The decarbonization of fossil fuel is crucial for balancing energy security and carbon management. Underground coal gasification (UCG) is a promising process because of its potential for mitigating ecological impacts, but its conventional autothermal approach with oxygen injection still generates substantial CO 2 . By utilizing thermal plasma to simultaneously provide heat and highly reactive species, can enable rapid coal seam heating and drive directed carbon-reduction pathways without carbon combustion, thereby effectively mitigating CO 2 generation. Experimental results demonstrate that, plasma-driven underground coal gasification (PUCG) initiates within 1 min, producing >94% effective gas and reducing CO 2 concentration by over 86%. Under plasma treatment, aromatic clusters in coal undergo cleavage and recombination into longer, more planar polycyclic assemblies, then carbon defect sites develop on these aromatic clusters, driving the carbon structure toward a more reactive state. The carbon defects serve as attack sites for reactive species, promoting the formation of oxygen-containing functional group (C O ). The activation energy for the reaction between a coal model compound and OH· species to form syngas is only 27.65 kJ·mol −1 . Compared with coal gasification by ground-state water, the activation energy is reduced by more than 80%. Therefore, PUCG offers a promising novel pathway for low-carbon transition of coal-based energy. • A novel in-situ enhanced coal seam gasification system via plasma for H 2 production • Plasma-driven gasification achieves >94% effective gas with >86% CO 2 reduction • Active carbon sites are continuously generated under conversion • Reactive oxygen-containing functional group (C O ) formed under plasma conditions • Activation energy for gasification is reduced by over 80%

Application of borehole-to-roadway DC resistivity tomography in detecting mining-induced floor damage in coal seams

Study of structure evolution and the promotion of coalbed methane extraction in hydrothermally fractured coal seam

Secondary fractal evolution mechanism of coal seam fractures and seepage under mechanical cavitation disturbance: Implications for CBM extraction

Fault slip induced by coal seam mining is a major hazard that can trigger dynamic disasters such as coal bursts. Understanding the evolution of shear and normal stress on the fault plane is essential for revealing the mechanical mechanism of mining-induced fault activation. Based on physical experiment and numerical simulation of the 21,221 mining face in Qianqiu coal mine, this study investigates the stress spatiotemporal response during mining face advancement. Results show that shear stress fluctuates in repeated cycles of abrupt drop followed by rapid rise, with cycles becoming less frequent and intense farther from the coal seam, indicating a distance-dependent disturbance effect. Mining activity also induces migrating stress relief and concentration zones, the relief zone expands toward the near coal seam side while the concentration zone shrinks and shifts away. A rise in normal stress accompanied by a drop in shear stress serves as a key slip precursor of fault, whereas a simultaneous sharp decline in both shear and normal stress marks fault instability and energy release. Therefore, coupled monitoring of normal and shear stress evolution law provides valuable early warning of fault slip risk.

In coal seams with high gas content, gas impeded pore wetting, resulting in a poor wetting effect and weak gas-liquid competitive adsorption during water imbibition. Although surfactant water was widely used to enhance coal wettability, existing studies mainly focused on surface wettability, while the internal imbibition-driven wetting and associated gas displacement in gas-bearing coal remain poorly understood, limiting its application in coal seam gas control. A gas-bearing coal imbibition testing system was developed to quantitatively characterize the imbibition characteristics of the surfactant water. For both water and surfactant water imbibition, the gas displacement amount increased with gas pressure, but the gas displacement ratio decreased. Under constant surfactant water concentration, higher gas pressure inhibited gas desorption in the pore with one end open and the other end closed (Pooc), but promoted it in a pore with both ends open (Pobe) due to restricted diffusion in the Pooc and enhanced gas migration in the Pobe. For the same gas pressure, higher surfactant concentrations increased both the gas displacement amount and the ratio. Phase-field simulations confirmed that surfactant water imbibed farther and exhibited stronger wetting effects compared with water. When coal seams primarily consisted of open-ended pores with limited connectivity, gas control could be achieved by synergistically combining permeability enhancement measures with surfactant-water injection technology. The results provided theoretical support for the application of high-gas coal seam surfactant water injection in gas control.

Retraction notice to "Study on the effect of SDBS and SDS on deep coal seam water injection" [Sci. Total Environ. 856 (2023) 158930

To address the ambiguity in the coupling mechanism between stress and gas seepage in the upper protected coal seam during deep and long-distance protective layer mining, this study takes the V8 coal seam (protective layer) and IV5-6 coal seam (upper protected coal seam) of Pingmei No.6 Mine as the research objects. Based on the theories of gas adsorption-desorption, rock mechanics and seepage mechanics, a fluid-solid coupling model for gas-bearing coal was constructed. The FLAC3D numerical simulation software was adopted to investigate the stress variation characteristics of the long-distance upper protected coal seam under different mining distances of the protective layer, while the COMSOL Multiphysics software was used to analyze the permeability evolution law of the aforementioned coal seam under the same mining conditions. The reliability of the established model was verified by field measurement data. The results show that with the advance of the protective layer working face, the pressure relief zone of the upper protected coal seam expands continuously, and the stress distribution gradually transitions from a "V" shape to a "U" shape. The permeability presents a "W"-shaped evolutionary trend with the characteristics of "reduction-increase-reduction-stabilization". After pressure relief, the simulated permeability value of the coal seam reaches 18.53 × 10⁻1⁷ m2, with a relative error of only 9.83% compared with the field measurement results. During gas extraction, the gas pressure in the coal seam shows an elliptical distribution pattern centered on the drilling hole, and the effective extraction radius increases with the extension of extraction time. The average error between the simulation results and actual measurement data of the effective extraction radius is 4.81%. The fluid-solid coupling model established in this study and the revealed evolution laws of stress, permeability and gas seepage provide accurate theoretical support and a solid technical foundation for the optimal design of pressure relief gas extraction in long-distance protected coal seams.

Goaf gas drainage and residual-coal spontaneous-combustion prevention are often designed independently, even though both are controlled by the same leakage-flow, oxygen-transport and heat-release fields in a longwall goaf. This decoupled design may reduce methane accumulation while unintentionally enlarging the oxidation zone. Taking the No. 1217 fully mechanized working face of Zhongxing Coal Mine, Shanxi Province, China, as an engineering prototype, this study develops an integrated laboratory-field numerical framework to quantify the drainage-induced evolution of the three zones of spontaneous combustion. Programmed temperature-rise experiments on the No. 2 coal seam were used to determine the oxygen-consumption rate, heat-release intensity and apparent activation energy under oxygen concentrations of 3–21%, yielding a critical oxygen concentration of 5.9%. Bundle-tube monitoring and distributed optical-fiber temperature sensing delineated the in situ three-zone boundaries, and a three-dimensional CFD model coupling porous-media seepage, species transport and Arrhenius-type heat generation was validated against the field data, with most relative errors below 5%. Parametric simulations for buried-pipe depths of 20, 30 and 50 m and negative pressures of 15 and 20 kPa reveal a pronounced asymmetric response: drainage compresses and advances the return-side oxidation zone toward the working face, but drives the inlet-side oxidation zone deeper into the goaf by enhancing oxygen-bearing leakage. Within the investigated parameter space, a buried depth of 30 m and a negative pressure of 20 kPa provide the best compromise, reducing the return-side oxidation-zone width from 32 to 21 m and the upper-corner methane concentration from 6.80% to 0.58%. The results demonstrate that drainage design should be constrained simultaneously by methane dilution and oxidation-zone control, and provide a quantitative basis for coordinating gas extraction with fire prevention in gas-rich, oxidation-prone longwall panels.

It has been confirmed that the source of germanium (Ge) in brown coal is the crystalline rocks and their weathering crusts of the Ukrainian Shield. As a result of erosion in the hypergenesis zone, mobile soluble compounds in the form of oxoanions and a solid phase in the form of suspensions were produced, entering the peat bog with groundwater and surface waters. In coal, Ge can occur in a non-mineral sorbed form, as simple and complex humates of the chelate type, and as organogermanium compounds. A minor amount of Ge enters the composition of accessory (garnet, pyrite) and rock-forming minerals (pyroxenes, feldspars) as isomorphic impurities. The relationship of Ge predominantly with gelified macerals of the lignitite and huminite groups has been established. A negative correlation between Ge content (on an ash basis) and ash yield was confirmed, along with the absence of a stable correlation when calculated on a whole-coal basis. This is explained by the ratio of the sorptive ash (the Ge carrier) to the terrigenous ash. The established 'sorption optimum' for the Dnipro Basin coal is observed at an ash content within 30–35%. Using the coal seam of the Verkhnyodniprovske field as an example, it was shown that the Ge content in coal on the periphery of the deposit is higher than in its central part, which is due to the relative proximity of the marginal parts of the field to the source area. Using the same field as an example, the germanium enrichment of the roof and floor parts of the seam, as well as of coal plies between intra-seam rock partings, was proven. An inverse relationship was recorded between the Ge content and the coal seam thickness. It is shown that open-pit coal mines within the hypergenic process zone are characterized by an uneven distribution of Ge. The high correlation of Ge with the resin content of the benzene extract suggests the possibility of recovering Ge (along with gallium (Ga)) during the technological processing of bitumen. In general, the bitumen-rich coal of the Dnipro Basin, with reserves estimated at 280 million tons, contains about 590 tons of Ge.

Under the condition of slicing mining in extra-thick coal seams, the presence of irregular coal pillars is likely to cause stress redistribution in the roadway region and induce rockburst. Taking the "3·22" rockburst event that occurred in the haulage roadway of the 250,101-2 working face in Huating Coal Mine as the engineering background, this paper investigated the occurrence mechanism of roadway rockburst under irregular coal pillar conditions by combining source mechanism inversion, numerical simulation, and theoretical analysis. The results show that the double-couple component is dominant in the moment tensor inversion results, indicating that the source type of this event was shear-type, and that the essence of the rockburst instability was the sudden shear slip of the coal-rock mass under high-stress conditions. The PFC simulation results show that, under the control of an L-shaped irregular coal pillar formed by a 20m residual section pillar and a 34m residual pillar, the overburden load developed a zonal load transfer pattern. Specifically, the 20m pillar constituted the main load transfer channel, while the compacted zone above the 34m pillar regained a certain bearing capacity after compaction of the caved rock mass and exerted an auxiliary reloading effect on the underlying surrounding rock, resulting in the haulage roadway not being in a fully destressed state. On this basis, a two-segment bearing model of the L-shaped irregular coal pillar was established, and the static stress distribution characteristics in the roadway region under the combined action of the two-segment loads were analyzed based on half-plane elasticity theory. Furthermore, by incorporating the attenuation law of vibration waves, a stress increment estimation model under dynamic loading disturbance was established, and the dynamic stress increment generated on the roadway surface by the "3·22" rockburst event was calculated to be about 4.06MPa. Finally, the stress concentration characteristics and stress deflection effect under the control of the L-shaped coal pillar structure were discussed. The results show that an increase in the right-wing thickness of the L-shaped coal pillar structure enlarges the stress deflection zone and enhances stress redistribution toward the roadway, thereby increasing the possibility of rockburst under the combined action of high static stress and dynamic disturbance. The research results reveal the occurrence mechanism of roadway rockburst under L-shaped irregular coal pillar conditions, and can provide a theoretical reference for identifying rockburst hazard zones and optimizing working face layout parameters under similar engineering conditions.

ABSTRACT Investigating the geochemical behavior of trace elements in coal under high-temperature conditions not only enables the reconstruction of their geochemical characteristics during thermal evolution but also holds great significance for the prevention and control of coal-derived pollution and the resource utilization of high-value-added elements. This study takes the No. 6 coal seam of the Guanbanwusu Mine in the Jungar Coalfield as the research object. Based on thermal simulation experiments at approximately 500°C, combined with sequential chemical extraction and multiple analytical methods, the modes of occurrence, migration, and enrichment of trace elements during thermal simulation are revealed. The results show that among 48 trace elements, except for Sr, U, and As, the concentration coefficients of the remaining elements in partings are higher than those in raw coal. In raw coal, 23 elements, including Li, Ga, Sr, Zr, In, Pb, Th, U, most rare earth elements and yttrium (REY), have contents higher than the average values for Chinese coals, warranting focused investigation. As the thermal simulation temperature increases, moisture and volatile matter decrease significantly, while ash yield increases. The trace element contents in semicoke show an increasing trend, indicating that trace elements do not readily migrate with gaseous and liquid products but are more likely to remain in the semicoke. Sequential chemical extraction reveals that in raw coal, most trace elements are primarily present in the silicate fraction, followed by carbonate and organic-bonded fractions. With increasing temperature, the proportion of organic-bonded trace elements in semicoke increases, while the proportion of silicate fractions decreases, which is attributed to enhanced complexation and physical encapsulation within the organic matrix. At each thermal simulation temperature, elements with mobility above the average are classified as highly mobile, whereas those below the average are considered weakly mobile. A comparison of relative migration capacities shows that Pb, Y, Li, Zr, Pr, Th, Tm, Er, Gd, Ga, and La exhibit stronger migration ability than the average, while Lu, Dy, Ho, Yb, Nd, Tb, Ce, In, Sm, U, Sr, and Eu exhibit weaker migration ability than the average. The migration process of elements is jointly influenced by temperature, raw coal properties, thermal stability of minerals, and modes of occurrence.

Recently, CH4 displacement from coal seams through flue gas injection has attracted increasing attention. However, the influence mechanism of different gases on the CH4 displacement behavior remains a matter of debate. This paper systematically studied the basic properties of coal, their adsorption behaviors toward gases, permeability, and the CH4 displacement behaviors through different gas injections under triaxial stress. The results showed that the CH4 displacement quantities through pure N2, N2-CO2 mixtures (85%:15% and 95%:5%), and pure CO2 injection from the tested bituminous coal were 798.3, 825.7, 868.7, and 875 mL at 2 MPa triaxial stress and 620.7, 711.4, 750.1, and 704.5 mL at 6 MPa triaxial stress, respectively. This indicated that pure CO2 injection achieved good displacement efficiency for the coal sample with a well-developed mesoporous structure under lower triaxial stress; however, but under higher triaxial stress, N2-CO2 mixture injection achieved greater displacement efficiency owing to the favorable permeability of N2 and the promotion of adsorbed CH4 desorption by coexisting CO2. Furthermore, as for lignite and anthracite with poor mesoporous structures, the displacement efficiency by pure CO2 injection decreased, respectively, from 705.2 and 1378.7 mL at 2 MPa triaxial stress to 513.2 and 783.8 mL at 6 MPa triaxial stress because of their poor migration ability, even being worse than that through pure N2 injection (527.4 and 1136.8 mL at 6 MPa triaxial stress). These results can provide a theoretical basis for selecting the appropriate gas when applying gas injection technology to different coal seams and confirm the feasibility and necessity of the N2-CO2 mixture injection.

Low permeability of China’s coal reservoirs and the high adsorption capacity of coalbed methane represent core constraints on coalbed methane extraction. Microwave heating technology has enhanced recovery rates in low-permeability reservoirs. However, existing studies typically simplify coal’s dielectric constant as a fixed value and adopt a single-point heating injection pattern. This study proposes a novel coalbed methane enhancement technique, based on directional drilling and continuous tubing technology, enabling large-scale coal seam heating. A fully coupled electromagnetic-thermal-fluid-solid model, incorporating the actual temperature-dependent dielectric properties of coal seams, was established. This model simulates and thoroughly investigates the enhancement and migration patterns of coalbed methane under varying microwave power levels. Results show microwave heating significantly promotes methane desorption and enhances reservoir permeability. 1600 W heating for 120 days, the effective desorption radius exceeded 3.6 m, with maximum permeability increases near the wellbore reaching 18.4%. Under 2400 W microwave irradiation, cumulative gas production reached 5781.5 m3, 15.7% higher than the 4996.3 m3 produced under the non-microwave heating. Due to excessively high power may cause overheating, 1600 W was determined as the optimal power level, balancing efficiency and safety. The study provides both theoretical foundations and practical strategies for optimizing microwave-enhanced coalbed methane recovery rates.

Coal seam gas content is a crucial parameter for ensuring coal mine safety and accurately assessing coalbed methane reserves. With the increasing demand for advancing gas control in mining faces, the depth of boreholes for gas content determination continues to extend, making it difficult to guarantee short sampling times. Consequently, existing gas loss compensation methods struggle to apply to prolonged exposure loss estimation. To address the limitations of current methods, which rely heavily on initial desorption data and are inadequate for long-duration loss estimation, a transient gas diffusion equation incorporating gas adsorption effects was established. The relationship between the dimensionless time number T and the dimensionless desorption quantity number Y was analyzed. Furthermore, novel loss compensation methods based on the tangent at the initial measurement point and the tangent at a dynamic point were proposed. The results indicate that the developed diffusion model can effectively characterize long-term gas diffusion behavior. The T-Y relationship conforms to a power-law function, but its exponent varies with the selected time interval; fixing the exponent leads to significant errors. Directly extrapolating the loss amount by fitting field desorption data to a power function introduces substantial inaccuracies. Therefore, the tangent at the initial point of the field desorption curve was used to substitute for the desorption curve during the exposure period for loss estimation (i.e., the Initial Measurement Point Tangent Method, CQSB). Comparative analysis revealed that the calculation error increases with longer coal sample exposure time, making this method suitable only for scenarios with short exposure times (less than 2 min). An improved method was proposed, which involves using the fitted power function to predict the slope at a dynamic point within the exposure period and then utilizing the tangent at this point to calculate the gas loss (i.e., the Dynamic Point Tangent Method, DQSB). This method enables accurate determination of gas loss under prolonged exposure conditions (3-30 min). This research outcome provides a new approach for estimating gas loss during deep hole drilling and sampling processes using either open or sealed core barrels. However, how to theoretically determine the location of the dynamic point without relying on experimental data, and whether the method proposed in this paper is applicable to lump coal samples containing fractures, require further research and experimental verification.

To address the insufficient bearing capacity of roadside backfill bodies and the tilting or failure induced by uneven pressure relief of the coal seam during gob-side entry retaining in thick coal seams with hard roofs, combined compression-shear loading tests incorporating rapid resistance build-up and varying inclination angles were performed. A novel Compression Shear Coupling Test system (CSCT) was developed, and a fitted relationship between backfill width and roof subsidence was established. The strength degradation behavior of backfill specimens subjected to different shear stress components was systematically investigated. The results reveal that the peak strength of the specimens declines with increasing shear stress component, and the failure mode transitions progressively from compressive to shear-dominated failure. The high-resistance backfill material derived from this study was implemented at the N2302 gob-side entry retaining working face, accompanied by an anti-tilting design for the backfill wall. The measured roof subsidence was reduced by 59.4% relative to the theoretically predicted value, and no evident signs of failure or deterioration were observed in the backfill body. These findings provide both data support and theoretical reference for gob-side entry retaining under similar mining conditions.

SiO2 nanofluid has significant potential for weakening coal mechanical properties and improving coal seam water injection efficiency, while prewetting time critically governs its modification outcomes. Accordingly, this study systematically investigated the effects of nanofluid prewetting time on coal mechanical performance, energy dissipation, and fracture propagation. Furthermore, Langmuir adsorption theory and Young's equation were employed to elucidate the fluid dynamic mechanism underlying time-dependent structural weakening of the modified coal. Results demonstrate that nanofluid prewetting treatment significantly reduces dynamic contact angles on coal surfaces and effectively enhances hydrophilicity. Nanofluid prewetting diminishes coal resistance to deformation, with the maximum reduction rate occurring at 2 h prewetting, corresponding to the lowest elastic modulus and peak stress alongside intensified lateral expansion. Post-treatment cumulative and maximum ring-down counts decrease substantially, indicating that internal cracks slide and propagate more readily during loading, thereby alleviating instantaneous elastic energy release upon failure. Moreover, fracture development reaches its maximum at 2 h prewetting, with failure mode transitioning from tensile-dominated to shear-dominated behavior. Nanoparticle adsorption on coal surfaces initially increases and subsequently stabilizes with prolonged prewetting time. However, prolonged prewetting time leads to aggregation and deposition of the nanofluid. The channels for water infiltration become narrowed. The enhancement of wetting modification is thereby restricted. In summary, prewetting time modulates nanoparticle adsorption and aggregation behavior, thereby influencing coal water absorption and structural weakening degree, ultimately determining the failure mode transition of the modified coal. The research findings provide foundations for optimizing prewetting time in coal seam water injection.

Rising atmospheric CO₂ emissions from fossil fuel-based industries have intensified the need for effective mitigation strategies, with Carbon Capture and Storage (CCS) in geological formations, particularly un-mined coal seams necessitates specialized research into numerous critical factors that are equivalent thorough characterization of coal properties such as porosity, permeability, uniaxial compressive strength (UCS), poisson ratio, and brittleness. This study serves as a preliminary assessment of the CCS potential of Coal Seam D in the Suban Burung Block, South Sumatra Basin, using drilling data from four wells (within area 960 m × 860 m). This study aims to describe the coal seam D strata for underground CO2 storage. Coal Seam D was identified from core sample data, with laboratory analyses conducted to determine density and porosity values. Then, in this study we predict velocity (Vp and Vs), UCS values, and BI using petrophysical and rock physics approaches. The analysis by integrates well correlation, estimation of elastic and mechanical parameters, and spatial interpolation of petrophysical properties. Results indicate that Coal Seam D has a thickness of 8.1–10.7 m and porosity ranging from 0.14–0.198. Mechanical properties show Uniaxial Compressive Strength (UCS) values of 9.5-9.7 MPa and a Brittleness Index 0.47–0.50 which indicate coal with low to medium strength and medium brittleness. Based on the preliminary analysis, the coal seam D in this region exhibits favorable candidate for implementation CO₂ storage in South Sumatra, Indonesia.

The combined use of hydraulic stimulation and gas-injection-enhanced drainage is an important technical approach to improve gas recovery from low-permeability coal seams. However, the coupling effect between residual water after hydraulic measures and the gas injection pressure on gas migration and distribution remains unclear. On the basis of a deep coal-rock stimulation and dynamic seepage simulation experimental platform, a large-scale 500 mm cubic water-bearing briquette coal sample was prepared. N2 displacement of CO2 experiments were conducted under different injection pressures (0.6 and 1.0 MPa) and coal moisture contents (4 and 6%). The spatiotemporal evolution of the gas pressure field and the concentration field was systematically investigated. The results show that along the gas flow direction, the pressure rise and CO2 concentration decline exhibit good synergy and hierarchical response. In the lateral direction, the farther from the injection port, the more delayed the pressure response and the slower the CO2 concentration decline. In the vertical direction, water migrates downward under gravity, significantly reducing the displacement efficiency in the lower part. Increasing the injection pressure effectively alleviates the inhibition of water on gas transport, especially in the early and middle stages of the displacement. Increasing the moisture content delays the pressure rise and concentration decline and prolongs the displacement cycle. The coupling effect exhibits significant directional differences. The combined effect of injection pressure and water content on gas pressure shows a "1 + 1 > 2" synergistic enhancement, whereas their effect on CO2 concentration decline shows "1 + 1 < 2" mutual inhibition. This reflects the fundamental difference in the sensitivity of the pressure field and concentration field to water. This study reveals the coupled evolution mechanism of gas pressure and concentration fields during gas injection displacement in water-bearing coal seams, providing a theoretical basis for optimizing injection parameters and improving the gas drainage efficiency.

Rockburst is a typical dynamic hazard in coal mining that severely affects safe production. Existing risk evaluation systems for rockbursts involve numerous factors and overlap in information, which affects the accuracy of judgment. This study combines theoretical analysis, mathematical modeling, and field testing to analyze the influencing factors of rockburst risk from multiple dimensions. Based on a factor reduction algorithm, seven influencing factors of rockburst are analyzed to identify the key disaster-causing factors and construct a hazard assessment method. The results indicate that coal seam thickness variation, geological structures, and floor coal thickness exert dominant control over the rockburst risk in coal mines. Using the ZF1420 working face of the Yadian coal mine in Shaanxi as a case study, the risk evaluation results align with the actual conditions. The study also proposes classification-based prevention and control measures for rockbursts. The research provides theoretical and methodological support for the evaluation and prevention of rockburst risks.