FINAL REPORT DEVELOPMENT OF A BAGASSE PELLETING MACHINE

USING A ROLLING PRINCIPLE

PART A SUGAR RESEARCH INSTITUTE COMPONENT

1985

PROJECT TITLE: DEVELOPMENT OF A BAGASSE PELLETING MACHINE

USING A ROLLING PRINCIPLE, 82/2435.

DATE OF ISSUE: 1982

AUTHORS: PART A. SUGAR RESEARCH INSTITUTE

R. CULLEN

v. MASON

PART B. UNIVERSITY OF QUEENSLAND

R. CULLEN

R. STALKER

P. KILLEN

NAMES OF ORGANISATIONS: SUGAR RESEARCH INSTITUTE

AND

DEPARTMENT OF MECHANICAL ENGINEERING,

UNIVERSITY OF QUEENSLAND.

TOTAL EXPENDITURE

UNDER GRANT: $93 960

DEVELOPMENT OF A BAGASSE PELLETING MACHINE USING A ROLLING PRINCIPLE

CONTENTS

PAGE

SUMMARY 1

PART A: SUGAR RESEARCH COMPONENT

CHAPTER 1. INTRODUCTION 5

CHAPTER 2. STAGES OF DEVELOPMENT 7

2.1 PRELIMINARY EXPERIMENTAL WORK 7

2.2 DESCRIPTION OF LABORATORY SCALE TWO

ROLL INSTALLATION 7

2.3 PROCEDURE 7

2.4 RESULTS 9

CHAPTER 3. THE PELLETING MILL 17

3.1 CONSTRUCTION 17

3.2 OPERATION AND MAJOR OPERATIONAL

DIFFICULTIES 17

3.3 EXPERIMENTAL RESULTS 24

3.3.1 Results with the Retention

Conveyor in Use 25

3.3.2 Results of Final Test Series 27

CHAPTER 4. ESTIMATION OF FORCES THROUGH ROLLERS AND

CHUTES 39

4.1 INTRODUCTION 39

4.2 MODEL FOR HORIZONTAL PRESSURES 39

4.3 DISCUSSION 42

CHAPTER 5. CONCLUSIONS 43

REFERENCES 45

APPENDIX I- EXPERIMENTAL RESULTS 46

1 .

SUMMARY

1. PROJECT OBJECTIVES

There were two basic objectives of the project. The first

was to investigate at a fundamental level the compaction mechan­

isms of bagasse particles to obtain a better understanding of the

influence that such factors as moisture content, particle size

and pressures have to play in obtaining stable and dense bagasse

pellets. This work was undertaken by the University of

Queensland, Department of Mechanical Engineering.

The second objective was to construct a pilot plant

pelleting machine based on a rolling concept similar to that used

in sugar milling to investigate the feasibility of such machines

producing bagasse pellets that can be stored and subsequently

reused as replacement fuel in sugar mill boilers and other uses.

2. MAIN FINDINGS AND MAIN CONCLUSIONS

The work carried out on the pilot plant pelleting machine

showed that pellets of typically up to 500 kg/m3 density could be

produced. However, difficulties were experienced with feeding

the light bagasse material and because of the lower than than

expected density obtainable in the feed chute throughputs at a

nominal roll surface speed were typically less than 70 per cent

of those predicted. It was found also that there were difficult­

ies in obtaining good scraper action of the nose plates of the

chutes between the rollers. This led to mechanical problems with

the scraping action of these chutes. Because of this the full

potential of the discharge chutes as a means of restraining

bagasse and increasing compaction could not be fully realised.

The work on energy of compaction established that over the

range of variables tested an increase in moisture content reduced

the compaction energy required; fine material required signific­

antly less energy for compaction than whole or coarse material

(coarse and whole bagasse however produced more durable pellets);

2.

and an increase in speed increased the compaction energy

required. This work also established that bagasse compression

energies ranged from 2.3 to 10.7 kJ/kg which was significantly-

less than typically 180 kj/kg required in operating a commer­

cially available machine.

One unexpected finding was the very high values of

coefficient of friction obtained for bagasse on steel or cast

iron. These high values of coefficient of friction were the

major reason for the operational problems that were experienced

with chutes of the experimental pelleting machine. The high

values of coefficient of friction influences the machine design

in two ways.

It means that significant compaction in the horizontal

direction could be developed within the chute. If this could

be combined with a high transverse pressure that there is

good potential for development of a three dimensional stress

situation in the chute with subsequent enhanced pellet

compaction.

It imposes significant design strength requirements on the

chutes and scrapers. Feeding problems experienced during the

tests did not allow the full potential of the unit to be

realised. Further work is required to understand the inter­

action between the geometry of the feed system and throughput

to be better understood so that rates can be predicted with

some degree of reliability.

3. WORK PROGRAMME DESCRIPTION

The work programme description is discussed in detail

under the different sections of the report. In brief it

consisted of:

UNIVERSITY OF QUEENSLAND COMPONENT

Further investigation of the influence of precompression and

temperatures on pellet density.

3.

. Investigation of energies of compaction for single and

multiple compressions under simulated rolling conditions.

. investigation of frictional characteristics of bagasse on

steel and cast iron surfaces.

. Preliminary investigation of rheological properties of

bagasse with particular emphasis on pressure relationships

under linear sinusoidal compression.

Preliminary investigation of a model for bagasse compression.

SUGAR RESEARCH INSTITUTE COMPONENT

. Design of the five roll pelleter and retention rig.

. Investigation of a range of parameters on throughput and

pellet densities achieved.

4. POTENTIAL FOR INDUSTRIAL APPLICATION

The fact that the unit as tested did not obtain the antic­

ipated production rate in no way reflects on the basic principle

of the machine. However, it is felt that more work is required

to understand the mechanics of the flow of bagasse through the

unit so that optimum operating geometries to maximise throughput

can be determined with confidence. At the present stage of

development, influences of small changes in feeding geometry on

throughput and performance cannot be accurately determined.

Towards the end of January preliminary discussions were

held with Walkers Limited of Maryborough and they expressed an

interest in the commercialisation of the machine. More detailed

discussions were scheduled for the middle of 1985 when the

project had been completed. Some more work is considered

necessary to further investigate some design aspects.

PART A: SUGAR RESEARCH COMPONENT

CHAPTER 1

INTRODUCTION

During the period 1979 to 1982 work was carried out at

Sugar Research Institute and at the University of Queensland,

Department of Mechanical Engineering on the NERDDC sponsored

project "Elimination of Use of Fuel Oil for Steam Generation in

the Sugar industry".

The University of Queensland work investigated the wide

range of variables that affect the pelleting characteristics of

bagasse. This work was the first basic study in the field and

quantified many aspects of the effect of temperature, moisture

content and particle size on the density of pellets produced and

their durability.

The work carried out at Sugar Research Institute

investigated a range of storage and compaction options suitable

for bagasse. The compaction options included baling and the use

of both reciprocating and rotating die pelleting machines. As a

result of these investigations it was established that whilst

pelleting of bagasse offered many advantages for storage -

principally bagasse density was increased by approximately eight

times - the existing commercial pelleting machines were too

expensive in initial cost and maintenance cost to allow pelleted

bagasse to compete economically with coal as a replacement fuel

for sugar factories (and potential other users).

The basic work at the University of Queensland established

that durable pellets could be produced over a range of operating

conditions but that the following conditions were typical of

those required for good, dense pellets.

6.

Moisture Approximately 10%

Temperature 80°C

Applied pressure 32 MPa

Hold time for applied pressure Between 1 and 8 seconds

Retention pressure (after applied pressure) 500 kpa

Retention time Between 1 and 8 minutes

An examination of how these conditions could be achieved in a

practical pelleting machine suggested the use of a basic rolling

device modelled on the principle of sugar milling.

The objectives of the present project were to further

examine the practicalities of a machine based on a rolling

principle. To this end a pilot plant machine was constructed at

Sugar Research Institute in Mackay and tested over a range of

operating conditions. The concept of the machine was to produce

slabs of compressed material typically 200 mm wide, 50 mm high

and continuous in length. It was believed that by using the

rolling action, considerable reduction in the energy required to

compress the material would be achieved and because of the

significant reduction in friction compared to existing pelleting

machines it was felt that this design of machine would be less

susceptible to die wear and therefore have significantly reduced

maintenance costs.

To supplement the practical work at Sugar Research

Institute, further detailed basic tests were planned at the

Mechanical Engineering Department of the University of Queensland

to investigate additional aspects of bagasse compaction, in

particular the factors affecting energy of compaction under

simulated rolling conditions. It was proposed also to further

investigate possible mechanisms to explain the compaction

phenomena observed for bagasse in an attempt to obtain more basic

data that could lead to a better design of rolling machine.

7.

CHAPTER 2

STAGES OF DEVELOPMENT

2.1 PRELIMINARY EXPERIMENTAL WORK

A series of preliminary tests were undertaken to

demonstrate the feasibility of pelleting bagasse using a

laboratory scale two roll cane crushing mill. Mechanical data on

the low and high compression characteristics of bagasse was

obtained for two typical moisture contents. This data coupled

with that from the University of Queensland work formed the basis

for the final design of the pelleting unit.

2.2 DESCRIPTION OF LABORATORY SCALE TWO ROLL INSTALLATION

The working unit consisted of two fixed rolls 474 mm mean

diameter, 203 mm wide, fed with an hydraulically operated ram

that pushed a lightly compressed block of bagasse along a

horizontal feed chute. Ram speed was controlled hydraulically

via a rotating servo-value so that its linear speed relative to

roll surface speed could be preset. The lightly compressed block

of bagasse of a given mass was prepared in a precompressor. This

apparatus enables the feed chute in the mill to be easily filled

as it imparted a tensile strength (albeit weak) to the block

during the compression process. The precompressor and the two

roll mill installation are shown diagrammatically in Figures 2.1

and 2.2.

2.3 PROCEDURE

The approach adopted was to obtain 'sighting' information

on the effect of bagasse moisture and nip compaction on roll

load, roll torque and resulting pellet density. Nip compaction

was varied by adjusting the quantity of bagasse used at the

precompressor and hence the mill feeder while keeping the work

opening between the rolls nominally constant. Table 2.1 shows

typical fibre masses required at discrete work openings to obtain

desired nip compactions.

FIGURE 2.2 Two ROLL MILL (DIAGRAMMATIC).

8 .

9.

TABLE 2.1 MASS OF FIBRE REQUIRED AT PARTICULAR WORK OPENINGS TO OBTAIN DESIRED NIP COMPACTION.

Two moisture contents were chosen for the test series - 10 and 20 per cent by weight. The desired moisture content was achieved by evaluating the average moisture content of a storage bag of bagasse and making up the difference to 10 or 20 per cent by misting the bagasse with water, mixing and leaving overnight before use in a test.

For each test the bagasse sample was lightly compressed in

the precompressor and the load deflection characteristics noted.

During a mill test which lasted about 18 seconds, roll load and

torque were sensed with appropriate transducers and the signals

recorded on an Esterline Angus three channel chart recorder.

2.4 RESULTS

Figures 2.3 and 2.4 show typical low pressure bagasse

compaction characteristics obtained with the precompressor

apparatus for a range of fibre masses. Pressure is shown plotted

against filling ratio which is defined as the no-void volume of

fibre divided by the actual volume. The no-void density of fibre

is assumed to be 1 53.0 kg/m3.

FIGURE 2.3 Low PRESSURE BAGASSE COMPACTION CHARACTERISTICS.

(10 per cent moisture bagasse).

FIGURE 2.4 LOW PRESSURE BAGASSE COMPACTION CHARACTERISTICS.

(20 per cent moisture bagasse).

1 0 .

The low pressure compression characteristics exhibited by bagasse at 10-20 per cent moisture is not unlike that of bagasse at higher moistures which in general give a trace which can be characterized by a polynomial of the form

p = A(Cf - B )2

where p = applied pressure kpa, Cf = filling ratio, (ratio of fibre density to no void

fibre density),

B = constant, namely the filling ratio of the sample at zero pressure, and

A = constant.

Over the range of tests conducted here, the low pressure compression characteristics of bagasse are not markedly affected by moisture content or sample weight.

The general shape of the roll load and roll torque traces measured for increasing sample weights of 20 per cent moist bagasse are shown in Figures 2.5 and 2.6. It can be seen that the traces exhibit a rapid increase in roll load and torque as the sample enters the nip of the mill. This is followed by a relatively flat top region where the sample is under maximum load followed by a sharp reduction in load as the sample exits the nip. Any irregularity in the flat top region is probably due to roll slip or feeding difficulties.

A summary of the resulting nip compaction, pellet density

and average torque and roll load data is given in Table 2.2.

The data are shown graphically in Figures 2.7 and 2.8.

For both roll load and torque, a linear relationship exists with

nip compaction. The lines of regression are based on the data

set with the three points having a nip compaction above 850 kg/m3

being treated as outliers. There is no evidence to suggest that

the relationships are not linear and at high roll loads and

torques it is extremely difficult to accurately calculate nip

F I G U R E 2.5 T Y P I C A L ROLL LOAD CURVES FOR 20 PER CENT M O I S T U R E

B A G A S S E .

FIGURE 2.6 TYPICAL ROLL TORQUE CURVES FOR 20 PER CENT MOISTURE

BAGASSE.

12.

300 400 500 600 700 800 900 1000 1100

NIP COMPACTION kg/cu m

F I G U R E 2*7 T H E E F F E C T O F N I P COMPACTION O N ROLL L O A D .

FIGURE 2.8 THE EFFECT OF NIP COMPACTION ON ROLL TORQUE.

14.

15.

compactions. Reference to Figures 2.5 and 2.6 also suggest the likelihood of roll slip on the blanket of bagasse which further complicates the accuracy of the nip compaction measurements. The best pellet density achieved in these preliminary tests was 522 kg/m3 at a nip compaction of 796 kg/m3, the average density was about 390 kg/m3 . At the time of completion of these tests, the results seemed quite promising. it was expected that an improvement in pellet density would result when the compressed mat of bagasse was held under load for a particular retention time as specified by the pelleting work carried out by the University of Queensland.

CHAPTER 3

THE PELLETING MILL

3.1 CONSTRUCTION

Functionally, the pelleting mill was based on the two roll

cane milling unit which was used in the initial feeding experi­

ments (see Chapter 2). For the pelleting mill, the hydraulic

pusher/feeder was removed and replaced with a three roll feeder

connected to the two roll mill by a pressure feeder chute. A

discharge chute and retention conveyor were designed for install­

ation after the main two roll mill. Figure 3.1 shows the funct­

ional layout of the complete pellet mill. Figure 3.2(a) shows

the completed pressure feeder and main mill assembly, while

Figure 3.2(b) shows the assembled retention conveyor. The values

for the major operation parameters and the dimensions of the

principal components are given in Table 3.1. The top main roll

and top pressure feed roll were supported in special strain

gauged bearing housings so that roll load could be measured. The

drive shafts to those roll were also instrumented to enable

torque to be measured. The pellet mill operated at a fixed

speed.

3.2 OPERATION AND MAJOR OPERATIONAL DIFFICULTIES

During each test, bagasse dried to the required moisture

level was fed manually onto a constant speed conveyor that moved

the bagasse up to the top of the feed chute, A levelling kicker

shaft was mounted above the conveyor to produce an even bagasse

height on the belt. Two bagasse sensors in the feed chute sensed

the height of the bagasse in the chute and switched the feed

conveyor on or off as required. Conventional conductivity

sensors do not adequately sense the presence of low moisture

bagasse. Hence two specially designed rotating arm sensors were

constructed which physically detected the presence of the bagasse

when the bagasse stopped them rotating.

1 9 .

FIGURE 3.2 THE COMPLETED PELLETING MILL.

(a) The pressure feeder and mill unit.

FIGURE 3.2 THE COMPLETED PELLETING M I L L .

(b) The retention conveyor.

20.

21.

TABLE 3.1 PELLET MILL COMPONENT SIZES.

(1) See Figure 3.1.

22.

The bagasse was dried in the Institute's experimental

bagasse dryer (Cullen and Mason, 1982) and stored in large

airtight plastic bags. When a warm bagasse feed was required,

the dryer and pellet mill were operated together. In general,

however, the bagasse was at ambient temperature. Bagasse dried

during both 1983 and 1984 seasons was used. No practical

operating differences between the bagasse from different years

was detectable in the results. In general, the very dry bagasse

(less than 10 per cent moisture) was from the 1983 season. This

bagasse had been stored in a dried condition for nearly one year.

The bagasse was drawn from the base of the feed chute and

passed through the pelleting mill. When reasonably dense pellets

were being made, a marked squeaking noise was produced in the

pressure feed chute (and delivery chute, if one was in use).

This noise is presumably associated with the usual slip/stick

motion of compressed bagasse over a metal surface.

The operation of the pellet mill was never entirely

problem free. The major difficulties involved poor feeding

behaviour, high frictional forces between the bagasse and the

stationary plates or chutes within the mill and problems with

scraper plate teeth.

Feeding Problems: It was very difficult to maintain a constant

flow through the mill under some conditions. The problem became

more severe with the very dry bagasses and as the feed height in

the feed chute decreased. The problem was exacerbated by the

very low density of dried bagasse and uneven filling of the feed

chute. The feed in the chute often contained voids or areas of

very low density on the "upper" surface of the inclined chute.

When these voids reached the pressure feed nip, there was often

insufficient feeding force to continue to push the bagasse into

the pressure feed chute, and bagasse flow would stop (i.e. the

rolls would slip on the bagasse). Considerable changes in mill

feed geometry (including an increase in underfeed roll size from

505 mm diameter to 750 mm and some weld roughening on the

pressure feed rolls) failed to cure the problem.

23 .

High Bagasse Friction Forces; The more fundamental experimental

work undertaken in the Mechanical Engineering Department,

Queensland University in parallel with this investigation, showed

that the coefficient of friction between dry bagasse and rust-

free, smooth steel is quite high and tends to increase as the

bearing pressure increases. This problem manifested itself in

the behaviour of the bagasse in the pressure feed and delivery

chutes.

In the pressure feed chute, if a test was performed with

the mill empty of bagasse at the start, the behaviour of the

bagasse changed noticeably as bagasse density increased (with a

corresponding decrease in bagasse speed along the chute). The

final (steady state) condition of the bagasse along the chute was

also quite sensitive to the details of the pressure feeder plate

shape. In early tests where there was quite marked convergence

toward the front of the pressure feed chute, the bagasse density

there built up to a level that required the use of a large wood

drill bit to remove it. Once the convergence was reduced to the

minimum required to enable the nose teeth on the plates to be

adequately fitted to the roll, the bagasse density decreased

markedly.

A further manifestation of the high friction forces

occurred when a short (100 mm) delivery chute was fitted to the

exit of the mill (no retention conveyor used). In this case, the

friction was sufficient to prevent the mill rolls from pushing

the bagasse through the delivery chute and caused the delivery

chute to be pushed away from the mill. Eventually, it caused the

top mill roll to "roll forward" on the bagasse blanket and

destroy both instrumented bearing housings.

Scraper Plate Teeth: It was very difficult to produce teeth on

the nose of the pressure feed and delivery chute plates that

would provide adequate scraping (groove cleaning) action and be

strong enough not to bend back under the action of the scraping

forces. The problem may have been due in part by the high

bagasse densities and large friction and pressure forces.

However the small size of the rig and the lack of ready access to

24.

fit the scraper teeth did not make the task any easier. The

final configuration for the nose of the pressure feed plates was

for chisel shaped (i.e. flat nosed) scrapers that fitted only the

top half of the grooving. On the delivery chute, the plates

finally used initially had no teeth but were fitted tightly to

the roll grooving so that when the roll was turned, the grooves

machined themselves into the leading edge of the delivery chute

plates.

The retention rig was used for a period in some of the

very early trials (late August 1984). However, the problems

outlined above, gradually became dominant and use of the delivery

chute and the retention rig was not achieved after that time.

3.3 EXPERIMENTAL RESULTS

The construction of the pelleting mill was completed

during August 1984 and initial commissioning runs undertaken

using the complete mill, including retention conveyor. As it

turned out, these were to be the only runs that were successfully

undertaken with the retention conveyor attached to the pelleting

mill.

The bagasse rate through the mill was below design

expectations. At the same time other feeding problems gradually

became apparent. Consequently an extended period was spent

investigating those problems and attempting to increase the feed

rate up to the design figure of 0.75 tonnes per hour. The main

effort concentrated on the feed rolls and feed chute area. At

one stage a larger (750 mm) diameter underfeed roll was fitted.

Unfortunately, this not only failed to increase the feed rate but

actually created additional feeding problems. Eventually the

original smaller (505 mm) diameter underfeed roll was placed back

into service and a decision made to investigate pelleting

performance with the five roll mill, initially without the

delivery chute and retention. These two items were to be added

as conditions permitted. Some tests were subsequently performed

with the delivery chute.

25.

3.3.1 Results with the Retention Conveyor in Use

The principal results for the test runs using the

retention conveyor are given in Table 3.2. It must be stressed

that these were intended to be commissioning tests. Hence the

results are not as complete and comprehensive as would be

desirable. However, they do show the effect of varying retention

time as illustrated in Figure 3.3.

FIGURE 3.3 VARIATION IN PELLET FIBRE DENSITY WITH RETENTION

TIME. (Retention Pressure 50 kPa)

Only one data point with "zero" retention time is

available. (This sample actually passed through the retention

conveyor, but no retention pressure was applied to it.) The

results suggest that when retention pressure is applied in the

(approximately) 90 seconds the bagasse pellet is in the retention

conveyor it has achieved about 60 to 70 per cent of the increase

TABLE 3.2 RESULTS WITH RETENTION RIG.

to the final density that it would have achieved if it had been

held in retention for an extended period (i.e. about 15 minutes).

The exponential curve shown on Figure 3.3 is placed there for

reference only. It is not a statistically produced curve of

"best" fit. It was chosen somewhat arbitrarily to pass through

the three points (375 kg/m3, 0 sec); (450 kg/m , 100 sec) and

(485 kg/m ; 1000 sec). It thus ignores the single data point at

2.3 x 10 seconds (64 hours).

No data are available for retention pressures other than

50 kPa and no attempts were made to optimise the setting of the

delivery chute. It is possible that the retention pressure in

the delivery chute dropped to near zero at times because no

attempt was made to optimise the chute settings. The University

of Queensland experimental data indicate the resultant pellet

density would be reduced under these circumstances.

3.3.2 Results of Final Test Series

Eight groups of test (covering a total of 52 individual

tests) were conducted in the final test programme. Brief details

of each ,group of tests will now be given. Detailed results are

tabulated in Appendix I.

Series 1 Tests

Tests A to H

These tests were conducted to ensure that the rig was

working adequately and were intended to be the last tests using

the one year old, very dry 1983 season bagasse. The majority of

the subsequent tests used the current season (1984) dried

bagasse. (The exception are those tests with bagasse moistures

below 10 per cent.) In practice, no difference was found between

results obtained with 1983 or 1984 bagasse.

Fibre feed rates and pellet densities from this series

were significantly above those obtained in the next few test

groupings.

Series 2 Tests

This series of tests was carried out to investigate a

series of variables on performance of the rolling machine.

Influence of Delivery Compaction on Pellet Fina1 Density

(Tests 1 - 7 )

This was a series of tests carried out at an average of

13.5 per cent moisture (range 10.6 to 18.1 per cent) in which the

delivery nip setting was gradually adjusted until delivery nip

fibre compactions were approximately equal to the nominal design

level of 1 100 kg/m3 . These tests were carried out without

retention and the relationship between delivery compaction and

pellet fibre density is shown in Figure 3.4.

Influence of Feed Bagasse Moisture on Pellet Final Density

(Tests 8 - 17)

These tests were designed to investigate a range of feed

bagasse moistures at nominally fixed delivery compaction.

Feeding problems again became apparent at low bagasse moistures.

The top and bottom surfaces of the pellet also began to adhere to

the rolls. When this adhesion became severe it was difficult to

take a pellet sample that was good enough to have its density

determined. During this test group, flat scrapers were fitted to

the delivery rolls 90 degrees around from the nip. These

scrapers ensured that no bagasse carried right around the roll

and reduced the effective nip opening on subsequent passage

through the nip. They also ensured that all bagasse passing

through the nip could be collected to enable accurate bagasse

rates to be determined. The influence of moisture content of

bagasse on average fibre density of the pellets for each test is

illustrated in Figure 3.5. Mean delivery compaction for these

tests was 1 020 kg/m with a delivery work opening of 13.5 mm.

FIGURE 3.4 RELATIONSHIP BETWEEN DELIVERY COMPACTION AND FINAL

PELLET DENSITY, AVERAGE 13.5% MOISTURE - No RETENTION.

FIGURE 3.5 RELATIONSHIP BETWEEN FEED BAGASSE MOISTURE AND FINAL

PELLET DENSITY - MEAN DELIVERY COMPACTION 1 020 KG/M3,

29.

30.

Influence of Delivery Nip Compaction on Pellet Final Density

(Tests 18 - 21)

A series of tests was carried out with increased delivery

nip compactions by reducing the delivery nip setting from 13.5 to

11.5 mm. Average delivery nip compactions increased from

1 020 kg/m3 to 1 330 kg/m3. The problem of the pellet adhering

to the roll became severe during these tests and the series was

therefore limited. The results obtained are shown in Figure 3.5

and show that the reduced setting had virtually no effect on

final pellet density.

Attempts to Overcome Roll Adhestion Problems

(Tests 22 - 27)

As a result of the severe roll adhesion problem, a

decision was made to fit a short (100 mm long) delivery chute in

an attempt to produce good quality pellets. The chute nose

setting was 30 mm with an exit setting of 31.5 mm. These

settings were chosen in an attempt to obtain pellet densities of

about 400 kg fibre/m3 . (For tests 19 and 21, where pellet

densities of about 365 kg fibre/m3 were achieved, the pellet

thickness averaged just under 35 mm. These tests used low

moisture (14 per cent and 7 per cent) bagasses. With higher

moistures, the pellets were thicker (e.g. Test 20, 23 per cent

moisture, 46 mm thick).

The nose of delivery chute plates was found to be turned

up after tests 23 and 25.

During these tests a problem occurred with the relative

movement of the top delivery roll and the associated

scraper/chute plates. This problem was eventually traced to the

failure (fracture) of the specially instrumented shaft bearing

housing. The bearing housing was subsequently repaired and made

'rigid'. Only the bottom plate of the delivery chute was used

for the remaining tests. The results of these tests are shown in

Figure 3.6.

31.

FIGURE 3.6 RESULTS OF TESTS WITH MODIFIED DISCHARGE CHUTE

FIGURE 3.7 RESULTS OF TESTS USING BAGASSE EX THE DRYER

32.

Influence of Temperature on Final Pellet Density

(Tests 28 - 37)

This was a group of tests intended to show up any effects

due to using warm bagasse (straight from the bagasse dryer)

rather than bagasse that had been dried for several weeks and was

at ambient temperature. However, the temperature losses between

the exit of the dryer and entry to the pellet mill vere larger

than expected and consequently, the average difference in pellet

temperature between the "cold" and "warm" bagasse was only 5

degrees Centigrade. There was no detectable difference in pellet

quality or density as illustrated in Figure 3.7. It was,

however, interesting to note that even the "cold" bagasse heated

up in its passage through the mill due to the amount of physical

work done on it. Thus "cold" bagasse feed temperatures were

generally about 31 degrees Centigrade but pellet temperatures up

to 39 degrees were recorded. Pellet temperatures increased as

each run progressed because the heat loss to the rolls and chute

plates decreased as they heated up during the test.

Investigations to Enhance Throughput

(Tests 38 - 46)

Following test 37, a major review of the experimental data

available at that time was undertaken and possible future work

reviewed. Three principal decisions were made.

(1) The first concerned further tests on the mill as it was then

being operated. It was noted that the early runs in the

test series (i.e. Series 1, Tests A to H) exhibited higher

fibre throughput rates and higher pellet densities than had

been achieved subsequently. Hence, this group of tests

(i.e. Tests 38 to 46) were conducted in an attempt to repeat

those higher values. The first part of the group (Tests 38

to 43) comprised pairs of tests in which runs with both high

and low feed chute levels were made on the same batch of

bagasse. Two of these tests with high feed chute levels

(see tests 41(a) and 42(a), Appendix 1) achieved fibre rates

above 500 kg/h which were quite close to the higher feed

33.

rates achieved in the Series 1 tests (see tests B, D and E).

However, pellet densities were nowhere near as high (280 to

320 kg fibre/m3 in tests 38 to 43 compared with 370 to 560

kg fibre/m3 in tests A to H).

In a final attempt to obtain the higher pellet densities,

the feed chute was changed to one that had divergence in

both cross-section dimensions from top to bottom. (The

chute used prior to this was nominally parallel in width.)

(2) The second decision was to check on the profile of the

divergence in the pressure feed chute and to adjust it to be

as nearly as possible the same as that which applied to the

Series 1 tests. For good scraping action, the teeth on the

nose of a scraper plate must be bent "into" the grooves so

that there must be a short convergent section. In the

Series 1 test situation, that convergent section was about

70 mm long and asymmetrical on the top and bottom plates.

Before the start of Series 2 tests, the plates were

"corrected" to give a smaller symmetric convergent length

(about 20 mm). For this final test group, the length of

convergence was again increased, although this time it was

nearly symmetric on top and bottom plates. It is awkward to

quantify that convergent-divergent shape because measure­

ments within the assembled pressure feed chute are very

difficult to make. The best indication of it was obtained

by placing a straight edge along the length of the plates.

For this final test group, the divergence from the straight

edge started 70 mm from the tip of the scraper nose and was

a maximum of about 10 mm at the nose.

The feed rates obtained in these tests (44 - 46) averaged

493 kg/m3 and final fibre densities averaged 354 kg/m3.

The results of all the tests performed are given in

Appendix 1. The major results are plotted in Figures 3.8 and

3.9. All data from this final test series are displayed on these

graphs.

FIGURE 3.8 VARIATION IN BAGASSE FEED FIBRE RATE WITH FEED

MOISTURE LEVEL

34.

FIGURE 3.9 VARIATION IN PELLET FIBRE DENSITY WITH PELLET

MOISTURE

35.

36.

Figure 3.8 plots the feed fibre rate against the moisture

content of the feed bagasse for all tests. It illustrates the

increasing feed rate as feed moisture decreases with a large

scatter with feed moistures below 10 per cent. The fibre rate

averaged over all tests was 410 kg/h. If the data had been

plotted as bagasse rate (rather than fibre rate), then above

about 10 per cent bagasse feed moisture, the feed rate was

approximately independent of moisture with an average of about

475 kg/h (range from 440 to 510 kg/h).

Figure 3.9 shows the variation in pellet

fibre density with pellet moisture content for

all tests. The linear regression line is shown (all available

data used). This has the

form

Pellet Fibre Density = 491.6 - 11.82 x (Pellet

Moisture (%))with r2 = 0.6193

and residual standard error = 45.94.

If a multiple regression of pellet density against pellet

moisture and delivery nip fibre compaction is carried out, the

resulting relationship is

Pellet Fibre Density = 428.3

+ 0.0660 x (delivery nip fibre

density)

- 13.46 x (pellet moisture)

with r2 = 0.6395

and residual standard error = 44.95.

This is not a very great improvement in precision compared to the

single regression. It indicates that a change of 100 kg fibre/m3

in the delivery nip density results in a 6.6 kg fibre/m3 change

in pellet fibre density under the pelleting conditions applying

to these tests. A ten per cent decrease in bagasse moisture

37.

results in an indicated 135 kg/m3 increase in fibre compaction.

(Note that in the single regression, a 10 per cent moisture

decrease only indicates a fibre density increase of 120 kg/m3).

39.

CHAPTER 4

ESTIMATION OF FORCES THROUGH ROLLERS AND CHUTES

4.1 INTRODUCTION

The results outlined in Chapter 3 have illustrated the

significant effect that the chute configurations have had on

performance of the pelleting machine. It was therefore decided

to undertake a fairly simplistic analysis of bagasse pressures

and frictional forces in the region of the delivery rolls and

delivery chute of the pelleting machine in an attempt to obtain a

better understanding of the influence of chute parameters on the

forces in the pellet and the subsequent pellet fibre density.

4.2 MODEL FOR HORIZONTAL PRESSURES

Computations of typical longitudinal bagasse pressures

attained during the pelleting experiments were made. The basis

of the calculations for the two roll mill was the relations found

in Chapter 2 of Murry and Holt (1967). These were developed for

the feeding region of a roll pair. These relationships are a

good approximation in the entry or feed region of such a roll

pair where the longitudinal pressures are low. The longitudinal

pressure however is significant in the region of the nip of the

rolls and the analysis of Murry and Holt is not generally

adequate because an analysis of the mechanics of rolling shows

that the phenomenon of "reabsorption" occurs. (The mechanism of

"reabsorption" is not fully understood and a detailed discussion

of it is beyond the scope of this exercise. However it is the

name used to describe the phenomena whereby more material passing

through the nip of a pair of rolls than is indicated by direct

geometric calculation.) Never-the-less, in this instance, the

theory of Murry and Holt was used partly because it was felt it

would give adequate precision for the purposes of this exercise

and also because an analysis of the data in Appendix I showed no

reabsorption was occurring except with the highest delivery

40.

compactions. (At a delivery work opening of 11.5 mm, average

delivery nip compaction 1 250 kg/m3, the average reabsorption

factor was only 1.09.)

On the assumption that the rolls can exert the full

frictional force on the bagasse during the rolling action it is

possible to calculate the maximum discharge pressure that can be

achieved from the roll pair. For the delivery chute, a one

dimensional chute theory was devised.

The purpose of these computations was to examine the

effects of varying some of the rig geometry parameters. Whilst

it is recognised that the model would overestimate pressures it

was felt that the models employed were sufficiently refined to at

least indicate major trends correctly.

None of the feeding configuration parameters was varied so

it was necessary only to examine the two roll mill itself and the

associated exit chute. A bagasse pressure zero was assumed at

the plane of the feed entry angle (30°) of the delivery rolls.

Computations were based on whole bagasse of 10 per cent moisture

at 20°C. Two values of mill work opening were considered

corresponding to delivery compactions of 1 100 and 900 kg/m3.

Three values of delivery angle (7, 12 and 15 degrees) and three

values of exit chute length were considered. The compression

characteristics of bagasse were those established during

sinusoidal tests described in Chapter 5 of the University of

Queensland component of the report. Peak pressures at 1 100 and

900 kg/m3 compaction were 37.4 and 16 MPa respectively.

Frictional data were those obtained during tests described in

Chapter 7 of the University of Queensland component of the

report.

Based on these data the results shown in Figures 4.1 and

4.2 were obtained. Figure 4.1 illustrates the very significant

effect that delivery compaction (and work opening) has on maximum

discharge pressure. As was anticipated, the predicted bagasse

pressures were higher for the smaller work opening. It is of

interest that the discharge pressure of the mill fell with rising

FIGURE 4.2 RELATIONSHIP BETWEEN LONGITUDINAL PRESSURE AT CHUTE

EXIT AND CHUTE LENGTH FOR TWO WORK OPENINGS AND A RANGE

OF DELIVERY ANGLES.

FIGURE 4.1 RELATIONSHIP BETWEEN LONGITUDINAL PRESSURE AT CHUTE

EXIT (CHUTE LENGTH ZERO) AND DELIVERY ANGLE OF

PELLETING ROLLS.

41.

42.

delivery angle. It must be remembered however that the

assumption of full friction always acting is made in the

representation of the mill. The longtudinal force of the bagasse

must thus always increase with the delivery angle. The

increasing delivery area means that there is actually a drop in

the discharge pressure as the delivery angle is raised in this

case. The effect of the chute length is seen to be a pressure

drop which increases with chute length as illustrated in Figure

4.2.

During the tests carried out with the pelleting plant it

was found that "reabsorption" was not occurring except at high

levels of delivery compaction (11.5 mm delivery work opening).

When horizontal pressures were calculated for the 11.5 mm work

opening as outlined above (i.e. ignoring reabsorption) a

discharge pressure of 197 MPa was predicted. The discharge

pressure of the mill was also calculated with acknowledgement of

the occurrence of reabsorption. It was assumed that the bagasse

flowed forward over the roll surfaces from the neutral plane to

one half of the angle of the neutral plane past the nip of the

mill. A constant pressure of that at the neutral plane and a

change in sign of the coefficient of friction were used to

represent this extrusion type process. The original model was

reverted to for the region from the estimated end of the

reabsorption to the delivery angle. In this manner, a discharge

pressure of only 42 MPa was predicted.

4.3 DISCUSSION

The rather simplistic approach to examination of the

potential horizontal forces developed in the rolling pelleting

machine has illustrated the sensitivity of the rolling unit to

chute configuration, delivery roll compaction, and potential

effects of "reabsorption". The analysis helps to qualitatively

explain some of the variation in compaction and throughput

obtained but offers little information on how to predict better

discharge densities. The analysis has illustrated the high

values of horizontal pressures developed relative to the vertical

pressure applied by the roller.

43.

CHAPTER 5

CONCLUSIONS

The design concept for the pelleting machine envisaged a

capacity for the pilot plant pelleter at its nominal design speed

of 36 mm/s of 0.75 tonne fibre per hour producing pellets at a

final fibre density of at least 500 kg/m3.

Tests on the machine that was constructed have shown that

stable pellets can be produced but that the best performance

under test conditions fall short of the design estimates. In the

tests carried out the maximum throughput achieved was 540 kg/m3

and maximum pellet final fibre densities was 530 kg/m3.

The sensitivity of the quality of pellets produced to the

moisture content of bagasse established in previous batch testing

was confirmed in the rolling pilot plant. In practice it would

not be practical to attempt to pellet by rolling, material with a

moisture content greater than 15 per cent, if high densities and

fibre rates were required.

Whilst the pilot plant pelleter has been able to produce

acceptable pellets the limiting factor in its ability to achieve

its nominal design capacity has been the inability to establish

the conditions to feed the low density dry bagasse at an accept­

able rate, to accommodate the high bagasse friction forces in the

pressure feed chute, and to optimise the geometry of the

discharge chute. The University of Queensland tests established

that high values of coefficient of friction existed for dry

bagasse and consequently high horizontal forces could be

developed. Whilst these forces can enhance the pellet density

further work is required to establish an effective nose plate

configuration which gives better removal of the bagasse from the

rollers.

Before the design can be fully commercialised further work

is required to optimise chute configurations in conjunction with

44.

retention. It is proposed to explore, in conjunction with a

potential manufacturer, the possibility of obtaining an

Australian Industrial Research and Development Incentives Board

project grant for this further development work.

45.

REFERENCES

Cullen, R.N. and Mason, V. (1982). Final Report. Elimination of

Use of Fuel Oil for Steam Generation in the Sugar Industry -

Part 1.

Murry, C.R. and Holt, J.E. (1967). The Mechanics of Crushing

Sugar Cane. Elsevier.

47.

APPENDIX I

EXPERIMENTAL RESULTS

Table I.1 lists all results obtained during the final test

series. Details of the test groupings within this series of

results are given in Section 3.3.2, and the major graphical

results are presented in Figures 3.8 and 3.9.