The compaction of partially dried lucerne

J. agric. Engng Res. (1987) 37, 73-80

The Compaction of Partially Dried Lucerne D . NASON*

The compaction of partially dried lucerne by a heated, piston-press process has been studied. Stable wafers with long-term densities of 650 kg/m3 and 350 kg/m3 can be pressed from lucerne at 30~ and 50~o moisture content w.b., respectively, using a pressure of 10 MPa applied at a temperature of 100~ for a few seconds, provided re-expansion is restrained for a few minutes after rressing. The dry matter specific energy of compression is nearly independent of moisture. The coefficient of sliding "friction for lucerne against steel at high pressure decreases with increzsing moisture content and pressure. The results show the feasibility of the compaction of lucerne in the partially dried condition. 1. Introduction Lucerne is a fodder crop which is usually compacted in the field at harvest time into bales with densities in the range of 150 to 250 kg/m 3. The most common bale is the so-called small rectangular bale, which typically has a length of about 1.0 m and width and height of less than 0.5 m, giving a volume of less than 0-25 m 3. Sometimes, larger bales are produced such as the large rectangular bale, which has a volume in excess of 2 m 3, or the round bale, which has a volume of 2 to 3 m 3. It is widely recognized that the cost of fodder storage and transport could be reduced substantially if bales of higher density, and perhaps smaller size, were produced. To this end, a considerable amount of investigation has been done into improving the compaction of fodder crops. The early work of Bruhn, 1 Butler and McColly,2 Pickard et al., 3 and others showed that factors such as compaction pressure, time under pressure, chopped length, crop moisture and maturity, and the size of the pressed units are important in relation to their ultimate density and durability. Studies by Reece, 4 Hall and Hall 5 and Orth and Lowe 6 showed that relaxed wafer density is increased by heating during compaction and by cooling the pressed units under load to restrain re-expansion. O'Dogherty and Wheeler 7 have recently summarized the work in these areas. The various studies of fodder compaction have been done under differing conditions, but some generalizations can be made. Crops such as lucerne can be compressed into durable pellets or wafers having unit densities exceeding 500 kg/m 3 if pressures up to 10 MPa are applied for a few seconds or more, but only if the crop moisture (m.c.) is in a range of 15-25~o w.b. At lower pressures, shorter compression times, or higher m.c., the pressed units have progressively lower density and reduced durability. The development of compaction technology has been based mainly on the use of well dried crops, which can be taken as 15 to 25~ m.c. This has occurred because the problems of spontaneous heating in storage are minimized if the compacted units are already at equilibrium m.c., and because dried crop can generally be compressed to higher densities, with resultant economic benefits in transport and storage. Conventional hay balers which produce rectangular bales operate best at 15 to 25~o m.c. Other compaction machines a which have been developed to produce smaller units such as wafers or cubes which have typical dimensions of a few centimetres also operate best in this m.c. range. An exception is * CSIRO Division of Textile Industry, Belmont, Geelong 3216, Australia Received 6 January 1986; accepted in revised form 23 May 1986

73 0021-8634/87/050073+ 08 $03.00/0

9 1987 The British Society for Research in Agricultural Engineering

74

THE

COMPACTION

OF P A R T I A L L Y

DRIED

LUCERNE

the roll-wafering machine) which was designed and underwent early development for the purpose of processing crops having a wide range of moisture contents. There are advantages to compacting crops in a partially dried rather th'an well dried state, and these are discussed below for lucerne. Lucerne is normally dried in the field after cutting to reduce the m.c. to a level suitable for baling. To attain the well dried 20~ m.c. from about the 75yo m.c. of freshly cut crop typically1~ requires 3 to 4 days in dry summer weather in temperate localities, and can take longer in humid conditions or if the drying period is interrupted by rain. There is a growing realization of the disadvantages of extensive field drying from a fodder quality standpoint. Studies have shown "-~3 that losses of nutrients and dry matter during field drying can be considerable. The alternative of artificial drying before compaction is unattractive because of the cost. One commercial method of minimizing field drying time is the use of mechanical crop conditioners to increase 14'" the drying rate. Another approach would be to compact the lucerne at a higher m.c. and allow the completion of drying to occur in storage. Nelson" has suggested that this may be the most economical approach under many conditions. There are known disadvantages in compacting higher m.c. material, such as mould growth and the possible loss of nutrients with any juice expressed during pressing. Also, the spontaneous heating which may occur during drying in storage can cause nutrient loss" and even spontaneous combustion in extreme cases unless precautions are taken in storage. Nevertheless, the potential nutritional advantages from compacting lucerne in a partially dried condition, and thereby minimizing field drying time, justify further investigation of the conditions required and the characteristics of the pressed units. This has been the objective of the present study. In the work reported in this paper, the compression of lucerne in the partially dried range of 25 to 65Yo m.c. was investigated using an instrumented laboratory press of the heated-die/ piston type. The factors examined included the applied load, the load dwell time, the press box temperature, and the restraint of the pressed wafers to prevent re-expansion following pressing. Where possible, pressing parameters were chosen at values which would be practicable for a full scale compactor. The relaxed, stable densities of the wafers were measured, and their acceptability to sheep as food was studied. Measurements were also made of the work of compression and the coefficient of sliding friction of lucerne against steel at high pressures for a range of lucerne moisture contents.

2. Experimental work The lucerne was an aphid resistant type WL 318 obtained from the Department of Agriculture Research Institute, Werribee, Victoria, Australia. The trials reported are from two lots. Lot 1 was a second cut in the flowering stage and Lot 2 was a third cut in a leafy, preflowering stage. The lucerne was hand cut and either paddock or rack dried to various moisture levels, which were determined using an oven drying technique. The crop as cut had 73 to 77Y/om.c. and typically dried to 60 to 65~ m.c. in one-half day, 45 to 55~ m.c. in one day, and 25 to 35~ m.c. in three days under rainless, summer conditions. The laboratory press was developed by Walls 17 at the CSIRO Division of Textile Industry, Australia for studies of the compression of wool and operates as shown schematically in Fig. 1. The steel pressbox was heated by oil circulated through jackets attached to the sidewalls. The load application was biaxial, with an initial compression by the air-activated vertical ram, followed by a final compression by the hydraulic horizontal ram. The press was instrumented with sensors to allow monitoring of the load applied to the horizontal ram in its direction of motion, the load normal to the endwall, the load normal to a sensor mounted in one of the sidewalls, and the sample length during compaction. The load on the vertical ram was not measured.

D. N A S O N

75

8~~.ql_. " 150 Eo o,,

VertiCOlrom

:-,, 1

Siclew~ sensor

n

Fig. 1. Schematic views of stages in press operation." (top left) before lucerne inserted, (top right) at full compaction, (bottom) in cooling frame The stages in forming a wafer are shown in Fig. 1. The pressbox was preheated to the required temperature, and a 90 g sample of lucerne of measured m.c. was inserted into the top of the press with the rams located as in Fig. 1 (top left). The vertical ram was then lowered to compress the lucerne in an initial stage to a fixed height of 50 mm. The horizontal ram then performed a final stage compression to a wafer length dependent on the ram load and the compressibility of the lucerne (Fig. 1, top right). The load was maintained for a selected dwell time, and the horizontal ram was then retracted momentarily to relieve the loading. A cooling frame was then installed as shown in Fig. 1 (bottom) and adjusted to the in-press sample height and length, and side plates (not shown) were attached to maintain the in-press width during cooling in the frame. The horizontal ram then pushed the wafer into the cooling frame and recompressed it to the in-press length (Fig. 1, bottom). At the completion of the selected cooling time, the wafer was released and stored for measurements and observations. For determinations of friction and work of compression, the horizontal ram load, sidewall load, endwall load, and wafer length were measured during the final stage of compaction

(Fig. 1, top right). 3. Results and discussion 3.1. Density The results in Table 1, for Lot 1--a second cut in the flowering stage, show the relaxed density as a function of pressbox temperature, dwell time and moisture content. Each result is the average of three or more replicates. The densities were measured eight weeks after pressing, when the wafers had stabilized. Measurements of the density in-press at the horizontal ram pressure of 10 MPa showed that compaction was essentially complete, with negligible in-press void volume in the wafer, so studies at higher pressures were not undertaken. The wafers typically expanded considerably just after release from the cooling

76

THE COMPACTION OF PARTIALLY DRIED LUCERNE

frame and dried out progressively to reach a stable density at a volume of 75 to 150 cm 3 after about 4 weeks. From handling the wafers manually, it was judged that wafers of less than about 250 kg/m 3 density would be unlikely to be durable in mechanical handling, so this was taken as an approximate lower limit to the useful density range. Table 1 shows that the density increased with pressing temperature at a given dwell time and moisture level. An extrapolation of the results to ambient temperatures indicates that wafer densities above 250 kg/m 3 would not be expected for any of the moistures or dwell times. Pressing at temperatures above 120~ was not assessed because charring of the sample surface was detected in preliminary trials at 140~ The density at a given moisture level and temperature was increased by lengthening the dwell time. However, useful densities were still achieved at the shortest dwell time of 1 min with the wettest, 65~ m.c. The reduction from 65~o m.c. to 33~ m.c. almost doubled the density for a 1 min dwell time. Measurements of the temperature inside the heated sample were not made, but from heat transfer considerations it is unlikely that the samples were heated through to their centres, especially at the short dwell times. The results in Table 1~were obtained with variable cooling frame times of from 2 to 60 min. Frame times approaching 60 min gave the highest 8-week densities, but typically more than 90~ of the 8-week density could be achieved with a 2 to 3 min frame time. Thus, frame time differences in the Table 1 trials were judged not to have significantly affected the main trends in the results. The results in Table 1 suggest that load dwell time could be shortened further without much reduction in density. This would be of great importance in any practical compactor, where throughput is economically critical. Thus, follow-up experiments were carried out with Lot 2 lucerne, a third cut, in a leafy pre-flowering stage, in which load dwell times were reduced to the practical limit of about 5 s by the design of the pressbox and the experimental procedure. Standard conditions were adopted for the other pressing parameters: 100~ pressbox temperature, 10 MPa horizontal ram load, 10 mm/s horizontal ram speed, and 3 min cooling frame time. Lucerne from samples which ranged from very moist (62~ m.c.) to nearly well dried (27~ m.c.) were used. The results of the dwell time trials are presented in Fig. 2. Useful densities of 250 kg/m 3 with the 62~ m.c. lucerne required pressing times of 30 s or more. With 39~ m.c., 350 kg/m 3 was achieved in 5 s or more. For 36~ and 27~o m.c., relatively high densities of above 550 kg/m 3 were achieved in 5 s or more. Most importantly, the results for moisture contents of up to 47~ suggest that densities above 300 kg/m 3 could be achieved at times less than the 5 s or 10 s limit in these experiments. This result is of particular interest because of the practical importance of minimizing in-press time. Further investigations of compaction with the type of process used here, but at times shorter than 5 s, would be worthwhile to show

Table 1 Relaxed wafer densities, kg/m 3 Pressbox temperature, ~

40 80 100 120

Dn~H time inpress, mmutes Moisture. %w.b.

50 50 33 50 65 50 65

2 to 3

470 380 260

5 to 6

15

240 450 550 500 320

360

410

420

630

D. N A S O N

77 800 700

.7

600

~

zT~ 36 O/o

500 400 -~ (~ 3 0 0 0

___________ ~ /

"

+ 39 % " 47 % "62%

200 I00

os,o 2'0

go ;o io 7b

Load dwell time, s

Fig. 2. Relaxed wafer density as a function of load dwell time at 10 MPa pressure for various lucerne moisture contents ( % w.b. )

how short the dwell times can be and still achieve useful densities. The anomalously low density results for 39~ m.c. may indicate something incongruous about the nature of that lucerne sample, but do not alter the conclusions drawn above. 3.2. Friction and work o f compaction An expression has been developed +8 which relates the coefficient of friction, the compressed length, the presswall pressures and the pressbox dimensions for material being compacted in a closed-end press. When adapted is to the apparatus used in the present study, the coefficient of friction of lucerne against steel, #, is given by

# = 2d(a+b) LL+JLLoJ

where As = d= a= b= s= Lo = Ls = Ld =

[LdJ

sidewall sensor area, 1194 mm 2 sample length under compression pressbox width, 25 mm pressbox height, 50 mm distance from horizontal ram face to centre of sidewall sensor, d - 2 7 . 5 m m horizontal ram load sidewall sensor load endwall load.

In the present study it is the coefficient of sliding friction, not static friction, which is of interest, so the ram and presswall loads and sample length measured during compaction were used in the expression above. Friction results for lucerne at various moisture contents and pressures are shown in Table 2. The listed pressures are averages of the endwall and ram pressures. The results in Table 2 indicate that the coefficient of friction increased progressively as lucerne moisture decreased, rising from very low values for crop with above 50Vo m.c. to values of around 0.20 for the driest crop at 27Vo m.c. For the wettest samples, it was noted that water had been squeezed from the lucerne onto the internal presswalls during compaction, and this may account for the lowered friction. At a given moisture level, the

78

THE COMPACTION

OF PARTIALLY

DRIED

LUCERI ~

Table 2

Coefficient of sliding friction Moisture, % w.b. 62 47 39 36 27

Pressure, MPa

la

10 5 9 8 4 8 4 8 4

0.01 0-03 0.06 0.11 0-19 0.11 0.19 0.17 0.23

Dry matter specific energy of compression, k Wh/t 0-9

1.1 1.0 1.1

friction at these already high pressures of 4 to 10 MPa decreased as the pressure increase:~ These trends in the friction with moisture and pressure changes were also seen in resul. reported 2~ for lucerne at similar pressures but lower moisture, 5 to 24~ m.c. Frictior" measurements for other crops of high moisture have been reported, zl but these refer to much lower pressures. A useful inference from Table 2 is that press wear due to presswall friction could be decreased by pressing crops in a partially dried rather than well dried condition. The specific energy of compression was calculated from the area under the curve of a plot of the horizontal ram load against wafer length during compaction. The contribution from the early stages of the horizontal ram stroke was negligible, so the contribution to the compaction energy from the vertical ram stroke which preceded it was ignored. The results in Table 2 show that the dry matter specific energy of compression was around 1.0 kWh/t (3.6 MJ/t) for lucerne from 27 to 62~ m.c., so the compression energy is essentially independent of moisture content over this range. The differences between the values calculated for the different moisture levels are not statistically significant. Other studies 7 using various presses and processes give compaction energies which vary considerably, but are generally higher than the values in Table 2. However, results 7 reported for dry matter specific energy of wheat straw over the 7~ to 44~ m.c. range are independent of moisture content, as are the present results. The values in Table 2 are only part of the total energy required for compaction, since crop heating was not taken into account. The total energy in a heated press process of this type would depend on heat losses in operation. These losses would include direct losses from the hot pressbox and indirect losses through producing wafers which exit the process at a higher temperature than the incoming crop. Thus, the total energy of compaction would depend on press design and, in particular, provisions for recycling heat from the cooling wafers to the incoming crop.

3.3.

Waferquality

The wafers were tested for their acceptability to sheep as a feed. Wafers of various texture, including some which were mouldy, were fed to a matched group of sheep at the Department of Agriculture Research Institute, Werribee. The rates of eating were compared for the wafers and the normal diet of lucerne chaff. The sheep clearly preferred their normal diet, as might be expected in a situation of an abrupt diet change. Wafers of loose or chewy texture and densities of 250 to 400 kg/m 3 were preferred to harder wafers of greater than 500 kg/m 3.

/e~,~SON

79 L the hard wafers were broken up into pellets of approximately 1 to 5 cm 3, 1 as readily as the looser wafers. Evidently, the 50 to 100 cm 3 wafers were ;heep if they had a chewy texture, but were too large if they had a hard g trials 1"22 using similarly sized wafers with larger animals such as cattle there are no problems with the higher densities. In fact, feed intake was r than with baled hay. here was some mould growth on wafer surfaces during the first weeks of rne pressed at above 50~ m.c. and traces of mould for lucerne at 35~ m.c. still visible but had lost its disagreeable odour after 3 to 4 months when the to the sheep. The sheep showed no preference between wafers which either d not been mouldy. Studies 23 have indicated that when mould is deemed to t can be suppressed by chemical additives. ; results indicated that lucerne wafers of 75 to 150 cm 3 and densities of 350 to : quite acceptable to sheep as feed, and prior mouldiness was not a problem if : allowed to dry out before feeding. For unit densities higher than 500 kg/m 3, cafers were preferred. 4. Conclusions

artially dried lucerne can be compressed into stable wafers under pressures of 10 M P a ied for a few seconds in a press heated to 100~ provided the wafers are restrained from .~-expansion for three minutes or more after pressing. Wafers with stable densities of ~pproximately 350 kg/m 3 and 650 kg/m 3 can be produced from crop of approximately 50~o and 30~ m.c., respectively, with press dwell times of l0 s and 5 s, respectively. Shorter press dwell times without loss of density may be possible. Higher density wafers are preferred for economical transport and storage and for durability in handling, while lower density wafers are preferred for palatability, and 350 to 400 kg/m 3 appears to be an optimum range of density, which can be achieved with lucerne pressed at 40 to 45~ m.c. G o o d ventilation during the drying period is advisable to minimize the known problems of moist storage. The coefficient of sliding friction against steel decreases from around 0.20 for well dried lucerne to less than 0.05 for lucerne with above 50~ m.c., and also decreases with increasing pressure. The dry matter specific energy of compression is essentially independent of crop moisture. These results indicate that the use of partially dried rather than well dried lucerne has the advantage of lower press wear and does not affect the mechanical energy required for compaction. The total energy required in the process would depend on press design and the control of heat losses. Acknowledgements

The author wishes to thank Dr G. W. Walls for helpful discussions and Messrs P. Krusic-Golub and G. K. Lee for assistance with the experiments. The assistance and advice of Dr R. Hodge and Messrs E.K. Simmons and H. Simpfendorfer of the DAV Werribee Research Institute are also greatly appreciated. References 1 Bruhn, H. D.; Zimmerman, A.; Niedermeier, R. P. Developments in pelleting forage crops. Agricultural Engineering 1959, 40(4): 204-207 2 Butler, J. L.; McColly, H. F. Factors affecting the pelleting of hay. Agricultural Engineering 1959, 40(8): 442-446 a Pickard, G. E.; Roll, W. M.; Ramser, J. H. Fundamentals of hay wafering. Transactions of the American Society of Agricultural Engineers 1961, 4(1): 65-68

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THE

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PARTIALLY

DRIED

L,

J r'x:~:"

4 Reeee, F. N. Temperature, pressure and time relationships in forming dense hay , . Transactions of the American Society of Agricultural Engineers 1966, 9(6): 744-751 s Hall, G. E.; Hall, C. W. Heated-die wafer formation of alfalfa and bermudagrass. TransaL. ,q the American Society of Agricultural Engineers 1968, 11(4): 578-581 ~J'[l e Orth, H. W.; Lowe, R. Influence of temperature on wafering in a continuous extrusion Journal of Agricultural Engineering Research 1977, 22:283-289 O'Dogherty, M. J.; Wheeler, J. A. Compression of straw to high densities in closed cylindri 5;" Journal of Agricultural Engineering Research 1984, 29:61-72 '~' a Mathies, H. J. Hay wafering in Europe. Farm Mechanization 1967, 14-15 o s Molitorisz, J.; McColly, H. F. Development and analysis of the rolling-compressing " ,t process. Transactions of the American Society of Agricultural Engineers 1969, 2(4): 419- ~'w lO Simpfendorfer, H. Private communication, Department of Agriculture, Victoria, Australia ,,1 11 Nelson, L. F. Spontaneous heating, gross energy retention and nutrient retention of high. q alfalfa hay bales. Transactions of the American Society of Agricultural Engineers 1968 9;. 595-600 12 Shepherd, J. B.; Wiseman, H. G.; Ely, R. E.; Melin, C. G.; Sweet-man, W. J.; Gordon,

Schoenleber, L.G.; Wagner, R.E.; Campbell, L.E.; Roane, R.D.; Hosertman, ..,u Experiments in harvesting and preserving alfalfa for dairy cattle feed. US Departme. ~ Agriculture, Technical Bulletin No. 1079, 1954 la Wilkinson, J. M. Losses in conservation and utilization of grass and forage crops. Annal..d Applied Biology 1981, 98(2): 365-375 Its 14 Savoie, P.; Rotz, C. A.; Bucholtz, A. F.; Brook, R. C. Hay harvesting system losses and drying r~ Transactions of the American Society of Agricultural Engineers 1982, 25(3): 581-589 f, is Rotz, C. A.; Sprott, D. J. Drying rates, losses and fuel requirements for mowing and condition. :. alfalfa. Transactions of the American Society of Agricultural Engineers 1984, 27(3): 715-720 is Simmons, E. K.; Simpfendorfer, H. H. What happens when hay heats. Agnote Order No. 2333/83, Department of Agriculture, Victoria, 1983 17 Walls, G. W. (To be published in the Journal of the Textile Institute) 18 Roberts, N. F.; Hammersley, M. J.; Barker, A.; Van Pelt, W.; Carson, J. M. Compression of wool bales: forces on the surface of the bale chamber. Wool Research Organisation of New Zealand, Communication No. 13, November 1973 lg Abbott, G. M.; Nason, D. The role of press-wall friction in the high-density compression of wool bales. CSIRO Division of Textile Industry, Belmont, Victoria, Australia, Textile Research Journal 1986, 56(12): 715-721 2o Menzies, D. R. Friction coefficients of alfalfa at high pressures. Canadian Agricultural Engineering 1976, 18(1): 16-17 zl Agricultural Engineers Year Book 1984, Friction coefficient of chopped fibres. American Society of Agricultural Engineers, D251.1, pp. 74-75 22 Dobie, J. B.; Curley, R. G.; Ronning, M.; Parsons, P. S. Feeding and economic value of wafered hay for dairies. Paper No. 65-640, presented at 1965 Winter Meeting of the American Society of Agricultural Engineers, Chicago Easson, D. L.; Nash, M. J. Preservation of moist hay in miniature bales treated with propionic acid. Journal of Stored Products Research 1978, 14:25-33