Coal Stage Research Articles

ON THE INFLUENCE OF FILTRATION AND MICROPORES ON THE GAS-DYNAMIC PROPERTIES OF COAL SEAMS

Purpose: to establish the closeness of the correlation between the volume of filtration pores of fossil coals for coal seams of the Donetsk and Lviv-Volyn basins with the degree of their metamorphic transformations. Methodology: the research methodology involves statistical processing by the least squares method of the values of total porosity, micropores and filtration pore volume, and establishing their empirical dependencies on the yield of volatile substances. Comparing indicators of the closeness of correlations between empirical curves and experimentally determined values of pore volumes allows us to establish the stages of formation of different types of pores. Results: Based on statistical processing, empirical dependences of the indicators of the volume of total porosity, micropores and filtration pores on the degree of metamorphic transformation of coal seams were obtained. Filtration pores, with volumes similar to or equivalent to micropores, were formed at earlier stages of coalification compared to metamorphism. As a result, there are no close correlations between the volume of filtration pores and the output of volatile substances. Approximately equal ratios were established between the average values of the volumes of filtration pores and micropores for the entire set of 776 coal seams. At the same time, the ranges of changes in micropores are significantly narrower than filtration ones, which indicates a purely individual relationship between the volumes of filtration pores and micropores, regardless of the degree of metamorphic transformations of a particular coal seam. Micropores determine the possibility of methane being in the main sorbed state, while filtration pores determine the possibility of methane being in a free state. The ratio of these forms of methane in coal, to a greater extent, in comparison with the metamorphic transformations of coal seams, is obviously characterized by their tendency to manifest gas-dynamic phenomena. Scientific novelty: for the first time, based on statistical processing of experimental data, the formation of micropores has been proven, mainly at the stage of metamorphic transformations of coal seams. This is confirmed by the close correlation between the volume of micropores and the release of volatile substances for a set of 776 coal seams. The obtained dependence is characterized by high values of the coefficient of determination and a relatively low root mean square deviation. Practical value: the research results make it possible to develop proposals for improving the regulatory framework for safe mining operations in terms of predicting the manifestation of hazardous properties of coal seams. Keywords: pores, volume, coal seams, hazardous properties, metamorphism, forecast, improvement.

On the transition to the use of modern lubricants in systems of energy facilities

The analysis of turbine oils currently in use is given. The classifications for petroleum base oils according to the systems developed at JSC VNIINP and the American Petroleum Institute are presented. The transition from the operation of turbine oils of the first group to more advanced oils obtained on the basis of base oils of the second group and above is justified. The problems that may arise when replacing oils of the first group with modern oils in connection with the processes of mixing energy oils of various formulations are outlined. The mixing process is complicated by the compatibility factors of both the commercial products themselves and the additive compositions used, as well as possible contamination with oil sludge of lubricants in operation and possible contamination with water and mechanical impurities of the new energy oil at the delivery stage. The necessity of analyzing the possibility of mixing is explained both by the compatibility of the energy oils being tested and by the compatibility of the additive package used in them. A list of indicators is presented by which it is possible to judge the possible compatibility of the energy oils under consideration. The results of the compatibility analysis are presented. In modern foreign policy conditions, for the Russian Federation, there is a need to activate Russian developments and provide modern lubricants for a new generation of energy facilities. A possible solution to this issue, developed by the specialists of the NRU MPEI, is presented. A system for the production of oil distillate with subsequent production of base (group III) and commercial oil for high-ash initial coals of the brown coal and early black coal stages of coalification of coal basins closed for development is presented.

Coal pore size distribution and adsorption capacity controlled by the coalification in China

The coalification process significantly influences the composition of coal material and the development of pore size, ultimately affecting the adsorption capacity of coal seams and the extraction of coalbed methane. This project aimed to assess the pore structure and gas adsorption capacity of coal samples from nine key sedimentary basins in China. The N2 test results revealed that during the initial stage of coalification, micropore, transition pore, and mesopore dominate in order of magnitude. In the subsequent coalification stages, the development of mesopore is most significant during the second jump, while the transition pore becomes more prominent in the third jump. Additionally, the findings from nuclear magnetic resonance tests indicate that the proportion of adsorbed pore demonstrates a decreasing-then-increasing trend throughout the coalification process. The first and second coalification jumps primarily contribute to the development of adsorption pore and seepage pore, whereas the third jump mainly focuses on adsorption pore development. The gas adsorption capacity is influenced by various factors such as pore structure, material composition, and chemical structure. During the first coalification jump, gas adsorption mainly occurs in the adsorption pore. In the second jump, medium gas adsorption capacity is achieved due to the combined effects of enhanced adsorption pore development and reduced moisture and ash content. Finally, the third coalification jump exhibits highly developed adsorption pores, the lowest moisture and ash levels, and a sufficient aromatic structure in the chemical composition, resulting in high adsorption capacity. These findings contribute to a deeper understanding of the variations in coalbed methane enrichment across different basins in China, with coalification playing a crucial controlling role.

Molecular structure characterization of coal with different coalification degrees: a combined study from FT-IR, Raman, 13C NMR spectroscopy and modelling

While extensive research has been conducted on the evolution of coal's molecular structure during coalification, the heterogeneity and complexity of coal continue to impede the accurate and precise quantitative characterization of its structural evolution mechanism. In this study, the organic matter source, coal quality, carbon skeleton structure, and functional group occurrence of coal at different coalification stages have been determined using maceral identification, vitrinite reflectance, industrial quality, elemental analysis, and spectral analysis. Four molecular models have been created through computer-aided molecular design. The results indicate that the hydrogen atoms in the benzene ring structure reduce as coalification increases. The content of aliphatic compounds gradually decreases, while the length of aliphatic side chains first decreases (Ro,max<1.81%) and then increases (Ro,max>1.81%). Moreover, the chemical structure of coal tends to mature and stabilize, as the degree of coal crystallization increases, and the oxygen-containing groups gradually decrease. Therefore, aromatization increases with the condensation reaction dominating in the late stage of coalification. After intensive attempts to determine the chemical bond mode between aromatic carbon, aliphatic side chain, and oxygen functional group, the final molecular structure models are presented as C188H165O13N3 (F-1), C182H161O9N5 (J-1), C195H153O10N3 (T-1), and C193H129O7N3 (W-1). Furthermore, the mechanism of coalification is discussed based on the variation of the side chains and chemical bonds. According to the adsorption heat of functional groups, the methane adsorption capacity increases as the degree of coalification increases. The inflection point of methane adsorption capacity is at Ro,max>1.80%. This study provides a theoretical framework to understand the molecular structure evolution of coals at varying degrees of metamorphism. Additionally, it proposes a reliable method for characterizing coal molecular structures with high accuracy.

The Overview of Indonesia Coal Bed Methane Resources

Coal Bed Methane (CBM) is generated by coal layers, there is a preceding process, which is the formation of coal (coalification). Coalification begins with the burial of organic material originating from plants. The burial, lasting hundreds of millions of years, results in increasing temperatures and pressures, causing physical and chemical changes to the organic material. Methane gas, the largest component of CBM, is formed. Two processes: biogenic and thermogenic. In the early stages of coalification, biogenic methane gas is produced as a byproduct of the activities of decomposing microorganisms (anaerobic bacteria), similar to what occurs in the decomposition of organic waste. Only about 5 to 20 percent is stored within coal. There are four mechanisms for gas storage in coal: absorption, adsorption, free gas, and gas dissolved in water. Among these four mechanisms, adsorption is the most significant, accounting for 98 percent of the stored CBM in a combined system of micropores (coal matrix) and cleats. Coal Bed Methane resources in Indonesia can be found across South Kalimantan, East Kalimantan, Sumatra, and Java.

Maturity Parameter of Carbonaceous and Coaly Matter Revealed through Raman Spectroscopy

Abstract This article presents the results of ten coal samples of sub- bituminous rank collected from working mines in Chhattisgarh, India, and subjected to proximate and ultimate analysis, organic petrology and Raman spectroscopic study to reveal their maturity parameters and structural characteristics of carbonaceous materials. Vitrinite reflectance measurement was also performed to understand the maturity parameter, with values ranging from 0.35% to 0.57%. The research is unique as it combines petrological, geochemical and spectroscopic technique on coals of Hasdeo basin. Conventional parametei; volatile matter ranges from 21.83 and 33.68% suggesting low maturity. The ash yield values range from 12.04 and 26.61%. Petrographically, the coals are vitrinite rich ranging from 53.95 to 63.47% while the inertinite group range from 17.9 to 29.2%. The liptinite concentration the samples remain below 10%. The maceral constitution of the coals indicates carbonaceous materials undergoing early diagenetic phase. The Raman results showed the presence of the D and G bands, two prominent spectral signatures, which are typical of natural materials rich in carbon. The Raman shift recorded for the D band ranges from 1348 to 1385 cm-1 and for the G band, the Raman shift was recorded between 1607 to 1616 cm-1. The ratio of the various parameters derived from the two bands aid to understand the material response to the changing physical and chemical variables in the geological setup. The intensity ratio of the ID/IG band ranged from 0.15 to 0.79 (average 0.50), while the (G-D1) parameter ranges from 225 to 260 indicating heterogeneity in the samples. However; the ratio of AD1/AG did not show any regularity, with values ranging from 0.07 to 1.85. The investigation elucidates that by integrating Raman spectroscopy analysis with vitrinite reflectance (VRo), we can discern distinctive structural attributes characteristic of immature kerogen. Furthermore, the petrological, proximate, and ultimate data collectively corroborate the carbonaceous, coaly material in its early stages of coalification.

Weathering alterations of coal mining wastes geochemistry, petrography, and mineralogy, a case study from the Janina and Marcel Coal Mines, Upper Silesian Coal Basin (Poland)

Weathering-caused alterations in coal mining waste geochemistry, petrography, and mineralogy occurring in the known time span of 1998–2018 in the temperate climate of Poland were investigated. A series of coal mining waste rocks representing two parts of the Upper Silesian Coal Basin (USCB), differing in coalification stage, i.e., corresponding to sub-bituminous and bituminous coals, have been selected to find how the level of organic matter conditionates the weathering. Differences in organic matter content, petrographic composition, rock mineralogy and inorganic geochemistry were also taken into account. Whereas organic petrography, including vitrinite reflectance values, do not show significant weathering signs, apart from irregular cracks and, in very rare cases, paler in color oxidation rims, organic geochemistry has been affected due to low-temperature secondary processes, including increased oxygen index values, particularly in less mature coal mining wastes. Biomarker distribution has been changed to the largest extent, with partial removal of n-alkanes possibly due to biodegradation of organic matter and water-washing of lighter polycyclic aromatic hydrocarbons, including alkyl naphthalenes. Organic matter of lower maturity in coal mining wastes from the USCB eastern part, as more reactive, showed a higher level of secondary transformations. Minerals formed during weathering due to time and storage on dumps were mainly oxidation products of iron sulphides: goethite and jarosite; however, most of the minerals have not been altered. Thus, despite physical changes in coal mining wastes, such as increased brittleness, decreasing fragment size and surface color change, there are only limited changes in inorganic chemistry and mineralogy. This indicates that low-temperature processes do not favour the inorganic elements release to the environment and points out that measures to prevent self-heating within the dumps are the most vital part of post-mining activity leading to the best scenario - slow low-temperature weathering of coal mining wastes.

Variation of δ15N in Indian coal, lignite and peat

Nitrogen (N) stable isotope ratio (δ15N) in coal organic matter (OM) provides information on the N source and dominant mechanisms affecting isotopic fractionation during coalification. However, published data on δ15N distribution in coal is rare. The present study is one of the first reports on the δ15N composition of peat, lignite, sub-bituminous and anthracite coals in India and one of the first attempts to understand the processes influencing δ15N composition at different stages of coalification from peat to anthracite. Peats were collected from the western coast of North Andaman Islands and Lake Loktak in Manipur. Plant samples were collected from the peat sampling locations. Cenozoic lignites were collected from Panandhro, Matanomadh, Umarsar and Tadkeshwar mines in Gujarat and Neyveli in Tamil Nadu. Cenozoic sub-bituminous and Permian anthracite coals were collected from Assam and Sikkim, respectively. Variation of δ15N in plants is attributed to the differences in rainfall, plant type and N sources. Lower δ15N values in peats (mean 1.19) compared to the plant samples (mean 2.77) indicate a nonlinear response of δ15N to the relative enrichment or loss of N during peat formation in Lake Loktak and decomposition of OM under anaerobic conditions leading to selective preservation of 14N in the Andaman Islands. The δ15N composition of the studied peat (−1.4–1.6), lignite (−1.4–1.8) and coals (−2.8–5.0) retains their OM source signature. Overall higher δ15N values of Cenozoic lignites compared to the Cenozoic sub-bituminous coal reflects regional differences in climate. Higher δ15N (1.3–5.0) values in Gondwana anthracites reflects the tectonic influence of Himalayan orogeny.

Relationship between Organic Sulfur Occurrence and Coalification Stage: Insights from Thermodynamic Simulation and X-ray Photoelectron Spectroscopy

Relationship between Organic Sulfur Occurrence and Coalification Stage: Insights from Thermodynamic Simulation and X-ray Photoelectron Spectroscopy

Geochemical signatures of potassium metasomatism in anthracite from the Himalayan fold-thrust belts of Sikkim, India

The present study focuses on the inorganic geochemical features of the bituminous coal samples from the Raniganj and the Jharia Basins, as well as the anthracite samples from the Himalayan fold-thrust belts of Sikkim, India. The SiO2 content (48.05 wt% to 65.09 wt% and 35.92 wt% to 50.11 wt% in the bituminous and anthracite samples, respectively) and the ratio of Al2O3/TiO2 (6.97 to 17.03 in the bituminous coal samples and 10.34 to 20.07 in the anthracite samples) reveal the intermediate igneous source rock composition of the minerals. The ratio of the K2O/Al2O3 in the ash yield of the bituminous coal samples (0.03 to 0.09) may suggest the presence of kaolinite mixed with montmorillonite, while its range in the ash yield of the anthracite samples (0.16 to 0.27) may imply the presence of illite mixed with kaolinite. The chemical index of alteration values may suggest the moderate to strong chemical weathering of the source rock under sub-humid to humid climatic conditions. The plot of the bituminous coal samples in the A–CN–K diagram depicts the traditional weathering trend of parent rocks, but the anthracite samples plot near the illite field and are a bit offset from the weathering trend. This may imply the plausible influences of the potassium-metasomatism at post coalification stages, which is further supported by high K2O/Na2O ratio (29.88–80.13). The Fourier transform infrared spectra further reveal the hydroxyl stretching intensity of illite in the anthracite samples substantiating the effect of the epigenetic potassium-metasomatism. The decrease in total kaolinite intensity/compound intensity of quartz and feldspar may provide additional evidence towards this epigenetic event.

Structural characterization of high fidelity for bituminous and semi-anthracite: Insights from spectral analysis and modeling

Although molecular structure evolution in response to coalification process has been reported to be prevalent recently, their characterization accuracy for coals of different ranks still needs to be enhanced. In this paper, the occurrence information of the carbon skeleton structure, functional groups, and nitrogen and sulfur heteroatoms in coals of different ranks (Ro, max = 0.99% for XZ-02, 1.39% for YT-09, 1.63% for YT-11, 2.29% for ZC-15) were extracted via ultimate and spectrum analysis. Then four molecular structure models were created using computer-aided molecular design (CAMD). Results show that the oxygen atoms in coal primarily exhibit phenolic hydroxyl groups, ether groups, and carbonyl groups. The nitrogen heteroatoms are present as pyrrole and protonated pyridine, and the sulfur heteroatoms mainly served as thiophene. The content of aliphatic structures gradually decreased as coalification continued. Their length first decreased (Ro, max ≤ 1.9%) and subsequently increased (Ro, max > 1.9%). The content of aromatic structures gradually increased accompanied by the gradually cleaving of oxygen functional groups and alkyl side chain from the aromatic clusters. Hence, the degree of aromatization gradually increased, and condensation reactions dominated in the late stage of coalification. The initial molecular formulas and structural models were determined for the four coals. Also, the bonding modes among the aromatic clusters, aliphatic side chains, and oxygen functional groups within the molecular structure were adjusted based on the comparison between experiment and calculation spectra, finally creating the following molecular representations: C174H128O8N2 (XZ-02), C167H114O5N2S2 (YT-09), C151H96O7N2 (YT-11), and C146H80O4N2 (ZC-15). The fitting carbon spectra of these molecular representations were consistent with the experimental carbon spectra whose error is within the allowable range (relative error < 15%), verifying the applicability of the molecular structure model and the above method. Furthermore, the mechanism of coalification was discussed based on the variations of the side chains and chemical bonds. The above research can provide a theoretical reference for the molecular structural evolution for coals of different ranks and a high fidelity method to create molecular structure representation.

Relationship between Organic Sulfur Occurrence and Coalification Stage：Insights from Thermodynamic Simulation and X-Ray Photoelectron Spectroscopy

Relationship between Organic Sulfur Occurrence and Coalification Stage：Insights from Thermodynamic Simulation and X-Ray Photoelectron Spectroscopy

Molecule structure and ultrasonic wave velocity of different ranking coals: An experimental perspective

Ultrasonic wave velocity is an important manifestation of coal elastic properties and is extremely useful for coal and coalbed methane exploration. In this study, we focused on analyzing how the molecular structure affects the elastic ultrasonic wave velocity. High-resolution transmission electron microscopy (HRTEM) and Fourier transform infrared spectroscopy (FT-IR) were used to explain the coal’s molecular structure. Ultrasonic wave velocity measurements were then used to determine the elastic parameters of coals of different ranks. From an elasticity theory point of view, the molecular structure of coal is a simple system consisting of two substructures. The results showed that the correlation between the ultrasonic wave velocity and the molecular structure differs in different coalification stages. In the first stage (Rr = 0.6%–∼1.6%), the ultrasonic wave velocity of coal keeps decreasing during the maturation process due to the aromatization of the coal molecular structure and the cleavage of alkyl side chains. In the second stage (Rr = 1.6%–∼3.0%), the average layer size (La) of the coal molecular structure increases and the average interlayer space decreases, which makes the ultrasonic wave velocity of coal increase continuously. The evolution of the molecular structure of coal is an important influencing factor for its elastic properties. The irregular parts (alkyl side chains and functional groups) play a major role in elastic ultrasonic wave velocity, followed by the average layer size (La) and the average interlayer spacing (d) of aromatic layers. It was determined that the relationship between the coal rank and ultrasonic wave velocity of coal without macro-fractured can be interpreted as the evolution of the molecular structure (two substructures), and the ultrasonic velocity measurement methods were applied to investigate the coal structure. These relations can be useful for geophysical exploration of coal and coalbed methane.

Trapping of uranium by organic matter within sandstones during mineralization process: A case study from the Shuanglong uranium deposit, China

Carbonaceous debris (i.e., CD), dispersed organic matter (i.e., DOM) within sandstones, has been studied in the Shuanglong uranium deposit of the southern Ordos Basin, northern China. Relations of uranium enrichment and organic matter (i.e., OM) were investigated through maceral analyses, proximate analysis, vitrinite reflectance measurements, and morphology analyses of uranium minerals. The results show that CD widely distributed in sandstones, is associated with pyrite and uranium minerals. Macerals are mostly composed of telinite, collotelinite, collodetrinite, semifusinite, and inertodetrinite, including that collotelinite is dominant (contents more than 60%). The CD is at the stage of medium coalification, and is of low moisture, high ash, high volatile matter, medium fixed carbon, and high sulfur. Uranium occurs in uranium minerals (i.e., pitchblende and coffinite) and amorphous uranium. Moreover, uranium minerals partly or completely replace macerals. Statistical analyses show that porous inertinite and higher organic matter maturity is more advantageous to uranium enrichment. Additionally, uranium is unevenly distributed in macerals along certain directions, which is related to the physicochemical properties of macerals. Porous inertinite could provide channel for uranium-bearing fluid migration and space for uranium preconcentration. The CD at the stage of gas coal is of reducibility to be favorable for the forming of uranium minerals. Moreover, the relative atomic content of uranium (IV) increases with the increasing organic matter maturity. Uranium-bearing fluid could permeate into macerals under certain physical and chemical conditions, and react, i.e., adsorption followed by reduction. It is beneficial to comprehend the mechanism of uranium mineralization, providing certain guides for prospection.

Pore structure, adsorptivity and influencing factors of high-volatile bituminous coal rich in inertinite

Bituminous coal rich in inertinite is widely distributed throughout the world, but the effect of the first coalification jump (FCJ) on the reservoir properties of high-volatile bituminous coal rich in inertinite, has rarely been studied. Therefore, nine coal samples were collected from the southwestern Ordos Basin and used in a series of experiments designed to study the adsorptivity of high-volatile bituminous coal rich in inertinite and factors affecting adsorptivity before and after FCJ. Such information is helpful for further understanding the evolution of coal physical properties in the key stage of early coalification. The pyrolytic parameters indicate that the FCJ of the coal samples was between 0.60% and 0.65% (Rr). The NMR-T2 spectrum type of the samples was bimodal differentiation before FCJ, and after FCJ, it was a bimodal connection or adsorption single peak. The fractal dimension of adsorption pore increased with increased Rr and inertinite content. The adsorptivity of coal were affected by coalification and macerals. Langmuir pressure was mainly controlled by the maceral composition and exhibited a positive linear relationship with the ratio of vitrinite to inertinite. Langmuir volume was mainly controlled by maceral composition, and coalification, before and after FCJ, respectively. The increase in the adsorption pore volume of high-volatile bituminous coal caused by the increase in inertinite content had little effect on the increase in the adsorption capacity of the coal. In the stage of high-volatile bituminous coal, bitumen is adsorbed by vitrinite, which blocks and fills the pores of vitrinite, thereby reducing the adsorption capacity of coal.

Enrichment of Li–Ga–Zr–Hf and Se–Mo–Cr–V–As–Pb Assemblages in the No. 11 Superhigh Organic Sulfur Coal from the Sangshuping Coal Mine, Weibei Coalfield, Shaanxi, North China

Superhigh organic sulfur(SHOS) coals have currently attracted great attention due to their typical depositional environments and formation history as well as their great negative impact on the ecosystem. This study investigated the geochemistry of the No. 11coalof the Late Carboniferous Taiyuan Formation from the Sangshuping coalmine, Hancheng miningarea, Weibei coalfield, Shaanxi, North China. The No. 11 coal is a high-sulfur coal with a large proportion of organic sulfur content (3.7 to 5.5%, avg. 4.4%) and belongs to typical SHOS coal. The high sulfur content in the Sangshuping coal mine has been mainly caused by the combined influences of seawater and hydrothermal fluids. The SHOS in No. 11 coal was formed in the Fe-poor and S-rich high-marine influenced occlusive environment. During the late coalification stage, a high proportion of pyritic sulfur was formed due to sufficient Fe supply from the Fe–S-rich epigenetic hydrothermal fluids. The No. 11 SHOS coal is enriched in Li–Ga–Zr–Hf and Se–Mo–Cr–V–As–Pb element assemblages. The sediment provenance of the Sangshuping coal mine is predominantly felsic–intermediate rocks from both the Yinshan and Qinling Oldland. However, the elevated concentrations of critical elements (Li, Ga, Zr, and Hf) in the No. 11 coal are primarily inherited from the Yinshan Oldland. The enrichment of the Se–Mo–Cr–V–As–Pb assemblage in No. 11 coal can be ascribed to the influence of both seawater and epigenetic hydrothermal activity.

Water-induced molecular changes of hard coals and lignites

Coal deposits comprise a huge proportion of carbon sequestered within the Earths crust. Many deposits are anthropogenically altered by mining operations. The changes in local hydrogeology include, first - water removal from the seam to enable excavations, second – restoration of natural water condition by cessation of water pumping. In each of these cases, there is a substantial change in the living conditions of microorganisms and coal sorption capacity towards gases. In the presented study, an attempt was made to connect results depicting structural changes (FT-IR), microbial community composition (NGS) and activity (GC) of four materials representing various stages of coalification upon hydratation. The issue is important as coal transformation often leads to generation of gases with high global warming potentials (CO2 and CH4). In all samples, hydratation led to coal dearomatization, decarboxylation and decompaction. The main gaseous product upon hydratation was carbon dioxide which was linked with both changes in coal sorption capacity upon hydratation and activity of the autochthonic microbiota. CO2 release was higher in lignites than in the hard coals. Within the experimental timeframes of 450 days, methane production was rather marginal. Furthermore, microbial communities composition suggests intrinsic turnover of that gas by the methanotrophic bacteria.

ON DETERMINING THE COAL CARBONIZATION INDICATOR FOR ESTABLISHMENT OF MINE FIRE HAZARD GROUPS

The article analyzes the dependence of endogenous fire hazards on the following factors: the degree of coalification, the total sulfur content, the thickness of the developed formations and their structure, the presence of tectonic disturbances. By the genetic propensity of coals to spontaneous combustion, three groups of coal sites are distinguished: І – Vdaf ≥ 41%, Сп ≤ 7, coals of marks D and partially G; ІІ – Vdaf = 40-30%, Сп ≤ 8, coals of marks D and partially F; ІІІ – Vdaf &lt; 36%, Сп &gt;8, coals of marks Zh, K, OS, T. Studies have shown that individual dependences of the decrease in the content of each component (Оo, Нo, No) significantly differ from each other. Decrease in oxygen content, as well as the sum of the main components, occurs with an increase in carbon content. The sum of the content of the main components decreases inversely to the increase in carbon content over the entire range of coal conversion from a small degree of their metamorphism (Со ≈ 70%) to anthracites (Со ≈ 98%). The dependence of decrease in oxygen content with increased carbon is nonlinear. With carbon content of more than 88%, there is a slight decrease in the rate of oxygen reduction in comparison to its decrease at earlier stages of metamorphism. Data analysis showed an almost functional dependence between the sum of the main components of the organic mass (0.995). A significant correlation of carbon with oxygen and hydrogen is observed. A significant scatter of points in experimental data is due to the diversity of composition of the source material, the conditions of its accumulation and transformation for different coal basins and individual coal sites. Studies have shown that the relationships between all the main components of the organic mass of coal during metamorphic transformations change significantly at different stages of these processes. The established dependence of the carbonization index on the carbon content at different stages of coalification allowed us to develop an engineering method for calculating the carbonization index for the entire range of coal metamorphism. The proposed method significantly clarifies the determination of the carbonization index, which is necessary not only for establishing the fire hazard of mine layers, but also for establishing other manifestations of the dangerous properties of coal sites during mining operations. Keywords: carbonization index, endogenous fire hazard, elemental composition, organic mass, metamorphism, coalification.

Adsorption pore structure and its fractal characteristics of coals by N2 adsorption/desorption and FESEM image analyses

The adsorption pore structure and its fractal characteristics of coals (0.69–2.79% Ro,m) were investigated by N2 adsorption/desorption and field emission scanning electron microscopy. First, organic matter (OM) pores and interparticle pores (InterP) with various shapes are distinguished in different rank coals. The OM pores present irregular shapes in low-/middle-rank bituminous coals, and circular, oval and slit shapes in high-rank bituminous coal and anthracite with width from 50 nm to 2 μm. In contrast, the InterP pores show polygonal/elongated shapes and exist between mineral grains or mineral and OM with width >100 nm. Second, the coals with H1 type hysteresis loop of N2 adsorption/desorption isotherms are favorable for gas adsorption due to large pore surface area, whereas coals with H2 and H3 types are conducive to gas desorption and diffusion because of open pores with good connectivity. Moreover, the fractal dimension of pore shape was obtained from FESEM images and ranges from 1.068 and 1.30, indicating the adsorption pore shape is relatively regular. The surface and volume fractal dimensions of micropores and mesopores were calculated from the N2 adsorption data on the basis of surface fractal model and FHH model, respectively. The fractal dimension of micropore and mesopore surface respectively ranges from 2.18 to 2.69 and 1.78 to 2.41, whereas that of micropore and mesopore volume respectively ranges from 2.18 to 2.67 and 2.29 to 2.79. The pore structure and coal composition during different coalification stages have specific effects on the fractal characteristics of pore shape, surface and volume.

Pore Structure of Coals by Mercury Intrusion, N2 Adsorption and NMR: A Comparative Study

Coalbed methane (CBM) mainly adsorb in massive pores of coal. The accurate characterization of pores benefits CBM resource evaluation, exploration and exploitation. In this paper, mercury intrusion porosimetry (MIP) and low temperature nitrogen adsorption (N2GA) combined with low field nuclear magnetic resonance (NMR) experiments were conducted to analyze the advantages and differences among different experimental techniques in pore characterization. The results show that the total porosity has a tendency to decrease first and then rise with the increase of coal rank, which is mainly caused by the compaction in early stage and the thermogenic gas produced in middle and late stages of coalification. The comparison between different techniques shows that NMR is superior to the conventional methods in terms of porosity and pore size distribution, which should be favorable for pore characterization. The N2GA pore size measurement, based on BJH model, is only accurate within 10‒100 nm in diameter. There is a peak misalignment between the NMR and MIP results in the pore size comparison. The reason for this phenomenon is that there is a centrifugal error in NMR experiment, which could cause a differential damage to the coal sample, resulting in partial loss of the nuclear magnetic signal.