Coal Rank Research Articles

Preparation and structural analysis of humic acid by co-thermal oxidation of wheat straw and Heilongjiang lignite

Combining with the characteristics of high yield of mineral humic acid (HA) and high activity of biochemical HA, co-thermal oxidation of low rank coal and biomass to produce complex HA (MIXHA) was newly proposed. The mixture (MIX) of Heilongjiang lignite (HL) and wheat straw (WS) was co-thermally oxidized in 10% HNO3 solution to prepare MIXHA. This work focused on comparison of the macro morphology and microstructure of MIXHA between HLHA and WSHA by SEM, FT-IR and 13C NMR analyses, and explored the synergistic effect between HL and WS during the co-thermal oxidation process. The results show that MIXHA content is higher than the theoretical value. Decomposition of HNO3 molecular produces active oxygen atoms and nitrogen oxides to attack the molecular structure of WS and HL. Due to hydrogen bond rearrangement, glycosidic bond rupture, and crosslinking, plenty of alkyl radicals generated in WS are combined with the condensation aromatic ring in HL. Thus, the protonated aromatic carbon is changed into aliphatic substituted aromatic carbon. The obtained MIXHA is rich in oxygen-containing functional groups, and has high activity. Obvious characteristic peaks are observed in FT-IR spectra of MIXHA. This work would provide a new idea for classification and resource utilization of low-rank coal and agricultural and forestry wastes.

Advanced bio-oil production from a mixture of microalgae and low rank coal using microwave assisted pyrolysis

This study aims to determine bio-oil production from a mixture of low rank coal (C) and microalgae Chlorella vulgaris (A) with 1 wt% HZSM-5 catalyst using microwave pyrolysis. In a batch reactor, pyrolysis was conducted at 500 °C under vacuum (−10 mmH2O). Operation variables included C/A ratio, pyrolysis time, and input power to obtain optimal bio-oil production. Bio-oil was analyzed by characterizing their physical properties, including its density and viscosity using GC–MS and FTIR. Optimal condition resulting in bio-oil yield of 33.8 wt% was found with C/A ratio of 4:6 and reaction time of 75 min. The outcomes of the present work can accelerate the development of biofuel production from microalgae to reduce carbon footprints from the massive use of conventional fuels.

Hydrogen sorption and diffusion in coals: Implications for hydrogen geo-storage

A full-scale hydrogen economy requires a bulk energy storage system to store the excess energy as a buffer and to fulfill the demand constantly. Hydrogen sorption capacity and diffusion behavior in coal quantify its potential to become a well storage candidate from geological formations. In this study, the sorption and diffusion behaviors of eight coals across the major coalfields in the United States in terms of their ranks, fixed carbon content, vitrinite reflectance, vitrinite or huminite content, O/C ratio, and H/C ratio were measured and analyzed. The sorption data shown that all eight coals have considerable sorption capacities, among which the LvB coal has the maximum adsorption capacity of ∼ 1.17 mmol/g, followed by An coal with the maximum hydrogen adsorption capacity of ∼ 0.95 mmol/g and the SemiAn coal of approximately 0.82 mmol/g. The fixed carbon content and typically the O/C ratio are obviously correlated with the maximum hydrogen adsorption capacity in coals, which may be attributed to the oxygen-containing functional groups. Hydrogen has superior diffusive gas deliverability defined by hydrogen effective diffusivity. The effective diffusivities of hydrogen in SemiAn coal decreases from ∼ 0.00156 to ∼ 9.26 × 10-41/s with pressure increases from ∼ 2.45 MPa to ∼ 10.07 MPa, among which the lowest diffusivity at ∼ 10.07 MPa is even ∼ 4 times higher than that of CH4 at equivalent pressure. This is a vantage for a promising field candidate of hydrogen storage in coal with maximized injectivity. The results reveal that the sorption and diffusion behaviors of hydrogen in different rank coals towards the depleted coalbed methane formation with injected hydrogen can serve as a geological “H2-Battery”.

Effect of supercritical CO2 extraction on pore characteristics of coal and its mechanism

Abundant pore space in coal is not only the place for the accumulation of coalbed methane (CBM), but also the tunnel for gas migration. In this study, five sets of coal samples before and after the second coalification were selected from the eastern margin of Ordos Basin to simulate supercritical CO2 (Sc-CO2) extraction in supercritical extraction equipment. The evolutions of pore structure and porosity were tested by mercury intrusion porosimetry and nuclear magnetic resonance spectroscopy to compare the changes of pore structure and porosity due to the Sc-CO2 extraction, and to explain the related mechanism. The results show that: (1) Pore volume, pore specific surface area, and connectivity characteristics changed significantly due to Sc-CO2 extraction, and the increment of pore volume and pore specific surface area presented a law of increase–decrease–increase with the increase in the coal rank, and the turning point was near the second coalification. (2) The porosity increment change trend due to Sc-CO2 extraction was increase–decrease–increase with increasing coal rank, and the turning point was again near the second coalification, which supports the mercury intrusion porosimetry results. (3) The changes were observed in the porosity characteristics due to Sc-CO2 extraction through pore-increasing and expanding effects. Before the second coalification, the pore-increasing and expanding effects co-existed in the micropores, and after the second coalification, the pore-expanding effect mainly existed in the transitional pores and above. (4) The variation model for the pore structure of coal due to Sc-CO2 extraction was established. The conclusions offer not only important theoretical significance for the CO2-enhanced CBM (CO2-ECBM) mechanism but also important significance for CO2-ECBM engineering.

A comprehensive insight into the effects of acidification on varied-sized pores in different rank coals

Elucidating the evolution law of coal pore structure under acidification is crucial for guiding the practical application of acidizing technology and improving the production of coalbed methane. To comprehensively investigate the influence of acidification on varied-sized pores in different rank coals, in this study, fat coal, meagre coal and anthracite coal were collected and acidified with a mixed solution composed of hydrochloric acid (9 wt%) and hydrofluoric acid (3 wt%). An approach integrating low-pressure CO2 adsorption (LPGA-CO2), low-temperature N2 adsorption (LTGA-N2) and Mercury intrusion porosimetry (MIP) was adopted to fully characterize the varied-sized pore structure before and after acidification to eliminate the limitations of single method. The results demonstrated that acid treatment improved the pore opening degree and connectivity in coal, but had essentially no effect on the pore shape. After acidification, all the coal samples showed significant increases in the porosity and total pore volume, which was mainly contributed by the numerous newly formed large mesopores and macropores, especially the macropores (with an average contribution rate of 74.59%). Taken as a whole, acid treatment had the largest impact on macropores, followed by mesopores, and the smallest impact on micropores. In addition, the variation trend of total specific surface area (SSA) under acidification was primarily determined by micropores. For the three different rank coals selected in this study, the total SSA of fat coal (PM) was more easily affected by acidification and had the largest percentage increase after acid treatment, followed by anthracite coal (YM), while that of meagre coal (LA) decreased slightly. This difference was driven primarily by the different variation trend of micropore SSA in different rank coals. After acidification, the SSA of ultra-micropores and super-micropores all increased in fat coal (PM) and anthracite coal (YM), whereas for meagre coal (LA), although ultra-micropores SSA increased, super-micropores SSA decreased, which ultimately led to the slight decrease of its micropore SSA. Moreover, the total pore volume increment of coal was closely related to the macropore volume increment under acidification, but not significantly related to the coal maturity,which might indicate that, compared with coal rank, the mineral content in coal might be a more important consideration when measuring the applicability of acidification technology.

Pore Structure Multifractal Characteristics of Coal Reservoirs in the Central and Eastern Qinshui Basin and Influencing Factors

The heterogeneity of the pore structure of coal reservoirs affects the desorption and diffusion characteristics of coalbed methane, and determining its distribution law is conducive to improving the theory of coalbed methane development. The central and eastern parts of the Qinshui Basin are rich in coalbed methane resources, but the heterogeneity characteristics of the pore structure of coal reservoirs are not clear. NMR has the advantages of being fast, non-destructive and full-scale, and multifractal can describe the self-similarity of NMR T2 curve at different scales so as to analyze the complexity of pore distribution. Based on this, 15 samples with different coal ranks were collected from the central and eastern Qinshui Basin (Ro,max between 1.54 and 2.78%), and quantitative pore characterization experiments such as low-field nuclear magnetic resonance (LF-NMR) and low-temperature liquid nitrogen adsorption (LTN2A) were conducted. Based on multifractal theory, the heterogeneity law of pore structure was quantitatively evaluated, and its influencing factors were elucidated. The results showed that the BJH pore volume of coal samples in the study area ranged from 0.0005–0.0028 cm3/g, with an average of 0.0014 cm3/g, and the BET specific surface area was 0.07–2.52 m2/g, with an average of 0.41 m2/g. The NMR T2 spectrum peaked at 0.1–1, 10–100 and 100–1000 ms, and the spectrum was mostly bimodal or trimodal, indicating that pores of different pore sizes were developed. There were great differences in the pore structure of different coal ranks; high-rank coal was dominated by micropores, and the proportion of mesopores and macropores of medium-rank coal was higher. The pore structure of coal samples showed obvious multifractal characteristics, and the fractal characteristics of the sparse region (low-value information) were more significant; they dominated the pore distribution and had a stronger influence on the distribution of pore space. Pore structure heterogeneity is closely related to the degree of coalification, and with the increase in coalification, it is closely related to coal lithotype and quality, and high mineral and inertinite contents lead to the enhancement of pore structure heterogeneity in coal reservoirs, while Ro,max, Mad and vitrinite group contents have opposite effects. The research results provide theoretical guidance for the subsequent exploration and development of coalbed methane in the region.

Investigation of cleat and micro-fracture and its aperture distribution in the coals of different ranks in North China: Relative to reservoir permeability

The apertures of cleats and micro-fractures in coal play an important role in the permeability of the coal bed. In this study, optical microscopy and scanning electron microscopy were used to investigate the morphology of cleats and micro-fractures and their apertures, distribution of minerals, and matrix/fracture interactions. The neighboring mineralized and unmineralized cleats suggest the possibility of multi-stage evolutionary processes of cleat formation during the coalification process. The micro-fracture distribution of coals is closely related to their components, including organic macerals and inorganic minerals. Micro-fractures are prone to developing at the junction surface of organic macerals or the surface of organic and inorganic minerals. A mineral-genetic micro-fracture can be classified as an intra-crystal micro-fracture, an extra-crystal micro-scale fracture, or a grain-edge micro-scale fracture. Compared with the low- and middle-ranking coals, cleat and micro-scale fractures in high-ranking coal are usually filled with carbonate minerals and clay minerals. Statistical analysis reveals that the aperture distribution of cleat and micro-fracture in coal shows a log-normal distribution. The aperture of cleat and micro-fracture shows a decreasing trend with increase in coal rank. For low-ranking coal, cleats contribute more to the permeability than micro-fractures. However, for the middle- and high-ranking coals, the contribution of cleats and micro-fractures to the coal reservoir permeability will be close. As the rank of coal increases, the degree of cleat contribution to reservoir permeability decreases, while the degree of micro-fracture contributing to the reservoir permeability increases. Possible reasons for the extremely low reservoir permeability in China may be the following: 1) subsurface cleats and micro-fractures close their apertures significantly due to the in situ geo-stress or 2) cleats and micro-fractures have better permeability in the geological history, which makes the precipitation of minerals decrease the coal reservoir permeability. Therefore, the acid solvent (e.g., HAc, HCl, and HF) added to the drilling or hydraulic fracturing fluid or the geo-stress relief technologies may be an effective way of enlarging the cleat or micro-fracture aperture and enhance the reservoir permeability for coalbed methane production.

Characteristics of Hybrid Coal from Co-Pyrolysis of Lignite and Corn Cob

Lignite is the lowest rank coal which has less economic value. Corn cobs are solid waste biomass as a by-product of corn processing. The processing of these two materials can produce a product in the form of hybrid coal through the co-pyrolysis process. This study aims to determine the optimum temperature and mixing ratio of co-pyrolysis of lignite and corn cob and to characterize the hybrid coal produced by co-pyrolysis. The lignite is dried and crushed to a particle size of 20-50 mesh. Corn cob was cleaned, cut into pieces, and sieved to a size of 0.4-2 mm. Then it was dried using an oven at 105oC for 24 hours. Lignite and corn cob were mixed with a ratio of 3:1, 1:1, and 1:3 (mass of lignite: mass of corn cob). The mixture of materials is inserted into the pyrolysis reactor as much as 400 grams. The pyrolysis process was carried out at temperatures of 350ºC, 400ºC, 450ºC, 500ºC, and 550ºC for 1 hour by flowing nitrogen gas into the reactor with a flow rate of 1.5 L/minute. The results showed that increasing the mixing ratio and co-pyrolysis temperature would decrease the yield of hybrid coal. Increasing the pyrolysis temperature will increase the calorific value of hybrid coal. Still, the effect of the mixing ratio of lignite and corn cob shows a decrease in heating value at a mixing ratio of 1:1 and an increase in a mixing ratio of 1:3. Other parameters such as moisture content and volatile matter content decreased with increasing temperature and mixing ratio. In contrast, ash content and fixed carbon content increased. So, an optimum temperature and mixing ratio of 450ºC and 1:3 is the best condition to get hybrid coal that met the requirements to be a solid fuel.

Molecular Simulation of Methane Adsorption Properties of Coal Samples with Different Coal Rank Superposition States.

Obvious differences exist in the structures of different coal rank coals, resulting in many differences and similarities in the amount of methane (CH4) gas adsorbed by coal and the control mechanism. In this study, we conducted adsorption simulations of three different coal rank coals in the superposition state using the Materials Studio software with simulated temperatures of 293.15, 313.15, and 333.15 K and adsorption pressures ranging from 0 to 10 MPa. We used the grand canonical ensemble Monte Carlo calculation method to calculate and study the adsorption amount, adsorption process, isosteric heat, and diffusion coefficient of CH4 in detail. We found that the adsorption of CH4 by coal samples of three coal rank coals (i.e., anthracite, bituminous coal, and lignite), which were mixed and stacked separately, was concentrated in the effective pores of coal molecules. Their effect on the amount of CH4 adsorption was dominated by the coal samples with more effective pores in the coal molecules. The isosteric heat of the coal samples showed an obvious exponential relationship with the adsorption pressure, and the isosteric heat of coal samples as a nonhomogeneous adsorbent gradually decreased with an increase in adsorption pressure. During the transformation of free-state CH4 into an adsorbed state, we obtained the best adsorption effect for bituminous-lignite superposition-state coal samples, and the diffusion coefficient of CH4 was the lowest.

Preservation of labile organic compounds in sapropelic coals from the Upper Silesian Coal Basin, Poland

Sapropelic coals were taken for study from coal mines and coal waste dumps in Upper Silesian Coal Basin (USCB), Poland. They showed unusual molecular compositions reflecting immature characteristics. The vitrinite reflectances 0.65-1.00% indicated mature organic matter (OM) and agreed with the Rock Eval Tmax (421-444oC). The OM maturity also was supported by the geochemical ratios, i.e., alkylnaphthalenes, dimethylphenanthrenes, methylbiphenyls and dibenzofuran or phenol and its methyl derivatives in the southern part of USCB. The petrographic features showed fine-grained and laminated OM, which is typical for sapropel with a dominance of vitrinite and inertinite macerals. The presence of fusinites and natural chars reflects paleowildfires prior to coal formation and/or peat fires. It is also confirmed by the presence of anthracene, methyl-anthracenes, and oxygen-containing aromatic compounds like anthrone, xanthone, fluorenone, and benzofluoren-11-ones. Rock Eval pyrolysis data showed very similar values as the humic coals from USCB, except for the elevated values of S2 (residual petroleum potential) and Hydrogen Index (HI) reflecting the presence of hydrogen-rich liptinite macerals. The distributions of n-alkanes, methylalkanes, alkylcyclohexanes and alkylbenzenes indicate a microbiological origin associated with microorganisms, including cyanobacteria or Botryococcus. In all samples 8β(H)-homodrimane dominated and indicated an anoxic environment of the deep lakes. The preservation of tetracyclic diterpenoids, diaster-13(17)-enes, ββ-hopanes, simonellite, bisnorsimonellite, cadalene, fatty acids, aromatic aldehydes and carboxylic acids from lignin degradation in sapropelic hard coals indicates an early diagenetic generation of these compounds from sapropels. Furthermore, during coalification, some the less thermally labile compounds can be preserved with increasing coal rank, while others isomerized or aromatized to more stable counterparts.

Tracing the origin and formation mechanisms of coalbed gas from the Fuxin Basin in China using geochemical and isotopic signatures of the gas and coproduced water

Coalbed gas (CBG) is an unconventional natural gas with a large resource potential. In order to determine its origins, formation pathways and mechanisms, studies on the geochemistry of CBG, coalbed water and coal have been carried out for years. However, the relationship between geochemistry of CBG and coalbed water is still not clear, especially with respect to the CO2 dissolution process. Here, a comprehensive study on the geochemistry of CBG and coproduction water in samples from the Fuxin Basin, China, is presented. Twenty-four gas and water samples were collected directly from the CBG producing well heads. C1/(C2 + C3) values are far >1000, δ13C1 values range between −62.8 and −57.6‰, δDCH4 values are from −252 to −225‰, ΔDH2O-CH4 values are from 148 to 178‰, and αCO2-C1 values are from 1.04 to 1.05. The composition characteristics, associated with the genetic diagrams of δ13C1 vs. δDCH4 and δ13C1 vs. δ13CCO2, and low coal rank (0.4–0.6% Ro), suggest that the CBG in the Fuxi Basin is mainly microbial gas. The Na-HCO3-Cl type of water in the coalbed is favorable for methanogenesis. δDH2O and δ18OH2O plot along the global meteoric water line (GMWL) and to the left of the local meteoric water line (LMWL), suggesting that the coalbed water is mainly from meteoric water recharge. However, isotope values in water may have been modified by methanogenesis and water-rock interaction. ΔDH2O-CH4 values of 148 to 178‰ suggest that methanogenic pathway is mainly CO2 reduction. However, αCO2-C1 values of 1.04 to 1.05 suggest that methanogenesis pathway might be acetoclastic or methylotrophic. This inconsistency may be caused by low δ13CCO2 values (−19.2 to −14.2‰) and thus low αCO2-C1 values due to high dissolution effect of CO2 and relative strong hydrodynamic activity in coal aquifers in the Fuxin Basin. Groundwater flow can carry away 13C-enriched CO2 dissolved in water, thus, the residual CO2 and DIC are depleted in 13C. Consequently, CO2 reduction is likely the main methanogenic pathway in the basin. Although CH4 and H2O may be close to isotope equilibrium for some samples, the CH4, CO2 and HCO3− are isotopically in disequilibrium at present reservoir conditions. Overall, kinetic processes largely control isotopic composition and distribution of the CBG and coalbed water in the Fuxin Basin. The CBG in the Fuxin Basin has been continuously generated, probably since the deposition of the coal-bearing formation. Although part of the CBG may have been lost during coalbed uplift stage, uplift to near surface allow re-inoculation of coalbeds with methanogenic microbial consortia via meteoric water recharge, which can accelerate the formation and accumulation of the microbial CBG. Consequently, most CBG in the present coalbed has been generated likely after the coal strata uplift.

Prediction of product distribution of low-medium rank coal pyrolysis using artificial neural networks model

In this study, artificial neural networks (ANNs) were used to build a correlation between the low-medium rank coal elements and the coal pyrolysis process parameters and product distribution. On the basis of numerous experimental data of different rank coals under various process conditions, a network training database was created in order to more correctly analyze the product distribution during coal pyrolysis. One ANN model was created for each of the six pyrolysis products, for a total of six models. The validation database validates the accuracy and applicability of the constructed ANNs model, and it also assesses the relative influence of each input parameter on the evolution of each pyrolysis product. The results show that the predicted values for the new inputs in the developed network model have strong correlation coefficients and are in good agreement with the experimental values of pyrolysis products, demonstrating its accuracy and robustness. Thus, it is demonstrated that the ANN based methodology is a viable alternative which can be used to predict the product yield of coal pyrolysis.

Experimental study on temperature change caused by ScCO2 adsorption–desorption during carbon sequestration in deep coal seam

Understanding the thermal effect of the coal induced by supercritical CO2 (ScCO2) adsorption and desorption is vital in evaluating the heat transfer behaviour during CO2 sequestration in deep coal seams. Therefore, the coal mass temperature (CMT) changes in anthracite and bituminous coal during ScCO2 adsorption–desorption under equilibrium temperature (ET) of 40 °C to 70 °C and maximum adsorption pressures of 12 MPa, were investigated experimentally. The Fourier-transform infrared spectroscopy (FTIR) method measures the functional groups of coal to reveal the mechanism behind the CMT change. The results indicate that the adsorption and desorption of CO2 increase and decrease the coal’s temperature, respectively. The induced CMT variation depends on the coal rank, CO2 pressure, and ET. The higher the coal rank, the higher the adsorption pressure; the lower the ET, and the greater the CMT change. At different ET, the CMT variation during adsorption is higher than in desorption, because of the desorption hysteresis phenomenon (DHP). An improved hysteresis index (IHI) was used to quantify this phenomenon. The IHI decreases linearly with the ET, and the value of anthracite is higher than that of bituminous coal. The DHP of ScCO2 during adsorption and desorption is correlated with the induced variation in the coal’s functional groups. With the increase in ET, the hydroxyl and oxygen-containing functional groups’ contents in coal decrease after ScCO2 adsorption and desorption, reducing the adsorption capacity and weakening the desorption hysteresis effect. A theoretical model was established to describe the influence of adsorption pressure on the CMT change induced by ScCO2 adsorption–desorption. The calculated data are consistent with the experimental data, and the results are meaningful to the ScCO2 sequestration in deep coal seams.

CO2 adsorption-deformation-percolation characteristics of coals with different degrees of metamorphism and improvement of dynamic prediction model

The deformation and permeability changes occurring during CO2 adsorption in the coal body are important factors affecting CO2 injectability. In order to reveal the characteristic laws of adsorption, deformation and permeability of coal in the process of CO2 adsorption by different coal rank coals, the test experiments of adsorption, deformation and permeability of different coal rank coals under the condition of constant external force were carried out. The adsorption capacity of different coal samples for CO2 was obtained as anthracite > coking coal > long flame coal; The radial deformation ɛ2 and axial deformation ɛ1 of coal after CO2 gas adsorption by anthracite are the closest (ɛ2/ɛ1 is about 1.37), coking coal is the second (ɛ2/ɛ1 is about 1.73), long-flame coal is the largest (ɛ2/ɛ1 is about 2.72), In other words, the higher the coal rank, the more significant the degree of internal homogeneity and isotropy of the coal body, and the mean value of the volume deformation of CO2 adsorbed by long-flame coal is 1.07 and 1.17 times higher than that of coking coal and anthracite. The permeability curves of CO2 adsorption in all three coal samples decreased first and then increased, and the permeability of CO2 in coking coal was the largest, and the sensitivity coefficient of CO2 permeability in coal to adsorption pressure showed a trend of rapid decrease and then small increase. Based on the CO2 adsorption-deformation-percolation variation law of coal, an improved dynamic prediction model of adsorption (fitting accuracy 0.9850 ∼ 0.9963), an improved dynamic prediction model of coal body deformation (fitting accuracy 0.9889 ∼ 0.9921) and an improved dynamic prediction model of permeability (fitting accuracy 0.9848 ∼ 0.9914).

Study on the evolution of the pore structure of low rank coal during spontaneous combustion.

Studying the evolution of the pore structure of coal during spontaneous combustion is of great value in further understanding the mechanism of coal spontaneous combustion (CSC) and its prevention. In this study, we selected three low-rank coals and used nuclear magnetic resonance (NMR) to visualize the macroscopic evolution of the pore structure of coal after heat treatment and to analyze the effect of temperature (25-500°C) on the pore structure of coal, including porosity, permeability, and fractal dimensions. The obtained results show that the overall NMR signal in coal increases with increasing temperature, indicating that heat treatment can induce the enlargement, opening, and interconnection of pores and fractures in coal. The equivalent average pore radius (rm) of coal shows a positive correlation with temperature, with a substantial increase in rm, especially after temperatures above 200°C. During heating, the porosity and permeability of all three coals tended to increase with temperature. At temperatures above 300°C, the permeability of coal dramatically increases, predicting a higher fluid transport capacity. Furthermore, NMR multifractal theory was proposed for quantitative pore space dimensional characterization. The obtained results show that the fractal dimensions of the adsorption space of coal pores increase and then decrease with temperature during heating, while the fractal dimensions of percolation space are negatively correlated with temperature. In addition, the dimensions of adsorption space vary more strongly than those of percolation space, meaning that the adsorption capacity of low-rank coals is more significantly influenced by temperature.

Carbon isotopic composition and organic petrography of thermally metamorphosed Antarctic coal: Implications for evaluation of δ13Corg excursions in paleo-atmospheric reconstruction

Large, globally documented negative δ13C excursions that occur across the Permian-Triassic transition are thought to represent the isotopic fingerprint of considerable releases of isotopically light carbon into the atmosphere, with the largest of these excursions (as much as −22.2‰) measured in bulk organic matter in coals and carbonaceous shales from Antarctica. Previous studies have attributed the carbon isotopic excursion (CIE) at the Permian–Triassic boundary to the release of CH4 from organic-rich rocks intruded by dikes and sills associated with the Siberian Traps flood basalts (Russia) and Longwood-Bluff (New Zealand) intrusions. However, any assessment of paleoatmospheric conditions based on this or other δ13Corg excursions must consider the multiple controls on isotopic composition, including maceral content and post-depositional diagenetic and maturation processes. Permian-age coal seams and associated sediments in Antarctica were extensively altered after burial by localized high heat flow and, in some cases, by contact metamorphism associated with Jurassic dikes and sills. Levels of organic maturation reach anthracite, meta-anthracite, and even graphite; such intense thermal alteration had the potential to change the carbon isotopic composition of the bulk organic matter (δ13Corg). To evaluate the Permian–Triassic isotopic excursion in Antarctica, it is important to understand the inherent variability in δ13Corg in Permian-age coal and organic-rich sedimentary rocks prior to the CIE; thus, 128 samples representing pre-boundary coals and shales were selected from the United States Polar Rock Repository for analysis. Petrographic (maceral analysis and vitrinite reflectance), geochemical (C, H, N), and carbon isotopic analyses were performed. Based on the presence of coarse-grained circular and fine-grained lenticular mosaic in the anisotropic cokes found in samples in close proximity to intrusions, the rank of the samples prior to intrusion is estimated to have been medium volatile bituminous (∼1.2% Rmax). These highly altered coals and cokes can have random vitrinite reflectances (Rr) exceeding 6%. Subsequent to intrusion, alteration beyond the contact aureoles continued due to high heat flow, producing high rank coals with Rr > 2%. Values of δ13Corg range between −26.5 and –21.1‰ (5.4‰ difference). Significant relationships (at the 1 and 5% levels) exist between δ13Corg and the level of organic maturity, organic carbon (Corg) content (i.e., size of the carbon reservoir), and total inertinite content, suggesting variability due to these factors should be considered when assessing isotopic excursions and interpretation of Permian-Triassic atmospheric conditions. These findings highlight the importance of incorporating organic petrography into δ13Corg studies.

Geochemistry of Late Permian coals from the Moxinpo mine, Chongqing, Southwestern China: Emphasis on petrology, minerals, and organic matter

The indicative significance of organic matter in fossil fuels, particularly polycyclic aromatic compounds (PACs), is a current research hotspot. Petrology, minerals, and organic matter in Late Permian coals from the Moxinpo (MXP) mine located in Chongqing, southwestern China, were analyzed to evaluate the coal rank, coal-forming material source and thermal maturity of organic matter, and sedimentary environment of the peat swamp. Vitrinite and inertinite were dominated by collotelinite, collodetrinite, inertodetrinite, fusinite, and semifusinite. Owing to the high maturity of the coals, liptinite was not observed under the microscope. Clay (kaolinite) and pyrite were the primary minerals in the MXP coals, followed by calcite, quartz, trace amounts of anatase, gypsum, xenotime, and bastnaesite. The minerals in the MXP coals were authigenic primarily in the epigenetic stage. The extract yield of organic matter in coals was low (0.30%), and the saturated and aromatic fractions accounted for approximately 60% of the extractable organic matter. Low ratios of saturated to aromatic compounds reflected the terrestrial higher plants input and high maturity level of the coals. The extracted yield in the host rock samples was much lower than in the coals, while the proportion of saturated hydrocarbons was relatively high. The marine-continental transitional sedimentary environment, distribution of n-alkanes, ∑ C21−/∑ C22+, and pristane (Pr)/n-C17 vs. phytane (Ph)/n-C18 indicated that the organic matter originated from the input of terrestrial higher plants and was accompanied by the contribution of marine lower phytoplankton and aquatic organisms. The coal-forming environment of the coal seam was reductive according to the Pr/Ph and sulfur-containing compounds of dibenzothiophenes and benzonaphthothiophenes. The thermodynamic stability controls the presence of polycyclic aromatic hydrocarbons in extractable organic matter. Isomeric compounds with high stability had higher concentrations, such as 2-methylnaphthalene, 2,6 + 2,7-DMN, 4- methylbiphenyl, and 2- methylphenanthrene.

Effects of oxidation degree of support and phosphorus modification on the catalytic cracking of coal pyrolysis tar by char-supported nickel catalysts

The low rank coal can be converted efficiently when it undergoes the pyrolysis process. The utilization of tar is modified and the problems such as pipeline blockage are solved by cracking and lightning heavy components in tar. In this study, the char-supported nickel (Ni) catalysts were prepared by different Ni loading methods, and the cracking of coal pyrolysis tar was improved by the coal char support with different oxidation degrees. The catalysts prepared by ion exchange method showed better performance for the tar cracking than other methods, which is related to their smaller Ni crystallite size (NCS). The oxidation of H2O2 and HNO3 significantly increased the number of oxygen-containing groups. The intrinsic correlation between oxidation degree of support and catalytic property indicated that the XH2O2-Ni/C (ZD coal pretreated by H2O2) series catalysts had better tar cracking activity than XHNO3-Ni/C (ZD coal pretreated by HNO3) series catalysts, which did not only depend on the increase of Ni loading amounts but also related to the smaller NCS and higher dispersion. Moreover, the non-metallic additive phosphorus (P) was added to modify the catalysts. It was found that the addition of P significantly improved the tar cracking performance of the 30%H2O2 P-Ni/C catalyst. The amounts of active components loaded and NCS contributed to the production of light tar.

New insight into the role of the emulsified diesel droplet size in low rank coal flotation

Low-rank coal (LRC) is difficult to float using oily collectors due to poor floatability resulted from its abundant oxygen-containing groups. In recent years, numbers of emulsified collectors have been used to enhance LRC flotation. In this study, diesel-Span80 mixed collectors were added to high shear disperser producing emulsified diesel (ED) with a droplet size ranging from 12.51 to 4.58 µm to discuss the effects of droplets size on LRC flotation. The adsorption and spreading of ED on coal particle surfaces was evaluated by total organic carbon analyzer (TOC) and fourier-transform infrared spectroscopy (FTIR). The interaction between the oil droplets and coal particles was conducted using adhesion force measurement system. The flotation results indicated clean product obtained by using ED as the collector had the higher combustible recovery and lower ash content than conventional diesel (CD). It was interesting to find an optimal droplet size (7.68 μm) at which the ED flotation performance was maximized and beyond which the flotation performance deteriorated. The FTIR, adsorption amount and adhesion force results were all well consistent with the flotation results. The effect of ED droplet size on LRC flotation is controlled by ED dispersibility and concentration of adsorbed surfactant molecules on each ED droplet surface, playing a critical role in determining the optimal droplet size which delivers the largest hydrophobicity, maximum adsorption amount, highest maximum adhesion force and hence the best flotation performance.

Prognosis of fine airborne coal dust formation at mechanical effects. Part 1. Effects of structure of different rank coals at their mechanical behavior at cyclic loading

Prognosis of fine airborne coal dust formation at mechanical effects. Part 1. Effects of structure of different rank coals at their mechanical behavior at cyclic loading