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Combustion and Flame xxx (2015) xxx–xxx
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Combustion and Flame
j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / c o m b u s t fl a m e
An observation of combustion behavior of a single coal particle entrained
into hot gas ﬂow
Hookyung Lee 1, Sangmin Choi ⇑
Department of Mechanical Engineering, Korea Advanced Institute of Science and Technology (KAIST), 291 Daehak-ro, Yuseong-gu, Daejeon 305-701, Republic of Korea
a r t i c l e
i n f o
Article history:
Received 17 December 2014
Received in revised form 12 March 2015
Accepted 13 March 2015
Available online xxxx
Keywords:
Coal
Single particle combustion
Cross-jet arrangement
Quantitative observation
a b s t r a c t
This experimental investigation undertakes visual observation of burning coal particles in a hot ﬂowing
gas environment, focusing on the initial stages of coal combustion. Pulverized single coal particles transported by cold carrier gas are injected perpendicularly into a high-temperature environment up to 1240 K
and 16.7-40.2 % of O2. High-speed photography with microscopic magniﬁcation in half-shadow conditions provided improved images of the combustion behavior of moving particles and ﬂames in time
and space. Spatial location of the particle along with a known time interval allowed positioning as related
with the mixing gas ﬂows. This approach allowed the timing of the characteristic sequence of the subprocesses. Observed images of burning particles were quantiﬁed in terms of changing size, shape, and
intensity of the apparent ﬂame emission. This method enabled an interpretation regarding the progress
of volatile ﬂame formation observed by the visual appearance of the particle and its associated ﬂame. The
size measurement of the apparent volatile ﬂames with varying particle size and oxygen concentration
also allowed instructive discussions along with model descriptions of spherically concentric formation
of the volatile ﬂames. The observation results of the burning particles starting from the injection,
heat-up, formation of volatile ﬂames as well as the progression of the heterogeneous reaction are
presented.
Ó 2015 The Combustion Institute. Published by Elsevier Inc. All rights reserved.
1. Introduction
The phenomenological understanding of combustion processes
of a single coal particle has formed the basis of the mathematical
description of coal particle combustion models, which include mixing and transport of particles in the gas ﬂow, heat transfer to and
from the particle, volatile release from the particle typically as a
function of temperature, oxidation of released volatiles, and char
oxidation. This modeling description of a single pulverized coal
particle, the typical size of which ranges on the order of a few tens
to hundreds of a micrometer in diameter, has been incorporated
into a comprehensive computational model [1], which is further
extended to now-commonly used programs as an associated package of computational ﬂuid dynamics (CFD) codes.
Description of combustion processes of a single coal particle has
likely beneﬁted from the more extensively studied topic of single
liquid fuel droplet combustion. A simpliﬁed geometry of the
⇑ Corresponding author. Tel.: +82 42-350-3030; fax: +82 42-350-1284.
1
E-mail addresses: [email protected] (H. Lee), [email protected] (S. Choi).
Tel.: +82 42 350 3070; fax: +82 42 350 1284.
spherical symmetry, the heat-up of a single droplet or particle in
a high-temperature gas environment, the evaporation or release
of volatile fuel, and the formation of a homogenous spherical ﬂame
are typically analogous [2,3]. Heat-up and the release of volatiles
by devolatilization in the early stages are often discussed with analogical reference [4–7], such as a concentric ﬂame sheet concept
with the liquid fuel droplets.
Describing the combustion processes of a single coal particle is
not simple because of its heterogeneous characteristics [8].
Overcoming the complexities of the inhomogeneous nature of
shape, composition, and reactive properties, extensive research
on devolatilization [9–11] and char combustion [12,13] has contributed to the formation of the modeling concept of coal combustion. The combustion of a pulverized coal particle is typically
associated with a series of sequential processes along with the
physical and chemical deformation of solid particles and the gassolid phase reaction [14]. The model concept associated with combustion behavior can be postulated based on visual observation.
Characteristic regions in the overall combustion process can be
identiﬁed based on the visual appearance of the particle and its
associated ﬂame, including the dynamic formation and extinction
of the volatile ﬂame and char oxidation.
http://dx.doi.org/10.1016/j.combustﬂame.2015.03.010
0010-2180/Ó 2015 The Combustion Institute. Published by Elsevier Inc. All rights reserved.
Please cite this article in press as: H. Lee, S. Choi, Combust. Flame (2015), http://dx.doi.org/10.1016/j.combustﬂame.2015.03.010
2
H. Lee, S. Choi / Combustion and Flame xxx (2015) xxx–xxx
The apparent ﬂame behavior of coal particles is closely related
with the hydrodynamic interaction of the solid and gas phases in
addition to the combustion-related properties of fuel coal. Coal
particles transported by a carrier gas are generally injected from
burners into a non-quiescent environment of the hot ﬂow ﬁeld in
conventional applications. In this case, the physical and thermal
mixing of the ﬂow ﬁelds is regarded as one of the important variables in determining the combustion status of the particle
entrained into hot gas ﬂow.
Direct observation of pulverized coal particles in a high-temperature gas environment has provided insight for phenomenological understanding. Experimental investigation through
observation of the apparent characteristics over the combustion
processes of single coal particles in a hot gas environment has been
documented over the years [15–20]. The heat-up and devolatilization processes are indelibly associated with the ignition characteristics of the volatile matter released from coal particles because the
heat-up region is generally recognized as a previous stage of
homogeneous ignition. McLean et al. [15] observed particles
entrained into a high-temperature gas environment ranging from
0-20 % O2 and 1100-1800 K in temperature in a co-ﬂow injection
arrangement between coal particle jets and post-combustion gas.
Devolatilizing coal particles were identiﬁed through bright yellow
emissions where the initial bright zone is thicker and more diffuse
than the following region. The location of the bright emission onset
was referred as the ignition point. The condensed phase released
from the coal particles was elongated, forming tails upwards
because of velocity slip and a faster burn rate at 20 % O2 compared
to 8 % O2 concentration at 1700 K. In a similar experimental
approach, devolatilization and ignition behavior were measured
in the middle of the upward trajectory of the single coal particle
through some observation schemes [16,17]. Molina and Shaddix
[16] deﬁned the coal ignition point as the onset of the CH⁄ chemiluminescence signal instead of the appearance of the visible ﬂame.
CH⁄ emission released from the coal particles was observed and
compared with blackbody emission in the 21 and 30 % O2 and at
approximately 1200 K in both the N2 and CO2 environments. To
improve limited sensitivity to the differences in particle ignition
and devolatilization in Ref. [16], Shaddix and Molina [17]
employed an intensiﬁed charge-coupled device (ICCD) to image
the individual particles. Ignition was characterized by the formation of a soot cloud surrounding the particle. The soot cloud around
the devolatilizing particles was observed through the particle
imaging of two coal types at a gas temperature of 1700 K over
a range of 12-36 % O2 in both N2 and CO2 diluent gases. As the
O2 concentration increased, the soot cloud shrunk in size and
formed more concentrically about the coal particle. In Refs.
[18–20], the particle free-falling condition was considered in hot
quiescent and active atmospheres of a drop-tube furnace electrically heated to a wall temperature of 1400 K. The results clearly
showed the apparent characteristics of the sequential combustion
processes of the coal particles. The particles fell down, forming
co-tails in the sooty wake of the particle trajectories. Volatile matter ignited forming diffusion ﬂames of a ring shape or of a heterogeneous behavior based on the apparent observation of the
different coal types and the biomass fuel in the ranges of 20-100
% O2 in both the N2 and CO2 environments. Experimental investigations have continuously attempted to describe the transient burning behavior of coal particles, especially in simulating the
combustion environment and also in providing quantitative results
in terms of time and space information.
This research investigated the early stages of the combustion
processes of a single coal particle entrained into a hot gas ﬂow
ﬁeld. Single coal particles transported by the cold carrier gas and
injected perpendicularly into a cross-stream of hot gas ﬂow are
observed in a lab-scale entrained ﬂow reactor. The objective of
injection method is to precisely deﬁne the starting points of particle heat-up and the subsequent processes leading to the char reaction in addition to the associated time and space scales. This will
help explain the entrainment behavior of coal particles and their
associated ﬂames in the ﬂow. The heat-up phase, the volatiles
release and oxidation phase, the ﬂame formation sequence, and
the char reaction phase are distinctly revealed while the particles
adjust to the main cross-stream. The successive sequence of combustion processes from the injection of the particles is represented,
and each region of the processes is quantitatively characterized in
terms of the size and intensity based on the observation of formation until extinction of the visible apparent ﬂame. Because the
reaction characteristics that affect ﬂame formation surrounding
the particles depends on the coal type, the particle size, and the
oxygen concentration of ﬂow ﬁeld, the effects on the sequential
regime are discussed. The methodical characteristics on the apparent images further enabled discussions on the mathematical
description associated with the combustion behavior of a single
coal particle entrained into a hot gas ﬂow.
2. Experimental Details and Procedure
2.1. Cross-jet injection of a single coal particle into a high-temperature
gas environment
Experiments were conducted in a post-combustion gas ﬂow
reactor with a coal particle injector arranged in a cross-ﬂow conﬁguration. Fig. 1 shows the facility, which provides optical access to
the early stages of combustion when a single coal particle is
entrained into a hot gas environment. A 40 40 mm square quartz
tube with thickness of 5 mm isolates the reacting particles and
combustion product gas from the surrounding atmosphere; it
allows optical access into the test section. The reactor operates at
atmospheric pressure and uses a stabilized ﬂat ﬂame to provide a
high temperature post-combustion gas. Burning a premixed gas
mix of commercial propane, air, and extra O2 at a burner produces
the combustion gas. The ﬂow rates of each gas are independently
adjusted in limited ranges producing the stable ﬂat ﬂame, for
which temperature and O2 concentration of the product are controlled. The hot product gas passes through a porous ﬂow straightener made of ceramic honeycomb to make a uniform ﬂow
environment in the test section. In this experimental approach,
Fig. 1. Schematic of the test section.
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conﬁrmation of the uniform ﬂow ﬁeld of the hot gas was visually
veriﬁed by introducing the solid coal particles, themselves, into
the hot gas ﬂow, as shown in Fig. 1. When introduced into the
high-temperature gas ﬂow with a carrier gas, the coal particles
passed through the hot ﬂow ﬁeld in a vertically straight line and
show a direct particle image velocimetry (PIV)-like effect.
Heating elements and insulation were used to cover the tube to
minimize heat loss to atmosphere. This guard heating system
enabled a steady hot gas ﬂow while minimizing the temperature
drop of the gas ﬂow stream. Additional guard heater covers were
installed in the downstream section after the optical window
whose height is adjustable. The monitoring system controlled the
target gas temperature near the heating elements up to 1023 K.
Coal particles were perpendicularly injected through a 2 mm
(i.d.) stainless-steel tube at the side of the quartz tube with air as
a carrier gas into the hot gaseous environment. The jet tube was
placed 10 mm inside the sidewall of the quartz tube to minimize
interference with the boundary layer of the gas ﬂow and the quartz
tube wall [21]. The coal particles drop down very slowly into a
glass funnel by a micro-syringe injector and then are entrained
with a carrier gas at room temperature. The ﬂow rate of the carrier
gas was independently controlled, regardless of the number of coal
particles falling into the glass funnel. This design allows the number density of particles and the jet ﬂow rate to be varied independently. The particle feeding rate was regularly kept to 25-30
particles/min (<0.01 g/min in the mass ﬂow rate) to observe the
behavior of nearly each single coal particle to guarantee that the
gas condition was not affected by particle combustion.
2.2. Combustion environment
Composition of ﬂue gas produced from the combustion of C3H8,
air and the extra O2 was typically O2 (16.7-40.2 %), N2 (51.7-75.2
%), CO2 (3.5 %), and H2O (4.6 %). In this study, the extra oxygen
was enriched as a variable to make an intense oxygen diffusion
environment where it is possible to observe the particle combustion up to the char reaction within the limited ﬁeld of view in
the current scope. The environment of the different oxygen concentration at a nearly constant temperature distribution was
achieved by controlling the air and extra oxygen ﬂow rates in a
ﬁxed total ﬂow rate, which is 20 standard liters per minute (slpm)
for each condition. The temperature of the hot gas ﬂow measured
at the inlet of the test section drops by 2.0 K as the oxygen fraction increases by 5.0-5.5 % in the lean post-combustion gas condition. Minor differences in ﬂow temperature exist for the different
gas composition, but it is not signiﬁcant enough to affect the phenomenological trends. The ﬂow rates were determined considering
the stability of the ﬂat ﬂame and the gas temperature associated
with the ﬂammability of the coal particles. The linear velocity of
the carrier gas at the exit of the nozzle of the coal injector and
the main gas stream was in the range of 1.3-2.7 m/s and 1 m/s,
respectively. Coal particle velocity depends on the particle size,
which is related to the initial particle inertia and the intermediate
reduction in the particle mass on the rate of volatile release during
combustion. The information can be directly obtained with the displacement per unit time of the moving particles based on the interval between the frames.
The temperature proﬁle of the gas ﬂow was measured at the
points on the centerline, vertically at a 5 mm interval over 40
mm (total measured points: 9) along the reactor height in the test
section with a type R thermocouple, for which uncertainty of the
measurements and radiative correction were evaluated.
Fluctuating temperature data were typically averaged over 30 seconds. Temperature of gas ﬂow introduced into the test section was
typically up to 1240 K at the exit (0 mm) of the injection tube, and
the gradient was formed along the reactor height, reaching up to
1150 K at the exit (40 mm) of the observation section. However,
the ﬁeld of view where the particle heat-up and subsequent processes until initiation of the char reaction occur is sufﬁciently narrow (vertically 0-11.0 mm), so the coal combustion stages are
mostly revealed inside the mixing region of the cross jet. An initial
hot gas ﬂow condition is maintained at approximately 1230-1240
K with little gas temperature variation. The vertical temperature
proﬁle along the centerline of the reactor height in this cross-jet
injection conﬁguration is noticeably different from the one in the
co-jet injection conﬁguration [16,17] because a single coal particle
transported by the cross jet is intensely mixed with the main
stream of the hot gas ﬂow. One of the focal concern of the current
phase of this research is to deﬁne the starting point for particle
heat-up and the effect of ﬂow mixing on the transient combustion
behavior of the particles; therefore, the gas ﬂow environment,
especially the stream mixing region in the early stages of combustion, would be characterized. This would help explain the ignition
delay reaching a certain temperature for volatile oxidation.
Detailed characterization results associated with the experimental
environment by a cold cross jet into a hot gas ﬂow will follow in
the forthcoming report, where detailed arguments will be presented, associated with the uncertain particle temperature history
and the ﬂame temperature in this cross jet mixing conﬁguration.
It is assumed that there is no radical species in the hot gas
stream and that the volatiles ignition depended only on the thermal conditions around the single particle. In this condition, the
particles reach the devolatilization temperature of over 600-700
K and the homogeneous ignition temperature of over 1000 K
within 20 ms [22]. However, it is not simple to unambiguously
determine a particle-heating rate due to the non-uniform temperature distribution because of the mixing between the cold carrier
gas jet and the hot main stream ﬂow until equilibrium is reached.
2.3. Coal type and properties
Two types of coal, one of high volatile bituminous and another
of subbituminous were used in this experiment. Both coals were
sieved to a size range of 75-90, 125-150, and 180-200 lm.
Proximate and ultimate analysis data of two coals are summarized
in Table 1. During the preliminary tests with coals of varying ranks,
while the combustion of the volatiles released from the bituminous
coal was sooty and luminous with tails forming in the wake of the
particle trajectories, lignite coal experienced extensive fragmentation, immediately followed by ignition of the char fragments
spread apart into a relatively large volume. The current paper
reports test results with higher rank coals, which can provide useful discussion on the model description of the apparent combustion behavior of coal particles.
Table 1
Coal properties.
Rank
Bituminous F
Subbituminous P
Proximate (air-dried) (wt%)
Ultimate (air-dried) (wt%)
Heating value (MJ/kg)
W
V.M.
F.C.
Ash
C
H
O
N
S
2.38
2.47
35.32
28.76
49.62
50.64
12.68
18.13
70.38
64.44
4.65
3.73
7.91
9.10
1.48
1.79
0.52
0.35
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27.98
24.82
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2.4. Observation scheme and uncertainty
The purpose of this report is to elucidate the apparent combustion characteristics obtained from visual observation. In this study,
the combustion behavior of single coal particles was observed
using a high-speed camera (Phantom V310, Vision Research) to
investigate the apparent changes on the time and space scales in
the short residence time (<40 ms at least) of the particles. The camera has a complementary metal oxide semiconductor (CMOS) sensor size of 25.6 16.0 mm and a pixel size of 20 lm at a 1280 800 resolution for observation through a normal lens. To produce
images at higher magniﬁcation in this study, however, an adjustable 50 mm photographic bellows and a 200 mm focal length f/32
camera lens with macro function (Nikon) were used to ﬁll the
image detector for a 14.6 11.0 mm ﬁeld of view. In the case of
this observation condition, the total depth of ﬁeld was calculated
to nearly 3 mm, and the size of the pixel was measured to 14.3
lm at a 1024 768 resolution. The pixel size was calibrated on
a circle and line-scale reticle with calibrated features that ﬁt completely inside the ﬁeld of view of the video for this magniﬁcation,
as indicated in Fig. 2. Larger ﬁelds of view with lower magniﬁcation
permit the observation of a larger portion until the end of char
reaction process for several particles simultaneously in the space
of a single frame. In the current study, a speed of 4200 frames
per second (fps) and exposure time of 10 ls per frame were
applied to each image with continuous backlighting by a halogen
lamp. More than 20 videos were recorded for each particular
experimental condition to obtain the averaged results considering
the repeatability.
Accurate measurement of the length scale is important in sizing
the apparent shape changes in detail. Furthermore, the outwards
appearance of the pulverized coal particles, which have a size on
the order of a few tens to hundreds of a micrometer in diameter,
is subject to a certain degree of uncertainty resulting from the
mechanical limitations of the observation apparatus. Current
recording size of the pixel might not be sufﬁciently precise in the
case of a smaller particle size. Fig. 2 quantitatively presents an
observational error of the diameter ranges of the particles considered in this study. It was calculated based on measurement of the
number of maximum and minimum pixels where the diameter of
the actual particles can occupy the ﬁeld of view. In the images, it
appears more diffuse than the actual size as the diameter of the
particle becomes smaller. Because the degree of uncertainty
decreases as the size of the objects increase, however, the enveloped volatile ﬂames, which are several times larger than the size
of the coal particle, could be measured with conﬁdence based on
the current set-up.
2.5. Image processing procedure
The images were processed to characterize the ﬂame intensity
as well as the size, both for the particle and the ﬂame. Successive
images were captured using the high-speed camera, which has
gray 8-bit resolution. At each capturing time in the same condition,
20 images were acquired to produce an averaged value to provide
quantiﬁed results in terms of sizing and receiving intensity. In the
case of char oxidation, for example, the surface of the char particle
is ﬁrst partially burned from edge of the non-spherical char surface
[23,24], and it was also observed in this study. Therefore, the raw
intensity values on the particles in each image have to be averaged
to present the characterized intensity values at a point in time. To
compare the images under the identical conditions, the gain value
and the exposure time of the camera were ﬁxed at all times, as
mentioned above. To quantitatively analyze the images, the data,
with numbers from zero to 255 corresponding to the intensity at
each image pixel, were normalized by the highest value in each
of the averaged image to reduce the effects of the contamination
on the quartz window or the reﬂected light from the quartz window. As there was backlighting source, a ﬁxed value was assigned
to the pixels, which occupy the background areas, except for the
coal particle and volatiles ﬂame in the raw images. The ﬂame
region and the particle region could then be distinguished. The
summary of the image processing procedure is represented in
Fig. 3.
The volatile ﬂame area (Af) was calculated by summing the
corresponding areas (dApixel) of the pixels in the image data and
the effective ﬂame radius (reff) value, which is assumed to be a concentric ﬂame shape, was estimated based on the conservation of
the ﬂame area according to the following equation, where rp corresponds to the coal particle radius.
reff
rﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃ
Z
1 ¼
Af þ pr 2p ; where Af ¼ dApixel
p
ð1Þ
The mean elongated distance (rc), which is a distance of the intensity-weighted ﬂame center from the particle center, and the mean
inclined angle (hc) were estimated from the image data to represent
the degree of asymmetric development of volatile ﬂames. It is
appropriate to consider the intensity-weighted values of the ﬂame
as a threshold because strong and weak values are distributed over
the ﬂame regions [25]. The radius of coal particle (rp) normalized
the mean elongated distance (rc). They are calculated in the following equations,
R
R
I x dA
I y dA
x ¼ R
¼ R
; y
I dA
I dA
rc ¼
Fig. 2. Quantiﬁed observational error. This shows the trend that the real size value
‘’ follows the minimum measurement value ‘s’ as the particle size increases, and
the degree of uncertainty associated with measurement gradually reduces on the
display screen as the size of the objects increases. In the range of particle size
considered, Max. 33.5 % and Min. 14.0 % (in the case of 75 lm) ? Max. 7.3 % and
Min. 0.1 % (in the case of 200 lm). Error values ‘h’ and ‘s’ would reach a zero level
for larger size objects.
pﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃ
x2 þ y
2
rp
hc ¼ cos1
=r p
y
rc
ð2Þ
ð3Þ
ð4Þ
where x, y, and I correspond to the horizontal coordinate in jet ﬂow
direction, the vertical coordinate in hot gas ﬂow direction, and the
mean the horizontal and
intensity value of ﬂame image.x and y
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Fig. 3. Representative image processing procedure: (a) raw images, (b) ﬂame image normalized by the highest pixel value, (c) a speciﬁc value, 70, is assigned to pixels in the
background having the intensity of the back-lighting source. The volatile ﬂame area (Af) and effective ﬂame radius (reff) are assumed to the concentric ﬂame based on the
conservation of the number of the pixels are calculated. For example, the values of rc and hc are printed on the bottom of the ﬂame image.
vertical coordinates of the intensity-weighted center of the volatile
ﬂame region. Large values in rc and hc mean that ﬂame is elongated
far from the particle center and that ﬂame is tilted much from the
vertical line, respectively. For rc=1, for example, it means that concentric volatile ﬂame is ideally formed around the particle.
3. Results and Discussion
3.1. Sequential regimes of combustion processes of entrained particles
Sequential combustion regimes were successively observed
from an injection point, t0=0 s and T0=298 K. From the tube tip
where particles are injected, heat-up and volatile oxidation processes of single coal particles were clearly identiﬁable, and subsequent char oxidation phase were visualized as shown in Figs. 4 and
5 (see Fig. S1, video image in the supplementary material, as representative behavior). These images display the initial stages of coal
particle combustion on different particle size and O2 concentration
in the bulk gas. Because elapsed time from the starting point and
displacement of particles till extinction of char reaction (possibly
till generation of ash) were relatively long (about 1.0 s and
150 mm, respectively), the visualization till ﬁnal particle burnout
region was limited in this experimental conﬁguration. It is possible
to extend the period of observation till the extinction of char reaction in lower magniﬁcation.
The sequential images provide information associated with the
corresponding status of the overall combustion process on the time
and spatial location of each particle. It was found that the residence
time and displacement of coal particles are variable depending on
the particle size and O2 concentration. The size of the particle is a
dominant factor in characterizing the physical displacement and
apparent combustion behavior of coal particles on time scale.
Time window for particle to stay inside the ﬁeld of view was typically 13-15 ms (75-90 lm), 15-20 ms (125-150 lm), and 25-28 ms
(180-200 lm) (see Fig. 4). Images of moving coal particles were
superimposed on a single frame. Distance of the particle location
between two successive images would correspond to the instantaneous speed of particle multiplied by the time interval. Arranged
images show the general trend of the cross-jet mixing conﬁguration. Particle, initially transported by the jet with the momentum only in the horizontal direction, gradually loses horizontal
momentum while attains vertical momentum. Entrainment process is typically dependent on the mass of the particle. Smaller particle of 75-90 lm loses almost of horizontal momentum in less
than 10 ms, while larger particle of 180-200 lm retains a large
amount of horizontal momentum even at 20 ms. Fully entrained
particle would follow the mean gas ﬂow, but observed inter-particle distance shows variation, also depending on the particle size;
for example 1.1-1.3 mm for 75-90 lm, 0.9-1.1 mm for 125-150
lm, and 1.8-2.0 mm (for 2.4 ms interval) for 180-200 lm. By looking at the images of coal particles, one can notice the shapes of particle are not spherical but of arbitrary natural rock-like form, which
is intuitively expected. The particle rotational motion is also
observable in addition to the translational motion in the horizontal
and vertical direction.
Fig. 5 shows the coal particle behavior with respect to oxygen
concentration. Now that ﬂow velocity and temperature conditions
are nearly identical to those cases shown in Fig. 4, particle motion
appears similar. However, shapes of the volatile ﬂames, as well as
the time-wise characterization of the volatile ﬂame, are strongly
inﬂuenced by the oxygen concentration. It is shown that the
spherically concentric shape of the volatile ﬂame is visible only
for the limited experimental condition, depending on the oxygen
concentration and the relative motion of the particles to the mean
gas stream. Now that the mass of coal particle is continuously
changing on reaction rate during the volatile release and char oxidation, it inﬂuences the entrainment behavior. The motion is
explained by particle velocity and relaxation time deﬁned as the
ratio of the particle aerodynamic response time to the ﬂow
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H. Lee, S. Choi / Combustion and Flame xxx (2015) xxx–xxx
Fig. 4. Sequential images of a single coal particle superimposed on a single frame for better understanding (all particles are introduced at the center of the tube). Three sets of
particle trajectories show the effect of particle size (75-90/125-150/180-200 lm) in the ranges of particle size groups on combustion behavior. Bituminous F coal, in O2 34.6 %,
1240 K. Time and displacement intervals are shown on the photo.
Fig. 5. Three sets of particles trajectory show the effect of oxygen concentration (21.1/29.8/40.2 %) in the bulk gas environment on combustion behavior. Bituminous F coal
with dp 125-150 lm, 1240 K. Time and displacement intervals are shown on the photo.
characteristic time. Aerodynamic behavior on changing reactivity
by being porous char particles of low density in combustion progress has been reported [26].
The direct observation shots of the particle behavior can be
speciﬁcally enumerated on time scale in more detail, as presented
in Fig. 6. Conceptual stages, which have been typically perceived
as particle heat-up, volatile release and combustion, and char
oxidation, are schematically depicted. Recorded images of black
particles represent shadow images and bright white images of the
volatile ﬂames are shown along with the shadow images of the particle in the center. And when the surface emission from the char is
visible, shadow image of the particle is not seen. Particles in the
heat-up phase are seen as a shadow. The images of volatile ﬂame
are believed to originate from the sooty volatile ﬂame, which is
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7
Fig. 6. Comparison of the combustion processes for two coal types: Bituminous F coal and Subbituminous P coal, dp 125 lm in O2 34.6 %, 1240 K. Particle heat-up phase
(0.72-3.6 ms for F coal and 0.72-5.04 ms for P coal), volatiles release and oxidation phase (4.32-10.08 ms for F coal and 5.76-12.24 ms for P coal), and char reaction phase (10.8
ms onwards for F coal and 12.96 ms onwards for P coal) in this condition. The increasing bar below the photo means the stream of time with 0.72 ms interval between the
frames.
considered as a volumetric radiative source. In between the extinction of the volatile ﬂame and the char ignition, char particles are
seen as a shadow. Burning char particles are typically seen with
bright emission. Intensity of char emission varies as char oxidation
progresses.
Sequential regimes of combustion for the two coal types of different rank are also compared in Fig. 6. Images of bituminous and
subbituminous show qualitatively similar progress of the sub-processes of the combustion, where experimental conditions were
maintained identical including the characteristic size of the coal
particles. Bituminous shows relatively faster progress of combustion compared to subbituminous. Fig. 6 shows one example set
of images from the large number of repetitive observation.
ignition delay of coal particle ﬂame, the time required from the particle injector tip to the appearance of the visible volatile ﬂame surrounding the particle. Time duration for the apparent volatile ﬂame
is deﬁned as the burning time of the volatile ﬂame (svol_oxidation).
The particle heat-up time and volatile burning time are summarized in Fig. 7, so that parametric inﬂuence of particle size and oxygen concentration on the particle heat-up times is shown. The
qualitative universality of particle heat-up and volatile burning
times for 75-90 lm particles is consistent with the experimental
results by Khatami et al. [19,27] and the theoretical description
by Maffei et al. [20]. This is also consistent with the typical mathematical description, a general form mpcp(dTp/dt)=hADT, where
particle mass mp, heat capacity cp, particle temperature Tp, heat
3.2. Time scales of particle heat-up, volatile release and char ignition
Conceptual representation of the time scales of particle heat-up,
volatiles and char combustion is shown in Fig. 6. From the serial
images, one can identify the time when the volatile release or oxidation is ﬁrst observed and the time last observed, as well as, the
time when the char combustion is ﬁrst observed. Although it is difﬁcult to minimize the uncertainty in determining the exact timing
because of its continuous changes, time duration for the heat-up
and volatile oxidation, and char oxidation can be deﬁned with
appearance of visible volatile and char ﬂames. Accordingly, discussion on the time duration for the heat-up, volatile release and oxidation can proceed.
Particle temperature increases as a result of convective and
radiative heating. The particle heating rate in this experimental
condition was calculated to be on the order of 105 K/s, which is
corresponding to the typical conditions of pulverized coal ﬂames
and previous experimental investigations [9–11,14,22]. Particle
heat-up time (sheat-up) is deﬁned as, which has been also called as
Fig. 7. Effects of oxygen concentration and particle size (of Bituminous F coal)
on particle heat-up time (Ignition delay), sheat-up, and volatile burning time,
svol_oxidation. In O2 21.1 and 16.7 %, volatile release was not terminated in this ﬁeld of
view in all particle size groups. In O2 25.4 %, volatile release of 180-200 lm particles
was not terminated.
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H. Lee, S. Choi / Combustion and Flame xxx (2015) xxx–xxx
Fig. 8. Effect of oxygen concentration on the effective radius of volatile ﬂame.
Bituminous F coal, dp 125 lm. Effective radius of the apparent ﬂame (reff) was
obtained through the processing of gray scale images. The heat-up and char
reaction regions are indicated through the initiation and ending points of the
volatiles oxidation region. In O2 21.1 and 16.7 %, volatile release was not terminated
in this time scale. Photo was taken when the ﬂame diameter reaches at its
maximum.
transfer coefﬁcient h, particle surface area A, and temperature
difference DT between the particle and the ambient gas environment [1,14].
As another way of representing the burning time of volatile
ﬂames, effective radius of the volatile ﬂame as deﬁned by Eq. (1)
is utilized, assuming that the image of the ﬂame is circular (volatile
ﬂame is spherical). Validity of this assumption is rather limited in
lower oxygen fraction environment, but interpreted result of the
time-wise variation of the size of the volatile ﬂame showing the
effect of oxygen concentration is informative as shown in Fig. 8.
The heat-up region and the initiation points of char oxidation
can be found based on the volatile oxidation region identiﬁed with
the initiation and ending points of reff/rp values on different oxygen
concentration. As the oxygen concentration decreases, it took relatively longer until the apparent volatile ﬂame appears on the particle surface. The results are consistent with previous experiments
[16,17] that devolatilization proceeds rapidly with higher oxygen
vol.%. The oxygen concentration is not generally related to the temperature rise during heat-up of coal particles in the simple mathematical models. However, experimental observations, including
those of this study, show the increasing time for appearance of
visual volatile ﬂame with decreasing oxygen concentration in the
bulk gas. It is also attributed to the longer burning time and the larger effective radius of the volatile ﬂame as the oxygen concentration decreases (see Figs. 5 and 8).
As already seen in Fig. 4, size of the volatile ﬂame is related to
the size of the coal particle. Fig. 6 shows the difference in dynamic
progress of the volatile ﬂames. Fig. 8 also shows the dynamic size
variation of the volatile ﬂame along with the effect of oxygen concentration in the mean gas stream. Although particle size and oxygen concentration are shown to affect the dynamic formation of
the volatile ﬂame in a plausible manner, quantitative explanation
was not attempted. It is natural to expect that formation and
shapes of the volatile ﬂame are directly related with the volatiles
release rate and the combustion rate of the volatiles, although
we have not yet speciﬁcally deﬁned the state and contents of the
volatiles. As related with formation of the volatile ﬂame for a coal
particle, several models [4–7] have been proposed. Basic modeling
approach resembles that of the liquid fuel droplet combustion, in
which a spherically symmetrical volume of the mantle is ﬁlled
with the devolatilized products, which will oxidize at a fast rate
and form an inﬁnitely thin ﬂame sheet. These mathematical models would require a complete set of information as related with the
properties of coal particle and environment conditions.
Shape of the volatile ﬂame is not, in general, spherical. As one
way of displaying the various shape of the volatile ﬂame, Fig. 9
shows ﬂame images with parametric variation of oxygen mole
fraction and the velocity ratio, Uj/U0, between the jet ﬂow and
the hot gas ﬂow. The nearly concentric and more intense ﬂame,
possibly with a higher ﬂame temperature, with a shorter radius
was commonly formed with increasing oxygen concentration.
This is consistent with the previous observation through particle
false-color imaging [17] where the ﬂame is described as the soot
cloud around the devolatilizing particles [28]. In relatively lower
oxygen concentrations, however, the volatiles were initially
released around the coal particles, and the ﬂame was less luminous
and immediately elongated, showing tail-like behavior. Because
one of the main parameters that would allow for the formation
of the concentric volatile ﬂame is obviously the oxygen concentration, an additional experiment was completed in the oxygen concentration of 45.0 and 50.0 %.
The vectorial behavior related to non-spherical appearance of
the volatile ﬂame was quantitatively interpreted to represent the
degree of asymmetric development of volatile ﬂame. The circularity of the volatile ﬂame formation was presented through the mean
elongated distance (rc) and the mean inclined angle (hc) values of
the volatile ﬂames, as shown in Fig. 10. Once the volatile matter
was released from the particle, the ﬂame was leaning to one side.
The direction of the tails was affected by the buoyancy and relative
velocity of the particle to the gas ﬂow, as shown in Fig. 9. In this
study, the tail was gradually tilted towards the left with an increasing velocity ratio. This is considered to result from the momentum
difference between the injection ﬂow and the combustion product
gas compared with the tail behavior in the previous co-ﬂow conﬁguration [15–17] and dropping into the quiescent gas [19,20].
Nearly concentric ﬂame appeared in the higher oxygen concentration at more than 45.0 % oxygen, regardless of the penetration
momentum of the jet ﬂow (Fig. 10).
3.3. Dynamic variation of the size and shape of the volatile ﬂames
3.4. Char oxidation (onset of apparent visible char ﬂame or ignition)
Existence of the volatile ﬂame in this observation is based on
the appearance of the visual images. The terminologies of ignition
and extinction of the volatile ﬂame are also used to represent the
initial appearance and ﬁnal disappearance of the envelop ﬂame.
As a ﬁrst step in discussing the dynamic variation of the size of
the volatile ﬂame, we have already utilized the assumption of
the spherically concentric volatile ﬂame. It has been seen that this
assumption would be acceptable only for the limited experimental
condition. We ﬁrst present experimental conditions when the volatile ﬂame appears to be spherically concentric and when not.
From the sequential observation of a single coal particle, the
onset of char combustion or ignition can be related with the formation of luminous intensity emitted from the particle surface, which
has already or simultaneously experienced the entire process of
volatile release and combustion (Fig. 6). It was generally found that
the oxidation of volatiles preceded the heterogeneous oxidation of
coal char, if all the volatiles released from the particle are involved
in the combustion reaction. However, overlapping of the volatile
oxidation and the char oxidation stages was often revealed, as indicated in Fig. 11. In the case of a particle size 75 lm and an oxygen
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H. Lee, S. Choi / Combustion and Flame xxx (2015) xxx–xxx
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Fig. 9. Formation dynamics of volatile ﬂame on different oxygen concentrations and ﬂow velocity ratios (Uj/U0). Bituminous F coal, dp 125 lm. Photo was taken when the
ﬂame diameter reaches at its maximum.
concentration of more than 34.6 % in this study, especially at the
particle surface of the shape with a sharp point, heterogeneous
ignition was almost overlapped with the apparent volatiles oxidation. The results are consistent with previous ignition regimes as a
function of heating rate and particle size [29,30]. It was shown that
a circumambient volatile ﬂame prevented the char reaction by
screening the solid from oxygen access on the left of the particle,
in contrast with the direct char oxidation on the right surface of
the particle (see Fig. 11(c)).
Different degrees of radiant intensity arising from the char surface oxidation are relevant to the temperature variation of the
luminous char particles. In a typical form, based on the wellknown gray body assumption and the Stefan-Boltzmann law,
describing the total energy radiated from the particle, i=reApT4p
where r is the Stefan-Boltzmann constant, e is the emissivity,
and Ap is the projected surface area of particles, the particle temperature Tp is directly inﬂuenced by intensity variation because
the emissivity is not precisely known as a function of temperature
or wave length for most coals or chars [31]. The projected surface
area of the moving particle can ﬂuctuate a little (because the particles are not perfectly spherical).
Contrary to the common modeling concept of uniform particle
char combustion, it was observed from many particle traces that,
in the initial stages, combustion on the particle surface is nonuniform. The phenomenon has been pyrometrically reported [24]
and a relevant model has been built to describe it in the previous
literature [23,24], now has been photographically conﬁrmed. It
was difﬁcult to deﬁne the onset points of char oxidation considering this state of affairs; however, the series of images of the
luminous char can be used for the discussion of char oxidation.
Fig. 12 presents the quantiﬁed averaged-intensity from the char
surface with parametric variation of the oxygen concentration
along with the associated time scale. In the present study, the
onset time of char oxidation, tchar_oxidation, is deﬁned to the point
where only char surface oxidation occurs after the simultaneous
hetero-homogeneous reaction (see Fig. 6). The normalized intensity on the gray scale would correspond to the averaged value
divided by the number of pixels of the projected surface of each
particle at each frame, although the particle was partially burned.
As a result, the values showed somewhat linearly increasing trends
along with the time elapse in the graph even though a more luminous intensity was detected at the point of the particle with the
increasing oxygen concentration.
Oxidation of the char particle following the extinction of the
volatile ﬂame occurs at a different rate. As the oxygen concentration increases, the char combustion takes shorter time based on
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H. Lee, S. Choi / Combustion and Flame xxx (2015) xxx–xxx
Fig. 12. Averaged global intensity during char oxidation (from tchar_oxidation, refer to
Fig. 6). Bituminous F coal, dp 125 lm. On the vertical axis, the normalized values 0
and 1 mean 0 (Min.) and 255 (Max.) in gray scale, respectively. The horizontal axis
means the time from the beginning of char ignition.
Fig. 10. Mean elongated distance (rc) and mean inclined angle (hc) to estimate the
degree of asymmetric development of volatiles ﬂame.
[32] as q/k(PO2)n, where q is the burning rate per unit external
surface, k is the kinetic rate, PO2 is the oxygen partial pressure
determined by the oxygen concentration in the bulk gas, and n is
the exponent. The parameters are subsequently related with the
global intensity of char surface. The higher luminous intensity indicates a relatively more intense burning situation [13,33], a possibly
higher particle temperature at the particle surface, which is
assumed to be the surface radiation of a gray body.
4. Concluding Remarks
Fig. 11. Overlapped combustion stages during particle entrainment. Typical
situation (a) heat-up, (b) volatile (homogeneous) oxidation, (c) simultaneous (both
homogeneous and heterogeneous) oxidation process, and (d) char (heterogeneous)
oxidation. dp 75 lm, O2 34.6 % (or higher).
the slope of the graph. The coal char particle with a non-spherical
appearance was burned from the edge of the particles because of a
different heat transfer gradient at the sharp point of the particle.
One of the typical expressions of burning rate of char is given
An experimental investigation on high magniﬁcation visual
observation was carried out to elucidate the combustion behavior
of single coal particles entrained into hot gas ﬂow. The direct
observation of instantaneous combustion behavior in the controlled environmental conditions is presented. Within the limits
of the present experimental conditions, following conclusions
and insights have been drawn:
By showing the sequential combustion processes of particle
heat-up, volatile release and oxidation, and char reaction in time
and space, the half-shadow visualization of the burning particles
was able to offer the positioning of the particles which led to the
timing of the characteristic sequence of the sub-processes including the particle heat-up time (delay until the visual ignition) and
the volatile burning time.
Imaging quantiﬁcation of the burning particles (coal being
heated, volatile ﬂame, and burning char) was conducted in terms
of the changes of the apparent size, shape, and intensity based
on its luminosity. Time-wise progress of the formation and extinction of the apparent volatile ﬂame was presented. Oxygen concentration directly affected the appearance of the volatile ﬂames. The
degree of asymmetric development of volatile ﬂames facilitated an
interpretation of the effect of oxygen concentration. In the highly
enriched oxygen range, volatile ﬂames were observed to be nearly
spherically concentric, with almost no effect of the relative speed
of the particle to the mean gas ﬂow.
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The onset of char combustion or ignition as evidenced by
luminosity from the char surface was observed in this study.
Initial appearance of luminosity was localized, and the char oxidation appeared to spread over the entire char surface. In some cases,
the combustion appeared from the edge of the excessively polygon-shaped particle, while the volatile ﬂame was still observable.
This observation results were presented with a hope to stimulate discussions on the fundamental understanding of combustion
stages of single coal particles. The limited set of quantitative observation data would help strengthen the capability of the predictive
modeling of coal combustion, which is ultimately beneﬁcial in utilizing the comprehensive computational codes.
Acknowledgments
The authors gratefully acknowledge support from the Korea
Advanced Institute of Science and Technology (KAIST) and the
Brain Korea 21+ project.
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