COGEL-01855; No of Pages 9
International Journal of Coal Geology xxx (2011) xxx–xxx
Contents lists available at ScienceDirect
International Journal of Coal Geology
j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / i j c o a l g e o
Transformation of aluminum-rich minerals during combustion of a bauxite-bearing
Chinese coal
Yongchun Zhao, Junying Zhang ⁎, Chuguang Zheng
State Key Laboratory of Coal Combustion, Huazhong University of Science & Technology, Wuhan 430074, China
a r t i c l e
i n f o
Article history:
Received 11 December 2010
Received in revised form 15 April 2011
Accepted 21 April 2011
Available online xxxx
Keywords:
Boehmite
Mineral transformations
Fly ash characteristics
Chinese high-aluminum coal
a b s t r a c t
Samples of two high-aluminum coals and an associated fly ash were collected from a coal-fired power plant
and a coalfield in Inner Mongolia, China. The mineralogy and physicochemical characteristics of lowtemperature ash (LTA), high-temperature ash (HTA), and fly ash from those coals were studied by X-ray
diffraction (XRD), X-ray fluorescence (XRF), and field scanning electron microscopy with energy dispersive
X-ray spectroscopy (FSEM-EDX). The transformation of typical aluminum-bearing minerals at high
temperature was investigated by systematic drop tube furnace (DTF) experiments and thermogravimetric
analysis. The results show that the aluminum-bearing minerals in the high-Al coal are mainly boehmite and
kaolinite. High temperature treatment transforms the aluminum-rich minerals to gamma alumina (γ-Al2O3),
corundum (α-Al2O3), and an amorphous phase. γ-Al2O3 is the main mineral in the HTA (17.4 wt.%), while
α-Al2O3 and mullite are the main minerals in the fly ash. The high-aluminum fly ash particles are irregular and
their shapes are related to their compositions. The degree of irregularity of the high-aluminum fly ash
particles is proportional to their aluminum content. The phase transformation of boehmite in the coal during
high temperature treatment appears to have involved four stages including: boehmite dehydroxylation,
transitional θ-Al2O3 formation, crystal nucleation and α-Al2O3 formation, and growth of α-Al2O3 crystals. The
DTF experimental results indicated that the growth of α-Al2O3 crystals has a significant impact on PM
emissions. Understanding the mineral transformation mechanism is therefore helpful in reducing PM
emissions.
© 2011 Elsevier B.V. All rights reserved.
1. Introduction
Coal is still the main energy resource in China. Coal production in
China was up to 3 billion tons in 2009, of which the coal-fired power
plants consumed 1.5 billion tons. With the increasing use of coal, a
large amount of coal combustion product material (ash) has been
generated, which occupies more land and causes serious environmental pollution (Zhao et al., 2008a). The amount of fly ash produced
by the coal-fired power plants in China was around 375 million tons
in 2009, while the utilization rate is only 30% (Yang et al., 2010).
Knowledge of fly ash characteristics is helpful to its utilization.
Inorganic matter in coal is the main source of fly ash; the occurrence,
distribution, and transformation of mineral matter can significantly
affect fly ash characteristics and the related environmental problems
(Vassilev and Vassileva, 1996; Vassileva and Vassilev, 2005, 2006;
Zhang et al., 2002, 2004a,b).
Aluminum is one of the main inorganic elements in coal-fired fly
ash. Usually the Al occurs in clay minerals in the coal. Recently, it has
been found that coals in the Jungar Coalfield, Ordos Basin, China are
⁎ Corresponding author. Fax: + 86 27 87545526.
E-mail address: [email protected] (J. Zhang).
unusually enriched in aluminum (Dai et al., 2006a,b). The No. 6 Coal at
the late Carboniferous Taiyuan Formation is about 30 m thick, with
coal mined at about 20 million tons per year. Dai et al. (2006a, 2008)
have investigated the mineralogy and geochemistry of this high
aluminum coal in detail. A high boehmite content (mean 6.1%) was
identified in the No. 6 Coal in the Jungar Coalfield (Dai et al., 2006a),
with the boehmite being derived from bauxite in the weathered crust
of the underlying Benxi Formation (Dai et al., 2006b). The Jungar
power plant is burning this coal, and every year it produces about
380,000 t high-aluminum fly ash (Shao et al., 2006). The enrichment
of aluminum-bearing minerals in the coal results in a high-Al2O3 fly
ash which has a potential economic value.
The physico-chemical characterizations of fly ash, as well as the
certain technological and environmental problems, have an important
relationship with the partitioning and high temperature behavior of
minerals during coal combustion (Vassileva and Vassilev, 2006; Zhao
et al., 2006). Dai et al. (2010) presented a systematic investigation on
mineralogical and chemical compositions of high-alumina fly ash
generated at the Jungar Power Plant. Many scientists have done
detailed research on the transformation of minerals, including ironbearing minerals (Srinivasachar et al., 1990a; Srinivasachar and Boni,
1989; Zhao et al., 2006), calcium-bearing minerals (Huffman et al.,
1990; Maes et al., 1997; ten Brink et al., 1995), illite (Srinivasachar
0166-5162/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.coal.2011.04.007
Please cite this article as: Zhao, Y., et al., Transformation of aluminum-rich minerals during combustion of a bauxite-bearing Chinese coal, Int.
J. Coal Geol. (2011), doi:10.1016/j.coal.2011.04.007
2
Y. Zhao et al. / International Journal of Coal Geology xxx (2011) xxx–xxx
Two typical coal samples were collected from the Jungar Coalfield
(ZG6-2) and Jungar Power Plant (ZGLQ, 200 MW), respectively. The
plant is a typical pithead power plant, and its feed coal comes from the
No. 6 Coal in the Jungar Coalfield. The samples were crushed and
pulverized in the laboratory using a bench-scale hammer mill and
passed through a sieve of 200 μm. The pulverized coal was subjected
to proximate analysis, ultimate analysis, as well as ash composition
analysis with an ARL-9800 X-ray fluorescence spectrometer (XRF).
Analytical data are given in Table 1. A fly ash sample was also collected
from the hoppers of the electrostatic precipitators (ESP) in the power
plant.
increment of 0.017° and a counting time of 10 s per step. The Rietveld
method was used to calculate semi-quantitatively the mineral
composition of the coal and LTA samples (Rietveld, 1969). The “X'Pert
HighScore” calculation software was used, which was developed by
PANalytical in 2003. The amorphous contents were determined by the
ratio of halo area to crystalline area of XRD (Vassilev et al., 2003). The
investigations by FSEM (field emission scanning electron microscopy)
were carried out on a Sirion200 microscope equipped with a GENESIS
EDX (energy dispersive X-ray spectroscopy) at HUST-ATC. FSEM-EDX
was used to study the morphology and composition of the highaluminum fly ash particles.
Thermogravimetric analysis (TG-DSC) of the coal and LTA samples
was conducted using a NETZSCH STA 409 thermogravimetric analyzer
in the State Key Laboratory of Coal Combustion (SKLCC) at the HUST.
The experiments were carried out using 16–20 mg of coal or LTA. The
samples were heated from ambient temperature to 1450 °C under a
static air atmosphere at a heating rate of 50 °C min − 1.
After characterization and thermogravimetric analysis, samples of
the ZG6-2 coal were burned in a drop tube furnace (DTF) located at
the HUST-SKLCC. The height of reactor tube was 200 cm and the inner
diameter was 56 mm. The raw coal at a feed rate of about 0.2 g/min
was entrained by air into the furnace, and a mixture of N2 and O2
gasses was used as the combustion atmosphere. The total gas flow
rate was 10 L/min. The residence time of the particles in the tube was
about 2 s. Under the given conditions, all the coals burnt completely.
The combustion gas, entraining the solid products, was first quenched
with N2 and simultaneously collected by a water-cooling probe. The
DTF experimental set-up has been described elsewhere in more detail
(Zhao, 2008). To understand the mineral transformations, the coal
was burned at different temperatures from 900 °C to 1300 °C in air.
The total ash was collected to analyze the particle size distribution
using a Master Mini particle size analyzer which was made by
Malvern Instruments Ltd.
2.2. Experiments and analytical methods
3. Results and discussions
Low-temperature oxygen-plasma ashing (LTA): Oxidizing the
organic matter and isolating the minerals without major alteration
can be achieved by ashing coal at low temperatures (120–150 °C) in
electronically-excited oxygen plasma (Gluskoter, 1965; Ward, 1999).
This probably represents the most reliable method for determining
the percentage of total mineral matter. The minerals isolated by this
technique can then be investigated by X-ray diffraction and similar
methods (Ward, 1999). The coal samples for the present study were
ashed using a K1050X low temperature asher in the State Key
Laboratory of Coal Combustion (SKLCC) of Huazhong University of
Science and Technology (HUST).
Major and minor minerals in the LTA and fly ash were identified
using XRD. XRD studies were carried out on a X'Pert PRO diffractometer equipped with a graphite diffracted-beam monochromator at the
Analytical Test Center (ATC) of HUST in China. The accelerating
voltage was 40 kV and the current was 40 mA. Diffraction patterns
were collected at 5–70°2θ using Cu-Kα radiation. The scans had an
3.1. Mineralogy of typical high-aluminum coal
et al., 1990b), and other phases. The transformations of inorganic
matter have significant effects on the emission of particulate matter
during coal combustion (Ninomiya et al., 2004; Zhao et al., 2008b).
However, little work has been done on the transformation of some
aluminum-rich minerals in coal combustion.
The present work addresses the thermal behavior of aluminumbearing minerals, particularly the transformation during combustion
of these high-aluminum coals. Two typical high-aluminum coals were
chosen to conduct systematic combustion experiments. The residue
ashes from the coal combustion experiments and fly ash from the
high-aluminum coal fired power plant were also sampled and
analyzed in detail by multiple methods to study the mineral
transformations. Our research, together with the systematic report
of Dai et al. (2010) on mineral and element distribution in this coal,
will provide further insight into the formation mechanism and
environmental problems caused by high aluminum ash in the Jungar
Coalfield, Inner Mongolia, China.
2. Samples and experiments section
2.1. Coal samples
Table 1
Proximate, ultimate analysis, and ash chemical composition of coals studied.
Sample
ZG-LQ
ZG6-2
Proximate analysis
Ultimate analysis (daf)
Mad
Vdaf
Ad
FCad
C
7.2
6.0
48.5
41.2
21.8
24.8
37.4
41.6
76.2
79.1
H
4.5
4.1
N
1.5
1.1
S
K2O
MgO
Na2O
TiO2
Other
0.4
1.3
0.3
b0.4
0.1
b 0.1
3.4
7.9
2.1
≥ 0.5
1.5
0.4
O
16.3
15.3
Chemical composition (organic free basis)
Sample
SiO2
Al2O3
ZG-LQ
ZG6-2
37.3
31.1
49.3
53.9
CaO
4.2
1.9
Fe2O3
2.9
2.9
The aluminum-bearing minerals which have been identified in coal
are summarized in Table 2 (Dai et al., 2006a, 2011; Martinez-Alonso
et al., 1992; Vassilev and Vassileva, 1996; Vassilev et al., 2005; Ward,
2002; Zhang et al., 2004a; Zhao et al., 2006). The main Al-bearing
minerals in coal are clays, including kaolinite, illite, montmorillonite,
etc. Aluminum hydroxide minerals have also been identified in coal
(Dai et al., 2006a), such as diaspore, gibbsite, and boehmite. Among the
aluminum-rich minerals, boehmite is more common in coal; the origin
of this mineral has been studied by Dai et al. (2006a, 2011). In addition,
Al-bearing minerals which have been identified in coal include
phosphate minerals, carbonate minerals, sulfate minerals, and other
minerals (Ward, 2002).
The mineralogy of two high-aluminum coals and their lowtemperature ash has been analyzed by XRD, and diffractograms are
shown in Figs. 1 and 2. The minerals in the high-aluminum coal are
mainly boehmite, kaolinite, calcite, and quartz. It is generally
considered that the boehmite is a weathering product of silicate
rocks, and is often associated with minerals such as gibbsite, diaspore,
kaolinite, dickite, chalcedony, and ammonium mica, but only kaolinite
was found to be associated with boehmite in the Jungar coal (Dai et al.,
2006a). The boehmite is cryptocrystalline and occurs as lumps in
collodetrinite or as fillings in fusinite cavities (Dai et al., 2006b). The
lumps may be discrete, irregular, or continuous. The size of the lumps
varies considerably, from 1 μm to more than 300 μm.
The proportion of LTA produced by the high-aluminum coal is
26.8%. Compared to the coal, the relative concentrations of aluminumbearing minerals decrease after low temperature ashing, while calcite
increases in the LTA, and the quartz content remains unchanged.
Please cite this article as: Zhao, Y., et al., Transformation of aluminum-rich minerals during combustion of a bauxite-bearing Chinese coal, Int.
J. Coal Geol. (2011), doi:10.1016/j.coal.2011.04.007
Y. Zhao et al. / International Journal of Coal Geology xxx (2011) xxx–xxx
3
Table 2
Aluminum-bearing minerals identified in coals (Dai et al., 2006a; Martinez-Alonso
et al., 1992; Vassilev et al., 2005; Vassilev and Vassileva, 1996; Ward, 2002; Zhang et al.,
2004a; Zhao et al., 2006).
Category
Minerals
Chemical formula
Relative
abundance
Silicates
Kaolinite
Illite
Mixed layer
clays
Montmorillonite
Chlorite
Halloysite
Orthoclase
Sanidine
Albite
Anorthite
Plagioclase
Dravite
Analcite
Clinoptilolite
Heulandite
Oblique chlirite
Al2Si2O5(OH)4
K1.5Al4(Si6.5Al1.5)O20(OH)4
[Al2Si4O10(OH)2·xH2O]
Very common
Very common
Common
Na0.33(Al1.67Mg0.33)Si4O10(OH)2
(MgFeAl)6(AlSi)4O10(OH)8
Al4Si4O10(OH)8
KAlSi3O8
KAlSi3O8
NaAlSi3O8
CaAl2Si2O8
NaAlSi3O8–CaAl2Si2O8
Na(Mg3)Al6(Si6O18)(BO3)3(OH)4
NaAlSi2O6·H2O
(NaK)6(SiAl)36O72·20H2O
CaAl2Si7O18·6H2O
(Mg,Fe)4.75Al1.25[Al1.25Si2.75O10]
(OH)8
(Ca,Ce)2(Fe3+,Fe2+)Al2[SiO4]
[Si2O7]O(OH)
KMg3AlSi3O10OHF
Ca2Fe2Al2Si2O4
KFe2Al(Al2Si2O10)(OH)2
KAl2(AlSi3O10)(OH)2
3Al2O3·2SiO2
(Fe,Mg)5Al(AlSi3O10)(OH)8
(Fe,Mg,Mn)Al2(SiO4)O(OH)2
Rare–common
Rare
Rare
Rare
Rare
Rare
Rare
Rare
Very rare
Very rare
Very rare
Very rare
Very rare
Epidote
Biotite
Hornblende
Siderophyllite
Muscovite
Mullite
Chamosite
Chloritoid
Amphibole
Vesuvianite
Zeolite
Andalusite
Vermiculite
Stilpnomelane
Oxides and
hydroxides
Phosphates
Carbonate
Sulfate
Other
Boehmite
Diaspore
Gibbsite
Spinel
Hercynite
Corundum
Svanbergite
Crandallite
Gorceixite
Goyazite
Dawsonite
Tschermigite
Alunite
Coquimbite
Ferrite
Al2(SiO4)O
(Mg,Fe,Al)3((Al,Si)4O10)
(OH)2·4H2O
(Fe2+,Mg,Mn)2(Al,Fe3+)Al3O2
[SiO4](OH)4
AlO(OH)
AlO(OH)
Al2O3·3H2O
MgAl2O4
FeAl2O4
Al2O3
SrAl3(PO4)(SO4)(OH)6
CaAl3(PO4)2(OH)5·H2O
BaAl3(PO4)2(OH)5·H2O
SrAl3(PO4)2(OH)5·H2O
NaAlCO3(OH)2
NH4Al(SO4)2·12H2O
KAl3(SO4)2(OH)6
Fe2 − xAlx(SO4)3·9H2O, x ~ 0.5
Ca4Al2Fe2O10
Very rare
Very rare
Very rare
Very rare
Rare
Very rare
Very rare–rare
Very rare
Very rare–rare
Very rare
Very rare–rare
Very rare
Very rare
Very rare
Rare
Very rare
Very rare
Very rare
Very rare
Very rare
Very rare
Very rare
Very rare
Very rare
Very rare
Very rare
Very rare–rare
Very rare–rare
Very rare
Fig. 1. X-ray diffraction patterns of high aluminum coal and its low temperature ash
(ZG-LQ). K: kaolinite; B: boehmite; Q: quartz; C: calcite.
mechanisms of aluminum-bearing oxides and sulfates minerals are
unclear.
The XRD pattern of the high temperature ash (815 °C) of the high
aluminum coal is shown in Fig. 3. The XRD trace shows that there are
obvious amorphous or noncrystalline peak profiles in these two high
temperature ashes. The crystalline minerals identified are quartz,
γ-alumina, α-alumina, and hematite. Corundum or α-alumina is the
thermodynamically stable structure of alumina, but there are also
many metastable polymorphs of alumina including γ-alumina
(Table 4). According to the arrangement of oxygen anions, the
metastable alumina structures can be divided into two broad
categories: a face-centered cubic (fcc) or a hexagonal close-packed
(hcp) arrangement (Levin and Brandon, 1998). The different
distributions of the aluminum cation within each subgroup result in
the different polymorphs (Levin and Brandon, 1998; Wefers and
Misra, 1987). The alumina structures based on fcc packing of oxygen
include γ, η (cubic), θ (monoclinic), and δ (tetragonal), whereas the
alumina structures based on hcp packing are represented by α
(trigonal), κ (orthorhombic), and χ (hexagonal) phases (Levin and
Brandon, 1998). The γ-alumina is derived from the dehydration of
boehmite, and α-alumina is derived from the further transformation
of crystal structure of γ-alumina (Yen et al., 2003).
The XRD pattern of the high-aluminum fly ash from the Jungar
Power Plant is shown in Fig. 4. The main component of the highaluminum fly ash is amorphous material; the crystalline minerals are
mainly mullite, α-Al2O3, and quartz. Mullite is the major mineral in
the fly ash, accounting for 73% of the crystalline fraction; followed by
During low-temperature ashing, the organic matter of coal was
oxidized, and part of the elements combined with the organic
components was released at the same time. The content of boehmite
in the two coals is 13% and 11.9%, respectively, which is close to Dai
et al.'s (2006a) results.
3.2. Mineral composition of high temperature products of
high-aluminum coal
A list of aluminum-bearing mineral species reported in different
high-temperature materials associated with coal utilization is given in
Table 3 (Martinez-Alonso et al., 1992; Vassilev et al., 2003; Vassilev
and Vassileva, 1996; Ward, 2002; Zhao et al., 2006). Aluminum often
occurs in aluminosilicates in high-temperature phases, but sometimes
also as oxides and sulfates. The aluminosilicate minerals are mainly
derived from the transformation of clays in coal, while the formation
Fig. 2. X-ray diffraction patterns of high aluminum coal and its low temperature ash
(ZG6-2). K: kaolinite; B: boehmite; Q: quartz; C: calcite; T: tridymite.
Please cite this article as: Zhao, Y., et al., Transformation of aluminum-rich minerals during combustion of a bauxite-bearing Chinese coal, Int.
J. Coal Geol. (2011), doi:10.1016/j.coal.2011.04.007
4
Y. Zhao et al. / International Journal of Coal Geology xxx (2011) xxx–xxx
Table 3
Principal Al-bearing minerals identified in high temperature phases associated with
coal utilization (Martinez-Alonso et al., 1992; Vassilev and Vassileva, 1996; Vassilev
et al., 2003; Ward, 2002; Zhao et al., 2006).
Catogery
Minerals
Chemical formula
Relative
abundance
Silicates
Metakaolin
Mullite
Feldspar
Pyroxene
Anorthite
Gehlenite
C3A
C12A7
Albite
Plagioclase
Sanidine
Ba-feldspar
Melilite
Illite
Montmorillonitemetamontmorillonite
Biotite
Chlorite
Al2O3·2SiO2
3Al2O3·2SiO2
(Na,Ca,Al)-silicate
(Mg,Fe,Ca,Al)-silicate
CaAl2Si2O8
Ca2Al2SiO7
Ca3Al2O6
12CaO·7Al2O3
NaAlSi3O8
NaAlSi3O8–CaAl2Si2O8
KAlSi3O8
BaAl2Si2O8
Ca2MgSi2O7–CaAl(SiAl)2O7
(KH2O)Al2(AlSi)Si3O10(OH)2
Rare–common
Common
Rare
Rare
Common
Common
Rare
Rare
Very rare
Rare–common
Very rare
Very rare
Very rare
Rare
Rare–common
KMg3AlSi3O10OHF
(MgFeAl)6(AlSi)4O10(OH)8
Very rare–rare
Very rare–
common
Very rare–rare
Very rare–rare
Very rare
Rare–common
Very rare–rare
Very rare–rare
Very rare
Rare–common
Very rare
Very rare
Very rare
Very rare
Rare
Oxides and
hydroxides
Sulfates
Phosphates
Amphibole
Zeolite
Andalusite
Muscovite
Boehmite
Gibbsite
Ferrian spinel
Hercynite
Corundum
Mg–Al spinel
Brownmillerite
Alunite
Coquimbite
Al-hauyne
Goyazite
Al2(SiO4)O
KAl2AlSi3O10(OH)2
AlO(OH)
Al2O3·3H2O
Mg(AlFe)2O4
FeAl2O4
Al2O3
MgAl2O4
Ca4Al2Fe2O10
KAl3(SO4)2(OH)6
Fe2 − xAlx(SO4)3·9H2O,
x ~0.5
C4A3S
SrAl3(PO4)2(OH)5·H2O
Very rare
Very rare
α-Al2O3 with 21% of the crystalline fraction (Fig. 5). The high
concentration of aluminum minerals in the fly ash is derived from
the high proportion of boehmite and kaolinite in the coal. Mullite is
the decomposition production of kaolinite and α-Al2O3 comes from
the transformation of boehmite (Sheng et al., 2007). Compared to the
high temperature ash (815 °C), the fly ash is enriched in mullite, and
without γ-Al2O3; this may be related to the higher formation
temperature of the fly ash.
Table 4
The structures of Al2O3 (Levin and Brandon, 1998).
Phases
γ
η
θ
δ
α
Al3+
O2−
Cubic
FCC (facecentered
cubic)
Cubic
FCC (facecentered
cubic)
Monoclinic
FCC (facecentered
cubic)
Tetragonal
FCC
(face-centered
cubic)
Trigonal
HCP
(hexagonal
colse-packed)
Fig. 4. X-ray diffraction patterns of high aluminum fly ash from Jungar power plant. M:
mullite, C: corundum (α-Al2O3), Q: quartz.
3.3. Chemical composition of high-aluminum fly ash
The chemical compositions of typical fly ashes from different coalfired power plants have also been compared, as shown in Table 5. The
alumina content of the high aluminum fly ash from Jungar is up to
52.7%; the ratio of Al/Si is as high as 1.5, which is higher than that in
the high iron fly ash from Qingshan (ratio Al/Si = 0.8) (Zhao et al.,
2006), the high calcium fly ash from Xiaolongtan (ratio Al/Si = 0.5)
(Zhao et al., 2010), and high silicon fly ash (ratio Al/Si = 0.2) (Zhao,
2008). This value is also about three times that of the fly ashes from
European coal-fired power plants (Moreno et al., 2005; Vassilev and
Vassileva, 2007). The other oxide contents in the Jungar ash are lower
than in other fly ashes. This extremely alumina-enriched fly ash is so
special that no sub-category has been found in the classification of the
American Society for Testing and Materials (ASTM C618-92a, 1992),
nor in other classification standards.
3.4. Morphology of high-aluminum ash
Scanning electron microscopy was used to investigate the
morphology of the high-aluminum ash particles, as shown in Fig. 6.
Most of the particles are irregular (Fig. 6a), and are different from the
Mullite
Corundum
Quartz
5.9%
20.9%
73.2%
Fig. 3. X-ray diffraction patterns of high temperature ash of high-aluminum coal
(815 °C). A:γ-Al2O3; Q: quartz; C:α-Al2O3; H: hematite.
Fig. 5. Semi-quantity composition of high aluminum fly ash.
Please cite this article as: Zhao, Y., et al., Transformation of aluminum-rich minerals during combustion of a bauxite-bearing Chinese coal, Int.
J. Coal Geol. (2011), doi:10.1016/j.coal.2011.04.007
Y. Zhao et al. / International Journal of Coal Geology xxx (2011) xxx–xxx
Table 5
Chemical composition of typical fly ashes from coal-fired power plants.
Fly ash
SiO2
Al2O3
Fe2O3
CaO
MgO
K2O
Na2O
SO3
Al2O3/SiO2
High
High
High
High
35.0
24.5
34.6
80.9
52.7
20.5
15.7
14.9
2.0
50.1
10.2
0.9
4.2
1.4
29.1
0.4
0.7
0.3
1.5
0.6
0.5
0.4
0.8
0.3
0.1
1.0
0.5
0.6
b0.1
0.7
6.6
b0.1
1.5
0.8
0.5
0.2
Al
Fe
Ca
Si
ferrospheres which have rough surfaces, but with shapes are close to
that of an ideal sphere (Zhao et al., 2006). The shape of the particles in
the Jungar ash is related to particle composition. A typical Si–Al
particle in the high-aluminum fly ash, which has a spherical shape, is
shown in Fig. 6b. The proportion of this type of particle is low in the
high-aluminum fly ash. A few strip-shaped mullite particles were also
found in the high-aluminum fly ash (Fig. 6c, d), with some nano-scale
crystallites on their surfaces (Fig. 6c). Fig. 6e shows a typical alumina
particle which is a composite of many melted crystallites (Fig. 6f).
Based on their microstructure the high-aluminum fly ash particles
can be divided into three main types of morphologies: irregular
particles, melted porous particles, and melted microcrystal particles.
Most of them are irregular particles (Fig. 6c, d), bigger than 10 μm,
which are probably derived from the fragmentation of discrete
5
aluminum minerals in the coal. McLennan et al. (2000) also found
that discrete kaolinite produced combustion residues with angular,
porous particles. The melted porous particles (Fig. 6b) are usually less
than 10 μm, and the obvious melting on the surface indicates that they
have undergone high-temperature calcination. The third type of
particles is represented by melted micro crystal particles (Fig. 6f),
with sizes ranging from 0.1 μm to 1 μm. EDX analysis shows that the
composition of the crystals is alumina, which may be derived from the
transformation of different types of alumina (Yu et al., 2004).
3.5. Transformation of aluminum-rich minerals during coal combustion
The enrichment of boehmite is rare in coal, and the transformation
of boehmite during coal combustion is still unknown. Two typical high
aluminum coals with enriched boehmite were used to investigate the
transformation process of aluminum-rich minerals.
3.5.1. Thermogravimetric analysis
High heating rate causes errors in quantification of decarboxilation
of dehidroxylation areas (in general, in all decomposition which
involves mass loss), and it was desirable to detect mineral transitions
and fusions with small heat exchange and without weight change
(Mayoral et al., 2001). Hence heating rates for all TGA experiments
Fig. 6. SEM images of high aluminum fly ash particles (a) a general view of high aluminum fly ash particles (scale bar 100 μm). (b) a typical spherical fly ash particle (scale bar 5 μm).
(c) a strip-shaped mullite particle with nano-scale crystallites (scale bar 20 μm). (d) a typical strip-shaped mullite particle (scale bar 20 μm). (e) a typical alumina particle combined
with melted micro crystallites (scale bar 20 μm). (f) melted micro crystallites (scale bar 2 μm).
Please cite this article as: Zhao, Y., et al., Transformation of aluminum-rich minerals during combustion of a bauxite-bearing Chinese coal, Int.
J. Coal Geol. (2011), doi:10.1016/j.coal.2011.04.007
6
Y. Zhao et al. / International Journal of Coal Geology xxx (2011) xxx–xxx
were set at 50 °C/min. The LTAs of the two high-aluminum coals were
used to conduct thermogravimetric analysis under these conditions,
and the TG-DSC curves of these two LTAs are shown in Fig. 7. Both
have an obvious weight loss between 400 and 600 °C, with the peak
located at about 500 °C. This may be derived from the dehydration of
crystal water from kaolinite and boehmite.
Kaolinite: Al2 Si2 O5 2 ðOH Þ4 723K Al2 O3 ⋅2SiO2 + 2H2 O
→
773K γ−Al2 O3 + H2 O
Boehmite: Al2 O3 ⋅H2 O→
Kaolinite is a common clay mineral in coal, the evolution of
kaolinite with thermal treatment has been described by Querol et al.
(1994). Kaolinite loses combined water from the hydroxyl groups
surrounding the aluminum atoms, yielding an amorphous phase,
metakaolin, at about 400 °C. After that, a fast reorganization of oxide
ions in the lattice structure develops into a spinel-like form of γ-Al2O3,
and further formation of mullite (3Al2O3·2SiO2) (Mayoral et al.,
2001). From the DSC curves in Fig. 7, there is an obvious exothermic
peak at about 1000 °C; this is the feature peak of spinel formation. At
about 1300 °C, there is also an exothermal trend which implies the
crystallization of mullite.
The characteristics of the spinel peaks for the two coals and their
LTAs at about 1000 °C are compared and summarized in Fig. 8 and
Table 6. The peaks are more clearly defined in the LTAs than the
original coals. The quantitative parameters of this exothermic peak
reflect the mineral behavior during heat treatment (O'Gorman and
Walker, 1973); the peak area represents the conversion heat, so the
mineral transformation degree can be evaluated from the comparison
of peak areas. The height of the peak indicates the crystallinity of the
Fig. 8. Typical exothermal peaks of mineral transformation.
product (Mayoral et al., 2001). The peak area can be calculated from
the following equation.
T
A=
∫
Hflow ðT ÞdT
Tonset
where, A is the peak area; T represents temperature; Tonset
represents the temperature at the onset point; Hflow represents the
heat flow.
For both the coals and the LTAs, the temperature of the exothermic
peak is about 1010 °C, which indicates that the spinel is formed at
about 1010 °C. The peak areas of the two LTAs are higher than their
original coals, which implies that the carbon in the coal inhibits the
formation of spinel. The high carbon coal sample ZG6-2 also has a
bigger peak area than the ZGLQ coal. The peak from the LTA is higher
than that from the coal, suggesting that the crystallinity of the spinel
from the LTA is better than that from the coal. All of these indicate that
the carbon in the coal will constrain the formation of spinel. The
formation of spinel is the key factor to the crystallization of mullite
(Mayoral et al., 2001), so aluminum-bearing minerals included with
carbon have more difficulty in forming the precursor of mullite than
discrete aluminum-bearing minerals.
3.5.2. Drop-tube furnace experiments
To understand the partition process of boehmite during coal
combustion, the evaluation of coal ZG6-2 was conducted on a drop
tube furnace (DTF). The total ash was collected to perform XRD
analysis, with the result at different temperatures shown in Fig. 9.
The minerals in the 900 °C ash are mullite and θ-Al2O3. Increasing
the temperature up to 1100 °C, θ-Al2O3 was transformed to α-Al2O3.
Since in the HTA (815 °C) the aluminum mainly occurs as γ-Al2O3 and
α-Al2O3, it can be concluded that boehmite dehydrates at low
temperature and transforms to γ-Al2O3 first, then with increasing
the temperature, γ-Al2O3 transforms to θ-Al2O3, and finally to αAl2O3.
The Rietveld method was used to calculate the semi-quantitative
mineral contents in LTA, HTA and total ash from DTF. The mineral
Table 6
Parameters of mineral transformation exothermal peaks.
Fig. 7. The TG-DTG-DSC curves of LTAs.
ZG6-2-Coal
ZG6-2-LTA
ZGLQ-Coal
ZGLQ-LTA
peak area(J/g)
Tonset(°C)
Tmax(°C)
△Hheight
14.62
25.38
8.39
20.59
982.7
993.9
994.0
986.7
1011.2
1010.4
1011.0
1009.2
1.05
1.84
0.41
1.26
Please cite this article as: Zhao, Y., et al., Transformation of aluminum-rich minerals during combustion of a bauxite-bearing Chinese coal, Int.
J. Coal Geol. (2011), doi:10.1016/j.coal.2011.04.007
Y. Zhao et al. / International Journal of Coal Geology xxx (2011) xxx–xxx
Fig. 9. X-ray diffraction patterns of high temperature ash from DTF. M: mullite; A:
θ-Al2O3; Q: quartz; H: hematite; Cr: cristobalite; C: α-Al2O3.
content in the LTA is about 73.9% (Table 7), aluminum-bearing
minerals are kaolinite and boehmite, of which the contents are 18%
and 22%, respectively. In the HTA, the main component is amorphous
material (up to 72%); γ-Al2O3 is the main aluminum-bearing mineral,
together with some α-Al2O3. From quantitative analysis of the
aluminum-bearing minerals, it can be concluded that γ-Al2O3 and
α-Al2O3 are mainly derived from boehmite, while kaolinite is
transformed to an amorphous phase during ashing at 815 °C. This
also validates the speculation of Querol et al. (1994).
Mullite in the ash mainly comes from the kaolinite in coal. In the
900 °C ash, the proportion of mullite is up to 49%, which is much
higher than that of kaolinite in the LTA. This implies that kaolinite is
not the only source of mullite. The proportions of amorphous material
and alumina minerals in the 900 °C ash are relatively low compared to
that in the HTA (Table 7), which indicates that both the alumina and
the amorphous phases are possible sources of mullite. In the 900 °C
ash, the alumina occurs as θ-Al2O3 derived from the lattice variation of
γ-Al2O3. Some authors describe the transitions between γ-Al2O3 and
θ-Al2O3 as increased ordering of the aluminum sublattice to produce
an intermediate between the defect spinel structure and corundum
(Zhou and Snyder, 1991).
In the 1100 °C ash, the metastable phase θ-Al2O3 transforms to
α-Al2O3 and an amorphous phase. The transformation from metastable alumina to α-Al2O3 is reported to occur by a nucleation and
growth mechanism, which must surmount high activation energy
7
(Youn et al., 1999). Yen et al. (2003) found that this transformation
occurred between 900 and 1300 °C, which is consistent with our
results. The transformation of θ-Al2O3 to α-Al2O3 involves a significant
change in the oxygen sublattice from cubic to hexagonal close
packing, and this large energy barrier generally requires a temperature above 1200 °C for completing conversion to the thermodynamically stable corundum phase (Bagwell and Messing, 1999; Youn et al.,
1999). In contrast, the transformation from boehmite to γ-Al2O3 and
θ-Al2O3 is topotactic, thus giving a considerably small activation
energy. During coal combustion, the temperature on the particle
surface is about 200 °C higher than environmental temperature,
because of the exotherm of carbon particle oxidation. So when the
furnace temperature is 1100 °C, the alumina has already been
transformed to α-Al2O3.
Recent studies on boehmite-derived nano-sized alumina powder
reveal that the phase transformation from θ-Al2O3 to α-Al2O3 is a
nucleation and coalescence process (Yen et al., 2003). There are
critical sizes for θ-Al2O3 and α-Al2O3 crystals during this phase
transformation, and transformation occurs once the θ-Al2O3 crystals
have grown to a size approaching 25 nm during the heat treatment
(Yen et al., 2002b). The nucleus then grows to a larger size with
continuous heating, at which point the phase transformation is
accomplished (Yen et al., 2002a). As indicated in Fig. 6f, there are lots
of 100 nm crystals melted and combined to the big particle. The
crystal sizes are larger than 50 nm, which is because of the
coalescence of the primary nucleus.
3.5.3. Transformation of typical aluminum-bearing minerals
The occurrences of minerals have significant influence on their
transformation during coal combustion. From the analysis above, the
main aluminum-bearing minerals in the Jungar coal are kaolinite and
boehmite. Kaolinite undergoes dehydration to form metakaolin, and
then transforms to pseudomullite and spinel at about 1010 °C during
coal combustion. It then crystallizes to form mullite with continual
heating, and ultimately may melt and transform to an amorphous
phase. The metakaolin phase allows accelerated diffusion, which has a
significant impact on slagging.
Boehmite undergoes dehydration to form metastable γ-Al2O3,
θ-Al2O3, and finally transforms to α-Al2O3. The transformation
between γ-Al2O3 and θ-Al2O3 results in the variation of lattice with
continued heating, and θ-Al2O3 grows to form stable crystalline
α-Al2O3. The α-Al2O3 crystallites aggregate to form bigger particles as
the temperature increases. Long term heat treatment promotes the
formation of α-Al2O3. The results of Gu and Lin (1997) also show that
non-crystalline alumina can transform from γ-Al2O3 to α-Al2O3
directly, without an intermediate product (θ-Al2O3), after calcinations
at 1200 °C. The nucleation process of stable α-Al2O3 is slow, resulting
in metastable alumina phases which may easily react with other
minerals. This is also the reason why there is so much mullite and
amorphous material in the 900 °C ash. In a coal-fired boiler, the Al2O3
transformation for the solid boehmite particles would be inactive in
the slagging process.
3.6. Formation and emission of aluminum-bearing particles
Table 7
Semi-quantitative aluminum-bearing mineral composition of ZG6-2 ash from DTF
(wt.%).
Amorphous
Minerals
Kaolinite
Boehmite
Mullite
γ-Al2O3
θ-Al2O3
α-Al2O3
Non-Al-bearing minerals
LTA
HTA(815 °C)
900 °C
1100 °C
1300 °C
26
74
18
22
72
28
38
62
40
60
42
58
49
32
45
2
26
7
6
17
7
34
5
6
6
To investigate the effect of mineral transformation on particle
emission, the total ashes from drop tube furnace combustion
experiments were collected, and the Malven particle size analyzer
was used to analyze the PM distribution. As shown in Fig. 10, the size
of the ash particles follows a typical trimodal distribution, with the
supermicron peak at 10 μm, the submicron peak at 0.7 μm, and the
nano-scale peak at about 100 nm. This is different to the typical
trimodal pattern of Seames (2003). With temperature increase, the
nano-scale peak disappeared, and the peak values of the submicron
and supermicron peaks increased simultaneously. This implies that
the nano-scale peak is not derived from the traditional vaporization of
Please cite this article as: Zhao, Y., et al., Transformation of aluminum-rich minerals during combustion of a bauxite-bearing Chinese coal, Int.
J. Coal Geol. (2011), doi:10.1016/j.coal.2011.04.007
8
Y. Zhao et al. / International Journal of Coal Geology xxx (2011) xxx–xxx
Fig. 10. Distribution of particle size in different temperatures.
inorganic elements. Integrated with the transformations of the
aluminum-bearing minerals, the nano-scale particles are possibly
derived from the transformations of the alumina phases. With
continuous heating, θ-Al2O3 nucleates and grows to form α-Al2O3,
resulting in an increase of particle size.
4. Conclusions
Aluminum-bearing minerals in the Jungar high aluminum coal are
kaolinite and boehmite. After high temperature treatment, the
aluminum-bearing minerals transform to mullite and alumina with
different crystal structures. The aluminum enriched ash particles
mainly have irregular shapes; a few nano-scale particles are also
derived from the phase transformation of alumina.
Kaolinite is the main source of mullite, spinel is formed at about
1010 °C during coal combustion, and the carbon in the coal appears to
inhibit the formation of spinel. The phase transformation of boehmite
in coal at high temperature involves four stages: boehmite dehydration, transition phase γ-Al2O3, θ-Al2O3 formation, crystal nucleation,
and α-Al2O3 formation and growth. In slagging the metakaolin is the
phase that allows accelerated diffusion, whereas the boehmite is
inactive in this process. The transformation from γ-Al2O3 to θ-Al2O3 is
a topologic change, while that from θ-Al2O3 to α-Al2O3 involves a
significant change in the oxygen sublattice which has important
influence on crystal particle size. The growth of α-Al2O3 crystals has a
significant impact on PM emissions, and understanding the mineral
transformation mechanism can greatly help the retention of particulate matter.
Acknowledgements
This project was supported by the National Natural Science
Foundation of China (NSFC) (50906031, 40972102, 50936001,
51021065), the National Key Basic Research and Development
Program (2011CB707301). We appreciate Prof. Shifeng Dai for
providing the coal and fly ash samples. The authors thank Dr. ChenLin Chou (Champaign, Illinois), Dr Robert Creelman, Prof. Colin Ward,
and an anonymous reviewer for their helpful and detailed comments.
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J. Coal Geol. (2011), doi:10.1016/j.coal.2011.04.007