Shifeng Dai

Distribution, isotopic variation and origin of sulfur in coals in the Wuda coalfield, Inner Mongolia, China

Abstract This paper describes coal petrographic characteristics, sulfur abundance, distribution and isotopic signature in coals in the Wuda coalfield, Inner Mongolia, northern China. Petrographic studies suggest that depositional environment influences petrographic composition. The No. 9 and No. 10 coal seams, which are thought to have formed on a tidal delta plain, have high collodetrinite contents (up to 66.1%) indicating enhanced gelification and bacteria activity during coal accumulation, and also have the highest sulfur content (3.46% and 3.42%). Both organic and pyritic sulfur isotope values (−12.3‰ to 5.8‰ and −18.7‰ to 1.1‰, respectively) are variable and generally tend to be more negative in high-sulfur coals than those in low-sulfur coals in the Wuda coalfield. The negative sulfur isotope values indicate that a large portion of sulfur in the high-sulfur coals has a bacterial origin. Sulfur isotopic compositions and variations within the section were used to propose a model to explain the origin of sulfur in these coals. The presence of pyritized rod-like bacteria, cyanophytes gelatinous sheaths and degraded algae organic matter suggests that bacteria, and perhaps algae, may play an important role in the formation of these high-sulfur coals.

Geochemical and mineralogical anomalies of the late Permian coal in the Zhijin coalfield of southwest China and their volcanic origin

Abstract This paper describes the influence of volcanic ash on the concentrations and occurrences of associated elements in coal in the Zhijin Coalfield in western Guizhou Province, China. Our studies reveal that the No. 9 coal seam in the Zhijin Coalfield has very high content of Fe (4.34%), Cu (369.90 μg/g), U (49.6 μg/g), Mo (63.10 μg/g), Zn (33.97 μg/g), and Zr (841.80 μg/g). The studies have also found that elements, such as Fe and Cu, do not occur as sulfides in this coal seam, in sharp contrast to many other coal seams in China. The geochemical and mineralogical anomalies of the coal seam are attributed to synsedimentary volcanic ash. In addition to normal macerals and minerals in coal, a volcanic-influenced material (VIM) derived from volcanic ash, detrital material of terrigenous origin and organic matter was identified under polarized-light reflectance microscopy and scanning electron microscopy equipped with energy-dispersive X-ray (EDX) analyzer. The volcanic-influenced material is the main carrier of the above elements in this coal. Six types of the volcanic-influenced material (VIM-1, VIM-2, VIM-3, VIM-4, VIM-5, and VIM-6) are further distinguished on the basis of their structures and compositions. To the best of our knowledge, this is the first report that presents a detailed classification of coal components with a high content of volcanic ash.

Petrology and geochemistry of the high-sulphur coals from the Upper Permian carbonate coal measures in the Heshan Coalfield, southern China

Abstract The Heshan coals, with very high organic sulphur content, are found in the Upper Permian marine carbonate successions (Heshan Formation) in the Heshan Coalfield, central Guangxi, southern China. The petrography, mineralogy, and geochemistry of coals and non-coal partings from the Suhe and Lilan coal mines of the Heshan Coalfield have been investigated using proximate, petrographic, inductively coupled plasma mass spectrometry (ICP-MS), X-ray fluorescence (XRF), X-ray diffraction (XRD), and scanning electron microscopy with an energy-dispersive X-ray (SEM-EDX) techniques. The sulphur content in the coals (with ash less than 50%) ranges from 5.3% to 11.6%, of which more than 90% is organic sulphur, reflecting a strong marine water influence on the palaeomire. The high vitrinite reflectance (1.89–2.18%Romax) indicates that the coals in the Heshan Coalfield are mainly low-volatile bituminous coal. Microscopic observation has revealed that the coal is mainly composed of vitrinite and inertinite macerals with relatively low TPI and high GI values, suggesting an unusual, strongly alkaline palaeomire, with high pH. XRD analysis plus optical and scanning electron microscopy show that the minerals in these coals are mainly quartz, calcite, dolomite, kaolinite, illite, and pyrite, although marcasite, strengite, and feldspar, as well as some oxidised weathering products such as gypsum, are also present. Most trace elements in the Heshan coals are enriched with respect to their world mean, with Mo, U, and W highly enriched, more than 10 times their world means. The trace elements are believed to be associated either with organic compounds (Mo and U) or minerals such as aluminium–iron-silicates (Sc, Ge, and Bi), aluminium-silicates (Cs, Be, Th, Pb, Ga, and REE), iron-phosphates (Zn, Rb, and Zr), iron-sulphides (As, Cd, Cr, Cu, Ni, Tl, and V), and carbonates (Sr, Mn, and W). Abnormally high organic sulphur content, high ash yields, relatively high GI values, very low TPI values, very high U contents, and very low Th/U ratios suggest that the Heshan coals accumulated in low-lying, marine-influenced palaeomires, developed on carbonate platforms. Many of these characteristics have also been recorded in the Tertiary coals of the circum-Mediterranean coal basins, where no marine influence is present. The similarities are thought to be produced by strongly alkaline groundwater chemistry, common to both environments.

Occurrence and origin of minerals in a chamosite-bearing coal of Late Permian age, Zhaotong, Yunnan, China

Abstract The minerals found in the no. 5 coal (Late Permian) from the Zhaotong Coalfield, Yunnan Province, southwestern China, have been examined and found to consist mainly of kaolinite, pyrite, chamosite, quartz, and calcite, with trace amounts of illite and mixed-layer illite-smectite. The proportion of chamosite in clay minerals ranges from 32 to 56 wt%, with an average of 46 wt%. Chamosite is distributed not only in collodetrinite, but also occurs as cellfillings in fusinite, semifusinite, and telinite. The high content and mode of occurrence of chamosite in this mine indicate its formation by interaction of kaolinite with Fe-Mg-rich Fluids during early diagenesis. Except for a minor amount of terrigenous quartz, most quartz is of authigenic origin and formed from kaolinite desilication. The calcite content of the no. 5 coal is 1.4.6.3% (with an average of 3%) and is distributed in collodetrinite and as cellfillings of coal-forming plants. Calcite originated from seawater invasion during peat accumulation. Pyrite occurs in several ways: as massive, framboidal, isolated enhedral/anhedral, and euhedral forms. In addition, the presence of a large amount of pyritized red algae provides strong evidence of seawater invasion during peat accumulation. The red algae may have played an important role in the enrichment of sulfur in the coal. The characteristic assemblage of minerals in this mine resulted from a unique basinal environment in which the mineral matter was derived from a basaltic source region, volcanic activity, and seawater transgression during coal formation

Surface analysis of pyrite in the No. 9 coal seam, Wuda Coalfield, Inner Mongolia, China, using high-resolution time-of-flight secondary ion mass-spectrometry

Abstract The chemical composition of pyrite in coal can be used to investigate its geological and mineralogical origin. In this paper, high-resolution time-of-flight secondary ion mass spectrometry (TOF-SIMS) was used to study the chemical composition of various pyrite forms in the No. 9 coal seam (St,d=3.46%) from the Wuda Coalfield, Inner Mongolia, northern China. These include bacteriogenic, framboidal, massive, cell-filling, fracture-filling, and nodular pyrites. In addition to Fe+ (54Fe+, 56Fe+, 57Fe+), other fragment ions were detected in bacteriogenic pyrites, such as 27Al+, Si+ (28Si+, 29Si+, 30Si+), 40Ca+, Cu+ (63Cu+, 65Cu+), Zn+ (64Zn+, 66Zn+, 67Zn+, 68Zn+), Ni+ (58Ni+, 60Ni+, 62Ni+), and C3H7+. TOF-SIMS images show bacteriogenic pyrites are relatively rich in Cu, Zn, and Ni, suggesting that bacteria probably play an important role in the enrichment of Cu, Zn, and Ni during their formation. Intense positive secondary ion fragments from framboidal aggregates, such as 27Al+, 28Si+, 29Si+, AlO+, CH2+, C3H3+, C3H5+, and C4H7+, indicate that formation of the framboidal aggregates may have occurred together with clay mineral and organic matter, which probably serve as the binding substance. The intense ions of 28Si+ and 27Al+ from massive pyrites also suggest that their pores incorporated clay minerals during crystallization. Together with the lowest 28Si+/23Na+ value, the intense organic positive secondary ion peaks from cell-filling pyrites, such as C3H3+, C3H5+, C3H7+, and C4H7+, indicate that pyrite formation may have accompanied dissolution or disintegration of the cell. The intense P+ peak was observed only in the fracture-filling pyrite and the highest 28Si+/23Na+ value of fracture-filling pyrite reflects its epigenetic origin. Together with XRD and REEs data, the stronger 40Ca+ in nodular pyrite than other pyrite forms shows seawater influence during its formation.

Geochemistry of ultra-fine and nano-compounds in coal gasification ashes: A synoptic view

The nano-mineralogy, petrology, and chemistry of coal gasification products have not been studied as extensively as the products of the more widely used pulverized-coal combustion. The solid residues from the gasification of a low- to medium-sulfur, inertinite-rich, volatile A bituminous coal, and a high sulfur, vitrinite-rich, volatile C bituminous coal were investigated. Multifaceted chemical characterization by XRD, Raman spectroscopy, petrology, FE-SEM/EDS, and HR-TEM/SEAD/FFT/EDS provided an in-depth understanding of coal gasification ash-forming processes. The petrology of the residues generally reflected the rank and maceral composition of the feed coals, with the higher rank, high-inertinite coal having anisotropic carbons and inertinite in the residue, and the lower rank coal-derived residue containing isotropic carbons. The feed coal chemistry determines the mineralogy of the non-glass, non-carbon portions of the residues, with the proportions of CaCO₃ versus Al₂O₃ determining the tendency towards the neoformation of anorthite versus mullite, respectively. Electron beam studies showed the presence of a number of potentially hazardous elements in nanoparticles. Some of the neoformed ultra-fine/nano-minerals found in the coal ashes are the same as those commonly associated with oxidation/transformation of sulfides and sulfates.

Valuable elements in Chinese coals: a review

ABSTRACT China is, and in the coming decades should continue to be, the largest producer and user of coal in the world. The high volume of coal usage in China has focused attention not only on the toxic trace elements that may be released from coal combustion but also on the valuable elements that may occur in the coal and associated ash. Valuable elements in several coals (or coal ashes) and some coal-bearing strata in China (e.g. Ge, Ga, U, rare earth elements and Y, Nb, Zr, Se, V, Re, Au, and Ag, as well as the base metal Al) occur at concentrations comparable to or even higher than those in conventional economic deposits. Several factors are responsible for these elevated concentrations: (1) injection of exfiltrational solutions during peat accumulation or as part of later epigenetic activity; (2) injection of infiltrational epigenetic solutions; (3) introduction of syngenetic alkali volcanic ashes into the peat-forming environment or into associated non-coal-forming terrestrial environments; (4) input of terrigenous materials into the coal-forming environment; (5) leaching of non-coal partings by groundwater/hydrothermal solutions; and (6) mixed processes involving both hydrothermal solutions and volcanic ash. The valuable elements in Chinese coals may be associated with either the organic matter or mineral matter, or have a mixed organic- and inorganic-affinity. For example, the Ge and U in coal-hosted ore deposits dominantly occur in the organic matter, with only traces of U-bearing minerals being present; gallium mainly occurs in boehmite and kaolinite, and to a lesser extent, in the organic matter. Rare earth elements and Y occur as carbonate-minerals (e.g. florencite, parisite), phosphate-minerals (e.g. rhabdophane, silico-rhabdophane, and xenotime), and in part are associated with the organic matter. Some metals (e.g. Ge, Al, Ga) have been successfully extracted at an industrial scale from Chinese coals, and others have significant potential for such extraction. Major challenges remaining for coal scientists include the development of economic extraction methods from coal ash, and the control of toxic elements released during the metal extraction process to protect human health and to avoid environmental pollution.

Concentrations and origins of platinum group elements in Late Paleozoic coals of China

The platinum group elements (PGEs) in 63 samples collected from the Late Paleozoic coal-bearing strata in western Guizhou Province and North China were determined using high-resolution inductively coupled plasma-mass spectrometry (HR ICP-MS). The study shows that PGEs in coal have five different sources: magmatic hydrothermal inputs, low-temperature hydrothermal fluids, synsedimentary volcanic ash, detrital minerals of terrigenous origin, and seawater. This study indicates that the first three sources can lead to enrichment of PGEs in these Chinese coals. The typical low contents of PGEs in most of the studied coals, defined as background contents in this work, mainly originate from detrital minerals. The background content of Pd (150 ng/g) is higher than that of Pt (30 ng/g), and much higher than that of Ru (5 ng/g), Rh (14 ng/g), and Ir (1 ng/g). The Pd content is nearly 15-fold higher than the crustal abundance. In contrast, Pt content is lower than its crustal abundance. The relatively good correlation between Pd, Pt, and ash contents suggests that the PGEs are associated with minerals.

Coal in a carbonate sequence stratigraphic framework: the Upper Permian Heshan Formation in central Guangxi, southern China

Microfacies and sedimentological analyses of the Upper Permian coal-bearing Heshan Formation in the Heshan coalfield of central Guangxi suggest that these coal measures formed in a marine carbonate platform setting. Low-volatile bituminous coals with very high organic sulphur overlie either palaeosols or locally developed cherts. The petrography and geochemistry of the coals suggest formation in low-lying mires, in which marine influence increased upwards. In the Heshan and overlying Dalong Formations, four third-order sequences are described based on the recognition of four sequence boundaries. Higher-order sequences within each of these third-order sequences are also documented and, within Sequence III, coal seams are developed above higher-order transgressive surfaces, representing the deposits formed during the lag time between initial flooding of the platform and the onset of carbonate production. In the Heshan Formation, the coals with greatest thickness occur immediately above third-order transgressive surfaces. It is argued that, in some coal-bearing siliciclastic-free marine carbonate-platform settings, accommodation creation rates and peat accumulation rates are balanced, hence greater coal accumulation can be achieved at the transgressive surface rather than at the maximum flooding surface.

Applied investigation on the interaction of hazardous elements binding on ultrafine and nanoparticles in Chinese anthracite-derived fly ash.

A multifaceted instrumental approach was employed to determine the chemistry and mineralogy of pulverized-coal-combustion fly ashes from two Chinese power plants. Techniques included traditional optical microscopy, X-ray diffraction, and chemical analysis along with a variety of electron beam methods. The aim is to demonstrate and bring together the wide variety of procedures dealing with F as the key element of concern, and determining its location in the mineral nanoparticles. The Hg content of the Anwen (Songzao coalfield) fly ashes is higher than that of the Diandong (East Yunnan) fly ashes, possibly owing to the greater C and Cl in the Anwen fly ashes. Both fly ash sources contain a variety of amorphous and nano-crystalline trace-element-bearing particles, both associated with multi-walled carbon nanotubes and as particles independent of carbons.