energies
Article
Effects of SiO2/Al2O3 Ratios on Sintering
Characteristics of Synthetic Coal Ash
Hongwei Hu 1 , Kun Zhou 1 , Kesheng Meng 1,2 , Lanbo Song 1 and Qizhao Lin 1, *
1
2
*
Department of Thermal Science and Energy Engineering, University of Science and Technology of China,
Hefei 230027, China; [email protected] (H.H.); [email protected] (K.Z.);
[email protected] (K.M.); [email protected] (L.S.)
Anhui Civil Aviation Airport Group, Hefei 230086, China
Correspondence: [email protected]; Tel.: +86-551-6360-0430; Fax: +86-551-6360-3487
Academic Editor: Mehrdad Massoudi
Received: 1 December 2016; Accepted: 14 February 2017; Published: 16 February 2017
Abstract: This article explores the effects of SiO2 /Al2 O3 ratios (S/A) on sintering characteristics
and provides guidance for alleviating ash depositions in a large-scale circulation fluidized bed.
Five synthetic coal ash (SCA) samples with different S/As were treated in a muffle furnace for
12 h at different temperatures (from 773 K to 1373 K, in 100 K intervals). The morphological and
chemical results of the volume shrinkage ratio (VSR), thermal deformation analysis by dilatometer
(DIL), scanning electron microscope (SEM), X-ray photoelectron spectrometer (XPS), and X-ray
diffraction (XRD) were combined to describe the sintering characteristics of different samples. The
results showed that the sintering procedure mainly occurred in the third sintering stage when the
temperature was over 1273 K, accompanied with significant decreases in the VSR curve. Excess
SiO2 (S/A = 4.5) resulted in a porous structure while excess Al2 O3 (S/A = 0.5) brought out large
aggregations. The other three samples (S/A = 1.5, 2.5, 3.5) are made up of an amorphous compacted
structure and are composed of low fusion temperature materials (e.g., augite and wadsleysite.).
Sintering temperatures first dramatically decrease to a low level and then gradually rise to a high
level as S/A increases, suggesting that Al2 O3 -enriched additives are more effective than SiO2 -enriched
additives in alleviating depositions.
Keywords: synthetic coal ash (SCA); sintering; dilatometer; eutectics
1. Introduction
Coal blending using coal slime or biomass in a large circulation fluidized bed (CFB, over 300 MW)
is widely utilized for its environmental and economic benefits. Meanwhile, because of high ash
content, fine particle size, and good mechanical properties, the fly ash of CFB can be applied as
alternative raw materials for construction materials (e.g., cement and geopolymer) [1–3]. However, ash
deposition on the surface of the cyclone separator or the heat exchanger is an operational obstacle in
combustion, which can adversely affect equipment performance. Previous studies have shown that ash
fusion temperature (AFT) is a key factor in determining the progress of coal combustion, gasification,
fluidization, and deposition [4]. Ash depositions formed at various ash fusion temperatures always
differ in chemical composition. Researchers have established that ash deposition can be alleviated by
changing chemical compositions through blending ash with proper additives [5–9]. Therefore, it is
important to investigate the relationship between ash fusion temperatures and chemical compositions
in order to better understand the deposition mechanism and the necessary operational adjustments.
Previous studies have shown that ash deposition rates depend on multiple factors,
including flue gas velocity, ash composition, ash particle size, atmosphere, and operating
temperatures [10–12]. Traditionally, the ash melting process is described by four temperatures:
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initial deformation temperature (IDT), softening temperature (ST), hemispherical temperature (HT),
and flow temperature (FT) [13,14]. These four temperatures are determined by the macroscopic
change in the cone shape, according to ISO 1995:540. Researchers have explored the microscopic
morphological variations compared to different temperatures (300–1873 K) and they have categorized
the deformation progress into three stages: sintering stage, primarily fusion stage, and free fluid
stage [15,16]. Furthermore, most researchers agree that the operation temperature should be above
FT [17,18]. However, the operation temperature of the circulation fluidized bed is 1073–1373 K, which
is lower than FT and falls under the sintering stage. Previous studies have also showed that coal ash
sintering is one of the dominant mechanisms of operational problems in the fluidized bed [19]. Few
prior studies have focused on the sintering stage when the temperature is below FT; therefore, it is
necessary to invest the sintering characteristics of coal ash.
However, some researchers have focused on exploring the effects of chemical composition on the
deposition characteristics of coal ash. Earlier studies have explored the influence of components on ash
fusibility or viscosity [20–22], but not much data is available on sintering characteristics. It has been
concluded that high SiO2 /Al2 O3 ratios (S/As) and low concentrations of basic oxides account for the
high ash fusion temperatures of most Australian bituminous coal ashes [23]. Recent related literature
has established ash chemical compositions under either an oxidizing atmosphere or a reductive
atmosphere, based on Jincheng coal, Xiaolongtan (XLT), and Huolinhe (HLH) coal, respectively [24,25].
However, the complex chemical compositions of industrial coal ashes bring obstacles in quantitatively
exploring the role of individual components. Liu B simplified the industrial coal ashes as synthetic
coal ashes (SCA) for quantitatively controlling ash composition [26]. Although other researchers have
adopted this simplification and found it to be satisfactory [27], the effects of individual oxides on
morphology and chemical compositions need to be explored more specifically.
The purpose of this paper is to investigate the effects of S/As on sintering characteristics through
various experimental conditions conducted in a muffle furnace. SCA samples with various S/As were
treated at different temperatures. The morphologic and chemical results of VSR, XRD, XPS, SEM, and
DIL were combined to explore the sintering characteristics of SCA versus temperatures and S/As. The
results of this study will offer guidance for the design, modification, and operation of CFB.
2. Experimental Apparatus and Procedures
2.1. Synthetic Coal Ash Samples Preparation
As shown in Table 1, the chemical compositions of the synthetic coal ash was determined by the
industrial fly ash of CFB (300 MW) burning coal blending with coal slime, located in Huaibei Linhuan
Power Station, Anhui, China. In order to explore the effects of SiO2 /Al2 O3 ratios (S/A) on sintering
characteristics and provide guidance for alleviating ash depositions in large scale CFB, we assembled
five SCA samples mixed with SiO2 -Al2 O3 -CaO-Fe2 O3 -MgO. The individual content is presented in
Table 1 on a m% basis. As shown, Sample (3) contains similar chemical composition with the industrial
sample (0) while the other four samples (1,2,4,5) are utilized as the contrastive sample by adjusting
SiO2 /Al2 O3 ratios. Since the furnace desulfurization is conducted in CFB by adding CaO into the
feeding system, the basic/acid ratio (B/A) of CFB fly ash is higher than that of the conventional
combustion. These mixed SCA samples were molded into a 4 × 4 × 12 mm cuboid sample before
sending the sample for further analysis. This preparation of SCA samples is deemed satisfactory, since
the properties of coal ashes at high temperatures might be similar to the properties of synthetic ashes
composed of primary oxides [22].
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Table 1. Chemical compositions of synthetic coal ash (SCA).
Sample
0
1
2
3
4
5
Chmical Compositions of SCA, m%
SiO2
Al2 O3
Fe2 O3
CaO
MgO
S/A
51.00
23.81
42.86
51.02
55.56
58.44
20.60
47.62
25.57
20.41
15.87
12.99
12.10
12.22
12.22
12.22
12.22
12.22
13.36
13.50
13.50
13.50
13.50
13.50
2.84
2.85
2.85
2.85
2.85
2.85
2.47
0.5
1.5
2.5
3.5
4.5
B/A
0.4
2.2. Volume Shrinkage Ratio (VSR) Test
In order to create a complete reaction, the cuboid samples were treated in a muffle furnace for
12 h under various temperatures (ranging from 773 K to 1373 K, in intervals of 100 K). The temperature
of the muffle furnace is below 1473 K and the accuracy of it is ±20 K. Volume shrinkage ratio (VSR)
represents the ratio between volume variation (∆V) and pre-reaction volume (Vpre ) of the cuboid
samples, as is shown in Equation (1).
VSR =
Vpos − Vpre
∆V
=
Vpre
Vpre
(1)
The VSR was examined by self-design filling method which consists of the following steps. First,
an empty square crucible was completely filled with Al2 O3 powders. The mass of the square crucible
combined with the powders was recorded as m1 . Second, the same crucible was completely filled with
a cuboid sample and a moderate amount of Al2 O3 powders and the total mass of the sample, crucible
and powders were documented as m2 . Third, the mass of the sample was recorded as m3 . Fourth,
after treating in the furnace and cooling down, the same square crucible was completely filled with a
sintered sample and moderate amount of Al2 O3 powders and the total mass of the sintered sample,
crucible, and powders were recorded as m4 . Lastly, the mass of sintered sample was documented
as m5 . The mass was tested on an electronic balance with an accuracy of 0.001 g. In order to reduce
experimental errors, the square crucible was treated in the furnace at 1373 K before each run to avoid
shape and density change. Further, the crucible was filled to the same scale with the same Al2 O3
powders and this operation repeated five times each run. The effective mass was the average value of
five measurements.
As described above, m1 is composed of 3 part: mbed ,m powder1 and m powder2 . Similarly, m2 is also
composed of 3 part: mbed ,m pre and m powder2 . Particularly, m powder1 takes over the same volume space as
the pre-reaction cuboid sample while m powder2 takes over the rest volume space of the square crucible.
m1 = mbed + m powder1 + m powder2 ⇒ m1 = mbed + ρ powder · Vpre + m powder2
(2)
m2 = mbed + m pre + m powder1
(3)
m3 = m pre
(4)
Equation (2) plus Equation (4) and then minus Equation (3), yield Equation (5).
m1 − m2 + m3 = ρ particle · Vpre
(5)
Thus, the volume of the pre-reaction cuboid sample (Vpre ) can be determined by Equation (6).
Vpre =
m1 − m2 + m3
ρ powder
(6)
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Vpre =
m1 − m2 + m3
(6)
ρ powder
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Similarly, the volume of the post-reaction sample ( Vpos ) can be determined by Equation (7).
Similarly, the volume of the post-reaction sample (Vpos ) can be determined by Equation (7).
m −m +m
Vpos =m 1− m 4+ m 5
1
Vpos =
4
ρ powder
(7)
(7)
5
ρ powder
Equation (6) and Equation (7) are substitute into Equation (1), yielding Equation (8).
Equations (6) and (7) are substitute into Equation (1), yielding Equation (8).
ΔV V − V pre m3 − m2 + m4 − m5
VSR =∆V =Vpospos− Vpre
= m −m +m −m
VSR = V pre=
V pre = 3m1 − 2m2 + 4m3 5
Vpre
(8)
(8)
m1 − m2 + m3
Vpre
Finally, VSR can be determined by measurable mass ratio instead of volume ratio.
Finally, VSR can be determined by measurable mass ratio instead of volume ratio.
2.3.
2.3. Sintering
SinteringTemperature
Temperature Test
Test
Sintering
Sintering temperature
temperature tests
tests were
were carried
carried out
out in
in aa horizontal
horizontal pushrod
pushrod dilatometer
dilatometer (NETZSCH
(NETZSCH
402C).
The
system
was
equipped
with
a
SiC
furnace,
allowing
measurements
to
be
carried
402C). The system was equipped with a SiC furnace, allowing measurements to be carried out
out at
at
temperatures
up
to
1573
K.
Measurements
were
taken
with
an
aluminum
sample
holder
and
pushrod
temperatures up to 1573 K. Measurements were taken with an aluminum sample holder and pushrod
with
with aa 0.20
0.20 N
N force.
force. Dilatometry
Dilatometry was
was carried
carried out
out using
using aa high
high temperature
temperature pushrod
pushrod dilatometer
dilatometer in
in aa
dry
nitrogen(gas
(gasflow
flow
50 mL/min)
atmosphere
mass spectrometer.
same
cuboid
dry nitrogen
50 mL/min)
atmosphere
with with
a massa spectrometer.
The sameThe
cuboid
specimens
specimens
(4
×
4
×
12
mm)
were
investigated
with
heat
rates
of
5
K/min.
The
line
deformation
(4 × 4 × 12 mm) were investigated with heat rates of 5 K/min. The line deformation ratio (LDR)ratio
was
(LDR)
was
plotted
against
time,
as
shown
in
Figure
1.
The
ash
sintering
temperature
(ST)
is
defined
plotted against time, as shown in Figure 1. The ash sintering temperature (ST) is defined as the abscissa
as
the intersection
abscissa of ofthe
intersection
of intwo
in the five
curve.
In this
article,with
fivedifferent
cuboid
of the
two
tangent lines
the tangent
curve. Inlines
this article,
cuboid
specimens
specimens
with
different
S/As
were
tested
with
a
dilatometer
to
achieve
sintering
temperatures.
S/As were tested with a dilatometer to achieve sintering temperatures.
dL/L
0.012
0
Tmin
ST
Line Deformation Ratio(LDR)
0.009
0.006
0.003
0.000
-0.003
-0.006
-0.009
Tmax
-0.012
700
800
900
1000
1100
1200
1300
1400
1500
1600
Temperature/K
Figure 1. Definition of sintering temperature (ST) in line deformation ratio (LDR) curve.
Figure 1. Definition of sintering temperature (ST) in line deformation ratio (LDR) curve.
2.4. Instrumental Analysis
2.4. Instrumental Analysis
After reacting in a muffle furnace at 1373 K, the sintered cuboid samples that were produced
After reacting in a muffle furnace at 1373 K, the sintered cuboid samples that were produced were
were sent for morphology analysis via a scanning electron microscope (SEM, XL30, Philips,
sent for morphology analysis via a scanning electron microscope (SEM, XL30, Philips, Amsterdam,
Amsterdam, The Netherland). Meanwhile, an X-ray photoelectron spectrometer (XPS, INCA300,
The Netherlands). Meanwhile, an X-ray photoelectron spectrometer (XPS, INCA300, OXFORD, Oxford,
OXFORD, Oxford, UK) was used to identify the element compositions in sintered SCA specimens.
UK) was used to identify the element compositions in sintered SCA specimens.
The sintered specimens were ground into powders of less than 10 µm before sending them for
further detections. The mineral compositions of sintered SCA samples were determined using X-ray
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The sintered specimens were ground into powders of less than 10 µm before sending them for
further detections. The mineral compositions of sintered SCA samples were determined using X-ray
diffraction
with aa scanning
scanning rage
rage of
of 3°
3◦ toto75°.
75◦ .The
TheXRD
XRDcurves
curves
were
diffraction(TTRAX3,
(TTRAX3, RIGAKU,
RIGAKU, Japan)
Japan) with
were
analyzed
using
the
computer
software
package
MDI
Jada
5.0.
analyzed using the computer software package MDI Jada 5.0.
3.3.Results
Resultsand
andDiscussion
Discussion
3.1.
3.1.Sintering
SinteringCharacteristics
Characteristicsat
at Various
Various Temperatures
Temperatures
3.1.1. Volume Shrinkage Ratio (VSR) Variations at Various Temperatures
3.1.1. Volume Shrinkage Ratio (VSR) Variations at Various Temperatures
The
Thecuboid
cuboidsamples
sampleswere
werekept
keptininaamuffle
mufflefurnace
furnacefor
for12
12hhat
atconstant
constanttemperatures,
temperatures, that
that were
were in
100
K
intervals
ranging
from
773
K
to
1373
K.
Then
the
volume
shrinkage
ratio
(VSR)
was
determined
in 100 K intervals ranging from 773 K to 1373 K. Then the volume shrinkage ratio (VSR) was
bydetermined
the self-design
method,
as described
the experiment
section. Figure
2 plots
the2volume
by thefilling
self-design
filling
method, asindescribed
in the experiment
section.
Figure
plots
shrinkage
ratios
(VSR) of
five(VSR)
different
SCA
samples
against
various
temperatures.
The VSR ofThe
each
the volume
shrinkage
ratios
of five
different
SCA
samples
against
various temperatures.
sample
gradually
decreases
as
the
temperature
increases,
and
at
the
same
time
the
volume
of
VSR of each sample gradually decreases as the temperature increases, and at the same time thethe
samples
When
the temperature
1073surpasses
K, the VSR
ofK,
each
a slight
volumedecreases.
of the samples
decreases.
When thesurpasses
temperature
1073
thesample
VSR of shows
each sample
increase
the curve,
that
the volume
samples
has
expanded.
There is There
a slight
shows ainslight
increasesuggesting
in the curve,
suggesting
that of
thethe
volume
of the
samples
has expanded.
decrease
when
the temperature
surpasses 1173
K, suggesting
the volume
thevolume
samplesofshrink.
is a slight
decrease
when the temperature
surpasses
1173 K,that
suggesting
thatofthe
the
When
the shrink.
temperature
1273 K,
the VSR1273
of each
sample
shows
dramatic
decrease
except
samples
When surpasses
the temperature
surpasses
K, the
VSR of
each asample
shows
a dramatic
decrease
except
low
S/A1.5).
onesThis
(S/A
= 0.5, 1.5).
This exception
is because
the low coefficient
thermal
the
low S/A
ones the
(S/A
= 0.5,
exception
is because
of the low
thermal of
expansion
coefficient that
of large
aggregations
that are
composedmaterials.
of Al2O3 enriched
materials.
ofexpansion
large aggregations
are mainly
composed
of mainly
Al2 O3 enriched
Two turning
points
Two
turning
points
are
obviously
observed
when
the
temperature
is
close
to
1073
K
or
above
1273
K
are obviously observed when the temperature is close to 1073 K or above 1273 in the VSR curve.
in the
VSR curve.
This
is because
the agglomerations
cataclasm of large
at 1073reactions
K and sintering
This
is because
of the
cataclasm
of of
large
at agglomerations
1073 K and sintering
at 1273 K.
reactions
at
1273
K.
Morphological
details
are
displayed
in
Section
3.2.
Although
the
expansion
or in
Morphological details are displayed in Section 3.2. Although the expansion or shrinkage differs
shrinkage
differs
in
extent
among
the
various
samples,
the
change
in
VSR
followed
a
consistent
trend
extent among the various samples, the change in VSR followed a consistent trend at the different
at the different
temperatures.
Therefore, onesample
representative
(S/A
= 2.5,
B/A
= 0.8) to
was
pickedthe
temperatures.
Therefore,
one representative
(S/A = sample
2.5, B/A
= 0.8)
was
picked
explore
to exploreinthe
variations in
morphology
amongspecimens.
the different specimens.
variations
morphology
among
the different
Figure2.
2. Volume
Volume shrinkage
shrinkage ratio
Figure
ratio (VSR)
(VSR)varies
variesatatdifferent
differenttemperatures.
temperatures.
3.1.2. Morphology Variations versus Different Temperatures
3.1.2. Morphology Variations versus Different Temperatures
Figure 3 depicts the VSR and morphology of the representative sample (S/A = 0.5, B/A = 0.8) at
Figure 3 depicts the VSR and morphology of the representative sample (S/A = 0.5, B/A = 0.8)
various temperatures. As shown in the Figure, the VSR curve first decreases as the temperature
at various temperatures. As shown in the Figure, the VSR curve first decreases as the temperature
increases and the porous structure can be detected in Figure 3h,i. This is because of the evaporation
increases
and theofporous
structure
can[28].
be detected
Figure 3h,i.increases,
This is because
of thestructure
evaporation
and transition
dissociative
water
As the in
temperature
the porous
is
and
transition
of
dissociative
water
[28].
As
the
temperature
increases,
the
porous
structure is
significantly reduced, as shown in Figure 3j,k. During this period, the small particles are transferred
significantly
reduced, as
shown
in Figure
3j,k.
Duringthe
thislarge
period,
the small particles
are transferred
by a thermophoresis
force
to fill
the void
between
agglomerates,
which accounts
for theby
a thermophoresis force to fill the void between the large agglomerates, which accounts for the volume
shrinkage of these samples. As the temperature increases further and the porous structure reappears,
Energies 2017, 10, 242
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2017,
242
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2017,
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66ofof1314
volume shrinkage
shrinkage of
of these
these samples.
samples. As
volume
As the
the temperature
temperature increases
increasesfurther
furtherand
andthe
theporous
porousstructure
structure
reappears,
as
is
shown
in
Figure
3l,m.
Meanwhile,
Figure
3k,l
show
that
the
large
agglomerates
are
asFigure
is shown
in Figure
3l,m. Meanwhile,
Figure
3k,lthe
show
that
the large agglomerates
asreappears,
is shown in
3l,m.
Meanwhile,
Figure 3k,l show
that
large
agglomerates
are broken are
into
broken into small particles. When the temperature surpasses 1273 K, the uniform porous structure is
broken
into small
particles.
When the temperature
surpasses
K, the
uniform
porousisstructure
small
particles.
When
the temperature
surpasses 1273
K, the 1273
uniform
porous
structure
present is
as
present as shown in Figure 3m. Finally, as shown in Figure 3n, the uniform porous structure turns
present
as
shown
in
Figure
3m.
Finally,
as
shown
in
Figure
3n,
the
uniform
porous
structure
turns
shown in Figure 3m. Finally, as shown in Figure 3n, the uniform porous structure turns into a dense
into a dense amorphous group, resulting in a dramatic decrease in the VSR curve.
into a dense
amorphous
group,
resulting in
a dramatic
decrease
in the VSR curve.
amorphous
group,
resulting
in a dramatic
decrease
in the
VSR curve.
Figure 3. VSR and morphology of SCA samples (S/A = 0.5, B/A = 0.8) at various temperatures.
Figure 3. VSR and morphology of SCA samples (S/A = 0.5, B/A = 0.8) at various temperatures.
Figure 3. VSR and morphology of SCA samples (S/A = 0.5, B/A = 0.8) at various temperatures.
Figure 4 establishes the chemical compositions of the large agglomerates and discrete particles,
Figure
4establishes
establishes
thechemical
chemical
compositionsare
of the
the
agglomerates
and
discrete
particles,
Figure
the
of
large to
agglomerates
and
discrete
particles,
which
are4detected
by XPS.
The
large compositions
agglomerates
referred
as Al2O3 while
the
small particles
which
detected
XPS.
The
large
agglomerates
referred
to Al
as2Al
O
3 while
small
particles
which
areare
detected
byby
XPS.
The
large
agglomerates
areare
referred
to as
O32The
while
thethe
small
particles
are regarded
as Ca-Al-Si
eutectics,
as
shown
in Figure
4a,b,
respectively.
large
agglomerates
areare
are
regarded
as
Ca-Al-Si
eutectics,
as
shown
in
Figure
4a,b,
respectively.
The
large
agglomerates
are
brokenasinto
small eutectics,
particles, as
resulting
the reappearance
of the The
porous
structure.
However,
the
regarded
Ca-Al-Si
shown in Figure
4a,b, respectively.
large
agglomerates
are broken
broken
small
particles,
the different
reappearance
of
the
porous
structure.
theof
resurgence
of
the resulting
porous structure
is in
quite
thestructure.
former
low
temperature
structure
into
smallinto
particles,
inresulting
the reappearance
of the from
porous
However,
theHowever,
resurgence
of the3hporous
structure
is
quite
from
the
formerstructure
low temperature
shown
Figure
Figure
3i. The from
high
temperature
porous
structure
occurs
because
ofin
Alstructure
2O3 has3h
theresurgence
porousinstructure
isorquite
different
thedifferent
former low
temperature
shown
Figure
in
3h or
Figure
3i. oxides
The
high
temperature
porous
structure
because
of Al
2Olow
3 has
aFigure
chemical
reaction
with
other
or structure
eutectics to
generate
Ca-Al-Si-O
eutectics
while
the
orshown
3i.Figure
The
high
temperature
porous
occurs
because
of Aloccurs
2 O3 has a chemical reaction
temperature
structures
are
mainly
formed
by
physical
evaporation
and
transition.
The
cataclasm
of
a
chemical
reaction
with
other
oxides
or
eutectics
to
generate
Ca-Al-Si-O
eutectics
while
the
low
with other oxides or eutectics to generate Ca-Al-Si-O eutectics while the low temperature structures
large
agglomerations
and
reappearance
of
porousevaporation
structure
accounts
theagglomerations
first
turning
point
structures
arethe
mainly
formedand
bythe
physical
and transition.
The
cataclasm
of
aretemperature
mainly
formed
by physical
evaporation
transition.
The cataclasm
offor
large
and
in
the
VSR
curve.
large
agglomerations
and
the
reappearance
of
the
porous
structure
accounts
for
the
first
turning
point
the reappearance of the porous structure accounts for the first turning point in the VSR curve.
in the VSR curve.
(a)
(a)
Figure 4. Cont.
Energies 2017, 10, 242
Energies 2017, 10, 242
7 of 14
7 of 13
(b)
Figure 4. SEM photos and elemental compositions of different structures at various temperatures: (a)
Figure 4. SEM photos and elemental compositions of different structures at various temperatures:
the large agglomerates in sintered SCA sample (S/A = 2.5, B/A = 0.8) at 1073 K; (b) discrete particles
(a) the large agglomerates in sintered SCA sample (S/A = 2.5, B/A = 0.8) at 1073 K; (b) discrete particles
in sintered SCA sample (S/A = 2.5, B/A = 0.8) at 1073 K.
in sintered SCA sample (S/A = 2.5, B/A = 0.8) at 1073 K.
As is described above, the morphologic characteristics agree with the VSR changing trend.
As is described above, the morphologic characteristics agree with the VSR changing trend. Further,
Further, the heating processes are categorized into three stages by temperature, according to the trend
the heating processes are categorized into three stages by temperature, according to the trend in VSR
in VSR change and morphology:
change and morphology:
1. Physical reaction dominant stage (temperature below 973 K). During this period, the volume of
1.
Physical reaction dominant stage (temperature below 973 K). During this period, the volume of the
the samples shrinks as the temperature increases, following the disappearance of the porous
samples shrinks as the temperature increases, following the disappearance of the porous structure.
structure. Further, physical reactions, such as dissociative water evaporation and particle
Further, physical reactions, such as dissociative water evaporation and particle transition, play a
transition, play a key role in the volume of the cuboid samples.
key role in the volume of the cuboid samples.
2. Expanding stage (temperature ranges from 973 K to 1273 K). During this period, the volume of
2.
Expanding stage (temperature ranges from 973 K to 1273 K). During this period, the volume
the samples expands as the temperature elevates, accompanied by the reappearance of the
of the samples expands as the temperature elevates, accompanied by the reappearance of the
porous structure. This stage has also been referred to as the beginning stage of sintering by
porous structure. This stage has also been referred to as the beginning stage of sintering by Wenjia
Wenjia Song [16]. At the same time, eutectics and oxide react with large agglomerations,
Song [16]. At the same time, eutectics and oxide react with large agglomerations, lancinating
lancinating large agglomerations into discrete particles.
large agglomerations into discrete particles.
3. Sintering stage (temperature above 1273 K). During this period, the volume of the samples
3.
Sintering stage (temperature above 1273 K). During this period, the volume of the samples shrinks
shrinks dramatically, with the production of the amorphous compact structure. Meanwhile,
dramatically, with the production of the amorphous compact structure. Meanwhile, chemical
chemical compositions and intermediate products react violently with each other.
compositions and intermediate products react violently with each other.
3.2. Sintering Characterization versus Different S/As
3.2. Sintering Characterization versus Different S/As
3.2.1. Volume
Volume Shrinkage
ShrinkageRatio
Ratio(VSR)
(VSR) Variations
Variationsversus
versusDifferent
DifferentS/As
S/As
3.2.1.
As has
has been
been stated,
stated, the
the sintering
sintering process
process mainly
mainly takes
takes place
place in
in the
the third
third sintering
sintering stage,
stage, at
at
As
temperatures
over
1273
K.
Therefore,
the
VSR
and
morphology
of
five
sintered
samples
with
various
temperatures over 1273 K. Therefore, the VSR and morphology of five sintered samples with various
S/As are
kept
inin
a muffle
furnace
forfor
12 h
K. When
the S/A
S/As
aredisplayed
displayedininFigure
Figure5,5,which
whichwere
were
kept
a muffle
furnace
12ath1373
at 1373
K. When
the
is small
(S/A(S/A
= 0.5),= the
agglomerations
dominate
the structure
of theofsintered
specimens.
The
S/A
is small
0.5),large
the large
agglomerations
dominate
the structure
the sintered
specimens.
porous
structure
is obviously
detected
in Figure
5a,u, 5a,u,
whichwhich
accounts
for thefor
slight
shrinkage
of the
The
porous
structure
is obviously
detected
in Figure
accounts
the slight
shrinkage
low
S/A
sample
in
the
VSR
curve.
As
S/A
increases
(S/A
=
1.5,
2.5,
3.5),
as
is
shown
in
Figure
5b–d
of the low S/A sample in the VSR curve. As S/A increases (S/A = 1.5, 2.5, 3.5), as is shown
in
and
Figure
5v–x,
amorphous
dense
agglomerations
are
clearly
observed.
This
is
because
the
acid
and
Figure 5b–d,v–x, amorphous dense agglomerations are clearly observed. This is because the acid and
basic oxide
oxide can
can react
react with
with each
each other
other to
to form
form low
low melting
melting eutectics,
eutectics, such
such as
as augite,
augite, wadsleysite,
wadsleysite, and
and
basic
calcium magnesium
magnesium iron
iron silicate.
silicate. This
This phenomenon
phenomenon will
will be
be explored
explored in
in the
the next
next section.
section. Although
Although
calcium
the
porous
structure
has
nearly
disappeared,
distinctions
can
be
observed
in
Figure
5v–x.
The large
large
the porous structure has nearly disappeared, distinctions can be observed in Figure 5v–x. The
agglomerations
skeleton
can
be
hazily
observed
in
Figure
5v
while
the
dense
amorphous
structure
agglomerations skeleton can be hazily observed in Figure 5v while the dense amorphous structure
can be
be viewed
viewed clearly
clearlyin
inFigure
Figure5x.
5x.As
AsS/A
S/A further
further increases
increases (S/A
(S/A == 4.5),
the
can
4.5), as
as is
is shown
shown in
in Figure
Figure 5y,
5y, the
porous
structure
reappears,
which
accounts
for
the
gradual
rise
at
the
end
of
the
VSR
curve.
porous structure reappears, which accounts for the gradual rise at the end of the VSR curve.
Energies
Energies2017,
2017,10,
10,242
242
Energies 2017, 10, 242
88 of
of 14
13
8 of 13
Figure
Figure5.5.VSR
VSRand
andmorphology
morphologyofofSCA
SCAsintered
sinteredsamples
samples(B/A
(B/A == 0.8,
0.8, 1373
1373 K)
K) at
at different
different S/As.
S/As.
Figure 5. VSR and morphology of SCA sintered samples (B/A = 0.8, 1373 K) at different S/As.
As shown in
in Figure 5a,u,
5a,u, the large
large agglomerations adhere
adhere to
to each
each other with
with small shot
shot rod
As
As shown
shown in Figure
Figure 5a,u, the
the large agglomerations
agglomerations adhere to
each other
other with small
small shot rod
rod
adhesives. Figure
Figure 66 establishes
establishes the
the chemical
chemical compositions
compositions of
of the
the shot
shot rod adhesives.
adhesives. It
It is
is observed
adhesives.
adhesives. Figure
6 establishes the
chemical compositions
of the
shot rod
rod adhesives. It
is observed
observed
that the adhesives
adhesives are mainly
mainly composed of
of Ca-Si-Al-O eutectics.
eutectics. These
These eutectics
eutectics have
have low
low fusion
that
that the
the adhesives are
are mainly composed
composed of Ca-Si-Al-O
Ca-Si-Al-O eutectics. These
eutectics have
low fusion
fusion
temperatures;
therefore,
they
would
liquefy
when
the
temperature
is
over
1273
K.
The
liquefied
temperatures;
therefore, they
they would
would liquefy
liquefy when
when the
the temperature
temperature is
is over
over 1273
1273 K.
K. The
temperatures; therefore,
The liquefied
liquefied
materials flow
flow to
to the vacuum
vacuum spaces among
among large agglomerations,
agglomerations, acting
acting as
as an adhesive
adhesive to hold
hold the
materials
materials flow
to the
the vacuum spaces
spaces among large
large agglomerations, acting
as an
an adhesive to
to hold the
the
shape
of
samples.
As
S/A
increases,
if
there
is
enough
SiO
2, it would produce more liquid eutectics
shape
of
samples.
As
S/A
increases,
if
there
is
enough
SiO
,
it
would
produce
more
liquid
eutectics
2
shape of samples. As S/A increases, if there is enough SiO2, it would produce more liquid eutectics
and thenthese
these
liquid
eutectics
would
completely
fill
the vacuum
space, resulting
in a dense
and
liquid
eutectics
would
completely
fill thefill
vacuum
space, resulting
a dense in
amorphous
and then
then these
liquid
eutectics
would
completely
the vacuum
space, in
resulting
a dense
amorphous
structure,
as shown
in Figure
5b–d
and
Figure
5v–x. Since
thethree
microsamples
structures
of =these
structure,
as
shown
in
Figure
5b–d,v–x.
Since
the
micro
structures
of
these
(S/A
1.5,
amorphous structure, as shown in Figure 5b–d and Figure 5v–x. Since the micro structures of these
three
samples
(S/A
=
1.5,
2.5,
3.5)
are
almost
unanimous,
their
VSRs
show
the
same
degree,
as
depicted
2.5,
3.5)
are
almost
unanimous,
their
VSRs
show
the
same
degree,
as
depicted
in
the
VSR
curve
in
three samples (S/A = 1.5, 2.5, 3.5) are almost unanimous, their VSRs show the same degree, as depicted
in the VSR
curve in Figure 5.
Figure
5.
in the VSR curve in Figure 5.
Figure
Figure 6.
6. Chemical
Chemical compositions
compositions of
of adhesives
adhesives among
among large
large agglomerations.
agglomerations.
Figure 6. Chemical compositions of adhesives among large agglomerations.
Energies 2017, 10, 242
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9 of 13
Figure 77 illustrates
illustrates the
the chemical
chemical composition
composition of
of the
the dense
dense amorphous
amorphous structure
structure and
and discrete
discrete
Figure
materials.
It
can
be
observed
that
both
the
dense
amorphous
structure
and
the
discrete
materials
are
materials. It can be observed that both the dense amorphous structure and the discrete materials
composed
of Ca-Al-Fe-Mg-Si-O
eutectics.
ThisThis
is consistent
withwith
Liang
Wang’s
result
that that
the iron
are
composed
of Ca-Al-Fe-Mg-Si-O
eutectics.
is consistent
Liang
Wang’s
result
the
bearing
compound
hematite
is
quick
to
react
with
quartz
and
aluminum
silicates
to
product
iron
iron bearing compound hematite is quick to react with quartz and aluminum silicates to product
silicates
and and
ironiron
aluminum
silicates
at at
low
temperatures,
of dense
dense
iron
silicates
aluminum
silicates
low
temperatures,which
whichexist
existinin the
the form
form of
amorphous structure
structure [29].
[29]. Comparing
Comparing Figure
Figure 66 with
with Figure
Figure7,7,ititisisdeduced
deducedthat,
that,the
theAl
Al2OO3 enriched
enriched
amorphous
2 3
large
agglomerations
can
obstruct
the
formation
of
Ca-Al-Fe-Mg-Si-O
eutectics,
which
will be
large agglomerations can obstruct the formation of Ca-Al-Fe-Mg-Si-O eutectics, which will be explained
explained
in
the
following
section.
in the following section.
(a)
(b)
Figure 7. Chemical compositions of dense amorphous structure and discrete materials: (a) dense
Figure 7. Chemical compositions of dense amorphous structure and discrete materials: (a) dense
amorphous structure; (b) discrete materials.
amorphous structure; (b) discrete materials.
Figure 8 establishes the morphology and chemical compositions of a sintered sample (S/A = 4.5,
establishes
the
morphology
chemical
of a sintered
sample
(S/A
= 4.5,
1373Figure
K). The8 cubic
crystal
and
slender rodand
crystal
twistcompositions
around each other,
dominating
the
structure
1373
K).entire
The cubic
crystal
slender few
rod crystal
twist around
each
other, dominating
structure
of
of the
sample.
Asand
is shown,
agglomerations
were
observed
while the the
vacuum
space
the
entire
sample.
As
is
shown,
few
agglomerations
were
observed
while
the
vacuum
space
repeated.
repeated. As is shown in Figure 8a, the slender rod crystal is mainly composed of Ca-Al-Si-O eutectics
As
is shown
in Figure
8a,isthe
slender
rod crystal
mainly silicates.
composedWhen
of Ca-Al-Si-O
whilewith
the
while
the cubic
crystal
mainly
composed
ofisCa-Si-O
Figure 8 eutectics
is compared
cubic
is mainly
composed
of excess
Ca-Si-O
silicates.
When Figure 8 is compared with Figure 7, it can
Figurecrystal
7, it can
be deduced
that the
SiO
2 prevents the Ca-Si-O silicates and Ca-Al-Si-O eutectics
be
deduced
that
the
excess
SiO
prevents
the
Ca-Si-O
silicates
Ca-Al-Si-O
eutectics
to forming
2
to forming Ca-Al-Fe-Mg-Si-O eutectics.
Further evidence
of thisand
is given
in the next
section.
Ca-Al-Fe-Mg-Si-O eutectics. Further evidence of this is given in the next section.
Energies 2017, 10, 242
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10 of 14
10 of 13
(a)
(b)
Figure 8. Chemical compositions of different micro structures: (a) slender rod crystal; (b) cubic crystal.
Figure 8. Chemical compositions of different micro structures: (a) slender rod crystal; (b) cubic crystal.
3.2.2. XRD Graph of SCA at Various Temperatures
3.2.2. XRD Graph of SCA at Various Temperatures
XRD graphs of different sintered SCA samples at 1373 K are established in Figure 9. Numbers
XRD graphs of different sintered SCA samples at 1373 K are established in Figure 9. Numbers 1–7
1–7 represent quartz, calcium magnesium iron silicate, augite, calcium silicate, wadsleyite, aluminum
represent quartz, calcium magnesium iron silicate, augite, calcium silicate, wadsleyite, aluminum oxide,
oxide, and magnesia, respectively. The chemical components of sintered specimens have been
and magnesia, respectively. The chemical components of sintered specimens have been determined by
determined by the chemical compositions of SCA and the same experimental conditions, thus each
the chemical compositions of SCA and the same experimental conditions, thus each species of eutectics
species of eutectics exists in every graph. However, distinctions can be detected among peak heights
exists in every graph. However, distinctions can be detected among peak heights of different species,
of different species, especially augite, calcium magnesium iron silicate, and wadsleyite, which
especially augite, calcium magnesium iron silicate, and wadsleyite, which qualitatively represent
qualitatively represent various relative contents of each species existing in various specimens. It is
various relative contents of each species existing in various specimens. It is obvious that the peak
obvious that the peak heights of each component of Sample 1(S/A = 0.5) and Sample 5 (S/A = 4.5) are
heights of each component of Sample 1 (S/A = 0.5) and Sample 5 (S/A = 4.5) are lower than the
lower than the others, as shown in Figure 9. This is because of the effects of excess Al2O3 and SiO2, as
others, as shown in Figure 9. This is because of the effects of excess Al2 O3 and SiO2 , as described
described previously. However, the Al2O3 and SiO2 mechanisms that affect the formation of eutectics
previously. However, the Al2 O3 and SiO2 mechanisms that affect the formation of eutectics are quite
are quite different. Al2O3, existing as large aggregations, as shown in Figure 5, has strong polarity
different. Al2 O3 , existing as large aggregations, as shown in Figure 5, has strong polarity and obstructs
and obstructs the formation of low melting eutectics [30], such as calcium silicate and calcium
the formation of low melting eutectics [30], such as calcium silicate and calcium magnesium iron
magnesium iron silicate. SiO2, in the form of cubic crystal and slender rod crystals, obstructs the
silicate. SiO2 , in the form of cubic crystal and slender rod crystals, obstructs the formation of low
formation of low melting eutectics by twining around the large aggregations and separating the
melting eutectics by twining around the large aggregations and separating the active components and
active components and products, as shown in Figure 5y. Excess Al2O3 and SiO2 both account for the
products, as shown in Figure 5y. Excess Al2 O3 and SiO2 both account for the porous morphology
porous morphology structure, as is documented in the previous section, although the mechanism is
structure, as is documented in the previous section, although the mechanism is different. The biggest
different. The biggest distinction among the other three samples is the peak height of wadsleysite (5),
distinction among the other three samples is the peak height of wadsleysite (5), as shown in Figure 9.
as shown in Figure 9. This is due to the strong polarity of excess Al2O3 that obstructs the formation of
This is due to the strong polarity of excess Al2 O3 that obstructs the formation of wadsleysite (5). The
wadsleysite (5). The products of calcium magnesium iron silicates (2), augite (3), wadsleysite (5), and
products of calcium magnesium iron silicates (2), augite (3), wadsleysite (5), and other eutectics that
other eutectics that are conducted by the interactions among acid oxide, basic oxide, and intermediate
are conducted by the interactions among acid oxide, basic oxide, and intermediate products, account
products, account for the amorphous dense morphology structure. The XRD results are in good
agreement with the morphologic and chemical composition results.
Energies 2017, 10, 242
11 of 14
for the2017,
amorphous
dense morphology structure.
Energies
10, 242
Energies
2017, 10, and
242 chemical composition results.
morphologic
The XRD results are in good agreement with
11 ofthe
13
11 of 13
1
1
1
1
1
Intencity
Intencity
1
1
1
2
2
1
2
1
2
2
2
1
1
2
2
1
2
1
2
20
25
1
1
1
1
1
1
3 4 2
3 4 2
2
4 2
3
4
3
2
4 2
3
4
3
2
4 2
3
4
3
2
4 2
3 4
3
30
3
3
5
55
6
6
3 5
3
6
6
35
35
6
6
3
3 5
5
6
6
35
35
6
6
35
40
1
1
7
6 8
1
5
2
7
6 8
1
5
2S a m p le75 ,S /A = 4 .5
5
S a m1 p le75 ,S /A = 41.5
7
7
7
7
7
7
7
7
68
68
1
1
5
1
68
68
1
68
68
1
8
1
5
1
5
50
55
6
8
6
45
1
5
5
5
5
1
7
1
2
1
1
7
2S a m p le
4 ,S /A = 3 .5
S a m1 p le 4 ,S /A = 31.5
2 1
7
1
7
S2a m p le 3 ,S /A = 2 .5
S a m1 p le73 ,S /A = 21.5
2
1
1
7
2
S a m p le 2 ,S /A = 1 .5
S am
1 p le 2 ,S /A = 1 .5
2
2
1
7
1
7
1
S a m p le 1 ,S /A = 0 .5
S a m p le 1 ,S /A = 0 .5
60
65
70
5
50
55
60
65
70
24 θ
2θ
Figure 9. XRD graphs of SCA versus various S/As at 1373 K: 1-quartz 2-calcium magnesium iron
Figure
of
versus
various S/As
S/As
at
1-quartz
2-calcium
silicate
3-augite
4-calcium
silicate
5-wadsleysite
6-aluminium
oxide
7-magnesium.
Figure 9.
9. XRD
XRD graphs
graphs
of SCA
SCA
versus
various
at 1373
1373 K:
K:
1-quartz
2-calcium magnesium
magnesium iron
iron
silicate
silicate 3-augite
3-augite 4-calcium
4-calcium silicate
silicate 5-wadsleysite
5-wadsleysite 6-aluminium
6-aluminium oxide
oxide 7-magnesium.
7-magnesium.
20
25
30
35
40
3.2.3. Sintering Temperature of SCA Samples versus Different S/As
3.2.3.
Sintering
Temperature of
Samples
versus Different
Different S/As
S/As
3.2.3.In
Sintering
of SCA
SCA
Samples
order toTemperature
further explore
the effects
of versus
S/As on sintering
characteristics of SCA, the sintering
In
order
to
further
explore
the
effects
of
S/As
on
sintering
characteristics
of
SCA,
temperatures
of
these
five
specimens
were
tested
by
dilatometer
(NETZSCH DIL
As sintering
is shown
In order to further explore the effects of S/As on sintering characteristics
of402C).
SCA, the
the
sintering
temperatures
of
these
five
specimens
were
tested
by
dilatometer
(NETZSCH
DIL
402C).
As
is
in
Figure 10, of
the
sintering
temperature
decreases(NETZSCH
to a low level
at first,
then
temperatures
these
five specimens
weresignificantly
tested by dilatometer
DIL 402C).
Asand
is shown
shown
in
Figure
10,
the
sintering
temperature
significantly
decreases
to
a
low
level
at
first,
and
then
gradually
rises
to
a
high
level
with
the
increase
of
S/As.
This
trend
of
sintering
temperature
is
in Figure 10, the sintering temperature significantly decreases to a low level at first, and then gradually
gradually
rises
to
a
high
level
with
the
increase
of
S/As.
This
trend
of
sintering
temperature
is
consistent
withlevel
the VSR
in Figure
As isThis
shown,
theofsintering
bywith
DIL
rises to a high
withcurve
the increase
of 5.
S/As.
trend
sinteringtemperatures
temperature measured
is consistent
consistent
within
the
VSR
curve
in shown,
Figureby5.
Assintering
is shown,
the
sintering
measured
by
DIL
are
K higher
than
those
the
VSR
curve.
This
is duemeasured
totemperatures
differences
during
theKheating
the 40
VSR
curve
Figure
5. determined
As is
the
temperatures
by DIL
are 40
higher
are
40those
K higher
than those
determined
bycontinuously
the
curve.
This
is dueduring
torate
differences
during
thewhile
heating
processes.
The
former
ones
were
heated
a heating
ofthe
5 K/min
inprocesses.
DIL
the
than
determined
by the
VSR
curve.
ThisVSR
is due
towith
differences
heating
The
processes.
The
former
ones
were
heated
continuously
with
a
heating
rate
of
5
K/min
in
DIL
while
the
latter
ones
were
kept
at a constant
temperature
in a muffle
The complete
reactions
require
a
former
ones
were
heated
continuously
with a heating
ratefurnace.
of 5 K/min
in DIL while
the latter
ones
latter
ones
were
kept
at
a
constant
temperature
in
a
muffle
furnace.
The
complete
reactions
require
a
set
period
of
time;
thus,
the
sintering
temperature
tested
by
DIL
is
higher
than
the
temperature
were kept at a constant temperature in a muffle furnace. The complete reactions require a set period of
set
period
ofbytime;
the sintering
temperature
tested than
by DIL
higher than
the temperature
determined
VSR.thus,temperature
time;
thus, the
sintering
tested
by DIL is higher
the is
temperature
determined
by VSR.
determined by VSR.
Figure
Sintering temperature
Figure 10.
10. Sintering
temperature of
of SCA
SCA samples
samples versus
versus different
different S/As.
S/As.
Figure 10. Sintering temperature of SCA samples versus different S/As.
When S/A is low (S/A = 0.5), the samples are constituted of large agglomerates and composed of
S/A isrefractory
low (S/A =materials.
0.5), the samples
are constituted
large
and composed
of
Al2OWhen
3 enriched
The strong
polarity ofofthe
Al2agglomerates
O3 enriched materials
and the
Al
2
O
3
enriched
refractory
materials.
The
strong
polarity
of
the
Al
2
O
3
enriched
materials
and
the
obstructed formation of low melting eutectics [30] account for the highest sintering temperature.
obstructed
of low(S/A
melting
[30]samples
accounthave
for the
highest sintering
temperature.
When S/As formation
are appropriate
= 1.5, eutectics
2.5, 3.5), the
an amorphous
dense structure
and
When S/As are appropriate (S/A = 1.5, 2.5, 3.5), the samples have an amorphous dense structure and
Energies 2017, 10, 242
12 of 14
When S/A is low (S/A = 0.5), the samples are constituted of large agglomerates and composed
of Al2 O3 enriched refractory materials. The strong polarity of the Al2 O3 enriched materials and the
obstructed formation of low melting eutectics [30] account for the highest sintering temperature. When
S/As are appropriate (S/A = 1.5, 2.5, 3.5), the samples have an amorphous dense structure and low
melting eutectics. Thus, the sintering temperatures are lower than the lowest S/A temperature. When
S/A is high (S/A = 4.5), the samples are constituted by a cubic crystal and slender rod crystal and
composed of eutectics and SiO2 enriched materials. Since SiO2 is itself a refractory material with a
high melting point, the sintering temperature of the high S/A sample is rather high.
The ST curve provides guidance for the operation of the CFB boiler. The dramatic decrease at the
beginning of the curve suggests that adding Al2 O3 enriched materials would significantly elevate the
sintering temperature, alleviating the ash depositions. The gradual increase in the end of the curve
indicates that adding SiO2 enriched materials would not make a difference. Considering the variety of
different coals and the complexity of the chemical composition of each ash, more ST data will be used
to improve future models.
4. Conclusions
In this work, five synthetic coal ashes were prepared as a mixture of SiO2 -Al2 O3 -MgO-Fe2 O3 -CaO
to quantitatively analyze the complex process of physical reactions, expansion, and sintering. Samples
with various S/A ratios were treated in a muffle furnace at different temperatures (from 773 K to
1373 K, at a 100 K interval) for 12 h to achieve sintered samples. The results of VSR, DIL, SEM, XPS, and
XRD comprehensively describe the morphological and chemical characteristics of sintered samples.
Using the VSR changing trend and morphology, the shrinkage process is categorized into three
stages at different temperatures: physical reaction dominated stage, expanding stage, and sintering
stage. The sintering procedure mainly occurred in the third sintering stage. During this period,
the sample volume shrinks dramatically, at the same time that an amorphous compact structure
is produced. Meanwhile, chemical compositions and intermediate products violently reacted with
each other.
There is great variation in the results of sintering characteristics in S/A ratios during the sintering
stage. Excess SiO2 (S/A = 4.5) results in a porous structure while excess Al2 O3 (S/A = 0.5) brings out
large aggregations. The other three samples (S/A = 1.5, 2.5, 3.5) have an amorphous compacted
structure and are composed of low fusion temperature materials (e.g., augite and wadsleysite).
Sintering temperatures first dramatically decrease to a low level and then gradually rise to a rather
high level as S/A increases, suggesting that Al2 O3 enriched additives are more effective than SiO2
enriched additives in alleviating depositions.
Acknowledgments: This work is supported by the National Natural Science Foundation of China (No. 51376171),
and the National Key Basic Research Program (No. 2010CB227300) founded by MOST (Ministry of Science
and Technology).
Author Contributions: All authors contributed to this work. Hongwei Hu is the first author. Hongwei Hu
designed the study and participated in the experiments and writing the manuscript. Kun Zhou and Kesheng Meng
conceived and designed the experiments; Lanbo Song analyzed the data and contributed to selecting the analysis
tools. The project was supervised by Qizhao Lin.
Conflicts of Interest: The authors declare no conflict of interest.
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