Bioresource Technology 101 (2010) 8859–8867

Contents lists available at ScienceDirect

Bioresource Technology
journal homepage: www.elsevier.com/locate/biortech

Case Study

Mechanical durability and combustion characteristics of pellets

from biomass blends
M.V. Gil, P. Oulego, M.D. Casal, C. Pevida, J.J. Pis, F. Rubiera *
Instituto Nacional del Carbón, CSIC, Apartado 73, 33080 Oviedo, Spain

a r t i c l e i n f o a b s t r a c t

Article history: Biofuel pellets were prepared from biomass (pine, chestnut and eucalyptus sawdust, cellulose residue,
Received 29 March 2010 coffee husks and grape waste) and from blends of biomass with two coals (bituminous and semianthra-
Received in revised form 4 June 2010 cite). Their mechanical properties and combustion behaviour were studied by means of an abrasion index
Accepted 9 June 2010
and thermogravimetric analysis (TGA), respectively, in order to select the best raw materials available in
the area of study for pellet production. Chestnut and pine sawdust pellets exhibited the highest durabil-
ity, whereas grape waste and coffee husks pellets were the least durable. Blends of pine sawdust with 10–
Keywords:
30% chestnut sawdust were the best for pellet production. Blends of cellulose residue and coals (<20%)
Biomass
Coal
with chestnut and pine sawdusts did not decrease pellet durability. The biomass/biomass blends pre-
Pellets sented combustion profiles similar to those of the individual raw materials. The addition of coal to the
Abrasion index biomass in low amounts did not affect the thermal characteristics of the blends.
Combustibility Ó 2010 Elsevier Ltd. All rights reserved.

1. Introduction tion of these materials would contribute to improving their behav-

iour as a fuel by increasing their homogeneity and allowing a wider
Recently there has been increasing interest in processes for pro- range of lignocellulosic materials to be used as fuel (Tabarés et al.,
ducing biofuel pellets both for domestic and industrial use. The 2000).
production of such pellets has grown rapidly in Europe, Northern Wang et al. (2009) have pointed out that there is a growing
America and China in the last few years (Peksa-Blauchard et al., market for biofuels in the production of briquettes and pellets for
2007; Samuelsson et al., 2009). At the same time, the need to im- domestic purposes, since biomass pellets can be used in grate fur-
prove the quality of the pellets has become increasingly important. naces and fluidized bed combustion while offering advantages,
Agricultural and forest wastes as well as industrial by-products such as easy storage and transport, lower pollution, lower dust lev-
are possible materials for biofuel pellet production. Although the els and higher heating values. Furthermore, the pellets offer the
chemical constituents and moisture content of biomass materials same advantages for automation and optimization as the petro-
vary, they all contain low amounts of polluting elements and ash leum-derived fuels, but with a higher combustion efficiency and
(Heschel et al., 1999). For this reason, the fabrication of pellets pre- a lower amount of combustion residues (Rhén et al., 2007).
pared with biomass is attracting increasing interest. According to Although the combustion characteristics of biomass may vary
Larsson et al. (2008), pelletized biomass is rapidly becoming an considerably depending on the composition of the raw material,
important renewable source of energy production. The utilization the use of biomass/coal blends could produce fuel pellets with
of biomass pellets will lead to a reduction in carbon dioxide emis- more suitable characteristics for combustion in industrial furnaces,
sions, as this source of energy is considered carbon neutral, i.e., the since coal has a higher carbon content and calorific value than bio-
carbon dioxide released during biomass utilisation is recycled as an mass (Heschel et al., 1999).
integral part of the carbon cycle. However, the use of different raw materials may have opposite
However, the combustion processes of biomass materials are effects on the final densified product. Pellet quality is usually mea-
complicated for three main reasons. Firstly, this fuel has a highly sured by means of bulk density and pellet durability. Mechanical
complex chemical and physical composition. Secondly, its combus- durability is a parameter that is defined by the Technical Specifica-
tion takes place in an uncontrolled environment and thirdly, the tion CEN/TS 14588:2003, as the ability of densified biofuels to re-
moisture content, density and heterogeneity of these materials main intact when handled, whereas durability refers to the
have a negative effect on the efficiency of combustion. Densifica- amount of fines that are recovered from pellets after these have
been subjected to mechanical or pneumatic agitation (Lehtikangas,
* Corresponding author. Tel.: +34 985 118 975; fax: +34 985 297 662. 2001; Thomas and van der Poe, 1996). The requirements and
E-mail address: frubiera@incar.csic.es (F. Rubiera). methods used for testing the mechanical durability of pellets are

0960-8524/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.biortech.2010.06.062
8860 M.V. Gil et al. / Bioresource Technology 101 (2010) 8859–8867

defined in the technical specification CEN/TS 15210-1:2005. Low Table 2

mechanical resistance leads to high dust emissions, feeding prob- Composition of the blends used for pellet preparation.

lems in boilers, and an increased risk of fire and explosions during Blends of two biomasses
pellet handling, storage and transport (Temmerman et al., 2006). % Pine + % % Pine + % % Eucaliptus + % % Pine + % Cellulose
The published research about biofuel pellets is mainly focused Chesnut Eucaliptus Chesnut residue
on the study of the factors affecting their mechanical durability 90PIN + 10CHE 70PIN + 30EUC 80 EUC + 20CHE 90PIN + 10CEL
(Bergström et al., 2008; Kaliyan and Morey, 2009; Samuelsson 85PIN + 15CHE 60PIN + 40EUC 80PIN + 20CEL
et al., 2009). However, few studies have been conducted in order 80PIN + 20CHE 50PIN + 50EUC
to compare different types of biomass for pellet fabrication, and 75PIN + 25CHE 40PIN + 60EUC
70PIN + 30CHE 30PIN + 70EUC
the main raw material for these kind of studies are wood residues.
On the other hand, the combustion behaviour of blends of biomass Blends of biomass and coal
and biomass and coal for pellet production has been scarcely stud-
% Bituminous % Bituminous coal + % % Bituminous coal + %
ied (Heschel et al., 1999; Rhén et al., 2007). Furthermore, the het- coal + % Pine Chesnut Eucaliptus
erogeneity of the raw materials used in pellet and briquette
5BCOAL + 95PIN 5BCOAL + 95CHE 5BCOAL + 95EUC
production, as well as the different processes used for biomass 10BCOAL + 90PIN 10BCOAL + 90CHE 10BCOAL + 90EUC
densification, makes it difficult to generalize from the published 15BCOAL + 85PIN 15BCOAL + 85CHE 15BCOAL + 85EUC
results and necessitates an individual study for each specific situ- 20BCOAL + 80PIN 20BCOAL + 80CHE 20BCOAL + 80EUC
ation. The primary objective of this work was to find a rapid way
of selecting the best raw materials from those available in the area % Semianthracite + % Pine % Semianthracite + % Chesnut

of study (i.e., Asturias, in NW Spain), in order to produce biofuel 5ACOAL + 95PIN 5ACOAL + 95CHE
pellets for industrial purposes. The experiments were designed to 10ACOAL + 90PIN 10ACOAL + 90CHE
15ACOAL + 85PIN 15ACOAL + 85CHE
evaluate the effect of different initial biomass materials, biomass/
20ACOAL + 80PIN 20ACOAL + 80CHE
biomass blends and coal/biomass blends on the mechanical dura-
Blends of three-components
bility and thermal characteristics of the biofuel pellets.
% Cellulose residue + % % Coffee husks + % % Semianthracite + %
(80PIN + 20CHE) (80PIN + 20CHE) (80PIN + 20CHE)
2. Methods 5CEL + 95 5COF + 95 5ACOAL + 95
(80PIN + 20CHE) (80PIN + 20CHE) (80PIN + 20CHE)
2.1. Materials

The types of biomass used in this work were pine sawdust (PIN),
ACOAL) were also used in small quantities to make up the blends.
chestnut sawdust (CHE), eucalyptus sawdust (EUC), cellulose resi-
This was done in order to supplement the seasonal availability of
due (CEL), coffee husks (COF) and grape waste (GRA). PIN and EUC
biomass, to improve the heating value, and to study the possible
are forest wastes that are available in large quantities in the area of
improvement in the mechanical durability of the pellet blends.
study, whereas CHE is less common. CEL, COF and GRA are minor-
The procedure employed was as follows. First, the raw materials
ity residues that could be used in low proportions in blends with
were dried at a constant temperature of 35 °C for 72 h. The samples
other biomasses for pellet production. Two coals were also used
were then ground and sieved in order to obtain a particle size frac-
in this work: a high-volatile bituminous coal (BCOAL) and a semi-
tion below 1 mm for the biomass samples and below 0.212 mm for
anthracite (ACOAL). Ultimate and proximate analyses together
the coals. Particle sizes higher than 1 mm will act as predeter-
with the heating values of the coal and biomass samples are pre-
mined breaking points in the pellets (Franke and Rey, 2006). The
sented in Table 1.
different blends were prepared in appropriate proportions and,
Different mixtures of two biomasses, one biomass and one coal,
manually, thoroughly mixed in order to assure a perfect homoge-
as well as mixtures of three different components, were used for
nization that guaranteed the effective composition of mixtures.
pellet preparation. It should be noted that the experimental design
was influenced by the availability of the different raw materials in
the area of study. Table 2 shows the composition of the blends em- 2.2. Pelletizing process and pellet characterisation
ployed. Pine sawdust and eucalyptus sawdust were added in high
percentages to the biomass blends, whereas other types of biomass The pellets were fabricated in a TDP benchtop press unit from
(CHE, CEL and COF) were included in lower percentages due to Tabletpress.net equipped with a single punch and die set. The bio-
their scarcity. The 80PIN + 20CHE blend was used in the three- masses, biomass/biomass and biomass/coal blends were pressed
component blends because of the good mechanical durability re- into cylindrical pellets of diameter 8.0 mm. In order to evaluate
sults attained with this binary blend. The two coals (BCOAL and the durability or mechanical resistance of the pellets, a procedure

Table 1
Ultimate and proximate analyses, and high heating values of the raw materials.

Sample Moisture content (%) Ultimate analysis (wt.%, db) Proximate analysis (wt.%, db) HHV (MJ/kg, db)
a a
C (%) H (%) N (%) O (%) S (%) Ash (%) FC (%) VM (%)
PIN 7.4 45.2 6.3 0.1 48.2 0.0 0.2 13.5 86.3 20.0
CHE 9.2 45.5 5.7 0.2 48.2 0.0 0.4 17.5 82.1 19.1
EUC 10.5 46.8 6.1 0.1 46.5 0.0 0.5 14.9 84.6 19.5
CEL 4.4 41.0 6.4 0.3 51.0 0.0 1.3 11.0 87.7 17.6
COF 6.7 43.2 6.3 2.6 43.2 0.2 4.5 16.1 79.4 20.1
GRA 6.4 50.0 6.0 2.0 34.4 0.1 7.5 24.6 67.9 22.1
BCOAL 1.4 77.9 5.1 1.7 6.2 1.5 7.6 54.7 37.7 32.4
ACOAL 0.8 66.8 2.5 1.1 3.6 0.5 25.5 67.0 7.5 25.6
a
Calculated by difference; db: dry basis.
 M.V. Gil et al. / Bioresource Technology 101 (2010) 8859–8867 8861

usually used to evaluate the mechanical strength of coke (MICUM

test) was adapted for this study (Pis et al., 1988). Samples of 40 pel-
lets were introduced in a rotating drum with an internal diameter
of 130 mm and a depth of 110 mm. The drum was equipped with
two opposite inner baffles (30  110 mm) arranged perpendicular
to the cylinder wall. The rotation speed was set at 35 rpm. Each
pellet sample was analysed after 3000 revolutions. After being re-
moved from the drum, the sample material was screened using a
2 mm sieve. Particles smaller than 2 mm were then weighed. The
abrasion index, Ia, was obtained by means of the MICUM test from
the mean value of three replications. It was calculated as the mass
percentage of pellets below 2 mm relative to the total initial sam-
ple mass after 3000 revolutions in the rotary drum. It is considered
that the lower the Ia is, the better the quality of the pellet.
Thermal characterization of the raw materials and blends was Fig. 1. Abrasion index for pellets formed from individual materials used in this
carried out by thermogravimetric analysis (TG) and derivative study.
thermogravimetry (DTG). Thermogravimetric analysis (TGA) is
one of the most common techniques available for rapidly investi-
gating and comparing thermal events and kinetics during the com- mechanical durability (Ia = 7%), followed by those made with pine
bustion and pyrolysis of solid raw materials, such as coal and sawdust (Ia = 12%), whose abrasion indices were very low. The pel-
biomass (Arenillas et al., 1999; Gil et al., 2010; Nowakowski lets from cellulose residue (Ia = 29%) and eucalyptus sawdust
et al., 2008; Pis et al., 1996; Rubiera et al., 1997, 2002). (Ia = 64%) showed higher abrasion indices. The values obtained
Non-isothermal TGA was performed using a Setaram TAG24 for the pellets from coffee husks and grape waste indicated that
analyser. The analyses were conducted under a 50 cm3 min 1 air their durability would be very poor (Ia > 90%). In the case of grape
flow rate at a heating rate of 15 °C min 1 from room temperature waste, the sample was almost totally destroyed during the abra-
to 1000 °C. Approximately 5 mg of sample was used for each sion test (Ia = 99%) and therefore it was eliminated from further
experiment. Only a small amount of sample and slow heating rate experiments. The pellets from BCOAL presented an intermediate
were used in order to avoid heat transfer limitations. The deriva- abrasion index value (Ia = 23%), while that of the ACOAL pellets
tive curves (DTG) of the samples were represented as a function was 100%, indicating that these pellets had been totally destroyed
of temperature. during the test.
The new European standard for solid biofuels ‘‘Fuel specifica-
tions and classes CEN/TS 14961:2005” is at present a classification
3. Results and discussion
standard, but general quality standards are about to be introduced
(Ståhl and Wikström, 2009). On the basis of the relative results ob-
3.1. Characteristics of raw materials
tained for all the samples used in these experiments, an abrasion
index of around 15% was established as the maximum value for
As can be observed in Table 1, the samples of sawdust (PIN, CHE
choosing the best biomasses and blends in this study. According
and EUC), which came from forest wastes, had a very low ash con-
to this standard, the biomasses that generated the most resistant
tent (<0.5%), whereas the minority residues (CEL, COF and GRA)
pellets were chestnut and pine. These biomasses, therefore, would
and the BCOAL coal had slightly higher ash contents (1–8%). The
be the most suitable raw materials for pellet production.
biomasses had a very low sulphur content (<0.2%). All of these
Fig. 2 shows the results of the abrasion index, Ia, for all the pel-
characteristics favour clean combustion conditions (Vamvuka
lets formed from blends of two biomasses (Fig. 2(a)), blends of
et al., 2003). The semianthracite (ACOAL) presented the highest
three-components (Fig. 2(b)) and blends of biomass and coal
ash content (25.5%), whereas BCOAL had the highest sulphur con-
(Fig. 2(c) and (d)). The pellets from the PIN + CHE blends all exhib-
tent (1.5%).
ited a similar mechanical durability, since the abrasion index val-
In comparison with the coals, the biomasses contained a higher
ues for the PIN and CHE pellets were very close (Fig. 2(a)). These
proportion of oxygen and hydrogen but less carbon (Table 1). These
values were very low for these samples, indicating that the pine
characteristics reduce their heating value since the energy con-
sawdust and chestnut sawdust blend would be suitable in the right
tained in carbon–oxygen and carbon–hydrogen bonds is lower
proportions (10–30% of chestnut sawdust) for pellet production.
than that of carbon–carbon bonds (Munir et al., 2009). However,
Moreover, it was found that the addition of chestnut sawdust im-
the higher oxygen content in the biomass indicates that this will
proved the mechanical durability of the pellets formed from pine
have a higher thermal reactivity than the coals (Haykiri-Acma
sawdust, despite the low abrasion index of PIN.
and Yaman, 2008).
Although the pellets from the PIN + CHE blends showed similar
The high heating value (HHV) on dry basis was found to be sim-
mechanical durability, the lowest abrasion index value was ob-
ilar for the different types of biomass (18–22 MJ kg 1). The coals
tained with pellets formed from the 80PIN + 20CHE blend (8%),
presented higher HHV values (26–32 MJ kg 1), which indicated
and this mixture was later used to make the three-component
that they would be the most appropriate additives for improving
blends. Thus, this sample was blended with a small percentage
the combustion characteristics of biomass.
(5%) of CEL, COF and ACOAL, but in the three cases, the abrasion in-
dex of the pellets formed using ternary blends was found to be
3.2. Abrasion index higher than that obtained with a the binary blend (Fig. 2(b)). It
can therefore be concluded that the addition of the third compo-
Fig. 1 shows the results of the abrasion index, Ia, for all the pel- nent did not improve mechanical durability, since no synergetic
lets from the raw materials used in the study. The mechanical behaviour was observed between the components. However, when
durability of the pellets was observed to decrease in the following a low percentage (5%) of CEL or ACOAL was added as third compo-
order: CHE > PIN > BCOAL > CEL > EUC > COF > GRA > ACOAL. The nent, the abrasion index only slightly increased, indicating that ter-
pellets formed from chestnut sawdust showed the highest nary mixtures could be used without significantly affecting the
8862 M.V. Gil et al. / Bioresource Technology 101 (2010) 8859–8867

(Fig. 2(a)). Thus, the addition of pine sawdust in a proportion equal

to or higher than 40% would be necessary to produce pellets from
eucalyptus sawdust in order to see a reduction in the abrasion in-
dex of eucalyptus pellets. The resultant durability, however, would
not be very satisfactory.
In the case of the chestnut and eucalyptus mixture (EUC + CHE),
chestnut sawdust was only added in proportions of up to 20%,
since this material is available in low quantities in the area of
study. However, although the mechanical durability of eucalyptus
pellets was improved when chestnut was added (Fig. 2(a)), the
abrasion index was found to be extremely high. This blend there-
fore was considered unsuitable for pellet production.
Finally, a mixture of pine sawdust with a small percentage of
cellulose residue (PIN + CEL) was assayed. The results indicated
that the addition of CEL in a percentage equal to or lower than
20% did not modify the abrasion index of the pine pellets
(Fig. 2(a)). Thus, these blends could be employed for pellet fabrica-
tion and the cellulose residue waste could be reused.
The pellets from blends BCOAL + PIN and BCOAL + CHE dis-
played a similar mechanical durability when BCOAL was added
to the mixture in a proportion of 5–20% (Fig. 2(c)). Their abrasion
index values were found to be very low and similar to those of
the raw biomasses, suggesting that a blend of pine sawdust or
chestnut sawdust with percentages of BCOAL of up to 20% would
be highly suitable for pellet production.
The blend of BCOAL with eucalyptus sawdust (BCOAL + EUC)
considerably improved the mechanical durability of the eucalyptus
pellets when the coal was added in a percentage equal to, or higher
than, 10% (Fig. 2(c)). However, the abrasion indices could then be-
come excessively high due to the already high values of the BCOAL
pellets.
In contrast, when pine sawdust was blended with ACOAL
(ACOAL + PIN), pine pellet durability was not affected if only 5%
of coal was added, whereas it increased slightly when the percent-
age of coal added was 10–15% (Fig. 2(d)), there being a dramatic in-
crease when additions reached 20%. When chestnut sawdust was
blended with ACOAL (ACOAL + CHE), chestnut pellet durability
was not affected if coal addition remained inside the proportion
of 5–15%, whereas it increased slightly when the percentage of coal
added reached 20% (Fig. 2(d)). Therefore blending ACOAL with
chestnut sawdust in a proportion of 5–15% would best suit the pur-
pose of producing pellets.

3.3. Thermal characteristics

To evaluate the effect of the amount and the type of raw mate-
rial (biomass and coal) on the combustion process, the blends were
subjected to thermogravimetric analysis under an oxidizing atmo-
sphere. In the case of the blends, the thermal analysis was carried
out only on those blends that presented the best abrasion index
values, i.e., the best mechanical durability. The DTG curves for
the biomass and coal samples and their blends, under air atmo-
sphere, are shown in Figs. 3–6.
Fig. 2. Abrasion index for pellets formed from blends of two biomasses (a), blends
of three-components (b), blends of biomass and BCOAL (c) and blends of biomass 3.3.1. Combustion behaviour of individual biomasses and coal pellets
and ACOAL (d). Fig. 3 shows the DTG combustion profiles for the pellets formed
from each individual biomass and both coal samples. In Fig. 3, the
DTG profile extends over the entire temperature interval (25–
mechanical durability of the pellets. Thus, pellet fabrication offers 700 °C) within which the thermogravimetric analysis was con-
the possibility of recycling minority wastes as an alternative to less ducted. From the curves, it can be seen that an initial mass loss
environmentally friendly ways of disposal. (stage A) occurred between the temperatures of 25 °C and 105 °C
When the pine was blended with eucalyptus (PIN + EUC), the for all samples, due to moisture loss. In this range, the biomass
abrasion index decreased proportionally as the proportion of pine samples experienced two-step mass losses (stages B and C), com-
increased up to 40%, whereas if the pine percentage was kept to be- pared to only a one-step mass loss (stage C) for the coal samples.
tween 40% and 70%, mechanical durability remained more or less In the case of the biomass samples, the mass loss in stage B,
constant, its values being slightly higher than those of pine pellets where the main mass loss occurred, is due to oxidative degradation
 M.V. Gil et al. / Bioresource Technology 101 (2010) 8859–8867 8863

The combustion of coal started at a higher temperature (312 °C

for BCOAL and 451 °C for ACOAL) than that of the biomass samples
(142–212 °C). Furthermore, the coal samples had a lower rate of
mass loss (DTGmax), 0.206% s 1 for BCOAL and 0.332% s 1 for
ACOAL, compared to the biomass samples (except for the COF sam-
ple) in stage B, 0.385–0.441% s 1.
Coal combustion was probably dominated by the oxidation of
char, while, in the case of the biomass samples, it was dominated
by the oxidation of the volatile matter, which was present in a
large proportion, i.e. approximately 80%. This caused the biomass
to burn at very low temperatures.
The pellets formed from COF generated more unburned residual
material (4.7%) at the end of the experiment than the other bio-
mass samples, in accordance with the higher ash content of the
raw material (Table 1). However, pellets made with PIN generated
Fig. 3. DTG curves for pellets formed from raw materials in an air flow rate of the lowest amount of unburned residue (0.6%). Likewise, Tabarés
50 cm3 min 1, at a heating rate of 15 °C min 1.
et al. (2000) found that briquettes made with pine and eucalyptus
generated the lowest amount of unburned material in their com-
bustion experiments.
– i.e., volatiles are released and then burned – whereas the mass In order to ensure a combination of efficiency and comfort for
loss in stage C is due to the combustion of the remaining char. the consumer of pellets in the domestic heating sector, it is neces-
Haykırı-Açma (2003) described the first stage as the burning region sary to avoid a high ash content, as this would remove the need to
in which volatiles are released and burned. Zheng and Koziński empty the ash box at regular interval, minimize the danger of slag
(2000) claimed that biomass combustion consisted of two main formation in the boiler and reduce soot emissions (Obernberger
steps, the first one characterised by the devolatilization process and Thek, 2004). In view of these risks, the forest sawdust samples
and burning of light organic volatiles and the second mass loss would appear to be the most suitable raw materials for pellet
resulting from the oxidation of char. production.
The combustion temperature interval, the mass loss, the final Taking into consideration that the temperature value at the
residue after combustion, the peak temperature and the maximum maximum rate of mass loss is considered inversely proportional
rate of mass loss (DTGmax) corresponding to the two different to reactivity (Haykırı-Açma, 2003), the coals proved to be the least
stages of mass loss (stages B and C) are presented in Table 3. The reactive materials, ACOAL being less reactive than BCOAL. In stage
initial temperature in stage B and the final temperature in stage B, the COF pellets were found to be the most reactive compared to
C were considered as the temperature values at which the rate of the other biomass samples. The COF pellets were also the most
mass loss was 0.005% s 1 (Rubiera et al., 1999). reactive biomass samples in stage C, the CEL pellets being the least

Fig. 4. DTG curves for pellets formed from blends of two biomasses in an air flow rate of 50 cm3 min 1
, at a heating rate of 15 °C min 1
: (a) PIN + CHE, (b) PIN + EUC, (c)
EUC + CHE and (d) PIN + CEL.
8864 M.V. Gil et al. / Bioresource Technology 101 (2010) 8859–8867

Fig. 5. DTG curves for pellets formed from blends of three-components in an air flow rate of 50 cm3 min 1
, at a heating rate of 15 °C min 1
: (a) CEL + (80PIN + 20CHE), (b)
COF + (80PIN + 20CHE) and (c) ACOAL + (80PIN + 20CHE).

reactive. It should also be noted that the peak temperature in stage ent stages of mass loss (stages B and C) for the pellets formed from
C for the CEL sample is higher than that of the other biomass sam- PIN + CHE, PIN + EUC, EUC + CHE and PIN + CEL.
ples and very close to that of the coals (Fig. 3). All these pellets from binary blends displayed similar tempera-
On the other hand, the COF sample exhibited the lowest DTG- ture intervals of combustion to each other (Table 4), for stages B
max in stage B, 0.154% s 1, and the highest DTGmax in stage C, and C, as well as similar intervals to those of the individual mate-
0.219% s 1, compared to the other biomass samples. This shows rials of each mixture (Fig. 4). Similarly, the biomass/biomass
that the volatile matter in the COF sample burned at a lower rate blends all displayed similar mass loss and residue values (Table 4),
and for a longer time than that in the other biomass samples, for stages B and C, as well as similar values to those of the individ-
whereas char combustion proceeded at a higher rate and at a lower ual raw materials. In the case of the 90PIN + 10CEL blend, the mass
temperature. In short the behaviour of COF was different to that of loss and residue values of the raw materials were very different.
the other biomass samples. This is in agreement with Rhén et al. However, the mass loss and residue values were also similar to
(2007), who claim that the char yield and char combustion rate those of the PIN sample, the largest component in the blend.
of a biofuel are correlated to the chemical composition of the The binary blends also had similar peak temperatures (Table 4)
biomass. to each other and to those of the individual materials, in both com-
bustion stages, except for the 90PIN + 10CEL sample in stage C,
which had a value very close to that of the PIN sample
3.3.2. Combustion behaviour of pellets formed from blends of raw (Fig. 4(d)), PIN being the largest component in the mixture. The
materials maximum rates of mass loss for the binary blends occupy an inter-
The DTG combustion profiles corresponding to the pellets mediate position among the raw materials (Table 4), the DTGmax
formed from biomass/biomass binary blends, under air atmo- value of the 90PIN + 10CEL sample in stage C being very close to
sphere, are shown in Fig. 4. The DTG curves corresponding to the that of the PIN sample (Fig. 4(d)).
pellets formed from the blends of three-components are repre- These results indicate that the thermal characteristics of the
sented in Fig. 5. Fig. 6 shows the DTG profiles corresponding to pellets formed from the biomass blends did not differ from those
the pellets from the biomass/coal blends. DTG curves are only of the individual biomasses which made up the mixture. Only
shown in the temperature range where sample combustion occurs, the PIN + CEL blend may have been affected by the presence of
because at lower temperatures there is only a minor initial mass CEL, but as CEL was present in such a low amount, the thermal
loss due to the loss of moisture in all the samples. characteristics of the blend were no different to those of the PIN
Fig. 4 shows the DTG combustion profiles for the pellets formed pellets. Therefore, in this study, the choice of raw materials for pel-
from binary blends of two biomass samples. The biomass/biomass let production should not be influenced by the thermal character-
blends displayed combustion profiles situated approximately half- istics of the pellets.
way between those of the individual raw materials. Table 4 pre- Fig. 5 shows the DTG combustion profiles for the pellets from
sents the combustion temperature interval, the mass loss, the the blends of three-components. These blends also had combustion
final residue after combustion, the peak temperature and the max- profiles that occupy an intermediate position among those of the
imum rate of mass loss (DTGmax) corresponding to the two differ- individual raw materials, although in this case, they were more
 M.V. Gil et al. / Bioresource Technology 101 (2010) 8859–8867 8865

Fig. 6. DTG curves for pellets formed from blends of biomass and coal in an air flow rate of 50 cm3 min 1
, at a heating rate of 15 °C min 1
: (a) BCOAL + PIN, (b) BCOAL + CHE.
(c) BOAL + EUC, (d) ACOAL + PIN and (e) ACOAL + CHE.

similar to that of the 80PIN + 20CHE blend. Table 5 contains the final residue after combustion, the peak temperature and the
combustion temperature interval, the mass loss, the final residue maximum rate of mass loss (DTGmax) corresponding to the two
after combustion, the peak temperature and the maximum rate different stages of mass loss (stages B and C) for the pellets
of mass loss (DTGmax) corresponding to the two different stages from the 20BCOAL + 80PIN, 20BCOAL + 80CHE, 20BCOAL + 80EUC,
of mass loss (stages B and C) for the pellets from the 5CEL + 95(80- 5ACOAL + 95PIN and 5ACOAL + 95CHE blends. The temperature
PIN + 20CHE), 5COF + 95(80PIN + 20CHE) and 5ACOAL + 95(80- intervals of combustion for the biomass/coal blends in stage B
PIN + 20CHE) blends. The ternary blends showed temperature (Table 6) were similar to those of the biomass sample. However in
intervals of combustion, mass loss, residue, peak temperatures stage C, although they started at approximately the same tempera-
and DTGmax values (Table 5), in stages B and C, that were similar ture as that of the biomass sample they finished at slightly higher
to those of the 80PIN + 20CHE blend, the principal component of temperatures, as previously mentioned (Fig. 6). The mass loss values
the mixture (Fig. 5). for the biomass/coal blends in stage B (Table 6) were slightly lower
Fig. 6 shows the DTG combustion profiles for the pellets formed than those of the biomass. However, their values, as well as the res-
from blends of the biomass and coal samples. The biomass/coal idue percentages, were higher in stage C, due to the effect of coal.
blends also presented combustion profiles in between those of Obernberger and Thek (2004) stated that a higher ash content in
the individual raw materials. All the biomass/coal blends showed the pellets might be acceptable if the pellets are destined for indus-
two combustion peaks, both of which were situated close to those trial use due to the greater robustness and sophistication of indus-
of the biomass sample, the second peak being slightly broader at trial combustion systems compared to domestic heating systems.
higher temperatures due to the influence of the coal. Table 6 Therefore, the pellets formed from blends of biomass and coal,
contains the combustion temperature interval, the mass loss, the which had higher values of residue content than the other samples,
8866 M.V. Gil et al. / Bioresource Technology 101 (2010) 8859–8867

Table 3
Temperature interval, mass loss, residue, peak temperature and maximum rate of mass loss (DTGmax) for the combustion stages B and C for pellets from individual materials
used in this study.

Sample Temperature interval (°C) Mass loss (%) Residue (%) Peak temperature (°C) DTGmax (%/s)
Stage B Stage C Stage B Stage C Stage B Stage C Stage B Stage C
PIN 192–353 353–490 62.6 30.1 0.6 322 457 0.385 0.132
CHE 209–360 360–487 64.1 24.3 1.5 313 460 0.441 0.093
EUC 212–339 339–487 64.4 25.4 1.0 309 454 0.473 0.077
CEL 205–377 377–537 69.4 25.0 1.9 313 490 0.462 0.076
COF 142–383 383–493 59.8 31.1 4.7 302 444 0.154 0.219
BCOAL – 312–600 – 98.3 8.3 – 500 – 0.206
ACOAL – 451–673 – 99.8 26.7 – 554 – 0.332

Table 4
Temperature interval, mass loss, residue, peak temperature and maximum rate of mass loss (DTGmax) for the combustion stages B and C for pellets from blends of two biomasses.

Sample Temperature interval (°C) Mass loss (%) Residue (%) Peak temperature (°C) DTGmax (%/s)
Stage B Stage C Stage B Stage C Stage B Stage C Stage B Stage C
90PIN + 10CHE 192–360 360–487 64.2 28.1 1.2 315 457 0.412 0.132
80PIN + 20CHE 192–339 339–490 60.6 30.9 1.1 315 460 0.423 0.119
70PIN + 30CHE 192–346 346–493 64.0 36.0 1.2 310 460 0.437 0.113
70PIN + 30EUC 189–342 342–484 63.5 28.3 0.9 316 460 0.448 0.096
50PIN + 50EUC 182–342 342–490 62.0 29.2 0.8 316 457 0.467 0.097
40PIN + 70EUC 206–346 346–500 62.5 28.5 0.9 312 457 0.475 0.092
80EUC + 20CHE 206–339 339–490 61.2 28.7 0.9 309 457 0.453 0.082
90PIN + 10CEL 209–346 346–500 62.1 30.9 0.9 319 463 0.430 0.108

Table 5
Temperature interval, mass loss, residue, peak temperature and maximum rate of mass loss (DTGmax) for the combustion stages B and C for pellets from blends of three-
components.

Sample Temperature interval (°C) Mass loss (%) Residue (%) Peak temperature (°C) DTGmax (%/s)
Stage B Stage C Stage B Stage C Stage B Stage C Stage B Stage C
5CEL + 95(80PIN + 20CHE) 199–356 356–512 62.9 29.8 0.9 311 467 0.429 0.106
5COF + 95(80PIN + 20CHE) 185–363 363–493 63.1 28.9 1.2 312 460 0.386 0.111
5ACOAL + 95(80PIN + 20CHE) 183–360 360–511 60.3 31.1 1.7 324 471 0.267 0.096

Table 6
Temperature interval, mass loss, residue, peak temperature and maximum rate of mass loss (DTGmax) for the combustion stages B and C for pellets from blends of biomass and
coal.

Sample Temperature interval (°C) Mass loss (%) Residue (%) Peak temperature (°C) DTGmax (%/s)
Stage B Stage C Stage B Stage C Stage B Stage C Stage B Stage C
20BCOAL + 80PIN 209–362 362–583 48.8 44.4 3.3 322 474 0.307 0.104
20BCOAL + 80CHE 199–349 349–560 51.6 40.5 2.7 302 474 0.285 0.083
20BCOAL + 80EUC 216–342 342–523 61.1 31.0 2.3 312 464 0.446 0.091
5ACOAL + 95PIN 210–361 361–501 61.2 29.8 2.5 327 468 0.367 0.105
5ACOAL + 95CHE 199–363 363–533 57.8 31.7 2.7 304 471 0.332 0.086

should be reserved for industrial use in large furnaces. The peak amounts will not affect the combustion characteristics of the
temperatures of the biomass/coal blends in stage B (Table 6) were pellets.
similar to those of the biomass samples, whereas the corresponding In view of the results obtained, further studies on pellet com-
DTGmax values were lower than those of the biomass. In stage C, the bustion in a small-scale combustor are being planned in order to
peak temperatures of the biomass/coal blends (Table 6) were extend our knowledge of the combustion behaviour of pellets be-
slightly higher than those of the biomass samples, whereas the fore they are produced at industrial scale. The selected raw mate-
DTGmax values, although close to those of the biomass samples, rials, i.e., those with the best mechanical durability, will be first
were far from those of the coal samples (Fig. 6). used for pellet production in a pilot-scale pellet press.
Thus, the thermogravimetric characteristics of the biomass/coal
blends differed only slightly in relation to the individual biomasses 4. Conclusions
due to the presence of coal in the mixture. Only the residue values
and the final combustion temperature showed slight increases. The most durable pellets were found with: chestnut sawdust
Thus, it can be concluded that the addition of coal in small (CHE), pine sawdust (PIN), CHE + PIN (630% CHE), cellulose residue
 M.V. Gil et al. / Bioresource Technology 101 (2010) 8859–8867 8867

(CEL) + PIN (620% CEL) and 5%CEL + 95% (80%PIN + 20%CHE). A Munir, S., Daood, S.S., Nimmo, W., Cunliffe, A.M., Gibbs, B.M., 2009. Thermal analysis
and devolatilization kinetics of cotton stalk, sugar cane bagasse and shea meal
bituminous coal (BCOAL) and a semianthracite (ACOAL) could be
under nitrogen and air atmospheres. Bioresour. Technol. 100, 1413–1418.
added to biomass for pellet production in the following propor- Nowakowski, D.J., Woodbridge, C.R., Jones, J.M., 2008. Phosporus catalysis in the
tions: CHE + BCOAL (620% BCOAL), CHE + ACOAL (615% ACOAL), pyrolysis behaviour of biomass. J. Anal. Appl. Pyrol. 87, 197–204.
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ACOAL + 95% (80%PIN + 20%CHE). Bioenerg. 27, 653–669.
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