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Corn stalk orientation effect on mechanical cutting
b i o s y s t e m s e n g i n e e r i n g 1 0 7 ( 2 0 1 0 ) 9 7 e1 0 6
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Research Paper
Corn stalk orientation effect on mechanical cutting C. Igathinathane a,b,*, A.R. Womac b, S. Sokhansanj c a
Department of Agricultural and Biosystems Engineering, North Dakota State University, 1221 Albrecht Boulevard, Fargo, ND 58102, USA Department of Biosystems Engineering and Soil Science, 2506 E. J. Chapman Drive, The University of Tennessee, Knoxville, TN 37996, USA c Oak Ridge National Laboratory, Environmental Sciences Division, Oak Ridge, P.O. Box 2008, TN 37831, USA b
article info
Research efforts that increase the efficiency of size reduction of biomass can lead to
Article history:
a significant energy saving. This paper deals with the determination of the effect of sample
Received 6 January 2010
orientation with respect to cutting element and quantify the possible cutting energy
Received in revised form
reduction, utilising dry corn stalks as the test material (15%e20% wet basis). To evaluate the
10 July 2010
mechanical cutting characteristics of corn stalks, a WarnereBratzler device was modified by
Accepted 14 July 2010
replacing its blunt edged cutting element with one having a 30 single bevel sharp knife edge. Cutting force-deformation characteristics obtained with a universal testing machine were analysed to evaluate the orientation effects at perpendicular (90 ), inclined (45 ), and parallel (0 ) orientations on internodes and nodes for cutting force, energy, ultimate stress, and specific energy of corn stalks. The corn stalks cutting force-displacement characteristics were found to differ with orientation, and internode and node material difference. Overall, the peak failure force, and the total cutting energy of internodes and nodes varied significantly (P < 0.05) with stalk cross-sectional area. The specific energy values (total energy per unit cut area) of dry corn stalk internodes ranged from 11.3 to 23.5 kN m1, and nodes from 8.6 to 14.0 kN m1. The parallel orientation (along grain) compared to perpendicular (across grain) produced a significant reduction of the cutting stress and the specific energy to onetenth or better for internodes, and to about one-fifth for nodes. Published by Elsevier Ltd on behalf of IAgrE.
1.
Introduction
Corn stover, among the crop-based residues, is considered as the most abundant biomass resource, while the straws of graminaceous crops such as wheat, barley, rice, oats, etc. follow the list of collectible biomass. Of the various corn stover components that are collectible from the field in a typical corn harvest, stalks dominate the available biomass (w73% fresh mass at a moisture content of w62% wet basis (w.b.)) followed by leaf (w20% fresh mass at w9% w.b.) and husk (w7% fresh mass at w11% w.b.) (Igathinathane, Womac,
Sokhansanj, & Pordesimo, 2006). On dry mass basis, about 56% of the total stover dry mass resided in the stalk, while its moisture ranged from 69% to 56% during the corn harvest; and the dry masses of other components were leaf at 21%, cob at 15%, and husk at 8% with total stover moisture content that ranged from 66% to 47% w.b. (Shinners & Binversie, 2007). These fibrous lignocellulosic materials, after meeting the field soil enrichment requirements, represent a substantial source of biomass that holds the promise for nonfoodbased biofuel for present and future energy needs. Biomass, especially in the raw form, has to go through a pretreatment
* Corresponding author. Department of Agricultural and Biosystems Engineering, North Dakota State University, 1221 Albrecht Boulevard, Fargo, ND 58102, USA. Tel.: þ1 701 667 3011; fax: þ1 701 667 3054. E-mail addresses: [email protected], [email protected] (C. Igathinathane). 1537-5110/$ e see front matter Published by Elsevier Ltd on behalf of IAgrE. doi:10.1016/j.biosystemseng.2010.07.005
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Nomenclature a A Ainc45 Apara Aperp
semi major-axis of the elliptical cross section of stalk, m cut sectional area based on the stalk orientation, m2 cut area generated when the stalk axis is inclined at 45 , m2 cut area generated when the blade is parallel to stalk axis, m2 cut area generated when the blade is perpendicular to stalk axis, m2
stage of size reduction, before any downstream energy or conversion procedure. Of the gross energy supplied to most hay-cutting devices, only a small portion was actually required for the size reduction as the rest was wasted due to inefficiencies (Chancellor, 1958). Size reduction/grinding is considered to be one of the most energy-intensive or energyinefficient operations (Mohsenin, 1986). Given the expected scale of future biomass energy utilisation, any improvement in efficiency or energy reduction in the size reduction process means substantial energy saving. Size reduction is a pretreatment that uses mechanical energy to produce a product which has better flowability, increased bulk density in general, and increased reactive surfaces. Studies have shown that the cutting energy is related to plant stems’ physical and mechanical properties, and type of cutting element and blade edge sharpness (Prince, Wheeler, & Fisher, 1958; Womac et al., 2005). Researchers have studied (Akritidis, 1974; Annoussamy, Richard, Recous, & Guerif, 2000; Burmistrova, Komolkova, Klemn, Panina, & Polonotshev, 1963; Chattopadhyay & Pandey, 1999; Dowgiallo, 2005; McRandal & McNulty, 1980; O’Dogherty, Hubert, Dyson, & Marshall, 1995; Prince, 1961) and reviewed (Miu, Womac, Igathinathane, & Sokhansanj, 2006; O’Dogherty, 1982; Yu, Womac, & Pordesimo, 2003) the size reduction process of plant materials. Shearing was shown to be the energy efficient method of size reduction, and was achieved by devices that used knives, shear bars, and linear knife grids with ram (Igathinathane, Womac, Sokhansanj, & Narayan, 2008, 2009). The reported mean ultimate shear stresses were approximately one-fourth of the ultimate tensile stresses for winter wheat (O’Dogherty et al., 1995) (8%e22% w.b.), one-fifth for switchgrass (Yu, Womac, Igathinathane, Ayers, & Buschermohle, 2006) (10%e 60% w.b.), and one-third for alfalfa stems (Galedar et al., 2008) (10%e20% w.b.). Thus size reduction equipment that predominantly uses shear mode for failure may hold promise for improved energy efficiencies. An experimental sheardominant linear knife grid consumed around one-fourth for corn stalks or more for switchgrass of the reported energy of an impact-dominant hammermill (Igathinathane et al., 2008, 2009). Distinguishing the difference in shear modes, Shinners, Koegel, Barrington, and Straub (1987) concluded that the longitudinal shear energy of alfalfa stems was less than one-tenth of the reported transverse shear values on a shear energy per unit area basis.
b Enet Et Ets Fsp l n xi yi su
semi minor-axis of the elliptical cross section of stalk, m net cutting energy, N m total cutting energy, N m total specific energy, N m1 peak cutting force, N length of stalk piece, m total number of observations of the forcedisplacement data, integer deformation at any instant i, m force at any instant i, N ultimate cutting stress, Pa
Accurate measurement of biomass cutting energy of plant stalks needs specialised fixtures or attachments, such as a WarnereBratzler or Kramer shear cell, or specially designed knife fixtures (Chen, Gratton, & Liu, 2004; Galedar et al., 2008; Prasad & Gupta, 1975). These devices were either used with universal testing machine (UTM) or transducers with dedi_ urluay, Gu¨zel, & cated data acquisition systems (Ince, Ug ¨ Ozcan, 2005). Since the WarnereBratzler or Kramer shear cell devices are meant for softer food materials, they need some modifications to handle tougher materials like plant stalks. It is also common knowledge that it takes less energy to cut along the grain (ripping) than across the grain (crosscutting), even with tougher biomaterial like wood. The difference in energy involved between these modes during size reduction presents opportunities to evolve new material feeding methods or develop new size reduction devices. In this paper, we study the effect of orientation of corn stalks on the cutting process with respect to the knife cutting plane. Therefore, the envisioned objective was to determine the mechanical cutting strength properties of corn stalks, such as peak force, peak energy, total energy, ultimate failure stress, and energy per unit area. Results of this research will determine the variation in mechanical cutting strength properties and quantify the benefits of material orientation in cutting.
2.
Materials and methods
2.1.
Corn stalks
Corn, variety Gamecorn (developed by the University of Tennessee (UT), USA, breeding program for wildlife food plots) was used as the test material. Although this is not a common corn variety, the crop was visually similar, and stalks had similar dimensions to common varieies (e.g. Dekalb) used in our other studies (Igathinathane et al., 2006, 2009). The corn was planted on April 21, 2005, ear harvested on October 5, 2005, and stalk harvested on December 7 and 14, 2005 from the plots (location: 35.897776 N, 83.961532 W) of Experiment Station, UT, Knoxville. The collected corn stover was stored (2 months) indoors until experiments in a laboratory having air conditions of about 23 C and relative humidity of about 55%, where the stover naturally dried and equilibrated with the prevailing ambient conditions. No degradation of the stalks was observed after the field stand and the indoor
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storage, as the stalks had maintained their structural integrity as was evident during material preparation cuts of the stalks.
2.2.
Preparation of test material
The corn stalk samples were prepared by stripping of the leaves and husks. The moisture content of the prepared stalks ranged from 15% to 20% w. b. (ASABE Standards, Sec. 358.2, 2008). Pieces of nodes and internodes of approximate length of 25 mm were cut using a bandsaw with fine blade. To determine the effect of size of corn stalks on various mechanical properties, three different cross-sectional sizes classified as small, medium, and large were prepared from the top, middle, and bottom sections of the stalks, respectively. The node pieces were prepared by aligning the nodular ring approximately at the centre of the piece. Five replications on each size category both for internode and node for perpendicular oriented cuts (across the grain), and ten replications each for parallel (along the grain) and inclined (45 ) oriented cuts were prepared.
2.3.
Modified WarnereBratzler shear device
WarnereBratzler shear device (Belew, Brooks, McKenna, & Savell, 2003) was originally meant for testing of typical softer food products. The cutting edge of the blade of the device is blunt and the blade comes in two configurations, the straight blade used for rectangular specimens, and the notched blade used for cylindrical specimens. The existing cutting edge of the blade being blunt was not suitable for cutting corn stalks or similar tougher biomass materials. Therefore, we modified the blunt edge of the notched blade into a sharp cutting edge; hence termed “Modified WarnereBratzler” shear device. New blades conforming to the
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specifications of the existing blade were fabricated from 3.175 mm A2 tool steel sheet with sharp cutting edge and were heat-treated. The cutting edge was given a single bevel angle of 30 that produced energy efficient cuts (Womac et al., 2005) and the notch angle was 60 (Fig. 1). The triangular notch of the blade self-centred the samples during cutting. The blade freely passed through the groove of the support block that served as the platform to hold the sample. Sharpness of the blade cutting edge was monitored by measuring the variation of cutting edge thickness using Scanning Electron Microscope (SEM) micrographs (Fig. 2).
2.4.
Shearing experiments in UTM
A two column UTM (MTS Alliance RT/30, MTS Systems Corporation, Eden Prairie, MN, USA; table-top model; 30 kN load cell-Serial: 100942) was used as the measurement platform in combination with the modified WarnereBratzler shear device (Fig. 3). TestWorks 4.05 (MTS Systems Corporation) application software controlled the UTM and acquired the data. A quasi-static cutting speed of 25.4 mm min1 was controlled by the cross head movement of the UTM. The blade attached to the crosshead moved down during cutting and the stationary support block was attached to the bed of the UTM. The presentation of the stalks to the cutting blade with different angles between the stalk axis and the plane of the blade gave different tested orientations (Fig. 3). It can be seen that the blade notch gap restricts the sample length that can be handled in the parallel (0 ) orientation (hence z 25 mm was used). However, this restriction was not present with the other two (90 and 45 ) orientations. Care was exercised while presenting the stalks at an inclined 45 orientation. A reference line marked on the platform aided repeatable positioning. In this 45 orientation, the stalk was manually held in position until the blade gripped the sample. Such holding of the stalk samples was not required with the two other orientations.
2.5.
Cut sections generated
The shape of the corn stalks was a tapered elliptical cylinder (Igathinathane et al., 2006). In other words, the cross section profile of the stalks was an ellipse. The cross-sectional area reduced gradually from the bottom to the top of the stalk. However, it was assumed that the variation is negligible for a small taper on a short length of samples. The dimensions of the major (2a e cross-sectional width) and minor (2b e crosssectional thickness) axes of the elliptical cross section were measured prior to testing using digital callipers and the mean values were recorded. From these dimensions and the length (l ) of the pieces, the cut sectional areas (90 and 45 e elliptical; or 0 e rectangular, through the stalk axis) were obtained according to the orientation of the stalks with respect to the blade movement from the following geometric formulae:
Fig. 1 e Modified blade with sharp cutting edge having a single bevel angle of 30 to be used for corn stalk cutting experiments.
Aperp ¼
p 2a 2b 4
(1)
Ainc45 ¼
p 1:414 2a 2b 4
(2)
Apara ¼ l 2b
(3)
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Fig. 2 e Blade cutting edge sharpness monitoring using SEM; (left) blade cutting edge at magnification 25X, and (right) at magnification 1000X showing the cutting edge thickness measured as 1.79 mm.
where: Aperp is the cut area generated when the blade is perpendicular to the stalk axis (m2), i.e. across the grain; 2a is the major-axis of the elliptical cross section of the stalk (m); 2b is the minor-axis of the elliptical cross section of the stalk (m); Ainc45 is the cut area generated when the stalk axis is inclined at 45 to the blade (m2); Apara is the cut area generated when the blade is parallel to the stalk axis (m2), i.e. along the grain; and l is the length of the stalk piece (m). The geometrical cut sectional areas generated (Eqs. (1)e(3)) can be visualised from the intersection of an elliptical cylinder (stalk) with a vertical plane (knife) at various orientations.
2.6.
UTM data collection and analysis
The UTM generated the force-displacement corresponding to the cutting characteristics of the corn stalks at different sizes and orientations. Usually, the net cutting energy expended was evaluated from the original data stream of the forcedisplacement characteristics using the following expression: Enet ¼
! n X yiþ1 þ yi ðxiþ1 xi Þ 2 i¼1 ! stalk n X yiþ1 þ yi ðxiþ1 xi Þ 2 i¼1
ð4Þ
considered as negligible. Furthermore, the TestWorks software was preprogrammed to directly output the peak load, peak energy, and total energy from the force-displacement characteristics. From these results, the ultimate cutting stress and specific energy were calculated from the cut sectional areas (Sec. 2.5) as: su ¼
Fsp A
(5)
Et A
(6)
Ets ¼
where: su is ultimate cutting stress (Pa); Fsp is the peak cutting force (N); A is the cut sectional area based on the stalk orientation (Eqs. (1)e(3)); Ets is the total specific energy (N m1); and Et is the total energy expended in cutting the stalks (N m). Statistical procedure of multiple ANOVA (Saxton, 2003) using SAS (ver. 9.2) was performed to determine the effect of size, sample material variation, and orientation on the various mechanical strength parameters involved in cutting of corn stalks.
3.
Results and discussion
idle
where: Enet is the net cutting energy (Nm); yi is the force at any instant i (N); xi is the deformation at any instant i (m); and n is the total number of observations of the forceedisplacement data. Since the blade had enough clearance and moved freely through the groove, the idle energy part of Eq. (4) was
3.1. Corn stalks cutting force-deformation characteristics Typical force-deformation characteristics of prepared corn stalks internodes and nodes at various orientations are shown in Fig. 4. Overall, the force-deformation curves show an initial
Fig. 3 e Experimental arrangement of corn stalk orientation effects using the modified WarnereBratzler device in the UTM.
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Fig. 4 e Typical force-displacement of cutting corn stalks internodes and nodes at different orientations; the inclined line indicates the slope of the linear portion of the force-displacement curve; the UTM operating software produced the English units, and the SI unit conversions are: Load 1 lbf [ 4.448 N, Extension 1 in [ 25.4 mm.
steep rise of cutting force, followed by an initial high peak, a sudden drop, a second low peak, and final insignificant force after stalk cutting. Difference in analytical components (internodes and nodes) and sample orientation (perpendicular and parallel) manifested clearly as distinct force-deformation characteristics (Fig. 4). Since the nodes were brittle, the forcedeformation curves of nodes were steep with narrower base; unlike flexible internodes curves differentiated by wider peak and base. Chancellor (1958) explained the nature of cutting process of fibrous biomass (timothy stems) by dividing the forcedisplacement into the following three identifiable regions: initial compression without cutting that takes most of the load, middle adjustment of pressure and initial failure of stem material adjacent to the blade, and final failure of stressed stems as blade progresses that mark a reduction in cutting load. Deviation from the reported general forcedisplacement characteristics will occur due to the differences in material property, orientations of material with respect to blade, number of stems tested, and blade sharpness.
3.2.
Orientation effects on nature of cutting process
Based on the heterogeneous morphology of the corn stalks (thick tough skin covering the inner soft pith core), blade geometry, simplified configuration of model corn stalk, and our direct observation during the sample cutting process, it was possible to identify five distinct regions on the observed force-displacement curves that explain the nature of cutting process (Fig. 5). Even though, the force-displacement
characteristics differed for perpendicular and parallel orientations, these regions were readily identified. The following was postulated for the nature of cutting process for individual stems of corn stalks: i. Compression of stalks by the blade e the blade edges deform and flatten the stalk, building up the strain energy in the stalk to a level of failure. ii. Initial cutting e the blade penetrates and makes the first cut (“pilot cut”) on a few fibres of the compressed skin. This usually corresponds to the initial peak or the start of observable change in the increasing trend of the curve. iii. Progress of skin and pitch cutting e the skin and pitch cutting progresses, the displacement of cut increases, as the blade descends additional sections of the blade that already compressed the stalk cut the skin, while the blade sections that cleared the skin continue to cut the soft compressed pith. The continuous failure of skin and pith fibres, the reinforcing structural component, gives the observed jagged variation of the load. The stress relief occurring while cutting was completed on some portions of the corn stalk, cutting of softer pith, and continuous reduction of cutting length once past the horizontal plane along the centre of stalk axis result in a substantial continuous reduction of cutting load. iv. Final cutting of skin e the bottom portion of the uncut skin after clearing most of the pith is cut; however, the skin being tougher, it produces the observed secondary peak. v. Completion of cutting and residual force e completion of cutting when the blade cutting edge clears the stalk
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Fig. 5 e Distinct regions of the corn stalk cutting force-displacement characteristics at different orientations explaining the nature of cutting process.
usually results in a fall in the load to the final minimum level. The observed residual loads correspond to the resistance offered by the combination of wedging, gripping, and rubbing of the stalk cut surfaces on the descending blade. These contribute to the parasitic forces that were present throughout the cutting operation. Difference in stalk orientation, especially when the stalk axis was parallel to blade, as well as the material difference between internode and node created different force-deformation characteristics having a major initial peak followed by sudden fall of load. Possible formation of cleavage or failure planes, due to the wedging action of the blade, may be attributed to the observed sudden fall in the load after initial peak. In the parallel orientation of the stalk the overall cutting force requirement was less compared to other orientations. The reason for this was that only the available structural fibres on the plane were split or ripped apart, whereas at other orientation angles (0 and 45 ) the whole bundle of fibres of the stalk was cut to complete the cutting. The formation of failure planes may not have readily occurred in any other stalk orientation other than the near parallel orientation. Orientation along the grain offered the path of least resistance
to the cutting blade. The reduced force requirements at parallel (along grain e 0 ) orientation may be advantageously utilised in the size reduction process by developing specific feeding arrangements to shear dominant size reduction machinery.
3.3. Effect of size and material variation of corn stalks on mechanical cutting parameters The variation in the mechanical cutting strength parameters along the tapering sections of the whole stalk from base to tip covered the entire spectrum of stalk sizes. Various mechanical strength properties were compared statistically among different sizes and between internode and node (Table 1). With internodes, among the stalk sizes, the means of dimensions (2a and 2b), cross-sectional area (data not shown), peak force, and peak and total energy were significantly different (P < 0.05). These values increased from small to large sizes. The ultimate cutting stress, being the peak force normalised over sectional area, was not significantly different; however, the specific energy was different between small and large sizes. With nodes among the stalk sizes, the dimensional parameters and peak force differed, while the energy and
Table 1 e Effect of size on corn stalk on the mechanical cutting parameters while cutting perpendicular to the stalk axis. Sample Size
Dimension (103 m) 2a
2b
Peak force (N)
Energy (N m) Peak
Specific energy (kN m1)
1.02 0.21 aA 1.06 0.13 aA 0.96 0.20 aA
11.27 2.26 bA 16.57 4.21 abA 23.49 8.93 aA
1.21 0.64 aA 1.30 0.36 aA 1.08 0.13 aA
8.64 4.37 aA 12.41 2.25 aA 14.00 12.22 aA
Total
Internode Small 12.40 1.21 cB 11.71 1.40 cB 115.60 25.24 cB 1.08 0.42 cA 1.30 0.46 cA Medium 16.37 1.30 bB 15.72 1.01 bB 215.80 52.12 bB 2.98 1.17 bA 3.44 1.37 bA Large 24.19 1.25 aB 21.48 1.40 aA 395.90 103.45 aA 7.60 3.56 aA 9.75 4.49 aA Node
Ultimate stress (MPa)
Small 15.13 1.73 cA 13.57 1.61 cA 194.40 99.60 bA 0.78 0.47 bA 1.46 0.90 bA Medium 20.06 2.49 bA 18.23 1.10 bA 381.80 153.57 aA 2.10 0.40 abA 3.59 0.95 abA Large 29.29 1.90 aA 25.78 1.78 aA 646.91 136.21 aA 6.20 8.64 aA 9.01 9.35 aA
Note: Values presented are mean standard deviation from the original data; number of replications ¼ 5; dissimilar lower case group labels (a,b, and c) represent a significant difference (P < 0.05) among size category; and dissimilar upper case group labels (A and B) represent a significant difference (P < 0.05) between internode and node.
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stress mean groups showed some overlap as the nodes were brittle and failed suddenly unlike the internodes. The specific energy values of internodes and nodes ranged from 11.3 to 23.5 and 8.6 to 14.0 kN m1 (Table 1), respectively. These values were in the range of reported values for corn stalks of 7.0e18.4 kN m1 (Burmistrova et al., 1963), 15.7 to 43.1 kN m1 (Akritidis, 1974), and 6.3 kN m1 (Prasad & Gupta, 1975). The specific cutting energy reported for other crops, such as field grass -9.8 kN m1, glasshouse grass -12.3 kN m1 (McRandal & _ McNulty, 1980), sunflower stalks -1.5 to 11.0 kN m1 (Ince et al., 2005), while for a number of other crops it varied from 4.3 to 24.1 kN m1 (Burmistrova et al., 1963), confirms the results belong to the reported value range in general. Between the internode and node materials, the dimensional parameters of nodes were significantly larger than those of internodes perhaps due to anatomical differences (Table 1). Although the force-displacement characteristics varied between internodes and nodes (Fig. 4), the peak force, cutting energy and stress parameters were usually smaller with internodes (except for specific energy), but were not significantly different. Overall, these results indicated that between internodes and nodes, other than the dimensional parameters difference, the mechanical cutting parameters were similar. Hence a cutting device designed based on the largest node portion of corn stalk, based on peak cutting force requirements, will work well for the whole stalk as it included already the variation arising from material (internode) and size. It has been well established that increased moisture increased the mechanical cutting stress and energy parameters (Annoussamy et al., 2000; Chen et al., 2004; Galedar et al., _ 2008; Igathinathane et al., 2009; Ince et al., 2005), and contributed to the increased parasitic forces (Persson, 1987). However, Yu et al. (2006) found that ultimate shear stress was not affected by moisture content (10%e60% w.b.).
3.4. Effect of orientation and material variation of corn stalks on mechanical strength Tests on different orientations (90 , 45 , and 0 ) of corn stalks revealed that force, energy, and stress vary significantly (P < 0.05) on overall basis (Table 2). A very clear reduction of force and energy of an order of magnitude was observed when
the cutting orientation changed from perpendicular (90 ) to parallel (0 ), even though parallel orientation produced significantly larger new cut surface area. Another reason for this reduction was at parallel orientation the cutting mostly proceeded through corn stalk soft pith. The variation of dimensional parameters (2a and 2b) was irrelevant as the samples of various sizes were pooled in the analysis. However, the change in orientation from 90 to 0 progressively increased the sectional area geometrically (ellipticalethe least to rectangularethe most) and produces significant differences. Among orientations, both internodes and nodes show significant (P < 0.05) variation on force and energy parameters (Table 2). Between materials, the parallel orientation gave more variation on force and energy parameters than at perpendicular orientation. It can be visualised that at parallel orientation of node samples, the cutting plane actually passed through dissimilar material (node sandwiched between lengths of internode), while at perpendicular orientation cutting plane passed through a similar material (node). Between the internode and node materials, the dimensional parameters obviously did not show any significant difference with orientation. Among the various cutting energy parameters, only the peak force and specific energy at parallel orientation varied between the materials. However, in parallel orientation all the force, energy and stress parameters varied significantly between internodes and nodes. The reason for the increased values with nodes for the parallel orientation can be attributed to the fact that the node section acted as resistance to the possible easy formation of a failure plane on the otherwise uniform bundle of straight fibres of internodes. This difference can also be appreciated from the fact that the material of the nodes is densely packed in the nodular disc (perpendicular to stalk axis), provided support to the leaves along the periphery, which is clearly different from pith core of internodes.
3.5. Overall comparison of corn stalks on mechanical cutting parameters An overall comparison of significant mechanical cutting parameters was determined by taking perpendicular
Table 2 e Effect of corn stalk orientation on the mechanical cutting parameters. Sample
Orientation
Dimension ( 103 m) 2a 17.65 5.2 bA
Peak force (N)
2b
Energy (Nm) Peak
Total
Ultimate stress (MPa)
Specific energy (kN m1)
16.3 4.32 aA 242.43 135.74 aB 3.89 3.48 aA 4.83 4.48 aA 1.01 0.17 aA 17.11 7.50 aA
Internode Perpendicular (90 ) Inclined (45 ) Parallel (0 )
28.73 5.17 a 19.05 3.39 a 127.01 37.99 b 21.52 5.06 bA 19.3 4.23 aA 46.54 14.29 cB
Node
21.49 6.37 aA 19.19 5.4 aA
407.7 227.59 aA 3.02 5.21 aA 4.69 6.03 aA 1.20 0.41 aA 11.68 7.42 aB
26.23 5.74 aA 21.05 4.01 aA
83.89 23.61 bA
Perpendicular (90 ) Parallel (0 )
2.37 1.28 a 2.99 1.49 a 0.30 0.07 b 0.24 0.08 bB 0.48 0.14 bB 0.10 0.03 cB
0.44 0.10 bA 0.93 0.13 bA 0.16 0.05 bA
6.64 1.69 b 1.00 0.22 cB
1.78 0.38 bA
Note: Values presented are mean standard deviation from the original data; number of replications ¼ 5; dissimilar lower case group labels (a,b, and c) represent a significant difference (P < 0.05) among size category; and dissimilar upper case group labels (A and B) represent a significant difference (P < 0.05) between internode and node.
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1.2
Peak force - ratio
1.0
1.00
Internodes (IN) Nodes (NO) Internode-Node Ratio (INR)
1.00
0.8 0.59
0.6
0.52
0.55
0.4
0.0
0.21
0.19
0.2
IN-90°
IN-45°
IN-0°
NO-90°
NO-0°
INR-90°
INR-0°
Component and cutting angle
Ultimate cutting stress - ratio
1.2 1.0
1.00
Internodes (IN) Internode-Node Ratio (INR) Nodes (NO)
1.00 0.85
0.8 0.61
0.6 0.4
0.30
0.2 0.0
0.14
0.10
IN-90°
IN-45°
IN-0°
NO-90°
NO-0°
INR-90°
INR-0°
Component and cutting angle 1.6
1.46
Internodes (IN) Internode-Node Ratio (INR) Nodes (NO)
Specific energy - ratio
1.4 1.2 1.0
1.00
1.00
0.8 0.56
0.6 0.39
0.4
0.15
0.2 0.0
0.06
IN-90°
IN-45°
IN-0°
NO-90°
NO-0°
INR-90°
INR-0°
Component and cutting angle Fig. 6 e Comparison of mechanical cutting parameters of corn stalk internodes and nodes at different orientations.
orientation as a reference and working out the ratios of other orientations, as well as between internode and node materials (Fig. 6). A clear significant (P < 0.05) reduction of ratios of peak load, ultimate failure stress, and specific energy reduced with reduction in angle of orientation for both internodes and nodes. These ratios for corn stalks vary in the range of 0.1e0.2 in the parallel orientation with reference to perpendicular orientation, for both internodes and nodes. The ultimate
failure stress ratios of corn stalk (internodes and nodes) in the range of 0.1e0.14 were consistent with the longitudinal (0 ) to transverse (90 ) ratios of about 0.1 reported for alfalfa stems (Shinners et al., 1987). The ratios of internode to node cutting parameters at parallel orientation were about 0.6 for peak load, ultimate stress, and specific energy, owing to the resistance the node offered to the cutting process (Fig. 6). However, for
b i o s y s t e m s e n g i n e e r i n g 1 0 7 ( 2 0 1 0 ) 9 7 e1 0 6
perpendicular cuts these ratios showed some deviations. Notwithstanding the variation, these ratios showed a consistent reduction from perpendicular to parallel orientation. These results collectively indicated that irrespective of material differences (internode and node), parallel orientation of the stalks (along grain or longitudinal) to the cutting plane reduced the load, stress, and specific energy requirements drastically (1/5) with respect to perpendicular orientation (across grain or transverse). This lower strength of the corn stalk in parallel orientation can be exploited advantageously to develop feeding mechanisms or new machines that takes orientation into consideration while presenting the material to the cutting elements, especially in the first stage of size reduction. It was also observed that any orientation other than the perpendicular also exhibited a reduction in the cutting energy requirements; therefore any practical deviation from the best parallel orientation will also produce the corresponding energy efficiency in size reduction. It should also be understood that for finer product sizes, it is inevitable to use random including the perpendicular orientation or other modes (impact or attrition) at a later stage of size reduction, as ideal parallel orientation is expected to produce a product of predominantly fibrous material without cross cutting of fibres. However, the parallel orientation as an initial stage of size reduction would have produced a loosened material that would not be expected to consume increased energy at subsequent stages, at any orientation including perpendicular. Further studies may be required to establish the advantage of parallel orientation in actual commercial size reduction machines and the size reduction characteristics of first stage parallel cut products on subsequent stage size reduction machinery, as well as address the challenges in scale up of parallel orientation size reduction devices.
4.
Conclusions
Five regions including initial compression, initial skin cutting, further skin and pith cutting, final skin cutting, and cutting completion were identified from the force-displacement characteristics of corn stalks. The cutting force-displacement characteristics of corn stalks were different with orientation, and internode and node material difference. Corn stalks orientation had significant effect (P < 0.05) on mechanical cutting force, energy, and stress, both for nodes and internodes. Parallel orientation (along grain) produced a reduction on the cutting stress and energy values to one-tenth or better for internodes and to about one-fifth for nodes compared to perpendicular orientation (across grain). The reduced energy requirements of the corn stalk in parallel orientation could be exploited advantageously to develop feeding mechanisms and new machines that consider orientation while presenting the material to the cutting elements.
Acknowledgements Funding for this research derived in part from the USDA-NRCS Grant Agreement 68-3A75-4-136 and USDA-DOE-USDA Biomass
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Research and Development Initiative DE-PA36-04GO94002. This financial support is gratefully acknowledged. We highly appreciate the fabrication help by Mr. Craig A. Wagoner, Lab Machinist, Department of Biosystems Engineering and Soil Science, University of Tennessee, Knoxville, TN, USA.
references
Akritidis, C. B. (1974). The mechanical characteristics of maize stalks in relation to the characteristics of cutting blade. Journal of Agricultural Engineering Research, 19, 1e12. Annoussamy, M., Richard, G., Recous, S., & Guerif, J. (2000). Change in mechanical properties of wheat straw due to decomposition and moisture. Applied Engineering in Agriculture, 16, 657e664. ASABE Standards. (2008). S358.2. Moisture measurementeforages. St. Joseph, Mich.: ASABE. Belew, J. B., Brooks, J. C., McKenna, D. R., & Savell, J. W. (2003). WarnereBratzler shear evaluations of 40 bovine muscles. Meat Science, 64, 507e512. Burmistrova, M. F., Komolkova, T., Klemn, N., Panina, M., & Polonotshev, I. (1963). Physico-mechanical properties of agricultural crops. Israel Program for Scientific Translations, TT 61-31216, 250. Chattopadhyay, P. S., & Pandey, K. P. (1999). Mechanical properties of sorghum stalk in relation to quasi-static deformation. Journal of Agricultural Engineering Research, 73, 199e206. Chancellor, W. J. (1958). Energy requirement for cutting forage. Agricultural Engineering, 39, 633e636. Chen, Y., Gratton, J. L., & Liu, J. (2004). Power requirements of hemp cutting and conditioning. Biosystems Engineering, 87, 417e424. Dowgiallo, A. (2005). Cutting force of fibrous materials. Journal of Food Engineering, 66, 57e61. Galedar, M. N., Jafari, A., Mohtasebi, S. S., Tabatabaeefar, A., Shariþ, A., O’Dogherty, M. J., Raþee, S., & Richard, G. (2008). Effects of moisture content and level in the crop on the engineering properties of alfalfa stems. Biosystems Engineering, 101, 199e208. Igathinathane, C., Womac, A. R., Sokhansanj, S., & Pordesimo, L. O. (2006). Mass and moisture distribution in aboveground components of standing corn plants. Transactions of the ASAE, 49, 97e106. Igathinathane, C., Womac, A. R., Sokhansanj, S., & Narayan, S. (2008). Knife grid size reduction to pre-process packed beds of high- and low-moisture switchgrass. Bioresource Technology, 99, 2254e2264. Igathinathane, C., Womac, A. R., Sokhansanj, S., & Narayan, S. (2009). Size reduction of high- and low-moisture corn stalks by linear knife grid system. Biomass Bioenergy, 33, 547e557. _ ¨ zcan, M. T. (2005). Bending and urluay, S., Gu¨zel, E., & O Ince, A., Ug shearing characteristics of sunflower stalk residue. Biosystems Engineering, 92, 175e181. McRandal, D. M., & McNulty, P. B. (1980). Mechanical and physical properties of grasses. Transactions of the ASAE, 23, 816e821. Miu, P. I., Womac, A. R., Igathinathane, C., & Sokhansanj, S. (2006). Analysis of biomass comminution and separation processes in rotary equipmentea review. ASAE Paper No. 066169. St. Joseph, Mich.: ASABE. Mohsenin, N. (1986). Physical properties of plant and animal materials. Amsterdam/The Netherlands: Gordon and Breach Publishers Inc.. pp. 1077, 1133. O’Dogherty, M. J. (1982). A review of research on forage chopping. Journal of Agricultural Engineering Research, 27, 267e289. O’Dogherty, M. J., Hubert, J. A., Dyson, J., & Marshall, C. J. (1995). A study of the physical and mechanical properties of wheat straw. Journal of Agricultural Engineering Research, 62, 133e142. Persson, S. (1987). Mechanics of cutting plant material. Michigan, USA: ASAE Publications.
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Prasad, J., & Gupta, C. P. (1975). Mechanical properties of maize stalk as related to harvesting. Journal of Agricultural Engineering Research, 20, 79e87. Prince, R. P., Wheeler, W. C., & Fisher, D. A. (1958). Discussion on “energy requirement for cutting forage”. Agricultural Engineering, 39, 638e639, 652. Prince, R. P. (1961). Measurement of ultimate strength of forage stalks. Transactions of the ASAE, 4, 208e209. Saxton, A. M. (2003). Mixed model ANOVA and regression analysis macros. Knoxville, TN, USA: The University of Tennessee. Shinners, K. J., Koegel, R. G., Barrington, G. P., & Straub, R. J. (1987). Evaluating longitudinal shear as a forage maceration technique. Transactions of the ASAE, 30, 18e22.
Shinners, K. J., & Binversie, B. N. (2007). Fractional yield and moisture of corn stover biomass produced in the northern US corn belt. Biomass and Bioenergy, 31, 576e584. Womac, A. R., Yu, M., Igathinathane, C., Ye, P., Hayes, D., Narayan, S., Sokhansanj, S., & Wright, L. (2005). Shearing characteristics of biomass for size reduction. ASAE Paper No. 056058. St. Joseph, Mich.: ASABE. Yu, M., Womac, A. R., & Pordesimo, L. O. (2003). Review of biomass size reduction technology. ASAE Paper No. 036077. St. Joseph, Mich.: ASABE. Yu, M., Womac, A. R., Igathinathane, C., Ayers, P. D., & Buschermohle, M. J. (2006). Switchgrass ultimate stresses at typical biomass conditions available for processing. Biomass Bioenergy, 30, 214e219.
Available at www.sciencedirect.com
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Research Paper
Corn stalk orientation effect on mechanical cutting C. Igathinathane a,b,*, A.R. Womac b, S. Sokhansanj c a
Department of Agricultural and Biosystems Engineering, North Dakota State University, 1221 Albrecht Boulevard, Fargo, ND 58102, USA Department of Biosystems Engineering and Soil Science, 2506 E. J. Chapman Drive, The University of Tennessee, Knoxville, TN 37996, USA c Oak Ridge National Laboratory, Environmental Sciences Division, Oak Ridge, P.O. Box 2008, TN 37831, USA b
article info
Research efforts that increase the efficiency of size reduction of biomass can lead to
Article history:
a significant energy saving. This paper deals with the determination of the effect of sample
Received 6 January 2010
orientation with respect to cutting element and quantify the possible cutting energy
Received in revised form
reduction, utilising dry corn stalks as the test material (15%e20% wet basis). To evaluate the
10 July 2010
mechanical cutting characteristics of corn stalks, a WarnereBratzler device was modified by
Accepted 14 July 2010
replacing its blunt edged cutting element with one having a 30 single bevel sharp knife edge. Cutting force-deformation characteristics obtained with a universal testing machine were analysed to evaluate the orientation effects at perpendicular (90 ), inclined (45 ), and parallel (0 ) orientations on internodes and nodes for cutting force, energy, ultimate stress, and specific energy of corn stalks. The corn stalks cutting force-displacement characteristics were found to differ with orientation, and internode and node material difference. Overall, the peak failure force, and the total cutting energy of internodes and nodes varied significantly (P < 0.05) with stalk cross-sectional area. The specific energy values (total energy per unit cut area) of dry corn stalk internodes ranged from 11.3 to 23.5 kN m1, and nodes from 8.6 to 14.0 kN m1. The parallel orientation (along grain) compared to perpendicular (across grain) produced a significant reduction of the cutting stress and the specific energy to onetenth or better for internodes, and to about one-fifth for nodes. Published by Elsevier Ltd on behalf of IAgrE.
1.
Introduction
Corn stover, among the crop-based residues, is considered as the most abundant biomass resource, while the straws of graminaceous crops such as wheat, barley, rice, oats, etc. follow the list of collectible biomass. Of the various corn stover components that are collectible from the field in a typical corn harvest, stalks dominate the available biomass (w73% fresh mass at a moisture content of w62% wet basis (w.b.)) followed by leaf (w20% fresh mass at w9% w.b.) and husk (w7% fresh mass at w11% w.b.) (Igathinathane, Womac,
Sokhansanj, & Pordesimo, 2006). On dry mass basis, about 56% of the total stover dry mass resided in the stalk, while its moisture ranged from 69% to 56% during the corn harvest; and the dry masses of other components were leaf at 21%, cob at 15%, and husk at 8% with total stover moisture content that ranged from 66% to 47% w.b. (Shinners & Binversie, 2007). These fibrous lignocellulosic materials, after meeting the field soil enrichment requirements, represent a substantial source of biomass that holds the promise for nonfoodbased biofuel for present and future energy needs. Biomass, especially in the raw form, has to go through a pretreatment
* Corresponding author. Department of Agricultural and Biosystems Engineering, North Dakota State University, 1221 Albrecht Boulevard, Fargo, ND 58102, USA. Tel.: þ1 701 667 3011; fax: þ1 701 667 3054. E-mail addresses: [email protected], [email protected] (C. Igathinathane). 1537-5110/$ e see front matter Published by Elsevier Ltd on behalf of IAgrE. doi:10.1016/j.biosystemseng.2010.07.005
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Nomenclature a A Ainc45 Apara Aperp
semi major-axis of the elliptical cross section of stalk, m cut sectional area based on the stalk orientation, m2 cut area generated when the stalk axis is inclined at 45 , m2 cut area generated when the blade is parallel to stalk axis, m2 cut area generated when the blade is perpendicular to stalk axis, m2
stage of size reduction, before any downstream energy or conversion procedure. Of the gross energy supplied to most hay-cutting devices, only a small portion was actually required for the size reduction as the rest was wasted due to inefficiencies (Chancellor, 1958). Size reduction/grinding is considered to be one of the most energy-intensive or energyinefficient operations (Mohsenin, 1986). Given the expected scale of future biomass energy utilisation, any improvement in efficiency or energy reduction in the size reduction process means substantial energy saving. Size reduction is a pretreatment that uses mechanical energy to produce a product which has better flowability, increased bulk density in general, and increased reactive surfaces. Studies have shown that the cutting energy is related to plant stems’ physical and mechanical properties, and type of cutting element and blade edge sharpness (Prince, Wheeler, & Fisher, 1958; Womac et al., 2005). Researchers have studied (Akritidis, 1974; Annoussamy, Richard, Recous, & Guerif, 2000; Burmistrova, Komolkova, Klemn, Panina, & Polonotshev, 1963; Chattopadhyay & Pandey, 1999; Dowgiallo, 2005; McRandal & McNulty, 1980; O’Dogherty, Hubert, Dyson, & Marshall, 1995; Prince, 1961) and reviewed (Miu, Womac, Igathinathane, & Sokhansanj, 2006; O’Dogherty, 1982; Yu, Womac, & Pordesimo, 2003) the size reduction process of plant materials. Shearing was shown to be the energy efficient method of size reduction, and was achieved by devices that used knives, shear bars, and linear knife grids with ram (Igathinathane, Womac, Sokhansanj, & Narayan, 2008, 2009). The reported mean ultimate shear stresses were approximately one-fourth of the ultimate tensile stresses for winter wheat (O’Dogherty et al., 1995) (8%e22% w.b.), one-fifth for switchgrass (Yu, Womac, Igathinathane, Ayers, & Buschermohle, 2006) (10%e 60% w.b.), and one-third for alfalfa stems (Galedar et al., 2008) (10%e20% w.b.). Thus size reduction equipment that predominantly uses shear mode for failure may hold promise for improved energy efficiencies. An experimental sheardominant linear knife grid consumed around one-fourth for corn stalks or more for switchgrass of the reported energy of an impact-dominant hammermill (Igathinathane et al., 2008, 2009). Distinguishing the difference in shear modes, Shinners, Koegel, Barrington, and Straub (1987) concluded that the longitudinal shear energy of alfalfa stems was less than one-tenth of the reported transverse shear values on a shear energy per unit area basis.
b Enet Et Ets Fsp l n xi yi su
semi minor-axis of the elliptical cross section of stalk, m net cutting energy, N m total cutting energy, N m total specific energy, N m1 peak cutting force, N length of stalk piece, m total number of observations of the forcedisplacement data, integer deformation at any instant i, m force at any instant i, N ultimate cutting stress, Pa
Accurate measurement of biomass cutting energy of plant stalks needs specialised fixtures or attachments, such as a WarnereBratzler or Kramer shear cell, or specially designed knife fixtures (Chen, Gratton, & Liu, 2004; Galedar et al., 2008; Prasad & Gupta, 1975). These devices were either used with universal testing machine (UTM) or transducers with dedi_ urluay, Gu¨zel, & cated data acquisition systems (Ince, Ug ¨ Ozcan, 2005). Since the WarnereBratzler or Kramer shear cell devices are meant for softer food materials, they need some modifications to handle tougher materials like plant stalks. It is also common knowledge that it takes less energy to cut along the grain (ripping) than across the grain (crosscutting), even with tougher biomaterial like wood. The difference in energy involved between these modes during size reduction presents opportunities to evolve new material feeding methods or develop new size reduction devices. In this paper, we study the effect of orientation of corn stalks on the cutting process with respect to the knife cutting plane. Therefore, the envisioned objective was to determine the mechanical cutting strength properties of corn stalks, such as peak force, peak energy, total energy, ultimate failure stress, and energy per unit area. Results of this research will determine the variation in mechanical cutting strength properties and quantify the benefits of material orientation in cutting.
2.
Materials and methods
2.1.
Corn stalks
Corn, variety Gamecorn (developed by the University of Tennessee (UT), USA, breeding program for wildlife food plots) was used as the test material. Although this is not a common corn variety, the crop was visually similar, and stalks had similar dimensions to common varieies (e.g. Dekalb) used in our other studies (Igathinathane et al., 2006, 2009). The corn was planted on April 21, 2005, ear harvested on October 5, 2005, and stalk harvested on December 7 and 14, 2005 from the plots (location: 35.897776 N, 83.961532 W) of Experiment Station, UT, Knoxville. The collected corn stover was stored (2 months) indoors until experiments in a laboratory having air conditions of about 23 C and relative humidity of about 55%, where the stover naturally dried and equilibrated with the prevailing ambient conditions. No degradation of the stalks was observed after the field stand and the indoor
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storage, as the stalks had maintained their structural integrity as was evident during material preparation cuts of the stalks.
2.2.
Preparation of test material
The corn stalk samples were prepared by stripping of the leaves and husks. The moisture content of the prepared stalks ranged from 15% to 20% w. b. (ASABE Standards, Sec. 358.2, 2008). Pieces of nodes and internodes of approximate length of 25 mm were cut using a bandsaw with fine blade. To determine the effect of size of corn stalks on various mechanical properties, three different cross-sectional sizes classified as small, medium, and large were prepared from the top, middle, and bottom sections of the stalks, respectively. The node pieces were prepared by aligning the nodular ring approximately at the centre of the piece. Five replications on each size category both for internode and node for perpendicular oriented cuts (across the grain), and ten replications each for parallel (along the grain) and inclined (45 ) oriented cuts were prepared.
2.3.
Modified WarnereBratzler shear device
WarnereBratzler shear device (Belew, Brooks, McKenna, & Savell, 2003) was originally meant for testing of typical softer food products. The cutting edge of the blade of the device is blunt and the blade comes in two configurations, the straight blade used for rectangular specimens, and the notched blade used for cylindrical specimens. The existing cutting edge of the blade being blunt was not suitable for cutting corn stalks or similar tougher biomass materials. Therefore, we modified the blunt edge of the notched blade into a sharp cutting edge; hence termed “Modified WarnereBratzler” shear device. New blades conforming to the
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specifications of the existing blade were fabricated from 3.175 mm A2 tool steel sheet with sharp cutting edge and were heat-treated. The cutting edge was given a single bevel angle of 30 that produced energy efficient cuts (Womac et al., 2005) and the notch angle was 60 (Fig. 1). The triangular notch of the blade self-centred the samples during cutting. The blade freely passed through the groove of the support block that served as the platform to hold the sample. Sharpness of the blade cutting edge was monitored by measuring the variation of cutting edge thickness using Scanning Electron Microscope (SEM) micrographs (Fig. 2).
2.4.
Shearing experiments in UTM
A two column UTM (MTS Alliance RT/30, MTS Systems Corporation, Eden Prairie, MN, USA; table-top model; 30 kN load cell-Serial: 100942) was used as the measurement platform in combination with the modified WarnereBratzler shear device (Fig. 3). TestWorks 4.05 (MTS Systems Corporation) application software controlled the UTM and acquired the data. A quasi-static cutting speed of 25.4 mm min1 was controlled by the cross head movement of the UTM. The blade attached to the crosshead moved down during cutting and the stationary support block was attached to the bed of the UTM. The presentation of the stalks to the cutting blade with different angles between the stalk axis and the plane of the blade gave different tested orientations (Fig. 3). It can be seen that the blade notch gap restricts the sample length that can be handled in the parallel (0 ) orientation (hence z 25 mm was used). However, this restriction was not present with the other two (90 and 45 ) orientations. Care was exercised while presenting the stalks at an inclined 45 orientation. A reference line marked on the platform aided repeatable positioning. In this 45 orientation, the stalk was manually held in position until the blade gripped the sample. Such holding of the stalk samples was not required with the two other orientations.
2.5.
Cut sections generated
The shape of the corn stalks was a tapered elliptical cylinder (Igathinathane et al., 2006). In other words, the cross section profile of the stalks was an ellipse. The cross-sectional area reduced gradually from the bottom to the top of the stalk. However, it was assumed that the variation is negligible for a small taper on a short length of samples. The dimensions of the major (2a e cross-sectional width) and minor (2b e crosssectional thickness) axes of the elliptical cross section were measured prior to testing using digital callipers and the mean values were recorded. From these dimensions and the length (l ) of the pieces, the cut sectional areas (90 and 45 e elliptical; or 0 e rectangular, through the stalk axis) were obtained according to the orientation of the stalks with respect to the blade movement from the following geometric formulae:
Fig. 1 e Modified blade with sharp cutting edge having a single bevel angle of 30 to be used for corn stalk cutting experiments.
Aperp ¼
p 2a 2b 4
(1)
Ainc45 ¼
p 1:414 2a 2b 4
(2)
Apara ¼ l 2b
(3)
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Fig. 2 e Blade cutting edge sharpness monitoring using SEM; (left) blade cutting edge at magnification 25X, and (right) at magnification 1000X showing the cutting edge thickness measured as 1.79 mm.
where: Aperp is the cut area generated when the blade is perpendicular to the stalk axis (m2), i.e. across the grain; 2a is the major-axis of the elliptical cross section of the stalk (m); 2b is the minor-axis of the elliptical cross section of the stalk (m); Ainc45 is the cut area generated when the stalk axis is inclined at 45 to the blade (m2); Apara is the cut area generated when the blade is parallel to the stalk axis (m2), i.e. along the grain; and l is the length of the stalk piece (m). The geometrical cut sectional areas generated (Eqs. (1)e(3)) can be visualised from the intersection of an elliptical cylinder (stalk) with a vertical plane (knife) at various orientations.
2.6.
UTM data collection and analysis
The UTM generated the force-displacement corresponding to the cutting characteristics of the corn stalks at different sizes and orientations. Usually, the net cutting energy expended was evaluated from the original data stream of the forcedisplacement characteristics using the following expression: Enet ¼
! n X yiþ1 þ yi ðxiþ1 xi Þ 2 i¼1 ! stalk n X yiþ1 þ yi ðxiþ1 xi Þ 2 i¼1
ð4Þ
considered as negligible. Furthermore, the TestWorks software was preprogrammed to directly output the peak load, peak energy, and total energy from the force-displacement characteristics. From these results, the ultimate cutting stress and specific energy were calculated from the cut sectional areas (Sec. 2.5) as: su ¼
Fsp A
(5)
Et A
(6)
Ets ¼
where: su is ultimate cutting stress (Pa); Fsp is the peak cutting force (N); A is the cut sectional area based on the stalk orientation (Eqs. (1)e(3)); Ets is the total specific energy (N m1); and Et is the total energy expended in cutting the stalks (N m). Statistical procedure of multiple ANOVA (Saxton, 2003) using SAS (ver. 9.2) was performed to determine the effect of size, sample material variation, and orientation on the various mechanical strength parameters involved in cutting of corn stalks.
3.
Results and discussion
idle
where: Enet is the net cutting energy (Nm); yi is the force at any instant i (N); xi is the deformation at any instant i (m); and n is the total number of observations of the forceedisplacement data. Since the blade had enough clearance and moved freely through the groove, the idle energy part of Eq. (4) was
3.1. Corn stalks cutting force-deformation characteristics Typical force-deformation characteristics of prepared corn stalks internodes and nodes at various orientations are shown in Fig. 4. Overall, the force-deformation curves show an initial
Fig. 3 e Experimental arrangement of corn stalk orientation effects using the modified WarnereBratzler device in the UTM.
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Fig. 4 e Typical force-displacement of cutting corn stalks internodes and nodes at different orientations; the inclined line indicates the slope of the linear portion of the force-displacement curve; the UTM operating software produced the English units, and the SI unit conversions are: Load 1 lbf [ 4.448 N, Extension 1 in [ 25.4 mm.
steep rise of cutting force, followed by an initial high peak, a sudden drop, a second low peak, and final insignificant force after stalk cutting. Difference in analytical components (internodes and nodes) and sample orientation (perpendicular and parallel) manifested clearly as distinct force-deformation characteristics (Fig. 4). Since the nodes were brittle, the forcedeformation curves of nodes were steep with narrower base; unlike flexible internodes curves differentiated by wider peak and base. Chancellor (1958) explained the nature of cutting process of fibrous biomass (timothy stems) by dividing the forcedisplacement into the following three identifiable regions: initial compression without cutting that takes most of the load, middle adjustment of pressure and initial failure of stem material adjacent to the blade, and final failure of stressed stems as blade progresses that mark a reduction in cutting load. Deviation from the reported general forcedisplacement characteristics will occur due to the differences in material property, orientations of material with respect to blade, number of stems tested, and blade sharpness.
3.2.
Orientation effects on nature of cutting process
Based on the heterogeneous morphology of the corn stalks (thick tough skin covering the inner soft pith core), blade geometry, simplified configuration of model corn stalk, and our direct observation during the sample cutting process, it was possible to identify five distinct regions on the observed force-displacement curves that explain the nature of cutting process (Fig. 5). Even though, the force-displacement
characteristics differed for perpendicular and parallel orientations, these regions were readily identified. The following was postulated for the nature of cutting process for individual stems of corn stalks: i. Compression of stalks by the blade e the blade edges deform and flatten the stalk, building up the strain energy in the stalk to a level of failure. ii. Initial cutting e the blade penetrates and makes the first cut (“pilot cut”) on a few fibres of the compressed skin. This usually corresponds to the initial peak or the start of observable change in the increasing trend of the curve. iii. Progress of skin and pitch cutting e the skin and pitch cutting progresses, the displacement of cut increases, as the blade descends additional sections of the blade that already compressed the stalk cut the skin, while the blade sections that cleared the skin continue to cut the soft compressed pith. The continuous failure of skin and pith fibres, the reinforcing structural component, gives the observed jagged variation of the load. The stress relief occurring while cutting was completed on some portions of the corn stalk, cutting of softer pith, and continuous reduction of cutting length once past the horizontal plane along the centre of stalk axis result in a substantial continuous reduction of cutting load. iv. Final cutting of skin e the bottom portion of the uncut skin after clearing most of the pith is cut; however, the skin being tougher, it produces the observed secondary peak. v. Completion of cutting and residual force e completion of cutting when the blade cutting edge clears the stalk
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Fig. 5 e Distinct regions of the corn stalk cutting force-displacement characteristics at different orientations explaining the nature of cutting process.
usually results in a fall in the load to the final minimum level. The observed residual loads correspond to the resistance offered by the combination of wedging, gripping, and rubbing of the stalk cut surfaces on the descending blade. These contribute to the parasitic forces that were present throughout the cutting operation. Difference in stalk orientation, especially when the stalk axis was parallel to blade, as well as the material difference between internode and node created different force-deformation characteristics having a major initial peak followed by sudden fall of load. Possible formation of cleavage or failure planes, due to the wedging action of the blade, may be attributed to the observed sudden fall in the load after initial peak. In the parallel orientation of the stalk the overall cutting force requirement was less compared to other orientations. The reason for this was that only the available structural fibres on the plane were split or ripped apart, whereas at other orientation angles (0 and 45 ) the whole bundle of fibres of the stalk was cut to complete the cutting. The formation of failure planes may not have readily occurred in any other stalk orientation other than the near parallel orientation. Orientation along the grain offered the path of least resistance
to the cutting blade. The reduced force requirements at parallel (along grain e 0 ) orientation may be advantageously utilised in the size reduction process by developing specific feeding arrangements to shear dominant size reduction machinery.
3.3. Effect of size and material variation of corn stalks on mechanical cutting parameters The variation in the mechanical cutting strength parameters along the tapering sections of the whole stalk from base to tip covered the entire spectrum of stalk sizes. Various mechanical strength properties were compared statistically among different sizes and between internode and node (Table 1). With internodes, among the stalk sizes, the means of dimensions (2a and 2b), cross-sectional area (data not shown), peak force, and peak and total energy were significantly different (P < 0.05). These values increased from small to large sizes. The ultimate cutting stress, being the peak force normalised over sectional area, was not significantly different; however, the specific energy was different between small and large sizes. With nodes among the stalk sizes, the dimensional parameters and peak force differed, while the energy and
Table 1 e Effect of size on corn stalk on the mechanical cutting parameters while cutting perpendicular to the stalk axis. Sample Size
Dimension (103 m) 2a
2b
Peak force (N)
Energy (N m) Peak
Specific energy (kN m1)
1.02 0.21 aA 1.06 0.13 aA 0.96 0.20 aA
11.27 2.26 bA 16.57 4.21 abA 23.49 8.93 aA
1.21 0.64 aA 1.30 0.36 aA 1.08 0.13 aA
8.64 4.37 aA 12.41 2.25 aA 14.00 12.22 aA
Total
Internode Small 12.40 1.21 cB 11.71 1.40 cB 115.60 25.24 cB 1.08 0.42 cA 1.30 0.46 cA Medium 16.37 1.30 bB 15.72 1.01 bB 215.80 52.12 bB 2.98 1.17 bA 3.44 1.37 bA Large 24.19 1.25 aB 21.48 1.40 aA 395.90 103.45 aA 7.60 3.56 aA 9.75 4.49 aA Node
Ultimate stress (MPa)
Small 15.13 1.73 cA 13.57 1.61 cA 194.40 99.60 bA 0.78 0.47 bA 1.46 0.90 bA Medium 20.06 2.49 bA 18.23 1.10 bA 381.80 153.57 aA 2.10 0.40 abA 3.59 0.95 abA Large 29.29 1.90 aA 25.78 1.78 aA 646.91 136.21 aA 6.20 8.64 aA 9.01 9.35 aA
Note: Values presented are mean standard deviation from the original data; number of replications ¼ 5; dissimilar lower case group labels (a,b, and c) represent a significant difference (P < 0.05) among size category; and dissimilar upper case group labels (A and B) represent a significant difference (P < 0.05) between internode and node.
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stress mean groups showed some overlap as the nodes were brittle and failed suddenly unlike the internodes. The specific energy values of internodes and nodes ranged from 11.3 to 23.5 and 8.6 to 14.0 kN m1 (Table 1), respectively. These values were in the range of reported values for corn stalks of 7.0e18.4 kN m1 (Burmistrova et al., 1963), 15.7 to 43.1 kN m1 (Akritidis, 1974), and 6.3 kN m1 (Prasad & Gupta, 1975). The specific cutting energy reported for other crops, such as field grass -9.8 kN m1, glasshouse grass -12.3 kN m1 (McRandal & _ McNulty, 1980), sunflower stalks -1.5 to 11.0 kN m1 (Ince et al., 2005), while for a number of other crops it varied from 4.3 to 24.1 kN m1 (Burmistrova et al., 1963), confirms the results belong to the reported value range in general. Between the internode and node materials, the dimensional parameters of nodes were significantly larger than those of internodes perhaps due to anatomical differences (Table 1). Although the force-displacement characteristics varied between internodes and nodes (Fig. 4), the peak force, cutting energy and stress parameters were usually smaller with internodes (except for specific energy), but were not significantly different. Overall, these results indicated that between internodes and nodes, other than the dimensional parameters difference, the mechanical cutting parameters were similar. Hence a cutting device designed based on the largest node portion of corn stalk, based on peak cutting force requirements, will work well for the whole stalk as it included already the variation arising from material (internode) and size. It has been well established that increased moisture increased the mechanical cutting stress and energy parameters (Annoussamy et al., 2000; Chen et al., 2004; Galedar et al., _ 2008; Igathinathane et al., 2009; Ince et al., 2005), and contributed to the increased parasitic forces (Persson, 1987). However, Yu et al. (2006) found that ultimate shear stress was not affected by moisture content (10%e60% w.b.).
3.4. Effect of orientation and material variation of corn stalks on mechanical strength Tests on different orientations (90 , 45 , and 0 ) of corn stalks revealed that force, energy, and stress vary significantly (P < 0.05) on overall basis (Table 2). A very clear reduction of force and energy of an order of magnitude was observed when
the cutting orientation changed from perpendicular (90 ) to parallel (0 ), even though parallel orientation produced significantly larger new cut surface area. Another reason for this reduction was at parallel orientation the cutting mostly proceeded through corn stalk soft pith. The variation of dimensional parameters (2a and 2b) was irrelevant as the samples of various sizes were pooled in the analysis. However, the change in orientation from 90 to 0 progressively increased the sectional area geometrically (ellipticalethe least to rectangularethe most) and produces significant differences. Among orientations, both internodes and nodes show significant (P < 0.05) variation on force and energy parameters (Table 2). Between materials, the parallel orientation gave more variation on force and energy parameters than at perpendicular orientation. It can be visualised that at parallel orientation of node samples, the cutting plane actually passed through dissimilar material (node sandwiched between lengths of internode), while at perpendicular orientation cutting plane passed through a similar material (node). Between the internode and node materials, the dimensional parameters obviously did not show any significant difference with orientation. Among the various cutting energy parameters, only the peak force and specific energy at parallel orientation varied between the materials. However, in parallel orientation all the force, energy and stress parameters varied significantly between internodes and nodes. The reason for the increased values with nodes for the parallel orientation can be attributed to the fact that the node section acted as resistance to the possible easy formation of a failure plane on the otherwise uniform bundle of straight fibres of internodes. This difference can also be appreciated from the fact that the material of the nodes is densely packed in the nodular disc (perpendicular to stalk axis), provided support to the leaves along the periphery, which is clearly different from pith core of internodes.
3.5. Overall comparison of corn stalks on mechanical cutting parameters An overall comparison of significant mechanical cutting parameters was determined by taking perpendicular
Table 2 e Effect of corn stalk orientation on the mechanical cutting parameters. Sample
Orientation
Dimension ( 103 m) 2a 17.65 5.2 bA
Peak force (N)
2b
Energy (Nm) Peak
Total
Ultimate stress (MPa)
Specific energy (kN m1)
16.3 4.32 aA 242.43 135.74 aB 3.89 3.48 aA 4.83 4.48 aA 1.01 0.17 aA 17.11 7.50 aA
Internode Perpendicular (90 ) Inclined (45 ) Parallel (0 )
28.73 5.17 a 19.05 3.39 a 127.01 37.99 b 21.52 5.06 bA 19.3 4.23 aA 46.54 14.29 cB
Node
21.49 6.37 aA 19.19 5.4 aA
407.7 227.59 aA 3.02 5.21 aA 4.69 6.03 aA 1.20 0.41 aA 11.68 7.42 aB
26.23 5.74 aA 21.05 4.01 aA
83.89 23.61 bA
Perpendicular (90 ) Parallel (0 )
2.37 1.28 a 2.99 1.49 a 0.30 0.07 b 0.24 0.08 bB 0.48 0.14 bB 0.10 0.03 cB
0.44 0.10 bA 0.93 0.13 bA 0.16 0.05 bA
6.64 1.69 b 1.00 0.22 cB
1.78 0.38 bA
Note: Values presented are mean standard deviation from the original data; number of replications ¼ 5; dissimilar lower case group labels (a,b, and c) represent a significant difference (P < 0.05) among size category; and dissimilar upper case group labels (A and B) represent a significant difference (P < 0.05) between internode and node.
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1.2
Peak force - ratio
1.0
1.00
Internodes (IN) Nodes (NO) Internode-Node Ratio (INR)
1.00
0.8 0.59
0.6
0.52
0.55
0.4
0.0
0.21
0.19
0.2
IN-90°
IN-45°
IN-0°
NO-90°
NO-0°
INR-90°
INR-0°
Component and cutting angle
Ultimate cutting stress - ratio
1.2 1.0
1.00
Internodes (IN) Internode-Node Ratio (INR) Nodes (NO)
1.00 0.85
0.8 0.61
0.6 0.4
0.30
0.2 0.0
0.14
0.10
IN-90°
IN-45°
IN-0°
NO-90°
NO-0°
INR-90°
INR-0°
Component and cutting angle 1.6
1.46
Internodes (IN) Internode-Node Ratio (INR) Nodes (NO)
Specific energy - ratio
1.4 1.2 1.0
1.00
1.00
0.8 0.56
0.6 0.39
0.4
0.15
0.2 0.0
0.06
IN-90°
IN-45°
IN-0°
NO-90°
NO-0°
INR-90°
INR-0°
Component and cutting angle Fig. 6 e Comparison of mechanical cutting parameters of corn stalk internodes and nodes at different orientations.
orientation as a reference and working out the ratios of other orientations, as well as between internode and node materials (Fig. 6). A clear significant (P < 0.05) reduction of ratios of peak load, ultimate failure stress, and specific energy reduced with reduction in angle of orientation for both internodes and nodes. These ratios for corn stalks vary in the range of 0.1e0.2 in the parallel orientation with reference to perpendicular orientation, for both internodes and nodes. The ultimate
failure stress ratios of corn stalk (internodes and nodes) in the range of 0.1e0.14 were consistent with the longitudinal (0 ) to transverse (90 ) ratios of about 0.1 reported for alfalfa stems (Shinners et al., 1987). The ratios of internode to node cutting parameters at parallel orientation were about 0.6 for peak load, ultimate stress, and specific energy, owing to the resistance the node offered to the cutting process (Fig. 6). However, for
b i o s y s t e m s e n g i n e e r i n g 1 0 7 ( 2 0 1 0 ) 9 7 e1 0 6
perpendicular cuts these ratios showed some deviations. Notwithstanding the variation, these ratios showed a consistent reduction from perpendicular to parallel orientation. These results collectively indicated that irrespective of material differences (internode and node), parallel orientation of the stalks (along grain or longitudinal) to the cutting plane reduced the load, stress, and specific energy requirements drastically (1/5) with respect to perpendicular orientation (across grain or transverse). This lower strength of the corn stalk in parallel orientation can be exploited advantageously to develop feeding mechanisms or new machines that takes orientation into consideration while presenting the material to the cutting elements, especially in the first stage of size reduction. It was also observed that any orientation other than the perpendicular also exhibited a reduction in the cutting energy requirements; therefore any practical deviation from the best parallel orientation will also produce the corresponding energy efficiency in size reduction. It should also be understood that for finer product sizes, it is inevitable to use random including the perpendicular orientation or other modes (impact or attrition) at a later stage of size reduction, as ideal parallel orientation is expected to produce a product of predominantly fibrous material without cross cutting of fibres. However, the parallel orientation as an initial stage of size reduction would have produced a loosened material that would not be expected to consume increased energy at subsequent stages, at any orientation including perpendicular. Further studies may be required to establish the advantage of parallel orientation in actual commercial size reduction machines and the size reduction characteristics of first stage parallel cut products on subsequent stage size reduction machinery, as well as address the challenges in scale up of parallel orientation size reduction devices.
4.
Conclusions
Five regions including initial compression, initial skin cutting, further skin and pith cutting, final skin cutting, and cutting completion were identified from the force-displacement characteristics of corn stalks. The cutting force-displacement characteristics of corn stalks were different with orientation, and internode and node material difference. Corn stalks orientation had significant effect (P < 0.05) on mechanical cutting force, energy, and stress, both for nodes and internodes. Parallel orientation (along grain) produced a reduction on the cutting stress and energy values to one-tenth or better for internodes and to about one-fifth for nodes compared to perpendicular orientation (across grain). The reduced energy requirements of the corn stalk in parallel orientation could be exploited advantageously to develop feeding mechanisms and new machines that consider orientation while presenting the material to the cutting elements.
Acknowledgements Funding for this research derived in part from the USDA-NRCS Grant Agreement 68-3A75-4-136 and USDA-DOE-USDA Biomass
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Research and Development Initiative DE-PA36-04GO94002. This financial support is gratefully acknowledged. We highly appreciate the fabrication help by Mr. Craig A. Wagoner, Lab Machinist, Department of Biosystems Engineering and Soil Science, University of Tennessee, Knoxville, TN, USA.
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