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Ash Formation from the Combustion of Coals with Maceral Concentrates at Various Pressures.

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Dev. Chem. Eng. Mineral Process. 13(3/4), pp. 415-422, 2005.
Ash Formation from the Combustion of
Coals with Maceral Concentrates at Various
Pressures
H. Wd’, C. Kong’, D.K. Zhang’ and T.F. Wall3
Centrefor Fuels and Energy, Curtin University of Technology,
GPO Box U1987, Perth, WesternAustralia 6845, Australia
School of Materials Science and Engineering, The University of New
South Wales, Sydney, New South Wales 2052, Australia
Chemical Engineering, School of Engineering, University of
Newcastle, New South Wales 2308, Australia
I
Two coal samples of different lithotypes and maceral-enriched fractions were
prepared (+63-90 pm) from samples of the same bituminous coal seam. Density
separations at a specific gravity of 2.0 were applied for the prepared coal samples in
order to remove the majority of the excluded mineral matter in the final coal samples.
These coal samples were then pyrolysed or combusted in drop-tube firrnaces at
0.1 and 1.5 MPa to produce char and ash samples at 1573K. The results indicate that
there are two main factors influencing char and ash formation under the experimental
conditions, i.e. pressure and coal petrographic property. In agreement with our
previous results, chars prepared at high pressure are more porous and ash particles
are finer. The structure of the porous char particles at 0.1 and 1.5 MPa are very
different, as evidenced by SEM surjace observation. The porous char particles
prepared at 1.5 MPa have a ‘Ifoam” structure, while those at atmospheric pressure
are mostly balloon-like structures. The vitrinite-rich coal produces a higher
proportion of porous char particles during pyrolysis. Although the mineral particles
in the vitrinite-rich coal sample are coarser than those in the inertinite-rich coal
sample, the ash particles produced from the vitrinite-rich coal afler combustion are
much finer. The results suggest that the extent of coalescence of included mineral
matter of the inernite-rich coal sample is more significant than that of the vitriniterich coal sample. Char particles from the vihl’nite-rich coal sample are more porous
compared to those from the inernite-rich coal sample, leading to more intensive char
fragmentation so that finer ash particles form during combustion. At 1.5 MPa, ash
particles produced from both coals during combustion are finer than at atmospheric
pressure, due to the char particle produced at 1.5 MPa being more porous. However,
the sensitivity of the effect ofpressure is lessfor the vitrinite-rich coal sample.
* Author for correspondence (h.wu@curtin.edu.au).
415
H. Wu, C.Kong, D.K.Zhang and T.F. Wall
Introduction
Coal accounts for over 70% of all fossil energy used worldwide, and is expected to
continue as a major source of energy into the future [l]. With the increasing
consumption of coal in the electricity generation industry, the emissions of gaseous,
liquid and solid wastes have received much attention. Over several decades, a number
of clean-coal technologies have been developed to address these issues, including
IGCC and PFBC [l]. High-pressure operation is a typical characteristic of these
systems, which provide great advantages over the conventional pulverised fuel (pf)
firing technologies [2].
The effects of pressure on ash formation during coal combustion and gasification
are of great interest, as the characteristics of the formed ash particles are an important
consideration in reactor design and operation. Our previous work [3, 5, 61
systemically investigated the effect of system pressure on ash formation from
included mineral matters in pulverised coal during combustion. It was found that ash
particles formed at higher pressure combustion were much finer, suggesting char
fragmentation is the dominant fine ash formation mechanism under pressure, due to
the more porous char particles formed at higher pressures [3-61. This paper
concentrates on the effect of pressure on ash formation from the combustion of coals
with maceral concentrates, collected from the same coal seam. A novel SEM surface
analysis technique [7] is also used for char structure characterisation, together with
the cross-section analysis [4, 61.
Experimental Details
Coal samples
Two bituminous coal samples (denoted as Coal A and Coal B) with relatively low ash
contents were collected from different bands in the same bituminous coal seam. The
two samples were sized by wet sieving to a +63-90 pm sue fraction. Density
separations at a specific gravity of 2.0 were applied for the coal samples in order to
remove the majority excluded mineral matter in the final coal samples. Proximate,
ultimate, ash, crucible and petrographic analysis of the coals used in this study were
performed according to the relevant Australian Standards. The results are presented in
Table 1. Although the two samples were collected from the same seam but different
band, there are significant differences in the coal properties. Coal A has very high
vitrinite content while Coal B is rich in intertinite.
Char and ash sample preparation
Two drop-tube furnaces (DTF) were employed in this study, including an
atmospheric-pressure drop-tube fiunace and a pressurised drop-tube furnace. Coal
samples were fed into the furnaces using a water-cooling feeding system while the
char or ash samples were collected at the reactor exit with both gas and water quench.
Char samples were prepared under nitrogen atmosphere and ash in air at two
pressures of 0.1 and 1.5 MPa and a temperature of 1573 K. Coal feeding rate was
about 5 gih.
416
Ash Formation fiom Coals with Maceral Concentrates at Various Pressures
Sample analysis
Coal samples were subjected to surface and cross-section analysis using scanning
electron microscopes (SEM). SEM surface analysis is done using an advanced
HITACHI S4500 field emission scanning electron microscope (FESEM) based on our
proposed procedure described elsewhere [7]. Briefly, a char sample was dispersed on
one side of a double-side sticky carbon tape, while the other side stuck to a sample
holder. Images of the sample were acquired at different accelerating voltages, ranging
fiom 1 to 15 kV. Particles in the images were analysed by an image processing
software. Char and ash samples were also subjected to cross-section analysis using a
Joel JSM-840 SEM, following the procedure published elsewhere [4, 61. In these
analyses, char or ash samples were set into resin stub, which was then polished after
hardening. SEM cross-section analysis was then performed on the stub after carbon
coating if necessary. The char particles were classified according to a proposed
structure classification system [4, 61. Particle size distributions of ash samples were
analysed following a procedure outlined elsewhere [B].
Table 1. Analysis of coals' used in the experimental program.
Proximate
(ad, wt.o/,)
A
B
Moisture a
Ash
2.8
5.7
40.8
50.7
2.2
7.5
32.2
58.1
Volatile Matter
Fixed Carbon
Ultimate
Add, wt%)
C
H
N
S
0
Inertinite
Crucible analysis
Swelling Index
Ash Analysis
Si02
82.0
5.61
1.53
1.18
9.68
82.9
4.84
1.72
0.92
9.62
34.47
7.21
58.32
B
3%
1
-
-
67.6
26.7
1.10
CaO
2.9
1.40
MgO
0.12
0.2
0.1
1.8
0.02
0.06
0.17
0.27
<0.01
1 .oo
A1203
Na20
K20
Ti02
Mn304
p205
60.65
12.17
27.18
A
56.7
33.6
3.1
Fe203
~
Petrographic
(vol%mmf)
Vitinite
Liptinite
-
0.01
0.13
c. AS 1038.6
b. AS 1038.3
d. A S 2856
e. AS1038.12.1
1:AS 1038.14.1
a. AS 1038. I
Results and Discussion
Structure of chars prepared
The cross-section char structure was characterised and classified into three groups
according to a simplified classification system [4,6]. Briefly, Group I particles have
larger size with a high porosity, Group I11 particles are dense but have smaller size,
417
H.Wu,C. Kong, D.K. Zhang and T.F. Wall
while Group Il particles are in the middle. The results on the structure of the char
particles from Coals A and B are presented in Figure 1. Data in Figure 1 indicate
that two factors influence the char structure. One is pressure, and it can be seen that
increasing pressure results in a higher proportion of the porous Group I type and
lower proportion of Group I11 type char particles. The other factor is the effect of
coal properties. Figure 1 indicates that the petrography of the parent coal influences
the structure of the char particles. The proportion of Group I type char particles
increases with vitrinite content, while that of Group I11 type char particles increases
with inertinite content at any pressure. Figure I also indicates that high pressure
favours Group I type particles. These results confirm the findings observed in our
previous results [4], and in other work using an optical microscope [9]. The porous
char particles have very different structures at 0.1 and 1.5 MPa, which can be
observed using an SEM surface characterisation technique [7]. The technique
utilises the fact that the penetration depth of electron beams into the char particles
increases with an increase in the accelerating voltage. Therefore, SEM images at a
low accelerating voltage show particle surface structure while the internal structure
of chars is seen at high accelerating voltage [7]. Figures 2 and 3 show the typical
SEM images of porous char particles prepared at 1.5 and 0.1 MPa using this
technique.
At 1.5 MPa, a large proportion of the char particles have a “foam” structure,
which consists of a cellular framework covered with thin surface films. This was
identified using SEM surface observation at different accelerating voltages. Point
counting of at least 250 particles was performed. When a char particle exhibits a
cellular framework at 20 kV, it was counted as a particle with “foam” structure.
Over 90% of Coal A char particles and 60% of Coal B char particles have a “foam”
structure. Although a foam structure is dominated in the char samples generated at
1.5 MPa, it should be noted that the foam structure varies from particle to particle.
As illustrated in Figure 2, char particles can have a closed surface (Particle l), a
fully open surface (Particle 2), or a partially open surface (Particle 3).
Coal A
Coal B
Figure I . Eflect ofpressure on the structure of chars prepared at 0.1 and 1.5 MPa
and 1573 K.
418
Ash Formation fiom Coals with Maceral Concentrates at Various Pressures
Pusllc~lrI
PiIStlCIL’
P ~ I S ~ 3I ~ L J
2
I
x- I’
5
kV
Figure 2. Char particles produced at 1.5 MPa; SEM image obtained at 1, 5 and 20 kV.
Pcirticlc I
-
Purttde 2
I
20 kV
Figure 3 Typical foam structures of char particles produced at I a m .
419
H. Wu, C. Kong, D.K.Zhang and T.F. Wall
2
80
e
a
60
0
0
10
20
30
40
50
60
Paticle size (pn)
Figure 4. PSD of mineral matter in coal and ash samples generated porn coals
A and B at the pressures indicated.
The structure of atmospheric-pressure chars is quite different from that of the
high-pressure chars. Char particles generated at atmospheric pressure from the same
coal show less foam structure, only -40% of char particles from Coal A, and -25%
from Coal B have a foam structure. The majority of the atmospheric pressure char
particles from all the three coals have a much thicker wall compared to the highpressure char particles, which prevents the electron beam from penetrating through
at accelerating voltage as high as 20 kV.Therefore, the SEM images do not show a
foam structure. Two typical char particles having a foam structure, observed in char
samples at atmospheric pressure, are shown in Figure 3. It can be seen that
compared to those produced at the high pressure, char particles produced at
atmospheric pressure have a foam structure, which is balloon-like with only one or
several large bubbles.
The difference in the foam structure of char particles formed at elevated and
atmospheric pressure provides an insight into the process of volatile release and
char formation during devolatilisation under these conditions. At atmospheric
pressure, the balloon structure of the char implies that there was significant
coalescence of the nucleated bubbles within the metaplast before bubbles escape
from the surface of the particle. This suggests that the bubble escape rate is
relatively slower than the bubble coalescence rate, so that bubbles have a chance to
coalescence to form large bubbles. At 1.5 MPa, the foam structure indicates that
there was much less coalescence of bubbles within the metaplast during
devolatilisation. The bubble coalescence rate is relatively slower compared to the
escape rate, probably due to high fluidity of the pyrolysing particles at elevated
pressure.
420
Ash Formation fiom Coals with Maceral Concentratesat Various Pressures
Ash formation during combustion
The particle size distribution (PSD) of both the mineral matter in coal and ash
samples prepared at 1573 K are shown in Figure 4. It shows that the PSD of ash,
formed after combustion at all pressures, are coarser than PSD of mineral matter
particles in respective coals, implying that char fragmentation and partial
coalescence of included mineral matter occurred during combustion. Figure 4 also
indicates that for both coals, ash samples formed from combustion at 1.5 MPa are
much finer than that at atmospheric pressure, in agreement with our previous results
[ 6 ] .It is suggested that the hgh-pressure char samples from both coals have more
porous char particles, as indicated in Figure 1. Compared to those at atmospheric
pressure, char fragmentation during combustion at high pressure is more severe,
thus reducing the coalesecence of included mineral matter, leading to smaller ash
particles at 1.5 MPa.
The data also illustrates the effect of coal petrography on the PSD of ash
samples produced at various pressures during combustion. The mineral matter
particles in Coal B are finer than that in Coal A. However at the same pressure,
PSD of ash samples of Coal B is coarser than that of Coal A, suggesting that the
coalescence extent of included mineral matter of Coal B is more significant than
that of Coal A. Furthermore, the PSD of ash formed after conversion is governed by
the limiting cases, i.e. one ash particle formed per coal particle (full coalescence)
and one ash particle formed per mineral grain (non-coalescence). Although ash
formed at higher pressures are finer for combustion of both Coals A and B, Figure 4
also suggests the sensitivity of the effect of pressure is less for Coal A of a higher
vitrinite content, while more significant for Coal B which has a higher inertinite
content.
Conclusions
There are two main factors, namely pressure and coal petrography, which
significantly influence char and ash formation under the experimental conditions.
Chars prepared at high pressure are more porous and ash particles are finer. The
porous char particles prepared at 1.5 MPa have a “foam” structure, while those at
atmospheric pressure are mostly balloon-like structures. The vitrinite-rich coal
produces a higher proportion of porous char particles during pyrolysis. Although
the mineral particles in the vitrinite-rich coal sample are coarser than those in the
inertinite-rich coal sample, the ash particles produced from the vitrinite-rich coal
after combustion are much finer. The results suggest that the extent of coalescence
of included mineral matter of inernite-rich coal sample is more significant than that
of the vitrinite-rich coal sample. This is due to chars from the inernite-rich coal
sample being less porous, leading to more intensive char fragmentation during
combustion. Similarly at 1.5 MPa, ash particles produced from both coals during
combustion are finer compared to atmospheric pressure. However, the sensitivity of
the effect of pressure is less for the vitrinite-rich coal sample.
42 I
H. Wu, C. Kong,D.K. Zhang and T.F. Wall
Acknowledgements
The authors gratefully acknowledge the financial and other support received for this
research from the Cooperative Research Centre (CRC) for Black Coal Utilisation,
which is established and supported under the Australian Government Cooperative
Research Centres program
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thesis, Department of Chemical Engineering, The University of Newcastle, Australia.
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