Fuel 135 (2014) 83–90
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Ash limitation of physical coal beneﬁciation for medium–high ash coal—A geochemistry perspective Wenfeng Wang a,b, Weiduo Hao a,b,⇑, Simon Xu c, Fuchang Qian a,b, Shuxun Sang a,b, Yong Qin a,b a
Department of Source and Earth Science, China University of Mining and Technology, Xuzhou 221008, Jiangsu Province, China Key Laboratory of Coalbed Methane Resource and Reservoir Formation Process, China University of Mining and Technology, Ministry of Education, Xuzhou 221008, Jiangsu Province, China c Department of Computer Science and Mathematics, Algoma University, 1520 Queen Street East, Sault Ste. Marie, Ontario P6A 2G4, Canada b
h i g h l i g h t s We studied the possibility of reducing the coal ash content geochemically. A signiﬁcantly correlation between ash yield and Al2O3 + SiO2 content is obtained. The intercept of regression equation on the ash axis is generally 2–5%. The intercept of 2–5% indicates an original inorganic component in coal-forming peat. 2–5% of inorganic components in the coal can hardly be separated by physical cleaning.
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Article history: Received 4 May 2014 Received in revised form 4 June 2014 Accepted 17 June 2014 Available online 2 July 2014 Keywords: Coal ash yield Physical coal beneﬁciation Correlation analysis Geochemistry
a b s t r a c t Nowadays the industrial coal beneﬁciation in China could only reduce the ash yield to about 10%, which could not meet the requirement or standard of environment protection. In this work, the possibility of reducing the ash yield was studied from the aspect of geochemistry. The channel samples were collected from two coal seams in Guizhou and Shanxi province, China and then conducted analysis by combining data from coals worldwide. The result reveals that the same coal seam or the coals deposited in the same peat swamp show a signiﬁcantly positive correlation between ash yield and Al2O3 + SiO2 content, and the intercept of regression equation on the ash axis is always less than 5% (generally 2–5%). Overall, the coal from China is featured with a higher intercept compared with that in the other countries. The intercept of 2–5% on the ash axis indicates an original inorganic component in coal-forming peat. The research result also presents a theoretical limitation of coal ash by coal cleaning, because 2–5% of inorganic components in medium–high quality ash coal could hardly be separated by traditional physical coal beneﬁciation. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction Coal plays an important part in the industrial production and daily life in China, with about 74% of total primary energy and 60% of chemical materials derived from coal production . At present, China has become the largest consumer and producer of coal in the world. Between 2011 and 2013, the total coal production in China was 3.52, 3.66 and 3.7 billion and its corresponding consumption was as high as 3.8, 3.91 and 3.61 billion, accounting for more than half of consumption in the world. ⇑ Corresponding author at: Department of Source and Earth Science, China University of Mining and Technology, Xuzhou 221008, Jiangsu Province, China. Tel.: +86 51683591000; fax: +86 51683590998. E-mail address: [email protected]
(W. Hao). http://dx.doi.org/10.1016/j.fuel.2014.06.041 0016-2361/Ó 2014 Elsevier Ltd. All rights reserved.
The energy structure featured as coal domination would not change in the near future , which, on the other hand, results in serious air pollution in China. To a great extent, the widely spread of fog and haze and the serious air pollution of PM2.5 in most Chinese cities recently is attributed to the combustion of coal [3–6]. According to recent studies, a number of human health problems are related to the volatilization of trace elements during coal combustion and coal gangue stacking [1,7–13]. Although traditional physical coal washing method could reduce the ash yield in coal to a great content, the coal beneﬁciation plant in China could only reduce the ash yield to about 10% at present [14,15]. On the other hand, the big environmental problem and limited transportation budget due to the utilization of this coal force us to seek a way to further reduce the ash yield in coal during production. According to Tomeczek and Palugniok ,
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the inherent minerals in coal are closely associated with organic matters and could not easily be separated, and the percentage of those inherent minerals in coal is usually below 10%, mostly 2– 4%. Unfortunately, the current technology used in coal processing could not produce coal with about 2–4% ash yield [17–19]. Coal is a complex heterogeneous mixture of organic and inorganic components with different abundance and origin [20–22]. The ash yield in the coal is the residue derived from the inorganic and organic matter during the incineration of the coal. The ashes from coals differ in type and origin, and those from identical coals differ in the ash yield, which often shows a diverse distribution of the essential components . In order to study the genesis of the inorganic matter during coaliﬁcation, the correlations between ash yield and ash-forming elements have been studied based on the samples from coal deposits in German, Russian, Ukrainian, Spanish, Canadian and Romanian . It is commonly accepted that the elements which are positively correlated with ash value are ‘imported’, whereas the elements which are negatively correlated with ash value are authigenic . Generally, Si and A1 are the major components of the ash, and the proportion of detrital minerals increases along with the increase of the ash yield . In other words, the composition of ash is close to the composition of the adjacent rocks with the increase of ash yield in the coal . In order to study the possibility of reducing the ash yield in coal from the aspect of geochemistry and the theoretical limitation of coal ash during physical coal beneﬁciation, samples were collected from two coal seams in China and conduct analysis on the compositions of ash yield and Al2O3 + SiO2 (or Al + Si). The data from other countries published previously are also included for analysis and comparison. 2. Geological background Five coal-distribution areas in China have been classiﬁed according to their geotectonic units and coal forming period . Among them, northern China (especially Shanxi province) and southwestern China (especially eastern Yunan and western Guizhou province) are the main coal production bases in China and have distinct characters in terms of sedimentary environment and original coal-forming plants. The Puan coal mine is located in the Puan County of western Guizhou province, Southern China (Fig. 1). The No. 17 coal seam where the samples were taken belongs to the Longtan Formation of the Late Permian age, with a thickness between 0.8 m and 6.2 m, average at 3.1 m. The geological background has been described in detail by Dai et al. . The Antaibao coal mine is located in the west part of the Pingshuo city, Shanxi province, northern China (Fig. 1). Its geological background is provided in our previous studies [27–29]. 3. Sampling and analytical method Eight incremental channel samples (approximately 15 cm across and 10 cm deep) were collected from the Puan coal mine, according to the macrolithotype of coal seam, of which ply two contains a thin layer of carbonaceous mudstone parting. In addition, a whole seam mixed sample was also collected. All samples were stored in plastic bags during transportation in order to avoid the contamination and oxidation. The samples were crushed and ground to less than 200 mesh for further test. Proximate analysis (ISO-1171 ) and chemical analysis were performed on the samples, and an X-ray ﬂuorescence in (XRF, Cu Ka source) analysis was performed in Advanced Analysis and Computation Center of China University of Mining and Technology, correspondingly (Table 1).
The speciﬁc sampling and analytical method of Antaibao samples could be seen in our previous studies [27,29].
4. Results and discussion The result of the ash yield and analysis of nine samples from Puan coal mine are shown in Table 1. Fig. 2a shows the relationship between ash yield and Al2O3 + SiO2 which were obtained by separating the anomaly samples. The anomaly samples are mainly high ash yield coal gangue. It can be seen from Fig. 2a that there is a positive correlation between ash yield and Al2O3 + SiO2 content in the coal. The analysis result of twenty-two channel samples from Antaibao mine is presented in Fig. 2b (the result of Plies 21 and 22 samples are not included due to their anomaly ). Attention has been paid to the occurrence and origin of minerals in coals by many researchers [31,32]. According to Stach et al. , the inorganic components of coal are divided in three groups: (1) inorganic matter from the original plant; (2) inorganic–organic complexes and minerals which formed during the ﬁrst stage of the coaliﬁcation process or were introduced by water or wind into the coal deposits as they were forming; (3) minerals deposited during the second phase of the coaliﬁcation process, after the consolidation of coal, by ascending or descending solutions in cracks, ﬁssures or cavities or by alteration of primarily deposited minerals. As seen in Fig. 2, the intercept of Al2O3 + SiO2–Ash regression equations on the ash axis is 3–5%. This value has the geological signiﬁcance since it represents the original inorganic component of the coal-forming peat swamp, including the minerals of the above ﬁrst two groups. It is possible that the content of the original inorganic components of the peat swamp is less than 5%, taking into account the inherent ash of plant generally less than 0.5% and the inorganic matter present in the peat swamp medium. The primary minerals are generally not more than 2% of the coal . In any case coal cleaning depends on mode of occurrence of the minerals. The above ﬁrst two groups, sometimes described as inherent mineral matter are closely associated with the organic matter of coal, including both intimately admixed minerals and nonmineral inorganics in the maceral components. The minerals of the third group derived from intra-seam noncoal bands or admixed roof or ﬂoor strata during coal mining are described as extraneous mineral matter. Inherent mineral matter could hardly be separated by the traditional physical beneﬁciation method, whereas extraneous mineral matter can be at least partly removed by the cleaning processes at coal preparation plants. Stach et al.  thought that the minerals which have formed together with the coal or have been introduced into the deposits are, as a rule, ﬁne-grained and intimately intergrown with the coal; by contrast, the minerals which formed during the second stage of the coalication process are neither ﬁne-grained nor intimately intergrown with the coal, because most of them were deposited in cracks and ﬁssures. In general, the secondary transformation, with or without the introduction of new material, of minerals which formed during the ﬁrst phase of the coalication process does not alter the intimate intergrowth between these minerals and the coal, and rarely leads to an increase in grain size. As shown in Fig. 2, even if the ash-forming elements Al and Si have been largely removed, there are still 3–5% of the coal ash-the intercept on the ash axis, indicating that the intercept of 3–5% could be the theoretical limitation of the coal ash with physical coal cleaning. As for these minerals deposited during the second phase of the coaliﬁcation process by ascending or descending solutions in cracks, ﬁssures or cavities directly lead to the increase of ash yield, but they can be separated by the traditional physical beneﬁciation method.
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Fig. 1. Stratigraphic column of coal-bearing strata in the two mining districts and sampling locations.
This observation could be veriﬁed in practice. According to previous studies, the ﬂoat and sink analysis data in coal cleaning plant show that the low density part of coal particles (lower than 1.3 g/
cm3) has an ash yield of about 4–6% [14,15,17,19,35]. This analysis method is performed by crushing coal into millimeter size, so it could separate all the epigenetic minerals and the 4–6% ash yield
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Table 1 Chemical analysis and lithotype of Puan coal (%). Samples
Mixed Ply 1 Ply 2 Ply 3 Ply 4 Ply 5 Ply 6 Ply 7 Ply 8
Semidull Dull Semibright Dull Semidull Semidull Bright Bright
22.48 23.72 38.32 11.64 35.33 26.42 26.58 8.94 9.33
Ad, ash yield; d, dry basis; Ro,
2.60 2.53 2.68 2.73 2.93 2.82 2.51
48.34 35.44 38.68 47.19 57.45 46.88 45.15 35.02 58.14
16.20 14.15 8.13 34.93 20.14 34.97 37.69 32.42 16.78
21.51 37.96 40.44 7.49 10.70 7.38 4.62 1.58 17.97
2.57 3.40 4.09 0.50 0.19 0.42 3.55 13.11 0.74
2.18 0.46 0.30 0.92 1.01 0.89 0.64 0.92 0.66
1.80 2.95 3.88 0.33 0.20 0.26 2.27 9.23 0.32
4.69 0.61 0.32 1.21 3.80 0.89 0.72 0.77 0.50
0.93 2.49 1.15 4.09 2.87 5.11 3.61 1.68 2.13
0.46 0.16 0.06 0.51 0.19 0.36 0.44 0.24 0.14
0.08 0.03 0.04 0.01 0.01 0.01 0.02 0.06 0.01
maximum vitrinite reﬂectiance; lithotype, following the Chinese standard method GB/T 18023-2000.
3.98% , and the pyrite is greatly contributed to the ash yield in form of iron oxides after coal combustion. 5.2. The northern Chinese coal Northern Chinese coals studied in this article were mainly formed in two periods: Late Carboniferous to Early Permian and Cretaceous. Most of them are located in Shanxi, Shaanxi, Shandong provinces and Inner Mongolia. The volatile matter of bituminous coal varies from about 15% to 40%. The depositional environments of coals in northern Shanxi province and center-southern Inner Mongolia mainly range from coastal to deltaic plain [29,43–48]. The intercept value of Al2O3 + SiO2–Ash line is mostly 2–4% (Fig. 4). 5.3. Coals from other Asian countries Fig. 2. Correlation analysis between ash yield and Al2O3 + SiO2 (Al + Si) in two Chinese coal seams.
is mainly formed by ﬁne-grained minerals and nonmineral inorganics in the maceral components. The Otisca method was developed in 1980s, which could reduce the ash yield to lower than 1% by crushing coal particles into smaller than 250 lm . This method is mainly used to produce ultra-cleaned coal for special usage and could not be widely used in coal washing plant for its high budget. The remaining 1% coal ash is mainly formed by nonmineral inorganics in the maceral components. Therefore, the ash limitation of coal with the traditional physical method of coal cleaning is about 4–6% for medium–high ash coal. This conclusion will be further discussed in next section. 5. Comparison with the existing data from other locations In this section, further discussion is performed on our observation based on the data from other countries or locations.
5.3.1. Iran coal Coal in Iran mainly was formed as sub-bituminous to bituminous with a high ash yield and variable sulfur content during the period between Late Triassic and Jurassic . Research on the coal from this area indicates a non-marine origin but coal deposited in lacustrine and swamp environment . The intercept value of Al2O3 + SiO2–Ash line is about 2% as shown in Fig. 5a. 5.3.2. Indonesia coal Most coal deposits in Indonesia are between Paleogene and Neogene in age, low to moderate rank and have low ash yield and sulfur . As they were formed late, coals in this area are characterized as from lignite to sub-bituminous to bituminous rank. But active igneous activities in this area contribute to the increase of coal rank in this area. In some basin, the thermal metamorphism leads to the formation of anthracite [1,52]. Similarly, the intercept is around 3% (Fig. 5b). 5.4. Coals from Europe
5.1. The southern Chinese coal As main coal production bases in China, the coal seams in eastern Yunnan, western Guizhou and Guangxi province occur in Late Permian in age and have vitrinite reﬂectance values between 1.20% and 2.00%, indicating a low to medium volatile bituminous coal. It is generally believed that the coal seams in this area are affected by a number of factors, such as multi-stage hydrothermal ﬂuids, sediment source and volcanic ash [26,37–42]. The relationships between the ash yield and the value of Al2O3 + SiO2 from those ﬁve coal mines in southern China are shown in Fig. 3, which shows 2–6% of the intercept on ash axis. The usage of Al2O3 + SiO2 + Sp,d but not Al2O3 + SiO2 of X-axis in Fig. 3b is based on the fact that the pyritic sulfur content (Sp,d) in the Fusui coal mine is extremely high: the pyritic sulfur content ranges from 1.04% to
Coal in Iberian Peninsula is characterized in geochemistry and mineralogy. It shows that many elements have aluminium-silicate afﬁnity. And active magmatic ﬂuids make contributions to the anomalous amount of Mn and V which show afﬁnity to carbonate . The intercept character of this coal shows about 4% of ash on ash yield axis (Fig. 5c). 5.5. Coals from America 5.5.1. Coal in USA It seems that the depositional environment of coal in U.S. is simpler than the coal discussed above. The coal-bearing strata in the Washington State were accumulated in the intertidal and deltaic environment along with a tidally inﬂuenced delta plain .
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Fig. 3. The correlation analysis between ash yield and Al2O3 + SiO2 (Al2O3 + SiO2 + Sp,d) in Southern China coal ((a) data come from Dai et al. ; (b) data come from Dai et al. ; (c) data come from Wang et al. ; (d) data come from Dai et al. ; (e) data come from Shao et al. ).
Fig. 4. The correlation analysis between ash yield and Al2O3 + SiO2 (Al + Si) in Northern China coal ((a) data come from Dai et al. ; (b) data come from Dai et al. ; (c) data come from Dai et al. ; (d) data come from Zhuang et al. ; (e) data come from Dai et al. ).
The apparent rank of the coal ranges from lignite A to anthracite and those coals were formed in late Cretaceous. Although extensive volcanism happened during the peat accumulation period and caused a large amount of tonstein parting in the coal seam, it seems that the intercept of Al2O3 + SiO2–Ash line is at a low level (lower than 1%) (Fig. 5d).
With regards to the coal in Kentucky, all of them are splint coal in high volatile B/A bituminous rank. With few notable exceptions, the major element chemistry of this area is dominated by the aluminosilicate chemistry typical of clay- and silt-rich detrital inﬂuences . As shown in Fig. 5e, the intercept is lower than 1%.
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Fig. 5. The correlation analysis between ash yield and Al2O3 + SiO2 (Al + Si) in coal worldwide.
5.5.2. Coal in Brazil A high volatile bituminous coal has been determined by Levandowski and Kalkreuth  in Brazil. The accumulation of this Permian coal is closely linked to transgressive/regressive circle with peat accumulation occurring in a predominantly back barrier/lagoonal setting . Therefore, many minerals like quartz, carbonate, and pyrite have been accumulated in the coal samples. The character of the Al2O3 + SiO2–Ash line (Fig. 5f and g) is the same as other coals we have discussed before [56,58].
5.7. Summary Based on the above correlation analysis, a column was made to show the average intercept of coal in Southern China, Northern China and other countries in the world (Fig. 6). As shown in Fig. 6, the Chinese coals generally have a high intercept on the Al2O3 + SiO2–Ash line in comparison with the coals from other countries and the intercept in Northern China coals seems to be higher than that in Southern China. This might be caused by the
5.5.3. Coal in Canada A coal mine in the middle-west of Canada was studied by Pollock et al. , which is divided into two cycles based on mineralogy and geochemical behavior. The difference between the two cycles indicates a decreasing stability of the peat-forming environment resulting from an increasing ﬂuvial inﬂuence and volcanogenic input. Correspondingly, the intercept of the line is as high as 4% (Fig. 5h). 5.6. Coal in Australia Zhao et al.  conducted a research on the coal from the Great Northern seam in the Sydney Basin, Australia. They found that the coal in this area was affected by volcanic activities and low temperature hydrothermal ﬂuid injection. The minerals in this coal are mainly composed of tonsteins, quartz feldspar and other clay minerals and are mainly epigenetic origin. Therefore, a low intercept about 1% is indicated on the line (Fig. 5i).
Fig. 6. The average intercepts of Al2O3 + SiO2–Ash regression equations on the ash axis.
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difference of coal-forming vegetables and environments. As mentioned above, coal seams in Southern China occur mostly in Late Permian. During the period, frequent volcanic activity was associated with the development of synsedimentary faults in southwestern China; as a result, the volcanic ash in the coal-bearing strata occurs in the roof, ﬂoor and partings of the coal seams . Dai et al.  also identiﬁed the presence of volcanic-inﬂuenced materials in a coal seam from the Zhijin Coalﬁeld in western Guizhou, and suggested that the geochemical and mineralogical anomalies associated with the seam could be attributed to the synsedimentary volcanic ash. In conclusion, the above research data were obtained from the same coal seam or the coals deposited in the same peat swamp, and mostly have the ash yields higher than 15%. Therefore, as to medium–high ash coal, its original inorganic component in coalforming peat is about 2–5%, which could also provide a theoretical limitation of the coal ash by the physical coal cleaning. 6. Conclusion In order to study the ash limitation of physical coal beneﬁciation for middle–high ash coal, we collected samples from two coal mines, one from Guizhou province and one from Shanxi province. In order to make comparison, a survey on the existing publications on the coals from other locations in China, and from other countries in the world was conducted. The research result demonstrates that: (1) The same coal seam or the coals deposited in the same peat bog show a signiﬁcantly positive correlation between the ash yield and the Al2O3 + SiO2 content. (2) The intercept of Al2O3 + SiO2–Ash line on the ash axis is always less than 5% (generally 2–5%). Overall, the Chinese coals are characterized as a high intercept compared with coals in other countries. (3) The intercept of 2–5% on the ash axis indicates the original inorganic components in coal-forming peat which could provide a theoretical limitation of coal ash by coal cleaning, i.e. 2–5% of inorganic components could hardly be separated by traditional physical coal beneﬁciation for medium–high ash coal.
Acknowledgments This study was supported by National Key Basic Research and Development Program of China (Nos. 2012CB214901 and 2014CB238905), the National Natural Science Foundation of China (Nos. 41330638 and 41372168), A Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions, Program for National Excellent Doctoral Dissertation Award (No. 201165), and the Fundamental Research Funds for the Central Universities (Nos. 2013ZCX006, 2013XK06 and 2014ZDPY25). We are grateful to anonymous reviewers for their valuable comments on the manuscript. References  Dai S, Ren D, Chou C-L, Finkelman RB, Seredin VV, Zhou Y. Geochemistry of trace elements in Chinese coals: a review of abundances, genetic types, impacts on human health, and industrial utilization. Int J Coal Geol 2012;94:3–21.  Dai GS, Ulgiati S, Zhang YS, Yu BH, Kang MY, Jin Y, et al. The false promises of coal exploitation: how mining affects herdsmen well-being in the grassland ecosystems of Inner Mongolia. Energy Policy 2014;67:146–53.  Ren A, Guo B, Du Z, Zhu Z, Gao J. Source identiﬁcation of inhalable particles in fog and haze. In: 3rd International conference on bioinformatics and biomedical engineering, vol. 1–11; 2009. p. 3641–4.
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