b i o m a s s a n d b i o e n e r g y 3 5 ( 2 0 1 1 ) 4 8 4 9 e4 8 5 4

Available online at www.sciencedirect.com

http://www.elsevier.com/locate/biombioe

Energy efficient pilot-scale production of wood fuel pellets

made from a raw material mix including sawdust and
rapeseed cake
Magnus Stahl*, Jonas Berghel
Department of Energy, Environmental and Building Technology, Faculty for Technology and Sciences, Karlstad University,
Universitetsgatan 2, SE-651 88 Karlstad, Sweden

article info

abstract

Article history:

Presently, most fuel pellets are made from sawdust or shavings. In Sweden, these mate-

Received 14 May 2008

rials are used to the maximum extent. As the demand for pellets increases, the supply of

Received in revised form

sawdust will be insufficient and other raw materials or mixes of raw materials will be used.

28 September 2011

This work investigates sawdust mixed with rapeseed cake. The latter is a residual product

Accepted 6 October 2011

from the production of chemically unmodified oil refined from cold-pressed rape oil. At the

Available online 27 October 2011

Department of Energy, Environmental and Building Technology at Karlstad University,

Sweden, a complete pilot-scale pellet production unit is located. The pellets are produced

Keywords:

and tested for mechanical durability, length, bulk density and moisture content according

Drying

to the Swedish Standard for pellets. During production, the load current, the die pressure

Wood fuel pellets

and the die temperature were measured along with other parameters. The main purpose

Quality

was to examine how the mixture of rapeseed cake and pine sawdust affected the energy

Energy efficiency

consumption of the pelletising machine and mechanical durability of mixed fuel pellets.

Additive

The results show that the energy consumption decreased and the amount of fines

Rapeseed cake

increased with increasing rapeseed cake in the wood fuel pellets. These results indicate
that we must compromise between a decrease in the use of energy and a decrease in
durability.
2011 Elsevier Ltd. All rights reserved.

1.

Introduction

According to Ljungblom [1], there are more than 442 pellet

plants in the world (excluding small plants) with a total
production capacity of 14 billion kilograms a year. The total
usage in Sweden accounted for 1.7 billion kilograms (with
a energy value of 8 TWh) of wood fuel pellets in 2006 [2]. The
pellets are used both in large and small-scale applications,
such as district and household heating. In Sweden, the use of
wood fuel pellets for household heating purposes increased
significantly between 2004 and 2006 with some 33% [2]. This is
the logical consequence of the increase in prices of oil and

electricity during recent years as well as of the taxes on carbon

dioxide emissions from fossil fuels, which makes renewable
energy sources economically favourable. Therefore, many
households have converted their heating systems into wood
fuel pellets heating systems.
As a result of the increased use of wood fuel pellets during
the last decade the preferred raw materials for production of
wood pellets, such as cutter shavings and sawdust, are used to
the maximum extent. Increased import could be a solution
but with a raised demand for pellets the supply of sawdust will
be insufficient and other raw materials or mixes of raw
materials have to be used. Martinsson [3] gives examples of

* Corresponding author. Tel.: 46 54 700 1000; fax: 46 54 700 1165.

E-mail address: Magnus.Stahl@kau.se (M. Stahl).
0961-9534/$ e see front matter 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.biombioe.2011.10.003

4850

b i o m a s s a n d b i o e n e r g y 3 5 ( 2 0 1 1 ) 4 8 4 9 e4 8 5 4

such raw materials, e.g., light thinning material, cull tree,

bark, peat, and logging residues. In a study of pellet production from woody biomass, such as light thinning material, cull
sterberg [4]
tree, logging residues and Salix, Martinsson and O
conclude that the quality properties of these pellets are the
same as of sawdust pellets, with the exception of the former
having a higher ash content.
Martinsson [3] predicts that in the long run both agricultural energy crops and lignin could come into use for pellet
production. These could be used on their own or by mixing
them with sawdust. One potentially new raw material is the
rapeseed cake, which is a residual product from rape oil
production. It is environmentally favourable to reuse waste
from one manufacturing line in another since the waste
product replaces virgin wood and, in addition, solves
a disposal problem. Concerning rapeseed pellets or rapeseed
mixed with sawdust in wood fuel pellets, no scientific
research has been made. However, Kukelko et al. [5] investigate the physical properties of rapeseed meal in order to
predict bin wall pressures. Furthermore, several authors have
studied the use of alternative raw materials for pellet
production. Paulrud and Nilsson [6] studied briquetting and
combustion of reed canary grass with respect to ash compo rberg et al. [7] investigated the pelletising and
sition. O
combustion of reed canary grass in order to make it more
mobile through the die, and Fasina [8] studied the physical
hman et al.
properties when producing peanut hull pellets. O
[9] showed that the quality of pellets may be improved by
using hydrolysis residue from a lignocellulosic ethanol
production. The improvements concern properties such as
higher heating values and lower ash contents as well as lower
slagging tendencies and lower emissions of fine particles than
is the case with stemwood pellets.
Several authors have studied the production of pellets in
order to investigate the pellets quality parameters, such as
those listed in national or international standards, e.g., SS
187120 [10] and CEN/TS 14691 [11]. Some studies have been
performed using single pellet production techniques [12], some
using laboratory scale (<50 kg per hour) appliances [13e15] and
some using a pilot scale (<300 kg per hour) machine [4,7].
Whatever the pelletising equipment used, the experimental
setups described in these articles have a few things in common,
for example: controlling the moisture contents going from raw
material to pellets, measuring die pressures and die temperatures, and storing pellets in plastic bags after production to
preserve the moisture content. After production, the pellets are
analysed in order to test various quality criterions. However,
a standardised production and a quality experimental set-up
do not seem to exist. Therefore, part of the method was
developed along the way. However, it was developed in
conformity with the studies mentioned above.
In this work I describe how wood fuel pellets were made
from sawdust mixed with various percentages of rapeseed
cake in a pilot-scale pelletising machine. The aim was to
investigate how the energy consumption of the pelletising
machine changes when increasing amounts of rapeseed cake,
within the test range (the optimum mix of rapeseed cake is not
investigated), are used. Furthermore, an analysis concerning
the pellet quality was made that investigated how physical
parameters, such as mechanical durability (amount of fines),

bulk density and length of pellets, are affected by the use of

rapeseed cake. In addition, a method to produce pellets in
a pilot-scale pelletising machine was developed and
improvements were presented. Finally, a discussion is held
concerning the magnitude of the energy changes and possible
future users of the produced pellets.

2.

Materials and method

The raw materials used for the production of pellets were

sawdust and rapeseed cake. The sawdust, from Scots Pine
(Pinus Sylvestris), was produced at a local sawmill that uses
frame saws. The wet sawdust had moisture content of
52e60%.1 Before pelletising, the sawdust was dried at
200e220  C in a pilot-scale steam dryer [16] to reach a moisture
content of 10.0e14.4%. Additionally, rapeseed cake (pulverous) from the Ecoil Company, with moisture content of
6.6e7.3% was mixed with the sawdust. In the diagonal mixer
(in which the materials are completely mixed after 5 min), the
rapeseed cake was mixed with sawdust in batches of about
60e100 kg (larger batches were needed during the starting up
period to raise the temperature to the desired level). The
rapeseed cake came from the production of Ecoil that is
chemically unmodified oil refined from cold-pressed rape oil.
An analysis was made to determine the amount of rape oil still
in the rapeseed cake. The rapeseed cake samples (0.010 kg)
were extracted using 50,106 m3 of acetone in a Soxhlet
extractor for 6 h. The extractive matter was then dried
according to SS 187170 [17]. The result showed that the level of
rape oil in the rapeseed cake was 18%.
The pellets were produced in a production unit (Fig. 1)
located at the Department of Energy, Environmental and
Building Technology at Karlstad University, Sweden. It
consists of: (1) a storage for wet raw material; (2) a pilot-scale
steam dryer; (3) a diagonal mixer, in which different materials
can be added; (4) a conveyor screw; (5) conditioning (if needed)
in a feeder; (6) an Amandus Kahl C33-390 pelletising press
with a flat die (The flat die has nine holes radius with 52 holes
en each row total 468 holes. The die has a, working width
0.075 m, hole diameter 0.008 m, effective compression length
0.042 m, total thickness 0.050 m, relief depth 0.02 m, hole inlet
diameter 0.00102 m, degree of inlet taper 17 , and no cutting
blade. The open area of die is 64%.) and a maximum output of
300 kg per hour; (7) a cooling tower.
The test method used was to some extent developed along
the way, mostly because the new equipment set-up needs
a running in period. For example, the new die has to be used
for some hours to work satisfactory. In accordance with
earlier studies, the tests should be performed under similar
steady conditions concerning die pressure, die temperature,
screw frequency (material flow control) and moisture content
of the raw material for reliable results. After a few initial tests,
it was decided that the moisture content of the pellet raw
material should be kept constant at approximately 12%, the
die pressure was set at a constant of approximately 8.5 MPa,
and the die temperature was kept at a constant level during
the tests quantity take out. Before running the last series of
1

All moistures are % wet mass basis.

4851

b i o m a s s a n d b i o e n e r g y 3 5 ( 2 0 1 1 ) 4 8 4 9 e4 8 5 4

where P is the average power consumption; UL is the line

voltage; I is the load current and cos f is the effect factor.
Line voltage (UL) for three-phase alternating currents and
star connection was calculated as:
p
UL UPN 3

(2)

where UL is the line voltage and UPN is the phase-to-neutral

voltage (average measured value 396 V).
For the presentation of the result, the power consumption
is converted into average energy consumption per kg (ds) as
follows:
_ DS
Q P=m
Fig. 1 e The pellet production line at Karlstad University.

tests, the pelletising machine, especially the die, was reconditioned (all the holes were cleaned out before the tests) in
order to investigate its potential importance. A reference test
was made with sawdust only at a screw frequency of 3.5 Hz.
The remaining tests were performed according to Table 1.
During the pellet production, the die pressure, die
temperature, screw frequency and load current of the pelletising machine were measured every 10 s. For each test (batch),
a quantity of approximately 15 kg of pellets was taken out for
analysis during steady conditions, i.e., when the die temperature and die pressure were steady at a constant level. The
test quantities were taken out during 5e10 min of production
(depending on the screw frequency), i.e., the material flow was
measured. The pellets were cooled down to ambient room
temperature and sieved before the analysis was made. They
were stored in plastic bags after cooling. The analysis was
done by testing and comparing the produced pellets with the
quality parameter settings in the Swedish Standard [10]. The
tested parameters were2:
1) The moisture content (%) determined according to SS
187170 [17]. Two samples of approximately 300 g each of
raw material, raw material mixture and cooled pellets were
examined.
2) The average length (mm) determined by measuring the
length of two samples of 30 randomly chosen cooled pellets
using a slide calliper (SS 187120 [10]).
3) The bulk density (kg m3) determined by measuring the
weight of a 0.002 m3 bucket filled with cooled pellets (three
samples).
4) The mechanical durability determined according to SS
187180 [18]. It is presented as the amount of fines/
percentage of fine particles < 3 mm (two samples).
From the measured load current, the average power
consumption for alternating currents was calculated according to:
P UL Icos4

(1)

(3)

where Q is the average energy consumption; P is the average

_
power consumption; and m DS is the average material flow (kg
dry substance) per second through the die.

3.

Results

3.1.

Data

The measured conditions under which the pellets were

produced are presented in Table 2.
The material flow and the power of the pelletising machine
were determined (see Table 3).

3.2.

Energy consumption

The total energy consumption of the pelletising machine is

the sum of the energy needed for (1) the no load operation, (2)
the mechanical pressing of the sawdust through the die (to
overcome the friction and the pressureevolume work), (3) the
water evaporation in the press and (4) the losses (conversion
of mechanical energy into heat; loss of heat due to friction and
the heating of pellets). The energy consumption is also
affected by parameters such as type of raw material and die
temperature. When the amount of rapeseed cake mixed in
with sawdust pellets increases, it seems to decrease the
energy consumption of the pelletising machine (Fig. 2). The
trend is also the same when the energy used for the evaporation of water in the pelletising press is excluded.
An increased amount of water will be transported out of
the raw material when the die temperature increases during
pelletising (Fig. 3). This is also the case when the material flow
is decreased. Increased amounts of evaporated water during
pelletising seem to imply that the energy consumption
increases. Fig. 3 also shows that the die temperature increases
when the amount of rapeseed cake in the raw material
mixture decreases.

Table 1 e Test matrix.

Amount of Rapeseed Cake (%)

The remaining quality parameters of Standard 187120 (1998), i.

e., contents of ash, moisture, Sulphur, Chlorine, additives, and
ash melting point, are not considered.

10
20
30

Screw Freq. (Hz)

3.5
3.5
3.5

4.0
4.0
4.0

7.0
7.0
7.0

4852

b i o m a s s a n d b i o e n e r g y 3 5 ( 2 0 1 1 ) 4 8 4 9 e4 8 5 4

Table 2 e Measured data during pellet production.

Test No.

Amount of
Rapeseed Cake (%)

P0
P10a
P10b
P10c
P20a
P20b
P20c
P30a
P30b
P30c
P30c
P30d

0
10
10
10
20
20
20
30
30
30
30
30

Die Pressure
(MPa)
8.65
8.80
8.56
8.85
8.63
8.68
8.19
8.77
9.07
8.29
8.22
9.50














0.04
0.06
0.09
0.14
0.03
0.04
0.06
0.12
0.06
0.03
0.03
0.07

Die Temperature
( C)
117.5
138.2
145.7
100.6
118.0
117.8
95.1
121.5
115.0
81.7
82.8
83.5














Dried Raw
Material MCa (%)

Pellet
MCa (%)

14.4
10.4
10.4
13.8
10.0
10.0
12.5
12.0
12.0
13.8
13.8
13.8

5.75
1.25
1.13
8.94
2.88
3.22
9.33
6.22
6.91
11.65
11.33
11.35

3.9
2.3
1.1
0.3
0.3
0.4
0.5
2.9
1.3
0.1
0.2
0.1

Screw Freq.b
(Hz)
3.7
3.6
4.0
7.0
3.5
4.0
7.0
3.6
4.0
7.0
7.0
7.0














0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.2
0.1
0.1
0.1
0.1

Load Current
(A)
25.4
30.0
31.2
29.5
25.8
26.0
24.3
23.2
24.0
23.8
23.8
23.7














1.1
0.6
1.0
0.7
0.7
0.9
0.5
0.5
0.4
0.4
0.4
0.5

a MC moisture content.
b Changes in screw frequency affect the material flow through the die. It varied from 0.020 kg s-1 to 0.060 kg s-1.

3.3.

Mechanical durability

3.5.

According to SS 187120 [10], the mechanical durability is

expressed as the amount of fines in the pellets. The amount of
fines seems to increase with an increased amount of rapeseed
cake in the pellets. The results, however, are not unambiguous (Fig. 4). Before running the tests at 7.0 Hz, the pelletising
machine was reconditioned and the tests were performed in
very similar conditions. Therefore, the curve at 7.0 Hz may be
the most reliable one, showing that the amount of fines
increases with an increased amount of rapeseed cake in the
pellets.
According to Standard 187120 [10], the amount of fines
should be below 0.8%. Two pellet assortments did not fulfil
this criterion. These pellets were produced at 3.5 Hz with 20
and 30% of rapeseed cake, respectively (Fig. 4).

3.4.

Bulk density

The bulk density of the pellets decreased with an increased

amount of rapeseed cake in the pellets (Fig. 5).

Table 3 e Calculated data from the pellet production.

Test No.

P0
P10a
P10b
P10c
P20a
P20b
P20c
P30a
P30b
P30c
P30c
P30d

Amount of
Rapeseed
Cake (%)

Power
Consumption
(kW)a

Material
Flow (kg ds/s)

0
10
10
10
20
20
20
30
30
30
30
30

9.80
11.54
12.02
11.37
9.95
10.02
9.35
8.94
9.24
9.18
9.18
9.11

0.019
0.023
0.026
0.045
0.024
0.029
0.054
0.023
0.033
0.044
0.044
0.044

a Maximum Power Consumption 21.56 kW.

Length

According to the Swedish Standard 187120 [10], the pellets

should, at a maximum, be 4 times the diameter of 8 mm
pellets. All pellets produced fulfilled this criterion, the average
lengths ranging from 0.0055 to 0.0115 m. The length of the
pellets seems to increase with an increased amount of rapeseed cake in the pellets and an increased moisture content,
which has also been observed by Lehtikangas [19].

3.6.

Method development

During method development, we found that the pelletising

machine and especially the die should preferably be reconditioned before an analysis of the test results is made. The
pelletising machine was reconditioned before running the
tests at 7.0 Hz. Fig. 3 shows that these tests were performed at
lower die temperatures than the tests at 3.5 and 4.0 Hz. The
reason for this could be that some of the holes in the die could
be clogged if the machine is not reconditioned, which could
cause the increase in temperature. Furthermore, for reliable
results the tests should be performed under similar steady
conditions concerning die pressure, die temperature, screw
frequency and moisture content of the raw material. In their
production of pellets, the Swedish manufacturers normally
dry the raw material to water content of 8e12% before the

Fig. 2 e Average energy consumption of the pelletising

machine vs. amount of rapeseed cake in the pellets.

b i o m a s s a n d b i o e n e r g y 3 5 ( 2 0 1 1 ) 4 8 4 9 e4 8 5 4

Fig. 3 e Amount of evaporated water from the pelletising

machine vs. die temperature.

pelletising process begins. We found that a moisture content

of about 14% in the raw material is needed if good quality
pellets with a moisture content of about 6e8% are to be
produced. These pellets fulfil the criterions tested in the
Swedish Standard 187120 [10].

4.

Discussion

The industry demands energy efficient solutions for their

production since these not only save energy but also money.
In the work presented in this article, we found that the energy
consumption of the pelletising machine decreased with an
increasing amount of rapeseed cake being mixed in with the
sawdust pellets, irrespective of whether the energy used for
the evaporation of water is excluded or not. It could be that the
oil in the rapeseed cake made the material flow easier through
the die due to less friction, which also decreases the wear of
the die. The coefficient of friction depends on the moisture
content and the roughness of the surface and it varies little
with species, except for those species that contain abundant
oily or waxy extractives [20]. This the amount of energy saved
by using rapeseed cake could be significant for a large-scale
producer of pellets. The potential energy savings (per kg of
dry material) when comparing a 0% rapeseed cake mix with
a 10% mix, all other parameters similar between the two tests,
amount to 5.6 MWh for our pelletising machine (Amandus
Kahl C33-390 pelletising press).

Fig. 4 e Amount of fines in the cooled pellets vs. amount of

rapeseed cake in the pellets.

4853

Fig. 5 e The bulk density of the pellets vs. the amount of

rapeseed cake in the pellets.

A large pellet plant in Sweden can produce 90 million kilograms of pellets a year (the total energy value of that amount of
pellets is about 420 GWh). For this kind of industry, the energy
saved would amount to 0.3 GWh a year. The amount of saved
energy is much greater between the most energy efficient test
(30%) and the test using pure sawdust. However, using as much
as 30% rapeseed cake in pellets is hardly possible. The price is
too high since it is used by other industries, i.e., it is exposed to
competition. Additionally, one should be careful making the
energy saving comparisons since many parameters affect the
energy consumption of the press, for example, the amount of
evaporated water during pelletising.
The tests demonstrated that an increased amount of
rapeseed cake in the pellets seems to decrease their
mechanical durability. Could it be that the rapeseed cake
interferes with the adherence mechanisms of wood? The
extractive matter in the rapeseed cake does not seem to act as
a binding agent. The reason could also be that this parameter,
i.e., the amount of fines caused by the rapeseed cake, is
sensitive in comparison with other parameters such as
moisture content [21,22] and performance of the pelletising
machine. If the amount of fines increases and pellets are to be
produced using rapeseed cake, precautions against a dusty
work environment have to be taken in the pellet plant.
The bulk density of pellets decreased with an increased
amount of rapeseed cake in the pellets. This could be due to
the difference in density, but we also found that it is possible
that increased moisture content decreases the bulk density.
This has also been observed by Fasina [8]. Which one of the
parameters that affects the bulk density the most is a question
that has to be examined further. In conformity with Lehtikangas [19], no correlation between length of pellets and bulk
density was found.
Concerning the length of the pellets, none of the pellets
produced were longer than stated in the standard [10]. On the
contrary, our production sometimes contained a large quantity of short pellets (<0.005 m). Pellets of too short length
might cause the same problems as fine particles do in feeding
screws and storage, e.g., stoppage. This could call for a change
in the standard, i.e., in addition to having a maximum length,
introducing a minimum pellet length.
Since the rapeseed cake affects the durability negatively, it
is necessary to be cautious should household users use these

4854

b i o m a s s a n d b i o e n e r g y 3 5 ( 2 0 1 1 ) 4 8 4 9 e4 8 5 4

pellets. Obernberger and Thek [21] even suggest that mixes of

raw materials should be avoided if the pellets are to be used in
small-scale plants. Presently, the household users represent
50% of the wood fuel pellet market in Sweden. Accordingly,
their demands of a high-grade fuel are even more important
than earlier to the pellet industry. Moreover, the households
demand quality since, e.g., they have relatively simple
conversion installations, most often without advanced
controls or professional management [23]. This means that
the household users cannot be as tolerant of differences in
pellet quality as large-scale users. The fact that pellets
represent a standardised fuel (which simplifies the construction and operation of burners) implies that if alternative raw
materials are to be mixed in the newly produced pellets, these
should fulfil the criterions in the standard, especially if the
household market is the target. Therefore, we need to make
more studies concerning alternative raw materials.

5.

Conclusions

The energy consumption of the pelletising machine decreases

with an increase in the amount of rapeseed cake in wood fuel
pellets.
The mechanical durability may decrease with an increased
amount of rapeseed cake in wood fuel pellets.
The bulk density of pellets seems to decrease with an
increased amount of rapeseed cake in pellets.
Tests done with a reconditioned die and cleaned pelletising
machine seem to be preferable. These test results displayed
the smallest standard deviation.

Acknowledgments
I would like to express my gratitude to Prof. Bengt Mansson for
fruitful discussions and comments on my work. I would also
like to thank Lars Pettersson for all his help during the
production of pellets and Dr. Karin Granstrom for all her help
with the rape seed cake tests. Thank you also to Ecoil for the
provision of rapeseed cake.

references

[1] Ljungblom L. The Pellets Map 2007 e the discussion.

Bioenergy Int 2007;29(6).
[2] The Swedish Association of Pellet Producers (PiR). Statistics
concerning wood fuel pellets in Sweden. Retrieved at:
http://www.pelletsindustrin.org/page/1/36.html.
Stockholm, Sweden. 2007-11-13 (In Swedish.)
[3] Martinsson L. Materials in Sweden for future production of
fuel pellets e A review of possible materials in short- and
medium long-term. Stockholm, Sweden: Varmeforsk; 2003.
Report 813 (In Swedish. Summary in English.).
sterberg S. Pelletising using forest residues
[4] Martinsson L, O
and Salix as raw materials e A study of the pelletising
properties. Stockholm, Sweden: Varmeforsk; 2004. Report
876 (In Swedish. Summary in English.).

[5] Kukelko DA, Jayas DS, White NDG, Britton MG. Physical
properties of Canola (Rapeseed) meal. Can Agric Eng 1988;
30(1):61e4.
[6] Paulrud S, Nilsson C. Briquetting and combustion of Springharvested reed canary-grass: effect of fuel composition.
Biomass Bioenergy 2001;20:25e35.
rberg H, Kalen G, Thyrel M, Finell M, Andersson LO.
[7] O
Pelleting of reed canary-grass. Umea, Sweden; unit of
biomass Technology and Chemistry. Swedish University of
Agricultural Sciences; 2006. BTK-report 2006:12.
[8] Fasina OO. Physical properties of peanut hull pellets.
Bioresour Technol 2008;99(5):1259e66.
hman M, Boman C, Hedman H, Eklund R. Residential
[9] O
combustion performance of pelletized hydrolysis residue
from lignocellulosic ethanol production. Energy Fuels 2006;
20:1298e304.
[10] SS 187120. Biobranslen och torv e Branslepellets e
Klassificering. (Biofuels and peat e Fuel pellets Classification). 3 pp. Stockholm, Sweden: Swedish Standards
Institute (SIS); 1998e06-10 (In Swedish.).
[11] SIS-CEN/TS 14961. Fasta biobranslen e Specifikationer och
klassificering. (Solid biofuels e Fuel specifications and
classes). 41 pp. Stockholm, Sweden: Swedish Standards
Institute (SIS); June 2005.
[12] Rhen C, Gref R, Sjostrom M, Wasterlund I. Effects of raw
material moisture content, Densification pressure and
temperature on some properties of Norway Spruce pellets.
Fuel Process Technol 2005;87:11e6.
hman M, Gref R, Wasterlund I. Effect of raw material
[13] Rhen C, O
composition in woody biomass pellets on combustion
characteristics. Biomass Bioenergy 2007;31:66e72.
[14] Pichler W, Greinocker C, Golser M. Pellet quality
Optimisation e Systematic analysis of Influencing factors
along the production-process and Microwave and H2O2
activation of the raw material. Proceedings of the 2nd world
Conference on pellets, Pellets 2006; 30 May e 1 June 2006.
Jonkoping, Sweden.
[15] Holm JK, Henriksen UB, Hustad JE, Srensen LH. Toward an
understanding of controlling parameters in softwood and
hardwood pellets production. Energy Fuels 2006;20:2686e94.
[16] Berghel J, Renstrom R. Controllability of product moisture
content when non-screened sawdust is dried in a Spouted
Bed. Drying Technol 2004;22(3):505e17.
[17] SS 187070. Biobranslen och torv e Bestamning av Total
fukthalt. (Biofuels and peat e Determination of total
moisture). 4 pp. Stockholm: Swedish Standards Institution
(SIS); 1997e12-04 [In Swedish.].
[18] 187080 SS. Fasta biobranslen och torv e Bestamning av
mekanisk hallfasthet hos pellets och briketter. (Solid biofuels
and peat e Determination of mechanical strength for pellets
and briquettes). 4 pp. Stockholm: Swedish Standards
Institution (SIS); 1999e12-01 [In Swedish.].
[19] Lehtikangas P. Quality properties of pelletised sawdust,
logging residues and bark. Biomass Bioenergy 2001;20:
351e60.
[20] Forest Products Laboratory. Wood Handbook e wood as an
Engineering material. General Technology Report FPL-GTR113, Forest Products Laboratory, Forest Service, U.S. Madison,
WI: Department of Agriculture; 1999.
[21] Obernberger I, Thek G. Physical characterisation and chemical
composition of densified biomass fuels with regard to their
combustion behaviour. Biomass Bioenergy 2004;27:653e69.
hman M, Vinterback J. Pellets quality effects of raw
[22] Jirjis R, O
material properties and manufacturing process parameters.
Report No. 14, ISSN 1651e0720. Uppsala, Sweden:
Department of Bioenergy; 2006.
[23] Langheinrich C, Kaltschmitt M. Implementation and
application of quality assurance systems. Biomass Bioenergy
2006;30:915e22.