Coal Lithotypes Research Articles

Lithotype-based modelling and simulations of coal degradation conditioned by both high and low energy breakage

The control of coal fragmentation and fines generation during mining and processing is important in coal production. A method to characterise, model and simulate coal size degradation and fines generation based on lithotypes has been developed. This method was refined to cover both high energy single impact to mimic blasting and crushing and low energy incremental breakage to mimic coal handling, transiting, stockpiling and processing. The JKRBT was utilised to characterise high energy single impact breakage and drop shatter tests were used to characterise low energy incremental breakage. X-ray Computed Tomography (XCT) scanning was used as an undisruptive technique to estimate size distributions of drill cores in the drop shatter tests. The JK size-dependent breakage model was applied for breakage characterisation, size degradation modelling and fines generation simulation. The results indicate that coal lithotype has a significant influence on coal degradation and fines generation. This paper has demonstrated that the adaption of two distinct breakage characterisation tests and linkage via the one model is a significant advance in quantifying coal degradation and fines generation during coal production.

Lignite oxidation under the influence of glacially derived groundwater: The pyropissite deposits of Zeitz-Weißenfels (Germany)

The Middle Eocene lignites of the Zeitz-Weißenfels coal mining area in central Germany hosted one of the most economically successful lithotypes of lignite deposits utilised in the paraffin industry of the 19th century, i.e. the pyropissite deposits. However, due to their economic significance, these rare lithotypes were almost completely mined out, such that presently only a few remnant deposits are known. Apart from the Zeitz-Weißenfels coal mining district, other pyropissite deposits were also encountered in other lignite mines, for instance, in the Hessian and Subhercynian Basins.Pyropissite is a whitish and bitumen-rich variety of soft brown coal (lignite) lithotypes, dominated by a homogeneous matrix with loosely embedded organic and inorganic components. Fresh exposure of pyropissite at Grana was logged and studied petrographically and geochemically, along with samples obtained from archival collections. Results were compared with those from the 19th and 20th century to derive general conclusions regarding origin and formation of pyropissite.Microscopically, the main constituent of this special lignite lithotype is amorphous to detrital in appearance. According to W. Schneider the term xanthinite is applied to this component, which forms the groundmass of the pyropissite. It contains high proportions of liptinitic substances with contributions of formless huminitic material and minor amounts of mineral matter. Geochemically, pyropissite is characterised by an increased H/C atomic ratio and a very high content of bitumen (in terms of toluene-soluble components) as well as low-temperature carbonisation tar.The obtained data revealed that the extraordinarily high content of liptinite and the paucity of huminitic material of the pyropissite deposits of the Zeitz-Weißenfels coal mining area are not related to primary depositional processes but rather are the result of dissipation of high condensed huminites and the consequent enrichment of liptinitic substances. The mechanism involves oxidation by groundwater and is related to glaciation processes operating during the Pleistocence. Such glacially derived processes also account for both vertical and horizontal distributions of the pyropissite.Thus, recent research activities may also serve as an innovative inspiration to fundamental understanding of the formation of lignites with extremely high liptinite content.

THE SIGNIFICANCE OF PEATLAND AGGRADATION IN MODERN AND ANCIENT ENVIRONMENTS

Abstract: Peats are commonly used in paleoenvironmental and paleoclimatic studies but detailed sedimentological and facies models for peatlands are poorly developed relative to other sedimentary settings. A comparison of the palynology and charcoal abundances in modern and ancient Cenozoic peats (i.e., brown coals) demonstrates that, in a single cycle, their respective flora commonly evolves from inundated wetland assemblages to more elevated and well-drained forest. The repetitive nature of this pattern suggests that the changing floral compositions result from changes in substrate wetness during peatland aggradation in high rainfall settings. In this scenario, floristic changes within the peat are suggested to represent peatland facies that were controlled by the local peat-forming environment. We suggest that peatland aggradation is an important process that may ubiquitously control the floral and environmental changes documented in modern and Holocene ombrogenous peats, brown coal lithotype cycles, and perhaps black coal dulling-upwards cycles.

Petrographic composition and forms of bituminous coal lithotypes in the Upper Carboniferous Formations of the Upper Silesian Coal Basin

Abstract Bituminous coal samples were collected from mine excavations of six mines in the Upper Silesian Coal Basin. I n the mentioned mining excavations, the stratighaphic sections, in the form of spot samples, were measured. Based on the macroscopic and microscopic observation, an attempt was made to determine the different lithotypes of coal. Vitrain coal is made of tellinite and collotelinite; the thickness of the layers varies from very thin, thin, medium, to coarse. Durain, which is dominated by macerals from the vitrinite group, is characterized by a darker, almost black color, genetically linked to heavily flooded peat areas, where the deposited phytogenic material is subjected to humification and gelification processes. A brighter durain, with a dark gray color, is dominated by macerals from the inertinite group that originated in the shallower areas of peat bogs where the water level was periodically lowered, which has led to the oxidation of the material deposited in the peat bog. Fusain is another coal component or constituent; it is produced as a result of peat bog fires. It is a charred (not burned) material deposited in the form of layers, lenses, usually with a thickness of up to several millimeters (or, less commonly, several centimeters), or dispersed in the form of shreds in the durain. The petrographic composition is dominated by fusinite and inertodetrinite. Fusain occurs in two varieties: soft (empty cellular spaces) and hard, usually mineralized with carbonates (siderite) or sulphides (pyrite, marcasite). The structure of bituminous coal is, due to its origin, most often laminated and consists of alternating dull and bright layers. Occasionally, such layering can be observed in bright coal, which is the result of layering of large parts of gelified plant materials. When it comes to larger sections of dull coal (without bright coal) in the profile, a solid structure can be observed. Some of the sections in the coal seam profiles show a distorted structure; warped, sometimes shredded layers of vitrain in durain, often containing lenses or shreds of fusain, can be observed.

Coal washability analysis using X-ray tomographic images for different lithotypes

Conventional coal washability analysis is achieved by conducting Float and Sink (FaS) test, which is sampling dependent, destructive and uses environmentally hazardous chemicals. With the improvement of modern 2D and 3D imaging technologies, image analysis methods of digital CT scanned images are being developed and trialled to predict coal washability in a more flexible, efficient and safe way. To support the validity of this approach, this study illustrates the fundamental differences in breakage and liberation behaviour of coal lithotypes at the same rank from a single seam. Nine lithotype samples were repeatedly CT scanned before and after a series of 2m drops to emulate fragmentation during the drop shatter test. The 3D watershed segmentation method was applied to scans to monitor changes in fragment size distribution and then assign a density to each fragment to obtain coal washability. Comparison between the processed image analysis results and FaS tests conducted after 31 drops show a very good match both in particle size distribution and coal washability for all the lithotypes. Results also show that dull coal or stone illustrate better agreement between CT image analysis and FaS test due to their very low percentage of small particles which may lead to underestimation when using CT image analysis. Possible ways of improving small particles estimation would be to use an XY plane H-maxima method and low merging coefficient in the image analysis process.

Digital coal: Generation of fractured cores with microscale features

Coal is highly heterogeneous at multiple scales, such that the characterisation of coal lags behind that of conventional reservoir rocks. This paper demonstrates the capability of a digital coal model that can be used to characterise the multiscale structure of coal as well as its petrophysical properties at the core scale (cm). X-ray micro-computed tomography (micro-CT) is applied to obtain the internal cleat structure of a heterogeneous fractured coal sample, based on which statistics of coal lithotype distributions and cleat geometrical properties are extracted. Digital coal models are constructed stochastically according to the measured statistics. Resulting models preserve the core scale (cm) heterogeneity and pore scale (µm to mm) cleat properties of the imaged coal sample. Furthermore, petrophysical evaluation is performed based on the stochastic digital coal models. We find that the generated digital coal models can provide permeability estimation with errors of 26.7% in comparison to that of the original micro-CT data. Since the presented model is able to preserve the multiscale heterogeneities as well as petrophysical properties of coal, our work provides an alternative to segmented micro-CT images. Therefore, we can avoid the challenges that are inherent to micro-CT images, such as segmentation errors, size and resolution limitations. The digital coal model can also be a tool of linking the rock microstructure to petrophysical properties at the core scale, such that we can predict the permeability of coal based on its geometrical measures rather than expensive laboratory experiments.

The effect of rank, lithotype and roughness on contact angle measurements in coal cleats

We report the effect of rank and lithotype on the wettability of coal in microfluidic experiments in two types of artificial microchannels: (1) reactive ion etched (RIE) channels and (2) die-cast channels prepared by pressing powdered lithotype concentrates. Five coals from the Bowen Basin with ranks in the Rmax% range 0.98–1.91% were examined. Contact angles and entry pressures of air and water in the artificial cleats were measured in imbibition experiments performed with a Cleat Flow Cell (CFC). The relative contact angles measured in CFC imbibition experiments were in the range 110–140° in the RIE channels and 85°–115° in the pressed discs, which are larger contact angles than measured on the flat bulk surfaces of these samples by the conventional sessile drop technique (58°–85°). The CFC observations also show the surface roughness of coal in inertinite-rich dull bands effects contact angle and the entry pressure of the air-water interface differently to the vitrinite-rich bright bands. Drainage experiments in the CFC revealed a thin residual water film on the inertinite cleat wall, yet not on the smooth vitrinite channel. The experimental observations are used to present a modified Cassie Equation model to predict coal contact angles based on the fractions of dull and bright bands, mineral content, and cleat surface roughness. The results of this study provide the basis to consider an improved relative permeability model that explicitly accounts for the effect of coal lithotype.

Maceral controls on porosity characteristics of lithotypes of Pennsylvanian high volatile bituminous coal: Example from the Illinois Basin

Porosity characteristics of vitrain, clarain, durain, and fusain lithotypes of the Springfield Coal Member of the Petersburg Formation and the Danville and Hymera Coal Members of the Dugger Formation from the Illinois Basin were investigated with a special emphasis on the control of coal macerals on pore-size distribution. These Pennsylvanian coals are of high volatile bituminous rank and have vitrinite reflectance ranging from 0.51 to 0.60%. The lithotypes studied show decreasing values of Brunauer-Emmett-Teller surface area, and micro- and mesopore volumes from vitrain through clarain and durain to fusain. Within the mesopore size range, vitrain and clarain are dominated by pore-size widths of 4 to 10nm, whereas durain and fusain have more volume for pores larger than 20nm. In contrast to mesopores, micropore sizes are very similar for all lithotypes, averaging 1.37 to 1.39nm. In addition to differences among lithotypes, there are significant differences in pore characteristics among the three coals studied, with largest surface areas and pore volumes documented for the Hymera, followed by the Danville and the Springfield. A strong relationship exists between surface area, mesoporosity, and microporosity and maceral composition, with vitrinite having a very strong positive correlation, liptinite having a weak positive correlation, and inertinite having a strong negative correlation. Negative correlations of total porosity with vitrinite and liptinite and positive correlations with inertinite suggest that among the maceral groups, pores in inertinite contribute most to the total porosity. The Fourier transform infrared spectrometry technique demonstrates that fusains from the three coals studied have higher aromaticity and a higher degree of aromatic ring condensation and lower hydrocarbon potential than the other lithotypes, whereas chemical differences between vitrain, clarain, and durain are less distinct. In addition, there is a relationship between aromaticity of the lithotypes and surface area and mesopore and micropore volumes.

Characterisation of lignite lithotypes from the “Kovin” deposit (Serbia) - implications from petrographic, biomarker and isotopic analysis

Four lignite lithotypes (matrix coal, xylite-rich coal, mixture of matrix and mineral-rich coal and mixture of matrix and xylite-rich coal), originating from the Kovin deposit, were investigated in detail. The paper was aimed to determine the main maceral, biomarker and isotopic (?13C) characteristics of investigated lithotypes. Based on these results the sources and depositional environment of organic matter in 4 lithotypes were established. These samples were also used as substrates for investigation of the influence of diagenetic alteration on ?13C signatures of biomarkers, as well as for assessment of the most convenient utilization for each lithotype. The investigated lithotypes differ in accordance with the composition of huminite macerals. Xylite-rich coal notably distinguishes from other lithotypes beacuse of the highest content of conifer resins vs. epicuticular waxes. The mixture of matrix and mineral-rich coal is characterised by the greatest contribution of algae and fungi and the most intense methanotrophic activity at the time of deposition. In all coal lithotypes diagenetic aromatisation influenced isotopic composition of individual biomarkers. Xylite-rich coal has the poorest grindability properties. However, this coal lithotype is the most suitable for fluidized bed gasification, whereas the mixture of matrix and mineral-rich coal has the lowest applicability for this process. The calorific value decreases in order: xylite-rich coal > matrix coal > mixture of matrix and xylite-rich coal > mixture of matrix and mineral-rich coal. The increase of organic carbon content and calorific value is controlled by the increase of contribution of wood vegetation vs. herbaceous peat-forming plants, as well as by stability of water table during peatification.

Influence of coal petrology on methane adsorption capacity of the Middle Jurassic coal in the Yuqia Coalfield, northern Qaidam Basin, China

The lower coalbed methane (CBM) adsorption capacity of the low rank coals is not only related to its lower maturity, but also determined by the coal maceral compositions. In this study, a total of 13 samples including 10 coals and 3 carbonaceous mudstones, were collected from the Middle Jurassic Dameigou Formation in the borehole YQ-1 of the Yuqia Coalfield, northern Qaidam Basin, NW China. Coal lithotypes, maceral compositions, coal ranks, coal facies and methane adsorption characteristics of these samples were investigated using microscopic observation, proximate analysis, porosity analysis, and isothermal adsorption experiments. The results show that the maceral composition has a great influence on the methane adsorption capacity, and the influence of vitrinite on the methane adsorption is generally stronger than that of inertinite at a similar coal rank. For most samples, there is a positive correlation between the vitrinite contents and the Langmuir volume (VL), a negative correlation between the inertinite contents and the VL, and no obvious correlation between the exinite contents and the VL. Furthermore, the vitrinite/inertinite (V/I) ratio also shows a positive relationship with the VL. However, some samples containing large amounts of unfilled fusinite and/or semifusinite have more pore spaces favoring methane adsorption and can also adsorb significant quantities of methane. Consequently, coals with higher vitrinite contents, in association with some unfilled fusinites or semifusinites, should have the greatest adsorption capacity. Coal maceral compositions vary with the types of coal facies, and thus the methane adsorption capacity of coals may be closely related to coal facies. It is found that the methane adsorption capacities of the coals in a wet forest swamp (Type I) and an intergradation forest swamp (Type II) are greater than those in a drained forest swamp (Type III) and an open water peat swamp (Type IV). It is suggested that the area developed with the wet forest swamp and in the intergradation forest swamp with high porosity should be the best target areas for the CBM exploration in the Yuqia Coalfield.

Electrofacies analysis for coal lithotype profiling based on high-resolution wireline log data

The traditional approach to coal lithotype analysis is based on a visual characterisation of coal in core, mine or outcrop exposures. As not all wells are fully cored, the petroleum and coal mining industries increasingly use geophysical wireline logs for lithology interpretation.This study demonstrates a method for interpreting coal lithotypes from geophysical wireline logs, and in particular discriminating between bright or banded, and dull coal at similar densities to a decimetre level. The study explores the optimum combination of geophysical log suites for training the coal electrofacies interpretation, using neural network conception, and then propagating the results to wells with fewer wireline data. This approach is objective and has a recordable reproducibility and rule set.In addition to conventional gamma ray and density logs, laterolog resistivity, microresistivity and PEF data were used in the study. Array resistivity data from a compact micro imager (CMI tool) were processed into a single microresistivity curve and integrated with the conventional resistivity data in the cluster analysis. Microresistivity data were tested in the analysis to test the hypothesis that the improved vertical resolution of microresistivity curve can enhance the accuracy of the clustering analysis. The addition of PEF log allowed discrimination between low density bright to banded coal electrofacies and low density inertinite-rich dull electrofacies.The results of clustering analysis were validated statistically and the results of the electrofacies results were compared to manually derived coal lithotype logs.

Zonation of deposits of hard coals of different porosity in the Upper Silesian Coal Basin

Abstract The article presents the results of tests of porosity of the Upper-Silesian Coal Basin (USCB) hard coals. The porosity was determined for various lithotypes of hard coal, collected in different areas of the Upper-Silesian Coal Basin. Samples of hard coal were collected in 60 seams of 16 coal mines, from the depth of approximately between 350 and 1200 m. There are also presented differences in values of open porosity of coal depending on the depth of occurrence, as well as chronostratigraphic and tectonic correlation of a seam. Uniaxial compression strength of the tested coals falls in the strength class from very low to very high (from 8.1 to 51.5 MPa), open porosity is 0.68–12.5% and total porosity is 3.29–17.45%. With an increase in depth, in general, open porosity of coals decreases. There is an apparent decrease in open porosity correlated with the age of hard coals. It was observed that the lower and upper limits of open porosity ranges of variability shift towards lower values the older the coals are, from the Łaziskie Beds to the Jaklowieckie Beds. The older the coals the greater the average drop in the share of open porosity in total porosity from 60% for the youngest coals to merely several per cent for the oldest ones. The highest values of total porosity (over 10%) were observed in the youngest coals (Cracow sandstone series and siltstone series). Older coals reached more diversified values of total porosity (3.29–17.45%). Based on the conducted tests, zones correlated with tectonics of the basin and with deposition of lithostratigraphic beds of specific age, where values of porosity of hard coals differ, were determined in the area of the USCB. However, in spite of their correlation to a structural unit, the coals demonstrated a decrease in open porosity with an increase in their geologic age. Thus, there is an apparent influence of the deposition depth of coal within a given structural unit on open porosity following the age of coal, i.e. correlation to an older and older chronostratigraphic unit. Porosity tests of hard coal are published in scientific journals yet, most often, there is no information on the type of porosity. Such detailed characteristics of open porosity for coal of the Upper Silesian Coal Basin, as in the article, have not been prepared yet, making our research novel. Such broad tests of porosity of hard coals, correlated with their strength and referring to the deposition depth of seams as well as a correlation to lithostratigraphic units within individual structural units in the USCB, had not been conducted yet. We believe it is a sufficient argument to start detailed research into the open porosity of hard coals. The results of the tests are important from both the research and practical points of view. Knowledge of coal porosity is indispensable in solving many engineering problems of geological engineering, mining geomechanics and mining e.g.: assessing behaviour of coal when it is mined and processed, assessing the possibility of capturing methane, which makes up over 90% of natural gas, from coal in the areas of methane-rich deposits; the possibility of storing various substances in abandoned hard coal mining areas (e.g. CO2 storage within the structure of coal), assessing risk of occurrence of certain natural hazards resulting from mining deposits (e.g. coal and rocks outburst hazard), as well as assessing the possibility of applying unconventional methods of mining hard coal deposits (e.g. underground coal gasification).

A workflow to build a model investigating coal cleat upscaling conditioned by the lithotype

Cleats are natural macroscopic fractures in coalbeds. The distribution of cleats inside network patterns is an important attribute as it influences mechanical behaviour, effective permeability and gas production performance. Various publications on geological and geotechnical investigations show relationships between rank class, lithotype and cleat frequency, and it is well known that the block size of coalbeds affect coal strength. These attributes are commonly tested for and numerically modelled independently without regard for any natural sequence (e.g. dulling upward, brightening upward, randomly distributed) within the coalbed. This paper presents a developing workflow with a methodology to set up a numerical modelling using the coal lithotype information as its basis. The contribution of each lithotype to the whole behaviour of the coalbed under transient stress conditions, and then to the larger rock mass, is considered a critical component. The main issues observed include: reliable information exists for individual bright bands but not for mixed lithotypes; neither a correlation between the individual bands and the lithotypes, nor between the lithotype and the coal seam, have been proposed; derived geotechnical properties are not assumed to be conditioned by the individual lithotypes; and, upscaling coal geotechnical properties from laboratory tests remains uncertain. These matters, therefore, should be pursued to allow some quantification of the heterogeneous nature of coal and to capture the combined effects on coalbed strength. It is anticipated that the workflow still requires validation by running a set of numerical experiments for proposed scenarios.

Porosity and permeability characterization of coal: a micro-computed tomography study

A unique contrast agent technique using X-ray micro-computed tomography (micro-CT) was developed for studying micrometer-sized features in coal. The technique allows for the visualization of coal fractures not visible with conventional imaging methods. A Late Permian medium volatile bituminous coal from Moura coal mine was imaged and the resulting three-dimensional coal cleat system was extracted for fluid flow simulations. The results demonstrate a direct relationship between coal lithotype and permeability, i.e. bright coals offer more permeability than dull coals. However, there was no direct relationship between porosity and permeability for any given lithotype. Scanning electron microscope and energy dispersive spectrometry (SEM–EDS) together with X-ray diffraction (XRD) methods were used for identifying mineral matter at high resolution. After segmentation of the micro-CT image, mineral phase was removed from the segmented data and it was found that permeability was significantly improved by increasing cleat void space and connectivity; suggesting that enhanced recovery methods could target de-mineralization techniques. Overall, only the epigenetic mineral phases in the bright band influenced permeability while lithotype had a stronger impact on permeability than porosity. Coal lithotype and mineralization are important evaluation criteria when considering coal seam gas development sites.

Electrofacies analysis using high-resolution wireline geophysical data as a proxy for inertinite-rich coal distribution in Late Permian Coal Seams, Bowen Basin

This paper examines the stratigraphic and geographic distribution of coal composition in main Late Permian coal measures in the Bowen Basin, using coal electrofacies as a proxy for inertinite maceral group content. Data for this research were derived from some 26 wells in the northern part of the Bowen Basin. A companion paper (Roslin and Esterle, 2015, in review) introduced the methodology, which is based on electrofacies analysis and uses high-resolution wireline data (including microresistivity from Compact Micro-Image (CMI) tool and Photoelectric Factor (PEF) data in addition to conventional gamma ray, density and laterolog resistivity) to obtain coal electrofacies. Validation of the methodology was performed by comparison of the weighted average proportion of coal electrofacies to the weighted average proportion of the corresponding coal lithotypes obtained from millimetre scale logging and to the percentage gathered from maceral content analysis. The weighted average proportion of the interpreted dull inertinite-rich coal electrofacies (independent of rank and heat effects) was then analysed to determine their stratigraphic and geographic distribution.From the base upward, the main Late Permian coal bearing units are the Moranbah Coal Measure (MCM), Fort Cooper Coal Measure (FCCM), and Rangal Coal Measure (RCM) and their stratigraphic equivalents across the basin. The proportion of inertinite-rich dull coal electrofacies increases upwards in the RCM, with some thick and merged seams showing distinctive electrofacies signatures. Within the study area, the coal measures and individual seams split from north to south, reflecting increased subsidence and sediment influx into basin depocentres. As the seams split, the proportion of inertinite-rich dull coal electrofacies decreases in the tested sample set.

Origin of banded structure and coal lithotype cycles in Kargali coal seam of East Bokaro sub-basin, Jharkhand, India: Environmental implications

The Kargali seam of Early Permian Barakar cyclothems of East Bokaro sub-basin of Jharkhand, India is 12–30 m thick, splits into two parts, and extends throughout the length of the basin. It is made up of interbedded sequences and variable proportions of Vitrain, Clarain, Durain and Fusain. Application of embedded Markov chain model rejects the phenomenon of randomness in the repetition of coal lithotypes. The preferential upward transition path for coal lithotypes that can be derived for the Kargali top coal seam is: Vitrain → Clarain → Durain ↔ Fusain → Vitrain, and for the Kargali bottom coal seam is: Clarain ↔ Vitrain → Fusain → Durain → Clarain. By and large, the cyclic repetition of coal lithotypes is similar in the Kargali bottom and top seams. Among the noteworthy features are two-way transitions between Durain and Fusian in Kargali top and between Clarain and Vitrain in the case of Kargali bottom coal seam. Entropy analysis corroborates Markov chain and indicates the presence of type A-4 asymmetrical cycles of coal lithotypes. It is suggested that the banded structure of a coal seam is not a random feature and follows a definite cyclic pattern in the occurrence of coal lithotypes in vertical order and is similar to that described in Australian and European coal seams. Asymmetrical cyclic sequences are a normal, rather than an unusual condition, within coal seams. It is visualized that a gradual decline of toxic environment and ground water level resulted in the coal lithotype cycles in the Kargali seam of East Bokaro sub-basin. The close interbedding of Vitrain and Clarain is suggestive of seasonal fluctuation in anaerobic and aerobic conditions during peat formation.

Specific Energy of Hard Coal Under Load

Abstract The article presents results of experimental tests of energy parameters of hard coals under loading, collected from research sites located within five main geologic structures of Upper Silesian Coal Basin (GZW) - Main Trough, Main Anticline, Bytom Trough, Rybnik Trough and Chwałowice Trough. Coals from12 mines were analysed, starting with seams of group 200, through groups 400, 500, 600 and, finally, seams of group 700. Coal of each of the groups of seams underwent uniaxial compression stress of the energy parameters, in a servo-controlled testing machine MTS-810NEW, for the full range of strain of the tested coal samples. Based on the tests the dependence of different types of specific energy of longitudinal strain of coals on the value of uniaxial compression strength was determined. The dependence of the value of dissipated energy and kinetic energy of coals on the uniaxial compression strength was described with a linear function, both for coals which due to their age belong to various bed sand for various lithotypes of coal. An increase in the value of dissipated energy and in kinetic energy was observed, which was correlated with an increase in uniaxial compression strength of coal. The share of dissipated energy is dominant in the total energy of strain. Share of recoverable energy in the total energy of strain is small, independent of the compression strength of coals and is at most a few per cent high. In coals of low strength and dominant share of dissipated energy, share of recoverable energy is the biggest among the tested coals. It was shown that following an increase in compression strength the share of recoverable energy decreases, while the share of dissipated energy in the total energy increases. Further studies of specific energy of longitudinal strain of rocks in the full-range strain will be the next step inperfecting methodology of research into natural rock burst susceptibility of Carboniferous rock mass and changes in the susceptibility resulting from mining activity.

Evaluation of gas sorption-induced internal swelling in coal

Gas sorption plays an important role in permeability evolution and gas flow in coal. Many efforts have been made to evaluate the effect of gas sorption on permeability. For example, the total gas sorption matrix swelling is divided in to two pars, one acts on cleat (internal swelling) and the other contributes to coal block deformation. Nevertheless, this effect has not been fully identified yet. This paper proposed a method that regressed the internal swelling by using a proposed permeability model. The internal swelling values under different boundary conditions were calculated by using the well-measured permeability from two published papers. The results indicated that the internal swelling was positively proportional to pore pressure and negatively proportional to confining stress for gaseous and supercritical methane and gaseous carbon dioxide. For supercritical carbon dioxide, the internal swelling increased with increasing pore pressure but was almost independent of confining stress. The internal swelling may be affected by sorbate type, coal structure and coal lithotypes in addition to pore pressure and confining stress. The internal swelling ratio that is defined as the ratio of the internal swelling to the gas sorption-induced swelling of coal matrix or coal block was also calculated. The internal swelling ratio may roughly keep constant during coalbed methane recovery when the in situ stress was greater than 5MPa. This assumption was practically useful for engineering application as it linked the effect of gas sorption on in situ permeability to the easy-measured sorption strain. Although the constant value cannot capture the full nature of the in situ internal swelling ratio, it was still capable of improving the accuracy of permeability prediction to some extent.

Reprint of “Petrological and organic geochemical properties of lignite from the Kolubara and Kostolac basins, Serbia: Implication on Grindability Index”

The influence of different coal lithotypes on grindability has been investigated using lignite from two of the most important Upper Miocene lignite basins in Serbia (Kolubara and Kostolac). Yellow xylite-rich types demonstrated the most negative impact on Hardgrove Grindability Index (HGI). All different types of xylite-rich coal, as well as total xylite-rich coal from the Kolubara basin have a negative influence on the grindability properties, while only the yellow type of xylite-rich coal from the Kostolac showed a negative impact on HGI. Matrix coal does not show a clear effect on HGI.A negative correlation between textinite content and HGI is observed in both basins, whereas contents of other macerals do not show influence on grindability properties.Content of total organic carbon demonstrated the negative impact on HGI. Correlation analysis indicates that the negative impact of the yellow type of xylite-rich coal and the sum of total xylite-rich coal on the grindability properties partly can be related to content of total organic carbon and high amount of soluble organic matter. Matrix lithotype does not show any significant correlation with bulk geochemical parameters in both basins.The peat-forming vegetation of all samples from both basins were dominated by decay-resistant gymnosperm (coniferous) plants, belonging to one or several of the families Taxodiaceae, Podocarpaceae, Cupressaceae, Araucariaceae, Phyllocladaceae and Pinaceae. Lignite from the Kolubara basin is characterized by a higher contribution of angiosperm vegetation than coal from the Kostolac basin. Peatification of the Kolubara coal occurred under more oxic conditions than the Kostolac one.Analysis of biomarkers indicated that the negative impact of all types of xylite-rich coal from the Kolubara on HGI can be related to the higher proportion of angiosperms, abundance of mid-chain n-alkanes and sesquiterpenoids, aromatization of non-hopanoid triterpenoids and hopanoids, and intense degradation of wood tissues in a more oxic environment. The positive impact of matrix coal on HGI in the Kolubara samples can be attributed to elevated content of non-aromatic hopanoids and low amounts of aromatic non-hopanoid triterpenoids and sesquiterpenoids, which seems to hinder the grindability properties.

Petrological and organic geochemical properties of lignite from the Kolubara and Kostolac basins, Serbia: Implication on Grindability Index

The influence of different coal lithotypes on grindability has been investigated using lignite from two of the most important Upper Miocene lignite basins in Serbia (Kolubara and Kostolac). Yellow xylite-rich types demonstrated the most negative impact on Hardgrove Grindability Index (HGI). All different types of xylite-rich coal, as well as total xylite-rich coal from the Kolubara basin have a negative influence on the grindability properties, while only the yellow type of xylite-rich coal from the Kostolac showed a negative impact on HGI. Matrix coal does not show a clear effect on HGI.A negative correlation between textinite content and HGI is observed in both basins, whereas contents of other macerals do not show influence on grindability properties.Content of total organic carbon demonstrated the negative impact on HGI. Correlation analysis indicates that the negative impact of the yellow type of xylite-rich coal and the sum of total xylite-rich coal on the grindability properties partly can be related to content of total organic carbon and high amount of soluble organic matter. Matrix lithotype does not show any significant correlation with bulk geochemical parameters in both basins.The peat-forming vegetation of all samples from both basins were dominated by decay-resistant gymnosperm (coniferous) plants, belonging to one or several of the families Taxodiaceae, Podocarpaceae, Cupressaceae, Araucariaceae, Phyllocladaceae and Pinaceae. Lignite from the Kolubara basin is characterized by a higher contribution of angiosperm vegetation than coal from the Kostolac basin. Peatification of the Kolubara coal occurred under more oxic conditions than the Kostolac one.Analysis of biomarkers indicated that the negative impact of all types of xylite-rich coal from the Kolubara on HGI can be related to the higher proportion of angiosperms, abundance of mid-chain n-alkanes and sesquiterpenoids, aromatization of non-hopanoid triterpenoids and hopanoids, and intense degradation of wood tissues in a more oxic environment. The positive impact of matrix coal on HGI in the Kolubara samples can be attributed to elevated content of non-aromatic hopanoids and low amounts of aromatic non-hopanoid triterpenoids and sesquiterpenoids, which seems to hinder the grindability properties.