EP0191763A1 - Combustible briquettes - Google Patents

Abstract

Briquette produced by combining sawdust and a charcoal material such as coal, coke, charcoal or animal in a ratio of 1: 1 to 1: 6 and compressing the mixture to a pressure exceeding 150 MPa and to a temperature between 120 and 285oC without using a binder. The moisture content determines the time required for the pressing operation and is preferably between 10 and 14% by weight.

Description

"COMBUSTIBLE BRIQUETTES"

THIS INVENTION relates to combustible briquettes formed of a mixture of sawdust or wood particles and a carbonaceous material such as coal, coke, charcoal or coalchar.

The majority of briquettes produced at the present time make use of a binder such as pitch or starch costing from $350.00 - $900.00 per tonne. As a result of extensive tests it has been found that by mixing the carbonaceous material with sawdust and pressing the mixture into briquettes at a suitable temperature and pressure it is possible to produce a satisfactory briquette without the use of a binder.

Thus in one form the invention resides in a method of producing a combustible briquette which comprises mixing sawdust or wood particles and a carbonaceous material such as coal, coke, charcoal or coalchar, pressing the mixture at a temperature and pressure sufficient to remove excess moisture from the mixture, to substantially eliminate the elasticity of the mixture, to substantially eliminate springback and to bind the components of the mixture together.

The invention will be better understood by reference to the following description of investigations carried out with mixtures of sawdust and coal.

Sawdust

The sawdust used was typical hardwood sawdust from kiln dried wood or green sawdust dried as required. Its moisture content was 13.5 percent although this was adjusted by the addition of water for some tests. The bulk density of the sawdust was 200 kilograms per cubic -3 metre (kg m ) when the container was loosely filled and

260 kg m -3 when the sawdust was lightly compacted by vibration. Briquettes made from sawdust alone had a _3 density of 1080 kg m . The size analysis of the sawdust was determined as:

Size Mass millimetre percent

- +2.36 0.7

-2.36 1.4

-1.70 3.8

-1.18 +1.70 9.3

-0.85 +1.18 26.1

-0.60 +0.85 58.6

+0.60

Coal

Coal from the Hebe seam in the Muja open cut in the Collie field was partly dried and then ground for use. The grinding was not to any particular degree of fineness and for some tests the moisture content of the coal was adjusted by the addition of water. Most of the testwork was done with coal containing 11.8 percent moisture and having the following size distribution:

Size Mass micrometres percent

+850 1.5

850 +600 12.0

600 +425 21.3

425 +300 19.7

300 45.5

The bulk density of coal lightly compacted by vibration was 740 kg πv~ . Equipment Used

Cylindrical briquettes were formed in two steel moulds with diameters of 28.64 and 41.54mm respectively. The maximum load that could be applied to the smaller mould was 25 tons, equivalent to a pressure of 386 MPa (56000 psig). The maximum load that could be applied to the large mould was 30 tons, equivalent to a pressure of 221 MPa (32000 psig) .

Briquettes were formed individually in the mould by manual application of hydraulic pressure. This mode of operation allowed the elasticity of the briquette to manifest itself in three ways:

i. by movement of the piston in the mould when the applied pressure was released;

ii. by expansion of the dimensions of the briquette as it was expelled from the mould;

iii. by an increase in the dimensions of the free¬ standing briquette as the stresses induced by the briquetting pressure was relieved over a few hours.

The first and greatest of these elastic movements is referred to as "springback" and was measured with a scale and pointer fixed to the base plate of the mould and the piston respectively. The second and third indications of elasticity were measured with vernier calipers and are much smaller than springback measurements.

The batch method of producing single briquettes used in this testwork does not parallel the industrial method of continuous production of sawdust logs currently available in the Australian market. Elasticity of Briquetting Materials

Elasticity is a physical property of matter which allows it to deform under pressure and return to its original dimensions (more or less) when the deforming stress is removed. However, elasticity values calculated for the purpose of this specification were derived in particular experimental conditions and should not be construed as absolute values of the elasticity of the materials under test.

The elasticity is presumed to cause planes of weakness in the briquettes parallel to the ends of the cylindrical shape. When the briquettes are stressed by being dropped onto a cement floor from a height of about two metres, they eventually fracture across such a plane. Similar planes are evident in the commercial sawdust logs although their frequency of occurrence along the length of a long is presumably related to the stroke of the extrusion mechanism as it compresses a slug of sawdust. The commercial logs possessed a hard, surface sheen which might be the result of heat generated by friction during pressing.

When mixtures of sawdust and coal in mass ratios 1:1 and 1:2 are briquetted, the volume of sawdust is much greater than the volume of the coal. Sawdust itself can be formed into a firm briquette but Collie coal by itself does not yield a competent briquette. As well, the coarser coal particules tend to fracture under high pressure. Hence, in briquetting these mixtures of sawdust and coal, it may be considered that the sawdust functions as the binder and the elastic properties of the mixture are more associated with the sawdust than with the coal.

OMPI Springback

Springback measurements were made as indicated above by measuring the movement of the piston in the mould when the pressure was released. It is expressed as a percentage of the height of the compressed cylindrical briquette. Most tests were made in duplicate and, generally, average results are reported even though replicate measurements may have differed by 2-4 percent. Hence, the inter¬ pretation of the results is qualified because of the lack of precision in the technique.

Effect of Particle Size of Sawdust

The sawdust as received, containing about 12-13 percent of moisture, was screened on square mesh laboratory screens of aperture 600 and 300 micrometres.

Measurements of springback for the three resultant size fractions were made at different pressures as indicated below and other values are included for comparison.

TABLE 1

Springback of Sized Fractions of Sawdust Etc.

Pressure MPa 77 154 232 309 286 Size of Sawdust Springback micrometres

+600 20.0 20.8 19.2 19.4 18.0 -600 +300 25.2 23.2 22.8 22.8 23.2 -300 22.9 22.8 22.0 21.6 21.4 As received 19.6 20.7 20.0 19.7 19.2 Pine Sawdust 22.6 22.5 23.4 22.4 22.6 Coal 16.8 19.3 18.9 19.4 19.0 The interpretation of these values is that the briquetting components have similar springback characteristics which are not destroyed by the pressures applied. There does not appear to be any benefit in sizing the sawdust to a particular size.

Effect of Moisture Content and Heating

The sawdust being used was hardwood sawdust with 12-13 percent of moisture. A quantity of sawdust with sufficient added water to raise the moisture content to 25 percent was left in a sealed plastic bag for about 18 hours to equilibrate. Portions of this wet sawdust were dried to different levels of moisture content and spring¬ back measurements were made for an applied pressure of 386MPa. Other portions of sawdust, were completely dried at 110 C or heated to 200 C and 300 C before similar springback measurements were made. The results are shown in Table 2.

TABLE 2

Effect of Moisture Content or Heating on Springback

Treated Sawdust Springback percent

Moisture -

25.0 percent 59.1

18.7 percent 25.8

15.5 percent 23.2

12.0 percent 19.3

5.7 percent 19.0

0.0 percent 24.1

Heated to -

200°c 22.9

300°C 33.6

OMPI These results indicate that springback is least with a mosture content in the sawdust in the range of 6-12 percent. Certainly, sawdust with moisture contents in the range 20-25 percent yields weak briquettes.

Expansion After Moulding

As they were expelled from the mould, briquettes expanded in both height and diameter. For sawdust (12-13 percent moisture) pressed at 386MPa, the expansion immediately after expulsion was about 5-10 percent in height and less than 1 percent in diameter. As the briquettes aged they expanded more, about 15 percent in height and less than 1 percent in diameter. This expansion was less when briquettes were pressed at 221MPa.

The expansions described above were presumably associated with the elastic recovery of the sawdust (12-13 percent moisture) from the stresses induced by the briquetting pressure. For dry and wet (25 percent moisture) briquettes, some of the dimensional changes were associated with gain or loss of moisture. There is an obvious need for the briquetted material to have a moisture content which is stable with respect to ambient conditions or to be protected from changes in its moisture content.

Total Expansion of Briquettes

The briquettes made with sawdust containing different levels of moisture (see Table 2) were observed over a period of six days. A briquette has its maximum specific gravity and minimum dimensions under pressure (386MPa) in the mould. The dimensions increase immediately on expulsion from the mould so that the specific gravity decreases. The dimensions increase again as the briquette ages and the final specific gravity is affected by these dimensional changes and any changes in moisture content. The following values were noted:

TABLE 3

Changes in Briquette Parameters with Time (a)

Moisture Content Specific Gravity Total Expansion (b) Initial Final Initial (c) Final ... ercent... ....percent

25.0 11.1 0.84 0.62 109.8

18.7 11.7 1.07 0.89 55.7

15.5 11.3 1.11 0.97 47.2

12.0 10.5 1.16 0.99 43.3

5.7 9.2 1.20 0.94 46.5

0.0 8.8 1.18 0.81 61.0

(a) Six days between making briquettes and final measure¬ ment.

(b) Based on increases in the height of the briquette expressed as a percentage of the minimum height under pressure.

(c) This is the specific gravity immediately after expulsion from the mould. Specific gravities in the mould under 386MPa varied over the much narrower range of 1.48-1.52.

These data show that dimensional changes in sawdust briquettes are least when the moisture content is 10-12 percent and stable and this may be the optimum level for briquetting. A high specific gravity is associated with high compressive strength in a briquette but none of these briquettes was sufficiently strongly bonded under pressure to retain the specific gravity of 1.48-1.52 developed in the mould. The decreases in specific gravity after expulsion from the mould and as the briquettes age indicate a weakening of the inter-particle bond and a loss of strength in the briquettes. Commercial sawdust logs had retained specific gravities of 1.2 and moisture contents of.8-9 percent even after prolonged exposure.

Sawdust-Coal Mixtures

As indicated above, the properties of sawdust-coal briquettes might be expected to be similar to those of sawdust briquettes because the major value component is sawdust and coal is also elastic in the briquetting conditions employed. Some briquettes were made with mixtures of sawdust and coal with various moisture contents. The briquetting details are shown in Table 4 and the properties of the briquettes are shown in Table 5 with corresponding item numbers to assist interpretation (see below).

This data confirms that sawdust-coal briquettes behave similarly to sawdust briquettes with respect to elasticity as defined in this report. There does not appear to be any significant difference between mass ratios of 1:1 and 1:2 (sawdust:coal) but dry sawdust and dry coal (dry = less than 5% by weight moisture) appear to yield weaker briquettes judged on appearance and specific gravity measurements. TABLE 4

Briquetting Mixtures of Sawdust and Coal

Item Moisture Content Mass Ratio Pressui

Sawdust Coal Sawdust : Coal

MPa

1 13.1 20.2 1:1 386

2 13.1 12.3 1:1 386

3 13.1 12.3 1:1 386

4 13.1 12.3 1:2 386

5 13.1 12.3 1:2 386

6 13.1 12.3 1:1 221

7 13.1 12.3 1:1 221

8 13.1 12.3 1:2 221

9 13.1 12.3 1:2 221

10 13.1 6.1 1:2 221

11 0.0 0.0 1:2 221

TABLE 5

Properties of Sawdust-Coal Briquettes

Item Specific Gravity Expansion - percent (a) Under After In On After

Pressure Expulsion Mould Expulsion 24 Hours

1 1.48 1.13 20.4 28.0 37.7

2 1.48 1.18 18.8 24.5 33.8

3 1.48 1.17 21.0 25.0 35.7

4 1.48 1.17 18.5 24.1 34.1

5 1.46 1.18 20.0 21.8 31.1

6 - 1.14

7 - 1.13 no results available

8 - 1.13

9 - 1.12

10 - 1.10

(a) This expansion refers to increases in the height of the briquette expressed as a percentage of the minimum height under pressure. These increases are additive in the three stages of pressure release (springback), expulsion from the mould and aging.

Conclusions

The testwork has shown that the elasticity of the materials increases the dimensions of sawdust-coal briquettes from the moment that the moulding pressure is released until the briquettes stabilise on free-standing. These dimensional changes are aggravated by changes in the moisture content of the briquettes until equilibria, with ambient conditions is attained. In the cirucmstances of this testwork, the dimensional changes due to both causes were minimised when the moisture contents of the sawdust and coal were each about 10-12 percent. It is probable that the moisture content of the sawdust is more critical than that of the coal because the sawdust is the major volume component of the mixutre. The particle size of the sawdust does not appear to be a significant factor.

Attempts were made to moderate the elasticity of the o o sawdust by heating it in air to 200 C and 300 C as a literature reference suggested that 270 C was an approp¬ riate temperature. These attempts were unsuccessful. The elasticity of the materials did not appear to be affected by the pressures attained (386MPa) in the briquetting press.

Commercially produced sawdust logs appear to have a stable moisture content of 8-9 percent and a stable specific gravity of 1.20-1.23. Although they are segmented

OMPI longitudinally, the logs have a hard surface sheen which may be induced by the commercial process - it was not achieved in the circumstances of the testwork now reported.

The main conclusion is that the least dimensional changes occur in sawdust-coal briquettes when the moisture contents of the sawdust and coal are each about 10-12 percent. As dimensional changes are minimised, the briquettes should be close to maximum strength although this will also be affected by the briquetting pressure.

The investigations and tests, described above establishes that satisfactorily strong briquettes could be made with mixutres of sawdust and Collie coal were compacted under a pressure of 220 megapascals.

Mixtures of sawdust and coal in mass ratios of in the range of 1:1 to 1:6 yielded firm briquettes with the potential to withstand reasonable handling stresses. However, the briquettes were not considered firm enough for handling and tranport in commercial application.

The results of additional tests described below establish that it is possible to produce a superior briquette with the introduction of greater pressure and heat, improving the handling capability of the briquette to a commercially acceptable level.

The aforementioned investigations established that a 1:1 mix of coal and sawdust, with a moisture content of 12-14% when compacted under a pressure of 368 MPa, produced a stronger briquette.

Further tests described below were carried out to determine what factors would enable the elasticity of the briquetting mixture to be controlled, to so avoid the briquettes expanding, and developing lateral cracks when expelled from the mould, which in turn would render the briquettes unacceptable for commercial use.

Three head samples of sawdust and coal mix were prepared.

Each bag contained approximately 5 kilograms of sawdust and coal mixed on a mass ratio of 1:1.

The moisture contents marked on the samples were 25%, 12% and 3% respectively.

Three separate sets of trials were conducted.

The first series of trials used a manually operated 100 tonne press.

A high tensile steel mould and piston were manufactured for the trials to facilitate the manufacture of the briquettes.

The external size of the mould was 150mm height x 149mm diameter, and the internal size was 150mm height x 82mm diameter, giving the mould wall a thickness of 33.5mm.

The piston measure 79mm height x 82mm diameter.

To allow moisture to escape during compaction, holes were drilled vertically down the piston from face to face. The six holes were drilled to 4mm in diameter at 60 pitch circule diameter, each at 60 points of the piston face. A seventh hole was drilled down the centre of the piston.

An Industrial welding blow torch was used to heat the mould. A hole was drilled into the side of the mould to

^ A. insert a thermometer -which measured the heat of the mould, approximately 8.5mm from the internal face of the mould.

A Second and Third series of Trials were conducted using the same equipment, with a foot operated mechanical 150 tonne press.

Vernier Calipers were used to measure the respective heights of briquettes during the trials.

Varying quantities of mixtures were pressed in the mould in order to determine the ideal pressure per square inch required to destroy elasticity of the mixture.

25% Moisture Content

In the First and Second series of Trials, it was established that the mix of 25% moisture content was unsatisfactory as moisture could not be expelled due to lack of breathing space for the moisture to escape.

A mixture of 25% moisture content slowly extruded under certain pressures and temperature, it is envisaged that the final product would still be neutralised without springback, and contain the ideal Gross Specific Energy, Specific Gravity, Moisture Level and Ash Content.

A fifth series of Trials, proved that 25% moisture mix could produce very satisfactory briquettes and these Trials are discussed later.

12% Moisture Content

Previous tests have established that an ideal moisture level was 12-14%, and because of the success of trials with the prepared sample of 12% moisture level, it was

'i - Wϊ?o decided to attempt only one test with the 3% moisture level mix due to the economic disadvantage of drying the mixture to such a level for commercial production.

The elasticity of 1:1 mixture of sawdust and coal is directly related to the pressure applied for compaction, and the moisture level of the mix, both during compaction and at completion of compaction.

The application of heat to the moulding process not only appears to extract the lignin content of the sawdust and so assist in the binding of the two materials, but also accelerates the removal of excess moisture from the mix by driving off steam.

The process of driving out moisture from the sawdust as steam appears to also release compounds from the sawdust such as lignins which act to bind the particles together.

Test results indicate that a variation of moisture level of the mixture, heat, compaction pressure, and duration of compaction pressure; directly cause a change in the hard¬ ness of the briquette and its Gross Specific Energy.

At particular pressures and heats with compaction sustain¬ ed for a period of time, the elasticity of the mixture appears to neutralise completely, producing a hard shelled, very dense and compact briquette, without any springback.

Springback

Springback has been isolated and can be removed, as ex¬ plained under Elasticity. The physical energy of sawdust that contributes to the elasticity of the coal/sawdust mixture, and in turn effects the springback percentage of the briquette produced, is directly controlled by the moisture level of the mix, heat and pressure to neutralise the elasticity, and duration of pressure of moulding to stabilise the mixture in its new briquetted form.

To avoid the use of costly abnormally high compaction pressure, a duration of compaction for a certain given period of time at particular pressures achieves the same result.

Expansion After Moulding

Expansion of briquettes after moulding occurred in certain Trials.

The expansion can be attributed to any one of the follow¬ ing:-

(a) Too much moisture still locked in mix.

(b) Too little moisture.

(c) Too low a compaction pressure.

(d) Not long enough period of compaction pressure.

(e) Not enough heat.

(f) Too much heat.

Total Expansion of Briquettes

Those briquettes which were considered successfully manu¬ factured had very little or not expansion after one, two and three days following manufacture. Even after a period of ten months there was no measurable expansion. The Specific Gravity of the similar sized briquettes compares favourably with each other when tested one, two, three and ten days after manufacture.

Gross Energy Values

Collie Coal as mined may contain 24-28 percent moisture and 3-8 percent ash. The Gross Specific Energy of that coal would be approximately 19.9 Mgj/kilogram.

Coal with a moisture content of 12.5 percent would have a Gross Energy Value of about 24.75 Mgj/kilogram.

To achieve its optimum Gross Energy Value of some 29.9 Mgj/kilogram, Collie Coal would have to be passed through the expensive process of drying to achieve nil moisture level and ash free state.

A commercial plant as envisaged for production of the briquettes, would need to dry economically the sawdust/coal mixture to about 12 percent moisture, before manufacture of the briquette.

Briquettes produced would have Gross Energy Values of between 23-24 Mgj/kilogram and an ash content of some 2 percent.

Much of the further drying process of the mix is carried out during the briquettes' manufacture, by the process of heat and pressure to produce a briquette containing a moisture level of some 5 percent.

It can be recognised that the moisture level of the briquette at time of expulsion from the mould, is critical as to the elasticity of the mixture, and so effects springback.

vrwri

WIPO In the Third and Fourth Series of manufacturing Trials, selected mould temperatures and compaction pressures were shown to remove elasticity and springback.

It was also discovered that the greater the volume of material to be moulded, the greater the period of time the compaction pressure was required, to neutralise elasticity.

In the Third series of Trials, a mass of 18oz of material at 25% moisture level, compressed in the mould for a period of ten minutes with pressure on the piston main¬ tained at 100 Tonne.

The briquette demonstrated great elasticity and spring¬ back, and finally broke into several pieces.

When a mass of 7oz of material at 12% moisture level was compressed in the mould for twenty minutes, at a commence¬ ment heat of 170 C and conclusion heat of 110 C, the briquette had no elasticity and no springback.

In the Fourth Trials, smaller quantities of mix were moulded to attempt less moulding time.

A lO^oz mixture was pressed at 100 Tonne for twenty seconds and left in the mould for three minutes without constant pressure being reapplied. When expelled from the mould, the briquette contained too much elasticity, and crazed laterally.

Very successful manufacturing results of the Third and Fourth series of Trials, were achieved when a series of 4oz of mixutres at 12% moisture level were compressed at a maintained pressure of 120 tonnes for a period of three and a half minutes and a commencement mould temperature of 195 C and conclusion mould temperature between 170°C and 180°C. The Fifth series of Trials was designed to prove or dis¬ prove the briquettability of a mix containing a moisture level of 25%.

The first of the Trials " 17" used 4oz of mix at 25% moisture content. The moisture escape holes blocked up, and when the briquette was expelled from the mould, the moisture content caused springback in the top section of the briquette. Cores of the mix were extruded from the moisture escape holes.

Trial "T20", used 4oz of 25% moisture mix, compacted at a pressure of 12 Tonnes for a period of eight minutes, at a o mould commencement temperature of 170 C and mould temper¬ ature of 136°C at completion.

The briquette showed no signs of springback after twenty-four hours following manufacture, prior to being pulped up for laboratory analysis.

Trial "T23", was designed to prove the duration of time of pressure under compaction was relative to the quantity of mix to be briquetted.

Allowing two minutes of compaction time for loz of mixture at 25% moisture level, 6 oz of mix was compacted at 120 Tonnes pressure for a period of twelve minutes, at a mould commencment temperature of 170 C and mould temper ature of 110 c at completion.

No springback was recorded and the briquette was submitted for laboratory analysis.

Trial "T24" was a repeat of Trial "T20", although the mould completion temperature had dropped to 118°C compared to 136°c in Trial "T20".

LEΛ

OMPI No springback was recorded, and the briquette has been kept.

Comparing the successfully manufactured briquettes in the Third, Fourth and Fifth Trials, the following conculsions can be drawn.

Moisture Level and Temperature Relativity

The moisture level of the mix determines at what pressure and temperature the mix needs to be manufactured, to destroy elasticity and springback.

3% Mositure Level

A moisture content of 3% will produce a briquette at 120 Tonnes pressure held for 3-5 minutes, at a temperature of 170 c at commencement of moulding, (see Trial 19).

A temperature of 120 C is not adequate for a 3% moisture mix and results in the briquette developing lateral cracks, (see Trial 22).

12% Moisture Level

Trial 18 produced a good briquette when 4oz of 12% moisture mix was compacted under 120 Tonne for a period of 3-5 minutes at a mould commencement temperature of 170 c.

Trial 21 demonstrated that a mould commencement temper¬ ature of 120 C was not satisfactory and the briquette produced, developed lateral cracks caused by springback.

25% Moisture Level

OJ&PI Trials 20, 23 and 24, proved that good briquettes can be produced from a mix containing 25% moisture, so long as a long enough period of time is given to expel1 the moisture content and so avoid springback.

The mix must be briquetted at a temperature of some 150 c to 170°C, and a compaction pressure of 12 Tonnes (32,480 p.s.i.) must be maintained for a period of two minutes per loz of mix.

Pressure

The optimum compaction pressure required to briquette a mixture of 1:1 sawdust and coal by mass (appears to be of the order of 12 Tonnes, (32,480 p.s.i.).

Temperature

The temperature of the mould should be in excess of 120 C and preferable no greater than 195°C although temperatures of up to 285 C may be required with coalchar.

Duration of Compaction Relative To Moisture Level

For mixtures containing a moisture content of between three and twelve percent, some fifty and sixty seconds should be allowed per loz of material.

For a 25% moisture content, 100 to 120 seconds should be allowed for every loz of material.

By compacting the mixture in accordance with the criteria discussed above it is possible to produce briquettes having a distinct surface sheen which contributes to the water resistance and hardness of the briquettes.

OMPI

Claims

THE CLAIMS defining the invention are as follows:

1. A method of producing a combustible briquette which comprises mixing sawdust or wood particles and a carbonaceous material such as coal, coke, charcoal or coalchar pressing the mixture at a temperature and pressure sufficient to remove excess moisture from the mixture, to substantially eliminate the elasticity of the mixture, to substantially eliminate springback and to bind the components of the mixture together.

2. A method as claimed in claim 1 wherein the ratio of sawdust to carbonaceous material was in the range of 1:1 and 1:6.

3. A method as claimed in claim 1 or 2 wherein the moisture content of the sawdust and the carbonaceous material is between.10% and 14%.

4. A method as claimed in claim 1 or 2 wherein the moisture content of the sawdust and the carbonaceous material is of 25% by weight.

5. A method as claimed in claim 1, 2, 3 or 4 wherein the compaction pressure is in excess of 150MPa.

6. A method as claimed in claim 1, 2, 3, 4 or 5 wherein the temperature at which the briquettes are pressed is within the range of 120 C to 285 C.

7. A method of producing a combustible briquette from sawdust and a carbonaceous material substantially as herein described.

8. A briquette comprising a mixture of sawdust having a moisture content of less than 15% by weight and a carbonaceous material such as coal, coke, charcoal or coalchar having a moisture content of less than 12% by weight in a mass ratio of 1:1 and 1:6 pressed at a pressure in excess of 150 megapascals.

9. Briquettes comprising a mixture of sawdust and a carbonaceous material substantially as herein described.