Toward an Understanding of Controlling Parameters in Softwood and

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Energy & Fuels 2006, 20, 2686-2694

Toward an Understanding of Controlling Parameters in Softwood and Hardwood Pellets Production Jens K. Holm,*,† Ulrik B. Henriksen,† Johan E. Hustad,‡ and Lasse H. Sørensen§ Energy Engineering Section, Department of Mechanical Engineering, Technical UniVersity of Denmark, Nils Koppels Alle´ 402, DK-2800 Kgs. Lyngby, Denmark, Department of Energy and Process Engineering, Norwegian UniVersity of Science and Technology, NO-7491 Trondheim, Norway, and ReaTech, c/o Centre for AdVanced Technology, Post Office Box 30, DK-4000 Roskilde, Denmark ReceiVed October 11, 2005. ReVised Manuscript ReceiVed August 8, 2006

Production of biofuel pellets from hardwood, such as beech, is often very troublesome as observed in largescale pellet production plants, where reduction of capacity and frequent blocking may be experienced. The problems may be reduced through a laborious optimization procedure of the process conditions and the utilization of softwood and adhesive materials. To optimize and facilitate such a procedure, a deeper understanding of the fundamental physical-chemical mechanisms that control the pelletizing process is sought after by combining small-scale experiments and an advanced pellet production model. Mixtures of beech rich in corrosive alkali chloride salt, pine softwood, brewers spent grains (BSG), and inorganic additives are experimentally tested using a small-scale pellet mill. It was possible to pelletize a beech/pine mixture containing up to 40% (wt) beech. The addition of 15% (wt) BSG to the beech dust significantly facilitated the pelletizing process. The addition of necessary inorganic anti-slag and anti-corrosive compounds into pellets made of beech may enhance the problems in an unpredictable way. However, if the inorganic additives are added directly into BSG before mixing it with the beech dust, the material can easily be pelletized. A pellet production model is developed that describes the pelletizing pressure variation along the press channels of the matrix. Equations based on differential control volumes are set up to describe the forces acting on the pellet in the matrix. Important model parameters are the sliding friction coefficient, the ratio of compression, and the material-specific parameters, such as the elastic modules and Poisson’s ratio. Model calculations show how the variation in the model parameters significantly changes the necessary pelletizing pressure. Using typical material parameters of beech and pine, it is illustrated why beech, in accordance with the experimental test results, is more difficult to pelletize than pine.

Introduction Biomass pellets are densified biomass particles formed to cylindrical pellets. Pellets of various wood residues and straw may be used for energy production in a broad range from private household appliances to full-scale power plants. At the highly efficient power plant Avedøre 2 in eastern Denmark, the energy company ENERGI E2 A/S utilizes up to 300 000 tons of wood pellets per year. In 2003, the Amager 2 power plant was redesigned to burn straw pellets from the biopellet factory of ENERGI E2 in Køge, south of Copenhagen. The biopellet factory can produce up to 180 000 tons of wood pellets per year and 150 000 tons of straw pellets per year. There are several advantages of densified fuel pellets compared to nondensified fuels. The higher bulk and energy densities result in lower transportation costs and higher energy efficiency, respectively. Moreover, the reduced moisture content (∼10%) increases the long-term storage capability. Major technical challenges concern the maintenance of a stable pelletization process with low-energy consumption and production of high-quality pellets. * To whom correspondence should be addressed. E-mail: [email protected]. Fax: +45-45935761. † Technical University of Denmark. ‡ Norwegian University of Science and Technology. § ReaTech.

The quality of biofuel pellets is evaluated on two major criteria: chemical and physical characteristics.1 The chemical characteristics include the ash content and composition (corrosion and slagging tendency) and the composition of the chemical components, e.g., the lignin and water content. The physical characteristics include criteria such as bulk and unit density, particle-size distribution, and durability. The physical characteristics of the biofuel pellets are highly influenced by the chemical and physical characteristics of the raw materials. In addition, the quality of the pellets depends upon the specific pellet mill, e.g., the dimensions of the ring matrix. Several process parameters influence the quality of the pellets: comminution of the raw material, steam and adhesive addition, and temperature. However, the present work will focus on how the pelletizing performance of pine and beech wood and mixtures thereof is influenced by the matrix dimensions and the materialspecific parameters, such as the sliding friction coefficient, the elastic modules, and Poisson’s ratio. Difficulties in terms of frequent blocking and even breakdown of the metal ring matrix during pelletization of pure beech pellets as well as beech pellets with inorganic additives have been observed by VTT Processes of Finland with a laboratory (1) Geisshofer, A., Hahn, B., Eds. Woodpellets in Europe. Industrial Network on Wood Pellets; Thermie B project DIS2043/98-AT, DGXVII. UMBERA GmbH: A-3100 St. Po¨lten, Austria, 2000.

10.1021/ef0503360 CCC: $33.50 © 2006 American Chemical Society Published on Web 09/09/2006

Softwood and Hardwood Pellets Production

California pellet mill (25 kg/h).2 Similar difficulties with blockage of the matrix have been observed at the Køge Biopellet Factory (KBF). In light of these observations, a series of pelletizing tests has been performed using a ring matrix California pellet mill with a compression ratio of 6.5 (ratio of matrix channel length/channel diameter). The initial tests confirm the difficulties in pelletizing pure beech as observed earlier at VTT and KBF. In contrast to beech dust, pine shavings are relatively easy to pelletize with the present pellet mill conditions. With the matrix used and in accordance with large-scale results obtained at KBF, it is possible to pelletize a 60% (wt) pine plus 40% (wt) beech mixture but nearly impossible to pelletize mixtures with a higher beech content.3 However, the addition of a moderate [i.e., around 10-15% (wt)] brewers spent grains (BSG) fraction to beech significantly improved the pelletizing process and additionally increased the pellet quality. The initial technical laboratory tests combined with the results obtained at KBF have focused on and progressed in creating specific solutions to practical problems relevant for the pellet production at KBF. However, no optimization has yet been made that considers the fact that several mixtures of wood, straw, and additives have to be handled in an optimized way whenever available. Therefore, in combination with the experimental work, efforts have been made to theoretically understand the fundamental physical and chemical mechanisms that control the production of biomass pellets and the pellet quality and durability. From a literature investigation, we, however, thus far found only limited information about the very fundamental aspects of pellet production using ring matrix pellet mills. To our knowledge, only a few reports on ring matrix pelletization of biomass have been published. In particular, the effects of raw material storage, temperature, and steam addition have been investigated.4,5 However, the results are ambiguous; e.g., there is some consensus about the positive effect of raw material storage on the pellet durability but not about the influence on the energy uptake of the pelletizing machinery. In contrast, an extensive number of textbooks and reports exist on the structure and compression of wood in general.6-8 Presently, numerical simulations of transverse compression and densification in wood are being published.9 These simulations are based on realistic wood structures and simulate the compression of wood in the entire range to full densification. However, (2) Aho, M.; Saastamoinen, J.; Moilanen, A. ReactiVity Study of PulVerized Biomass Fuels; VTT Processes test report, project number PRO 24/C2SU02060, VTT Processes: FIN-40101 Jyvaskyla, Finland, 2003. (3) Holm, J. K.; Henriksen, U. B.; Hustad, J. E.; Sørensen, L. H. Fundamentals of Biomass Pellet Production; MEK-ET-2005-01, Department of Mechanical Engineering, Energy Engineering Section, Technical University of Denmark: Lyngby, Denmark, 2005. (4) Abrahamsson, K.; Zethræus, B.; Oskarsson, J. Utveckling av tra¨pelletstillverkningsråmaterialhanteringens påverkan på tillverkningsoch produktegenskaper. DESS slutrapport (in Swedish), Inst. fo¨r Biovetenskaper och Processteknik, Va¨xjo¨ Universitet, Sweden, 2002. (5) Nielsen, N. P. K. Wood Pellet Production and the Importance of Raw Material Storage. M.S. Thesis, Danish Centre for Forest, Landscape and Planning, The Royal Veterinary and Agricultural University, Copenhagen, Denmark, 2004. (6) Tsoumis, G. Science and Technology of Wood; Van Nostrand Reinhold: New York, 1991. (7) Kultikova, E. V. Structure and Properties Relationships of Densified Wood. M.S. Thesis, Wood Science and Forest Products, Virginia Polytechnic Institute and State University, Blacksburg, VA, 1999. (8) Morsing, N. Densification of Wood: The Influence of Hygrothermal Treatment on Compression of Beech Perpendicular to the Grain. Ph.D. Thesis, Department of Structural Engineering and Materials, Technical University of Denmark, Lyngby, Denmark, 2000. (9) Nairn, J. A. Wood Fiber Sci. 2006, manuscript submitted.

Energy & Fuels, Vol. 20, No. 6, 2006 2687

Figure 1. Laboratory California pellet mill at MEK, DTU.

these simulations are not directly aimed at explaining the fundamental mechanisms of the pelletization process. Some modeling approaches have been put forward to understand the mechanical forces in play during the compression process. However, these models consider the compression of the material against a fixed backstop like in a single pelleter unit9-12 and not against a continuously moving backstop as for ring matrix pelletization. An empirical model has been presented on the basis of data obtained from compression tests with a single pelleter unit,11 but a fundamental theoretical model for ring matrix pelletization has, to our knowledge, not been published thus far. To obtain an initial theoretical approach, a novel model is therefore developed. The model qualitatively describes the pelletizing pressure variation along the press channels of the matrix. Equations based on differential control volumes are set up to describe the forces acting on the pellet in the matrix. Important model parameters are the sliding friction coefficient, the ratio of compression, and the material-specific parameters, such as the elastic modules and Poisson’s ratio. Model calculations show how the variation in the characteristic parameters significantly changes the necessary pelletizing pressure. Using typical material parameters of the hardwood beech and the softwood pine, it is illustrated why beech, in accordance with the experimental test results, is more difficult to pelletize than pine. Experimental Section The laboratory pellet mill (California Pellet Mill Co.) is based on extrusion for the pellet production (see Figure 1). It is equipped with a vertical ring-type matrix, and the biomass is forced outward through the cylindrical holes by the action of an eccentrically mounted roller (see Figure 2). Some commercial mills have conical inlets to the holes, but the one described in the present paper has straight holes without conical inlets. A stationary knife mounted next to the matrix cuts the pellets after the extrusion. A data-logger ammeter of the pellet mill motor functions as a load indicator. The ring matrix has 40 holes. Each cylindrical hole has a diameter of 7.7 mm and a length of 50 mm, giving a ratio of compression for the pellet mill of 6.5. The optimal ratio of compression depends upon the specific material to be pelletized. A model describing the (10) Mani, S.; Tabil, L. G.; Sokhansanj, S. Can. Biosystems Eng. 2004, 46, 3.55-3.61. (11) Faborode, M. O. Biol. Wastes 1989, 28, 61-71. (12) Bhattacharya, S. C.; Sett, S.; Shrestha, R. M. Energy Sources 1989, 11, 161-182.

2688 Energy & Fuels, Vol. 20, No. 6, 2006

Holm et al.

Figure 2. Schematic drawing showing the zone of biomass compression in the ring matrix.

forces exerted on a pellet in the matrix is presented in the Theoretical Basis. The basic principle of pelletization is shown in Figure 2. When the matrix is rotating, the adjacent roller forces the raw material into the channels of the matrix. The actual compression of the material is likely only to take place in a minor compression zone at the very beginning of the channel. The compressed material in the matrix then functions as a continuously moving backstop, set up by the friction with the channel walls. Samples. The following samples have been tested: pine shavings (pine), Junckers beech dust (beech), 80% (wt) pine plus 20% (wt) beech, 60% (wt) pine plus 40% (wt) beech, 40% (wt) pine plus 60% (wt) beech, 20% (wt) pine plus 80% (wt) beech, and 85% (wt) beech plus 10-15% (wt) BSG. The final moisture content of the samples was adjusted to approximately 12% (wt). Mixed samples were prepared in a Bjo¨rn varimixer for 15 min. Junckers beech dust refers to disintegrated beech wood chips from a Danish wood floor producer (Junckers Industries A/S). Pine shavings are purchased as a commercial product. Pine and beech are examples of softwood and hardwood, respectively. BSG is a waste material from the brewing industry with the following approximate composition: 30% (wt) protein, 8% (wt) fat, 58% (wt) fiber, and 4% (wt) ash.13 Particle-size distributions of the tested materials are shown in Figure 3.

Theoretical Basis Pure Elastic Case. First, consider the case where elastic material is to be pressed through the press channel of a matrix. The following derivation gives an expression for the pelletizing pressure-variation changes along a pellet in the press channel of the matrix. The differential volume element dV is defined as shown in Figure 4. The volume element has the shape of a flat disk of thickness dx and radius r. The center of the disk is situated on the center axis of the pellet. The differential pressure difference dPx across dx is given by

dPx )

dFx πr 2

T dFx ) dPx πr 2

(1)

The differential force dFx is assumed constant over the cross(13) Grøndal, J. Utilization of Brewers Spent Grains Fractions as Ingredients in the Food and Feed Industry; An Industrial Research Education Program under the Danish Academy of Technical Sciences, Project number 244, United Milling Systems A/S; Carlsberg Research Laboratory, Technical University of Denmark: Lyngby, Denmark, 1990.

Figure 3. Particle-size distributions of tested samples of pine shavings, beech dust, and BSG.

Figure 4. Schematic drawing of the forces and pressures exerted on a differential volume element dV within the press channel of the matrix. A pellet with fibers oriented perpendicularly to the press channel is shown.

sectional area πr2. Because the press channel has no conical inlet, this is a valid assumption. As the pressure needed to press the material through the matrix is decreasing through the channel from the inlet to the outlet, dPx will be positive in the direction of the x axis, as defined in Figure 4. The friction force dFµ related to the differential area 2πr dx is given by

dFµ ) µ dFN ) µPN2πr dx

(2)

We now wish to relate the differential force dFx to the differential friction force dFN. The following derivation is based on the assumption that wood is an orthotropic material; i.e., the mechanical properties are independent in the directions parallel and perpendicular to the fibers (grain) (see Figure 5). It is assumed that the fiber bundles within the pellet orient themselves perpendicularly to the long direction of the press channel (see Figure 4). This is likely to be a valid assumption when shavings and needlelike particles are pelletized, but in the case of particles characterized as dust, it might be necessary to introduce an orientational distribution. However, this is outside the scope of the paper. Furthermore, the material is assumed to have elastic properties; i.e., there is a linear relationship between the stress and strain (Hooke’s law). Plastic deformations are not considered in the model. However, in a slightly modified model presented later in this section, an inelastic prestress of the pellet is taken into account.



Figure 5. Principal axes of wood with respect to the grain direction.14

In the general notation of νxy, x and y denote the direction of applied stress and the direction of transverse deformation, respectively. As mentioned earlier, the differential force dFx is assumed to be constant over the cross-sectional area of the pellet. Moreover, the wood material is assumed to be homogeneous over the same cross-section with all fiber bundles lying parallel to the plane. Hence, dV/dr is equal to ∆r/r, where r is the radius of the pellet and ∆r is the radial extension of the pellet crosssectional area. In the case of a spatial-restricted pellet in a press channel, the radial extension is prohibited and, hence, the strain shows up as a stress on the walls of the press channel. With reference to Figure 4, Hooke’s law can be written for the longitudinal and radial directions of the fiber bundles

∆r r

(5)

du FR ) ERAR dx

(6)

FL ) ELAL

where EL and ER are the elastic modules for the longitudinal and radial directions of the fiber bundles, respectively. AR ) πr2 is the cross-sectional area of the volume element, and AL ) 2πr dx is the area of the side surface of the volume element. It is now possible to obtain the ratio between the magnitudes of the forces FR and FL, given in terms of the elastic modules of the specific material in the directions parallel and perpendicular to the fiber orientation and in terms of Poisson’s ratio and the size of the volume element

FL AL EL ∆r du 2dx EL / ) ) ν FR AR ER r dx r ER RL

(7)

The corresponding pressure ratio can be written as Figure 6. Schematic drawing showing the deformation when an infinitesimal volume element is loaded perpendicularly to the long direction of a bundle of fibers by a force FR. The fibers are oriented along the y axis.

For materials obeying Hooke’s law, the ratio between the stress and strain is constant

stress )E strain

If PL is equated with PN and PR is equated with Px, the relationship between PN and Px can be given as

PN ) GνRLPx (3)

where E is the constant of proportionality known as the modulus of elasticity or Young’s modulus. The stress is defined as the load or force per unit area and has the unit N/m2. The stress may be tensile (a pull) and positive or compressive (a push) and negative. The strain is defined as the percentage deformation of an infinitesimal element and is dimensionless. Strain as a result of an extension is considered positive, whereas strain as a result of a contraction is considered negative. If the extension or contraction in a component because of an applied force disappears upon removal of the force, the material is described as elastic. The longitudinal strain of a bundle of wood fibers is related to the radial strain by a constant that is unique for a particular material and referred to as Poisson’s ratio. Poisson’s ratio is defined as the ratios of strains for an infinitesimal volume element as shown in Figure 6. Hence, the ratio can be written as

longitudinal strain dV du ) / νRL ) radial strain dr dx

PL FL FR FL r EL r 2dx EL ) / ) νRL ν (8) ) ) PR AL AR FR 2dx r ER 2dx ER RL

where G ) EL/ER. If eqs 1 and 2 are set equal, a differential equation in Px is obtained

dFx ) dFµ T dPx πr2 ) µPN2πr dx ) µGνRLPx2πr dx (10) The equation can be rewritten as

2µGνRL 1 dx dPx ) Px r

(11)

with the solution

2µGνRL dx r

(12)

2µGνRL (x - x0) r

(13)

∫PP P1xdPx ) ∫xx x

x0

ln Px - ln Px 0 )

0

Px ) Px0e2 µGνRL(x-x 0)/r (4)

(9)

for x - x0 ) L p

(14)


Holm et al.

Px ) Px0e2µGνRLLp/r

(15)

The model is only valid when Px is different from 0 (see eq 11). This introduces the initial pressure Px0, called the start back pressure, as a pressure necessary to start the pelletizing. In the following subsection, a prestressing pressure is introduced, which allows Px to start from 0. Prestressed Case. The relations above assume that the material is elastic; i.e., deformations are recoverable after the load is removed. In terms of pelletizing, this would imply that the pellets should slip out easily when the pelletizing pressure was released. However, as expected, this does not happen in the pellet mill, where the pellets following compression are tightly packed within the matrix. This effect is taken into account by the introduction of an additional prestressing pressure term PNo, which gives a constant contribution to PN (see Figure 7). Hence, eq 9 can be extended as

PN ) GνRLPx + PNo

Figure 7. Schematic drawing of the forces and pressures exerted on a differential volume element within the press channel of the matrix. A pellet with fibers oriented perpendicularly to the press channel is shown. PNo denotes the permanent prestress of the material when compressed in the matrix.

GνRL )

(16)

dFx ) dFµ T dPxπr2 ) µPN2πr dx ) µ(GνRLPx + PNo)2πr dx (17) or

(

)

2µPNo 2µGνRL Px + dx r r

(18)

2µGνRL 2µPNo and b ) r r

(19)

by substituting

a)

x

(20)

with the solution

[a1ln(aP + b)] x

Px 0

1 1 ) ln(aPx + b) - ln b a a

(21)

or

a ax ) ln Px + 1 b

(

)

a b b eax ) Px + 1 or Px ) eax b a a Px )

PNo 2µGνRLx/r PNo e GνRL GνRL

(22) (23)

(24)

The modules of elasticity and Poisson’s ratios are related by expressions of the form

νij νji ) , i* j, i and j ) L, R, and T Ei Ej

Px )

PNo 2µνLRx/r PNo e νLR νLR

(27)

The equation derived above holds very useful information related to the pelletizing of different wood species. It shows how the parameters PNo, µ, νLR, r, and x are related to the pelletizing pressure. The expression given by eq 27 is valid for all x. In the limit of small x, the expression can be expanded as follows using a Taylor expansion

e2µνLRx/r/rx ) 1 +

2µνLR x for small x r

(28)

Equation 27 can then be rewritten as

the following integral equation is obtained

∫0P aPx1+ bdPx ) ∫0xdx

(26)

Equation 24 may then be written as

The differential equation in Px can now be written as

dPx )

EL ν ) νLR ER RL

(25)

where L, R, and T denote the longitudinal, radial, and tangential directions, respectively. Using eq 25,

Px )

(

)

PNo 2µνLR PNo 2PNoµ x x 1+ ) νLR r νLR r

(29)

Hence, in the limit of small x, the pelletizing pressure Px is independent of Poisson’s ratio. This has implications for the initiation of the pelletizing process. Upon start-up of the pelletizing process, no back pressure has been established and, hence, the fiber bundles will not deform according to Poisson’s ratio when pushed into the channel. Only when a back pressure is obtained because of a buildup of material in the matrix, the fibers will start to deform. Obviously, the initiation of the back pressure depends upon the material in terms of porosity and the ability of self-packing. Hence, oil-containing materials, such as corn or oats, are often used to start the pelletizing. Model Parameters. The friction coefficient µ depends upon the moisture content and the roughness of the surface. There are only small variations among different wood species, except for species containing high concentrations of oily or waxy extractives.15 The room temperature friction coefficient increases when the moisture content increases from oven dry to the fiber saturation point. Hereafter, it remains constant until considerable water is present. When the surface is highly saturated with water, (14) http://www.ggi-myanmar.com/wood/. (15) Forest Products Laboratory. Wood HandbooksWood as an Engineering Material; General Technology Report FPL-GTR-113; Forest Products Laboratory, Forest Service, U.S. Department of Agriculture: Madison, WI, 1999.



Table 1. Material Properties of Selected Hardwoods and Softwoodsa

species softwood pine spruce Douglas fir hardwood beech oak

modulus of elasticity, EL (MPa)

modulus of elasticity, ER (MPa)

EL/ER

density (kg/m3)

Poisson’s ration, νLR

Poisson’s ration, νRL

side hardnessb (N)

6634 5038 6057

320 433 399

20.7 11.6 15.2

383 409 438

0.337 0.255 0.666

0.016 0.022 0.044

1900 2300 3200

9563 5267

1486 1481

6.4 3.6

674 597

1.112 0.767

0.174 0.216

5800 5400

a Literature data are from ref 15, and side hardness data are from ref 16. b The side hardness represents the force required to embed a 11.28 mm steel ball to one-half its diameter and is given as average values of radial and tangential observations at 12% (wt) moisture content.

the friction coefficient decreases.15 The sliding coefficient of friction typically varies with the wood moisture content as given below15 • Smooth dry wood on hard smooth surface: 0.3-0.5 • At intermediate moisture content: 0.5-0.7 • Near fiber saturation: 0.7-0.9 Typical elastic modules and Poisson’s ratios of selected hardwoods and softwoods are tabulated in Table 1. The table also shows the density and the side hardness of the different wood species. It should be emphasized that the values defer somewhat between different references.15,16 However, generally hardwoods have higher Poisson’s ratios than softwoods. It is important to emphasize that the classification of hardwood and softwood refer to the biological origin of the material and do not imply anything about the properties of the wood. Nonetheless, it is seen that in the case of the selected hardwoods and softwoods the density and side hardness is generally higher for hardwoods compared to softwoods. The indices L and R refer to the longitudinal and radial directions of the fibers as shown in Figure 5. The ratio between the length of the pellet and the diameter is often called the ratio of compression. Equations 15 and 27 show that the pelletizing pressure is increased as the length of the pellet increases. In contrast, the pressure is decreased as the diameter of the pellet increases. Hence, the dimensions of the press channels in the matrix have a strong influence on the pressure needed to press the pellets through the matrix. To initiate the pelletizing process, a start back pressure is necessary. The start back pressure is produced by the buildup of material in the press channel, which sets up a demand for a pressure to overcome the friction with the channel walls. In the prestressed case, the matrix dimensions, friction coefficient, and the prestressing pressure PNo (see eq 29) determine the initial pelletizing pressure. Results and Discussion Pine shavings could be pelletized without problems (see Figure 8). The temperature of the pellets in the matrix was ∼90 °C and was measured in the following way. Immediately after the press was stopped, a small hole was made in the pellet while it was still in the matrix. A thermocouple was placed in the hole, and after stabilization, the temperature was determined. The pellet mill was running at a low load corresponding to a matrix motor current of 2.5-3 A. Pelletizing of pure Junckers beech dust resulted in the blockage of the matrix, and the holes had to be drilled out. Moreover, problems with the feeding system were also observed. The feeding lead to blockage at the (16) Bucur, V.; Archer, R. R. Wood Sci. Technol. 1984, 18, 255-265.

Figure 8. Pellets pressed from pine shavings.

Figure 9. Pellets pressed from a mixture of 60% (wt) pine plus 40% (wt) beech.

point where the material enters the matrix. The consequence of the blockage was that the feeder screw was pushed backward. A mixture with 80% (wt) pine shavings and 20% (wt) Junckers beech dust could be pelletized without problems. The pellet temperature in the matrix was ∼105 °C. At a mixture ratio of 60% (wt) pine shavings and 40% (wt) Junckers beech dust, pellets could still be produced (see Figure 9). The pellet temperature in the matrix was ∼115 °C. At a mixture ratio of 40% (wt) pine shavings and 60% (wt) Junckers beech dust, the pellet mill was running at a high load (4 A) and the mill was blocked after a few minutes. At this point, the pellet temperature in the matrix was ∼120 °C. Adding 3% (wt) Wafolin or 3% (wt) rape oil to the last mixture did not solve the problems with the blockage. When the matrix was disassembled after the blockage, dark burnt material was seen in the matrix ring. As a possible organic additive, BSG were tested in combination with Junckers beech dust. A mixture of 85% (wt) beech


Figure 10. Pellets pressed from a mixture of 85% (wt) beech plus 15% (wt) BSG.

Figure 11. Normalized pelletizing pressure as a function of the compression ratio for different wood species. PNo ) 1, r ) 4 mm, and µ ) 0.5 (×, beech; O, oak; ], fir; 4, pine; 0, spruce).

and 15% (wt) BSG could be pelletized, and the durability of the pellets was increased (see Figure 10). Further, when BSG was added to the beech dust inorganic powder, additives [30% (wt) dry basis] could also be added without any problems. These inorganic additives could not have been added to the beech dust alone without causing great problems in the pellet mill. Theoretical Calculations. Figure 11 visualizes how the material-specific parameters of the different wood species influence the pelletizing pressure. Equation 27 is used to calculate the curves with PNo ) 1, r ) 4 mm, and µ ) 0.5. Material-specific parameters are the same as shown in Table 1. The necessary pelletizing pressure is shown to increase exponentially for all wood species as a function of the increased pellet length (channel length). By necessary pressure, it is understood that it is the pressure necessary to overcome the back pressure built up in the press channel. The pressure is presented normalized to the fixed prestressing pressure PNo. The pelletizing pressures of the hardwoods, oak and beech, are increasing more rapidly than the corresponding pressures of the softwoods, spruce, fir, and pine, as a function of the increased pellet length. As an example, one can assume that the pellet mill can give a maximum normalized pelletizing pressure of 1000. According to the curves shown in Figure 11,

Holm et al.

Figure 12. Normalized pelletizing pressure as a function of the compression ratio for different friction coefficients of beech and pine. PNo ) 1, and r ) 4 mm (Beech: 4, µ ) 0.8; 0, µ ) 0.6; O, µ ) 0.4. Pine: +, µ ) 0.8; ], µ ) 0.6; ×, µ ) 0.4).

it should be possible to pelletize beech up to a pellet length of approximately 25 mm, whereas a pellet length of 70 mm should be achievable for pine. If the channel length is increased beyond 25 mm in the case of beech, the press will block, because the pelletizing pressure will not be able to overcome the back pressure. Figure 12 shows the necessary pelletizing pressures of beech and pine for three different friction coefficients. As expected, higher friction coefficients demand higher pelletizing pressures for fixed channel length and radius. As before, if one assumes a maximum normalized pelletizing pressure of 1000, beech pellets up to a length of approximately 15 mm can be pressed if the friction coefficient is 0.8. The corresponding length for pine is 43 mm. Under identical mill conditions, a friction coefficient of 0.4 for pine will not demand a normalized pelletizing pressure higher than 1000 over the length range from 0 to 80 mm. Hence, the channel length can be changed at will in this range without blockage of the matrix. It should be noted that different materials are likely to have different optimal pelletizing pressures, i.e., pressures that produce pellets with the desired quality and durability but without unnecessary energy uptake. This optimal pressure should be between the lower and upper pelletizing pressure limit, given by the back pressure and the maximum mill pelletizing pressure, respectively. The effect of varying the pellet radius on the pelletizing pressure is shown in Figure 13. When the definition of the ratio of compression, Lp/D, is recalled, it is seen that an increase in the pellet radius corresponds to a decrease in the ratio of compression and, hence, to a decrease in the necessary pelletizing pressure for a particular pellet length. A significant difference in the pelletizing performance was observed between pine shavings and Junckers beech dust. Whereas pine shavings could easily be pelletized, Junckers beech dust rapidly led to the blockage of the matrix. The presented model suggests that the difference in the pelletizing performance can be explained in terms of the different elastic properties of beech and pine, as given by Poisson’s ratios. The exponential behavior of the pelletizing pressure curves implies that even small differences in Poisson’s ratios have a great impact on the pressure. For every different biomass material, the necessary



Figure 13. Normalized pelletizing pressure as a function of the channel length for different pellet radii of beech and pine. PNo ) 1, and µ ) 0.5 (Beech: O, r ) 3 mm; 0, r ) 4 mm; 4, r ) 5 mm. Pine: ×, r ) 3 mm; ], r ) 4 mm; +, r ) 5 mm).

pelletizing pressure depends upon the material-specific elastic properties, the friction, and the matrix dimensions. Hence, the optimal pelletizing conditions for each material can be archived by optimizing the matrix dimensions or by changing the friction, e.g., by adding lubricants. As an example, pelletizing experiments at KBF showed that a compression ratio of 7 was too high for pelletization of Junckers beech dust. In contrast, a ratio of compression around 3 yielded loose, poor-quality pellets. Because the pellet mill at MEK has a ratio of compression of 6.5, this also appears as too high of a value for pelletization of pure Junckers beech dust. The temperature of the material during the pelletizing process is another important parameter. Preheating the raw material has been shown to lower the work and pressure of compression, likely as an effect of thermal softening of the fibers.17 In addition, the temperature determines the phase behavior of polymers present in the biomass. It is known that the lignin part of the cell wall undergoes a glass transition at temperatures down to ∼60 °C, depending upon the moisture content and the specific material.18 Above the glass transition temperature, the lignin is more flexible and, hence, it might more effectively take part in the interactions between the cells. Added organic binders containing proteins or other polymers are likely to have similar properties, as may be the case for BSG. Preliminary experiments with BSG turned out very promising. The pelletizing performance of “problematic” wood, such as beech dust, could be improved markedly by the addition of a small amount of BSG [down to ∼10% (wt)]. BSG could furthermore be pelletized in pure form with high amounts of inorganic additives [30% (wt)]. Moreover, BSG seems to improve the pellet quality of straw pellets, which otherwise show poor binding properties with the matrix used. Conclusions Beech dust has experimentally proven to be much more difficult to pelletize than pine. Under the present mill conditions, it was not possible to obtain a stable production of pellets of pure beech. In contrast to this finding, a stable production of (17) Reed, T. B.; Trezek, G.; Diaz. L. Thermal ConVersion of Solid Wastes and Biomass; American Chemical Society: Washington, DC, 1980; pp 169-177. (18) Irvine, G. M. Tappi J. 1984, 67, 118-121.

Figure 14. Normalized pelletizing pressure variation through a matrix channel.

pellets from pine shavings could easily be obtained. Upon mixing, pellets could be made with the ratio 60% (wt) pine and 40% (wt) beech. With these observations in mind, a model has been developed with the aim of explaining the observed differences in terms of material properties and dimensions of the pellet mill. The observed differences in the pelletizing ability of beech and pine are nicely correlated with the presented model calculations. In these calculations, the pelletizing ability is shown to be a function of the specific material (prestressing pressure and Poisson’s ratio) and pellet mill properties (sliding friction coefficient and ratio of compression). The model determines the pelletizing pressure for matrix pelletization, i.e., densification of the material without a fixed backstop. Hence, one cannot only consider the compressive properties of the material without taking into account the friction with the channel walls. The model utilizes the property of wood that a stress in one of the principle directions is accompanied by an expansion in the transverse direction. The magnitude of the expansion is given by Poisson’s ratio. For simplicity, the fibers are assumed to orient perpendicularly to the long direction of the press channel. The result of this mechanism is that a stress parallel to the press channel is transformed into a transverse stress. This transverse stress gives rise to a friction force that opposes the unhindered movement of material down the channel; i.e., a back pressure has been established. This back pressure increases exponentially as the pellet increases in length (see Figure 14). For a fixed channel radius and friction coefficient, the channel length must be adjusted, so that the back pressure does not exceed the upper limit of the achievable pelletizing pressure. The particle-size distribution may also influence the pelletizing performance. The pine shavings are characterized by a relatively higher percentage of larger particles than the Junckers beech dust (see Figure 3). Therefore, the packing density of the pine shavings is correspondingly lower. Hence, in the case of pine shavings, it is likely that the amount of material entering the matrix chamber is lower than in the case of Junckers beech dust for the same feeding rate, because of the lower density of


pine shavings before compression. Thus, the risk of blockage because of the packing of material in the matrix chamber and in the matrix may be lower for pine shavings compared to Junckers beech dust. However, at KBF, spruce and beech are comminuted in the same hammer mill and beech is still significantly harder to pelletize, thus suggesting that the fundamental structural differences between softwoods and hardwoods are of major importance. The effect of BSG on the pelletizing ability of Junckers beech dust is an intriguing experimental finding. It is likely that BSG has a dual role that affects both the friction and the binding of the material. The effect of co-added organic additives including various waste materials should be tested further. In particular, the effect of polymers and proteins on the binding properties should be addressed. It has been suggested that polymers can penetrate the pits in wood fibers and thus bind the fibers together.19,20 The unfolding of proteins upon heating might make them suitable as polymeric binders. BSG is one likely polymeric binder because of the high protein content, and a number of cheap waste materials may have similar properties. Several questions still need answers. To obtain quantitative information from the model calculations, the values of the different parameters have to be obtained experimentally from the specific materials under investigation. Studies should be made of friction coefficients, the elastic ratio G, and Poisson’s ratio νRL for mixtures under the relevant conditions. Afterward, we intend to further develop the model with respect to the (19) Fujii, T.; Sung-Jae, L.; Kuroda, N.; Suzuki, Y. IAWA J. 2001, 22, 1-14. (20) Fujii, T.; Te-fu, Q. Bull. FFPRI 2002, 1, 115-122.

Holm et al.

plasticity effect, because it is generally accepted that the lignin part of the cell wall shows a glass transition at temperatures above approximately 60 °C, depending upon the moisture content; i.e., the material is likely to change properties during the pelletizing process. Nomenclature r ) radius of press channel (mm) D ) diameter of press channel (mm) Lp ) length of press channel (mm) AL ) side surface area of volume element (mm2) AR ) cross-sectional area (mm2) µ ) sliding friction coefficient ν ) Poisson’s ratio E ) modulus of elasticity (N/m2) G ) EL/ER Pxo ) start back pressure PNo ) prestressing pressure BSG ) brewers spent grains KBF ) Køge biopellet factory Subscripts L ) longitudinal fiber direction R ) radial fiber direction Acknowledgment. Financial support from ENERGI E2 A/S and the Norwegian Research Council, RENERGI program, is highly acknowledged. Extensive cooperation with the staff at Køge Biopellet Factory is furthermore acknowledged. Carlsberg Breweries are acknowledged for providing the BSG. EF0503360

Toward an Understanding of Controlling Parameters in Softwood and

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