Selected Papers on Coal Petrology and Coal Geochemistry in Ren Deyi
Coal accounts for 76. 2% of China's primary energy structure, and this structure will not change much for a long time. Sulfur-rich coal and high-sulfur coal occupy a considerable proportion in China's coal reserves, and the SO2 emission from coal combustion has reached 16.89 mt, which has attracted attention at home and abroad. At the same time, the sulfur in coal also seriously affects the quality of coal processing products such as coke and synthetic gas. Therefore, it is of great practical significance to study the occurrence law of sulfur in coal and the new desulfurization technology.
Southwest China is the region with the highest sulfur content in China coal, with high sulfur coal accounting for 43. 438+0% coal reserves. Acid rain caused by coal burning in Chongqing, Guiyang and other cities directly affects the ecological environment and people's health. In this paper, coal petrology, geochemistry, sedimentology and other methods are comprehensively used to study the occurrence law of sulfur in late Permian coal in southwest China, focusing on the form, composition and magnetism of pyrite in coal and the composition, structure and genesis of high organic sulfur coal.
1. Sulfur content in late Permian coal in southwest China.
The sulfur content in late Permian coal in southwest China changed greatly, and the total sulfur content could be from 0. 5% to 10% or more. It can be seen from Table L that the sulfur content is mainly controlled by the paleogeographic environment of coal accumulation. The total sulfur content of coal formed in alluvial plain environment is lower than 1. 0%. The coal formed in the upper delta plain environment has a total sulfur content of less than 65438 0.5%, which belongs to low sulfur coal. The total sulfur content of coal formed in tidal flat environment of limited carbonate platform is 8. 89% ~ 13. 14%, mainly organic sulfur, which is rare in the world. The total sulfur content of coal formed in lagoons, tidal flats and other environments is different, ranging from medium sulfur coal to high sulfur coal, and most of them are sulfide sulfur.
Second, pyrite in coal
Pyrite is the main sulfide mineral in late Permian coal in southwest China. In addition, there are sphalerite, galena, chalcopyrite, white iron ore and galena. The macroscopic structure of pyrite is mainly disseminated fine particles, nodules, veins and lumps. Microscopically, in the study of sulfur in late Permian high-sulfur coal in southwest China since 1985, pyrite was observed by optical microscope and scanning electron microscope, and combined with the macroscopic characteristics of pyrite, the microscopic classification of pyrite as shown in Table 2 was put forward. The classification basis of this classification is: firstly, it is divided into two categories according to the existence of biological fabric, and then the pyrite of biological fabric is subdivided according to the biological category; Abiotic fabric classes are subdivided according to their morphological structure. Microscopic statistics show that the sulfur-rich and high-sulfur coals in Nantong No.5 and No.6 coal seams in Sichuan are mostly cellular and nodular (75% and 89% in the two coal seams respectively), and pyrite particles larger than 100μm are more than those in other coal seams, with higher dissociation degree (above 60%). However, pyrite in Wuyi coal mine in Sichuan Province is mainly composed of authigenic crystals and aggregates (45%), raspberries and their groups (3 1%) and white iron ore (2 1%), with few other types (only 3%). Authigenic crystals are mostly octahedral, with particles smaller than 65438 00μ m, and pyrite with particles smaller than 20 meshes has a very low degree of dissociation (less than 8%), which reflects that Wuyi coal mine was formed in a coal-forming environment obviously influenced by seawater. Because high fine pyrite content and high organic sulfur coal are difficult to desulfurize, it is of great significance to study the occurrence law and washability of pyrite in coal for desulfurization technology and coal processing and utilization.
Table 1 Sulfur Content in Late Permian Coal in Southwest China
Table 2 Microscopic classification of pyrite in late Permian coal in southwest China.
Third, the magnetism of coal and pyrite.
High gradient magnetic separation (HGMS) is a method to separate weakly paramagnetic particles. Combined with physical methods such as flotation, it is a potential new desulfurization technology for fine coal. Therefore, 10 coal samples with different coal ranks, coal-derived pyrite and common mineral samples associated with coal were selected, and the magnetic susceptibility of the samples was tested by the superconducting magnet sample vibrometer (VSMSCON2) of Institute of Physics, Chinese Academy of Sciences. The experimental results are shown in Table 3 to Table 5.
Table 3 Magnetic Susceptibility of Coal
Table 4 Magnetic susceptibility of pyrite in coal seam
Table 5 Magnetic susceptibility of common minerals in coal
As can be seen from Table 3, the magnetic susceptibility of Yunnan insurable soft lignite is lower than the background noise, which can be regarded as zero value, while from bright lignite to coking coal in gas coal, fat coal and bituminous coal, even anthracite and ultra-anthracite are diamagnetic. The magnetic susceptibility of pyrite in coal and coal seam roof and floor is positive, and its value is in the range of (1.12 ~ 58.0) ×10-7 emu/g, and the magnetic susceptibility of pyrite with different forms and occurrences is different, and the magnetic susceptibility of pyrite crystals with complete crystals and pure components is low. The magnetic susceptibility of pyrite is (1.26 ~ 2.63) ×10-7 emu/g, and the average value is, and the magnetic susceptibility of pyrite in spherical and sandy layered aggregates with poor crystal shape and mixed components is higher, which are (3.01~ 30.9/) ×/respectively.
The diamagnetism of coal and the paramagnetism of coal pyrite provide a theoretical basis for the development of high gradient magnetic separation to remove sulfur from fine coal. High gradient magnetic separation shows a good prospect for desulfurization of high sulfur coking coal. At the same time, as can be seen from Table 5, the magnetic susceptibility of calcite and siderite, which are often associated with coal, is also positive, so it is possible to effectively separate from coal together with pyrite in high gradient magnetic separation.
The magnetism of pyrite is mainly related to the content of its associated paramagnetic elements. According to instrumental neutron activation analysis and correlation analysis of associated elements of pyrite in coal measures, its magnetic susceptibility is positively correlated with paramagnetic elements Mn, V, Cr, Ti, Ca and Mg [1].
Petrological and geochemical characteristics of high organic sulfur coal
Ultra-high organic sulfur coal in southwest China was formed in tidal flat environment of limited carbonate platform. Taking K3 coal seam in Guiding, Guizhou as an example, the direct floor of the coal seam is exposed pine algae limestone, which belongs to the cause of algae mat and has no root rock, and the roof of the coal seam is also exposed pine algae limestone. The petrological and geochemical characteristics of coal seams are unique.
Matrix vitrinite in (1) coal is the main component of macerals. Although its reflectance VRmax is 1.48%, it belongs to medium bituminous coal, but it still has dark red fluorescence under the excitation of blue light. Observation by transmission electron microscope showed that there were a large number of ultraplastids and ultrabacteria, and the contrast was sharp. There are microscopic components such as structural algae, layered algae and shell fragments in chitin group. In addition, the coal also contains a large number of pyritized bacteria and algae fossils, indicating that the coal-forming plants in Guiding are not only higher plants such as Lepidoptera and Hibiscus, but also lower plants rich in lipids and protein. Bacteria and algae visible in coal are only a small part of the original, and more have been degraded into ultrafine components dispersed in matrix vitrinite.
(2) The minerals in coal are mainly clay minerals and quartz, as well as pyrite, carbonate minerals and a small amount of apatite. Clay minerals are mainly illite and montmorillonite; Calcite and dolomite are dispersed in matrix vitrinite in different sizes, which is rare in coal. * * * There are a large number of autogenous fine grains, pyrite polycrystals and raspberries, reflecting that high organic sulfur coal was formed in a moderately weak alkaline and reductive ancient peat swamp. The apatite in coal is flaky and lenticular, and there are also pentagonal crystals. After absorbing phosphorus from the surrounding media in a calcium-rich environment, it is further transformed into apatite by bacteria, algae and lower organisms.
In the upper, middle and lower parts of Guiding coal seam, calcareous algae (quasi-gymnosperms) and fragments of calcium spheres were found. Gymnosperms are marine red algae distributed in the subtidal zone, and modern calcium globes are umbrella algae spores living in the extremely shallow waters of the bay with the water depth less than1m. There are also calcareous biological fragments of brachiopod and cladocera, as well as relatively complete foraminifera and ostracod fossils in Guiding coal, which indicates that these organisms living in shallow water were transported to peat swamp by tidal current, and their hydrodynamic energy was weak, so calcareous biological fossils were preserved in weak alkaline medium.
(3) Micro-area analysis of matrix vitrinite by SEM shows that associated elements such as K, Na, Mg, V and Mo in Guiding coal are 3 ~ 10 times higher than those in Panxian coal formed in the upper delta plain, and the U content (instrumental neutron activation method, INAA) is 50 times higher than that in Panxian coal, and the B content (ICP method) is 65,433 times higher than that in Panxian coal.
Table 6 Comparison of Associated Elements between Guiding Coal and Panxian Coal Unit: 10-6
The concentration of magnesium, boron and other elements in seawater is much higher than that in fresh water. The enrichment of these elements in coal guiding is a strong evidence that peat bogs are often submerged by seawater. The enrichment of U, Mo, V and other elements is related to the adsorption of bacteria, algae and other lower organisms and their degradation products and humus, and the reducing environment creates favorable conditions for the preservation of these elements.
On the profile of guiding coal seam, bright and dark thin layers alternate. Microscopically, the matrix rich in clay minerals and the thin layer dominated by matrix vitrinite frequently alternate, showing wavy bedding and lenticular bedding, and micro-deformation bedding can be seen locally. This does not mean the alternation of dry and wet stages, which is common in forest swamps, but may be the expression of tidal movement. Most of the cell walls of sericite and semifusinite in coal have expanded to varying degrees, and even become coarse particles, indicating that they have experienced strong gelation before undergoing oxygen oxidation brought by tidal current; There are many large and round cell cavities in some filaments, which are similar to aeration tissues, indicating that peat bogs are seriously submerged.
Verb (abbreviation of verb) Enrichment mechanism of organic sulfur in high organic sulfur coal.
The origin of organic sulfur in coal is complex, and it can be generally divided into two types according to its sources: primary organic sulfur and secondary organic sulfur provided by coal-forming plants, and most of the organic sulfur in high organic sulfur coal is secondary [5]. Taking Guiding coal as an example, the main factors of high organic sulfur enrichment are analyzed.
(1) The participation of bacteria and algae in coal-forming plants. Organic sulfur mainly comes from sulfur-containing amino acids of protein in coal-forming plants. Due to different living environments, the contents of protein and sulfur in the organic composition of different coal-forming plants are obviously different. The sulfur content of modern Lycopodiaceae plants is 0. 10% ~ 0. 14%, that of Pinaceae plants is only 0.05%, and that of mangrove plants growing in coastal salt marshes is mostly 0.30% ~ 0.40%. The coal-forming plants in Guiding coal are well preserved, well ventilated, and massive vitrinite is developed (its predecessor may be tannin). Combined with the overall coal-accumulating environment, it is likely that a considerable part of them have a living environment similar to mangroves, which makes the primary organic sulfur slightly higher. It is worth noting that there are a large number of bacteria, algae and other low-grade plants and their degradation products in Guiding coal. Protein accounts for 50% ~ 80% of the dry weight of modern bacteria, and protein accounts for 20% ~ 30% of algae, much more than higher plants. It can be seen that bacteria and algae are an important source of primary organic sulfur in coal, especially in Guiding coal, which is likely to involve a considerable number of gymnosperms, belonging to red algae, with sulfur content of 2.63%(Bowen, 1979).
(2) Continuous supply of sulfate in seawater. Seawater contains a large amount of sulfate ion SO2-4 (2.7g/L), which is more than 200 times higher than that in fresh water. The coal in Guiding is accumulated in the tidal flat of limited carbonate platform, and the periodic reciprocating infiltration of tidal water brings a lot of SO2-4 to the peat swamp. The roof of the coal seam is tidal flat limestone, that is, after the peat layer is formed, seawater can still penetrate into the peat layer continuously, so that the SO2-4 in the pore water always keeps the concentration conducive to sulfur accumulation.
(3) The culture medium has weak alkalinity, reducibility and microbial activity. Petrological research shows that the medium in the peat swamp in Guiding is weakly alkaline and reductive, and this medium condition, combined with tropical and subtropical climate conditions, is very conducive to the reproduction of sulfate-reducing bacteria and other microorganisms. Sulfate-reducing bacteria reduce SO2-4 in swamp water medium to reduced inorganic sulfur, which provides a material basis for the enrichment of high organic sulfur. Pyritized Vibrio, cocci, bacilli, filamentous fungi and their colonies are common in coal seams, which is their physical evidence. The isotope δ34S of organic sulfur in Guiding coal is -7.4 ‰ ~-7.7 ‰, and that of pyrite is -28.2 ‰ ~-30.6 ‰. The fact that sulfur isotopes are extremely light indicates that peat bogs have strong and continuous microbial activities. Sulfate-reducing bacteria continuously reduce sulfate with 32S H2S in seawater, and preferentially combine with iron ions to form light isotope iron sulfide minerals, and then combine with organic matter.
(4) The active iron ions are limited. As we all know, active iron ions are more competitive in reducing sulfur than organic compounds. When active iron ions exist, sulfur ions will preferentially combine with them to form iron sulfide minerals. Only when iron ions are limited, excess H2S will combine into organic molecules [4]. Guiding coal is formed in tidal flat of limited carbonate platform, and the supply of terrigenous detritus is relatively poor, and the coal-bearing rock series is also composed of iron-poor carbonate rocks. Therefore, it is impossible for the swamp to have sufficient iron ion supply. The analysis shows that the total iron content in Guiding coal is 8400× 10-6, and the total sulfur content is 8.89%, while the total iron content in Liuzhi Dizong coal (St, d=4.57%) with much lower total sulfur content is 18000× 10-6, which shows that
In a word, in the limited tidal flat environment of carbonate platform, algae mats developed into peat bogs. With the participation of coal-forming plant bacteria and algae, seawater often flooded to form weak alkaline media, and sulfate-reducing bacteria were very active, which promoted the formation and high enrichment of organic sulfur in Guiding coal.
Six, the structure of organic sulfur compounds in coal
Organic sulfur in coal mainly exists in the form of mercaptan, sulfide and disulfide, thiophene and its derivatives. [2]
The photoelectron spectroscopy (ESCA) analysis shows that the main organic sulfur in Nantong coal and Guiding coal with high organic sulfur is thiophene, accounting for 55.9% and 75.4% respectively, followed by lipid sulfide sulfur and sulfone. This roughly reflects the state of organic sulfur in coal macromolecular network phase. [3]
The structure of organic sulfur in coal can be accurately identified by detecting the organic solvent extract of coal by gas chromatography-mass spectrometry. Chloroform was used as organic solvent to extract high organic sulfur coal from Wuyi mine, Guiding mine and Yanshan Ganhe mine in Anxian county. The content of aromatic fraction was more than 60% of asphalt "A". The ligand exchange thin layer chromatography was used to analyze Guiding coal, and pure sulfur-containing compounds were separated. GC-MS analysis shows that the organic sulfur-containing compounds in Guiding coal are mainly alkyl-substituted series compounds, among which dibenzothiophene series is the main one, accounting for 65.5% of the total sulfur-containing compounds, among which C 1- dibenzothiophene and C2- dibenzothiophene with stable methyl side chain are absolutely dominant.
According to GC-MS analysis, the structure of organic sulfide in coal samples of Wuyi Mine in Anxian County and Ganhe Mine in Yanshan County is similar to that of Guiding Coal. C 1- dibenzothiophene and C2- dibenzothiophene account for 62.3% and 72.3% of sulfur-containing compounds, respectively, indicating that dibenzothiophene series compounds have an increasing trend with the increase of coal rank (the vitrinite reflectance of Wuyi coal is 0. Low-rank Wuyi coal contains many compounds containing ethyl and metastable methyl.
This result is similar to the composition of sulfur compounds obtained by White et al. (1990) by analyzing the world-famous Rasa high organic sulfur coal in Croatia with low pressure and high resolution mass spectrometry.
This work was supported by the National Natural Science Foundation of China, and the GC-MS analysis was supported by the State Key Laboratory of Organic Geochemistry of China Academy of Sciences, and was guided by researcher Sheng. LETLC analysis is directed by researcher Gu Yongda from Institute of Coal Chemistry, China Academy of Sciences; Thanks to Academician Han Dexin for his guidance.
Take the exam and contribute.
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[2] Zhou Chenglin, 1990. Geochemistry of sulfur in coal. Orr W L. and White C M. Geochemistry of Sulfur in Fossil Fuels. ACS seminar series 429, chapter 2, pages 30 ~ 52. American Chemical Society, Washington D.C.
Tang Yuegang, Shao, Lei, et al. Journal of Fuel Chemistry,1993,21(4)
[4] Bernard R. A. Geochim. Cosmic chim. Acta， 1984，48( 4)
[5] casagrande, doctor of law. Coal and coal-bearing strata: latest progress, Geological Society, London,1987,87 ~105.
Study on sulfur occurrence law and pyrite magnetism in late Permian coal in southwest China
Ren Deyi Tang Yuegang Lei Jiajing Mao Liuhe Lin Fuqin Ai Tianji
(Beijing Graduate School of China University of Mining and Technology)
Abstract: The sulfur content of late Permian coal in southwest China ranks first in China. The change of sulfur composition in coal during peat accumulation is mainly controlled by paleogeographic environment. High organic sulfur coal is formed in peat bogs developed in tidal flat of limited carbonate platform. It has special characteristics of coal petrology and coal geochemistry, and its sulfur-containing organic compounds are mainly thiophene. The macroscopic and microscopic forms or types of pyrite in late Permian coal are diverse. Bituminous coal and anthracite are diamagnetic, but pyrite is paramagnetic. The magnetic susceptibility of pyrite depends on the content of accessory mineral elements associated with pyrite.
Key words: high sulfur coal, pyrite, organic sulfur, occurrence law, magnetism
(This paper was co-authored by Tang Yuegang, Lei, Mao, Liu and Ai Tianjie, and originally published in Collection of Coalfield Geology: Celebrating Professor Gao's 80th Birthday and 60 Years of Geological Work 1996).