Coal Micropores Research Articles

Influence of Pore Structure Characteristics of Primary Coal on Coalbed Methane Adsorption

In order to better discuss the influence of pore structure characteristics of primary coal on coalbed methane adsorption, suburban coal and Zhaozhuang coal in Henan Province were systematically sampled. The analysis method of pore structure characteristics of primary coal is divided into shooting research and experimental analysis. Scanning electron microscope (SEM) was used to analyze the formation reason of coal micropores, pore shape, pore connectivity, pore size, and filling effect on coalbed methane adsorption. Experimental analysis is mainly based on CO2 adsorption experiments to obtain specific surface, pore volume, and distribution of coal samples. The pore-specific surface area, pore volume, and adsorption-desorption curve of coal samples were obtained by ultralow-temperature N2 adsorption experiment. Finally, the influence of pore structure characteristics of primary tectonic coal on coalbed methane adsorption was discussed by fusion of phase study and experimental analysis.

Pore Structure Characteristics and Adsorption and Desorption Capacity of Coal Rock after Exposure to Clean Fracturing Fluid.

In the hydraulic fracturing process, fracturing fluid contacts coal rock and physical and chemical reactions occur, which inevitably damage the pore structure of the coal rock and affect the adsorption and desorption capacity of the coal rock. In this paper, a low-temperature N2 adsorption method and scanning electron microscopy (SEM) were used to characterize coal samples. Using gas adsorption/desorption tests, high-, medium-, and low-rank coal samples before and after the clean fracturing fluid treatment were systematically studied. According to the relationship between coal pore structure parameters and gas adsorption/desorption characteristics, a correlation between the microscopic pore structure and the macroscopic gas adsorption/desorption characteristics of coal was obtained. The results show that the number of closed pores in high-, medium-, and low-rank coal samples increased after the clean fracturing fluid treatment. The micropore volume increased by 0.0009, 0.00143, and 0.0035 mL/g, respectively, and the specific surface area increased by 4.87, 9.06, and 57.60%. The fractal dimension also increased compared with that of raw coal. SEM analysis indicated that the influence degree of clean fracturing fluid treatment on the pore structure of different-rank coal samples was Gengcun low-rank coal > Pingba middle-rank coal > Jiulishan high-rank coal. The experimental results of methane adsorption and desorption showed that the adsorption capacity of the coal samples after clean fracturing fluid treatment was enhanced, which is related to increases in the micropore proportion, micropore volume, and specific surface area of the coal. The desorption capacity of the coal samples was also enhanced. The desorption rate of medium- and high-rank coal samples increased after the clean fracturing fluid treatment but that of low-rank coal samples decreased. The main reason is the increase in the number of micropores in low-rank coal, which enhances the gas adsorption ability and makes gas desorption difficult. Therefore, clean fracturing fluid is suitable for medium- and high-grade metamorphic coalbed methane mines. These research results provide a theoretical basis for the application of clean fracturing fluid in different coalbed methane wells.

Feasibility investigation of enhanced coalbed methane recovery by steam injection

Heat injection is an effective technique to enhance unconventional oil and gas recovery. As an excellent heat transfer medium, steam has a great potential for coalbed methane (CBM) recovery enhancement. To investigate the feasibility of enhanced CBM recovery by steam injection, infrared thermal imaging (ITI) and thermocouples were employed to study the heating efficiency of steam on three ranks of coal, and nuclear magnetic resonance (NMR) was applied to analyze water transport within the coal after steam treatment. The results showed that the internal temperature of the coal samples reached about 80 °C in 17 min of steam treatment and thermal cracking of coal were induced, hence steam injection could facilitate gas desorption and provide more channels for gas migration. NMR test results indicated that steam preferentially condensed in coal micropores due to the capillary condensation effect, hence steam might have the potential to displace methane. The T2 spectra of the coals after steam treatment exhibited only the first two peaks without the third peak, indicating that steam injection might not cause the water blocking effect. The outcomes of this study are expected to provide valuable insights into the field application of steam injection to enhance CBM production.

Study on the Microscopic Pore Structures of Coal Measure Reservoirs in the Shanxi Formation, Eastern Ordos Basin

The Carboniferous-Permian coal measures in China contain abundant natural gas resources. Shale, coal and tight sandstone reservoirs are developed in coal measures, and the quantitative characterization of the pore structures of different types of reservoirs can provide scientific guidance for the sweet spot prediction of tight reservoirs. In this study, taking the Shan 2 Member coal measure of the Shanxi Formation in the eastern Ordos Basin as an example, the pore structures of shale, coal rock and tight sandstone were systematically studied based on organic geochemistry, scanning electron microscopy, high-pressure mercury injection, and low-temperature N2 and CO2 adsorption experiments. The results show that the microscopic pore structures of different types of reservoirs in the Shan 2 Member coal measures are quite different. Shale and tight sandstone mainly develop clay mineral pores at mesopore scale, followed by intragranular and dissolution pores developed in quartz and feldspar minerals, while organic pores are rarely developed. A large number of macro-scale clay mineral pores and micro-fractures are developed in tight sandstone, meanwhile the pore connectivity of tight sandstone is better than that of shale. A large number of micro to nano-scale organic pores are developed in coal, and the specific surface area of micropores in coal is much larger than that of mesopores in shale and tight sandstone. Sandstone, shale, and coal are frequently interbedded in coal measure strata. Tight sandstone provide the main storage space for free gas, and pores in shale and coal absorb a large amount of natural gas. Sandstone-shale-coal assemblages and sandstone-coal assemblages are the key targets for the exploration of hydrocarbons in the Shanxi Formation coal measures in the study area.

Liquid carbon dioxide drying and subsequent combustion behavior of high-moisture coal at high pressure

• LCO 2 drying increases combustion rate by 37% for coal with 20% water at 500 °C. • The faster combustion of LCO 2 –coal slurry produces 35% higher particle temperature. • Both NMR and weight loss data support LCO 2 drying mechanism in high moisture coal. • The high pressure boiler with LCO 2 drying cycle increases power output by 11.3%. In this work, the combustion and radiation characteristics of high-moisture coal treated with liquid carbon dioxide (LCO 2 ) at high pressure were examined. The coal conversion and gas yield were measured to determine the combustion rate constants at four different temperatures. The particle temperature and radiative heat flux emitted from the burning coal bed were measured using a two-color pyrometer and radiometer, respectively. The changes in the moisture content of the coal microstructure after LCO 2 treatment were measured using nuclear magnetic resonance (NMR) and thermogravimetric analysis (TGA). The combustion kinetic results showed faster LCO 2 –coal slurry combustion, which frequently occurred at low temperatures. The particle temperature and radiative heat flux of the LCO 2 –coal slurry increased faster to higher steady-state values than those of raw coal. The influence of LCO 2 on the combustion of high-moisture coal was not observed on graphite devoid of water or volatile content. This indicates an effective exchange between LCO 2 and water in coal micropores when coal is exposed to LCO 2 . In addition to the TGA data, the LCO 2 drying mechanism in high-moisture coal was confirmed using NMR relaxation data. The performance of the LCO 2 drying process was compared with that of the conventional boiler with oven drying unit in terms of the water removal effect, combustion, and radiation behavior. An energy analysis was undertaken to calculate the amount of total energy produced from three power cycles occurring in a high-pressure boiler, including one combining the LCO 2 drying cycle with the steam power cycle.

Molecular Simulation on Competitive Adsorption Differences of Gas with Different Pore Sizes in Coal.

Micropores are the primary sites for methane occurrence in coal. Studying the regularity of methane occurrence in micropores is significant for targeted displacement and other yield-increasing measures in the future. This study used simplified graphene sheets as pore walls to construct coal-structural models with pore sizes of 1 nm, 2 nm, and 4 nm. Based on the Grand Canonical Monte Carlo (GCMC) and molecular dynamics theory, we simulated the adsorption characteristics of methane in pores of different sizes. The results showed that the adsorption capacity was positively correlated with the pore size for pure gas adsorption. The adsorption capacity increased with pressure and pore size for competitive adsorption of binary mixtures in pores. As the average isosteric heat decreased, the interaction between the gas and the pore wall weakened, and the desorption amount of CH4 decreased. In ultramicropores, the high concentration of CO2 (50–70%) is more conducive to CH4 desorption; however, when the CO2 concentration is greater than 70%, the corresponding CH4 adsorption amount is meager, and the selected adsorption coefficient SCO2/CH4 is small. Therefore, to achieve effective desorption of methane in coal micropores, relatively low pressure (4–6 MPa) and a relatively low CO2 concentration (50–70%) should be selected in the process of increasing methane production by CO2 injection in later stages. These research results provide theoretical support for gas injection to promote CH4 desorption in coal pores and to increase yield.

Influence of sequestered supercritical CO2 treatment on the pore size distribution of coal across the rank range

The influence of supercritical CO2 (SC-CO2) fluids on pore structure characteristics of coal plays a vital role in geo-sequestration of CO2 into deep coal seams. Changes in coal pore structure and mineral content after exposed to SC-CO2 fluids are quantitatively evaluated by N2/CO2 adsorption and X-ray diffraction (XRD) for a lignite, bituminous and anthracite coals. The SC-CO2 exposure has a notable effect on coal pore distribution. Affected by SC-CO2, the micropore volume within coal is reduced by 12.6 to 34.8%, while both the average pore diameter and macropore volume are remarkably increased. XRD analysis shows that no newly-formed mineral compositions were detected in treated coals, but the mineral content is altered. As the main carbonate minerals of coal, both calcite and dolomite are consumed during SC-CO2 treatment. Due to the strong extraction capacity for small organic molecules, the influence of SC-CO2 is related to coal rank, and the largest increment in macropore volume is observed in lignite. The greatest reduction of micropore volume is identified in anthracite treated sample, indicating that SC-CO2 has the most impact on micropores in high-rank coal. Mineral dissolution likely accounts for this reduction in micropore contribution by increasing mesopores and macropores.

Construction of Buertai Coal Macromolecular Model and GCMC Simulation of Methane Adsorption in Micropores.

With the increase in high gas mines in the low coal rank mining area in the northwestern part of China, high gas mines in the low-rank coal mining area have caused many gas emission accidents. Coal is a porous material, containing a large number of micropores (<2 nm), which can absorb large amounts of methane, so it is necessary to explore methane adsorption in micropores of low-rank coal. In this work, FTIR, HRTEM, and 13C-NMR were used to test the macromolecular structural parameters of Buertai coal, which was a kind of low-rank Jurassic coal in northwestern China. The results showed that the aromatic structural units in the Buertai coal structure mainly consist of naphthalene, anthracene, and phenanthrene. The fat structure mainly occurs in the form of aliphatic side chains, cycloalkanes, and other compounds. The oxygen atoms are present in the form of carbonyl groups, ether bonds, and phenol groups with a ratio of about 6:4:9. The nitrogen atoms are present in the form of pyrrole and pyridine compounds. Finally, the macromolecular structure model of Buertai coal was built, and the calculated NMR spectrum from the model was very consistent with the experimental NMR spectrum of Buertai coal. The relationship between the macromolecular density and energy of Buertai coal was explored using the Amorphous Cell module in the simulation software, Materials Studios 8.0 (MS 8.0), and the density value at the lowest energy was determined to be about 1.23 g/cm3. The pore structure parameters of Buertai coal were also calculated. It was found that both pore volume and void fraction decreased evenly as the diameter of the probe molecule increased, but the surface area decreased rapidly when the diameter of the probe molecule was 3.46 Å. All pore sizes were found to be smaller than 10 Å from the pore size distribution (PSD) curve of Buertai coal, which provided a lot of adsorption sites for methane (CH4). The results of the CH4 adsorption simulation from Grand Canonical Monte Carlo (GCMC) showed that CH4 is adsorbed inside the micropores of coal, and the adsorption capacity of CH4 depends on the diameters of micropores when the micropores are less than 8.5 Å. There are many micropores where CH4 did not appear because these micropores are closed and did not provide a channel for CH4 to enter. The results of experimental methane adsorption indicate that the excess adsorption capacity from the GCMC simulation was very close to the experimental results of Buertai coal. This work provides a new perspective to study the methane adsorption behavior in micropores of coal.

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.

Research on the fractal characteristics of the micro/nano-mesoscopic pore structure of high-rank coal under impact loads

ABSTRACT To reveal the mechanism of impact load on coal micropores and improve the extraction efficiency of high-rank coal gas, a separate Hopkinson pressure bar (SHPB) impact test system was used to simulate the shock and stress waves of the impact stress in different attenuation processes. Based on mercury intrusion and low-temperature liquid nitrogen test data before and after impact, fractal theory was applied to study the micropore fractal characteristics of high-rank coal in different directions before and after impact in the Chengzhuang Mine. The results show that pores smaller than or equal to 100 nm based on mercury intrusion data and pores larger than or equal to 100 nm based on low-temperature liquid nitrogen data do not follow fractal rules. Impact loads reduce the seepage and adsorption pore fractal dimensions. Although the seepage and adsorption pore fractal dimensions do not decrease with increasing impact load, a minimum fractal dimension occurs. The impact load corresponding to the minimum seepage and adsorption pore fractal dimensions in the same direction is the same. Impact loads increase gas migration velocities of the seepage pores, reduce the adsorption capacity of the adsorption pores, and promote gas desorption. The research results are important for determining the optimal drainage hole layout and examining the impact cracking and infiltration mechanism.

Experimental study on the effects of electrode materials on coal breaking by plasma

The extensively applied plasma technology boasts potential application prospects in the field of coal seam permeability enhancement. However, the effect of electrode materials on coal breaking by plasma (CBP) has not been reported. For this reason, the effects of four kinds of electrode materials, namely aluminum, copper, brass and stainless steel, on CBP were investigated by using a self-built CBP experimental system. The experimentally obtained statistics show that the sizes of coal fragments differ under the action of different electrodes. Specifically, the coal sample tends to be broken into small particles under the action of aluminum electrode, whereas the broken fragments generally have a diameter of larger than 15 mm under the action of brass electrode. The scanning electron microscopy results suggest that the different electrode materials correspond to different modes of crack initiation on coal surface. The results of liquid N2 adsorption show that the aluminum electrode exerts the poorest effect on improving micro-pores in coal, while the copper electrode achieves the best effect. Furthermore, the effects of electrode materials on the current and voltage waveforms at different voltages were analyzed by using a Rogowski coil and a HV test rod, and the effects of electrode materials on deposited energy generated by plasma in the coal were also discussed. The results indicate that at low voltages, the voltage and current waveforms under the action of four electrode materials differ obviously; in contrast, at high voltages, the voltage and current waveforms become almost identical. At the same voltage, the deposited energies formed by different electrode materials also differ due to the difference in breakdown voltages and current waveforms. The deposited energies formed by the aluminum electrode, the brass electrode and the stainless steel electrode inside the coal fall first and then rise with the increase of breakdown voltage, whereas that formed by the copper electrode goes up gradually with the increase of breakdown voltage. Based on the photograph of plasma discharge process shoot by a high-speed camera, the law of plasma propagation inside the coal was analyzed, and the effects of electrode materials on plasma development were discussed. A plasma streamer which is accompanied by dazzling white light and extremely high temperature is formed inside the coal at the moment of discharge, and it continuously expands outward as time passes by. The brightness degrees of plasma streamers formed under the action of electrode materials are different. Among the four electrode materials, the brass electrode forms the brightest light and generates the greatest energy.

A comparative study of the nanopore structure characteristics of coals and Longmaxi shales in China

AbstractBoth of the coalbed methane (CBM) and shale gas reservoirs are dominated by nanometer‐scale pores with their nanopore structures controlling the occurrence, enrichment, and accumulation of natural gas. Low‐pressure nitrogen gas adsorption (LP‐N2GA), low‐pressure carbon dioxide gas adsorption (LP‐CO2GA), high‐pressure methane adsorption (HPMA), and field emission scanning electron microscope (FE‐SEM) experiments were conducted on 14 different‐rank coal samples and nine Longmaxi shale samples collected from various basins in China to compare their nanopore characteristics. The FE‐SEM results indicate that the pore structures of both the coal and shale samples consist of nanometer‐sized pores that primarily developed in the organic matter. The types of their isothermal adsorption curves are similar. However, the coal and shale samples possess various hysteresis loops, which suggest that the nanopores in shale are open‐plated, whereas those in coal are semi‐open. Furthermore, the specific surface area (SSA) and pore volume (PV) of the micropores in coal are much larger than those of the mesopores, with the micropore SSAs accounting for 99% of the total SSA in the coal samples. However, the micropore SSAs in the shale samples only account 42.24% of the total SSA. These different nanopore structures reflect their different methane adsorption mechanisms. The methane adsorption of coal is primarily controlled by the micropore SSA, whereas that of shale is primarily controlled by the mesopore SSA. If we use mesopore SSA to analyze its impact on methane adsorption capacity of coal and shale, it will be mismatched. However, no mismatching relationship exists between the total SSAs and adsorption capacities of coal and shale. This study highlights the controlling effect of total SSA on methane adsorption capacity.

A Multi-Scale Modeling of CH4 and H2O Adsorption on Coal Molecules and the Water Blocking Effect in Coalbed Methane Extraction

Coalbed methane (CBM) is of great economic value. However, at the same time, CBM is facing a multitude of technological challenges. The water blocking effect (WBE) is one of the physical effects that controls the production of CBM. To alleviation WBE, it is necessary to study its mechanisms at the molecular level. In this study, we used a combined first-principles calculation and molecular simulation approach to investigate the adsorption and diffusion of both methane and water in coal. The results suggest that water does not compete with methane in the adsorption on coal surfaces, yet the presence of water significantly slows down the diffusion of methane within the micropores of coal. This work not only explains the fundamental mechanisms of the WBE but also provides a simulation framework for building strategies to alleviate WBE.

Porosity changes in bituminous and anthracite coal with ultrasonic treatment

Improving pore structures is key for enhancing coalbed methane (CBM) recovery. The variations in pores of Chinese bituminous and anthracite coal were analysed by using low-pressure N2 gas adsorption and CO2 gas adsorption during 20 kHz and 50 kHz ultrasonic treatment. Although the shape of the physisorption isotherm and hysteresis loop, that indicate the pore type of coal, were not changed after ultrasonic treatment, the pore size distribution (PSD) of the treated coal differed from that of the raw coal, especially for bituminous coal with tissue pores, intercrystal pores, and gas pore clusters. An obvious increase in bituminous coal pores was noted, although only a slight change was noted in anthracite pores except for macropores during ultrasonic treatment. The 50 kHz ultrasonic treatment contributed to pore development more than 20 kHz, and created sub-micropores, mesopores, and macropores in bituminous coal fractures, after rapid growth in the treatment process. In the 20 kHz ultrasonic field, these ranges of pores represent continuous and monotonous growth. The range of micropores in bituminous coal increased rapidly before 36 h of ultrasonic treatment, followed by a slow increase. Throughout the ultrasonic treatment process, the largest growth of micropores, sub-micropores, mesopores, and macropores in the bituminous coal were about 148.5%, 418.4%, 228.3%, and 43.6%, respectively, with the higher frequency of ultrasonic treatment; for the lower frequency of the ultrasonic field, the increment values were 115.0%, 179.9%, 103.9%, and 76.4%, respectively. The cumulative pore volume of anthracite showed a 61.9% increase during the 50 kHz ultrasonic treatment after macropore fracturing.

Experimental study the influences of geochemical reaction on coal structure during the CO2 geological storage in deep coal seam

Deep unminable coal seams are suitable geological bodies for carbon dioxide capture and storage (CCS). In order to discuss the geochemical reaction between the injected carbon dioxide (CO2) and deep coal seam, and study its influence on the structure of reservoir, the high-ash anthracite, high-volatile anthracite, low-volatile bituminous coal obtained from Qinshui basin, China, and high-volatile bituminous coal obtained from Bohaiwan basin, China, were processed to 4–8 mm. Then the simulation experiments for the CO2 injection were conducted for 10 days at the burial depth of 1500 m under in-situ reservoir conditions. The conclusions were obtained based on the changes of minerals, elemental concentrations and pore structure parameters of coal before and after the reaction. Different minerals underwent various geochemical reactions and mechanisms after CO2 injection. These geochemical reactions can transform the reservoir structure, which was mainly reflected in the connection of pores and fissures, and the blockage on the pore throat. The reactions between carbonate minerals and the CO2 acid fluid were violent and quick, which can increase the volume of macropores and fractures. While the weak reactions of silicate minerals may affects the micropores in coal. The chlorite and feldspar could continuously react with the CO2 acid fluid, with the slow dissolution of potassium, sodium and silicon, which makes it possible to use these characteristics to monitor the geochemical reaction. Besides, the volumes of the micropores (1–2 nm) in the anthracite were significantly increased, making it a potential geological body for CCS.

Effects of supercritical CO2 on micropores in bituminous and anthracite coal

The effects of supercritical CO2 (ScCO2) on coal micropores play a critical role in determining the CO2 capacity of coalbeds. To investigate the effects of ScCO2 on micropores in coals of different ranks under a range of temperature and pressure conditions, CO2 sequestration processes are replicated using a ScCO2 geochemical reactor. Four samples of coal of different ranks are exposed to ScCO2 and water for 240 h at 62.5 °C and 15 MPa, and CO2 adsorption tests, Fourier transform infrared spectroscopy, and X-ray diffraction are performed to determine the volume and pore size distribution of the micropores, the types and contents of the organic groups, and the aromatic crystallite structures of the coal samples before and after the ScCO2-H2O treatment. The influence of the chemical structure and the organic groups of the coal on the coal micropores is then studied. Micropores with widths <0.46 nm are mainly associated with the inter-layer spacing of the aromatic layers. Swelling caused by ScCO2 detaches heterocycles and non-condensed polynuclear aromatics of high-volatile bituminous coal, resulting in slight increases in the volume of micropores with widths <0.46 nm. The falling off of cross-links with low bond energies caused by swelling and the formation of CArCAr cross-links between the aromatic layers caused by polyaddition reactions together cause a decrease in the volume of micropores with widths <0.46 nm in semi-anthracite and anthracite coal. Micropores with widths >0.46 nm are pores in the macromolecular structure of coal and are known as intermolecular pores, and their volumes are determined by the directional arrangement of aromatic crystallites and the aliphatic hydrocarbon chain length. The broken bonds caused by ScCO2 in high-volatile bituminous coal form new aliphatic or aromatic compounds via addition and polyaddition reactions. These processes increase the directional arrangement of aromatic crystallites, resulting in a decrease in the volume of micropores with widths >0.46 nm. As the coal rank increases, the aliphatic hydrocarbon chain length gives priority to the macromolecular structure of coal after the ScCO2 treatment, resulting in an increase in the volume of micropores with widths >0.46 nm in semi-anthracite and anthracite coal.

Molecular Simulation of H2O, CO2, and CH4 Adsorption in Coal Micropores

Based on the chemical model of coal, slit micropores with different pore sizes are established and structures are optimized in the software of materials studio. As the temperature rises, absolute adsorption capacities of H2O are slightly affected, while absolute adsorption capacities of CO2 and CH4 gradually decrease. As the fugacity rises, excess adsorption curves of CO2 experience increase-decrease-gentle three stages, while the curves of CH4 gradually decrease. With the increase of pore size, adsorption capacities of H2O increase, while adsorption capacities of CO2 and CH4 gradually decrease. H2O firstly adsorbs on the oxygen-containing functional group, so the walls of pore are the preferential area for H2O, while CO2 and CH4 choose to adsorb on–C–C–, therefore the walls are the primary area for CO2 and CH4. Strong potential in micropores and hydrogen bond among water molecules will promote the water adsorption, while the adsorptions of CO2 and CH4 are only induced by the Van der Waals interaction, but the difference between adsorption density and bulk density of CO2 and CH4 decides the change of excess adsorption capacity.

Adsorption heat features of coalbed methane based on microcalorimeter

Basic knowledge of the adsorption thermodynamic process of coalbed methane is conducive to a deep understanding of its adsorption mechanism. In this study, we directly measure the adsorption heats of methane on coals using the microcalorimetry with volumetric method. The adsorption content and adsorption heat of four types of coals and their oxidized and extracted coals were measured at 308.15 K in the 0–3.5 MPa pressure range. The results show that extracted coal has lower adsorption content and adsorption heat than raw coal because of the decrease in micropore volumes, which indicates that the micropores of coal significantly affect the adsorption properties of coal. The release heat from the adsorption process increases the coal temperature, and the change in temperature can be more than 20 K. In the experimental pressure range, the isosteric heat decreases with the increase in adsorption content, and because of the enhanced adsorption potential of micropores, methane is preferentially adsorbed in smaller micropores. In addition, a new method to estimate the pseudo-saturation vapor pressure of supercritical adsorption has been introduced according to the experimental results of isosteric heats of adsorption.

Coal pores and fracture development during CBM drainage: Their promoting effects on the propensity for coal and gas outbursts

This study explored the occurrence of coal-rock dynamic disasters during the low-temperature oxidation of coalbed methane (CBM) reservoirs for different coal ranks by investigating the dynamic development of pores in coal during low-temperature oxidation. This research monitored dynamic changes in the diameters and numbers of pores in different rank coals during low-temperature oxidation using nuclear magnetic resonance (NMR) and the P-wave rock measurement system. Microscopically, by utilizing industrial component analysis technology and gas chromatography, this study determined the dynamic changes occurring in the composition of coal and the concentrations of representative gases in the low-temperature oxidation of different rank coals. By adopting a new comprehensive index K, which is a combination of the coal-rock hardness (f) and the initial velocity (△p) of a gas emission, to predict a gas outburst, the authors predicted coal-rock dynamic disaster hazards in the low-temperature oxidation of different rank coals. The experiment showed that there are similarities and differences in pore development during the low-temperature oxidation of different rank coals. The similarities are illustrated in the consistencies underlying fracture development. Specifically, in the early stage of low-temperature oxidation of coal (30–130 °C), due to the evaporation of water, the dehydration of compounds containing crystalline water and the decomposition and volatilization of volatiles, micro-pores in coal expand and connect to form meso-pores. In the late stage of oxidation (130–230 °C), the macromolecular compounds and volatiles in coal oxidize and decompose such that meso-pores expand and connect to form macro-pores and micro-fractures. However, as the metamorphic degree of the coal increases, the oxidation resistance and thermal stability of the coal improves, and the initial temperature for the development of pores with the same diameter increases. Based on the concept of the comprehensive prediction index K, the probability of a coal-rock dynamic disaster occurring in a CBM reservoir gradually increases as the oxidation temperature increases. The lower the metamorphic degree of the coal, the faster the growth rate. Therefore, the oxidation temperature at which the probability of a coal-rock dynamic disaster increases by 50% is used as the critical temperature for taking fire prevention measures, i.e., 90 °C and 130 °C for lignite and bituminous coals, respectively.

DFT study of CO2 and H2O co-adsorption on carbon models of coal surface.

The moisture content of coal affects the adsorption capacity of CO2 on the coal surface. Since the hydrogen bonds are formed between H2O and oxygen functional group, the H2O cluster more easily adsorbs on the coal micropore than CO2 molecule. The coal micropores are occupied by H2O molecules that cannot provide extra space for CO2 adsorption, which may leads to the reduction of CO2 adsorption capacity. However, without considering factors of micropore and oxygen functional groups, the co-adsorption mechanisms of CO2 and adsorbed H2O molecule are not clear. Density functional theory (DFT) calculations were performed to elucidate the effect of adsorbed H2O to CO2 adsorption. This study reports some typical coal-H2O···CO2 complexes, along with a detailed analysis of the geometry, energy, electrostatic potential (ESP), atoms in molecules (AIM), reduced density gradient (RDG), and energy decomposition analysis (EDA). The results show that H2O molecule can more stably adsorb on the aromatic ring surface than CO2 molecule, and the absolute values of local ESP maximum and minimum of H2O cluster are greater than CO2. AIM analysis shows a detailed interaction path and strength between atoms in CO2 and H2O, and RDG analysis shows that the interactions among CO2, H2O, and coal model belong to weak van der Waals force. EDA indicates that electrostatic and long-range dispersion terms play a primary role in the co-adsorption of CO2 and H2O. According to the DFT calculated results without considering micropore structure and functional group, it is shown that the adsorbed H2O can promote CO2 adsorption on the coal surface. These results demonstrate that the micropore factor plays a dominant role in affecting CO2 adsorption capacity, the attractive interaction of adsorbed H2O to CO2 makes little contribution.