Phase–mineral and chemical composition of composite samples from feed coals, bottom ashes and fly ashes at the Soma power station, Turkey

International Journal of Coal Geology 61 (2005) 35 – 63 www.elsevier.com/locate/ijcoalgeo

Phase–mineral and chemical composition of composite samples from feed coals, bottom ashes and fly ashes at the Soma power station, Turkey Stanislav V. Vassileva,*, Christina G. Vassilevaa, Ali I. Karayigitb, Yilmaz Bulutb, Andres Alastueyc, Xavier Querolc a

Central Laboratory of Mineralogy and Crystallography, Acad. G. Bonchev Str., Bl. 107, Bulgarian Academy of Sciences, 1113 Sofia, Bulgaria b Department of Geological Engineering, Hacettepe University, Beytepe, 06532 Ankara, Turkey c Institute of Earth Sciences bJaume AlmeraQ, CSIC, C/Lluis Sole i Sabaris s/n, 08028 Barcelona, Spain Received 9 February 2004; accepted 28 June 2004 Available online 3 October 2004

Abstract The phase–mineral and chemical composition of feed coals (FCs) and their bottom ashes (BAs) and fly ashes (FAs) produced in the Soma thermo-electric power station (TPS), Turkey, was characterized. FCs are high-ash Soma subbituminous coals abundant in moisture and Ca, and depleted in S. The inorganic composition (in decreasing order of significance) of FCs includes calcite, quartz, kaolinite, illite+muscovite, chlorite, plagioclase, gypsum, pyrite, montmorillonite, K-feldspar, dolomite, siderite, ankerite, opal, and volcanic glass. The results for 57 elements studied show that CaNNbNCsN(V, Li) have significantly higher contents in FC ashes than the respective Clarke values for coal ashes. The water-soluble residues isolated from FCs include gypsum, calcite, inorganic amorphous matter, Ca–Mg–Na–K phase, and opal. These residues are enriched in NaNSeNSNBNMgNMoNSrNCaNK. The phase–mineral composition of BAs and FAs includes mainly glass, quartz, char, mullite, plagioclase, calcite, and portlandite; and, to a lesser extent, illite+muscovite, melilite, hematite, anhydrite, lime, cristobalite, kaolinite, and magnetite. Minor amounts of K-feldspar, dolomite, ankerite, Fe-spinel, gypsum, and Ca–K–Na phase also occur in BAs and FAs. FAs are enriched in inorganic matter, glass, cristobalite, mullite, Fe oxides, lime, and anhydrite, and depleted in mineral matter, char, quartz, clay minerals, melilite, portlandite, and carbonates in comparison with BAs. Only Se is significantly enriched in BAs and FAs compared to FC ashes. Most of the trace elements (in particular As, Bi, Cd, Ge, Pb, Sn, Tl, and W) are more abundant in FAs, while BAs are more enriched in Ca, Cs, Fe, Ho, Mn, P, Sc, Se and Tb. Significant percentages (11–59%) of elements initially present in FCs, namely SNSbNSnNTaNMoNBiNZnNNiNNaN(Lu, Tm)NB, were emitted by stack emissions and not

* Corresponding author. Tel.: +359 2 9797055; fax: +359 2 9797056. E-mail address: [email protected] (S.V. Vassilev). 0166-5162/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.coal.2004.06.004

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captured by the cleaning equipment in the Soma TPS. Some genetic features, properties, possible environmental concerns, and potential utilization directions related to FCs, BAs, and FAs are also discussed. D 2004 Elsevier B.V. All rights reserved. Keywords: Mineral and chemical composition; Trace elements; Coal; Bottom ash; Fly ash; Turkey

1. Introduction Coal is used dominantly for burning in thermoelectric power stations (TPSs) worldwide. Fly ash (FA) and, to a lesser extent, bottom ash (BA) are the combustion residues produced and collected during coal burning in a TPS. Intensive investigations on the composition and properties of solid combustion wastes (in particular FAs) originated from various coal-fired TPSs worldwide have been carried to elucidate some utilization, environmental, and technological problems. Detailed and summarized data on the chemical and phase–mineral composition of FAs and BAs have been reported (Fisher and Natusch, 1979; Hulett and Weinberger, 1980; Lauf, 1982; Ramsden and Shibaoka, 1982; Mattigod and Ervin, 1983; Panteleev et al., 1985; Raask, 1985; Grossman et al., 1988; McCarthy et al., 1988; Shpirt et al., 1990; Vassilev, 1992, 1994; Querol et al., 1995, 1996; MartinezTarazona and Spears, 1996; Vassilev and Vassileva, 1996a, 1997; Karayigit et al., 2001; Vassilev et al., 2003). These investigations show that the combustion waste products exhibit complex (unique, polycomponent, heterogeneous, and variable) composition depending mostly on coal types and technological processes used in TPSs. The combustion residues are generated from various detrital and authigenic inorganic and organic constituents in feed coals (FCs) and the phase–mineral composition of BAs and FAs includes: (1) inorganic constituent represented by various non-crystalline (amorphous) and crystalline (mineral) phases; (2) organic constituent represented by char phases; and (3) fluid constituent represented by liquid (moisture), gas and gas–liquid inclusions associated with the inorganic and organic matter (Vassilev and Vassileva, 1996a). Despite the numerous studies, systematic mineralogical and geochemical investigations simultaneously of FCs, BAs, and FAs are still restricted (Vassilev, 1992, 1994; Querol et al., 1995, 1996; Vassilev and Vassileva, 1996a, 1997; Karayigit et al., 2001; Vassilev et al., 2001). Such parallel studies

are important in the elucidation of mineral and element behaviour, products’ formation, evaluation of possible utilization directions, and prediction of some environmental and technological impacts related to coal burning in TPSs. These problems may differ significantly for the various coals used. The coal combustion may lead to some serious environmental pollution of the air, water, soil, and plants in the areas surrounding TPSs by toxic and potentially toxic (TPT) elements and compounds such as Ag, As, Ba, Be, C (CO), Cd, Cl (HCl), Co, Cr, Cu, F (HF), Hg, Mn, N (NOx ), Ni, Pb, S (SO2), Sb, Se, Sn, Th, Tl, U, V, Zn, others. These elements and compounds are regulated as potentially hazardous emissions from combustion systems (Harrop, 1994; Linak and Wendt, 1994). TPT elements may be: (1) released directly to the atmosphere via stack emissions; (2) concentrated in the combustion residues; and (3) liberated from the waste products during their transport, storage, and utilization (Clarke and Sloss, 1992; Martinez-Tarazona and Spears, 1996; Vassilev and Vassileva, 1997). The partitioning of elements between BAs, FAs and stack emissions can be investigated by mass balancing techniques and such data were used to identify possible environmental hazard (Clarke and Sloss, 1992; Linak and Wendt, 1994; Meij, 1994; Ratafia-Brown, 1994; Vassilev, 1994; Vassilev and Vassileva, 1997; Vassilev et al., 2001; Xu et al., 2003). The concentration of trace elements in the solid products from TPS depends on the occurrence of these elements in FCs. The trace elements were incorporated in coal by the detrital material, syngenetically during the development of the coal-forming peat mire or epigenetically as a result of fluids migrating through the coal seams. The bearing and concentrating phases of trace elements in coal are organic matter and various minerals and inorganic phases, which take part in the formation of BAs and FAs. Therefore, some coals are more prone to high concentrations of trace elements with environmental concern than other coals. On the other hand, certain valuable elements, minerals or

S.V. Vassilev et al. / International Journal of Coal Geology 61 (2005) 35–63

phases could be present in sufficient concentrations in coal or combustion waste products to be commercially recovered and utilized (Valkovic, 1983; Shpirt et al., 1990). The preliminary studies (Karayigit et al., 2000) have shown that some industrial Turkish coals have trace element contents higher than the World average for coals and in some cases concentrations even above the recorded maximums for World coals (Swaine, 1990). However, limited data are currently available on the trace elements in Turkish FCs, BAs, and FAs, or on their behaviour and fate during coal combustion in TPSs. The Soma basin is one of the most productive lacustrine coal basins of western Anatolia-Turkey (Fig. 1). This basin has a coal potential of 626 Mt, and producing run-of-mine coals of about 11 Mt per year are consumed for domestic heating due to relatively low-sulphur content and high calorific value, and as FCs in the coal-fired Soma TPS. The geology of the Soma coal basin has been described and illustrated in detail earlier (Karayigit and Whateley, 1997; Karayigit, 1998; Inci, 2002). Briefly, this basin contains two mineable coal seams, lower (k1) and upper (k3), which are currently exploited in underground and mainly open-cast mines. The lower seam is located in the Soma Formation of Early-Middle Miocene age and the upper seam in the Denis Formation of Late Miocene age. The Soma and Denis Formations, which are characterized by non-marine, laterally extensive limestone–marl–claystone dominated units, form the

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sedimentary sequence of the basin. Pyroclastic materials are more common in the Denis Formation. Preliminary results relating to the characterization of FCs, BAs, and FAs from the Soma TPS (Turkey) have been reported by Bulut et al. (2002). Additional studies on the variations in composition for individual FA samples from two power units in the Soma TPS have been also performed (Karayigit et al., in press). The present investigation on composite FCs, BAs, and FAs from the same two power units was undertaken to address the following objectives: (1) to characterize the phase–mineral and chemical composition of FCs, BAs, and FAs; (2) to determine the partitioning of elements between BAs, FAs, and stack emissions; (3) to predict some possible environmental concerns and potential utilization directions related to FCs, BAs, and FAs.

2. Material and methods Three types of composite samples, comprising two FCs and their respective BAs and FAs generated from the pulverized coal combustion in two boiler units (Units B1–4 and Units B5–6) at the Soma TPS (Turkey) were examined. Units B1–4 use coals dominantly from the lower seam k1 in the central mines at the Soma basin (southern Soma). Units B5–6 burn coals mainly from the lower k1 and upper k3 seams in the Denis mines at the Soma basin (northern Soma). The composite samples include FC1, BA1,

Fig. 1. Location of the Soma and other Turkish coal-fired thermo-electric power stations.

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S.V. Vassilev et al. / International Journal of Coal Geology 61 (2005) 35–63

FA1 from Units B1–4 and FC2, BA2, FA2 from Units B5–6. Each composite sample includes eight individual samples collected every week in the period 25 June 2001–13 August 2001. The individual FC, BA and FA samples were taken simultaneously from TPS (HU, Ankara). The respective six composite samples were prepared from the individual samples by mixing and homogenizing equal weight quantities (at CLMC, Sofia). FC samples were taken from the coal mills in Soma TPS. Bulut et al. (2002) noted that the investigated FCs from Units B1–4 and B5–6 have relatively high moisture contents (av. 15.7% and 17.0%, as-received), high ash yields (av. 40.0% and 49.3%), low total sulphur contents (av. 0.8% and 0.8%), and low calorific values (av. 2918 and 2161 kcal kg
1), on an air-dried basis. BA samples were collected from the water tanks (under the dry-ash discharged boilers) and then dried at 105 8C at the laboratory of TPS. FA samples were taken from the hoppers at different fields of the electrostatic precipitator chains in TPS. The produced BAs and FAs from Units B1–6 are transported hydraulically and deposited together in the disposal ponds near TPS. Some separation procedures such as sieving and water leaching of composite FCs and FAs were applied (at CLMC, Sofia). Water-soluble residues from FCs (CWRs) and FAs (WRs) were isolated by evaporation and crystallization of the water solutions leached. The leachates from water extraction procedures were generated from FCs (30 g) and FAs (100 g) placed in glass tanks with distilled water (solid–liquid weight ratio 1:10) for 24 h at ambient temperature. The suspensions were periodically stirred and after that decanted and filtered through 0.1-Am membrane filters. The pH value was potentiometrically measured in the water solutions generated, and then the leachates were placed in a drying furnace at 80 8C for evaporation and crystallization until dry CWRs and WRs were produced. The high-temperature ashes (HTAs) of composite FCs and the incineration of composite BAs, FAs and their fractions were conducted in an electric furnace (in atmosphere of static air) at 750 8C for 4 h with a heating rate of 15 8C min
1 (at CLMC, Sofia). The phase–mineral composition of composite FC, BA and FA samples, and their separated fractions were investigated by light microscopy, scanning electron microscopy (SEM), powder X-ray diffraction (XRD), differential thermal analysis (DTA), and thermo-

gravimetric analysis (TGA) (at CLMC, Sofia). An ordinary stereomicroscope and a polarizing microscope under transmitted light for glycerin immersions were used for optical observations. XRD patterns were recorded by a DRON 3M diffractometer and collected at 5–658 2h using CoKa radiation. The investigation by SEM was carried out on a Philips SEM-515 microscope equipped with an EDAX 9100 energy dispersive X-ray analyzer (EDX). DTA and TGA analyses were conducted by a Stanton Redcroft STA 760 apparatus. Air atmosphere with a constant carrier flow rate of 30 ml h
1 at 20 8C was used. The experiments were carried out for 10-mg samples heated from ambient temperature up to 1400 8C in corundum crucibles. The heating rate was 10 8C min
1. Fifty-seven major, minor and trace elements in the composite air-dried FCs, BAs, FAs, and their separated fractions were analysed using inductively coupled plasma (ICP)-mass spectroscopy (MS) for As, Be, Bi, Cd, Ce, Co, Cs, Dy, Er, Eu, Ga, Gd, Ge, Hf, Ho, La, Li, Lu, Mo, Nb, Nd, Ni, P, Pb, Pr, Rb, Sb, Sc, Se, Sm, Sn, Ta, Tb, Th, Tl, Tm, U, W, Y, Yb, Zn, and Zr, and ICPatomic emission spectroscopy (AES) for Al, B, Ba, Ca, Cr, Cu, Fe, K, Mg, Mn, Na, S, Sr, Ti, and V (at IES, Barcelona). The samples were previously digested using a two-stage extraction procedure devised to retain volatile elements (such as As and B) during the analysis. A detailed description of the digestion procedure has been given earlier (Querol et al., 1997). Digestion of international reference materials (SARM 19 and NBS 16633b) and blanks were prepared using the same procedure. Analytical errors were estimated at b5% for most of the elements. The individual FC, BA and FA samples were also characterized by standard proximate and ultimate analyses according to procedures of the American Society for Testing and Materials (ASTM) (1991) (at HU, Ankara). The mineralogical classification of elements by Solodov et al. (1987) is used for grouping of elements.

3. Results and discussion 3.1. Characterization of feed coals 3.1.1. Common characteristics Some common characteristics of the FCs studied are listed in Table 1. The data are similar to those

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Table 1 Some common characteristics (air-dried basis) of individual and composite feed coals (FCs), bottom ashes (BAs) and fly ashes (FAs) from Soma TPS, wt.% Characteristic

FC1

FC2

Mean

BA1

BA2

Mean

FA1

FA2

Mean

Mean values for individual samples (HU, Ankara) Moisture, as-received (105 8C) Moisture, air-dried (105 8C) Ash yield at 750 8C Volatile matter Fixed C Total S Calorific value (kcal kg
1)

15.7 8.2 40.0 32.8 19.0 0.8 2918

17.0 9.5 49.3 26.6 14.6 0.8 2161

16.4 8.9 44.7 29.7 16.8 0.8 2540

1.3 81.2 15.0 2.5 0.2 535

1.1 89.3 8.6 1.0 0.3 559

1.2 85.3 11.8 1.8 0.3 547

0.2 97.9 1.9 V0.1 0.4

0.2 98.7 0.9 0.1 0.7

0.2 98.3 1.4 V0.1 0.6

41.5 41.5

51.6 51.6

46.6 46.6

82.2 71.4 10.8

91.0 70.4 20.6

86.6 70.9 15.7

97.3 63.1 34.2

98.2 61.1 37.1

97.7 62.1 35.6

58.5

48.4

53.4

17.8 0.91 1.11

13.4 0.84 0.97

7.2

7.8

7.5

2.7 0.99 1.18 11.9

1.8 0.87 1.05 12.1

2.3 0.93 1.12 12.0

0.5

0.6

0.6

1.7

2.6

2.2

Values for composite samples (CLMC, Sofia) Inorganic matter (ash at 750 8C/4 h) Mineral matter (crystalline area by XRD) Inorganic amorphous matter (halo area by XRD-LOI at 750 8C/4 h) Organic matter (LOI at 750 8C/4 h) Bulk density (g cm
3) Bulk density, compacted state (g cm
3) pH for FC and FA soaked in distilled water (weight ratio 1:10) in a tank for 24 h Water-soluble residue from FC (CWR) and FA (WR)

described in the literature (Karayigit et al., 2000) and show that the Soma coal is low-rank subbituminous coal with high ash yield, abundant in moisture and depleted in total S and fixed C. A comparison between both FCs shows that FC1 (from the central mines) has greater values of organic matter, volatile matter, fixed C, and calorific value, and lower values of moisture, inorganic matter, pH, and CWR in comparison with FC2 (from the Denis mines). Despite these distinctions, both FCs normally do not show great variations in the above characteristics. 3.1.2. Chemical composition The element concentrations of FCs are given in Table 2. The data show that 50 of the 57 elements studied (excluding Be, Bi, Mo, S, Sb, Se, and Tl) in these FCs have mean contents greater than the respective Clarke values for lignite and subbituminous coals worldwide (Yudovich et al., 1985) or USA coals (Finkelman, 1994). This comparison is evaluated by the enrichment/depletion factor (EDF) defined as a ratio of the mean element content in coal samples to the respective Clarke value in coals (Table 2). The highest over Clarke concentrations (EDF=2.0–16.7) in FCs reveal: CaNPbNCsNNbNRbN(U, Cd)NVN(Li,

9.0 0.77 0.83

Ni)NCrNZnNTaNTiN(B, P)N(Cu, Mg)NAlN(Sc, Sn)NKNHfNAsNNaN(Ba, Y)NZrN(Co, Lu)N(Nd, Pr, Tm). However, the element contents in coal ashes are more important for the present study because they are comparable with the combustion wastes. The calculated data for FC ashes show that 19 elements, namely lithophile (Ca, Cs, Li, Mg, Nb, Rb, Ta), chalcophile (As, Cd, Cu, Sn, Zn), siderophile (Cr, Ni, Ti, V), non-metal (B, P) and radioactive (U) elements have average contents greater than the respective Clarke values for coal ashes (Table 2). Lithophile elements (excluding V) such as CaNNbNCsN(V, Li) in FC ashes reveal the highest (EDF=2.4–4.7) over Clarke concentrations. The data for both FCs (ash basis) are relatively similar, excluding the greater contents (by factor 2.0–2.5) of Ca, Mn, and Mo in FC1 and higher concentrations (by factor 1.5–2.1) of Be, Bi, Cd, Ce, Cu, Dy, Gd, Ge, Hf, La, Li, Na, Nb, Nd, Pb, Pr, Sm, Ta, Tb, Th, Ti, V, W, Yb, and Zr in FC2. Hence, FC2 from the Denis mines is more abundant in trace elements than FC1 from the central mines in the Soma basin. This is probably related to the higher content of inorganic matter in FC2 than FC1. An association of the Denis coal with volcanic inputs

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Table 2 Element concentrations (air-dried basis) of composite feed coals (FCs), FC ashes and water-soluble residues (CWRs) separated from composite FCs at Soma TPS, ppm (indicated otherwise) Element FC1 Coal

FC2 Ash

Ash (%) 41.5

Coal

Clarkea

Mean Ash

51.6

Coal

Ash

46.6

13.1

Non-metal elements B 308 742 340 659 324 695 P (%) 0.05 0.12 0.05 0.10 0.05 0.11 S (%) 0.9 2.2 1.0 1.9 1.0 2.1 Lithophile Al (%) Ba Be Ca (%) Ce Cs Dy Er Eu Gd Hf Ho K (%) La Li Lu Mg (%) Mo Na (%) Nb Nd Pr Rb Sm Sr Ta Tb Tm W Y Yb Zr

elements 3.6 8.7 253 610 1.1 2.7 9.7 23.4 24 58 9 22 1.9 4.6 1.0 2.4 0.5 1.2 2.3 5.5 1.2 2.9 0.4 1.0 0.4 1.0 15 36 53 128 0.2 0.5 0.4 1.0 2 5 0.1 0.2 5 12 12 29 3.1 7.5 28 67 1.8 4.3 138 333 0.6 1.4 0.3 0.7 0.2 0.5 1.2 2.9 13 31 1.0 2.4 43 104

6.2 311 2.1 5.7 52 11 3.5 1.7 0.9 4.7 2.6 0.7 0.5 31 124 0.3 0.4 1 0.2 12 25 6.4 43 3.6 169 1.2 0.6 0.3 2.5 21 1.8 94

12.0 603 4.1 11.0 101 21 6.8 3.3 1.7 9.1 5.0 1.4 1.0 60 240 0.6 0.8 2 0.4 23 48 12.4 83 7.0 328 2.3 1.2 0.6 4.8 41 3.5 182

4.9 282 1.6 7.7 38 10 2.7 1.4 0.7 3.5 1.9 0.6 0.5 23 89 0.3 0.4 2 0.2 9 19 4.8 36 2.7 154 0.9 0.5 0.3 1.9 17 1.4 69

Coal

EDFb Ash

e

CWR1 CWR2 Mean

Coal Ash adb

adb

adb

3.6

85 560 0.013 0.1 1.8e 13.7e

3.8 3.8 0.6

1.2 1310 3220 2270 1.1 b0.03 b0.03 b0.03 0.2 9.6 11.2 10.4

10.5 1.5e 11.5e 3.3 605 120 890 2.4 3.4 2.4 11 0.7 16.5 0.46e 3.51e 16.7 82 21e 160e 1.8 21 1.1e 8.4e 9.1 14.5e 1.4 5.8 1.9e 3.0 1.0e 7.6e 1.4 1.5 0.4e 3.1e 1.8 13.7e 1.9 7.5 1.8e 4.1 0.7e 5.6e 2.7 1.3 0.35e 2.7e 1.7 1.1 0.18e 1.37e 2.8 92e 1.9 49 12e 191 20 80 4.5 0.6 0.14e 1.1e 2.1 0.9 0.11e 0.8e 3.6 4 2.4 13 0.8 0.4 0.08e 0.61e 2.5 19 1 5 9.0 41 9.5e 73e 2.0 18.3e 2.0 10.3 2.4e 77 5 46 7.2 5.8 1.7e 13.0e 1.6 330 130 1100 1.2 1.9 0.22e 1.7e 4.1 1.1 0.30e 2.3e 1.7 0.6 0.15e 1.2e 2.0 7.6e 1.9 4.1 1.0e 36 7.0 37 2.4 3.0 0.9 5 1.6 148 30 160 2.3

0.9 b0.03 b0.03 b0.03 0.7 128 113 121 0.3 0.03 0.01 0.02 4.7 23.3 18.2 20.8 0.5 0.2 0.4 0.3 2.5 1.0 0.4 0.7 0.4 b0.02 b0.02 b0.02 0.4 b0.01 b0.01 b0.01 0.5 b0.03 b0.03 b0.03 0.5 b0.01 b0.01 b0.01 0.7 b0.1 b0.1 b0.1 0.5 b0.01 b0.01 b0.01 0.8 0.7 1.9 1.3 0.5 0.1 0.2 0.2 2.4 60 154 107 0.5 b0.2 b0.2 b0.2 1.1 2.7 2.9 2.8 0.3 7 8 8 0.7 1.6 4.3 3.0 3.8 0.1 0.1 0.1 0.6 0.1 0.1 0.1 0.6 b0.1 0.1 V0.1 1.7 23 38 31 0.4 b0.02 b0.02 b0.02 0.3 587 439 513 1.1 0.1 0.1 0.1 0.5 b0.01 b0.01 b0.01 0.5 b0.2 b0.2 b0.2 0.5 0.4 0.7 0.6 1.0 0.1 0.1 0.1 0.6 b0.01 b0.01 b0.01 0.9 1 2 2

Siderophile elements Co 6 14 8 16 7 15 3.4 Cr 39 94 67 130 53 114 12 Fe (%) 1.3 3.1 1.5 2.9 1.4 3.0 1.3e Mn 206 496 130 252 168 361 100 Ni 26 63 45 87 36 77 8 Sc 4 10 7 14 6 13 2.0 Ti 1310 3160 2720 5270 2020 4330 500 V 93 224 172 333 133 285 23

EDFc Leachedd, %

20 70 9.9e 510 51 15 2600 120

2.1 4.4 1.1 1.7 4.5 3.0 4.0 5.8

0.8 1.6 0.3 0.7 1.5 0.9 1.7 2.4

1 5 b1.0 80 24 b2 41 6

1 4 b1.0 88 24 b2 34 19

1 5 b1.0 84 24 b2 38 13

3.3

4.2

5.0

6.2

0.2 0.3 b0.1 b0.1 1.3 1.6 b0.1 b0.1 b0.1 b0.1

1.2 1.6 b0.1 b0.1 0.6 0.7 3.1 4.2 2.0 2.4 7.5 9.0 b0.1 b0.1 b0.1 b0.1 b0.1 b0.1 0.4 0.5 1.6 2.0 0.1 b0.1

0.1 0.2 b0.1 b0.1 b0.1 b0.1

0.1 b0.1

0.1 0.1

0.2 0.3

0.3 0.4

b0.1 b0.1 b0.1 0.1

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Table 2 (continued) Element FC1 Coal

FC2 Ash

Coal

Chalcophile elements As 38 92 Bi 0.3 0.7 Cd 1 2 Cu 19 46 Ga 8 19 Ge 1 2 Pb 16 39 Sb 1 2 Se 1 2 Sn 2 5 Tl 0.4 1.0 Zn 67 161

36 0.6 2 35 14 2 33 1 1 3 0.7 89

Radioactive elements Th 5 12 U 13 31

13 15

a b

Clarkea

Mean Ash 70 1.2 4 68 27 4 64 2 2 6 1.4 172

25 29

Coal 37 0.5 2 27 11 2 25 1 1 3 0.6 78

9 14

Ash

Coal

EDFb Ash

79 1.1 4 58 24 4 54 2 2 6 1.3 167

14 b1.0e 0.3 7.5 7 1.5 2.5 1.3 3 1 1.2e 18

60 b7.6e 3 48 36 9 53 8 20 4.1 9.2e 100

19 30

6.3 2.1e

22 16.0e

CWR1 CWR2 Mean

Coal Ash adb 2.6 1.3 14 ~0.5 ~0.1 b0.1 6.7 1.3 0.3 3.6 1.2 21 1.6 0.7 0.02 1.3 0.4 0.03 10.0 1.0 5.2 0.8 0.3 0.3 0.3 0.1 11 3.0 1.5 1 0.5 0.1 b0.1 4.3 1.7 107

1.4 6.7

0.9 1.9

0.1 20

adb

adb

EDFc Leachedd, %

34 b0.1 0.3 26 0.10 0.10 2.0 1.0 16 1 0.1 43

24 0.3 0.4 b0.1 0.3 0.1 0.1 24 0.4 0.5 0.06 b0.1 b0.1 0.07 b0.1 b0.1 3.6 0.1 0.1 0.7 0.4 0.4 14 7.0 8.4 1 0.2 0.2 V0.1 V0.1 V0.1 75 0.4 0.6

b0.1 12

V0.1 16

b0.1 b0.1 0.5 0.7

Clarke for lignite and subbituminous coals or their ashes worldwide (Yudovich et al., 1985). Enrichment/depletion factor—a ratio of the mean element content in coal or coal ash samples to the respective Clarke value in coal or coal

ash. c Enrichment/depletion factor—a ratio of the mean element content in CWRs to the respective mean element concentration in FCs (ash basis). d Mean leached percentage of the element from FCs. e Clarke for USA coals or coal ashes (Finkelman, 1994).

probably also has some contribution for increased concentrations of trace elements in this FC. Such a preliminary geochemical comparison indicates elements that may have some possible environmental and technological impacts, and/or potential resource recovery during FC utilization. These geochemical features reveal that most of the regulated TPT elements are more or less concentrated in the coals or coal ashes studied. Hence, the enriched and/or hazardous elements (in particular chalcophile, siderophile and radioactive ones) should have a more detailed environmental and economical concern. The bulk chemical composition of FCs is an important characteristic, but quite insufficient for a reliable explanation of an elemental behaviour during FC utilization. Therefore, the abundance, origin, and behaviour of modes of element occurrence (minerals and phases) in FCs and their products have a leading role in coal use. The most important fraction in coal from an environmental point of view is the water-soluble leachate. The total mineralization of water solutions

leached from FCs (0.5–0.6%) is classified as salty according to the CWR yields (Perelman, 1989). These solutions have pH values of 7.2–7.8 (neutral to slightly alkaline) as FC2NFC1 (Table 1). The reason for that could be again the higher content of inorganic matter in FC2 than FC1. It is well known that the more acidic environment increases the leaching behaviour of most trace elements in comparison with the more alkaline media. Hence, the above environment is relatively favourable for depressed leaching of most heavy metals. The element contents in CWRs leached from FCs are listed in Table 2. These data show that most of the elements (excluding Al, Bi, Dy, Er, Eu, Fe, Gd, Hf, Ho, Lu, P, Sc, Sm, Tb, Tm, and Yb) are in detectable amounts in CWRs and occur more or less as watersoluble forms in FCs. The data for both CWRs are relatively similar, excluding the higher contents (by factor 1.7–3.0) of Be, Cs, Pb, U, and Zn in CWR1 and higher concentrations (by factor 1.5–5.0) of As, B, Ce, Ga, Ge, K, La, Li, Na, Rb, Sb, Se, V, W, and Zr in CWR2. Hence, CWR2 produced from coal of

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the Denis mines is more abundant in trace elements than CWR1 generated from coal of the central mines in the Soma basin. This could be also explained by the greater content of inorganic matter in FC2 than FC1. It was found that elements such as NaNSeNSNBNMgNMoNSrNCaNK (mostly non-metal and lithophile ones) in CWRs show average concentrations greater (EDF=1.2–7.5) than FC ashes (Table 2). The highest amounts (1.6–9.0%) leached from FCs show the above nine elements in similar order, namely NaNSeNSN(B, Mg)NMoNSrN(Ca, K) (Table 2). 3.1.3. Mineralogy The literature data (Karayigit et al., 2000; Bulut et al., 2002) show that the mineral composition (in decreasing order of significance) of Soma FCs

includes calcite, kaolinite/chlorite, quartz, pyrite, illite, smectite, gypsum, dolomite, feldspar, and siderite. The present study (Tables 3 and 4, and Figs. 2–6) confirmed the occurrence of the above minerals plus some additional minerals identified. The mean mineral composition (in decreasing order of significance) of FCs includes calcite, quartz, kaolinite, illite+muscovite, chlorite, plagioclase, gypsum, pyrite, montmorillonite, K-feldspar, dolomite, siderite, ankerite, opal, and volcanic glass. Additional accessory minerals may also occur in FCs, but they are not under consideration due to their complicated identification and quantification. For example, Cu–Zn and Cu–Zn– Pb sulphides, barite enriched in Sr, Cr oxide, halite, and Cu–Zn chloride were identified in the Soma FCs from Denis mines (Karayigit et al., 2000). SEM studies (Figs. 5 and 6) illustrate the occurrence of

Table 3 Occurrence and abundance of minerals and inorganic phases identified (F—forming, N10%; M—major, N1–10%; Mi—minor, 0.5–1%; A— accessory, b0.5%) and their probable origin (D—detrital; S—syngenetic; E—epigenetic; —dominant; o—subordinate) in composite feed coals (FCs) from Soma TPS

.

Mineral, phase

Formula

FC1

FC2

Origin D

S

Sulphides Pyrite

FeS2

Mi

Mi

.

Carbonates Calcite Dolomite Ankerite Siderite

CaCO3 CaMg(CO3)2 Ca(MgFe)(CO3)2 FeCO3

F A A A

F Mi

. . . .

Sulphates Gypsum

CaSO4d 2H2O

M

M

Silicates Quartz Opal Kaolinite Illite+ Muscovite Montmorillonite Chlorite Plagioclase K-feldspar

SiO2 SiO2d nH2O Al2Si2O5(OH)4 (KH2O)Al2(AlSi)Si3O10(OH)2 KAl2AlSi3O10(OH)2 (NaCa)0.3(AlMgFe)2Si4O10(OH)2d xH2O (MgFe)5Al2Si3O10(OH)8 NaAlSi3O8–CaAl2Si2O8 KAlSi3O8

M A M M

F

.

F M

o

.

A M M A

Mi M M Mi

. . . .

A

.

Others Volcanic glass Ca–Mg–Na–K phasea Inorganic amorphous mattera a

Identified in CWRs.

A A

Mi

A

E

. . . o o o

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43

Fig. 2. X-ray diffraction patterns of FC1, BA1 and FA1. Mineral abbreviations: cps=counts per second, A=anhydrite, Ak=ankerite, Cc=calcite, Ch=chlorite, Cr=cristobalite, D=dolomite, G=gypsum, H=hematite, I=illite, K=kaolinite, Kf=K-feldspar, L=lime, M=mullite, Me=melilite, Ms=muscovite, Mt=magnetite, P=pyrite, Pl=plagioclase, Po=portlandite, Q=quartz, S=siderite.

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Fig. 3. X-ray diffraction patterns of FC2, BA2 and FA2. Mineral abbreviations: cps=counts per second, A=anhydrite, Ak=ankerite, Cc=calcite, Ch=chlorite, Cr=cristobalite, D=dolomite, G=gypsum, H=hematite, I=illite, K=kaolinite, Kf=K-feldspar, L=lime, M=mullite, Me=melilite, Ms=muscovite, Mt=magnetite, P=pyrite, Pl=plagioclase, Po=portlandite, Q=quartz, S=siderite.

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45

Fig. 4. X-ray diffraction patterns of CWR1 and CWR2. Mineral abbreviations: cps=counts per second, Cc=calcite, Cf=Ca–Mg–Na–K phase, G=gypsum, O=opal.

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Fig. 5. SEM images of FC (secondary electrons). (a) Syngenetic framboidal pyrite coated with kaolinite film in FC1. (b) Syngenetic calcite brosesQ in FC2. (c) Epigenetic gypsum lens in FC2. (d) Prismatic and needle epigenetic gypsum crystals in FC2 [a detail from (c)].

minerals and phases such as pyrite (Fig. 5a), kaolinite (Fig. 5a), calcite (Fig. 5b), gypsum (Figs. 5c–d), opal (Fig. 6a), and volcanic glass (Fig. 6b) in FCs. The

mineral composition of both FCs reveals a relatively similar qualitative composition and limited quantitative variations, despite the different regional coal

Fig. 6. SEM images of FC and CWR (secondary electrons). (a) Syngenetic opal grains (1) filling and coating cell cavity structures in FC2. (b) Aluminosilicate volcanic spheres and spheroids in FC2. (c) General view of CWR1. The sample is composed mainly of gypsum, calcite, and inorganic amorphous matter. (d) Gypsum (1), calcite (2), calcite coated with Ca–Mg–Na–K phase (3), and opal (4) in CWR1.

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47

minerals are commonly finely dispersed in organic matter. Epigenetic gypsum lenses and lenticles (up to 300 Am in diameter) composed of prismatic and needle crystals and aggregates were also identified (Fig. 5c–d). The detrital minerals and phases such as quartz, feldspars, volcanic glass (Fig. 6b), and some clay and mica minerals (mainly illite, muscovite, montmorillonite, chlorite, and, to a lesser extent, kaolinite) are present as individual grains, crystals, aggregates or rock lumps mainly in the size interval 1–100 Am among organic matter. The higher contents of quartz, clay minerals and volcanic material, and lower concentrations of calcite in FC2 than FC1 are in accordance with the lithology of the Soma and Denis Formations. DTA–TGA profiles of both FCs in air are quite similar (Figs. 7 and 8) and show: (1) loss of adsorbed water up to 100 8C (endothermic effects with maximums at 77 and 85 8C); (2) organic matter oxidation at 200–520 8C (large exothermic effects with maximums at 410 and 435 8C) accompanied by the greatest mass loss; (3) decomposition of calcite at

types used. The distinctions between them includes the occurrence of ankerite and opal in FC1, presence of volcanic glass in FC2, as well as higher contents of calcite and lower concentrations of quartz, kaolinite, illite+muscovite, montmorillonite, K-feldspar, dolomite, and siderite in FC1 than FC2. However, FC1 shows slightly higher proportions of kaolinite, chlorite, plagioclase, pyrite, and gypsum than FC2 when compared on crystalline matter basis (Table 4). The present study reveals that FCs have moderate detrital mineral abundance and increased authigenic mineralization dominated by carbonate than sulphide–sulphate tendency (Tables 3 and 4). The authigenic minerals are syngenetic pyrite, carbonates, opal, some clay and mica minerals (mainly kaolinite, and, to a lesser extent, illite, muscovite, montmorillonite, and chlorite), and epigenetic gypsum. For example, syngenetic minerals such as biogenetic framboidal pyrite coated with kaolinite film (Fig. 5a), biogenetic calcite brosesQ (Fig. 5b), and opal filling and coating cell cavity structures in organic matter (Fig. 6a) were observed. The syngenetic

Table 4 Phase–mineral composition and content (determined by XRD and LOI) of composite feed coals (FCs) and water-soluble residues (CWRs) separated from composite FCs at Soma TPS Mineral, phase

FC1 adb

Inorganic matter Mineral matter Inorganic amorphous matter Organic matter Mineral matter Quartz Opal Kaolinite Illite+Muscovite Montmorillonite Chlorite Plagioclase K-feldspar Calcite Dolomite Ankerite Siderite Pyrite Gypsum Ca–Mg–Na–K phase Total

FC2 cb

adb

Mean cb

adb

41.5 41.5

51.6 51.6

46.6 46.6

58.5

48.4

53.4

CWR1 cb

adb

14

16

31

11

22.5

10 3 b0.5 3 2 b0.5 13 b0.5 b0.5 b0.5 1 2

25 6 1 6 4 1 32 1 1 1 3 5

12 4 0.5 3 2 0.5 11 0.5

23 7 1 5 3 1 22 1

0.5 1 2

1 2 4

11 3.5 V0.5 3 2 V0.5 12 V0.5 b0.5 V0.5 1 2

24 6.5 1 5.5 3.5 1 27 1 ~0.5 1 2.5 4.5

V42.5

100

101

V48

adb

100.0 82.9 17.1

6

53

CWR2 cb

~100.5

The mineral and phase abundance is on air-dried (adb) and crystalline basis (cb), wt.%.

Mean cb

100.0 82.8 17.2

2

2

31

37

41

46 5 84

55 6 100

adb

cb

100.1 82.9 17.2

1

1

49

36

43

42

51

83

100

44 2.5 83.5

53 3 100

48

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Fig. 7. DTA–TGA profiles of FC1, BA1 and FA1.

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Fig. 8. DTA–TGA profiles of FC2, BA2 and FA2.

49

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Table 5 Sieve analysis of composite bottom ashes (BAs) and fly ashes (FAs) from Soma TPS, wt. % Sample

N200 Am

100–200 Am

63–100 Am

b63 Am

BA1 BA2 Mean FA1 FA2 Mean

67.8 55.9 61.8 13.5 8.9 11.2

17.1 24.7 20.9 20.6 16.1 18.3

6.3 9.6 8.0 16.1 15.3 15.7

8.8 9.8 9.3 49.8 59.7 54.8

680–750 8C (endothermic effects with maximums at 710 and 729 8C) associated with intensive mass loss; and (4) decomposition of anhydrite at 1150–1220 8C (endothermic effects with maximums at 1176 and 1205 8C) accompanied by limited mass loss. CWRs isolated from FCs contain dominantly sulphates and carbonates (Table 4 and Figs. 4 and 6c–d). Their yields are 0.5–0.6% as CWR2NCWR1 (Table 1) due to the higher content of inorganic matter in FC2 than FC1. The mean phase–mineral composition (in decreasing order of significance) of CWRs includes gypsum, calcite, inorganic amorphous matter, Ca–Mg–Na–K phase, and opal (Table 4). SEM investigations illustrate a general view of CWR (Fig. 6c) and occurrence of calcite, gypsum, opal, and Ca– Mg–Na–K phase (Fig. 6d). The phase–mineral composition of CWR1 and CWR2 is similar, excluding the occurrence of opal and Ca–Mg–Na– K phase in CWR1. CWRs are more enriched in mineral matter, inorganic amorphous matter, opal, calcite, gypsum, and Ca–Mg–Na–K phase than bulk FC samples (Table 4). The minerals and phases in CWRs are newly formed species produced during water treatment of FCs and subsequent evaporation of leachates and storage of CWRs. Some of these minerals were also found as original species in FCs (calcite, gypsum, opal). However, inorganic amorphous matter (CWR1 and 2) and Ca–Mg–Na–K phase (CWR1) were not directly identified in FCs (Table 4). The crystalline Ca–Mg–Na–K phase is probably an organic oxalate mineral from the whewellite–weddelite group (CaMgNaK(C2O4)n d nH2O) according to the XRD pattern (Fig. 4), SEM investigation (Fig. 6d), chemical (Table 2) and mineral (Table 4) composition of CWR1. The inorganic amorphous matter is represented by cryptocrystalline, metacolloid or gel phases with compo-

sition similar to the above-mentioned minerals based on SEM studies. The occurrence of inorganic amorphous matter and Ca–Mg–Na–K phase in CWRs could be explained by the presence of identical or similar phases in coal, their dissolution in water and subsequent precipitation from the solutions leached. Hence, the highly enriched B, Ca, K, Mg, Mo, Na, S, Se, and Sr in CWRs associate with the above water-soluble minerals and phases in FCs. Additionally, the identified water-soluble halite and barite in Soma FCs (Karayigit et al., 2000) could also contribute for the leaching behaviour of Ba, Na, S, and Sr. 3.2. Characterization of bottom ashes and fly ashes 3.2.1. Common characteristics Some common characteristics of the BAs and FAs studied are listed in Tables 1, 5 and 6. General views of BA and FA are illustrated in Fig. 9a–c. BA1 shows relatively higher values of char, fixed C, volatile matter, and bulk density, and lower values of inorganic matter and inorganic amorphous matter in comparison with BA2. On the other hand, FA1 reveals

Table 6 Loss on ignition (LOI) determined at 750 8C/4 h of composite bottom ashes (BAs) and fly ashes (FAs) from Soma TPS (air-dried basis), wt.% Sample

LOI

Sample

LOI

BA1 BA1 BA1 BA1 BA1 BA1 BA2 BA2 BA2 BA2 BA2 BA2

17.8 21.2 6.7 6.5 17.9 12.1 9.0 11.5 5.8 5.0 9.3 7.8

FA1 FA1 FA1 FA1 FA1 FA1 FA2 FA2 FA2 FA2 FA2 FA2

2.7 6.2 1.3 1.5 2.8 2.2 1.8 5.5 2.0 1.5 2.6 0.9

13.4 16.4 6.3 5.8 13.6 10.0

Mean values FA1+2 FA1+2 (N200 Am) FA1+2 (100–200 Am) FA1+2 (63–100 Am) FA1+2 (N63 Am) FA1+2 (b63 Am)

(N200 Am) (100–200 Am) (63–100 Am) (N63 Am) (b63 Am) (N200 Am) (100–200 Am) (63–100 Am) (N63 Am) (b63 Am)

Mean values BA1+2 BA1+2 (N200 Am) BA1+2 (100–200 Am) BA1+2 (63–100 Am) BA1+2 (N63 Am) BA1+2 (b63 Am)

(N200 Am) (100–200 Am) (63–100 Am) (N63 Am) (b63 Am) (N200 Am) (100–200 Am) (63–100 Am) (N63 Am) (b63 Am)

2.3 5.9 1.7 1.5 2.7 1.6

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51

Fig. 9. SEM images of BA and FA (secondary electrons). (a) General view of BA1. The sample consists mainly of char, calcite, quartz, glass, portlandite, feldspars, and mullite. (b) General view of FA1. The sample contains mainly glass, quartz, feldspars, mullite, lime, and anhydrite. (c) General view of FA2. The sample is composed mainly of glass, quartz, mullite, feldspars, lime, and anhydrite. (d) Spongy char highly enriched in glass (1) and semi-fused quartz crystal (2) in FA2.

relatively higher values of mineral matter, char, volatile matter, and bulk density, and lower values of inorganic matter, inorganic amorphous matter, S, pH, and WR in comparison with FA2. However, most of the characteristics for both couples BA1–2 and FA1–2 normally do not show great quantitative variations, despite the different regional coal types used. FAs reveal relatively higher values of inorganic matter, inorganic amorphous matter, and bulk density, and lower values of moisture, mineral matter, char, fixed C, volatile matter, and calorific value in comparison with BAs. FAs are finely dispersed materials with grain size mainly b63 Am (50–60%), while BAs are coarser-grained products with grain size dominantly N200 Am (56–68%) (Table 5 and Fig. 9a–c). The couple BA2–FA2 has relatively finergrained size than the couple BA1–FA1. The higher ash and lower calorific value Denis coal shows better combustion performance in TPS than the central coal according to the loss on ignition (LOI) data for BAs and FAs (Table 6). LOI values commonly decrease with decreasing grain sizes of BA and FA fractions (Table 6), excluding higher values in the finest fraction b63 Am (BAs and FA1).

3.2.2. Chemical composition The data (ash basis) reveal that seven elements in BAs, namely siderophile (Fe, Mn, Sc), lithophile (Cs, Tb), non-metal (P), and chalcophile (Se) elements have average concentrations slightly greater (EDF=1.1–1.3, excluding Se) than FC ashes (Table 7). The element highly enriched in BAs (EDF=5.0) in comparison with FC ashes is only Se. The enrichment of the above elements in BAs could be related mainly to the preferable particle partitioning in the combustion chamber (see below). The results for both BAs show some differences. For example, BA1 has higher concentrations (by factor 1.5–1.6) of Ca and Mn, while BA2 has greater contents (by factor 1.5–3.2) of Be, Ce, Gd, Ge, Hf, La, Na, Nb, Pb, Sb, Sn, Th, Ti, Tl, V, W, Zn, and Zr (mainly lithophile and chalcophile elements). Hence, BA2 produced from coal of the Denis mines is more abundant in trace elements than BA1 generated from coal of the central mines in the Soma basin. The data (ash basis) show that 25 elements in FAs, namely lithophile (Al, Be, Ce, Cs, Eu, Gd, La, Li, Nd, Pr, Rb, Sm, W, Y), chalcophile (As, Cu, Ga, Pb, Se, Tl), siderophile (Cr, Sc, Ti) and radioactive (Th, U)

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Table 7 Element concentrations (air-dried and ash basis) of composite bottom ashes (BAs) and fly ashes (FAs) from Soma TPS, ppm (indicated otherwise) Element BA1 adb

BA2 ab

Ash (%) 82.2

adb

EDFa FA1

Mean ab

91.0

adb

ab

adb

86.6

FA2 ab

97.3

adb

EDFb FA/BAc

Mean ab

98.2

adb

ab

97.7

Non-metal elements B 321 391 467 513 394 455 0.7 P (%) 0.10 0.12 0.09 0.10 0.10 0.12 1.1 S (%) 0.4 0.5 0.5 0.5 0.5 0.6 0.3

584 600 715 728 650 665 1.0 0.11 0.11 0.10 0.10 0.11 0.11 1.0 0.7 0.7 1.1 1.1 0.9 0.9 0.4

1.5 0.9 1.5

Lithophile Al (%) Ba Be Ca (%) Ce Cs Dy Er Eu Gd Hf Ho K (%) La Li Lu Mg (%) Mo Na (%) Nb Nd Pr Rb Sm Sr Ta Tb Tm W Y Yb Zr

9.9 612 2.9 18.3 73 30 5.3 2.7 1.4 7.1 3.7 1.0 1.0 46 168 0.5 1.0 3.0 0.2 16 39 9.6 79 5.6 335 1.2 0.9 0.5 5.7 33 2.7 129

elements 7.3 8.9 428 521 1.0 1.2 17.4 21.2 49 60 23 28 3.5 4.3 2.0 2.4 1.1 1.3 4.6 5.6 2.1 2.6 0.9 1.1 0.7 0.9 30 36 93 113 0.3 0.4 0.8 1.0 1.5 1.8 0.2 0.2 10 12 25 30 6.6 8.0 51 62 4.1 5.0 266 324 0.7 0.9 0.9 1.1 0.3 0.4 1.3 1.6 24 29 1.8 2.2 76 92

10.1 523 3.5 11.8 81 24 5.5 2.5 1.5 7.5 3.7 1.1 0.8 50 147 0.4 0.7 1.8 0.3 18 39 10.5 69 5.5 282 1.1 1.0 0.4 2.7 34 2.5 136

11.1 575 3.8 13.0 89 26 6.0 2.7 1.6 8.2 4.1 1.2 0.9 55 162 0.4 0.8 2.0 0.3 20 43 11.5 76 6.0 310 1.2 1.1 0.4 3.0 37 2.7 149

8.7 476 2.3 14.6 65 24 4.5 2.3 1.3 6.1 2.9 1.0 0.8 40 120 0.4 0.8 1.7 0.3 14 32 8.6 60 4.8 274 0.9 1.0 0.4 2.0 29 2.2 106

10.0 550 2.7 16.9 75 28 5.2 2.7 1.5 7.0 3.3 1.2 0.9 46 139 0.5 0.9 2.0 0.3 16 37 9.9 69 5.5 316 1.0 1.2 0.5 2.3 33 2.5 122

1.0 0.9 0.8 1.0 0.9 1.3 0.9 0.9 1.0 0.9 0.8 0.9 0.8 0.9 0.7 0.8 1.0 0.5 0.8 0.8 0.9 1.0 0.9 0.9 1.0 0.5 1.1 0.8 0.6 0.9 0.8 0.8

Siderophile elements Co 10 12 13 14 12 14 Cr 65 79 99 109 82 95 Fe (%) 3.0 3.6 2.7 3.0 2.9 3.3 Mn 402 489 295 324 349 403 Ni 34 41 53 58 44 51 Sc 10 12 15 16 13 15 Ti 2590 3150 4180 4590 3390 3910 V 168 204 277 304 223 258

0.9 0.8 1.1 1.1 0.7 1.2 0.9 0.9

10.2 629 3.0 18.8 75 31 5.4 2.8 1.4 7.3 3.8 1.0 1.0 47 173 0.5 1.0 3.1 0.2 16 40 9.9 81 5.8 344 1.2 0.9 0.5 5.9 34 2.8 133

11.7 621 4.2 12.7 103 20 6.4 3.1 1.7 8.9 4.7 1.2 1.0 62 223 0.5 0.8 2.0 0.3 22 50 12.6 87 7.1 313 1.7 1.1 0.5 4.7 40 3.2 171

11.9 632 4.3 12.9 105 20 6.5 3.2 1.7 9.1 4.8 1.2 1.0 63 227 0.5 0.8 2.0 0.3 22 51 12.8 89 7.2 319 1.7 1.1 0.5 4.8 41 3.3 174

10.8 617 3.6 15.5 88 25 5.9 2.9 1.6 8.0 4.2 1.1 1.0 54 196 0.5 0.9 2.5 0.3 19 45 11.1 83 6.4 324 1.5 1.0 0.5 5.2 37 3.0 150

11.1 632 3.7 15.9 90 26 6.0 3.0 1.6 8.2 4.3 1.1 1.0 55 201 0.5 0.9 2.6 0.3 19 46 11.4 85 6.6 332 1.5 1.0 0.5 5.3 38 3.1 154

1.1 1.0 1.1 1.0 1.1 1.2 1.0 1.0 1.1 1.1 1.0 0.8 0.9 1.1 1.1 0.8 1.0 0.7 0.8 1.0 1.1 1.1 1.1 1.1 1.0 0.8 0.9 0.8 1.3 1.1 1.0 1.0

1.1 1.1 1.4 0.9 1.2 0.9 1.2 1.1 1.1 1.2 1.3 0.9 1.1 1.2 1.4 1.0 1.0 1.3 1.0 1.2 1.2 1.2 1.2 1.2 1.1 1.5 0.8 1.0 2.3 1.2 1.2 1.3

14 14 15 15 15 15 109 112 124 126 117 120 3.0 3.1 2.7 2.7 2.9 3.0 330 339 263 268 297 304 59 61 72 73 66 68 14 14 13 13 14 14 3890 4000 5150 5240 4520 4630 266 273 316 322 291 298

1.0 1.1 1.0 0.8 0.9 1.1 1.1 1.0

1.1 1.3 0.9 0.8 1.3 0.9 1.2 1.2

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53

Table 7 (continued) Element BA1 adb

BA2 ab

adb

Chalcophile elements As 25 30 Bi 0.1 0.1 Cd 0.5 0.6 Cu 33 40 Ga 14 17 Ge 1 1 Pb 16 19 Sb 0.4 0.5 Se 9 11 Sn 0.9 1.1 Tl 0.5 0.6 Zn 59 72

30 0.1 0.6 49 20 2 29 0.7 9 1.8 0.9 99

Radioactive elements Th 10 12 U 18 22

19 20

EDFa FA1

Mean ab

adb

33 0.1 0.7 54 22 2 32 0.8 10 2.0 1.0 109

21 22

ab

adb

FA2 ab

adb

EDFb FA/BAc

Mean ab

adb

ab

28 0.1 0.6 41 17 2 23 0.6 9 1.4 0.7 79

32 0.1 0.7 47 20 2 27 0.7 10 1.6 0.8 91

0.4 0.1 0.2 0.8 0.8 0.5 0.5 0.4 5.0 0.3 0.6 0.5

114 0.7 3.0 56 24 3 50 1.0 4 4.0 1.4 130

117 0.7 3.1 58 25 3 51 1.0 4 4.1 1.4 134

76 1.1 4.0 64 28 4 82 1.0 5 5.0 1.8 165

77 1.1 4.1 65 29 4 84 1.0 5 5.1 1.8 168

95 0.9 3.5 60 26 4 66 1.0 5 4.5 1.6 148

97 0.9 3.6 61 27 4 68 1.0 5 4.6 1.6 151

1.2 0.8 0.9 1.1 1.1 1.0 1.3 0.5 2.5 0.8 1.2 0.9

3.0 9.0 5.1 1.3 1.4 2.0 2.5 1.4 0.5 2.9 2.0 1.7

15 19

17 22

0.9 0.7

17 37

17 38

27 31

27 32

22 34

23 35

1.2 1.2

1.4 1.6

a Enrichment/depletion factor—a ratio of the mean element content in BAs (ash basis) to the mean respective element concentration in FCs (ash basis). b Enrichment/depletion factor—a ratio of the mean element content in FAs (ash basis) to the mean respective element concentration in FCs (ash basis). c Ratio of the mean element content in FAs (ash basis) to the mean respective element concentration in BAs (ash basis).

elements have average concentrations slightly greater (EDF=1.1–1.3, excluding Se) than FC ashes (Table 7). The element significantly enriched in FAs (EDF=2.5) in comparison with FC ashes is again Se. On the other hand, elements such as SNSbNMoN(Bi, Ho, Lu, Mn, Na, Sn, Ta, Tm)N(Cd, K, Ni, Tb, Zn) show significant depletions in FAs (EDF=0.4–0.9) in comparison with FC ashes mainly due to their volatilization behaviour in TPS (see Section 3.3). The trace elements in FA2 produced from the Denis coal commonly have slightly higher concentrations (by factor 1.1–1.6) than FA1 generated from the central coal (similar to FC2 and BA2). Most of the trace elements are slightly (by factor 1.1–1.7) more abundant in FAs compared to BAs (Table 7). However, some trace elements such as BiNCdNAsNSnNPbNWN(Ge, Tl) (mainly chalcophile ones) are highly more enriched (by factor 2.0–9.0) in FAs than BAs. On the other hand, some depletion (by factor 0.5–0.9) in FAs compared to BAs (Table 7) shows SeN(Mn, Tb)N(Ca, Cs, Fe, Ho, P, Sc). The enrichment of elements in FAs is related mainly to: (1) preferable particle partitioning of fine-grained phases

containing these elements to FAs; and (2) volatilization behaviour of these elements in the boiler and their subsequent significant condensation from flue gases on FA particles. The depletion of elements in FAs is connected mostly with: (1) preferable particle partitioning of coarse-grained phases containing these elements in the combustion chamber to BAs; and (2) their volatilization in the combustion chamber and subsequent limited condensation from the flue gases on FA particles, as explained later. 3.2.3. Mineralogy 3.2.3.1. Phase–mineral composition. The phase– mineral composition of BAs is given in Figs. 2 and 3, and Tables 8 and 9. BAs contain inorganic matterNmineral matterNinorganic amorphous matterN organic matter (BA2) or inorganic matterNmineral matterNorganic matterNinorganic amorphous matter (BA1). The inorganic amorphous matter is composed mostly of glass, to a lesser extent amorphous unfused clay and mica products, and some poorly crystallized

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Table 8 Occurrence and abundance of minerals and phases identified (F—forming, N10%; M—major, N1–10%; Mi—minor, 0.5–1%; A—accessory, b0.5%) and their probable origin (P—primary; S—secondary; T—tertiary; !—dominant; o—subordinate) in composite bottom ashes (BAs) and fly ashes (FAs) from Soma TPS Mineral, phase

Formula

BA1+2 (mean)

FA1+2 (mean)

Silicates Quartz Cristobalite Kaolinite–metakaolinite Illite+ Muscovite Mullite Plagioclase K-feldspar Melilite

SiO2 SiO2 Al2Si2O5(OH)4–Al2Si2O5 (KH2O)Al2(AlSi)Si3O10(OH)2 KAl2AlSi3O10(OH)2 Al6Si2O13 NaAlSi3O8–CaAl2Si2O8 KAlSi3O8 Ca2MgSi2O7–CaAl(SiAl)2O7

F M M M

F M M M

M M Mi M

M M M M

Oxides and hydroxides Magnetite Hematite Ferrian spinel Lime Portlandite

FeFe2O4 a-Fe2O3 Mg(AlFe)2O4 CaO Ca(OH)2

Mi M

M M A M M

Carbonates Calcite Dolomite Ankerite

CaCO3 CaMg(CO3)2 Ca(MgFe)(CO3)2

F M Mi

Mi A

Sulphates Anhydrite Gypsum

CaSO4 CaSO4d 2H2O

Mi

M A

Others Glass Char Ca–K–Na phase

M

F F

F M A

Origina P

S

!

o ! o o

! !

o !

o o o

T

! ! o !

! ! ! ! o

!

o o o

! ! !

! !

o

! ! !

a

Primary—original (coal) mineral or phase in BAs and FAs that has undergone no phase transformation during coal combustion; secondary—new phase or mineral in BAs and FAs formed during coal combustion; tertiary—new mineral or phase formed during water treatment, drying and storage of BAs and FAs.

phases. The average mineral matter content is represented by silicates (43%)Ncarbonates (16%)Noxyhydroxides (11%)Nsulphates (1%). The mean phase–mineral composition (in decreasing order of significance) of BAs includes glass, calcite, char, quartz, mullite, plagioclase, portlandite, melilite, illite+muscovite, kaolinite–metakaolinite, hematite, dolomite, cristobalite, K-feldspar, magnetite, anhydrite, and ankerite. Both BAs reveal similar qualitative mineral composition, excluding the occurrence of dolomite and ankerite in BA2. Higher contents of mineral matter, char, kaolinite–metakaolinite, magnetite, hematite,

portlandite, and calcite, and lower concentrations of inorganic matter, inorganic amorphous matter, cristobalite, illite+muscovite, mullite, plagioclase, and melilite in BA1 than in BA2 were also found. The phase–mineral composition of FAs is given in Figs. 2 and 3, Figs. 9–11, and Tables 8 and 9. The FAs contain inorganic matterNmineral matterNinorganic amorphous matterNorganic matter, whereas the mean mineral matter content is represented by silicates (39%)Noxyhydroxides (17%)Nsulphates (6%)Ncarbonates (1%). Hence, inorganic matter, glass, oxyhydroxides, and sulphates are more characteristic of FAs,

S.V. Vassilev et al. / International Journal of Coal Geology 61 (2005) 35–63

55

10d), anhydrite (Fig. 10d), microsphere (Fig. 11a), plerosphere (Fig. 11b), and dermasphere (Fig. 11c) are illustrated. There are significant qualitative and quantitative phase distinctions between the composition of FAs and BAs (Tables 8 and 9). For example, ankerite was not found in FAs, while Fe-spinel, lime, gypsum, and Ca–K–Na phase were not identified in BAs. FAs are enriched in cristobalite, mullite, Fe oxides (magnetite, hematite), lime, and anhydrite, and depleted in quartz, clay and mica minerals (kaolinite–metakaolinite, illite, muscovite), melilite, portlandite, and carbonates (calcite, dolomite, ankerite) in comparison with BAs. On the other hand, FAs are slightly enriched in quartz and feldspars in comparison with BAs on crystalline matter basis (Table 9). The present study cannot explain the modes of trace element occurrence in BAs and FAs. However, most of the elements (excluding Ca, Cs, Fe, Ho, Lu, Mg, Mn, Na, P, Sc, Se, Tb, and Tm) show a predominant association with fine-grained glass, lime,

while mineral matter, char, silicates, and carbonates are more typical of BAs. The mean phase–mineral composition (in decreasing order of significance) of FAs includes glass, quartz, mullite, plagioclase, lime, anhydrite, hematite, portlandite, illite+muscovite, melilite, magnetite, char, cristobalite, kaolinite–metakaolinite, K-feldspar, calcite, dolomite, Fe-spinel, gypsum, and Ca–K–Na phase (Tables 8 and 9). Both FAs reveal similar qualitative and quantitative phase composition. Slightly higher contents of mineral matter, char, feldspars, magnetite, hematite, and portlandite, and slightly lower concentrations of inorganic matter, inorganic amorphous matter, quartz, and kaolinite in FA1 than in FA2 were found. SEM investigations of FAs confirmed the results achieved by XRD and optical microscopy and gave additional information. For example, the occurrence of various minerals, phases and particles such as char (Figs. 9d, 10d, and 11d), glass (Figs. 9d and 11d), quartz (Fig. 9d), metakaolinite (Fig. 10a), plagioclase (Fig. 10b), melilite (Fig. 10c), cenosphere (Fig. 10c), lime (Fig.

Table 9 Phase–mineral composition and content (determined by XRD and LOI) of composite bottom ashes (BAs) and fly ashes (FAs) from Soma TPS Mineral, phase

BA1

Inorganic matter Mineral matter Inorganic amorphous matter Organic matter

82.2 71.4 10.8 17.8

adb

Mineral matter Quartz Cristobalite Kaolinite–metakaolinite Illite+Muscovite Mullite Plagioclase K-feldspar Melilite Magnetite Hematite Lime Portlandite Calcite Dolomite Ankerite Anhydrite Total

BA2 cb

adb

Mean cb

91.0 70.4 20.6 9.0

adb

FA1 cb

86.6 70.9 15.7 13.4

adb

FA2 cb

97.3 63.1 34.2 2.7

13 1 4 3 7 7 1 4 1 3

18 2 5 4 9 9 2 6 2 4

13 2 2 6 8 8 1 6 0.5 2

18 3 3 8 11 11 2 8 1 3

13 1.5 3 4.5 7.5 7.5 1 5 1 2.5

18 2.5 4 6 10 10 2 7 1.5 3.5

9 17

13 23

1 71

2 99

6 10 4 1 1 70.5

8 14 5 2 2 99

7.5 13.5 2 0.5 1 71

10.5 18.5 2.5 1 2 99

adb

Mean cb

98.2 61.1 37.1 1.8

adb

cb

97.7 62.1 35.6 2.3

10 2 1 3 9 8 2 3 4 5 6 4 1

16 3 2 4 14 12 3 5 6 8 10 6 2

13 2 2 3 9 6 1 3 2 4 6 3 1

22 3 3 5 14 10 2 5 4 6 9 5 2

11.5 2 1.5 3 9 7 1.5 3 3 4.5 6 3.5 1

19 3 2.5 4.5 14 11 2.5 5 5 7 9.5 5.5 2

6 64

9 100

6 61

10 100

6 62.5

9.5 100

The mineral and phase abundance is for air-dried (adb) and crystalline basis (cb), wt.%.

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Fig. 10. SEM images of FA (secondary electrons). (a) Semi-fused metakaolinite aggregate in FA2. (b) Plagioclase spheroid in FA2. (c) Melilite sphere (1) and aluminosilicate cenosphere (2) in FA2. (d) Spongy char highly enriched in lime and anhydrite in FA2.

mullite, magnetite–hematite, anhydrite, and accessory minerals in FAs. On the other hand, a preferable association of Ca, Cs, Fe, Ho, Mn, P, Sc, Se, and Tb

with coarser-grained carbonates, glass, char, clay minerals, portlandite, and melilite in BAs can be also emphasized (Tables 7 and 9). The characterization of

Fig. 11. SEM images of FA (secondary electrons). (a) Aluminosilicate glass microspheres in FA1. (b) Aluminosilicate plerosphere in FA2. (c) Aluminosilicate dermasphere in FA2. (d) Inertinite char enriched in glass in FA2.

S.V. Vassilev et al. / International Journal of Coal Geology 61 (2005) 35–63 Table 10 Balance factor (see the text) for elements during coal combustion in Soma TPS, wt.% Element

Units B1–4

Units B5–6

Units B1–6

Non-metal elements B
25 P
7 S
70

4 0
48


11
4
59

Lithophile elements Al 14 Ba 0 Be
2 Ca
18 Ce 24 Cs 38 Dy 13 Er 13 Eu 15 Gd 27 Hf 23 Ho 2 K
2 La 24 Li 26 Lu
4 Mg 0 Mo
43 Na 0 Nb 27 Nd 31 Pr 27 Rb 15 Sm 31 Sr 2 Ta
19 Tb 34 Tm
4 W 74 Y 6 Yb 12 Zr 20


2 3 2 17 1 1
6
6
1
2
7
14
2 2
11
20 0 0
25
6 3 1 4
1
3
30
8
20
8
2
9
7

6 2 0
1 13 20 4 4 7 13 8
6
2 13 8
12 0
22
13 11 17 14 10 15
1
25 13
12 33 2 2 7

Siderophile elements Co
3 Cr 12 Fe 3 Mn
26 Ni
10 Sc 36 Ti 21 V 16


8
6
5 11
20
3
3
4


6 3
1
8
15 17 9 6

Chalcophile elements As 8 Bi
17


3
25

3
21

57

Table 10 (continued) Element

Units B1–4

Units B5–6

Units B1–6

Chalcophile elements Cd 30 Cu 18 Ga 23 Ge 30 Pb 14 Sb
55 Se 170 Sn
30 Tl 24 Zn
24


15
8 2
10 15
52 200
25 17
9

8 5 13 10 15
54 185
28 21
17

Radioactive elements Th 33 U 12

3 3

18 8

fractions separated from FAs in another publication will give additional data about the modes of trace element occurrence. 3.2.3.2. Genetic features. The genesis of minerals and phases in BAs and FAs is: (1) primary (original coal minerals or phases that have undergone no phase transformation during coal combustion); (2) secondary (new phases or minerals formed during coal combustion); and (3) tertiary (new minerals or phases formed during water treatment, drying and storage) (Table 8). This differentiation has been discussed in detail earlier (Vassilev and Vassileva, 1996a). The silicates in BAs and FAs (Table 8) are both primary (quartz, kaolinite, illite, muscovite, plagioclase, K-feldspar) and secondary (quartz, cristobalite, metakaolinite, metaillite, mullite, plagioclase, K-feldspar, melilite). The new-formed silicates are a result of partial dehydroxylation and destruction (metakaolinite, metaillite), solid phase reactions (mullite, anorthite, melilite) or crystallization from silicate melts (quartz, cristobalite, mullite, plagioclase, K-feldspar, melilite). SEM images of semi-fused primary quartz crystal (Fig. 9a), semi-fused secondary metakaolinite aggregate (Fig. 10a), secondary plagioclase spheroid (Fig. 10b), and secondary melilite sphere (Fig. 10c) are illustrated. The oxides, namely magnetite, hematite, Fe-spinel, and lime (Fig. 10d) in BAs and FAs are secondary due to the decomposition of pyrite and Ca–Mg–Fe carbo-

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nates, crystallization of Fe and Ca components liberated from organic matter of coal, and crystallization from melts (Table 8). The formation of portlandite is both secondary and tertiary in BAs and FAs. The latter origin is dominant due to the lime hydration. For example, BAs (fallen in water tanks under the boilers) show a characteristic portlandite occurrence, while FAs reveal a typical lime presence as a result of their collection under dry conditions (Table 9). The carbonates (calcite, dolomite, ankerite) in BAs and FAs (Table 8) are primary, secondary, and, particularly, tertiary in origin. They could be original coal minerals that were not transformed in the boilers due to their occurrence as coarse grains, limited residence time in combustion chambers, and lower combustion temperatures (b900 8C) in some parts of the boilers. For instance, the carbonates show a more characteristic presence in BAs than in FAs (Table 9). BAs are coarser-grained products (Table 5) and contain some original unchanged calcite lumps that drop faster under the combustion chambers and quench in the water tanks. In contrast, the finergrained carbonates are transformed easily to lime in boilers and pass mostly into FAs (Table 9). Additionally, FAs experience longer residence time at relatively high temperatures together with hot flue gases in TPS. It should be noted that some new-formed secondary carbonates in FAs and BAs could also occur as a result of reaction between Ca–Mg–Fe oxides (liberated from organic matter, carbonates, and sulphides) and CO2 (released from organic matter and carbonates) during combustion process. However, the tertiary carbonates in BAs and FAs are dominant and their formation is due to the reaction between the above listed oxides and CO2 (from air and water) during water treatment, drying, and storage of BAs and FAs. For instance, the tertiary calcite occurrence is common after water treatment of FAs at the expense of lime and portlandite carbonatization (Vassilev and Vassileva, 1996a; Vassilev et al., 2003). The sulphates in BAs and FAs are represented by new-formed anhydrite (Fig. 10d) and gypsum. The secondary anhydrite is a result of gypsum dehydration, pore-water crystallization, and a reaction between Ca and S oxides liberated mostly from organic matter, carbonates, and pyrite. The gypsum occurrence is tertiary in FAs due to anhydrite hydration (Table 8).

The occurrence of inorganic amorphous matter in BAs and FAs is mostly secondary as a result of: (1) glass formation from silicate melts; (2) to a lesser extent dehydroxylation and destruction of clay and mica minerals; and (3) probable limited formation of some non-crystalline or poorly crystallized matter from organic matter and inorganic matter of coal. The phases resulted from processes 2 and 3 were not undergone complete fusion (Fig. 10a) in the boilers in contrast to the minerals and phases forming glass (Figs. 9a–d, Fig. 10c, and Figs. 11a–d). The primary glass in BAs and FAs could be related to some original volcanic glass occurrence in coal (Table 3 and Fig. 6b). BAs are depleted in inorganic amorphous matter in comparison with FAs. The reason for that seems to be the dominant occurrence of coarser-grained quartz, clay and carbonate particles in BAs and their limited decomposition and melting during combustion process. The glass phases have dominantly aluminosilicate composition enriched in Ca and occasionally in Fe. They were observed mainly as angular to rounded particles (Figs. 9a–d and 11d) and solid microspheres (Figs. 9a–c and 11a and d), and, to a lesser extent, cenospheres (Figs. 9b and 10c), plerospheres (Fig. 11b), and dermaspheres (Fig. 11c). These glass types have been described earlier (Vassilev and Vassileva, 1996a; Vassilev et al., 2003). The char is represented by secondary semicoked and coked coal particles (Figs. 9d, 10d, 11d) in BAs and FAs. The coarser-grained size of organic particles in FCs is the major reason for the char enrichment in BAs (Tables 5 and 6). Such particles exhibit depressed oxidation due to their size and short residence time in the boilers during combustion process in TPS. The WR yields from FAs (Table 1) are 1.7–2.6% and higher than CWR yields. Hence, the combustion process in TPS enhances the formation of water-soluble phases in FAs. The abundance of Cabearing minerals such as oxides (lime), hydroxides (portlandite), carbonates (calcite), sulphates (anhydrite, gypsum), and silicates (anorthite, melilite) play a leading role for the properties of these FAs and BAs. The formation of Ca silicates was described above, while the mechanisms for lime (1) formation, (2) sulphatation, (3) hydration and

S.V. Vassilev et al. / International Journal of Coal Geology 61 (2005) 35–63

sulphatation, (4) hydration and (5) subsequent carbonatization and dehydration with formations of the most stable calcite are: CaCO3 YCaO þ CO2

ð1Þ

CaO þ SO2 þ 1=2O2 YCaðSOÞ4

ð2Þ

CaO þ 2H2 O þ SO2 þ 1=2O2 YCaðSOÞ4 d2H2 O ð3Þ CaO þ H2 OYCaðOHÞ2

ð4Þ

CaðOHÞ2 þ CO2 YCaCO3 þ H2 O

ð5Þ

3.2.3.3. Thermal behaviour. The thermal behaviour of BAs and FAs in air is illustrated in Figs. 7 and 8. The DTA–TGA profiles of both BAs (Figs. 7 and 8) are relatively similar and show: (1) loss of adsorbed water up to 150 8C; (2) char combustion at 200–550 8C (large exothermic effects with maximums at 437 and 458 8C) accompanied by intensive mass loss; (3) portlandite dehydroxylation (endothermic effects at 500–650 8C) associated with significant mass loss; (4) calcite decomposition at 650–750 8C (endothermic effects with maximums at 710 and 737 8C) accompanied by intensive mass loss; (5) anhydrite decomposition at 1150–1250 8C (endothermic effects with maximums at 1196 and 1216 8C) associated with significant mass loss; and (6) fluid ash-fusion of BA2 at about 1279 8C (endothermic effect) accompanied by some mass loss. The DTA–TGA profiles of both FAs (Figs. 7 and 8) are also similar, but have some differences up to 750 8C in comparison with the thermal behaviour of BAs. The profiles of FAs show: (1) adsorption of some moisture (from air) by highly reactive lime and subsequent hydroxylation to portlandite up to 600 8C (large exothermic reactions with maximums at 410 and 440 8C) accompanied by small increasing of mass; (2) char combustion at about 440–550 8C (exothermic effects with maximums at 488 and 489 8C) associated with significant mass loss; (3) portlandite dehydroxylation and calcite decomposition at 600–690 8C (endothermic effects with maximums at 654 and 670 8C) accompanied by intensive mass loss;

59

(4) anhydrite decomposition at 1150–1230 8C (endothermic effects with maximums at 1179 and 1213 8C) associated with intensive mass loss; and (5) fluid ashfusion of FA2 at about 1272 8C (endothermic effect) accompanied by some mass loss. The above data show that a problem occurs with the determination of organic matter content in combustion residues (in particular BAs) based on the standard LOI measurements at 750 8C for this type of coal products (highly enriched in Ca). The abundance of portlandite and calcite in BAs, and, to a lesser extent, in FAs (Table 9) causes this problem. The DTA–TGA profiles reveal that the dehydroxylation and decarbonatization of these minerals occur in the temperature interval 500–750 8C and the char values (based on LOI experiments) are overestimated. Hence, the present results for char contents based on LOI measurements should be used with caution (in particular for BAs). Additional methods, namely C measurement and/or incineration up to 500 8C, are required for the determination of actual organic matter contents. 3.3. Elemental balance during coal combustion The balance factor (BF) by Egorov et al. (1979) was used in order to determine the element proportions (in %), which did not bind in the collected solid waste products (Table 10). This factor is based on the formula: BF ¼

CFA K1 þ CBA K2
CFC  100 CFC

where C FA is the element content in FA (ash basis); C BA is the element content in BA (ash basis); C FC is the element content in FC (ash basis); K 1 (0.8) and K 2 (0.2) are coefficients showing the ratio between FA and BA produced from the Soma TPS, respectively. Such calculations reveal elements that have the susceptibility to liberate into the atmosphere. It was found that many elements initially present in FCs were partly emitted by stack emissions (in solid, liquid and gas states) during coal combustion in the Soma TPS. For example, significant proportions (11– 59%) of elements such as SNSbNSnNTaNMoNBiNZnN NiNNaN(Lu, Tm)NB were volatilized and not captured by the cleaning equipment (Table 10). These elements (with negative imbalance of 10–70%) are

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S.V. Vassilev et al. / International Journal of Coal Geology 61 (2005) 35–63

SNSbNMoNSnNMnNBNZnNTaNCaNBiNNi for the Units B1–4; and SbNSNTaN(Bi, Na, Sn)N(Lu, Ni, Tm)NCdNHoNLiNGe for the Units B5–6 (Table 10). It can be seen that most of them belong to TPT elements. The reasons for this negative imbalance of elements could be: (1) partial volatilization (evaporation and sublimation) of elements in the combustion chamber and their incomplete condensation on FA; (2) migration of elements trough the finest ash particles that were not caught in the cleaning equipments; (3) capture and remaining of elements on the TPS’s metal surfaces (Vassilev et al., 2001). The first two reasons are dominant, while the last one has negligible importance for the total balance of elements during coal combustion. Some principle factors for the element behaviour (partitioning, volatilization, condensation, capture, and retention) in TPS have been discussed earlier (Vassilev et al., 2001). The modes of element occurrence in FCs, BAs, and FAs are the fundamental guide for the volatilization behaviour of elements during combustion. The most volatile Bi, Mo, Ni, S, Sb, Sn, Ta, and Zn in the Soma TPS probably show a tendency for concentration in the easily decomposing organic matter and authigenic coal minerals (sulphides, carbonates, sulphates, and some hydrosilicates). For instance, the Soma coal reveals increased authigenic mineralization. These bearing phases of trace elements exhibit high vapour pressure developed during the phase decomposition. Such unstable phases favor the trace element mobility together with volatile C, S, N and other gases, water vapour or their condensed products in the flue emissions generated during coal combustion in the Soma TPS. Proportions of extremely volatile gas species with low initial dewpoint temperatures (probably some of the most volatile elements) may leave the Soma TPS as vapours without condensation and capture in FAs. On the other hand, solid phases with a dimension b10 Am in FAs may also escape easily the particulate control systems in TPS (Fisher and Natusch, 1979). Some of the volatile elements (B, Bi, Ca, Cd, Ge, Ho, Li, Lu, Mn, Mo, Na, Ni, S, Sb, Sn, Ta, Tm, and Zn) in the Soma TPS may occur as original or newly formed finely dispersed (b10 Am) discrete phases and particles in FAs. For example, proper phases of Ca, Mn, Na, S, Sb, and Zn with particle size of 0.01–10 Am were found in FAs (Vassilev and Vassileva, 1996a;

Vassilev et al., 2003). Such micrometer and dominantly submicrometer in size solid phases are difficult to be caught in the cleaning equipment at TPS. In contrast, the non-volatile or slightly volatile elements in the Soma TPS probably occur predominantly in original or newly formed coarser-grained (10–100 Am) relatively refractory phases and particles in FA. These minerals and phases include glass, kaolinite– metakaolinite, mica, feldspars, Al, Fe, Ca, Mg and Ti oxides, apatite, zircon, and other inert species, which undergo weak changes during combustion process at 1200–1600 8C (Vassilev and Vassileva, 1996b; Vassilev et al., 2001, 2003). Hence, the behaviour of elements during coal combustion strongly depends on the boiler temperatures and modes of element occurrence in coals and their combustion residues. 3.4. Possible environmental concerns and potential utilization directions The present results show that elements such as B, Ca, K, Mg, Mo, Na, S, Se, and Sr occur as watersoluble forms in the Soma coals and significant amount of them are leached during water treatment. These highly mobile elements may contaminate the environment (surface, subsoil and drinking waters, soils, and plants) in the areas surrounding the coal mines and fuel depositories at TPSs during mining and storage of the Soma coals. On the other hand, the high concentrations of Ca, Cs, Li, Nb, and V in FCs, and the large amounts of the above-listed water leachable elements from FCs may have some resource recovery potential. The present study also provides a preliminary prediction of some possible environmental concerns during combustion of the Soma coals in TPSs. For example, FAs are more abundant in trace elements than BAs in the Soma TPS and the former products should have more detailed environmental and utilization concerns. Additionally, significant proportions of elements such as B, Bi, Ca, Cd, Ge, Ho, Li, Lu, Mn, Mo, Na, Ni, S, Sb, Sn, Ta, Tm, and Zn were emitted by stack emissions and not captured by the cleaning equipment in the Soma TPS. These elements may pose some air, water, soil and plant threats in the area surrounding the Soma TPS and may provoke environmental problems. On the other hand, some elements show relatively high retention effects in the combustion residues from the Soma TPS. For example,

S.V. Vassilev et al. / International Journal of Coal Geology 61 (2005) 35–63

elements such as Cs, Ga, La, Nb, Pb, Rb, Sc, Se, Th, Tl, W, and most of the rare-earth elements show high positive imbalance (BF=10–185%) in the solid combustion products (Table 10). The reason for this high retention behaviour of the above listed elements could be their favorable: (1) modes of element occurrences in FCs, BAs and FAs; (2) particle partitioning in TPS; and (3) volatilization–condensation mechanism in TPS. Some of these trace elements (Pb, Se, Th, Tl) belong to TPT elements, which is favorable for the Soma TPS due to their high capture by the cleaning equipment. Calcite, lime, portlandite, char, and clay minerals in the Soma FCs, BAs, and FAs probably play an important sorbent role for the high retention behaviour of these elements. The present results also indicate some potential utilization directions of the combustion residues. For example, FAs exhibit low bulk density and char contents, and valuable self-cementation, pozzolanic and size properties. These characteristics make FAs favorable materials in the building industry. On the other hand, some fractions isolated from BAs or FAs may contain valuable minerals, phases, particles or elements that could be in sufficient concentrations to be commercially recovered and utilized. For instance, Se shows significant enrichments in BAs and FAs. Some fractions separated from the Soma FAs will be characterized in another paper.

4. Conclusions FCs are high-ash Soma subbituminous coals abundant in moisture and Ca, and depleted in S. The phase–mineral and chemical composition of FCs, BAs, and FAs for both power facilities (Units B1–4 and Units B5–6) does not show great variations, despite the different regional coal types used. However, FC, WCR, BA, and FA samples originated from the Denis mines are relatively more abundant in trace elements than the respective samples generated from the central mines in the Soma basin. FCs have moderate detrital mineral abundance and increased authigenic mineralization dominated by carbonate than sulphide–sulphate tendency. The inorganic composition (in decreasing order of significance) of FCs includes calcite, quartz, kaolinite, illite+muscovite, chlorite, plagioclase, gyp-

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sum, pyrite, montmorillonite, K-feldspar, dolomite, siderite, ankerite, opal, and volcanic glass. Elements such as CaNNbNCsN(V, Li) have significantly higher contents in FC ashes than the respective Clarke values for coal ashes. The composition (in decreasing order of significance) of CWRs isolated from FCs includes gypsum, calcite, inorganic amorphous matter, Ca–Mg–Na–K phase, and opal. These CWRs are enriched in NaNSeNSNBNMgNMoNSrNCaNK compared to FC ashes and such mobile elements could contaminate the environment during coal mining and processing. The phase–mineral composition of BAs and FAs includes mainly glass, quartz, char, mullite, plagioclase, calcite, and portlandite; and, to a lesser extent, illite+muscovite, melilite, hematite, anhydrite, lime, cristobalite, kaolinite, and magnetite. Minor amounts of K-feldspar, dolomite, ankerite, Fe-spinel, gypsum, and Ca–K–Na phase also occur in BAs and FAs. FAs show higher values of bulk density, inorganic matter, glass, cristobalite, mullite, Fe oxides, lime, and anhydrite, and lower values of mineral matter, char, quartz, clay minerals, melilite, portlandite, and carbonates in comparison with BAs. Primary minerals in BAs and FAs are some quartz, clay and mica minerals, feldspars, carbonates, and glass. Secondary minerals and phases formed during combustion process are quartz, cristobalite, metakaolinite, metaillite, mullite, feldspars, melilite, Fe oxides, lime, portlandite, carbonates, anhydrite, glass, and char. Tertiary minerals and phases are portlandite, carbonates, and gypsum. The data for BAs and FAs reveal that only Se has significantly higher contents compared to FC ashes. Most of the trace elements (in particular As, Bi, Cd, Ge, Pb, Sn, Tl, W) are more enriched in FAs, while BAs are more abundant in Ca, Cs, Fe, Ho, Mn, P, Sc, Se, and Tb. The enrichment or depletion of such elements is related to particle partitioning and volatilization–condensation behaviour of elements during combustion process. Significant percentages (11–59%) of elements initially present in FCs, namely SNSbNSnNTaNMoNBiNZnNNiNNaN(Lu, Tm)NB, were emitted by stack emissions and not captured by the cleaning equipment in the Soma TPS. Possible environmental pollutions of the air, water, soils, and plants in the area surrounding the Soma TPS could be supposed for these elements. The potential utilization direction of FAs and BAs is

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related mainly to their application in the building industry. Nomenclature AES atomic emission spectroscopy BA bottom ash CLMC Central Laboratory of Mineralogy and Crystallography, Bulgaria CWR water-soluble residue from coal DTA differential thermal analysis EDX energy dispersive X-ray analysis FA fly ash FC feed coal HTA high-temperature ash HU Hacettepe University, Turkey ICP inductively coupled plasma IES Institute of Earth Sciences, Spain MS mass spectroscopy SEM scanning electron microscopy TGA thermo-gravimetric analysis TPS thermo-electric power station WR water-soluble residue from fly ash XRD X-ray diffraction

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