- Email: [email protected]
Experimental study about the effects of disc chipper settings on the distribution of wood chip size
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 ) 8 4 3 e8 5 2
Available at www.sciencedirect.com
http://www.elsevier.com/locate/biombioe
Experimental study about the effects of disc chipper settings on the distribution of wood chip size Rami Abdallah*, Se´bastien Auchet, Pierre Jean Me´ausoone Laboratoire d’Etudes et de Recherche sur le Mate´riau Bois, Ecole Nationale supe´rieure des technologies et Industrie du Bois, 27 rue Philippe SE´GUIN, BP 1041, 88051 Epinal Cedex 9, France
article info
abstract
Article history:
Nowadays wood should be of principal sources of biomass. This wood is transformed into
Received 23 February 2010
chips in order to increase automatic operations and to decrease the technical effort needed
Received in revised form
at the energy conversion plant. Typical high quality chips, which are used to feed small
19 October 2010
woodchip boilers, vary in size from 10 10 5 mm to 15 15 8 mm. Chips that are
Accepted 1 November 2010
relatively square and flat are easily conveyed, augured, and fed into the system smoothly.
Available online 3 December 2010
We are mainly interested in the raw material of inferior quality. A disc chipper test bench was constructed in our laboratory to study the chipping process in cutting conditions
Keywords:
which are similar to those used in the industry. The test bench design allows many factors
Disc chipper
to be varied include cutting speed, feed per tooth, cutting angles, anvil height and cutting
Cutting conditions
direction. In this paper, we attempt to understand the effect of several factors on chip size
Chip size
distribution. Four feeds per tooth, four cutting angles, two sharpness angles and three
Wood
cutting speeds were chosen to cut wet logs of oak and fir wood, while the other factors
Biomass
remained constant. The results are similar for both oak and fir. The proportion of small chips decreases when we increase the feed per tooth, the cutting angle and the sharpness angle, whereas it increases when the cutting speed is increased. The feed per tooth and the cutting speed have a linear effect on the variations in the size distribution, while the cutting angle has a non-linear effect on these variations. ª 2010 Elsevier Ltd. All rights reserved.
1.
Introduction
In the last ten years, wood chips have been used increasingly as solid biofuels for both large-scale and small-scale applications. For examples, wood chips are used in power plants, in CHP-plants, in biogas stations, in large heating plants and in small combustion units. Wood chipping serves many purposes such as upgrading waste forest wood, decreasing the production cost of renewable energy, promoting the development of unproblematic supply chain from technical and economic viewpoint and giving possibilities to automate the operation in the end use technology [1]. Trees damaged by
storms, harvesting residues and wood from thinning or pruning represent the main sources of the raw material for chipping [2]. There are three different devices which can produce wood chips: the cone-screw wood chunker, the drum chipper and the disc chipper. We are interested in the disc chipper because it is the most widely used due to its ability to process a variety of wood feedstock, its relative simplicity, ruggedness, low cost and ease of maintenance [3]. In the pulp and paper industry, many studies have been carried out on wood chipping to understand the formation process of the chips. This industry looks for suitable chip
* Corresponding author. Tel.: þ33 3 29296100x89752; fax: þ33 3 29296138. E-mail address: [email protected] (R. Abdallah). 0961-9534/$ e see front matter ª 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.biombioe.2010.11.009
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Fig. 1 e Chipping configuration and compressed area [3].
dimensions with high interest in chip thickness [4], whereas in the biomass field, chips are classified according to highest dimension [5,6]. The analysis of the chip formation is quite complicated, because wood is an anisotropic, heterogeneous and hygroscopic material [7], and because during the wood chipping process, variables interact in an unexpected manner [8]. The chip formation process depends on the mechanical properties of the wood. The most important being the cleavage strength, the shear strength parallel to the grain, and the compressive strength parallel to the grain [3,9]. McLauchlan [3] explains that during the chip formation process, compressive stresses are developed in an area adjacent to the knife surface (Area A e Fig. 1). Components of these stresses react with the cleavage or splitting strength of the
wood, or with the shear strength parallel to the grain to cause chip formation. A higher compressive strength or a lower cleavage or lower shear strength will therefore produce thinner chips. Hellstro¨m et al. [10] have identified three different types of chip formation processes: opening mode, forward shear mode and remote opening mode. The type of process is highly dependent on the friction between the wood material and the chipping tool. The influence of friction ranges from low friction in the opening mode to high friction in the remote opening mode. A constant chip thickness distribution will be obtained if only the opening mode is active, inducing that the friction is as low as possible. Edelma and Stuart [11], Hartler [12], Hernandez and Jacques [13] all agree that increasing cutting speed leads to the production of a higher percentage of both small chips and fines, since other variables remain constant. Hernandez and Jacques [13] explain that chip thickness is determined by longitudinal wood splitting. As the slice cut by the knife hits against the knife clamp, the change in direction due to the angle of the knife clamp produces tensile stresses perpendicular to the grain of the slice made by each knife. As cutting speed increases, the slice hits with increasing force against the knife clamp. The maximum tensile stress is reached more consistently as cutting speed increases. This leads to more splitting and therefore a thinner chip. In light of the studies performed by Buchanan and Duchinicki [14] and Monico and Soule [15], we understand that decreasing the cutting angle also increases the chip thickness. As the cutting angle decreases, the force applied perpendicular to the grain increases, therefore splitting is delayed and thick chips are produced. Moisture content influences chip formation significantly. The higher the moisture content, the thinner the chip [16], while the chip width tends to be greater. There have not been numerous studies on chip width [14], because it is affected by many factors such as the strength properties of the wood, moisture content, temperature of the wood, cutting speed and cutting direction. The average chip width can also be noticeably reduced after being formed by the impact between the chips and the chipper enclosure [17].
Fig. 2 e Test bench components.
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Fig. 3 e Identification of the feeding direction.
We would like to better understand the chip formation process in order to produce the suitable chip size. We want to avoid producing oversized chips, which are called “coarse particles”, and small ones (under 1 mm diameter), which are called “fines”. Coarse chips cause two problems in the feeding unit of the automatic woodchip boiler: first of all, they could jam the feeding screw [18] and secondly these chips could encourage what is called the “bridging tendency” [19]. The latter represents the formation of hollow cavities in the hoppers and bins as the material below is removed. Thus the “bridging” of materials can cause some systems to shut down because it seems that the bin is out of chips when it is not. An unusually large amount of wood “fines” can also present issues when moisture content is either too low or too high, as it reduces the energy conversion efficiency and creates more emissions [20].
2.
Materials and methods
2.1.
Test bench
Our main objective is to study the chipping of harvesting residues in industrial conditions, which is why we have constructed a full-size test bench. This test bench is composed of
two independent units (Fig. 2): a disc chipper and a feeding mechanism. The disc weighs 182 kg and measures 950 mm in diameter. A variable speed driver allows us to vary the disc’s rotation speed from 1 rpm to 1000 rpm. Thus we have a cutting speed variable of 0.03e32 m/s. The chipper has an asynchronous motor with the power of 21 KW. It is possible to use either two or four knives. The feeding unit has an independent motor. The feeding speed is controlled by a second variable speed driver and a gearbox. It can be varied from 2 m/min to 25 m/min.
2.2.
Samples
Two species: fir (Abies alba) and oak (Quercus petraea) were chosen as representatives of both softwood and hardwood in the French forest. We chipped over a hundred wet samples from thirty trees that we harvested from the Dogneville forest (Vosges, France). These trees are 8e15 m long. The small diameter of the trunk varies from 70 mm to 100 mm and the large diameter varies from 100 mm to 160 mm. The samples have an average diameter of 90e115 mm, taper from 1 mm/m to 10 mm/m and length of 1000 mm. Although all logs are green, the moisture content is not the same in all the samples. The
Fig. 4 e Cutting angles.
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a way that we can identify the feeding direction and knife angles independently [21]. Feeding direction can be defined by the cutting direction definition of McKenzie [22], in which two angles are concerned: r and e (Fig. 3), r: Located on the horizontal plane between the cutting edge and the grain direction (cutting edge is always parallel to the front face of the disc). e: Located on the vertical plane between the cutting plane and the grain direction (cutting plane is always parallel to the front face of the disc). This angle is also called the spout angle. In Fig. 4(1) we can identify the traditional cutting angles: the cutting angle which is also called the rake angle (g), the sharpness angle (b), the clearance angle (a). The sum of these three angles is always 90 . Using these angles make it possible to know the angles between the knife faces and the disc. We have added a fourth angle (Fig. 4(2)), which we have called the real cutting angle (gr). It is the angle between the cutting face and the grain direction. This angle has an important effect on both chip formation and cutting forces [23]. Chip length is determined by the equation (1), [3]:
Fig. 5 e Cutting configurations.
L ¼ H=sin3
Table 1 e Values of the tested parameters. fz (mm)
g ( )
Vc (m/s)
b ( )
3.5 4.8 6 8
45 50 52.5 55
19 26 32.4
25 34
moisture content ranges from 44% to 55% wet basis for fir specimens and from 32% to 40% wet basis for the oak specimens. The samples don’t come from the same tree, which implies different mechanical properties and variable morphology.
2.3.
Cutting configurations
Based on previous works done in the wood machinery field, we designated the cutting angles and cutting direction in such
(1)
Where H is the knife height; 3 is the spout angle. But this equation is only applicable when the in-feeding wood touches the disc face. For example, in the case of a dropfeed, the chips length can be adjusted by changing the knife height [24]. Our test bench and most of the whole tree chippers have an external feeding system, which allows us to control the feeding speed. This keeps the in-feeding wood from touching the disc, and gives us the ability to eliminate the energy consumed due to friction. When a feeding system is used, equation (1) is not applicable anymore; therefore it is much more appropriate to use equation (2): L ¼ fz =sin3
(2)
where fz is feed per tooth (mm), fz ¼ (Vf/Z N) 1000. Vf is feeding speed (m/min); Z is number of knives; N is disc rotation speed (rpm). The positioning coordinate of the anvil center (K ) remained constant during our experiments in the reference
Fig. 6 e Screening devices.
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Table 2 e Values of the cutting angles sets. Angles sets
a ( )
b ( )
g ( )
Set-1 Set-2 Set-3 Set-4
11 6 3.5 1
34 34 34 34
45 50 52.5 55
the loss. Then, the moisture content (wet basis) is measured using the procedure described in the Norm NF B 51-004 [25], - screening the wood chips:
Fig. 7 e Effect of variation in feed per tooth on chip size distribution e Oak.
(OXY), with point O as the disc center (Fig. 5). This is why the cutting forces located in the plane (OXY) always move along the same path. Hence, the samples tend to move towards the axis OY when they are not held in place by the feeding wheel. The feeding direction was (r ¼ 90 , 3 ¼ 90 ) and the number of knives (two) remained constant during the experiments. The parameters tested are the feed per tooth ( fz), the cutting angle (g ¼ gr), the sharpness angle (b) and the cutting speed (Vc), which is calculated at the knife middle (Table 1).
2.4.
Experimental protocol
We carried out our experiments using the following procedure: - measuring the sample weight, length, average diameter and taper, - chipping the sample, - measuring the weight of the wood chips and comparing it with the sample weight, in order to know the proportion of
Fig. 8 e Effect of variation in feed per tooth on chip size distribution e Fir.
Six sieves with square holes were used to classify the chip sizes. The sieves holes have the following dimensions: 1, 2, 4, 6.3, 8and 10 mm. These sieves are placed in three screen shakers (Fig. 6). The screening operation lasts approximately 1 h for each test. There are many possible factors which cause variation in the size distribution measurements, therefore the margin of error in the screening results must be determined. To test our sieves and sieving method, 21 L of fir chips and 21 L of oak chips were screened four times at constant thermal conditions. Standard deviations of these screenings range from 0.0% to 0.4% and the percentage of lost chips was less than 2%. We decided to find the margin of error when we chip similar samples in the same cutting configurations. We gathered three Fir samples from several different trees, as well as three Oak samples. These samples were chipped in the following cutting configurations: Vc ¼ 19.5 m/s, g ¼ gr ¼ 50 , b ¼ 34 and fz ¼ 8 mm. The screening results showed that the margin of error of the chip size distribution is less than 5%. These results prove that our screening process and our chipping procedure are accurate.
3.
Results and discussion
Most of the wood chips that were produced had a square shape with constant thickness. The cracks which define chip thickness are clearly initiated from the vessels. It was noticed that when chipping oak, the largest chips come from slivered bark, whereas the largest chips are from knots when chipping fir. Moreover, it was noticed that the higher the gap between the knife and the anvil, the worse the cutting effects: knives’ wear and cutting noise increase, cutting surface becomes rougher. The movement between the knife and anvil is similar to a scissor action, therefore the gap should be as small as possible (0.5e1 mm) in order to avoid bad chipping processes. The effect of four cutting parameters (feed per tooth, cutting speed, cutting angle, and sharpness angle) on chip size distribution was specifically observed. First of all, we were interested in finding out the feed per tooth effect on the chip size distribution. Four values of feed per tooth ( fz) (3.5, 4.8, 6, 8 mm) were chosen at a constant cutting speed (26 m/s), sharpness angle (25 ) and cutting angle (50 ). Three sieves (1, 2 and 4 mm square holes) were used to determine the size distribution. Chips were then classified into four classes (<1, 1e2, 2e4, >4). The results are shown in
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Fig. 9 e Chip size distribution for different cutting speeds e Fir.
Fig. 7 for oak wood chips and in Fig. 8 for fir wood chips. Increasing the feed per tooth leads to an increase in chip size for both oak and fir wood. The percentage of the size class (>4 mm) rises while the percentage of the size classes (1e2 and 2e4 mm) decreases with greater feed per tooth. It was noticed that most of the chips had at less one dimension equal to the cutting length, which is a logical outcome according to equation (2). The cutting length (L) is equal to the feed per tooth ( fz) and not to the knife height perpendicular to the disc face (H ). Quite predictably, when one chip dimension increases, the other dimensions will proportionately increase as well. As can be seen in Figs. 7 and 8, the variation in size distribution is linear with the feed per tooth one, meaning that
the greatest percentage of the class size (>4) will be produced with the highest feed per tooth. Other trials were conducted in order to evaluate the effect of the cutting speed and the cutting angle. Four cutting angles sets were tested (Table 2). For each set of angles, three cutting speeds (19.5e26e32.4 m/s) were tested. The feed per tooth had a constant value of 8 mm, implying the feeding speed to follow respective values of 9.6, 12.8 and 18 m/min. Five sieves were chosen (1e4e6.3e8e10 mm) to classify the chip sizes into six classes (<1, 1e4, 4e6.3, 6.3e8, 8e10, >10 mm). Due to the interaction between the effects of Vc and g, both (Vc, g) must be analyzed together in order to accurately understand their effects.
Fig. 10 e Chip size distribution for different cutting speeds e Oak.
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Fig. 11 e Chip size distribution for different cutting angles e Fir.
The average values of the size distribution can be seen in Fig. 9 and in Fig. 11 for fir wood, and in Fig. 10 and in Fig. 12 for oak wood. Each column represents the chip size distribution for a different cutting configuration. There were large percentages for the following size classes (1e4), (4e6.3), (6.3e8) mm but the variations in the chips percentage are most visible in the classes (1e4), and (6.3e8) mm. When discussing our results we often refer to the chip size classes {(<1), (1e4), (4e6.3)} as “small chip size”, whereas the chip size classes {(6.3e8), (8e10), (>10)} are referred to as “big chip size”. Effect of variation in cutting speed on the size distribution for softwood and hardwood is respectively shown in Fig. 9 and in Fig. 10. When cutting speed increases, the percentage of the
“small chip size” increases too, while the percentage of the “big chip size” decreases, this tendency is apparent in fir and oak with different cutting angles. We can also notice that the variation in size percentage is linear, meaning that the greatest percentage of “big chip size” will be produced with the lowest cutting speed, this result is in accordance with the literature. But, if a higher chip size is wanted, we can not decrease the cutting speed regardless of other parameters. For example, if we decrease the cutting speed to under 500 rpm, the knives wear will be faster and the chipper productivity will fall. Therefore a compromise between the important parameters needs to be found, which could significantly affect the chipping process from economic viewpoint.
Fig. 12 e Chip size distribution for different cutting angles e Oak.
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formation. Chips are obviously produced by shear and crack. The crack mechanism is influenced by the friction between the wood and the knife cutting face. The cracks e friction theory can be summarized in few words: crack initiation and crack propagation are highly influenced by the friction magnitude. When friction between wood and tool edge increases the number and the depth of the cracks will increase as well. In our cutting configurations the cutting friction coefficient (m) can be easily computed from the principal and normal forces, respectively Fp and Fn (Fig. 13), by equation (3): m ¼ tanðarctanðFn=FpÞ þ gÞ
Fig. 13 e Crack propagation and cutting forces.
Effect of variation in cutting angle on the size distribution is shown in Fig. 11 for fir and in Fig. 12 for oak. In the first figure, there is no clear variation tendency, whereas for oak, if we omit the irregular result for (Vc: 19.5 m/s and g: 52.5 ), we can distinguish that the percentage of “big chip size” tends to be higher, with non-linear variation, when the cutting angle increases. There is a rising threshold between 50 and 52.5 . We believe that the non-linear variation is caused from one part by the mechanical behavior of wood, which is highly nonlinear [26], and for the other part by the mechanisms of chip
(3)
The value of Fp and Fn are affected by the variation of the instantaneous rotation angle of the disc qc (Fig. 5), qc is continuously changed due to the disc rotation, which implies variation in friction value. Friction is also affected by the cutting speed and the moisture content [27]. Moreover, from equation (3) we can deduce that friction evolves when the cutting angle is varied. For all these reasons, we can suppose that the non-linear variation of chip distribution is due to the effect of friction on cracks initiation and propagation. Then, Chip formation and size distribution are influenced by variation in cutting angle. In some cutting conditions, the variations in size distribution have a clear tendency for oak, and in other conditions they have not for fir. The last parameter to be analyzed is the sharpness angle (b). Two sharpness angles (25 , 34 ), three cutting speeds (19.5, 26, 32.4) m/s and one cutting angle (g: 45 ) are chosen. The results are shown in Fig. 14 for fir and in Fig. 15 for oak. The percentage of “big chip size” and size class (4e6.3 mm) increases as the sharpness angle increases. This tendency is for both fir and oak, and with all tested values of the cutting speed. The size distribution is analyzed for two values of the sharpness angle. With two points of measurement we can not infer whether or not the variation is linear. Further investigation must be carried out under other chipping conditions in
Fig. 14 e Chip size distribution for different sharpness angles e Fir.
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 ) 8 4 3 e8 5 2
851
Fig. 15 e Chip size distribution for different sharpness angles e Oak.
order to disclose the effect of the sharpness angle on the chip size distribution in all possible cutting conditions.
4.
Conclusion
In this study, the effects of different cutting configurations on chip size distribution were analyzed by using a chipping test bench similar to industrial disc chippers. The results were similar for both fir and oak wood: the proportion of small size classes in the produced chips increases when the cutting speed increases, whereas chips tend to have a higher proportion of large size classes with a greater value of feed per tooth, cutting angle and sharpness angle. A compromise between chipping parameters should be found in order to get the desirable chip size with maximum productivity and minimum energy consumption. The results analysis is a complicated task, because in wood chipping, the mechanisms of the chip formation are resulted from the competition between shear mechanism and crack propagation. This study offers some important highlights and suggestions for further investigations. The mechanism of the chip formation defined by other researchers lacks of information about the effects of several factors (cutting angle, sharpness angle, and cutting speed) on the chip size distribution and the energy conception. Our future study will focus on the reduction of the energy consumption in order to produce the suitable chip size with the minimum of energy.
references
[1] Martin K, Weber M. Markets for solid biofuels within the EU15. Biomass & Bioenergy 2006;30:897e907.
[2] Poue¨t JC. Mise en place d’une chaufferie au bois. EDP Sciences. ADEME e´ditions; 2007. [3] McLauchlan TA, Lapointe JA. Production of chips by disc chippers. In: Hatton JV, editor. Chips quality monograph. Pulp and paper technology Series. Atlanta: TAPPI; 1979. p. 15e32. [4] Borlew PB, Miller RL. Chip thickness: a critical dimension in Kraft pulping. Tappi 1970;53(11):2107e11. ¨ NORM M 7133: Holzhackgut fu¨r energetische zwecke, [5] O Anforderungen und Pru¨fbestimmungen. Vienna, Austria: Austrian Standards Institute; 1998. [6] CEN/TS 14961:2005 (E), Solid biofuels e Fuel Specification and classes, CEN/TC 335 e Solid Biofuels; April 2005. p. 40. [7] Franz N. An analysis of the wood cutting process [Ph d thesis]. Ann Arbor: University of Michigan; 1958. [8] Uhmeier A. Some fundamental aspects of wood chipping. TAPPI J 1995;78(10):79e86. [9] Twaddle A. The influence of species, chip length, and ring orientation on chip thickness. TAPPI J 1997;80(6):123e31. [10] Hellstro¨m LM, Gradin PA, Carlberg T. A method for experimental investigation of the wood chipping process. Nordic Pulp & Paper Res J 2008;23(3):339e42. [11] Edelma JS, Stuart WB. Effect of disk speed on sawmill residue wood chip quality. In: Proceeding of TAPPI pulping conference; 1992. p. 375e80. Boston, MA, USA. [12] Hartler N. Achievement and significance of optimal chip quality. TAPPI J 1986;79(2):259e64. [13] Hernandez R, Jacques B. Effect of the rotation speed on the size distribution of black spruce pulp chips produced by chipper-canter. For Prod J 1997;47(4):43e9. [14] Buchanan JG, Duchinicki TS. Some experiments in lowspeed chipping. Pulp & Paper Magazine canada; 1963 May: T235e45. [15] Monico JA, Soule HM. A machine to harvest and chip brush stands. Presented at the 1979 Winter Meeting. New Orleans LA: American Society of Agricultural Engineers; Dec 11e14 1979. Paper No. 79e1595. [16] McKenzie WM. The relationship between the cutting properties of wood and its physical and mechanical properties. For Prod J; 1962 June:287e97.
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[17] Engelgau WG. What is new and old in chipping? Tappi 1978; 61(8):77e80. [18] Biomass Energy Resource Center. Woodchip fuel specifications and procurement strategies for the black hills. Prepared by BERC, Montpelier, Vermont. For: South Dakota Department of Agriculture, Resource Conservation & Forestry Division; 2007. p. 31. [19] Jensen PD, Mattsson JE, Kofman PD, Klausner A. Tendency of wood fuels from whole trees, logging residues and roundwood to bridge over openings. Biomass & Bioenergy 2004;26:107e13. [20] Hartmann H, Bo¨hm T, Jensen PD, Temmerman M, Rabier F, Gloser M. Methods for size classification of wood chips. Biomass & Bioenergy 2006;30:944e53. [21] Abdallah R, Auchet S, Meausoone PJ, Triboulot P. Effect of disc chipper settings and wood properties on the chips size distribution. In: Zhou H, Zhu N, Ding T, editors. IWMS 19:
[22] [23]
[24]
[25] [26]
[27]
Proceeding of the 19tn International Wood Machining Seminar; 2009 Oct. 31e23; 2009. p. 32e42. Nanjing, China. McKenzie WM. Fundamental analysis of the wood cutting process [Ph d thesis]. Ann Arbor: University of Michigan; 1961. Martin P, Sales C. Processus de coupe et la scie a` ruban. In: Jodin P, editor. Le bois mate´riau et d’inge´nierie. ARBOLOR; 1994. p. 290e332. Jones TG, Song GG, Richardson JD. Effect of chipper setting on chip size distribution and mechanical pulp properties. Appita J 2005;58(1):56e63. Nf B 51e004: De´termination de l’humidite´. Paris, France: Association franc¸aise de normalisation; 1985. Holmberg S, Persson K, Petersson H. Nonlinear mechanical behaviour and analysis of Wood and fibre materials. Comput Struct 1999;72:459e80. McKenzie WM, Karpovich H. The Frictional behaviour of wood. Wood Science & Technology 1968;2:139e52.
Available at www.sciencedirect.com
http://www.elsevier.com/locate/biombioe
Experimental study about the effects of disc chipper settings on the distribution of wood chip size Rami Abdallah*, Se´bastien Auchet, Pierre Jean Me´ausoone Laboratoire d’Etudes et de Recherche sur le Mate´riau Bois, Ecole Nationale supe´rieure des technologies et Industrie du Bois, 27 rue Philippe SE´GUIN, BP 1041, 88051 Epinal Cedex 9, France
article info
abstract
Article history:
Nowadays wood should be of principal sources of biomass. This wood is transformed into
Received 23 February 2010
chips in order to increase automatic operations and to decrease the technical effort needed
Received in revised form
at the energy conversion plant. Typical high quality chips, which are used to feed small
19 October 2010
woodchip boilers, vary in size from 10 10 5 mm to 15 15 8 mm. Chips that are
Accepted 1 November 2010
relatively square and flat are easily conveyed, augured, and fed into the system smoothly.
Available online 3 December 2010
We are mainly interested in the raw material of inferior quality. A disc chipper test bench was constructed in our laboratory to study the chipping process in cutting conditions
Keywords:
which are similar to those used in the industry. The test bench design allows many factors
Disc chipper
to be varied include cutting speed, feed per tooth, cutting angles, anvil height and cutting
Cutting conditions
direction. In this paper, we attempt to understand the effect of several factors on chip size
Chip size
distribution. Four feeds per tooth, four cutting angles, two sharpness angles and three
Wood
cutting speeds were chosen to cut wet logs of oak and fir wood, while the other factors
Biomass
remained constant. The results are similar for both oak and fir. The proportion of small chips decreases when we increase the feed per tooth, the cutting angle and the sharpness angle, whereas it increases when the cutting speed is increased. The feed per tooth and the cutting speed have a linear effect on the variations in the size distribution, while the cutting angle has a non-linear effect on these variations. ª 2010 Elsevier Ltd. All rights reserved.
1.
Introduction
In the last ten years, wood chips have been used increasingly as solid biofuels for both large-scale and small-scale applications. For examples, wood chips are used in power plants, in CHP-plants, in biogas stations, in large heating plants and in small combustion units. Wood chipping serves many purposes such as upgrading waste forest wood, decreasing the production cost of renewable energy, promoting the development of unproblematic supply chain from technical and economic viewpoint and giving possibilities to automate the operation in the end use technology [1]. Trees damaged by
storms, harvesting residues and wood from thinning or pruning represent the main sources of the raw material for chipping [2]. There are three different devices which can produce wood chips: the cone-screw wood chunker, the drum chipper and the disc chipper. We are interested in the disc chipper because it is the most widely used due to its ability to process a variety of wood feedstock, its relative simplicity, ruggedness, low cost and ease of maintenance [3]. In the pulp and paper industry, many studies have been carried out on wood chipping to understand the formation process of the chips. This industry looks for suitable chip
* Corresponding author. Tel.: þ33 3 29296100x89752; fax: þ33 3 29296138. E-mail address: [email protected] (R. Abdallah). 0961-9534/$ e see front matter ª 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.biombioe.2010.11.009
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Fig. 1 e Chipping configuration and compressed area [3].
dimensions with high interest in chip thickness [4], whereas in the biomass field, chips are classified according to highest dimension [5,6]. The analysis of the chip formation is quite complicated, because wood is an anisotropic, heterogeneous and hygroscopic material [7], and because during the wood chipping process, variables interact in an unexpected manner [8]. The chip formation process depends on the mechanical properties of the wood. The most important being the cleavage strength, the shear strength parallel to the grain, and the compressive strength parallel to the grain [3,9]. McLauchlan [3] explains that during the chip formation process, compressive stresses are developed in an area adjacent to the knife surface (Area A e Fig. 1). Components of these stresses react with the cleavage or splitting strength of the
wood, or with the shear strength parallel to the grain to cause chip formation. A higher compressive strength or a lower cleavage or lower shear strength will therefore produce thinner chips. Hellstro¨m et al. [10] have identified three different types of chip formation processes: opening mode, forward shear mode and remote opening mode. The type of process is highly dependent on the friction between the wood material and the chipping tool. The influence of friction ranges from low friction in the opening mode to high friction in the remote opening mode. A constant chip thickness distribution will be obtained if only the opening mode is active, inducing that the friction is as low as possible. Edelma and Stuart [11], Hartler [12], Hernandez and Jacques [13] all agree that increasing cutting speed leads to the production of a higher percentage of both small chips and fines, since other variables remain constant. Hernandez and Jacques [13] explain that chip thickness is determined by longitudinal wood splitting. As the slice cut by the knife hits against the knife clamp, the change in direction due to the angle of the knife clamp produces tensile stresses perpendicular to the grain of the slice made by each knife. As cutting speed increases, the slice hits with increasing force against the knife clamp. The maximum tensile stress is reached more consistently as cutting speed increases. This leads to more splitting and therefore a thinner chip. In light of the studies performed by Buchanan and Duchinicki [14] and Monico and Soule [15], we understand that decreasing the cutting angle also increases the chip thickness. As the cutting angle decreases, the force applied perpendicular to the grain increases, therefore splitting is delayed and thick chips are produced. Moisture content influences chip formation significantly. The higher the moisture content, the thinner the chip [16], while the chip width tends to be greater. There have not been numerous studies on chip width [14], because it is affected by many factors such as the strength properties of the wood, moisture content, temperature of the wood, cutting speed and cutting direction. The average chip width can also be noticeably reduced after being formed by the impact between the chips and the chipper enclosure [17].
Fig. 2 e Test bench components.
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Fig. 3 e Identification of the feeding direction.
We would like to better understand the chip formation process in order to produce the suitable chip size. We want to avoid producing oversized chips, which are called “coarse particles”, and small ones (under 1 mm diameter), which are called “fines”. Coarse chips cause two problems in the feeding unit of the automatic woodchip boiler: first of all, they could jam the feeding screw [18] and secondly these chips could encourage what is called the “bridging tendency” [19]. The latter represents the formation of hollow cavities in the hoppers and bins as the material below is removed. Thus the “bridging” of materials can cause some systems to shut down because it seems that the bin is out of chips when it is not. An unusually large amount of wood “fines” can also present issues when moisture content is either too low or too high, as it reduces the energy conversion efficiency and creates more emissions [20].
2.
Materials and methods
2.1.
Test bench
Our main objective is to study the chipping of harvesting residues in industrial conditions, which is why we have constructed a full-size test bench. This test bench is composed of
two independent units (Fig. 2): a disc chipper and a feeding mechanism. The disc weighs 182 kg and measures 950 mm in diameter. A variable speed driver allows us to vary the disc’s rotation speed from 1 rpm to 1000 rpm. Thus we have a cutting speed variable of 0.03e32 m/s. The chipper has an asynchronous motor with the power of 21 KW. It is possible to use either two or four knives. The feeding unit has an independent motor. The feeding speed is controlled by a second variable speed driver and a gearbox. It can be varied from 2 m/min to 25 m/min.
2.2.
Samples
Two species: fir (Abies alba) and oak (Quercus petraea) were chosen as representatives of both softwood and hardwood in the French forest. We chipped over a hundred wet samples from thirty trees that we harvested from the Dogneville forest (Vosges, France). These trees are 8e15 m long. The small diameter of the trunk varies from 70 mm to 100 mm and the large diameter varies from 100 mm to 160 mm. The samples have an average diameter of 90e115 mm, taper from 1 mm/m to 10 mm/m and length of 1000 mm. Although all logs are green, the moisture content is not the same in all the samples. The
Fig. 4 e Cutting angles.
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a way that we can identify the feeding direction and knife angles independently [21]. Feeding direction can be defined by the cutting direction definition of McKenzie [22], in which two angles are concerned: r and e (Fig. 3), r: Located on the horizontal plane between the cutting edge and the grain direction (cutting edge is always parallel to the front face of the disc). e: Located on the vertical plane between the cutting plane and the grain direction (cutting plane is always parallel to the front face of the disc). This angle is also called the spout angle. In Fig. 4(1) we can identify the traditional cutting angles: the cutting angle which is also called the rake angle (g), the sharpness angle (b), the clearance angle (a). The sum of these three angles is always 90 . Using these angles make it possible to know the angles between the knife faces and the disc. We have added a fourth angle (Fig. 4(2)), which we have called the real cutting angle (gr). It is the angle between the cutting face and the grain direction. This angle has an important effect on both chip formation and cutting forces [23]. Chip length is determined by the equation (1), [3]:
Fig. 5 e Cutting configurations.
L ¼ H=sin3
Table 1 e Values of the tested parameters. fz (mm)
g ( )
Vc (m/s)
b ( )
3.5 4.8 6 8
45 50 52.5 55
19 26 32.4
25 34
moisture content ranges from 44% to 55% wet basis for fir specimens and from 32% to 40% wet basis for the oak specimens. The samples don’t come from the same tree, which implies different mechanical properties and variable morphology.
2.3.
Cutting configurations
Based on previous works done in the wood machinery field, we designated the cutting angles and cutting direction in such
(1)
Where H is the knife height; 3 is the spout angle. But this equation is only applicable when the in-feeding wood touches the disc face. For example, in the case of a dropfeed, the chips length can be adjusted by changing the knife height [24]. Our test bench and most of the whole tree chippers have an external feeding system, which allows us to control the feeding speed. This keeps the in-feeding wood from touching the disc, and gives us the ability to eliminate the energy consumed due to friction. When a feeding system is used, equation (1) is not applicable anymore; therefore it is much more appropriate to use equation (2): L ¼ fz =sin3
(2)
where fz is feed per tooth (mm), fz ¼ (Vf/Z N) 1000. Vf is feeding speed (m/min); Z is number of knives; N is disc rotation speed (rpm). The positioning coordinate of the anvil center (K ) remained constant during our experiments in the reference
Fig. 6 e Screening devices.
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Table 2 e Values of the cutting angles sets. Angles sets
a ( )
b ( )
g ( )
Set-1 Set-2 Set-3 Set-4
11 6 3.5 1
34 34 34 34
45 50 52.5 55
the loss. Then, the moisture content (wet basis) is measured using the procedure described in the Norm NF B 51-004 [25], - screening the wood chips:
Fig. 7 e Effect of variation in feed per tooth on chip size distribution e Oak.
(OXY), with point O as the disc center (Fig. 5). This is why the cutting forces located in the plane (OXY) always move along the same path. Hence, the samples tend to move towards the axis OY when they are not held in place by the feeding wheel. The feeding direction was (r ¼ 90 , 3 ¼ 90 ) and the number of knives (two) remained constant during the experiments. The parameters tested are the feed per tooth ( fz), the cutting angle (g ¼ gr), the sharpness angle (b) and the cutting speed (Vc), which is calculated at the knife middle (Table 1).
2.4.
Experimental protocol
We carried out our experiments using the following procedure: - measuring the sample weight, length, average diameter and taper, - chipping the sample, - measuring the weight of the wood chips and comparing it with the sample weight, in order to know the proportion of
Fig. 8 e Effect of variation in feed per tooth on chip size distribution e Fir.
Six sieves with square holes were used to classify the chip sizes. The sieves holes have the following dimensions: 1, 2, 4, 6.3, 8and 10 mm. These sieves are placed in three screen shakers (Fig. 6). The screening operation lasts approximately 1 h for each test. There are many possible factors which cause variation in the size distribution measurements, therefore the margin of error in the screening results must be determined. To test our sieves and sieving method, 21 L of fir chips and 21 L of oak chips were screened four times at constant thermal conditions. Standard deviations of these screenings range from 0.0% to 0.4% and the percentage of lost chips was less than 2%. We decided to find the margin of error when we chip similar samples in the same cutting configurations. We gathered three Fir samples from several different trees, as well as three Oak samples. These samples were chipped in the following cutting configurations: Vc ¼ 19.5 m/s, g ¼ gr ¼ 50 , b ¼ 34 and fz ¼ 8 mm. The screening results showed that the margin of error of the chip size distribution is less than 5%. These results prove that our screening process and our chipping procedure are accurate.
3.
Results and discussion
Most of the wood chips that were produced had a square shape with constant thickness. The cracks which define chip thickness are clearly initiated from the vessels. It was noticed that when chipping oak, the largest chips come from slivered bark, whereas the largest chips are from knots when chipping fir. Moreover, it was noticed that the higher the gap between the knife and the anvil, the worse the cutting effects: knives’ wear and cutting noise increase, cutting surface becomes rougher. The movement between the knife and anvil is similar to a scissor action, therefore the gap should be as small as possible (0.5e1 mm) in order to avoid bad chipping processes. The effect of four cutting parameters (feed per tooth, cutting speed, cutting angle, and sharpness angle) on chip size distribution was specifically observed. First of all, we were interested in finding out the feed per tooth effect on the chip size distribution. Four values of feed per tooth ( fz) (3.5, 4.8, 6, 8 mm) were chosen at a constant cutting speed (26 m/s), sharpness angle (25 ) and cutting angle (50 ). Three sieves (1, 2 and 4 mm square holes) were used to determine the size distribution. Chips were then classified into four classes (<1, 1e2, 2e4, >4). The results are shown in
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Fig. 9 e Chip size distribution for different cutting speeds e Fir.
Fig. 7 for oak wood chips and in Fig. 8 for fir wood chips. Increasing the feed per tooth leads to an increase in chip size for both oak and fir wood. The percentage of the size class (>4 mm) rises while the percentage of the size classes (1e2 and 2e4 mm) decreases with greater feed per tooth. It was noticed that most of the chips had at less one dimension equal to the cutting length, which is a logical outcome according to equation (2). The cutting length (L) is equal to the feed per tooth ( fz) and not to the knife height perpendicular to the disc face (H ). Quite predictably, when one chip dimension increases, the other dimensions will proportionately increase as well. As can be seen in Figs. 7 and 8, the variation in size distribution is linear with the feed per tooth one, meaning that
the greatest percentage of the class size (>4) will be produced with the highest feed per tooth. Other trials were conducted in order to evaluate the effect of the cutting speed and the cutting angle. Four cutting angles sets were tested (Table 2). For each set of angles, three cutting speeds (19.5e26e32.4 m/s) were tested. The feed per tooth had a constant value of 8 mm, implying the feeding speed to follow respective values of 9.6, 12.8 and 18 m/min. Five sieves were chosen (1e4e6.3e8e10 mm) to classify the chip sizes into six classes (<1, 1e4, 4e6.3, 6.3e8, 8e10, >10 mm). Due to the interaction between the effects of Vc and g, both (Vc, g) must be analyzed together in order to accurately understand their effects.
Fig. 10 e Chip size distribution for different cutting speeds e Oak.
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Fig. 11 e Chip size distribution for different cutting angles e Fir.
The average values of the size distribution can be seen in Fig. 9 and in Fig. 11 for fir wood, and in Fig. 10 and in Fig. 12 for oak wood. Each column represents the chip size distribution for a different cutting configuration. There were large percentages for the following size classes (1e4), (4e6.3), (6.3e8) mm but the variations in the chips percentage are most visible in the classes (1e4), and (6.3e8) mm. When discussing our results we often refer to the chip size classes {(<1), (1e4), (4e6.3)} as “small chip size”, whereas the chip size classes {(6.3e8), (8e10), (>10)} are referred to as “big chip size”. Effect of variation in cutting speed on the size distribution for softwood and hardwood is respectively shown in Fig. 9 and in Fig. 10. When cutting speed increases, the percentage of the
“small chip size” increases too, while the percentage of the “big chip size” decreases, this tendency is apparent in fir and oak with different cutting angles. We can also notice that the variation in size percentage is linear, meaning that the greatest percentage of “big chip size” will be produced with the lowest cutting speed, this result is in accordance with the literature. But, if a higher chip size is wanted, we can not decrease the cutting speed regardless of other parameters. For example, if we decrease the cutting speed to under 500 rpm, the knives wear will be faster and the chipper productivity will fall. Therefore a compromise between the important parameters needs to be found, which could significantly affect the chipping process from economic viewpoint.
Fig. 12 e Chip size distribution for different cutting angles e Oak.
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formation. Chips are obviously produced by shear and crack. The crack mechanism is influenced by the friction between the wood and the knife cutting face. The cracks e friction theory can be summarized in few words: crack initiation and crack propagation are highly influenced by the friction magnitude. When friction between wood and tool edge increases the number and the depth of the cracks will increase as well. In our cutting configurations the cutting friction coefficient (m) can be easily computed from the principal and normal forces, respectively Fp and Fn (Fig. 13), by equation (3): m ¼ tanðarctanðFn=FpÞ þ gÞ
Fig. 13 e Crack propagation and cutting forces.
Effect of variation in cutting angle on the size distribution is shown in Fig. 11 for fir and in Fig. 12 for oak. In the first figure, there is no clear variation tendency, whereas for oak, if we omit the irregular result for (Vc: 19.5 m/s and g: 52.5 ), we can distinguish that the percentage of “big chip size” tends to be higher, with non-linear variation, when the cutting angle increases. There is a rising threshold between 50 and 52.5 . We believe that the non-linear variation is caused from one part by the mechanical behavior of wood, which is highly nonlinear [26], and for the other part by the mechanisms of chip
(3)
The value of Fp and Fn are affected by the variation of the instantaneous rotation angle of the disc qc (Fig. 5), qc is continuously changed due to the disc rotation, which implies variation in friction value. Friction is also affected by the cutting speed and the moisture content [27]. Moreover, from equation (3) we can deduce that friction evolves when the cutting angle is varied. For all these reasons, we can suppose that the non-linear variation of chip distribution is due to the effect of friction on cracks initiation and propagation. Then, Chip formation and size distribution are influenced by variation in cutting angle. In some cutting conditions, the variations in size distribution have a clear tendency for oak, and in other conditions they have not for fir. The last parameter to be analyzed is the sharpness angle (b). Two sharpness angles (25 , 34 ), three cutting speeds (19.5, 26, 32.4) m/s and one cutting angle (g: 45 ) are chosen. The results are shown in Fig. 14 for fir and in Fig. 15 for oak. The percentage of “big chip size” and size class (4e6.3 mm) increases as the sharpness angle increases. This tendency is for both fir and oak, and with all tested values of the cutting speed. The size distribution is analyzed for two values of the sharpness angle. With two points of measurement we can not infer whether or not the variation is linear. Further investigation must be carried out under other chipping conditions in
Fig. 14 e Chip size distribution for different sharpness angles e Fir.
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Fig. 15 e Chip size distribution for different sharpness angles e Oak.
order to disclose the effect of the sharpness angle on the chip size distribution in all possible cutting conditions.
4.
Conclusion
In this study, the effects of different cutting configurations on chip size distribution were analyzed by using a chipping test bench similar to industrial disc chippers. The results were similar for both fir and oak wood: the proportion of small size classes in the produced chips increases when the cutting speed increases, whereas chips tend to have a higher proportion of large size classes with a greater value of feed per tooth, cutting angle and sharpness angle. A compromise between chipping parameters should be found in order to get the desirable chip size with maximum productivity and minimum energy consumption. The results analysis is a complicated task, because in wood chipping, the mechanisms of the chip formation are resulted from the competition between shear mechanism and crack propagation. This study offers some important highlights and suggestions for further investigations. The mechanism of the chip formation defined by other researchers lacks of information about the effects of several factors (cutting angle, sharpness angle, and cutting speed) on the chip size distribution and the energy conception. Our future study will focus on the reduction of the energy consumption in order to produce the suitable chip size with the minimum of energy.
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