Bagasse Compaction - Interim Report Research Report No 5 81

8/22/2019 Bagasse Compaction - Interim Report Research Report No 5 81

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U N I V E R S I T Y O F Q U E E N S L A N D

Department of Mechanical Engineering

BAGASSE COMPACTION - INTERIM REPORT

D.S. MacARTHUR

Research Report No. 5/81


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QUT

Library

BAGASSE COMPACTION - INTERIM REPORT

D.S. MacARTHUR

Research Report No. 5/81


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INTRODUCTION

The aim of this report is to gather together in one document all the inform

ation collected to date on the University of Queensland Bagasse Compaction

Project, because of the impending departure from Australia of the author.

Most of the experimental work has been reported in detail in a paper to be

presented at the 1981 ASSCT Conference, attached as Appendix II. A much

broader review of the whole bagasse compaction project, including much of

the work done at the Sugar Research Institute, is to be presented at the 1981

ANZAAS Congress, and a copy of this paper is attached as Appendix I. It is

suggested that any reader unfamiliar with the background to the project should

read this paper first.

The main body of this report therefore serves to record as much as possible of

the subsidiary information gathered during the project, such as some of the

minor experiments, the format of the stored data of the compression tests (if

future analysis of this is desired), and some of the opinions or "feel" developed

by the author during the project.

TRI-AXIAL COMPACTION

Early in the project, it was suggested that ideally, the compaction should be

tri-axial rather than uni-axial; it was thought that higher densities should

be possible because of better packing, and that the pellets should be more

stable and stronger because of the better interlocking of the fibres. This was

supported by the evidence that uni-axially compacted pellets were very weak in

the plane perpendicular to the compaction axis. Three attempts at tri-axial

compaction were made:

(a) True hydrostatic compaction was attempted by enclosing the bagasse in

a plastic bag inside a pressure vessel full of water. The mouth of thebag was tied around a tube leading out to the atmosphere, to allow thevoid air to escape while water was pumped into the pressure vessel. Thiswas not successful, as the plastic bags always split before high pressureswere reached.

(b) Pseudo-hydrostatic compaction, in 6 steps (two steps on each of three axes)

was attempted using an arrangement of plattens and a frame to give com

paction down to a cube of side 50 mm. This gave encouraging results, with

slightly higher final densities than for the equivalent uni-axial compact

ion. More importantly, the pellets were much more robust. These twoexperiments are described in Lab. Report by M. Just, R. Cressey, and

J.G. Loughran.

(c) Much later in the project, when the low speed compaction rig was operational,

a further attempt was made. This time tri-axial compaction was approx

imated by a first stage of radial compaction of a cylindrical sample,

followed by axial compression. The apparatus was simply a sheet of thin

steel, wrapped around itself to make a cylinder of larger diameter than

the 76 mm diameter test cylinder. It was filled with loose bagasse at

this starting diameter, and then compressed circumferentially (and hence

radially) by means of slowly screwing up a number of hose clamps around it,until the bagasse charge could be pushed out into one of the standard test

cylinders. The results of this experiment are shown in Figure 1, and

indicate that a slight improvement in both density and durability is achieved

with a moderate amount of radial pre-compaction, and no further improvement

is gained with a greater amount of pre-compaction.


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COMMENTS on PRESSURE, PACKING and WALL FRICTION

The low speed compaction apparatus used a platten and load cell to measure

the pressure over a central circular area of half the pellet diameter, on

the bottom face of the pellet. This was done for two reasons. Firstly, a

5-tonne load cell was available, and secondly, it was thought that by measur

ing the pressure over this central area only, wall effects due to frictionand different packing of fibres would be avoided.

In retrospect, this was not the best arrangement. While this platten will

indeed measure the true pressure on that central area of bagasse, which may

well be different from the mean pressure over the whole base because of these

effects, it is not possible to use this "improvement" in measuring technique

to derive a more accurate pressure - density relationship, because one must

assume uniform packing in order to calculate the density! Thus two effects

of unknown magnitude but probably opposite sign are affecting this measured

central bottom pressure: wall friction will tend to make the bottom pressure

less than the top pressure, while the effect of the walls on the packing ofthe cylinder is thought to result in higher density (and hence higher pressure)

in the centre.

However, the recorded results indicate that these two effects are either small

or roughly equal, as the bottom pressure is usually within -13% to +5% of the

top pressure. Since the results show a linear dependence of density on log

pressure, the error is very slight. Nevertheless, it would have been preferable

to have been able to measure the average pressure over the whole bottom face,

as then some measurements of wall friction could have been made.

PRESSURE - DENSITY DATA

The recorded pressure - displacement data for some 150 pellets is stored on

floppy discs on the Apple microcomputer system. This data was recorded with

two aims in mind: firstly, to give the simple results of density under

pressure, actual maximum pressure, and actual dwell time used in the main

experimental analysis, and secondly, to provide a data base for subsequent

analysis of visco-elastic behaviour, which has not yet been carried out. This

section of the report will give enough information on the storage format of

this data to allow later analysis if required.

Two channels of data were recorded via the Biomation Waveform Recorder:

Channel 1: The output of the displacement transducer ( i . e . pelletheight)

Channel 2: The output of the load cell, measuring the force on the

platten in the base of the cylinder (i.e. bottom pressure).

The Biomation unit is a very flexible digital data recorder, capable of extrem

ely fast sampling rates (5 MHz), though these were not needed for this applic

ation. Unfortunately, it is more suited to its intended task of high speed,

semi-quantitative waveform recording, than low speed, accurate data recording.Offsets are not easy to adjust or. reset, and the nominal scaling varies between

channels and ranges, by up to 6%. With only 8-bit resolution, care must be

taken to use as much of the dynamic range as possible. Incidentally, the

timebase was found to be consistently out by a factor suspiciously close to 1.024


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.3.

for all ranges over 1 ms, while being accurate at the faster sampling rates!

For almost all the pellet data recorded, the sampling rate was a constant

50 ms/sample, which with 2048 samples per channel, gave about 105 secs of

recording time. This covered the compression and dwell at reasonable time

resolution, and relaxation for over 70 secs, while avoiding the complexities

of the dual timebase facility.

The raw data was transferred to the Apple microcomputer, checked visually and

by the machine for errors, and stored as a binary file, using the program

INFAST, the author's modification of Ken Staib's program FAST INTERFACE (which,

among other things, plotted upside down!). Both programs call the assembly

language routine ASS1, renamed ASS1 A$16FC L$52 by the author to give the

relocation information needed for copying.

These binary files of pellet data are known as P-files, with names such as P1234.

The pellet numbering code is given in Figure 2. They consist of 4096 8-bitnumbers in sequence, being the 2048 samples from Channel 1 (displacement) followed

by the 2048 from Channel 2 (pressure). However, because of the way ASS1 was

written, in the interest of fast execution, the whole data is actually stored

backwards on disc. To use this data, the appropriate P-file should be BLOADed

with a suitable starting address (e.g. A24576) and then accessed backwards,

starting at the location 4095 higher. Thus:

Channel 1 (displacement element I1 is at (28672 - I1) for I1 = 1 to 2048 and

Channel 2 (pressure) element T2 is at (26624 - I2) for 12 = 1 to 2048.

Two other files of data exist for each pellet, the D-file and the E-file. These

are essentially the same, the E-file being a copy of the D-file with some

further data calculated by the program PD*. These files are sequential text

files of 61 elements. The elements and their units are identified by two common

files, DATA INDEX and UNIT INDEX. The data stored in the D-and E-files includes

pellet weight, moisture content, nominal (and in the E-files, actual) pressure

and dwell, and calibration data. This calibration data, in elements 44 to 51,

consists of 4 pairs of numbers, giving high and low inputs to the recording set-up

(pellet heights of 83 and 23 mm, from setting pieces, and set voltages correspond

ing to 35.04 and s MPa), and the corresponding 8-bit integers recorded on the

Biomation. Linear calibration equations can therefore be set up (see lines

3000 to 3090 in the program PD*). The 8-bit integers were determined using theprogram BIOCAL, as described below. A further useful data element in the E-file

is element 52, the sample number of zero time (defined as the end of the dwell).

The above information should enable full access to this recorded data for any

future analysis. Using the calibration data in the E-file, the P-file can be

converted into pairs of pressure and height/density measurements at 50 ms

intervals during the compression and relaxation of the pellet. An example of

the procedure is given in the program LOGP-DMD, which does this, and then plots

log pressure against density.

PROGRAMS

The programs and files written by the author which may be of interest to later

users are listed below with brief comments:


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Biomation operation:

INFAST Reads data from Biomation unit. Plots each channel for visual

checking, then writes to a named file on disc. Performs error

check by re-reading Biomation, and reading data from disc, and

comparing.

Calls assembly routine ASS1 A$16FC L$52.Data is stored backwards. (See comments under "Pressure -

Density Data" above).

BIOCAL Used to calibrate the Biomation unit. It reads a binary file

from disc and displays/prints the actual numbers (for brevity,

the sums of 10 consecutive samples). To use for calibration, a

recording is made on the Biomation of some known inputs (e.g.

using the setting pieces for displacement and preset voltages

for pressure). This data is transferred to disc using INFAST,

and then examined using BIOCAL. The corresponding pairs of

calibration numbers are then calculated and stored in a CAL-fileusing the program MAKE CALXX.

P-COPY Copies a P-file or any other 4 k-byte binary file.

Pelleting Data:

DFILE* Creates a D-file for each pellet by keyboard entry of data.

Calibration data, common for several pellets, is copied from

a CAL-file by specifying the CAL-file number.

DATA INDEX Sequential text file of 61 elements, identifying the corres

ponding elements in D- and E-files.

UNIT INDEX Ditto, giving the corresponding units.

MAKE CALXX Creates a CAL-file from keyboard entry of Biomation set-up and

calibration data. DFILE* copies this data into each D-file.

The main data-processing program used. For each pellet, it loadsthe P-file and the D-file. By examination of the data, it cal

culates quantities such as the sample no of zero time (the end

of the dwell), the actual dwell period, the mean bottom pressure

during the dwell, the density at the end of the dwell and at

various times during relaxation. These results are printed,

and also filed as an E-file. The raw data from the two channels

are plotted for visual assessment. (As the Apple will only plot

Y-coordinates up to 191, and the raw data ranges up to 255,

values from 192 to 255 are reflected from the top of the plot.

This is preferable to scaling the data, as one-to-one corres

pondence is maintained, with the full resolution available, and .

no truncation ambiguities). The printed outputs from thisprogram are filed along with the pellet data sheets.

PD*


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LOGP-DMD Plots log (bottom pressure) against dry matter density for

the compression record of selected pellets. It inputs the

P-file and E-file for each pellet, with a pause and a bell

to allow disc changing if necessary.

P-DMD~Q As above, but plots pressure against dry matter density raised

to the power Q (input from the keyboard).

W0RK4 Produces a plot of compression energy vs. pressure from keyboard

input of DMD's at 2 and 32 MPa, derived from the output of LOGP-DMD.

It assumes that log P is linear with DMD in this range.

General Purpose:

CTRL FIND The program given in the Apple manual to display normally invisible control characters.

COPY TEXT Lists, prints and/or copies sequential text files. The number ofelements must be known.

PRESSURE - DENSITY RELATIONSHIPS

Some analysis of the compression behaviour of the test pellets was carried out,

using a program P-DMD, P-DMD Q, and LOGP-DMD. Sample results are shown in

Figures 3-6, for a selected group of 6 pellets, nos. 5500, 5200, 5300, 5100,

5400 and 5000. These all had a moisture content of 10.7% and a nominal maximum

pressure of 32 MPa, at temperatures of 40, 60, 80, 100, 120 and 140C respect -

ively.

Figure 3 is a straightforward plot of pressure vs. dry matter density, showing

the expected steeply rising curves (indicating exponential or power law relation

ships) . It is seen that the pressure required to reach a given density falls

rapidly with temperature.

Figure 4 shows the same data plotted on scales of log pressure vs. density. The

lines are fairly straight, particularly at the high temperatures, indicating an

approximately exponential relationship between pressure and density. The lines

are also roughly parallel, showing that at any given density, the pressure

required at 40C is roughly 2.5 times higher than at 100 - 140C.

Figures 5 and 6 show the same data plotted on scales of pressure vs (density)Q

from Q = 3 and 4 respectively. These power law relationships also fit the data

well, with an index of 3 being a better fit at 40-80C, while an index of 4 is

closer at 100-140C.


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COMPRESSION AND EXTRUSTION ENERGIES

Some estimates of compression energy were made. Because of the low resolution

of the Biomation unit, the pellet data records do not cover densities below

300 kg/m3. An early test on dry bagasse at room temperature gave a pressure-

density curve for the low density region, which on integration gave a nearly lin

ear relationship between compression energy and pressure at pressures up to 2 MPa:

Energy = 0.9 kJ/kg per MPa

This will be an over-estimate, as at higher temperatures and moistures, thepressure/density curve will almost certainly be lower.

The energy at the higher pressures was calculated by the program W0RK4, using

the assumption that log pressure is linearly related to density, and using data

measured from the plotted outpt of LOGP-DMD. This produced the lower set of

three curves shown in Figure 7, representing an estimate of the energy required

to compress 10% moist bagasse to a given maximum pressure, with no side wall

friction. These curves must be regarded as approximate, and probably over

estimate the true compression energy, because of the low pressure part of the

curve, which is a significant part of the total (20%). However, at 100C, the

total compression energy to 32 MPa will be around 8- 1 0 kJ/kg.

Also shown on the same figure is the extrustion energy required by a Hausmann-

type open-ended die extrusion machine. It is assumed that the bagasse is com

pressed against a rigid plug of previously compacted bagasse, until the maximum

pressure is reached, whereupon the rest of the stoke is completed at that con

stant back pressure (set by the hydraulic loading on the die). The extrusion

energy at pressue P, giving a density at that pressure of pp, is therefore

simply P/p . As p decreases with pressure, curves of the form shown are

obtained, and the extrusion energy is seen to be some 2 -2.5 times the compaction

energy.

The sum of these two curves is also shown. It represents an estimate of the min

imum total energy for an extrusion machine, with no friction losses other than in

the extrusion itself. For the pressures of interest, the power required by such

a machine can be approximated by

PELLETING MACHINE DESIGN CONSIDERATIONS

The experimental program has given considerable data on the effects of temper

ature, pressure, moisture content and dwell time. Before a machine can be

designed, two things must be specified - the pellet size and the required density.

From a user viewpoint, the pellets should ideally be as small and as dense as

possible. As small pellets are more difficult to make (at the same tonnage

throughput), and as higher densities are also more difficult to achieve, some

compromise must be made. Without full design studies and prototype testing of

different options, the "optimum" size and density must be subjectively assessed.


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Pellet size:

The pellets should be as large as possible consistent with easy bulk hand

ing. It is thought that a diameter of 75 mm (as in the experimental rig)

is a comfortable maximum. Larger sizes than this would not flow so easily,

would probably be less durable (as a square/cube law should apply to the

strength/mass of pellets), and would involve much larger machine forces.Smaller pellets, say 50 mm diameter, would be better, but would require

almost four times the production rate. There is also a strong argument in

favour of designing a prototype machine to the same dimensions as the test

results, to avoid any ambiguity over scale effects. Thus a pellet diameter

of 75 mm should be chosen. The pellets should have a final L/D ratio of

between 0.5 and 1 for durability, good packing, and good flow behaviour.

Pellet Density:

The "optimum" density is more difficult to specify. Densities of 5-600 kg/m3are very easy to achieve, and probably adequate for within-mill storage,

giving a bulk dry-matter density of 3-400 kg/m3, a four-fold improvement on

loose bagasse. These densities would also be adequate for short range road

transport, i.e. between neighbouring mills. However, for an export market,

much higher densities are required to reduce shipping costs. It may be that

two different machines are required for the two duties. Experiments currently

in progress indicate that densities of 750 - 950 kg/m3 are attainable with

moderate compression parameters (32 MPa at 80 - 100C) provided that the

pellet is restrained from expansion by a relatively light pressure (0.1 - 2 MPa)

for a long dwell period ( 2 - 8 minutes). This would appear to be the best

process for high densities, and should be attainable by either the proposed

roller-conveyor or piston extrustion type machines. The author strongly favours

| the extrustion-type machine, primarily because of the problems associated with

breaking up a continuously formed slab into small, durable pellets. The

roller-type machine is in principle more energy-efficient, as there are no

friction losses. However, as shown in Figure 8 in Appendix I, although extrusion

uses about three times more energy than the theoretical minimum, the total is

still low compared with the drying and/or heating energies, and represents only

a minor operating cost.


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D e n s i t y

( k g / r n 3 )


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PELLET NUMBERING CODE

Digit 1 Baaasse Batch Number (hence moisture content)

Digit 2 Temperature 0 140

1 100

2 60

3 80

4 120

5 40

6 160

D i g i t 3 Top P r es su re 0 32 MPa

1 16 MlPa2 8 MPa

3 4 MPa

4 2 MP a

D i g i t 4 R e p l i c a t i o n s


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/


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APPENDIX I

(Figures 4, 5, and 6 hove been omitted, as they are

Figures 2, 1, arid 4 re sp ec ti ve ly in Appendix II)

PELLETING BAGASSE

D . S . MacARTHUR

To be presented at the 51st ANZAAS Congress, Brisbane, May 11-15, 1981.

Bagasse, the fibrous residue from sugar cane

milling, is an important energy and fibre resource,

the usefulness of which is limited by a very low

bulk density. If bagasse could be economically

compacted into high density pellets, mills could

store more of it, reducing their fuel oil consump

tion, and potential markets for surplus bagasse

as a fuel or fibre source would become viable.

The limitations of existing pelleting machines arementioned, and the results of an experimental

investigation of bagasse Compaction discussed,

giving an insight into the mechanisms of pellet

formation and guidelines for the design of

improved machines.


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INTRODUCTION

This paper is about bagasse, a by-product of sugar manufacture.

I will be telling you

what bagasse is,

how much of it is produced in Australia,

what it can be used for,

why pelleting is desirable, and

the problems involved.

The results of an experimental investigation of its compaction behaviour

will be briefly discussed, giving an insight into the mechanisms of pellet

formation, and guidelines for pelleting machine design.

I will close by giving some figures on the relative energies involved in

various parts of the process.

WHAT IS BAGASSE?

Bagasse is the fibrous residue from sugar cane milling. Sugar cane consists

of roughly 1/7 sugar and 1/7 fibre, the remainder being water. The Australian

Sugar Industry comprises 7200 growers, cultivating 350,000 hectares along

2100 km of coastal strip from Northern New South Wales to the North of

Queensland, and 33 mills producing raw sugar. See Figure 1. The milling

process consists of shredding the cane, and then crushing it through a series

of rollers, with intermediate washings to extract. 95% of the sucrose. The

final bagasse comprises 49% fibre, 48% water, and 3% dissolved solids.

Bagasse "fibre" actually consists of 2/3 "true" fibre, and 1/3 pith. Both

pith and true fibre have similar chemical compositions, being mainly cellulosic

material, with around 20% lignin and 1-2% ash.


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Bagasse is an excellent fuel, even in the moist state, with a net calorific

value of approximately 7700 kJ/kg (wet basis). However, it is also

potentially useful as a fibre resource, for paper-making, or for animal feed.

Production figures vary with world market demand, but typically 21 M-tonne/yr

of cane is harvested to produce 3 M-tonne/yr of raw sugar (of which 75% is

exported). Annual production of bagasse is therefore also around 3 M-tonne/yr

(of dry matter). To give an idea of the scale of this resource, if it were

all burned in power stations, it could supply 40% of Queensland's electricity

consumption.

At present, all bagasse is burnt as a fuel in the sugar mill boilers, supplying

97% of the energy requirements of raw sugar manufacture. In fact, the sugar

industry is an almost perfect example of self-sufficiency in energy - the

principal waste product of the operation is used as a fuel to raise steam,

which supplies turbines to directly drive the mills, and other turbines to

generate electricity. The low pressure exhaust steam supplies the process

heat required to evaporate the water from the sugar juice. It is a fortunate

industry to have a free supply of fuel, and nicely compatible requirements

for power and process heat.

In principle, there is enough bagasse to supply all the energy needs of the

sugar factory. Yet sugar mills have historically burnt a significant amount

of fuel oil - some 40,000 tonnes/yr, or 1% of the national total. Why is this?

Unfortunately, although the total supply of bagasse is ample for all the

energy needs, there are variations in both supply and demand for a number

of reasons:

(a) The percentage of fibre in the cane varies from week to week, and

from the start to the end of the crushing season, typically from


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13% to 15%, which is a 15% change in fuel rate.

(b) The percentage of fibre in the cane also varies from area to area,

averaging 13.3% in the wetter northern areas, and 14.6% in the

south. Southern mills therefore have about 10% more fuel available.

(c) Short term fluctuations in both supply and demand are caused by

mill stoppages, wet cane, and starting and stopping production

at weekends.

Although most mills now have on-line storage of bagasse with capacities

typically 400-1000 tonnes (wet), this is only enough to supply the boilers

for a few hours, even if initially full. Thus fuel oil is burnt when the

bagasse supply runs out, generally

(a) at the start of the season;

(b) at the start and finish of each week's crushing;

(c) when the fibre in the cane is low, and

(d) when wet weather or crushing problems result in wet bagasse.

It should be noted that fuel oil is also burnt for operational convenience

rather than absolute necessity in some cases. For example, at the start

of a week's crushing, even if the bagasse bin contains ample bagasse, it is

much easier and quicker to start up a boiler with fuel oil than to start

and build up a fire with bagasse alone.

If the mills could store large amounts of bagasse, particularly over the

"slack" to use at the start of the new season, it would be possible to

eliminate almost entirely the need for fuel oil. As well as saving money,

this would give the mills independence from a commodity not always reliably

available.

Bagasse is a loose, fibrous material with an extremely low bulk density,

typically around 80 kg/m3 dry matter density. The average sugar mill burns


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fuel oil equivalent to some 7000 tonnes of moist bagasse in a season. Not

all of this would need to be stored at once, but a large proportion of it,

say 2500 tonnes, would need to be stored between seasons. This would

3require a storage volume of some 16,000 m , or a pile 3 m high covering

half a hectare! Compaction of the bagasse into a high density, easily handled

form has obvious advantages.

At present, "surplus" bagasse is an embarassment, as no viable market for

it exists, and it presents a serious disposal problem. Sugar mill boilers,

and the whole energy balance of the sugar factory, are set up to burn all

the bagasse produced. The requirement for minimum fuel oil consumption is

balanced by the need to avoid surplus bagasse production. If bagasse could

be compacted and stored cheaply, a temporary surplus could be stored for

later burning, saving fuel oil.

If a viable market for bagasse existed, sugar mills could adjust their

operations to produce considerable quantities. A small amount, up to perhaps

10% of production, could be made available immediately at almost no cost,

simply by altering boiler and process settings appropriately. Larger amounts,

25% of production or more, could be made available at some cost and at some

future date, by installing new equipment (particularly boilers) designed for

minimum energy (and bagasse) consumption. And if the market demanded it,

all 3 M-tonne/yr of fibre could be made available by installing coal fired

boilers.

The main potential markets for surplus bagasse are:

(a) as a fuel - probably limited to small scale, local use, or possibly

within the sugar industry, from high fibre to low fibre areas;

(b) mixed with molasses and used as animal feed, where the high fibre

content is desirable;


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(c) papermaking. This is potentially a very large market, and could

conceivably take the entire supply.

For all of these, and particularly for export for papermaking, the economics

depend heavily on transport and storage costs, and effective and cheap

compaction of bagasse is essential.

Thus bagasse compaction offers immediate benefits to sugar mills by reducing

their fuel oil consumption, and potentially large long term benefits by

opening up new markets for bagasse.

BAGASSE COMPACTION

The Sugar Research Institute has investigated several alternative methods of

bagasse storage, with and without compaction:

Loose storage in the open is straightforward and cheap, if the land is

available. Covering the bagasse piles with tarpaulins is desirable to

protect the bagasse from rain and moderate winds. This is at present the

most practical method for mills to use for bulk storage for their own use,

though the effects of a cyclone on one of these piles has yet to be experienced.

Bagasse can be baled and stored in stacks, in the open or under cover. Baling

machines vary from simple presses with manual strapping of the bales, to

fully automatic but expensive machines. Large bales of around one tonne

(wet) are convenient for handling. Dry matter densities of 270 kg/m are

easily achieved.

Disadvantages of bales include:

(a) they must be individually handled and stacked;

(b) they must be strapped with special high strength straps, which must

be cut on reclaiming;


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(c) they must be broken up on reclaiming, though this is relatively-

easy, and

(d) they still require covering to prevent rainwater increasing the

moisture content too much.

However, tests have shown that fears of spontaneous combustion are unfounded,

and microbiological deterioration is not too severe.

Baling is, therefore, a less attractive alternative than loose storage

under tarpaulins, unless storage space is severely restricted.

Pellets are the ideal form of bagasse storage. If small, high density,

durable pellets could be made cheaply, they would offer the following

advantages:

(a) easy bulk handling using conveyor belts, chutes, etc.;

(b) minimum storage volume, rendering storage in sheds, under cover,

economic, and

(c) lower transport costs, especially for shipping overseas.

The Sugar Research Institute has conducted trials on two different types of

commercially available pelleting machines:

One type of machine is the American Sprout Waldron design, which produces

10 mm diameter pellets by compressing the material between a roller and the

inside of an annular die with radial holes (See Figure 2). 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 version of this machine

is reported to produce pellets at 3 tonne/hr using very finely hammer-milled

bagasse and 18% molasses, but with severe wear and maintenance problems.

It seems that the fibrous nature and high friction of bagasse conflict with


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the operating principle of such machines, which requires the compressed

material to separate and flow down a large number of small holes.

The second type of machine, the Hausmann briquetting press (see Figure 3),

produces much larger pellets (50 to 110 mm diameter). 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

31100 kg/m were produced using bagasse at a moisture content of 12-6.

However, the throughput (1000 kg/hr for a 50 kW, 90 mm bore press) was too

low to be economic for this relatively expensive ($150,000) machine.

Although the product 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.

The conclusion from these trials was that neither type of machine would

produce bagasse pellets at a low enough cost to compete with either loose

storage or baling, as far as within-mill storage is concerned. It was

also apparent that the effects of parameters such as temperature, pressure and

moisture content were poorly understood. A research project was therefore

initiated at the Department of Mechanical Engineering at the University of

Queensland, to carry out a fundamental investigation of the compaction

behaviour of bagasse, and to use the results to devise an improved pelleting

process.

One further aspect of the S.R.I. investigation should be mentioned here.

Bagasse will not form stable pellets at high moisture contents (e.g. 50%

as milled), and though the optimum moisture content was not known, it seemed to

be well below 20%. Thus drying of the bagasse is an essential prerequisite

for pelleting. The S.R.I. conducted tests on existing driers, designed,


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built and tested two prototype driers, and have now completed a design

study for a 5 tonne (fibre) per hour drier, using flue gas as the

energy source.

THE UNIVERSITY OF QUEENSLAND BAGASSE COMPACTION PROJECT

The major part of the work to date has been an experimental study of the

compaction behaviour of bagasse under controlled conditions of temperature,

pressure, moisture content and dwell time. The major problem with this

investigation was the control of moisture content at elevated temperatures,

when obviously the bagasse tends to dry out. As loose bagasse is an

excellent insulator, the long heating periods required to achieve uniform

temperatures will result in a significant loss of moisture. This problem

was overcome by designing special test cylinders as shown in Figure 4.

These were gas tight, and could be heated in an oven to a uniform temperature

without loss of moisture. As the seal was broken at the start of compaction,

some moisture was inevitably lost during compression, particularly at the

higher temperatures. However, this still reproduces the behaviour of

bagasse which has that temperature and moisture content at the entry to the

chamber of a pelleting machine.

The compaction rig itself, as shown in Figure 5, was a fairly straightforward

arrangement of a long stroke hydraulic ram, with displacement and pressure

transducers, and a pneumatic cylinder and lever arrangement to eject the

pellet after pressing. The arrangement of a floating top piston, attached

to the displacement transducer, was devised to measure the relaxation behaviour

of the pellet, particularly just after ejection. The density of the pellet

was also measured at 32 minutes and 24 hours after ejection, and the durability

determined by tumbling for 3 minutes in a slowly rotating wire cage, and

measuring the weight loss.


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A series of experiments was conducted, covering pressures of 2 - 32 MPa,

temperatures of 40 - 160C, moisture contents from 9 - 22%, and dwell

periods of 1 - 64 seconds.

The results of these experiments will not be presented in detail here.

Very briefly, the main findings were:

(a) both density under pressure, and final density, increase linearly

with log pressure. Doubling the pressure increases the density

by 60 - 80 kg/m3.

(b) The final density increases roughly linearly with log (dwell time).

3Doubling the dwell increases the density by 20 - 24 kg/m .

(c) Within the range 9 - 22% moisture content (wet basis), increasing

moisture content reduces the final density, at a rate of 20 - 30

3kg/m per 1% increase in moisture content.

(d) Increasing temperature increases the density, in two distinct

stages. (See Figure 6). This behaviour will be discussed in

more detail in a moment.

(e) Durabilities generally improved with parameter changes which

increased final densities. In particular, moisture contents over

20%, temperatures below 60C, and pressures below 4 MPa gave very

weak pellets.

These results can be summarised by saying that stable, durable pellets can

be formed under quite moderate conditions (e.g. 10% moisture content, 8 sec

dwell, 60 - 100C, 8 MPa), though the densities are not high (440 - 480 kg/m3

3

dry matter density). Densities of 600 kg/m require 32 MPa, while higher

densities than this would require some combination of much higher pressures,

much longer dwell times, and higher temperatures.

In looking for some insight into the mechanism of pellet formation, the shape

of the curves of final density vs. temperature is interesting (see Figure 6):


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There is a rapid increase in final density as the temperature rises from

40C to 60C, and then little change until another rapid rise at temperatures

over 120C. This suggests two different mechanisms of pellet formation.

At temperatures below 120C, the mechanism is one of plastic deformation of

the fibres into an interlocked, compacted configuration. The pellet is held

together by a mixture of friction and positive interlocking of the fibres.

Actual adhesion between the fibres is slight or non-existent. However, at

higher temperatures, it is thought that constituents of the bagasse, e.g.

lignin, soften, and adhesion then plays a significant part.

This is supported by references in the literature to the softening point of

lignin as being around 136C, and by a crude experiment conducted to test this:

Pairs of bagasse fibres were clamped together and heated in an oven. They

were then pulled apart by hand and the adhesion between them rated subjectively

on an arbitrary scale. The results showed that adhesion is almost non-existent

at temperatures below 130C, and moderate to good at temperatures above

160C (see Figure 7).

Thus one way to get good pellets is to exploit this effect by using high

temperatures. However, high temperatures consume energy and constitute

a fire risk.

Examining the lower temperature end of these curves more carefully, it is

seen that there is no increase in density between 60C and 100C. Indeed,

most of the tests at moisture contents over 10% showed that densities were

higher at 60 than at 100. This leads to the suggestion that there is a

critical softening temperature around 60C. Temperatures above this make

the fibres easier to deform (hence the observed increase in density under

pressure), but also leave them more able to relax, allowing the pellet to

expand. Perhaps better pellets can be made by a cycle consisting of heating


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.11.

to over 60C, pressing, and then cooling under pressure to well below 60

before releasing the pressure?

Tests showed that such treatment did indeed give much higher final densities,

some 50% higher in fact at 880 kg/m3 . However, as the bagasse takes some

10 - 30 minutes to cool, it is difficult to separate this effect from that

of the increased dwell time alone. By keeping the temperature constant

while holding under pressure for similar lengths of time, it is found that

most (but not all) of this dramatic improvement in densities is indeed due

to these unrealistically long dwell times.

However, the relaxation forces are very much weaker than the compression

forces. Quite low pressures suffice to restrain the pellet from significant

expansion, and it is not necessary to maintain the full compression pressure

during this long dwell period. Thus very good pellets can be made by taking

them through a cycle comprising a short period (1 to 8 seconds) at high

pressure (32 MPa) followed by a long dwell (2 to 8 minutes) at a very much

3lower pressure (0.5 to 2 MPa). Densities range from 760 to 930 kg/m ).

Investigations along these lines are continuing, but the most significant

feature of this behaviour is that the open-ended die extrusion pelleting

machines such as the Hausmann briquetting press do, in fact, put the bagasse

through a similar cycle. The bagasse spends a few seconds in the main die,

where it is subjected to repeated compactions at high pressure. The pressure

rapidly reduces as the pellet is extruded against, friction from the rest of

the "log", and the pellet then passes into a long cooling line, where it is

still lightly restrained for several minutes. Perhaps such a machine is the

best way to compact bagasse after all, and only needs improved feeding arrange

ments and careful control of pressures, temperatures, and moisture contents

to be successful.


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RATE EFFECTS AND AIR ESCAPE LIMITATIONS

Obviously, the faster a given machine operates, the higher its output, and

hopefully, the lower the unit cost. What limitations are there to the

speed of compaction of the bagasse?

Loose bagasse is 95% air by volume. On compaction from 80 to 1000 kg/m3 ,

which requires a pressure of around 16 MPa, the void volume decreases by a

factor of 34. This would result in an internal gas pressure of 3.4 MPa if

the compression were isothermal and no air escaped. An associated research

project investigated this phenomenon and its importance.

Measurements made of the permeability of compressed dry bagasse indicated

3that it is very low indeed - at densities over 600 kg/m it is comparable to

that of sandstone. Numerical modelling of the airflow, supported by some

experimental measurements, shows that if the air can only escape axially,

rapid compaction results in such a rapid decrease in permeability that most

of the void air is trapped inside at high pressure, and takes a significant

time (one or two seconds) to diffuse away.

The importance of this effect for a real machine is unclear, because it is

difficult to estimate how much effect internal gas pressures of this magnitude

would have on the expansion of the pellet. However, it would seem desirable

to arrange for the air to escape radially. The transverse permeability is

thought to be much lower than in the axial direction because of the layering

effect.

The other major limitation to the speed of operation of a bagasse compaction

machine would seem to be the need for long dwell times (albeit at low pressures)

Feeding problems also become severe at very short cycle times. A large,

low speed, low power machine therefore seems preferable to a smaller, high

speed, high power one.


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ENERGY CONSIDERATIONS

In closing, it is of interest to compare the energies involved in different

parts of the pelleting process. Figure 8 shows these energies, converted

to common units of kilojoules per kilogram of dry matter.

ACKNOWLEDGEMENTS

As noted above, much of the work on this project has been carried out by the

Sugar Research Institute, and the research on Bagasse Compaction at both

S.R.I. and the Department of Mechanical Engineering at the University of

Queensland has been funded by a grant from the National Energy Research,

Development and Demonstration Council.


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FIGURE 1

AUSTRALIAN BAGASSE PRODUCTION

720D GROWERS on 3 5 0 , 0 0 0 h e c t a r e s

p r o d u c e

SUGAR CAME 2 1 M - t o n n e / y r ( 1 / 7 s u g a r , 1 / 7 f i b r e , 5 / 7 w a t e r )

f r o m w h i c h

33 SUGAR WILLS

p r o d u c e

RAW SUGAR 3 M - t o n n e / y r (90% r e c o v e r y - 75% e x p o r t e d )

a n d a l s o

BAGASSE 3 l Y l - t o n n e / y r ( d r y m a t t e r )

W h i c h c o n s i s t s o f

4&% w a t e r

3 % d i s s o l v e d s o l i d s

49% " f i b r e " - ( i n f a c t 2 / 3 t r u e f i b r e , 1 /3 p i t h , b o t h m a i n l y

c e l l u l o s e s , 2 0 % l i g n i n , 1-2% a s h )

H i g h e r C a l o r i f i c V a l u e ( d r y , a s h f r e e ) 1 9 , 5 0 0 k j / k g

L ow e r C a l o r i f i c V a l u e ( 4 8% m o i s t u r e ) 7 , 7 0 0 k j / k g

( E q i v a l e n t t o 2 M - t o n n e / y r o f c o a l , W h i c h i s

4 0 % o f Q u ee n s l a nd ' s c o n su m p t i o n f o r e l e c t r i c i t y g e n e r a t i o n )


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FIGURE 2

ROTATING DI E PELLETING MACHINE (Sprout Wa ld ro n) .

DIE SPEED 450rpm p E L L E T D I E

"(Approx 500mm t.D.)

9-5mm DIEHOLES

PELLETBREAKER

_ FEED ROLLERS(on sta tionary shaf t)

MATERIAL TO BE PELLETTED

(fed into this spa ce)


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FIGURE 3


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Adhesion of Bagasse Fibres


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ENERGY CONSIDERATIONS - SOME RELATIVITIES

NERGY VALUES ARE IN KILOJOULES p e r KILOGRAM of DRY MATTER

ALORIFIC VALUES:

.C.V. o f d r y f i b r e 19 ,5 00

.C.V . a t 50% mo is tu re con tenb (a s m i l l e d ) 15 ,5 00

RYING ENERGY:

om 5 0 % mo is tu re to 1 0 % mo i s tu re ( l a t e n t h e a t , n o re c o v e r y . 2 , 3 0 0

o r 1 % c h an g e a ro u n d 1 0 % mo i s tu re ( i n c l u d i n g s o r b t i o n e n e rg y ' 4 0

EATING ENERGY:

To heat 10% (20%) moist bagasse from (no evaporation) 90 (120)

ELLETING ENERGY - THEORETICAL:co mp re ss io n to 32 MPa ( SpdV ) 10

Ex tr us io n at 32 MPa ( P/p ) 25

T o ta l t h e o r e t i c a l e n e rg y fo r a n e x t r u s io n ma c h in e a t 3 2 MPa 3 5

PELLETING ENERGY - ACTUALLY ACHIEVED:

Rotating die (Sprout Waldron) } S.R.I, tests, Miliaquin: 900

(Not including hammer milling) } Florida, reported Foster: 180

Low speed extrusion (Hausmann Briquetting press, S.R.I, tests) 180

FIGURE 8


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ASSCT PAPER

APPENDIX II

PELLETING BEHAVIOUR OF BAGASSE

D.S. MACARTHURDepartment 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 (40C - 160C), 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 60C and very rapidly at temperatures

over 120C, with little change from 60C to 120C.

Stable, durable pellets can be made at very moderate conditions. At 10%

moisture, 60-100C, 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.

by


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. 1 .

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, 1980).

(All densities and production rates in this paper are reported on a dry

fibre basis, and moisture contents on a wet basis).

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 18%

molasses, but with severe wear and maintenance problems (Foster, 1980) . 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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. 2 .

The second type of machine, the Hausmann briquetting press, produces much

larger pellets (50 to 110mm dia.). 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 kg/m 3

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 hay),

(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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.3.

Increasing temperature improves both density and durability, even at

quite moderate temperatures (e.g. 60C).

(Butler & McColly, 1959, Smith et al, 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 MPa 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 dwell, 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. The 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, but leaves this

piston resting lightly on the now unrestrained pellet).


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.4.

The base of the cylinder incorporates a load-sensing piston, which bears on a

str ain -ga uge load c e l l when the cyl ind er is in po si t i on in the pr es s. This

ef fe ct iv el y measures the bagasse pre ssu re over the area of t h i s pi st on , which

is half the diameter of the cy li nd er . Four id en ti ca l cyl in de rs were made.

The displacement and pressure signals were recorded on a micro-computer

based d ig i ta l da ta- logging sys tem.

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 3-5C

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


39/51

.5.

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 160C, 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, 60-100C, 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

140C 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 100C 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%)

while the ranges within each batch are 0.6%, 2.9%, 6.2%and 1.8% respectively


40/51

.6.

(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, 100C 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 60C and 140C, 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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.7.

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 40C and 60C, followed by a plateau until another rapid

rise at temperatures over 120C.

(b) The durability of pellets made at temperatures below 60C 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 100C at 8 MPa.

(c) The curve of 3-minute moisture content vs. temperature shows

that at temperatures up to 100C, very little moisture is lost

during and immediately after compaction. However, at 140C

or 160C, 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 140C). 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


42/51

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 60C appear higher than those at 100C,

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

60C, up to perhaps 120C, the fibres are more able to relax, giving lower

final densities, despite higher compressed densities.

EFFECT OF DWELL

Two series of tests at 100C, 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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.9.

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

100C, the moisture lost in the first three minutes is negligible (< 1.6%).

However, the large amount of moisture lost at temperatures of 140C and

160C 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 120C. 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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.10.

temperatures. It is suggested that at temperatures below 120C, the

mechanism is one of plastic deformation of the fibres into an interlocked,

compacted configuration. A mini-mum temperature of 60C 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 60C, 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

60C is higher than for those at 100C.

At temperatures over 120C, 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 160C, 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 - 100C 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 160C 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.


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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 60C is needed.

A process in which pellets are formed at 80C, followed by rapid cooling

under pressure to 40C 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.


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.12.

REFERENCES

Butler, J.L., and McColly, H.F. (1959). Factors affecting the pelleting

of hay. Agricultural Engineering, 40, 442-6.

Cullen, R.N., Murray, C.R., and Mason, V. (1980). The elimination of

fuel oil usage in sugar factories. Aust. Inst, of Energy, 2nd Nat. Conf.

on "Petroleum, Policies and People", Melbourne, May 1980.

Dobie, J.B. (1975). Cubing tests with grass forages and similar roughage

sources. Trans. A.S.A.E., 18, 864-6.

Foster, D.H. (1980) Bagasse storage, drying and pelleting at overseas

mills. S.R.I. Tech. C i r c , 55, 14-23.

Hall, G.W., and Hall, C.W. (1968). Heated die wafer formation of alfafa

and Bermudagrass. Trans. A.S.A.E., 1JL, 578-81.

Smith, I.E., Probert, S.D., Stokes, R.E., and Hansford, R.J. (1977).

The briquetting of wheat straw. J. Agric. Eng. Res., 22 105-111.

Sugar Research Institute (1980) Elimination of use of fuel oil for steam

generation in the sugar industry. Progress report to N.E.R.D.D.D.C.,

Nov. 1979 - June 1980.

Tromp, L.A. (1936) Machinery and equipment of the sugar cane factory.

London, Norman Rodger, 1936.


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LOW SPEED COMPACTION RIG

FIGURE 1


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BAGASSE CYLINDER

Fi ure 2


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FIGURE 3

EFFECT OF PRESSURE ON PELLET CHARACTERISTICS


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Figure 4.

Effects of Temperature and pressure on pellet characteristics


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FIGURE 5

Effect of Moisture Content on Pellet Characteristics

Documents

Bagasse Compaction - Interim Report Research Report No 5 81