Degree Of Coal Metamorphism Research Articles

Migration characteristics of constant elements in the process of coal dissolution by liquid CO2

The effect of liquid CO2 on coal dissolution plays a key role in improving coal bed methane (CBM) recovery and CO2 geological storage. In order to study the effect of liquid CO2–H2O reaction on the migration of mineral constant elements in coal, three kinds of coal samples (lignite, bituminous coal, and anthracite) with different degrees of metamorphism were selected as the research objects. The one-factor experiments of dissolution of liquid CO2 and water-bearing coal samples were carried out by simulating the temperature and pressure of the stratum. X-ray fluorescence spectroscopy (XRF) was used to test the changes in the relative content of constant elements in coal samples after liquid CO2 injection under different pressure conditions. Scanning electron microscopy (SEM) was utilized to observe the evolution pattern of the surface features of the coal before and after the dissolution of liquid CO2. A quantitative analysis of coal pore development characteristics was conducted based on low-temperature N2 adsorption/desorption experiments, and isothermal adsorption experiments were carried out before and after etching the coal samples to understand the adsorption characteristics of CH4 and CO2 by coal under different pressure conditions. The research results indicated that the migration ability of constant elements in coal showed a trend of first decreasing and then increasing with the increasing degree of coal metamorphism. Among the three selected coal samples, the migration ability of constant elements was found to be strongest in anthracite, followed by lignite and bituminous coal. Most of the element migration in coal was carried out through chemical reactions (hydrolysis, carbonation, redox), while some elements were primarily transported through physical means (such as the low-temperature impact of liquid CO2). The dissolution process expands the original pore space of coal and generates new pores, greatly increasing the coal's capacity for CO2 adsorption. These findings deepen our understanding of mineral dissolution in coal by liquid CO2 and provide important insights for achieving efficient and long-term coalbed CO2 geological storage and enhanced methane recovery techniques.

Pyrolysis characteristics and product distribution of low-rank coal with heat-carrying particles adopting TG-FTIR and a novel self-mixing down tube reactor

A novel self-mixing down tube (SDT) pyrolyzer has been developed, which can separate the heat carrying particles preheated in the fluidized bed from the coal, and the two can be mixed through a specific structure to instantly heat the coal particles to a predetermined temperature. The present study investigated the pyrolysis behavior of low-rank coal with hot silica sand heat-carrying particles adopting TG-FTIR. Identifies optimal conditions for maximum tar production in a laboratory-scale SDT using a significantly larger range of coal feed rates (4 kg/h). An increase in the degree of coal metamorphism increased the initial temperature and the yield of char. The tar yield from Huolinhe lignite coal (6.56 %) and Daliuta bituminous coal (7.48 %) showing a trend of first increased and then decreased with elevating the operating temperature and optimal temperature at 520 °C, respectively. The effect of coal particle size on tar yield mainly comes from the different degrees of secondary cracking caused by contact with solid heat-carrying particles as well as the differences in the content of macerals. GC‒MS spectra of tar and deposited carbon on silica sand indicated that heavy tar can easily deposit on the surface of sand and form char through polymerization reactions.

A micro-macro coupled permeability model for gas transport in coalbed methane reservoirs

It is of great scientific and engineering significances to reveal the coupling mechanism of microscale and macroscale permeabilities in coalbed methane (CBM) reservoirs. In this paper, firstly, the characteristics of the pore-fracture system of coal matrix, such as gas content, permeability, connectivity, geometric size difference, and direction of micro-fracture development, are revealed. Secondly, a coupled permeability model is established, and the coal rank control mechanism on the transformation of microscale and macrosclae permeabilities is clarified. Thirdly, based on the microscale gravitational field model, the coal rank-pressure coupling influence mechanism on slippage/diffusion/jump of adsorbed gas in the surface control field is clarified. Results show that: (a) under a certain temperature, the microscale permeability and macroscale permeability of coal matrix have a statistical correlation, and the degree of coal metamorphism is the only variable controlling the transformation of micro-macro permeabilities. (b) With the increase of coal rank from 0.5% to 3.5%, the conversion coefficient of bulk gas permeability decreases by 15 orders of magnitude. (c) With the increase of coal metamorphism, the porosity and the density of oxygen-containing functional groups decrease, leading to the proportion of the area of adsorbed water film decreases at first and then increases.

The identification of coal and gangue and the prediction of the degree of coal metamorphism based on the EDXRD principle and the PSO-SVM model

The identification of coal and gangue and the prediction of the degree of coal metamorphism based on the EDXRD principle and the PSO-SVM model

Pore and gas desorption characteristics of primary coal with different degrees of metamorphism

AbstractPore difference characteristics and adsorption/desorption experiments of primary coal with different degrees were explored to study the characteristics of pore structure and gas desorption. The results show that the pore volume increases with the increase in the degree of metamorphism. In primary coal with different degrees of metamorphism, the distribution of micropores, small pores, and medium‐sized pores (by volume) is WY < PM < CY. The distribution of large pore volume and visible pore volume was WY > PM > CY. According to the pore volume distribution law of different metamorphic primary coals, the average pore diameters of WY, PM, and CY were 68.40, 45.60, and 30.50 nm, respectively. The porosity and specific surface area of primary coal with different degrees of metamorphism show a distribution pattern such that WY > PM > CY. For large pore and visible pore, the pore volumes of WY, PM, and CY coal were 0.0785, 0.0587, and 0.0300 mL/g, respectively, and the proportions were 81.86%, 74.49%, and 55.25%, respectively. The porosity of WY, PM, and CY were 11.57%, 9.03%, and 6.57%, respectively, and the specific surface areas were 6.917, 5.826, and 5.611 cm3/g. According to the desorption characteristics of coal bodies with different metamorphic degrees at different time nodes found that the gas desorption at different time nodes shows similar changes under the same adsorption equilibrium pressure. The higher the degree of coal metamorphism, the greater the amount of gas desorption, and the faster the desorption amplitude. The desorption intensity of WY coal is significantly higher than that of PM and CY coal with the increase of desorption time which also verifies the measurement results of pore structure and adsorption constant.

Study of the Oxidation Characteristics and CO Production Mechanism of Low-Rank Coal Goaf

Affected by an insufficient understanding of oxidation characteristics and the CO production mechanism in low-rank coal goaf, the safety management of coal spontaneous combustion (CSC) faces severe challenges. In this study, in-depth research was conducted using ambient temperature oxidation (ATO), temperature-programmed, in situ FTIR experiments and DFT simulation after analyzing the oxidation scenario characteristics of low-metamorphic coal goaf. The results show the oxidation of low-rank coal goaf includes two processes of ATO in the dissipation zone and CSC in the oxidation zone. The CO production of ATO increases with a decrease in coal metamorphic degree, and the risk of CSC is influenced by ATO, with an inhibitory effect before the critical temperature, and an encouraging effect after that. The CO production mechanism of low metamorphic coal goaf from ATO to CSC is established. Before the critical temperature, CO mainly comes from the primary aldehyde functional groups, then peroxide-free radicals participate in the reaction, resulting in the production of a large number of secondary aldehyde functional groups, which leads to the sudden change in CO output. The problem of the abnormal, continuous exceedance of CO in the tailgate corner can be solved by developing an ATO inhibitor, which plays an inhibiting role at ambient temperature and decomposes in the event of CSC.

Thermodynamic characteristics of methane adsorption on coals from China with selected metamorphism degrees: Considering the influence of temperature, moisture content, and in situ modification

A better understanding of the thermodynamic characteristics of methane (CH4) adsorption on coal surfaces is needed for the high-efficiency coalbed methane (CBM) preextraction or prevention of coal and gas outbursts. In this study, atomistic representations of selected coal samples with different degrees of metamorphism, such as long flame coal, coking coal, and anthracite coal obtained from China were created; CH4 adsorption isotherms considering the temperature, moisture content, and microwave in situ modification for CBM preextraction were simulated and analysed. The coal molecular structural results showed that as the degree of coal metamorphism increased from long flame coal to coking coal and anthracite coal, the functional groups in the molecular structure of the coal reduced, while the degree of graphite increased. In situ modification of coal facilitated this process. The saturation adsorption capacity of CH4 decreased and then increased with increasing coal metamorphism degree and decreased linearly with increasing temperature. When water molecules were present, the adsorbed CH4 showed different degrees of decreasing trends with increasing moisture content. The isosteric heat of CH4 adsorption followed the order of coking coal > anthracite coal > long flame coal. The isosteric heat gradually decreased with increasing methane adsorption, and the presence of water molecules had a limited effect on the isosteric heat. In situ modification disrupted the self-conjugated hydrogen bonds, reduced the oxygen-containing functional groups, and decomposed some associated minerals in the coal samples. After in situ modification, the amount of methane adsorption decreased due to a greater contribution from the functional group change than that from the pore volume change, and the modification effect was more evident for the low-rank coal samples than for the middle and high metamorphism degree coal samples. In situ modification applied in the coal seams could contributed to designs for CBM exploration, and the results of this experiment and simulation could provide a novel theoretical basis for in situ modification technology.

Construction of different metamorphic degree coal models and corresponding adsorption characteristics based on molecular simulation

Coal samples with different metamorphic degrees have different gas-adsorption capacities. Most of the existing studies analyze differences in gas-adsorption characteristics from a macro perspective, based on experimental methods, and there has been limited research on the micro level. Using industrial and elemental analyzes, Fourier-transform infrared spectroscopy, 13C-NMR, and X-ray photoelectron spectroscopy, we tested and analyzed the molecular-structure characteristics, such as elemental composition, atomic ratio, and functional-group type and distribution, of three coal samples: anthracite (WYM), coking coal (JM), and lignite (HM), with different metamorphic degrees and constructed their macromolecular-structure models. The carbon contents and densities of the models were consistent with the measured values. We also constructed a coal matrix model using a molecular simulation based on the constructed coal molecular model. By comparing and analyzing the trends of adsorption amount, isosteric heat, and selective adsorption coefficient, we explored the differences in the adsorption characteristics of single and binary mixtures on coals with different metamorphic degrees. The adsorption of CH4 and CO2 on the coal matrix conformed to the Langmuir adsorption model. With increasing metamorphic degree, the adsorption amount increased in the order WYM > JM > HM, and the adsorption capacity of the coal matrix for CO2 was higher than that for CH4. The isosteric adsorption heat of CH4 was proportional to the degree of coal metamorphism; however, its relationship with pressure was not clear. Under different temperature and pressure conditions, the selective adsorption of CH4/CO2 on coal with various metamorphic degrees decreased with an increasing CO2 molar ratio. Conditions of 303.15 K (30 °C), 2–4 MPa, and CH4/CO2 = 0.1 were more conducive to the competitive adsorption of CO2 than that of CH4.

Tectonic control law and zoning division of coal seam gas occurrence in Sichuan Province

Understanding the coal seam gas occurrence law is the premise and foundation of gas disaster prevention and production planning. The tectonic control law and zoning division of gas occurrence in Sichuan Province have been discussed, based on the gradual tectonic control theory for gas occurrence, combined with coal mine gas-geology data and a geological survey in Sichuan Province. The obtained results indicate that the generation, migration, and preservation of gas are controlled by the formation and development, cracking, subduction collision, and intracontinental orogeny of the upper Yangtze block. The gas occurrence and distribution characteristics in Sichuan Province are significant. Gas occurrence of Late Permian coal seams is significantly higher than that of Late Triassic coal seams, as 85.7% of the outburst mines in the Sichuan Province are distributed into the Late Permian coal seams. The Late Permian coal seams in the Xuyong-Junlian superimposed fold belt and Huayingshan detachment fold belt are high gas occurrence zones. These zones had been under strong tectonic compression for several years, thereby resulting in interlayer sliding, formation of stratified tectonic coal, and high degree of coal metamorphism and high gas content, which represent the fundamental reasons why 84.3% of outburst mines in Sichuan Province are located in these two regions. According to the tectonic control law and characteristics of coal seam gas occurrence in Sichuan Province, the gas occurrence zones of coal seam in Sichuan Province were divided into three high gas and outburst zones, three high gas zones and one low gas zone; in addition, their tectonic control characteristics and gas occurrence characteristics were analysed.

Using indices of the current industrial coal classification to forecast hazardous characteristics of coal seams

Purpose is to identify changes in the basic indices of vitrinite reflection as well as content of volatile matter in the process of seam underground transformation influenced by high temperature and pressure and define nature of the changes in the classification indices and their correspondence to grades, groups, and subgroups of similar metamorphic coal transformations to determine hazardous characteristics of seams while mining. Methods. Selection of the most adequate basic index, evaluating metamorphism degree of coal seam and forecasting of their hazardous characteristic demonstration while mining, is based upon the average data of coal composition in Donbas depending upon the stages of seam metamorphism. Findings.Analysis of indices of the current industrial classification and normative base has shown the following. They cannot characterize directly the coal metamorphism connected with changes in composition and properties of the original organic substance. It has been mentioned that volatile matter do not demonstrate specifically ultimate composition of the organic matter and phase coal composition while fluid extraction from seams during their metamorphic transformations. Only the total release of gaseous products without identification of the released gases is defined by its value. Average vitrinite reflection has been defined experimentally along with other indices to identify the coal grades. For that reason, values of the index not always correspond to coal metamorphism degree in their grade ranking. Originality. Nature of changes in the classification indices of industrial coal ranking has been defined to identify hazardous characteristics of coal seams while mining. Practical implications. Possibilities to improve a normative base for safe mining have been defined while determining the classification indices characterizing directly the changes in ultimate composition during metamorphic processes.

Prediction of spontaneous combustion tendency of coal based on acetaldehyde release

ABSTRACT Acetaldehyde is one of the crucial gases with odor released during the early stage of spontaneous combustion of coal, and it is essential for predicting the spontaneous combustion tendency of coal. In this study, we systematically investigated the effects of coal metamorphic degree, coal temperature, coal particle size, and oxygen concentration on the release of acetaldehyde using three coal samples with different metamorphic degrees and a combination of oxidation and warming processes as well as gas and liquid chromatography. The results showed that with the increase in temperature, the amount of acetaldehyde released first increased and then stabilized or slightly decreased. The amount of acetaldehyde released at low temperatures was linearly related to oxygen consumption. The higher the degree of coal deterioration, the lower the amount of acetaldehyde released. As the particle size decreased, the amount of acetaldehyde released increased and then decreased. Finally, four warning levels were established based on the concentrations and trends of acetaldehyde together with other index gases.

НАУКОВІ ОСНОВИ РОЗРОБКИ МЕТОДУ ПРОГНОЗУ НЕБЕЗПЕЧНИХ ВЛАСТИВОСТЕЙ ВУГІЛЬНИХ ШАХТОПЛАСТІВ

Purpose: to develop a method of coal gradation to predict the hazardous properties of coal seams during mining. Methodology: based on the study of the interdependence between indicators of the degree of coal metamorphism, which characterize various aspects of its transformation in geological processes. Results: the dependence of the manifestation of any dangerous property of coal seams during mining operations on the influencing factors of the three blocks is considered. In the general case, the most dangerous properties of coal seams include the release of explosive and flammable gases, sudden emissions of coal and gas, the tendency to spontaneous combustion and the occurrence of endogenous fires, increased dust formation, explosiveness of coal dust and other negative phenomena. To prevent emergencies during mining operations, it is necessary to take into account the influence of factors of all three units. Factors of the first block determine the genetic predisposition of mine shafts to the appearance of dangerous properties under the influence of geological processes and metamorphic transformation of the source material. The factors of the second block include mining and geological conditions of coal seams. On the basis of data on parameters of the first two blocks at stages of designing and operation of the coal enterprise, mining indicators of the third block of factors are put. In contrast to the mining-geological and mining conditions of the second block, the factors of the first block are the least studied and not always reliably established. They must determine under the influence of metamorphism changes in the chemical composition, structure and physical properties of coal in the bowels of the Earth, mainly under the influence of elevated temperature and pressure. Currently, more than thirty factors are known, which in different ways characterise the metamorphic transformations of the starting material. There is a practice when in normative documents for the characteristic of degree of metamorphic transformations of layers in the vast majority of cases one indicator is used – an exit of volatile substances at thermal decomposition of coal without access to air. One indicator can not simultaneously and on all sides characterise the content, structure, chemical and physical and mechanical properties of the organic mass of coal and mineral impurities. It is necessary to proceed from the position that each dangerous property of mine layers depends on a certain influence of several factors of metamorphism. Studies have shown that the carbon content directly controls the overall change in the sum of the main components of organic matter (hydrogen, nitrogen, sulphur, oxygen), and the reflection rate of vitrinite – reflects structural changes in petrographic composition. The chemical activity of coal is also affected by the presence of moisture in different states, composition and properties of mineral impurities. The use of each auxiliary indicator must be justified taking into account the purpose of application and the method of its determination Scientific novelty: the carbon content and the rate of reflection of vitrinite in the range of ranking coal by their degree of metamorphic transformations have a reliable quantitative determination, which allows their use in establishing the hazardous properties of coal seams as the main classification indicators. Practical value: the results obtained allow developing general principles of scientific substantiation of the method of forecasting the dangerous properties of coal seams in combination with mining-geological and mining conditions of work on the basis of carbon content and vitrinite reflection index.

Effect of anionic/nonionic surfactants on the wettability of coal surface

To study the effect of different surfactants on the wettability of coal surface, the adsorption simulation of different coal-surfactant-water model systems was carried out by using materials Studio 8.0 molecular simulation software. Combined with the contact angle experiments between different surfactants and coal samples, the effect law of different surfactants on the wettability of three coal samples was obtained. The adsorption simulation results were analyzed from the aspects of adsorption spacing, adsorption sites and “reverse adsorption” phenomenon, and the mechanism of the influence of different surfactants on the wettability of coal surface was revealed from a microscopic point of view. The research results show that: the higher the degree of coal metamorphism, the lower the water absorption and the worse the wettability of coal. Anionic surfactant SDBS has better effect on improving the wettability of Hami (HM) coal with low metamorphic degree, while nonionic surfactant AEO9 has better effect on improving the wettability of Yuwu (YW) and Tunliu (TL) coal with high metamorphic degree. By comparing intermolecular distances, it is shown that nonionic surfactants are more likely to aggregate on the surface of coal molecules and have a greater effect on the wettability of the coal surface. The middle position of the benzene ring structure of coal is the best adsorption site for the hydrophobic tail of the surfactant. By calculating the adsorption energy of different simulation systems, it is confirmed that the phenomenon of “reverse adsorption” between nonionic surfactants and low metamorphic coal is caused by hydrogen bonding. The results of the study provide a theoretical basis for the effective application of different surfactants in coal dust control.

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.

The feasibility of CO2 emission reduction by adsorptive storage on Polish hard coals in the Upper Silesia Coal Basin: An experimental and modeling study of equilibrium, kinetics and thermodynamics

Carbon dioxide storage in unmineable coal seams is advantageous in the highly industrialized areas, such as the Upper Silesia Coal Basin (USCB), Poland, where heavy industry constitutes the source of huge CO2 emissions and coal mines will be closed in the future, due to unprofitability. The paper presents the results of experimental and theoretical research of CO2 capture on medium rank C and B bituminous coals coming from three mines located in the USCB. The porous texture of the investigated adsorbents was analyzed using SEM images and the N2 and CO2 isotherms at −196 °C and 0 °C, respectively. Qualitative studies using DRIFT spectroscopy showed that band intensity attributed to the functional groups of coals changed after CO2 adsorption. The analyses encompassed the equilibrium, kinetics and thermodynamics of CO2 adsorption on coals at 25, 50 and 75 °C (up to 2000 kPa). The adsorption isotherms were obtained by the static gravimetric method and described by means of the Langmuir, Freundlich, Dubinin-Radushkevich and Dubinin-Astakhov models. The highest CO2 uptakes were obtained for medium rank C bituminous coals at 25 °C; the values were 1.600 mol/kg and 1.274 mol/kg. The adsorption kinetics was better characterized by the Avrami fractional-order model rather than by the pseudo-first and pseudo-second order models. The results reveal that the adsorption process is the fastest for medium rank C bituminous coals. The isosteric heats of adsorption were calculated in the following two ways: based on the multi-temperature Toth isotherm and the Clausius-Clapeyron equations. Depending on degree of coal metamorphism, the heat of adsorption ranged from 18 to 26 kJ/mol. The estimated maximum temperature increase due to heat accumulation in the insulated coalbed during CO2 adsorption was 6 °C and did not reach the self-ignition temperature in any of the tested adsorption systems.

Quantitative study on the influence of the difference of the molecular structure on the spontaneous combustion characteristics of the coal metamorphism process

Neimeng lignite, Huajin bituminous coal, Sitai bituminous coal, and Changcun lean coal with different degrees of metamorphism were analyzed via infrared spectrum curve fitting analysis and thermal analysis experiments, and the influence of the change in the active functional groups in the coal structure on the coal spontaneous combustion characteristics was explored. After the spectral curve was fitted, it could be divided into four areas—hydroxyl, C–H stretch band, oxygen-containing functional group, and aromatic structure—and then, the molecular structure content of different functional groups was quantitatively calculated. In the thermal analysis experiment, the characteristic value and the combustion activation energy of the coal were obtained through the thermogravimetric analysis curve, and the calorific value of the coal could be calculated using the differential scanning calorimetry curve. After combining the two experiments to obtain a linear relationship, the results showed that as the degree of coal metamorphism deepened, the activation energy of the coal combustion gradually increased, and the amount of heat released decreased. For the self-associated hydroxyl group in the hydroxyl group, the methylene group in the aliphatic hydrocarbon, the C=C double bond in the oxygen-containing functional group, and the three-substituted aromatic ring in the aromatic structure, the content of these functional groups is the highest for coal, with increases of 11.01%, 31.81%, 14.34%, and 22.9%, respectively, and the effect of inhibiting the spontaneous combustion of coal is more significant.

Characterization of Nanostructure Evolution in Coal Molecules of Different Ranks.

Four coals samples at different ranks were analyzed by Fourier transform infrared (FTIR), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), and solid-state 13C nuclear magnetic resonance (NMR). The calculated coal molecular model was constructed according to the experimental data. The mode of evolution of four coal molecules with different metamorphic degrees was explored. The results indicate that the nanostructures of these four coal molecules mainly consist of aromatic structures, aliphatic structures and oxygen-containing functional groups. The coal metamorphic degree is the most important factor affecting the evolution of the coal molecular nanostructure. By increasing the coal rank, the aromatic carbon content and aromatic system increase, while the aliphatic carbon content and aliphatic system decrease, and the species and content of oxygen containing functional groups are also reduced. During the evolution of the molecular microcrystalline structure, the degree of vertical order of the aromatic structural unit, the flatness of the aromatic structural unit (La), the average crystallite stacking height (Lc), and the average number of crystallites in a stack (n) increase, while the interlayer distance between aromatic sheets (d002) decreases; the short-range ordering of the coal structure is mainly caused by changes in the orientational arrangement from intramolecular aromatic layers to intermolecular aromatic layers when low-rank coal molecules evolve to high rank coal molecules. The structural evolution mechanism of coal molecules of different ranks has been revealed through the analysis of the mode of evolution of the molecular structure the coal. This study enables us to better understand the nanostructure evolution mechanism of coal molecules at different ranks.

СТАТИСТИЧНІ МОДЕЛІ КЛАСИФІКАЦІЙНОГО ПОКАЗНИКА СТУПЕНЯ МЕТАМОРФІЗМУ ВУГІЛЛЯ ДЛЯ ПРОГНОЗУ НЕБЕЗПЕЧНИХ ВЛАСТИВОСТЕЙ ШАХТОПЛАСТІВ

Purpose: Based on statistical models of volatile matter yield during thermal decomposition of coal without air access (Vdaf) to establish probable relative errors of its measurement of absolute values which would allow to use the indicator with predetermined accuracy Vdaf as the primary classification criterion metamorphism of coal beds for the prediction of dangerous properties when led mining. Methods. It is used to create several sets of Vdaf values that have some features that are essential for their characteristics, followed by analysis of the statistical characteristics of these sets with the establishment of differences or similarities between statistical models. Results. Six sets of coal beds, which have different features of their creation according to the Vdaf criterion. There were from 114 to 412 variants in each set. As a general conditionally accepted set of 2091 coal beds of different coal deposits. For all (7) sets tested, the hypothesis of a normal distribution of variants according to Pearson’s criterion χ2 was tested. One part of the sets tested was close to the normal distribution of random variables, and the other did not correspond to the normal law. It is established that one of the reasons for the inconsistency of parts of the population with the normal law is the various errors in determining Vdaf depending on the absolute value of this indicator. In addition, individual samples from the conditionally general population have the same nature of the distribution of relative frequencies: with an increase in Vdaf to 10%, they decrease; in the range from 10 to 40% slightly increase; when Vdaf > 40% – again there is a decrease. Based on the close correlation dependences of the relative standard deviations of the volatile matter yield from the averaging curves, the validity of the possible use of Vdaf in engineering calculations of the forecast of dangerous properties of shafts only at its values more than 15-20% is proved. The possible influence of the distance of the coal sampling site to the boundaries of gas weathering zones and geological disturbances, and the high dependence of coal (more than 20%) on the additional error of Vdaf determination was also established. The established possible error values do not guarantee high accuracy of forecasting the dangerous properties of coal beds when using Vdaf as the main indicator of the degree of metamorphism. Improving the regulatory framework for safe mining of coal seams requires justification and application of other indicators of the degree of coal metamorphism. Scientific novelty: For the first time on the basis of statistical models, ambiguous possible errors in determining the yield of volatile substances during thermal decomposition of coal without air access and the inexpediency of its use to predict the hazardous properties of coal beds in mining. Practical significance. The necessity of using other indicators of the degree of metamorphism to improve the regulatory framework for safe mining of coal beds, which directly reflect the content and properties of coal in the process of geological transformations, has been proved.

Structure feature and evolution mechanism of pores in different metamorphism and deformation coals

Coal pore structure plays a crucial role in the adsorption, migration and production of coalbed gas, and its feature and evolution are mainly controlled by metamorphism and deformation. In this study, the original and tectonically deformed structures of middle- and high-rank coal samples (Ro,max = 1.14–3.44%) were obtained through low-temperature nitrogen adsorption (LTNA), small-angle X-ray scattering (SAXS) and scanning electron microscopy (SEM) to analyze the pore structure of both open pores and closed pores. Moreover, we investigate the evolution features and mechanism of pore structure with different metamorphism and deformation degrees by comparing the pore structures of these samples with those of low-rank tectonically deformed coals (Ro,max = 0.88–0.96%). The differentiation and inheritance are summarized as the distinctive evolution feature of pore structure of coal with different metamorphism and deformation degree. The pore size of tectonically deformed coals decreases with increase in the degree of deformation under the action of the tectonic stresses. This phenomenon is in line with the metamorphism evolution phenomenon of the original structure that the macro-pores decrease and the micro-pores develop as the coal metamorphism degree increases. Tectonic stress has a dual effect on the transformation between closed pores and open pores of coal in the process of deformation. Closed pores increase is controlled by local uneven shrinkage of the matrix, reduction of pore throat caused by tectonic stress, and partial dislocation of open and semi-open pores. Larger closed pores may open more easily due to deformation caused by shear stress.

Nanopore Characteristics of Coal and Quantitative Analysis of Closed Holes in Coal.

Methane gas is mainly present in coal in two forms: free and adsorbed. There are a large number of closed pores inside the coal, which makes it difficult to measure the gas content of the coal. Therefore, studying the nanoscale closed pores of coal is of great importance for gas control. To study the pore structure characteristics of coal with different deformation degrees and to analyze the volume fraction of closed pores in coal, various coal samples were analyzed by the low-temperature liquid nitrogen adsorption method (LT-N2GA), the carbon dioxide adsorption method, and small-angle X-ray scattering (SAXS). The variation of parameters such as the pore size, pore volume, specific surface area, and degree of metamorphism was compared by using different methods to obtain the proportion of the closed pore volume of different coal samples. The results show that with the increase of the degree of coal metamorphism, the total pore volume and specific surface area of coal samples show a decreasing trend first and then an increasing trend, while the average pore diameter of coal samples gradually increases first and then decreases sharply. When the degree of deterioration of coal is low (volatile content > 20%), the closed pores of coal account for more than 48% of the open pores. When the degree of deterioration of coal samples is relatively high (volatile content <20%), the proportion of large pores in coal bodies decreased from 59.47 to 29.07%, and the proportion of pores in mesopores decreased from 12.15 to 11.09% and finally increased to 11.65%, and the proportion of micropore diameter increased from 28.38 to 59.28%. The volume fraction of the coal sample measured by the SAXS experiment shows that when the coal quality is high, the volume of the mesopores is large, which is consistent with the results of the low-temperature liquid nitrogen and CO2 adsorption experiments. Judging from the number of holes, the number of closed holes is 1 to 3 orders of magnitude greater than the number of open holes, and the number of closed holes of coal samples accounts for more than 94% of the total number of holes. It shows that the number of closed holes in the coal is far greater than the number of open holes, so the gas in the coal is mainly concentrated in the closed holes, and the formation of closed pores is partly because of the collapse of the internal structure and partly because of the volatilization of unstable substances. The research combined with LT-N2GA, the carbon dioxide adsorption method, and SAXS test methods can better analyze the number of closed pores of coal and characterize the nanopore fracture structure of coal. The novelties of this article are that this is a quantitative analysis performed using a scientific method of SAXS. The findings of this study can lead to a better understanding of the coal and gas outburst mechanism and the existence of gas to adopt better prevention measures.