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The characterization of coal quality from the Jining coalfield
Energy 30 (2005) 1903–1914 www.elsevier.com/locate/energy
The characterization of coal quality from the Jining coalfield Guijian Liua,b,*, Liugen Zhenga, Lianfen Gaoa, Haoyuan Zhanga, Zicheng Penga,b a
Department of Earth and Space Science, University of Science and Technology of China, Hefei, Anhui 230026, People’s Republic of China b Key Laboratory of Loess and Quaternary Geology, Institute of Earth and Environment, CAS, Xi’an, Shanxi 710075, People’s Republic of China Received 2 June 2003
Abstract The Jining coalfield in the west Shandong Province contains coal of Permian and Carboniferous age. The 31 and 32 seams of the Permian Shanxi Formation and seams 6, 10, 15, 16 and 17 of the Carboniferous Taiyuan Formation were analyzed for coal petrogrophy, mineralogy and geochemical parameters. The coal rank parameters indicate that the coal grade is a high volatile bituminous rank. The coal of the Taiyuan Formation is characterized by high vitrinite, low to medium inertinite and liptinite contents, lower ash yield and higher sulfur content than the Shanxi Formation. These properties may be related to the coal forming environment from more reducing conditions in a marine influenced lower delta plain environment for the early Taiyuan coals to more oxidizing paleoenvironmental conditions in an upper delta plain for the upper Shanxi coal seams. The major mineral phases present in the coal are quartz, kaolinite, pyrite and calcite. Sulfur is one of hazardous elements in coal. The major form of sulfur in coal is pyritic sulfur. The sulfur content of the Taiyuan coal seams is considerably higher than that of the Shanxi coals. The sulfur content is positively correlated with pyritic sulfur. q 2004 Elsevier Ltd. All rights reserved.
1. Introduction Coal is one of the most important sources of energy. Its worldwide use will continue to expand during the next several decades, particularly in rapidly developing China. Unfortunately, the use of coal brings with it several kind of environmental and health hazard problems. Some of these problems could be minimized or even avoided if coal quality information is available to decision makers. Coal quality information must be investigated in order to minimize environmental pollution during coal use. Coal quality studying mainly provide information about contents of ash, moisture, volatile matter, * Corresponding author. Tel.: C86 551 3603714; fax: C86 551 3621485. E-mail address: [email protected] (G. Liu). 0360-5442/$ - see front matter q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.energy.2004.09.003
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Fig. 1. Location of Jining coalfield in Shandong Province, China.
minerals, C, H, N, S, trace elements, ash composition and calorific value, among others. This data in turn will provide information on the technological performance of coal. This information may also have value in estimating atmospheric emissions of sulfur, carbon and nitrogen and the contribution of these elements to environmental problems, such as acid rain and global climate change [1]. A large coal resource occurs in the Jining coalfield in Shandong Province, China (Fig. 1). The coal resource is concentrated in six working underground coalmines and five exploration areas for which additional mines are being designed and built in the Jining coalfield. The annual production of coal of this area is estimated to be about 16 Mt for 2002. The larger mines in the coalfield are the Jining-2 mine (with annual production is about 5 Mt), the Jining-3 mine (with annual production is about 4 Mt), the Xuchang mine (with annual coal production is about 3 Mt), and the Tangkou exploration area where a mine is being built (Fig. 2). Coal is one of the most important resource in Eastern China and mainly used for power generation with a production of coke in minor quantity. 2. Geological setting of the Jining coalfield The major coal beds of the Jining mining district occur in the Upper Carboniferous Taiyuan Formation and the Lower Permian Shanxi Formation (Fig. 3). The coal-bearing formations are underlain by the Middle Carboniferous Benxi Formation and overlain by the Upper Permian Xiashihezi Formation. These formations contain minor coal seams. The coal seams are numbered in ascending sequence from 17 to 1, with seams 17–4 in the Taiyuan Formation (Upper Carboniferous) and seams 3–1 in the Shanxi Formation (Lower Permian). The Taiyuan Formation ranges from 130 to 190 m in thickness and is composed primarily of sandstone, limestone, mudstone, and coal (Fig. 3). Out of a total of 20 coal seams, 2–4 are mineable throughout the area and 5–7 are mineable locally.
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Fig. 2. Location of the coal mines and the exploration areas in the Jining coalfield.
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Fig. 3. Stratigraphic section of the upper Carboniferious Taiyuan Formation and the lower Permian Shanxi Formation in the Jining coafield.
The Shanxi Formation consists of sandstone, mudstone and coal (Fig. 3). Its total thickness ranges from 50 to 120 m. The formation contains one to three coal seams, with a cumulative workable coal thickness of 3–8 m. Of these seams, only coal seam 3 of the Shanxi Formation is mined in the Yanzhou mining district. Coal seam 3 is divided into two sub-seams, referred as coal seams 31 and 32. Coal rank in western Shandong is mainly high volatile bituminous, although medium volatile bituminous coal occurs in the deeper areas of the coalfield.
3. Sampling The samples were collected for analysis from the coal seams 31, 32, 6, 10, 15, 16 and 17 (See Fig. 3). Some of them were sampled from working underground coal mine, and few from the Tangkou
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exploration area. Samples consist primarily of full bed core and channel samples from borehole cores. The samples number of the coal seams 31, 32, 6, 10, 15, 16 and 17 are 72, 13, 48, 3, 20, 28 and 29, respectively.
4. Methods The bulk coal samples were air-dried, milled and split until a representative 0.5 kg sample milled to pass through a 0.250 mm sieve, was obtained for mineralogical, proximate, ultimate, and chemical analyses. Other splits were made at different size fractions (!0.20 mm) for petrological studies. Proximate and ultimate analyses were performed following ASTM [28] standard procedures. Ashes and coals were subjected to mineralogical using X-ray diffraction (XRD) and scanning electron microscopy (SEM). Mineral identification was carried out by electron microscope. Maceral composition was determined and random vitrinite reflectance was measured on telovitrinite using Leitz MPV-compact and MPV-3 microphotometers. The petrographic and chemical analysis results are listed in Tables 1–3.
5. Results and discussion Table 1 shows the results of the proximate and ultimate analyses of the coal seams. Maceral and mineral matter contents of the profile samples and average values of the some coal seams as determined by coal petrographic study are listed in Table 2. The variations are graphically shown in Fig. 4. 5.1. Moisture Moisture in coal is one of the most basic parameters which can evaluate coal economic value and effect coal using. During coal combustion, the moisture absorbs heat and turns it into vapor. When coal are processed, stockpiled and translated, lower moisture content in coal is good thing. According to Table 1, the moisture in the coal seams range from 0.96 to 3.41 wt%, the max value is 3.41 wt% in the coal seam 15. The average values of the main workable coal seams 31, 32, 6, 10, 15, 16 and 17 are 2.49, 2.54, 2.27, 2.09, 2.15, 2.22 and 2.14 wt%, respectively. The moisture content of the coals in the Shanxi Formation (seam 3) and Taiyuan Formation (seams 6–17) is less than 3% (Tables 1 and 2). The moisture in the Shanxi Formation coal seams is slightly higher than the Taiyuan Formation coals (see Table 1). 5.2. Calorific value According to Table 1, the calorific value in the coal seams ranges from 20.97 to 32.92 MJ/kg. The average values of the studied coal seams 31, 32, 6, 10, 15, 16 and 17 are 27.61, 26.71, 29.00, 25.91, 28.47, 29.43 and 29.03 MJ/kg, respectively. The effect factors on calorific value are organic content, moisture, minerals degree of coal rank and petrological composition. In the study area, because of low moisture, the calorific value is relatively
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Table 1 The main parameters of coal quality from the coal seams in Jining coalfield Items
31
32
6
10
Min
Max
Ave
Num
Min
Max
Ave
Num
Min
Max
Ave
Num
Min
Max
Ave
Mad (%) A d (%) V daf (%) S t,d (%) P d (%) Q b,ad (MJ/Kg) C daf (%) H daf (%) N daf (%)
72 72 72 72 42 65
1.69 10.08 36.07 0.22 0.002 22.96
3.27 21.93 40.76 0.8 0.053 30.09
2.49 14.63 38.57 0.55 0.0162 27.61
12 12 12 12 10 13
1.95 11.51 38.28 0.44 0.004 20.97
2.91 23.51 41.93 0.74 0.022 29.42
2.54 16.69 39.72 0.64 0.0095 26.71
47 48 48 47 37 45
1.6 1.52 40.97 1.66 0.003 26.25
2.98 22.55 47.27 5.5 0.036 31.17
2.27 13.85 43.29 2.92 0.0141 29
3 3 3 3 3 3
1.86 17.66 45.17 3.71 0.013 23.53
2.26 26.79 46.64 5.96 0.016 27.34
2.09 21.34 46.04 4.72 0.015 25.91
46 46 46
81.84 5.12 1.43
83.89 6.11 1.63
83.06 5.47 1.54
9 9 9
81.50 5.22 1.53
83.77 5.72 1.76
82.73 5.45 1.60
35 35 35
81.19 5.29 1.43
83.72 7.40 1.73
82.34 5.80 1.58
2 2 2
80.95 5.22 1.51
82.14 5.88 1.55
81.55 5.55 1.53
Items
15
Mad (%) A d (%) V daf (%) S t,d (%) P d (%) Q b,ad (MJ/Kg) C daf (%) H daf (%) N daf (%)
16
Num
Min
20 19 19 20 10 18 14 14 14
Max
Min
Max
0.96 3.13 2.15 27 10.67 24.67 14.66 28 41.94 50.12 45.27 28 2.59 6.15 3.87 27 0.003 0.011 0.0053 17 26.56 30.07 28.47 26
1.63 6.5 42.05 2.98 0 25.38
79.98 5.37 1.33
80.23 4.71 1.28
82.26 6.06 1.63
Ave
81.19 5.68 1.52
Num
17
24 24 24
Ave
Min
Max
Ave
3.41 2.22 29 23.2 12.54 28 46.39 43.94 27 6.74 4.02 28 0.006 0.0032 13 32.92 29.43 24
1.23 5.32 43.34 2.67 0.003 24.94
2.88 21.18 46.97 6.66 0.045 32.19
2.14 13.94 44.63 4.26 0.0096 29.03
83.49 6.02 1.54
79.64 5.10 1.28
83.92 6.08 1.53
82.45 5.66 1.41
82.06 5.51 1.38
Num
17 17 17
Notes: Num, Number of the samples; Min, Minimum value; Max, Maximum value; Ave, Average value.
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Table 2 Maceral and mineral matter contents of profile samples as determined by coal petrology analysis in vol.% Samples
Vitrinite
Semiintertinite
vitrinite
Semiintertinite
Liptinite
Mineral matter (%)
The value of the some samples from the vertical profile in the same drilling borehole 31K1 top 60.62 12.51 1.03 10.65 8.59 31–2 54.69 12.62 1.07 13.78 7.34 31–3 48.39 13.53 2.97 20.02 7.79 31–4 bottom 51.50 13.81 2.05 14.79 6.95 16 Top 75.34 1.30 1.95 5.48 6.60 16 Bottom 71.72 2.05 2.23 5.26 7.84 17 Top 72.48 1.61 2.06 8.70 3.95 17 Bottom 70.54 5.06 4.57 11.97 4.87 Average values for the Jining coalfield 31 52.3 10.0 4.2 16.5 8.8 32 53.4 6.5 7.4 12.3 13.7 6 55.0 6.4 5.6 11.9 15.3 15 75.2 3.4 3.1 8.9 4.5 16 74.3 4.0 2.8 7.3 4.0 17 74.6 3.2 3.7 6.2 2.4
Pyrite
Clay
Carbonate
0.80 0.41 0.75 1.30 1.70 4.41 2.60 0.71
0.31 0.44 0.73 1.61 2.90 1.80 0.60 0.30
5.50 9.32 5.21 7.70 1.10 2.90 3.90 1.70
0.3 0.1 2.0 1.7 1.7 2.3
6.4 5.6 2.4 0.8 2.4 2.8
1.0 0.6 1.1 1.9 2.0 1.9
high, and the calorific value of the coals in the Shanxi Formation is slightly lower than the Taiyuan Formation (see Table 1). 5.3. Ash yield and composition The ash yield of coal is the residue derived from the inorganic and organic matter during incineration and combustion of the coal Yield, content and geochemical characterization were dependent on coal quality and the conditions of coal formation, which is further controlled by the temperature and manners of coal combustion. The chemical composition of coal ashes varies widely depending on the mineral and organic constituents associated with coals of the area. The experimental studies reported that most of the trace and minor elements in coal except K, Mg, Na and halogens do not volatilize during coal combustion [2–5]. They constitute the matrix of ash in the form of a homogeneous melt and crystalline, formed through a series of physic-chemical processes. Table 3 Ash composition of the coal seams in the Jining coalfield Coal seams
SiO2 (%)
Al2O3 (%)
Fe2O3 (%)
CaO (%)
MgO (%)
SO3 (%)
TiO2 (%)
K2O (%)
Na2O (%)
MnO2 (%)
P2O5 (%)
31 32 6 10 15 16 17
45.95 46.60 39.12 38.89 26.94 17.63 19.78
35.18 35.82 24.86 21.68 23.25 17.00 16.10
4.52 3.94 23.51 23.53 4.86 32.49 34.88
8.39 7.89 6.31 7.70 34.67 19.38 14.99
0.68 1.11 0.86 1.42 1.31 2.61 2.08
1.60 1.58 3.05 3.95 4.14 9.11 10.465
1.58 1.74 1.01 1.03 0.93 0.80 0.48
0.35 0.56 0.36 0.05 0.23 0.71 0.74
0.50 0.48 0.45 0.40 0.81 0.14 0.37
0.07 0.08 0.14 0.29 0.18 0.06 0.05
0.16 0.19 0.33 1.06 0.75 0.07 0.07
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Fig. 4. Vertical variation in proximate and ultimate analysis properties of the workable coal seams from the Tangkou Exploration area.
The formation of ash particles in power plant greatly influences the heat and mass transfers in a coalfired boiler and the particles emitted from combustion, become a hazardous source to human health and environment [2,5–7]. According to the samples collected from the coal seams 31, 32, 6, 10, 15,16 and 17 in the Jining coalfield, the yield of ashes are 10.08–21.93, 11.51–23.51, 7.52–22.55, 17.66–26.79, 10.67–24.67, 6.5–23.20 and 5.32–21.18 wt%, the averaged by 14.63, 16.69, 13.85, 21.34, 14.66, 12.54 and 13.94 wt%, respectively. In general, the upper coal seams in the Shanxi Formation have a relatively higher ash yield than that in the Taiyuan Formation coals (see Table 1). A mineralogical analysis by using semi-quantitative X-ray diffraction and electron microscope gives the major composition of ashes. The ashes are mixture of various kinds of particles, including noncrystalline material, crystal materials formed during the course of combustion, leftover minerals and unburnt coal found in ashes. Their characteristics depended on the composition of the each kind of particle. It is commonly known that ashes are built up of inorganic and organic matters. In ashes, inorganic matter mainly come from minerals originally contained in coal and those formed during combustion in coal. During coal combustion, inorganic matters in ashes inherited the minerals from coal. Organic matter in ash is chiefly unburn coal particles during coal combustion. In ashes, crystalline matters formed through a series of physico-chemical processes such as fusion or partial melting of discrete mineral matter, coalescence of melted mineral in coal burning processes. Normally under microscope and X-ray, the major minerals identified in crystalline matter of the ash samples examined are silicate glass, quartz, magnetite feldspars, anhydrite, hematite and lime.
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The glassy matters in ashes, made up of spheres and microspheres of Fe, Mg, Si, Al oxide, are production from part of minerals unfinished crystallizing during coal combustion. They can be identified by SEM. The glassy matters is much lower than crystalline matters. According to its chemical composition, the ashes belong to CaO–Fe2O3–Al2O3–SiO2 system. The composition of studied ash samples is similar to that in the Zibo coalfield [8]. The chemical composition usually fluctuates much due to the different mineral components contained in the coals. The results of the analysis showed that the content of the main minerals in coal has a positive correlationship with the chemical and mineral composition of ashes. The coal ashes consist mainly of SiO2 and Al2O3, with CaO, Fe2O3 and SO3, and minor proportions of MgO, TiO2 and other oxides. SiO2 and Al2O3 make up 42.5–82.8% of the total ash. SiO2 and Al2O3 (Table 3) are more abundant in the Shanxi Formation coal samples than the Taiyuan coals, because of the presence of more definite minerals (especially clay minerals) in the Shanxi coals. The Taiyuan coals contain much higher SO3 than Shanxi coals, due to its high sulfur contents. 5.4. Mineral matter The major mineral phases as identified by XRD and SEM, is quartz, kaolinite, pyrite and calcite. However, traces of other minerals like dolomite, ankerite, illite, opal, feldspar and marcasite, and weathering products such as gypsum, and melanterite are also observed in several coals. Hematite in ash was also detected using electron and optical microscopy. Pyrite is more abundant in the coals of the lower Taiyuan Formation than in those of the Shanxi Formation, reflecting the marine influence on the depositional environment. Pyrite is the major sulphide mineral, but marcasite is also detected. Pyrite in the coals occurs as typical syngenetic framboidal, euhedral and massive cell-filling forms. Carbonate minerals like calcite and dolomite occur as the dominant carbonate species in the coal seams. Because the roof of most of the Taiyuan coal seams is limestone, especially seam 16, the occurrence of calcite in these coal seams is fiequent. Hence, the calcite content of the Taiyuan coal seams is higher than that of the Shanxi coals. Clay is the main inorganic mineral matter and are only inorganic constituent in the studied coal. In the study area, the deposition environments of the Shanxi Formation are fluvial [9–11]. Many kaolinite, illite and other clay minerals major comes from weathering and oxidation products in Shanxi coal seams. However, the Taiyuan coal seams are deposited in marine environment, so clay mineral, which is present at trace levels in the coals studied, is lower than that of the Shanxi coals. 5.5. Sulfur The major forms of sulfur in coal are pyretic sulfur, organic sulfur and sulfate sulfur. Pyritic and organic sulfur generally account for the bulk of sulfur in coal. Elemental sulfur also occurs in coal, but only in trace to minor amounts and it is not determined in routine coal analysis. During combustion, almost all organic sulfur, elemental sulfur, most pyritic sulfur and part of sulfate sulfur in coal are emitted into the atmosphere as aerosols and gaseous products of flue gas emissions [12,13]. But the pyritic sulfur and the sulfur from gypsum during combustion can form anhydyite (CaSO4) with CaO from calcite, Fe from pyrite with oxygen form hematite. Bassanite
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may also be formed from gypsum. SOx emissions may have a noxious effect on air [13,14], water [15– 18] and living organisms, including humans [1,19,20]. Several authors suggest that minor elements associated to organic and sulfide fraction also gets volatilized when coal is burnt [12,13]. The sulfur content of the coal seams of the Shanxi Formation is ranging from 0.22 to 0.74 wt%. And all sulfur content of the Shanxi Formation coal seams is less than 1 wt%. Consequently this is a low sulfur coal. But the sulfur content in the coal seam 6, 10, 15, 16 and 17 range from 1.66 to 5.50, 3.71–5.96, 2.59–6.15, 2.98–6.75 and 2.67–6.66 wt%, the average values are 2.92, 4.72, 3.87, 4.02 and 4.47 wt%, respectively. All the sulfur content from samples of the Taiyuan Formation coal seams is always more than 1.6 wt%, and the sulfur in many samples of the Taiyuan Formation coal seams are more than 3 wt%. The sulfur contents in the Taiyuan coals are much higher than in the Shanxi coals because of pyritic sulfur content. Tables 1 and 2 show pyritic sulfur is correlated closely with sulfur, and pyrite is present at very low concentrations in the Lower Permian coals (Shanxi Formation coals) (Table 2). However, pyrite concentrations in the Carboniferous coal seams (Taiyuan Formation) are much higher (Table 2). 5.6. Volatile matter In the high volatile bituminous coals (Fig. 4), vitrinite reflectance ranging from 0.6 to 0.8%, however, vitrinite reflectance shows a downward decrease from the Shanxi coals to the Taiyuan coals. This may be due to impregnation of the vitrinite by hydrocarbons [2,21–23]. However, it may also reflect marine influence on the coals in the lower part of the section [24,25]. The reverse trend is apparent for volatile matter, which is higher (O43% on a dry ash-free basis) in the Taiyuan Formation coal seams (6–17) than that in the Shanxi Formation coal seam (!40% daf). Such an inverse trend is impressive because coal samples from the Shanxi and Taiyuan Formations were taken from the same vertical profile (Table 2). However, the benches within seam 3 show significant variation, and the lowest volatile matter content is observed in coal seam 3. 5.7. Macerals The coal in the Tankou exploration area is characterized by high vitrinite and low to medium inertinite and liptinite contents (Table 2). The major macerals of the vitrinite group are telinite, telocollinite and desmocollinite. The coal seams of the Shanxi Formation contain mainly desmocollinite, with secondary telinite and minor telocollinite, while the coals of the Taiyuan have maceral composition characterized by telocolliniteOdesmocolliniteOtelinite. The vitrinite content of the Taiyuan Formation coals is higher than that of the Shanxi Formation coal seams, but the contents of intertinite and liptinite in the Shanxi Formation coal seams are higher than in the Taiyuan Formation coals. Based on the results of this work and previous studies [2,26,27], the telinite content is higher at the bottom than at the top of the coal seams. The top of coal seams 16 and 17 is characterized by a lower telinite content and higher telocollinite and desmocollinite concentrations compared to the bottom parts. The proportions of vitrinite group macerals in coal seam 16 do not show any significant variation [26]. As with desmocollinite, inertinite macerals are enriched at the top of coal seam 3, compared to the bottom of the seam. Both coal seams 3 and 16 from the Tangkou exploration area show a high inertinite content. The proportion of liptinite macerals is relatively low. The major liptinite macerals are sporinite,
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cutinite, and suberinite. The progressive increase in the inertinite content from the lower to the upper coal seams could be attributed to the presence of more reducing conditions of the sedimentary environments for the Taiyuan coals and more oxidizing paleoenvironments for the Shanxi seams. A similar evolution could account for the vertical increase in inertinite abundance from bottom to top of coal seam 3. The stratigraphic trends are very consistent, with evidence of a lower delta plain in the basal Taiyuan Formation, with marine influence, high subsidence, and probably more reducing environmental conditions, and a progressive upwards gradation to a more proximal upper delta plain environment in the Shanxi Formation.
6. Conclusions (1) The moisture in the Shanxi Formation coal seams is slightly higher than that in the Taiyuan Formation coals. Because of the above, the calorific value of the coals in the Shanxi Formation is slightly less than that in the Taiyuan Formation. (2) The coal of the study area is of high volatile bituminous rank. Petrographic studies indicate a downward decrease in vitrinite reflectance values from the Shanxi coals to the Taiyuan coals, and an inverse trend for volatile matter content, which is higher in the Taiyuan coal than in the Shanxi coal seams. This reflects a greater aboundance of vitrinite and a suppression of vitrinite reflectance associated with the marine influence. (3) The Shanxi coal seams have slightly higher ash yield than the Taiyuan coal seams. The major composition of ashes consist of crystallize (mineral matter), with minor glassy and organic matter. The major composition of ashes are SiO2, Al2O3, with Fe2O3, CaO and SO3, little content of P2O5, Na2O, K2O and TiO2. The minerals in the coal seams are chiefly quartz, kaolinite, pyrite, and calcite. (4) The sulfur content of the Shanxi Formation coal seams is considerably low, and the sulfur content of the Taiyuan coal seams is considerably higher.
Acknowledgements This work was supported by National Natural Science Foundation of China (40133010). We thank reviewers and editors for giving us many constructive comments.
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[5] Liu GJ, Peng ZC, Yang PY. Characteristics of coal ashes in Yanzhou mining district and distribution of trace elements in them. Chin J Geochem 2001;20(4):357–67. [6] Liu GJ, Yang PY. The advance of environmental study on trace and minor elements in coal. Coal Mine Environ Prot 1999; 13(5):17–19 (in Chinese with English abstract). [7] Yan R, Gauthier D, Flamant G, Badie JM. Thermodynamic study of the behavior of minor coal elements and their affinities to sulfur during coal combustion. Fuel 1999;78:1817–29. [8] Liu GJ, Yang PY, Peng ZC. Comparative study of the quality of some coals from the Zibo coalfield. Energy 2003;28(1): 969–78. [9] Liu GJ, Hou CX, Liang TQ. Sedimentary environments and its development of Shaxi Formation of the Liangbaosi area. Coal Geol Explorat 1995;23(4):14–17 (in Chinese with English abstract). [10] Liu GJ, Yang PY, Peng ZC, Chou CL. Petrographical and geochemical contrasts and environmentally significant trace elements in marine-influenced coal seams, Yanzhou mining area, China. J Asian Earth Sci 2004;23(4):491–506. [11] Liu GJ, Yang PY. Geochemistry of trace elements in Jining coalfield. Geol Geochem 1999;27(4):77–82 (in Chinese with English abstract). [12] Gayer RA, Rose M, Dehmer J, Shao LY. Impact of sulphur and trace element geochemistry on the utilization of a marineinfluenced coal—case study from the South Wales Variscan foreland basin. Int J Coal Geol 1999;40(2–3):151–74. [13] Liu GJ, Peng ZC, Yang PY. Sulfur in coal and its environmental impact from Yanzhou mining district, China. Chin J Geochem 2001;20(3):273–81. [14] Block C, Dams R. Inorganic composition of Belgian coals and coal ashes. Environ Sci Technol 1975;9:147–50. [15] Gupta DC. Environmental aspects of selected trace elements associated with coal and natural waters of Pench Vally coalfield of India and their impact on human health. Int J Coal Geol 1999;40:133–49. [16] Finkelman RB, Palmer CA, Krasnow MR. Combustion and leaching behavior of elements in the Argonne premium coal samples. Energy Fuels 1990;4:755–66. [17] Liu GJ. Experiment of coal leaching and study of the separation of trace elements. Acta Geol Sin 2000;74(2):386–90. [18] Wang YQ, Ren DY, Zhao FH. Comparative leaching experiments for trace elements in raw coal, laboratory ash, fly ash and bottom ash. Int J Coal Geol 1999;40:103–8. [19] Fyfe WS. Clean energy for 10 billion human in the 21st century: is it possible? Int J Coal Geol 1999;40:85–90. [20] Zheng BS, Ding ZH, Huang RG. Issues of health and disease relating to coal use in southwestern China. Int J Coal Geol 1999;40:119–32. [21] Goodarzi F, Davies GR, Nassichuk WW, Snowdon LR. Organic petrology and rock eval analysis of the lower carboniferous Emma Fiord formation in Sverdrup basin, Canadian Arctic archipelago. Mar Petroleum Geol 1987;10(4): 383–98. [22] Gentzis T, Goodarzi F. Reflectance suppression in some Cretaceous coals from Alberta, Canada. In: Mukhopadhyay PK, Dow WG, editors. Vitrinite reflectance as a maturity parameter applications and limitations. ACS symposium series, vol. 6, 1994. p. 93–111. [23] Suarez-Ruiz I, Jimenez I, Iglesias MJ, Laggoun F, Prado JG. Influence of the resinite on huminite properties. Energy Fuels 1994;8(6):1417–24. [24] Gurba LW, Ward CR. Elemental composition of macerals in relation to vitrinite reflectance, Gunnedah Basin, Australia, as determined by electron microprobe analysis. Int J Coal Geol 2000;44:127–47. [25] Gurba LW, Ward CR. Vitrinite reflectance anomalies in the high volatile bituminous coals of the Gunnedah Basin, New South Wales, Australia. Int J Coal Geol 1998;36:111–40. [26] Chen P, Wang JY. Organic sulfur and its sistribution in macerals of Yanzhou coal. J Fuel Chem Technol 1993;21(3): 298–302. [27] Querol X, Alastuey A, Lopez-Soler A, Plana F, Zeng RS, Zhao J, Zhuang XG. Geological controls on the quality of coals from the West Shandong mining district, Eastern China. Int J Coal Geol 1999;42(1):63–88. [28] American Society for Testing and Materials. Annual book of ASTM standards. Section 5: petroleum products, lubricants, and fossil fules. Gaseous fuels: coal and coke; 1992.5.05.
The characterization of coal quality from the Jining coalfield Guijian Liua,b,*, Liugen Zhenga, Lianfen Gaoa, Haoyuan Zhanga, Zicheng Penga,b a
Department of Earth and Space Science, University of Science and Technology of China, Hefei, Anhui 230026, People’s Republic of China b Key Laboratory of Loess and Quaternary Geology, Institute of Earth and Environment, CAS, Xi’an, Shanxi 710075, People’s Republic of China Received 2 June 2003
Abstract The Jining coalfield in the west Shandong Province contains coal of Permian and Carboniferous age. The 31 and 32 seams of the Permian Shanxi Formation and seams 6, 10, 15, 16 and 17 of the Carboniferous Taiyuan Formation were analyzed for coal petrogrophy, mineralogy and geochemical parameters. The coal rank parameters indicate that the coal grade is a high volatile bituminous rank. The coal of the Taiyuan Formation is characterized by high vitrinite, low to medium inertinite and liptinite contents, lower ash yield and higher sulfur content than the Shanxi Formation. These properties may be related to the coal forming environment from more reducing conditions in a marine influenced lower delta plain environment for the early Taiyuan coals to more oxidizing paleoenvironmental conditions in an upper delta plain for the upper Shanxi coal seams. The major mineral phases present in the coal are quartz, kaolinite, pyrite and calcite. Sulfur is one of hazardous elements in coal. The major form of sulfur in coal is pyritic sulfur. The sulfur content of the Taiyuan coal seams is considerably higher than that of the Shanxi coals. The sulfur content is positively correlated with pyritic sulfur. q 2004 Elsevier Ltd. All rights reserved.
1. Introduction Coal is one of the most important sources of energy. Its worldwide use will continue to expand during the next several decades, particularly in rapidly developing China. Unfortunately, the use of coal brings with it several kind of environmental and health hazard problems. Some of these problems could be minimized or even avoided if coal quality information is available to decision makers. Coal quality information must be investigated in order to minimize environmental pollution during coal use. Coal quality studying mainly provide information about contents of ash, moisture, volatile matter, * Corresponding author. Tel.: C86 551 3603714; fax: C86 551 3621485. E-mail address: [email protected] (G. Liu). 0360-5442/$ - see front matter q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.energy.2004.09.003
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Fig. 1. Location of Jining coalfield in Shandong Province, China.
minerals, C, H, N, S, trace elements, ash composition and calorific value, among others. This data in turn will provide information on the technological performance of coal. This information may also have value in estimating atmospheric emissions of sulfur, carbon and nitrogen and the contribution of these elements to environmental problems, such as acid rain and global climate change [1]. A large coal resource occurs in the Jining coalfield in Shandong Province, China (Fig. 1). The coal resource is concentrated in six working underground coalmines and five exploration areas for which additional mines are being designed and built in the Jining coalfield. The annual production of coal of this area is estimated to be about 16 Mt for 2002. The larger mines in the coalfield are the Jining-2 mine (with annual production is about 5 Mt), the Jining-3 mine (with annual production is about 4 Mt), the Xuchang mine (with annual coal production is about 3 Mt), and the Tangkou exploration area where a mine is being built (Fig. 2). Coal is one of the most important resource in Eastern China and mainly used for power generation with a production of coke in minor quantity. 2. Geological setting of the Jining coalfield The major coal beds of the Jining mining district occur in the Upper Carboniferous Taiyuan Formation and the Lower Permian Shanxi Formation (Fig. 3). The coal-bearing formations are underlain by the Middle Carboniferous Benxi Formation and overlain by the Upper Permian Xiashihezi Formation. These formations contain minor coal seams. The coal seams are numbered in ascending sequence from 17 to 1, with seams 17–4 in the Taiyuan Formation (Upper Carboniferous) and seams 3–1 in the Shanxi Formation (Lower Permian). The Taiyuan Formation ranges from 130 to 190 m in thickness and is composed primarily of sandstone, limestone, mudstone, and coal (Fig. 3). Out of a total of 20 coal seams, 2–4 are mineable throughout the area and 5–7 are mineable locally.
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Fig. 2. Location of the coal mines and the exploration areas in the Jining coalfield.
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Fig. 3. Stratigraphic section of the upper Carboniferious Taiyuan Formation and the lower Permian Shanxi Formation in the Jining coafield.
The Shanxi Formation consists of sandstone, mudstone and coal (Fig. 3). Its total thickness ranges from 50 to 120 m. The formation contains one to three coal seams, with a cumulative workable coal thickness of 3–8 m. Of these seams, only coal seam 3 of the Shanxi Formation is mined in the Yanzhou mining district. Coal seam 3 is divided into two sub-seams, referred as coal seams 31 and 32. Coal rank in western Shandong is mainly high volatile bituminous, although medium volatile bituminous coal occurs in the deeper areas of the coalfield.
3. Sampling The samples were collected for analysis from the coal seams 31, 32, 6, 10, 15, 16 and 17 (See Fig. 3). Some of them were sampled from working underground coal mine, and few from the Tangkou
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exploration area. Samples consist primarily of full bed core and channel samples from borehole cores. The samples number of the coal seams 31, 32, 6, 10, 15, 16 and 17 are 72, 13, 48, 3, 20, 28 and 29, respectively.
4. Methods The bulk coal samples were air-dried, milled and split until a representative 0.5 kg sample milled to pass through a 0.250 mm sieve, was obtained for mineralogical, proximate, ultimate, and chemical analyses. Other splits were made at different size fractions (!0.20 mm) for petrological studies. Proximate and ultimate analyses were performed following ASTM [28] standard procedures. Ashes and coals were subjected to mineralogical using X-ray diffraction (XRD) and scanning electron microscopy (SEM). Mineral identification was carried out by electron microscope. Maceral composition was determined and random vitrinite reflectance was measured on telovitrinite using Leitz MPV-compact and MPV-3 microphotometers. The petrographic and chemical analysis results are listed in Tables 1–3.
5. Results and discussion Table 1 shows the results of the proximate and ultimate analyses of the coal seams. Maceral and mineral matter contents of the profile samples and average values of the some coal seams as determined by coal petrographic study are listed in Table 2. The variations are graphically shown in Fig. 4. 5.1. Moisture Moisture in coal is one of the most basic parameters which can evaluate coal economic value and effect coal using. During coal combustion, the moisture absorbs heat and turns it into vapor. When coal are processed, stockpiled and translated, lower moisture content in coal is good thing. According to Table 1, the moisture in the coal seams range from 0.96 to 3.41 wt%, the max value is 3.41 wt% in the coal seam 15. The average values of the main workable coal seams 31, 32, 6, 10, 15, 16 and 17 are 2.49, 2.54, 2.27, 2.09, 2.15, 2.22 and 2.14 wt%, respectively. The moisture content of the coals in the Shanxi Formation (seam 3) and Taiyuan Formation (seams 6–17) is less than 3% (Tables 1 and 2). The moisture in the Shanxi Formation coal seams is slightly higher than the Taiyuan Formation coals (see Table 1). 5.2. Calorific value According to Table 1, the calorific value in the coal seams ranges from 20.97 to 32.92 MJ/kg. The average values of the studied coal seams 31, 32, 6, 10, 15, 16 and 17 are 27.61, 26.71, 29.00, 25.91, 28.47, 29.43 and 29.03 MJ/kg, respectively. The effect factors on calorific value are organic content, moisture, minerals degree of coal rank and petrological composition. In the study area, because of low moisture, the calorific value is relatively
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Table 1 The main parameters of coal quality from the coal seams in Jining coalfield Items
31
32
6
10
Min
Max
Ave
Num
Min
Max
Ave
Num
Min
Max
Ave
Num
Min
Max
Ave
Mad (%) A d (%) V daf (%) S t,d (%) P d (%) Q b,ad (MJ/Kg) C daf (%) H daf (%) N daf (%)
72 72 72 72 42 65
1.69 10.08 36.07 0.22 0.002 22.96
3.27 21.93 40.76 0.8 0.053 30.09
2.49 14.63 38.57 0.55 0.0162 27.61
12 12 12 12 10 13
1.95 11.51 38.28 0.44 0.004 20.97
2.91 23.51 41.93 0.74 0.022 29.42
2.54 16.69 39.72 0.64 0.0095 26.71
47 48 48 47 37 45
1.6 1.52 40.97 1.66 0.003 26.25
2.98 22.55 47.27 5.5 0.036 31.17
2.27 13.85 43.29 2.92 0.0141 29
3 3 3 3 3 3
1.86 17.66 45.17 3.71 0.013 23.53
2.26 26.79 46.64 5.96 0.016 27.34
2.09 21.34 46.04 4.72 0.015 25.91
46 46 46
81.84 5.12 1.43
83.89 6.11 1.63
83.06 5.47 1.54
9 9 9
81.50 5.22 1.53
83.77 5.72 1.76
82.73 5.45 1.60
35 35 35
81.19 5.29 1.43
83.72 7.40 1.73
82.34 5.80 1.58
2 2 2
80.95 5.22 1.51
82.14 5.88 1.55
81.55 5.55 1.53
Items
15
Mad (%) A d (%) V daf (%) S t,d (%) P d (%) Q b,ad (MJ/Kg) C daf (%) H daf (%) N daf (%)
16
Num
Min
20 19 19 20 10 18 14 14 14
Max
Min
Max
0.96 3.13 2.15 27 10.67 24.67 14.66 28 41.94 50.12 45.27 28 2.59 6.15 3.87 27 0.003 0.011 0.0053 17 26.56 30.07 28.47 26
1.63 6.5 42.05 2.98 0 25.38
79.98 5.37 1.33
80.23 4.71 1.28
82.26 6.06 1.63
Ave
81.19 5.68 1.52
Num
17
24 24 24
Ave
Min
Max
Ave
3.41 2.22 29 23.2 12.54 28 46.39 43.94 27 6.74 4.02 28 0.006 0.0032 13 32.92 29.43 24
1.23 5.32 43.34 2.67 0.003 24.94
2.88 21.18 46.97 6.66 0.045 32.19
2.14 13.94 44.63 4.26 0.0096 29.03
83.49 6.02 1.54
79.64 5.10 1.28
83.92 6.08 1.53
82.45 5.66 1.41
82.06 5.51 1.38
Num
17 17 17
Notes: Num, Number of the samples; Min, Minimum value; Max, Maximum value; Ave, Average value.
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Table 2 Maceral and mineral matter contents of profile samples as determined by coal petrology analysis in vol.% Samples
Vitrinite
Semiintertinite
vitrinite
Semiintertinite
Liptinite
Mineral matter (%)
The value of the some samples from the vertical profile in the same drilling borehole 31K1 top 60.62 12.51 1.03 10.65 8.59 31–2 54.69 12.62 1.07 13.78 7.34 31–3 48.39 13.53 2.97 20.02 7.79 31–4 bottom 51.50 13.81 2.05 14.79 6.95 16 Top 75.34 1.30 1.95 5.48 6.60 16 Bottom 71.72 2.05 2.23 5.26 7.84 17 Top 72.48 1.61 2.06 8.70 3.95 17 Bottom 70.54 5.06 4.57 11.97 4.87 Average values for the Jining coalfield 31 52.3 10.0 4.2 16.5 8.8 32 53.4 6.5 7.4 12.3 13.7 6 55.0 6.4 5.6 11.9 15.3 15 75.2 3.4 3.1 8.9 4.5 16 74.3 4.0 2.8 7.3 4.0 17 74.6 3.2 3.7 6.2 2.4
Pyrite
Clay
Carbonate
0.80 0.41 0.75 1.30 1.70 4.41 2.60 0.71
0.31 0.44 0.73 1.61 2.90 1.80 0.60 0.30
5.50 9.32 5.21 7.70 1.10 2.90 3.90 1.70
0.3 0.1 2.0 1.7 1.7 2.3
6.4 5.6 2.4 0.8 2.4 2.8
1.0 0.6 1.1 1.9 2.0 1.9
high, and the calorific value of the coals in the Shanxi Formation is slightly lower than the Taiyuan Formation (see Table 1). 5.3. Ash yield and composition The ash yield of coal is the residue derived from the inorganic and organic matter during incineration and combustion of the coal Yield, content and geochemical characterization were dependent on coal quality and the conditions of coal formation, which is further controlled by the temperature and manners of coal combustion. The chemical composition of coal ashes varies widely depending on the mineral and organic constituents associated with coals of the area. The experimental studies reported that most of the trace and minor elements in coal except K, Mg, Na and halogens do not volatilize during coal combustion [2–5]. They constitute the matrix of ash in the form of a homogeneous melt and crystalline, formed through a series of physic-chemical processes. Table 3 Ash composition of the coal seams in the Jining coalfield Coal seams
SiO2 (%)
Al2O3 (%)
Fe2O3 (%)
CaO (%)
MgO (%)
SO3 (%)
TiO2 (%)
K2O (%)
Na2O (%)
MnO2 (%)
P2O5 (%)
31 32 6 10 15 16 17
45.95 46.60 39.12 38.89 26.94 17.63 19.78
35.18 35.82 24.86 21.68 23.25 17.00 16.10
4.52 3.94 23.51 23.53 4.86 32.49 34.88
8.39 7.89 6.31 7.70 34.67 19.38 14.99
0.68 1.11 0.86 1.42 1.31 2.61 2.08
1.60 1.58 3.05 3.95 4.14 9.11 10.465
1.58 1.74 1.01 1.03 0.93 0.80 0.48
0.35 0.56 0.36 0.05 0.23 0.71 0.74
0.50 0.48 0.45 0.40 0.81 0.14 0.37
0.07 0.08 0.14 0.29 0.18 0.06 0.05
0.16 0.19 0.33 1.06 0.75 0.07 0.07
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Fig. 4. Vertical variation in proximate and ultimate analysis properties of the workable coal seams from the Tangkou Exploration area.
The formation of ash particles in power plant greatly influences the heat and mass transfers in a coalfired boiler and the particles emitted from combustion, become a hazardous source to human health and environment [2,5–7]. According to the samples collected from the coal seams 31, 32, 6, 10, 15,16 and 17 in the Jining coalfield, the yield of ashes are 10.08–21.93, 11.51–23.51, 7.52–22.55, 17.66–26.79, 10.67–24.67, 6.5–23.20 and 5.32–21.18 wt%, the averaged by 14.63, 16.69, 13.85, 21.34, 14.66, 12.54 and 13.94 wt%, respectively. In general, the upper coal seams in the Shanxi Formation have a relatively higher ash yield than that in the Taiyuan Formation coals (see Table 1). A mineralogical analysis by using semi-quantitative X-ray diffraction and electron microscope gives the major composition of ashes. The ashes are mixture of various kinds of particles, including noncrystalline material, crystal materials formed during the course of combustion, leftover minerals and unburnt coal found in ashes. Their characteristics depended on the composition of the each kind of particle. It is commonly known that ashes are built up of inorganic and organic matters. In ashes, inorganic matter mainly come from minerals originally contained in coal and those formed during combustion in coal. During coal combustion, inorganic matters in ashes inherited the minerals from coal. Organic matter in ash is chiefly unburn coal particles during coal combustion. In ashes, crystalline matters formed through a series of physico-chemical processes such as fusion or partial melting of discrete mineral matter, coalescence of melted mineral in coal burning processes. Normally under microscope and X-ray, the major minerals identified in crystalline matter of the ash samples examined are silicate glass, quartz, magnetite feldspars, anhydrite, hematite and lime.
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The glassy matters in ashes, made up of spheres and microspheres of Fe, Mg, Si, Al oxide, are production from part of minerals unfinished crystallizing during coal combustion. They can be identified by SEM. The glassy matters is much lower than crystalline matters. According to its chemical composition, the ashes belong to CaO–Fe2O3–Al2O3–SiO2 system. The composition of studied ash samples is similar to that in the Zibo coalfield [8]. The chemical composition usually fluctuates much due to the different mineral components contained in the coals. The results of the analysis showed that the content of the main minerals in coal has a positive correlationship with the chemical and mineral composition of ashes. The coal ashes consist mainly of SiO2 and Al2O3, with CaO, Fe2O3 and SO3, and minor proportions of MgO, TiO2 and other oxides. SiO2 and Al2O3 make up 42.5–82.8% of the total ash. SiO2 and Al2O3 (Table 3) are more abundant in the Shanxi Formation coal samples than the Taiyuan coals, because of the presence of more definite minerals (especially clay minerals) in the Shanxi coals. The Taiyuan coals contain much higher SO3 than Shanxi coals, due to its high sulfur contents. 5.4. Mineral matter The major mineral phases as identified by XRD and SEM, is quartz, kaolinite, pyrite and calcite. However, traces of other minerals like dolomite, ankerite, illite, opal, feldspar and marcasite, and weathering products such as gypsum, and melanterite are also observed in several coals. Hematite in ash was also detected using electron and optical microscopy. Pyrite is more abundant in the coals of the lower Taiyuan Formation than in those of the Shanxi Formation, reflecting the marine influence on the depositional environment. Pyrite is the major sulphide mineral, but marcasite is also detected. Pyrite in the coals occurs as typical syngenetic framboidal, euhedral and massive cell-filling forms. Carbonate minerals like calcite and dolomite occur as the dominant carbonate species in the coal seams. Because the roof of most of the Taiyuan coal seams is limestone, especially seam 16, the occurrence of calcite in these coal seams is fiequent. Hence, the calcite content of the Taiyuan coal seams is higher than that of the Shanxi coals. Clay is the main inorganic mineral matter and are only inorganic constituent in the studied coal. In the study area, the deposition environments of the Shanxi Formation are fluvial [9–11]. Many kaolinite, illite and other clay minerals major comes from weathering and oxidation products in Shanxi coal seams. However, the Taiyuan coal seams are deposited in marine environment, so clay mineral, which is present at trace levels in the coals studied, is lower than that of the Shanxi coals. 5.5. Sulfur The major forms of sulfur in coal are pyretic sulfur, organic sulfur and sulfate sulfur. Pyritic and organic sulfur generally account for the bulk of sulfur in coal. Elemental sulfur also occurs in coal, but only in trace to minor amounts and it is not determined in routine coal analysis. During combustion, almost all organic sulfur, elemental sulfur, most pyritic sulfur and part of sulfate sulfur in coal are emitted into the atmosphere as aerosols and gaseous products of flue gas emissions [12,13]. But the pyritic sulfur and the sulfur from gypsum during combustion can form anhydyite (CaSO4) with CaO from calcite, Fe from pyrite with oxygen form hematite. Bassanite
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may also be formed from gypsum. SOx emissions may have a noxious effect on air [13,14], water [15– 18] and living organisms, including humans [1,19,20]. Several authors suggest that minor elements associated to organic and sulfide fraction also gets volatilized when coal is burnt [12,13]. The sulfur content of the coal seams of the Shanxi Formation is ranging from 0.22 to 0.74 wt%. And all sulfur content of the Shanxi Formation coal seams is less than 1 wt%. Consequently this is a low sulfur coal. But the sulfur content in the coal seam 6, 10, 15, 16 and 17 range from 1.66 to 5.50, 3.71–5.96, 2.59–6.15, 2.98–6.75 and 2.67–6.66 wt%, the average values are 2.92, 4.72, 3.87, 4.02 and 4.47 wt%, respectively. All the sulfur content from samples of the Taiyuan Formation coal seams is always more than 1.6 wt%, and the sulfur in many samples of the Taiyuan Formation coal seams are more than 3 wt%. The sulfur contents in the Taiyuan coals are much higher than in the Shanxi coals because of pyritic sulfur content. Tables 1 and 2 show pyritic sulfur is correlated closely with sulfur, and pyrite is present at very low concentrations in the Lower Permian coals (Shanxi Formation coals) (Table 2). However, pyrite concentrations in the Carboniferous coal seams (Taiyuan Formation) are much higher (Table 2). 5.6. Volatile matter In the high volatile bituminous coals (Fig. 4), vitrinite reflectance ranging from 0.6 to 0.8%, however, vitrinite reflectance shows a downward decrease from the Shanxi coals to the Taiyuan coals. This may be due to impregnation of the vitrinite by hydrocarbons [2,21–23]. However, it may also reflect marine influence on the coals in the lower part of the section [24,25]. The reverse trend is apparent for volatile matter, which is higher (O43% on a dry ash-free basis) in the Taiyuan Formation coal seams (6–17) than that in the Shanxi Formation coal seam (!40% daf). Such an inverse trend is impressive because coal samples from the Shanxi and Taiyuan Formations were taken from the same vertical profile (Table 2). However, the benches within seam 3 show significant variation, and the lowest volatile matter content is observed in coal seam 3. 5.7. Macerals The coal in the Tankou exploration area is characterized by high vitrinite and low to medium inertinite and liptinite contents (Table 2). The major macerals of the vitrinite group are telinite, telocollinite and desmocollinite. The coal seams of the Shanxi Formation contain mainly desmocollinite, with secondary telinite and minor telocollinite, while the coals of the Taiyuan have maceral composition characterized by telocolliniteOdesmocolliniteOtelinite. The vitrinite content of the Taiyuan Formation coals is higher than that of the Shanxi Formation coal seams, but the contents of intertinite and liptinite in the Shanxi Formation coal seams are higher than in the Taiyuan Formation coals. Based on the results of this work and previous studies [2,26,27], the telinite content is higher at the bottom than at the top of the coal seams. The top of coal seams 16 and 17 is characterized by a lower telinite content and higher telocollinite and desmocollinite concentrations compared to the bottom parts. The proportions of vitrinite group macerals in coal seam 16 do not show any significant variation [26]. As with desmocollinite, inertinite macerals are enriched at the top of coal seam 3, compared to the bottom of the seam. Both coal seams 3 and 16 from the Tangkou exploration area show a high inertinite content. The proportion of liptinite macerals is relatively low. The major liptinite macerals are sporinite,
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cutinite, and suberinite. The progressive increase in the inertinite content from the lower to the upper coal seams could be attributed to the presence of more reducing conditions of the sedimentary environments for the Taiyuan coals and more oxidizing paleoenvironments for the Shanxi seams. A similar evolution could account for the vertical increase in inertinite abundance from bottom to top of coal seam 3. The stratigraphic trends are very consistent, with evidence of a lower delta plain in the basal Taiyuan Formation, with marine influence, high subsidence, and probably more reducing environmental conditions, and a progressive upwards gradation to a more proximal upper delta plain environment in the Shanxi Formation.
6. Conclusions (1) The moisture in the Shanxi Formation coal seams is slightly higher than that in the Taiyuan Formation coals. Because of the above, the calorific value of the coals in the Shanxi Formation is slightly less than that in the Taiyuan Formation. (2) The coal of the study area is of high volatile bituminous rank. Petrographic studies indicate a downward decrease in vitrinite reflectance values from the Shanxi coals to the Taiyuan coals, and an inverse trend for volatile matter content, which is higher in the Taiyuan coal than in the Shanxi coal seams. This reflects a greater aboundance of vitrinite and a suppression of vitrinite reflectance associated with the marine influence. (3) The Shanxi coal seams have slightly higher ash yield than the Taiyuan coal seams. The major composition of ashes consist of crystallize (mineral matter), with minor glassy and organic matter. The major composition of ashes are SiO2, Al2O3, with Fe2O3, CaO and SO3, little content of P2O5, Na2O, K2O and TiO2. The minerals in the coal seams are chiefly quartz, kaolinite, pyrite, and calcite. (4) The sulfur content of the Shanxi Formation coal seams is considerably low, and the sulfur content of the Taiyuan coal seams is considerably higher.
Acknowledgements This work was supported by National Natural Science Foundation of China (40133010). We thank reviewers and editors for giving us many constructive comments.
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