Geochemical characteristics of the Rajmahal coals in Dhulia North Block, Eastern India: implication to their utilization and environment.

The borehole coal samples of Dhulia North Block from the Rajmahal Basin, Eastern India, were systematically analyzed based on the chemical composition and concentration of major and trace elements (including rare earth elements, REEs) to assess the distribution of REEs and their environmental implications with utilization potential. The Dhulia North Block coals are characterized by the predominant major oxides of SiO2, Al2O3, and Fe2O3, accounting for 94% of the total ash composition, indicating the presence of quartz, clay-rich minerals, and pyrite. Compared with the average world coal ash, the total REE content in the analyzed samples ranged from 341.0 to 810.4 ppm, which is substantially higher. Hot humid climate conditions with intermediate igneous source rocks of the basin were demonstrated by the major oxide ratios (Al2O3/TiO2 < 20) and plots of TiO2 with Al2O3 and Zr. The redox-sensitive elements such as V, Ni, Cr, and Co found in the Dhulia North Block coal indicate that an oxic sedimentary environment existed in the basin when coal was formed. The low sulfur content (1% in most samples) indicates freshwater conditions in the basin at the time of organic matter deposition. The outlook coefficient (Coutl) varies between 0.7 and 1.6, indicating that the Dhulia North Block coals are a prospective source of REEs. The Dhulia North Block coals are characterized by low H/C and O/C atomic ratios ranging from 0.56 to 0.90 and 0.10 to 0.22, respectively, and contain type-III kerogens, indicating gas-prone source rock. Further, the basic-to-acid oxide ratio suggested that Dhulia North Block coals were suitable for utilization during combustion processes.

Chromium in Chinese coals: geochemistry and environmental impacts associated with coal-fired power plants.

Chromium (Cr), one of the prime hazardous trace elements in coals, may engender adverse effects on eco-environment and threaten human health during utilization of coal. Based on the samples obtained in our laboratory and published literature, the abundance and modes of occurrence of Cr in Chinese coals, and the environmental impacts associated with coal-fired power plants (CFPPs) were elucidated in this study. With a total of 1397 sets of data, the mean concentration of Cr in Chinese coals was calculated as 21.33 µg/g by the "reserve-concentration" weighted calculation method. Spatially, the average Cr contents increased gradually from North China to South China. Temporally, coals from T3, E-N and P2 were relatively enriched in Cr compared to the other geological time. The Cr concentration in coal varied with different coal ranks. The geological factors accounted for Cr enrichment in coals could be divided into the primary, secondary and epigenetic processes. Higher percentages of organically Cr occurred in low-rank coals, while inorganically associated Cr was mainly found in clay minerals. After coal combustion, most of Cr was enriched in solid wastes (e.g., fly ash and bottom ash). The leaching of Cr from solid wastes in the rainy season (especially acid rain) needs to be a concern for CFPPs. It was estimated that the atmospheric emission of Cr from CFPPs increased annually from 2015 to 2019 and reached approximately 159 tons in 2019.

A complete atmospheric emission inventory of F, As, Se, Cd, Sb, Hg, Pb, and U from coal-fired power plants in Anhui Province, eastern China.

Anhui Province is the most important energy production base for eastern China. Many large pithead coal-fired power plants are being operated in the coal-rich Huainan and Huaibei coalfields in northern Anhui. To assess the environmental risks of local coal-fired power plants, a complete atmospheric emission inventory of F, As, Se, Cd, Sb, Hg, Pb, and U from coal-fired power plants in Anhui was compiled by a simple mass-balance-based method. The results indicated that the atmospheric emissions of F, As, Se, Cd, Sb, Hg, Pb, and U in 2017 from the Anhui coal-fired power plants were 578 t, 2.01 t, 15.3 t, 0.57 t, 0.18 t, 2.80 t, 23.7 t, and 0.099 t, respectively. The emission factor is the major contributor to the uncertainties in this inventory. With increasing energy demand by the more developed eastern China region, the atmospheric emissions of volatile hazardous elements will continue to increase in the near future.

Geochemistry of vanadium (V) in Chinese coals.

Vanadium in coals may have potential environmental and economic impacts. However, comprehensive knowledge of the geochemistry of V in coals is lacking. In this study, abundances, distribution and modes of occurrence of V are reviewed by compiling >2900 reported Chinese coal samples. With coal reserves in individual provinces as the weighting factors, V in Chinese coals is estimated to have an average abundance of 35.81 µg/g. Large variation of V concentration is observed in Chinese coals of different regions, coal-forming periods, and maturation ranks. According to the concentration coefficient of V in coals from individual provinces, three regions are divided across Chinese coal deposits. Vanadium in Chinese coals is probably influenced by sediment source and sedimentary environment, supplemented by late-stage hydrothermal fluids. Specifically, hydrothermal fluids have relatively more significant effect on the enrichment of V in local coal seams. Vanadium in coals is commonly associated with aluminosilicate minerals and organic matter, and the modes of V occurrence in coal depend on coal-forming environment and coal rank. The Chinese V emission inventory during coal combustion is estimated to be 4906 mt in 2014, accounting for 50.55 % of global emission. Vanadium emissions by electric power plants are the largest contributor.

Spectroscopic characterizations of silicate fertilizers prepared by chemical deashing of coals.

Quality silicate fertilizers should be in great demand, and yet the production has been limited due to strict regulations on heavy metals, despite many raw materials and activation methods being used. In the chemical deashing of coals for the production of ultraclean coals, the silica gels of high purity were precipitated with little heavy metals from the acid deashing solutions, which could be used to produce quality silicate fertilizers by pulping with CaO or MgO under mild conditions. By varying the Ca/Si molar ratios, silicate fertilizers with different chemical compositions were prepared, and the active silica contents were measured and validated by ICP and colorimetric methods. For the curve of the active silica contents versus the Ca/Si molar ratios, four regions could be clearly marked with unique patterns, and quality silicate fertilizers occurred with the Ca/Si molar ratios from ∼1.10 to ∼3.50. The pH values of the silicate fertilizers could also be divided into the same four regions with respect to the Ca/Si molar ratios, and the highest active silica content occurred at the pH value of ∼11.30 with the Ca/Si molar ratio of ∼1.50. With the XRD investigations of the silicate fertilizers selected from the four regions, the water-insoluble 1.5CaO·SiO2•xH2O was identified as the contributor of active silica in the silicate fertilizers. By replacing full or part of CaO with MgO in the preparation of silicate fertilizers, the silica gels were found to preferably react with CaO, and the active silica contents grew with the increase of CaO. By referring to the model silicate fertilizers prepared in this work by varying the (Ca + Mg)/Si molar ratios, 1.5CaO·SiO2•xH2O was also identified as the dominant in one commercial slag silicate fertilizer. Silicate fertilizers by silica gels can be helpful for secondary pollution elimination and cost reduction of coal deashing.

Study on the mechanism of methane "solid-liquid-gas" conversion controlled by the evolution of coal micro- and nanopore structure.

Currently, the utilization of coalbed methane resources in the Guizhou region faces challenges such as complex reservoir structure, high gas content, and microporous development. Based on these, the pore structure and adsorption capacity of Guizhou tectonic deformed coals (TDCs) were evaluated using a suite of integrated diagnostic techniques including low-temperature nitrogen adsorption (LT-N2A), mercury intrusion porosimetry (MIP), methane isothermal adsorption. Through the above methods, the pore structure and adsorption characteristics of the samples were characterized; The samples were divided into the range of joint pores by combining the results of MIP and LT-N2A; Using the molecular simulation software, the 2 nm, 4 nm, 10 nm pores affecting the methane endowment state were investigated respectively, and from the perspective of the heat of adsorption and energy, the concept of the three-phase transition of methane was proposed, and explore the change of the pore spacing affecting the endowment state of methane from the solid state pore to the gas state pore. The results provide new ideas for the in-depth study of gas storage in tectonic coal reservoirs in Guizhou Province.

Experimental study on CO and CO2 emissions from spontaneous heating of coals at varying temperatures and O2 concentrations.

Laboratory experiments were conducted to investigate carbon monoxide (CO) and carbon dioxide (CO2) emissions from spontaneous heating of three U.S. coal samples in an isothermal oven at temperatures between 50 and 110 °C. The oxygen (O2) concentration of an oxygen/nitrogen (N2) mixture flowing through the coal sample was 3, 5, 10, 15, and 21%, respectively. The temperature at the center of the coal sample was continuously monitored, while the CO, CO2, and O2 concentrations of the exit gas were continuously measured. The results indicate that the CO and CO2 concentrations and the CO/CO2 ratio increased when the initial temperature was increased. As the inlet O2 concentration increased, the CO and CO2 concentrations increased, while the CO/CO2 ratios tended to converge to the same value. The ratio of CO/CO2 was found to be independent of coal properties, approaching a constant value of 0.2. The maximum CO production rate correlated well with the maximum coal temperature rise. The apparent order of reaction for coal oxidation was estimated to be between 0.52 and 0.72. The experimental results in this study could be used for early detection and evaluation of a spontaneous heating in underground coal mines.

Quantifying climate conditions for the formation of coals and evaporites.

Coals and evaporites are commonly used as qualitative indicators of wet and dry environments in deep-time climate studies, respectively. Here, we combine geological records with climate simulations to establish quantitative relationships of coals and evaporites with temperature and precipitation over the Phanerozoic. We show that coal records were associated with a median temperature of 25°C and precipitation of 1300 mm yr-1 before 250 Ma. Afterwards, coal records appeared with temperatures between 0°C and 21°C and precipitation of 900 mm yr-1. Evaporite records were associated with a median temperature of 27°C and precipitation of 800 mm yr-1. The most remarkable result is that net precipitation associated with coal and evaporite records remained constant across time. The results here have important implications for quantifying climate conditions for other lithologic indicators of climate and for predicting exogenetic ore deposits.

Exploring the potential of sulphur forms in Northeastern Indian coals: Implications in environmental remediation and heavy metal sensing.

The Sampar Coalfield in Northeastern India is a source of plentiful coal reserves, which are burnt for energy production and industrial applications, resulting in the release of pollutants such as sulphur , arsenic, and lead, which are hazardous to the environment and public health. In this work, samples from the Sampar coalfield have been analyzed to understand the origin, distribution, and various forms of sulphur and their ability to detect toxic heavy metals. The total sulphur concentration ranged from 4.31% to 6%, with organic sulphur being the predominant form at 69.21%, followed by pyritic sulphur at 16.49% and sulphate sulphur at 14.28%. With high sulphur content, this coal indicates a marine influence in the peat-forming swamps. The samples have also been examined for petrographic and elemental analysis, which have revealed the presence of vitrinite, liptinite, inertinite, carbon, hydrogen, nitrogen, oxygen, and mineral matter. In addition, the same coal sample has also been used for electrochemical sensing-based detection of toxic heavy metals like arsenic and lead, and the findings indicate an improved efficacy. These results are expected to have significant implications in the development of effective bio-based remediation strategies in the region to mitigate the harmful effects of coal-related pollution.

Infrared spectroscopy quantification of functional carbon groups in kerogens and coals: A calibration procedure.

The determination of the abundances of the CHx, C = O and aromatic groups in chondritic Insoluble Organic Matter (IOM) and coals by Infrared (IR) spectroscopy is a challenging issue due to insufficient knowledge on the absorption cross-sections and their sensitivity to the molecular environment. Here, we report a calibration approach based on a 13C synthetic model material whose composition was unambiguously determined by Direct-Pulse/Magic Angle Spinning Nuclear Magnetic Resonance (DP/MAS NMR). Ratios of the cross-sections of the CHx, C = O and aromatic groups have been determined, and the method has been applied to IOM samples extracted from four chondrites as Orgueil (CI), Murchison (CM), Tagish Lake (C2-ungrouped) and EET 92042 (CR2), and to a series of coals. The estimate of the aliphatic to aromatic carbon ratio (nCHx/nAro) in IOM samples from Orgueil, Murchison and Tagish Lake chondrites is in good agreement with Single-Pulse/NMR estimates earlier published, and is lower by a factor of 1.3 in the case of the CR chondrite EET 92042 (but the error bars overlap). In contrast, the aliphatic to carbonyl ratio (nCHx/nC=O) is overestimated for the four chondrites. These discrepancies are likely due to the control of the absorption cross-section of the C = O and C = C bonds by the local molecular environment. Regarding coals, the use of published NMR analyses has brought to light that the integrated cross-section ratio ACHx/AAro varies with the vitrinite reflectance over an order of magnitude. Here as well, the local oxygen speciation plays a critical control in AAro, which decreases with increasing the vitrinite reflectance. We provide an analytical law that links ACHx/AAro and vitrinite reflectance, which will allow the determination of nCHx/nAro for any coal sample, provided its vitrinite reflectance is known.

Study of the pyrolysis of coals of different rank using the ReaxFF reactive force field.

Based on the general coal model, the processes of pyrolysis of coals of different rank were studied by molecular dynamics simulations with the ReaxFF reactive force field. Simulations were performed over wide temperature ranges both for heating from 300 K to 3000 K and a constant temperature range from 2000 K to 3500 K. The pyrolysis of coals of different coal rank under different heating conditions was analyzed. The temperature at which the structure units began to be consumed heavily and the temperature at which the consumption was complete were elevated. In terms of the time scale, the rise of the heating rate caused the reactions to occur earlier. It was found that the pathway of coal pyrolysis at different heating rates was not affected, and the initial reaction pathways of five kinds of coal pyrolysis were obtained. Under the pyrolysis conditions of 2000 K to 3500 K, we observed the change in the total molecular number of different coal rank coal systems with temperature, reflecting the degree of cracking. At the same time, carbon monoxide and hydrogen were chosen as the typical products. The change rules of the temperature were obtained, and the formation mechanisms of different stages were discussed. The comparison of coals of different coal rank showed that the temperature had more influence on the lower rank coal. The results observed in this work are broadly in agreement with the literature. This work demonstrates a new methodology for investigating the overall behaviors and the underlying complex mechanisms of the pyrolysis of coals of different rank.

Hydrogeochemistry signatures of produced waters associated with coalbed methane production in the Southern Junggar Basin, NW China.

The Southern Junggar Basin (SJB) in China is an emerging coalbed methane (CBM) development area with abundant low-rank CBM resources. CBM development is accomplished by pumping significant volumes of water from the aquifer, and this water is commonly termed as produced water, which has great utilization values for the water-deficient areas like SJB. Geochemistry signatures are prerequisites in the management of the produced water. Meanwhile, geochemistry surveys of this produced water could also help study the basin hydrogeology and then serve the CBM development. In this study, geochemical compositions of the produced waters, including major ions, stable isotopic compositions, trace elements, and rare earth elements, were analyzed. Results show that produced waters from CBM wells in the SJB are of Na-HCO3 type and have wide total dissolved solid (TDS) ranges from 963 to 11,916 mg/L (avg. 7417 mg/L). Cl-, Na+, and HCO3- are the principal determinates of the TDS contents of the produced waters, and their concentrations all increase with greater depth of the produced waters. Overall, the net results of groundwater-aquifer mineral-bacteria interactions with groundwater flowing along the flow path are to deplete Ca2+, Mg2+, and SO42- and increase Na+, Cl-, HCO3-, and TDS. Stable isotopic values of the CBM produced waters (δDH2O and δ18OH2O) cluster along or below the local meteoric water line (LMWL), and the shift of stable isotopic values to the right side of LMWL was affected by a joint effect of evaporation and mixing with near-surface water. Trace elements that exceed the regulated concentrations for drinking water of China include As, Fe, Mn, Ba, and Ni, among which Ba and Fe need to be most concerned because over 50% of the CBM produced waters exceed the regulated values. Through principal component analysis, the trace element associations in the CBM produced waters and their potential origins were analyzed. The ∑REY concentrations of the CBM produced waters increase exponentially with the increase of pH and present a certain correlation with TDS. The relationship between ∑REY concentrations and TDS reflects different water-rock reaction degrees and hydrogeological backgrounds.

Geochemical characteristics of n-alkanes and isoprenoids in coal seams from Zhuji coal mine, Huainan coalfield, China, and their relationship with coal-forming environment.

Ten coal seams in Upper Shihezi Formation, Lower Shihezi Formation, and Shanxi Formation from the Zhuji mine, Huainan coalfield, China, were analyzed for n-alkanes and isoprenoids (pristine and phytane) using gas chromatography-mass spectrometry (GC-MS), with an aim of reconstructing the coal-forming plants and depositional environments along with organic carbon isotope analyses. The total n-alkane concentrations ranged from 34.1 to 481 mg/kg. Values of organic carbon isotope (δ13Corg) ranged from - 24.6 to - 23.7‰. The calorific value (Qb,d), maximum vitrinite reflectance (Romax), proximate, and ultimate analysis were also determined but showed no correlation with n-alkane concentrations. Carbon Preference Index (CPI) values ranged from 0.945 to 1.30, suggesting no obvious odd/even predominance of n-alkane. The predominance of C11 and C17 n-alkanes implied that the coal may be deposited in the fresh and mildly brackish environment. According to the contrary changing trend of pristine/phytane (Pr/Ph) ratio and boron concentrations, Pr/Ph can be used as an indicator to reconstruct the marine transgression-regression in sedimentary environment of coal formation. The influence of marine transgression may lead to the enrichment of pyrite sulfur in the coal seam 4-2. C3 plants (- 32 to - 21‰) and marine algae (- 23 to - 16‰) were probably the main coal-forming plants in the studied coal seams. No correlation of the n-alkane concentration and redox condition of the depositional environment with organic carbon isotope composition were found.

Thermal degradations of wood biofuels, coals and hydrolysis lignin from the Russian Federation: Experiments and modeling.

The thermal degradation of wood biofuels (spruce, pine), of coals from different fields of the Russian Federation and of hydrolysis lignin is investigated using a thermogravimetric analyzer under different heating conditions and under non-oxidative or oxidative atmospheres. The samples are indeed submitted to a linear temperature ramp of 10K/min or to a temperature ramp of 200K/min up to a residence temperature between 250 and 450°C where they are maintained during 4h (isothermal conditions). The values of the kinetic parameters are determined for these different samples in both thermal conditions, either using the differential isoconversional method or by means of an Extended Independent Parallel Reaction (EIPR) model. The values of the kinetic parameters obtained with this EIPR model for spruce trunk are also compared with that of its main constituents (hemicellulose, cellulose and lignin).

Effective removal of sulfur components from Brazilian power-coals by ultrasonication (40kHz) in presence of H2O2.

The present investigation reports a preliminary attempt of using ultrasonic energy (40kHz) to clean some low rank high sulfur Brazilian power-coal samples in presence of H2O2 solution. All types of sulfur components (i.e. pyritic, sulfate and organic) could be removed from the coal samples by this process. The raw and ultrasonicated coal samples were characterized by chemical analysis, Fourier Transformation Infrared (FT-IR) spectroscopy, X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), focused ion beam (FIB), high-resolution transmission electron microscope (HR-TEM) with selected area electron diffraction (SAED) and/or microbeam diffraction (MBD), scanning transmission electron microscopy (STEM), energy-dispersive X-ray spectrometer (EDS), and Thermogravimetry (TG-DTG) techniques to evaluate the clean-coal quality. The FT-IR spectroscopic analysis demonstrated the formation of oxidized sulfur species (SO and -SO2) and their subsequent removals after ultrasonication. The XRD profiles supported the presence of mineral matters in the coals. The TG-DTG profiles of the beneficiated coals revealed their improved quality for using in thermal plants with better combustion efficiency.

Ambient air quality and emission characteristics in and around a non-recovery type coke oven using high sulphur coal.

The objective of this study is to determine the concentrations of gaseous species and aerosols in and around a non-recovery type coke making oven using high sulphur coals. In this paper, physico-chemical properties of the feed coal sample are reported along with the collection and measurement of the emitted gases (SO2, NO2, and NH3) and aerosol particles (PM2.5, PM10) during the coal carbonization in the oven. The coals used are from northeast India and they are high sulphur in nature. The concentrations of the gases e.g., SO2, NO2 and NH3 emitted are observed to be within the limit of National Ambient Air Quality Standard for 24h. The mean PM10 and PM2.5 concentrations are found to be 125.4 µg/m(3) and 48.6 µg/m(3) respectively, as measured during three days of coke oven operations. About 99% of the SO2 in flue gases is captured by using an alkali treatment during the coke oven operation. A Principal Component Analysis (PCA) after Centred Log Ratio (clr) transformation is also performed to know the positive and negative correlation among the coal properties and the emission parameters.