Biomass peletization

1. ORIGINAL ARTICLE
Characterization and quality analysis of wood pellets: effect
of pelletization and torrefaction process variables on quality
of pellets
Asif Ali Siyal1,2
& Yang Liu1
& Xiao Mao3
& Babar Ali2
& Sakhawat Husaain4
& Jianjun Dai1
& Tianhao Zhang1
& Jie Fu1
&
Guangqing Liu1
Received: 12 September 2020 /Revised: 26 November 2020 /Accepted: 21 December 2020
# The Author(s), under exclusive licence to Springer-Verlag GmbH, DE part of Springer Nature 2021
Abstract
A lab-scale single-channel press was employed for producing pellets and the effect of pressure, die temperature, moisture content
(MC), particle size, and binding agent on pellet quality was investigated. Meanwhile, torrefaction subsequent to pelletization was
performed and the quality of pellets was evaluated under different torrefaction conditions. From the analysis of variance (at
p < 0.05), temperature (70–200 °C), pressure (70–160 MPa), and MC (5.5–28%) were found most significant factors for density,
strength, and compression energy of pellets. Particle density and strength of raw wood pellets were significantly affected by
temperature and pressure. The highest particle density and strength (1.307 g/cm3
, 11.8 N/mm2
) for particles of 0.25–0.5 mm and
(1.300 g/cm3
and 10.7 N/mm2
) for particles of 0.5–1.41 mm were found at a temperature of 200 °C (pressure ≥ 100 MPa and MC
5.5%), indicating that pelletization at a temperature of 200 °C was beneficial. The relaxed density of pellets remained the same as
of particle initial density after storage time of 2 weeks. The expansion ratio of the pellet was found lower. Use of synthetic resin as
a binder in the proportion of 1:9 was found optimum, the particle density increased from 1.19 to 1.24 g/cm3
(0.25–0.5 mm) and
from 1.17 to 1.22 g/cm3
(0.5–1.41 mm) and energy consumption reduced by 9.39% (0.25–0.5 mm) and 8.7% (0.5–1.41 mm).
Pellets made at a temperature of 200 °C were found water-resistive as compared to those made at other temperatures. The highest
lower heating value, i.e., 26.76 MJ/kg of torrefied pellets (TOPs) was achieved at 300 °C and 120 min. Torrefaction process
parameters adversely affected the particle density, volumetric energy density, strength, and durability of TOPs. The highest true
density (1.85 g/cm3
) and porosity (65 v %) for TOPs were achieved at 300 °C and 120 min, much higher than those of raw pellets.
Moisture uptake of TOPs at 300 °C was 2.0–2.8 wt.%, showing strong water-resistant ability. From the results of FTIR, O–H
bond was destroyed after torrefaction.
Keywords Pelletization . Torrefaction . Pellet quality
1 Introduction
Among all renewable energy sources, biomass is abundantly
available and has the great potential to replace fossil fuels,
producing a clean and carbon-neutral energy fuel (e.g., pellets)
to meet future energy demands in a sustainable manner [1, 2].
Raw biomass is marked by several drawbacks such as highly
dispersed distribution, asymmetrical shape, high moisture
content (MC), low bulk density, low heating value, poor
flowability, and potential risk of degradation during transpor-
tation and storage [3], which results in an intricate supply
chain and higher costs of storage, handling, and transportation
[4]. Pelletization is a well-known densification process for
compressing milled fine particles of biomass under the appli-
cation of temperature and pressure, resulting in more stable
* Jianjun Dai
daijianjun@hotmail.com
* Guangqing Liu
guangqingliu1@mail.buct.edu.cn
1
Biomass Energy and Environmental Engineering Research Center,
State Key Laboratory of Organic-Inorganic Composites, College of
Chemical Engineering, Beijing University of Chemical Technology,
15 Beisanhua East Road, Chaoyang District, Beijing 100029, China
2
Quaid-e-Awam University of Engineering Science and Technology,
Nawabshah, Sindh, Pakistan
3
Shanghai Boiler Works Ltd., 250 Huaning Road, Minhang District,
Shanghai 200245, China
4
Department of Mathematics, Quaid-e- Azam University,
Islamabad, Pakistan
https://doi.org/10.1007/s13399-020-01235-6
/ Published online: 7 January 2021
Biomass Conversion and Biorefinery (2021) 11:2201–2217

2. and uniform product. Fuel (e.g., pellet) quality is standardized
by pelletization, as pelletization technology enhances energy
density of pellet, decreases the pellet’s handling, storage, and
transportation costs, and improves the overall biomass quality,
stability, durability, and strength [5]. These properties stimu-
lated the rapid development of the pellet industry around the
world and have the potential to substitute coal in many appli-
cations [6–8]. In recent years, pellets are produced from var-
ious biomass materials such as woody biomass, herbaceous
biomass, and sewage sludge [9–12]. Among all these, woody
biomass sources are extensively used because of their avail-
ability as a renewable energy source [2].
Pellet quality is largely dependent on process variables
such biomass types and properties (e.g., MC and particle size),
pelletizer types and configurations (e.g., flat die or ring die
pellet press, die size, die speed), and binding agents and pro-
cess conditions (e.g., compaction pressure, temperature, and
compaction time) [13]. All these significantly affect the phys-
ical forces that bond or interlock mechanically between bio-
mass particles during densification. Furthermore, pellet qual-
ity depends on the effectiveness of the inter-particle bonds and
interaction between the feedstock and die surface. Due to the
application of relatively high pressures and temperatures, solid
bridges are developed by diffusion of molecules from one
particle to another at the points of contact [14]. Die tempera-
ture, pelletizing pressure, MC, and particle size of raw feed-
stock, all influence the binding phenomena. MC should be
adjusted based on pelletizing technology and the type of raw
material [15]. Finely ground particles (e.g., 0.25–1 mm) have
more surface area or contact area and usually facilitate better
binding [16]. It is essential to determine the optimum process
conditions for pellets of superior quality with minimum ener-
gy consumption.
Many studies have been performed to examine the influ-
ence of process variables on pellet density (e.g., particle or
bulk) and durability [9, 17–20], and strength [19, 21].
Densification of woody biomass (e.g., chips) and agricultural
biomass (grasses and straws) enhances the bulk density, i.e.,
600–800 kg/m3
and 40–200 kg/m3
[22], thus reduces the over-
all handling and transportation costs associated with biomass
processing [23] and dust emission during handling and trans-
portation. Densification of biomass produces solid fuel (e.g.,
pellet) with a highly dense and uniform structure. It improves
the physical (e.g., particle density), mechanical (e.g., strength
and durability) and reduces the moisture content (MC) (e.g.,
less than 10%) depending upon the MC of feedstock being
compressed in an environmentally friendly manner [24, 25].
For example, Nguyen et al. [21] performed the pelletization of
sugar maple trees and found the highest particle density of >
1107 kg/m3
and strength of > 62 N/mm2
for pellets at a tem-
perature of 125 °C and MC of > 8.1%, respectively. Pak Yiu
Lam et al. [26] reported that Douglas fir pellets made at
100 °C (pressure of 63–190 MPa) were found with the highest
range of particle density 1000–1230 kg/m3
and mechanical
strength, i.e., 3.53–6.43 MPa. Similarly, Lee et al. [27] for
(larch, beech, and Scot pine), Nielsen et al. [28] for (Norway
spruce), and Stelte et al. [29] for straws found that pelletization
enhances the durability using different process conditions.
Though pelletization enhances the pellet density for exam-
ple bulk density, i.e., 500–650 kg/m3
for biomass pellets and
lower the MC (7–10% w.b), pellet energy density is still very
low (7.8–10.5 GJ/m3
) due to higher oxygen content compared
to that of coal (1.75–1.86 times higher) for co-firing applica-
tions. Furthermore, wood pellets have the tendency to absorb
water from the atmosphere and their grindability is poor com-
pared to that of coal [30]. All these barriers have limited the
applications of pellets on industrial scales. Torrefaction after
pelletization can address these issues and produce pellets with
good characteristics (e.g., higher hydrophobicity and heating
value) [31–34]. Torrefaction is a thermal pre-treatment pro-
cess for biomass which usually involves heating of biomass at
a temperature of 200–300 °C in the absence of oxygen [35,
36]. Torrefaction is a well-established method to improve car-
bon content, heating value, and grindability of pellets and
improves the resistance to water absorption and bacterial
growth [37, 38]. By degradation of hemi-cellulose and remov-
al of volatile compounds [35, 39], the torrefaction technique
provides a fuel (e.g., pellets) with higher carbon content and
more characteristics resembling to coal [39]. Pellet quality can
be enhanced and the cost associated with storage and trans-
portation can be reduced [38, 40]. The heating value of com-
mercial wood pellets is reported as approximately 19 MJ/kg,
lower than 28–30 MJ/kg of coal [41].
Pellet quality indicators (e.g., density, hydrophobicity,
strength, and durability) are highly dependent on torrefaction
process conditions such as temperature and residence time.
Torrefaction subsequent to pelletization has rarely been re-
ported [31, 42–45]. Manouchehrinejad and Mani [31] carried
out research on torrefaction of wood pellets by employing
batch scale reactor at process conditions, i.e., temperature
(230, 250, 275, and 290 °C) and residence time 30 min.
Increasing torrefaction temperature from 230 to 290 °C result-
ed in enhancement of heating value and hydrophobicity of
pellets. However, in contrary to this, the pellet density, hard-
ness, and durability were decreased with increasing tempera-
ture. The volumetric energy density (VED) of torrefied pellets
(TOPs) remained the same until the torrefaction temperature
of 270 °C; above this temperature, it dropped drastically.
Torrefaction at a higher temperature (e.g., 300 °C) and resi-
dence time of 120 min resulted in the highest lower heating
value (~ 24 MJ/kg) of torrefied furfural residue pellets.
Moisture uptake of the corresponding pellet was only
1.4 wt.%, found at torrefaction temperature of 300 °C and
residence time of ≥ 30 min. Increasing torrefaction tempera-
ture from 200 to 300 °C and residence time from 15 to 30 min,
the VED of torrefied furfural residue pellets increased from
2202 Biomass Conv. Bioref. (2021) 11:2201–2217

3. 25.69 to 27.59 kJ/m3
. However, pellet density and strength
were adversely affected by severe torrefaction conditions
[42]. Brachi et al. [45] reported that the heating value of wood
pellets increased from 18.66 to 23.02 MJ/kg with increasing
torrefaction temperature. A slight decrease in bulk and appar-
ent pellet densities was observed at mild torrefaction condi-
tions due to the release of volatiles. VED decreased slightly by
2–3%.
This study was designed to systematically examine the in-
fluence of pelletization process variables on the quality param-
eters (e.g., particle, relaxed, and true density, pellet strength,
and water resistivity) and compression energy. Furthermore,
true and relaxed densities of wood pellets have rarely been
reported. Information pertaining to relaxed and true densities
is essential for the industrial design of a silo for pellet storage.
This is why the findings of the current study are expected to
provide very useful information about true density and relaxed
density.
The study also aimed to investigate the effects of the
torrefaction process variables (i.e., temperature and residence
time) on quality parameters (e.g., heating value, VED,
strength, particle density, durability, and hydrophobicity) of
TOPs. The results are expected to provide the optimum con-
ditions and useful information to produce pellets of higher
quality, which can be used as an alternate source of green
energy.
2 Materials and methods
2.1 Material preparation
Sawdust was obtained from a furniture plant located in the
suburb of Beijing. Prior to pelletization, sawdust was air-
dried at room temperature until its moisture reached to equi-
librium with ambient air. Biomass material was then sieved
for 15 min to obtain the desired particle size of 0.25–0.5 mm
and 0.5–1.41 mm. MC of raw material was adjusted to the
desired moisture levels (e.g., 5.5, 8%, 13%, 18, and 28%) by
mixing deionized water evenly with sawdust. Bulk density,
proximate, and ultimate analyses of raw sawdust were tabu-
lated in Table 1. The binding agent adopted in this study was
one type of synthetic resin made from waste cooking oil col-
lected from a local restaurant in Beijing.
2.2 Experimental setup and procedures
2.2.1 Pelletization
A computer-controlled lab-scale single-channel press (Model
LYWN-W50KN, Jinan lingyue, precision instrument Co.
Ltd., China) with a single die opening was employed for mak-
ing pellets. The cylindrical die of 12 mm inner diameter (ID)
and 130 mm long was first heated to the desired temperature.
After the temperature reached a steady-state, about 2–2.5 g
sample of sawdust was put into the cylinder and compressed
by the piston at a speed of 0.2 kN min−1
to the desired pres-
sure. A holding time of 300 s was employed at the desired
pressure for each pellet; then, the pellet was pressed out of the
cylinder by removing the bottom plate underneath the die. The
compression force and displacement data were recorded si-
multaneously for the complete cycle of compaction and ejec-
tion of pellets by MaxText control system connected to the
pelletizing unit. Table 2 shows the experimental settings for
pelletization. At each die temperature, i.e., 70, 100, 130, 160,
and 200 °C, the influence of compaction pressures (70, 100,
130, and 160 MPa) and MC of 5.5, 8, 13, 18, and 28%,
respectively, were investigated. At least 10 pellets were made
at each condition.
2.2.2 Torrefaction of pellets
Pellets made at a temperature of 130 °C, the pressure of
130 MPa, MC of 5.5%, and particle size of 0.25–0.5 mm were
selected for torrefaction. Sawdust pellets were stored for
1 week at room temperature in an airtight plastic bag for later
torrefaction. After reaching equilibrium with the environment,
the MC of raw pellets was measured as 2.5 wt.%. Table 2
Table 1 Properties of raw biomass
Analysis Sawdust
Ultimate analysisa
(wt.%)
C 48.66 ± 0.001
H 5.5 ± 0.001
N 3.19 ± 0.002
S 0.53 ± 0.004
Ob
42.12 ± 0
Lower heating value (MJ/kg) 17.36 ± 0.03
Proximate analysisc
(wt.%)
Volatile matter 75.4 ± 0.002
Fixed carbon 16.69 ± 0.001
Ash 3.06 ± 0.002
Moisture content 4.85 ± 0.003
Chemical analysis (wt.%)c
Ligninb
11.98 ± 0
Cellulose 50.17 ± 0.003
Hemicellulose 21.3 ± 0.002
Acid-insoluble ash 1.36 ± 0.005
Impurity 15.18 ± 0.003
Bulk density (kg/m3
)
0.25–0.5 (mm) 202 ± 0.005
0.5–1.41 (mm) 193 ± 0.003
abc
Air-dry basis; calculated by difference; oven-dry basis
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Biomass Conv. Bioref. (2021) 11:2201–2217

4. represents the torrefaction process settings. For every experi-
ment, one pellet was placed in the sample boat, which was slid
into the central section of the tube reactor (Model GSL-
1100X-S, Hefei Kejing material technology Co., Ltd.). The
heating rate was set at 10 °C/min until reaching the desired
torrefaction temperature of 200, 250, and 300 °C. The flow
rate of the carrier gas (N2) was 100 ml/min to provide an inert
environment. For each torrefaction temperature, torrefaction
reaction times of 15, 30, 45, 60, and 120 min were selected. At
least five replicates were made at each operating condition to
ensure repeatability. After torrefaction, the tubular reactor fur-
nace was turned off for cooling until the biomass pellets
reached to room temperature. The flow of nitrogen gas was
kept to continue until the temperature of the reactor reached
below 50 °C. After each experiment, TOPs were weighed,
sealed in airtight plastic bags, and stored at room temperature
for commencing later analysis. TOPs at 200 °C, 250 °C, and
300 °C were named as TOP-200, TOP-250, and TOP-300,
respectively.
2.3 Characterization of pellets
Proximate analyses were performed according to GB/T
28731–2012 (proximate analysis methods for solid biofuels).
An elemental analyzer (Vario EL cube) was used to analyze
the elemental analyses (C, H, N, and S) of samples, whereas
the O content was obtained by difference (i.e., O = 100-C-H-
N-S-ash). The chemical analyses (lignin, cellulose, hemicel-
lulose, acid-insoluble ash, and impurity) were determined by
an AMKOM 2000 fiber analyzer (AMKOM, USA) [46]. The
particle density of individual pellet in the current study was
determined according to previous literature [47].
Particle density ¼
mp
Vp
ð1Þ
Vp ¼
π
4
D2
L ð2Þ
where mp, Vp, D, and L are mass (g), volume (cm3
), diameter
(mm), and length (mm) of individual pellet, respectively.
Determination of mass, length, and diameter was repeated
after 2-week storage time to determine the relaxed density
(RD). An automatic density analyzer (Quantachrome,
Boyton Beach, FL, USA) was used to determine the true den-
sity (TD) of individual pellet and the porosity was calculated
in terms of particle density and TDs [48]. Force-displacement
data was noted during the compression and ejection of the
pellet. The total energy needed to produce pellets was the
sum of energy required for compressing biomass and energy
required for extruding the pellet from the die. Energy con-
sumption associated with compression and ejection of the in-
dividual pellet was determined by integrating the force-
displacement curve employing Eq. 3. SEC was the total ener-
gy divided by the mass of the pellet.
Ec ¼ ∫
x
0 F x
ð Þdx ð3Þ
The tensile strength of pellets was measured by a
computer-controlled testing machine according to ASTM
standard D-3967 [49]. A durability test was performed by
employing the methods of sieving the randomly selected num-
ber of pellets in a sieve of larger size of 2 mm and pellets were
vibrated for ~ 15 min consistent to the previous study [50].
This method was based on mass loss of pellets and initial mass
(m1) of pellets before test, and final mass (m2) of pellets after
the test was recorded, and durability was calculated by using
Eq. 4.
Durability ¼ 100
m2
m1
ð4Þ
The Water-resistance test was performed by following the
procedure adopted in previous literature [41, 51–53]. Raw
pellets were soaked in deionized water for more than
Table 2 Experimental settings of
process variables Pelletization process parameters
Temperature (°C) 70 100 130 160 200
Pressure (MPa) 70 100 130 160
Moisture content (%) 5.5 8 13 18 28
Particle size (mm) 0.25–0.5 0.5–1.41
Additive (SR/FR/S) 1:9 2:8
Torrefaction process parameters
Torrefaction process parameters Torrefaction temperature (°C) Residence time (min)
200 15
250 30
300 45
60
120
2204 Biomass Conv. Bioref. (2021) 11:2201–2217

5. 30 min. After that pellets were removed, excessive water was
drained by placing pellets on an absorbent paper. Pellets were
then exposed to a controlled environment under the conditions
such as room temperature (18-22 °C) and humidity (50–58%)
for 4 h. The initial (m1) and final mass (m2) of pellets were
recorded and the change in mass was expressed as;
Water resistance ¼
m2−m1
m1
100 ð5Þ
Similarly, for TOPs, pellets made at each torrefaction tem-
perature and residence time were selected and soaked into
deionized water in a beaker for 2 h; later, the pellet was taken
out and placed on an absorbing paper at room temperature for
4 h. Next, the pellet was dried in an oven at 105 °C for 24 h to
determine its MC. A bomb calorimeter (ZDHW-A9) was
employed to determine the heating value of pellets on a dry
basis. VED was determined from the product of lower heating
value (LHV) and particle density. XRD (X-Ray diffraction,
Ultima IV, Rigaku, Japan) and FTIR (Fourier transform infra-
red spectroscopy, Bruker vector 22, Bruker) tests were con-
ducted to analyze the crystalline phases and functional groups
of raw pellets and TOPs.
2.4 Statistical analyses
Analysis of variance (ANOVA) and Tukey’s multiple range
tests were performed and results of ANOVA were tabulated in
(Supplementary Table 1). The non-linear surface fitting was
performed to describe the influence of pressure and tempera-
ture on particle density of raw pellets as shown in Eq. 6. Non-
linear surface fitting was performed to simulate the relation-
ship between torrefaction process variables (i.e., temperature
and residence time) and hydrophobicity as shown in Eq. 7.
Y ¼
Zo þ Aox þ Boy þ Boy2
þ Boy3
1 þ A1x þ A2x2 þ A3x3 þ B1y þ B2y2
ð6Þ
Y ¼ a þ b*exp −T=c−R=d
ð Þ ð7Þ
where in Eq. 6, Y is response (dependent variable), Zo, Ao, Bo,
A1, A2, A3, B1, and B2 are model parameters, x and y are
independent variables, in Eq. 7, Y is the response (dependent
variable), a, b, c, and d are model constants, and T and R are
temperature and residence time, respectively.
3 Results and discussion
3.1 Particle, relaxed, and true densities
The pelletization process variables such as pressure and tem-
perature and biomass material variables (e.g., MC and particle
size) affect not only the quality parameters (e.g., density,
strength, and durability) but also the energy consumption of
pelletizing process.
Figure 1 demonstrates the nonlinear surface fitting of par-
ticle density versus different pressures, temperatures, and par-
ticle sizes. From the analysis of variance (ANOVA), pressure
and temperature were the most significant factor (p 0.05) to
the particle and relaxed densities (Supplementary Table 1).
Pressure is applied to promote cohesion and adhesion by
increasing molecular contact between adjacent molecules
[11]. Higher pressure generally makes solid particles closer
to each other, leading to higher pellet density. The particle
density of pellets made from particles of 0.25–0.5 mm in-
creased from 1.03 to 1.273 g/cm3
with pressure increasing
from 70 to 160 MPa at temperatures of 70–160 °C and MC
of 5.5% (Fig. 1). However, at 200 °C, the particle density of
pellets made from particle size of 0.25–0.5 mm increased with
increasing pressure from 70 to 130 MPa and then decreased at
pressure 130 MPa. For particle size of 0.5–1.41 mm, pellets
were not formed at a pressure of 70 MPa and temperature of
70 °C, and the extruded pellets were immediately crashed
mainly due to the weaker binding strength of particles.
Particle density increased as temperature increased, and at
temperature 200 °C, particle densities were higher than those
made at other temperatures (e.g., 70, 100, 130, 160 °C). The
highest particle density of 1.307 g/cm3
and 1.300 g/cm3
was
achieved at 200 °C for pellets made from 0.25–0.5 and 0.5–
1.41 mm, respectively (Fig. 1). At higher temperatures (e.g.,
200 °C) and pressure (e.g., 130 MPa), significant elastic and
plastic deformation occurred and the particles were forced to
squeeze into gaps and voids, increasing the contact area of
particles [19]. Moreover, protein denaturation and softening
of lignin enhanced the binding of particles, leading to a more
condensed structure and higher particle density. Vaporized
free water at higher temperature could easily diffuse into the
middle lamella to activate the lignin which acts as a binder for
increasing the binding ability of the pellets [29].
Non-linear surface fitting was performed as shown in Fig. 1
and the fitting model function (Eq. 6) was also plotted. The
adjusted R-squares for pellets made from both particle sizes
were 0.97, respectively. It showed that the proposed equation
had good adaptabilities to experimental results of pellets pre-
pared from 0.25–0.5 mm and 0.5–1.41 mm, respectively.
Table 3 represents the RD of pellets at different tempera-
tures. Pellets formed at a lower pressure (e.g., 70 MPa) and
temperature (e.g., 70 °C) have a higher tendency of expansion
after extrusion from the die and during storage. The term is
known as the spring back effect [54–56]. RDs of pellets were
examined after a storage time of 2 weeks. The highest de-
crease in RD of 0.55–0.77% and 0.47–0.63% was found at a
lower temperature (e.g., 70 °C). The binding ability of indi-
vidual particles within a pellet at relatively low pressure and
the temperature is weaker and the formed pellets had a pro-
pensity to expand, thereby resulted in a decrease of pellet
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Biomass Conv. Bioref. (2021) 11:2201–2217

6. density. Pellets made at a higher temperature (e.g., ≥ 130 °C)
remained dimensionally stable than those made at a lower
temperature (e.g., 70 and 100 °C). RDs of pellets were almost
similar to those of initial particle density. Information
pertaining to RD of pellets was a significant finding of this
study and would be helpful for the industrial design of a silo
for pellet storage [26].
TD or skeletal density is closely related to biomass types
and properties (e.g., texture and particle size), and operating
conditions for making these pellets. TD of pellets was evalu-
ated after 2 h as shown in Table 3. From the results of
ANOVA, the temperature was significant (p 0.05) to the
TD of pellets for both particle sizes. The maximum TD of ~
1.53 g/cm3
was achieved at a temperature of 130 °C and
pressure of 70 and 100 MPa as indicated in Table 3. The
evaporation of moisture from sawdust caused the shrinkage
of the micropores of the micro-fibrils at a temperature of ~
130 °C, resulting in higher TDs [26], while the highest TD of
pellets made from the particle size of 0.5–1.41 mm was ob-
tained at die temperature of 70 °C and pressure of 160 MPa.
As the temperature increased from 70 to 200 °C, TDs of
pellets significantly decreased and the lowest being achieved
at a temperature of 200 °C. TD of pellets made from both
particle sizes at 200 °C found very close to each other. At
higher temperature (e.g., 200 °C), the more condensed struc-
ture and softening of lignin may block the pores and inhibit
the intrusion of N2, leading to a reduction in TDs of pellets.
Figure 2 demonstrates the influence of MC on particle, RD,
and true density of pellets made from particles of 0.25–0.5 and
0.5–1.41 mm. From the results of one-way ANOVA, MC was
significant to the particle density, RD, and TD
(Supplementary Table 1). Particle density of pellets increased
with MCs increasing from 5.5 to 18% for both particle sizes
(at 130 °C and 100 MPa) and the highest particle densities of
1.243 and 1.23 g/cm3
for particles 0.25–0.5 and 0.5–1.41 mm
were obtained at MC of 18%.
The increase in particle density was mainly due to the lu-
bricating and binding effects of moisture, and lignin-moisture
interaction during compression, leading to a more compact
structure and escalation in binding forces between the
Fig. 1 Influence of temperature and pressure on particle density of pellets made from particle size of 0.25–0.5 mm and 0.5–1.41 mm
Table 3 Relaxed and true densities (g/cm3
) of wood pellets made at
different temperatures and pressures
a
DT/P 0.25–0.5 mm 0.5–1.41 mm
1
RD (g/cm3
) 2
TD (g/cm3
) 1
RD (g/cm3
) 2
TD (g/cm3
)
70/70 1.027 ± 0.01 1.512 ± 0.01 N/A
70/100 1.083 ± 0.01 1.51 ± 0.01 1.057 ± 0.01 1.493 ± 0.02
70/130 1.121 ± 0.01 1.489 ± 0.02 1.102 ± 0.02 1.498 ± 0.02
70/160 1.134 ± 0.01 1.458 ± 0.01 1.126 ± 0.01 1.516 ± 0.01
100/70 1.127 ± 0 1.525 ± 0.02 1.109 ± 0.01 1.474 ± 0.01
100/100 1.162 ± 0 1.522 ± 0.02 1.153 ± 0 1.471 ± 0.01
100/130 1.187 ± 0 1.498 ± 0.01 1.178 ± 0 1.473 ± 0.02
100/160 1.196 ± 0.01 1.462 ± 0.01 1.193 ± 0 1.46 ± 0.01
130/70 1.137 ± 0.01 1.527 ± 0.02 1.142 ± 0 1.419 ± 0
130/100 1.190 ± 0.01 1.528 ± 0.01 1.18 ± 0 1.427 ± 0.01
130/130 1.227 ± 0.01 1.5 ± 0.01 1.215 ± 0 1.435 ± 0.01
130/160 1.245 ± 0.01 1.473 ± 0.01 1.226 ± 0.01 1.438 ± 0
160/70 1.160 ± 0.01 1.474 ± 0.03 1.148 ± 0 1.416 ± 0.01
160/100 1.220 ± 0.01 1.48 ± 0.01 1.206 ± 0 1.43 ± 0.02
160/130 1.249 ± 0.01 1.473 ± 0.04 1.24 ± 0.01 1.486 ± 0.01
160/160 1.270 ± 0.01 1.473 ± 0.03 1.252 ± 0.01 1.449 ± 0.04
200/70 1.272 ± 0.01 1.392 ± 0.01 1.27 ± 0 1.389 ± 0.01
200/100 1.307 ± 0 1.406 ± 0 1.295 ± 0.01 1.398 ± 0.02
200/130 1.307 ± 0 1.384 ± 0.01 1.3 ± 0 1.374 ± 0.01
200/160 1.301 ± 0 1.389 ± 0.01 1.298 ± 0.01 1.389 ± 0.01
a12
DT/P die temperature (°C)/pressure (MPa), RD relaxed density, and
TD true density, N/A not available
2206 Biomass Conv. Bioref. (2021) 11:2201–2217

7. individual particles [57]. Moisture lowered the glass transition
temperature of lignin and increased the contact area of parti-
cles as it acted as film type binder with hydrogen bonding.
Furthermore, a thin film of water around the particles would
enhance bonds through capillary sorption between particles
[14]. Higher MC (e.g., 28%) could reduce pellet density due
mainly to the incompressibility of water and moisture stuck
within the particles preventing the complete release of natural
binders from the biomass particles [19]. Moreover, higher
MCs increased the degrees at which pellets relaxed after ejec-
tion from the die, which can considerably affect pellet quality
[58].
RD of pellets was considerably effected by MC. The
highest percentage of decrease (i.e., 1.43% and 1.16%) in
RD of pellets consistent to a particle size of 0.25–0.5 and
0.5–1.41 mm was observed at MC of 28% (Fig. 2). Due to
poor cohesion and adhesion of particles at relatively high MC
(e.g., 28%), the RD of pellets made from both particle sizes
decreased. Higher MC led to water absorption and expansion
of pellets during storage. It should be noted that the RD of
pellets made from MC of 5.5% at a temperature of 200 °C did
not change with time and remained almost the same for both
particle sizes.
TD of pellets (0.25–0.5 mm) slightly increased with in-
creasing MC from 5.5 to 8% then decreased at MC 13 and
18%. A similar trend was observed for particles 0.5–1.41. At
MC of 28%, a slight increase in TD of pellets made from both
particle sizes was observed (Fig. 2). The variations of TDs
were related to an increase in pellet volume, adsorption of
water and other substances, and development of pore volume
and structure [59].
Particle size has a significant influence on pelletization and
the quality of pellets is intrinsically associated with the particle
size, which significantly influences the compression, contact
between the adjacent particles, flowability, and friction in the
pelletizer die. Two particle sizes (i.e., 0.25–0.5 and 0.5–
1.41 mm) were selected for the fabrication of pellets. Pellets
made from small particles of 0.25–0.5 mm achieved higher
densities (particle, relaxed, and true) compared to particles of
0.5–1.41 mm. Small particles of sawdust provided larger spe-
cific surface area and had the tendency to fill in empty spaces
and voids by rearrangement of particles during compression,
resulted in better binding of particles and hence denser and
stronger pellets. At higher temperature (e.g., 200 °C), particle
density of pellets made from both particles was very close to
each other, indicating that particle size was not significant to a
higher temperature.
3.2 Expansion ratio and compression ratio
Expansion ratio (ER) in biomass pelletization is considered an
important factor, as it helps us to understand the diametrical
expansion of extruded biomass pellets. Lower ER is desirable
as expanded pellets usually have higher porosity, lower pellet
density, and weaker strength, which is detrimental to storage
and transport of pellets. Supplementary Table 2 shows the ER
at different applied pressures, temperature, and MCs right af-
ter extrusion and storage time of 2 weeks. Process parameter
such as pressure, temperature, and MC typically affect the ER
during storage [20, 60]. ER of pellets formed particularly at
higher temperature (200 °C) was found lower compared to
those pellets made at other temperatures, indicating higher
quality of pellets.
The quality of pellets is also affected by compression ratio
(CR), which is the ratio of volume before pressure applied to
the pellet volume. CR was dependent on the applied pressure,
temperature, and MC as shown in Supplementary Table 2. It
showed that higher temperature corresponded to larger ER
and CR, and CR increased with increasing pressure. ER was
slightly affected by pressure at low temperature (e.g., 70 °C).
However, ER remained almost the same particularly at high
temperature (e.g., 160 °C), indicating that pressure influence
was not significant at higher temperature. CR increased as MC
increased from 5 to 18%, reaching maximum (i.e., 6.687) at
MC of 18%, then decreased as MC further increased.
However, MC did not demonstrate any significant influence
on ER. The highest CR of 7.098 and 7.032 was found at a
temperature of 200 °C (pressure of 100 MPa and MC 18%) for
5 10 15 20 25 30
1.17
1.20
1.23
1.26
1.29
Particle density (g/cm3
)
Relaxed density (g/cm3
)
True density (g/cm3
)
Moisture content (%)
0.25-0.5 mm
1.17
1.20
1.23
1.26
1.29
1.32
1.38
1.44
1.50
5 10 15 20 25 30
1.17
1.20
1.23
1.26
1.29 Particle density (g/cm3
)
Relaxed density (g/cm3
)
True density (g/cm3
)
Moisture content (%)
0.5-1.41 mm
1.17
1.20
1.23
1.26
1.29
1.36
1.40
1.44
1.48
Fig. 2 Influence of MC on the
particle, relaxed, and true
densities of pellets made from
particle size of 0.25–0.5 mm and
0.5–1.41 mm
2207
Biomass Conv. Bioref. (2021) 11:2201–2217

8. pellets made from particles of 0.25–0.5 mm and 0.5–1.41 mm,
respectively. The temperature in comparison to pressure was
found a more dominant factor, greatly affected the CR and
ER. The incompressibility of water and incomplete release
of natural binders from the biomass may be responsible for
lower CR at higher MC (e.g., 28%) [19]. ER of pellets after
extrusion and 2 weeks remained almost equal to 1 which in-
dicated the higher quality of pellets.
3.3 Strength of pellets
Mechanical property (e.g., strength) of pellet is the key quality
parameter, reflecting the resistance to deformation and break-
age of individual pellet during handling, transport, and storage
[61]. The strength of the pellets is presumably influenced by
feedstocks characteristics, MC, particle size, and process pa-
rameters (e.g., pressure and temperature). Figure 3a–c
represents the strength of pellets at different applied pressures,
temperatures, and MCs. In general, the strength of pellets
increased with increasing pressure. Higher pressure lowers
the porosity and increased the bonding area, resulting in
strong pellets [62]. However, pellet strength was also associ-
ated with temperature. At higher temperature (e.g., 160,
200 °C), pellet strength decreased as pressure increased from
130 to 160 MPa and 100 to 160 MPa respectively for both
particle sizes, probably due to larger water loss rate and re-
duced interactions between water and binding agents (e.g.,
lignin) during compression. As temperature further increased
from 160 °C to 200 °C, pellet strength significantly increased
up to 10 N/mm2
, depending on different pressure, MC, and
particle sizes (Fig. 3a–b). Higher pellet strength could be the
result of strong binding mechanisms of adjacent particles at
higher temperature, whereas lower strength at the lower tem-
perature (e.g., 70 °C) was mainly due to weaker bonding be-
tween the particles.
Small particles (0.25–0.5 mm) usually achieved greater
pellet strength in comparison with larger particles (e.g., 0.5–
1.41 mm) as indicated in Fig. 3a. The strengths of 11.8 N/mm2
and 10.7 N/mm2
were achieved at a temperature of 200 °C
(pressure of 100 MPa and MC of 5.5%) for pellets made from
particles of 0.25–0.5 and 0.5–1.41 mm, respectively. These
high strengths were 50–56% and 44–53.5% higher than those
of pellets made at a temperature of 160 °C (pressure of 70–
160 MPa and MC of 5.5%), respectively, as indicated in Fig.
3a–b. Small particles achieved a larger surface area, larger
contact areas, and better binding characteristics, leading to
stronger pellets.
It seemed that MC of 18% corresponded to higher pellet
strength depending on temperature, pressure, and particle
sizes as shown in Fig. 3c.
3.4 Energy consumption
One of the crucial factors in the biomass pellet industry is
energy consumption and associated costs with higher energy
consumption. From the results of ANOVA, pressure, temper-
ature, and MCs were significant (p 0.05) to SEC. Figure 4a–
c shows the SEC of pellets at different applied pressure, tem-
perature, and MCs. For MC of 5.5%, SEC was highly corre-
lated with pressure and increased from 21.04 to 33.4 J/g (for
particles 0.25–0.5 mm) with increasing pressure from 70 to
160 MPa (Fig. 4a) and from 27.1 to 34.7 J/g (for particles 0.5–
1.41 mm) (Fig. 4b) with pressure increasing from 100 to
160 MPa at a temperature of 70 °C.
Energy consumption of compressing sawdust was signif-
icantly affected by die temperature. In the current study,
SEC of pellets made at a temperature of 70 °C and pressure
of 160 MPa was found higher, indicating that natural bind-
ing agent (e.g., lignin) present in sawdust could not be soft-
ened at this lower temperature. As the temperature increased
60 80 100 120 140 160 180 200
2
4
6
8
10
12
14
70 MPa
100 MPa
130 MPa
160 MPa
Strength
(N/mm
2
)
Die temperature (o
C)
(a) 0.25-0.5 mm
60 80 100 120 140 160 180 200
2
4
6
8
10
12
Die temperature (o
C)
70 MPa
100 MPa
130 MPa
160 MPa
Strength
(N/mm
2
)
(b) 0.5-1.41 mm
5 10 15 20 25 30
1.5
3.0
4.5
6.0
7.5 0.25-0.5 mm
0.5-1.41 mm
h
t
g
n
e
r
t
S
(
N/mm
2
)
Moisture content (%)
(c)
Fig. 3 Influence of temperature, pressure, and MC on strength of pellets
made from the particle size of 0.25–0.5 mm and 0.5–1.41 mm
2208 Biomass Conv. Bioref. (2021) 11:2201–2217

9. to 100–160 °C, SEC of pellets decreased and was found
lower than those made at a temperature of 70 °C and the
same pressures, indicating higher temperature resulted in
the decrease of pelletizing pressure, consistent with previ-
ous studies from Tumuluru [9] and Stelte et al. [10]. It
looked like a die temperature of 100 °C achieved lower
SEC (23.11 J/g) for particles of 0.25–0.5 mm and (24.24 J/
g) for particles of 0.5–1.41 mm in comparison with other
temperatures adopted (70, 130, and 160 °C). The vaporiza-
tion of water during heating was a primary reason in
achieving the low SEC as water acted as a lubricating agent
during compression at 100 °C, while faster moisture loss
rate during compaction at higher temperatures (e.g., 130–
160 °C) and ineffective binding properties of lignin at a
lower temperature (e.g., 70 °C) increased SEC of pellets.
SEC of pellets made from both particle sizes at a tempera-
ture of 130 °C and 160 °C in the pressure range of 70–
160 MPa were close to each other (Fig. 4a–b).
SEC of pellets was found lower at higher temperature (e.g.,
200 °C) and pressure of 70–160 MPa than those made at other
60 80 100 120 140 160 180 200
5
10
15
20
25
30
35
40
45
70 MPa
100 MPa
130 MPa
160 MPa
)
g
/
J
(
n
o
i
t
p
m
u
s
n
o
c
y
g
r
e
n
E
Temperature (o
C)
(a) 0.25-0.5 mm
60 80 100 120 140 160 180 200
5
10
15
20
25
30
35
40
45
(b) 0.5-1.41 mm 70 MPa
100 MPa
130 MPa
160 MPa
Energy
consumption
(J/g)
Temperature (o
C)
5 10 15 20 25 30
20
22
24
26
28
)
g
/
J
(
n
o
i
t
p
m
u
s
n
o
c
y
g
r
e
n
E
Moisture content (%)
0.25-0.5 mm
0.5-1.41 mm
(c)
Fig. 4 Influence of temperature,
pressure, and MC on energy
consumption of pellets made from
a 0.25–0.5 mm, b 0.5–1.41 mm, c
both
2209
Biomass Conv. Bioref. (2021) 11:2201–2217

10. temperatures (Fig. 4a–b). Sawdust has higher hemicellulose
content and thermal decomposition of hemicellulose promot-
ed pore formation of small particles, making sawdust easy to
compress [63] and decreasing SECs of pellets. Furthermore,
the increasing temperature decreased the friction as depicted
by Nielsen et al. [64] depending upon the types of biomass.
Infrared spectra of the pellet surface produced at higher tem-
perature showed hydrophobic extractives, which might have
acted as a lubricant, reducing the friction between biomass
particles and between biomass and an inner surface of the
die channel. Softening of lignin during the heating process
to a relatively higher temperature (e.g., 200 °C) may also
contribute to low SEC.
SEC of pellets decreased from 25.44 to 20.5 J/g (0.25–
0.5 mm) and 26.24 to 20.22 J/g (0.5–1.41 mm) with increas-
ing MC from 5.5 to 28% (Fig. 4c). Higher MC decreased SEC
by bonding sawdust particles together and sawdust easily
compressed, reducing friction between sawdust particles.
Higher MC (e.g.,18–28%) was generally beneficial to reduce
SEC due mainly to lubricating effects of water and interac-
tions between water and binding agents (e.g., lignin) at certain
temperature and pressure (e.g., 130 °C and 100 MPa).
However, low SECs were observed at higher temperature
(e.g., 200 °C) and pressure 70–160 MPa for both particle
sizes.
In fact, particle sizes did not considerably contribute to
SEC of pellets. SEC of pellets made from 0.5–1.41 mm was
slightly higher than those from particle size of 0.25–0.5 mm.
The relatively large particles (e.g., 0.5–1.41 mm) increased
mechanical resistance for compression, reduced contact areas
of particles, decreased effects of lubricants (e.g., water) and
binding agents (e.g., lignin), enhanced moisture evaporation
rate due to the large void fraction in comparison with that of
small particles (e.g., 0.25–0.5 mm), thus required more energy
to compress.
3.5 Use of binder
Additional binders are used to enhance the quality of the pellet
and to reduce the energy consumption of the pelletizing pro-
cess. The synthetic resin (SR) made from waste cooking oil
was used as a binder at mass ratios (synthetic resin/sawdust,
i.e., SR/S) of 1:9 and 2:8. Using SR/S of 1:9, particle density
of pellets increased from 1.19 to 1.24 g/cm3
(0.25–0.5 mm)
and from 1.17 to 1.22 g/cm3
(0.5–1.41 mm), respectively. At
SR/S proportion of 2:8, particle densities remained quite sim-
ilar to those obtained from SR/S of 1:9 (Table 4). It means SR
as a binder increases the particle density of pellets at a certain
mass ratio (e.g., SR/S = 1:9). RD remained almost the same as
particle density. The addition of a binder decreased the TD of
pellets. The use of the binder slightly increased the strength of
pellets (Table 4). At SR/S proportion of 1:9, the strength of
pellets made from both particles was higher than those obtain-
ed at SR/S proportion of 2:8.
For SR/S of 1:9 and 2:8, SEC of pellet made from particles
of 0.25–0.5 mm decreased by 9.39%, and 6.8%, respectively.
Correspondingly, SEC of pellets made from particles of 0.5–
1.41 mm decreased by 8.7% and 7.5%. Hence, as the binding
and lubricating agent, SR reduced the friction between bio-
mass particles and between particles and inner surface of die
during compression, which ultimately decreased SEC of the
pelletizing process.
3.6 Water resistivity
Raw biomass has a tendency to absorb moisture from the
atmosphere and high moisture content influences the growth
of fungal, resulting in the rot of biomass with time. Pellets
should be water repellent and biologically impervious for bet-
ter handling during storage and transportation [65]. In this
regard, pelletization was performed at a higher temperature
(e.g., 200 °C) and MC of 5.5%. Results of the water-
resistance test were shown in Table 5. Pellets made at a tem-
perature of 200 °C were found water-resistive as compared to
those made at a temperature of 130–160 °C, which immedi-
ately disintegrated as immersed in water (Fig. 5). The highest
percentage of resistance (95.3%) to water penetration was
found for pellets made from particles of 0.25–.0.5 mm at
MC of 5.5%, which could be the result of lower porosity
and higher density of pellets for small particles compared to
relatively large particles. Pellets produced at a temperature of
200 °C stayed intact in water without disintegration for more
than 30 min. Hence, it is possible to enhance the resistivity of
pellets to water penetration by performing pelletization at
higher temperature (e.g., 200 °C).
Table 4 Effect of binder on
quality parameters and energy
consumption of pellets
SR/
SDa
Particle size Particle density
(g/cm3
)
Relaxed density
(g/cm3
)
True density
(g/cm3
)
Strength
(N/mm2
)
SEC (J/g)
1:9 0.25–0.5 mm 1.24 ± 0.02 1.238 ± 0.02 1.391 ± 0.01 3.952 ± 0.03 23.25 ± 0.58
0.5–1.41 mm 1.219 ± 0.02 1.239 ± 0.01 1.376 ± 0.01 3.452 ± 0.04 23.38 ± 0.34
2:8 0.25–0.5 mm 1.239 ± 0.01 1.220 ± 0.01 1.392 ± 0.01 3.914 ± 0.02 23.8 ± 0.4
0.5–1.41 mm 1.220 ± 0.02 1.218 ± 0.02 1.371 ± 0.01 3.436 ± 0.01 23.66 ± 0.48
a
SR/SD synthetic resin/sawdust (at die temperature of 130 °C and pressure of 100 MPa)
2210 Biomass Conv. Bioref. (2021) 11:2201–2217

11. 3.7 Characteristics of TOPs
LHV of raw pellets was found to be 18.37 MJ/kg, respective-
ly. With increasing torrefaction temperature from 200 °C to
300 °C and residence time from 30 to 120 min, the LHVs of
TOPs increased from 18.75 to 26.76 MJ/kg (Fig. 6a).
Vaporization of water and elimination of oxygen during
torrefaction enhanced the LHVs of TOPs consistent with
[66]. LHVs of TOPs first increased, and then remained unaf-
fected at higher residence time (i.e., 45 min) although there
were some variations (Fig. 6a). The heating value of TOPs in
the current study was found higher and comparable to the
heating values, i.e., 18.72 MJ/kg for torrefied wheat straw
pellets [67], 21.8 MJ/kg for torrefied oat hulls pellets [43],
and 24.34 MJ/kg for torrefied scot pellets [68]. Further
LHVs were found similar to coal (i.e., 25–30 MJ/kg) [69].
Improved LHVs showed a higher quality of TOPs; the formed
pellets have the potential to substitute coal or mixed with coal
in various applications.
VED of TOPs decreased significantly from 20.81 to
16.96 kJ/m3
as shown in Fig. 6b with increasing temperature,
lower than raw pellets (21.1 kJ/m3
). After torrefaction, the loss
of hemicellulose present in pellets and retention of fibrous
skeleton made the volume reduction much less than energy
loss, leading to reduction of VED.
Biomass pelletization improves the quality of pellets (e.g.,
strength and particle or bulk density) and the torrefaction pro-
cess further improves the other quality parameters (e.g., hy-
drophobicity, heating value, grindability, and VED) of pellets.
Untreated pellets became loose and disintegrated after 2 h of
soaking in deionized water. Conversely, TOPs retained their
original regular shape though there were minor elongation and
swelling caused by water immersion (Fig. 6c). After 2 h
soaking, TOP-200 became loose and wet and their length
and volume increased noticeably. Pellets were found dimen-
sionally stable and no changes in the appearance of TOP-250
and TOP-300 were observed. Moisture uptake (MU) of TOPs
made at different torrefaction conditions was measured to de-
termine the hydrophobicity of TOPs. Non-linear surface
fitting was performed as shown in Fig. 6d and fitting model
function (Eq. 7) was also plotted.
MU of TOPs decreased with increasing temperature from
200 °C to 300 °C (Fig. 6d). With increasing residence time,
the MU of TOPs did not change considerably at the same
temperature. For TOP-200, the MU of 35.60–46.70 wt.%
was higher than those of TOP-250 (i.e., 5.40–7.20 wt.%)
and TOP-300 (i.e., 2.0–2.80 wt.%), and MU of TOP-200
was lower than that of raw pellets (i.e., 74.0 wt.%). The
torrefaction process enhanced the hydrophobicity of pellets.
The destruction of hydroxyl groups in the pellets during
torrefaction obstructed the formation of hydrogen bonds,
hence eliminated or reduced the hygroscopic nature of the
pellet [70]. Torrefaction subsequent to pelletization was ben-
eficial as it did not destroy the compacted and smooth outer
layer of the pellets; hence, the corresponding TOPs had capa-
bility to resist water from being absorbed [41].
Torrefaction subsequent to pelletization can improve pellet
characteristics, advantageous for transport and storage with
improved safety and environmental friendliness. Based on
Eq. 7, the fitting equation was established as follows:
MU TSP
ð Þ ¼ 2:40 þ 400000*exp
−T=21:71
þ R= 3:96E þ 143
ð Þ ð8Þ
The proposed model equation was well fitted to the exper-
imental results with adjusted R-squares of 0.97.
Physical (e.g., particle density) and mechanical (e.g.,
strength, durability) characteristics of TOPs were largely de-
pendent on torrefaction conditions. Particle density decreased
from 1.10 g/cm3
(at 200 °C, 15 min) to 0.63 g/cm3
(at 300 °C,
120 min) as shown in Table 6, much lower than raw pellets
Table 5 Water resistivity of raw
pellets Process settings Particle size (mm) m1 (g) m2 (g) % of change in mass
200/100/5.5 0.25–0.5 2.5118 2.6312 4.7
0.5–1.41 2.5266 2.9266 15.8
Fig. 5 Water immersion test of
pellets made from a 0.25–
0.5 mm, b 0.5–1.41 mm
2211
Biomass Conv. Bioref. (2021) 11:2201–2217

12. (1.23 g/cm3
). Strength is considered one of the most important
quality parameters; torrefaction temperature was the most
dominant factor compared to a residence and affected the
strength largely. The strength of TOPs decreased from
2.23 N/mm2
(at 200 °C, 15 min) to 0.47 N/mm2
(at 300 °C,
120 min). Moreover, the strength of TOPs was found lower
than those of raw pellets (i.e., 3.23 N/mm2
). During the
torrefaction process, natural binding agents in biomass were,
at least partly, destroyed, and hence caused the decrease of
strength of TOPs [71]. Though the torrefaction process
lowered the strength and particle density of TOPs, pellets
remained in their original shape, consistent with the previous
study [41].
The durability of pellets is defined as the ability of the
pellet to endure destructive loads and forces, during handling,
transportation, storage or during feeding into specific unit op-
erations (e.g., combustors) [72–74]. The durability of raw pel-
lets and TOPs was determined for randomly selected pellets
(e.g., pellets made at a temperature of 130 °C, the pressure of
130 MPa, MC of 5.5%, and torrefaction temperature 200–
300 °C, and residence time 30 min) as indicated in Table 6.
The durability of pellets decreased (from 94.22 to 75.88%)
with increasing torrefaction temperature from 200 to 300 °C
lower than those of raw pellets (97.56%). The decrease in
durability could be due to the loss of more volatiles and higher
thermal degradation of chemical constituents particularly at
higher torrefaction temperature (e.g., 300 °C). Furthermore,
porosity development by severe thermal treatment resulted
in a decrease of durability. TOPs did not meet the requirement
of ISO/TS 17225-8 ( 95%) [75]; hence, the formed TOPs
25.93
19.29
25.10
24.27
23.44
22.61
21.78
20.12
20.95
25.93
19.29
25.10
24.27
23.44
22.61
21.78
20.12
20.95
25.93
19.29
25.10
24.27
23.44
22.61
21.78
20.12
20.95
25.93
19.29
25.10
24.27
23.44
22.61
21.78
20.12
20.95
25.93
19.29
25.10
24.27
23.44
22.61
21.78
20.12
20.95
25.93
19.29
25.10
24.27
23.44
22.61
21.78
20.12
20.95
25.93
19.29
25.10
24.27
23.44
22.61
21.78
20.12
20.95
25.93
19.29
25.10
24.27
23.44
22.61
21.78
20.12
20.95
25.93
19.29
25.10
24.27
23.44
22.61
21.78
20.12
20.95
25.93
19.29
25.10
24.27
23.44
22.61
21.78
20.12
20.95
25.93
19.29
25.10
24.27
23.44
22.61
21.78
20.12
20.95
25.93
19.29
25.10
24.27
23.44
22.61
21.78
20.12
20.95
25.93
19.29
25.10
24.27
23.44
22.61
21.78
20.12
20.95
25.93
19.29
25.10
24.27
23.44
22.61
21.78
20.12
20.95
25.93
19.29
25.10
24.27
23.44
22.61
21.78
20.12
20.95
25.93
19.29
25.10
24.27
23.44
22.61
21.78
20.12
20.95
25.93
19.29
25.10
24.27
23.44
22.61
21.78
20.12
20.95
25.93
19.29
25.10
24.27
23.44
22.61
21.78
20.12
20.95
25.93
19.29
25.10
24.27
23.44
22.61
21.78
20.12
20.95
200 220 240 260 280 300
20
40
60
80
100
120
26.76*
25.04*
20.15*
19.55*
19.48*
Residence
time
(min)
Temperature ( )
(a)
18.75*
17 93
20.34
20.34
19.86
17.44
19.37
18.89
17.93
18.41
17 93
20.34
20.34
19.86
17.44
19.37
18.89
17.93
18.41
17 93
20.34
20.34
19.86
17.44
19.37
18.89
17.93
18.41
17 93
20.34
20.34
19.86
17.44
19.37
18.89
17.93
18.41
17 93
20.34
20.34
19.86
17.44
19.37
18.89
17.93
18.41
17 93
20.34
20.34
19.86
17.44
19.37
18.89
17.93
18.41
17 93
20.34
20.34
19.86
17.44
19.37
18.89
17.93
18.41
17 93
20.34
20.34
19.86
17.44
19.37
18.89
17.93
18.41
17 93
20.34
20.34
19.86
17.44
19.37
18.89
17.93
18.41
17 93
20.34
20.34
19.86
17.44
19.37
18.89
17.93
18.41
17 93
20.34
20.34
19.86
17.44
19.37
18.89
17.93
18.41
17 93
20.34
20.34
19.86
17.44
19.37
18.89
17.93
18.41
17 93
20.34
20.34
19.86
17.44
19.37
18.89
17.93
18.41
17 93
20.34
20.34
19.86
17.44
19.37
18.89
17.93
18.41
17 93
20.34
20.34
19.86
17.44
19.37
18.89
17.93
18.41
17 93
20.34
20.34
19.86
17.44
19.37
18.89
17.93
18.41
17 93
20.34
20.34
19.86
17.44
19.37
18.89
17.93
18.41
17 93
20.34
20.34
19.86
17.44
19.37
18.89
17.93
18.41
17 93
20.34
20.34
19.86
17.44
19.37
18.89
17.93
18.41
200 220 240 260 280 300
20
40
60
80
100
120
)
n
i
m
(
e
m
i
t
e
c
n
e
d
i
s
e
R
Temperarure (o
C)
(b)
21.03*
20.80* 17.79*
17.53* 16.96*
17.59*
(c)
Fig. 6 a LHV, b volumetric
energy density of TOPs, c TOPs
made from 02.5–0.5 mm soaked
in deionized water for 2 h, and d
MU of TOPs after 4 h air-drying
under different conditions
2212 Biomass Conv. Bioref. (2021) 11:2201–2217

13. may possibly cause the release of dust particles during han-
dling and transportation. One way to deal with this issue is to
develop a pelletization and torrefaction unit near to power
plant site or near to pellet consumer’s site. The durability of
pellets can be improved by employing a binder either before
torrefaction or after torrefaction.
The porosity of TOPs varied from 22 to 65 v%, much
higher than raw pellets (i.e., 16 v%), due mainly to
volatilization of hemicellulosic components and development
of pore structure during torrefaction [41]. The TDs of TOP-
200 and TOP-250 were found lower than those of raw pellets,
while TDs of TOP-300 were higher than those of raw pellets
with the maximum TD of 1.85 g/cm3
was achieved at this
temperature. Thermal decomposition of hemicellulose at 200
and 250 °C resulted in the release of some light volatile com-
ponents and increased pore volumes of TOPs [63]. At 300 °C,
larger loss of volatiles and more severe devolatilization led to
shrinkage of torrefied matrices and collapse of void structure
to form highly carbonaceous materials, resulting in higher
TDs [63].
3.8 XRD analysis of TOPs
For analysis of crystal plane and crystallinity of cellulose in
TOPs, XRD test was conducted and results are shown in Fig.
7a. According to previous literature [76], crystallinity of cel-
lulose could be determined by using the following formula
(9).
CrI ¼ I 002−I am
ð Þ=I 002 100% ð9Þ
where CrI, I002, and Iam are defined as crystallinity, the inten-
sity of the 002 diffraction crystal plane (at 2θ = 22o
), and the
intensity of scattering peaks (at 2θ = 18o
). In Fig. 7a, 2θ =
34.5o
, 22o
resembles to 002 and 004 crystal planes of type I
cellulose, respectively. The crystal plane corresponding to
2θ = 16o
was the composite crystal planes of 101 and 10ī
crystal planes of type I cellulose.
The crystallinity of cellulose in raw, TOP-200, TOP-250,
and TOP-300 was 44.39%, 50.44%, 47.42%, and 28.62%,
respectively (Fig. 7a). The crystallinity of cellulose in TOPs
increased first and then decreased with increasing torrefaction
temperature. Thermal decomposition of hemicellulose result-
ed in an increase of crystallinity of cellulose in TOPs at
200 °C. Increasing temperature from 200 °C to 250 °C and
300o
, the cellulose began to decompose gradually, resulting in
a decrease of the cellulose crystallinity in TOPs. Meanwhile,
the disappearance of 004 crystal plane of type I cellulose in
TOP-300 also showed that higher temperatures (e.g., 300 °C)
destroyed the crystal structure of cellulose.
3.9 FTIR analysis of raw pellet and TOPs
Figure 7b represents the infrared spectrums of raw pellets and
TOPs. The peak position of infrared spectra of raw and TOPs
was the same, though there are some deviations. The peak at
3431 cm−1
and 2854 cm−1
represented to O–H stretching vi-
bration of alcohol. With increasing temperature, the peak in-
tensity at 3431 cm−1
did not change and at 2854 cm−1
de-
creased slightly, showing that O–H bond was destroyed after
torrefaction, hence confirming the enhancement in
Table 6 Characteristics of raw and TOPs
Quality parameters Torrefaction temperature (°C)
200 250 300
Raw pellets TOPs
Particle density (g/cm3
)
1.23 ± 0.01
Residence time (min)
15 1.10 ± 0.01 0.91 ± 0.04 0.70 ± 0.02
30 1.10 ± 0.03 0.90 ± 0.05 0.68 ± 0.02
45 1.07 ± 0 0.89 ± 0.01 0.67 ± 0.01
60 1.05 ± 0.04 0.89 ± 0.04 0.64 ± 0.05
120 1.08 ± 0 0.87 ± 0.01 0.63 ± 0.02
Strength (N/mm2
)
3.23 ± 0.22
Residence time (min)
15 2.35 ± 0.22 1.36 ± 0.22 0.58 ± 0.22
30 1.99 ± 0.22 1.27 ± 0.22 0.49 ± 0.22
45 2.33 ± 0.22 0.92 ± 0.22 0.50 ± 0.22
60 1.52 ± 0.22 0.87 ± 0.22 0.50 ± 0.22
120 1.38 ± 0.22 0.93 ± 0.22 0.47 ± 0.22
Durability (%)
97.56
Residence time-30 min 94.22 86.44 75.88
True density (g/cm3
)
1.48 ± 0.22
Residence time (min)
15 1.42 ± 0.01 1.4 ± 0.01 1.52 ± 0.03
30 1.41 ± 0.01 1.38 ± 0.02 1.59 ± 0.04
45 1.41 ± 0.02 1.37 ± 0 1.61 ± 0
60 1.38 ± 0.02 1.39 ± 0.01 1.69 ± 0.11
120 1.40 ± 0.01 1.39 ± 0.02 1.85 ± 0
Porosity
0.16 ± 0.01
Residence time (min)
15 0.22 ± 0.01 0.37 ± 0.02 0.54 ± 0.02
30 0.22 ± 0.03 0.36 ± 0.04 0.57 ± 0
45 0.24 ± 0.012 0.35 ± 0.01 0.58 ± 0.01
60 0.24 ± 0.02 0.34 ± 0.02 0.62 ± 0
120 0.23 ± 0 0.35 ± 0.02 0.65 ± 0
*RT residence time
2213
Biomass Conv. Bioref. (2021) 11:2201–2217

14. hydrophobicity of pellets. The peaks at 2923 cm−1
,
1735 cm−1
, and 1503 cm−1
were attributed to the stretching
vibration of C–H (alkane), C=O (carboxylic acid), and N-O
(nitro compound), respectively. The peak at 1626 cm−1
was
associated to C=C stretching vibration of alkene, conjugated
alkene, cyclic alkene, and/or N–H bending vibration of amine.
The peak at 1061 cm−1
was due mainly to the stretching vi-
bration of C–N (amine), C–O (i.e., aliphatic ether and/or sec-
ondary alcohol), and/or S=O (i.e., sulfoxide, sulfonic acid).
The peak at 584 cm−1
was associated with C–R stretching
vibration of the halo compound. The peak intensity at
1735 cm−1
decreased as the temperature increased indicating
that C=O of carboxylic acid was unstable. Conversely, with
increasing temperature, the peak intensity at
1626 cm−1
,1503 cm−1
,1061 cm−1
, and 584 cm−1
was actually
the same and did not increase, indicating that C=C (i.e., al-
kene, conjugated alkene and/or cyclic alkene), N–H (amine),
C–N (amine), C–O (i.e., aliphatic ether and/or secondary al-
cohol), S=O (i.e., sulfoxide, sulfonic acid), N–O (nitro com-
pound), and C–R (halo compound) were relatively stable.
4 Conclusion
Recently, pelletization in combination with torrefaction has
greater importance in the research and industry fields.
Pelletization integrated with torrefaction enhances the char-
acteristics such as of density, strength, heating value, and
hydrophobicity of pellets and makes pellets suitable for
combustion, gasification, and pyrolysis. Die temperature,
pressure, and MC were found most significant factors to
the pellet quality. The highest particle density (1.307 g/
cm3
) and strength (11.8 N/mm2
) for particles of 0.25–
0.50 mm were found at a temperature of 200 °C (pressure ≥
100 MPa and MC 5.5%). Similarly, for particles of 0.5–
1.41 mm, the highest particle density (1.300 g/cm3
) and
strength (10.7 N/mm2
) were achieved at same process con-
ditions, indicating that pelletization at a temperature of
200 °C was beneficial. Relaxed density of pellets at higher
temperature and pressure and lower MC (≥ 130 °C, ≥
100 MPa and 5.5%) was found almost the same as of initial
particle density. The expansion ratio of pellet was found
lower. Use of synthetic resin as a binder in the proportion
of 1:9 was found optimum; the particle density increased
from 1.19 to 1.24 g/cm3
(0.25–0.5 mm) and from 1.17 to
1.22 g/cm3
(0.5–1.41 mm) and energy consumption reduced
by 9.39% (0.25–0.5 mm) and 8.7% (0.5–1.41 mm). Pellets
made at temperature of 200 °C were found water resistive.
MC of 18% was found optimum for particle and relaxed
densities and strength of raw pellets. The highest lower
heating value 26.76 MJ/kg of TOPs was achieved at
300 °C and 120 min. Severe torrefaction conditions ad-
versely affected the particle density, volumetric energy den-
sity, strength, and durability of TOPs. The highest TD
(1.85 g/cm3
) and porosity (65 v %) for TOPs were achieved
at 300 °C and 120 min, much higher than those of raw
pellets. MU of TOPs at 300 °C was 2.0–2.8 wt.%, showing
strong water-resistant ability. FTIR showed that O–H bond
was destroyed after torrefaction.
Optimum conditions for producing pellets of higher
quality (e.g., higher pellet density and strength) were
found at temperature 200 °C, pressure 130 MPa and MC
18%, and particle size 0.25–0.5 mm respectively.
Similarly, for torrefaction, the optimum process condi-
tions where pellets of higher quality in terms of hydro-
phobicity and heating value were found as temperature
300 °C and residence time ≥ 30 min.
Supplementary Information The online version contains supplementary
material available at https://doi.org/10.1007/s13399-020-01235-6.
Authors’ contributions Asif Ali Siyal: conceptualization, investigation,
writing—original draft, and writing—review and editing. Yang Liu:
methodology and investigation. Xiao Mao: writing—review and editing.
Babar Ali: formal analysis and investigation. Sakhawat Hussain: data
curation. Tianhao Zhang: methodology. Jianjun Dai and Guangqing
Liu: project administration, supervision, funding acquisition, and
writing—review and editing.
Funding This work was supported by Ministry of Science and
Technology of the People’s Republic of China (2017YFE0124800).
Data availability Findings of the current study are included within the
article and in supplementary file.
10 20 30 40 50 60
101
004
101
2θ (o
)
WPs TWPs-200
TWPs-250 TWPs-300
002
(a)
4000 3500 3000 2500 2000 1500 1000 500
C-R
O-H
Wavenumber (cm-1
)
WPs TWPs-200
TWPs-250 TWPs-300
O-H C-H
C=O
C=C N-H
N-O
C-N
C-O
S=O
(b)
Fig. 7 a Diffraction patterns of
raw and TOPs. b FTIR results for
raw and TOPs at different
torrefaction temperatures
2214 Biomass Conv. Bioref. (2021) 11:2201–2217

15. Compliance with ethical standards
Conflict of interest The authors declare that they have no conflict of
interest.
Code availability None.
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