Coal Structure Research Articles

The influence of coal mining subsidence on the movement and deformation of loess slope in the loess gully area of Northern Shaanxi

Introduction: How to solve the contradiction between coal mining and soil and water conservation is a key scientific problem to realize ecological environment protection and high-quality development in the middle reaches of the Yellow River.Methods: Using FLAC3D numerical simulation experiment method, the influence of loess slope surface shape and coal seam overburden structure coupling on slope movement and deformation is studied.Results: Under any surface slope shape, the average slope subsidence coefficient (q slope average) increases with the increase of sand layer coefficient after coal mining subsidence. When the sand layer coefficient is less than 0.71, the q slope average increases rapidly, with an increase of more than 2.86%, and when the sand layer coefficient is greater than 0.71, the q slope average tends to be stable. Under any surface slope shape, the q slope average decreases with the increase of sand-mud ratio. When the overburden structure characteristics of any coal seam and the natural slope of the surface slope are less than or equal to 5°, the q slope average of the convex slope is the largest, and the q slope average of the four slope types is ranked as follows: convex slope > straight slope ≈ composite slope > concave slope; When the structural characteristics of overlying strata in any coal seam and the natural slope of surface slope are more than 5°, the q slope average of concave slope is the largest, and the q slope average of four slope types is in the order of concave slope > straight slope ≈ composite slope > convex slope. With the increase of the natural slope of the surface slope, the q slope average first decreases and then increases, and the inflection point is 15°. The influence law of loess slope surface morphology and coal seam overburden structure on the average horizontal movement of slope surface is similar to that of average subsidence of slope surface.Discussion: The results can provide scientific basis for surface movement and deformation and soil and water conservation in the mining subsidence area of northern Shaanxi in the middle reaches of the Yellow River Basin in China.

Effect of supercritical CO2 transient high-pressure fracturing on bituminous coal microstructure

Carbon dioxide phase change fracturing is an effective permeability enhancement technology, and its influence mechanism on the coal microstructure remains to be further investigated. To explore the impact of CO2 fracturing on the coal chemical structure, the self-developed transient high-pressure fracturing of CO2 test platform was independently developed. Based on X-ray photoelectron spectrometer (XPS), Fourier transform infrared spectroscopy (FTIR) and X-ray diffraction (XRD), the elemental morphology, functional group and crystal structure of the two bituminous coal samples before and after fracturing were analyzed. The results showed that the fracturing process resulted in the disruption of hydrogen bonds within coal, leading to a reduction in the cross-linking density of the coal structure. Moreover, it facilitated the breaking of fatty alkyl side chains and oxygen-containing functional groups, causing the detachment of side chains and shortening of the fatty chains. The interlayer spacing of aromatic layers (d200) of fractured coal decreased, and the average transverse dimension (La), the average stack height (Lc) and the aromaticity (fa) increased. With the improvement of aromaticity, the arrangement between aromatic layers was looser. The volume and basic unit structure of coal crystal nucleus became larger, and the flattening degree increased. The research results contribute to the understanding of the change of coal microstructure under supercritical CO2 transient high-pressure fracturing and provide theoretical support for exploring the methane adsorption capacity of fractured coal.

Investigation of cross-scale characterization of porous structure and fluid index in bituminous coal via microwave-LN2 freeze-thaw cycles

Microwave-LN2 cyclic processing (MLCP) is a clean and high-efficiency anhydrous fracturing technology. The change in the porous structure of coal after freeze-thaw cycles significantly affects development efficiency of coalbed methane (CBM) in low porosity and permeability reservoirs, while there is a lack of in-depth macro-meso-micro studies. The damage mechanism of coal under microwave-LN2 cold and thermal shock and the evolution of pore fluid reservoir space is based on cross-scale pore characterisation. This paper evaluates the multi-scale porous structure variations of coal before and after MLCP was assessed by hydrogen nuclear magnetic resonance (1H NNR) and field emission scanning electron microscopy (FE-SEM). The evolutionary trends in coal pore fluid fugacity space under freeze-thaw cycles was indirectly characterised by T1-T2 spectra. Additionally, utilizing T2 distribution, peak area and pore throat changes reflect the evolution trend of coal pore space. The results indicate that as the cycles increased, the coal free water and total fluid reservoir space gradually expanded beyond the cycle threshold (n > 10), the free-fluid index rose to 25.68 %, and the free fluid saturation increased by 20.39 %. Additionally, the NMR permeability of the coal increased by 98.37–1471.65 %, and the pore throat distribution increased by 28.05 %. Comprehensive analysis showed that as the cycles progressed, the free water space in the pore space increased significantly, the proportion of bound water gradually decreased, and the internal pores and fluid space of the coal were improved. This study provides fundamental support for multidimensional evaluation of the porous structure of microwave-LN2 freeze-thaw cycles.

The genetic characteristics of intermittency and continuity in peat formation and accumulation processes

The formation and accumulation of peat shows the characteristics of continuity and intermittency. The continuity and intermittency of peat formation and accumulation are actually short-cycle features in the evolution of peat swamp environments. In this study, two basins which were formed during the same period of the Paleogene in China. The coal-forming Huangxian Basin in Shandong Province and the Qiongdongnan Basin were selected for comparative analysis using such methods and techniques as drilling lithology observations and logging comparison data. The Huangxian Basin is a continental lake basin, and the Qiongdongnan Basin is a marginal sea basin in the northern part of the South China Sea. The research results revealed that the Paleogene peat formation and accumulation processes in Qiongdongnan Basin were accompanied by intermittency. As a result, thin coal seams were formed, and the content levels of inorganic substances reached as high as 40%. Those findings indicated that the peat swamp environment of the Qiongdongnan Basin had been unstable and had suffered constant damage. However, the peat formation and accumulation in the Huangxian Basin had a high continuity. As a result, the formed coal seams were relatively thick. The intermittency was manifested as interference caused by changes in the peat accumulation environment, without reaching the level of complete destruction. In addition to the ash yield in the coal, the changes in the sulfur content, ash composition index, and elements in the coal had also reflected the characteristics of sediment and water medium changes during the formation of the coal seams. It was determined that those characteristics had marked the sequential changes of the peat and non-organic matter alternation during the peat accumulation processes, as well as revealing the changes in the peat swamp environmental conditions. This was also one of the manifestations of the continuity and intermittency of the peat accumulation process. It could be ascertained from this study’s experimental results that the intermittency and continuity of the formation and accumulation processes of the peat in the study areas were actually caused by the destruction and reconstruction processes of the peat swamps. Those repetitive processes had formed the observed alternations of peat and non-peat substances, finally resulting in the complexity of the coal seam structures, as well as the overall complexity of the coal compositions in the regions.

Structural characterization analysis and macromolecular model construction of coal from Qinggangping coal mine

Understanding the molecular structure characteristics of coal at the molecular level is of great significance to realize the rational utilization and efficient conversion of coal. This paper gives insights into the acquisition of characterization parameters of coal molecular microstructure by testing and analyzing the long flame coal from Qinggangping (QGP) Coal Mine through proximate analysis, ultimate analysis, vitrinite reflectance determination, fourier transform infrared Spectroscopy test (FTIR), X-ray photoelectron epectroscopy test (XPS), carbon nuclear magnetic resonance (13C-NMR) and X-ray diffraction (XRD). The results show that benzene rings in the QGP coal are mainly connected in a disubstituted way, accounting for 36.48%. Oxygen atoms mainly exist in the oxygen-containing functional groups such as the ether C–O, C=O and –COO. Aliphatic hydrocarbons in the aliphatic group are mainly of symmetrical -CHx stretching vibration. Hydroxyl groups are mainly composed of OH–OH and OH–O hydrogen bonds, accounting for 29.21% and 21.53%, respectively. Nitrogen atoms exist in the form of C4H5N. The coal molecular is mainly of aromatic carbon structure, where the ratio of bridge aromatic carbon to peripheral carbon is 0.198. There are benzene, naphthalene and anthracene in the coal molecular structure, and the former two chemicals play a dominating role. According to the analysis results, the molecular formula of the QGP coal is finally determined as C205H181O29N3S. On this basis, the two-dimensional and three-dimensional macromolecular models are constructed with the assistance of simulation software. In addition, the 13C-NMR spectra and densities of the constructed molecular models are calculated, which verifies the rationality of the models. The macromolecular structure model of bituminous coal constructed in this study provides a theoretical model basis for the optimal surfactant.

COMPREHENSIVE INVESTIGATION OF THE FEATURES OF MOLECULAR COMPOSITION AND SUPRAMOLECULAR STRUCTURE OF COAL AT DIFFERENT METAMORPHISM STAGES

The features of the molecular and supramolecular structure of brown and bituminous coals at different metamorphisms stages have been determined using a complex of methods: IR spectroscopy, X-ray diffraction, the kinetics of swelling in solvents, and sorption of molecular iodine. The relations of the supramolecular structure of coals with the indicators of molecular composition are established, the evolution of coal structure in the series of metamorphism, the mechanisms of transport of solvent molecules into the volume of the organic mass of various coals are considered.

Modified network kinetic model for coal pyrolysis with high-value products and low carbon emissions

Pyrolysis is a clean and efficient technology for the conversion of low-rank coal. Its key product is tar with high value. The complex composition of tar limits its further effective utilization. Although most kinetic models can provide the final distribution of three-phase products in coal pyrolysis, an effective prediction method for the specific composition of tar and gas has not yet been developed. Most numerical models serve to improve the total yield of volatile matter and cannot provide theoretical guidance for the improvement of tar quality or the reduction of carbon emission. Based on the basic assumptions of the CPD (Chemical Percolation Devolatilization) model for product molecules structure and generation mechanism, this paper proposed a more sophisticated method for dividing tar composition, which can achieve the division of light, medium, and heavy fractions of tar components according to boiling point. Light gas composition could be obtained with the combination of FG(Functional Group) submodel. The influence of pyrolysis final temperature, heating rate, and environmental pressure was explored for the yield and distribution of products of 18 coals. The simulation results showed that the total yield of tar was affected by both coal structure characteristics and operating parameters. Under conditions favorable for evaporation, such as low pressure and high temperature, the total yield of tar was positively correlated with the average aromatic core size in coal. Under other conditions, tar release was limited by its volatility. Coal with larger average aromatic core size corresponded to less tar yield instead. The pressure in the operating parameters had the most significant influence on the total yield of tar. The heavy fractions of tar are more sensitive to changes in operational parameters, making working conditions that increase the total yield of tar reduce tar quality. The overall carbon emission of pyrolysis is <1 kg CO2/kg tar, which is far lower than the 2–4 kg CO2/kg tar and 3–9 kg CO2/kg tar of direct and indirect liquefaction processes. Under the conditions of sufficient insulation time and no consideration of secondary reactions, the carbon emission of pyrolysis only depended on the content of carboxyl groups in the precursor functional groups. Therefore, the carbon emission intensity decreased with the increase of coal rank. For the optimization for total tar yield and tar quality, 1 atm pressure and 723–923 K temperature was optimal for low rank coals, while low temperature and heating rate with 10–20 atm pressure or high temperature and heating rate with 45–50 atm pressure were optimal for medium rank coals. For the optimization of light tar yield and tar quality, 6–9 atm pressure was optimal for low-rank coals, and 1–3 atm pressure was optimal for medium-rank coals.

Study on the Influence of Coal Structure and Oxidation Performance by Endogenous Bacterium

In order to solve the defects of traditional coal spontaneous combustion prevention technology in a closed goaf, a strain of aerobic endogenous bacteria was isolated from coal and used as a blocking raw material. Based on the metabolic and reproductive characteristics of microorganisms, the experimental study on the inhibition of coal spontaneous combustion by microorganisms was carried out. The colonies were isolated and purified by the dilution concentration plate method and the scribing plate method. The growth morphology of microorganisms was analyzed, and the growth curve was determined. The strains were identified by seamless cloning technology for high-throughput sequencing. The surface morphology of coal was analyzed by SEM, the differences of oxidation characteristic temperature points were analyzed by TG–DTG–DSC images, a programmed heating experiment was used to analyze the concentration of the indicator gas CO, and the changes in microscopic groups before and after microbial action were analyzed by FTIR and XPS spectra. Therefore, the inhibition of coal oxidation by endogenous bacteria was verified from macroscopic and microscopic perspectives. The results show that the coal bacteria isolated from the coal is Lysinibacilus sp. After the culture of Lysinibacilus sp., the surface of the coal demonstrated less detritus, and was relatively smooth. In the early stage of low temperature oxidation of coal spontaneous combustion, the characteristic temperature point of coal oxidation and the reaction between coal and O2 could be delayed by Lysinibacilus sp., and the total heat release was reduced in the combustion process. Not only that, Lysinibacilus sp. could also reduce the CO concentration during coal heating. After the coal was decomposed by Lysinibacilus sp., the C=C thick ring skeleton structure had little effect; however, the aromatic substitution pattern changed. This bacterium had an effect on the C-O bond, reducing the percentage of -CH2- and increasing the percentage of -CH3. It might also use the crystalline water in coal for life activities. The carboxyl carbon in coal changed the most, with a decrease of 12.03%, so it might become the carbon source required for microbial growth. The reproductive metabolism of microorganisms also affected the form of nitrogen, and the percentage of pyridine nitrogen in coal was reduced. The ratio of single-bond carbon to double-bond carbon in raw coal was about 3:2, but after this bacterial action, the ratio of the two was about 1:1. The analytical conclusions of XPS and FTIR spectra were consistent, and the results supported each other.

H2, CH4 and CO2 adsorption on Cameo coal: Insights into the role of cushion gas in hydrogen geological storage

Large-scale hydrogen underground storage is widely considered as an important solution for energy transition to mitigate global warming, however, our understanding on the various chemical and physical processes leading to H2 loss during geo-storage remains largely unknown. We conducted adsorption isotherms measurements of H2, CH4 and CO2 on Cretaceous Cameo coal at 35, 50 and 65 °C, to explore adsorption mechanisms of H2 and deciphering the effect of cushion gas (e.g., CH4, CO2) on H2 loss in the subsurface. The adsorbed amounts of H2 are just 12% and 6% of CH4 and CO2 at temperature of 35 °C and pressure of 2.3 MPa, respectively, and H2 adsorption behavior on Cameo coal can be described well by Langmuir monolayer theory. The extremely high Langmuir pressures (35–49 MPa) for H2 indicate very weak affinity with coal, compared to CH4 and CO2. Differences of Langmuir adsorption capacities suggest that the gas adsorption of H2 and CH4 are mainly by physical interactions while both physical and chemical bonding with coal structures occurs for CO2. The affinity with solid materials for the three gases can be quantitatively evaluated by the heat of adsorption, which was determined to be ∼9.2 kJ/mol for H2. Extrapolation of our experimental results to natural coal seams indicates that H2 presents much smaller adsorption amounts at depth up to 2000 m, corresponding to temperatures lower than 65 °C and pressure less than 30 MPa, geological conditions that are suitable for hydrogen underground storage. This study confirms that the injection of CH4 or CO2 as cushion gas can largely reduce the H2 loss by adsorption in the subsurface. Empirical calculation suggests H2 adsorption will be insignificant with chemical composition of CH4 above 8% or CO2 above 2% at the storage sites (e.g., abandoned mines, depleted coal seams). Our study lays a foundation on the understanding of the risk of H2 loss in large-scale hydrogen underground storage.

Effect of low-temperature pre-oxidation on the self-heating of coal

In order to investigate the effect of low-temperature pre-oxidation on the self-heating of coal, research was made on the temperature rise characteristics of coal samples with different pre-oxidation temperatures, duration and numbers. Besides, in situ FTIR and ESR experiments were also carried out to study changes in active groups and free radicals of raw and pre-oxidized coal. Results show that the pre-oxidation in the 30–60 °C temperature range markedly reduces the oxidation activity of coal and inhibits the oxidation reaction. TTR (total temperature rise) of the pre-oxidized coal is inversely proportional to pre-oxidation temperature, duration and number. The hydroxyl and aliphatic hydrocarbons continue to decrease in content during the oxidation reaction. The C-O group and carbonyl group, as the main oxidation products, show an increasing trend. The trend of group content in the pre-oxidized coal samples is significantly smaller than that of the raw coal, indicating the activity of active groups is weakened. The variation in Ng for coal samples decreases eventually after pre-oxidation, suggesting that initial oxidation makes the coal molecular structure more stable. This study helps to understand further the effect of low-temperature pre-oxidation on the self-heating of coal and provides a theoretical basis to use the pre-oxidation method to prevent coal spontaneous combustion.

Investigation on the influence of the macropores in coal on CBM recovery

As an important component of coal structure, the macropores have a great influence on CBM recovery. In this paper, the macropores characteristic of two coal samples collected from 3# coal seam in the south of Qinshui Basin, China was analyzed on the basic of mercury intrusion porosimetry, and a fully coupled triple-porosity/double permeability mathematical model for CBM recovery was established according to the physical structure of coal and the non-Darcy flow of methane in macropores. Then, the various factors affecting the macropores permeability were discussed and the influence of size distribution and connectivity of macropores on CBM recovery was investigated. The following conclusions have been drawn from these efforts: (1) in 3# coal seam of the south of Qinshui Basin, the macropores have an extremely heterogeneous pore size distribution with the high variation, and their connectivity is not good because they are mainly composed of the conical and cylindrical pores with one dead end and the open pores, the structure characteristics of macropores are not conducive to CBM recovery; (2) the fully coupled triple-porosity/double permeability mathematical model containing the non-Darcy flow of methane in the macropores includes the methane occurrence-migration field, hydraulic field, thermal field and stress field as well as the complex intercoupling between them, and the model was verified by the fitting of methane production history, with an average error of 3.24%; (3) the macropores permeability is closely related to the Knudsen number controlled by methane pressure and temperature in macropores and the intrinsic permeability which is an internal attribute of macropores affected by size distribution and connectivity of pore; (4) the pressure drop of reservoir plays a major role in the macropores permeability, which promotes the increase of macropores permeability with time, and the high intrinsic permeability of macropores corresponding to the good pore size distribution and connectivity is more conducive to the improvement of fracture permeability and methane production rate of coal reservoir during CBM recovery. It is recommended that coal seams including the macropores with uniform size distribution and good connectivity should be preferentially used for the development of CBM.

Study on Molecular Model Construction and Gas Adsorption Simulation of Anthracite in the Qinshui Basin.

Uncovering gas adsorption characteristics of coal at the molecular scale is of great theoretical significance for the study of gas occurrence, coalbed methane exploitation, and carbon dioxide sequestration. In this study, based on proximate analysis, ultimate analysis, 13C nuclear magnetic resonance, and Fourier-transform infrared spectroscopy experiments, the existence forms and relative contents of elements of anthracite in the Qinshui Basin were tested and analyzed, and a macromolecular structure model was constructed. Besides, three types of acidic oxygen-containing functional groups, namely, carboxyl groups, phenolic hydroxyl groups, and lactone groups, were added to the molecular model. Furthermore, CH4 adsorption simulation was conducted on the original molecular model of anthracite and models with three types of acidic functional groups added. The following research results were obtained. The molecular formula of the constructed macromolecular model of anthracite in the Qinshui Basin is C193H138N2O7. The molecular structure of coal becomes more compact and curved after structural optimization and annealing optimization. For the four models, the CH4 adsorption characteristics of coal molecules all conform to the Langmuir equation under the same simulation conditions. Among them, the original model has the largest CH4 adsorption capacity, while the addition of oxygen-containing functional groups reduces the CH4 adsorption capacity to varying extents. The reduction of CH4 adsorption capacity follows the order: adding carboxyl groups > adding phenolic hydroxyl groups > adding lactone groups, which is mainly attributed to the different adsorption heats and adsorptive potential wells triggered by the addition of acidic functional groups in molecules.

The hydro-modification mechanism of subbituminous coal

The hydro-modification mechanism of subbituminous coal was investigated by the isotope trace method. The results show that the yield and caking index (GRI) of modified coal increase to 95.96 wt% and 81 under H2-THN, respectively. Both of them are higher than that under H2-1-MN or N2-THN, which also is consistent with the hydrogen consumption. Meanwhile, the linear regression between the GRI of modified coal and the yield of extracts is 0.9841, which indicates the presence of bonding components in the extracts. Furthermore, the IRMS analysis of modified coal under different conditions further confirmed that more hydrogen atoms from H2 and THN enter into the modified coal with high GRI. Through the 2H NMR and IRMS analyses of the extracts and calculation of hydrogen contribution rate, more hydrogen atoms enter into the possible bonding components, and most of them arrive in that in THF soluble. The hydrogen atoms from H2, transferred by THN, are combined with the macromolecular structure of coal, and the hydrogen atoms from THN enter into possible bonding components. Moreover, the hydrogen atoms in α positions of THN are abstracted at first, and then β positions, which are found from the 2H NMR analyses of THN-D4 before and after reaction. Based on the above, the hydro-modification mechanism of subbituminous coal is proposed.

A systematic exploration of mapping knowledge domains for free radical research related to coal

In order to better grasp the development and trends of free radical research related to coal. Based on 820 publications collected from the core collection database of Web of Science (WoS), this paper systematically analyzed the free radicals research related to coal through the method of bibliometrics, and used VOSviewer and CiteSpace to plot knowledge domain maps. The results indicate that this research topic has gone through four stages of budding, slow development, accelerated development, and rapid development, the current number of papers is increasing exponentially. The better development and utilization of coal resources is the driving force for the development of this research field, and the exploration of coal structure and reactivity is a key first step. Coal spontaneous combustion and prevention, co conversion and utilization of coal-biomass, coal pyrolysis, environmental hazard and coal geochemistry are the five main research branches in this field, which have expanded from the macro level to the micro level. The first three research branches among them are the most popular and will continue to receive more attention in the future. In addition, we further revealed the emergence and evolution of various research branches and identified their research frontiers.

Analysis of the Pore Structure and Fractal Characteristics of Coal and Gas Outburst Coal Seams Based on Matrix Compression Correction

The comparative analysis of coal samples under different gas occurrence conditions systematically reveals the pore structure characteristics of coal and gas outburst coal seams. The functional relationship between R0,max and Kc was clarified using mathematical statistical methods, and the pore structure and fractal characteristics of coal and gas outburst coal seams were analyzed on the basis of modified mercury pressure data and fractal analysis. The results show that the functional relationship between R0,max and Kc is consistent with y = 1.59x−0.48, and when the mercury inlet pressure is 10~120 MPa, the coal sample is the most affected by the matrix compression effect. The average pore volume and average specific surface area of the outburst coal samples were 41.71% and 23.09%, which is greater than those of the non-outburst coal samples, respectively, and the specific surface area and pore volume provided by the outburst mining micropores were 46.24% and 81.67%, respectively. The fractal dimension, D, of the coal seam increased with the increase in metamorphism, and compared with low gas mines, the fractal dimension of coal samples in the coal and gas outburst mines was higher, the influence of the matrix compression effect was more obvious, and the heterogeneity was stronger.

Deep learning interprets failure process of coal reservoir during CO2-desorption by 3D reconstruction techniques

This work aims to investigate the failure and mechanical behaviors of coal rocks during CO2-desorption. The unconfined and confined compression experiments with certain depletion pressure-stress path and desorption of CO2 are conducted, in which the cracking process is captured by X-ray CT imaging. A deep learning-based coal fine-crack segmentation (CFCS) network with 3D reconstruction technique is proposed to study the cracking behaviors. In addition, the CO2 evolution effects on the failure process and mechanical responses are also analyzed. Results show that the proposed CFCS can effectively segment the crack-CO2-soid structure in coal with the errors all less than 10% at the range of 3.5620%–9.9967%. The failure process of coal samples can be divided into the extremely fast CO2-desorption + microcracks formation stage, fast CO2-desorption + stable microcracks propagation stage and slow stable CO2-desorption + microcracks coalescence to form macrocracks stage. The shear failures of coal samples are caused by the reduction of lateral effective stress due to CO2-desorption and drainage, which increases the anisotropy of coal samples with microcracking behaviors. The proposed method provides better understanding of failure mechanism and mechanical responses of coal reservoirs, which are helpful for the CO2 storage-based resource explorations.

Molecular insights on influence of CO2 on CH4 adsorption and diffusion behaviour in coal under ultrasonic excitation

Decoupling the molecular structure from the pore structure is challenging under experimental conditions; this hinders the independent investigation of the effect of the changes in molecular structure on methane (CH4) transport. In this study, the molecular model of coal was reconstructed based on the test results of X-ray photoelectron spectroscopy (XPS) and 13C nuclear magnetic resonance (13C NMR). A grand canonical Monte Carlo (GCMC) based molecular dynamics (MD) simulation method was used to independently investigate the effect of CO2 on CH4 adsorption and diffusion behaviour in coal under changes in the molecular structure of coal after ultrasonic excitation. The experimental results show that, under ultrasonic excitation, the proportion of oxygen content in the coal increases and the oxygen-containing functional groups (carboxyl groups, ether bonds/hydroxyl groups) increase compared to the original coal. The simulation results show that the amount of CH4 adsorbed, the isosteric heat and the proportion of low-energy CH4 in the coal sample decrease after ultrasonic excitation for the same pore size. Furthermore, for the same pore size and CO2 partial pressure ratio, adsorption selectivity factor increases after ultrasonic treatment. Additionally, for the same pore size and the same CO2 partial pressure ratio, the interaction energy between CH4 and coal in the CH4–CO2–coal mixed system decreases and the density of high-energy CH4 increases after ultrasonic excitation; this leads to an increase in CH4 self-diffusion and transport diffusion coefficient. The presence of oxygen-containing functional groups causes a significant increase in the electrostatic potential difference between CO2 and coal as compared to that between CH4 and coal, which is the fundamental reason for the change in the effect of CO2 on the adsorption-diffusion behaviours of CH4. Our findings revealed the micro-mechanism of ultrasonic-assisted coalbed methane extraction and demonstrated the effectiveness of ultrasonic-CO2 collaborative technology.

Study on the influence of different-voltage plasma breakdowns on functional group structures in coal

Promoting gas desorption is the key to improving gas extraction efficiency, physical discharge-based plasma carries massive joule heat, and inevitably affects the structures of functional groups in coal, which has a certain impact on the adsorption and desorption of gas, but existing literature lacks in-depth research on this. This study explores the changes of functional groups on GHS high metamorphic anthracite and HL low metamorphic bituminous coal samples under different breakdown voltages (BVs), from 15.3 kV to 17.3 kV, which serves as the key to determining the amount of energy injected into plasma channels. This study also explains the influence of BV on functional groups in coal from the perspective of electrical parameters such as voltage, current waveform and deposited energy. For two kinds of coal samples, the results reveal the following: (1) With the breakdown voltage increases, the coal is more likely to be polarized and the pre-breakdown period is shorter; accordingly, the coal can be broken down more easily. (2) The peak current increases linearly with the rise of BV, and the energy conversion efficiency jumps first and then tends to stabilize; (3) The plasma-induced oxidation reaction degree of the coal surface gradually strengthens as the BV rises, because more energy is used for reactions on the coal surface. Specifically, the higher the BV is, the faster the contents of hydroxyl and oxygen-containing functional groups decline, and the faster the aliphatic hydrocarbons in the coal decompose.

A preliminary study on the quality evaluation of coking coal from its structure thermal transformation: Applications of fluidity and swelling indices

Evaluating the coking coal quality from coal structure thermal transformation was crucial to control the coking process. The coal quality indices of ash composition, volatile matter, element distribution and caking indices were redefined based on the coal structural characteristics. Five structure parameters derived from volatile matter and element distribution, including carbon aromaticity (fa), average number of condensation rings (NCR), ring condensation index (2(NCR −1)/C), aliphatic hydrogen content (AlH) and number of bridge bonds (AlB), were proposed to evaluate the comprehensive structure properties of coals. The rationality of these parameters was verified through 13C NMR analysis. The superiority of fluidity and swelling indices in reflecting the structure transformation of individual and blended coals was clarified. In addition, a novel mothed for measuring swelling pressure was developed, taking into account the impact of heat and mass transfer on the coking process. The results show that the bridge bonds content (AlB) affects the coal particle softening which attributes to the thermal cleavage of Cal-Cal/O and Car-O. The high aliphatic hydrogen content (AlH) relating to the stabilization of pyrolytic fragments decreases maximum fluidity temperature. The formula of F(MD) captures the relevance among maximum dilatation, volatile matter, caking and plastic layer indices, which aids in comprehending the swelling mechanism based on the balance between gas release and permeability evolution. The fluidity and swelling indices serve as indicators of the structural transformation during individual coal coking, contributing to the quality evaluation of blended coals with similar coke performance. The self-designed device provides a more accurate prediction of pushing current based on the maximum swelling pressure, compared to the Audibert-Arnu dilatation. This work contributes to the scientific evaluation of coking coals and the precise control of the coking process.

Variations in the pore structure and fluid mobility under anionic surfactant assisted matrix acidification of coal based on nuclear magnetic resonance T1-T2 spectra

Matrix acidification is a potential method for increasing the permeability of coalbed methane reservoirs. However, current research on the acid corrosion process is still unclear, and acid solutions can cause water blocking damage to reservoirs. Therefore, this article obtained a T1-T2 joint spectrum through nuclear magnetic resonance (NMR) test and conduct contact angle (CA) test to analyze the pore structure and fluid mobility evolution of bituminous and anthracite coal under sole acidification and Sodium secondary alkyl sulfate (SAS) surfactant assisted acidification. The results show that the acidification process exhibits two distinct stages. Stage 1: acidification mainly expand the pore space of the free fluid, while stage 2: acidification significantly increases both the bound fluid and free fluid space. After sole acidising, the porosity of bituminous and anthracite coal increased by 4.44% and 31.31%, while increased by 18.28% and 38.35% under SAS assisted acidification, respectively. The adding of SAS resulted in better connectivity of coal samples, with pore throat (>0.1 μm) of bituminous and anthracite increasing by 56.02% and 351%, respectively. Under SAS assisted acidification, the maximum decrease of T2cutoff value of bituminous and anthracite coal is 62.79% and 20.28% respectively. Surfactants can effectively alleviate the negative impact of acidification on coal surface wettability. The results can provide references for field application of matrix acidification in coalbed methane reservoir.