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Energy & Fuels 2007, 21, 2446-2449
Experimental Verification of Novel Pellet Model Using a Single Pelleter Unit Jens K. Holm,*,† Ulrik B. Henriksen,† Kim Wand,† Johan E. Hustad,‡ and Dorthe Posselt§ Energy Engineering Section, Department of Mechanical Engineering, Technical UniVersity of Denmark, Nils Koppel’s Alle´ 402, DK-2800 Kgs. Lyngby, Denmark, Department of Energy and Process Engineering, Norwegian UniVersity of Science and Technology, NO-7491 Trondheim, Norway, and ReAddit, UniVersitetsparken 7, Post Office Box 30, DK-4000 Roskilde, Denmark ReceiVed March 27, 2007. ReVised Manuscript ReceiVed May 23, 2007
Pelletization of biomass for bioenergy purposes has established itself as an important step toward a reduction in the emissions of greenhouse gases. A novel pellet model describing the pressure forces in a press channel of a pellet mill has previously been published. The model gives a theoretical explanation of how the biomassspecific parameters, such as the friction coefficient and Poisson’s ratio, influence the pelletizing pressure. The model showed that the pelletizing pressure increases exponentially as a function of the channel length. In the present paper, the pellet model is verified experimentally. When the back pressure needed to press pellets of different lengths out of the press channel is measured, it is shown that the pelletizing pressure does increase exponentially as a function of the pellet length. Second, the back pressures of the hardwood beech are higher than the corresponding pressures of the softwood pine for all tested pellet lengths. Least-squares fit of the model to the data shows that the fitted parameters are in agreement with values from the literature. The procedure for using a single pelleter unit as a means for simulating an industrial pelletizing process in a controllable way is described.
Introduction Biomass compacted to fuel pellets has gained in popularity worldwide over the past decade. The pellets offer easy handling and storing, and standardized pellets can be used in small household boilers and pellet stoves as well as in full-scale power plants. Pellets are produced mainly in pellet mills of the ring matrix type.1 An eccentrically mounted roller forces the material out through the cylindrical channels of the matrix, as shown in Figure 1. The friction between the material and the walls of the channels sets up a back pressure that in combination with the roller pressure densifies the material to pellets. The back pressure, Pback, obtained at the top of the pellet is equal in magnitude to the oppositely directed pressure, Proller, needed to make the pellet move in the channel, as seen in Figure 1. To optimize the pellet production capacity and lower the energy consumption during the pressing, it is important to understand how the forces leading to densification are established within the channels of the matrix. Furthermore, it is of great importance to know how these forces are influenced by the process parameters, such as the temperature and matrix dimensions, and the material-specific parameters, such as the moisture content, modulus of elasticity, and Poisson’s ratio. A theoretical model describing the building up of forces along the channels of the matrix has previously been published.2 In * To whom correspondence should be addressed. Fax: +45-45935761. E-mail:
[email protected]. † Technical University of Denmark. ‡ Norwegian University of Science and Technology. § ReAddit. (1) Bhattacharya, S. C.; Sett, S.; Shrestha, R. M. Energy Sources 1989, 11, 161-182. (2) Holm, J. K.; Henriksen, U. B.; Hustad, J. E.; Sørensen, L. H. Energy Fuels 2006, 20, 2686-2694.
Figure 1. Matrix channel and roller. The pressures at the top of the pellet are depicted.
the model, it was shown how the friction coefficient, prestressing pressure, compression ratio (ratio of the matrix channel length, Lp, to the channel diameter, D), and Poisson’s ratio of the material influence the magnitude of the forces. In particular, it was shown that the pelletizing pressure, Proller, necessary to overcome the established back pressure in the matrix channel increases exponentially as a function of the increased channel length; i.e., the back pressure increases exponentially as a function of the increased channel length. Hardwoods, such as beech and oak, were shown to give rise to higher pelletizing pressures than softwoods, such as pine and spruce, for the same compression ratio and friction coefficient, because of different Poisson’s ratios of the two types of wood. This theoretical prediction is in accordance with the experimental experience obtained with a laboratory pellet mill and at largescale pellet factories. However, to verify the model, one needs
10.1021/ef070156l CCC: $37.00 © 2007 American Chemical Society Published on Web 06/27/2007
Experimental Verification of NoVel Pellet Model
Energy & Fuels, Vol. 21, No. 4, 2007 2447
Figure 4. Pellet length versus pellet weight with single or sequential loading. (]) One loading. (b) Sequential loading. (‚‚‚) Linear trend line of sequential loading. The equation and R2 of the trend line for sequential loading are given in the figure. Figure 2. Single pelleter unit. Setup for pellet production (A). Setup for the measurement of back pressure (B).
Figure 3. Single pelleter pressure curve illustrating the yield point obtained when the applied pressure exceeds the back pressure.
to be able to systematically measure the pelletizing forces for different materials and matrix dimensions. In addition, the sample temperature and moisture content should be controlled. This is not possible right away for the ring matrix pellet mill, but it is possible to simulate the process by using a single pelleter unit. In the present paper, it is described how single pellets should be prepared to mimic the pelletization of the ring matrix mill and it is shown that the experimental data obtained with the single pelleter unit is in accordance with the theoretical model described and with the practical experiences from the ring matrix pellet mills. Experimental Section The single pelleter unit is shown in Figure 2. It consists of a cylindrical press channel and a tightly fitted piston. The pressure is applied by a hydraulic press. Compression of the material is obtained by pressing against a fixed backstop. In comparison to the pellet mill, the backstop plays the role of a fully compressed pellet with a length equal to the channel length. The pelleter unit is placed on a load cell stand to measure the pressures needed to make the pellet and to press the pellet out of the press channel. When the pellet is built-up to the desired length, the back stop is removed and the pressure on the top of the pellet is increased from 0 until the pellet starts moving downward in the press channel (the yield point in Figure 3). The minimum pressure necessary to start the movement is measured by the load cell. The pressure needed to make the pellet move is equal to the back pressure arising from friction along the pelleter walls. This is a model of the process taking place in the rotating pellet mill, where the magnitude of the pressure exerted on the pellet by the roller is equal to the back pressure under steady-state conditions. The steady-state condition is defined as the situation where pellets have been built-up in the channels and the highest obtainable back pressure under the specific conditions has been reached at the inlet of the channels. Hence, if neither the material nor the process
parameters are changed, the pellet production will be stable. Because of the high back pressure at the inlet, the compression of the material is likely only to take place at the inlet to the channel, as indicated in Figure 1. To simulate the pelletizing process of the ring matrix mill, the pellet in the single pelleter has to be built-up sequentially; i.e., a small amount of sample is loaded and then compressed to a predefined pressure (simulating the roller pressure) against a fixed backstop at the bottom of the pelleter unit. The pressure is then released, and a new sample portion is loaded and compressed to the same pressure. This step is repeated until the pellet has the desired length. The procedure results in a pellet with a laminated structure, where each thin layer has been exposed to the same pressure. Consequently, the density of the pellet will be uniform under sequential loading, as illustrated by the linear relationship between the pellet weight and length in Figure 4. If the pellet is produced by loading the entire amount of sample at the same time, the pressure will decrease down through the pellet and the density will decrease as well. In Figure 4, this is seen as a deviation from the linear relationship between the pellet weight and length. The sequential loading of the single pelleter unit is a suitable model for the loading taking place in the ring matrix pellet mill. Every time the roller passes a channel, a small amount of material is compressed and forced into the channel. As for the single pelleter, this results in a laminated structure that for most materials is visible right away. All samples were loaded sequentially by adding approximately 0.15 g of wood grains in each step. The pelletizing pressure was 4400 bar at ambient room temperature. The cylindrical press channel has a diameter of 15.6 mm. To obtain full correspondence with the pellet mill, the pressure applied in each step of the single pellet pressing should be equal to the pressure needed to press the pellets out of the ring matrix in the pellet mill. However, because this pressure is not known, the magnitude of the pressure is not related to the pressure in the pellet mill for the present study. The pressure is chosen to ensure that stable pellets are formed at the present conditions. Samples. The following samples have been tested: Scotch pine shavings (Pinus sylVestris) and European beech shavings (Fagus sylVatica). The final moisture content of the samples was adjusted to approximately 12% (wt), wet basis. Particle-size distributions of the tested materials are shown in Figure 5.
Theoretical Section We have recently presented a theoretical model for the pressure established within the press channel and relating this pressure to the dimensions of the channels and to the materialspecific parameters, such as the friction coefficient, modulus of elasticity, and Poisson’s ratio.2 The model considers the case where fibers, oriented perpendicularly to the long direction of the press channel, are elastically deformed when exposed to a
2448 Energy & Fuels, Vol. 21, No. 4, 2007
Holm et al.
Figure 6. Back-pressure measurements as a function of the compression ratio of beech (+) and pine (O). Corresponding model fits are shown as solid and dashed lines (µ ) 0.3, and r ) 7.8 mm). Figure 5. Particle-size distributions of Scots pine shavings and European beech.
radial pressure. The radial pressure gives rise to a longitudinal elongation of the fibers, but as the channel walls are fixed, the potential elongation is converted into a transverse pressure on the channel walls. An oppositely directed normal pressure compensates the transverse pressure that ultimately gives rise to a friction force, prohibiting the free flow of material through the channel. Under steady-state pelletization, the pelletizing force produced by the action of the roller and the friction force are equal but opposite in direction. The magnitude of the pelletizing pressure is given by eq 1.
Proller(x) )
PNo 2µνLRx/r (e - 1) νLR
Table 1. Modeling Parameters species
PNo (bar)
νLR
µa
ra (mm)
beech pine beech pine beech pine
173 ( 10 61 ( 8 129 ( 8 46 ( 6 104 ( 6 37 ( 5
0.56 ( 0.02 0.66 ( 0.03 0.42 ( 0.02 0.49 ( 0.03 0.34 ( 0.01 0.39 ( 0.02
0.3 0.3 0.4 0.4 0.5 0.5
7.8 7.8 7.8 7.8 7.8 7.8
a
Table 2. Poisson’s Ratio νLRa
Results and Discussion The experimental back pressures from the single pelleter unit for beech and pine are shown in Figure 6. The data points represent the pressure needed to press out a pellet of a specific length, given in terms of the compression ratio. Each data point corresponds to an individual pellet; i.e., for each compression ratio tested, a new pellet has been built and pressed out. The solid and dashed lines are fits of the model to the data. In accordance with the model, the experimental data are shown to follow an exponential growth. The pressure needed to release a pellet of beech is higher than the corresponding pressure for pine for all pellet lengths. The model fits were carried out as least-squares fits of the model given by eq 1 to the pressure data. The friction coefficient µ was taken as a constant in the fits, while PNo and νLR were left free to vary. The model fits to the presented data sets are given for three different friction coefficients in Table 1. As the model fits are carried out on the same data sets for the three selected values of the friction
νLR
species
(1)
PNo is a prestressing pressure term that incorporates inelasticity in the model. x is the distance from the channel outlet to the inlet; r is the radius of the channels; µ is the friction coefficient; and νLR is Poisson’s ratio. The first index of ν denotes the direction of applied stress, and the second index denotes the direction of transverse deformation. L and R correspond to the principal longitudinal and radial fiber axes, respectively. In the model, the pellet is constructed as a stack of differential volume elements in the form of flat discs of fixed radius r and thickness dx. For an elaborate description of the model, the reader is referred to ref 2.
µ and r are entering as constants in modeling.
a
hardwood
beech birch
0.45 0.49
softwood
scots pine douglas fir spruce
0.42 0.29 0.41 ( 0.03b
Literature data from ref 2, 9-13% moisture. b Average of six values.
coefficient, the ratios PNo/νLR and µνLR will evidently be constant. The results of the fits for µ ) 0.3 are shown in Figure 6. The values of the friction coefficients are chosen as typical values for dry wood on a hard smooth surface, falling in the range between 0.3 and 0.5.3 It should be emphasized that the aim of the modeling is not to derive detailed values of the parameters. The purpose is to show the exponential increase in the back pressure as a function of the increasing compression ratio and to show that by using typical values for the friction coefficient one can obtain values for Poisson’s ratio that are within the typical range of published values. Typical values of Poisson’s ratio, νLR, fall in the range between 0.2 and 0.6.4 The value depends upon the wood species in question, and significant differences are seen between Poisson’s ratios from different literature sources,4,5 probably because of the difficulty in measuring Poisson’s ratios accurately. Poisson’s ratio, νLR, of a few selected hard- and softwoods are given in Table 2. The fitted values for Poisson’s ratio are shown to be in the expected range, according to the published values in Table 2. Evidently, the obtained values depend upon the choice of friction (3) 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. (4) Hearmon, R. F. S. Elasticity of Wood and Plywood; Special Report on Forest Products Research, No. 7; His Majesty’s Stationery Office: London, U.K., 1948. (5) Bucur V.; Archer, R. R. Wood Sci. Technol. 1984, 18, 255-265.
Experimental Verification of NoVel Pellet Model
coefficient. However, the friction coefficient represents typical values obtained from the literature. The model thus describes the experimental data in an excellent way. The fitted Poisson’s ratio of beech is lower than that of pine, when the friction coefficient is set equal for both species. However, the assumption that the friction coefficient is equal for both wood species might be too strong of a simplification. Even small changes in this parameter can result in a fitted Poisson’s ratio that is higher for beech than pine (see Table 1). The prepressing pressure, PNo, is consistently higher for beech as compared to pine. Besides introducing inelasticity, the exact nature of this parameter is outside the scope of the present paper. However, the higher value for beech might be related to the particle-size distributions, where a higher percentage of fines are seen in the beech sample as compared to the pine sample. The packing of larger shavings in the pine sample is likely to be more elastic than the packing of predominantly smaller particles in the beech sample. One of the major differences between the rotating pellet mill and the single pelleter unit is how the pelletizing pressure is controlled. In the single pelleter unit, the pressure is controlled by the user and is hence independent of the material and the matrix dimensions. In the pellet mill, the specific matrix dimensions and the material in question control the pelletizing pressure. Hence, the pressure necessary to press out a pellet made of softwood is likely to be lower than the corresponding pressure for a hardwood pellet, when the pressure is measured in the channel of the pellet mill. If this is the case, the applied pressure in the single pelleter unit should be lower for softwood than for hardwood. Conclusions The aim of the paper was to verify the published theoretical pellet model to predict the back pressures in a pellet mill for different wood species. The verification was archived by using a single pelleter unit, where back pressures can be measured for pellets with a predefined length. The determined back-pressure curves of beech and pine show the expected exponential behavior, as suggested by the model.
Energy & Fuels, Vol. 21, No. 4, 2007 2449
Second, the curve of beech exceeds the curve of pine for all tested compression ratios, again in agreement with the model. The results of the model fits to the data show that the fitted values of Poisson’s ratios are in good agreement with literature data. Dependent upon the exact value of the friction coefficient, Poisson’s ratios can be matched very well to the literature data. As discussed in the previous section, the prestressing pressure might be related to the particle-size distributions, where the larger shavings in the pine sample may contribute less to the inelasticity than the smaller particles in the beech sample. Finally, the paper describes how pellets should be prepared in the single pelleter unit to mimic the pelletization process taking place in a rotating pellet mill. It is shown that the sequential loading, i.e., the buildup of the pellet in small steps, is necessary to ensure that each thin disc of the pellet has been exposed to the same high pressure as is the case in the rotating pellet mill. This pelletizing pressure used in the single pelleter press should be equal to the pressure needed to press a pellet out of the channel in the rotating pellet mill. Acknowledgment. Financial support from Dong Energy A/S and the Danish Energy Agency is highly acknowledged.
Nomenclature r ) radius of the press channel (mm) D ) diameter of the press channel (mm) Lp ) length of the press channel (mm) µ ) friction coefficient ν ) Poisson’s ratio PNo ) prestressing pressure (bar) Proller ) pelletizing pressure (bar) Pback ) back pressure (bar) Subscripts L ) longitudinal fiber direction R ) radial fiber direction EF070156L
