Journal of Sustainable Bioenergy Systems, 2013, 3, 265-271
Published Online December 2013 (
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Fluidized Bed Superheated Steam Dryer for Bagasse:
Effects of Particle Size Distribution
Luz Stella Polanco1*, Vadim Kochergin2, Jose F. Alvarez3
1Audubon Sugar Institute, Louisiana State University AgCenter, Saint Gabriel, USA
2Amalgamated Research LLC, Boise, USA
3Sugar Cane Growers Cooperative of Florida, Belle Glade, USA
Email: *,,
Received July 6, 2013; revised August 5, 2013; accepted September 3, 2013
Copyright © 2013 Luz Stella Polanco et al. This is an open access article distributed under the Creative Commons Attribution Li-
cense, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
Fluidized bed superheated steam drying is one of the technologies successfully applied to drying pulp in the sugar beet
industry. It has the technological advantages of energy efficiency and safety (inert environment) required for use in
drying bagasse. A comparison of the particle size distribution of bagasse and beet pulp was evaluated in terms of flu-
idization. The size distribution of bagasse particles is from 2 to 10 times broader than the equivalent distribution of beet
pulp particles. The mean particle size of the bagasse is 1/3 of the mean size of the beet pulp. Fluidization tests proved
that bagasse fluidization is possible. It was found that beet pulp and bagasse particles clearly differ on shape and size
distribution which in turn will affect the design of the ancillary equipment and the fluidization systems if sugarcane
bagasse is to be dried using superheated steam technology.
Keywords: Biomass; Bagasse Drying; Particle Size; Fluidized Bed
1. Introduction
Feasibility studies of proposed biomass processing plants
for production of biofuels or biochemicals require serious
consideration of energy balances, especially steam gen-
eration. As in the cane sugar industry, biorefineries
processing grassy biomass materials, such as energy cane
and sweet sorghum, will extract juice as the first stage.
The residue left from this stage—bagasse, would be ex-
pected to contain about 50% moisture and can be used as
a fuel to satisfy power requirements of the biorefinery.
Bagasse in surplus of energy requirements can be avail-
able for further conversion to other products. Reduction
of bagasse moisture to a range between 10% - 40% in-
creases fiber preservation during storage, gives efficient
and stable operation to those boilers using it as a fuel,
lowers emission, and gives higher energy efficiency and
higher product quality for thermochemical conversion
technologies [1]. This moisture content cannot be at-
tained with conventional mechanical/milling methods.
Alternative technologies for bagasse drying deserve con-
sideration prior to biorefinery construction.
Physical properties of bagasse that are related to its
origin make it a difficult material to process. Both the
particle size distribution and the behavior of assemblies
of bagasse particles are important considerations for the
selection and design of equipment for feeding, collection,
burning, depithing, pneumatic transportation, separation
of dust, pelletizing and drying.
Bagasse drying has not been widely used because of
additional costs and fire hazards; however, it is justified
by plants with high energy demand. Flue gas dryers such
as the rotary and the most recently pneumatic and cyc-
lonic dryers have been the favored technologies for dry-
ing bagasse to a moisture content of 30% - 40% [2-4].
The fluid bed pressurized superheated steam dryer (Fig-
ure 1), which is successfully used to dry beet pulp, offers
a high evaporation capacity, reduction of emissions from
the dryer, reduction of fire and explosion hazards and the
possibility of heat recovery to increase the overall energy
efficiency of factory operations. These features and the
possibility of final moisture between 10% - 20% show
the advantage of a superheated steam dryer over conven-
tional bagasse dryers [3,4].
In a fluid bed dryer, the bagasse bed is expanded by the
drying medium (pressurized superheated steam) which is
*Corresponding author.
Figure 1. Pressurized superheated steam dryer and basic operation scheme showing the main components of the dryer such
as: inlet and outlet, fluidization cells, dust separation system, heat exchanger and fan [5,6].
injected upward. Steam and bagasse are transported to-
gether (like a single phase fluid) passing through several
fluidization cells to a collection point where the dried ba-
gasse particles are separated from the combined streams
of superheated steam and evaporated moisture. The re-
leased vapor still superheated, entrains fine particles (dust)
which are separated by internal cyclones [7]. Tempera-
ture, pressure and velocity of the gas as well as particle
size, density change (moisture content change), height
and bed void are parameters that affect the stability of the
fluid bed. Geometrical configuration and dimensions of
the fluidization chambers, recirculation of coarse parti-
cles, dust separation system and design of grid plates and
gas spargers are design details for the success of a dryer
on a large scale.
Particle size distribution affects handling, influences
the retention time and causes elutriation in the fluid bed
and at the cyclones [8]. There are several publications on
empirical correlations, based on particle size, for han-
dling properties and fluidization parameters for wet and
dry bagasse—in equilibrium with the relative air humid-
ity ~10% moisture content [9-14]. These empirical flu-
idization correlations are intended to establish a mini-
mum entrainment speed for pneumatic transport. The
correlations give only proximate information for the de-
sign of a fluid bed; however the performance of the fluid
bed cannot be predicted from these correlations. The
quantification of bagasse characteristics and comparison
with the materials that are successfully dried by super-
heated steam dryers (beet pulp) is important information
that is currently missing. Comparative analysis of parti-
cle size distribution between beet pulp and bagasse can
yield an insight for designinga pressurized superheated
steam dryer for bagasse.
Particle size distribution is a mathematical function
which describes the relative frequency of a given particle
size with respect to the whole sample [8]. Normal and
log-normal or Weibull distribution functions are used to
describe the relative frequency of a given particle size
[15]. Typically, the statistical parameters which describes
particle size distribution are mean, mode, standard devia-
tion, skewness (the symmetry or preferential spread to one
side of the average—tail), kurtosis (concentration relative
to the average), and several cumulative percentile values
(the size at which a determined percentage of particles
are larger or coarser) D10, D50, D90. Other relations are
used also to describe the distribution such as D90/D10,
D90-D10, D75/D25 and D75-D25; the span or width of the
distribution can be calculated from (D90-D10)/D50 [16].
For a sample with a wide particle size distribution,
sieve analysis is carried out for the coarse fraction while
for the finer fraction another type of size measurement is
required [8]. Sieve analysis has been the traditional siz-
ing technique applied to bagasse, but bagasse fibers on
sieve analysis can pass through the screen with longer
sieving times and these fibers can also interlock with
high volume of samples. Therefore, a standardized pro-
cedure is necessary that fixes both the sample volume
and the sieving time [12]. Particle size distribution of
bagasse is expected to vary from each factory due to dif-
ferent knifing, shredding and milling as well as sampling
techniques. Natural segregation of particles according to
size and particle density is common during storage [17].
Particle size distribution of bagasse is in some degree
reproducible when the feedstock is well prepared, such as
for a preparation index ~90 (following the International
Commission for Uniform Methods of Sugar Analy-
sis-ICUMSA procedure). The goal of this document is to
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compare the particle size distribution of sugarcane ba-
gasse with beet pulp relative to its implications for using
a fluidized bed superheated steam drying system.
2. Materials and Methods
2.1. Materials
The Amalgamated Sugar Company LLC (season 2009/10)
supplied two samples of dried beet pulp and several ba-
gasse samples from different locations and pretreatments
were taken during the 2010 sugarcane season in Louisi-
Enterprise factory bagasse piled in 2009 with one
year of outdoor storage (EPM09-P)—Cane Prepara-
tion: 2 sets of revolving knives, first with 30 blades at
600 rpm and the second with 70 blades at 750 rpm,
and one Tongaat shredder at 1200 rpm. Milling Tan-
dem of 24 rolls [18].
Enterprise factory diffuser bagasse piled in 2009 with
one year outdoor storage (EPD09-P)—Cane Prepara-
tion: one set of knives and one heavy duty shredder.
Two set of dewatering mills with 4 rolls each [18].
Saint Mary factory bagasse piled in 2009 with one
year outdoor storage (SMM09-P)—Cane Preparation:
2 sets of knives conventional rotation, first with 63
blades at 750 rpm and the second with 60 blades at
750 rpm [18].
LaFourche factory bagasse, piled in 2007 & 2009,
with 3 years and 1 year of outdoor storage (LFM07-P
& LFM09-P) and fresh bagasse taken from conveyor
in 2010 (LFM10-C)—Cane Preparation: One single
revolving knife set with 52 knives and a Fiberizer at
1200 rpm [18].
2.2. Methods
Bagasse samples (~5 kg) were carefully homogenized in
sample bags and subsamples of approximately 100 g
were taken and placed in disposable aluminum foil cups.
Wet samples were dried in a convection oven at 75˚C for
12 hours and then were left inside a hood for 24 hours, to
reach the moisture in equilibrium (~10%) with the rela-
tive humidity of the air in the laboratory. Bagasse mois-
ture was determined in a Sartorius Mark 3 Moisture
Analyzer before sieving. An absolute total weight loss
less than 2% the weight initial sample is considered ac-
ceptable [19].
Sieving tests were performed on a Retsch Sieving
Machine Type AS200, dividing each test in two consecu-
tive sections of 30 minutes (maximum number of sieves
is 6). The first section had 5 sequential sieves with open-
ings of 9.5, 4.0, 2.8, 2.0 and 1.4 mm. The samples that
passed through the smallest screen were further separated
in a round of sieving with 6 sequential sieves with open-
ings of 1.0, 0.85, 0.60, 0.425, 0.250 and 0.125 mm. Sie-
ving parameters were amplitude 2.5 mm/g, acceleration
10 g (98.1 m/s2) and intervals 10 sec.
The fraction of the bagasse particles with sizes below
125 µm (0.125 mm) were dispersed in water and ana-
lyzed in the CILAS 1180 Particle Size Analyzer. The
instrument measurement range goes from 0.04 to 2500
µm; the measurement technique is based on the laser
diffraction of the light source and in the wet mode, the
results can be given in number or in volume.
3. Results and Discussion
A qualitative comparison (Figures 2(a) and (b)) shows
clearly that the shape of the beet pulp particle is like a
flake and the small particles are almost spherical. The
shape of bagasse particles is more complex, some parti-
cles are just single strings with variable L/D, other parti-
cles are conglomerates of strings with rectangular shapes
(rind) and some small particles that are spherical (pith).
These different shapes are an important factor for the
design of a feeding and collection system of a dryer. Ba-
gasse fibers tend to entangle and choke, being a chal-
lenge for the design of the feeding valves (rotary valves)
Figure 2. Pictures of particles retained between sieves during sieve analysis. (a) Beet pulp and (b) sugarcane bagasse.
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to keep pressure and a uniform feed to the bed. It is also
important that rind and pith particles have different den-
sity and will behave differently during fluidization.
The particle size distribution in sugarcane bagasse de-
pends on cane preparation and number of mills used for
the juice extraction process. [2] states that the average
particle size of bagasse ranges between 1 - 5 mm but can
be as high as 25 mm when the cane preparation is poor.
Some differences were found for the bagasse samples
from different locations, pretreatments and sampling
points in the relative weight of material retained by the
sieves. The logarithmic graphs (Figures 3(a) and (b))
showed that the distributions are almost flat (low kurtosis)
without a defined bell shape (no normal distribution
The logarithmic graph (Figure 4) for the particle size
distribution of beet pulp is skewed (tail to the left) and
shows higher relative frequencies for the sieves with
openings larger than 2.0 mm. The shape of the distribu-
tion is approximately normal.
Data in Table 1 shows that the bagasse samples have a
very wide distribution (D90/D 10 ratio from 10 to 50 and
(D90-D10)/D50 span from 3 to 8) compared to the beet pulp
(D90/D10ratio ~5 and D90-D10)/D50 span ~2). The range of
Figure 3. Relative and cumulative-logarithmic graphs for particle size distribution of bagasse by sieving. Primary y axes: p3
[%] relative frequencybars, secondary y axes: Q3 [%] cumulative frequencylines and x axes: x [mm] particle size in mm.
(a) SMM09-Pred, EPM09-P blue and EPD09-P green. (b) LFM07-P red, LFM09-P blue and LFM10-C green.
Figure 4. Relative and cumulative-logarithmic graphs for particle size distribution of beet pulp. Primary y axes: p3 [%] rela-
tive frequencybars, secondary y axes: Q3 [%] cumulative frequencylines and x axes: x [mm] particle size in mm.
P09-10-S1 & S2 analysis by duplicate (red & blue). B
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values for the statistical parameters derived from the par-
ticle size distribution of bagasse can be due to differing
sugarcane processing for each location. The median D50
for these bagasse samples ranged from 0.7 to 1.3 mm
compared to beet pulp for which D50 was around 3.0 mm.
These differences indicate that lower velocities can be
expected for bagasse fluidization compared to beet pulp.
Mass percentage of bagasse with size below 0.125 mm
ranged from 4% to 10% while for beet pulp it was 0.6%.
Amount and particle size distribution of this fraction is
an important consideration for the design of the dust se-
paration system of a superheated steam dryer. Retained
particles will be recycled to the fluid bed as it is desirable
a very small amount of entrained particles leaving the
dryer with the vapors.
The particle fractions below 0.125 mm retained by the
sieve were analyzed using the laser diffraction instrument.
Figure 5 shows the particle size distribution results for
(a) beet pulp and (b) bagasse. In general, the distributions
show a sharp peak with a tail to the left for both the beet
pulp and the bagasse particles. It may be expected that
some of the particles from the left tail of the distribution
(<10 µm) will be entrained in any vapor leaving the
Table 2 summarizes the particle size distribution de-
termined by laser diffraction method. Although the sam-
ples presented to the instrument supposedly were below
125 μm (from sieve analysis) the irregularity on the
shape of both beet pulp and bagasse particles resulted in
a fraction of larger particles present in the samples (D90
percentiles were above 200 μm). Ten percent (D10) of the
particles that are expected to be handled by dust separa-
tion systems are below 13 μm for bagasse compared to
16 μm for beet pulp. Coefficient of variation (CV) and
span (D90-D10)/D50 are similar for all the samples.
Particle size distribution has a major impact on fluidi
Table 1. Particle size distribution of sugarcane bagasse and beet pulp. Sieving tests with samples at equilibrium moisture
Restch Sieving Machine (0.125 - 9.5 mm) Particles
D10 D
50 D
90 D
90/D10 (D90-D10)/D90 0.125 mm
Sample Description
mm mm mm Ratio Span mass%
Bagasse EPD09-P 0.16 0.90 5.36 34.1 5.8 8.6
Bagasse EPM09-P 0.12 0.71 4.90 40.8 6.7 10.4
Bagasse SMM09-P 0.12 0.78 6.36 52.6 8.1 10.4
Bagasse LFM07-P 0.24 0.95 6.63 28.0 6.7 4.6
Bagasse LFM09-P 0.30 1.28 3.92 13.1 2.8 4.0
Bagasse LFM10-C 0.21 0.80 4.45 20.8 5.3 4.0
Beet pulp BP09-10-S1 1.42 2.89 6.75 4.8 1.8 0.6
Beet pulp BP09-10-S2 1.55 3.00 6.86 4.4 1.8 0.5
(a) (b)
Figure 5. Particle size distributions by laser diffraction of the fraction below 0.125 mm. Primary y axes: Q3 [%] cumulative
frequency, secondary y axes: q3 [ln x] relative frequency and x axes: particle size in μm. (a) Beet Pulp BP09-10 analysis by
duplicate (red & blue) and (b) Bagasse EPM09-P red, EPD09-P green and SMM09-P blue.
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able 2. Particle size analysis by laser diffraction for beet T
pulp and bagasse particles below 0.125 mm.
CILAS Particle Size Analyzer (<0.125 mm)
D10 D
50 D
90 (D90-D10)/D90
µm µm µm span
EPM09-P 0.93 13 68 201 2.8
EPD09-P 12 44 118 0.87 2.4
SMM09-P 14 55 131 0.79 2.1
BP09-10-S1 16 69 204 0.92 2.7
BP09-10-S2 17 76 211 0.88 2.6
Noteumf pri
ation, plugging of the fluidization cells, feeding, product
and reduce the drag [9].
have more shape variation than beet
rticles can have a string like shape
Cane Growers Cooperative
eports from the fluidization
[1] S. Pang and Ay Biomass
for Bioenergyd Optimization for
: reported as a vole% oarticulate mateal.
collection and dust separation in the fluidized super-
heated steam dryer for bagasse. Shape of the bottom of
the cell and distribution of the air passages are the key
features that assure stability to the fluidized bed. Bagasse
particles are expected to behave differently compared to
beet pulp since the mean particle size is ~1 mm for ba-
gasse compared to ~3 mm for beet pulp, and the distribu-
tion span is ~6 for the bagasse particles compared to an
span ~2 for the beet pulp particles. A concern is the
higher percentage on weight of particles less than 125
μm for bagasse ~7% compared to beet pulp ~0.5%.
From tests performed for fluidized bed combus
BC), it was concluded that bagasse cannot fluidized
under normal conditions, rather it requires mixing with
other inert fluidizing materials [20]. However, EnerDry
(Denmark) in collaboration with Sugar Cane Growers
Cooperative (SCGC) of Florida built a fluidization unit
(a section of a full size dryer) and performed preliminary
fluidization tests for both beet pulp and bagasse. To
achieve good fluidization, improved gas distribution was
required. It was achieved through modification of the
geometry and open area of the perforated bottom. Finally,
air velocity of 1.8 m/sec was required to fluidize bagasse
at 50% moisture compared to an air velocity of 2 m/sec
to fluidize beet pulp at the same moisture content. The
required air velocity to fluidize bagasse ranged from 2.2
m/s for approximately 60% moisture content to 1 m/sec
for approximately 10% moisture content [21]. Consider-
able entrainment of large particles with a flat shape was
noticed, as then probably behaved as airfoils. [10] de-
scribed the shape of bagasse fibers as a rectangular prism
with flat parallel faces; and [9] stated that the aerody-
namic behavior of the bagasse particles depends on the
position of the particle respect to the air stream, thus,
when the area in front of the airflow is the maximum the
‘terminal velocity’ is minimum. The roughness of the
surface and the ends of the bagasse fibers will produce a
flutter movement which will increase the drag of the par-
ticle, therefore smoothness and roundness of the ends
4. Conclusion
will affect the orientation
Bagasse particles
particles. Bagasse pa
(fiber) or a spherical shape (pith) and they can be glued
together in a flat shape (rind), while beet particles have
mainly a flat rounded shape. The size distribution of ba-
gasse particles is wider than that of the beet pulp parti-
cles, with a coarse-to-fine (D90/D10) ratio that can be 2 to
10 times higher than that for the beet pulp particles
whose shape distribution is closer to a bell (Gaussian or
Normal distribution). The mean size of the beet particles
(~3 mm) is three times higher than the mean size of ba-
gasse particles (~1 mm). A stable fluid bed of bagasse
was achieved in tests performed in Denmark by Enerdry,
by modifying geometry and increasing the differential
pressure of the bottom air distributor. Air velocities of
1.8 m/s were required to fluidize bagasse at 50% mois-
ture content compared to air velocity of 2 m/s required
for beet pulp with the same moisture content. The per-
centage of the small particles fraction (<0.125 mm) was
as high as 10% for bagasse while for beet pulp the per-
centage of this fraction was just ~0.5%. According to
laser diffraction, the size distribution of this fraction of
particles is similar for both bagasse and beet were 0.13
mm, which was the mean particle size for bagasse and
0.16 mm, which was the particle size for beet pulp, re-
spectively. Besides the entrainment of the small particles,
larger particles with a flat shape were entrained during
the fluidization tests, which was a factor that has to be
considered for the design of the dust system of the dryer.
5. Acknowledgements
Special thanks to the Sugar
of Florida for sharing the r
tests performed by EnerDry Aps in Denmark. Also, spe-
cial thanks to Iryna Tishechkina for the results of particle
size analysis by Laser Diffraction of the small particle
fraction. This work was funded by a grant from the Ame-
rican Sugar Cane League and the USDA Agriculture and
Food Research Initiative Competitive Grant Award No.
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