmer micropores and the length of the descending branch of the bearing pressure diagram were varied. As a result of the approximation of these calculations, both pairwise dependences of the Volmer diffusion coefficient on the listed parameters and its multifactorial relationship with them were established. Therefore, it is concluded that the process of methane diffusion in the elastic zone of bearing pressure is not blocked by the rock pressure, as previously thought, but is actively developing. The diffusion of free methane will be determined by the established regularity of changes in the Volmer diffusion coefficient in the elastic zone of the coal seam bearing pressure. The calculations show that as the distance from the maximum of the bearing pressure increases, the Volmer diffusion coefficient of methane in the coal seam increases, which is due to a decrease in the pressure of rocks in the descending branch of the bearing pressure diagram. However, this growth is not great due to the weak compressibility of pores. Therefore, for pores of the same diameter, the Volmer diffusion coefficient in the elastic zone of the coal seam bearing pressure for the given mining geological conditions can be considered a constant. For depths of, for example, 1000 m and pore diameters of 10 Å, the value of the Volmer diffusion coefficient will be approximately 3.77·10-8 m2/s. This confirms that methane gas release is caused not only by filtration of free gas, but also by Volmer diffusion of adsorbed methane. In turn, the reserves of the latter are known to be the main reserves of methane in coal. Therefore, the established regularity makes it possible to more accurately calculate the volumes of methane, which will be released from the coal massif during mining operations, in order to assess safety of conditions for coal deposits mining and to develop technologies for coal mine methane production.

ДИНАМИКА ЗМІН КОЕФІЦІЄНТА ФОЛЬМЕРІВСЬКОЇ ДИФУЗІЇ МЕТАНУ, АДСОРБОВАНОГО У МІКРОСОРБЦІЙНІЙ СТРУКТУРІ ПРУЖНОЇ ЗОНИ ОПОРНОГО ТИСКУ ВУГІЛЬНОГО ПЛАСТА

Purpose. Set the possible change in the coefficient of Volmer diffusion of methane in the microsorption structure of the elastic zone of the supporting pressure of the coal seam. Methodology. The energy of the sorption bond of methane with the coals, the activation energy of the Volmer diffusion in the porous expanse of coal, as well as the stresses of the elastic zone and its effect on the change of the Volmer porosity, was taken into account. In the calculations, such parameters as the diameter of the Volmer micropores and the length of the descending branch of the support pressure diagram were varied. Results. Approximation of calculations made it possible to establish both the pairwise dependences of the Volmer diffusion coefficient on the listed parameters and its multifactorial relationship with them. It was concluded that the diffuse process of methane in the elastic support pressure zone is not blocked by rock pressure, as previously thought, but is actively developing. In this case, the diffusion of free methane will be determined by the established pattern of change in the Volmer diffusion coefficient in the elastic zone of the coal seam bearing pressure. Calculations showed that as the distance from the maximum reference pressure increases, the Volmer diffusion coefficient of methane in the coal seam increases, which is due to a decrease in rock pressure in the descending branch of the reference pressure graph. However, this growth is not strong due to the weak compressibility of pores. Therefore, for pores of the same diameter, the Volmer diffusion coefficient in the elastic zone of the supporting pressure of the coal seam for the given mining geological conditions can be considered a constant. For depths of, for example, 1000 m and pore diameters of 10 Å, the value of the Volmer diffusion coefficient will be approximately 3.77×10-8 m2/s. Scientific novelty. Calculations of the Volmer diffusion coefficient of methane adsorbed in the micropores of coal in the elastic zone of the supporting pressure of the coal seam, which, of course, is under significant compressive stresses, have been performed. The established phenomenon confirms that methane gas release is determined not only by free gas filtration, but also by Volmer diffusion of adsorbed methane. Practical significance. Since adsorbed methane reserves are known to be the main methane reserves in coal, the regularity established in the article makes it possible to more accurately calculate the volumes of methane that will be released from the coal massif during mining operations for the assessment of safe working conditions for coal deposits and for the development of technologies for the production of shale methane.

Spontaneous combustion characteristics and mechanism of water-immersed and air-dried brown coal

ABSTRACT To reveal the influential laws and mechanism of water immersion on the spontaneous combustion traits of lignite in the low-temperature oxidation stage. Raw coal (HL), water-immersed for 30 days (HL-S30), 90 days (HL-S90) and 150 days (HL-S150), were taken as the research objects. The pore evolution characteristics and functional group migration law of coal were explored by Scanning Electron Microscopy (SEM), Low-temperature Nitrogen Adsorption Test (LNAT) and Fourier Transform Infrared Spectroscopy (FTIR). The low-temperature oxidation procedure was simulated by Programmed Heating Test (PHT), and the gas release characteristics, exothermicity and ultimate spontaneous combustion parameters of the relic coal were compared. These findings demonstrated that the discriminant indexes of spontaneous combustion tendency of four specimens change periodically with temperature, but generally the spontaneous combustion intensity of HL-S30 and HL-S150 is greater than HL, and HL-S90 is the least. The micro reasons for increasing oxidation activity of HL-S30 and HL-S150 are as follows: the number of micropores of water-immersed coal is almost 3 ~ 4 times that of HL, and the pores expanded with immersion which helps to enhance the adsorption of oxygen; the content of broken aromatic hydrocarbon -C = C- reached 50%, the total amount of hydroxy -OH decreased to 2/3 of HL, while the content of carboxyl -COOH and ether-oxygen bond -C-O- of active group doubled which are beneficial to accelerate the process of free radical chain reaction, and promote the gas release and the temperature of residual coal to reach the ignition point. The research findings enrich the basic theory of spontaneous combustion traits of water-impregnated coal, which help reduce the fire accidents and hidden dangers in actual production.

Effects of Liquid CO2 Phase Transition Fracturing on Mesopores and Micropores in Coal

Effects of Liquid CO₂ Phase Transition Fracturing on Mesopores and Micropores in Coal

Study of Adsorption Characteristics of CO2, O2, and N2 in Coal Micropores and Mesopores at Normal Pressure

Study of Adsorption Characteristics of CO₂, O₂, and N₂ in Coal Micropores and Mesopores at Normal Pressure