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The mining of thin coal seam resources is seriously affected by the overlying coal seam gas. The detection of three zones of goaf roof is of great significance to the safety production. In this paper, we carried out loading coal and rock resistivity testing experiment, analyzed the loading damage in the process of coal and rock resistivity change and distribution, and carried on the theoretical analysis and numerical simulation of overlying strata three zones according to the actual embedded depth, and then carried out field test. Direct current method has been used to test the forward three-zone apparent resistivity distribution. The main conclusions are as follows: the resistivity changes due to the crack propagation of coal samples, and the resistivity surge point corresponds well with the failure point of coal and rock; in the process of coal and rock failure under load, there are abnormal areas in the resistivity cloud diagram under load, and it migrates with the uneven deformation of coal and rock under load; the collective subsidence of the goaf after the roof is cut in the coal seam mining will form a large sectioned area in thefracture area of the upper part of the goaf. There are large cracks in this part of the sectioned area, and it will extend to the roof rock of the return air roadway, resulting in the increase of cracks in the rock; the electrical detection results show that the horizontal distance from the working face to the caving zone is about 20 m, and the influence resistance value of the fracture zone becomes larger and its height is 17 m, which is consistent with the theoretical calculation and numerical simulation results.

As an important basic energy and industrial raw material in China, the safe mining of coal is the key to ensuring the national energy security and supporting the stable development of economy and society. With the gradual depletion of shallow coal resources, deep mining has gradually become the new normal of coal resource development in China, and the risk of coal- rock dynamic disasters has risen, which seriously restricts the safety and efficiency of coal mining, and has caused adverse effects on the sustainable development of coal industry. Accurate and reliable monitoring methods are considered as the prerequisite for the prevention and control of coal-rock dynamic disasters. Geophysical methods are widely used in the early warning of coal and rock dynamic disasters by sensing the acoustic, electrical, magnetic and other physical signals released during the deformation and failure process to retrieve the damage and failure state of coal and rock mass. Among those methods, the electromagnetic radiation method is a non-invasive, non-destructive, real-time and strong precursory technique suitable for the development requirements of mine informatization and intellectualization. Therefore, the development of electromagnetic radiation monitoring technology is of great significance for the prevention and control of coal and rock dynamic disasters and can provide important technical support for the construction of intelligent mines. The research achievements of electromagnetic radiation theory and technology are summarized in three aspects: experimental phenomena, mechanism models and technical methods. The discovery process of electromagnetic radiation phenomenon is reviewed. The signal characteristics, influencing factors, electromechanical coupling effects and other electromagnetic radiation characteristics of coal-rock are analyzed. According to the correlation between typical electromagnetic radiation mechanism models, stress action and crack propagation process, a classified review is made. The existing coal and rock electromagnetic radiation monitoring and early warning equipment and technical methods are then briefly introduced. On this basis, the new progress made in recent years is described in detail, including the research on vector characteristics of electromagnetic radiation, micro-scale verification of electromagnetic radiation mechanism, and electromagnetic positioning technology of coal-rock fracture. It summarizes the two current theoretical and technical research bottlenecks that the mechanism is not fully revealed and the location of disaster-prone areas cannot be achieved. Finally, a new target for the future development of electromagnetic radiation theory and technology is proposed.

Compared with the energy dissipation of coal and rock failure, the more intuitive stress drop has certain practical significance for assisting the study of coal and rock failure characteristics and variation law of geophysical signals. And the stress drop is the macroscopic manifestation of energy dissipation. Most of the current research on energy dissipation and stress drop is only for one-sided research, without a comprehensive study on the relationship between the two. The relationship between the two is not clear, which restricts the application of the stress drop index. In order to solve the above problems, YAW−600 microcomputer controlled electro-hydraulic servo pressure testing machine is used to carry out uniaxial compression experiments on coal samples. The evolution characteristics of energy dissipation index and stress drop index in the whole process of coal sample failure under load and the relationship between them are studied and analyzed. Based on theoretical analysis, the relationship between energy dissipation index and the product of stress level and stress drop index is further studied. The results show that the energy dissipation index and stress drop index in the uniaxial compression failure process of coal sample have good response to the significant fracture of coal sample. And the indexes are in the same order of magnitude in numerical value, but they are not completely proportional. The energy dissipation rate is linearly related to the product of stress level and stress drop rate. And the energy dissipation is linearly related to the product of stress level and stress drop. The goodness of fit is 1 and 0.997 5 respectively. Compared with the energy dissipation index, the more intuitive product of stress level and stress drop index is more sensitive to the significant damage of coal samples, and has a burst characteristic. The research results clarify the relationship between energy dissipation and stress drop, and provide a certain theoretical support for the application of stress drop index. The relationship can be used for in-depth research on the coal rock failure characteristics and the variation law of geophysical signals such as acoustic emission, electromagnetic radiation and infrared radiation.

In order to solve the problem of difficult prevention and control of coal and gas outburst in thin coal seam, the distribution characteristics of effective extraction area in thin coal seam are analyzed. Due to the limitation of the thickness of the thin coal seam, the expansion of the effective gas extraction area in the vertical direction is hindered, and it tends to extend in the horizontal direction, resulting in the effective extraction radius in the horizontal direction is much larger than the thickness of the coal seam. The effective extraction area is elliptically distributed. The gas seepage field mainly focuses on the direction and inclination of the coal seam. According to the characteristics, it is pointed out that the outburst elimination technology of shield tunneling in middle roadway based on the coal seam extraction mode can make the extraction area connected together. The technology is more suitable for gas extraction in thin coal seam. This paper analyzes the advantages and technical principles of applying the outburst elimination technology of shield tunneling in the middle roadway to block outburst elimination in thin coal seam. The gob-side entry retaining technology is adopted to make the return airway roadway of the previous working face as the air inlet roadway of the next working face. The gas extraction boreholes are constructed ahead of the air inlet roadway, and the extraction range covers and exceeds the predetermined middle roadway by more than 20 m. The gas extraction is used to eliminate the outburst danger of the middle roadway. tunneling the middle roadway. The gas extraction boreholes are constructed at the predetermined position in the middle roadway to the return airway roadway, and the extraction range covers and exceeds the predetermined return airway roadway by more than 20 m. The gas extraction is used to eliminate the outburst danger of the middle roadway. Finally, the return airway roadway is excavated to form the working face. Taking the thin coal seam of 9305 working face of a mine as the research object, the numerical simulation is carried out. The results show that when the extraction time is between 10 d and 30 d, the increase of the effective extraction radius is the largest. With the increase of the extraction time, the increase of the effective extraction range gradually decreases. When the borehole spacing is 3 m, the effective extraction radius between the two holes is almost tangent, and the extraction effect is the best. The extraction pressure can basically make most of the coal seam gas to be effectively diffused, resolved and passively extracted. The gas extraction in the middle roadway reduces the gas pressure between the return airway roadway and the progressive middle roadway area effectively. The field measurement results show that the optimal extraction borehole spacing for shield tunneling in the middle roadway of outburst coal seam in 9305 working face is 3 m, the borehole diameter is 94 mm, the effective extraction diameter is not more than 5 m, and the drilling depth is 107 m. The outburst elimination technology of shield tunneling in middle roadway reduces the gas volume fraction of the thin coal seam by about 70%, and the outburst elimination effect is remarkable.

To investigate the variations in wave velocity fields between impact and outburst coal seams, we analyzed the fluctuations in wave velocity under loading conditions for both coal types. A comprehensive methodology was developed to correct coal wave velocities in response to stress and gas presence, which was then applied to field assessments of hazardous regions. Our findings reveal significant differences in wave velocity alterations between impact and outburst coal seams during loading-induced failure. Gas pressure exhibits a negative correlation with wave velocity in outburst coal (correlation coefficient R2 = 0.86), whereas wave velocity in impact coal demonstrates a positive correlation with stress (R2 = 0.63). A robust methodology for correcting coal wave velocities in response to stress and gas presence was established to enable more accurate measurement of wave velocity changes. In field applications, seismic wave computed tomography identified stress anomalies that closelycorresponded with geological structures and mining operations, effectively pinpointing hazardous zones. The abnormal wave velocity coefficient ranges for outburst coal seams and impact coal seams are −0.6 to 0.25 and −0.35 to 0.16, respectively, which correspond well with the field stress distribution. [ABSTRACT FROM AUTHOR]

In order to explore the rule of the duration of the acoustic electric signal when the coal sample is damaged by loading, the full waveform synchronous acquisition of the acoustic electric signal during the uniaxial loading failure process of the coal sample under different loading rates is carried out, the time sequence characteristics of the acoustic electric signal during the coal sample failure process are compared and analyzed, and the waveform characteristics and duration of the acoustic emission and electromagnetic radiation in the complete failure stage are emphatically analyzed. The results show that: in the process of uniaxial loading failure of coal samples, the response of acoustic and electrical signals has a good synergy, the higher the loading rate is, the denser and stronger the acoustic and electrical signals are generated during the main failure; the results show that the AE signal changes with the loading stage, and there is no obvious AE signal in the compaction and elastic stage; in the accelerated failure stage, the AE signal is gradually enriched and the amplitude increases; in the complete failure stage, the amplitude of the AE signal reaches the maximum when the main failure occurs, and decays synchronously after the peak; under different loading rates, the duration of AE signal is in millisecond level, and the duration of AE is 2 to 3 times longer than that of electromagnetic radiation. With the increase of loading rate, the duration of AE signal and electromagnetic radiation increase significantly, and the multiple increases with the increase of loading rate.

The catalytic conversion of CO2 into high value‐added chemical products driven by electricity was an effective strategy to reduce atmospheric CO2 concentration. The development of high efficiency catalysts had always been the core for electrocatalytic reduction of CO2. The single transition metal sites anchored on nitrogen‐doped carbon showed great potential in CO2 reduction reaction (CO2RR). In this paper, the atomically dispersed Ni−N sites are constructed on the two‐dimensional mesoporous carbon nanosheets by the secondary pyrolysis method. The rich mesoporous structure of the two‐dimensional nanosheets is beneficial to CO2 diffusion and adsorption during CO2RR, and the high nitrogen contents could increase the Ni single atom loadings. The nitrogen content and type could be controlled by regulating the ratio of nitrogen source to carbon source, and the catalyst activity is improved as the proportion of nitrogen sources increases. When the ratio of nitrogen source to carbon source is four, the oxidation state of Ni single atoms Ni@NMCN4 is between 0 and +2, and Ni single atoms exists as Ni−N4 structure with pyridinic N of 32.78 %, which exhibits the best electrochemical CO2RR activity. The active site of Ni@NMCN4 is confirmed as the highly stable Ni single atoms. Moreover, the pyridinic N species are proved to play an important role in enhancing the CO2 electroreduction performance. This work provides a new perspective to adjust the nitrogen content and type in transition metal monatomic‐N‐doped carbon electrocatalysts for CO2RR. [ABSTRACT FROM AUTHOR]

A multisignal nanosecond synchronous acquisition system to measure acoustic emission (AE) and electromagnetic radiation (EMR) generated during the process of loading and failure of coal and rock samples is established. The correlation between the energy of the AE and EMR signals and the loading stress of outburst coal-rock samples was studied, and the characteristics of the AE and EMR signals during the process of loading and fracturing the outburst coal and rock samples were analyzed. The results show that (1) before the failure of the outburst coal and rock samples, the fluctuation of the AE and EMR signals is the largest, with the same rising and falling trend, and the intensity is not strictly positively correlated, with the phenomenon of low EMR when the AE intensity is high; (2) the EMR and AE deviation degree and frequency exhibit a good response to coal and rock fracturing. The correlation between EMR and stress drop is stronger than that of AE, and the AE signal is richer than the EMR signal. The results show that it is feasible to develop combined AE and EMR early warning technology to improve the early forecasting accuracy of coal and gas outbursts. [ABSTRACT FROM AUTHOR]