Characteristics Of Coal Samples Research Articles

Experimental study on dynamic mechanical properties of multidirectional constrained water-bearing coal samples under dynamic-static coupling loading

The objective of this study is to investigate the dynamic mechanical properties of coal and rock under deep water conditions. The research employs an enhanced Split Hopkinson Pressure Bar (SHPB) testing system. Five sets of dynamic impact experiments were conducted on coal samples under varying loading conditions to analyse the changes in dynamic strength, energy dissipation, fractal dimension and other characteristics of coal samples under different water content states were analyzed. The experimental results demonstrate that: (1) Under specific strain rate conditions, the dynamic strength of saturated coal samples is lower than that of natural coal samples. As the strain rate gradually increases, the bonding force generated by free water and the Stefan effect jointly act, and the peak strength of saturated coal samples under high strain rate loading conditions is higher than that of natural coal samples. (2) Under certain strain rate conditions, the absorption energy of saturated coal samples is approximately 10% to30% lower than that of natural coal samples, and deformation hysteresis phenomenon occurs in natural coal samples, thereby improving the dynamic strength of natural coal samples relative to saturated coal samples; (3) The fractal dimension of saturated coal samples with a specific strain rate under three-dimensional dynamic static combination loading is higher than that of natural coal samples, and the percentage of small particle coal samples with debris is higher than that of natural coal samples; Finally, based on the HJC model, some coal samples were selected to simulate the coal rock failure characteristics during the triaxial loading process using ANSYS/LS-DYNA, and their stress–strain curves and failure morphology diagrams were obtained. The discrepancy between the numerical simulation and the experimental results was less than 10%, thereby further elucidating and corroborating the coal failure process and dynamic mechanical characteristics.

Study on Damage Characteristics of Water-Bearing Coal Samples under Cyclic Loading–Unloading

For underground water reservoirs in coal mines, the complex water-rich environment and changing overburden stress can damage coal pillar dams. In this paper, the coal samples from coal seam 22 of Shangwan coal mine were taken as research objects and the damage mechanism and characteristics of coal samples with different moisture content and wetting-drying cycles under cyclic loading were investigated. The results show that as the moisture content and wetting-drying cycles increase, the post-peak stage of the coal samples under cyclic stress becomes obvious, and the hysteresis loop changes from dense to sparse. Compared to the uniaxial compression experiment, when w = 5.28% (the critical water content), mechanical parameters such as peak strength and modulus of elasticity decrease the most. Under cyclic loading, the damage mode of both sets of coal samples was tensile damage, but the increase in wetting-drying cycles promotes the development of shear fractures. For evaluating fracture types, the RA-AF density map is more applicable to wetting-drying cycle coal samples, whereas for the coal samples with different moisture contents this should be carried out with caution. This study can provide some theoretical basis for the stability evaluation of coal pillar dams in underground water reservoirs.

Characteristics of Coal Porosity Changes before and after Triaxial Compression Shear Deformation under Different Confining Pressures.

It is important to explore the changes in coal pores in response to triaxial compression and shear deformation for coal mine gas drainage and efficient coalbed methane mining. To study the variation in coal pores depending on stress, first, a mechanical analysis was carried out, and then the characteristics of coal samples before and after triaxial compression were quantitatively analyzed combined with low-temperature nitrogen adsorption experiments. The compressive strength of the coal samples with a high elastic modulus is significantly greater than that of coal samples with a low elastic modulus. Sihe coal samples with a larger elastic modulus experienced higher peak stress and strain during compression than those from the Chengzhuang Mine with a smaller elastic modulus. With the exception of the coal sample from the Chengzhuang Mine with a confining pressure of 15 MPa, the peak strength and axial strain of the coal samples gradually increased with an increase in confining pressure. The larger the elastic modulus, the greater the axial strain. After triaxial compression, pores with diameters ranging from 2 to 5 nm exhibited a significant change. After the compression of coal with a high elastic modulus, the pore volume and pore specific surface area decreased with the increase in confining pressure, by 60.7 and 59.7%, respectively (compared with raw coal). The complex pore structure consisting of mesopores and macropores (>11 nm) became simpler. The volume and specific surface area of the pores of the coal samples with a low elastic modulus first increased, then decreased, and then increased again with the increase in confining pressure, and after compression, the roughness and complexity of macropores of coal samples are greater than those of micropores. The changes induced in the coal samples of the two mining areas in response to compression differ, which are related to the mechanical properties of the coal bodies.

Study on Spontaneous Combustion Characteristics of Coal Samples with Different Pre-oxidized Temperatures in Secondary Oxidation Process

ABSTRACT The paper used thermogravimetric experiments to study the spontaneous combustion characteristics of coal samples with different pre-oxidized temperatures during the secondary oxidation process and focused on the calculation of kinetic parameters during the oxygen-absorbing and weight-increasing stage. The results showed that the total reaction course of spontaneous combustion of raw coal and pre-oxidized coal were similar; the difference in pre-oxidized temperature only had a greater impact on the critical temperature and dry cracking temperature of the coal sample, and then had a greater impact on the water evaporation and desorption stage and the oxygen-absorbing and weight-increasing stage. And the oxygen-absorbing and weight-increasing stage had a more violent reaction than the water evaporation and desorption stage. In the oxygen-absorbing and weight-increasing stage, the difference of pre-oxidized temperature had little effect on the reaction mechanism of each coal sample. The reaction functions all followed the Jander equation, and the mechanism was three-dimensional diffusion. The energy of activation of pre-oxidized coal samples was smaller than that of raw coal, and decreased with the rise of pre-oxidized temperature. When the pre-oxidized temperature was less than 160°C, the pre-factor of the pre-oxidized coal was larger than that of the raw coal, and when the pre-oxidized temperature was greater than 160°C, the pre-exponential factor of the pre-oxidized coal was smaller than that of the raw coal. The research results can lay a foundation for the subsequent prediction of coal spontaneous combustion reaction process, determination of critical spontaneous combustion conditions and evaluation of inhibitory effect. At the same time, it can also be used as a reference and supplement for the existing research on coal oxidation kinetic parameters.

Effect of Acid and Alkaline Environment on Dynamic Strength and Porosity Characteristics of Bursting Liability Coal

In order to investigate the influence of acid and alkaline environment on dynamic strength and porosity characteristics of bursting liability coal, scanning electron microscopy (SEM) and X-ray diffraction (XRD) analysis were used to compare the microstructures of coal with different bursting liabilities. A split Hopkinson bar (SHPB) was used to test the dynamic compressive strength and tensile strength of coal samples with different bursting liabilities. The results show that the surface micromorphology and structure characteristics of coal samples with different bursting liabilities are representatives, which can be used as an auxiliary basis to determine the bursting liability of coal seam. The microstructure of coal with strong bursting liability is characterized by mylonitic, fragmentary, and brecciated structure, and the microstructure is diverse and complex. However, the microstructure of no bursting liability coal is single and uniform. Coal with strong bursting liability shows tensile, compressive, and shear cracks produced by tectonic action, and the distribution of cracks is complicated. The development of fissures is greatly affected by the degree of coal metamorphism, organic components, minerals, and other factors. Under acidic and alkaline environments, the decrease amplitude of tensile strength of coal is obviously larger than that in neutral solution, which indicates that under the action of acid-based solution soaking, the easily soluble minerals in coal react with hydrogen ions and hydroxyl ions in solution obviously. Porosity increment in acidic environment is much larger than that in alkaline and neutral environments. The strong bursting liability coal is more sensitive to acidic environment, while the no bursting liability coal is more sensitive to alkaline environment.

Study of Pyrolyzates from a Variety of Indian Coals and Their Dependency on Coal Type and Intrinsic Properties – An Analytical Fast Pyrolysis Study

ABSTRACT Pyrolysis is a promising technique used to produce oil, syngas, and char of high utility. This work investigates the pyrolytic products and char yields of nine different Indian coals collected from various coal subsidiaries through analytical fast pyrolysis. Coal pyrolysis was performed at 750°C with a single-shot micro pyrolyzer interfaced with gas chromatograph mass spectrometer (GCMS). The characteristics of coal samples were studied and the mean higher heating value (HHV) of coals was determined as 26.26 ± 1.81 MJ/kg. The main pyrolyzates in coals in terms of area composition percentage are identified as C0- to C3-alkylbenzenes (13.83 ± 3.76%), C0- to C3-alkylphenols (12.843 ± 8.18%), C0- to C3-alkylnaphthalene (6.378 ± 3.491%), C0- to C3-alkylindenes (0.98 ± 0.301%), and alkylphenanthrenes and alkylanthracenes (1.054 ± 1.655%). Among all the pyrolyzates, alkyl phenol was found to be most widely distributed in the coal samples. The pyrolytic product dependency on various intrinsic properties of coal was analyzed. A second order correlation equation between the coal type number and main pyrolyzates was established that fitted well (R2 = 0.97) with the obtained data. The char yields from pyrolysis of Indian coals were determined in the range 58.07–80.55% with a mean value of 70.00 ± 6.29%. The carbon contents of the chars were found to be higher than the original coals and the mean HHV of chars was estimated to be 24.47 ± 2.26 MJ/kg. Overall, this study gave an insight into the coal pyrolyzates produced from a variety of Indian coals and their interrelations with the intrinsic coal properties. The results of this study can serve as a reference material for furthering research on coal pyrolysis and establishing relations between the coal properties and pyrolytic products generation.

Microwave absorption properties of organic sulfur compounds in coal: application of desulfurization

ABSTRACT According to the analysis of the characteristics of coal samples, organic sulfur compounds with different structures were selected and their microwave absorption performance was observed in the range of 2.0 GHz to 8.0 GHz. The relationship between absorption performance and desulfurization efficiency was studied by the microwave desulfurization experiment. Results showed that microwave frequency change slightly affects the real part of the complex dielectric constants of organic sulfur compounds. In contrast, it had a significant effect on the imaginary part of complex dielectric constants, dielectric loss tangent, microwave penetration depth and the absorption coefficient, except for the imaginary part of complex dielectric constants and dielectric loss tangent of thiophene compounds. The polarization ability of compounds in the microwave field followed the order: thiophene > mercaptan > thioether, while the efficiencies of mercaptan and thioether for absorbing electromagnetic energy and transforming it into thermal energy were more substantial than that of thiophene. The stronger microwave absorbing performance, the higher the desulfurization rate. Mercaptan and thioether compounds were more accessible to remove than thiophene from coal by microwave radiation and chemical additives.

First investigation of microbial diversity and biogenic methane potential in coal mines located in the Red River Basin, Vietnam

The generation of biogenic methane from coal mines is considered a sustainable approach for enhancing the recovery of natural gas from coalbeds. This study aims to provide the first quantitative data about the characteristics of coal samples, microbial community, and biogenic methane potential in coal mines located in the Red River Basin (RRB), Vietnam. Coal samples from the RRB were characterized and classified into Lignite B and Subbituminous A. Formation water (FW) and coal samples from the investigated boreholes were also analyzed and characterized for microbial communities. Bacterial communities were found to be more dominant (65.8%) than archaea (34.2%) in the FW sample. Four bacterial phyla, including Proteobacteria, Firmicute, Actinobacteria, and Bacteroidetes, were found in the FW sample, in which Proteobacteria were the most abundant phylum, accounting for 56% of total identified bacteria. In the Proteobacteria phylum, only Alphaproteobacteria, Deltaproteobacteria, and Gammaproteobacteria were found in the FW sample; and Gammaproteobacteria were the most abundant class (50.5%). At the genus level, more than 30 microbial genera were detected in the FW sample with the relative abundance >0.1%, in which Shewanella was the most abundant genus, followed by uncultured Rhodobacteraceae, Paracoccus, and Pseudomonas. The microbial population in the coal sample was less diverse than that in the FW sample. The final methane yields after 21 days of incubation with coal samples varied from 2.51 to 5.21 mL of methane per gram coal, exhibiting the great potential for the biotransformation of coal to methane. The results of this study have shown that the presence of indigenous microbial consortia along with available substrates can play an important role in the generation of biogenic methane in coal mines.

Geochemical Alteration and Mineralogy of Coals under the Influence of Fault Motion: A Case Study of Qi’nan Colliery, China

Geochemical characteristics of rocks in fault zones have been extensively studied, while there are limited studies on coal occurring in fault zones of underground coal mine. In this study, five coal samples were carefully collected from a reverse fault zone in Qi’nan colliery. Systematical detection methods were employed to analyze the different chemical and physical characteristics of fault-related coal samples. Through comparative analysis, the following insights are obtained. Three subdivided fault zones were classified according to the deformation characteristics of coal samples. Frictional heat and strong ductile deformation generated by fault motion led to the dissociation of phenol and carboxyl groups in coal molecules, which sharply decreased the concentrations of elements Co and Mo bound to these functional groups in zone I. The modified pore-cleat system in zone I with higher pore volume and lower permeability allowed solutions containing enriched trace elements to migrate through zone I locally. Concentrations of HREE, MREE and related elements associated with the invasive solutions showed significant positive anomalies in zone I. Precipitation and smearing of clay minerals in zone I led to poorer connectivity. Disruption and delamination of laminar clay minerals by strong compression-shear stress significantly increased the adsorption sites for related elements, especially the HREE and MREE. Nano-scale clay minerals resulting from stress-induced scaly exfoliation also enhanced the retention capability of REE in zone I.

Mechanical response characteristics and permeability evolution of coal samples under cyclic loading

AbstractUnderstanding the stress‐cracks‐permeability evolution law inside coal body under cyclic loading is significant to optimize the gas extraction technology and gas disasters prevention technology. In order to explore the influence of mining cyclic stress on the mechanical behavior and permeability variation of gas‐bearing seams, the experiments on permeability evolution, deformation law, and acoustic emission (AE) characteristics of coal samples under cyclic loading with different loading stress level and different loading frequency were carried out. These results indicate that the higher loading stress level, the shorter the fatigue life of coal sample. With the increase of the loading stress level, the change rate of permeability became higher and the relation curves between permeability and loading cycles developed from “U” to “V,” which can be defined and divided into three stages, for example, decrease stage, stable stage, and increase stage. And the permeability in the decrease stage can be modeled as a power function of the loading cycles, while the permeability in the increase stage can be modeled as an exponential function of the loading cycles. Besides, the loading stress level has a significant on the permeability, while the loading frequency has little impact on the permeability evolution. In the process of coal deformation under cyclic loading, the strain development of strain coal samples was in the shape of inverted “S,” while the variation of the peak values of corresponding AE ring counts presented “U” type. In addition, there was a good correlation between the permeability and the AE parameters of coal samples, which provide a new sight into the dynamic variation process of the stress‐cracks‐permeability evolution inside coal body under cyclic loading.

Investigation of gasification characteristics of some coals in Turkey by the thermogravimetric analysis method

ABSTRACTWhen the coal is heated, conversion process starting with moisture loss passes through pyrolysis, burning and gasification processes depending on the atmosphere, temperature rise rates, final temperature and other parameters. Since coal has a heterogeneous structure, interpretation of these phases is difficult. Thermogravimetric analysis, on the other hand, has a wide range of applications and allows valid approaches to the physical and chemical properties of coal. Therefore, the thermogravimetric analysis makes it possible to analyze the characteristics of the coal gasification phase in a practical and rapid manner.In this study, the gasification characteristics of coal samples taken from Ilgın, Ermenek and Zonguldak regions in Turkey were determined. Conversion time and gasification rates of Ilgın and Ermenek coals were investigated using thermogravimetric analyzer at 700°C, 750°C, 800°C and 850°C in CO2 atmosphere and comparisons were made between samples. In addition, conversion and gasification rates of coal samples taken from Zonguldak region at 1150°C were investigated. It was observed that the conversion time of the Ilgın coal at 800°C and the Ermenek coal at 850°C was shorter than the other temperatures. When the 80% conversion rates of Ilgın coal at 800°C, Ermenek coal at 850°C, and Zonguldak at 1150°C, are compared; it is observed that there exist a 1,2 min difference between Ilgın-Zonguldak, 1,05 min between Ilgın-Ermenek and 1,96 min between Ermenek and Zonguldak coals.

Study on the microwave effect on the physical and mechanical properties of coal

In this paper, we study the microwave effect on the physical and mechanical properties of coal samples. Firstly, the analysis of material composition change of coal samples was done. Then, the micro characteristics of coal samples were analysed. Finally, the mechanical and AE properties were tested under uniaxial compression and the causes of coal samples influenced by microwave are studied based on FTIR. The results show that, with the increase of microwave time, the contents of industrial composition in the coal samples greatly change. The content of O2 gradually reduces, while the contents of CO, CO2 and other gases gradually increase. The porosity of coal samples slowly decreases and then increases. The surface physical characteristics of small coal samples are more and more obvious. The compressive strength and the maximum AE pulsing counts of coal samples gradually decrease. The causes of coal samples influenced by microwave are explained based on FTIR. [Received: July 6, 2017; Accepted: December 13, 2017]

Study on the microwave effect on the physical and mechanical properties of coal

In this paper, we study the microwave effect on the physical and mechanical properties of coal samples. Firstly, the analysis of material composition change of coal samples was done. Then, the micro characteristics of coal samples were analysed. Finally, the mechanical and AE properties were tested under uniaxial compression and the causes of coal samples influenced by microwave are studied based on FTIR. The results show that, with the increase of microwave time, the contents of industrial composition in the coal samples greatly change. The content of O2 gradually reduces, while the contents of CO, CO2 and other gases gradually increase. The porosity of coal samples slowly decreases and then increases. The surface physical characteristics of small coal samples are more and more obvious. The compressive strength and the maximum AE pulsing counts of coal samples gradually decrease. The causes of coal samples influenced by microwave are explained based on FTIR. [Received: July 6, 2017; Accepted: December 13, 2017]

The experimental research on response characteristics of coal samples under the uniaxial loading process

In order to study the response characteristics of infrasound in coal samples under the uniaxial loading process, coal samples were collected from GengCun mine. Coal rock stress loading device, acoustic emission tested system and infrasound tested system were used to test the infrasonic signal and acoustic emission signal under uniaxial loading process. The tested results were analyzed by the methods of wavelet filter, threshold denoise, time–frequency analysis and so on. The results showed that in the loading process, the change of the infrasonic wave displayed the characteristics of stage, and it could be divided into three stages: initial stage with a certain amount infrasound events, middle stage with few infrasound events, and late stage gradual decrease. It had a good consistency with changing characteristics of acoustic emission. At the same time, the frequency of infrasound was very low. It can propagate over a very long distance with little attenuation, and the characteristics of the infrasound before the destruction of the coal samples were obvious. A method of using the infrasound characteristics to predict the destruction of coal samples was proposed. This is of great significance to guide the prediction of geological hazards in coal mines.

Characterization of Coal Samples from Godavari Kani Deposits Using Fourier Transform Infrared Spectroscopy

Coal is one of the most abundant organic fuels available in India. Understanding the properties of minerals and ash in coal will be helpful in providing a better knowledge of the factors that control their performance in furnaces. The convention tool for the analysis of mineral content is X-ray diffraction (XRD) method. The capability of XRD for quantitative analysis is restricted to the minerals with good crystal structures. For the amorphous and low crystalline minerals, XRD cannot provide accurate quantitative information. At the same time, even for minerals with a good crystal structure, XRD measurement is a time consuming technique. However, studies on minerals have suggested that FTIR spectroscopy is a superior alternative with respect to speed, accuracy and lower cost. FTIR technique has been used widely in the study of coal,minerals,fly ash and rock. Fourier transform infrared (FTIR) spectra of sub bituminous coal samples, collected from a borehole located at Godavari kani deposit, Andhra Pradesh were recorded between 400 and 4000cm-1. A comparative study of mineral content in coke sample imported from China is also performed using FTIR spectroscopy.

97/01617 DRIFT spectroscopic characterization of coal samples modified by chemical treatments

97/01617 DRIFT spectroscopic characterization of coal samples modified by chemical treatments

Dextrin adsorption by oxidized coal

Dextrin can be used as a depressant to varying degrees of success in the differential flotation of pyrite from coal. The objective of this investigation has been further identification of physico-chemical factors that control the adsorption of dextrin by coal. The research program involved oxidation of coal samples (both natural and demineralized), determination of oxygen functional groups (carboxylic and phenolic), characterization of coal samples in terms of their hydrophobic character and in terms of their electrokinetic behavior, and measurement of dextrin adsorption densities as related to the oxygen group content. Experimental results support previous suggestions that dextrin exhibits Langmuir adsorption behavior with hydrophobic bonding. The more hydrophobic demineralized coal was found to exhibit an adsorption density twice the adsorption density of natural coal. In both cases the extent of adsorption decreased with an increased level of oxidation but was largely independent of pH. Adsorption free energies of about −5.5 kcal/mol, independent of the extent of oxidation, were calculated.