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Mechanisms of the central mode particle
formation during pulverized coal combustion
Dunxi Yu, Minghou Xu
*
, Hong Yao, Xiaowei Liu,
Ke Zhou, Lin Li, Chang Wen
State Key Laboratory of Coal Combustion, Huazhong University of Science and Technology,
1037 Luoyu Road, Wuhan, Hubei 430074, China
Abstract
Ash particles produced from pulverized coal combustion are considered to be tri-modally distributed.
These include the well-known ultrafine and coarse modes, and a central mode that is less reported but
attracts increasing attention. This work presents a preliminary study on the formation mechanisms of
the central mode particles during pulverized coal combustion. Experiments of four sized and density-sep-
arated coal samples were carried out in a laboratory drop-tube furnace under various controlled condi-
tions. Experimental data show that the ash particle size distributions have an evident central mode at
4lm for all coal samples. Increasing combustion temperature leads to an increase in the central mode
particle formation, which is thought to be due to enhanced char fragmentation. The small-size coal sample
produces a larger amount of the central mode particles, reasonably due to abundant fine particles in the
parent coal sample. Under similar combustion conditions, both the Heavy (>2.0 g/cm
3
) and Light
(<1.4 g/cm
3
) coal fractions produce a central mode, indicating that not only the included minerals but also
the excluded minerals contribute to the formation of the central mode particles.
Ó2009 The Combustion Institute. Published by Elsevier Inc. All rights reserved.
Keywords: Coal combustion; Formation mechanism; Central particle mode; Fragmentation
1. Introduction
Particulate matter (PM) from pulverized coal
combustion (PCC) is conventionally considered
to be bimodally distributed, including an ultrafine
mode (0.1 lm) formed primarily by the vapori-
zation–condensation mechanism [1–4] and a
coarse mode (1–20 lm) produced mainly by the
coalescence of molten ash droplets and char frag-
mentation [5–7]. However, some experimental and
modeling studies also reported an additional
central particle mode (0.3–5 lm) between the
commonly observed two modes [5,8–17]. The cen-
tral mode contributes a large portion to PM
2.5
(PM with an aerodynamic diameter of less than
2.5 lm) and has some practical implications. For
example, the central mode particles may have sub-
stantial influence on gas-particle mass transfer
and be responsible for a significant fraction of het-
erogeneous Hg
0
transformation within the electro-
static precipitator (ESP) [18]. Furthermore, these
particles seem to have higher surface-to-volume
ratios than typical smooth spheres [11,17] and
contain a larger mass fraction of some transition
metals [9] and trace elements [11,19,20] than
coarse mode particles. Once suspended in the
1540-7489/$ - see front matter Ó2009 The Combustion Institute. Published by Elsevier Inc. All rights reserved.
doi:10.1016/j.proci.2008.07.037
*
Corresponding author. Fax: +86 27 87545526.
E-mail address: mhxu@mail.hust.edu.cn (M. Xu).
Available online at www.sciencedirect.com
Proceedings of the Combustion Institute 32 (2009) 2075–2082
www.elsevier.com/locate/proci
Proceedings
of the
Combustion
Institute
Author's personal copy
atmosphere and inhaled by humans, the particles
in the central mode size range can be retained in
the lung [21] and are expected to cause adverse
health effects.
So far, the origins of the central particle mode
have not been made as clear as those of the ultra-
fine and coarse modes. Therefore, sufficient
knowledge of the formation of these particles is
essential to the understanding of the effects of fly
ash on toxicant partitioning and the development
of advanced technologies for PM
2.5
emission con-
trol. The central mode particles have been implied
to be formed through char fragmentation
[5,13,15,22,23], ash bursting [24] and/or shedding
[8,11]. Fine particles [25,26] and excluded minerals
[27] in the raw coal are also suggested to be possi-
ble sources. Collision-induced fragmentation is
suspected to contribute as well. However, none
of them has been demonstrated by purposely
designed experiments.
This work is intended to explore several possi-
ble mechanisms contributing to the formation of
the central particle mode by burning pretreated
coal samples from a bituminous coal in a well-
designed drop tube furnace (DTF). Due to diffi-
culties in measurements, ash bursting and/or
shedding are not covered presently. In addition,
the role of collision-induced fragmentation is not
evaluated since there are insufficient particles in
the DTF for this purpose. Three possible origins,
including char fragmentation, transformation of
fine particles and excluded minerals, are investi-
gated under well-controlled conditions. Combus-
tion temperatures have significant effects on char
structures and hence particle fragmentation [5].
To verify the contribution of char fragmentation
to the central particle mode, one size-classified
coal sample was burnt at different temperatures.
Fine particles in the raw coal are also suggested
to be another source of the central mode [25].
To validate this, two size-classified coal samples,
i.e. the fine and coarse size fractions, were com-
busted under the same conditions. In order to
evaluate the role of excluded minerals, the raw
coal was density separated by using heavy liquids.
The light and heavy density fractions were burnt
under the same conditions.
2. Experimental
2.1. Coal sample preparation and their character-
istics
A Chinese bituminous coal (Pingdingshan) was
crushed, ground and separated into two batches
for subsequent sample preparations. One batch
was sieved into two size fractions, i.e., <63 and
100–200 lm (hereafter named as Fine and Coarse,
respectively). The other batch was further sepa-
rated into two density fractions using heavy
liquids, i.e., <1.4 and >2.0 g/cm
3
(hereafter named
as Light and Heavy, respectively). This method is
conventionally used to isolate excluded minerals
from the raw coal.
The proximate and ultimate analyses of the
coal samples are listed in Table 1. The data show
that particle sizing seems to have little effect on
coal properties, but density separation has a sig-
nificant influence. The Light coal fraction has a
very low ash content of 4.4% and is expected to
contain only fine included minerals. In contrast,
the Heavy coal fraction has a much higher ash
content of 79.5% and should mainly contain
excluded minerals. For further illustration, a small
amount of the density fractions was mounted in
an epoxy resin, polished, carbon coated, and then
examined by a Sirion 200 field emission scanning
electron microscope (SEM). SEM micrographs
in Fig. 1 do show that mineral particles in the
Light coal fraction (Fig. 1a) are fine and domi-
nantly included in nature, while those in the
Heavy coal fraction (Fig. 1b) are coarse and
mainly excluded in nature.
The particle size distributions (PSDs) of the
four coal samples, the low temperature ash
(LTA) of the Heavy coal fraction, as well as char
samples prepared (see next section) were analyzed
using a Malvern particle-size analyzer (Model:
MAM 5004).
2.2. Char and PM sample preparation and char-
acterization
A high temperature drop-tube furnace (DTF)
was used to prepare PM samples from the com-
bustion of the four coal samples. To help explain
the role of char properties in the central mode for-
mation, pyrolysis experiments were also carried
out in the N
2
atmosphere. The obtained char sam-
ple is assumed to be comparable to that formed at
the devolatilization stage of coal combustion.
Table 1
Characteristics of coal fractions
Size (lm) Density (g/cm
3
)
Fine Coarse Light Heavy
<63 100–200 <1.4 >2.0
Proximate analysis
a
% (wt, dry)
VM 41.8 36.4 27.3 16.5
FC 39.6 44.7 68.3 4.0
A 18.6 18.9 4.4 79.5
Ultimate analysis % (wt, dry)
C 69.5 67.8 81.3 6.7
H 5.2 5.4 5.4 0.5
N 1.1 1.1 1.4 0.2
S 3.9 3.9 3.3 7.2
O
b
1.7 2.9 4.2 5.9
a
VM, volatile matter; FC, fixed carbon; A, ash.
b
By difference.
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Details of the DTF system have been pro-
vided elsewhere [16,28]. Briefly, coal particles
were fed via a Model MFEV-10 Micro Feeder
(Sankyo Piotech Co., Ltd.) and introduced into
a laminar flow tube reactor through a water-
cooled injection probe. The tube reactor has a
diameter of 56 mm and a length of 2000 mm
and is heated electrically. PM or char samples
were extracted isokinetically via a water-cooled
sampling probe and quenched immediately by
a stream of nitrogen at the inlet of the probe
to prevent further reactions. For PM collection,
a Dekati cyclone (Model SAC-65) was first used
to remove particles larger than 10 lm. The
entrained particles less than 10 lm were then
size-segregated by a Dekati low pressure impac-
tor (DLPI) into 13 fractions. More information
can be found elsewhere [16,28]. For char collec-
tion, glass fiber filters were used and the proce-
dure has been detailed previously [29].
A coal feeding rate of about 0.2 g/min was
used in all experiments. The particle residence
time in the heated zone of the reactor was esti-
mated to be 1–2 s. Pyrolysis and combustion
experiments of the Fine coal fraction were con-
ducted at 1373 and 1673 K, while those of the
other coal samples were carried out at 1673 K.
Pyrolysis experiments were completed in the N
2
atmosphere. Combustion experiments were car-
ried out in a 50% O
2
in N
2
mixture to ensure com-
plete combustion. Loss-on-ignition (LOI)
measurements indicated that negligible organic
matter was detected and PM samples consisted
almost entirely of inorganic matter. The porosity
and pore size distributions of all char samples
were analyzed using a Micromeritics ASAP 2000
Analyser. Some char samples were prepared and
observed in the above-mentioned SEM. PM sam-
ple collected on each DLPI stage was weighed
using a Sartorius M2P Microbalance (0.001 mg).
3. Results and discussion
3.1. The modality of particle mass fraction size
distributions
Differential PM mass fraction size distributions
for the sized and density-separated coal fractions
are plotted in Figs. 2 and 3, respectively. Three
distinct modes are clearly observed for all coal
fractions. An ultrafine mode occurs at 0.1 lm
and a coarse mode beyond 6 lm. Besides the
two commonly reported particle modes, a central
mode is also observed at 4lm for all samples.
This further demonstrates the tri-modal nature
of coal fly ash particle size distributions, as sug-
gested previously [5,8–13,15,17]. Since these size
distributions are based on mass fraction rather
than mass concentration, they should not be used
to indicate relative changes in particle modes for
different cases. The formation of the ultrafine
and the coarse modes are believed to be due to
Fig. 1. SEM micrographs of density fractions: (a) Light, <1.4 g/cm
3
; (b) Heavy, >2.0 g/cm
3
.
Fig. 2. PM mass fraction size distributions for sized coal
fractions.
D. Yu et al. / Proceedings of the Combustion Institute 32 (2009) 2075–2082 2077
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vaporization–condensation [3,6,7] and char frag-
mentation [22,23], respectively. However, the for-
mation mechanisms of the central mode particles
are still not clear. For the convenience of the fol-
lowing discussion, the three observed modes are
separated by the transition point from one mode
to another [12], as shown by the vertical lines in
Figs. 2 and 3. The central mode particles are
defined as particles in 0.5–6 lm and their mass is
estimated as the sum of particle mass collected
on impactor stages 7–11. The central mode con-
centration (conc.) used in related figures is repre-
sented as the mass of the central mode particles
produced per gram of coal (mg/g_coal).
3.2. Formation of the central particle mode
3.2.1. Char fragmentation
Char fragmentation has been shown to control
the formation of residual ash particles [22,23,30–
32]. However, there are limited reports emphasizing
its role in the formation of the central mode. Several
studies [5,13,15,22,23] have suggested that char
fragmentation most likely contributes to the central
mode, which has not yet been illustrated in detail by
experiments. To justify the role of char fragmenta-
tion, the Fine coal fraction (<63 lm) was com-
busted in the DTF at 1373 and 1673 K,
respectively. The concentrations of the generated
central mode are compared in Fig. 4. It is clear that
the central mode concentration measured at
1673 K is much higher, approximately 4 times that
at 1373 K. This shows that the higher temperature
leads to an increase in the central mode concentra-
tion. Such phenomenon is most possibly due to dif-
ferent fragmentation patterns during char
combustion.
It has been shown that char particle size [23]
and the porosity of macropores (>50 nm) [13,22]
are important variables in controlling the extent
of fragmentation. Figure 5 compares the average
particle size of all coal fractions and their resul-
tant char samples. The two char samples prepared
from the Fine coal fraction have a much larger
average particle size than their parent coal sample,
suggesting significant swelling during coal pyroly-
sis. They generally contain a large number of par-
ticles with structures as shown in Fig. 6. These
char particles are primarily spherical in shape
and have a large central void surrounded by a
very thin shell with a non-uniform distribution
Fig. 3. PM mass fraction size distributions for density-
separated coal fractions. Fig. 4. Central mode concentrations for the Fine coal
fraction at 1373 and 1673 K.
Fig. 5. Average particle size of all coal and char
samples.
Fig. 6. SEM micrographs of char cenospheres: (a)
surface; (b) cross-section.
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of macropores, often known as cenospheres. This
type of char particles is believed to fragment
extensively during combustion [5,8,11,13,22,23,
32] and possibly contributes to the formation of
the central mode.
As shown in Fig. 5, the 1673 K char prepared
from the Fine coal fraction has a larger average
particle size than the 1373 K char, indicating more
significant swelling at 1673 K. Though quantifica-
tion is not available, the 1673 K char is observed
to qualitatively contain more cenospheres under
SEM. As a consequence, the 1673 K char pre-
pared from the Fine coal fraction has a higher
porosity of macropores (>50 nm) as shown in
Fig. 7, nearly twice as much as the 1373 K char.
Baxter [23] found that the extent of char fragmen-
tation was strongly dependent on particle size and
coal rank. Larger bituminous char particles frag-
mented more extensively and generated much
more fine ash particles. Both experimental and
modeling results [5,22] showed that when the
macroporosity was increased, char fragmentation
also increased and more fine particles were pro-
duced. Therefore, the higher concentration of
the central mode at 1673 K (Fig. 4) is believed
to be primarily due to more extensive fragmenta-
tion of char particles caused by their larger size
and higher macroporosity. These results show
that char fragmentation is an important contribu-
tor to the central particle mode.
3.2.2. Contribution of original fine particles
Relative to char fragmentation, the role of fine
particles in the raw coal in the central mode forma-
tion has received much less concern. But the utility-
grind coal generally has a wide size distribution
and often contains a large number of fine particles,
which are also expected to contribute to the central
mode after combustion. The lack of such informa-
tion is probably due to the unavailability of a quan-
titative measurement technique [25] or narrowly
size-classified samples used [22].
To examine the contribution of the original
fine particles, the Coarse coal fraction (100–
200 lm) assumed to have eliminated the effects
of fine particles was also combusted in the DTF
at 1673 K. The obtained central mode concentra-
tion for the Coarse coal fraction is compared in
Fig. 8 with that for the Fine coal fraction com-
busted at 1673 K. It is evident that the central
mode for the Fine coal fraction has a much higher
concentration, more than 3 times that for the
Coarse coal fraction. This demonstrates that the
Fine coal fraction enriched in fine particles con-
tributes more to the central mode than the Coarse
coal fraction depleted of fine particles. Kramlich
and Newton [14] separated both the raw and
cleaned coals into different size cuts and measured
their resultant ash size distributions. A central
mode at approximately 2 lm was observed for
all cases. The more important result was that the
mode height for the smaller size fraction was
higher than that for larger size fractions, indicat-
ing a higher concentration of the central mode
for the smaller size fraction. It is qualitatively con-
sistent with the present work.
Char fragmentation has been shown previously
to be an important source of the central mode
particles. It is strongly dependent on particle size
[23].Figure 5 indicates that the 1673 K char from
the Coarse coal fraction has a larger average par-
ticle size than that from the Fine coal fraction. Its
lower macroporosity (Fig. 7) may be partially due
to more violent fragmentation of larger coal par-
ticles during pyrolysis [29]. It has been pointed
out that larger char particles fragment more
extensively and a larger number of fine fly ash
would be produced [23]. If assuming that char
fragmentation is the only mechanism for the cen-
tral mode formation, the 1673 K char from the
Coarse coal fraction should fragment more vio-
lently and generate more central mode particles
due to its larger average particle size (Fig. 5).
But it is not the case. As indicated in Fig. 8, the
Fig. 7. Macroporosity of all char samples.
Fig. 8. Central mode concentrations for sized coal
fractions at 1673 K.
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Fine coal fraction produced more central mode
particles at the same combustion condition. This
obvious inconsistency is thought to be due to no
consideration of the contribution of the original
fine particles.
There are at least two studies that have
addressed the contribution of original fine parti-
cles to fine fly ash. For example, Sadakata et al.
[26] studied the formation of the submicron fly
ash (SFA) between 0.1 and 1 lm during PCC,
and concluded that approximately half of the
SFA was simply carried over from the submicron
coal fragments, while the remaining SFA was
newly formed through the breakup of larger coal
particles in the devolatilization region. Measure-
ments by Holve [25] showed that the convention-
ally ground power plant coal had a high number
density of very fine particles, whose size distribu-
tion and composition were similar to that of the
fly ash in 0.1–2 lm range. It implied that the
resulting fine fly ash size distribution was derived
directly from the input size distribution. This
again suggested that the original fine particles
were reasonably another important source of fine
fly ash. In the present study, the Fine coal fraction
is abundant in fine particles while the Coarse frac-
tion depleted of fine particles. Therefore, in addi-
tion to char fragmentation, the much larger
number of fine particles in the Fine coal fraction
may also contribute to the central mode. This
can explain why the Fine coal fraction produced
more the central mode particles (Fig. 8).
It is of great practical significance for quantita-
tively determining the contributions of fragmenta-
tion and original fine particles to the central mode
formation. If char fragmentation controls the cen-
tral mode formation, it suggests that major efforts
should be concentrated on combustion processes
and PM control equipments. In contrast, if the
original fine particles are the dominant source as
shown by Holve [25], then pre-combustion
removal of the fine materials might be an efficient
PM emission control choice. However, for the
present work, the degree to which the central
mode formation is induced by fragmentation
and the combustion of the original fine particles
is hard to quantify. This remains a challenge for
future research.
3.2.3. Contribution of excluded minerals
Previous studies of ash formation often focus
on the behavior of included minerals that are
embedded in the carbon matrix. Excluded miner-
als that are outside the carbon matrix behave
quite differently during combustion. They are con-
sidered to contribute to ash particles larger than
1lm either through evolving independently or
fragmentation [33–36]. However, very limited
data are available regarding the role of the
excluded minerals in the central mode formation.
In one study, Kramlich and Newton [14] con-
cluded that the observed central mode around
2lm was not due to excluded minerals, but due
to included minerals that escaped agglomeration.
In contrast, our data in Fig. 3 also show an evi-
dent central mode for the Heavy coal fraction
(>2.0 g/cm
3
) that is comprised primarily of
excluded minerals, similar to that for the Light
coal fraction (<1.4 g/cm
3
) consisting mainly of
included minerals. This indicates that excluded
minerals also contribute to the central mode. In
a later report [27], the data for various cleaned
fractions from an Illinois bituminous coal showed
an appreciable central mode for all coal fractions,
including the heaviest fraction (ash content, 61%)
enriched in excluded minerals. It implied that the
central mode could also be produced from
excluded minerals, consistent with the results
obtained in this study.
The central mode concentrations for the Light
and Heavy coal fractions are compared in Fig. 9.
It can be seen that the central mode concentration
for the Heavy coal fraction is comparable to that
for the Light coal fraction. It should be noted that
this is in terms of the amount of the central mode
particles per gram of coal. But the Heavy fraction
has a much higher ash content of 79.5% (Table 1),
nearly 18 times that of the Light fraction (4.4%,
Table 1). Simple calculation indicates that the cen-
tral mode particles produced from per gram of
excluded minerals is only one twentieth of that
from per gram of included minerals. In this point,
the excluded minerals contribute less to the central
mode than the included minerals. But they may
have a noticeable contribution for utility coals
with a high content of the excluded minerals.
The central mode formation for the Light coal
fraction is reasonably due to particle fragmenta-
tion and combustion of original fine coal particles
as proposed previously. As shown in Figs. 5 and
7, the char prepared from the Light coal fraction
at 1673 K has a larger average particle size than
Fig. 9. Central mode concentrations for density-sepa-
rated coal fractions at 1673 K.
2080 D. Yu et al. / Proceedings of the Combustion Institute 32 (2009) 2075–2082
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its parent coal sample and a macroporosity of
0.011 cm
3
/g, indicating particle swelling during
pyrolysis. The produced macroporous or even
cenospherical char particles are believed to frag-
ment extensively during combustion and form
the central mode particles. Since the Light coal
fraction was prepared without particle sizing, it
possibly contains a large number of fine coal par-
ticles that also contribute directly to the central
mode. The Heavy coal fraction has been shown
to consist primarily of excluded minerals, and
therefore its average particle size remains almost
unchanged after pyrolysis at 1673 K (Fig. 5).
The very low macroporosity of the pyrolyzed sam-
ple (Fig. 7) is possibly due to fractures and defects
in large particles. The central mode for the Heavy
coal fraction is likely formed through simple car-
ryover of fine minerals and fragmentation of large
particles. The particle size measurement of the
LTA prepared from the Heavy coal fraction
shows that approximately 5% on a volume basis
of the total minerals is less than 6 lm. These par-
ticles may be transformed directly into the central
mode [25,37]. On the other hand, some kinds of
minerals (e.g., pyrite and calcite) can fragment
during combustion due to thermal shock or gas
evolution [33,35,38–40] and likely generate the
central mode particles.
The obtained results also have important prac-
tical implications. If excluded minerals contribute
significantly to the central mode, then pre-com-
bustion separation by coal cleaning techniques
would reduce fine PM emissions greatly, especially
for coals with a significant fraction of excluded
minerals.
4. Conclusions
This work conducted a preliminary study on
the formation mechanisms of the central particle
mode during pulverized coal combustion. Com-
bustion experiments were purposely designed
using sized and density-separated coal fractions
and well-defined conditions. The results showed
that the fly ash particle size distributions had a
general central particle mode at 4lm for all coal
samples. A larger amount of the central mode par-
ticles was produced at the higher combustion tem-
perature due to enhanced char fragmentation. The
higher concentration of the central mode particles
for the small-size coal sample suggested that fine
particles present in the parent coal also contrib-
uted to the formation of the central mode. Exper-
imental results for two density-separated coal
samples indicated that the central mode particles
could also be produced from the excluded miner-
als. But their contribution was less than the
included minerals if in terms of per gram of ash
contained in the coal samples. The quantification
of the contributions of individual formation
mechanisms to the central mode is essential to
the prediction of fine PM emissions and the selec-
tion of appropriate control strategies, which is
deserving of future efforts.
Acknowledgments
This work was supported by the National Key
Basic Research and Development Program of
China (Grant No. 2002CB211602) and the Na-
tional Natural Science Foundation of China
(Grant Nos. 50706013 and 50720145604). The
authors thank Dr. Hongwei Wu at Curtin Univer-
sity, Australia for beneficial discussions and his
help in polishing this manuscript.
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