Fragmentation Effect on Batches of Pine Wood Char Burning in a ...

Energy Fuels 2010, 24, 318–323 : DOI:10.1021/ef900806z
Published on Web 11/09/2009
<strong>Fragmentation</strong> <strong>Effect</strong> on Batches of Pine Wood Char Burning in a Fluidized Bed
Nelson Rangel and Carlos Pinho*
Centro de Estudos de Fenomenos de Transporte (CEFT)-Departamento de Engenharia Mec^anica (DEMec),
Faculty of Engineering, University of Porto, Rua Dr. Roberto Frias s/n, 4200-465 Porto, Portugal
Received July 29, 2009. Revised Manuscript Received October 20, 2009
Batches of Pinus pinea char particles with average diameters of 2.2, 2.8, and 3.6 mm were burned in a
bubbling fluidized bed at temperatures of 600, 700, and 750 °C, with a velocities ratio U/U mf = 9. The
results show that only primary fragmentation occurs with an average fragmentation ratio of 1.5. No effect
of the secondary fragmentation phenomena is observed. The increase in the number of particles in the bed
because of fragmentation is accounted over the burning time, and its effect is evaluated on the
determination of the global combustion resistance.
Introduction
The fragmentation of combustible particles during the
burning in a fluidized bed was studied for the first time by
Campbell and Davidson. 1 These authors have studied the
breaking that occurs during the burning process, which is
defined as secondary fragmentation. This type of fragmentation
results from the physical shocks that occur between
particles during their burning in the bed and produces fragments
of non-elutriable dimension. The breaking of the
particles occurs because of the fragility of their internal
structure, because of the internal burning that destroys the
structural links of the particles. If a particle burns only at the
surface, it is not supposed to experience the phenomenon of
secondary fragmentation.
The so-called primary fragmentation is due to thermal
shock that exists when the particles of fuel are released initially
into the bed. From the contact of the particles with the bed at
high temperature, there are thermal stresses accompanied by
an increase in the pressure of the volatile matter contained
within the particles that cause the breakup. This type of
breaking produces relatively large fragments that remain in
the bed; they are not elutriated.
Besides these two types of fragmentation, there is the
attrition resulting from collisions of the particles between
themselves and with the walls of the reactor, as well as the
fragmentation that results from percolation, associated with
the loss of the structure of the particles resulting from the
internal burning. Both the attrition and the fragmentation
associated with the phenomenon of percolation produce easily
elutriable fragments.
Beer et al. 2 have studied the combustion of coal in a
fluidized bed, searching for a relationship between the type
of coal and the amount of elutriated fines. These authors have
suggested that the attrition of the particles competes directly
with the combustion itself in reducing the size of the particles
*To whom correspondence should be addressed. Telephone: þ351-
225081400. Fax: þ351-225081440. E-mail: ctp@fe.up.pt.
(1) Campbell, E. K.; Davidson, J. F. Institute of Fuel Symposium
Series Number 1: Fluidised Combustion, London, U.K., 1975; paper A2.
(2) Beer, J. M.; Massimilla, L.; Sarofim, A. F. Institute of Energy
Symposium Series Number 4, Fluidised Combustion: Systems and
Applications, London, U.K., 1980; paper IV-5.
r 2009 American Chemical Society 318 pubs.acs.org/EF
and that this relative effect is strongly affected by the reactivity
of the particles. The more reactive particles have greater
immunity to the effects of erosion by staying in the bed for a
shorter time, because of the shorter burning time. The study
was fulfilled in two facilities, and one of them had heattransfer
elements within the bed, which contributed to attrition.
The fluidization velocities were of the order of 40 times
the minimum fluidization velocity, which also promoted the
attrition by contact between the particles. D’Amore et al. 3
have continued the study of the influence of the reactivity of
the fuel particles in the distribution of sizes, using fossil fuels
and other carbonaceous materials, concluding that there is
more attrition during the combustion of less reactive coals.
Chirone et al. 4 have studied the rates of fines elutriation of
carbon, which resulted from attrition during the combustion
of a South African mineral coal, for various sizes of particles
fed to the bed. Still focused on the problem of abrasion,
Salatino and Massimilla 5 have proposed a model that took
into account the interaction between the burning inside the
pores, the weakening of the mechanical structure of the
particle, and the attrition. In comparison of the measured
rates of elutriated carbon from the combustion of the char of a
bituminous coal to those provided by the model, a good
correlation was found between them, suggesting that the
elutriated fragments are originated from attrition between
particles and are also the result of the burning inside the pores,
which may have caused their coalescence accompanied by the
release of carbon fragments that left the particle and bed
without burning. Arena et al. 6 have also studied the phenomenon
of attrition, obtaining the attrition rate constants of
carbon for various operating conditions of the bed and for
chars obtained from a bituminous coal.
The studies mentioned above focused mainly on the phenomenon
of elutriation of small carbon particles because of
(3) D’Amore, M.; Donsi, G.; Massimilla, L. Proceedings of the 6th
International Conference on Fluidized Bed Combustion, Atlanta, GA,
1980; pp 675-685.
(4) Chirone, R.; Cammarota, A.; D’Amore, M.; Massimilla, L.
Proceedings of the 19th International Symposium on Combustion,
The Combustion Institute, Pittsburgh, PA, 1982; pp 1213-1221.
(5) Salatino, P.; Massimilla, L. Chem. Eng. Sci. 1985, 40, 1905–1916.
(6) Arena, U.; D’Amore, M.; Massimilla, L.; Meo, S.; Miccio, M.
AIChE J. 1986, 32, 869–871.

Energy Fuels 2010, 24, 318–323 : DOI:10.1021/ef900806z Rangel and Pinho
attrition. However, other authors, such as Dakic et al., 7 were
concerned with the phenomenon of primary fragmentation,
saying that it is very complex and insufficiently studied and that
the changes in size and shape of the particles during this process
may affect the subsequent combustion stage. Stubington and
Wang 8 have also turned to the study of the elutriation of small
particles of unburned carbon resulting from the combustion of
Australian black coals in a pressurized fluidized bed, indicating
that the attrition within the bed is the dominant mechanism for
generation of carbon fines, and observed primary and secondary
fragmentation phenomena for particles larger than 2 mm.
Cui and Stubington 9 have focused on the study of secondary
fragmentation, whereas Zhang et al. 10 have concluded that the
main reason for the particle fragmentation is the primary
fragmentation, stating that this is the dominant type of fragmentation.
In a study on the fragmentation effects of carbonized
biomass particles, Scala et al. 11 have shown that,
dependent upon the type of biomass, the particles may experience
significant changes in size because of the primary and
secondary fragmentation phenomena, thus influencing the
distribution of particle sizes in the bed.
As mentioned above, for several years, the studies on the
phenomena of fragmentation focused on the problem of
particle attrition and the consequent reduction of the combustion
efficiency by elutriation of unburned fines. Gradually, the
importance of the primary and secondary fragmentation
effects has been recognized, particularly the primary fragmentation,
in the combustion process of batches of carbon
particles in fluidized beds.
In the present study, the importance of fragmentation
during the combustion of nut pine char particles in a laboratory-scale
fluidized-bed reactor was assessed and the experimental
data were analyzed according to the fragmentation
model of Pinho. 12 As such, a simple overview of the mentioned
fragmentation model is presented herein.
<strong>Fragmentation</strong> Model
In the work referred above, the fragmentation effect on the
combustion rate of batches of particles in the fluidized bed was
discussed. In that work, the following expression was presented
to obtain the equivalent average diameter dcorr of the
fragmented particles composing a batch:
dcorr ¼ di
ð1 - f Þ 1=3
P
1=3
Nj
j Nc
where di is the initial diameter of the particles and f is the
consumed mass fraction of the batch
f ¼ 1 -
P
j mj
with mc being the initial mass of the batch of char particles. Nj
is the number of particles for the j-size fraction
Nj ¼ 6mj
Fcπdj 3
ð3Þ
(7) Dakic, D. V.; Grubor, B. D.; Oka, S. N. Proceedings of the 41st
IAE Fluidized Bed Conversion Meeting, Salerno, Italy, 2000.
(8) Stubington, J. F.; Wang, A. L. T. Proceedings of the 41st IAE
Fluidized Bed Conversion Meeting, Salerno, Italy, 2000.
(9) Cui, Y.; Stubington, J. F. Fuel 2001, 80, 2245–2251.
(10) Zhang, H.; Cen, K.; Yan, J.; Ni, M. Fuel 2002, 81, 1835–1840.
(11) Scala, F.; Chirone, R.; Salatino, P. Energy Fuels 2006, 20,91–102.
(12) Pinho, C. Chem. Eng. J. 2006, 115, 147–155.
mc
ð1Þ
ð2Þ
319
where F c is the density of the fuel particles and d j is the average
diameter for the j-size fraction. N c is the number of particles
initially in the batch, i.e., before fragmentation occurred
Nc ¼ 6mc
Fcπdi 3
ð4Þ
Equation 1 suggests that, because of the occurrence of fragmentation,
the diameter of the particles for a given consumed
fraction of the batch, which in the absence of fragmentation is
given by
d ¼ dið1 - f Þ 1=3
ð5Þ
should be corrected by dividing it by the factor
0 11=3
X Nj @ A
j
Nc
obtained from experimental data of fragmentation, namely,
the values of N j.
Global Combustion Resistance Corrected by the <strong>Fragmentation</strong>
<strong>Effect</strong>. To apply the fragmentation model, it is necessary
to find an equation to calculate the value of the
corrected global combustion resistance. Ross and Davidson
13 have found that the global combustion resistance could
be given by
1
K ¼ 12d2 mc
F c di 3 AtUk 0
From eqs 4 and 7, it can be written that
1
K ¼ 2d2Ncπ AtUk0 where the quantity N cπd 2 is the superficial area of the
reaction for a number of N c particles of diameter d, A t is
the cross-sectional area of the bed, U is the superficial
velocity of the fluidizing air, and k 0 is the dimensionless
constant for the oxygen consumption rate in the bed. When
the fragmentation effect is taken into account, the total
number of particles in the bed is different from the initial
value, with the global combustion resistance 1/K corr being
corrected by the fragmentation effect, given by
1
Kcorr
¼ 2dcorr 2 Nπ
AtUk 0
where N is the total number of particles in the bed after
fragmentation and d corr is the average diameter of those
particles given by eq 1; N = P jN j.
Dividing eqs 8 and 9 gives a relationship between the
corrected and noncorrected values of the global reaction
constant, K corr and K
Kcorr ¼ d2Nc dcorr 2 N
ð6Þ
ð7Þ
ð8Þ
ð9Þ
K ð10Þ
Taking into account eq 1 for dcorr and eq 5 for d, it can be
written that
1
Kcorr ¼ K ð11Þ
1=3
ðN=NcÞ
(13) Ross, I. B.; Davidson, J. F. Trans. Inst. Chem. Eng. 1981, 59, 108–
114.

Energy Fuels 2010, 24, 318–323 : DOI:10.1021/ef900806z Rangel and Pinho
Considering the definition of the multiplication factor of the
particles or fragmentation ratio σ, as defined by Pinho, 6
σ ¼ X Nj
¼
Nc
N
ð12Þ
Nc
then
j
Kcorr ¼ 1
K ð13Þ
σ1=3 The relation between Kcorr and K is only a function of the
fragmentation ratio σ, which is obtained from experimental
data.
Experimental Section
The experiments were carried out in an electrically heated
fluidized bed reactor with 54.5 mm of internal diameter operating
in bubbling regime. The bed was fluidized by air, and the inert
material of the bed was silica sand (200-250 μm). The combustible
particles were fed above the free surface of the bed, which
had a height of 100 mm. The tests were performed through
interrupted combustion of 5 g batches of nut pine (Pinus pinea)
char particles, with average diameters of 2.2, 2.8, and 3.6 mm, for
superficial velocities of 9Umf and bed temperatures of 600, 700,
and 750 °C. The properties of nut pine char are shown in Table 1.
The combustion was stopped after 30, 60, 120, and 180 s, when
burning smaller particles, whereas for particles of larger size (di=
3.6 mm), experiments were carried out until 240 s; this additional
stoppage is due to the greater burning time of these larger
particles. The freezing of the reaction was achieved by replacing
the fluidization air by nitrogen and subsequently cooling the bed.
After the cooling of the bed, the particles were extracted by
suction (particles of carbon and inert material) and passed
through a sieve set, which varied according to the size of particles
that were being tested. The sieve sets used for the three initial sizes
of studied particles are detailed in Table 2.
After a sieving time of around 2 min, the particles retained in
the sieves were weighed, obtaining the mass of particles mj
corresponding to the four average diameters of the sieve set.
The average diameters were obtained from the arithmetic mean of
the sieve meshes, in which the particles were retained at the end of
the sieving process.
Knowing the experimental values of average diameters and the
masses corresponding to them, for each value of combustion
time, it is possible through the fragmentation model presented
above to evaluate the evolution of the number of particles in
the bed along the burning process as well as to obtain the
correction factors for the fragmentation effects along the burning
process.
Results and Discussion
<strong>Fragmentation</strong> Ratio. The degree of fragmentation of the
particles for different test conditions is shown in Figure 1. It
represents the evolution of the total number of particles N in
the bed along the combustion for the studied bed temperatures
and initial particle diameters.
For all studied cases, as shown in Figure 1, the increase in
the number of particles occurs only during the first 30 s of
burning. Afterward, the number of particles remains unchanged
or decreases. This suggests that the particle fragmentation
occurs just after the release of the batch in the bed
because of thermal shock undergone by them, i.e., primary
fragmentation. There is not, therefore, experimental evidence
of further fragmentation throughout the combustion.
If fragmentation did occur during the combustion process,
the number of particles in the bed when the reaction was
frozen at 60 and 120 s should be greater than at 30 s.
320
Table 1. Proximate Analysis and Density Value of Nut Pine Char
particle density a
(g/cm 3 )
moisture at
105 °C
proximate analysis (wt %, as received)
ashes at
500 °C
volatile matter at
900 °C
fixed
carbon
0.77 7.7 0.7 17.7 73.9
a
Density was obtained with a mercury porosimeter.
Table 2. Sieve Sets Used To Obtain the Batch Granulometric
Characterization after the Freezing of the Reaction with Nitrogen
di (mm) DIN standard sieve sets (mm)
3.6 -4 þ 3.15 -3.15 þ 2.5 -2.5 þ 2 -2 þ 0.8
2.8 -3.15 þ 2.5 -2.5 þ 2 -2 þ 1.6 -1.6 þ 0.8
2.2 -2.5 þ 2 -2 þ 1.6 -1.6 þ 0.8 -0.8 þ 0.5
Figure 1. Evolution of the total number of particles in the bed over
time. Bed temperatures were (a) 750 °C, (b) 700 °C, and (c) 600 °C,
and particles had initial diameters of 3.6, 2.8, and 2.2 mm. The zero
instant corresponds to the moment before the release of the particles
into the bed (N = N c).
The results are inconclusive with respect to the correlation
of the primary fragmentation ratio with both the particle size
and bed temperature. The limited range of tested sizes and
temperatures can explain this. The reduced number of
particles at the end of burning is due to the disappearance
of smaller particles, which are the first to burn out. The
experimental values of the fragmentation ratios are presented
in Table 3.

Energy Fuels 2010, 24, 318–323 : DOI:10.1021/ef900806z Rangel and Pinho
Considering the values for t=30 s as those referring to the
primary fragmentation ratio, by Table 3, it can be concluded
that, after releasing the particles into the bed, their number
increases about 46% on average.
The average values of the fragmentation ratio for the three
bed temperatures tested in this work are compared to some
data published in the literature in Table 4. The present results
are comparable to data published by Chirone et al. 4 for
mineral coal particles with initial diameters (1-3 mm)
equivalent to those found in the present study. Larger
particles in the Chirone et al. 4 data present higher fragmentation
ratios. As far as the data from the work of Scala et al. 11
are concerned, only for one type of biomass char tested by
these authors is there similarity on the fragmentation ratio,
despite an initial diameter of char particles 3 times higher
than the present ones. In this second case, the proximate
analysis and consequently the volatile matter content are not
available for comparison.
It should be stressed that the bed temperatures studied
here were lower than those considered by the other referred
authors, with this fact having major importance on fragmentation
as is consensually accepted, although this phenomenon
was not observed in the narrow temperature range
tested, as mentioned. Probably the effect of the bed temperature
on the primary fragmentation ratio is not relevant when
a range of relatively low bed temperatures is taken into
consideration. A similar phenomenon on the dependence
of primary fragmentation with an initial diameter of particles
was observed by Zhang et al. 10 These authors have shown
that, for diameters between 1 and 4 mm, there is a very weak
dependence of primary fragmentation with the particle
diameter, whereas for diameters larger than 4 mm, the
t (s) T (°C)
Table 3. <strong>Fragmentation</strong> Ratios
di = 3.6 mm;
Nc = 270
fragmentation ratio (σ)
di = 2.8 mm;
Nc = 547
di = 2.2 mm;
Nc = 1083
30 600 1.61 1.15 1.59
700 1.38 1.27 1.49
750 1.68 1.38 1.63
60 600 1.54 1.13 1.25
700 1.42 1.22 1.43
750 1.64 1.38 1.57
120 600 1.51 1.18 1.15
700 1.39 1.31 1.20
750 1.68 1.23 0.93
180 600 0.95 0.78 0.59
700 1.25 0.81 0.47
750 1.09 0.39 0.25
240 600 0.67 not available not available
700 0.85
750 0.41
Table 4. Comparison of Primary <strong>Fragmentation</strong> Ratios
321
dependence is exponential; the particle breakage soars with
the increase of the particle size. The data in Table 4 confirm
the results of Zhang et al. 10 Accordingly, for smaller particle
diameters, the fragmentation ratio is relatively low and does
not vary considerably with the initial particle size; thus,
within the range of diameters studied in this work, the values
of σ 1 are approximately the same.
<strong>Fragmentation</strong> <strong>Effect</strong>. From the experimental data, it was
possible to calculate the diameter of particles corrected by
the fragmentation effect through eq 1 and the global combustion
resistance through eq 9 for each instant in which the
combustion was stopped and compare them to the corresponding
uncorrected values (Figure 2). Such comparisons
in the figure are for the three tested bed temperatures.
As Figure 2 shows, the incorporation of the fragmentation
effect in the model for calculating the global combustion
resistance affects the values of 1/K and d in a different way at
the beginning or at the end of burning.
At the beginning and after the fragmentation, there are
more particles in the bed than in the absence of such an event
and the corrected diameters are lower than those obtained
without considering the fragmentation. However, when this
fragmentation event is ignored, such an increase on the
reaction area will be interpreted as an apparent increase on
the reactivity of the particles or, in other words, as a reduction
of the global resistance.
At the end of combustion, the inverse phenomenon occurs;
there are fewer particles in the bed as a consequence of
the burnout of the smaller ones created during the primary
fragmentation process, and the corrected diameters are now
greater than when the effect of particle breakage is not
considered. It should be remembered that, when particle
breakage is ignored, their number is considered constant
throughout the combustion experiment and equal to the
initial value. Therefore, in practical terms, when the fragmentation
phenomenon is ignored, it appears as if there is a
decrease of the global resistance at the end of combustion,
while the true situation is a reduction of the global reactive
surface.
<strong>Effect</strong> of the Primary <strong>Fragmentation</strong> on the Global Reaction
Constant. With the experimental data from primary
fragmentation (Table 3), it is possible to quantify the influence
of this phenomenon on the values of the global reaction
constant by eq 13.
To account for the influence of primary fragmentation,
only the number of particles remaining in the bed 30 s after
the start of the combustion process are considered for the
correction procedure. It is supposed that such a new number
of particles existing in the bed after the above-mentioned
primary fragmentation remain constant until the end of the
proximate analysis (% on a dry basis)
T (°C) di (mm) density (g/cm 3 ) fuel tested fixed carbon volatile matter ash σ1 Chirone
et al. 4
850 1-3 mineral coal 60 25 15 1.5
3-6 3.1
6-9 7.0
Scala
et al. 11
850 8.5 0.49 biomass chars 1.6
10.4 0.17 4.5
5.2 0.17 2.5
4.6 0.40 1.0
this work 600-750 2.2 0.77 nut pine char 80 19.2 0.8 1.6
2.8 1.3
3.6 1.6

Energy Fuels 2010, 24, 318–323 : DOI:10.1021/ef900806z Rangel and Pinho
Figure 2. <strong>Fragmentation</strong> effect on the global combustion resistance
and the diameter of particles. The superficial velocity was 9U mf,
and the static bed height was 100 mm. The bed temperatures were
(a) 600 °C, (b) 700 °C, and (c) 750 °C.
combustion process, which is not effectively true, as explained
in the previous section, when considering the final
stages of the combustion process.
Table 5 presents the ratios of corrected and uncorrected
K values. According to the experimental data presented in
the table, it is possible to conclude that the primary fragmentation
leads on average to a 12% reduction on the global
reaction constant (Kcorr = 0.88K).
For the range of temperatures and particle diameters being
tested and for nut pine char, ignorance of the primary
fragmentation phenomenon has some influence on the
322
Table 5. <strong>Effect</strong> of the Primary <strong>Fragmentation</strong> on the
Global Reaction Constant
K corr/K
T (°C) di = 3.6 mm di = 2.8 mm di = 2.2 mm
600 0.85 0.95 0.86
700 0.90 0.92 0.88
750 0.84 0.90 0.85
global reaction constant obtained through the experiments
and, consequently, uncorrected K values can be used without
great concern. However, such may not be true for higher
combustion temperatures when the thermal shock subsequent
to the introduction of the char particles in the bed can
be more severe, enhancing the primary fragmentation process.
It is expected that, in such circumstances, neglecting
the fragmentation impact can lead to serious experimental
errors.
Conclusions
The primary fragmentation phenomenon was observed
with an average fragmentation ratio of 1.5; i.e., after releasing
the particles into the bed, their number increases 50%
because of the breakage caused by the thermal shock that
they have undergone. This value is in agreement with some
results 4 published in the literature for fragmentation of
particles with the same initial diameter, whereas for biomass
chars, 11 agreement was only obtained for one particle size.
This combination of agreement and discrepancies leads
to the conclusion that, besides the particle size, the type of
char might have an important role on the degree of fragmentation.
No effect was observed resulting from the secondary fragmentation
of the particles, suggesting that, for the studied
char, this phenomenon is not present.
Without considering the effect of primary fragmentation in
calculating the global combustion resistance, the value of the
reactivity of the char particles appears to be higher than it
actually is, because of the deceptive effect of the increased
surface area for the reaction, as a result of the increased
number of particles caused by the breakage, which contributes
to the increase of the burning rate.
For the rate of primary fragmentation found for this type of
char, there is a reduction of 12% on the value of the reaction
rate constant when the impact of primary fragmentation is
taken into account. Therefore, ignoring the influence of
primary fragmentation on the evaluation of the global reaction
constant, for the temperature range covered in the
experiments, has some impact on this parameter.
Nomenclature
A t = cross-sectional area of the bed (m 2 )
D = bed diameter (m)
d = diameter of char particles (m)
d corr = corrected particle diameter by the fragmentation
effect (m)
di = initial diameter of char particles (m)
f = burned fraction
f c = mass fraction of carbon in the batch
K = global reaction constant (m/s)
k 0 = dimensionless constant for the oxygen consumption
rate

Energy Fuels 2010, 24, 318–323 : DOI:10.1021/ef900806z Rangel and Pinho
K corr = corrected global reaction constant (m/s)
m c = mass of carbon in a batch of coal particles (kg)
mj = mass of particles in j-size fraction (kg)
N = total number of particles in the bed
N c = number of particles in a carbon or char batch
N j = number of particles for the j-size fraction
T = temperature (°C orK)
323
U = superficial air velocity (m/s)
Umf=superficial air velocity at incipient fluidization (m/s)
Greek Letters
Fc = mass of carbon per unit volume of particle (kg/m 3 )
σ = fragmentation ratio or particle multiplication factor
σ1 = primary fragmentation ratio