FUEL

PROCESSING
TECHNOLOGY
Fuel Processing Technology 5 I (I 997) 19-45

Geochemistry of coals, coal ashes and combustion

wastes from coal-fired power stations
Stanislav V. Vassilev *, Christina G. Vassileva
Central Laborato~ of Mineralogy and Cqvtallography, 92 Rakouski Str., Bulgarian Acudemy c~fSciences.
So@ 1000, Bulgaria

Received 7 March 1996; accepted 7 November 1996

Abstract

Contents, concentration trends, and modes of occurrence of 67 elements in coals, coal ashes,
and combustion wastes at eleven Bulgarian thermoelectric power stations (TPS) were studied. A
number of trace elements in coal and coal ash have concentrations greater than their respective
worldwide average contents (Clarke values). The highest values in coal ash are displayed by
elements such as Rb, Cs, Ba, Cu, Sb, Bi, U, and Ag. Trace elements are concentrated mainly in
the heavy accessory minerals and organic matter in coal. In decreasing order of significance, the
trace elements in coal may occur as: element-organic compounds; impurities in the mineral
matter; major components in the mineral matter; major and impurity components in the inorganic
amorphous matter; and elements in the fluid constituent. A number of trace elements in the waste
products, similar to coal ashes, exceed known Clarke contents. Trace elements are mainly
enriched in non-magnetic, heavy and fine-grained fractions of fly ash. They are commonly present
as impurities in the glass phases, and are included in the crystalline components. Their accessory
crystalline phases, element-organic compounds, liquid and gas forms, are of subordinate impor-
tance. Some elements from the chalcophile (Cu, Zn, Ga, Ge, Pb, As, Sb), lithophile (Be, Ba, Ce,
Hf, Sr, La, Zr, MO, U) and siderophile (SC, Cr, V) groups may release into the atmosphere during
coal burning. For others, the combustion process appears to be a powerful factor causing their
relative enrichment in the fly ash and rarely in the bottom ash and slag. Considerable amounts of
Hf, some chalcophile elements (Zn, As, Tl, Pb) and Ag from stack emissions have probably
entered the soil near TPS. Trace elements can also occur in water soluble forms in the fly ash (Li,

* Corresponding author. Present address: Universiti Libre de Bruxelles, Service de Chimie G&t&ale et
Carbochimie, Facultt des Sciences Appliqkes, CP 165, 50 Avenue F.D. Roosevelt, B-1050 Bruxelles,
Belgium.

0378-3820/97/$17.00 0 1997 Elsevier Science B.V. All rights reserved.

PII SO378-3820(96)01082-X
20 S. V. Vassileu, C.G. Vassileva / Fuel Processing Technology 51 (1997) 19-45

Sr, MO, Cs, V, Cr, Mn, As, Bi, B, F, Cl, Br, I> and probably contaminate the surface and subsoil
waters. Some trace elements (Sr, Ba, Yb, SC, Cd, Tl, Pb, Bi) may accumulate in the vegetation
near TPS. 0 1997 Elsevier Science B.V.

Keywords: Coal, coal ash and fly ash; Chemical composition; Trace elements

1. Introduction

Extensive studies on contents, concentration trends, distributions, organic-inorganic

affinities and modes of occurrence of elements in different coals and their low-tempera-
ture (LTA) and high-temperature ashes (HTA) have been conducted by many authors.
Summarized data on major, minor and trace elements in coal have been reported [l-9].
Studies on the content of elements in fly ash (FA), and to a lesser extent in bottom ash
CBA), slag and lagooned ash-slag (LAS) from coal burning at thermoelectric power
stations (TPS) worldwide have been repeatedly performed. Additional attention has been
given to trace elements in different particle-sized [ 10-171, density [ 16,171 and magnetic
fractions [17-201. The concentration of trace elements in glass phases, quartz-mullite
matrix, magnetic spine1 matrix [211, pyrite [22], and surface salts of glassy material
[23,24], as well as the affinity of elements to glass, mullite, feldspar, Fe oxide, Ca oxide,
Ca sulphate, and unburnt coal particles [17] have been characterized to a certain extent.
Special attention has been also given to the volatilization-condensation process [ 11,13-
15,17,25-34] and material balance of trace elements in TPS [11,25,26,28,35-371.
Surprisingly, simultaneous studies on the geochemistry and mineralogy of coals and
their various by-products are restricted. An advanced systematic study on the trace
elements in coal and combustion wastes in a large power station has been published
recently [17]. Such investigations are important in the elucidation of elemental be-
haviours during coal burning, and in the prediction and evaluation of possible utiliza-
tions and environmental impacts of the combustion products.
This paper generalizes a systematic characterization of contents, concentration trends,
behaviours and modes of occurrence of different major, minor and especially trace
elements in coals and waste products related to coal burning at eleven Bulgarian TPS.
The present work is a summary of studies reported earlier [38-501 and also includes
additional unpublished data. The paper is a continuation of mineralogical studies on
coals and their combustion wastes which have been described recently [51,52].

2. Material and methods

Three major types of samples from solid waste products, generated from dry-ash and
slag discharge boilers in TPS, were collected and examined: BA and slag from under the
combustion chambers; FA from the hoppers of electrostatic precipitators and mechanical
collectors; and lagooned ash (LA) and LAS taken from a set of points at different depths
from disposal sites near TPS. The samples from BA, slag, FA, LA and LAS were
composed of a large number of single samples (30-70). The samples studied are from
 S. V. I/ass&r. C. G. Vassiler,a / Fuel Processing Technology 51 (I 9971 19-45 21

eleven large Bulgarian TPS with pulverized coal-fired systems. These TPS use Bulgar-
ian lignitic coals from Maritza-West (Maritza-3 TPS), Sofia (Kremikovtzi TPS) and
Maritza-East (Maritza-East 1, 2 and 3 TPS) deposits; Bulgarian sub-bituminous coals
from Bobov Do1 (Bobov Do1 TPS) and Pernik (Republica TPS) deposits; and Ukrainian
bituminous and anthracitic coals from Donbass deposit (Russe, Svishtov, Vama and
Devnya TPS).
For parallel investigations, composite samples of coals from bunkers of the aforesaid
TPS and some coal preparation plants were taken. In addition, soils near TPS (200-2000
m from the chimney-stacks), grass vegetation from the LA and LAS disposals and waste
waters from the disposal ponds, were also collected and studied.
LTA and HTA from coal were prepared in an oxygen plasma asher and in an electric
furnace, respectively. The temperature for oxygen plasma ashing was estimated to be
below 200°C while the ashing temperature in the electric furnace was 450-815°C.
Sieving, heavy liquids, and magnetic separations, as well as hand picking under a
binocular stereomicroscope were applied to concentrate the minerals and phases present
in coal and waste products. Chemical analyses for elements were performed by atomic
emission spectroscopy (AES), inductively coupled plasma-atomic emission spectroscopy
(ICP-AES), X-ray fluorescence (XRF), potentiometric, wet chemical and instrumental
neutron activation (INA) analyses of bulk samples and separated fractions. Various
element determinations were also carried out with scanning @EM) and transmission
electron microscopes equipped with energy dispersive X-ray analyzers. Semi-quantita-
tive and quantitative determinations of the mineral and phase proportions in samples
were performed using separation procedures, light microscopy (point counting method),
X-ray diffraction (semi-quantitative analysis) and SEM (micromorphometric particle
analysis).
The mineralogical classification of elements [53] was used during element grouping
and description in the present work. Clarke values for sub-bituminous coals and their
ashes [7] were applied for comparison of element concentration in the samples studied.
Clarke values for shales [54], and clays and shales [55] were used for elements without
Clarke values for coals and coal ashes.

3. Results and discussion

3.1. Chemical composition of coal and coal ash

3.1.1. Major and minor elements in coal and coal ash

The elements found in coals are commonly classified as major (> 1 wt%), minor
(1-O. 1 wt%) or trace ( < 0.1 wt%) elements. These elements may occur in both organic
and inorganic constituents of coal and each element has dominant associations and
affinities with different phases in coal. Major elements, in order of decreasing signifi-
cance, are normally C, 0, H, Si, Al, S, N, and Fe; while minor elements are respectively
Ca, IS, Mg, Ti, Na, and occasionally P, Mn, Cl, Ba, and Sr. The other identified
elements in coal are mostly in trace concentrations. However, there are many instances
where this order does not hold true.
22 S.V. Vassileu, C.G. Vassileua / Fuel Processing Technology 51 (1997) 19-45

Major and minor ash-forming elements in LTA and HTA are commonly 0, Si, Al,
Fe, Ca, S, K, Mg, Ti, Na, and occasionally elements such as H, C, N, P, Mn, Cl, Ba, Sr,
and F. A conventional classification system [49] based upon the quantities of five main
oxides in HTA from 41 coal deposits of Bulgaria, Australia, the United States, Japan,
Canada, China, South Africa and Ukraine, and combustion wastes from eleven Bulgar-
ian TPS, was carried out (Fig. 1). The classification demonstrates that the coal ashes
belong commonly to sialoferricalcic and sialic types (- 60%); to a lesser extent to
sialocalcic, sialoferric, calsialic, ferricalsialic and ferrisialic types; and rarely to calcic
type. The modes of major and some minor elements occurrence in coal and coal ash are
comparatively well known [51,52], while the occurrences of trace elements are still less
known and poorly understood.

3.1.2. Trace elements in coal and coal ash

3.1.2.1. Elemental contents and concentration trends. A number of trace elements in the
coals and coal ashes studied have concentrations greater than the respective Clarke
values. For instance, the highest contents in Bulgarian and Ukrainian coal ashes (Table
1) show elements such as Rb, Cs, Ba, Cu, Sb, Bi, U, and Ag. However, each deposit has
individual plant constituents, regional, depositional and paleoenvironmental conditions
which cause an enrichment or depletion of certain elements. Thus, the concentration and
mode of occurrence of trace elements may vary widely for different coals, even for a

SiO2+Al203

Fig. 1. Chemical classification system based upon main composition of 420 high-temperature coal ash and
combustion waste samples (wt%).
 S. V. Vassilw. C.G. Vassilrra / Fuel Processing Technology 51 ( 19971 19-45 23

single coal deposit. For example, differences in mean trace element contents in the
studied coals are illustrated in Table 1. Table I demonstrates that most of the trace
elements increase their concentrations in HTA with increasing coal rank, respectively
carbon contents (dry-ash-free basis) in coal. In addition, it can be seen that HTA of the
lowest-ash Donbass coal is enriched in most trace elements.
The total ash and mineral contents, as well as the correlation trends of an element
with ash content are informative, but they have restricted significance in understanding
the element affinity in coal. The ash content of coal comprises a sum of various genetic
ash classes and subclasses, including detrital, authigenic (syngenetic, epigenetic), bio-
genetic, sorption, chemigenetic, etc, and their distribution in coal have to be considered
separately.
The HTA has generally higher trace element concentrations and a greater number of
elements above their respective Clarke values, in comparison with the non-coaly rocks
(ash basis) associated with a given coal [43]. This is due to the influence of organic
matter in coal, which contains biophilic trace elements and is a favorable environment
for the capture, fixation and accumulation of various trace elements from circulating
solutions. The organic matter may act as a sorbent, ion-exchanger, ion and molecular
“sieve” under most favourable conditions. For example, wood and coal inclusions in
sedimentary rocks can be anomalously enriched in some trace elements, namely Ge, U,
Sb, Sm, V, MO, Ce, Hf, Y, Yb, Zn, As, Tb, Zr [48,56]. The qualitative composition and
mode of occurrence of the minerals in coal show that sulphides, sulphates, carbonates,
chlorides, and some phosphates and clay minerals (mainly kaolinite) are commonly
authigenic in origin. Minerals such as quartz, feldspars, mica, Fe oxyhydroxides, other
silicates and oxyhydroxides, and some clay minerals (mainly montmorillonite and illite)
are considered be mostly detrital in coal [47,48,51]. The detrital minerals in coal are
normally rock-forming minerals in the non-coaly rocks associated with that coal.
Therefore, many of the trace elements show an affinity towards the organic matter and
authigenic minerals in coal.
It was found that elements exhibit different concentration trends in the density and
magnetic fractions of crushed (< 500 km> coal samples. The generalized trends are
listed in Table 2. Most trace elements are concentrated in the heavy (> 2.9 g cm-‘) and.
to a lesser extent, in the medium (1.6-2.9 g cm-‘) density fractions. For instance,
elements highly enriched in heavy fractions (the smallest fraction) are Sr, MO, Ba, La.
Mn, Co, Cu, Zn, As, Pb, and Ag. They associate predominantly with minerals such as
sulphides, carbonates, sulphates, rutile, chromite, zircon, apatite, scheelite, Fe oxyhy-
droxides, and other mainly accessory minerals. A significant amount of organic matter
also occurs in these fractions due to finely dispersed heavy minerals in the petrographic
ingredients. Elements enriched in the light fractions (< 1.6 g cm-“) associate with the
organic matter, as well as with some finely dispersed and lighter authigenic minerals
(clay minerals, sulphates and carbonates) in organics. The medium density fractions are
commonly the largest fraction. Elements concentrated in them associate with: organic
matter; detrital minerals such as clay, quartz, feldspars, and mica; and some coarse-
grained authigenic minerals such as gypsum, calcite, dolomite, and aragonite.
Organics, authigenic, heavier, and especially accessory minerals in coal are concen-
trating phases for most of the trace elements. However, the accessory minerals are
24 S.V. Vassileu, C.G. Vassileua/Fuel Processing Technology 51 (1997) 19-45

Table 1
Mean trace element contents of high-temperature coal ashes from eleven Bulgarian TPS (ppm)
Element Ma&a-3 Kremikovtzi Maritza-East Bobov Republica Russe, Clarke f
1,2and3 Do1 Svishtov,
Varna and
Devnya

Lithophile elements
Li a < 70 < 70 70 70 100 160 80
Bea 2 3 2 5 5 9 11
Rb b 30 40 61 104 130 180 46
Sr a 630 750 440 230 250 680 1100
Ya 20 < 10 15 30 20 25 37
Zr a 135 25 150 150 105 200 160
Nba < 10 < 10 lib 10 < 10 10 5
Mea 5 1 15 9 3 30 13
cs b 2 3 5 10 11 20 5s
Bab 400 3500 785 600 450 1400 890
Lab 25 26 33 36 62 73 92 g
Ce b 66 50 65 80 135 127 59 s
Smb 5.1 4.9 5.0 5.7 9.0 13.4 6.4 s
Eu b 1.0 0.7 1.6 1.7 2.5 2.3 1s
Tbb 1.0 0.3 0.9 1.2 1.4 1.9 1s
DY b 4.0 4.9 4.6 s
Ybb 0.7 0.9 2.8 3.2 2.1 5.6 5
Lub 0.6 < 0.5 0.7 s
Hf b 3.0 2.6 2.6 3.3 4.5 7.3 2.8 s
Tab 0.4 0.3 0.8 0.9 0.6 2.0 0.8 s

Siderophile elements
SC b 11 10 19 17 34 24 15
Va 35 50 152 b 165 b 90 290 120
Cr b 35 55 84 82 115 130 70
Cob 12 10 29 18 27 53 20
Ni b 30 35 96 74 40 140 51

Chalcophile elements
cu a 35 20 120 110 75 165 48
Znb 70 140 143 290 100 170 100
Gas 15 9 20 30 20 35 36
Ge a <l <l <l 2 1 12 9
As b 38 59 36 83 37 122 60
Sn a 1 <l 1 3 3 5 4.1
Sb b 1.5 1.9 1.7 5.1 1.6 19.5 1.5 s
Tla <l <l <l <l 2 2 1.4 s
Pb a 15 70 15 30 25 100 53
Bi a 1 2 0.01 h

Radioactive elements
Thb 13 7 16 18 23 26 22
Ub 11 11 9 21 12 20 3.2 h
 S. V. Vassileu, C. C. Vassileca / Fuel Processing Technology 51 (I 997) 19-45 25

Table 1 (continued)
Element Ma&a-3 Kremikovtzi Maritza-East Bobov Republica Russe, Clarke f
1,2and3 Dol Svishtov.
Varna and
Devnya

Noble elements
Ag a 0.1 0.4 < 0.2 0.2 0.2 3 I
Au ’ < 0.001 0.002 0.002 m 0.001 < 0.001 0.002 0.001 h
AC in coal (%) 54 37 33 46 60 14d
C daf in coal (%‘c) 62.9 63.0 65.5 73.9 74.3 86.6
ns 2 2 6 3 2 8

’ On AES data; h On INA data; ’ Ash (dry basis) obtained at 450°C; d Ash (dry basis) obtained at 800°C;
’ Number of samples and analyses; f Clarke for sub-bituminous coal ashes [7]; p Clarke for shales [54]: ’
Clarke for clays and shales [55].

commonly present in trace quantities. Their contribution to the enrichment of trace

elements in the whole coal is rarely of great significance. Therefore, the distribution of a
number of trace elements is controlled mostly by the organic matter, clay minerals, and
in some cases by sulphides, which are dominant carriers of trace elements in coal.
Quartz and feldspars are the main minerals which act as diluents of trace element
concentrations in the system.

Table 2
Characteristic element concentration trends for density and magnetic fractions separated from coals of six
Bulgarian and Ukrainian deposits (in brackets-element with minor affinity)
Elements Fractions

Light a Medium a Heavy a Magnetic

Lithophile Be (Sr) (Ba) Al Li (Be) (Mg) (Ca) (Rb) MO Ce Sm
(Ce) Mg Ca Rb Sr Y Zr
(Y) (Zr) Nb MO (Cs) Ba
Cs (La) (Sm) La Ce Sm
(Eu) (Tb) (Yb) Eu Th Yb
(Hf) (Ta) Hf Ta

Siderophile SC Ti (V) (SC) (Ti) V SC Ti V

Cr (0) Mn Fe Cr Mn Fe
Co Ni Co Ni

Chalcophile (Zn) Ge Ga (Sb) Cu Zn (Ca) Zn As Sb

As Sn Sb Pb
Pb

Radioactive (Th) u Th (U) U

Noble Au Ag (Au) Au

Non-metal Si (Si)

a Light, < 1.6 g cm-j; Medium, 1.6-2.9 g cm-j; Heavy, > 2.9 g cm-‘.
26 XV. Vassiler, C.G. Vassileua / Fuel Processing Technolog?; 51 (1997) 19-45

3.1.2.2. Modes of occurrence. Trace elements may be present in both the organic and
inorganic parts of the coal (Table 3). According to the abundance of different fractions,
phases and minerals in coal, and to the determined elemental concentrations in them, it
seems that trace and minor elements in coal occur in decreasing order of significance as:
organic compounds; major and impurity components in the inorganic constituent; and
elements in the fluid constituent.
Element-organic compounds and impurities in the organic minerals. Elements can
be adsorbed in organic matter or bound in metal-organic complexes [57] and certain
specific groups, namely carboxylic acid, phenolic, hydroxyl, imino and mercapto [9,58].
Some elements may have exchanged with some of the hydrogen of carboxylic acid
groups in low-rank coals [58]. Trace elements may also occur in the crystalline organic
matter (minerals from whewellite-weddelite, hartite-evenkite and mellite-kladnoite
groups) or other amorphous organic phases (amber). Most of the elements in the coals
studied have been documented to have an organic association, especially Li, Be, Y, Zr,
Nb, MO, rare earth elements (REE), Hf, SC, Ti, V, Cr, Co, Ni, Zn, Ga, Ge, As, Sb, U,
Au, and non-metals [43,47,48]. However, the exact organic location of these elements is
not certain.
Elements as impurities in the mineral matter. Elements occur isomorphously or in
defect sites of crystal structures. They are also present as ion-exchanged and adsorbed
elements of the mineral matrix, or adsorbed onto mineral surfaces. This may be the
origin of different elements identified in the coal minerals and fractions enriched in
some minerals [47,48]:
- clay minerals, mica and feldspars-Li, Be, Rb, Sr, Cs, Ba, SC, Ti, V, Cr, Mn, Co, Ni,
Zn, Ga, Ge, U, F, P, and Cl;
- Fe sulphides and oxyhydroxides-Rb, Y, Zr, MO, Cs, Ba, REE, Hf, Ta, SC, Ti, V,
Cr, Mn, Co, Ni, Cu, Zn, Ga, As, Cd, Sn, Sb, Pb, Bi, Th, Ag, and Au;
- carbonates-Sr, Y, Ba, REE, SC, Ti, V, Mn, Ni, Cu, Zn, As, Pb, Bi, Ag, P, and Cl;
- sulphates-Sr, Y, MO, Ba, REE, SC, Ti, V, Cr, Mn, Co, Ni, Cu, Zn, Ga, As, Sn, Bi,
P, and Cl;
- zircon-Hf and U; stolzite-Ni; monazite-Nd, Zn, Pb, and U; gamagarite-Ni and
As; etc.
Some of the aforesaid elements may also occur as fine, discrete phases (mainly below
1 pm in size) in different host mineral matrices.
Elements as major components in the mineral matter. Elements such as Sr, Y, Zr,
MO, Ba, La, Ce, Hf, W, Ti, V, Cr, Mn, Cu, Zn, As, Sn, Sb, Pb, U, Au, N, F, P, and Cl
[47,48,51] are present as proper and mainly finely-dispersed accessory minerals in coal.
Elements as major and impurity components in the inorganic amorphous matter. The
volcanic glass present in coal has dominantly an aluminosilicate or titanium-rich
composition with minor contents of Ca, Fe, Mg, K, Na, P, S, and Cl [47]. Some trace
elements may also associate with volcanic glass and glassy cosmic material, as well as
with metamict, metacolloid and gel phases characteristic of the clay minerals, zircon,
phosphates, Fe and Mn hydroxides.
Elements in the fluid constituents. Elements such as Ca, Mg, Na, K, S, C, halogen
elements, P, Mn, Fe, and other trace elements associated with them may occur as ions
dissolved in the pore water of coal. These elements can form water soluble sulphate,
 S. V. Vassilec. C. G. Vassilet,a / Fuel Processing TechdoRy 51 C1997) 19-45 71

carbonate and chloride minerals as a result of crystallization from these highly mineral-
ized solutions during the coal formation and storage of coal. Minerals with probable
pore-water crystallization during coal storage were identified in coal [47,51]. Some
elements may also occur in gas-liquid inclusions of apatite, quartz, zircon and other
minerals and phases. Detrital apatite and quartz crystals, as well as volcanic glass phases
with fluid inclusions were often observed in coal [47,48,51].
It seems that siderophile, chalcophile, non-metal and some lithophile trace elements
are bound basically to the organics and authigenic minerals, while mainly lithophile and
some siderophile trace elements are linked preferentially to the detrital minerals in coal
(Table 3).

3.2. Chemical composition of solid waste products

3.2.1. Major and minor elements in solid waste products

Major and minor ash-forming elements in the solid waste products are the same as in
LTA and HTA, however, their proportions can be changed significantly. The oxide
distributions in the products are illustrated in Table 4. The results show that SiO,, K,O,
CO,, and occasionally P20, are commonly enriched in BA, while Fe,O,, K20, and to
some extent CaO are more abundant in slag, with respect to their FA. The other
ash-forming oxides are normally enriched in FA. The listed changes are related to the
element redistribution among BA, slag, FA and stack emissions during combustion of
coal. A volatilization and subsequent partial condensation of some elements, as well as
particle separations and fractionations in the combustion chambers, electrostatic precipi-
tators and mechanical collectors, take place during coal burning. A considerable
proportion of some major and minor volatile elements in coal, namely C, H, N, S. P, Cl,
F. and Na, were emitted and did not bind in the LTA, HTA, BA, slag, FA. LA. and LAS
products. For example, an average of about 52% of SO, and 14% of Na,O determined
in LTA were lost in produced HTA, according to data for seventeen coal samples
worldwide [59]. Respectively, non-volatile elements such as Si, Al, Fe, Ca. Mg, K, and
Ti can relatively increase their proportions in the products.
LA and LAS demonstrate mainly an intermediate chemical composition between
BA/slag and FA, however, there are some changes in the contents of alkaline and
alkaline-earth oxides, Fe,O,, CO,, and SO,, in particular for LA. These changes are
related to some solution and fractionation during the transport, deposition and storage of
BA, slag, and FA. The modes of major and some minor elements occurrence in BA,
slag, FA, LA, and LAS are comparatively well known [40.52].

3.2.2. Trace elements in solid wa.ste products

3.2.2.1. Elemental contents and concentration trends. Similar to coal ashes, normally the
same lithophile, siderophile and chalcophile trace elements have greater than Clarke
contents in waste products (Tables 5-7). The concentration order of most trace elements
analyzed is generally FA > (LA,LAS) > (BAslag), which is normally in accordance
with the total trace element contents. This is a result of the aforesaid fractionation,
volatilization, condensation, and solution of some phases during coal burning and
28 XV. Vassilev, CC. Vassileva / Fuel Processing Technology 51 (1997) 19-45

Table 3
Determined and suggested modes of element occurrence in Bulgarian coals, in a probable decreasing order of
significance
Element Mode of occurrence

Lithophile elements
Li Organics, clay minerals, mica, amphibole?
Be Organics, clay minerals, mica, Fe oxyhydroxides, apatite, beryl?
Na Plagioclase, clay minerals, organics, halite, sylvite, jarosite, volcanic glass, amphibole, zeolite,
charoite
Mg Clay minerals, organic& dolomite, siderite, mica, chlorite, magnesite, hexahydrite, brucite,
spinel, polyhalite, volcanic glass, amphibole, chloritoid, olivine, enstatite, vermiculite, talc,
vesuvianite, garnet, huntite
Al Clay minerals, mica, K feldspar, plagioclase, organics, chlorite, diaspore, boehmite, gibbsite,
ahmite, corundum, goyazite, spinel, volcanic glass, zeolite, amphibole, andalusite, mullite,
svanbergite, garnet
K K feldspar, clay minerals, mica, organics, sylvite, halite, jarosite, alunite, polyhalite, zeolite,
volcanic glass, siderite, charoite
Ca Organic& calcite, gypsum, clay minerals, plagioclase, dolomite, ankerite, siderite, polyhalite,
volcanic glass. zeohte, aragonite, barite, anhydrite, apatite, amphibole, portlandite, pyroxene,
scheelite, vesuvianite, garnet, charoite, huntite
Rb, Cs Clay minerals, feldspars, mica, organics, Fe oxyhydroxides
Sr Organics, clay minerals, aragonite, celestine, strontianite, calcite, gypsum, svanbergite,
goyazite, barite, feldspars, apatite, charoite
Y Organic% clay minerals, xenotime, zircon, Fe sulphides and oxyhydroxides, gypsum, aragonite
Zr Organics, zircon, oxyhydroxides
Nb, Ta Organics, zircon, monazite, Fe oxyhydroxides
MO Organics, pyrite, marcasite, molybdates, molybdenite, Fe oxyhydroxides, gypsum
Ba Barite, witherite, calcite, barytocalcite, organic& clay minerals, feldspars, celsian, aragonite,
gypsum, siderite, gamagarite, mica, Fe sulphides and oxyhydroxides, cbaroite
REE Organics, clay minerals, monazite, xenotime, apatite, zircon, mica, feldspars, Fe sulphides and
oxyhydroxides, gypsum, siderite, calcite, aragonite
Hf Organics, clay minerals, zircon, oxyhydroxides, Hf-Sn oxide
W Scheelite, stolzite, tunstates, organics, molybdates, oxyhydroxides
Re Fe sulphides?, organics?

Siderophile elements
SC Organics, clay minerals, silicates, siderite, Fe sulphides and oxyhydroxides, gypsum, aragonite
Ti Organic& clay minerals, rutile, anatase, brockite, ilmenite, titanite, volcanic glass, mica, Fe
sulphides and oxyhydroxides, gypsum, calcite, aragonite
V Organics, pyrite, marcasite, clay minerals, magnetite, Fe oxyhydroxides, dolomite, aragonite,
gamagarite, vanadates, gypsum
Cr Organic% pyrite, marcasite, clay minerals, mica, magnetite, Fe oxyhydroxides, cbromite,
gypsum, pyroxene
Mn Pyrite, marcasite, organics, clay minerals, siderite, dolomite, calcite, mica, manganocalcite,
rhodochrosite, Fe oxyhydroxides, alabandite, hauerite, aragonite, apatite, chalcophanite, Ca-Mn
silicate, gypsum, garnet, cosmic materials?
Fe Pyrite, marcasite, siderite, ankerite, clay minerals, organics, jarosite, pyrrhotite, magnetite,
hematite, goethite, lepidocrocite, dolomite, ferrodolomite, szomolnokite, rozenite, melanterite,
coquimbite, roemerite, mica, chlorite, volcanic glass, vivianite, spinel, ilmenite, amphibole,
pyroxene, chromite, chalcophanite, olivine, rutile, anatase, brockite, polyhalite, sphalerite,
chalcopyrite, arsenopyrite, delafossite, gamagarite, cosmic materials?
 S.V. Vassileo. C.G. Vassileua / Fuel Processing Technology 51 (19971 19-45 29

Table 3 (continued)
Element Mode of occurrence
Siderophile elements
co Organic% clay minerals, carbonates, Fe sulphides and oxyhydroxides, gypsum, garnet, cosmic
materials?
Ni Organic% pyrite, marcasite, clay minerals, siderite, dolomite, Fe oxyhydroxides. sphalerite.
vanadates, tunstates, gypsum, millerite?. cosmic materials?
Chalcophile elements
CU Pyrite, marcasite, organics, chalcocite, chalcopyrite, siderite, calcite, aragonite, jarosite. Fe
oxyhydroxides, gypsum, delafossite
Zn Organics, sphalerite, magnetite, clay minerals, pyrite, gypsum, siderite, calcite, smithsonite.
galena, jarosite, chalcophanite, anglesite, delafossite, monazite
Ga Organic& clay minerals, pyrite, marcasite, Fe oxyhydroxides, gypsum, sphalerite?
Ge Organic% clay minerals, pyrite, jarosite, Fe oxyhydroxides. monazite, vanadates, sphalerite,
galena
As Organic% arsenopyrite, orpiment, realgar, clay minerals, Fe sulphides, magnetite, jarosite,
sphalerite, galena, calcite, gypsum, vanadates, molybdates, native S
Se, Te Fe sulphides?, organics?, native S, galena?
Cd Sphalerite, organics, oxyhydroxides, Fe sulphides, galena
In Sphalerite?, organics?, Fe sulphides?, galena?, carbonates?
Sn Oxyhydroxides, sphalerite, galena, organic% zircon, Fe sulphides, gypsum, Hf-Sn oxide
Sb Organics, Pb-Sb sulphosalts, Fe sulphides and oxyhydroxides, galena
Hg Fe sulphides?, native Hg?, cinnabar?, sphalerite?, organics?
Tl Fe sulphides, sphalerite, galena, organics
Pb Galena, Pb-Sb sulphosalts, organics, Fe sulphides. magnetite, anglesite. oxyhydroxides,
cerussite, calcite, aragonite, monazite, stolzite
Bi Sphalerite, galena, organics, Fe sulphides, gypsum, siderite, calcite, aragonite, tunstates

Radioactive elements
Th Clay minerals, zircon, apatite, monazite, xenotime, organics, Fe oxyhydroxides
U Organic% clay minerals, zircon, monazite. xenotime, magnetite, phosphates, sulphates

Noble elements
Ag Fe sulphides, organic& galena, sphalerite, aragonite, sulphosalts, native Au
AU Organic% magnetite, native Au, Fe sulphides?, quartz?

Non-metal elements
H, 0 Organics, oxyhydroxides, silicates, sulphates, carbonates, phosphates, etc.
B Organics?, tourmaline?, clay minerals?, mica?, quartz?, feldspars?, phosphates?
C Organics, calcite, siderite, dolomite, ankerite, aragonite, witherite, magnesite, manganocalcite,
barytocalcite, ferrodolomite, rhodochrosite, smithsonite. huntite, graphite
N Organic& clay minerals, mica, nitrates, chlorides
F Organic% clay minerals. mica, apatite, amphibole, charoite, carbonates, fluorite?, tourmaline?,
topaz?
Si Quartz, clay minerals, K feldspar, plagioclase, mica, organic& opal, cristobalite, chalcedony,
chlorite, volcanic glass, zeolite, amphibole, pyroxene, zircon, enstatite, garnet, olivine
P Organic& clay minerals, apatite, goyazite, vivianite, monazite, xenotime, svanbergite, siderite,
gypsum, calcite, volcanic glass, jarosite, anglesite, polyhalite
30 S.V. Vassilev, C.G. Vassileva/ Fuel Processing Technology 51 (1997) 19-45

Table 3 (continued)
Element Mode of occurrence

Non-metal elements
s Organics, pyrite, marcasite, gypsum, clay minerals, native S, jarosite, szomolnokite, rozenite,
melanterite, coquimbite, roemerite, volcanic glass, alunite, hexahydrite, barite, anhydrite,
polyhalite, siderite, dolomite, pyrrhotite, galena, chalcocite, sphalerite, As and Mn sulphides,
Pb-Sb sulphosalts, svanbergite, anglesite, celestine, chalcopyrite, arsenopyrite, molybdenite,
sylvite
Cl Organics, halite, sylvite, polyhalite, clay minerals, mica, volcanic glass, gypsum, siderite,
apatite
Br,I Oraanics?, clay minerals?, mica?, phosphates?

Table 4
Mean chemical composition of bottom ashes CBA), slags (S), fly ashes (FA), lagooned ashes (LA) and
lagooned ashes-slags (LAS) from eleven Bulgarian TPS, normalized to 100% without loss of ignition (wt%)

TPS Product SiO, TiO, Al,O, Fe,O, MnO MgO CaO NasO K,O P,O, CO, SO, n a

A. From dry-ash discharge boilers

Maritza-3 BA 41.45 0.50 16.49 8.70 0.07 2.29 16.53 0.74 0.85 0.30 3.51 8.57 12
FA 37.16 0.55 14.60 11.45 0.23 2.51 20.11 1.21 0.88 0.32 1.77 9.21 19
LA 46.41 0.52 17.11 9.79 0.15 1.83 13.39 1.82 0.99 0.29 3.24 4.46 3
Kremikovtzi BA 49.83 0.68 12.23 6.33 0.14 2.03 23.29 0.56 1.11 0.20 2.94 0.66 1
FA 37.74 0.57 16.23 6.04 0.16 2.67 29.36 0.99 0.87 0.21 1.70 3.46 18
Maritza-East 1 BA 49.31 0.60 23.62 12.55 0.05 2.19 5.14 0.91 1.10 0.21 0.33 3.99 7
FA 47.24 0.73 23.37 13.82 0.12 2.51 7.49 1.16 0.92 0.07 0.09 2.48 38
LA 49.07 0.59 22.10 13.52 0.10 2.17 7.43 1.41 1.09 0.08 0.25 2.19 5
Maritza-East 2 BA 46.79 0.78 23.23 15.30 0.04 2.25 5.55 1.23 1.15 0.07 0.10 3.51 9
FA 45.25 0.89 21.27 15.21 0.09 3.07 8.47 1.37 0.96 0.08 0.11 3.23 20
LA 48.05 0.92 22.67 15.88 0.07 2.79 5.75 1.28 1.10 0.08 0.14 1.27 2
Ma&a-East 3 BA 51.35 0.77 26.16 11.56 0.02 1.93 5.02 0.76 0.99 0.04 0.03 1.37 4
FA 48.64 0.91 24.07 11.98 0.07 2.28 6.95 0.76 0.95 0.06 0.65 2.68 9
LA 52.32 0.88 25.94 11.31 0.05 1.82 4.18 0.68 0.94 0.05 0.19 1.64 1
Bobov Do1 BA 59.00 0.98 23.24 8.15 0.07 2.13 2.31 0.89 2.43 0.07 0.15 0.58 2
FA 57.09 0.88 23.43 9.27 0.06 2.17 2.96 1.04 2.07 0.09 0.03 0.91 31
LA 57.67 1.01 23.75 8.53 0.07 2.16 3.13 0.82 1.99 0.08 0.21 0.58 4
Republica BA 54.78 0.77 23.80 10.42 0.09 1.97 2.81 0.48 2.19 0.11 1.28 1.30 8
FA 56.90 0.91 26.31 7.91 0.03 1.94 2.68 0.61 1.67 0.06 0.02 0.96 48
LA 57.75 0.58 25.60 8.38 0.08 1.91 1.87 0.44 2.28 0.09 0.43 0.59 1

B. From slag discharge boilers

Russe S 50.68 0.83 23.10 16.83 0.04 1.37 3.35 0.73 2.66 0.02 0.04 0.35 7
FA 51.46 1.08 22.01 13.46 0.08 2.73 3.98 1.44 1.86 0.17 0.11 1.62 27
LAS 48.35 0.62 23.27 17.53 0.08 1.39 3.91 1.67 1.84 0.10 0.50 0.74 7
Svishtov S 48.87 0.91 20.45 19.79 0.06 1.43 4.82 0.76 2.09 0.02 0.04 0.76 7
FA 44.14 1.03 22.69 18.38 0.09 1.78 5.46 1.36 1.71 0.20 0.54 2.62 32
LAS 42.50 0.97 22.84 20.07 0.06 1.67 5.34 1.12 2.38 0.06 0.61 2.38 6
Vama S 52.10 1.09 18.42 16.29 0.14 2.43 4.65 0.90 3.15 0.03 0.02 0.78 7
FA 52.93 0.97 22.09 12.78 0.10 1.92 3.68 1.61 2.60 0.20 0.17 0.95 39
LAS 51.29 0.82 24.51 12.54 0.13 1.70 3.43 2.45 1.64 0.15 0.52 0.82 4
Devnya S 40.29 1.00 25.76 22.13 0.09 1.71 4.99 0.92 2.69 0.07 0.19 0.16 1
FA 51.91 0.99 23.40 13.37 0.12 1.35 3.74 1.47 2.09 0.29 0.16 1.11 8

a Number of samples and analyses.

 S.V. Vassileu, C.G. Vassileua / Fuel Processing Technology 51 (1997) 19-45 31

ash-slag transport and storage. Trace elements showing no or weak fractionation

between FA and BA/slag are predominantly from the lithophile group which preferen-
tially remain dissolved in the melts of BA and slag. A relatively high enrichment in
siderophile and certain chalcophile trace elements associated with Fe sulphides and
oxyhydroxides in coal, was detected in slags and some BA.
The trace elements in FA and separated fractions show different concentration trends
(Table S), however, they are mainly enriched in non-magnetic, heavy (> 2.9 g cm-“)
and fine-grained fractions which are abundant in accessory minerals [40,43,52]. They
frequently follow closely the behaviour of their geochemical analogues among the major
and minor elements during coal burning. Trace elements, mainly from chalcophile and
siderophile groups, could considerably increase their contents in the separated heavy
fractions.
Lithophile trace elements associate with the glass and crystalline non-magnetic
phases, those of high density being preferred. Molybdenum is an impurity often found in
magnetite and maghemite. Accessory phases of Sr, Zr, Nb, Ba, and W [43.52] were
identified, and the same could be inferred for Y, MO, La and Ce because of their high
values in some heavy and fine-grained fractions in FA, as well as their modes of
occurrence in coal [51].
Siderophile trace and minor elements follow the trends of Fe. and have high
concentrations in Fe oxyhydroxides and ferrospheres. Magnetite is commonly enriched
in Ti and Cr, while Ni content reaches up to 1.6% in some ferrospheres. Siderophile
elements were found as accessory phases of Ti, Cr, Mn, and probably of V [40,43.52].
They also occur in glass phases and in aluminosilicates.
Chalcophile elements are typical of the fine-grained and heavy fractions. Their strong
enrichment trends lead to the assumption that elements, namely Cu, Zn, Ga, Ge, As, Sn,
Sb. and Pb may occur as proper accessory phases. For instance, mineral phases of Cu,
Zn, Sb, and Pb were identified with particle sizes mainly in the range of 0.01-5 pm
[40,43,52]. In addition, elements such as Cu, Zn, and Ge frequently show a high level of
concentration in the magnetic fraction, similar to siderophile elements. For example,
about 0.6% Cu occurrence in a ferrosphere was detected.
Radioactive elements show a weak enrichment in the density, size and magnetic
fractions separated. Their modes of occurrence tend to be rather various and connected
with fractions enriched in glass phases, kaolinite-metakaolinite, feldspars, sulphates,
phosphates and organic constituent. Noble elements also demonstrate weak concentra-
tion trends. Their occurrence is probably related to glass phases in the first place, or can
be found to a lesser extend in fine-grained magnetite and maghemite. or occur as native
metals.

3.2.2.2. Modes of occurrence. The problem related to the modes of trace element
occurrence is complex, however, the results of this study show that these elements are
present in decreasing order of significance as: impurities in different inorganic phases;
proper inorganic phases; organic compounds; and elements in the fluid constituent.
Elements as impurities in the glass phases. These phases are represented by particles
with variable elemental composition. Their origin is mainly secondary, but could be
primary as well [40,43,52]. The secondary glass phases are generated from quartz, clay
32 S.V. Vassileo, C.G. Vassileva/ Fuel Processing Technology 51 (1997) 19-45

Table 5
Mean trace element contents of bottom ashes/slags from eleven Bulgarian TPS (ppm)
Element Maritza-3 Kremikovtzi Maritza-East Bobov Republica Russe,
1,2and3 Do1 Svishtov,
Varna and
Devnya

Lithophile elements
Li a < 70 < 70 70 100 150 145
Be a 3 2 4 6 2 6
Rb b 30 58 70 120 100 160
Sr a 750 250 370 450 225 350
Y” 20 10 15 30 15 15
Zr a 190 15 110 150 135 135
Nba < 10 < 10 13 b 10 < 10 < 10
Mea 2 1 9 4 2 10
cs b 2 3 6 12 8 15
Bab 900 700 623 565 400 1300
Lab 26 30 31 40 38 110
Ce b 51 59 56 65 76 186
Ndb 27
Smb 3.7 5.5 4.0 6.8 8.0 15.8
Ellb 0.4 0.8 1.4 1.9 2.1 3.7
Tbb 0.4 1.1 1.0 1.5 1.7 2.7
DY b 3.9 5.5
Yb b 0.5 1.1 2.8 3.3 3.4 6.2
LUb < 0.4 0.5
Hf b 2.1 2.2 2.7 3.0 2.0 12.3
Tab 0.4 0.5 0.7 0.9 0.6 2.3

Siderophile elements
SC b 8 14 16 17 17 37
Va 40 70 <2OOb 202 b 70 290
Cr b 40 63 72 81 55 360
co b 9 16 16 19 20 53
Ni b 10 a 30 86 100 50 160

Chalcophile elements
cu a 25 25 65 45 70 120
Znb 50 60 45 90 60 140
Cia” 15 15 25 20 20 20
Ge a <l <l 1 <l <l <l
As b 28 38 22 16 28 25 a
Sn a 2 <1 2 2 3 2
Sb b 1.3 1.0 0.7 2.0 1.2 1.7
Tl a <l <l <l <l 2 <l
Pb = 15 5 25 20 20 20

Radioactive elements
Thb 10 7 17 21 17 28
Ub 7 8 8 9 10 12
 S. V. Vassileu, C.G. Vassileva / Fuel Processing Technology 51 (I 9971 19-45 33

Table 5 (continued)
Element Maritza-3 Kremikovtzi Maritza-East Bobov Republica Russe,
1, 2 and 3 Do1 Svishtov,
Vama and
Devnya

Noble elements
Ag a 0.1 < 0.2 < 0.2 < 0.2 < 0.2 < 0.2
Au h 0.001 0.002 < 0.001 < 0.001 0.001 0.001
A’(‘%) 76 96 82 94 90 100
nd 2 2 6 2 2 8

’ On AES data: h On INA data; ’ Ash (dry basis) obtained at 1000°C: ’ Number of samples and analyses.

minerals, feldspar, mica, chlorite, and other easily melted minerals [49], where the
typical trace elements are lithophile ones. These elements are commonly dissolved in the
melts and inherited in the glass. This is clearly seen by the increased contents of
lithophile elements in BA, slag and FA fractions built up mainly of glass phases. In
addition, different trace element-bearing accessory minerals in coal could also be
dissolved in the melt. Some volatile trace elements can be adsorbed onto glass surfaces.
The enrichment of alkali, rare-earth, and many transition elements in the glass phase has
been emphasized [2 11.
Elements as impurities in the crystalline components. Elements can occur isomor-
phously or in defect sites of crystal structures. They can also be present as ion-ex-
changed elements of the mineral matrix, or adsorbed onto mineral surfaces. The
minerals of primary origin remain as carriers of the same trace elements. The new
phases formed from solid-phase transformations and reactions may also contain retained
trace elements from the original minerals in coal, but along with certain changes in the
concentrations. The newly-crystallized phases from silicate melts contain trace elements
dissolved in these melts. Different trace elements may be surface enriched in particles
(for example in the wall of hollow spheres and dermaspheres) during crystallization
processes. A similar explanation of the trace elements being frozen out and concentrated
in the glass phases has been described elsewhere [21]. Other trace elements may be
enriched on the surface of particles as a result of condensation process and subsequent
crystallization during cooling of stack gasses [52]. Certain changes in the concentrations
of trace elements in the crystalline components may occur as a result of recrystallization
and solution during transport and storage of the products. Trace elements present in the
crystal structure of different mineral phases may refer to: siderophile elements, MO, Zn
and Cu in iron oxyhydroxides; Li, Be, Rb, Cs, Ba, siderophile elements, Cu, Zn, F in
kaolinite-metakaolinite, mullite and mica; Sr, Ba, As, Cl in gypsum and anhydrite; Be,
Sr, Y, Nb, REE, Hf, Mn, Sn, Th, U, Au, F, Cl in zircon and apatite; Nb, Ta, Sn in rutile;
Mn, Ni, Zn in olivine; Rb, Sr, Cs, Ba, REE, Ti, Mn, Pb, B in feldspars, etc. Some
possible isomorphic substitutions in magnetic spine1 and quartz-mullite matrices have
been discussed earlier [21].
Elements as proper accessory crystalline phases. A number of trace and minor
elements occur in this form. This is characteristic of Sr, Ba, Zr, Nb, W, Ti, Cr, Mn, Cu,
34 S. V. Vassilev, C.G. Vassileva /Fuel Processing Technology 51 (1997) 19-45

Table 6
Mean trace element contents of fly ashes from eleven Bulgarian TPS (ppm)
Element Maritza-3 Kremikovtzi Maritza-East Bobov Republica Russe,
1.2 and 3 Do1 Svishtov,
Varna and
Devnya

Lithophile elements
Li a < 70 < 70 70 100 100 150
Be a 4 2 5 4 5 13
Rb b 30 60 70 146 140 130
Sr a 1250 630 550 430 280 930
Ya 20 10 15 25 20 25
Zr a 25 15 110 110 125 225
Nb” < 10 < 10 10 b < 10 < 10 8
MO= 8 4 20 7 2 8
cs b 2 4 6 15 11 12
Bab 900 500 1063 875 450 900
Lab 29 30 43 55 48 53
Ce b 65 62 75 117 91 102
Ndb 23
Smb 5.4 5.7 8.0 8.0 9.6
Eu b 1.0 0.9 1.6 2.3 1.6
Tbb 1.4 0.8 1.7 1.9 1.9
DY b 5.2 6.0
Ybb 0.9 2.1 3.4 4.8
LUb < 0.6 0.7
Hf b 2.0 2.0 3.3 3.9 5.2
Tab 0.8 0.5 1.1 1.4 1.0

Siderophile elements
SC b 8 15 22 26 24 18
Va 30 45 202 b 244s 90 135
Cr b 30 60 97 118 80 110
Cob 13 16 30 25 22 33
Ni b 40 30 123 110 45 113

Chalcophile elements
Cu” 25 40 110 55 75 155
Znb 100 130 147 155 75 300
Ga” 25 25 25 25 20 30
Ge a 5 <1 4 1 1 7
Ash 34 116 78 56 37 79
Sn” 2 <1 2 3 4 5
Sb b 1.7 1.3 2.9 3.4 2.0 12.5
Tl = <l <1 <l <1 2 3
Pb a 25 45 30 40 25 85
Bi a 1

Radioactive elements
7 25 25
;bb 148 11 10 13
 S. V. Vassilev, C. G. Vassileva /Fuel Processing Technology 51 (1997) 19-45 35

Table 6 (continued)
Element Maritza-3 Kremikovtzi Maritza-East Bobov Republica Russe,
I,2 and 3 Do1 Svishtov.
Vama and
Devnya

Noble elements
Ag a 0.4 0.4 0.2 0.2 0.2 0.6
Au ’ < 0.001 0.002 0.00 1 0.001 < 0.001 0.001
A’(%) 95 95 97 99 96 81
n* 2 2 17 4 2 8

’ On AES data: b On INA data; ’ Ash (dry basis) obtained at 1000°C; d Number of samples and analyses.

Zn, Sb, Pb, U, and probably of Y, MO, La, Ce, Hf, V, Ga, Ge, As, Sn, Ag [52]. The
accessory phases are original coal minerals or they are newly-formed crystalline and
noncrystalline phases generated during coal burning, transport and storage of the
products. These phases occur: on the surface of the coarser particles; as inclusions into
matrix phases; and especially as discrete particles, aggregates, crystals, and grains,
generally below 5 pm in size [40,52]. The last occurrence could explain the increased
concentration of most trace elements in the finer FA. A significant part of accessory
finely-dispersed phases, mainly surface enriched oxyhydroxides, hydrates and salts, may
originate from the condensation of volatilized species on FA particles during cooling of
combustion gases.
Element-organic compounds. It can be assumed that some elements with organic
affinity in coal, namely Be, Sr, REE, Ti, V, Cr, chalcophile, Au, and non-metals (Tables
2 and 3), occur originally or with certain changes in this form. The organic attachment
may be more significant in some products because the contents of unburnt coal
components are normally (wt%): BA = 4-22, FA = l-5, and LA = 4- 11 from dry-ash
discharge boilers; slag = I 0.3, FA = 9-25, and LAS = 7-23 from slag discharge
boilers.
Elements in the jluid constituents. Some of the elements occur in liquid and gas states
in refractory primary minerals and phases [47,48,51]. Different volatile elements in the
combustion chamber, mainly chalcophile and non-metals, may also be present as
gas-liquid inclusions (gas and condensed gas) in new-formed cenospheres, plerospheres
and other porous phases. The locked liquid subsequently precipitates in films.

3.2.2.3. Volatilization. In order to establish the approximate element proportion which

does not bind in the discharged FA, slag and BA, the balance factor, K,, [35] has been
used. The calculations indicate that some trace elements show susceptibility to release
into the atmosphere. For example, there could be emitted from some TPS into the
atmosphere maxima of up to: 10% of Nb, Ta, Sn, and Th; lo-20% of Y, Cs, Eu, Co, Ni,
and Ga; 20-30% of Sr, La, and V; 30-40% of Be, Ce, Hf, SC, Cr, As, and Sb; and
40-60% of Zr, MO, Ba, Cu, Zn, Ge, Pb, and U [45]. Most typical is the escape of
chalcophile and some lithophile and siderophile elements characteristic of fine-grained
and heavy fractions in FA (Table 8). The reasons for that could be: (1) their partial
volatilization (evaporation and sublimation) in the combustion chamber and their
36 S. V. Vassilev, C.G. Vassileva / Fuel Processing Technology 51 (1997) 19-45

Table 7
Mean trace element contents of lagooned ashes/lagooned ashes-slags from nine Bulgarian TPS (ppm)
Element Maritza-3 Maritza-East Bobov Do1 Republica Russe, Svishtov
1,2and3 and Vama
Lithophile elements
Li a < 70 70 100 100 250
Be a 3 5 3 5 10
Rbb 50 88 175 95 120
Sr = 1250 420 400 330 1100
Ya 20 15 30 20 20
Zr a 125 150 85 85 160
Nb” < 10 < 10 < 10 < 10 10
Mea 2 10 6 3 5
cs b 5 6 13 8 12
Bab 720 943 650 365 800
Lab 46 48 45 43 47
Ce b 80 93 89 82 81
Smb 6.5 7.0 6.7 7.0 8.8
Eu b 1.0 1.5 2.1 1.5 0.9
Tbb 0.9 1.0 1.1 1.3 1.3
DY b 3.9 6.0
Ybb 1.5 3.7 3.8 2.5 2.0
Lub 0.7 0.6
Hf b 4.2 3.4 3.5 2.2 3.5
Tab 0.7 0.9 1.1 0.6 1.0

Siderophile elements
SC b 14 26 21 23 14
Va 30 172 b 196 b 95 185
Cr b 50 110 110 78 75
co b 16 26 24 18 26
Ni b 20 77 90 30 70

Chalcophile elements
Cu” 35 110 45 95 80
Znb 190 60 130 110 60
Cia” 20 25 20 20 20
Ge a <I <l <l <l 3
As b 63 39 45 33 45
Sna 1 2 3 3 3
Sb b 1.7 1.6 3.3 1.2 6.5
Tl a <l <l <l <l <l
Pb a 15 20 35 25 65

Radioactive elements
Thb 18 19 21 19 14
Ub 13 14 14 10 8

Noble elements
Ag a < 0.2 0.2 0.2 0.2 0.1
Aub 0.002 0.001 < 0.001 0.001 0.002
 XV. Vassileu, C.G. Vassileua / Fuel Processing Technology 51 (1997) 19-45 37

Table 7 (continued)
Element Maritza-3 Maritza-East Bobov Do1 Republica Russe, Svishtov
1,2and3 and Vama

Noble elements
A’ (%l 82 93 96 93 82
n* 2 6 2 2 6

a On AES data: b On INA data; ’ Ash (dry basis) obtained at 1000°C; ’ Number of samples and analyses.

incomplete condensation in FA; (2) migration by the finest ash particles that have not
been caught in the cleaning equipment; and (3) their capture and remaining on the TPS’s
metal surfaces. The last reason has a negligible importance for the balance of trace
elements. The technological processes of fuel preparation, burning and waste removal,
as well as the modes of element occurrence, the size of minerals and phases containing
trace elements and their physico-chemical properties are principal factors for trace
element migration.
The study indicates that the modes of trace element occurrence in coal are a guide for
the behaviour of these elements during combustion. Trace elements with volatile
behaviour show a tendency for concentration in the easily decomposing organic matter
and authigenic minerals containing S, C, P, Cl, H anions and anionic groups (sulphides

Table 8
Characteristic element concentration trends for fractions separated from fly ashes of eleven Bulgarian TPS (in
brackets-element with minor affinity)
Elements Fractions

Light a Medium a Heavy ’ Fine-grained b Magnetic

Lithophile Ca (Zr) (Nb) Li Be Rb (Be) Mg (Sri Be Mg Sr Mg MO Ce

(Csl (La) (Ce) Sr Y Zr NbMoBa Zr Nb MO Sm Tb Dy
(Eu) (Tal Cs (Ba) La Ce (Sm) (Yb) Ba Yb Hf
SmEuTb Hf
Yb (Hf) Ta

Siderophile (SC) SC Ti (V) (Ti) V Mn VCrMn Cr Mn Fe

Cr Fe Co Ni Fe Co Ni Co Ni

Chalcophile (Zn) Sb (Bi) Ga As Sn Cu Zn (Gal Cu Zn Ge Cu Zn Ge

(Pb) Bi Ge (As) (Sn) As Sn Sb Pb
(Sb) Pb Tl Pb Bi

Radioactive (U) U

Noble Ag Au Au

Non-metal (Sil Si P PS S

’ Light, < 1.6 g cm -3; Medium, 1.6-2.9 g cmm3; Heavy, > 2.9 g cmm3.
b Fraction < 63 pm is: > 55 wt% for dry-ash discharge boilers; and > 80 wt% for slag discharge boilers.
38 S.V. Vassileu, C.G. Vassileua/Fuel Processing Technology 51 (1997) 19-45

Table 9
Mean element contents of waste waters from eleven Bulgarian TPS (mg l- ’)
Element Maritza-3 Kremikovtzi Maritza-East Bobov Republica Russe, Clarke *
1, 2 and 3 Do1 Svishtov,
Varna and
Devnya

Lithophile elements
Li a 0.81 0.0025
Be a 0.002 0.001 e
Nab 168 440 210 108 144 > 2557 6.3 e
Mgb 105 38 67 144 20 30 4.1 e
Kb 82 65 42 26 36 108 2.3 e
Cab 3000 5600 5100 2540 1720 4040 15e
Sr a 0.99 0.05
Y= < 0.006 0.007
Zr a <O.l 0.0026
Nb” < 0.05 0.00001
Mea 0.39 0.001
cs b 145 0.00003
Ba” 0.03 0.02
Las < 0.06 0.0002

Siderophile elements
SC = < 0.06 0.00001
Ti a 0.0011 0.0050 0.0220 0.0021 0.0018 0.0032 0.003
Va 0.110 0.128 0.140 0.120 0.069 0.082 0.001
Cr a 0.022 0.005 0.018 < 0.020 < 0.005 < 0.013 0.001
Mna 0.0019 0.0033 0.0095 0.1100 0.0026 0.0035 0.01
co = < 0.02 0.0003
Ni a < 0.05 0.0025

Chalcophile elements
Cu” 0.004 0.013 0.010 < 0.030 < 0.001 < 0.001 0.007
Zn a 0.037 0.058 0.032 0.020 0.190 0.028 0.02
Gas < 0.2 O.OQOl
Ge a < 0.12 0.00007
As a 0.13 0.0014
Se a < 0.05 0.0002
Cd a < 0.002 < 0.002 < 0.002 < 0.002 < 0.002 < 0.002 o.OQO2
Sn a < 0.02 0.00004
Sb a < 0.1 0.001
Te a < 0.1 O.OOl?
Tl” < 0.4 0.001
Pb a < 0.01 < 0.01 < 0.01 < 0.01 < 0.01 < 0.01 0.001
Bi a 0.23 0.00002?

Noble elements
Ag a < 0.03 0.0002
 S. V. Vassileu, C.G. Vassileoa/ Fuel Processing Technology 51 (1997) 19-45 39

Table 9 (continued)
Element Maritza-3 Kremikovtzi Maritza-East Bobov Republica Russe, Clarke ’
1,2and3 Dol Svishtov,
Varna and
Devnya

Non-metal elements
% :’ 4.1 0.013 c
Fh 0.1 0.1 5.8 13.3 3.2 0.6 0.09 c
Cl b 19 120 73 20 38 58 7.8 ’
Br h 28 950 210 213 64 233 0.02 1 c
Ih > 0.1 0.002
PH ’ 10.2 Il.1 7.3 7.9 X.0 8.1
nL 1 I 3 3 1 4

d On ICP-AES data: ’ On potentiometric analysis data; ’ Number of samples and analyses; ’ Clarke for
surface waters [60]; ’ Clarke for surface waters [61].

and sulphates, carbonates, phosphates, chlorides, hydrates, hydroxides), and in pore

water of coal. This attachment favours their mobility during combustion by volatilization
together with water vapors, sulphur, carbon, nitrogen, chlorine and other gases gener-
ated. Elements which occur as volatilized species in the furnace partially condense and
adsorb downstream, preferentially on the finer FA particles (with high specific surface),
as the temperature of the flue gas drops. Such a volatilization-condensation mechanism
has been repeatedly discussed [ 11,13- 15,17,25-341. The important role of anhydrite and
lime in the sorption of trace elements (mainly chalcophile elements) from the combus-
tion gas has been also emphasized [ 171. The volatile elements form secondary surface-
bonded oxyhydroxides, hydrates, sulphates, phosphates, carbonates, etc, or they produce
fine and discrete accessory phases identified in FA [40,43,52]. The newly-formed solid
phases with submicrometer sizes, namely minerals of Sr, Ba, Nb, W, Ti, Cr, Mn, Cu,
Zn, Pb, U and glass spheres, may escape particulate control systems. Certain extremely
volatile species of halogens, non-metals and chalcophile elements, may also leave TPS
as fluid phases. Weakly volatile trace elements, predominantly lithophile and siderophile
ones, are more characteristic of the original, refractory and mainly detrital minerals, such
as silicates and oxides in coal. Their migration mostly parallels that of the pre-existing
fine particles (for example, minerals of Y, Zr, REE, Hf, W, Ti, V, Cr, Mn and Sn) that
frequently are below 5 pm in size and identified in coal [47,48,51], which makes them
difficult to catch in the cleaning equipment at TPS. Therefore, the element behaviour
and migration by stack emissions (in solid, liquid and gas states) during combustion is
complex, but strongly depends on modes of element occurrence in coal and FA.
The combustion process is a possible powerful factor for high relative enrichment of
certain trace elements, mainly from the chalcophile and siderophile groups in the FA at
some TPS. Such data are of interest because FA may contain economically significant
amounts of some elements. They may be concentrated in certain discrete crystalline and
non-crystalline phases that can be readily separated from products by physical and/or
chemical means.
40 S. V. Vassilev, C.G. Vassileva / Fuel Processing Technology 51 (1997) 19-45

3.3. Trace elements in water, soil and vegetation near TPS

The concentration of elements, namely Li, Na, Mg, K, Ca, Sr, MO, Cs, V, Cr, Mn,
As, Bi, B, F, Cl, Br, and I, in pond waters (Table 9) may exceed (> 10 times) the
respective Clarke content for surface waters [60,61]. These elements can occur as

Table 10
Mean trace and minor element contents of soils from ten Bulgarian TPS (oum)
Element Maritza-3 Ma&a-East Bobov Do1 Republica Ruse, Svishtov, Clarke d
1,2and3 Vama and Devnya

Lithophile elements
Li a < 70 < 70 70 70 < 70 30
Be a 3 3 5 6 3 6
Sr a 6.50 170 370 350 280 300
Y= 20 20 25 30 25 50
Zr” 255 150 180 200 480 300
Nba < 10 < 10 15 10 - 10 19?
Mea 1 2 2 2 2 2
Baa 1500 850 1300 1000 850 500e
Las 50 35 65 50 55 30
Yba 2 2 3 3 2 3?
Hf ’ 100 <loo 160 100 -100 6

Siderophile elements
SC a 7 10 12 20 15 7
Ti a 5000 4000 3900 b 7000 5500 5000
V” 35 60 110 115 45 100
Cr a 35 30 65 80 80 100
Mna 1000 800 700b 2000 850
coa 6 5 15 15 4 8
Ni a 15 15 25 30 25 40

Chalcophile elements
cu a 35 20 45 35 55 20
Zn a 200 40 110 60 35 50
Gas 20 15 30 20 15 30
Ge a <l (1 <l <l <l 1
As a <loo <loo 100 <lOO -100 6
Sna 5 2 3 9 2 10
Tl a <l <l 4 <I <l 0.1
Pb = 105 20 40 45 50 10 e

Noble elements
Ag a 0.1 0.1 0.2 < 0.2 0.4 0.1

Non-metal elements
Pb 1200 650
Sb 1400 700
nc 2 6 9 2 8

a On AES data; b On XRF data; ’ Number of samples and analyses; d Clarke for soils [61]; e Clarke for
soils [64].
 S. V, Vassileu, C.G. Vassileca / Fuel Processing Technology 51 (19971 19-45 41

chlorides, sulphates, carbonates and other water soluble salts and compounds in FA,
where those phases commonly have surface particle associations. The mobile elements
migrate inside and out of the pond disposals, and probably contaminate the surface and
subsoil waters. According to the EC [62], USA [63] and other standards in some
European countries for drinking water, it can be seen that the following elements may
exceed the maximum permissible concentrations (mg 1-l): Be (0.00021, Mg (801, Ca
(1501, V (O.Ol), Mn (0.051, Zn (0.11, As (0.021, B (1.01, F (0.51, Cl (25). In addition,
waste waters from Maritza-3 TF’S and Kremikovtzi TPS producing respectively calsialic
and calcic FA (Fig. 1) have pH values (Table 9) exceeding the standard limits (6.5-8.51.

Table 11
Mean trace and minor element contents of grass vegetation (ash, obtained at 450°C) from nine Bulgarian TPS.
Data are based on AES analysis (ppm).
._
Element Maritza-3 Maritza-East Bobov Do1 Republica Russe, Svishtov Clarke h
1,2and3 and Vama

Lithophile elements
Li < 70 < 70 < 70 70 II
Be <I 2 <l 2.1
Sr 1500 1500 700 900 30
Y 3 L 20 15 10
Zr 50 20 50 50 20 64 ’
MO 100 20 10 70 20
Ba 500 650 1000 1000 730 100
Yb I -1 2 2 < 0.15 ‘

Siderophile elements
SC 3 I 15 5 I 0.8 c
Ti 1000 500 3000 700 1000
V 50 60 70 200 30 61
Cr 2 50 50 20 250
Mn 500 800 700 1000 800 7500
co 7 5 5 2 15
Ni 5 15 20 50 10 50

Chalcophile elements
CU SO 85 150 150 65 200
Zn 30 140 300 1000 105 900
Ga 2 10 30 20 4 50
Ge <I 10 -I
Cd < 10 < IO < 10 10 < 10 0.01
Sn <I 4 5 <I 5
Tl <I <l <1 <I 2 0.005
Pb 15 25 100 200 25 10
Bi <I <I <l <I 0.0005

Noble elements
Ag 0.5 2 1.5 0.7 0.2 1
na I 3 1 1 3

a Number of samples and analyses; ’ Clarke for vegetation (ash basis) [65]; ’ Clarke for angiospermae
plants [61], transformed to ash basis at 1% ash.
42 XV. Vassileu, C.G. Vassileua/ Fuel Processing Technology 51 (1997) 19-45

Mineral sorbents like mixed-layered clay, montmorillonite and clinoptilolite may be

effectively used for purification of contaminated waste waters from TPS [50]. They can
be applied individually, and especially in a combination, depending on the chemical
specification of different waste waters. The sorbents used may have a secondary
application in industry. The water purification and recycling could improve the environ-
mental state around the pond disposals of TPS. On the other hand, a number of elements
such as Li, MO, Cs, V, Bi, B, F, Br, I, alkaline and alkaline-earth elements could be of
interest in terms of their extraction from waste waters and utilization.
The concentration of elements including Li, Sr, Zr, Ba, La, Hf, SC, Ti, V, Mn, Co,
Cu, Zn, As, Tl, Pb, Ag, P, and S in soils (Table 10) near TPS may be higher than the
respective Clarke value for soils [61,64]. Significant proportions of many of these
concentrations, especially for Hf, Zn, As, Tl, Pb, and Ag, have probably come from
stack emissions. According to standards in some European countries for soils, As and Pb
may provoke certain problems. FA particles are always present in soils, even at 8 km
distance from the chimney-stack at Bobov Do1 TPS. In addition, all elements (0, Cu,
Ag, F, Cl, Br, and I) analyzed after redistilled water leaching from the soils near that
TPS, were easily released into solution [43]. The results confirm some of the calcula-
tions made for the elements balance in TPS. The discrepancies could be explained by:
the unclear behaviour and fate of the elements from stack emissions after their discharge
into the atmosphere; their original excess Clarke content in these soils; the effect of
other side contaminants; and the migration of elements from the soils.
Elements such as Li, Sr, Y, MO, Ba, Yb, SC, Ti, V, Zn, Cd, Sn, Tl, Pb, Bi, and Ag,
may have higher concentrations in grass vegetation (ash) taken from the disposals (Table
11) than the respective Clarke content for vegetation (ash basis) [61,65]. Probably, some
proportions of the aforesaid elements have been accumulated, in particular Sr, Ba, Yb,
SC, Cd, Tl, Pb and Bi, from the waste products and waters.
The present results indicate some possible environmental pollution of the air, water
and soil with toxic and potentially toxic trace elements in the areas surrounding the large
coal-fired power stations. Therefore, a better understanding of the trace elements
behaviour during coal burning and waste storage, and of the fate of the trace elements
after their discharge into the atmosphere, are required.

4. Conclusions

A number of trace elements in Bulgarian and Ukrainian coals and coal ashes have
concentrations greater than the respective Clarke content. The highest values in coal ash
are displayed by elements such as Rb, Cs, Ba, Cu, Sb, Bi, U, and Ag. Trace elements
are concentrated mainly in the heavy accessory minerals and organic matter in coal. In
decreasing order of significance, the trace elements in coal may occur as: element-
organic compounds; impurities in the mineral matter; major components in the mineral
matter; major and impurity components in the inorganic amorphous matter; and ele-
ments in the fluid constituent. A number of trace elements in the waste products, similar
to coal ashes, exceed known Clarke contents. Trace elements are mainly enriched in
non-magnetic, heavy and fine-grained fractions of FA. They are commonly present as
 XV, Vassileu, C.G. Vassilera / Fuel Processing Technology 51 (19971 19-45 43

impurities in the glass phases, and are included in the crystalline components. Their
accessory crystalline phases, element-organic compounds, liquid and gas forms, are of
subordinate importance. Some elements from the chalcophile (Cu, Zn, Ga, Ge, Pb, As,
Sb), lithophile (Be, Ba, Ce, Hf, Sr, La, Zr, MO, U> and siderophile (SC, Cr, V) groups
may release into the atmosphere during coal burning. For others, the combustion process
appears to be a powerful factor causing their relative enrichment in the FA and rarely in
the BA and slag. Considerable amounts of Hf, some chalcophile elements (Zn, As, Tl,
Pb) and Ag from stack emissions have probably entered the soil near TPS. Trace
elements can also occur in water soluble forms in the FA (Li, Sr, MO, Cs, V, Cr, Mn,
As, Bi, B, F, Cl, Br, I> and probably contaminate the surface and subsoil waters. Some
trace elements (Sr, Ba, Yb, SC, Cd, Tl, Pb, Bi) may accumulate in the vegetation near
TPS. The combustion waste products may be considerable pollutants of the air, water
and soil near TPS. Therefore, an effective control of the content and a better understand-
ing of the behavior and fate of trace elements during coal burning and waste products
storage at large power plants, are required.

References

111 Yurovskii, A., Mineral Components in Caustobioliths, Nedra, Moscow, 1968, 215 pp. (in Russian).
121 Gluskoter, H., R. Ruth, W. Miller, R. Cahill, G. Dreher and J. Kuhn, J., Trace elements in coal:
occurrence and distribution, Illinois State Geological Survey, Urbana, Circular No. 499, 1977, 154 pp.
[3] Yudovich, Y., Geochemistry of Coal, Nauka, Moscow. 1978, 262 pp. (in Russian).
[4] Bouska, V., Geochemistry of Coal, Academia, Prague, 1981, 284 pp.
[5] Krejci-Graf, K., in S. Augustithis (Ed.), The Significance of Trace Elements in Solving Petrogenetic
Problems and Controversies, Theophrastus Publications, Athens, 1983, pp. 533-597.
161 Valkovic, V., Trace elements in coal, Vols. I and II. CRC Press, Boca Raton, FL, 1983, Vol. I, 210 pp.;
Vol. II, 281 pp.
171 Yudovich, Y., M. Ketris and A. Mertc, Trace Elements in Coal, Nauka, Leningrad, 1985, 239 pp. (in
Russian).
181 Kler, V., G. Volkova, E. Gurvich, A. Dvomikov, Y. Jarov, D. Kler, V. Nenahova, F. Saprikin and M.
Shpirt, Metallogeny and Geochemistry of Coal and Shale Bearing Stratum in USSR: Geochemistry of
Elements, Nauka, Moscow, 1987, 240 pp. (in Russian).
191 Swaine, D., Trace Elements in Coal, Butherworths, London, 1990. 296 pp.
[lOI Davison, R., D. Natusch and J. Wallace, Env. Sci. Techn., 8 (1974) 1107-1766.
[ill Kaakinen, J., R. Jorden, M. Lawasany and R. West, Env. Sci. Techn., 9 (1975) 862-869.
[121 Gladney, E.. J. Small, G. Gordan and W. Zoller, Atm. Env., 10 (1976) 1071-1077.
[I31 Campbell, J., J. Laul, K. Nielson and R. Smith, Anal. Chem., 50 (1978) 1032-1040.
(141 Coles, G., R. Ragaini, J. Ondov, G. Fisher, D. Silberman and B. Prentice, Env. Sci. Techn., 13 (1979)
455-459.
[151 Quann, R.. M. Neville, M. Janghorbani, C. Mims and A. Sarofim, Env. Sci. Techn.. 16 (19821 776-781.
[161 Furuya, K., Y. Miyayima, T. Chiba and T. Kikuchi, Env. Sci. Techn., 21 (1987) 898-903.
[171 Querol. X., J. Fernandez-Turiel and A. Lopez-Soler, Fuel, 74 (1995) 331-343.
[181 Hansen, L., D. Silberman and G. Fisher, Env. Sci. Techn., 15 (1981) 1057-1062.
[191 Valkovic, V., J. Makjanik, M. Jaksic, S. Popovic, A. Bos, R. Vis, K. Wiederspahn and H. Verheul, Fuel.
63 (19841 1357-1362.
[201 Norton, G., R. Markuszewski and H. Shanks, Env. Sci. Techn., 20 (19861409-413.
[211 Hulett, L., A. Weinberger, K. Nortmcutt and M. Ferguson, Science, 210 (1980) 1356-1358.
[22] Spears, D., M. Tarazona and S. Lee, Fuel, 73 (1994) 1051-1055.
[23] Raask, E. and L. Goetz, J. Inst. Energy, 54 (1981) 163-173.
44 S. V. Vassileu, C.G. Vassileua / Fuel Processing Technology 51 (1997) 19-45

[24] Raask, E., Mineral Impurities in Coal Combustion, Hemisphere, Washington, 1985, 484 pp.
[25] Billings, C., A. Sacco, W. Matson, R. Griffin, W. Coniglio and R. Harley, .I. Air Poll. Cont. Assoc., 23
(1973) 773-777.
]26] Andren, A., D. Klein and Y. Talmi, Env. Sci, Techn., 9 (19751 856-858.
[27] Block, C. and R. Dams, Env. Sci. Techn., 9 (1975) 147-150.
[28] Klein, D., A. Andren, J. Carter, .I. Emery, C. Feldman, W. Fulkerson, W. Lyon, .I. Ogle, Y. Talmi, R.
Van Hook and N. Bolton, Env. Sci. Techn., 9 (1975) 973-979.
1291 Linton, R., A. Loh, J. Natusch, C. Evans and J. Williams, Science, 191 (1976) 852-854.
[30] Linton, R., P. Williams, C. Evans and J. Natusch, Anal. Chem., 49 (1977) 1514-1520.
1311 Smith, R., J. Campbell and K. Nielson, Atm. Env., 13 (1979) 607-617.
[32] Smith, R., J. Campbell and K. Nielson, Fuel, 59 (1980) 661-665.
[33] Smith, R. and D. Baer, Atm. Env., 17 (1983) 1399-1409.
[34] Clarke, L., Fuel, 72 (1993) 731-736.
[35] Egorov, A., N. Laktionova, N. Popinako and I. Novosselova, Toploenergetika, 2 (1979) 22-25 (in
Russian).
[36] Conzemius, R., T. Welcomer and H. Svec, Env. Sci. Techn., 18 (1984) 12-18.
[37] Shpirt, M., V. Kler and I. Pertsikov, Inorganic Components of Coals. Chimia, Moscow, 1990, 240 pp. (in
Russian).
[38] Vassilev, S., Rev. Bulg. Geol. Sot., 51(2) (1990) 35-45 (in Bulgarian with English abstract).
[39] Vassilev, S., Petrol. Coal Geol., 28 (1991) 46-51 (in Bulgarian with English abstract).
[40] Vassilev, S., Fuel, 71 (1992) 625-633.
1411 Vassilev, S., Compt. Rend. Acad. Bulg. Sci., 46(6) (1993) 57-59.
[42] Vassilev, S., in K. Skarzynska (Ed.), Proceedings of the Fourth Intematinal Symposium on the
Reclamation, Treatment and Utilization of Coal Mining Wastes, Krakow, Poland, September 6-10, 1993,
Vol. I, pp. 203-210.
[43] Vassilev, S., Fuel, 73 (1994) 367-374.
[44] Vassilev, S., Ann. Univ. Sofia, I-Geol., 84 (1995) 85-108.
[45] Vassilev, S., Compt. Rend. Acad. Bulg. Sci., 48(4) (1995) 45-47.
[46] Vassilev, S. and B. Vassileva, Compt. Rend. Acad. Bulg. Sci., 45(7) (1992) 49-52.
[47] Vassilev, S., M. Yossifova and C. Vassileva, Int. J. Coal Geol., 26 (1994) 185-213.
[48] Vassilev, S., G. Eskenazy, M. Tarassov and V. Dimov, Geol. Bale., 25(3-4) (1995) 111-123.
[49] Vassilev, S., K. Kitano, S. Takeda and T. Tsurue, Fuel Proc. Techn., 45 (1995) 27-51.
[50] Vassileva, B., S. Vassilev and C. Vassileva, Compt. Rend. Acad. Bulg. Sci., 49(41 (1996) 59-62.
[51] Vassilev, S. and C. Vassileva, Occurrence, abundance and origin of minerals in coals and coal ashes. Fuel
Proc. Techn., 48 (1996) 85-106.
[52] Vassilev, S. and C. Vassileva, Mineralogy of combustion wastes from coal-fired power stations. Fuel
Proc. Techn., 47 (1996) 261-280.
[53] Solodov, N., E. Semenov and V. Burkov, Geological Handbook on Heavy Lithophilic Trace Metals,
Nedra, Moscow, 1987, 438 pp. (in Russian).
[54] Beus, A. and C. Grigorian, Geochemical Methods of Prospecting and Exploration of Solid Mineral
Resource Deposits, Nedra, Moscow, 1975, 279 pp. (in Russian).
[55] Vinogradov, A., Geochimia, 7 (19621 555-572 (in Russian).
[56] Yudovich, Y., Gramme is More Expensive Than Tons: Trace Elements in Coal, Nauka, Moscow, 1989,
160 pp. (in Russian).
[57] Van der Flier-Keller, E. and W. Fyfe, Fuel, 67 (1988) 1048-1052.
[58] Swaine, D., in S. Augustithis (Ed.), The Significance of Trace Elements in Solving Petrogenetic Problems
and Controversies, Theophrastus Publications, Athens, 1983, 521-532.
[59] Takeda, S., H. Unuma, T. Tsurue, S. Sayama and S. Ito, Reports of the Government Industrial
Development Laboratory, Hokkaido, 37 (1985) 6-12 (in Japanese).
[60] Dobrovolskii, V., Geography of Trace Elements: Worldwide Scattering, Misl, Moscow, 1983, 272 pp. (in
Russian).
[61] Bowen, H., Trace Elements in Biochemistry, Academic Press, New York, London, 1966, 241 pp.
[62] Council of the European Communities. Council Directive of 15 July 1980 relating to the quality of water
intended for human consumption. 80/778/EEC. Off. J. Eur. Commun., 23(L229) (19801 11.
 S. V. Vassiler, C.G. Vassilera / Fuel Processing Technology 51 f I9971 19-45 4s

[63] Clarke, L.B. and I.M. Smith, 9th International Ash Use Symposium, 1991, EPRI GS-7162. Vol. 3.
70-l-70-15.
[641 Vinogradov. A.. Geochemistry of Trace Elements in Soils, Izdatelstvo Academii Nauk SSSR, Moscow.
1957, 238 pp. (in Russian).
[65] Tkalitch, S., in Biogeochemical Research of Ore Deposits. Wan-Ude. Izdatelstvo Academii Nauk SSSR,
Moscow. 1969, pp. 83-90 (in Russian).