A kiln

Abstract

A kiln 2 comprises a base 4 and side walls 6 which define a kiln chamber 2a. A conveyor comprises a housing 10 that passes through an aperture in the side wall with a first open end located within the

Description

The present invention relates to a kiln and, in particular, to a kiln for use in the production of charcoal. The charcoal produced by the kiln is suitable for use as biochar. Biochar is a solid carbonaceous material that results from the thermochemical conversion (thermal decomposition) of biomass in a low-oxygen or oxygen-free environment. It is a stable solid that is rich in pyrogenic carbon. Biochar has many uses, such as a soil supplement, as part of animal fodder and as a carbon sink. For example, it may be used as part of a compost material. Biochar is typically produced in a batch process in which a pyrolytic chamber is loaded with a biomass material, which is then combusted in a low-oxygen or oxygen-free environment to produce biochar as the main product. However, batch process tend to be labour-intensive and tend to only be operated at certain times of the day as a result of this. It is desired to provide an apparatus that is able to produce biochar on a continuous and / or automated basis. According to a first aspect of the invention, there is provided a kiln comprising a base and one or more side walls which together define a kiln chamber; a conveyor apparatus including a housing that defines opposed open ends, wherein a first open end of the conveyor apparatus is located within the kiln chamber adjacent to the kiln base, the housing passes through an aperture defined by the or one of the side walls, and a second open end of the housing is external to the kiln chamber; and wherein the kiln includes a water inlet and / or a water outlet to maintain a predetermined depth of water at the bottom of the kiln chamber in use. When using the kiln of the present invention, the formed biochar material may be removed via the conveyor apparatus. This permits a continuous operation of the kiln. In the context of the present invention, "continuous" means that removal of the biochar may occur without interrupting the operation of the kiln. This is different to the known batch-production of biochar in which the operation of kiln is stopped to enable removal of the biochar material. Accordingly, the kiln may be operated continuously, even if the biochar material is not removed continuously. In an embodiment of the invention, the or each side wall tapers outwards from the base. Thus, the width dimension or diameter of the kiln decreases towards the base. For example, the side wall(s) of the kiln may be frustoconical or frustopyramidal, wherein the conic section or pyramidal section is narrower towards its bottom. It is believed that kilns that include outwardly tapering walls have a more efficient burn chamber. However, it is possible that the advantages of having outwardly tapering walls are not great. Accordingly, kilns with substantially vertical walls are also within the scope of the invention as defined herein. It will be appreciated that kilns having substantially vertical side walls may be less complicated and therefore may be cheaper and easier to construct. Suitably, the conveyer apparatus includes an auger disposed within the housing. In such embodiments, rotation of the auger about its longitudinal axis urges the biochar material from the first open end of the housing to the second open end of the housing. Thus, the conveyor may be an auger conveyor (also known as a screw conveyor). In such embodiments, the housing may be a cylindrical housing. The auger of an auger conveyor tends to comprise a central shaft about which a helical flight (or helical screw blade) is arranged. In such arrangements, a first end of the central shaft may be rotatably supported within a bearing and the opposite end (second end) of the central shaft may be connected directly or indirectly (e.g., via a gearbox) to a drive motor. In this way, the auger may be driven to rotate within the conveyor housing. In an embodiment of the invention, the bearing includes a bearing housing and the bearing housing is carried by the kiln base. The first end of the central shaft may extend beyond the open first end of the conveyor housing. In a further embodiment of the invention, the second open end of the housing is higher than the first open end, whereby the housing slopes upwards from its first open end to its second open end. In the context of the present invention, the term "higher" means that the second end of the housing has a greater gravitational potential energy than the first end. By arranging the second end of the conveyor housing higher than the first end, the biochar material must be urged or driven out of the kiln chamber by the conveyor apparatus. Such an arrangement permits the controlled removal of the biochar material from the kiln chamber. It may desired to reduce the particle size of the biochar material, for example to produce "pellets" of biochar material that have one or more maximum dimensions, for example width or diameter. In order to achieve this, a plate may be disposed over an outlet port defined by the second open end of the conveyor housing, wherein the plate defines therethrough a plurality of apertures or bores. In this way, the conveyor apparatus forces the biochar material through the apertures of bores within the plate. This has the effect of grinding or extruding the biochar material such that it has one or more maximum dimensions, for example a diameter or width dimension. The skilled person will appreciate that biochar is a relatively brittle material. Accordingly, the "pellets" produced using such a plate have a maximum dimension (e.g., width or diameter), but tend to have a range of dimensions smaller than the maximum dimension permitted by the plate. In embodiments in which the conveyor apparatus includes an auger and the auger is driven to rotate via a motor, the motor suitably includes a controller which controls the operation of the motor. In such embodiments, the motor may be controlled to drive the auger to rotate continuously, in which case, the biochar material is removed from the kiln chamber in a continuous manner. Alternatively, the motor may be controlled to drive the auger to rotate periodically, in which case, the biochar material may be removed from the kiln chamber periodically. It is desired to cool the biochar material before it is removed from the kiln chamber and / or to quench the thermal decomposition process. Accordingly, the kiln chamber includes a depth of water at its base in use, which acts to quench and / or cool the biochar material at the base of the kiln chamber. Accordingly, the kiln includes a water inlet and / or a water outlet in order to maintain the desired depth or volume of water within the kiln chamber. In certain embodiments of the invention, the kiln includes both a water inlet and a water outlet. The water outlet suitably includes an outlet conduit which passes through an aperture defined by the kiln base or the or one of the kiln side walls. The presence of a water layer at the bottom of the kiln chamber has the added benefit of preventing or limiting air entering the kiln chamber via the conveyor apparatus. Thus, the desired depth of water may be such that the open first end of the conveyor housing is covered by the water. Water may be added to the kiln chamber batchwise or continuously. For example, the water inlet may include an inlet conduit. The inlet conduit may define a port disposed within the kiln chamber. Suitably, a valve controls a flow of water through the water inlet. In a further embodiment of the invention, the kiln further includes a water level sensor, which senses the depth of water within the kiln chamber. Suitably, the valve is controlled by the water level sensor. Thus, the sensor and the valve together may maintain a minimum level of water within the kiln chamber. The level of water is suitably higher than the first open end of the conveyor housing. The water level sensor and valve may be a relatively simple float valve. In an embodiment of the invention, the top of the kiln is open or the kiln includes a roof panel which defines therein an opening. The opening into the kiln permits the addition of biomass to the kiln. The opening at the top of the kiln does not impact significantly on the low-oxygen or oxygen-free environment within the kiln chamber, as the thermal decomposition of the biomass takes place within a layer below the newly added biomass. However, the oxygen level within the kiln chamber may be controlled by providing the kiln with a closable lid or by providing the opening defined by the roof panel with a closable lid. Thus, the kiln may include a closable lid or the kiln may include a roof panel which defines therein an opening and the roof panel may further include a closable lid which selectively closes the opening. According to a second aspect of the invention, there is provided a method of producing charcoal that may be used as biochar, the method comprising: (i) providing a kiln according to the first aspect of the invention as defined anywhere herein; (ii) thermally decomposing biomass in a reduced oxygen environment within the kiln chamber to produce charcoal; (iii) and removing charcoal from the kiln chamber via the conveyor apparatus. Suitably, the charcoal is removed from a lower or bottom portion of the kiln chamber. As noted above, the kiln of the invention is intended to operate continuously. Accordingly, the step of removing the charcoal from the kiln chamber is carried out while step of thermally decomposing the biomass is being carried out. Although the second step of the method is a continuous step, the step of removing the charcoal from the kiln chamber may be performed continuously or periodically. As noted above, it is desired to stop the thermal decomposition of the biomass prior to removing the charcoal material from the kiln chamber. Accordingly, the method may include the steps of maintaining a pre-determined level of water within the kiln chamber and quenching the produced charcoal material with water prior to removal from the kiln chamber. The skilled person will appreciate that removal of the charcoal material from the bottom of the kiln chamber via the conveyor apparatus results in charcoal material above the removed material moving downwards under the action of gravity. Thus, the charcoal material that has undergone thermal decomposition within a decomposition zone defined above the water layer drops downwards into the water layer, where it is quenched and the thermal decomposition is stopped. As the thermal decomposition process is quenched prior to removal of the charcoal material from the kiln chamber, the method may further include the step of drying the charcoal material after it has been removed from the kiln chamber. In an embodiment of the invention, the conveyor apparatus includes a plate disposed over an outlet port defined by the second open end of the conveyor housing; the plate defines therein a plurality of apertures; and the method includes the step of reducing the particle size of the biochar by urging it through the plate. It will be appreciated that the biochar material is extruded through the apertures and thus has a diameter or width dimension according to the diameter or width dimension of the apertures. If the apertures are provided with an equal width dimension or diameter, then the extruded biochar material will have a corresponding width dimension or diameter. The addition of a rotating blade on the outlet side of the plate would result in pellets of biochar material being produced having a substantially equal particle size. Thus, gravity ensures the following cycle takes place within the kiln chamber: (1) addition of biomass to the kiln chamber; (2) thermal decomposition of the biomass to produce charcoal / biochar; (3) quenching of the charcoal / biochar material with water; and (4) removal of the quenched charcoal / biochar material from the kiln chamber. The cycle may be considered to be "gravity driven", as the fresh biomass is added to the top of the kiln chamber and descends downwards towards the base of the kiln chamber under the action of gravity. An embodiment of the invention will now be described, by way of example only, with reference to the accompanying drawing in which: Figure 1 shows a cross-sectional view through an embodiment of a kiln according to the first aspect of the invention. For the avoidance of doubt, the skilled person will appreciate that in this specification, the terms "up", "down", "front", "rear", "upper", "lower", "width", etc. refer to the orientation of the components as found in the example when installed for normal use as shown in the Figures. A kiln 2 is shown in Figure 1, wherein the kiln includes a base 4 and four side walls 6. The base 4 and the side walls 6 together define therein a kiln chamber 2a. The side walls 6 taper outwards and form a frustopyramidal configuration extending upwards from the base 4. A conveyor apparatus comprising an auger screw 8 disposed within a cylindrical conveyer housing 10 extends through one of the side walls 6. The auger screw 8 includes an elongate screw shaft 8a from which extends a helical flighting 8b. A proximal end (i.e. the end of the shaft which is closest to the base 4 of the kiln 2) of the elongate screw shaft 8a is rotatably housed within a bearing 12 carried on the base 4. A distal end of the elongate screw shaft 8a is coupled to an electric motor (not shown) which rotates the shaft 8a, and thus the auger screw 8, when the motor is energised. As seen in Figure 1, a proximal end of the conveyor housing 10 is disposed within the kiln chamber 2a and a distal end of the conveyor housing 10 projects beyond the kiln chamber 2a. A distal end of the conveyor housing 10 includes a plate 14. The plate 14 defines therethrough a plurality of cylindrical bores each having the same diameter. The kiln 2 further includes a water inlet pipe 16 and a water outlet pipe 18, which together maintain a pre-determined minimum depth d of water at the bottom of the kiln chamber 2a. A valve 20 is connected to a level sensor 22, which controls the inlet of water into the kiln chamber 2a. The water outlet pipe 18 allows a flow of water through the kiln chamber 2a. As shown in Figure 1, the depth d of water maintained within the kiln chamber 2a is sufficient to cover the proximal end of the conveyor housing 10. This prevents air entering the kiln chamber 2a via the conveyor housing 10. A biochar collector 24 collects biochar 26 as it exits the distal end of the conveyor housing 10 via the plate 14. In use, the kiln chamber 2a is filled with biomass 28 which is pyrolysed in a low oxygen or oxygen-free environment. Additional biomass 28 is added to the kiln chamber 2a during the operation of the kiln 2 and forms a burn zone b towards the top of the kiln chamber 2a. The biomass below the burn zone is pyrolyzed to form biochar 26. As biochar 26 is removed from the bottom of the kiln chamber 2a by the conveyor apparatus 8,10, the biochar above falls downwards. When the biochar reaches the water level, it is quenched and cooled prior to being transported out of the kiln chamber 2a. The auger screw 8 forces the biochar 26 disposed within the conveyor apparatus through the plate 14. This has the effect of providing the biochar exiting the conveyor apparatus with a maximum diameter dimension. Although the kiln 2 is described herein with outwardly tapering side walls 6, the skilled person will appreciate that a kiln having substantially vertical sides may be constructed in an alternative embodiment.

Claims

1. A kiln comprising a base and one or more side walls which together define a kiln chamber; a conveyor apparatus including a housing that defines opposed open ends, wherein a first open end of the conveyor apparatus is located within the kiln chamber adjacent to the kiln base, the housing passes through an aperture defined by the or one of the side walls, and a second open end of the housing is external to the kiln chamber; and wherein the kiln includes a water inlet and / or a water outlet to maintain a depth of water at the bottom of the kiln chamber in use.

2. A kiln according to Claim 1, wherein the or each side wall tapers outwards from the base.

3. A kiln according to Claim 1 or Claim 2, wherein the conveyer apparatus includes an auger disposed within the housing.

4. A kiln according to Claim 3, wherein the auger includes a central shaft; one end of the central shaft is rotatably supported within a bearing and the opposite end of the central shaft is operatively connected to a drive motor.

5. A kiln according to Claim 4, wherein the bearing includes a bearing housing and the bearing housing is carried by the kiln base.

6. A kiln according to any of Claims 1 to 5, wherein the second open end of the housing is higher than the first open end, whereby the housing slopes upwards from its first open end to its second open end.

7. A kiln according to any of Claims 1 to 6, wherein the conveyor apparatus includes a plate disposed over an outlet port defined by the second open end of the conveyor housing; and the plate defines therein a plurality of apertures.

8. A kiln according to any of Claims 4 to 7, wherein the drive motor includes a controller which controls the operation of the drive motor.

9. A kiln according to any of Claims 1 to 8, wherein the water outlet includes an outlet conduit which passes through an aperture defined by the kiln base or by a lower portion of the or one of the kiln side walls.

10. A kiln according to any of Claims 1 to 9, wherein the water inlet includes an inlet conduit which defines a port disposed within the kiln chamber.

11. A kiln according to any of Claims 1 to 10, wherein the water inlet includes a valve which controls a flow of water through the water inlet.

12. A kiln according to Claim 11, wherein the kiln further includes a water sensor which senses the depth of water within the kiln chamber and the valve is controlled by the depth sensor.

13. A kiln according to any of Claims 1 to 12, wherein the top of the kiln chamber is open or the kiln includes a roof panel which covers the kiln chamber and the roof panel defines therein an opening.

14. A method of producing charcoal for use as biochar, the method comprising providing a kiln according to any of Claims 1 to 13; thermally decomposing biomass in a reduced oxygen environment within the kiln chamber to form charcoal; and removing the charcoal from the kiln chamber via the conveyor apparatus.

15. A method according to Claim 14, wherein the charcoal is removed continuously or periodically from the kiln chamber via the conveyor apparatus.

16. A method according to Claim 14 or Claim 15, wherein a water level is maintained within the kiln chamber at a pre-determined depth and the method further includes the step of quenching the produced charcoal prior to removal from the kiln chamber.

17. A method according to any of Claims 14 to 16, wherein the conveyor apparatus includes a plate disposed over an outlet port defined by the second open end of the conveyor housing; the plate defines therein a plurality of apertures; and the method includes thestep of reducing the particle size of the charcoal by urging it through the plate.

518. A method according to any of Claims 14 to 17, wherein the method further includes the step of drying the charcoal after it has been removed from the kiln chamber.11

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