GB2322439A - Plant and process for carbonising vegetable matter - Google Patents

Abstract

A vegetable matter carbonisation plant comprising a kiln (100) adapted to accommodate vegetable matter, the kiln comprising a gas inlet port (104) for introducing gas into the kiln and a gas outlet port (105) for discharging gas from the kiln; hot gas supply means including a hot gas main pipe in fluid communication with the gas inlet port, the hot gas supply means being adapted to supply hot gas to the kiln for carbonising the vegetable matter to produce charcoal; and, cooling means for cooling the charcoal in the kiln after carbonisation. The vegetable matter is preferably peat and is introduced into the kiln in a metal basket (110). Warm gas for drying is introduced before the hot gas.

Description

PLANT AND PROCESS FOR CARBONISING VEGETABLE MATTER The present invention relates to a plant and process for carbonising vegetable matter and particularly, though not exclusively, to a plant and process for carbonising peat to produce charcoal.

The use of simple ring-type kilns to carbonise vegetable matter such as wood, nut shells or peat to charcoal is well known. Such kilns, however, use substantial amounts of raw materials (at least 6 tonnes of wood for every tonne of charcoal produced) and are known to cause considerable pollution, particularly in the form of hydrocarbons which are considered to be significant environmental carcinogens.

Factory scale production of charcoal by the Lambiotte process is also known. This process requires high structures, hoists to load the kilns and elaborate discharge and cooling arrangements for the product. This, and other continuous processes tend to degrade the charcoal particle size which reduces its value.

The present invention provides a vegetable matter carbonisation plant which includes a kiln, a gas supply means for supplying gas to the kiln and a cooling means for cooling charcoal in the kiln.

Accordingly, in one aspect, the invention provides a vegetable matter carbonisation plant comprising: a kiln adapted to accommodate vegetable matter, the kiln comprising a gas inlet port for introducing gas into the kiln and a gas outlet port for discharging gas from the kiln; hot gas supply means including a hot gas main pipe in -fluid communication with the gas inlet port, the hot gas supply means being adapted to supply hot gas to the kiln for carbonising the vegetable matter to produce charcoal; and cooling means for cooling the charcoal in the kiln after carbonisation.

The vegetable matter carbonisation plant of the current invention has the advantage that the vegetable matter is not disturbed during the carbonisation and cooling steps. This increases the yield of large particles of charcoal so increasing the charcoal value.

The vegetable matter carbonisation plant of the current invention also has the advantage that it can be loaded and unloaded easily without the need for specialised equipment.

Preferably, the carbonisation plant further comprises drying gas supply means including a drying gas main pipe in fluid communication with the gas inlet port of the kiln, the drying gas supply means being adapted to supply drying gas to the kiln for drying the vegetable matter prior to carbonisation, the drying gas being cooler than the hot gas.

This enables vegetable matter to be introduced into the kiln, dried, carbonised and cooled without being disturbed. This further increases the yield of charcoal having a large particle size.

Preferably the drying gas supply means further comprises a drying gas return pipe in fluid communication with the gas outlet port, the drying gas supply means comprising a gas blower for circulating the drying gas between the main pipe and return pipe of the drying gas supply means via the kiln.

The cooling means of the vegetable matter carbonisation plant can be a water spray cooling means. This efficiently cools the charcoal so reducing cooling time.

The cooling means of the vegetable matter carbonisation plant can be a cooling gas supply means comprising a cooling gas main pipe in fluid communication with the gas inlet port of the kiln and being adapted to supply cooling gas to the kiln to cool the charcoal.

The cooling gas supply means can further include a cooling gas return pipe in fluid communication with the gas outlet port of the kiln, the cooling gas supply means being adapted to cool at least a portion of the gas received by the cooling gas return pipe to provide the cooling gas. This circulation of cooling gas reduces pollution to the surrounding air and prevents air from entering the kiln whilst the charcoal is still hot.

The cooling gas supply means can further comprise a gas blower for circulating the cooling gas between the main and return pipes via the kiln.

Preferably, the hot gas supply means further comprises a hot gas return pipe in fluid communication with the gas outlet port of the kiln, the hot gas supply means further comprising a hydrocarbon extractor for extracting hydrocarbons from the gas received by the hot gas return pipe to produce off gas. The hydrocarbon extractor can comprise a tar cyclone. The hydrocarbon extractor can further comprise a tar pot and tar tank. This extraction of hydrocarbons from the hot gas before recirculation through the kiln prevents tar build up throughout the carbonisation plant.

The hot gas supply means can further comprise a furnace for at least partly oxidising the off gas to produce hot gas.

This oxidation of off gas to produce hot gas which is then repassed through the kiln reduces fuel consumption and pollution during the carbonisation stage.

Preferably, the furnace comprises a second chamber for burning the off gas in excess air to produce drying gas, the second chamber being in fluid communication with the drying gas main pipe.

Preferably, the tar tank of the hydrocarbon extractor is connected to the furnace so that the tar stored in the tar tank is supplied as fuel to the furnace. This reduces the fuel consumption during the carbonisation process as the hydrocarbons produced by the carbonising vegetable matter are used to fuel the furnace.

The hot gas supply means can comprise a conduit extending between the hydrocarbon extraction means and the hot gas main pipe for controllably diluting the hot gas added to the kiln with off gas. This increases charcoal yield and reduces carbonisation temperature.

The carbonisation plant can further comprise a vegetable matter basket adapted to accommodate the vegetable matter within the kiln. The basket can be adapted so as to enable gas to flow through the vegetable matter. The basket can have a grating bottom. Such a basket improves the drying of the vegetable matter by the gas flowing through the kiln.

The carbonisation plant can'further comprise a plurality of kilns, the hot gas supply means and drying gas supply means being arranged such that the heat generated by the oxidation of off gas from one kiln can be used to heat the drying gas of another kiln. By operating the kilns out of phase so that one kiln is drying vegetable matter whilst another is carbonising vegetable matter one can significantly reduce the amount of energy required by the kiln. This is because all of the excess heat generated by the oxidation of the off gas can be used to heat drying gas rather than it being dissipated to the plant's surroundings. The plant can be made self sufficient in fuel terms.

The carbonisation plant can further comprise a transfer conduit for transferring drying gas from one kiln to the hot gas of another kiln. This enables the hot gas temperature to be controlled precisely and easily.

The kiln can be a ring kiln.

Each kiln can include a plurality of gas nozzles in fluid communication with the gas inlet port, support means supporting the vegetable matter basket and sealing means to seal the vegetable matter basket into the kiln so that the gas entering the kiln via the gas inlet port passes through the vegetable matter basket.

The vegetable matter basket can be a right cylindrical metal basket having solid walls and a hinged releasable grating for a base.

The vegetable matter basket can be rectangular in cross section.

The kiln can include a loading aperture for loading the vegetable matter basket into the side of the kiln.

According to a further aspect of the invention there is provided a vegetable matter carbonising process comprising the steps of placing vegetable matter in a kiln, carbonising the vegetable matter by passing hot gas through it to produce charcoal and, cooling the charcoal in the kiln.

The carbonising process of the invention can further include the step of drying the vegetable matter in the kiln by passing drying gas through it before the vegetable matter is carbonised.

The carbonising process of the invention can further include the step of collecting the gas produced by carbonising the vegetable matter and combusting this collected gas to provide further gas to carbonise vegetable matter.

The carbonising process of the invention can further include the step of mixing the hot gas with the drying gas to control the temperature of the hot gas.

The carbonising process of the invention can further include the step of mixing the hot gas with the collected gas to control the carbonising temperature of the vegetable matter.

The carbonising process of the invention can further include the step of mixing the hot gas from one kiln with the drying gas of another kiln to control the temperature of the hot gas.

The carbonising process of the invention can further include the step of extracting hydrocarbons from the gas produced by carbonising the vegetable matter and supplying the hydrocarbons as fuel to a furnace which produces hot gas used in carbonising the vegetable matter.

The carbonising process of the invention can further include the step of using the heat generated by combusting the gas collected from one kiln to heat the drying gas of another kiln.

The present invention will now be described by way of example only, with reference to the accompanying drawings in which: Figure 1 is a sectioned view of a kiln for use in the process of the invention; Figure 2 is a schematic diagram of a first embodiment of a carbonisation plant according to the present invention; Figure 3 is a flow chart illustrating a process according to the invention; Figure 4 is a schematic illustrating the mean gas flow through the pipelines of a second embodiment of a carbonisation plant according to the invention; Figure 5 is a schematic illustration of a third embodiment of a carbonisation plant according to the invention.

The mean gas flow in the pipelines of this plant is shown; Figure 6 is a schematic illustration of a fourth embodiment of a carbonisation plant according to the invention; and Figure 7 is a schematic illustration of a fifth embodiment of a carbonisation plant according to the invention.

With reference to Figure 1, there is shown a preferred ring kiln adapted for use in a plant operating by the process according to the invention. The kiln, designated generally by reference numeral 100, is intended for use in the carbonisation of vegetable matter. The kiln 100 is typically a ring kiln, having a circular wall 101, floor 102 and removable roof 103.

The roof may include a spring loaded explosion relief valve (not shown) and be mounted on guides which allow it be swung aside when access is required to the kiln. The kiln is typically made from mild steel lined by a thermally insulating, refractory materials well known in the art. There is provided a gas inlet port 104, near the base of the kiln and a gas outlet port 105 near the roof 103 of the kiln. A number of gas nozzles 116 connected to the inlet port 104 are located in the base of the kiln. A first thermocouple 106 is provided near the base of the kiln and a second thermocouple 107 is provided in the roof. Pressure sensors 108 may also be provided in the roof. Support means 109 are located on the floor of the kiln and a soft seal 115 is provided around the base of the kiln.

The kiln is constructed such that when the roof is in place, it is substantially air tight. The kiln also includes a loading door in its side wall which can be used when loading or emptying the kiln.

A removable metal basket 110 is also provided. Typically it is right cylindrical and formed so that it fits flush to the internal surface of the kiln wall, while its base 113 engages the support means 109 and the soft seal 115. The basket comprises a solid wall 111 and has a rolled ring 112 running around an open top which is further supported in a metal ring 114 provided near the top of the kiln to locate the basket correctly.

The base of the basket 113 is made from a grate, typically an industrial standard grating of appropriate pitch and depth. Preferably, the base is provided with hinges or other such devices so that it can be opened. Typically the grate is constructed from lateral members and transverse cross members, with the cross members being less deep than the lateral members. The basket typically has a diameter of approximately two and a half metres and is approximately two metres deep. Sufficient members are provided in the grate to support any load in the basket but as open a grate structure as possible is preferred so as not to hinder gas flow through the bottom of the basket.

In an alternative embodiment the vegetable matter basket is rectangular in cross section.

A preferred embodiment of a plant according to the invention will now be described with reference to Figure 2.

The plant is designated generally by reference numeral 200 and comprises a plurality of similar kilns 201a to 201f as previously described with reference to Figure 1. The plant further comprises a drying gas supply means comprising a drying gas main pipe 202, a drying gas return pipe 203 and a drying gas blower 203; a hot gas supply means comprising a hot gas main pipe 205, a hot gas return pipe 206, a tar extractor 207 and a central furnace 208; and a cooling gas supply means comprising a cooling gas main pipe 209, a cooling gas return pipe 210, a gas cooler 211 and a cooling gas blower 212. The main pipes of the supply means are the parts of the supply means that supply gas to the kilns and the return pipes are the parts of the supply means that take gas from the kilns.

Further subsidiary pipes are provided together with valves to control the flow of gas so as to selectively provide a flow path through each kiln for each of the three pipes. For example, for kiln 201a there are provided an input pipe 213 connected to kiln gas inlet port 104 and an output pipe 214 connected to kiln gas outlet port 105 and numerous valves, designated generally by reference numeral 215, to control which gas pipeline, or pipelines, the kiln is in communication with.

The tar extractor 207 is located in the hot gas pipeline.

It comprises a cyclone 216, a tar pot 217, a tar tank 218 and a hot gas blower 219. The cyclone and tar pot act on gases passing through them to extract tars which are then stored in the tar tank. The hot gas blower causes gas in the hot gas pipeline to circulate in a generally clockwise direction and feeds gas from the tar extraction system to the central furnace 208. The central furnace 208 is also located in the hot gas pipeline. The central furnace 208 comprises a burner 220, a first combustion chamber 221, a second combustion chamber 222, a stack 223 and a forced draft fan 224.

A pipe 225 with a valve provides selective communication between the hot gas pipeline and the drying gas pipeline and a pipe 226 with a valve provides selective communication between the furnace and the drying gas pipeline.

By way of example only the operation of the plant by the process will be described in respect of the carbonisation of peat to produce charcoal with reference to Figures 2 and 3.

There are three main steps to the carbonisation cycle: a drying step, a carbonisation step and a cooling step.

The drying step is a consequence of the considerable proportion of peat that is moisture. Raw peat is typically harvested by a bog extrusion process. Peat is scooped up, macerated and extruded under low pressure as pellets having typical diameters of approximately 50mm to 70mm, when wet, and a length to diameter ratio of approximately 1.5. At this stage the peat is approximately 80% water by mass. The peat is left to air dry but this only reduces the water content to approximately 35% to 45%. (Previous processes have utilised a further external drying step, using a conventional industrial drier in a tunnel, whereas in the Lambiotte process, peat is pre-dried in situ in a kiln.) In the drying step, a basket is filled with air dried peat pellets and the basket is placed in a kiln so that the kiln seals internally owing to the soft seal. The roof of the kiln is closed so that the kiln is substantially air tight.

The peat pellets are then dried in situ in the kiln by passing warm drying gas from the furnace system through the pellets by way of the drying gas pipeline. If no other kilns are currently being operated, then a fuel, typically propane or oil, is burned in the furnace system and the hot combustion gases, typically carbon dioxide and nitrogen, are passed to the drying gas pipeline by either of pipes 225 or 226 to provide the warm drying gas. (The warm drying gas is actually a hot gas but is nominally warm so as to distinguish it from the hot carbonising gas described later.) The kilns can process the vegetable matter simultaneously. This is relatively simple to build. Alternatively, the kilns may be out of phase with different kilns performing different stages of the process. In this case the gasses resulting from carbonisation being carried out in the one kiln may be re-directed from the hot gas pipeline to the drying gas pipeline by pipe 225 to provide warm drying gas.

The warm drying gas circulates in the drying gas pipeline and enters the base of a kiln by way of its gas inlet port and is distributed uniformly through the peat pellets by the gas nozzles 116 in the base of the kiln. A large empty space above and below the basket will allow for even gas distribution within the peat. As the basket is sealed at the base of the kiln and has solid walls, it forms a conduit causing all the gases entering the base of the kiln to flow through the material located in the basket. The warm drying gas passes upwardly through the peat pellets extracting moisture from them and so drying them. The lower most layer of pellets dries first, rigidifying the lower strata and leaving voids through which the drying gas passes to progressively dry the upper layers. The warm drying gas temperature is controlled so that the peat is dried at a temperature below 180 C so as to prevent carbonisation before the peat is sufficiently dry. The warm drying gas is recirculated around the drying gas pipeline and maintained at a temperature of approximately 1800C by introducing further hot gas either from the hot gas pipeline or directly from the furnace.

As the temperature of the drying gas when it exits the kiln is approximately 10-50"C, part of this gas is used to cool hot charcoal concurrently being produced in a different kiln before passing back into the drying gas pipeline. Cool drying gas that has left a kiln may also be directed into the hot gas pipeline so as to cool the hot carbonising gas (as described later) to prevent coking in the pipelines and to allow the tars to condense.

The drying stage is quite a slow process and it takes approximately 8 hours to dry the peat pellets. Preferably the peat is dried until it has a water content of approximately 5%10% by weight. The thermocouple in the roof of the kiln is used to monitor the temperature of the gas passing through the peat and an increase in the monitored temperature, to approximately 100"C, indicates the completion of the drying stage.

The carbonisation step is carried out by recirculating hot carbonising gas around the hot gas pipeline via the kiln, the tar extractor and the furnace. As carbonisation is a mildly exothermic reaction, most of the energy input to the carbonisation cycle goes into the evaporation of water from the peat in the drying stage. If no other kilns are being operated then the hot carbonising gas is provided by the combustion products of burning fuel, such as propane or oil as used previously in the drying stage, in the furnace. The peat is brought up to the carbonising temperature by increasing the temperature of the hot carbonising gas entering the kiln.

There are two gas stream entering the kiln: the hot gas stream from the furnace via the hot gas pipeline and warm gas via the drying gas pipeline. The temperature of the hot carbonising gas entering the kiln can be controlled by varying the proportions of hot gas and warm gas in the hot carbonising gas.

The temperature of the hot carbonising gas is increased to approximately 400-450"C while the gas exiting the kiln stays at a temperature of approximately 100"C until the carbonisation starts. At this time the temperature of the gas exiting the kiln increases and its constitution changes from predominantly steam to a rich off gas. The rich off gas contains the products of the carbonisation reaction which are threefold: tar, incondensible gases (such as carbon dioxide, carbon monoxide, hydrogen and methane) and pyroligneous vapours. The temperature of the hot carbonising gas is controlled to maintain the carbonisation temperature in the kiln above 1800C with a maximum carbonisation temperature of approximately 260"C. Changing the carbonisation temperature changes the carbon content, yield and flammability of the charcoal produced. The actual heat provided by the hot carbonising gas flowing through the peat is low compared to the heat capacity of the peat. An exothermic reaction occurs in the peat during carbonisation and the presence of some water (approximately 5t) in the peat helps to control the temperature of the rich off gas: i.e. the latent heat of vaporisation of the water in the peat helps to control the exit temperature of the rich off gas produced by carbonisation. The temperature of the rich off gas and kiln pressure are monitored during the carbonisation process by the thermocouple and pressure sensors 108 mounted in the roof of the kiln. The pressure is monitored to check for air leaks which would result in the potentially hazardous presence of oxygen leading to spontaneous combustion of the charcoal.

The rich off gas leaves the kiln via its gas outlet and circulates around the hot gas pipeline. The rich off gas passes through the tar extractor 207. Tar is extracted from the rich off gas by means of a cyclone 216 and a tar pot 217 and stored in a tar tank 218. The stored tar may be kept as a resource as it is, or it is preferably used in the furnace as fuel. Once the rich off gas has had its tar removed it is nominally off gas and it is passed on to the furnace 208 by an off gas blower 219. In the furnace 208, the off gas is burned which oxidises some of the organic pollutants. As the sulphur content of peat is low (normally about 0.4%), the major acidic pollutant is nitric acid and this burning of the off gas helps reduce the generation of pollutant oxides of nitrogen. The burning of the off gas to reduce the production of oxides of nitrogen is a two-stage combustion process in which much of the enthalpy of reaction in the first stage is removed as process heat. In the second stage a lower temperature is used which reduces the amount of oxides of nitrogen formed as the formation of oxides of nitrigen is an endothermic reaction. By recirculating the off gas through the hot gas pipeline and burning it in the furnace system, further hot gas is produced for the kilns and the amount of pollutant produced is reduced.

The temperature of the hot carbonising gas in the hot gas pipeline can be further controlled by changing the proportions of hot gas from combustion in the furnace and off gas from the tar extractor which has not been burned in the furnace. It is necessary to extract the tars from the rich off gas in order to avoid them condensing where they are not wanted. When tars are extracted from the rich off gas, the yield of charcoal may be enhanced and the carbonisation temperature reduced by diluting the hot carbonising gas with some of the off gas thereby ensuring that reducing conditions prevail in the kiln during carbonisation.

Tar extracted from the rich off gas is used as fuel for a burner 220 in the furnace system 208. A two stage burning process is used. Some of the tar is burned to oxidise the off gas in less than stoichiometric air in a first combustion chamber 221. Then the off gas is burned in a second combustion chamber 222 in an atmosphere of excess air, provided by a forced draft fan 224. The combustion products from the first combustion chamber 221 provide hot gas in the hot gas pipeline, while the combustion products from the second combustion chamber 222 provide hot gas for the drying gas pipeline, or are allowed to vent to the atmosphere via the stack 223. The furnace system may have more than two stages, so that it is a multi stage burner in which hot gases for further use in the process are extracted before the injection of secondary air and further combustion. Those waste gases from combustion which can not be used further in the process may be allowed to disperse in the environment through the stack which is of a suitable height. Alternatively these hot waste gases may be used to dry raw or other materials.

The carbonisation step takes approximately 4 hours and results in pellets, shrunk form their original dimensions, of charcoal which is predominantly carbon. There t-hen follows a cooling step.

The charcoal must be cooled to below approximately 80"C before the kiln can be opened and the charcoal exposed to air, otherwise the charcoal will spontaneously combust with the oxygen present in air. For this reason it is vital to keep air out of the kiln during the carbonisation step. The kiln is removed from the hot gas pipeline and drying gas pipeline and connected into the cooling gas pipeline. Preferably the charcoal is cooled until its temperature is below 50 C. Cool gas at a temperature of approximately 30"C is passed from the cooling gas main line into the kiln via its gas inlet port, passes through the charcoal and out of the kiln via its gas outlet port and into the cooling gas return line. The cool gas may be cooled drying gas that has passed through a kiln as mentioned earlier or the cool gas may be cooled combustion products from the furnace. After approximately 4 hours the temperature of the charcoal is reduced to approximately 80"C as heat has been transferred to the cooling gas, increasing the temperature of the cooling gas as it passes into the cooling gas return line. The warmed cooling gas passes around the cooling gas pipeline and through a gas cooler 211 which utilises cooling water to cool the warmed cooling gas. The gas cooler also acts as a cooling gas cleaner and removes some of the waste products picked up form the charcoal by the cooling gas. The cooling water is further cooled by a conventional fan (not shown). The cooled cooling gas is circulated around the cool gas pipeline by a gas blower 212. Alternatively, the cooling gas may be drawn entirely from the drying gas return line when peat is being dried in other kilns.

When the charcoal has cooled to a temperature below 80 C, as measured by the thermocouples provided in the kiln, the roof of the kiln is removed and the metal basket containing the charcoal is removed by conventional lifting equipment, e.g. a fork lift truck. The charcoal is then removed from the basket and conditioned by leaving it under cover for 24 hours to come to equilibrium with the atmosphere, during which time it absorbs approximately 7% by weight moisture from the atmosphere. The provision of a hinged base to the basket eases removal of the charcoal from the basket. The basket can be positioned above a storage hopper, the base of the basket released and the charcoal allowed to pass into the storage hopper. Such a method of removing the charcoal from the kiln helps to reduce the amount of fines generated by handling to approximately 5%. Previous charcoal production processes, such as Lambiotte's, use a continuous downflow process which requires the charcoal to pass down a tall structure generating approximately 30% fines. Fines are a far less commercially valuable product and so their generation should be minimised.

Fines may be processed to make charcoal briquettes but briquettes provide less than half the revenue of solid charcoal.

Shown in figure 4 is a schematic showing the typical gas flow through the pipelines of a second embodiment of a carbonisation plant according to the invention during the drying, heating, carbonising and cooling stages of the process.

Shown in figure 5 is a schematic illustration of a third embodiment of the carbonisation plant of the invention. Unlike the previous embodiment this embodiment lacks a tar cyclone.

Mean values are shown for the flow of gas through the pipelines in minimum flow and maximum flow configurations. This embodiment comprises two kilns which can be operated independently.

Shown in figure 6 is a schematic illustration of a fourth embodiment of the invention. This embodiment comprises four kilns which can also be operated independently. Again, mean flows through the pipelines are shown in minimum and maximum flow configurations.

Shown in figure 7 is a schematic illustration of a fifth embodiment of the invention. Unlike the previous embodiments this embodiment comprises an external dryer which dries the vegetable matter before it is placed in the kilns. As the kilns of this embodiment are only used for carb sized product, seasonal variations in demand for a certain charcoal product can be accommodated.

The plant also provides environmental and cost advantages. The separation of tars from the rich off gas provides a cheap source of fuel for use in the furnace and also helps to reduce pollution by tars which are considered to be major potential carcinogens. The tars may also be used as fuel for a multi-fuel engine or for a steam generator so as to generate power as well as heat. Further, the combustion of the remaining off gas helps to further reduce the amount of pollution generated especially as nitrous precursors for acid rain. Indeed, the closed pipeline plant which allows the various gases used in the process and produced by the process to be recirculated in different pipelines helps to reduce the amount of pollution generated by the plant over other plants which merely use high chimneys to exhaust pollutants to the atmosphere. The two combustion chamber furnace also helps to reduce the amount of pollution by providing atmospheres having the optimum combustion conditions to most efficiently burn the gases used in the process.

Claims (29)

1. A vegetable matter carbonisation plant comprising: a kiln adapted to accommodate vegetable matter, the kiln comprising a gas inlet port for introducing gas into the kiln and a gas outlet port for discharging gas from the kiln; hot gas supply means including a hot gas main pipe in fluid communication with the gas inlet port, the hot gas supply means being adapted to supply hot gas to the kiln for carbonising the vegetable matter to produce charcoal; and cooling means for cooling the charcoal in the kiln after carbonisation.

2. A plant as claimed in claim 1, further comprising a drying gas supply means, the drying gas supply means including a drying gas main pipe in fluid communication with the gas inlet port of the kiln, the drying gas supply means being adapted to supply drying gas to the kiln for drying the vegetable matter prior to carbonisation, the drying gas being cooler than the hot gas.

3. A plant as claimed in claimed in claim 2, the drying gas supply means further comprising a drying gas return pipe in fluid communication with the gas outlet port, the drying gas supply means comprising a gas blower for circulating the drying gas between the main pipe and return pipe of the drying gas supply means via the kiln.

4. A plant as claimed in claim 1 wherein the cooling means is a water spray cooling means.

5. A plant as claimed in claim 1 wherein the cooling means is a cooling gas supply means comprising a cooling gas main pipe in fluid communication with the gas inlet port of the kiln and is adapted to supply cooling gas to the kiln to cool the charcoal.

6. A plant as claimed in claim 5, the cooling gas supply means further comprising a cooling gas return pipe in fluid communication with the gas outlet port of the kiln, the cooling gas supply means being adapted to cool at least a portion of the gas received by the cooling gas return pipe to provide the cooling gas.

7. A plant as claimed in either of claims 5 or 6, the cooling gas supply means further comprising a gas blower for circulating the cooling gas between the main and return pipes via the kiln.

8. A plant as claimed in claim 1, the hot gas supply means further comprising a hot gas return pipe in fluid communication with the gas outlet port of the kiln, the hot gas supply means further comprising a hydrocarbon extractor for extracting hydrocarbons from the gas received at the hot gas return pipe to produce off gas.

9. A plant as claimed in claim 8, the hot gas supply means further comprising a furnace for at least partially oxidising the off gas to produce hot gas.

10. A plant as claimed in claim 9, the furnace further comprising a second chamber for burning the off gas to produce drying gas, the second chamber being in fluid communication with the drying gas main pipe.

11. A plant as claimed in claim 8, the hydrocarbon extractor further comprising a tar tank for storing the extracted hydrocarbons.

12. A plant as claimed in claim 11 wherein the tar tank of the hydrocarbon extractor is connected to the furnace so that the tar stored in the tar tank is supplied as fuel to the furnace.

13. A plant as claimed in claim 8 further comprising a conduit extending between the hydrocarbon extraction means and the hot gas main pipe for controllably diluting the hot gas added to the kiln with off gas.

14. A plant as claimed in any one of claims 1 to 13, further comprising a vegetable matter basket adapted to accommodate the vegetable matter within the kiln.

15. A plant as claimed in claim 2 further comprising a plurality of kilns, the hot gas supply means and drying gas supply means being arranged such that the heat generated by the oxidation of the off gas from one kiln can be used to heat the drying gas of another kiln.

16. A plant as claimed in claimed in claim 2 further comprising a transfer conduit for transferring drying gas from one kiln to the hot gas of another kiln.

17. A plant as claimed in claim 1 wherein the kiln is a ring kiln.

18. A plant as claimed in claim 14 wherein the kiln comprises a plurality of gas nozzles in fluid communication with the gas inlet port, support means for supporting the vegetable matter basket and sealing means to seal the vegetable matter basket into the kiln so that the gas entering the kiln via the gas inlet port passes through the vegetable matter basket.

19. A plant as claimed in claim 14, wherein the basket is a right cylindrical metal basket having solid walls and a hinged releasable grating for a base.

20. A plant as claimed in claim 14, wherein the basket has a rectangular cross section.

21. A plant as claimed in claim 1, the kiln further comprising a loading aperture for loading the vegetable matter basket into the side of the kiln.

22. A vegetable matter carbonising process comprising the steps of placing vegetable matter in a kiln, carbonising the vegetable matter by passing hot gas through it to produce charcoal and, cooling the charcoal in the kiln.

23. A carbonising process as claimed in claim 22 further including the step of drying the vegetable matter in the kiln by passing drying gas through it before the vegetable matter is carbonised.

24. A carbonising process as claimed in claim 22 further including the step of collecting the gas produced by the carbonising vegetable matter and combusting the collected gas to provide further gas to carbonise vegetable matter.

25. A carbonising process as claimed in claim 23 further including the step of mixing the hot gas with the drying gas to control the temperature of the hot gas.

26. A carbonising process as claimed in claim 24 further including the step of mixing the hot gas with the collected gas to control the carbonising temperature of the vegetable matter.

27. A carbonising process as claimed in claim 23 further including the step of mixing the hot gas from one kiln with the drying gas of another kiln to control the temperature of the hot gas.

28. A carbonising process as claimed in claim 22 further including the step of extracting hydrocarbons from the gas produced by carbonising the vegetable matter and supplying the hydrocarbons as fuel to a furnace which produces hot gas used in carbonising the vegetable matter.

29. A carbonising process as claimed in claim 23 further including the step of using the heat generated by combusting the gas collected fro one kiln to heat the drying gas of another kiln.