MATERIAL TO BE PELLETTED (fed into this space)
D. S. MACARTHUR
Department of Mechanical Engineering University of Queensland
SUMMARY
Pelleting bagasse would reduce storage and handling costs, reducing fuel oil consumption at the mill and making alternative uses more viable.
Existing pelleting machines are expensive and do not work well on bagasse.
This paper reports the results of an experimental study of the compaction behaviour of bagasse under controlled conditions of pressure (2 to 32 MPa), temperature (40°C - 160°C), moisture content (9% to 22%) and dwell time
(1 to 64 seconds). Dry matter densities at 32 min and 24 hr were found to be nearly identical. The final pellet density increases linearly with log pressure and linearly with log dwell time, and decreases roughly linearly with increasing moisture content above 10%. The density increases with temperature in two stages, from 40° to 60°C and very rapidly at temperatures over 120°C, with little change from 60°C to 120°C.
Stable, durable pellets can be made at very moderate conditions. At 10%
moisture, 60-100°C, 8 sec dwell, and pressures from 8 MPa to 32 MPa, densities range from 450 to 600 kg/m3. If higher densities than this are
required much higher pressures and temperatures are indicated, increasing the cost of the machine and its energy consumption.
b y
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INTRODUCTION
Loose bagasse has a very low dry matter density, (around 80 kg/m3 ) making it difficult and expensive to store, handle and transport. Compaction of bagasse into high density pellets has potential benefits in two areas
(Cullen et al 1980) :
(a) Reduced storage and handling costs would enable mills to store large amounts of bagasse cheaply, eliminating the need to burn fuel oil (b) Reduced transport costs would make more viable the utilisation of surplus bagasse for alternative purposes such as fuel, papermaking, or animal feed.
Briquetting machines for bagasse have been described as far back as 1936 (Tromp, 1936) but recent trials by CSIRO and the Sugar Research Institute on two commercially available machines were not encouraging, (SRI, 1 9 8 0 ) .
(All densities and production rates in this paper are reported on a dry fibre basis, and moisture contents on a wet b a s i s ) .
One type of machine is the American Sprout Waldron design, which produces 10mm dia. pellets by compressing the material between a roller and the inside of an annular die with radial holes. Although such machines work well with feedstuffs such as oats, the production rate with bagasse is relatively low. In most of the S.R.I. tests, a pelleting rate of less than 0.3 tonne/hr was achieved, using a 75 kW machine with fuel oil or molasses as additives. In Florida, a 150 kW version of this machine is reported to produce pellets at 3 tonne/hr using very finely hammer-milled bagasse and 1 8 % molasses, but with severe wear and maintenance problems (Foster, 1 9 8 0 ) . It seems that the fibrous nature and high friction of bagasse conflict with the operating principle of such machines, which requires the compressed material to separate and flow down a large number of small holes.
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The second type of machine, the Hausmann briquetting press, produces much larger pellets (50 to 110mm d i a . ) . It has a reciprocating plunger, extruding material as a 'log' against the friction of a hydraulically loaded, open- ended die. In trials, good pellets with densities of 900 to 1100 k g / m3
were produced using bagasse at a moisture content of 12%. However, the throughput (lOOOkg/hr for a 50 kW, 90 mm bore press) was too low to be
economic for this relatively expensive ($150,000) machine. Although the production rate achieved in the trials was limited mainly by feeding problems, the
power consumption was near the maximum and much higher rates could not have been reached.
These trials indicated the need for a fundamental study of the compaction properties of bagasse, to give an understanding of the compaction process and data to assist the design of a better machine,,
Although very little seems to have been published on the pelleting of bagasse, there is a large body of literature on the pelleting (or 'watering' or 'cubing') of forage materials. There are two main types of paper:
(a) experimental studies of the effects of one or more variables (pressure, moisture content, dwell time, temperature, e t c ) on the wafer properties (density and durability) of particular materials (mostly h a y ) ,
(b) modelling of the stress-strain behaviour of a material as a visco- elastic solid.
The results of these wafering experiments generally indicate that:
• Final density increases with both pressure and dwell time (in a roughly logarithmic manner).
• Moisture content affects the compression behaviour somewhat, and the expansion behaviour considerably. Increasing moisture content in the range 10 to 30 per cent generally reduces the final density but in the range 0 to 10 per cent, conflicting trends are noted by
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• Increasing temperature improves both density and durability, even at quite moderate temperatures (e.g. 60°C).
(Butler & McColly, 1959, Smith et a l , 1977, Dobie, 1975, and Hall and Hall, 1968)
The experimental study described in this paper had the following aims:
(a) To investigate the effects of pressure, temperature, moisture content and dwell time on the density and durability of bagasse pellets.
(b) To collect compression and relaxation data for later analysis of the mechanics of compaction (not reported here).
EXPERIMENTAL COMPACTION APPARATUS
The apparatus is essentially a long stroke hydraulic press (see Fig. 1 ) . The hydraulic cylinder is controlled by solenoid valve and the maximum pressure regulated by a relief valve and measured with a semi-conductor pressure transducer. Pellet pressures of up to 36 M P a can be achieved, with a stroking time of 10 secs. Dwell periods up to 30 sec. are controlled by a preset timer. At the end of the d w e l l , the hydraulic ram starts to rise slowly, and simultaneously a large pneumatic cylinder forces the bagasse containing cylinder rapidly upwards, ejecting the pellet and allowing it to expand without restraint from either side wall friction or end pressure.
The bagasse is contained inside a steel cylinder, with internal dimensions 76.2 ram dia x 381 mm long (L/D = 5 ) . This cylinder (see Fig.2) is designed to be gas-tight, with 0-ring seals, allowing a moist bagasse charge to be heated to a uniform temperature without loss of moisture. T h e seal is broken as soon as pressing starts. The sliding piston is a loose fit inside the cylinder, allowing air and steam to escape during compression. It has a hook for the wire of a long stroke displacement transducer (of the wire-wound drum type) to provide a continious record of the pellet height during compression and relaxation. (The hydraulic ram retracts, b u t leaves this piston resting lightly on the now unrestrained p e l l e t ) .
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The base of t h e c y l i n d e r i n c o r p o r a t e s a l o a d - s e n s i n g p i s t o n , which b e a r s on a s t r a i n - g a u g e load c e l l when t h e c y l i n d e r i s i n p o s i t i o n i n t h e p r e s s . T h i s e f f e c t i v e l y measures t h e b a g a s s e p r e s s u r e over t h e a r e a o f t h i s p i s t o n , which i s h a l f t h e d i a m e t e r o f t h e c y l i n d e r . Four i d e n t i c a l c y l i n d e r s were made.
The d i s p l a c e m e n t and p r e s s u r e s i g n a l s were r e c o r d e d on a micro-computer based d i g i t a l d a t a - l o g g i n g system.
PELLETING EXPERIMENTS - METHODS
The Bagasse used was supplied by the Sugar Research Institute, having been dried to a very low moisture content in their experimental flue gas drier. It had been stored for several months at an equilibrium moisture content of around 10%. Batches of 3-4 kg were made up to the required moisture content by thoroughly mixing while adding water from a spray bottle, and left to equilibrate for 3-7 days with several re-mixings.
Samples were then taken and dried in a microwave oven to determine the actual moisture content of the batch.
Each cylinder was tared, filled loosely with bagasse to a nominal dry matter density of around 80 kg/m3 (139 g of fibre) and the cylinder sealed and re- weighed, before heating in an oven for 2 hours. (Thermocouple measurements had shown that after this time, the centre temperature would be within 3-5°C of the oven temperature).
The relief value on the hydraulic supply was adjusted to give the required pellet top pressure. (The pressure transducer in the oil line had been calibrated against a proving ring in the press to measure the axial load directly).
The hot cylinder was positioned in the rig, and the pellet pressed, and ejected after the pre-set dwell. The relaxation of the pellet was recorded for 100 secs, after which the pellet was removed. Its height was measured in a jig with a dial gauge and platten at 2 minutes, 8 minutes, 32 minutes and 24 hours. It
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was weighed at 3 minutes and 24 hours, and was then tumbled in the durability testing machine (a rectangular wire mesh cage, rotated at 13 rpm about a diagonal axis for 3 minutes - ASAE standard S 269.2) and re-weighed. The durability is expressed as the percentage of the initial weight remaining inside the 12.7 mm mesh cage.
RESULTS
Some 150 pellets were made in this apparatus, covering a range of pressures from 2-32 MPa, temperatures from 40 to 160°C, moisture contents from 8.9% to 22.3%,and dwell periods from 1 to 64 secs. Obviously, it was not feasible to try every possible combination of parameters.
However, preliminary tests had shown that reasonable pellets could be made at 10% moisture content, 60-100°C, and pressures above about 10 MPa for a dwell of 10 seconds. Principal values of the controlled variables were therefore chosen as 8 MPa and 32 MPa for pressure, 60°, 100° and 140°C for temperatures, and 8 seconds for dwell. These parameter values were used to investigate the effects of moisture content. The effects of other temperatures, pressures and dwell times were investigated at moisture contents around 10% only.
With a non-uniform material such as bagasse, some variability in the results is to be expected. However, it was generally found that for nominally identical conditions, pellets made from the same batch of bagasse had very consistent measurements, replicate pairs usually agreeing within about 3% for densities at 32 mins/24 hrs (with even closer agreement for the density under pressure), and with differences seldom exceeding 5%.
On the other hand, there were considerably larger discrepancies between the results obtained from different batches of bagasse, even at very similar moisture contents. For example, the 32-minute densities of nine pellets made at 8 MPa, 8 sec dwell and 100°C from four batches of bagasse at 8.9, 10.0, 10.3 and 10.7% moisture content, span a range from 433 to 510 kg/m (16.2%) w h i l e t h e ranges w i t h i n each b a t c h a r e 0.6%, 2.9%, 6.2%and 1.8% r e s p e c t i v e l y
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(See Fig. 3 ) . It is unlikely that these differences are more than partly caused by these small changes in moisture content, and it is more likely that much of the difference is due to variations in uncontrolled properties of the bagasse such as particle size distribution and pith/fibre ratio.
EFFECT OF PRESSURE:
Figure 3 shows the density under pressure, the density at 32 mins, and the durability of pellets made at around 10% moisture content, 100°C and 8 sec dwell plotted against Log pressure . Most of the points plotted (the circles) are for one batch at 10.3% moisture content.
Considering only the points from this batch, it is seen that both the density under pressure, and the density at 32 mins are very nearly linearly related to Log pressure , at least over the range 4-32 MPa. The densities at 24 hr are so close to the 32 min densities, being generally 1-2% higher, that they have not been plotted. As might be expected, the scatter in the durability measurements is large, but the points generally indicate a curve of the form shown.
Very similar relationships were found to hold at 60°C and 140°C, and these results have been combined with others in Figure 4.
EFFECT OF TEMPERATURE
Figure 4 shows the effects of both temperature and pressure on pellet densities at 10.0% moisture content and 8 sec. dwell. Individual experimental points are not shown, because this is a composite graph combining the results from several series of tests on four batches of bagasse, from 8.9% to 10.7% moisture content. Hypothetical results for 10.0% moisture content were obtained by interpolation of plots against moisture content.
As most of the results were at pressures of 32 or 8 MPa, the curves shown
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for 16 and 4 MPa are in part derived by assuming that the Linear-Log relationships of Figure 3 hold, and interpolating and extrapolating from the results at 32 and 8 MPa.
Durability curves at 8 and 32 MPa only are shown, and also a curve show- ing the average moisture content at 3 minutes from ejection.
Several interesting aspects of bagasse compaction behaviour are evident from this figure:
(a) The curves of final density against temperature show a rapid rise between 40°C and 60°C, followed by a plateau until another rapid rise at temperatures over 120°C.
(b) The durability of pellets made at temperatures below 60°C is very low.
At low pressure (8 MPa), the durability increases steadily with temperature. The minimum acceptable durability is probably around 90%, indicating a minimum temperature of 100°C at 8 MPa.
(c) The curve of 3-minute moisture content vs. temperature shows that at temperatures up to 100°C, very little moisture is lost during and immediately after compaction. However, at 140°C or 160°C, most of the moisture escapes as steam immediately pellet pressing starts, and the pellet is actually formed at a moisture content well below the nominal figure.
EFFECT OF MOISTURE CONTENT:
Figure 5 shows density under pressure, density at 32 min and durability plotted against moisture content for two pressures (8 and 32 MPa) and three temperatures (60, 100 and 140°C). For clarity, only the mean values for the two or three pellets made at each set of condition have been plotted.
Despite the scatter of points, particularly around 10% moisture content, the major trend in these results is quite clear: at all temperatures and pressures, the final density of the pellets falls off markedly with
increasing moisture content above about 10%. Durabilities also fall off with increasing moisture content, particularly above 20%.
The density under pressure increases with moisture content at 8 MPa, but appears to decrease with moisture content at 32 MPa. This is at least partly due to "solid" densities being reached at higher pressures and moisture contents, with the water filling all the void volume. //surpris- ingly, the final densities at 60°C appear higher than those at 100°C, at both pressures and all moisture contents above approx. 10%. The differences are large enough (5 - 18%) and consistent enough that this is probably a real effect and not just experimental error. This would tend to support the hypothesis suggested above, that at temperatures over 60°C, up to perhaps 120°C, the fibres are more able to relax, giving lower final densities, despite higher compressed densities.
EFFECT OF DWELL
Two series of tests at 100°C, 8.9/10.0% moisture, and 32/8 MPa applied pressure, examined the effect of dwell times from 1 to 64 secs. The results (not shown) indicate that the final density increases roughly linearly with Log dwell time . At 32 MPa, the final density increases by 20 kg/m3 (3%) per doubling of the dwell time, while at 8 MPa, the rate of change is slighly higher at 24 kg/m2 (5%) per doubling. The effect on density under pressure is similar but smaller. The durability at 32 MPa was unaffected by dwell (being over 97% at all values of dwell) while the durability at 8 MPa fell markedly at dwell times less than 8 seconds.
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DISCUSSION
Two limitations of these results should be borne in mind. Firstly, the variation between nominally identical batches indicates that the bagasse supply was not sufficiently uniform, and that the pith/fibre ratio or the particle size distribution may be important variables, the effects of which also require investigation. Thus in the interpretation of these results, it is felt that although considerable confidence can be placed in the trends indicated for changes in pressure, temperature and dwell time, as these are generally derived from tests on one batch, the results presented for changes in moisture content are necessarily derived from different batches, and the trends indicated will be less accurate.
The second limitation concerns the control of moisture content at elevated temperatures. Although the sealed cylinders ensured that no moisture was lost during heating, they could not prevent loss of moisture during and immediately after pressing. Figure 4 shows that at temperatures up to 100°C, the moisture lost in the first three minutes is negligible (< 1.6%).
However, the large amount of moisture lost at temperatures of 140°C and 160°C raises the question of whether the much higher final densities at these temperatures are caused directly by temperature, or indirectly by its effect on moisture content. Both factors probably contribute to the increased densities, but the important point is that Figure 4 represents the effect of temperature at a moisture content of 10% at the start of compaction, i.e. at entry to the die of a pelleting machine. The increase of density at high temperatures is therefore real enough, whatever the mechanism involved.
This may partly explain the rapid increase in final density at temperatures over 120°C. However, the shape of the final density - temperature curves in Figure 4 suggests that "two different mechanisms are involved at different
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temperatures. It is suggested that at temperatures below 120°C, the mechanism is one of plastic deformation of the fibres into an interlocked, compacted configuration. A mini-mum temperature of 60°C is needed to allow the fibres to deform plastically, but at higher temperatures the fibres, although easier to deform, giving the higher densities under pressure, are also more able to relax. The two effects balance out giving no increase in final density. It may be that there is a critical softening temperature around 60°C, and if the fibres cool below this quickly enough, the pellet will set at a higher density than otherwise. Further evidence for this hypothesis is given by the anomalous behaviour noted in the comments on Fig. 5. At moisture contents above 10%, the density of pellets made at 60°C is higher than for those at 100°C.
At temperatures over 120°C, the dominant mechanism is thought to be one of adhesion, with constituents of the bagasse fibre such as lignin softening and bonding the fibres together. At 160°C, the pellets are discoloured and have a resin-like smell.
CONCLUSIONS
The compaction behaviour of bagasse has been examined under controlled conditions, and the effects of pressure, temperature, moisture content and dwell time determined. Stable, durable pellets can be formed under quite moderate conditions (around 10% moisture content, 8 second dwell, 60 - 100°C and pressures as low as 8 MPa), although the densities achieved are not high (440 - 480 kg/m3}. Densities around 600 kg/m3 require around 32 MPa, while increasing the temperature to 140 or 160°C causes adhesive bonding, giving densities up to 940 kg/m3. Doubling the dwell time increases densities by 20-24 kg/m3 while doubling the pressure increases densities by 60-80 kg/m3. Moisture contents much over 10% give much lower densities and durabilities, with final density falling by about 20-30 kg/m3 per 1% increase in moisture content.
IMPLICATIONS FOR PELLETING MACHINE DESIGN
The design of a pelleting machine and the pelleting parameters needed will depend almost entirely on the required pellet density, which in turn will depend on the intended use. Pellet dry matter densities around 550-600 kg/m3 are easily achievable with moderate pressure, low temperatures and even quite short dwell times. The corresponding bulk densities of around 300-350 kg/m may be adequate for local storage and road transport, but for shipping, much higher pellet densities would be desirable. These would seem to require much higher pressures (100 MPa or so) and/or much higher temperatures. Such high pressures make the pelleting machine heavy and expensive, and consume much energy in an extrusion process, while high temperatures also consume energy, cause a fire risk and may render the bagasse useless for paper-making or animal feed.
Further investigation of the "softening" behaviour around 60°C is needed.
A process in which pellets are formed at 80°C, followed by rapid cooling under pressure to 40°C may be desirable.
ACKNOWLEGEMENTS
This project has been funded by the National Energy Research, Development and Demonstration Council. The assistance of various members of staff at the Sugar Research Institute and the Department of Mechanical Engineering, University of Queensland is also gratefully acknowledged.