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Now showing items 33 - 48 of 72

  • Influence of supercritical CO2 on pore structure and functional groups of coal:Implications for CO2 sequestration

    Zhang, Kaizhong   Cheng, Yuanping   Li, Wei   Wu, Dongmei   Liu, Zhengdong  

    To better understand the effects of CO2 sequestration and long-term storage, it is worth studying the interactions between supercritical CO2 (SC-O-2) and coal, and its influence on coal properties or, more specifically, the changes in coal pore structure and functional groups caused by SC-CO2. In this study, three different metamorphic grades of coal were sampled and exposed to SC-CO2 (similar to 40 degrees C and 10 MPa) for 120 h through a geochemical reactor, simulating CO2 storage in deep coal seams. The functional groups and pore structure of different coal ranks before and after SC-CO2 treatment were measured by Fourier Transform Infrared Spectroscopy (FTIR), Mercury Intrusion Porosimetry (MIP) and physical adsorption method. The results show that the absorption peak intensity of -OH groups, with intramolecular association and C-H stretching vibrations, clearly changed for anthracite compared to others. Compounds with weakly polar functional groups, such as hydrocarbons, epoxy and lipid compounds (ether or ester), decreased significantly, whereas strongly polar functional groups exhibited only a slight change. Pore structure and distribution of each pore phase showed the diversity present in different coal ranks. The development of seepage-flow pores (mesopore and macropore) was promoted by SC-CO2. For high rank and medium rank coals, the degree of pore development was significantly altered by SC-CO2, while pore development in low rank coal was largely unaltered. The results of this study contribute to the understanding of coal structure evolution and its effects on coal reservoir during long-term geological sequestration. (C) 2017 Elsevier B.V. All rights reserved.
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  • The dual deformation and remodeling of coal powders:implications for obtaining reliable stress-formed coal samples

    Dong, Jun   Cheng, Yuanping   Guo, Pinkun  

    The preparation of suitable specimens is important for obtaining credible mechanical and methane migration parameters for tectonic coal, which help to guide methane extraction and disaster prevention. In this study, a dual-deformation mechanism for porous media was introduced along with two powder compression models, and the issues that should be considered in the preparation of coal specimens were analyzed. By compression tests, the relationship between bed relative density and the applied stress in the compression of coal particles was obtained. The method of coal specimen preparation was introduced in detail. The results indicated that the Kawakita model is suitable for describing the compressive process of tectonic coal powders and guiding the preparation of tectonic coal specimens. The key parameters a and b in the Kawakita model are 0.411 and 0.108, respectively. The bed relative density shows a slight increasing trend followed by an obvious rising tendency with an increase in the applied stress. A compressive stress of 150 MPa was determined to be suitable for preparation of the tested coal specimens.
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  • Experimental study of the mechanical properties of intact and tectonic coal via compression of a single particle

    Dong, Jun   Cheng, Yuanping   Hu, Biao   Hao, Congmeng   Tu, Qingyi   Liu, Zhengdong  

    Mechanical properties of coal are key factors that influence coal mining and methane extraction. Considering the difficulties in obtaining the mechanical properties of the tectonic coal and some intact coal, uniaxial compression tests were conducted on both types of coal particles in the size range of 0.2-4.0 mm. The force-displacement curves, effective elastic moduli and tensile strengths of the intact and tectonic coal particles were obtained. The power functions were used to describe the distributions of the effective elastic moduli and tensile strengths with the diameters of both coal particles. Statistical distributions of the effective elastic modulus and tensile strength for each coal sample with different particle size intervals were also plotted using a logistic function. The test results revealed that the intact coal shows obvious brittleness, whereas the tectonic coal has a smaller brittleness. The obtained effective elastic modulus and tensile strength of the intact coal are 2.72-4.57 and 2.86-6.35 times those of the tectonic coal, respectively, with particle diameters of 0.2-4.0 mm. The characteristics of low strength and large deformation of the tectonic coal would result in greater difficulty of methane extraction and increased risk during coal mining. Some considerations on the structural model of the tectonic coal and measures to enhance the methane extraction efficiency and reduce the risk of mining were also analyzed. (C) 2017 Elsevier B.V. All rights reserved.
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  • Enhanced high-rate performance of sub-micro Li4Ti4.95Zn0.05O12 as anode material for lithium-ion batteries

    Wu, Dongmei   Cheng, Yuanping  

    Zn-doped Li4Ti5O12 was prepared by a ball milling-assisted solid-state method, and the characters were determined by X-ray diffraction, Raman spectroscopy, scanning electron microscopy, cyclic voltammetry, and galvanostatic charge-discharge testing. The results show that Li4Ti5-x Zn (x) O-12 (x = 0, 0.05) exhibits the pure phase structure, and Zn doping does not change the electrochemical reaction process and basic spinel structure of Li4Ti5O12. The particle size of both samples is about 300-500 nm. The prepared Li4Ti4.95Zn0.05O12 presents an excellent rate capability and capacity retention. At the charge-discharge rate of 1C, the initial discharge capacity of Li4Ti4.95Zn0.05O12 is 268 mAh g(-1). After 90 cycles at 5C, the discharge capacity of Li4Ti4.95Zn0.05O12 is obviously higher than that of Li4Ti5O12. The excellent electrochemical performance of the Li4Ti4.95Zn0.05O12 electrode could be attributed to the improvement of reversibility by doping zinc and the sub-micro particle size.
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  • Apparent and True Diffusion Coefficients of Methane in Coal and Their Relationships with Methane Desorption Capacity

    Dong, Jun   Cheng, Yuanping   Liu, Qingquan   Zhang, Hao   Zhang, Kaizhong   Hu, Biao  

    The diffusion coefficient of methane in coal is a key parameter for the prediction of coalbed methane production. The apparent diffusion coefficient is different from the true diffusion coefficient, which would result in the deviation of methane production. In this study, the particle method using the unipore model and the counterdiffusion method is adopted to measure the methane diffusion coefficients. The results indicated that the true diffusion coefficient obtained by the counterdiffusion experiment decreases first and then increases with increasing methane pressure. The apparent diffusion coefficients obtained by the particle method with two different grain sizes are lower than the true diffusion coefficient. The relationship between the apparent diffusion coefficient and the true diffusion coefficient is analyzed, and the desorption capacity factor (DCF) is proposed to reflect the gap between them. The apparent diffusion coefficient is closer to the true diffusion coefficient when the DCF is small. When using the particle method to estimate the methane diffusion coefficient, experiments with large coal particles and a small methane concentration gradient should be adopted.
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  • Analysis of pulverized tectonic coal gas expansion energy in underground mines and its influence on the environment.

    Wang, Zhenyang   Cheng, Yuanping   Wang, Liang   Wang, Chenghao   Lei, Yang   Jiang, Zhaonan  

    Production of a large amount of gas during outbursts will cause greenhouse effects, which will impact the atmospheric environment. In this study, some inherent properties of pulverized tectonic coal were investigated. The results indicate that tectonic coal was more broken and exhibited a higher gas adsorption volume. No obvious changes were found in the micropore and mesopore volumes, whereas the macropore volume and pulverized tectonic coal porosity were significantly increased compared with those of intact coal. Additionally, the initial gas desorption capacities of pulverized tectonic coal were enhanced by tectonism, which might be related to the development of macropore structures and porosity. Analysis of gas expansion energy at the same particle size showed that the values increased with the increasing pressure. Pulverized tectonic coal had a higher gas expansion energy, which could result in a larger outburst of potential energy. Almost all outbursts occurred in tectonic development zones and released a large amount of gas, which greatly damaged the ecological environment. From the perspective of environmental protection, attention should be paid to gas control in the tectonic development zone.=20
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  • Establishment of the equivalent structural model for the tectonic coal and some implications for the methane migration

    Dong, Jun   Cheng, Yuanping   Wang, Liang   Guo, Pinkun  

    The establishment of the equivalent structural model (ESM) is the foundation to simplify the structures of the coal matrix and fractures for the study of methane migration. The ESM of the intact coal has been proposed and widely used by many scholars, but the ESM of the tectonic coal has not been found in the literature. The application of the ESM of the intact coal to the tectonic coal is not reasonable for the study of the methane migration properties, so the establishment of the ESM for the tectonic coal is necessary and meaningful. In this study, a tectonic coal specimen was remodeled from collected coal powders, and methane permeability tests were conducted. Then the ESM of the tectonic coal was established by analyzing the fracture structure. The results show that the tectonic coal can be idealized as a model containing a cubic matrix with square section fractures located at the twelve edges of the matrix. Because of the variation of the ESM, the permeability and the fracture porosity has a square relation for the tectonic coal, rather than the cubic relation for the intact coal. The matrix shape factor of the tectonic coal has also been proposed, which has a value of 480b(f)/7a3m and is much smaller than that of the intact coal.
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  • Experimental observations of matrix swelling area propagation on permeability evolution using natural and reconstituted samples

    Liu, Qingquan   Cheng, Yuanping   Ren, Ting   Jing, Hongwen   Tu, Qingyi   Dong, Jun  

    Many researchers have concluded that the permeability should be measured after the adsorption equilibrium state is reached and in many permeability models, it is normally assumed that pm (pressure in the coal matrix) is equalized with p(f)(pressure in the fractures). However, when the dynamic balance between pm and pf is broken which exists in most coalbed methane engineering, the differences between the two pressures induced by gas diffusion and matrix shape factor will generate nonuniform matrix swelling which have not been well learned to date, thus influence the permeability evolution. It is of great importance to investigate this issue on account of it will play a more important role in CO2-enhanced coalbed methane and CO2 sequestration engineering as the coal adsorption capacity for CO2 is considerably higher than that for CH4. To achieve this target, a series of novel coal permeability tests and simultaneous strain measurements were conducted using both natural coal and reconstituted coal. In these experiments, the radial strain evolution characteristics and the permeability evolution characteristics as a function of the adsorption equilibrium time and confining pressure were obtained. Based on the experimental results, the effects of the differences between pm and pf on radial strain evolution and coal permeability evolution were discussed, and a new matrix swelling area propagation theory was achieved. In addition, the differences between natural coal and reconstituted coal in terms of their matrix swelling and stress sensitivity properties were obtained. (C) 2016 Elsevier B.V. All rights reserved.
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  • Gas outburst disasters and the mining technology of key protective seam in coal seam group in the Huainan coalfield

    Wang, Haifeng   Cheng, Yuanping   Yuan, Liang  

    Coal and gas outburst disasters in coal seams are becoming more serious as coal mines extend deeper underground in China. To aid gas control in high-gas outburst coal seam group, this study performed research based on the geological conditions of the Xinzhuangzi coal mine in the Huainan coalfield. The laws of gas occurrence, the strength of the coal outburst, and the regional partition were studied. Simultaneously, we introduced the key protective seam mining technology and confirmed the mining sequence of coal seam groups. The results indicate that (1) each seam absorbs gas well, and the currently measured gas content is up to 15.0 m(3)/t. (2) Although some differences about coal seams outburst intensity remain, the differences in the same group are very small. (3) The coal seam B10 was chosen as the key protective seam and was mined first; then adjacent seams were mined from bottom to top by layer within the roof of B10 and from top-to-bottom within the floor of B10 to guarantee each adjacent coal seam received the good effects of pressure-relief and increasing permeability. (4) The main methods of gas extraction in each protected seam are surface boreholes and net-like penetrating boreholes in the floor roadway, and related technical parameters were determined according to the degree of pressure-relief in coal seam. This in situ experiment indicates a method aiding the gas control problem and guaranteeing safe and highly efficient exploitation of high-gas outburst seams.
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  • Mechanical failure mechanisms and forms of normal and deformed coal combination containing gas:Model development and analysis

    Lu, Shouqing   Li, Lei   Cheng, Yuanping   Sa, Zhanyou   Zhang, Yongliang   Yang, Ning  

    In coal mining, coal seams often contain several normal coal layers and deformed coal layers in the same region. However, the existence of deformed coal may promote the occurrence of coal and gas outbursts. To analyze the role of failure of different combinations of deformed coal on coal and gas outbursts, force analysis of normal and deformed coal combinations containing gas was carried out, and the expressions of interfacial stress induced by uncoordinated horizontal deformation were first derived in this paper. The results showed that the additional interfacial stresses are affected by the external stresses, internal gas pressure, gas sorption-induced swelling deformation and mechanical parameters of coal and that the additional interfacial stresses on the normal coal and deformed coal are equal and opposite. Then, based on the Mohr-Coulomb criterion and uncoordinated deformation, the mechanical failure mechanisms and forms of combinations containing gas under ideal conditions were analyzed. We found that there are seven failure forms for the combination containing gas, and in most cases, the destruction of the combination is caused by the structural failure of deformed coal. Finally, the failure mechanisms of the combination due to mining were discussed using the parameters from the Qinshui Basin. The results showed that failure is more probable at the interface between the deformed coal and normal coal bodies and that the existence of deformed coal can promote the damage of normal coal. These research results can help us to better understand the role of mechanical failure in coal and gas outbursts within combinations, and provide a theoretical basis for the control of coal and gas outbursts.
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  • Experimental investigation of the constant and time-dependent dynamic diffusion coefficient:Implication for CO2 injection method

    Liu, Zhengdong   Cheng, Yuanping   Wang, Liang   Pang, Bin   Li, Wei   Jiang, Jingyu  

    Gas diffusion in coal is an important transport mechanism that plays a crucial role in carbon dioxide (CO2) storage in and methane (CH4) extraction from coal seams. Studies on coal diffusion are largely based on experimental work on different coal particles and the establishment of uni-pore or bidisphere theoretical models. However, problems remain in both experimental and theoretical analyses. Therefore, the initial desorption time of adsorbed gas, something that is not done using conventional experimental devices, was revealed in this study. The initial desorption time was obtained by calculating the relationship between the amount of gas collected and the theoretical amount of free gas. An improved diffusion model considering lost gas was established based on the initial desorption time of adsorbed gas. The mpdel provides diffusion coefficients for different equilibrium pressures and particle sizes on the basis of a unipore diffusion model. By correlating diffusion coefficient and time, an empirical time-dependent diffusion model was established to solve the dynamic diffusion coefficient under different conditions. Finally, in order to substantiate that the variation in diffusion length is the primary cause of the variation of diffusion coefficients with time, the variation of diffusion length with desorption time was obtained. As a result, a stepwise pressure-rasing method was proposed. This new injection method can effectively improve CO2 storage and CH4 recovery from coal seams. The study has a certain guiding significance in engineering practice.
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  • Investigation of the formation mechanism of coal spallation through the cross-coupling relations of multiple physical processes

    Tu, Qingyi   Cheng, Yuanping   Liu, Qingquan   Guo, Pinkun   Wang, Liang   Li, Wei   Jiang, Jingyu  

    Coal spallation is a visible product of outburst, and it represents a unique failure type for coal. A theoretical analysis of coal spallation was constructed to study its formation mechanism. Then, physical experiments and numerical simulations were performed to determine the physical features of coal spallation and evaluate the theoretical analysis. In addition, the effect of gas, stress and coal strength on coal spallation was discussed. The results show that coal spallation is a comprehensive product of multiple physical factors, including stress, gas and the coal strength. The formation process is related to stress transfer and mechanical behavior changes, gas migration and the evolution of the coal strength. Coal is eventually subjected to tensile failure, and the mechanical condition for coal spallation requires that the gas pressure difference (p(i) - p(a)) near the exposed surface is enough to overcome the tensile strength (sigma(t)). The gases in coal provide the power necessary for coal spallation; thus, coal gas promotes plastic failure and is the leading factor for tensile failure. Stress is the leading factor for plastic failure and has a great influence on the extent of coal spallation. Meanwhile, with an increase in the coal strength, which represents the resistance to coal spallation, the acreage of the plastic area decreases, and thus, a greater pressure difference is required for tensile failure.
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  • Pore structure characterization of coal particles via MIP,N-2 and CO2 adsorption:Effect of coalification on nanopores evolution

    Jiang, Jingyu   Yang, Weihua   Cheng, Yuanping   Zhao, Ke   Zheng, Shaojie  

    To study the pore structure characteristics of middle-high rank coals (MHRC), five different kinds of coal samples from northern China were selected and pulverized, and the pore volume, pore size distribution, and fractal dimension were analyzed using N-2/CO2 adsorption and mercury intrusion porosimetry. The results show that the nanopores size of MHRC tends to decrease with the increase of coal rank, especially the ultra-micropores dominate in semi-anthracite coal. In addition, the fractal dimensions of the adsorption pore (pores >0.5 nm and pores <0.5 nm) are calculated, respectively. Both of these fractal dimension-fitting curves exhibit a U-shaped parabolic relationship with the R-o. It can be found that the nanopores is affected by the coalification jumps by analyzing the relationship between the pore structure parameters and R-o. The third coalification jump has a turning effect on the micropores development, and the fifth coalification jump significantly promoted the mesopores volume. (C) 2019 Elsevier B.V. All rights reserved.
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  • Molecular structure characterization of middle-high rank coal via XRD, Raman and FTIR spectroscopy: Implications for coalification

    Jiang, Jingyu   Yang, Weihua   Cheng, Yuanping   Liu, Zhengdong   Zhang, Qiang   Zhao, Ke  

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  • Microscale Research on Effective Geosequestration of CO2 in Coal Reservoir:A Natural Analogue Study in Haishiwan Coalfield,China

    Zhang, Kaizhong   Li, Wei   Cheng, Yuanping   Dong, Jun   Tu, Qingyi   Zhang, Rong  

    Anatural analogue study in CO2-rich coalfield (Haishiwan, China) provides a strong support for safe, reliable, and long-term storage by analyzing the mechanism of CO2 migration, entrapment, and storage in coal reservoir. Thus, effects of geological tectonism on reservoir properties were investigated. Simultaneously, coal and oil shale samples before and after supercritical CO2 (SCCO2) treatment via geochemical reactor were collected to analyze changes in pore structure, functional group distributions, and SCCO2 extraction. Observations from in situ properties of coal seam indicate that there is a positive relationship with CH4 contents and F19 fault whereas CO2 and carbonate contents decrease as the distance from F19 increases. Analysis of pore properties reveals that SCCO2 enlarges the development of coal pore and facilitates the diffusion and seepage channel of coal reservoir, while no changes in larger pores are found in oil shale, which may restrain fluids from passing through. Then, oxygen-containing functional groups are mobilized by SCCO2 from oil shale, associated with a decrease in sorption sites. The sealing capacity of cap rock (oil shale) and geological tectonism(F19 fault), as the major contributors to CO2 enrichment and accumulation, provides insights into the suitable selection of CCGS site for long geological time.
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  • Experimental Investigation on the Effects of Supercritical Carbon Dioxide on Coal Permeability:Implication for CO2 Injection Method

    Li, Wei   Liu, Zhengdong   Su, Erlei   Cheng, Yuanping  

    CO2 storage in deep unrecoverable coal seams has become an effective method to curb greenhouse gas emission, which also contributes to an increase in coalbed methane (CBM) production. Due to high temperature and pressure in deep coal seams, the injected CO2 remains in a supercritical state. However, the influence of injecting supercritical CO2 into coal seams on coal permeability is not particularly clear at present. Therefore, this paper conducted a series of studies through high-pressure triaxial setups on the naturally fractured coal, the coal adsorbing supercritical CO2 for different times, and supercritical CO2-treated coal, respectively. In the experiment, the laws of coal permeability variation with different adsorption times of supercritical CO2 under different gas injection pressure were first determined. Results indicate that there is a decline in coal permeability in the initial stage because of the great swelling deformation induced by adsorption. Besides, the coal may find a rebound in permeability in the later phase if its mechanical properties have been changed after repeated adsorption. Meanwhile, He was applied to measuring the permeability of original coal and the coal completely desorbing supercritical CO2, respectively. On the basis of a comparison between their permeability values, the latter has higher permeability. It directly indicates that supercritical CO2 has extraction and dissolution effects on organic matter and inorganic minerals in coal, respectively, thus enhancing the permeability. To further support this point of view, pore characteristics of such two kinds of coal were determined through mercury intrusion method. It is demonstrated that the supercritical CO2-treated coal has well developed and connected pore system, with a rise in the proportion of meso- and macro-pores as well as total pore volume. In addition, the coal with different adsorption times varies in permeability evolution laws under the same effective stress. Inspired by such variation, this paper proposed a new gas injection method, that is, injection pressure rose gradually after cyclic adsorption of supercritical CO2, which provides a certain reference for efficient gas injection.
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