Improving the physical, mechanical and energetic properties of Quercus spp. wood pellets by adding pine sawdust

Abstract

Background. Biomass usage for energy purposes has emerged in response to global energy demands and environmental problems. The large amounts of by-products generated during logging are rarely utilized. In addition, some species (e.g. Quercus spp.) are considered less valuable and are left in the cutting areas. Production of pellets from this alternative source of biomass may be possible for power generation. Although the pellets may be of lower quality than other types of wood pellets, because of their physical and technological properties, the addition of different raw materials may improve the characteristics of the oak pellets.

Methods. Sawdust from the oak species Quercus sideroxyla, Q. rugosa, Q. laeta and Q. conzattii was mixed with sawdust from the pine Pinus durangensis in different ratios of oak to pine (100:0, 80:20, 60:40, 40:60 and 20:80). Physical and mechanical properties of the pellets were determined, and calorific value tests were carried out. For each variable, Kolmogorov-Smirnov normality and Kruskal-Wallis tests were performed and Pearson’s correlation coefficients were determined (considering a significance level of p < 0.05).

Results. The moisture content and fixed carbon content differed significantly (p < 0.05) between the groups of pellets (i.e. pellets made with different sawdust mixtures). The moisture content of all pellets was less than 10%. However, volatile matter and ash content did not differ significantly between groups (p ≥ 0.05). The ash content was less than 0.7% in all mixtures. The addition of P. durangensis sawdust to the mixtures improved the bulk density of the pellets by 18%. Significant differences (p < 0.05) in particle density were observed between species, mixtures and for the species × mixture interaction. The particle density was highest in the 80:20 and 60:40 mixtures, with values ranging from 1,245 to 1,349 kg m-3. Bulk density and particle density of the pellets were positively correlated with the amount of P. durangensis sawdust included. The mechanical hardness and impact resistance index (IRI) differed significantly (p < 0.05) between groups. The addition of pine sawdust decreased the mechanical hardness of the pellets, up to 24%. The IRI was highest (138) in the Q. sideroxyla pellets (100:0). The mechanical hardness and IRI of the pellets were negatively correlated with the amount of P. durangensis sawdust added. The bulk density of the pellets was negatively correlated with mechanical hardness and IRI. The calorific value of mixtures and the species × mixture interaction differed significantly between groups. Finally, the mean calorific value was highest (19.8 MJ kg-1) in the 20:80 mixture. The calorific value was positively related to the addition of P. durangensis sawdust.

Full text

Improving the physical, mechanical and
energetic properties of Quercus spp. wood
pellets by adding pine sawdust
Víctor Daniel Núñez-Retana
1
, Rigoberto Rosales-Serna
2
,
José Ángel Prieto-Ruíz
3
, Christian Wehenkel
4
and
Artemio Carrillo-Parra
4
1Maestría Institucional en Ciencias Agropecuarias y Forestales, Universidad Juárez del Estado de
Durango, Durango, Durango, Mexico
2Campus Valle del Guadiana, Instituto Nacional de Investigaciones Forestales, Agrícolas y
Pecuarias, Durango, Durango, México
3Facultad de Ciencias Forestales, Universidad Juárez del Estado de Durango, Durango, Durango,
Mexico
4Instituto de Silvicultura e Industria de la Madera, Universidad Juárez del Estado de Durango,
Durango, Durango, Mexico
ABSTRACT
Background: Biomass usage for energy purposes has emerged in response to global
energy demands and environmental problems. The large amounts of by-products
generated during logging are rarely utilized. In addition, some species (e.g., Quercus
spp.) are considered less valuable and are left in the cutting areas. Production of
pellets from this alternative source of biomass may be possible for power generation.
Although the pellets may be of lower quality than other types of wood pellets, because
of their physical and technological properties, the addition of different raw materials
may improve the characteristics of the oak pellets.
Methods: Sawdust from the oak species Quercus sideroxyla, Q. rugosa, Q. laeta and
Q. conzattii was mixed with sawdust from the pine Pinus durangensis in different
ratios of oak to pine (100:0, 80:20, 60:40, 40:60 and 20:80). Physical and mechanical
properties of the pellets were determined, and calorific value tests were carried out.
For each variable, Kolmogorov–Smirnov normality and Kruskal–Wallis tests were
performed and Pearson’s correlation coefficients were determined (considering a
significance level of p< 0.05).
Results: The moisture content and fixed carbon content differed significantly
(p< 0.05) between the groups of pellets (i.e., pellets made with different sawdust
mixtures). The moisture content of all pellets was less than 10%. However, volatile
matter and ash content did not differ significantly between groups (p≥0.05).
The ash content was less than 0.7% in all mixtures. The addition of P. durangensis
sawdust to the mixtures improved the bulk density of the pellets by 18%. Significant
differences (p< 0.05) in particle density were observed between species, mixtures
and for the species × mixture interaction. The particle density was highest in the
80:20 and 60:40 mixtures, with values ranging from 1,245 to 1,349 kg m
−3
. Bulk
density and particle density of the pellets were positively correlated with the amount
of P. durangensis sawdust included. The mechanical hardness and impact resistance
index (IRI) differed significantly (p< 0.05) between groups. The addition of pine
sawdust decreased the mechanical hardness of the pellets, up to 24%. The IRI was
How to cite this article Núñez-Retana VD, Rosales-Serna R, Prieto-Ruíz JÁ, Wehenkel C, Carrillo-Parra A. 2020. Improving the physical,
mechanical and energetic properties of Quercus spp. wood pellets by adding pine sawdust. PeerJ 8:e9766 DOI 10.7717/peerj.9766
Submitted 22 January 2020
Accepted 29 July 2020
Published 20 August 2020
Corresponding author
Artemio Carrillo-Parra,
acarrilloparra@ujed.mx
Academic editor
Scott Wallen
Additional Information and
Declarations can be found on
page 15
DOI 10.7717/peerj.9766
Copyright
2020 Núñez-Retana et al.
Distributed under
Creative Commons CC-BY 4.0

highest (138) in the Q. sideroxyla pellets (100:0). The mechanical hardness and IRI of
the pellets were negatively correlated with the amount of P. durangensis sawdust
added. The bulk density of the pellets was negatively correlated with mechanical
hardness and IRI. The calorific value of mixtures and the species × mixture
interaction differed significantly between groups. Finally, the mean calorific value
was highest (19.8 MJ kg
−1
) in the 20:80 mixture. The calorific value was positively
related to the addition of P. durangensis sawdust.
Subjects Biotechnology, Natural Resource Management, Forestry, Green Chemistry
Keywords Pellets, Quercus, Forest residues, Sawdust, Bioenergy
INTRODUCTION
The increase in the world’s population has created a greater demand for fossil fuels, which
has led to a scarcity of these materials and to unstable prices. According to the United
Nations Population Fund (UNFPA, 2019) the world’s population was 7,000 million in 2010
and close to 7,715 million in 2019, representing an average annual rate of population
change of 1.1% (for the period 2010–2019). Plant biomass has become an important
renewable resource and currently covers approximately 15% of total energy consumption
in the world (Holubcik, Jandacka & Durcansky, 2016).
Mexican temperate forests are dominated by pine-oak species (Galicia, Potvin &
Messier, 2015). Mexican pine-oak forests, which cover an area of 31.8 million hectares
(FAO, 1998), are commonly uneven-aged mixed forests (Wehenkel et al., 2011;Maciel-
Nájera et al., 2020). Pinus wood production reached 5.0 million of m
3
of roundwood in
the last decade (SEMARNAT, 2016). Although Quercus spp. represent the second most
important Mexican forest timber resource, covering an area of about 8.4 ha and yielding
annual wood production of about 738,000 m
3
of roundwood (SEMARNAT, 2016),
these species remain almost underutilized (Bárcenas-Pazos et al., 2008;Villela-Suárez et al.,
2018).
Current methods of forest harvesting usually select pine species for harvesting, and
logging activities also generate large amounts of by-products in the form of tree branches,
tips, bark and sawdust. Logging thus changes the forest structure and species composition
promoting the dominance of some species of low economic value, such as Mexican
oaks in pine-oak forests (Moreno-Lopez, Alarcón-Herrera & Martin-Dominguez, 2017).
The dominance of oak trees interferes with natural restoration of pine populations under
intensive wood production in temperate forests. The presence of some Quercus species
has been associated with negative effects such as shading (Puértolas, Benito & Peñuelas,
2009), allelopathy, restrained seed germination and seedling radicle growth and inhibition
of nitrifying bacteria, thus affecting the self-restoration of ponderosa pine and the herb
understory (Li, Jia & Li, 2007).
In Mexico, the disposal of solid timber by-products can create problems in forestlands
and sawmills as it can lead to forest fires during periods of intense heat, generate dust in
the air and block spaces in production installations (Fregoso-Madueño et al., 2017).
Furthermore, pine wood is destined for the production of firewood, pulp, resin, edible
Núñez-Retana et al. (2020), PeerJ, DOI 10.7717/peerj.9766 2/20

seeds and other products such as furniture and boards (Sánchez, 2008) and therefore it
should not be used to produce bioenergy. By-products and poorly formed stems and
mature wood from oak trees could be used as an alternative source of material to produce
bioenergy. Nevertheless, oak material is rarely transformed into pellets, because of
technical problems due to the anatomical, physical, mechanical and drying characteristics
of the timber (Miranda et al., 2011). The density of oak wood is considered medium to
high (401–800 kg m
−3
)(Herrera-Fernández et al., 2017), which may lead to machining
problems in sawmill systems during conversion (Zavala Zavala, 2003;Herrera-Fernández
et al., 2017).
Pressing biomass into pellets has emerged as an efficient means of creating a renewable
energy resource. However, not all species are easily pelletized, and the quality of the
pellets is determined by the physical, mechanical, chemical and energetic properties.
Mechanical properties such as strength and durability can be measured by compressive
resistance testing, the tumbling can method, the Holmen Ligno tester and by impact
resistance and water resistance methods (Kaliyan & Morey, 2009).
When the physical, mechanical and energetic properties of the pellets do not reach
international standards, the quality can be improved by the use of mixtures of material to
make the pellets. Indeed, researchers such as Kaliyan & Morey (2009) and Harun & Afzal
(2016) recommend using mixtures of raw materials. Thus, Wilson (2010) mixed pine
sawdust with white oak and red oak sawdust, thereby improving the durability of the
pellets. Miranda et al. (2009) showed that pellets made from Quercus pyrenaica residues
were suitable for energy applications. The same researcher used mixtures of Pyrenean
oak and washed grape pomace to make pellets, which proved to have good physical and
thermal properties (Miranda et al., 2011). Arranz et al. (2015) compared commercial
pellets and an experimental type of Pyrenean oak pellet made in a semi-industrial pelletizer
and found that the calorific values produced by some pellets were sufficient. These
researchers therefore recommend taking specific actions to improve the pellet quality and
optimize the operations in relation to collecting and handling the raw material. Similarly,
Monedero, Portero & Lapuerta (2015) recommended the addition of pine sawdust to
poplar chips (Populus spp.) before pelletizing to improve the pellet quality and enable
compliance with the established requirements of the standard EN 14961-2 (Spanish
Association for Standarization (UNE), 2012).
The aim of this study was to improve the physical, mechanical and energetic properties
of oak wood pellets (without bark) by mixing the oak sawdust with pine (P. durangensis)
sawdust in different proportions before pelletizing the material.
MATERIALS AND METHODS
Raw materials and experimental design
Specimens of the oak species Quercus sideroxyla, Q. rugosa and Q. laeta were collected in
the cutting areas in the Llano Blanco (SG.FO-08/-2014/91), El Nopal (SG.FO-08-2014/
129), Chinatú (SG.FO-08-2014/52), El Pinito (SG.FO-08/2015/40) and El Tule y Portugal
(SG.FO-08-2014/82) forest communities located in the municipality of Guadalupe y
Calvo, state of Chihuahua, Mexico. Specimens of another oak species, Quercus conzattii,
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were collected from the Nicolás Romero forest area (SG/130.2.2.2/002203/17), and Pinus
durangensis specimens were obtained from the El Regocijo forest community (SG/
130.2.2.2/002243/11), both in the municipality of Durango, state of Durango, Mexico.
The material was collected by motor-manual harvesting, as follows: four logs were
cut from pest- and disease-free specimens of each species, of diameter at breast height
(dbh) ≥25 cm; the stem was required to be straight at least until the proposed height for
cutting (1.30 m) due to the complexity for chipping.
Preparation of biomass raw material and pellet manufacturing
The logs were seasoned under laboratory conditions and debarked, cut and chipped
using an Industrial Duty (SD4P25T61Y) machine. The sawdust was produced in a
hammer mill (TFS 420) with a 3.15 mm mesh. The sawdust from each oak species was
mixed with pine sawdust without bark in the following proportions (oak:pine): 100:0,
80:20, 60:40, 40:60 and 20:80. Ten kg of each mixture was prepared for pellet
manufacturing. The sawdust was placed in rubber bags and mixed homogeneously.
The sawdust was conditioned by controlling the temperature of the boiler, by means
of a digital controller, until 10% humidity was reached. The sawdust was mechanically
transported to the entrance of the ZLSP-R300 pelletizer to form the pellets (Fig. 1).
The pelletizer consists of a flat disc with channels 8 mm long and 6 mm wide and
produces pellets at a rate of 400 kg h
−1
. Before the samples were pelleted, the temperature
of the pelletizer was increased by processing pine sawdust only. The pelletizer was then
constantly fed with the sawdust mixture until 8 kg of material per mixture was formed.
The pellets were cooled by holding at room temperature for 24 h (Fig. 2).
Figure 1 Schematic representation of the pelletizing process used in the Instituto de Silvicultura e Industria de la Madera of the Universidad
Juárez del Estado de Durango (made by Víctor Daniel Núñez-Retana). Full-size

DOI: 10.7717/peerj.9766/fig-1
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Proximate analysis
The cooled pellets were subjected to proximate analysis. The moisture content of the
pellets was determined according to EN 18134-3 (Spanish Association for Standarization
(UNE), 2016a). Samples were weighed on a 1 mg precision weighing scale before and after
drying in an oven for 4 h at 105 ± 2 C. The volatile matter was measured following
standard EN 18123 (Spanish Association for Standarization (UNE), 2016b) in which the
samples are heated at 900 ± 10 C for 7 min.
The ash content was measured according to standard EN 18122 (Spanish Association for
Standarization (UNE), 2016c). Thus, the samples were initially weighed and placed in a
muffle at 250 C for 1 h, and the temperature was then increased to 550 C for 2 h.
The final weight was determined after cooling the samples in a desiccator. The amount of
fixed carbon was calculated by subtracting the sum of moisture content, volatile matter and
ash from 100% (Carrillo-Parra et al., 2018).
Figure 2 Pellets of different mixtures of oak-pine sawdust. Horizontally (A–D) Mixtures of 100:0.
(E–H) Mixtures of 80:20. (I–L) Mixtures of 60:40. (M–P) Mixtures of 40:60. (Q–T) Mixtures of 20:80.
Vertically (A–Q) Q. sideroxyla.(B–R) Q. rugosa.(C–S) Q. laeta.(D–T) Q. conzattii species.
Full-size
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DOI: 10.7717/peerj.9766/fig-2
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Physical properties
Bulk density tests were carried out in triplicate in a 600 mL metal cylinder, according to the
procedure outlined in standard EN-17828 (Spanish Association for Standarization (UNE),
2016d), in which the pellets were poured into the cylinder until it was full. A debris
cone was then formed. The cylinder was then struck three times on a hard surface from a
height of 150 mm to consolidate the pellets, and excess pellets were removed from the edge
of the cylinder.
The pellet particle density (kg m
−3
) was estimated by measuring the weight and volume
of 20 pellets.
Mechanical properties
The mechanical hardness was estimated by means of the drop test. The test consists
of measuring the weight of each pellet before and after being dropped twice from a
height of 1.85 m onto a concrete floor. Twenty repetitions were carried out per treatment.
The impact resistance index (IRI) was then calculated as described by Richards (1990),
(IRI) = (100 × N)/n, where Nis the number of drops, and nis the total number of pieces
after Ndrops. The maximum IRI value is 200. Small pieces weighing less than 5% of
the total pellet weight were not considered.
Energetic properties
The pellet calorific value was calculated in a semi-automatic isoperibol calorimeter
(LECO model AC600) in TruSpeed
Ò
mode and according to standard EN-14918 (Spanish
Association for Standarization (UNE), 2011a). The sample for analysis was burned with a
high oxygen pressure in a calorimetric pump under specified conditions. The tests were
carried out in triplicate on an anhydrous basis. The calculation was performed
automatically by the calorimeter.
Statistical analysis
Kolmogorov–Smirnov normality tests and analysis of variance were performed for all
the variables according the assumption of normality. Statistical analysis of the bulk
density, particle density, calorific value, mechanical hardness and IRI data were
performed according to a factorial design (4 × 5), for the factors species (4) and mixture (5).
A Kruskal–Wallis test was applied for non-normally distributed variables. Pearson’s
correlation coefficients were calculated in order to evaluate the strength of association
between the addition of Pinus durangensis sawdust and bulk density, mechanical hardness,
IRI and calorific value, as well as between bulk density and mechanical hardness and
IRI. All tests were performed considering a significance level of p< 0.05 and were
implemented in the statistical program RStudioÒversion 3.2.2 R (Bolker, 2012).
RESULTS
Proximate analysis
The moisture content differed significantly between species (p= 4.09 × 10
−5
), mixtures
(p= 1.14 × 10
−7
) and for the species × mixture interaction (p= 0.001) (Table 1).
Núñez-Retana et al. (2020), PeerJ, DOI 10.7717/peerj.9766 6/20

Table 1 Proximal analysis of pellets made from different mixtures of oak and P. durangensis
sawdust.
Factor MC (%) MV (%) AC (%) FC (%)
Species
Q. sideroxyla 2.14 C 90.64 A 0.54 6.65 B
Q. rugosa 3.22 A 87.33 C 0.55 8.88 A
Q. laeta 2.90 B 89.63 A 0.51 6.93 B
Q. conzattii 2.35 B 88.64 B 0.57 8.44 A
P. durangensis*4.80 80.68 0.53 13.91
Mixture
100:0 3.78 A 87.54 0.53 8.13
80:20 2.09 B 88.98 0.51 8.40
60:40 2.24 B 89.70 0.55 7.48
40:60 2.35 B 89.99 0.55 7.09
20:80 2.78 B 89.11 0.57 7.51
Q. sideroxyla–P. durangensis
100:0 3.71 b 89.76 0.36 6.15 k
80:20 1.10 m 91.34 0.47 7.07 j
60:40 1.21 l 90.82 0.60 7.36 i
40:60 2.11 j 91.08 0.65 6.13 k
20:80 2.57 h 90.23 0.62 6.56 k
Q. rugosa–P. durangensis
100:0 3.36 e 84.00 0.74 11.88 a
80:20 3.28 f 87.48 0.46 8.76 d
60:40 3.38 e 87.99 0.53 8.08 e
40:60 3.43 d 88.01 0.42 8.12 e
20:80 2.65 g 89.18 0.57 7.58 g
Q. laeta–P. durangensis
100:0 4.00 a 88.45 0.52 7.01 j
80:20 2.64 g 88.67 0.51 8.15 e
60:40 2.66 g 89.91 0.48 6.92 j
40:60 1.72 k 90.83 0.55 6.89 j
20:80 3.50 c 90.31 0.51 5.66 l
Q. conzattii–P. durangensis
100:0 4.05 a 87.95 0.49 7.49 h
80:20 1.35 k 88.42 0.59 9.62 c
60:40 1.73 k 90.07 0.60 7.58 f
40:60 2.14 j 90.03 0.58 7.23 i
20:80 2.42 i 86.72 0.58 10.26 b
Notes:
*
Pine values are added as a comparison parameter.
MC, Moisture content; VM, Volatile matter; AC, Ash content; FC, Fixed carbon. Different letters correspond to
significant statistical differences p< 0.05. Capital letters corresponds to species and mixtures; lowercase letters
correspond to species × mixture interaction.
Núñez-Retana et al. (2020), PeerJ, DOI 10.7717/peerj.9766 7/20

The moisture content was lowest in the pellets made from Q. sideroxyla (2.0%), followed
by those made from Q. conzattii (2.3%) and Q. laeta (2.7%) and Q. rugosa (3.3%).
The moisture content was highest in the pellets made from oak sawdust only (for all
species). The pellets made from Q. sideroxyla mixed with pine in ratios of 80:20 and 60:40
had the lowest moisture contents, of 1.1% and 1.2%, respectively. The moisture content of
P. durangensis pellets (4.8%) was higher than that of the pellets made from any of the
mixtures.
The volatile matter content differed significantly between species (p= 0.001), but not
between mixtures or for the species × mixture interaction (p= 0.07 and 0.63, respectively)
(Table 1). The mean values were highest in Q. sideroxyla and Q. laeta pellets (90.6%
and 89.6%, respectively), followed by Q. conzattii (88.6%) and Q. rugosa (87.3%) pellets.
The values were in the range 81–91%, and the volatile matter content of the pine pellets
(80.6%) was lower than in the pellets made from any of the mixtures.
There were no significant differences in ash content between species (p= 0.83), mixtures
(p= 0.86) or for the species × mixture interaction (p= 0.30) (Table 1). However, in all
mixtures including P. durangensis, the ash content was below 0.7% (except Q. rugosa
100–0).
Fixed carbon differed significantly between species (p= 0.002) and for the species ×
mixture interaction (p= 0.001), while there were no significant differences for mixtures
(p= 0.40) (Table 1). The mean fixed carbon content was highest in Q. rugosa (8.8%)
and Q. conzattii (8.4%) followed by Q. laeta (6.9%) and Q.sideroxyla (6.6%) pellets. On the
other hand, the fixed carbon content was highest in Q. rugosa pellets (100:0) (11.8%)
and lowest in the Q. laeta:P. durangensis 20:80 mixture (5.6%). The fixed carbon content
of P. durangensis pellets (13.9%) was higher than in all mixtures.
Physical properties
Bulk density varied in the range 557–703 kg m
−3
. The bulk density did not vary
significantly between species (p= 0.18) or for the species × mixture interaction (p= 0.99),
but it did differ significantly between the mixtures (p= 1.04 × 10
−6
)(Fig. 3A). The value
was highest in all 20:80 mixtures (>646 kg m
−3
) and lowest in the oak-only pellets
(100:0) (<580 kg m
−3
). The bulk density of the P. durangensis pellets was 647 kg m
−3
.
The particle density of pellets differed significantly between species (p= 0.01), mixtures
(p= 1.08 × 10
−8
) and for the species × mixture interaction (p= 1.49 × 10
−11
)(Fig. 3B).
The mean particle density was highest in Q. laeta (1,282 kg m
−3
) followed by Q. sideroxyla
(1,257 kg m
−3
) and Q. rugosa (1,256 kg m
−3
) and Q. conzattii (1,246 kg m
−3
) pellets.
The particle density was highest in the 80:20 and 60:40 mixtures and varied in the
range 1,245–1,349 kg m
−3
. The particle density was highest in the 80:20 mixture of Q. laeta
and P. durangensis. The particle density of the P. durangensis pellets was 1,227 kg m
−3
.
The pellet bulk density was positively correlated with the amount of P. durangensis
sawdust added (Fig. 3A). The bulk density of Q. laeta was most closely correlated (r= 0.82)
with the amount of P. durangensis sawdust added, while that of Q. conzattii was least well
correlated (r= 0.71). On the other hand, except for Q. rugosa, the particle density was
poorly correlated with the amount of P. durangensis sawdust added (Fig. 3B).
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Mechanical properties
The mechanical hardness differed significantly between species (p= 0.01), mixtures
(p= 7.32 × 10
−7
) and for the species × mixture interaction (p= 0.001) (Fig. 4A). The mean
percentage of retained mass was highest in Q. conzattii (69.1%), followed by Q. laeta,
Q. sideroxyla and Q. rugosa pellets: 63.7%, 61.4% and 61.0%, respectively.
The mean percentage of retained mass was highest in all oak-only pellets (100:0) (71%)
and lowest in the 20:80 mixtures (54.3%). The percentage of retained mass was highest in
the Q. sideroxyla-only pellets (100:0) (78.2%). The mean retained mass in the
P. durangensis pellets was 58.4%.
Figure 3 Addition of P. durangensis sawdust correlated with pellets bulk density (A) and particle density (B) of four oak species.
Full-size
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DOI: 10.7717/peerj.9766/fig-3
Figure 4 Addition of P. durangensis sawdust correlated with pellets mechanical hardness (A) and Impact Resistance Index (B) of four oak
species. Full-size
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DOI: 10.7717/peerj.9766/fig-4
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The IRI values also differed significantly between species (p= 0.01), mixtures (p= 3.64 ×
10
−6
) and for the species × mixture interaction (p= 0.01) (Fig. 4B). The mean value
was highest in Q. conzattii (116), followed by Q. laeta (107), Q. sideroxyla (106) and
Q. rugosa (99) pellets. The mean IRI value was highest in the oak-only pellets (100:0)
(120) and lowest in the 20:80 mixtures. On the other hand, the value was highest in the
Q. sideroxyla-only pellets (100:0) (138) and lowest in the Q. rugosa: P. durangensis 40:60
and Q. laeta:P. durangensis 20:80 pellets (80). The corresponding value of the index in
the P. durangensis pellets was 98.
The mechanical hardness of the pellets was negatively correlated with the amount of
P. durangensis sawdust added (Fig. 4A). The hardness of the Q. rugosa pellets was most
closely correlated with the amount of P. durangensis sawdust added (r=−0.92), while
that of the Q. conzattii pellets was the least well correlated with the same parameter
(r=−0.09). The IRI was also negatively correlated with the amount of P. durangensis
sawdust added (Fig. 4B), and the correlation was most closely for Q. sideroxyla (r=−0.95).
The pellet bulk density was also negatively correlated with mechanical hardness
and IRI (Fig. 5). The bulk density of Q. sideroxyla pellets most closely correlated with
mechanical hardness (r=−0.92) and IRI (r=−0.92), while that of Q. sideroxyla pellets was
least closely correlated with the same parameters (r=−0.34 and r=−0.07, respectively).
Energetic properties
The calorific value did not differ significantly between species (p= 0.24), but there
were significant differences between mixtures (p= 1.82 × 10
−5
) and for the species ×
mixture interaction (p= 4.25 × 10
−6
)(Fig. 6). The calorific value was highest in the 20:80
mixtures (above 19.7 MJ kg
−1
). The values for the mixtures with the four oak species
were in the range 19.0–19.8 MJ kg
−1
. The calorific value of the Pinus durangensis pellets
was highest (19.9 MJ kg
−1
).
Figure 5 Pellets bulk density correlated with mechanical hardness (A) and Impact Resistance Index (B) of four oak species.
Full-size
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DOI: 10.7717/peerj.9766/fig-5
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The calorific value was positively correlated with the amount of P. durangensis sawdust
added. All values for the oak pellets were highly correlated with the amount of
P. durangensis sawdust (r> 0.83).
DISCUSSION
Proximate analysis
The moisture content was below 10% for all species and mixtures, which are therefore
classified as M10 class according to EN 14961-2 (Spanish Association for Standarization
(UNE), 2012). Values for the oak-only pellets (100:0) were higher than for all mixtures
with pine sawdust. This is because oak wood has fewer empty voids than pine wood,
but large numbers of tyloses, which occlude the vessels and slow down the drying process
(De la Pérez-Olvera et al., 2015). In general, the moisture content was below the 6.5%
reported by Zamorano et al. (2011) for oak wood. It was lower than the range of
7.0–7.4% reported by Miranda et al. (2011) for mixtures of Pyrenean oak wood and
washed grape pomace. Moisture content is a physical property that can be controlled by
means of natural or artificial drying, and the values could therefore be optimized.
Volatile matter was lower in all oak-only pellets (100:0) than in the mixtures with
pine sawdust. Values were similar to those reported by Arranz et al. (2015) for Pyrenean
oak and pine forest residues (83.6 and 84.2%, respectively) and higher than those reported
by Miranda et al. (2011) (range 67.8–83.6%). Values of volatile matter in pine were
slightly lower than reported by Rollinson & Williams (2016) for pine pellets (83.6%) and by
Qin, Keefe & Daugaard (2018), who reported values in the range 81.2–82.6% for pellets
from green beetle-killed and burned lodgepole pine. High volatile matter contents reveal
important thermal properties. For example, pellet ignition is facilitated at low
Figure 6 Pellets calorific value correlated with the addition of P. durangensis sawdust.
Full-size

DOI: 10.7717/peerj.9766/fig-6
Núñez-Retana et al. (2020), PeerJ, DOI 10.7717/peerj.9766 11/20

temperatures, which increases the efficiency during the combustion process (Torres-Ramos
et al., 2015). However, a low volatile matter content will hinder ignition of biofuel
(Vassilev, Vassileva & Vassilev, 2015).
The ash content was generally lower in oak-only pellets than in the mixtures, except
in the Q. rugosa (100:0) and Q. laeta (100:0) pellets. However, the ash contents of each
species and mixtures were in accordance with the requirement specified by the standard
EN- 14961-2 (Spanish Association for Standarization (UNE), 2012) for non-industrial
use class A1 (≤0.7%). Quercus rugosa pellets were classified as A2 (≤1.5%), while
P. durangensis pellets were classified as A1, and the values were similar to those reported by
Filbakk et al. (2011) for Pinus sylvestris (0.47%).
The ash content was lower than the values reported by Zamorano et al. (2011)
(3.3%) and by Serrano et al. (2011) (3.1%). Herrera-Fernández et al. (2017) mentioned
that the ash content of the wood of some oak species reached 1.0%. Variations in ash
content can be attributed to several factors, including physiological adaptations of the
species (Bárcenas-Pazos et al., 2008), collection method, drying and handling of logs
(Zamorano et al., 2011) and the proportion of bark in the wood (Filbakk et al., 2011;
Herrera-Fernández et al., 2017;Lerma-Arce, Oliver-Villanueva & Segura-Orenga, 2017;
Núñez-Retana et al., 2019). High ash contents cause slag formation, fouling and sintering
(Vega-Nieva et al., 2016), negatively affecting the maintenance cost for both household and
industrial users (Carrillo-Parra et al., 2018).
Lower values for fixed carbon were obtained for pellets made from each of the four oak
species (100:0) than that reported by Miranda et al. (2011) (12.65%). The fixed carbon
content of the pine pellets was similar to that reported by Poddar et al. (2014) (13.0%),
and it was below the range of 16.9–18.43% reported by Qin, Keefe & Daugaard (2018).
Fixed carbon is an important bioenergy parameter due to its relationship with potential
energy of solid fuels, and high fixed carbon contents are associated with high calorific
power and with low moisture content and volatile matter content (Chen, Peng & Bi, 2015;
Forero-Nuñez, Jochum & Sierra, 2015).
Physical properties
The bulk density of the oak-only pellets (100–0) classified these as class BD550
(≥550 kg m
−3
) according to standard EN 14961-1 (Spanish Association for Standarization
(UNE), 2011b). These pellets are therefore of lower quality than pellets including pine
sawdust, and classified as BD600, but complied with the specifications established by
EN 14961-2 (Spanish Association for Standarization (UNE), 2012)(≥600 kg m
−3
) for wood
pellets destined for non-industrial use. The P. durangensis pellets were also classified as
suitable for non-industrial use.
The bulk density was higher in the oak pellets containing pine sawdust than in those
made from oak sawdust only (Fig. 3A). A similar response was observed by Monedero,
Portero & Lapuerta (2015). This can be explained by the higher bulk density of pine pellets
relative to oak pellets.
Values of the bulk density of oak pellets were within the range reported for other
species. For example, Miranda et al. (2015) reported bulk density values in the range
Núñez-Retana et al. (2020), PeerJ, DOI 10.7717/peerj.9766 12/20

620–824 kg m
−3
for pellets made from different raw materials, and they were higher
than those established by Carrillo-Paniagua (2015) for residues of Hieronima
alchorneoides and Eucalyptus spp. (in the range 480–603 kg m
−3
).
Bulk density is an important factor due to its relationship with the space required
for storage and transport of the pellets (Lehtikangas, 2001;Lerma-Arce, Oliver-
Villanueva & Segura-Orenga, 2017), as well as with the costs associated with these activities
(Zamorano et al., 2011;Garcia-Maraver et al., 2015). Higher bulk density corresponds
to more energy per unit volume, thus indicating greater economic benefits (Rollinson &
Williams, 2016).
Particle density was lower in oak pellets (100:0) than in the oak-pine pellets, and
the particle density was highest in the 80:20 mixtures. In a similar study carried out by
Liu et al. (2016) with bamboo, the values were reduced by the addition of pine to the
pellet mixture. In this study, the values increased in the (80:20) mixture and then
decreased with the addition of P. durangensis sawdust until almost reaching the values
obtained for the oak pellets (100:0). These researchers also reported values in the range
990–1,300 kg m
−3
. These differences can be attributed to the relatively lower density of
bamboo (540 kg m
−3
) than of oak. However, the values obtained in the present study
were higher than those reported by Monedero, Portero & Lapuerta (2015) (range
1,020–1,120 kg m
−3
). The differences in density can be attributed to the fact that these
researchers used poplar chips with particle density of 790 kg m
−3
in the mixtures, and
in the present study the particle density was highest in the oaks, in the range
1,118–1,271 kg m
−3
.
Similar values of pellet particle density were obtained by García et al. (2019) and
Bergström et al. (2008), who reported values in the range 1,259–1,276 kg m
−3
.Lehtikangas
(2001) reported values in the range 1,146–1,350 kg m
−3
for pellets made with different
varieties of sawdust, logs and bark residues. Jamradloedluk & Lertsatitthanakorn (2017)
reported particle density values in the range 1,300–1,800 kg m
−3
, which is much higher
than the values reported here. However, the differences may be due to the use of adhesives.
Particle density is an important parameter due to its influence on the apparent density
and combustion behavior. Low density particles are needed in order to increase the
burning period and energy production (Qin, Keefe & Daugaard, 2018). Particle density
is also related to the moisture content of the raw material at the time of pelletizing, as
at lower moisture content the friction increases through the plate in the matrix, which
affects the movement of the particles and therefore, increases compression and density
(García et al., 2019).
Mechanical properties
The durability, expressed as the percentage mass retained after dropping the pellets twice,
was within the range 67–78% for all oak pellets (100:0), that is, higher than the 51–74%
obtained for the pellets containing P. durangensis sawdust. The lower durability of the
pellets containing pine sawdust can be attributed to the very low abrasion index reported
for this material (12%) (Gil et al., 2010).
Núñez-Retana et al. (2020), PeerJ, DOI 10.7717/peerj.9766 13/20

Abedi, Cheng & Dalai (2018) used the drop test to determine the mechanical durability
of spruce and oat hull pellets with additives (lignin and proline) and without additives.
For pellets without additives, the durability ranged from 55% to 61%, and for pellets with
additives, the values ranged from 60% to 90% (values similar to those reported here).
Carrillo-Parra et al. (2018) also observed significant differences (p< 0.05) in the retained
mass in pellets made from three common tropical species. These researchers reported
values of 49.4% for Havardia pallens, 61.7% for Ebanopsis ebano, and 66.2% for Acacia
wrightii, respectively, without explaining the reason for these differences. These values
were lower than reported here for oak species.
The impact resistance test, also known as the “drop resistance”or “shattering
resistance”test (Kaliyan & Morey, 2009), enables estimation of the degree of compaction of
the pellets and the resistance to breakage. These are important factors, as pellets must be
able to support the transport, charge, discharge, storage and combustion processes to
which they are subjected and will affect the efficiency of a pellet burner stove or burners
(Hu et al., 2015). The impact resistance also enables evaluation of the mechanical
durability through the shock and/or friction of densified fuels (Temmerman et al., 2006),
as well as the strength of inter-particle bonds (Forero-Nuñez, Jochum & Sierra, 2015).
The values of the Impact Resistance Index (IRI) for the oak-only pellets (100:0) were
in the range 113–138, and they were higher than in all mixtures, except for Q. rugosa
80:20 mixture and for the of Q. laeta and Q. conzattii 60:40 and 40:60 mixtures. Overall,
the IRI was lower in the pellets containing P. durangensis sawdust. This can be attributed
to the increase in the amount of P. durangensis sawdust which probably produced a
pressure change in the pelletization process. This contrasts with the observations made
by Forero-Nuñez, Jochum & Sierra (2015) for pellets made from cocoa shell mixtures and
coal, in which the increase in cocoa shell mixture improved the impact resistance values, as
with the use of fine particles (<0.297 mm). On the other hand, similar IRI values were
reported by Carrillo-Parra et al. (2018) for A. wrightii (116 and 160).
The inverse correlation between the bulk density of the pellets and mechanical
properties can be attributed to the higher bulk density as the pellets harden. Thus,
when high-density pellets are dropped twice onto a concrete floor they will not absorb
the impact and will break, while lower-density pellets are more likely to absorb the impact
and not break. However, future studies should analyze the variations in shape and size
distributions in relation to durability of bulk materials, in specific, bulk modulus and
elastic response of pellets, as these factors are relatively poorly understood (Wilson, 2010).
The variations in the findings of several laboratories regarding the mechanical
properties of pellets may be due to the different methods used to determine the
characteristics of the pellets or the different devices used to produce the pellets. Further
studies should be carried out to compare different methods used to determine the
mechanical properties.
Energetic properties
The calorific value of all pellets containing P. durangensis sawdust was slightly higher than
that of the oak-only pellets. A similar pattern was described by Serrano et al. (2011)
Núñez-Retana et al. (2020), PeerJ, DOI 10.7717/peerj.9766 14/20

because of the higher calorific value of pine. The values were above the limit
established by standard EN 14961-2 (Spanish Association for Standarization (UNE),
2012) (16.5–19.0 MJ kg
−1
), and the pellets can therefore be used for residential or industrial
applications.
The calorific values obtained here were within the range reported for hardwoods
(17.63–20.80 MJ kg
−1
)byTelmo & Lousada (2011). They are also similar to those reported
by Monedero, Portero & Lapuerta (2015) for poplar and pine mixtures, and by Miranda
et al. (2011) for Pyrenean oak waste. However, the addition of this raw material to
washed grape pomace decreased the calorific value of the pellets. On the other hand,
Liu et al. (2016) reported a value of 18.2 MJ kg
−1
for pine pellets, which is lower than the
value reported here. The difference in values can be attributed to the physical and chemical
properties, which can vary widely among different species (Miranda et al., 2015) and
which are also influenced by the location, tree age, genetics and wood section in the canopy
(Dos Santos Viana et al., 2018).
CONCLUSIONS
The addition of P. durangensis sawdust to Q. sideroxyla,Q. rugosa,Q. laeta and
Q. conzattii sawdust improved the bulk density and calorific value of the pellets made
with the material. Making pellets with mixtures of oak and pine sawdust is therefore
a potentially valuable alternative means of disposing of the by-products Quercus
material generated by the forestry industry. On the other hand, the moisture and ash
contents of the oak-pine pellets were in accordance with the limits established by
standard EN 14961-2 (≤10% and ≤0.7%, respectively). Addition of the pine sawdust also
improved the bulk density, with values reaching 703 kg m
−3
, so that the pellets met the
requirements specified by EN 14961-2 (≥600 kg m
−3
). The mechanical hardness and
IRI were lower in the pellets containing pine sawdust than in the other pellets. The calorific
value of all mixtures increased with the addition of pine sawdust, reaching a maximum of
19.8 MJ kg
−1
. Mixing oak and pine sawdust produced pellets with acceptable values for
important traits included in the international standards, which are used as quality
parameters.
ACKNOWLEDGEMENTS
We thank Dr. Claudia Edith Bailón-Soto for assistance with the translation of this
manuscript.
ADDITIONAL INFORMATION AND DECLARATIONS
Funding
This research was funded by Fondo de Sustentabilidad Energética, grant number
SENER-CONACYT 2014 246911 Clúster de Biocombustibles Sólidos para la generación
térmica y eléctrica y CONACYT project 166444. The funders had no role in study design,
data collection and analysis, decision to publish, or preparation of the manuscript.
Núñez-Retana et al. (2020), PeerJ, DOI 10.7717/peerj.9766 15/20

Grant Disclosures
The following grant information was disclosed by the authors:
Fondo de Sustentabilidad Energética: SENER-CONACYT 2014 246911.
Clúster de Biocombustibles Sólidos para la generación térmica y eléctrica y CONACYT:
166444.
Competing Interests
Christian Wehenkel is an Academic Editor for PeerJ.
Author Contributions
Víctor Daniel Núñez-Retana conceived and designed the experiments, performed the
experiments, analyzed the data, prepared figures and/or tables, and approved the final
draft.
Rigoberto Rosales-Serna performed the experiments, authored or reviewed drafts of the
paper, and approved the final draft.
José Ángel Prieto-Ruíz analyzed the data, authored or reviewed drafts of the paper, and
approved the final draft.
Christian Wehenkel analyzed the data, prepared figures and/or tables, authored or
reviewed drafts of the paper, and approved the final draft.
Artemio Carrillo-Parra conceived and designed the experiments, performed the
experiments, analyzed the data, authored or reviewed drafts of the paper, and approved
the final draft.
Field Study Permissions
The following information was supplied relating to field study approvals (i.e., approving
body and any reference numbers):
The forest harvest permissions were approved and provided by the Secretariat of the
Environment and Natural Resources on cutting areas Chinatú (SG.FO-08-2014/52),
El Nopal (SG.FO-08-2014/129), El Pinto (SG.FO-08/2015/40), El Tule y Portugal
(SG.FO-08-2014/82) and Llano Blanco (SG.FO-08/-2014/91) on Chihuahua State, Nicolas
Romero (SG/130.2.2.2/002203/17) and El Regocijo (SG/130.2.2.2/002243/11) on Durango
State.
Data Availability
The following information was supplied regarding data availability:
The raw data are available in the Supplemental File.
Supplemental Information
Supplemental information for this article can be found online at http://dx.doi.org/10.7717/
peerj.9766#supplemental-information.
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