Maritza III”, „Republika - Institute of Physical Chemistry

Trace elements in fly ashes from “Varna”, “Bobov dol”, “Maritza Iztok
I”, “Maritza ”, „Republika” and “Rousse Istok” power plants,
Bulgaria
A. Shoumkova and V. Stoyanova*
University of Chemical Technology and Metallurgy,
8 “Kliment Ohridski” Blvd, 1756 Sofia, Bulgaria, e-mail: annnie@abv.bg
*Institute of Physical Chemistry, Bulgarian Academy of Sciences,
“Acad.G.Bonchev” bl.11, 1113 Sofia, Bulgaria, e-mail: valeria@ipc.bas.bg
ABSTRACT
A comparative study of the content of five macro- and about fifty micro- and
trace elements in fly ashes from six coal-burning power plants in Bulgaria is
carried out by means of a combination of Chemical Analysis, AAS, X-Ray
Luminescence and X-Ray Fluorescence EDA. The elements’ distribution
among and within single particles is investigated on the base of the
comparison of data from bulk analysis, micro-analyses, and the results from
acid leaching tests using 0.1M HNO 3 . It is established that many heavy
metals and other toxic trace elements (Ba, Cu, Sr, Pb, Tl, Cd, some
Lantanides and rare elements) exist predominantly in singe particles and/or
acid-soluble forms and up to 70% of them could be extracted, when just 3-
15% of alumino-silicate material is dissolved.
The results are discussed from chemical and ecological point of view.
Keywords: Coal fly ash, trace elements, EPXMA.
INTRODUCTION
Fly ashes from coal burning power plants represent one of the most massive
industrial solid wastes produced worldwide. The quantity of annually
generated fly ashes in Bulgaria is about 6 million tones and only 15%-20%
of them are being utilized, but the rest is being disposed, predominantly in
open landfills. From chemical point of view, fly ashes represent
aluminosilicate matter, consisting mainly of Si, Al, Fe, Ca, Mg. Therefore
they are often classified and treated as inert, and hence innocuous, materials
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[1]. But actually, many investigations [2-4] have shown that fly ashes
contain also a number of heavy metals and other toxic elements, which
extraction in the environment may create serious air, soil, and water
pollution problems [3-9]. The fact that some of the harmful ingredients are
usually highly enriched in respiratory size particles (diameter < 10 μm) [10-
14] involves an additional health risks.
Tat is why the main aim of the present study was to determine not only the
chemical composition of the fly ashes, generated in some of the biggest
Bulgarian power plants, but also to obtain information about the elements
distribution among and within particles, and the possibility for their
extraction in acid conditions.
EXPERIMENTAL
1. Materials
In the present investigation fly ashes from six coal-burning power plants in
Bulgaria– “Varna” (3582MWth), “Bobov dol” (1716MWth), “Maritza Iztok
I” (currently „Brikel”, 865MWth), “Maritza ” (300MWth), „Republika”
(502MWth) and “Rousse Istok” (1450MWth) are used.
All the fly ashes except that from TPS “Rousse Istok” are collected in the
period 2002-2004 from the gas cleaning utilities - electrostatic precipitators
and cyclones in the power stations. The sample from TPS “Rousse Istok”
represents a mixture of fly ash and slag.
2. Methods
The content of macro elements in the fly ashes - Si, Al, Fe, Ca, Mg is
determined by Silicate analysis (SA). The content of alkaline metals K and
Na is defined by Atomic Absorption Spectroscopy (AAS), while trace
elements concentrations are studied X-Ray Luminescence and X-Ray
Fluorescence Energy Dispersive Analyses. The chemical composition of
individual particles is examined mainly by JEOL Electron Probe X-Ray
Micro-Analizer JXA-733 (EPXMA) in both SEI and BEI regimes, more
particularly described in [15-18]. For some of these analyses also Scanning
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(Jeol JSM 5800LV) and Transmission (Hitachi HNAR) Electron
Microscopes, equipped with Energy Dispersive X-ray Analyzers are used.
For the acid leaching tests extraction with 0.1M HNO 3 (24h, 1g ash/ 100 ml
acid) is carried out. After that the suspensions are filtered and analyzed by
Inductively Coupled Plasma Mass Spectroscopy (Thermo electron
Corporation, Model IRIS Intrepid II XSP). On the base of the concentration
of the elements in the filtrate, the quantity of each element (in ppm), which
is extracted from the sample is calculated.
The particle size distribution is determined by Laser Analizer Beckman
Coulter LS 230. The specific weight is pycnometricaly determined by
Micrometrics AccuPyc 1330. The specific surface and pore volume are
defined by Micrometrics ASAP 2010 on the base of inert gas absorption.
Loss on ignition is defined thermogravimetrically (Setaram TGA) in air
atmosphere.
RESULTS AND DISCUSSION
The chemical composition, expressed as oxides (not normalized to 100%)
and some physical properties of the fly ash samples are shown in table 1.
Table 1. Chemical composition and physical properties.
Component/ Parameter
Varna
Bobov
dol
Maritza
istok I
Content, wt. %
Maritza
III
Republika
Rousse
iztok
SiO 2 46.7 50.9 23.4 42.1 57.1 54.6
Al 2 O 3 23.5 20.8 13.5 19.9 27.6 29.5
Fe 2 O 3 8.6 8.5 26.9 12.9 5.8 4.6
CaO 5.2 13.6 7.1 10.7 3.2 3.5
MgO 3.0 3.4 1.4 3.1 1.0 2.3
K 2 O 2.9 2.1 1.1 1.2 2.5 1.8
Na 2 O 0.9 0.6 0.4 1.1 0.3 0.7
LOI 1273 9.8 1.0 22.9 6.3 1.1 0.3
Specific weight, kg/m 3 2456 2177 2594 2363 2180 2635
Specific surface, m 2 /g 3.0 1.9 12.7 15.44 15.34 6.81
Pore volume, m 3 /g 0.68 0.43 2.92 3.55 3.52 1.56
d 50 by volume, μm 5.3 44.2 20.1 35.7 16.3 34.6
d 50 by number, μm 0.59 0.58 0.58 0.64 0.56 0.58
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As it can be seen from the table, all the fly ashes investigated are of acidic
type, having ratio (CaO+MgO)/(SiO 2 +Al 2 O 3 +Fe 2 O 3 )

Element
Varna
ble 2. Trace elements content, ppm.
Bobov
dol
Maritza
istok I
Maritza III
Republika
Rousse
iztok
Ag 4 4 5 5 1 5
As 458 1032 298 177 71 32
Ba 1773 1461 1227 523 663 929
Bi 101 73 108 108 0 130
Br 61 9 107 0 7 0
Cd 4 2 20 5 0 2
Ce 236 107 13 59 56 111
Cl 136 59 109 0 10 22
Co 10 70 84 41 35 10
Cr 140 165 90 40 73 63
Cs 48 36 67 6 2 7
Cu 740 590 295 613 180 240
Ga 164 110 100 87 82 113
Ge 20 4 3 0 2 6
Hf 280 182 352 376 121 343
Hg 1.3 0.044 0.407 0.187 0.33 0.086
I 18 2 16 5 7 15
In 0 0 0 1 1 2
La 106 80 41 95 11 43
Mn 1100 600 790 1165 400 550
Mo 57 55 106 36 7 16
Nb 76 57 27 32 22 37
Nd 19 0 0 34 0 0
Ni 140 122 111 80 65 62
P 4100 2033 68 2350 720 465
Pb 426 310 67 82 101 210
Pr 34 30 0 0 0 0
Rb 653 526 182 281 361 618
S 18000 25400 72900 22400 17100 8100
Sb 24 17 21 2 3 9
Sc 317 704 378 875 66 176
Se 64 5 577 20 9 6
Sn 56 45 62 37 13 43
Sr 3926 1942 2081 2445 350 1089
Ta 119 44 104 67 14 102
Te 10 1 4 0 2 1
Th 20 22 13 0 44 9
Ti 17300 15416 7300 13200 13600 8600
Tl 36 17 51 26 28 0
U 32 12 28 0 3 35
V 132 115 216 249 272 346
W 193 96 72 151 25 215
Y 194 124 124 83 82 132
Zn 979 819 602 469 301 1869
Zr 1090 529 301 434 113 437
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Generally the concentration of trace elements in fly ashes depends on the
composition of parent coals, which is not a subject of the present study, but
it also is a function of the burning regime, which defines their sharing
between slag or bottom ash and fly ash, and their distribution within and
among particles. As the interactions taking part during the burning process
are too complicated, it is almost impossible a proper prediction of the
elements distribution to be made. Therefore in order to obtain reliable
information about trace elements distribution, an investigation of the
individual particles composition is carried out.
The chemical composition of about 780 (“Varna” - 270, “Bobov dol” - 240,
“Maritza Iztok I” - 140, “Maritza ” - 50, „Republika” - 50 and “Rousse
Istok” - 30) individual particles, selected by morphological in (SEI mode) or
“brightness” (in BEI mode) criterion is determined. The investigation shows
that all the samples consist mainly of aluminosilicate particles, having
different content of Fe, Ca, Mg, Na and K, as well. In the samples “Varna”,
“Bobov dol”, “Maritza Iztok I”, “Maritza ” also particles having high
(>65%) iron, calcium or silicon content are often observed. The macroelements
in these samples are more uneven spread among the particles,
while in fly ashes from “Republica” and “Rousse Istok” the chemical
composition of the most individual particles analyzed is very close to the
composition of the bulk material. Some relationships between
morphological characteristics – shape, surface relief, texture, etc. and
chemical composition are also observed [15,16]. The investigation of the
composition of heavy metals rich particles show that among “bright”
particles iron rich ones predominate in all the samples. Another widely
spread element, concentrated in individual particles is Ba, which is often
observed in a combination with Cu, Sr, Ca and Tl. Strontium is also
frequently detected in individual particles accompanied by S, Ti, Ca. The
forth trace element, which trend to be enriched in individual particles is Cu.
Lead is also often met, sometimes as Pb-rich spots in the surface of
aluminosilicate particles, where it appears probably in result of
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condensation from vapors during the cooling of the ash. Many other
elements, including some rare elements and Lanthanides are detected in
high concentrations. More detailed description of some of the observed
“bright” particles is given in [15-17], but it can be summarized that some of
them are usually found in typical combinations (as shown in Table 3) and
almost always in fine particles of a respiratory size (

Contrawise, the low degree of extraction of Ti could be explained by the
incorporation of this element in the aluminosilicate glass phase.
Element
ble 4. Quantity of acid leached elements, ppm
Varna
Bobov
dol
Maritza
istok I
Maritza III
Republika
Rousse
iztok
Al 7846 18340 8366 21400 10900 4140
As 115.6 98.6 8.6 34.4 29.4 24.4
Ba 303 205.4 72.4 166.2 284.6 93.2
B 300.4 277 214.6 256.8 231.6 199.8
Ca 13224 50400 15910 48800 6300 8440
Cd 1.6 2.8 7.4 4.6 1 1.4
Co 3.8 4.2 5 5 3 2.8
Cr 40 22.4 27.2 10.8 7.4 8.4
Cu 27.8 33.2 41.2 91.8 24.6 19.2
Fe 5452 10440 32600 15640 3480 4580
K 1609.8 2606 242 1686 1984 352
Li 25 24 9.4 16 11 3.6
Mg 2330 3488 1986 4820 1570 1042
Mn 254.4 216 262 536 67.4 112
Mo 16.2 6.8 3 5.2 1.8 1.4
Na 1168 1302 1390 1978 664 344
Ni 10.2 33.6 29.8 28 8.8 8.3
P 1376 610.6 30 690 159.2 354
Pb 10.6 7.8 21.2 25.2 12.4 23.6
Rb 7 15.8 2.4 12.6 13.4 1.4
S 6584 5530 22860 8564 1736 171.4
Si 6314 25920 3540 27800 6860 7180
Se 12.8 3.4 10 0.4 1.2 0
Sr 511.4 212.4 144.8 318.6 110.8 58
Ti 218.8 784 341 643.6 194.4 164.6
V 218.2 132.2 92.8 127.6 101.6 54.8
W 7 4.6 2.8 6 2.8 3.8
Zn 39.2 45 43.6 51.4 52 36.8
Zr 5.4 9.8 2.4 8.8 0.8 1.4
Total
trace el.*
3.5 kg/t 2.8 kg/t 1.0 kg/t 3.0 kg/t 1.3 kg/t 1.2 kg/t
Al+Si
dissolved
3.8 % 12.6 % 6.1 % 15.2 % 4.0 % 2.5 %
Total ash
dissolved
4.6 % 12.0 % 8.8 % 13.3 % 3.5 % 2.7 %
*Total quantity of trace elements (Al, Si, Fe, Ca, Mg, K, Na and S) extracted.
Among the trace elements As, B, Ba, Mn, Sr, Ti, V and P present in highest
concentration in the solution, while the most significant degree of extraction
8

is reached by Cd (up to 90%), P, As, Se and Cu (up to 70-75%), Ba, Mn, V
and Mo (up to 45-50%), Pb, Ni, Cr and Co (up to 30-35%).
As it can be seen from the comparison of the data obtained with those for
the total trace elements content, the quantity of trace elements, extracted in
the solution does not depend only from their concentration in the ash and its
solubility. For example, the fly ash from TPS “Republica” has almost 3
times lower total content of trace elements than “Maritza iztok I” fly ash,
but the extracted quantity is 60% higher, regardless of the fact that the
dissolved material is 2.5 time less. This could be explained by the surface
enrichment of some elements and/or their existence in individual particles in
well soluble forms.
CONCLUSSIONS
On the base of the provided investigations of the chemical composition, the
distribution of micro- and trace elements, and their acid leaching, it could be
concluded that all the fly ashes studied have relatively high content Ba, Mn
and Sr (above 500 ppm), and some of them are rich of As, P, Pb, Cu, Rb,
Sc, Se, Zn and Zr, as well. Many toxic elements, such as Ba, Sr, Cu, Cd, Pb,
Lanthanides, etc. are concentrated preferably in fine individual particles.
They often coexist each to other and with sulfur and chlorine, which
enlarges the risk of extraction in the environment. A significant part (up to
30-90%) of these and other toxic trace elements could be extracted by nitric
acid leaching, while only 3-15% of aluminosilicate material is dissolved.
This leads to the conclusion that the risk of trace elements distribution in the
environmental could be higher than it could be predicted on the base of the
bulk analyses, and should not be neglected.
The fact that more than half of the particles are of submicron size facilitates
the wind erosion of the open landfills, their transport by the air flows, the
inhalation by the animals and the people, and thus additionally enlarges the
risk of toxic trace elements distribution and extraction in the environment
and organisms.
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ACNOLEDGEMENTS
The authors express their personal thanks to Mr. Rogovetz for the
performing of the acid leaching tests and to Mrs. Tzacheva for EPXMA. The
financial support received in the frame of a Joint research project of
Bulgarian Academy of Sciences and Czech Academy of Sciences is also
highly appreciated.
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