Acidification is a commonly used chemical modification method to ameliorate the coal seam permeability. Current research on acidified coal mainly focuses on mineral composition, pore structure, and permeability. However, as one of the key stages of coalbed methane (CBM) development, there is still insufficient research on adsorption in acidified coal. The study selected HCl as the modification reagent and explored the pore and adsorption evolution characteristics through pore-combined testing technology and isothermal adsorption experiments. The results demonstrate that HCl acidification has a pore-enlarged effect on the macropore, with an increase in pore volume and a decrease in pore area. Mesopore displays pore-enlarged and transformed effect, with a decrease in pore volume and area. But HCl acidification has a weak effect on the micropore structure with chemical properties. The changes in the pore structure influence the Langmuir constant. Specifically, the decrease in macropore and mesopore area reduces the gas adsorption sites and weakens the adsorption capacity. The Langmuir constants generally show a decrease in Langmuir volume V L and an increase in Langmuir pressure P L. The study clarified the targeted control effect of HCl acidification on reducing coal adsorption and enhancing desorption efficiency and revealed the mechanism of CBM increasing production from the perspective of adsorption and desorption, which provides theoretical guidance and an evaluation basis for the HCl application in reservoir modification and CBM increasing production.

Adsorption temperature, pressure, and pore structure parameters are the key factors affecting gas adsorption in porous media. However, an adsorption dynamics model has not been established that comprehensively couples adsorption temperature, adsorption pressure, and pore structure parameters. In this study, the methane adsorption and pore structure measurements are conducted, and a novel fractal dynamics model of gas adsorption in porous media is proposed. The research results show the proposed model is a function of adsorption temperature (T), adsorption pressure (P), pore size parameters (γmax and γmin), fractal dimension (Df) for pore size, and fractal dimension (DT) for pore tortuosity. The error analysis indicates the predicted adsorption amount from the proposed model matches the measured adsorption amount. The proposed model achieves the determination of the critical point of gas absorption's temperature and pressure effects through a temperature-pressure dynamics analysis. This model provides a methodological basis for clean production based on the adsorption principle and also establishes a significant foundation for the fractal design strategies of novel materials.

A large amount of coal dust is produced in coal mine excavation, which has a serious impact on the working environment and health of underground workers. For this problem, through numerical simulation to understand the temporal and spatial evolution law of coal dust in the excavation tunnel, that is, the coal dust velocity first increases and then decreases along the diffusion direction, and the maximum velocity reaches 8 m/s, and fills the entire tunnel at 50 s. In this regard, the an ultrafine water mist partition multistage dust suppression system was proposed, and numerical simulation studies were carried out on the wind flow distribution and water mist particle transportation law, and the results showed that the air flow velocity in the 0-5 m area was between 6-14 m/s and varied a lot, and the speed of the spray jet in the 10-30 m area decayed faster, and eventually stabilized at 1 m/s. The water mist particles surrounded the cutting head with a speed of 8 m/s in 2 s, and spread to the working face. After 10 s, the water mist particles cover the whole roadway, and the speed is also stabilized at 2 m/s. Through field application and measurement in the I030409 excavation face of Qipanjing Coal Mine, the average reduction efficiency for total dust and respirable dust in the tunnel reached 91.74% and 93.4%, respectively, which effectively controls the problem of dust pollution in the excavation tunnel.

To analyze the transformed effect of three-dimensional (3D) fracture in coal by CO2 phase transition fracturing (CO2-PTF), the CO2-PTF experiment under a fracturing pressure of 185 MPa was carried out. Computed Tomography (CT) scanning and fractal theory were used to analyze the 3D fracture structure parameters. The fractal evolution characteristics of the 3D fractures in coal induced by CO2-PTF were analyzed. The results indicate that the CO2 phase transition fracturing coal has the fracture generation effect and fracture expansion-transformation effect, causing the maximum fracture length, fracture number, fracture volume and fracture surface area to be increased by 71.25%, 161.94%, 3970.88% and 1330.03%. The fractal dimension (DN) for fracture number increases from 2.3523 to 2.3668, and the fractal dimension (DV) for fracture volume increases from 2.8440 to 2.9040. The early dynamic high-pressure gas jet stage of CO2-PTF coal influences the fracture generation effect and promotes the generation of 3D fractures with a length greater than 140 μm. The subsequent quasi-static high-pressure gas stage influences the fracture expansion-transformation effect, which promotes the expansion transformation of 3D fractures with a length of less than 140 μm. The 140 μm is the critical value for the fracture expansion-transformation effect and fracture generation effect. Five indicators are proposed to evaluate the 3D fracture evolution in coal caused by CO2-PTF, which can provide theoretical and methodological references for the study of fracture evolution characteristics of other unconventional natural gas reservoirs and their reservoir stimulation.

Faults, as a kind of fracture tectonics, play a role in reservoir closure or provide oil and gas transportation channels. The accurate understanding of the distribution characteristics of faults is significant for oil and gas exploration. The traditional fractal dimension for fault number (Df3) cannot comprehensively characterize the complexity and heterogeneity of fault network distribution. In this paper, a fractal characterization method on three-dimensional (3D) tortuosity of fault tectonics is proposed based on 3D seismic exploration. The methodology is described in detail to establish the model on the fractal dimension for the 3D tortuosity of fault tectonics. The results show the proposed method of estimation of the DT3 displaying high accuracy and rationality. Compared with the traditional fractal dimension Df3, the proposed DT3 can comprehensively characterize the fractal characteristics of faults network systems in the 3D space. This study achieves a breakthrough in the fractal characterization of the 3D tortuosity of fault tectonics. It is worth further study for establishing an analytical fractal equation based on the DT3 and oil or gas transfer, which can provide the theoretical foundation and technical support for oil and gas exploration.

Mechanized coal extraction generates substantial dust, adversely affecting the physical and mental wellbeing of underground workers and degrading the operational environment. To address the limitations of traditional spray systems, which often suffer from poor atomization effects and limited ability to withstand wind interference, the innovative vortex atomizing nozzle has been developed specifically for dust suppression. This advanced solution was rigorously tested through detailed numerical simulations and comprehensive experimental investigations. These studies focused on examining the impact of critical parameters, including the velocity at the airflow director's exit, the angle of the exit hole, and its radius, on the efficacy of dust suppression. Findings highlighted the paramount importance of the airflow director's exit velocity in enhancing dust removal, with the exit hole's radius having the least impact. Optimal conditions for dust suppression were identified as an airflow director exit velocity of 20 m/s, an angle of 60 degrees, degrees , and a radius of 0.6 mm. Field validations under these optimum parameters demonstrated unparalleled dust control efficiency, achieving a reduction in coal dust concentration exceeding 90 %. This apparatus offers a highly effective, user-friendly solution for comprehensive coal dust management across various mine locations.

Coal dust is a significant health hazard for coal workers, as prolonged inhalation can lead to severe diseases such as Coal Workers' Pneumoconiosis. Water spray with surfactants is commonly used as a precontrol strategy to control coal dust in underground mines. The zeta potential test is a widely used static test to evaluate surfactants with different ionicities. However, there are still some arguments about the evaluation criteria of zeta potential on surfactant performance between different researchers. Moreover, whether zeta potential can be used as an index to evaluate the surfactant suppression performance is inconclusive. To address these problems, this study conducted the zeta potential test and the wind tunnel test (a dynamic test) using various surfactants with different ionicities. It was found that the zeta potential test and the wind tunnel test gave consistent results for the anionic surfactant. However, this trend was not achieved for the non-ionic and cationic surfactants. Therefore, the zeta potential is only suggested to be used as an index to evaluate the surfactant's dust suppression efficiency, while dynamic tests are still recommended for these two types of surfactants to evaluate dust suppression efficiency. (c) 2023 The Society of Powder Technology Japan. Published by Elsevier BV and The Society of Powder Technology Japan. This is an open access article under the CC BY license (http://creativecommons.org/ licenses/by/4.0/).

The current studies primarily analyze the heterogeneity and complexity of mesopore structures based on low-temperature nitrogen (N2) adsorption curves and the Frenkel–Halsey–Hill (FHH) fractal model. However, these studies ignore the fact that the low-temperature N2 desorption curve can also reflect the desorption performance of the mesopore structure. In this research, novel fractal indicators for characterizing the adsorption–desorption performance of mesopores based on the fractal dimension from the N2 adsorption curves and N2 desorption curves are proposed. The novel fractal indicators I1 and I2 are applied to evaluate the adsorption–desorption performance of mesopores with pore size 2–5 nm and pore size 5–50 nm, respectively. The fractal indicator I1 shows an increasing trend with coalification, reflecting that the gas adsorption performance of 2–5 nm mesopores is enhanced with coalification. The fractal indicator I2 exhibits a trend of first increasing and then decreasing with coalification, indicating the gas desorption performance of mesopores with pore size 5–50 nm decreases first and then increases. The proposed indicators provide novel analytical parameters for further understanding the gas adsorption–desorption mechanism of porous coal-based or carbon-based materials.

In this study, liquid CO2 phase transition fracturing treatment (LCPTF) and CH4 adsorption-desorption experiments were carried out on coal to explore the effects of LCPTF on the methane desorption capacity in coal. The results show that the LCPTF treatment caused about 35% reduction of desorption, indicating the transformed effect of coalbed methane (CBM) from the adsorbed state to the free state during the LCPTF process. Also, the enhanced effect of LCPTF on the effective desorption range and capacity was revealed. This effect can further enhance CBM production by prolonging the steady gas production period and delaying the arrival of the depletion stage of CBM production. This study provides an access to understanding the mechanism of LCPTF for enhancing CBM recovery from the perspective of the dynamic desorption process. The five-index method on the effect evaluation of the LCPTF technique for enhancing the CBM recoverability was proposed. The expected results are conducive to improving the evaluation system and optimizing the field application of the LCPTF technique.

The conventional Langmuir equation based on the homogeneous surface assumption is failed to characterize the gas adsorption in coal due to the obvious heterogeneity of the coal pore structure. Therefore, a novel fractal Langmuir adsorption equation on coal is proposed based on the fractal characteristics of pore heterogeneity in this paper. The pore heterogeneity in coal displays obvious fractal characteristic, which can be characterized by the fractal dimension for the pore tortuosity (DT) and the fractal dimension for the pore size (Df). The principles and methodology of the proposed equation are deduced and summarized in detail. The results indicate that the proposed fractal Langmuir adsorption equation displays the higher accuracy than the conventional Langmuir equation. In addition, the proposed equation is the function of the fractal dimension for the pore tortuosity (DT), the fractal dimension for the pore size (Df), and the pore structural parameters (λmin, λmax, and L), which more comprehensively reflects the influence of pore structure on adsorption. Therefore, the proposed equation achieves a breakthrough in the calculation of the methane adsorption capacity in pores with different pore diameter ranges, which can more accurately reflect the methane adsorption in coal pores compared with the conventional Langmuir equation.

The research and analysis on the heterogeneous characterization of adsorbed methane in coal at the molecular scale has not yet been fully explored. In this paper, the complex system of the coal molecular structure adsorbed methane was constructed based on the Langmuir adsorption equation and molecular simulation. A fractal characterization model to analyze the heterogeneous distribution of adsorbed methane in coal molecular structure was established, and the principle and method of the proposed model is specifically derived and summarized. The quantization relation between the fractal dimension (D-f) for the methane adsorption of coal molecular structure and the adsorption pressure (P) was established, which is similar to the variation of adsorption capacity with adsorption pressure of the Langmuir equation. This research achieved a breakthrough in the fractal characterization of methane adsorption by coal at the molecular scale and further revealed the control of fractal structure over fractal behavior. The proposed analysis and characterization method provides insight into a deeper understanding of the gas adsorption mechanism of coal-based or carbon-based materials, which is conducive to creating a significant basis for the novel fractal design strategies of coal-based materials.

The excavation of coal mines generates substantial dust, posing environmental and health risks for underground coal mine workers. To address the dust issue, a novel Vortex Air Flow Negative Pressure Entrainment Dedusting Device was developed. Through numerical simulations and a comprehensive experiment, factors influencing dust removal efficiency, such as the number of air ducts, wind speed, and pressure duct layout angle, were analysed. The result showed that the number of air ducts had the most significant effect on the suppression efficiency, while the pressure duct layout angle had the least significant impact. Six ducts with 25 m/s air velocity and 30 degrees pressure duct angle were determined as the optimum operational parameters for dust removal. Field tests with the optimum setting were also conducted. The results demonstrated the best dust control performance compared to other settings, which resulted in a 90% reduction in coal dust concentrations. The developed device provides underground coal mines with an effective method to control coal dust efficiently during the excavation.

Currently, two multifractal characterization methods on the complexity and heterogeneity of micropores by the low-temperature CO2 adsorption measurement exhibit systematic differences due to the different studied objects that include the relation curves of the CO2 adsorption capacity vs relative pressure and micropore volume vs micropore size distribution. This situation directly affects the accuracy and applicability of multifractal characterization of micropores. In this article, the four high-rank coal samples are selected to conduct the low-temperature CO2 adsorption experiment. The multifractal parameters are derived and compared to examine the accuracy and applicability of the two multifractal characterization methods of the CO2 adsorption capacity vs relative pressure and micropore volume vs micropore size distribution. The results show that the curves of the partition function, mass scaling function, and singular fractal display the classical multifractal existence characteristic by the methods of the CO2 adsorption capacity vs relative pressure and micropore volume vs micropore size distribution. The generalized fractal dimension parameters of the CO2 adsorption capacity vs relative pressure and micropore volume vs micropore size distribution display a similar variation trend but with different values, by which it is insufficient to determine the relative merits of the two methods. The multifractal singularity parameters (R-dR and R-dS, Delta f(R)(alpha(R)) and Delta f(S)(alpha(S))) reflect the impact of the differences in quantity within regions of the same dimension, which present an opposite trend. The multifractal characterization method of micropore volume vs micropore size distribution exhibits greater consistency with the micropore structure in reality, supporting the evidence that the micropore volume vs micropore size distribution method can provide a more accurate and applicative multifractal characterization of CO2 adsorption in micropores than the CO2 adsorption capacity vs relative pressure method. This paper proposes a targeted strategy for improving the accuracy of multifractal characterization of CO2 adsorption in micropores, which provides a scientific basis for the multifractal design strategy of nanoporous materials.

The traditional Langmuir equation displays drawback in accurately characterizing the methane adsorption behavior in coal, due to it assuming the uniform surface of coal pores. Additionally, the decay law of gas adsorption capacity with an increasing coal reservoir temperature remains unknown. In this study, the fractal adsorption model is proposed based on the fractal dimension (Df) of coal pores and the attenuation coefficient (n) of the adsorption capacity. The principles and methods of this fractal adsorption model are deduced and summarized in detail. The results show that the pore structures of the two coal samples exhibit obvious fractal characteristics, with the values of fractal dimensions (Df) being 2.6279 and 2.93. The values of adsorption capacity attenuation coefficients (n) are estimated as −0.006 and −0.004 by the adsorption experiments with different temperatures. The proposed fractal adsorption model presents a greater theoretical significance and higher accuracy than that of the Langmuir equation. The accuracy of the fractal adsorption model with temperature effect dependence is verified, establishing a prediction method for methane adsorption capacity in deep coal reservoirs. This study can serve as a theoretical foundation for coalbed methane exploration and development, as well as provide valuable insights for unconventional natural gas exploitation.

To characterize the microcomponents and pore structure of high-rank coals under supercritical CO2 pulsation, the effects of supercritical CO2 pulsation on the mineral composition, organic functional groups, pore structure and pore fractal characteristics of high-rank coals were investigated on the basis of a combination of X-ray diffraction (XRD), Fourier transform infrared (FTIR), high-pressure mercuric pressure, low-temperature liquid nitrogen, and low-field nuclear magnetic resonance experiments. The results showed that after supercritical CO2 pulsation, the mineral compositions and organic functional group contents of the coals changed to different degrees, among which the contents of calcite, ilmenite and illite decreased, the contents of ferro-dolomite and kaolinite increased, and the characteristic peaks of oxygen-containing, benzene-ring-five-substituted H, and aliphatic functional groups decreased. The pore and fractal structure of the coals after supercritical CO2 pulsation was significantly altered, the total pore volume of the coal body increased by 188%, the percentage of seepage pore volume increased by 13 times, and the porosity increased by 8.272%. The fractal dimensions of the coal samples after supercritical CO2 pulsation all showed a decreasing trend, and the complexity of their pore structure and surface roughness decreased.

Prolonged exposure to high concentrations of respirable coal mine dust causes coal workers’ pneumoconiosis and silicosis. Underground coal mine roof bolter operators are more prone to elevated exposure to coal and silica dust. The canopy air curtain (CAC) was developed by NIOSH to protect roof bolter operators from the exposure. The CAC supplies filtered air over the breathing zone of the operator. This dilutes the high coal dust concentrations and provides for an impenetrable air curtain. Many studies have been carried out to improve CAC efficiency. However, field test has shown variable dust control efficiencies indicating room for further improvement due to the non-uniform airflow distribution across the plenum and ineffective perimeter flow. This study therefore redesigns the CAC with optimized flow distribution that effectively protects roof bolters from coal dust exposures. The Simplex Evolutionary Operational (EVOP) optimization algorithm was applied to optimize the configuration of the new CAC. Computational fluid dynamics (CFD) simulations are run at each iteration of the algorithm. The combination of these methodologies led to the optimization of the uniformity of airflow distribution across the plenum to achieve the best possible uniformity. A lab experiment using a physical model of the optimized CAC was used to validate the CFD model and confirm the ability of this design to protect roof bolter operators from excessive coal dust.

Submerged waterjets often yield low coal-breaking efficiency and have limited permeability. This study introduced a novel approach to enhance submerged waterjets by harnessing the annular fluid hydraulic energy of a borehole. A new waterjet modulator was developed using large eddy simulation (LES) combined with the response surface method (RSM). The reliability of the method was verified by submerged waterjet high-frequency pulsation pressure test experiments. The results indicated that the pulsation amplitude of the annular fluid enhanced self-excited oscillation pulsed waterjet (AFESOPW) was 330.05%, 45.17%, and 32.56% higher than those of the continuous waterjet, self-excited oscillation pulsed waterjet (SOPW), and pre-optimized waterjet, respectively. Additionally, the pulsating pressure peak increased by 42.84%, 19.60%, and 25.81%, respectively. Under the submerged condition, the outlet pulsating pressure peak of the AFESOPW was 1.32–1.58 times the inlet pressure. The self-priming annular fluid weakened the hinderance of the submerged environmental pressure and enhanced the waterjet pulsation effect.

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

From the perspective of plugging air leakage paths and preventing coal spontaneous combustion in the goaf of coal mines, filling and plugging the ends of the working face is crucial. Generally speaking, rapidly solidified foam slurry is beneficial to plugging air leakage paths. However, researchers have not yet clarified the accumulation and diffusion rules of rapidly solidified foam, resulting in low efficiency in plugging air leakage paths. To solve the above problems, this study takes the goaf of 1301 working faces of the Shandong Lilou Coal Industry as the research object, uses the computational fluid dynamics modeling method to construct a physical model of pressure injection foam slurry, and optimizes the on-site application process parameters of plugging the end of the working face. The numerical simulation results show that coagulant can improve the accumulation characteristics and plugging efficiency of foam slurry, and the optimal addition amount is 3 wt%. In addition, as the perfusion flow increases from 5 to 15 m3/h, the airflow rates on the working face gradually increases and the oxidation zone gradually shrinks. Therefore, the grouting flow is preferably 15 m3/h. Field test results show that rapid-setting inorganic solidified foam blocks the airflow at the air inlet corner from leaking into the goaf and weakens the composite reaction of coal and oxygen. The temperature at each monitoring point first decreased, then stabilized, and the final temperature was maintained at 27-28 degrees C. The CO concentration gradually decreased and finally remained below 20 ppm. The CO concentration of the mine's total return airflow is stable below 10 ppm. This study can provide theoretical guidance for the technological design of using solidified foam to rapidly plug the air leakage paths at the end of the fully mechanized caving working face.