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The briquetting of wheat straw
J. agric. Engng Res. (I 977) 22, 105-l 11
The Briquetting
of Wheat
Straw
I.E. SMITH*; S. D. PROBERT*; R. E. STOKES*; R. J. HANSFORD* Limited tests have been carried out which demonstrate that wheat straw can be compressed to a relative density greater than unity, and stabilized at that density without binding or other mechanical aids. This represents a reduction in the volume of the material by an order of magnitude, so the straw could be transported and stored more economically than is possible at present. In order to achieve stability the application of heat as well as pressure is necessary. However the total energy input to the process is only about 6% of the calorific value of the material. 1.
Introduction
During 1973, 3.6 x lo6 tonnes of cereal straw surplus to requirements were produced in England and Wales.’ The disposal of this straw in the fields is achieved either by burning or sometimes by ploughing it back into the soil. Because the calorific value of straw is about one-half that of high-grade coal, the surplus straw is equivalent to about 1.8 million tonnes of coal equivalent or nearly 2 % of the primary energy consumed by all the electricity-generating stations in the U.K. The straw stores solar energy acquired during the growing season, and in contrast to fossil fuels, it represents an annually renewable source of energy. However, to consider this straw solely as a fuel would be wrong. Other more profitable uses already exist, such as incorporation after processing into animal feedstuffs, and there are possibilities of using it either (a) for paper making or (b) in pressed board form as an internal thermal insulant, or (c) by pyrolysis, hydrolysis or fermentation to liquid and gaseous fuels or other chemicals2 Whatever further use is contemplated for straw, its processing or treatment should be carried out on a continuous basis throughout the year if the best return on the capital invested in processing facilities is to be achieved. However as straw is harvested in the U.K. for only two months of the year, storage facilities for periods of up to ten months then become necessary. The density of conventional bales is about 120 kg m-3, and because the cost of weatherproof storage is about &l per cubic metre, it can be shown that the cost of storing would halve the value of the straw, considered on the basis of a fuel. A further penalty arising from the low density of the bales is the high cost of their transport, since the volume limitation of carriers is reached before the load limit. Road transport vehicles and railway trucks could be loaded to capacity if the baling density were to be increased respectively by factors of two and four .* Such compactions would reduce the transportation costs appropriately. 2.
Previous studies of crop compaction
Although it has been recognized that compaction of crops such as grass have storage and handling advantages, the end use for these products has generally been as animal forage where too high density may render the material unsuitable. Bruhr? suggested a maximum density of 300 to 500 kg m-s, and it is towards a figure within this range that most research has been devoted. Many of the studies have used piston-and-die type presses in which the pressure is either developed against the closed end of the die, or as the result of friction between the material and the wall of the die. Reece4 showed that by heating the material to between 60°C and 70°C a more stable product with a lower recovery of original dimensions could be obtained than was *School of Mechanical Engineering, Cranfield Institute of Technology, Cranfield, Received 14 November 1975; accepted in revised form 26 October 1916
105
Bedford
106
BRIQUETTlNG
OF
WHEAT
STRAW
possible with unheated material. From a series of tests with lucerne and bermuda grass over a temperature range of 32°C to 165”C, Hall and Hall’ found that the higher the temperature, the lower the pressure needed to provide a given degree of compaction. They also concluded that grass having a relatively high moisture content could be stably compacted at an elevated temperature, whereas this was not possible at ambient conditions. The optimal moisture content to facilitate stable compaction was found to lie between 16% and 23 “i, by weight (w.b.). Rehkugler,6 Reece“ and Pickard’ considered that the protein content of the grass was primarily responsible for bonding and stabilization, and suggested that one effect of the load was to squeeze the protein and pectins out of the stem and leaf walls which subsequently acted as the bonding agent. The compaction and stabilization of straw may have a mechanism that differs from that occurring during the compaction of grass. In the first place the proportion of leaf is considerably smaller, and secondly the moisture content is usually lower than that of grass because straw is a dead rather than a living material. The purpose of the present study was therefore, to determine to what extent straw could be stably compacted and to identify those factors responsible for the stability. 3. Experimental procedure Only wheat straw was investigated and the bales were taken from a single field. The initial experiments were carried out using a simple closed-end die press which could be loaded, locked in position and then heated. The diameter of the die was 26 mm and it had a capacity of 20 g of chopped straw. Fresh wheat straw having a moisture content of 15 “/, by weight (w.b.), was employed. As the heat was applied externally by means of an electrical element surrounding the die, time was required for heat to penetrate to the centre of the sample. After a period of 20 minutes the centre temperature rise measured by means of a thermocouple, was 90% of that at the surface and this heating period was used throughout the test series. In order to assess the stability of the briquetted straw, measurements of the length were taken immediately on removal from the die, after one week of exposure to the atmosphere and again after eight weeks exposure. The durability of the briquettes was determined by a modified version of the A.S.A.E. standard test* which was intended to assess the durability of “pellets and crumbles”. The rig consisted of a box 230 mm x 230 mm x 110 mm which was rotated about an axis lying normal to the centre of the square side for 10 min at 50 rev/min. In operation it was filled with 10 test briquettes and the durability rating was expressed as the percentage of the original mass of the briquettes remaining as agglomerates after tumbling. Thus a durability figure of 100 implies that no disintegration occurred. The principal variables studied were the temperature of the straw and the pressure applied, but in addition the influence of moisture content and the presence of soluble wax were examined. 4. Compression experiments The effect of applied load on straw heated to a temperature of 80°C is shown in Fig. I with the ordinate indicating both briquette length and relative density. Each column represents the average of three tests. It must be emphasized that length was the only dimension change during the compression and subsequent expansion process, the briquette diameter remaining invariant at all times. The final density tended to an asymptotic value of around 1250 kg m-3 at very high loadings. The recovery length with the load removed, both in the short and long term, increased with increasing initial load. In order to examine the effect of temperature, the load was standardized at 56.3 MN m-2 and the die heated to various temperatures for 20 min. The average result of 3 tests at each temperature is shown in Fig. 2, from which it will be observed that not only does an increase in temperature lead to a greater degree of compaction, but that
I.
E.
SMITH
ET
107
AL.
the length recovery of the briquette is less when the initial applied temperature was in the range 90°C to 140°C. Thus in order to compact to a given density there is some interchangeability between temperature and load, although the higher the temperature (within the limits examined) the greater will be the stability of the briquette. It will also be observed from the number at the head of each column in Fig. I that the durability of all specimens produced was high, only falling to 90:/, when an exceedingly high load was employed. 41 40 39 36 37 p
36
L
35
5
34
5
33
f
32
_$
31
aI
30 29 26 27 26
:: 18.
AmI pressureduring heatmg
Fig. 1. Effect of axial die pressure
-
(MN m-?
upon the expansiotl
qf‘straw
briquettes
33 f
32
z
31
$ L30 5 29 _p
28
m
27 26 25 60
65
70
60
85
90
Die temperature
Fig. 2. Effect of die temperature
loo
II0
120
I30
140
(“0
on the expansiotl
of straw briquettes
That the stability was enhanced by employing a temperature in excess of 80°C during compression led to the suggestion that the thin layers of wax which surround the stems of the straw fibres might be responsible-its melting and subsequent solidification serving to provide adhesion between individual fibres. To examine this, a sample of straw was carefully de-waxed by refluxing in a benzene/ethanol azeotrope until no further extraction occurred. Because this treatment also removed most of the water from the straw, the sample was carefully re-hydrated until it regained its original moisture content of 15% by weight. The result, as may be seen from the second column of Fig. 3 again demonstrated that the removal of wax (approximately 2 % of total weight) made a negligible difference to the bonding of the sample. In order to determine whether moisture was responsible for the stabilization, samples of straw were carefully dried according to the A.S.A.E. method8 and compressed at 80°C. The third and fourth columns of Fig. 3 indicate
108
RRIQUETTIN(i
OF
WHEAT
STRAW
0.8
I.0 I.2
C = $
1.4
s g
I.6
-$
IO
0
Fig. 3. Results of tests on virgin and treated straw (BO”C, 56 MN m-‘)
Furthermore, convincingly the beneficial effect of moisture upon the stability at this temperature. by taking samples which had been dried before compression, adding moisture, and then recomof the samples pressing them, stable briquettes were obtained (column 5, Fig. 3). Photographs are shown in Fig. 4. Although these tests showed that the moisture content was critical in influencing the stability at temperatures below lOO”C, it could only contribute at temperatures above this if the steam given off during the heating cycle affected the stabilization. Accordingly tests were carried out on completely dried straw with the standard load of 56.3 MN m -~2 at higher temperatures. The results in Fig. 5 show that the subsequent recovery in briquette length is very much greater at temperatures of 90°C to 105°C than for straw having a normal water content (see Fig. 2), but that as the temperature was increased above 110°C the initial moisture content had little influence on the stability.
Fig. 4. (a) Wax and water removed:
(b) wax removed, rehydrated and recompressed
to 14%:
(c) wafer only removed:
(d) rehydrated
I. E. SMITH
109
ET AL.
At temperatures above 110°C some discoloration of the briquettes was observed and at the It is possible that the products of this highest t.emperatures some surface charring ensued. chemical degradation were responsible for the stabilization of the material, although this has not yet been verified.
50 7 2
I
Water
only
removed
’ 150°C
12OT
110°C
105T
IOOT
90°C
40
f P : z s P l%
30
20
IO
n
Fig. 5. Resucs qfbriquetting tests on dried straw (axial die pressure ~~ 56.3 MN m-‘)
5.
Extrusion of chopped straw
To eliminate the long heating and cooling times when the sample was under load, some preliminary tests were carried out using a Glomera Type 156/412 reciprocating extrusion press. This machine had a 50 mm bore and a stroke of 400 mm. The straw was fed by means of an auger into the breech of the press, restraint being effected by means of an adjustable water-cooled, tapered die. For tests on heated straw, the material was heated separately in a humidified oven and then was transferred as rapidly as possible to the press. Because cooling ensued during transfer from the oven, the temperatures were monitored by means of thermocouples inserted in the straw as it was fed to the breech. With a throughput of 75 kg h-l and a relative density of about 1.22, the period of residence for the material loaded in the press was about 53 s, as compared with 40 min in the previous experimental series. Tests were carried out on samples having five different pre-compaction treatments described in Table 1. Jt is concluded that straw can be satisfactorily compacted to a relative density exceeding unity with or without the application of heat. However, even at the relatively modest temperature of 75°C the durability of the briquette was considerably enhanced as compared with those TABLE1 Average results of tests on extruded straw briquettes
I Description of sample
Straw heated in humid atmosphere at 75°C prior to compression 15 % moisture straw Moisture-reduced straw Dried straw Dried straw
I Temperature (“C) at which compression occurred
I Percentage During compaction
moisture content At durability test
Relative density
Durability rating
75
12
10.5
1.19
90
21 21 21 120
15 10 0 0
12
1.20 1.13 0.80 0.70
81 73 0 0
9 0
110
BRIQUETTING
OF
WHEAT
STRAW
produced at room temperature. Furthermore the importance of a reasonable moisture content in achieving stability is clearly evident. The result obtained for dried straw, pre-heated to 12O”C, which had a low density and a zero durability rating appears to be at a variance with that shown by Fig. 5, column 2. An explanation may lie in the shorter high temperature residence period in the continuous process, but further investigation is clearly required. 6.
Re-constitution
of straw briquettes
Samples of briquettes produced both in the stationary die press and the extrusion press were stored for a period of eighteen months in the laboratory where the ambient temperature averaged about 17°C and the relative humidity 50 “/, during which period they were apparently unchanged. However, upon immersion in water, they underwent a rapid swelling and disintegrated completely within a few minutes. The resultant product, after drying, resembled the original straw in every way, even regaining its original tubular structure. Fig. 6 illustrates a sample of “reconstituted” straw alongside the briquette from which it originated. This abilitv to reconstitute mav move to be of value if the use of the material as a feedstuff although it does Lindicate that the compressed straw must be shielded from is c:ontemplaied dri\ /ing rain during transport and storage.
Fig. 6
7.
Conclusions
Tests have shown that wheat straw can be compressed and stabilized to a density of the order of ten times that of normal bales by the application of pressures of between 20-60 MN m-2 after heating to a temperature of between 80°C to 140°C. The material in this compressed form is resistant to attrition, but may be reconstituted to its original form by soaking in water. The possibility of stabilizing straw at these high densities opens up the possibility of utilizing it as an animal feedstuff or as an industrial material since the economics of handling, transport and storage are vastly improved.
I.
E.
SMITH
ET
III
AL.
Although the mechanism of stabilization has been shown to depend upon the presence of moisture within the straw, further work is required in order to obtain a complete understanding of the process. Acknowledgements The authors would like to express their thanks to the National Lnstitute of Agricultural Engineering and the Grassland Research Institute for the loan of the Glomera extrusion press, and for assistance in operating it. REFERENCES ’
N.F.U. Working Party on the Use and Disposal of Straw. Report on rhe use and disposal of sfruw.
Cycle: 1186/95/73, T.M.66 * Smith, D. L. 0.; Rutherford, I.; Radley, R. W. The use of energy in ugricufrure: straw productron. Paper presented to the Institute of Agricultural Engineers, 1973 ’ Bruhn, W. D.; Zummerman, A.; Neidermeier, R. P. Developments in pelleting forage crops. Agric. Engng, 1959 40 442446 A Reece, F. N. Temperature, pressure and time relationships in forming dense hay wafers. Trans. A.S.A.E., 1966 9 749-751 ’ Hall, G. E.; Hall, C. W. Heated die wafer formation of alfalfa and Bermuda grass. Trans. A.S.A.E., 1968 11 578-581 6 Rehkugler, G. E.; Buchele, W. F. Bio-mechanics offorage wafering. Trans. A.S.A.E., 1969 12 1-8 ’ Pickard, G. E.; Roll, W. M.; Ramser, J. H. Fundamentals of hay wufering. Trans. A.S.A.E., 1961 4 65-68 and Methods for determining Specific Weight, Durability ’ Wafers, Pellets and Crumbles-Definitions and Moisture Content. Agricultural Engineers Yearbook, 1970. Publ. A.S.A.E., 1970 306-308
The Briquetting
of Wheat
Straw
I.E. SMITH*; S. D. PROBERT*; R. E. STOKES*; R. J. HANSFORD* Limited tests have been carried out which demonstrate that wheat straw can be compressed to a relative density greater than unity, and stabilized at that density without binding or other mechanical aids. This represents a reduction in the volume of the material by an order of magnitude, so the straw could be transported and stored more economically than is possible at present. In order to achieve stability the application of heat as well as pressure is necessary. However the total energy input to the process is only about 6% of the calorific value of the material. 1.
Introduction
During 1973, 3.6 x lo6 tonnes of cereal straw surplus to requirements were produced in England and Wales.’ The disposal of this straw in the fields is achieved either by burning or sometimes by ploughing it back into the soil. Because the calorific value of straw is about one-half that of high-grade coal, the surplus straw is equivalent to about 1.8 million tonnes of coal equivalent or nearly 2 % of the primary energy consumed by all the electricity-generating stations in the U.K. The straw stores solar energy acquired during the growing season, and in contrast to fossil fuels, it represents an annually renewable source of energy. However, to consider this straw solely as a fuel would be wrong. Other more profitable uses already exist, such as incorporation after processing into animal feedstuffs, and there are possibilities of using it either (a) for paper making or (b) in pressed board form as an internal thermal insulant, or (c) by pyrolysis, hydrolysis or fermentation to liquid and gaseous fuels or other chemicals2 Whatever further use is contemplated for straw, its processing or treatment should be carried out on a continuous basis throughout the year if the best return on the capital invested in processing facilities is to be achieved. However as straw is harvested in the U.K. for only two months of the year, storage facilities for periods of up to ten months then become necessary. The density of conventional bales is about 120 kg m-3, and because the cost of weatherproof storage is about &l per cubic metre, it can be shown that the cost of storing would halve the value of the straw, considered on the basis of a fuel. A further penalty arising from the low density of the bales is the high cost of their transport, since the volume limitation of carriers is reached before the load limit. Road transport vehicles and railway trucks could be loaded to capacity if the baling density were to be increased respectively by factors of two and four .* Such compactions would reduce the transportation costs appropriately. 2.
Previous studies of crop compaction
Although it has been recognized that compaction of crops such as grass have storage and handling advantages, the end use for these products has generally been as animal forage where too high density may render the material unsuitable. Bruhr? suggested a maximum density of 300 to 500 kg m-s, and it is towards a figure within this range that most research has been devoted. Many of the studies have used piston-and-die type presses in which the pressure is either developed against the closed end of the die, or as the result of friction between the material and the wall of the die. Reece4 showed that by heating the material to between 60°C and 70°C a more stable product with a lower recovery of original dimensions could be obtained than was *School of Mechanical Engineering, Cranfield Institute of Technology, Cranfield, Received 14 November 1975; accepted in revised form 26 October 1916
105
Bedford
106
BRIQUETTlNG
OF
WHEAT
STRAW
possible with unheated material. From a series of tests with lucerne and bermuda grass over a temperature range of 32°C to 165”C, Hall and Hall’ found that the higher the temperature, the lower the pressure needed to provide a given degree of compaction. They also concluded that grass having a relatively high moisture content could be stably compacted at an elevated temperature, whereas this was not possible at ambient conditions. The optimal moisture content to facilitate stable compaction was found to lie between 16% and 23 “i, by weight (w.b.). Rehkugler,6 Reece“ and Pickard’ considered that the protein content of the grass was primarily responsible for bonding and stabilization, and suggested that one effect of the load was to squeeze the protein and pectins out of the stem and leaf walls which subsequently acted as the bonding agent. The compaction and stabilization of straw may have a mechanism that differs from that occurring during the compaction of grass. In the first place the proportion of leaf is considerably smaller, and secondly the moisture content is usually lower than that of grass because straw is a dead rather than a living material. The purpose of the present study was therefore, to determine to what extent straw could be stably compacted and to identify those factors responsible for the stability. 3. Experimental procedure Only wheat straw was investigated and the bales were taken from a single field. The initial experiments were carried out using a simple closed-end die press which could be loaded, locked in position and then heated. The diameter of the die was 26 mm and it had a capacity of 20 g of chopped straw. Fresh wheat straw having a moisture content of 15 “/, by weight (w.b.), was employed. As the heat was applied externally by means of an electrical element surrounding the die, time was required for heat to penetrate to the centre of the sample. After a period of 20 minutes the centre temperature rise measured by means of a thermocouple, was 90% of that at the surface and this heating period was used throughout the test series. In order to assess the stability of the briquetted straw, measurements of the length were taken immediately on removal from the die, after one week of exposure to the atmosphere and again after eight weeks exposure. The durability of the briquettes was determined by a modified version of the A.S.A.E. standard test* which was intended to assess the durability of “pellets and crumbles”. The rig consisted of a box 230 mm x 230 mm x 110 mm which was rotated about an axis lying normal to the centre of the square side for 10 min at 50 rev/min. In operation it was filled with 10 test briquettes and the durability rating was expressed as the percentage of the original mass of the briquettes remaining as agglomerates after tumbling. Thus a durability figure of 100 implies that no disintegration occurred. The principal variables studied were the temperature of the straw and the pressure applied, but in addition the influence of moisture content and the presence of soluble wax were examined. 4. Compression experiments The effect of applied load on straw heated to a temperature of 80°C is shown in Fig. I with the ordinate indicating both briquette length and relative density. Each column represents the average of three tests. It must be emphasized that length was the only dimension change during the compression and subsequent expansion process, the briquette diameter remaining invariant at all times. The final density tended to an asymptotic value of around 1250 kg m-3 at very high loadings. The recovery length with the load removed, both in the short and long term, increased with increasing initial load. In order to examine the effect of temperature, the load was standardized at 56.3 MN m-2 and the die heated to various temperatures for 20 min. The average result of 3 tests at each temperature is shown in Fig. 2, from which it will be observed that not only does an increase in temperature lead to a greater degree of compaction, but that
I.
E.
SMITH
ET
107
AL.
the length recovery of the briquette is less when the initial applied temperature was in the range 90°C to 140°C. Thus in order to compact to a given density there is some interchangeability between temperature and load, although the higher the temperature (within the limits examined) the greater will be the stability of the briquette. It will also be observed from the number at the head of each column in Fig. I that the durability of all specimens produced was high, only falling to 90:/, when an exceedingly high load was employed. 41 40 39 36 37 p
36
L
35
5
34
5
33
f
32
_$
31
aI
30 29 26 27 26
:: 18.
AmI pressureduring heatmg
Fig. 1. Effect of axial die pressure
-
(MN m-?
upon the expansiotl
qf‘straw
briquettes
33 f
32
z
31
$ L30 5 29 _p
28
m
27 26 25 60
65
70
60
85
90
Die temperature
Fig. 2. Effect of die temperature
loo
II0
120
I30
140
(“0
on the expansiotl
of straw briquettes
That the stability was enhanced by employing a temperature in excess of 80°C during compression led to the suggestion that the thin layers of wax which surround the stems of the straw fibres might be responsible-its melting and subsequent solidification serving to provide adhesion between individual fibres. To examine this, a sample of straw was carefully de-waxed by refluxing in a benzene/ethanol azeotrope until no further extraction occurred. Because this treatment also removed most of the water from the straw, the sample was carefully re-hydrated until it regained its original moisture content of 15% by weight. The result, as may be seen from the second column of Fig. 3 again demonstrated that the removal of wax (approximately 2 % of total weight) made a negligible difference to the bonding of the sample. In order to determine whether moisture was responsible for the stabilization, samples of straw were carefully dried according to the A.S.A.E. method8 and compressed at 80°C. The third and fourth columns of Fig. 3 indicate
108
RRIQUETTIN(i
OF
WHEAT
STRAW
0.8
I.0 I.2
C = $
1.4
s g
I.6
-$
IO
0
Fig. 3. Results of tests on virgin and treated straw (BO”C, 56 MN m-‘)
Furthermore, convincingly the beneficial effect of moisture upon the stability at this temperature. by taking samples which had been dried before compression, adding moisture, and then recomof the samples pressing them, stable briquettes were obtained (column 5, Fig. 3). Photographs are shown in Fig. 4. Although these tests showed that the moisture content was critical in influencing the stability at temperatures below lOO”C, it could only contribute at temperatures above this if the steam given off during the heating cycle affected the stabilization. Accordingly tests were carried out on completely dried straw with the standard load of 56.3 MN m -~2 at higher temperatures. The results in Fig. 5 show that the subsequent recovery in briquette length is very much greater at temperatures of 90°C to 105°C than for straw having a normal water content (see Fig. 2), but that as the temperature was increased above 110°C the initial moisture content had little influence on the stability.
Fig. 4. (a) Wax and water removed:
(b) wax removed, rehydrated and recompressed
to 14%:
(c) wafer only removed:
(d) rehydrated
I. E. SMITH
109
ET AL.
At temperatures above 110°C some discoloration of the briquettes was observed and at the It is possible that the products of this highest t.emperatures some surface charring ensued. chemical degradation were responsible for the stabilization of the material, although this has not yet been verified.
50 7 2
I
Water
only
removed
’ 150°C
12OT
110°C
105T
IOOT
90°C
40
f P : z s P l%
30
20
IO
n
Fig. 5. Resucs qfbriquetting tests on dried straw (axial die pressure ~~ 56.3 MN m-‘)
5.
Extrusion of chopped straw
To eliminate the long heating and cooling times when the sample was under load, some preliminary tests were carried out using a Glomera Type 156/412 reciprocating extrusion press. This machine had a 50 mm bore and a stroke of 400 mm. The straw was fed by means of an auger into the breech of the press, restraint being effected by means of an adjustable water-cooled, tapered die. For tests on heated straw, the material was heated separately in a humidified oven and then was transferred as rapidly as possible to the press. Because cooling ensued during transfer from the oven, the temperatures were monitored by means of thermocouples inserted in the straw as it was fed to the breech. With a throughput of 75 kg h-l and a relative density of about 1.22, the period of residence for the material loaded in the press was about 53 s, as compared with 40 min in the previous experimental series. Tests were carried out on samples having five different pre-compaction treatments described in Table 1. Jt is concluded that straw can be satisfactorily compacted to a relative density exceeding unity with or without the application of heat. However, even at the relatively modest temperature of 75°C the durability of the briquette was considerably enhanced as compared with those TABLE1 Average results of tests on extruded straw briquettes
I Description of sample
Straw heated in humid atmosphere at 75°C prior to compression 15 % moisture straw Moisture-reduced straw Dried straw Dried straw
I Temperature (“C) at which compression occurred
I Percentage During compaction
moisture content At durability test
Relative density
Durability rating
75
12
10.5
1.19
90
21 21 21 120
15 10 0 0
12
1.20 1.13 0.80 0.70
81 73 0 0
9 0
110
BRIQUETTING
OF
WHEAT
STRAW
produced at room temperature. Furthermore the importance of a reasonable moisture content in achieving stability is clearly evident. The result obtained for dried straw, pre-heated to 12O”C, which had a low density and a zero durability rating appears to be at a variance with that shown by Fig. 5, column 2. An explanation may lie in the shorter high temperature residence period in the continuous process, but further investigation is clearly required. 6.
Re-constitution
of straw briquettes
Samples of briquettes produced both in the stationary die press and the extrusion press were stored for a period of eighteen months in the laboratory where the ambient temperature averaged about 17°C and the relative humidity 50 “/, during which period they were apparently unchanged. However, upon immersion in water, they underwent a rapid swelling and disintegrated completely within a few minutes. The resultant product, after drying, resembled the original straw in every way, even regaining its original tubular structure. Fig. 6 illustrates a sample of “reconstituted” straw alongside the briquette from which it originated. This abilitv to reconstitute mav move to be of value if the use of the material as a feedstuff although it does Lindicate that the compressed straw must be shielded from is c:ontemplaied dri\ /ing rain during transport and storage.
Fig. 6
7.
Conclusions
Tests have shown that wheat straw can be compressed and stabilized to a density of the order of ten times that of normal bales by the application of pressures of between 20-60 MN m-2 after heating to a temperature of between 80°C to 140°C. The material in this compressed form is resistant to attrition, but may be reconstituted to its original form by soaking in water. The possibility of stabilizing straw at these high densities opens up the possibility of utilizing it as an animal feedstuff or as an industrial material since the economics of handling, transport and storage are vastly improved.
I.
E.
SMITH
ET
III
AL.
Although the mechanism of stabilization has been shown to depend upon the presence of moisture within the straw, further work is required in order to obtain a complete understanding of the process. Acknowledgements The authors would like to express their thanks to the National Lnstitute of Agricultural Engineering and the Grassland Research Institute for the loan of the Glomera extrusion press, and for assistance in operating it. REFERENCES ’
N.F.U. Working Party on the Use and Disposal of Straw. Report on rhe use and disposal of sfruw.
Cycle: 1186/95/73, T.M.66 * Smith, D. L. 0.; Rutherford, I.; Radley, R. W. The use of energy in ugricufrure: straw productron. Paper presented to the Institute of Agricultural Engineers, 1973 ’ Bruhn, W. D.; Zummerman, A.; Neidermeier, R. P. Developments in pelleting forage crops. Agric. Engng, 1959 40 442446 A Reece, F. N. Temperature, pressure and time relationships in forming dense hay wafers. Trans. A.S.A.E., 1966 9 749-751 ’ Hall, G. E.; Hall, C. W. Heated die wafer formation of alfalfa and Bermuda grass. Trans. A.S.A.E., 1968 11 578-581 6 Rehkugler, G. E.; Buchele, W. F. Bio-mechanics offorage wafering. Trans. A.S.A.E., 1969 12 1-8 ’ Pickard, G. E.; Roll, W. M.; Ramser, J. H. Fundamentals of hay wufering. Trans. A.S.A.E., 1961 4 65-68 and Methods for determining Specific Weight, Durability ’ Wafers, Pellets and Crumbles-Definitions and Moisture Content. Agricultural Engineers Yearbook, 1970. Publ. A.S.A.E., 1970 306-308