Synthesis strategies of hard carbon anodes for sodium-ion batteries

Peer reviewe

Effect of disturbed coal pore structure on gas adsorption characteristics: mercury intrusion porosimetry

Studying pore structures of disturbed coal and their influences on adsorption characteristics is conducive to in-depth understanding of occurrence and migration of gas in reservoirs in areas prone to coal and gas outbursts. A mercury porosimeter and a high-pressure gas adsorption instrument were separately used to investigate pore structures and measure adsorption characteristics of disturbed coal and undisturbed coal in Ningtiaota Coal Mine and Xigu Coal Mine (Shaanxi Province, China). In addition, pore structures and gas adsorption characteristics of coal samples were studied. The Menger’s sponge model was adopted to calculate fractal dimensions of coal samples, to estimate influences of pore structures and fractal features on the gas adsorption characteristics of disturbed and undisturbed coal. Results show that the pore volume of undisturbed coal is mainly contributed by micropores and transitional pores, while that of disturbed coal arises mainly from macropores and mesopores. Micropores and transitional pores account for large proportions of the specific surface area of pores in both disturbed and undisturbed coal. The adsorption isotherms of disturbed and undisturbed coal conform to the Langmuir equation and tectonism increases the limiting adsorption quantity of coal. The fractal dimensions D1 of the four types of coal samples in the experiments are in the range of 2.7617–2.9961, while the fractal dimensions D1 and D2 of disturbed coal are both larger than those of undisturbed coal, indicating that disturbed coal is more likely to collapse under high pressure. The total pore volume, total specific surface area of pores, and fractal dimensions are positively correlated with the adsorption constant a, while they have U-shaped correlations with the adsorption constant b of coal samples. The adsorption constant a of disturbed coal is always greater than that of undisturbed coal, while no obvious trend is observed between the adsorption constant b and tectonism. The research results can provide theoretical basis for further study of gas occurrence in disturbed coal seams

Research on hydration mechanism of nano-alumina modified cementing cement

With the accelerated development of surface well cementing technology, the engineering site has higher requirements for the early fluidity and coagulation performance of surface well cementing water slurry. In order to improve the top-off efficiency of cement injection operation in cementing engineering and improve the application of cementing engineering field operation, the method of nanoalumina particles modified cementing cement was proposed, and the NMR technique was used to study the change characteristics of the hydration reaction of nanoalumina-modified cement slurries (with the water-cement ratio of 0.44) with the mass fractions of 0.00% wt, 0.05% wt, 0.10% wt, 0.15% wt, and 0.20% wt at different hydration ages were investigated to characterize the changes in hydration reactions, and to probe the aqueous phase distribution, changes in the total signal amount of physically bound water, peak shape index, and migration rate of the nanoalumina-modified cements. The results showed that the T2 patterns of nano-alumina-modified cement pastes at the hydration stage would show three relaxation peaks, corresponding to the flocculated structure-filled water (0.1−10 ms), capillary water (10 ms), and free water (800−1000 ms), respectively; and the total signal of the physically bound water decreased gradually with the increase of the mass fraction of nanoalumina, in which the nanoalumina-modified cement pastes with the mass fraction of 0.20%wt. Alumina-modified cement paste has the fastest reduction rate of signal; the peak shape index shows the trend of increasing and then decreasing, when the hydration reaction is carried out to 600 min, the modified cement paste with mass fraction of 0.20%wt has the fastest rate of change, and the rate of the relaxation peak moving to the short relaxation direction is accelerated; the combination of the migration rate of the nanoalumina-modified cementing cement hydration reaction stage division, and the hydration mechanism of its hydration mechanism respectively acted in the four Hydration period: ① initial hydration period (5−60 min), the hydration rate did not change significantly; ② accelerated hydration period (60−600 min), nano-alumina to make the physical binding water in the cement paste into chemical binding water in a shorter period of time, the hydration process is accelerated to achieve the effect of promoting the hydration of the cement; ③ stabilized hydration period (600−1200 min), the cement system tends to stabilize; ④ Delayed hydration period (1200−1800 min), the rate of conversion of physically bonded water to chemically bonded water slows down, and the hydration process of cement slows down. It was concluded that the hydration period of nanoalumina-modified cement paste can be divided into four stages, and nanoalumina plays the roles of physical filling, accelerating, stabilizing, and retarding for the cement paste, respectively. In the accelerating period, nanoalumina promotes the early flow of cement, which is conducive to the pumping and flow of cement paste, and in the retarding period, nanoalumina retards the further hydration process of cement, which is conducive to the development of early strength. Among them, the nano-alumina modified cement paste with a mass fraction of 0.20% wt has the greatest influence on the cement hydration process, promotes the conversion of filler water to bound water, and accelerates the hydration reaction of cement paste to the greatest extent during the accelerated hydration period, which serves as the basis for providing guidance for the preparation and application of cementing cements in the field

Asymmetric characteristics of “three-field” in overburden of inclined coal seam groups and target extraction mechanism

The three-field (stress, displacement and fracture field) evolution laws of the inclined coal seam group are complex, which are important for the transport and storage of pressure-relief gas. In order to investigate the three-field evolution law of inclined coal seam group, the study carried out a similar simulation experiment of multiple mining in inclined coal seam group with the 1930 coal mine in Xinjiang as an object. The collapse pattern of the overlying strata was analyzed, the stress evolution characteristics of overlying strata was obtained, the overlying strata displacement distribution and movement direction characteristics were analyzed, and the characteristics of mining-induced fracture distribution were elucidated. The influence of the three-field evolution law on gas migration was further investigated, and a directional borehole gas extraction field test was carried out for veri-fication. The study results shown that, the rectangular ladder platform of mining-induced fracture shown obvious asymmetric characteristics under multiple mining of inclined coal seam group. The overlying strata stress of the lower side was more variable, and the pressure-relief effect was more obvious with increasing mining frequency, while the overlying strata stress of the higher side was less variable. Combined with the gravity-dip effect, the overlying strata of the higher side was more prone to be damaged, and the collapse order was preferred with asymmetric characteristics. The displacement distribution of overlying strata was asymmetric, with significant displacement on the high side and large changes in the movement direction. The frequency of mining-induced fractures in the high side fractured area was significantly higher than that in the low side. The high side fracture area had more fracture distribution and fracture aperture was bigger. The mining-induced fractures shown the asymmetric characteristics of “high expansion-low compression”. The multiple mining made the asymmetric characteristics of three-field more significant. In addition, there was a “slow decreasing-fast decreasing” in the overlying strata penetration. Based on the relationship between three-field evolution characteristics and gas mi-gration, the mechanism of preferential gas extraction targeting was revealed. Combined with the experimental results, the process of determining the target area for gas extraction in the fractured zone based on the three-field evolution law was constructed. The gas extraction effect in the field was great, which ensured the safe and efficient recovery of the working face. The results of this study provide a theoretical reference for the accurate extraction of pressure-relief gas in the inclined coal seam group, aiming to improve the gas extraction from the inclined coal seam group, prevent gas overlimit in the upper corner, and achieve safe and efficient mining of the inclined coal seam group

Research status and reduction strategies of methane emissions from closed/abandoned coal mines

With the global transition to a low-carbon energy structure, coupled with the depletion of coal mine resources, gas disasters, and other problems, the number of closed/abandoned coal mines is increasing rapidly. After the coal mine closes, the residual methane in the goaf escapes to the ground continuously, becoming an essential source of greenhouse gas emissions. In view of a series of issues such as methane residual stock, emission rate and emission reduction measures in closed/abandoned coal mines, the number of closed/abandoned coal mines and the distribution of high-gas mines at home and abroad were clarified through a large number of literature review, and the sources and estimation methods of methane residual stock were summarized. At the same time, the theory and research method of methane geological leakage in the field of natural gas accumulation was used for reference, and the emission mechanism and monitoring means of residual methane were analyzed. Finally, the countermeasures and challenges of residual methane emission reduction were put forward. The study found that there are many closed/abandoned coal mines in Shanxi, Guizhou, Chongqing, Hunan, Jiangxi, etc., and the residual coal is mainly anthracite with strong methane adsorption capacity, resulting in a large amount of residual methane in the mine, which will become a critical methane emission source. Methane from closed/abandoned coal mines is desorbed and released to the goaf, and then discharged to the atmosphere through channels such as wellhead and mining-induced fractures. Methane monitoring in coal mines can be realized by means of satellite remote sensing, flux chamber method, geochemical probe method, micro-meteorological technology, etc. Based on the methane emission prediction model of closed/abandoned coal mines, the methane emissions from closed/abandoned coal mines may account for more than 20% of total methane emissions from coal mining operations by 2050, so it is urgent to solve the problem of methane emissions from closed/abandoned mines. Therefore, the countermeasures of emission reduction, such as extraction and utilization, in-situ deflagration power generation, microbial degradation of methane, water flooding, and methane emission channel closure are put forward. Considering the limitations of cost, treatment time, groundwater contamination, and other limitation factors, it is concluded the mineralized remediation method can be used to seal large-scale mining-induced fractures in overlying rocks, which can achieve the methane emission reduction goal of closed/abandoned coal mines at a low cost

Experimental Research into the Evolution of Permeability in a Broken Coal Mass under Cyclic Loading and Unloading Conditions

The permeability characteristics of a broken coal mass under repeated loading and unloading conditions exert significance on spontaneous combustion of coal in goaf during the mining of coal seam groups. Considering this, by using the seepage test system for broken coal-rock mass, seepage tests under cyclic loading and unloading conditions, were carried out on broken coal masses. The test results show that the fitting curves between permeability and effective stress, strain and porosity are a logarithmic function, cubic function and power function, respectively. Besides, the permeability of a broken coal sample under cyclic loading and unloading conditions is determined by its porosity, which conforms to the cubic law. With increased cyclic loading and unloading times, the permeability loss, stress sensitivity and the crushing amount of the broken coal sample were gradually reduced, but the particle size gradation of the broken coal sample gradually became better. During one loading and unloading cycle, the stress sensitivity of the permeability of coal samples in the loading stage was far higher than that in the unloading stage. In the loading stage, the re-arrangement, breakage and compressive deformation of coal particles can lead to a reduction in porosity, consequently resulting in a decreased permeability. In the unloading stage, only the permeability reduction of coal samples due to particle deformation can be recovered

Cause Analysis of Coal Mine Gas Accidents in China Based on Association Rules

Coal mine gas accidents will cause great economic losses and casualties. It is of great significance to find out the essential causes of coal mine gas accidents and put forward measures to prevent them. In this paper, 110 coal mine gas accidents which occurred in China from 2001 to 2022 are selected to analyze the causes of the accidents by extracting the keywords of human factors, equipment factors, environment factors, and management factors from the accident investigation reports. Firstly, the accident statistical analysis is carried out from three dimensions of factor frequency, accident type, and accident grade. Secondly, the Apriori algorithm is used for data mining to obtain frequent item sets and association rules of coal mine gas accident factors. Finally, the coal mine gas accident cause chains which are obtained by using the association rule. The frequent terms of 9 factors, 23 association rules, and 3 coal mine gas accident cause chains are obtained. The results show that the production of coal mine enterprises by illegal organizations is an important reason for the occurrence of coal mine gas accidents. The lack of good management culture easily leads to habitual violations of personnel and decision-making errors, and then causes equipment problems, reflected in the ventilation system which is not perfect, resulting in gas accumulation. The occurrence of coal mine gas accidents can be prevented to a large extent by preventing the absence of good management culture in enterprise management and the occurrence of illegal production behaviors

Seepage and Damage Evolution Characteristics of Gas-Bearing Coal under Different Cyclic Loading–Unloading Stress Paths

The mechanical properties and seepage characteristics of gas-bearing coal evolve with changes in the loading pattern, which could reveal the evolution of permeability in a protected coal seam and allow gas extraction engineering work to be designed by using the effect of mining multiple protective seams. Tests on gas seepage in raw coal under three paths (stepped-cyclic, stepped-increasing-cyclic, and crossed-cyclic loading and unloading) were carried out with a seepage tester under triaxial stress conditions. The permeability was subjected to the dual influence of stress and damage accumulation. After being subjected to stress unloading and loading, the permeability of coal samples gradually decreased and the permeability did not increase before the stress exceeded the yield stage of the coal samples. The mining-enhanced permeability of the coal samples in the loading stage showed a three-phase increase with the growth of stress and the number of cycles and exhibited an N-shaped increase under the stepped-cyclic loading while it linearly increased under the other two paths in the unloading stage. With the increase of peak stress and the accumulation of damage in coal samples, the sensitivity of the permeability of coal samples to stress gradually declined. The relationship between the damage variable and the number of cycles conformed to the Boltzmann function

Improving Coal Permeability using Microwave Heating Technology -- A Review

Microwave heating is a promising non-aqueous technology in coal seam enhancement. Recently, it has been considered as an alternative technology to hydraulic fracturing and Enhanced Coal Bed Methane recovery, which may be inapplicable due to environmental and geological restrictions. In this paper, a critical review of microwave heating (MH) applications in coal permeability enhancement is presented. The mechanisms of both microwave heating and microwave-induced permeability enhancement are explained. Most of the experimental studies and numerical simulations in the related area are reviewed. The influencing factors for microwave heating/fracturing effect are then discussed in detail. After discussing the challenges in applying microwave on-site, potential field applications were suggested. As no field application is reported till now, further studies, especially experiments in the field-scale are in badly demand to test its technical and economic feasibility

Laboratory Study on Changes in the Pore Structures and Gas Desorption Properties of Intact and Tectonic Coals after Supercritical CO₂ Treatment: Implications for Coalbed Methane Recovery

Tectonic coals in coal seams may affect the process of enhanced coalbed methane recovery with CO2 sequestration (CO2-ECBM). The main objective of this study was to investigate the differences between supercritical CO2 (ScCO2) and intact and tectonic coals to determine how the ScCO2 changes the coal’s properties. More specifically, the changes in the tectonic coal’s pore structures and its gas desorption behavior were of particular interest. In this work, mercury intrusion porosimetry, N2 (77 K) adsorption, and methane desorption experiments were used to identify the difference in pore structures and gas desorption properties between and intact and tectonic coals after ScCO2 treatment. The experimental results indicate that the total pore volume, specific surface area, and pore connectivity of tectonic coal increased more than intact coal after ScCO2 treatment, indicating that ScCO2 had the greatest influence on the pore structure of the tectonic coal. Additionally, ScCO2 treatment enhanced the diffusivity of tectonic coal more than that of intact coal. This verified the pore structure experimental results. A simplified illustration of the methane migration before and after ScCO2 treatment was proposed to analyze the influence of ScCO2 on the tectonic coal reservoir’s CBM. Hence, the results of this study may provide new insights into CO2-ECBM in tectonic coal reservoirs