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To explore the application prospects of higher-molecular-weight gases (HMWGs) in forecasting coal spontaneous combustion (CSC), the emission characteristics of HMWGs during the oxidation of seven bituminous coals with different degrees of metamorphism were studied using gas chromatography. By analyzing the gas samples released from the oxidation of experimental samples, a total of 20 HMWGs with hydrocarbons as the main component was detected. The concentration of HMWGs changed regularly during the oxidation of experimental samples. The concentrations of most representative hydrocarbon gases increased significantly with increasing temperature but plateaued or even decreased when the temperature exceeded about 210°C. The relationship between HMWG production and the metamorphic degree indicators was nonlinear. The volatile matter content and vitrinite reflectance of coal cannot be used directly to evaluate HMWG production. The initial appearance temperatures of HMWGs during the oxidation of different coal samples also varied. The accuracy of CSC forecasting technology can be significantly improved if HMWGs are used as complementary index gases to forecast the different development stages of bituminous CSC. [ABSTRACT FROM AUTHOR]

Black shale-hosted vanadium (V) deposits account for about 80% vanadium resources in the world, but only <2% vanadium in the black shale can be extracted mainly due to insufficient recognition on the occurrence mode of vanadium. It is commonly agreed that most vanadium in the black shale is hosted in clay minerals and organic matters, but it is not clear how the other parts of vanadium exist and whether there exists a vanadium mineral, which has limited our understanding of metallogenic mechanism of black shale-hosted vanadium deposits. The Jiujiang Basin at the Lower Yangtze Block is a significant black shale-hosted vanadium metallogenic district. In this work, we conducted systematic studies of mineralogy, lithology and geochemistry on the occurrence of vanadium hosted in the black shales. Electron probe microanalysis (EPMA), Raman spectroscopy, and X-ray diffraction (XRD) show that the main vanadium-hosting mineral in the black shale is mannardite, with a structural formula of B a 0.96 ⋅ H 2 O T i 5.87 V 1.87 3 + V 0.11 4 + S i 0.07 C r 0.07 F e 0.02 3 + O 16.00 , space group I41/a, unit-cell parameters a = b = 14.346(7) Å, c = 5.899(1) Å, α = β = γ = 90°, Z = 4. Data from EPMA, TESCAN integrated mineral analyzer (TIMA), and whole-rock geochemistry indicate that 12.32–44.06% (average 24.95%) vanadium exists in mannardite. Most vanadium atoms in mannardite occupy its structural sites as trivalent vanadium (V3+), forming chemical bonds with O atoms as V O 2 − , whereas a minor amount of vanadium atoms replace titanite atoms (Ti4+) as quadrivalent vanadium (V4+) by isomorphism. Mannardite precipitates under a strong reductive condition with sufficient trivalent vanadium species, titanium and biogenic barium (bio-barite). Our first identification of mannardite in black shale-hosted vanadium deposits thus sheds light on the occurrence mode of vanadium and the metallogenic mechanism of black shale-hosted vanadium deposits. [ABSTRACT FROM AUTHOR]

Decomposition of oxygen-containing functional groups simultaneously produces CO and CO2, accompanied by the generation of active sites. Two different methods were applied in the study to speculate the kinetics of gas generation and the formation of active sites in coal pyrolysis. The evolution of CO and CO2 gas generation during isothermal pyrolysis under different temperatures was first analyzed, and the generation of CO and CO2 in the pyrolysis process was divided into three stages: the rapid decline stage, slow decline stage, and stabilization stage. In the early stage of the pyrolysis reaction, functional groups exhibited a rapid decomposition rate and a high conversion rate ($$a$$ a), which rapidly decreased the gas concentrations. With increasing pyrolysis time, the decomposition of functional groups declined with a low conversion rate ($$a$$ a) and showed a steady reaction rate in the later stage. On this basis, a method to calculate the activation energy during pyrolysis based on the data of gases generated in a steady period with a continuous cooling temperature was proposed. Activation energies of 70.12–148.39 kJ/mol for the CO formation and 64.47–127.38 kJ/mol for the CO2 formation were obtained from four different coals. In addition, a thermogravimetric analyzer was used to explore the apparent activation energies in the decomposition process via Coats-Redfern (C-R) method; activation energies of 39.33–126.65 kJ/mol and pre-exponential factors of 1.31–5.15E+08 s−1 were obtained for different coals. The results revealed that the CO had a greater generation activation energy than CO2, and a higher metamorphism degree implies a higher decomposition activation energy, which indicates that the decomposition of carboxyl groups and low-rank coals result in higher active sites. [ABSTRACT FROM AUTHOR]