Petrographic characteristics and depositional conditions of Permian coals of Pench, Kanhan, and Tawa Valley Coalfields of Satpura Basin, Madhya Pradesh, India

This paper attempts to characterize the coals of Satpura Gondwana basin using a large number of pillar coal samples drawn from the working coal mines of Pench, Kanhan, and Tawa (Pathakhera) Valley Coalfields of this basin. This westernmost Gondwana basin of Peninsular India is graben/half-graben type and occupies an area of 12 000 km² with sedimentary fills (>5000 m) ranging in age from Permian to Cretaceous. The Barakar Formation (Permian) is exclusively coal-bearing with a total coal reserve of nearly 2000 Mt. The results show that the coals of this basin are equally rich in inertinite (22.8–58.7%, 24.5–62.0% mmf basis) and vitrinite (24.4–52.4%, 24.4–56.0% mmf basis). The concentration of liptinite ranges from 8.8% to 23.2% (9.0–26.0% mmf basis). The dominant microlithotypes of these coals are inertite and vitrite with comparatively low concentrations of vitrinertite and clarite. The vitrinite reflectance (Rom% values) suggests that the Pench Valley (0.30–0.58%) coals are subbituminous C to high volatile C bituminous in rank, while the Kanhan and Tawa Valley coals (0.52–0.92%) are subbituminous A to high volatile A bituminous in rank. The localized enhancement of rank in the latter two basins has been attributed to the extraneous heat flow from deep-seated igneous intrusions in the basin. The microlithotype composition of these coals is suggestive of their evolution in limno-telmatic zones, under fluvio-lacustrine control with the development of upper deltaic and lower deltaic conditions near the fresh water lacustrines. The floral input is characteristic of forest swamps with intermittent floods, leading to the development of reed moor and open moor facies, particularly in the Pench Valley basin. The Gelification Index (GI) and Tissue Preservation Index (TPI) are suggestive of terrestrial origin with high tree density. Further, moderately high GI and exceedingly high telovitrinite based TPI along with high ash content, particularly for the coals of Kanhan and Tawa Valley Coalfields, are indicative of the recurrence of drier conditions in the forested swamps. Furthermore, lateral variation in TPI values is indicative of increase in the rate of subsidence vis-à-vis depth of the basin from east to west (Pench to Tawa Valley Coalfield). The Ground Water Index (GWI) suggests that these coals have evolved in mires under ombotrophic to mesotrophic hydrological conditions. The Vegetation Index (VI) values are indicative of the dominance of herbaceous plants in the formation of Pench Valley coals and comparatively better forest input in the formation of Kanhan and Tawa Valley coals.     

A microscopic study of the combustion residues of subbituminous and bituminous coals from Alberta, Canada

Chars produced by the combustion of a set of three coals from Alberta, Canada, were classified morphologically using reflected light microscopy. Produced chars are different in morphological features, pore thickness, anisotropy and degree of reactivity, because of differences in the vitrinite and inertinite contents.The subbituminous A coal produced the highest percentage of unreactive or slightly reactive components due to its high inertinite content (20.0%), followed by isotropic cenospheres. Isotropic cenospheres, both thin- and thick-walled, and exploded cenospheres are the characteristic chars produced by the high-voltile bituminous B coal, whereas the low-volatile bituminous coking coal produced cenospheres with granular anisotropy (mosaic) on the walls and abundant coke fragmentsOptical microscopy is useful in differentiating the performance of a series of coals during combustion based on petrographic composition and rank and can aid in understanding the relationship between ‘reactive’ and ‘non-reactive’ coal macerals to burnout performance.     

Properties of thermally metamorphosed coal from Tanjung Enim Area, South Sumatra Basin, Indonesia with special reference to the coalification path of macerals

Thermally metamorphosed Tertiary age coals from Tanjung Enim in South Sumatra Basin have been investigated by means of petrographic, mineralogical and chemical analyses. These coals were influenced by heat from an andesitic igneous intrusion. The original coal outside the metamorphosed zone is characterized by high moisture content (4.13–11.25 wt.%) and volatile matter content (> 40 wt.%, daf), as well as less than 80 wt.% (daf) carbon and low vitrinite reflectance (VR_max = 0.52–0.76%). Those coals are of subbituminous and high volatile bituminous rank. In contrast the thermally metamorphosed coals are of medium-volatile bituminous to meta-anthracite rank and characterized by low moisture content (only < 3 wt.%) and volatile matter content (< 24 wt.%, daf), as well as high carbon content (> 80 wt.%, daf) and vitrinite reflectance (VR_max = 1.87–6.20%). All the studied coals have a low mineral matter content, except for those which are highly metamorphosed, due to the formation of new minerals.The coalification path of each maceral shows that vitrinite, liptinite and inertinite reflectance converge in a transition zone at VR_max of around 1.5%. Significant decrease of volatile matter occurs in the zone between 0.5% and 2.0% VR_max. A sharp bend occurs at VR_max between 2.0% and 2.5%. Above 2.5%, the volatile matter decreases only very slightly. Between VR_r = 0.5% and 2.0%, the carbon content of the coals is ascending drastically. Above 2.5% VR_r, the carbon content becomes relatively stable (around 95 wt.%, daf).Vitrinite is the most abundant maceral in low rank coal (69.6–86.2 vol.%). Liptinite and inertinite are minor constituents. In the high rank coal, the thermally altered vitrinite composes 82.4–93.8 vol.%. Mosaic structures can be recognized as groundmasss and crack fillings. The most common minerals found are carbonates, pyrite or marcasite and clay minerals. The latter consist of kaolinite in low rank coal and illite and rectorite in high rank coal. Change of functional groups with rank increase is reflected most of all by the increase of the ratio of aromatic C–H to aliphatic C–H absorbances based on FTIR analysis. The Oxygen Index values of all studied coals are low (OI < 5 mg CO₂/g TOC) and the high rank coals have a lower Hydrogen Index (< 130 mg HC/g TOC) than the low rank coals (about 300 mg HC/g TOC). T_max increases with maturity (420–440 °C for low rank coals and 475–551 °C for high rank coals).Based on the above data, it was calculated that the temperature of contact metamorphism reached 700–750 °C in the most metamorphosed coal.     

The occurrence of anthracites in an area characterized by lower rank coals in the Upper Silesian Coal Basin of Poland

The results of petrographical-geological and chemical examinations on anthracites, semianthracites and medium-low volatile bituminous coals from Jastrzebie in the Upper Silesian Coal Basin of Poland are presented. The coking coals mined in this region exhibit volatile matter V^daf = 18–26%, free swelling index FSI = 3–8 and reflectance R_m = 1.10–1.35% and are inertiniterich coals (I = 25–63%).Coal Seam 504 of the Anticlinal beds (Namurian B) has been affected by thermal metamorphism and contains both coking coals and coals of higher rank. According to the criterion of Polish Standards this coal seam varies from anthracite (V^daf <10%) to semianthracite (V^daf = 10–14%) in rank. The carbon content is slightly lower and the hydrogen content a little higher than those of typical anthracites and semianthracites. The reflectance values (R_m = 1.56–2.62%) are generally lower than the R_m values proposed by the International Committee for Coal Petrology as boundary values for anthracites and bituminous coal. The magnitude of anisotropy and microhardness were also examined. Examinations of optical properties prove that the metamorphism exhibited by the coals is the result of elevated temperature and variable pressure. The analyses of the maceral composition indicate that there is a decrease in the inertinite content in anthracites. Vitrinite exhibits the features of thermally altered coal. The micrinite content shows a little variation. In coking coals, a strongly fluorescing bituminous substance with the optical features of exsudatinite was found. The constructed geological section of Coal Seam 504 shows distinct regular changes in chemical and physical properties as well as the petrographic composition which may be caused by the heat flux of a magma intrusion, not localized so far.     

On the effects of petrographic composition on coalbed methane sorption

The effect of petrographic composition on the methane sorption capacity has been determined for a suite of coals and organic-rich shales. Subbituminous and bituminous coals were separated into bright and dull lithotypes by hand-picking. The methane sorption capacities range between 0.5 and 23.9 cm³/g at a pressure of 6 MPa. The low volatile bituminous Canmore coal and the anthracite sample have the highest capacities with the “natural coke” having the lowest. For low-rank coals there is no significant difference between bright and dull samples except for one coal with the dull sample having a greater sorption capacity than its bright equivalent. For higher-rank coals, the bright samples have a greater methane capacity than the dull samples and the difference between sample pairs increases with rank. The boghead coal samples have the highest sorption capacities in the liptinite-rich coals suite and are higher than subbituminous to medium volatile bituminous samples. Pore size distribution indicates that methane is held as solution gas in liptinite-rich coals and by physical sorption in micropores in liptinite-poor coals. These contrasting processes illustrate that liptinite-rich samples need to be independently assessed. The positive relationship between reactive inertinite content and methane sorption capacity occurs within the subbituminous to medium volatile bituminous coals because the reactive inertinite is structurally similar to vitrinite and have a higher microporosity than non-reactive inertinite. Reactivity of inertinite should be assessed in CBM studies of dull coals to provide a better understanding of petrographic composition effects on methane capacity.     

Organic geochemical characteristics and depositional environments of the Jurassic coals in the eastern Taurus of Southern Turkey

In this study, organic matter content, type and maturity as well as some petrographic and physical characteristics of the Jurassic coals exposed in the eastern Taurus were investigated and their depositional environments were interpreted.The total organic carbon (TOC) contents of coals in the Feke–Akkaya, Kozan–Gedikli and Kozan–Kizilinc areas are 24.54, 66.78 and 49.15%, respectively. The Feke–Akkaya and Kozan–Kizilinc coals have low Hydrogen Index (HI) values while the Kozan–Gedikli coals show moderate HI values. All coal samples display very low Oxygen Index (OI) values. The Kozan–Gedikli coals contain Type II organic matter (OM), the Feke–Akkaya coals contain a mixture of type II and type III OM; and the Kozan–Kizilinc coals are composed of Type III OM. Sterane distribution was calculated as C₂₇ > C₂₉ > C₂₈ from the m/z 217 mass chromatogram for all coal samples.T_max values for the Feke–Akkaya, Kozan–Gedikli and Kozan–Kizilinc coals are 439, 412 and 427 °C. Vitrinite reflectance values (%R_o) for the Feke–Akkaya and Kozan–Kizilinc coal samples were measured as 0.65 and 0.51 and these values reveal that the Feke–Akkaya and Kozan–Kizilinc coals are at subbituminous A or high volatile C bituminous coal stage. On the basis of biomarker maturity parameters, these coals have a low maturity.The pristane/phytane (Pr/Ph) ratios for the Feke–Akkaya, Kozan–Gedikli and Kozan–Kizilinc coals are 1.53, 1.13 and 1.25, respectively. In addition, all coals show a homohopane distribution which is dominated by low carbon numbers, and C₃₅ homohopane index is very low for all coal samples. All these features may indicate that these coals were deposited in a suboxic environment.The high sterane/hopane ratios with high concentrations of steranes, low Pr/Ph ratios and C₂₅/C₂₆ tricyclic ratios > 1 may indicate that these coals formed in a swamp environment were temporarily influenced by marine conditions.     

The role of coal petrographic characteristics in evaluating the non-coking nature of the coals of ramagundam and Kothagudem coalfields,Godavari Valley Basin,Andhra Pradesh,India

The maceral and microlithotype compositions of coals representative of the different coal seams of the Ramagundam and Kothagudem coalfields, Godavari Valley Basin, are compared with those of the Ib River, Talcher, South Karanpura, Hura, and Brahmani coalfields. The vitrite + clarite—“Intermidiates”—durite + fusite + shale (<20%) triangular diagram places these coals in the area of non-coking coals, clearly distinct from the coking and semi-coking coals. The vitrinite reflectance is low (R_ormoil^aver: 0.38–0.71%), far below the coking-coal range. Thus, based on petrographic composition and rank, these coals are of non-coking nature. A triangular diagram is proposed delineating the coking, semi-coking and non-coking coal areas for the Gondwana coals of India.     

Mongolian coal-bearing basins: Geological settings, coal characteristics, distribution, and resources

This paper presents geological settings, stratigraphy, coal quality, petrography, reserves and the tectonic history of the Mongolian coal-bearing basins. This is based on a synthesis of the data from nearly 50 coal deposits. The results of ultimate and proximate analyses, and calorific value, maceral composition and vitrinite reflectance data is given.The coal deposits of Mongolia tend to become younger from west to east and can be subdivided into two provinces, twelve basins, and three areas. Main controlling factor of coal rank is the age of the coal bearing sequences. Western Mongolian coal-bearing province contains mostly high rank bituminous coal in strata from Late Carboniferous. The basins in southern Mongolia and the western part of central Mongolia have low rank bituminous coal in strata from the Permian. The northern and central Mongolian basins contain mainly Jurassic subbituminous coal, whereas the Eastern Mongolian province has Lower Cretaceous lignite. The Carboniferous, Permian and Jurassic coal-bearing sequences were mainly deposited in foreland basins by compressional tectonic event, whereas Cretaceous coal measures were deposited in rift valleys caused by extensional tectonic event. Petrographically, Mongolian coals are classified as humic type. Vitrinite/huminite groups of Carboniferous, Permian, and Cretaceous coal range from 44.9% to 82.9%. Inertinite group varies between 15.0% and 53.3%, but liptinite group does not exceed more than 7%. Jurassic coals are characterized by high percentages of vitrinite (87.3% to 96.6%) and liptinite groups (up to 11.7%). This might be explained by paleoclimatic conditions. Mongolian coal reserves have been estimated to be 10.2 billion tons, of which a predominant portion is lignite in the Eastern Mongolian province and coking coal in the South Gobi basin.     

Studies of the relationship between coal petrology and grinding properties

The maceral and microlithotype composition of selected coals has been investigated with respect to the grinding properties, specifically Hardgrove grindability index (HGI), of the coals. The study expands upon previous investigations of HGI and coal petrology by adding the dimension of the amount and composition of the microlithotypes. Coal samples, both lithotypes and whole channels, were selected from restricted rank ranges based on vitrinite maximum reflectance: 0.75–0.80% R_max, 0.85–0.90% R_max and 0.95–1.00% R_max. In this manner, the influence of petrographic composition can be isolated from the influence of rank. Previous investigations of high volatile bituminous coals demonstrated that, while rank is an important factor in coal grindability, the amount of liptinite and liptinite-rich microlithotypes is a more influential factor. In this study, we provide further quantitative evidence for the influence of microlithotypes on HGI and, ultimately, on pulverizer performance.     

Study of physical and chemical properties of vitrinites. Inferences on depositional and coalification controls

A detailed study of petrological, geochemical, textural and coking properties was carried out on vitrains from the Puertollano, Blanzy–Montçeau, Asturias and Teruel. The objective was to determine in depth the physical and chemical properties of a series of natural and pure vitrinites of different rank, and the influence that the sedimentary and post-sedimentary conditions had on them. It is demonstrated that although vitrains are almost entirely made up of the huminite/vitrinite maceral group they have a different composition, thermal behaviour and physical properties. Thus, geochemical and textural properties of Blanzy–Montçeau vitrains can be considered to be representative of the telinite, the major component in both samples (75% vol.) at the beginning of the bituminization stage (subbituminous/high volatile C bituminous coals). The characteristics of the Puertollano vitrains described here can also be attributed to the telocollinite (>80% vol.) for the high volatile C bituminous coal. Variations in physico-chemical properties between Puertollano and Blanzy–Montçeau vitrains are due to the differences in the initial composition of organic matter. Slight differences (i.e., bed moisture content or porosity) between vitrains from the two coal seams in the same basin can be attributed to their stratigraphic position. Several parameters such as S₂, HI, oil and extraction yield and fluorescent properties suggest that the Puertollano and Blanzy–Montçeau vitrains have a lower hydrocarbon potential. The relationships between geochemical and textural properties make it possible to distinguish between ‘normal' and perhydrous vitrains. The two different hydrogen-enrichment processes that occur in vitrains from the Teruel and Asturias basins can be distinguished from the extraction yield data.     

Scientific classification of bituminous coals

Proposals for the new international classification systems of bituminous coals, at present being prepared by the respective U.N. ECE group of experts, presuppose applying the micropetrographic parametes — vitrinite reflectance, as a measure of rank, and inertinite content. These parameters, the utilizability of which is indisputable from the scientific point of view, are not sufficient for a complete characterization of the coal mass properties in thermo-chemical transformations, particularly in the coking process. These proposals, therefore, assume the use of further parameters, specially the swelling index and volatile matter, to eliminate the limitation of the micropetrographic parameters.On the basis of an extensive sampling base of bituminous coals produced in the Ostrava-Karviná coal district (OKR coals), an evaluation was carried out of different parameters characterizing the most important properties of the coal mass and respective classifications were proposed. Selected classification parameters were: vitrinite reflectance, as a measure of rank, associated with the properties of the ordered part of vitrinite macromolecules, the H/O_at ratio, reflecting the properties of their disordered part, inertinite content and coking property, expressed by the swelling index (SI). The scientific character of this classification consists in the fact that it reflects significant physical and chemical properties which can be measured by means of the instrumental techniques available at present and, moreover, that it is associated with coal mass genesis.In order to evaluate coal reserves in seams, this classification has been modified to a less complicated variant, using three parameters, which is applicable to evaluating coal blends for coke production on the basis of expressing the coal-blend composition according to rank.     

Petrographic prediction of coking properties for the Curragh coals of Australia

Curragh Queensland Mining Limited, Australia, produces a high quality medium volatile bituminous coking coal from the Orion, Pollux and Castor seams from the upper Permian Rangal Coal Measures. It is one of the lowest ash, prime hard coking coal blends produced in Australia. It is also low in sulfur and produces very strong coke when carbonized alone and in blends. Early attempts to predict coking properties of the coals from petrographic data produced predicted coke stabilities that were significantly lower than those determined from coke tests. There is some question as to how much of the ‘inertinite’ in these and other southern hemisphere coals is truly inert during carbonization and how much is reactive. The current study characterized the Curragh coals in terms of physical, chemical and petrographic characteristics and also involved the production of test oven cokes for characterization and strength testing. As part of the work effort a series of suggested techniques for improving predictions of coke strength from petrographic data were examined and a new and improved technique was developed for the Curragh coals. How broadly the technique can be applied to other coals needs to be determined.     

Characterization of Turkish coals: a nationwide perspective

The U.S. Geological Survey (USGS) and the Turkish General Directorate of Mineral Research and Exploration (Maden Tetkik ve Arama Genel Müdürlügü, MTA) are working together to provide a better understanding of the chemical properties of Turkish coals from major Turkish lignite producing areas.The coals in Turkey are generally low rank (lignite or subbituminous) formed in several different depositional environments at different geologic times and have differing chemical properties. Eocene coals are limited to northern Turkey; Oligocene coals, found in the Thrace Basins of northwestern Turkey, are intercalated with marine sediments; Miocene coals are generally located in Western Turkey. The coal deposits, which have limnic characteristics, have relatively abundant reserves. Pliocene–Pleistocene coals are found in the eastern part of Turkey. Most of these coals have low calorific values, high moisture, and high ash contents.Analysis of 143 coal channel samples (most are lignite and subbituminous in rank, but a few are bituminous and one is anthracitic in rank) has been completed for up to 54 elements and other coal properties using a variety of analytical techniques, including inductively coupled plasma emission and mass spectrometry, instrumental neutron activation analysis, and various single element techniques and ASTM standard procedures. Many of these coals have elemental concentrations similar to U.S. lignites found in the Gulf Coast and Fort Union regions. However, maximum or mean concentrations of B, Cr, Cs, Ni, As, Br, Sb, Cs, and U in Turkey are higher than the corresponding maximum or mean values found in either the Fort Union or Gulf Coast regions.     

Fossil fuel energy resources of Ethiopia: Coal deposits

The gravity of Ethiopian energy problem has initiated studies to explore various energy resources in Ethiopia, one among this is the exploration for coal resources. Studies confirmed the presence of coal deposits in the country. The coal-bearing sediments are distributed in the Inter-Trappean and Pre-Trap volcanic geological settings, and deposited in fluvio-lacustrine and paludal environments in grabens and half-grabens formed by a NNE–SSW and NNW–SSE fault systems. Most significant coal deposits are found in the Inter-Trappean geological setting. The coal and coal-bearing sediments reach a maximum thickness of 4 m and 300 m, respectively. The best coal deposits were hosted in sandstone–coal–shale and mudstone–coal–shale facies. The coal formations of Ethiopia are quite unique in that they are neither comparable to the coal measures of the Permo-Carboniferous Karroo Formation nor to the Late Devonian–Carboniferous of North America or Northwestern Europe. Proximate analysis and calorific value data indicated that the Ethiopian coals fall under lignite to high volatile bituminous coal, and genetically are classified under humic, sapropelic and mixed coal. Vitrinite reflectance studies confirmed 0.3–0.64% Ro values for the studied coals. Palynology studies confirmed that the Ethiopian coal-bearing sediments range in age from Eocene to Miocene. A total of about 297 Mt of coal reserve registered in the country. The coal reserve of the country can be considered as an important alternative source of energy.     

Petrographic characteristics and depositional conditions of Eocene coals of platform basins, Meghalaya, India

The coal deposits of Meghalaya occur in the Lakadong Sandstone (25–250 m thick) of Eocene age. The coal-bearing formations are understood to have been deposited over platform areas in estuarine and lagoonal environments and subjected to recurrent marine transgressions and regressions during the Eocene period. There are three major groups of coalfields in Meghalaya, viz. Garo Hills (West Daranggiri and Siju Coalfields), Khasi Hills (Langrin and Mawlong–Shella Coalfields) and minor coalfields (Laitryngew, Cherrapunji and Bapung Coalfields). Pillar coal samples have been collected from 10 seams at 15 locations and have been subjected to a detailed petrographic examination for their characterization. An effort has been made to trace the path of their evolution based on coal petrography-based models. The quantitative petrographic analysis shows that these coals are vitrinite rich (45.0–92.9%, mean 73.4% mmf basis) with low concentration of inertinite (0.0–13.8%, mean 3.0% mmf basis), whereas the liptinite occurs in appreciable concentration (5.5–53.1%, mean 22.5% mmf basis). Further, these coals are rich in vitrite (51.6–100%, mean 78.3% mmf basis). The volatile matter (from 38.5% to 70.0%, d.a.f.) and vitrinite reflectance (Rom from 0.37% to 0.68%) characterize these coals, as per German (DIN) and North American classification, approximately as sub-bituminous ‘C' to high volatile ‘C' bituminous. The occurrence of teleutospore (single, double and triple celled) suggests that these coals have originated from a characteristic Tertiary flora. The maceral and microlithotype composition in the coal petrography-based depositional models suggest that the coals of Garo Hills were formed in reed to open water swamps in telmatic to limnic conditions. The coals of Khasi Hills were dominated by forest swamps and telmatic to limno-telmatic conditions. In addition, the occurrence of large-size resins suggests prolific growth of conifers in the swamps.     

Iodine in Chinese coals and its geochemistry during coalification

To determine the I distribution in Chinese coals, a nationwide survey was undertaken based on the distribution, periods of formation, rank and production yields of various coal deposits. A total of 305 coal samples were collected and their I contents were determined by catalytic spectrophotometry with pyrohydrolysis. The geochemistry of I during coalification (including both peat diagenesis and coal metamorphism) was assessed. It was found that the I contents of Chinese coals range from 0.04 mg kg^–1 to 39.5 mg kg^–1 and exhibit a lognormal distribution, with a geometric mean of 1.27 mg kg^–1. Statistical correlation analysis and the observation that I contents increase with coal rank indicate that coal I is chalcophile in nature, and not generally organically bound. When peat developed into lignite through diagenesis, 95–99.9% of the original I was lost. The composition and structure of clay minerals present in the coal were controlled by the original depositional environment. The higher the I content of coals, the more likely the original sediments were affected by a marine environment. Iodine contents increased from lignite through sub-bituminous and bituminous coals to anthracite. This indicates that coal absorbed excess I from hydrothermal fluids during metamorphism (including geothermal metamorphism and telemagmatic metamorphism). The telemagmatic metamorphism was caused by magmatic activities that depended on the specific geological structure of the region. In China, most high-rank coals were formed by telemagmatic metamorphism.     

Possibilities of the utilization of micropetrographic parameters in the scientific classification of bituminous coals

Proposals for new scientific classifications of bituminous coals are based on micropetrographic parameters, i.e. vitrinite reflectance as a criterion of the coalification and maceral composition, presupposed to express the connection between the genetic peculiarities and physical, chemical, and technological properties of the coal mass. In the case of coals with high inertinite contents, however, the utilizability of these parameters meets with difficulties resulting from the subjectivity of determining the different transitional material and from insufficient knowledge of inertinite behaviour at higher temperatures. In the case of the maceral-variable bituminous coals produced in the Ostrava-Karviná Coal Basin, these insufficiencies are not important since it is especially the expression of the variability of the properties of isometamorphic vitrinites, which has decisive effects up-on the course of the thermo-chemical transformations, that is of principal importance to the scientific classification of these coals.In the first approximation, the properties of isometamorphic vitrinites may be expressed by the parameter (H/O)_at, closely connected with fluidity. While the micropetrographic parameters reflect in particular the peculiarities in the chemical structure of the aromatic parts of coal macromolecules, the parameter (H/O)_at expresses the properties of the non-aromatic structures of vitrinite, significantly affecting the course of its thermal degradation. The experimental results show that the value of the parameter (H/O)_at, fluidity and the course of degassing the coal of a lower coalification are independent of the maceral composition and vitrinite reflectance; also that the caking and coking properties of low-rank coals are especially dependent on the parameter (H/O)_at and partially on the micropetrographic parameters. All these facts should be taken into consideration in preparing new scientific classifications of bituminous coals.     

The petroleum generation potential and effective oil window of humic coals related to coal composition and age

A worldwide data set of more than 500 humic coals from the major coal-forming geological periods has been used to analyse the evolution in the remaining (Hydrogen Index, HI) and total (Quality Index, QI) generation potentials with increasing thermal maturity and the ‘effective oil window’ (‘oil expulsion window’). All samples describe HI and QI bands that are broad at low maturities and that gradually narrow with increasing maturity. The oil generation potential is completely exhausted at a vitrinite reflectance of 2.0–2.2%R_o or T_max of 500–510 °C. The initial large variation in the generation potential is related to the original depositional conditions, particularly the degree of marine influence and the formation of hydrogen-enriched vitrinite, as suggested by increased sulphur and hydrogen contents. During initial thermal maturation the HI increases to a maximum value, HI_max. Similarly, QI increases to a maximum value, QI_max. This increase in HI and QI is related to the formation of an additional generation potential in the coal structure. The decline in QI with further maturation is indicating onset of initial oil expulsion, which precedes efficient expulsion. Liquid petroleum generation from humic coals is thus a complex, three-phase process: (i) onset of petroleum generation, (ii) petroleum build-up in the coal, and (iii) initial oil expulsion followed by efficient oil expulsion (corresponding to the effective oil window). Efficient oil expulsion is indicated by a decline in the Bitumen Index (BI) when plotted against vitrinite reflectance or T_max. This means that in humic coals the vitrinite reflectance or T_max values at which onset of petroleum generation occurs cannot be used to establish the start of the effective oil window. The start of the effective oil window occurs within the vitrinite reflectance range 0.85–1.05%R_o or T_max range 440–455 °C and the oil window extends to 1.5–2.0%R_o or 470–510 °C. For general use, an effective oil window is proposed to occur from 0.85 to 1.7%R_o or from 440 to 490 °C. Specific ranges for HI_max and the effective oil window can be defined for Cenozoic, Jurassic, Permian, and Carboniferous coals. Cenozoic coals reach the highest HI_max values (220–370 mg HC/g TOC), and for the most oil-prone Cenozoic coals the effective oil window may possibly range from 0.65 to 2.0%R_o or 430 to 510 °C. In contrast, the most oil-prone Jurassic, Permian and Carboniferous coals reach the expulsion threshold at a vitrinite reflectance of 0.85–0.9%R_o or T_max of 440–445 °C.     

Supercritical gas sorption on moist coals

The effect of moisture on the CO₂ and CH₄ sorption capacity of three bituminous coals from Australia and China was investigated at 55 °C and at pressures up to 20 MPa. A gravimetric apparatus was used to measure the gas adsorption isotherms of coal with moisture contents ranging from 0 to about 8%. A modified Dubinin–Radushkevich (DR) adsorption model was found to fit the experimental data under all conditions. Moisture adsorption isotherms of these coals were measured at 21 °C. The Guggenheim–Anderson–de Boer (GAB) model was capable of accurately representing the moisture isotherms over the full range of relative pressures.Moist coal had a significantly lower maximum sorption capacity for both CO₂ and CH₄ than dry coal. However, the extent to which the capacity was reduced was dependent upon the rank of the coal. Higher rank coals were less affected by the presence of moisture than low rank coals. All coals exhibited a certain moisture content beyond which further moisture did not affect the sorption capacity. This limiting moisture content was dependent on the rank of the coal and the sorbate gas and, for these coals, corresponded approximately to the equilibrium moisture content that would be attained by exposing the coal to about 40–80% relative humidity. The experimental results indicate that the loss of sorption capacity by the coal in the presence of water can be simply explained by volumetric displacement of the CO₂ and CH₄ by the water. Below the limiting moisture content, the CO₂ sorption capacity reduced by about 7.3 kg t^− 1 for each 1% increase in moisture. For CH₄, sorption capacity was reduced by about 1.8 kg t^− 1 for each 1% increase in moisture.The heat of sorption calculated from the DR model decreased slightly on addition of moisture. One explanation is that water is preferentially attracted to high energy adsorption sites (that have high energy by virtue of their electrostatic nature), expelling CO₂ and CH₄ molecules.     

An overview of the Permian (Karoo) coal deposits of southern Africa

The coal deposits of southern Africa (Botswana, Malawi, Mozambique, Namibia, South Africa, Swaziland, Tanzania, Zambia and Zimbabwe) are reviewed. The coal seams formed during two periods, the Early Permian (Artinskian–Kungurian) and the Late Permian (Ufimian–Kazanian). The coals are associated with non-marine terrestrial clastic sedimentary sequences, most commonly mudrock and sandstones, assigned to the Karoo Supergroup. The Early Permian coals are most commonly sandstone-hosted while the younger coals typically occur interbedded with mudstones. The sediments were deposited in varying tectono-sedimentary basins such as foreland, intracratonic rifts and intercratonic grabens and half-grabens. The depositional environments that produced the coal-bearing successions were primarily deltaic and fluvial, with some minor shoreline and lacustrine settings. Coals vary in rank from high-volatile bituminous to anthracite and characteristically have a relatively high inertinite component, and medium- to high-ash content. In countries where coal is mined, it is used for power generation, coking coal, synfuel generation, gasification and for (local) domestic household consumption.