GB2188916A - Method and apparatus for producing charcoal - Google Patents

Abstract

A method of and apparatus for producing charcoal comprises feeding plant fibre 41 into a primary feed mechanism 40 which passes the fibre through a conduit 43 within which the fibre is heated and converted to charcoal to emerge at an outlet 48. The outlet 48 is adjustable in size to determine the rate of flow of plant fibre/charcoal through the conduit 43. The conduit 43 is shown as being heated by hot gases within a jacket 44 but may also be heated electrically. The hot gases maybe derived by combustion of plant fibre. Volatile by-products resulting from the conversion of the fibre to charcoal are vented through outlets 50, 51 to be condensed. In a modification charcoal emerging from the outlet of the conduit 43 passes into a mixing chamber where it is quenched with water and a binding agent to be subsequently compressed into cohesive form. <IMAGE>

Description

SPECIFICATION A method of, and apparatus for, producing charcoal Technical field and background art This invention relates to a method of, and apparatus for, producing charcoal.

Conventionally charcoal is manufactured from wood by roasting whereby a wood pile is ignited and covered by, for example, turfs to restrict the supply ofairto support combustion. The heat generated by part of the wood burning drives off the water and other volatile components to produce the charcoal.

Eventual Iy the wood ceases to burn because of the restricted air supply and the charcoal pile is allowed to cool before removal.

There are several disadvantages with this conventional technique of charcoal production.

Primarily the method employs a batch process where each batch requires a iong time cycle to complete (because of the relatively long period required for the pileto heat-up and cool-down).

Much ofthe heat given off by the burning wood is wasted to the surroundings. Wood, as the raw material from which the charcoal is formed, is regarded as an expensive commodity. Considerable time and expertise is required in constructing the pile to operate effectively.

It has also been proposed to produce charcoal buy a continuous process in which, for example forest products are forced under pressure into and through a roasting chamberwith a restricted air atmosphere to be converted to charcoal - International Patent Publication WO 83/01781 relates to such a proposal.

With such a proposal it is difficultto control, and to compensate for changes in, the flow rate of material through the roasting chamber and thereby adequately to control the quality of characteristics of the charcoal which results.

It is an object of the present invention to provide a method and apparatus for the production of charcoal which alleviates the disadvantages of the above described prior proposals.

Statement ofinvention & advantages According to the present invention there is provided a method of continuously producing charcoal which comprises feeding plant fibre through a roasting chamber; heating said fibre in the roasting chamber during its progression therethrough and within a restricted air atmosphere to effect its conversion to charcoal; separating and removing from the charcoal in the roasting chamber volatile products of the conversion, and expelling the charcoal from the roasting chamber and in which the conversion of plant fibre to charcoal within the roasting chamber is controlled by controlling the rate at which charcoal is permitted to emerge from the roasting chamber.

Further according to the present invention there is provided apparatus for cqntinuously producing charcoal which comprises a roasting chamber; means for continuously feeding plant fibre into and through the roasting chamber; means for heating the fibre in the roasting chamber during its progression therethrough and within a restricted air atmosphere to effectthe conversion of the fibre into charcoal; means for separating and removing volatile products ofthe conversion from the charcoal, and means for expelling the charcoal from the roasting chamber; and wherein the roasting chamber has an aperture through which the charcoal emerges, said aperture having associated therewith feed control means by which the rate of flow of charcoal therethrough and thereby the rate of flow of plantfibre/charcoal through the roasting chamber is controlled to control the rate of conversion ofthe plant fibre into charcoal.

Preferably the plantfibre is one or more of straw, hay, rice and other husks, corn cobs, wood dust or chippings orothersimilarfibrous material which may generally be regarded as waste produce.

A primary feed mechanism can continuously supplyrawplantfibretotheroasing chamberand force thatfibre through the roasting chamberwhile it is subjected to heat within a restricted airsupplyto effect the conversion to charcoal.

The primary feed mechanism is conveniently a conventional conveyor unit, auger or ram as is well known in the agricultural industry to supply bales of straw or chopped straw for stoking a furnace and may be readily adjustable or modified to handle a wide range of plant fibres.

The primary feed mechanism will supplythe plant fibre at an appropriate rate (depending on the type of fibre) to the roasting chamber. It is envisaged thatthe roastingchamberwill usually be formed by a thermally insulated conduit through which the plant fibre is displaced. Heat is applied to the plant fibre during its passage through the conduitto effect the conversion to charcoal. One possible heat source is provided by electrical heaters mounted on the conduit. However, a preferred heat source is the hot flue gases formed by combustion of plantfibre and/or charcoal in a separate combustion chamber.

In this case the flue gases are passed over the roasting chamber which is contained in an insulated jacket. It is envisaged for commercial purposes that an array of conduits in parallel will usually be provided which are supplied with plant fibre from a single primaryfeed mechanism, thereby increasing the production rate of the charcoal. Where such an array is used heating by means offlue gases is particularly advantageous since all the conduits may be contained in a single insulated jacket and supplied with flue gases from a single combustion chamber.

The conversion of plant fibre to charcoal in useful form is believed to occur in three process stages during its progression through the roasting chamber. During a first stage the compressed plant fibre is progressively heated until it reaches a conversion temperature in the region of 400 C. This conversion temperature is likely to vary for different forms of the plantfibre. Conversion ofthe plantfibre to charcoal can occur rapidly, however such conversion is unlikely to occur uniformly across the conduit since the temperature adjacent to the walls oftheconduitwill usuallybehigherthanthatatthe centre of the conduit. The conversion to charcoal is the second process stage and the third process stage results in a useful form of the charcoal.During the progression of the fibre through the roasting chamber and its conversion to charcoal there will be produced a quantity of volatile products (such as tar and water/steam) which are mixed with the charcoal to form what may be considered as an undesirable sludge. This mixture istherefore heated inthe roasting chamber to vapourise the volatile products which are then drawn off or vented from the roasting chamber.

A large decrease in thevolume of solids occurs during the conversion from plant fibre to charcoal; the actual magnitude of the change in volume varies since the plantfibre is not generally homogeneous but, typically, a compaction change in the ratio 9 to 1 and aweightchange in the ratio 2to 1 maybe experienced. It has been found thatthe heterogenous nature ofthe plant fibre is likelyto cause variations in the change of volume ofthe contents of the roasting chamberwhich, since the primary feed mechanism is usually a constant volume feed device, can cause undesirable changes in the flow rate of the charcoal and plantfibre through the roasting chamber.Changes in the flow rate can also be adversely affected by volatile products which can unpredictably change the magnitude of friction between internal surfaces of the roasting chamber and the charcoal and plant fibre. The effect of a change in flow rate tends to disturb the equilibrium in the roasting chamber.

Where the flow rate is increased the stages of the conversion move downstream and this usually results in incomplete conversion of the plant fibre to charcoal. Where the flow rate is decreased the stages move upstream but this is likely to be an unusual occurrence since, for maximum efficiency during normal operation, the stages of conversion will be as far upstream as possible within the roasting chamber. In practice it has been found that it is difficult or impractical to control the flow rate ofthe charcoal and mixture through the roasting chamber/conduit by adjusting the feed rate of plant fibre into the roasting chamber.The flow rate of plantfibre/charcoal through the roasting chamber is therefore, in accordance with the present invention, determined by flow control means controlling the outlet rate of charcoal from the roasting chamber; this flow control is conveniently achieved by providing an outlet aperture the size of which is adjustable and through which the charcoal emerges from the roasting chamber. The outlet aperture may be controlled manually but is preferably controlled thermostatically and automatically by temperature sensitive means located, say, at the second process stage in the roasting chamberwhereby a drop in temperature atthis stage may indicatethatthe conversion stage has moved upstream in the roasting chamber or conduit as a result of an undesirable increase in the flow rate of the material in the roasting chamber.To correct or compensate for such an increase in the flow rate the outlet aperture may be adjusted manually or in responseto the thermostatic control to reduce in size and thereby reduce the flow rate through the roasting chamber.

This constriction to flow may be maintained by the aperture until the desired conversion temperature is again achieved at the second process stage (or elsewhere in the roasting chamber depending upon the location of the temperature sensitive means).

With many types of plant fibre the charcoal which is formed as a result of being heated and compressed during the third process stage will fuse and be expel led from the roasting chamber in a coherent form. However, some types of plantfibre may produce charcoal in a particulate form at the third process stage; such a particulate form is generally considered less useful than the cohesive form. Preferably therefore the particulate form of the charcoal from the third process stage is converted to a cohesive form by expelling the hot particulate charcoal from the roasting chamber into a mixing or cooling chamberwhere it is cooled orquenched,for example, with water. The mixing chamber preferably comprises a hopper into which the hot charcoal falls from the roasting chamber, and a jet or nozzle which sprays on water and a binding agent.The water spray cools the hot charcoal and together with the binding agent forms a paste-like mixture which may be displaced by a secondary feed mechanism. The secondary feed mechanism preferably comprises an Archimedean screw which displaces the above mentioned charcoal mixture along a pipe which narrows towards an open end through which the charcoal is extruded. As the mixture is forced along the pipe towards the open end it is compacted so that excess water is removed and the charcoal is bound together. The compacted charcoal as extruded from the end of the secondary feed mechanism can be cut into convenient sized brickets. The charcoal may then be allowed to stand to further dry out before use.

As mentioned above, a mixture ofvolatile products will be produced during conversion of the plantfibre; these products will usually bevapourised to separate them from the charcoal. Also if the charcoal produced is quenched by mixing with water a considerable quantity of steam may be produced.

These hot vapours and steam can, advantageously, be used to improvethethermal efficiencyofthe method and apparatus (particularlyforthe roasting chamber) for example, by utilising the thermal energy of hot gases and vapours by mixing them with the hot flue gases in the combustion chamber.

Alternatively, or additionally, the heatfrom these gases and vapours could be used for space heating, for example in a drying room forthe compact charcoal. Asignificantquantity of acetone may be found in the volatile products and this may be separated from the other products to form a saleable by-product. Some of the aforementioned vapours and gases may be combustible and it is also possible to separatethese and usethemasafuelsource.

Among the advantages ofthe present invention arethatthe characteristics ofthecharcoal which is produced can be monitored and the method and apparatus controlled accordingly to provide substantially uniform qualityforthe charcoal; the apparatus can be made relatively compact to be mounted,forexample,onatrailer,sothatisis portable; the time and skill required for the construction of a wood pile as previously described is alleviated; the charcoal production can reasily be adjusted for a wide range of different types of plant fibre, it is continuous, and is likely to be more energy efficient than the wood pile technique.

Drawings Several embodiments of an apparatus for the continuous production of charcoal from plant fibre and constructed in accordance with the present invention will now be described, by way of example only, with reference to the accompanying illustrative drawings, in which: Figure lisa sectional view of a first embodiment in which the roasting chamber is heated by hotflue gases and has a variable size outlet aperture for controlling flow rate; Figure2 is a sectional view of a second embodiment in which the roasting chamber is heated by electrical heaters and a mixing chamber is provided to produce a coherent form of charcoal from a particulate form;; Figure 3shows a perspective view, in part section, of a third embodiment ofthe apparatus which is mounted on atrailerto be mobile and has an array of conduits providing separate roasting chambers contained in a common heating jacket and to which conduits plantfibre is supplied buy a common feed mechanism, and Figures4A and 4B show enlarged perspective views of a variable size outlet aperture for controlling the flow rate of charcoal and/orfibrethrough the roasting chambers of the embodiments shown in Figures 1 to 3.

Detailed description of drawings The apparatus shown in Figure 1 has a substantially conventional primaryfeed mechanism 40 which receives bales 41 of plant fibre (such as straw or hay) on a conveyor belt 42. The primaryfeed mechanism 40 is a commercially available agricultural furnace stoker(usually employed to feed straw to a burner). The plant fibre is forced bythe primary feed mechanism 40 into and through a tubularconduit43. The conduit 43 is enclosed forthe major part of its length within a thermally insulating jacket 44. An inlet pipe 45 located at an end ofthe jacket close to the primaryfeed mechanism connects the interior of the jacket 44 with a supply of hot flue gases derived from a separate combustion chamber (not shown).An exhaust pipe 46 located at the end of jacket 44 remote from the primary feed mechanism 40 is provided through which the flue gases can be vented or extracted. Extending helically around the outer surface of the conduit 43 are heattransferfins orflanges 47 which promote the transfer of heat fromthehotfluegasestotheinterioroftheconduit 43 and hence to the plantfibre passing therethrough.

The end of conduit 43 remote from the mechanism 40 projects from the jacket44 and is provided with an outlet aperture 48 the cross sectional area or size of which is adjustable by an electric motor. A detailed description of the outlet aperture 48 is given in the description of Figures 4A and 4B below. The operation of the electric motor is controlled by a thermostatic switch 49.

The conduit 43 can be considered as comprising three longitudinally extending sequential regions A, B and C (which are not separated by any barriers) from the mechanism 40 and which correspond to particular process stages ofthe conversion ofthe plant fibre to charcoal. In the first process stage A, the plant fibre is heated by the hot flue gases in the jacket so that its temperature risesgradua lly as it progresses through the conduit 43 until it reaches a conversion temperature at the end of that process stage (that is at the end of region A remote from the primaryfeed mechanism).At the conversion temperature and within the restricted oxygen atmosphere ofthe conduit 43, the plant fibre undergoes a rapid conversion to charcoal while generating a mixutre of gases, volatile liquids and water or steam. The conversion temperature is likely to be in the order of 400"C but it will be appreciated thatthis temperature will vary depending upon several factors such as the type and quality ofthe plantfibre (quality being, for example, whether the fibre is dry or wet, and its state of decomposition).

This conversion to charcoal is intended to occur in region B ofthe conduit 43 and the thermostatic switch 49 is located in this region to sense the conversion temperature. The conversion of the plant fibre to charcoal is unlikely to occur simultaneously throughout the fibre which is located in aflatplane extending across the conduit; most likely the conversion will be effected in what may approximate to a parabolic of bell-shaped plane projecting upstream in the conduit 43. This is because the temperature of plant fibre at the centre of the cross section or bore of conduit 43 is likely to be lowerthan that atthe wall oftheconduit.

Conversion ofthe plantfibre in region B results in a sludgy mixture of charcoal and volatile liquids, (such as tar, acetone and water). This mixture then progresses (by pressure from the input ofthefeed mechanism 40) into the region C ofthe conduit 43 where it is continually heated to vapourise the volatile products in a third process stage. The vapours and gases generated in the region C are separated from the charcoal in the conduit by extracting them through vents 50 and 51 which extend from the interior of the conduit and emerge through the jacket 44. The vents 50 and 51 are conveniently connected to a condenser (not shown) which may separate particular constituents ofthe vapours or gases.

The charcoal, which remains following the extraction ofthevapours and gases, is compressed as it progresses through the region C and this assists in its fusion into a cohesive form in which form it is finally extruded through the aperture 48.

As the charcoal emerges from the aperture 48 it may be broken or cut into conveniently sized pieces.

The compression and flow rate of the materials within the conduit 43 are primarily controlled by adjustment in the size of the cross sectional area of the outlet aperture 48 rather than by changing the rate at which the plant fibre is fed into the conduit 43 by the mechanism 40. This is because ofthe large decrease in volume experienced by the solids during the conversion from fibre to charcoal; this decrease is unlikely to be consistent and may be in the order of 9:1.The outlet aperture 48 is adjusted automatically in response to control from the thermostatic switch 49 that if switch 49 senses a fall in temperature (belowthedesired conversiontemperature),the outlet aperture 48 is reduced in size by the electric motor.Such a fall in temperature may be caused by the region B advancing towards the aperture 48 of the conduit (possibly because of a reduction in volume of charcoal in the region C and a consequential increase in flow rate) with the possible effectthatthe plantfibre may not be fully converted to charcoal on emerging from the aperture 48.

However, the contraction of the aperture 48 will result in a reduction in the flow rate within the conduit 43so that the region B is displaced downstream until the conversion process stage is again correctly sensed by the thermostatic switch 49.

Should the thermostatic switch 49 sense a higher temperaturethan that required ofthe region then the aperture 48will respond accordingly by enlarging to permit an increase in the flow rate. If required the size of the outlet aperture 48 can be adjusted manually and appropriately in response to temperature readings taken at the region B; also the size ofthe outlet aperture 48 may be adjusted (manuallyorautomatically) in response to the mean of several temperature sensingstaken at intervals along the length ofthe conduit 43.

In a practical example ofthe arrangement shown in Figure 1 the conduit 43 is a cylindrical steel tube 3 metres in length and with a bore of 18 cms; straw is fed from the mechanism 40 to provide a throughput time of approximately 36 hours; the exterior ofthe tube 43 is heated to approximately440 C and charcoal emerges from the outlet48at approximately 35 cms/hour.

In the embodiment of Figure 2 plant fibre is supplied in theform of straw or hay bales 1, by means of a conveyor belt 2, to the primary feed mechanism 3. The primaryfeed mechanism 3 is a commercially available agricultural furnace stoker (usualiy employed to feed straw to a burner). The plant fibre is forced by the primary feed mechanism into a conduit4 having afire damper 5, the latter serving as a safety device to close off the conduit 4 in the eventthatfire encroaches through the fibre in an upstream direction (that is towards the primary feed mechanism). The conduit4 communicatesthrough the fire damperwith a roasting chamber comprising a conduit6 which may conveniently be regarded as having consecutive part lengths A and B.The conduit 6 is constructed of a heat resistant material with good heattransfer properties. Located on the exterior ofthe part length A (that part length nearest to the fire damper) are spirally extending heat exchange fins 33. Located on the exterior of the part length B ofthe conduit 6 are electrical heaters 10, also this part length B is provided with vents 12 to allow the passage of gases and vapoursfrom the conduit. The conduit 6 opens at 14 and by way of a variable size aperture 38 in to a hopper 15. A detailed description of the aperture 38 is given below with reference to Fig ures 4A and 4B. Substantially surrounding the whole length of the conduit 6 is an air-tightandthermally insulating jacket 7. The jacket 7 is secured at its ends 8 and 9 to the conduit 6.The jacket7 has an exhaust vent 13 nearto the damper 5 and a port 34 nearto the conduit end 14. The port 34 is coupled to a port 20 in the hopper 15 by means of a pipe21.

Theplantfibreisfed into the conduit 6from the conduit 4 and during the passage ofthe plantfibre through the first part length A ofthe conduit it is preheated. The heatforthis pre-heating is provided by hot gases and vapours (which include steam) developed in the apparatus and the source of which will be descriped hereinafter. These hot gases and vapours pass in a downstream direction through a chamber 35, (formed between the outer surface of the conduit 6 and the inner surface ofthe jacket 7) and during their passage over the part length A transfer heat th rough the conduit 6to the plantfibre; the efficiency of this heattransfer is improved by the heat exchange fins 33.

The preheated plant fibre passes to the part length B where it is heated by the electric heaters 10 to a temperature in the range of 400"C and 800 C,the working temperature depending on the type of plant fibre and the flow rate ofthatfibre through the conduit 6. Inthis part B ofthe conduit6 hot gases and vapours will evolve from the fibre and these are passed into the chamber 35 through the vents 12.

The charcoal formed by roasting the plant fibre in the conduit part B passes from the end 14 of conduit6 into the hopper 15.

At its stage of entry to the hopper 15 the charcoal is hot and in a loose and powdery state so that it is necessary to cool it and to form it into a more easily handled form. The hopper 15 forms part of a mixing mechanism and has a base 18 which tapers downwardlyto an opening 24 in its lower end. The hopper is closed at its upper end by means of a plate 16, and sidewalls of the hopper extend above this plate to form atank 17. Stored within the tank 17 is a liquid mixtureofwateranda binding agentsuchas starch, this liquid is sprayed onto the aforementioned hot charcoal (through a flow valve 19 located in the plate 16) to cool the charcoal. The flow valve 19 is adjustable to vary the flow rate ofthe liquid in accordance with the temperature and flow rate of the charcoal through the hopper. As the liquid cools the charcoal a considerable quantity of steam is evolved, and this is exhausted from the hopper 15 by way of the port 20 and pipe 21 into the jacket 7to provide an additional source of heat for pre-heating the plant fibre. Eventually both this steam and the vapours and gases are exhausted from the assembly through the exhaust vent 13. If required an outlet (not shown) can be provided in the bottom of the jacket7 to draw-offwaterformed by condensed steam.

As a result of spraying the charcoal with the liquid mixture a charcoal paste is developed in the lower part 18, of the hopper 15. This paste falls through the opening 24 into a cylindrical pipe 26 rotatably mounted within which is an Archimedean screw 27.

The pipe 26 is closed atone end 36throughwhich a drive shaft 37 of the screw 27 passes. The screw 27 extends along the pipe 26 over the opening 24, and the opposite end of the pipe is tapered at 28 to converge towards an outlet 29. As the paste falls into the pipe 26 it is fed along the pipe towards the outlet 29 by rotation of the screw 27. The tapered end 28 of the pipe 26 causes the paste to be compacted to expel excess water therefrom and cause the charcoal to bind together as it is extruded from the outlet 29.

The water expelled from the paste can be recycled to the tank 17 and the extruded charcoal cut into conveniently sized brickets which can then be stored and dried before use.

The mobile form of the apparatus shown in the embodiment of Figure 3 has a primaryfeed mechanism 60 which is supplied with plantfibre from a hopper 61. The primary feed mechanism 60 feeds compressed plant fibre into an array of roasting chambers in the form of parallel cylindrical tubularconduits 62. The conduits 62 are enclosed within a common thermally insulating jacket 63 and are each provided with vents (not shown) which may be linked together and byway of which the volatile products can be removed from the respective roasting chambers. The ends ofthe conduits 62 remote from the primary feed mechanism 60 project from the jacket 63 and each is provided with an adjustably sized outlet aperture 64 (similar to the aperture 48 in Figure 1).The jacket 63 is mounted on a combustion chamber64so that an apertured top or grid 65 ofthe combustion chamber forms a base of the chamber enclosed by jacket 63. The combustion chamber 64 is supplied with plant fibre from the primary feed mechanism 60 which fibre is then burned in the combustion chamber 64and the hot gases generated thereby pass, by way of apertures 66 in the grid 65, into the jacket 63. The hot gases heat plant fibre passing through the conduits 62 to convert the fibre to charcoal in the manner previously described with reference to Figure 1.The flue gases from the jacket 63 are exhausted to atmosphere by way of a vent 67.

The flow rate of the emerging charcoal is controlled in the manner described forthe embodiment of Figure 1 by adjusting the size of outlet apertures 69 from the roasting chambers either manually or in response to thermostatic switch devices (not shown) on the respective conduits 62. A detailed description of the aperture 69 is given below with reference to Figures 4A and 48.

Charcoal emerging from the apertures 69 can fall onto an inclined conveyor 70 to be delivered thereby into a hod or other container (not shown).

The apparatus shown in Figure 3 is advantageous in that itcan be made compact, and the large number of conduits 62 permit a high production rate of charcoal. By providing such a compact apparatus it can be mounted on a mobile base such as a trailer68.

In the embodiments of Figures 1 and 3 it will be apparent that the charcoal emerging from the outlet apertures 48 and 64 will be hot. If required therefore, one or more cooling chambers (not shown) can be provided into which the emergent charcoal is displaced and cooled, for example by the application thereto of a non-flammable cooling liquid, gas or vapour. Conveniently the cooling chamber or chambers are formed by extensions of the roasting tubes. In the embodiment of Figure 2 the hopper 15 effectively forms a cooling chamberwithin which the charcoal is quenched.

The variable size aperture shown in Figures 4A and 4B can conveniently be used with any of the embodiments shown in Figures 1 to 3, for exam pl e at 48 (in Figure 1 ),38 (in Figure 2) or69 (in Figure 3).The tube in which the variable size aperture is formed may be separably secured to the end of the conduit 43 (in Figure 1), 6 (in Figure 2) or 62:(in Figure 3); however, the variable size aperture is conveniently and preferably formed integral with an end ofthe conduit forming the roasting chamber as shown in Figure 2 (therebyalleviating a possible problem of ensuring that the conduit and variable size aperture are co-axial which is preferred to achieve smooth progress of fibre/charcoal through the roasting chamber.

The variable size aperture shown in Figures 4A and 4B comprises the end of a cylindrical conduit 70 having a plurality of co-extensive slots 71 cut to extend longitudinally from orifice 73 in the end ofthe conduit 70so that the end of the conduit is formed from a plurality ofcircumferentially spaced strips 72.

The slots 71 extend radiallythrough the wall of the conduit and are equally spaced from each other around the circumference ofthe conduit.

Aflexible, extendable and contractible band 74 is located around the external circumference of the conduit 70 to overlie the strips 72. The band 74 may be extended or contracted by rotation of a screw 75 (or other gear) and resembles a conventional jubilee clip so that it can be tightened or loosened around the strips 72. The rotation of the screw 75 is effected by an electric motor 76 which responds automatically to changes in temperature sensed by temperature sensitive devices such as the thermostat49 described with referenceto Figure 1.

Thus when a fall in temperature is sensed,the electric motor 76 rotates the screw 75 causing the band 74to contract. The contraction of the band 74 causes the strips 72 to bend or flex inwardly from their end regions 77 (where they join the body ofthe conduit 70) so that the diameter ofthe orifice 73 is reduced (as shown in Figure 4B) and thereby a constriction or restriction is provided to the flow of charcoal from the roasting chamber. When the temperature sensed rises to a predetermined value (for example, the conversion temperature) the band 74 is loosened or extended by rotation of the screw 75. The pressure of charcoal being expelled through the orifice 73,togetherwith the resilience of the material of the strips 72 causes the strips 72 to move radiallyoutwardlyto increasethediameterofthe orifice 73 and thereby reduce the constriction or restriction to the flow of charcoal from the roasting chamber. The advantage of this variable size aperture shown in Figures 4A and 48 is that it can readily be formed in or on to the end of a conduitfor the roasting chamber; it is relatively simple and inexpensive; it permits the outlet aperture 73 to be co-axial with the bore of the conduit 70, and it provides a smooth internal transition surface from the bore ofthe conduit 70 to the outlet 73. In practice a smooth, progressive change in diameterfrom the bore ofthe conduit 70 to the outlet 73 is preferred to alleviate possible blockage of the charcoal in the conduit (for example by abutment ofthe charcoal against a shoulder in the conduit).

It has been found in experimental practice,thatthe change in diameter of the outlet aperture 73 which is necessary to effectively control the flow rate ofthe charcoal and fibre mixture through the roasting chamber conduit are, on a 20 cm diameter conduit 70, lessthan 1 cm (and not normally more than 0.3 cm). It will be appreciated that the variable size aperture is not necessarily located at an end of a conduit forming the roasting chamber and can be located partway along such a conduit.

Claims (42)

1. A method of continuously producing charcoal which comprises feeding plant fibre through a roasting chamber; heating said fibre in the roasting chamber during its progression therethrough and within a restricted airatmosphereto effect its conversion to charcoal; separating and removing from the charcoal in the roasting chamber volatile products ofthe conversion, and expelling the charcoal from the roasting chamber; and in which the conversion of plant fibre to charcoal within the roasting chamber is controlled by controlling the rate atwhich charcoal is permitted to emerge from the roasting chamber.

2. A method as claimed in claim 1 which comprises compressing the charcoal within the roasting chamber by the pressure applied thereto from the plant fibre which is fed into the roasting chamberforthe charcoal to adopt a cohesive form on its emergence from the roasting chamber.

3. A method as claimed in either claim 1 orclaim 2 which comprises expelling the charcoal from the roasting chamber and into a cooling chamberwithin which the charcoal is cooled.

4. A method as claimed in claim 3 which comprises expelling the charcoal from the roasting chamber into the or a cooling or mixture chamber within which the charcoal is quenched, bound and compacted into a cohesive form.

5. A method as claimed in any one ofthe preceding claims which comprises heating the plant fibre within the roasting chamber by electrical means.

6. A method as claimed in any one of the preceding claims which comprises heating the plant fibre within the roasting chamber by hot gases generated by combustion.

7. A method as claimed in claim 6 in which the hot gases are generated by combustion of plant fibre.

8. A method as claimed in any one of the preceding claims which comprises utilising derivativesfromthehotvolatile products for heating the plantfibre.

9. A method as claimed in any one ofthe preceding claims which comprises condensing the hotvolatile productsto provide specific derivatives therefrom.

10. A method as claimed in any preceding claims which comprises controlling the said conversion by adjusting the size of an aperture thrnugh which the charcoal emerges from the roasting chamber.

11. A method as claimed in claim 10 which comprises detecting the temperature in which the roasting chamberwhere conversion ofthe plant fibre to charcoal is to be effected and controlling the size of said aperture through which the charcoal emerges to decrease that size in response to a fall in the detected temperature and to increase that size in response to a rise in the detected temperature.

12. A method of continuously producing charcoal as claimed in claim 1 and substantially as herein described.

13. Charcoal when produced by the method as claimed in any one of the preceding claims.

14. Apparatus for continuously producing charcoal which comprises a roasting chamber; means for continuously feeding plant fibre into and through the roasting chamber; means for heating the fibre in the roasting chamber during its progression therethrough and within a restricted air atmosphere to effectthe conversion of the fibre into charcoal; means for separating and removing volatile products of the conversion from the charcoal, and means for expelling the charcoal from the roasting chamber; and wherein the roasting chamber has an aperture through which the charcoal emerges, said aperture having associated therewith feed control means by which the rate of of flow of charcoal therethrough and thereby the rate of flow of plantfibre/charcoal through the roasting chamber is controlled to control the rate of conversion ofthe plant fibre into charcoal.

15. Apparatus as claimed in claim 14 in which the roasting chamber comprises a tubular conduit through which the plant fibre passes for its conversion to charcoal.

16. Apparatus as claimed in either claim 14 or claim 15 and comprising a primaryfeed means by which the plant fibre is fed under pressure into the roasting chamberfor its passage therethrough.

17. Apparatus as claimed in any one of claims 14 to 16 in which the feed control means comprises means for adjusting the size of the aperture through which the charcoal emerges.

18. Apparatus as claimed in claim 17 wherein the means for adjusting the size of the aperture provides for a smooth and progressive internal transition surfaceforthe roasting chamber.

19. Apparatus as claimed in either claim 17 or claim 18 wherein the means for adjusting the size of the aperture comprises a plurality of peripherally spaced strips extending longitudinally ofthe roasting chamber and forming the aperture, said strips being flexible radially, and means being provided to effect radial displacement of the stips to vary the diameterofthe aperture.

20. Apparatus as claimed in claim 19 wherein the means to effect radial displacement of the strips comprises an extendible and contractible band located around the periphery of the strips and means to effect extension and contraction of that band.

21. Apparatus as claimed in claim 20 wherein the means to effect extension and contraction ofthe band is driven by an electric motor capable of being automatically actuated.

22. Apparatus as claimed in any one of claims 14 to 21 and comprising temperature sensing means for indicating the temperature of the plantfibre/charcoal within the roasting chamber.

23. Apparatus as claimed in claim 22 in which the feed control means is responsive to said temperaturesensing means and reactsto decrease the rate at which charcoal emerges from the roasting chamberwhen said temperature sensing means senses a decrease in temperature below a predetermined temperature and vice versa.

24. Apparatus as claimed in either claim 22 or claim 23 in which the temperature sensing means comprises a thermostatic switch located to be responsive to temperature ata region of the roasting chamber at which the plant fibre is expected to undergo conversion to charcoal.

25. Apparatusasclaimed in anyoneofclaimsl4 to 24 in which the heating means comprises an insulated enclosure having inlet and outlet ports and within which the roasting chamber is located, and means for developing hot gases generated by combustion of a fuel in a combustion chamber which hot gases are fed to said enclosure by way ofthe inlet port to heatthe roasting chambers

26. Apparatus as claimed in claim 25 and comprising a heattransferfin or flange extending from the exterior of the roasting chamber to promote the transfer of heat from the hot gases to the roasting chamber.

27. Apparatus according to any one of claims 14 to 26 and comprising electrical heating meansforthe roasting chamber.

28. Apparatus as claimed in anyoneofclaimsl4 to 27 in which the roasting chamber is provided with vents through which vapour or gases derived from volatile by-products in the conversion of the plant fibre to charcoal are extracted from the roasting chamber.

29. Apparatus according to claim 28 wherein said vapours or gases derived from the by-products are vented to heat the external surface ofthe roasting chamber.

30. Apparatus as claimed in any one of claims 14 to 28 and comprising condenser means for condensing gases or vapours derived from the volatile by-products in the roasting chamberwhich results from the conversion of the plant fibre to charcoal.

31. Apparatus as claimed in claim 30 in which the condenser fractionally separates constituents ofthe by-products.

32. Apparatus as claimed in anyoneofclaimsl4 to 31 in which the heating means is intended to comprise hot gases derived from the combustion of plant fibre, said plant fibre being derived from a source which is common to that from which plant fibre is derived for feeding into the roasting chamber.

33. Apparatus as claimed in any one of claims 14 to 32 in which the roasting chamber comprises an array oftubular conduits within each of which plant fibre is converted to charcoal.

34. Apparatus as claimed in claim 33 in which the tubular conduits are located in a common enclosure to be heated by hot gases.

35. Apparatus as claimed in either claim 33 or claim 34 in which plant fibre is fed to all ofthetubular conduits in the array from the common feed mechanism.

36. Apparatus as claimed in any one of claims 14 to 35 and comprising a cooling chamber into which the charcoal is displaced from the roasting chamber and means for cooling the charcoal in the cooling chamber.

37. Apparatus as claimed in claim 36 in which the cooling chamber comprises means for quenching the charcoal and feed means capable of displacing the quenched charcoal from said cooling chamber in compressed cohesive form.

38. Apparatus as claimed in claim 37 in which the quenching means comprises a binding agent.

39. Apparatus as claimed in any one of claims 14 to 38 and mounted to be mobile.

40. Apparatus for continuously producing charcoal substantially as herein described with reference to Figure 1 and Figures4Aand 4B ofthe accompanying illustrative drawings.

41. Apparatus for continuously producing charcoal substantially as herein described with reference to Figure 2 and Figures 4A and 4B ofthe accompanying illustrative drawings.

42. Apparatus substantially as herein described with reference to Figure 3 and Figures 4A and 4B of the accompanying illustrative drawings.