Energy & Fuels 2009, 23, 3211–3216
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Effect of Fiber Orientation on Compression and Frictional Properties of Sawdust Particles in Fuel Pellet Production Niels Peter K. Nielsen* Forest & Landscape, Faculty of Life Sciences, UniVersity of Copenhagen; RolighedsVej 23, DK 1958, Frederiksberg C, Denmark
Jens Kai Holm Chemical Engineering, DONG Energy Power A/S; A.C. Meyers Vænge 9, DK 2450, Copenhagen SV, Denmark
Claus Felby Forest and Landscape, Faculty of Life Sciences, UniVersity of Copenhagen; RolighedsVej 23, DK 1958, Frederiksberg C, Denmark ReceiVed October 23, 2008. ReVised Manuscript ReceiVed May 3, 2009
The increasing production of wood pellets has increased the importance of optimizing the raw materials. The pelletizing process is affected by differences in the raw materials, but knowledge of which of the wood’s properties that cause these differences is limited. The present study investigates the effect of fiber orientation in the raw material particles of sawdusts from European beech (Fagus sylVa´tica L.) Quaking aspen (Populus tremula´ L.) and Scots pine (Pinus sylVestris L.) on their pelletizing properties. Sawdusts with the fibers oriented along the plane of the particles (longitudinal fiber orientation) and across the plane of the particles (transverse fiber orientation), respectively, were prepared, and the effect was quantified by measuring the compression and frictional properties of the sawdust in single-pellet productions, along with measuring the pellet strength. The results showed that sawdust with transverse fiber orientation required less energy to compress, and to press through the die, while producing pellets with the same or higher strength, compared to the longitudinal fiber orientation. Also, the frictional properties of pellets made from beech and aspen with longitudinal fiber orientations were significantly higher than pine, although no significant difference in friction was shown between species, when the pellets were made with transverse fiber orientation in the particles. The study shows that in the preparation of raw materials for wood pellet production, a transverse fiber orientation in the particles should be preferred to optimize the production capacity and minimize the energy consumption of the pellet mills. In addition, the study shows that the fiber orientation must be considered when analyzing the pelletizing properties of sawdust.
Introduction Sawdust from primary and secondary wood processing industries is used as raw materials for wood pellets. The pellets are cylindrical (diameter 6-12 mm, length of 5-30 mm) and are used for residential and centralized heat and power production. In 2007, the annual demand for wood pellets worldwide was ∼8 million tons, which is expected to increase to 15 million tons within 2010.1,2 The pellets are produced in pellet mills, by extruding the raw material through cylindrical channels in a ring-shaped steel die.3 The die is rotating and two rolls run on the inner surface of the die. As the raw material is added in front of the rolls on the inner surface of the die, the rolls force * Corresponding author e-mail:
[email protected]. (1) Vinterba¨ck, J. Internationell PelletsmarknadsutVeckling. In Conference presentation at SVEBIO Pellets 08, Sundsvall, Sweden, Jan 30-31, 2008. (2) Ljungblom, L. Bioenergy Int. 2007, 29, 9–23. (3) Leaver, R. H. Wood pellet fuel and the residential market. In Conference proceeding at The Ninth Biennial Bioenergy Conference, Bioenergy 2000 s Moving the technology into the marketplace, Oct. 1-2, 2000, Buffalo, NY, Jan. 10. 2000.
the raw material through the channels as they roll over the press channel openings. The friction between the sawdust and the press channel generates a force in the opposite direction, which results in compression of the sawdust.4 The die temperature may reach 120-130 °C, caused by the friction only, and the pressure may reach 200-450 MPa.3,5 The sawdust is continuously added in front of the rolls, and a flow of compressed sawdust thus passes through the channels. At the end of the channels, the compressed sawdust breaks off and forms the pellets. The pellets maintain their shape and density due to bonding that occurs between the particles at the high pressure in press channels. The durability of the pellets is an important quality parameter because a minimum of dust and debris formation is wanted during transport and handling. The durability may be closely related to the strength of the bonding between the particles, as the bonding will contribute to the physical stability of the pellets. (4) Holm, J. K.; Henriksen, U. B.; Hustad, J. E.; Sorensen, L. H. Energy Fuels. 2006, 20, 2686–2694. (5) Nielsen, N. P. K.; Gardner, D. J.; Poulsen, T.; Felby, C. Wood Fiber Sci. 2009, Accepted for publication.
10.1021/ef800923v CCC: $40.75 2009 American Chemical Society Published on Web 05/22/2009
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The compression and frictional properties of the raw material are essential for the load on the pellet mill, because energy is required to compress the raw material and to extrude the compressed raw material through the press channels. The mill load influences the costs of production by the energy requirements (kWh) and capacity (kg/h) of the mill, and by the lifespan of the die and rolls. Significant differences in the load are caused by differences in the raw material’s properties, which vary according to, for example, wood species, temperature, moisture content, three-dimensional shapes of the particles, production method, and storage of the raw material. In particular, experience from the pellet industry shows that hardwood sawdust often implies a higher load than softwood sawdust. The increasing pellet production and limited local sawdust resources imply that other raw materials than sawdust are sought as alternatives. Therefore, methods for producing raw materials on site of the pellet factories are being implemented, but little is known about which raw material properties should be preferred to maximize the capacity of the pellet mills. Wood is an anisotropic material because its mechanical properties are strongly linked to the orientation of its fibers.6,7 Because of the high tensile strength of the fiber’s crystalline cellulose microfibrils, the strength of wood is highly dependent on the angle between the fiber orientation and the applied compression or tensile stress.6,8 Therefore, the fiber orientation may also affect the energy required to pelletize wood, because the pelletizing process involves significant changes in shape and dimensions of the raw material. On the basis of the Poisson’s ratios and friction coefficients of solid wood from the literature, Holm et al. developed a model to describe pressure required in the pelletizing process of different wood species, which showed good correlation with experimental work.4,9 The model assumes a perpendicular orientation of the fibers to the orientation of the press channel. This could be a reasonable assumption for some types of raw material particles (e.g., fiber bundles), but for sawdust particles, which can be been cut from different orientations on the fibers, this assumption is less appropriate. The importance of this difference between fiber bundles and sawdust for the pelletizing is not known, and the objective of the present study was therefore to investigate the effect of the raw material’s fiber orientation in the press channel. The effect was measured on the compression, the friction, and the strength of the pellet of different wood species to provide knowledge for optimizing the production and analysis of wood pellet raw materials. Materials and Methods Materials. Sawdusts from European beech (Fagus sylVa´tica, L.) (heart wood), Quaking aspen (Populus tre´mula, L.) (heart wood), and Scots pine (Pinus sylVestris, L.) (splint wood) were prepared with a chainsaw (Stihl MS 260, 0.325′′ chain) from boards purchased at a local lumberyard. The chainsaw was operated without chain oil, to prevent sample contamination. Sawdusts with two different fiber orientations were prepared by cutting parallel and across the fiber (6) Kollmann, F. F. P. Mechanics and Rheology of Wood. In: Principles of Wood Science and Technology; Kollmann, F. P. P., Coˆte´, W. A., Eds.; Springer-Verlag, Berlin: Heidelberg, 1968; pp 292-420. (7) Ellis, S.; Steiner, P. IAWA J. 2002, 23, 201–211. (8) Fengel, D.; Wegener, G. WOOD Chemistry Ultrastructure Reactions; Walter de Gruyter: Berlin, 1989. (9) Holm, J. K.; Henriksen, U. B.; Wand, K.; Hustad, J. E.; Posselt, D. Energy Fuels. 2007, 21, 2446–2449.
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Figure 1. Illustration of sawdust particles showing the difference in fiber orientation between T particles (A) and L particles (B). The black bars illustrate fibers.
orientation of the boards, respectively. In the following, transverse particle orientation (T particles) refers to fibers lying parallel to the particle’s z-axis, and longitudinal fiber orientation (L particles) refers to fibers lying parallel to the xy plane (Figure 1). The scanning electron microscope (SEM) images in Figure 2 further illustrate the difference in fiber orientations. The images also illustrate that the lumens of the T particles were collapsed, whereas the cell structures in the L particles were more-or-less intact. The sawdust was size fractionated in a sieve shaker (Retsch, AS 200 and Retsch sieves). For samples of beech and pine, the 0.5-1.0 and 2.0-2.8 mm fractions were used for further analysis, and for aspen, the 0.5-1.0, 1.0-1.4, and 1.4-2.0 mm fractions were used. Five grams of each sample were conditioned to 12% moisture content (based on dry matter) in a desiccator with 75% relative humidity, for 14 days at 23 °C. The samples were kept sealed at -18 °C prior to further analyses. Methods. A 10 mm diameter pellet die (Specac, Ltd., part nr 03100, modified) and an Instron 5566 materials testing system with a 10 kN loadcell were used for compression and friction measurements. The pellet die was fitted with a heated (HSS braid coil, HT 30 regulator, Horst GmbH) aluminum cylinder to set the temperature of the die to 120 °C. The compression of a pellet and subsequent extrusion were made in a two-step procedure (see Figure 3). In the first step, 250 mg of sample was poured into the pellet die and compressed at a rate of 25 mm/min. When the compression force reached 9000 N (115 MPa), the compression was stopped and the press piston position was kept for 18 s. In the second step, the base and stop piston were removed and the pellet was pushed out of the channel, at a speed of 25 mm/min. This formed a 3 mm high pellet, and this process was repeated six times for each sample. The force/displacement data was continuously logged on a computer. The examples in Figure 4 illustrate the data analysis. The areas under the compression and friction force/displacement plots were calculated and used for the work for compression Wcomp (in Joules) and the work for friction Wfric (mJ), respectively. By placing the cylindrical pellet horizontally (laying the pellet on its side), compression force could be applied by the Instron to the side of the pellet (Figure 5). A value for the pellet strength was measured as the maximum force the pellet could withstand before crushing. Compression speed was 25 mm/min and pellet temperature was 23-25 °C. The sawdust samples extractives contents were determined by 3 h of Soxhlet extraction (∼6 cycles/h) with HPLC grade acetone. The content was calculated from the decrease in dry matter content caused by extraction as percentage of the dry matter content before extraction. Duplicates of the species were extracted. Results and Discussion Compression Properties. The 115 MPa compressions densified the samples to ∼1.4 g/cm3 for all three species. Table 1 presents the measured Wcomp (J). The separations in low and high density compression are described below. The table shows
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Figure 2. Scanning electron microscope (SEM) images of sawdust particles from European beech (particle size fraction 2.0-2.8 mm). 1A and 1B show T particles having the fibers oriented parallel to the particle’s z-axis. 2A and 2B show L particles having the fibers oriented parallel to the particle’s xy plane.
Figure 3. The pellet die and the two-step pelletizing procedure. The force/displacement data (see Figure 4) of the compression and friction measurements were used for calculation of the energy required for each process. d ) 0 marks the 0.0 mm position in the plots in Figure 4.
that fiber orientation, species, and for the longitudinal fiber orientation, also particle size had a significant influence on Wcomp. Figure 6 further illustrates the differences in compression properties related to fiber orientations. From Table 1 and Figure 6, it is seen that Wcomp was lower for the T particles than the L particles, and that the T and L plots are clearly separated during the low density compression. However, at a specific density (∼0.8 for beech, ∼0.9 for aspen, and ∼0.8
Figure 4. Illustrations of data from the two-step pelletizing process and the plot areas used for sample characterization. In both plots, 0.0 mm refers to the d ) 0 position in Figure 3 The vertical dashed line in the friction plot (B) marks the point where the static friction is exceeded and the pellet starts to move. Compression was stopped at approximately 2 mm pellet height, and relaxation after pressure release is illustrated in the friction plot as springback. At -2 mm in the friction plot, the force declines as the pellet is pushed out of the channel.
for pine), the T and L plots join and the slope shifts upward, showing an increase in stress acceleration for further compression and also that the high density compression hereafter is less influenced by the fiber orientation. Two reasons are suggested for the difference in Wcomp low density between the T and L particles. First, the plots show that the compression stress of the L particles starts to build up earlier than the T particles, meaning that the initial density of the T particles were higher than the L particles. The SEM images in Figure 2 show collapsed lumens in the T particles,
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Figure 5. The test for the internal bonding strength of the pellet. The pellet was placed on its side (the pellet’s cylinder shape oriented horizontally) and force was applied to the pellet’s side with a press piston. The plot shows that a well-defined value for the strength could be found.
which indicates that the cutting action precompressed the particles. This is only seen for the T particles, where the cutting action had occurred across the microfibrils, which are primarily longitudinally oriented in the fibers.7,8 This is contrary to the L particles, which were cut parallel to the microfibrils. It requires a higher force to cut across compared to parallel of microfibrils;10 therefore, the lumens in the T particles were more collapsed, giving the T particles the highest initial density. Second, the T and L particles may have different strength properties. The initial sample density (∼0.2 g/cm3) (see Figure 6) shows that a large part of the uncompressed sample volume was made up by air cavities and gaps between the particles, because the normal density of solid wood is from ∼0.4 g/cm3 and upward depending on species. Therefore, the first part of the compression was as a bending and deformation process of the randomly oriented particles, in which the energy requirement was influenced by the particles strength properties. Due to the anisotropic nature of wood, the strength of the particles is highly dependent on the fiber orientation; and because the T particles had the fibers oriented across the plane of the particles (see Figure 1), they required less energy to bend and deform compared to the L particles. For the L particles, the angle between the bending stress and the fiber orientation would be close to 90°, giving the particles higher strength.6 Also, the process conditions exceeded the lignin glass transition temperatures.11-13 The strength properties of the T particles were therefore further weakened because the bending of the particles occurred along the lignin-rich middle lamella, which constitute the adhesive between the fibers, and which extends through the length of the z-axis of the T particles. Figure 6 also shows that the slope increases for all samples in the high density phase. This shift may reflect a transition from the process of macroscopic scale bending and deformation of the particles, to a microscopic scale process of deformation of the cell walls. The Wcomp high density in Table 1 shows that this phase also involves a difference between T and L particles. The reason may be related to the reinforcing properties of the microfibrils in the lignocellulose polymer matrix, where long fibers would add more strength to the matrix than short fibers. Due to the fiber orientation of the T particles in the short z-axis, they have shorter microfibril lengths than the L particles. This may be related to the importance of particle size for Wcomp, where it is seen that only for the L particles, increasing particle size correlated to the increasing Wcomp (Table 1). In the T particles, the microfibril lengths did not vary with particle size because the z-axis length (approximately 0.2 mm) for all particles was set by the cutting action of the chainsaw. For the L (10) (11) (12) (13)
McMillin, C. W.; Lubkin, J. L. For. Prod. J. 1959, 9, 361–366. Goring, D. A. I. Pulp Paper Mag. Canada. 1963, 64, 517–527. Back, E. L.; Salmen, N. L. Tappi J. 1982, 65, 107–110. Irvine, G. M. Tappi J. 1984, 67, 118–121.
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particles instead, increasing particle size increased the microfibril lengths because the xy plane increased with the particles size. Frictional and Strength Properties. Table 2 and Figure 7 show that the pellets made from L particles required more energy to push in press channel compared to the T particles, except for the 0.5-1.0 mm fraction of pine. Also, the biggest differences between species were seen with the L particles. The viscoelastic properties of the sawdust particles may explain these observations. Relaxation was observed after the compression procedure indicating that the compressed deformation of the fibers was not entirely plastic, which corresponds to the literature.14 After the compression, the pellets would therefore apply a stress on its surroundings, including a stress on the press channel wall (stresswall), which contributes to the friction force, when the pellet is pushed through the channel. Therefore, the Wfric data indicates that stresswall was highest for the L particles, or that the coefficient of friction was higher for the L particles. The anisotropic nature of wood cause the stress/strain ratios to be dependent on the fiber orientation,6,15,16 which may explain this difference between the T and L particles. When compression stress is applied to any elastic material, a positive strain will occur perpendicular to the compression stress (toward the press channel wall in this case) along with the negative strain caused by the compression (the decrease in pellet height). The ratio between these strains is referred to as the Poisson’s ratio, which for wood is highly dependent on the fiber orientation. It is assumed that the sawdust, which was initially poured randomly into the press channel, would gradually organize the xy plane of the particles horizontally, when force was applied by the press piston. The following Poisson’s ratio can therefore describe the differences between the T and L particles: ν)
strainxy-plane strainz-axis
(1)
If the sawdust particles are considered as small boards of solid wood, the strainxy-plane refers to the increase in board area (xy plane) when stress is applied parallel to the z-axis of the board, and strainz-axis refers to the decrease in board thickness. This ratio is smaller when the fibers are oriented parallel to the stress (the T particles) than if the fibers are oriented in the xy plane (the L particles).16 This means that the T particles expand less in the xy plane than the L particles, that is, they apply less stresswall. A compression stress along the particles z orientation will be the dominating orientation when the xy planes of the particles are oriented horizontally in press channel. Due to plastic deformation of the particles, some of the stresswall were preserved after the compression, and the L particles therefore had a higher friction because they implied a higher stresswall. Also, the force itself for pressing the pellet through the press channel will generate stress on the press channel wall, which is controlled by the same Poisson’s ratios. That Wfric is connected to plastic strain as a result of the compression is supported by the differences in pellet strengths (14) Shiraishi, N. Wood Plasticization. In: Wood and Cellulosic Chemistry; Hon D. N. S., Shiraishi, N., Eds.; Marcel Dekker, Inc.: New York, 2001; pp 655-700. (15) Green, D. W.; Winandy, J. E.; Kretschmann, D. E. Mechanical Properties of Wood. In: Wood Handbook-Wood As an Engineering Material. Gen. Tech. Rep. FPL-GTR-113; U.S.D.A., Forest Services, Forest Products Laboratory: Madison, WI, 1999; pp 4.1-4.45.
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Table 1. Energy Required for Compression (Wcomp) for All Samplesa compression works Wcomp Wcomp total (J) species
SFb
Beech
0.5-1.0 2.0-2.8 0.5-1.0 1.0-1.4 1.4-2.0 0.5-1.0 2.0-2.8
Aspen Pine
Tc 6.28 6.27 5.56 5.58 5.66 5.08 5.47
Wcomp low density (J) L
(0.02) (0.02) (0.02) (0.02) (0.03) (0.01) (0.04)
6.45 6.99 6.47 6.62 6.66 5.48 6.32
T
(0.04) (0.06) (0.02) (0.04) (0.03) (0.03) (0.06)
0.79 0.79 1.22 1.23 1.27 1.20 1.37
Wcomp high density (J) L
(0.01) (0.01) (0.01) (0.01) (0.02) (0.00) (0.02)
0.93 1.06 1.73 1.84 2.08 1.36 1.71
T
(0.01) (0.03) (0.05) (0.03) (0.04) (0.02) (0.03)
5.49 5.48 4.34 4.35 4.38 3.88 4.10
(0.01) (0.01) (0.01) (0.01) (0.01) (0.01) (0.03)
L 5.52 5.93 4.74 4.78 4.58 4.12 4.61
(0.01) (0.06) (0.06) (0.04) (0.05) (0.02) (0.04)
a The W comp total refers to the area under the plot illustrated in Figure 4. Low and high density are illustrated in Figure 6. Brackets numbers are standard error of the mean (SEM). b SF: Size fraction (sawdust particles). c T and L refer to transverse and longitudinal particle fiber orientations.
Figure 6. Semilog plots of stress for compression illustrating the differences between the T and L particles, and the linearity of the low and high density phases. The vertical dotted lines separate the low and high density compression phases and a merging point between the T and L plots. Note that the force/displacement data is converted into stress (MPa) vs press piston position. Density is plotted on the linear secondary axis. Table 2. Energy Required to Push the Pelletized Samples in the Press Channel (Wfric), Pellet Strengths, and the Acetone Extractives Contenta friction Wfric (mJ)
pellet strength (N)
extractives
species
SFb
Tc
L
T
L
% of dry matter
Beech
0.5-1.0 2.0-2.8 0.5-1.0 1.0-1.4 1.4-2.0 0.5-1.0 2.0-2.8
48.3 (3.3) 48.2 (3.3) 60.5 (2.8) 50.2 (1.7) 51.5 (1.9) 51.3 (0.8) 43.4 (1.4)
115.8 (4.2) 141.6 (5.4) 99.4 (3.3) 98.6 (1.5) 100.6 (4.6) 51.9 (2.4) 67.3 (2.7)
99.1 (2.3) 113.1 (3.7) 50.3 (0.06) 50.2 (0.05) 50.3 (0.06) 26.3 (0.4) 28.4 (1.5)
86.5 (2.0) 92.0 (4.5) 51.9 (1.0) 50.8 (0.4) 52.9 (1.3) 25.7 (0.3) 24.2 (1.1)
0.0 (0.0)
Aspen Pine
a Numbers in brackets are standard error of the mean (SEM). particle fiber orientations.
b
1.1 (0.2) 5.6 (0.1)
SF: size fraction (sawdust particles). c T and L refer to transverse and longitudinal
Figure 7. Plots of the friction force vs pellet position in press channel as the pellet is pushed out. The plots illustrate the differences in frictional properties caused by fiber orientation and species.
between species (see Table 2). Apart from the variation within species caused by T and L fiber orientation, it is also seen that Wfric,beech > Wfric,aspen > Wfric,pine for the L particles (Table 2). This order of species is also seen for the pellet strength and indicates that high pellet strength implies high stresswall, which could be due to the pellet’s ability to maintain the stresswall after the compression.
This connection between pellet strength and friction was not seen for the T particles, indicating that the stresswall for the T particles was too small for the differences in pellet strength to influence the importance of plastic strain. The differences in the pellet strength between species may be related to the bonding mechanisms that contribute to the
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strength of the pellets. The bonding occurs without adhesives, and the mechanisms behind can therefore be dominated by hydrogen bonding between the hydroxyl groups on the surfaces of the lignocellulosic particles.17,18 This type of bonding is sensitive to surface contamination by extractives, which prevent close contact between the surfaces.19-23 Contamination with extractives is a time-dependent surface accumulation of various natural substances from the wood and is referred to as surface inactivation22,24 or the formation of a “weak boundary layer”.25 The handling of samples in the present study did not prevent this type of contamination, and the differences between the species in types and amount of extractives could cause various degrees of extractives accumulations. In particular, the amount of low molecular weight lipophilic substances (e.g., fatty acids, waxes, resin acids) could have a significant effect on this.22,26 In Table 2, it is seen that the increasing extractives contents between the species correlated to decreasing pellet strength, and this indicates that extractives affect the bonding between the particles and thus the pellet strength. Nielsen et al.27 further documents this. Conclusion The study showed that the fiber orientation in sawdust particles is an important property for its pelletizing properties, because the energy requirements for compression and friction of the pelletized sawdust are affected by the fiber orientation. Sawdust particles from European beech, Quaking aspen and Scots pine with fiber orientation parallel to the particle z-axis (T particles), required less energy to compress and to press through the channel compared to particles with the fibers oriented parallel to the particles xy plane (L particles). It is (16) Bodig, J.; Goodman, J. R. J. Wood Sci. 1973, 5, 249–264. (17) Back, E. L. Holzforschung 1987, 41, 247–258. (18) Gardner, D. J. Adhesion mechanisms of durable wood adhesive bonds. In: Characterization of the Cellulosic Cell Wall; Groom L. H., Stokke, D. D. Eds.; Blackwell Publishing: Ames Iowa, 2006; pp 254265. (19) Hse, C. Y.; Kuo, M. I. For. Prod. J. 1988, 38, 52–56. (20) Christiansen, A. W. Wood Fiber Sci. 1991, 23, 69–84. (21) Nussbaum, R. M. Holz. Roh. Werkst. 1999, 57, 419–424. (22) Back, E. L. For. Prod. J. 1991, 41, 30–36. (23) Christiansen, A. W. Wood Fiber Sci. 1990, 22, 441–459. (24) Nielsen, N. P. K.; Nørgaard, L.; Strobel, B. W.; Felby, C. Eur. J. Wood Prod. 2009, 67, 19-26. (25) Stehr, M.; Johansson, I. J. Adhes. Sci. Technol. 2000, 14, 1211– 1224. (26) Sundberg, A.; Holmbom, B.; Willfor, S.; Pranovich, A. Nord. Pulp Pap. Res. J. 2000, 15, 46–53. (27) Nielsen, N. P. K.; Gardner, D. J.; Felby, C. Fuel 2009, Accepted for publication.
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discussed that the lower energy requirements for compression of the T particles can be related to precompression occurring in the production of the sawdust, lower particle strength properties, shorter fiber lengths, and softening of the middle lamella that weakens the T particles more than the L particles due to the lamella’s orientation. The lower friction in the press channel with pellets made from T particles may be related to a lower stress applied by the pellet to the press channel wall, due to lower Poisson’s ratios of the T particles compared to the L particles. The study also showed differences in internal pellet bonding strength between the species, which can be due to different degrees of particle surface contamination by extractives that prevent optimal bonding between the particles in the pellet. The presented results indicate that sawdust particles produced by cutting across the fiber orientation may maximize the pelletizing machinery’s capacity compared to particles cut parallel to the fiber orientation. This can be of importance when comparing the pelletizing properties of sawdust particles with the pelletizing properties of fiber bundles (slivers) made by hammermilling of wood chips, because the fiber bundles have a fiber orientation similar to sawdust particles cut parallel to the fiber orientation. Also, it must be noted that for wood pellet raw material analyses like the one presented, the fiber orientation and particle structure must be taken into account. Acknowledgment. DONG Energy Power A/S; Statoi1Hydro A/S; and The Danish Ministry of Science, Technology, and Innovation are gratefully acknowledged for financing. The assistance by Jan B. Kristensen, Forest and Landscape, Faculty of Life Sciences, University of Copenhagen with SEM image recording is also gratefully acknowledged.
Nomenclature L particles ) sawdust particles with the fibers oriented in the xy plane of the particles. MC ) moisture content as percentage of dry matter content. stress ) pressure (Pa or MPa). stresswall ) the pressure applied by the pellet to the press channel wall in the die. strain ) deviation from initial dimension as a result of stress. (∆dimension/dimensioninitial) T particles ) Sawdust particles with the fibers oriented parallel to the (short) z axis. Wcomp ) energy (J) used for compressing a pellet of 250 mg sawdust to 115 MPa pressure. Wfric ) energy (mJ) used to push the pellet out of the die. Quantifies the friction between the pellet and the die. EF800923V
