State Key Laboratory of Coal Resources and Safe Mining

Freezing and thawing cycles occur with cyclic liquid nitrogen (LN2) injection in coal. The freeze–thaw treatment damages the pore structure of coal and thus increases its permeability. In this study, NMR and strain monitoring were employed to investigate the changes in coal structure when the coal specimens were under cryogenic treatment using LN2. We classified freeze–thaw process into four stages; stages I and III are dominated by seepage pore development, and stages II and IV are dominated by adsorption pore development. It was found that LN2 freeze–thaw cycles can cause structural deterioration in the coal so as to improve both fracture density and overall permeability. The results demonstrate that the rate of increase of both the effective porosity and total porosity of the coal are positively correlated with the LN2 freezing time and the number of freezing cycles but negatively correlated with the residual porosity. For the same absolute LN2 freezing time, cyclic freeze–thawing has a greater effect on the rate of growth of pore spaces and reduction of P-wave velocity in the coal compared with single freeze–thaw treatment. It was also found that the number of freeze–thaw cycles is a very important factor for the creation of larger pores, pores that can connect the fracture network. The results suggest that appropriate control of the number of freeze–thaw cycles can result in effective fracturing of coal.

Rapid development of world economy has been gradually exhausting the shallow mineral resources. More and more attentions have been paid to the mineral exploitation in deeper subsurface of the earth. For examples,current coal mining has reached 1 500 m in depth; geothermal exploitation has reached over 3 000 m,the depth for ferrous metal mining over 4 350 m and for oil and gas development 7 500 m. Therefore,deep miningis an on-going mining industry. However,more and more engineering accidents or hazards have occurred in deep mining. These accidents or hazards are difficult to be predicted with the current theories or experiences because the mechanical behaviors in deeper ground have not been well understood. On the other hand,current research and development in rock mechanics cannot solve the practical problems in deep ground engineering. A novel research and development scheme should be specially designed for deep ground engineering. Particularly,fundamental concepts and basic theory of rock mechanics should be revisited for deep ground engineering. For examples,what is the deep ground engineering?Is it measurable by the depth of ground?What is the essential difference of the mechanical properties of rock in shallower and deeper subsurface?Can the classical rock mechanics be applied to describe the mechanical behaviors of rocks in deep ground?How can the black box of rock mechanics in deep ground be revealed during the mining induced disturbance ? How does engineering activity such as the development and storage of energy,CO2 sequestration and nuclear waste disposal in deep environments(such as earthquake,geology,geochemistry,geothermal environment as well as microbial environment) affect the micro-scale variation of rock ? These are the fundamental questions in rock mechanics for deep ground engineering. This paper revisits these fundamental concepts by taking the rock mechanics in coal mining as an example. Our results showed that hydrostatic pressure was a typical stress state in deep ground engineering. Deep ground was defined by its stress state instead of its depth. Based on this concept of stress state,we proposed a framework of rock mechanics for mining process in terms of stress paths. Such a framework is different from the classical rock mechanics. Furthermore,a visualization technology was then developed for the investigation of rock mechanics in deep ground. This technology combined the CT scanning,the 3D printing,the reconstruction technology with fractal analysis and the stress freezing technology. This visualization technology provided transparent observation of the disturbance of stress during mining,the development of fractures,the volumetric breaking,the plastic instability and the micro-seepage.

The unique sandwich-like CNTs/Si/C nanotubes show superior cycling and rate performances.

Microplastics (MPs) are emerging pollutants that exist in different environmental media. Because of their wide range and large potential environmental hazards, they have attracted widespread attention in recent years. At present, the research on MP is mostly concentrated on the water ecosystems, and the impact on soil ecosystems is less studied. In this study, 12 typical soil samples from southeastern suburbs of Baoding city were investigated and characterized by time-of-flight secondary ion mass spectrometry (TOF-SIMS) combined with mass high resolution mode and positive and negative ion imaging mode. Four types of MPs, poly (propylene) (PP), poly (vinyl chloride) (PVC), poly (ethylene terephthalate) (PET), and poly (amide 6) (PA6), were quickly identified, of which PET and PA6 accounted for the largest proportion of both up to 30.2%; the particle size of the obtained MPs ranged from 0 to 35 μm, of which the proportion of <10 μm MPs was more than 26.3%, while that of 20-25 μm and 25-35 μm MPs was relatively small (17.83% and 9.3%, respectively). Risk assessment results of the MP in the soil showed that the risk level of MPs in the non-ferrous metal industrial parks and in concentrated with small workshops areas is relatively high, and attention should be paid to such areas. In addition, the study provides a reference method for the investigation and risk assessment of MPs in terrestrial soils, coastal beaches, and sediments.

This study explores how liquid nitrogen (LN2) freezing affects the physical pore and fracture structure of coal. Under lab-controlled conditions, coal specimens were frozen with LN2 under different conditions and thawed, and then the uniaxial compressive strengths, acoustic emissions, and ultrasonic wave velocities of the different specimens were compared. After 60 min of freezing for one set of specimens and 30 freeze–thaw cycles for another set, the elastic moduli of the coal specimens decreased by 47.8% for the 60 min freezes and by 76.2% for the 30 cycles. For the tested two sets of the same specimens, the uniaxial compressive strengths and longitudinal wave velocities dropped by 13.4% and 40.2% and by 47.8% and 76.2%, respectively. At the same time, the coal porosities and Poisson’s ratios increased by 17.5% and 68.1% and by 7.14% and 28.6%, respectively. Owing to the reduction of the coal’s mechanical strength, the elastically straining stage was shortened and the peak yield point and the plastic deformation were accelerated. By establishing a relational model for an elastic modulus based damage variable D and the LN2 freezing conditions, it was found that variable D increased to and stabilized at 0.12 with the single freezing experiments. However, the damage to the coal caused by cyclic freezing and thawing was continuous and damage accelerated after 20 freeze–thaw cycles. By modeling the state of stress in fractures of LN2 treated coal, the theoretical governing equations for the tension in a single fracture were derived. In addition, the expression regarding the volumetric strain of ice under the effect of tension for a single fracture was obtained. The results showed that the proposed model and expressions were in good agreement with the experimentally obtained data.

To explore the reaction mechanism of the low-temperature oxidation of coal and the law of active groups producing CO through oxygenolysis, this article analyzed the reaction characteristics of free radicals and functional groups during low-temperature oxidation by using electron spin resonance and Fourier transform infrared technologies. Based on gas chromatograph analysis, the index gas CO produced by active groups during coal spontaneous combustion was analyzed by studying the reaction of free radicals and oxygen-containing functional groups. The experiments showed that, with rising oxidizing temperatures from 30°C to 230°C, the g-value increased first and then decreased, while the concentration of free radicals constantly rose by 48.3%. When the oxidation temperatures rose to 100°C, the maximum g-value occurred, and the concentration of free radicals changed from slowly increasing by 10.2% to dramatically rising by 31.7%. Moreover, the relative intensities of various oxygen-containing functional groups, including –OH, C=O, C–O, and –COOH, exhibited different change laws with rising temperatures. The –OH constantly declined by 67.0% and began to slowly decrease when the temperatures reached 100°C, while C=O declined first, then increased, and began a rapidly rising stage after the temperatures increased to 80°C. The C–O was nearly unchanged below 180°C, while the growth was accelerated after reaching 180°C, and –COOH decreased at first, then rapidly increased after 80°C. Based on the free radical theory of coal spontaneous combustion, it was revealed that carbonyl radicals are the important active groups to produce CO. It can be assumed that a C=O functional group can also produce CO by taking phenylacetaldehyde as an example. A C–O functional group can produce CO by generating carbonyl radicals, so carbonyl radicals are the direct active groups to produce CO during low-temperature oxidation. CO concentration sharply increased after the oxidizing temperatures reached 100°C, which is consistent with the change of concentration of free radicals. There is a significant inflection point of oxygen-containing functional groups at 80°C, so 80–100°C (especially 100°C) is the allowable maximum temperature of the low-temperature oxidation of coal mass.

BACKGROUND: The Pearl River Origin area, Qujing District of Yunnan Province, has one of the highest female lung cancer mortality rates in China. Smoking was excluded as a cause of the lung cancer excess because almost all women were non-smokers. Crystalline silica embedded in the soot emissions from coal combustion was found to be associated with the lung cancer risk in a geographical correlation study. Lung cancer rates tend to be higher in places where the Late Permian C1 coal is produced. Therefore, we have hypothesized the two processes: C1 coal combustion --> nanoquartz in ambient air --> lung cancer excess in non-smoking women. METHODS/DESIGN: We propose to conduct a retrospective cohort study to test the hypothesis above. We will search historical records and compile an inventory of the coal mines in operation during 1930-2009. To estimate the study subjects' retrospective exposure, we will reconstruct the historical exposure scenario by burning the coal samples, collected from operating or deserted coal mines by coal geologists, in a traditional firepit of an old house. Indoor air particulate samples will be collected for nanoquartz and polycyclic aromatic hydrocarbons (PAHs) analyses. Bulk quartz content will be quantified by X-ray diffraction analysis. Size distribution of quartz will be examined by electron microscopes and by centrifugation techniques. Lifetime cumulative exposure to nanoquartz will be estimated for each subject. Using the epidemiology data, we will examine whether the use of C1 coal and the cumulative exposure to nanoquartz are associated with an elevated risk of lung cancer. DISCUSSION: The high incidence rate of lung cancer in Xuan Wei, one of the counties in the current study area, was once attributed to high indoor air concentrations of PAHs. The research results have been cited for qualitative and quantitative cancer risk assessment of PAHs by the World Health Organization and other agencies. If nanoquartz is found to be the main underlying cause of the lung cancer epidemic in the study area, cancer potency estimates for PAHs by the international agencies based on the lung cancer data in this study setting should then be updated.

mitigation policies and practices in China.

The hard roof difficult to collapse easily causes gas accumulation, which threatens the production safety of coal mine. Therefore, roof pre-cracking is required. Although blasting and hydraulic fracturing can also crack the roof, blasting can easily induce rock bursts, whereas hydraulic fracturing needs complex equipment. In contrast, soundless cracking demolition agents (SCDAs) with noise-free, dust-free, and safe characteristics have obvious advantages. The main component of SCDA is calcium oxide, which reacts with water to produce higher expansion pressure. In this paper, focused on the angles of the borehole, the effect of SCDA is analyzed by numerical simulation based on Pingdingshan coal mine. The research results showed that the azimuthal angle α (between borehole projection and the roadway direction) does not significantly affect the efficacy of SCDAs, whereas the influence of borehole elevation angle β is far more significant than that of the azimuthal angle. Therefore, the angle β is a dominant factor influencing the effect of SCDAs. Based on different effects of SCDAs at different angle of boreholes, the weakening unit was established, so the SCDAs could give full play to roof fracturing. Moreover, field tests validated the importance of borehole angle on weakening the hard roofs.

For coal mines, rock, coal, and rock bolt are the critical constituent materials for surrounding rock in the underground engineering. The stability of the “rock-coal-bolt” (RCB) composite system is affected by the structure and fracture of the coal-rock mass. More rock bolts installed on the rock, more complex condition of the engineering stress environment will be (tensile-shear composite stress is principal). In this paper, experimental analysis and theoretical verification were performed on the RCB composite system with different angles. The results revealed that the failure of the rock-coal (RC) composite specimen was caused by tensile and shear cracks. After anchoring, the reinforcement body formed inside the composite system limits the area where the crack could occur in the specimen. Specifically, shearing damage occurred only around the bolt, and the stress-strain curve presented a better post-peak mechanical property. The mechanical mechanism of the bolt under the combined action of tension and shear stress was analyzed. Additionally, a rock-coal-bolt tensile-shear mechanical (RCBTSM) model was established. The relationship (similar to the exponential function) between the bolt tensile-shear stress and the angle was obtained. Moreover, the influences of the dilatancy angle and bolt diameter of the RCB composite system were also considered and analyzed. Most of the bolts are subjected to the tensile-shearing action in the post-peak stage. The implications of these results for engineering practice indicated that the bolts of the RCB composite system should be prevented from entering the limit shearing state early.

Microplastics pollution is becoming one of the most serious threats to the surface ecosystem of the earth; it is widespread in oceans, rivers, sediments, soils, and organisms. It is a growing concern as an environmental pollutant, which currently has no clear detection standard. Detection methods still need to be constantly supplemented and improved. This study explored a novel method called time‐of‐flight secondary ion mass spectrometry (ToF‐SIMS) in this field. Four types of microplastics in farmland soils, namely, polypropylene, polyvinyl chloride, polyethylene terephthalate, and polyamide 6, were successfully identified in terms of particle size and abundance by combining the high molecular specificity with ion imaging capability of ToF‐SIMS. The procedure based on ToF‐SIMS analysis also provides a methodological reference and basic data for the investigation and research of microplastics in soil, coastal beaches, and sediment.

Foam ceramics as a porous medium have demonstrated huge utilization potentiality to suppress multiple and continuous gas explosions in coal mines. Explosion suppression ability and thermal conductivity of different foam ceramics was analyzed. Explosion energy that combines propagating velocity and overpressure was introduced to evaluate suppressing effect. It was indicated that the overpressure was significantly attenuated by foam ceramics. Besides, for two types of foam ceramics, Al2O3 foam ceramic showed a better suppressing performance on explosion energy than SiC. Foam ceramic of bigger pores exhibited a better inhibition effect on explosion energy. It was found that, as foam ceramic become thicker, thickness was becoming the dominant influential factor suppressing gas explosion. The results suggest foam ceramics with larger pores present a higher thermal conductivity, which account for why larger pores foam ceramic exhibits a better suppressing ability. It is a wall effect that prevents chemical chain reactions, the heat of flame would be transferred to the skeleton, and free radicals are reduced.

ABSTRACT Clarifying and locating small-scale discontinuities or inhomogeneities in the subsurface, such as faults and collapsed columns, plays a vital role in safe coal mining because these discontinuities or inhomogeneities may destroy the continuity of layers and result in dangerous mining accidents. Diffractions carry key information from these objects and therefore can be used for high-resolution imaging. However, diffracted/scattered waves are much weaker than reflected waves and consequently require separation before being imaged. We have developed a Mahalanobis-based diffraction imaging method by modifying the classic Kirchhoff formula with an exponential function to account for the dynamic differences between reflections and diffractions in the shot domain. The imaging method can automatically account for destroying of reflected waves, constructive stacking of diffracted waves, and strengthening of scattered waves. The method can overcome the difficulties in handling Fresnel apertures, and it is suitable for high-resolution imaging because of the consistency of the waveforms in the shot domain. Although the proposed method in principle requires a good migration velocity model for calculating elementary diffraction traveltimes, it is robust to an inaccurate migration velocity model. Two numerical experiments demonstrate the feasibility of the proposed method in removing reflections and highlighting diffractions, and one field application further confirms its efficiency in resolving masked faults and collapsed columns.

China is the world’s largest coal producer country. However, large-scale coal mining has led to severe environmental pollution issues such as surface subsidence and gangue piling up. The gangue discharging amount has ranked the first in the world and coal mine enterprises are facing enormous discharging reduction pressure. This paper summarizes the research progress of the solid backfilling mining technology and then illustrates the realistic demands and significance of implementing underground coal-waste separation. It also focuses on the technical principles, systems and key equipment of the common underground coal-waste separation methods, such as the selective crushing method, the dense medium shallow groove method, the vibro-assisted jigging method and full-size water separation method and ray identification method. In addition, the selection steps of underground coal-waste separation method, the design process of large section separation chamber and the design principle of separation and backfilling system are proposed, finally, the mining-separating-backfilling + X for coal mining is put forward. By combining the technology of mining-separating-backfilling with other technologies, such as gob-side entry retaining with non-pillar mining, gas extraction, solid waste treatment, water protection mining, mining under buildings, railways and water bodies, the integrated mining methods, mining-separating-backfilling + setting pillars, gas drainage, treatment, protection and prevention methods are formed. It also introduced the ‘mining-separating-backfilling + gas extraction’ technology’s whole idea, system arrangement, separation equipment and practical engineering application effects based on the specific engineering case of pingmei no. 12 coal mine. The results indicate that the integration of underground coal-waste separation and solid backfilling technology could achieve gangue discharging reduction, underground washing and surface subsidence control. It is effective at realizing green mining.

CO2 capture and storage (CCS) is an important strategy in combatting anthropogenic climate change. However, commercial application of the CCS technique is currently hampered by its high energy expenditure and costs. To overcome this issue, CO2 capture and utilization (CCU) is a promising CO2 disposal method. We, for the first time, developed a promising method to mineralize CO2 using earth-abundant potassium feldspar in order to effectively reduce CO2 emissions. Our experiments demonstrate that, after adding calcium chloride hexahydrate as an additive, the K-feldspar can be transformed to Ca-silicates at 800°C, which can easily mineralize CO2 to form stable calcium carbonate and recover soluble potassium. The conversion of this process reached 84.7%. With further study, the pretreatment temperature can be reduced to 250°C using hydrothermal method by adding the solution of triethanolamine (TEA). The highest conversion can be reached 40.1%. The process of simultaneous mineralization of CO2 and recovery of soluble potassium can be easily implemented in practice and may provide an economically feasible way to tackle global anthropogenic climate change.

ABSTRACT Seismic diffractions are the responses of small-scale discontinuous structures. They contain subwavelength geologic information. Thus, diffractions can be used for high-resolution imaging. The energy of diffractions is generally much weaker than that of reflections. Therefore, diffracted energy is typically masked by specular reflected energy. Diffraction/reflection separation is a crucial preprocessing step for diffraction imaging. To resolve the diffraction-separation problem, we have developed a method based on the multichannel singular-spectrum analysis (MSSA) algorithm for diffraction separation by reflection suppression. The MSSA algorithm uses the differences in the kinematic and dynamic properties between reflections and diffractions to suppress time-linear signals (reflections) and separate weaker time-nonlinear signals (diffractions) in the common-offset or poststack domain. For the time-linear signals, the magnitudes of the singular values are proportional to the energy strength of the signals. The stronger the energy of a component of the linear signals is, the larger the corresponding singular values will be. The singular values of reflections and diffractions have dissimilar spatial distributions in the singular-value spectrum because of the differences in their linear properties and energy. Only the singular values representing diffractions are selected to reconstruct seismic signals. Synthetic data and field data are used to test our method. The results reveal the good performance of the MSSA algorithm in enhancing diffractions and suppressing reflections.

Safety of coal mine and workers is seriously threatened by coal dust, which tends to inhale and explode. Based on several kinds of commonly used surfactants, the physical and chemical properties and the performance of wetting coal of magnetized surfactant solution were experimentally researched. The relationship between surfactant concentration and surface tension was analyzed, the best surfactant for dust suppression effects was defined according to the wettability, the influence of different concentrations of surfactant solution under the condition magnetization on contact angle of coal and PH value of solution were further researched and analyzed. The researches drew the following conclusions: the surface tension of solution decreases as surfactant concentration increases within a certain range; Triton was the best surfactant in the experiment; contact angle of coal decreases greatly after surfactant solution is magnetized, contact angle of coal is the minimum and wetting effect is the best when concentration increases from 0.02% to 0.03%; magnetization surfactant solution has great prospect in mine coal dust suppression.

A rockburst is a dynamic disaster that occurs during underground excavation or mining which has been a serious threat to safety. Rockburst prediction and control are as important as any other underground engineering in deep mines. For this paper, we tested electromagnetic radiation (EMR) signals generated during the deformation and fracture of rock samples from a copper mine under uniaxial compression, tension, and cycle-loading experiments, analyzed the changes in the EMR intensity, pulse number, and frequency corresponding to the loading, and a high correlation between these EMR parameters and the applied loading was observed. EMR apparently reflects the deformation and fracture status to the loaded rock. Based on this experimental work, we invented the KBD5-type EMR monitor and used it to test EMR signals generated in the rock surrounding the Hongtoushan copper mine. From the test results, it is determined the responding characteristics of EMR signals generated by changes in mine-generated stresses and stress concentrations and it is proposed that this EMR monitoring method can be used to provide early warning for rockbursts.

The integration of hydraulic slotting and gas drainage techniques has become a mainstream technique for enhancing permeability in coal seams with low permeability. However, the mechanism of action of this process is unclear. In this paper, field experiment and laboratory tests are described that aim at elucidating this process. Given the sensitivity and accuracy of test methods and their corresponding determination principles, a combination of mercury intrusion porosimetry and nitrogen gas adsorption was proposed as a complementary technique and the pore-size distribution (PSD) was obtained. It is shown that the proportion of minipores decreases remarkably, whereas that of the macropores gradually increases with the decrease in the distance from the slotted borehole. By contrast, the mesopores and micropores present insignificant changes. Meanwhile, the adsorption pore and the seepage pore show a similar variation in tendency with the minipores and macropores, respectively. Moreover, the specific surface area decreases substantially with the decrease in borehole distances. The integration of hydraulic slotting and gas drainage can lower the gas-adsorption properties and enhance the gas-seepage capacity within the disturbed zone significantly. The paper highlights the guiding factors for improving the enhanced coal bed methane recovery.

ABSTRACT Seismic weak responses from subsurface small-scale geologic discontinuities or inhomogeneities are encoded in 3D diffractions. Separating weak diffractions from a strong reflection background is a difficult problem for diffraction imaging, especially for the 3D case when they are tangent to or interfering with each other. Most conventional diffraction separation methods ignore the azimuth discrepancy between reflections and diffractions when suppressing reflections. In fact, the reflections associated with a specific pair of azimuth-dip angle possess sparse characteristics, and the diffractions adhering to Huygens’ principle behave as low-rank components. Therefore, we have developed a 3D low-rank diffraction imaging method that uses the Mahalanobis-based low-rank and sparse matrix decomposition method for separating and imaging 3D diffractions in the azimuth-dip angle image matrix. The advantages of our 3D diffraction imaging method not only includes the handling of interfering events but also includes ensuring a better protection of weak diffractions. The numerical experiment illustrates the good performance of our method in imaging small-scale discontinuities and inhomogeneities. The field data application of carbonate reservoirs further confirms its potential value in resolving the masked small-scale cavities that can provide storage spaces and a migration pathway for petroleum.