Maximum Vitrinite Reflectance Research Articles

Re-Calibrating the Mercury-Intrusion-Porosimetry-Measured Pore Size Distribution of Coals: A Novel Method for Calculating the Matrix Compression Coefficient

Accurate measurement of the pore size distribution (PSD) in coals is crucial for guiding subsequent coalbed methane (CBM) engineering practice. Currently, mercury intrusion porosimetry (MIP) measurement has been widely used as a PSD testing method due to its effectiveness and convenience. Nevertheless, it is worth noting that the elevated pressure during the MIP experiments can lead to matrix compressibility, potentially causing inaccurate estimations of PSD in coals. Therefore, correction methods are used to modify the PSD in the high-pressure segment to improve the accuracy of MIP data. This study proposed a novel method with higher accuracy and convenience for calculating the matrix compressibility coefficient compared to the traditional calculation methods. Firstly, the matrix compressibility coefficients of six coal samples were calculated by using low-temperature nitrogen adsorption (LTNA) data. Subsequently, by utilizing the mathematical correlation between Kc (the compressibility coefficient of the coal matrix) and Ro,max (the maximum vitrinite reflectance) from prior research, a novel statistical method was designed to determine the matrix compressibility coefficient of the samples. Finally, the statistical matrix compressibility coefficient determination method was used to examine the fractal characteristics of the actual PSD. The results indicate that when the pressure exceeds 24 MPa, the volume obtained from mercury intrusion exceeds the pore volume measurement. The Kc calculated using the traditional correction method is in the range of 0.876–1.184 × 10−10 m2/N, while the Kc values of our proposed statistical correction method range from 0.898 × 10−10 to 1.233 × 10−10 m2/N, with a comparison error rate of ~0.11–5.25%. The MIP data greater than 24 MPa were effectively corrected using the statistical correction method, thus reducing the mercury intrusion volume error by 91.75–96.40%. Additionally, the corrected pore fractal dimension (D2) values fall within the range of 2.792 to 2.975, which are closer to the actual values than the pore fractal dimension range of 3.186 to 3.339.

Physical Characteristics of Coal Reservoirs and CBM Favorable Area Prediction in the Huaibei Coalfield, Northern China.

To quantitatively characterize middle-high-ranked coal reservoirs, the physical characteristics of seven coal samples from the Huaibei Coalfield in northern China were investigated in detail based on experiments including proximate analysis, coal petrology, low-temperature nitrogen adsorption (LTNA), mercury intrusion porosimetry (MIP), nuclear magnetic resonance (NMR), and methane isothermal adsorption. The results show that coal maceral in the Huaibei Coalfield is dominated by vitrinite, with a large change in the maximum vitrinite reflectance ranging from 0.7 to 2.5%. The various coal metamorphisms can be attributed to the combined influence of magmatic activities of the Tanlu and Taihang Mountains during the Yanshanian. The coal quality can be characterized by medium-high ash yield, low to very low sulfur content, low phosphorus, and medium-high calorific value. From low-ranked coals to medium-ranked coals, the volume of adsorption and seepage pores decreases but the fracture volume increases due to the stronger dehydration and coal matrix shrinkages. From medium-ranked coals to high-ranked coals, adsorption pores have a significant advantage, suggesting a stronger CH4 adsorption capacity. There is a positive correlation among the fixed carbon content, coal rank, and Langmuir volume, which can be attributed to the transformation of coal chemical composition and structure by coal metamorphism. The deep Xiaoxi in the Suixiao coal mining area, deep Nanping, deep Taoyuan-Qinan, deep Pengqiao, northern Zhuxianzhuang in the Suxian coal mining area, deep Renlou-Zhaoji, and Xutuan deep in the Linhuan mining area are predicted to be favorable areas for CBM exploration in the Huaibei Coalfield.

Characteristics and Influence Factors of Natural Desorption in Coal Bodies from Fukang Mining Area, Xinjiang, China.

Coal body desorption characteristics are one of the key factors that influence the development of coalbed methane (CBM). In this study, 91 coal core samples from 11 CBM wells in the Fukang mining area were collected from Xinjiang, China, and the coal quality, high-pressure mercury compression, gas content, and natural desorption characteristics measurements were launched. With the detailed analyses of the differences in cumulative desorption volume, desorption ratio, and on-site average desorption rate for the coal samples with different body structures and macrolithotypes, the influence of the maximum reflectance of vitrinite, microscopic coal rock composition, and coal quality and pore characteristics on CBM desorption characteristics were discussed. The results showed that the cumulative desorption volume, desorption ratio, and desorption rate of cataclastic structure-bright coal are higher than those of primary structure-semibright coal. With the increase of RO,max and vitrinite content, the adsorption capacity of coal increases, and the increased methane concentration difference during desorption leads to an increase in cumulative desorption volume and on-site average desorption rate. The higher contents of moisture and ash yield would occupy the adsorption sites and hinder gas diffusion, which would decrease the desorption of coalbed methane. The greater porosity/pore volume ratio of medium and large pores can enhance the connectivity of pores, which increases the desorption ratio and the average desorption rate, while the higher micropore porosity/pore volume ratio can increase the gas adsorption space and the cumulative desorption volume. The pore characteristics have the most significant effect on the cumulative desorption volume and desorption ratio. The results of the study can help guide coal mine gas management and CBM development from middle-and low-rank coal reservoirs in Xinjiang.

Characteristics of Deep Coal Reservoir and Key Control Factors of Coalbed Methane Accumulation in Linxing Area

Deep coalbed methane (CBM, commonly accepted as >1500 m) has enormous exploration and development potential, whereas the commercial development of deep CBM exploration areas wordwide has been quite limited. The Linxing area, with coals buried approximately 2000 m deep, shows great development potential. Based on a basic geological analysis of structural and hydrodynamic conditions, combining field tests of reservoir temperature and pressure and indoor measurements of maceral composition, proximate analysis, thermal maturity, porosity and permeability, the factors controlling deep CBM accumulations were discussed. The results show that the present burial depth of the No. 8 + 9 coal seam, mainly between 1698 and 2158 m, exhibits a high reservoir temperature (45.0–64.0 °C) and pressure (15.6–18.8 MPa), except for the uplift area caused by the Zijinshan magma event (with coal depth approximately 1000 m). The maximum vitrinite reflectance (Ro,max) of the coal varies from 1.06% to 1.47%, while the magma-influenced areas reach 3.58% with a relatively high ash content of 31.3% (air-dry basis). The gas content calculated by field desorption tests shows a wide range from 7.18 to 21.64 m3/t. The key factors controlling methane accumulation are concluded from regional geological condition variations. The north area is mainly controlled by structural conditions and the high gas content area located in the syncline zones. The center area is dominated by the Zijinshan magma, with relatively high thermal maturity and a high gas content of as much as 14.5 m3/t. The south area is developed with gentle structural variations, and the gas content is mainly influenced by the regional faults. Furthermore, the groundwater activity in the eastern section is stronger than that in the west, and the hydrodynamic stagnant areas in the western are more beneficial for gas accumulation. The coals vary from 3.35% to 6.50% in porosity and 0.08 to 5.70 mD in permeability; thus, hydrofracturing considering high temperature and pressure should be applied carefully in future reservoir engineering, and the co-production of gas from adjacent tight sandstones also should be evaluated.

A method to classify coal pore system by using cumulative amplitude ratio and its dynamic variation

AbstractAs coal pore development is decisive for choosing the engineering site and predicting the CO2 storage capacity, this paper provides a new method to define the double T2 cutoff values by using cumulative amplitude ratio measured by nuclear magnetic resonance measurements, classifies the coal pore systems, and analyzes the influences on cumulative amplitude ratio. The following cognitions are achieved. The minimum ratio always varies narrowly and ranges from 0.9 to 1.1, which is quite stable and approximately equals to 1. Ranges of maximum and average ratios are 1.2–3.5 and 1.1–1.8, respectively. T2c1 represents the dividing point of diffusion pore and permeation pore, and its average value is about 4.1 ms. T2c2 represents the dividing point of permeation pore and cleat, with an average value of about 81.9 ms. The volumetric proportions of diffusion pore range from 1.5% to 76.2%, with an average value of 34.6%; the volumetric proportions of permeation pore are from 14.9% to 98.5%, with an average of 46.8%; while the volumetric proportions of cleat are between 8.4% and 57.5%, with an average of 26.6%. According to the different influencing degrees on maximum and average ratios, three types of parameters can be divided. The first type is strong correlation parameters and includes permeability, volumetric percentage of cleat, and relative volumetric percentage of cleat. The second type is medium correlation parameters, such as volumetric percentage of diffusion pore. The third type is weak correlation parameters, including T2 cutoff values, porosity, and maximum vitrinite reflectance. © 2023 Society of Chemical Industry and John Wiley & Sons, Ltd.

Fractal characteristics for coal chemical structure: Principle, methodology and implication

Despite coal chemical structure's importance as the organic backbone for hydrocarbon production, the fractal characteristics for coal chemical structure remains unexplored. Therefore, this paper proposed a novel method to calculate the fractal dimension Df for coal chemical structure. The principle and methodology of the proposed method were deduced and summarized in detail. This method was applied to calculate the fractal dimension Df for the coal chemical structure from lignite to anthracite. The research results indicate that the fractal dimension Df for coal chemical structure decreases first and then increases with coalification. The turning point of the variation trend of fractal dimension Df and coalification corresponds to the second coalification jump (maximum vitrinite reflectance Ro, max = 1.30 %). This study has achieved the micro characterization on coal chemical structure at molecular scale, which has potential application value on the functional properties analysis and molecular synthesis design of coal-based material research fields.

Was coal metamorphism an influence on the minor element chemistry of the Middle Pennsylvanian Springfield (No. 9) coal in Western Kentucky?

Coal rank in the Middle Pennsylvanian Carbondale Formation Springfield coal in the Western Kentucky coalfield, part of the Illinois Basin, ranges from high volatile A bituminous in southern Union and Webster counties to high volatile C bituminous in the rest of the region. The chlorine content closely follows the coal rank, being highest in the high volatile A bituminous coals. High concentrations of V and Cr are evident in the top benches at all sites and is particularly strong in the higher rank sites. Zinc and other elements (Ni, Mo, among others) are enriched in some benches, but the trend is not as consistent as with V and Cr. Zirconium (+ Nb) and TiO2, both serving as proxies for a detrital influx, show differentiation among the quadrangles, with the high volatile C bituminous Spottsville quadrangle coals having the highest Zr + Nb content. The correlation between Y and Zr indicates that the highest Y occurs in the Spottsville samples. The Spottsville samples have highest REE. Principal components analysis using both a rank parameter (vitrinite maximum reflectance) and the set of geochemical parameters showed that the coal rank axis (Rmax) (with the highest rank being in the Providence quadrangle) is orthogonal to the overlapping LREE/HREE and GdN/GdN* axes and is opposite to the REE, Zr + Nb, and Zn axes. Hydrothermal metamorphism certainly influenced coal rank, the chlorine content of the coals, and, perhaps, some aspects of the minor element chemistry. Coal metamorphism, however, was not the sole influence and not even the dominant influence on the concentration of REE in the Springfield coal in western Kentucky since other factors, such as the greater concentrations of detrital minerals in the Spottsville quadrangle samples, seem to have had a greater influence on the REE chemistry.

Metallurgical performance and structural characteristics of cokes of hypercoal prepared from the mixture of low-rank coal and biomass residue

This study aimed to effectively utilize low-rank coal and biomass residue in the coking process. Hypercoal was produced through co-thermal extraction, and its performance as a component in the coking process was investigated. The effects of the amount of hypercoal extracted from 80 wt% low-rank coal and 20 wt% biomass residue on the coke strength and structure were investigated. Raman spectroscopy, optical microscopy, and scanning electron microscopy were used to analyze the carbonaceous, optical, and pore structures of the hypercoal coke. The optimal ratio of the coal blend contained 15 wt% hypercoal, with compressive and drum strengths of 7.76 MPa and 86 %, respectively. The vitrinite content and average maximum vitrinite reflectance of the hypercoal were 95 % and 0.352, respectively. High vitrinite contents in hypercoal contribute to the formation of mosaics within the coke, which can improve its cold strength. Moreover, the evolution of the hypercoal coke structure at various addition ratios was explored.

Biogenic Methane Accumulation and Production in the Jurassic Low-Rank Coal, Southwestern Ordos Basin

Geological conditions are the key for coalbed methane (CBM) accumulation and production. However, the geological feature of CBM accumulation and production in the Jurassic of Ordos Basin lacks systematic and detailed evaluation, resulting in poor CBM production in this area. This study has determined the genetic types of gas according to geochemistry characteristics of the gas, the geological factors to control CBM accumulation and production performance were revealed, and a comprehensive method was established to evaluate favorable areas based on 32 sets of CBM well production data from Jurassic Yan’an Formation. The results show the coal macerals are rich in inertinite (41.13~91.12%), and the maximum reflectance of vitrinite (Ro,max) in coal is 0.56~0.65%. According to gas compositions and carbon isotopes analysis, the δ13C(CH4) is less than −55‰, and the content of heavy hydrocarbon is less than 0.05%. The value of C1/(C2 + C3) is 6800~98,000, that is, the CBM is a typical biogenic gas of low-rank coal. The CBM accumulation model is the secondary biogenic on the gentle slope of the basin margin, in which gas content is closely related to buried depth and hydrodynamic environment, i.e., the high gas content areas are mainly located in the groundwater weak runoff zone at the burial depth of 450 m~650 m, especially in the syncline. Meanwhile, gas production mainly depends on the location of the structure. The high gas production areas of vertical wells were distributed on the gentle slope with high gas content between anticline and syncline, and the horizontal wells with good performance were located near the core of the syncline. According to the above analysis combined with the random forest model, the study area was divided into different production favorable areas, which will provide a scientific basis for the CBM production wells.

Fractal characteristics of pulverized high volatile bituminous coals with different particle size using gas adsorption

To better understand the effect of particle size on fractal characteristics of pores in coal, three high volatile bituminous coals with mean maximum vitrinite reflectance of 0.82–0.90% collected from the Naryksko-Ostashinskaya coalbed methane field in Kuznetskiy Basin in Russia were crushed and sieved to five progressivelydecreasing particle size fractions: 1.0–0.5 mm, 0.50–0.25 mm, 0.250–0.125 mm, 0.125–0.063 mm and 0.063–0.032 mm. For all particle size fractions, particle size distribution measurement, proximate analysis, maceral analysis and low-pressure N2 adsorption and CO2 adsorption analyses were conducted. Fractal dimensions D1 (pore surface roughness) and D2 (pore structural irregularity) were determined from relative pressure intervals of 0–0.45 and 0.45–1 of N2 adsorption isotherm with the application of the fractal Frenkel-Halsey-Hill theory, respectively. The results demonstrate significant differences in fractal dimensions in different particle size fractions. As particle size of coal samples decreases, D1 decreases, while D2 increases. Some constricted mesopores are transformed into non-constricted mesopores during crushing. The non-constricted mesopores increasingly concentrate in smaller pore sizes, which results in smoother pore surface (decreasing D1) and more heterogeneous pore structure (increasing D2). Mesopores with pore width between 2 nm and 5 nm are the key to fractal dimensions. Individual mesopore structure parameters closely associated with particle size have universal correlations with fractal dimensions. The increasing mesopore specific surface area and volume and decreasing average mesopore width lead to a decrease in D1 and an increase in D2, which further confirms the effect of particle size on fractal dimensions.

Study on the Applicability of Reservoir Fractal Characterization in Middle-High Rank Coals with NMR: Implications for Pore-Fracture Structure Evolution within the Coalification Process.

In order to evaluate the applicability of the pore-fracture structure fractal characterizations in coal reservoirs and confirm the internal relationships between the porosity, permeability, coal metamorphic grade, and pore-fracture structure, the pore-fracture features of 21 middle–high rank coal samples from Anhe, Jiaozuo, and Huaibei coalfields in northern China were investigated using a low-field nuclear magnetic resonance (NMR). All the coal samples are characterized by low moisture content (Mad), low and medium ash yield (Aad), and high vitrinite (V) in coal maceral. The adsorption space fractal dimension (DA) is positively correlated with the Langmuir volume (VL) under the three-peak transverse relaxation time (T2) spectrum. The fractal dimension of all effective T2 points under saturated water (DNMR) is positively correlated with VL and the adsorption pore volume, but negatively correlated with the volume ratio of seepage pores and fractures. The free flow space fractal dimension (DM) is negatively correlated with the porosity of full saturated water (ΦF) and the porosity of movable water (ΦM). There is a negative correlation between ΦF and the seepage space fractal dimension (DS) in the coal samples with one-peak and two-peak T2 spectra, but a positive correlation can be found with the three-peak T2 spectrum. Therefore, it is necessary to consider the types of T2 spectral peak as a prerequisite to analyze the correlations between pore-fracture parameters and NMR fractal dimensions. With the increase of coal rank, the adsorption pore content, ΦF, and bulk volume immovable (BVI) fraction first increase and then decrease, whereas the seepage pore content, fracture development, bulk volume movable (BVM) fraction, and BVM/BVI first decrease and then increase. The inflection points of these changes correspond to the maximum vitrinite reflectance (Ro,max) at 2.6–2.8%, which would be attributed to the third coalification jump. Generally, DA is the fractal dimension representing the coal pore surface, and DS and DM are closely related to the pore structure. Furthermore, DNMR not only represents the roughness of the pore surface but also the complexity of the pore structure.

HRTEM observation of morphological and structural evolution of aromatic fringes during the transition from coal to graphite

The maximum reflectance of vitrinite or graphite microcrystals in Yongan Coalfield, Fujian Province, China, ranges from 5.33% to 10.21%, comprising meta-anthracite, semi-graphite, and graphite. The morphological characteristics of aromatic fringes during different evolution stages were quantitatively analyzed using high-resolution transmission electron microscopy and image processing techniques. The results show that a nonlinear evolution of aromatic fringe morphology in the process of coal graphitization is manifested in four aspects, involving the rapid connection and growth, enhancement of ordering alignment, increase of stacking and development of straightening. In meta-anthracite samples, 3 × 3 aromatic rings representing short aromatic fringes are dominated, a majority of aromatic fringes were curved, disjoint or separate, and show a turbostratic layers, with poorly aligned. The spacing between adjacent aromatic fringes exceeded the range of graphitized carbon, and structural defects, such as nanopores were generally visible. In semi-graphite samples, in addition to 3 × 3 aromatic rings, the proportion of >8 × 8 aromatic rings represented longer aromatic fringes increasing significantly, along with ordering alignment and straightening. Stacking with two more layers increased and the spacing between adjacent aromatic fringes decreased to the range of graphitized carbon, and the structural defects decreased considerably. In graphite sample, the proportion of >8 × 8 aromatic rings increase significantly, the length, stacking, and ordering alignment of aromatic fringes increased abruptly, the spacing between adjacent aromatic fringes remained within the range of natural graphite, and a rapid graphite-like structure with flat aromatic fringes formed. A stage evolution of aromatic fringe morphology, whereas remaining almost constant of mainly organic elements, which imply the evolution of aromatic fringes in meta-anthracite originated from ring condensation dominated by late coalification, graphitization starts at semi-graphite stage and enhanced during graphite stage. The making up, originating from linkup and pileup, considered as a special mechanism for the graphitization of aromatic fringes.

Relationship between average maximum and average arbitrary reflection values of coals vitrinite

For a quantitative assessment of the degree of coalification of coals in different countries, either average arbitrary (R0,r) vitrinite reflectance or average maximum (R0,max) vitrinite reflectance are mainly used. Since these indicators differ markedly in magnitude, in many countries, equations of the relationship between them have been established relatively long time ago in order to compare correctly the technological properties of different coals of the same type by indicators R0,max or R0,r. For Russian coals, the relationship between the vitrinite reflection indices R0,max and R0,r has not yet been established. Based on the studies of 40 samples of coals from Russia (Kuznetsk Basin), equations of the relationship between the average maximum (R0,max) and average arbitrary (R0,r) reflection values of coals vitrinite were obtained, taking into account the degree of their genetic recovery. The accuracy of calculating the indicator R0,max through R0,r at that is higher when taking into account the degree of genetic recovery of coals, which was 0.03%, versus 0.04% without accounting it. To compare technological properties of Russian coals by the same type of indicators R0,max or R0,r with the coals of Australia, USA, Canada, Poland, Germany, the equations of the relationship between the average maximum (R0,max) and the average arbitrary (R0,r) vitrinite reflectance presented for the coals of these countries according to publications in scientific journals.

Chemical Composition Variations of Altered and Unaffected Coals from the Huaibei Coalfield, China: Implications for Maturity

The composition characteristics of altered coals in the Huaibei Coalfield, China, was investigated through a comparative analysis between altered and unaffected coals from the Wolonghu, Taoyuan and Renlou coal mines. Results indicated that the altered coals in Wolonghu coal mine are mostly anthracite coals, with a maximum vitrinite reflectance of 1.6–3.9% (average of 2.9%). Coals from Wolonghu coal mine were mainly consisted of vitrinite (66.2–97.0%), followed by inertinite (2.0–4.0%) and exinite (0.4–6.9%). Differences in volatile matter content were observed between the altered coals in Wolonghu coal mine and unaffected coals from neighboring coal mines, implying that the chemical composition and maturity of coals were changed after magmatic alteration. In addition, differences in hydrogen element were noted among the coals from Wolonghu, Renlou and Taoyuan coal mines, and the phenomenon of “deficient in hydrogen element” was observed in Wolonghu coals. The aliphatic hydrocarbon structure parameters suggested that the aliphatic chain lengths of Wolonghu coals are shorter than those of coal samples from the Renlou and Taoyuan coal mines. In addition, maturity is positively correlated with hydrogen enrichment degree, but negatively related with aliphatic hydrocarbon structure. Coals from Renlou and Taoyuan coal mines showed great weight loss with various heating rates at temperatures of 0–1000 °C, whereas those from Wolonghu coal mine had less weight loss.

Differential graphitization of organic matter in coal: Some new understandings from reflectance evolution of meta-anthracite macerals

Middle Permian coal from the Yongan Coalfield, Fujian Province, China, with a maximum vitrinite reflectance (VRmax) of 5.33%–8.95%, indicating a typical meta-anthracite stage, enables the study of differential graphitization of macerals. The composition and optical properties of macerals were analyzed using optical (under non-polarized light, polarized light with gypsum test plate (1λ), and cross-polarized light) and scanning electron microscopy (SEM). The reflectance anisotropy and reflectance-indicating surface (RIS) were also analyzed to investigate the differential graphitization of macerals. The results show that although the morphology and structure of the original macerals were mostly preserved, the path of maceral evolution was differentiated. With increasing VRmax, the minimum reflectance of inertinite increased, while that of vitrinite and liptinite inverted from increasing to decreasing at approximately 8.0% and 7.5% VRmax, respectively. The reflectance anisotropy parameters of macerals changed accordingly, further enhancing the optical differentiation between macerals. Individually, VRmax does not adequately represent the evolution of liptinite and inertinite. The three-dimensional RIS ellipsoid revealed that liptinite and vitrinite have a biaxial negative optical character with strong optical anisotropy, while inertinite has a biaxial neutral to positive optical character with optical isotropy. A gradual unidirectional path of optical evolution from liptinite to graphite was observed, indicating that the evolution of macerals from coal to graphite followed a monotonic function model. Based on optical anisotropy, and considering the maximum maceral reflectance (XRmax) of 8.0% as the demarcation point and approximately 10.0% XRmax as the indicator of graphitization, the late coalification of macerals was resolved into two stages: pre-graphitization and quasi-graphitization. The conversion of coal to graphite largely depends on the transformation potential of the individual macerals, rather than on the total organic matter in coal. The graphitization of macerals in the Yongan coal samples was significantly differentiated; most of the liptinite was transformed into graphite and vitrinite was converted to semi-graphite, while all the inertinite remained in the anthracite stage. Biaxial negative macerals, therefore, presented a more prominent graphitization tendency than biaxial positive macerals.

Hrtem Observation of Morphological and Structural Evolution of Aromatic Fringes During the Transition from Coal to Graphite

The maximum reflectance of vitrinite or graphite microcrystals in Yongan Coalfield, Fujian Province, China, ranges from 5.33% to 10.21%, comprising meta-anthracite, semi-graphite, and graphite. The morphological characteristics of aromatic fringes during different evolution stages were quantitatively analyzed using high-resolution transmission electron microscopy and image processing techniques. The results show that a nonlinear evolution of aromatic fringe morphology in the process of coal graphitization is manifested in four aspects, involves the rapid connection and growth, enhancement of ordering alignment, increase of stacking and development of straightening. The stage evolution of aromatic fringe morphology and remained almost constant of mainly organic elements, implying the evolution of aromatic fringes in meta-anthracite originated from ring condensation dominated by late coalification, graphitization starts at semi-graphite stage and enhanced during graphite stage. The making up, originating from linkup and pileup, considered as a special mechanism for the graphitization of aromatic fringes.

Prediction of Gray-King coke type from radical concentration and basic properties of coal blends

Metallurgical coke is mainly produced from coal blends. The coke qualities have been related with or predicted by numerous polynomials with one or a few coal parameters from the proximate and ultimate analyses, maximum vitrinite reflectance (Rmax) and quantity of plastic matters. More fundamental and intrinsic prediction of coke quality, such as that required by artificial intelligence in the future, calls for relations between coke quality and its intermediate state, such as the coke type (CT) determined by the well-known Gray King (GK) assay, and consequently the relations between GKCT with the basic properties of coal blends. This work studies the GKCT of 68 coal blends and predicts the G type coke (G-coke), the best coke form defined by GK, with the parameters from the ultimate and proximate analyses, and the radical concentration (Cr) of coals because Cr is found correlating well with Rmax. The prediction methods include the traditional single- and multi-parameter range (MPR) methods and 3 machine learning models, namely K-Nearest Neighbors (KNN), Linear Discriminant Analysis (LDA), and Support Vector Machine (SVM). It is found that the readily measurable Cr of coals is an important parameter in GKCT prediction. MPR, KNN, LDA and SVM are capable to predict G-coke with no more than 5 parameters, and SVM is more effective than other models.

Diffusion characteristics of methane in various rank coals and the control mechanism

The methane diffusion behavior is one of the key factors that influence the production potential of coalbed methane (CBM) reservoirs. In this study, low temperature nitrogen, carbon dioxide adsorption and methane isotherm adsorption experiments for various rank coal samples were carried out to investigate the characteristics of pore size distribution and gas adsorption-diffusion behavior. A mathematical model similar to the Langmuir equation was established to describe the methane diffusion in coals. The variation law of the Langmuir effective diffusion coefficient (DL) with coal rank and its influencing factors, control mechanism were analyzed. With the increase of coal rank, the development degree of micropore rises, and both the specific pore volume and specific surface area first decrease and then increase. The diffusion coefficient of methane in coals generally rises with the rise of pressure. The Langmuir volume rises with the rise of Ro,m (maximum vitrinite reflectance with oil). The influence of moisture and coal molecular structure on gas diffusion varies with coal rank. The moisture content has a more significant influence on the gas diffusion in the medium-rank coals than in the low- and high-rank coals. The outcomes may deepen our understanding of gas transport in coals and benefit the exploration of CBM.

Pore Structure Characteristics of Coal and Their Geological Controlling Factors in Eastern Yunnan and Western Guizhou, China.

Coalbed is the carrier for coalbed methane (CBM) enrichment and migration. The pore structure characteristics of coal and their main geological controlling factors are critical to the exploration and development of CBM. In this paper, 20 coal samples were collected from eastern Yunnan and western Guizhou, China. Based on vitrinite reflectance, proximate analysis, maceral analysis, and low-temperature N2 adsorption/desorption (LT-N2GA) experiments, the hysteresis coefficient of low-temperature N2 desorption was proposed, the types of pore structure were identified, and the effects of coal facies and rank on the pore structure were revealed. The results show that the Ro,max values of the 20 coal samples are between 0.74 and 3.38%, which belong to medium- and high-rank coal. In the coal macerals, the vitrinite is mainly collodetrinite. The inertinite is dominated by semifusinite, and some coal samples contain exinite. The coal samples investigated can be divided into two types. Type A samples mainly contain open pores, while type B samples are rich in bottle-shaped pores. Compared with type A coal samples, type B samples have the characteristics of smaller total pore volume (TPV), smaller average pore diameter (APD), larger specific surface area (SSA), and larger hysteresis coefficient. The coal samples are located in three regions of different coal facies, including low-level swamp (reed) facies, wetland herbaceous swamp facies, and wet forest swamp facies. The tissue preservation index (TPI) values of most coal samples are less than unity, which indicates that herbaceous plants have absolute dominance in the coal-forming plants in eastern Yunnan and western Guizhou. The maximum vitrinite reflectance (Ro,max), gelification index (GI), TPI, vitrinite content (V), inertinite content (I), Barrett-Joyner-Halenda pore volume (VBJH), Brunauer–Emmett–Teller SSA (SBET), and low-temperature N2 desorption total hysteresis coefficient (Ht) were clustered using the R-type cluster analysis method. It is found that TPI is the main controlling factor of the pore structure of type A coal samples, while the pore structure of type B coal samples are jointly controlled by TPI and coal rank. Type B coal samples are mainly located in Zhuzang and Laochang high-rank coal research areas, while the distribution of type A coal samples is mainly in other medium–high-rank coal research areas. These results will contribute to the exploration and development of CBM and also guide the study of pore structures of other unconventional gas reservoirs.

Revisiting the thermally metamorphosed coals of the Transantarctic Mountains, Antarctica

Petrographic and geochemical data for Late Permian coals and carbonaceous shales from the Transantarctic Mountains, Antarctica (source: Polar Rock Repository, PRR), were used to evaluate maturation levels and assess the effects of contact metamorphism. Coals were evaluated for locations in the Southern Transantarctic Mountains, the Central Transantarctic Mountains, and South Victoria Land, including samples from the Buckley, Mt. Glossopteris, and Queen Maud formations and the Weller Coal Measures. These formations have been intruded by sills and dikes of the Jurassic Ferrar Group (177–183 Ma) associated with the breakup of Gondwanaland. Proximate (129 samples), total sulfur (69) analyses, vitrinite reflectance analysis (92), and petrographic composition (34) were determined. One third of the samples have >50% ash yields (dry basis). A subset of samples (87) with <50% ash (dry basis) was treated with 6 N HCl to remove any carbonates formed during the intrusive events. Acid treatment did not significantly reduce ash, suggesting silicates are a major component; sulfur contents of 1–2% (dry basis) decreased to <0.8% reflecting gypsum dissolution (as confirmed by XRD). Volatile matter (VM) contents (dry, ash-free or daf basis) for samples with <50% ash range from 3 to 43%. Based on VM, samples range from high volatile bituminous to anthracite; however, reflectance analysis indicates anthracite to meta-anthracite, with some reflectances >7%. Thus, VM does not reveal true rank of the Antarctic coals. Vitrinite reflectance (Rr) typically surpasses that of inertinite. The VM–Rr relationship for these coals does not follow that of coals matured by normal burial maturation, but more closely follows that of intruded coals. Coke textures, including isotropic coke and anisotropic mosaics, vacuoles, pyrolytic carbon, and coked bitumen are observed, indicating alteration by contact metamorphism and providing insights to the rank of the coal at the time of intrusion. Coarse-grained circular and fine-grained lenticular mosaic textures suggest coal rank at the time of intrusion was medium volatile bituminous coal (maximum vitrinite reflectance ~1.2%). This would imply a burial depth by time of intrusion of ~5–5.5 km (assuming 25 °C/km) or ~ 4 km (assuming 34 °C/km). Modern-day background reflectance levels of ~2.5% Rr indicate continued post-intrusion maturation, possibly due to exposure to higher regional heat flow. Coals and carbonaceous shales from the Polar Rock Repository (PRR) can provide reliable petrographic and maturation data (using reflectance) to help decipher the burial history for various parts of the Transantarctic Mountains. However, geochemical data must be used with caution due to the high original inorganic content, and possible formation of gypsum and changes in VM during long-term storage. HCl-treatment removes some of the neoformed minerals, but samples should be treated extensively with HCl-HF to remove all silicate minerals prior to proximate and ultimate analysis to ensure more reliable data.