Patent Publication Number: US-2015083574-A1

Title: Retort

Description:
The present invention relates generally to a retort for producing charcoal and a method of use thereof, and finds particular, although not exclusive, utility in charcoal retorts. 
     Charcoal is obtained by heating wood until its complete pyrolysis (carbonization) occurs, leaving only carbon. Conventional charcoal manufacture involves burning wood that has been mostly covered with mud or bricks, to reduce the amount of oxygen available. The heat generated by burning part of the wood pyrolyzes the rest of the wood, forming charcoal. The limited supply of oxygen prevents the charcoal from burning. A more modern alternative is to heat the wood in an airtight metal vessel, which is much less polluting and allows volatile wood gas to be collected. 
     A conventional charcoal kiln produces about 100 kg of charcoal for every 700 kg of wood; that is a ratio of 1:7. 
     According to a first aspect of the present invention, there is provided a retort, for producing charcoal, comprising: an inner vessel, for receiving wood to be converted into charcoal; an outer vessel containing the inner vessel and defining an intermediate region between the inner vessel and the outer vessel, the intermediate region comprising a burning region configured for burning fuel to heat the inner vessel; a flue from the inner vessel to a region outside the outer vessel, for conveying gas from the inner vessel to the atmosphere; a feed pipe from the flue to the intermediate region, for conveying gas from the inner vessel to the intermediate region, wherein the entire length of the feed pipe is located within the outer vessel. 
     In this way, wood gas produced in the inner vessel may be conveyed to the intermediate region, in which it may be burnt. In particular, such wood gas may be maintained at an elevated temperature during its passage to the intermediate region, in order to improve its combustion. 
     Accordingly, less pollution may be released into the atmosphere than with conventional methods of making charcoal. Because the wood inside the inner vessel is heated in the absence of air, all of the wood present may be available for pyrolysis. 
     There may be no wood wasted. There may be no ash produced in the inner vessel. Any type of wood may undergo pyrolysis within the retort of the present invention, for instance scrap wood, logs, brush, twigs, sawdust or any other type of wood. 
     The retort may be a charcoal-producing retort, specifically, a charcoal retort. 
     The retort may be able to produce 100 kg of charcoal from 400 kg of wood; that is a ratio of 1:4. The retort may be able to convert wood to charcoal at a ratio of between 1:3 to 1:5. 
     The inner chamber may have a volume of between 1 m 3  and 5 m 3 . Specifically between 1.5 m 3  and 2 m 3 , and in particular 1.63 m 3 . 
     The retort may be configured to accept a maximum of between 100 kg and 1,000 kg of wood. Specifically, between 300 kg and 800 kg, and in particular 400 kg, 600 kg or 750 kg. 
     The retort may be configured to produce between 25 kg and 250 kg of charcoal per burn. Specifically between 100 kg and 200 kg, and in particular 150 kg. 
     The retort may be configured to convert wood to charcoal in (i.e. have a burn time of) between 2 and 10 hours from lighting to shut down. Specifically, between 4 and 8 hours, and in particular 6 hours. 
     Upon heating wood, moisture is initially driven off as steam and/or water vapour. It is undesirable to convey such moisture into the intermediate region, as this would inhibit burning of fuel to heat the inner vessel. After between approximately 1 and 4 hours, the majority of moisture may have been driven out of the wood, the temperature within the inner vessel may reach approximately 270° C., 350° C. 375° C. or 500° C., and wood gas may start to be produced. 
     The retort may further comprise a flue valve for controlling flow of gas through the flue. The flue valve may be arranged to control flow of gas into and/or out of the flue. In this way, the amount of gas conveyed from the inner vessel to the atmosphere can be regulated. In particular, the amount of gas conveyed from the inner vessel to the atmosphere can be regulated, in response to the composition of the gas given off by the wood. For instance, the flue valve may be a sliding shutter, a lid, a cap, an iris diaphragm, or any other type of valve. In particular, the flue valve may be a removable cap configured to be restable on an open upper end of the flue. The removable cap may comprise a seal disposed around an internal periphery. The seal may be arranged to be engageable with the periphery of the open upper end of the flue. In this way, the cap may form an air-tight seal with the flue. The seal may be a ceramic rope seal. Such a seal may be incorporated on any other part of the retort where an air-tight seal is desired, such as joins between components, doors, hatches, or vents. The flue valve may be configured to substantially stop all gas flow to the atmosphere from the inner vessel. The flue valve may allow controlled adjustment of flow of gas through the flue, over a continuous range of aperture sizes. Alternatively, the flue valve may allow controlled adjustment of flow of gas through the flue over a discrete set of aperture sizes. In particular, the flue valve may have only two states, for instance, open and closed. 
     By restricting the amount of gas conveyed from the inner vessel to the atmosphere, gas may be diverted into the feed pipe. Gas in the feed pipe may be conveyed to the intermediate region, for burning as fuel in the burning region, to heat the inner vessel. The gas conveyed to the intermediate region from the inner vessel may produce sufficient heat when burned to maintain the inner vessel at a temperature sufficient for wood gas to continue to be driven off. 
     The feed pipe may be arranged such that, in use, gas flow from the inner vessel may be required to substantially change direction within the flue to enter the feed pipe. The change of direction may be a substantial reversal of direction. For instance, gas moving upward within the flue may be required to move downward to enter the feed pipe. In one embodiment, an inlet of the feed pipe may be arranged inside the flue and face an outlet of the flue. Alternatively, the change of direction may be a change of direction of substantially 90 degrees. In one embodiment, an inlet of the feed pipe may be arranged inside the flue and face a longitudinal wall of the flue. In this way, the majority of gas flowing within the flue, in use, will not enter the feed pipe unless flow of gas out of the flue is restricted. 
     The retort may further comprise a feed pipe valve for controlling flow of gas through the feed pipe. The feed pipe valve may be arranged to control flow of gas into and/or out of the feed pipe. In this way, the amount of gas conveyed from the inner vessel to the intermediate region can be regulated. In particular, the amount of gas conveyed from the inner vessel to the intermediate region can be regulated, in response to the composition of the gas given off by the wood. For instance, the feed pipe valve may be a sliding shutter, a lid, a cap, an iris diaphragm, or any other type of valve. The feed pipe valve may be configured to substantially stop all gas flow to the intermediate region from the inner vessel. The feed pipe valve may allow controlled adjustment of flow of gas through the feed pipe, over a continuous range of aperture sizes. Alternatively, the feed pipe valve may allow controlled adjustment of flow of gas through the feed pipe over a discrete set of aperture sizes. In particular, the feed pipe valve may have only two states, for instance, open and closed. 
     The burning region may be located substantially under the inner vessel. In this way, heat from burning fuel may be more efficiently transferred to the inner vessel. 
     The feed pipe may be configured to convey gas into a substantially central part of the burning region. In this way, gas from the inner vessel may be burned in the most economical part of the burning region. 
     The retort may further comprise a second feed pipe from the flue to the intermediate region, for conveying gas from the inner vessel to the intermediate region. The second feed pipe may be configured to convey gas into the burning region in a direction substantially opposite to that of the first feed pipe. In this way, gas from one feed pipe may be used to slow the flow of gas from the opposing feed pipe in order for burning of the gas to be restricted to a substantially central part of the burning region. 
     The retort may further comprise a deflector coupled to an outlet end of the feed pipe. The deflector may be configured to direct gas, as it exits the feed pipe, in any desired direction. The direction that gas, as it exits the feed pipe, is directed by the deflector may be adjustable. In this way, damage to the retort from burning gas from the feed pipe may be reduced, by being able to adjustably direct gas away from vulnerable regions. The deflector may be an angled deflector plate. The deflector may be configured to clamp onto the outlet end of the feed pipe. The deflector may be configured to clamp onto the outlet end of the feed pipe by means of a securing nut. The deflector may be configured to couple to the feed pipe at a plurality of orientations. The plurality of orientations may be at 90 degree intervals. 
     The retort may further comprise a trailer and/or skids for supporting the retort. Alternatively, the retort may be mountable on a trailer and/or skids. In this way, the retort may be more easily movable. 
     The retort may further comprise a temperature sensor within the inner vessel and/or the flue, for measuring a temperature of the inner vessel, the wood to be converted into charcoal and/or the gasses passing through the flue. The retort may further comprise a temperature indicator, for indicating a temperature measured by the temperature sensor to an operator. The temperature sensor may be a digital thermometer. In this way, an operator may monitor the temperature of the inner vessel in order to manage effective conversion of wood to charcoal. 
     The retort may further comprise a first replaceable sacrificial member located on an inside surface of the outer vessel, adjacent the burning region. In this way, the outer vessel may be protected from the effects of burning within the burning region. The retort may further comprise a second replaceable sacrificial member located on an outer surface of the inner vessel, adjacent the burning region. In this way, the inner vessel may be protected from the effects of burning within the burning region. In addition, the sacrificial members may be conveniently replaced. The sacrificial members may be sacrificial baffle plates. The sacrificial members may be disposed longitudinally within the intermediate region. 
     The retort may further comprise an extraction pipe from the lower side of the inner vessel. The extraction pipe may be arranged to remove fluid from the inner vessel to a location external to the retort. The extraction pipe may be configured to extract moisture, condensed fluid and/or wood gas from the inner chamber. In this way, moisture given off when the wood is heated may condense inside the inner vessel and be removed via the extraction pipe as a liquid. Extraction of moisture from the inner vessel may improve pyrolysis of wood inside the inner vessel. Additionally, wood gas may also be extracted from the inner vessel for storage and later use, including powering a combustion engine and/or distilling into various products. Furthermore, fractions of the wood gas having relatively low boiling points may condense within the inner vessel and be removed via the extraction pipe as a liquid, for storage and later use. The retort may further comprise an extraction pipe valve for controlling the flow of fluid through the extraction pipe. The extraction pipe valve may be a stop valve, tap, or any other type of fluid valve. The extraction pipe valve may be configured to substantially stop all fluid flow from and/or into the extraction pipe. The extraction pipe valve may allow controlled adjustment of flow of fluid through the extraction pipe, over a continuous range of aperture sizes. Alternatively, the extraction pipe valve may allow controlled adjustment of flow of fluid through the extraction pipe over a discrete set of aperture sizes. In particular, the extraction pipe valve may have only two states, for instance, open and closed. 
     The retort may further comprise an access door substantially at each end of the retort. Each access door may be configured to provide access to an interior of the inner vessel and/or the intermediate region. In this way, easy access to the interior of the inner vessel and the intermediate region may be provided. The retort may comprise a pair of access doors substantially at each end of the retort. One of each pair of access doors may be configured to provide access to an interior of the inner vessel and the other of each pair of access doors may be configured to provide access to the intermediate region. Alternatively, the retort may comprise an access door at only one end of the retort. The retort may comprise a pair of access doors at only one end of the retort. Each access door may be hingably connected to the outer vessel or the inner vessel, for instance by means of a lift off hinge. At least one access door may further comprise a respective firebox door therein; each firebox door may be configured to provide access to the intermediate region only. In this way, operation of the firebox doors may allow regulation of air flow into the burning region, thus providing a mechanism for controlling the temperature of the inner vessel. The firebox door may be of the hammer gate type or may be of a hinged type, such as having a vertical hinge. Alternatively, at least one access door may be configured to provide access to an interior of the inner vessel only, and/or at least one access door may be configured to provide access to the intermediate region only. The retort may comprise at least one firebox door spatially separate from the access door. 
     The flue may be located closer to a first end of the retort than to an opposing second end. The retort may comprise a second flue located closer to the second end of the retort than to the first end. In this way, even distribution of heat may be effected by convection of exhaust gasses from the inner vessel. The flue may be located substantially at one end of the retort. A second flue may be located substantially at an opposing end of the retort. The or each flue may be located approximately one third, one quarter or one fifth of the way along the retort from one end. The flue may be located centrally, or off-centre of the retort. 
     The inner and outer vessels may be substantially cylindrical. Accordingly, heat loss can be reduced for a given internal volume of retort by minimising the reducing the surface area of the retort. Access doors may be provided on each phase of the cylinder. 
     The retort may be made from metal, in particular steel, for instance mild steel or stainless steel. The metal may have a thickness in the range of 3 mm to 10 mm, in particular 4 mm. 
     The retort may be insulated. The outer vessel may be insulated. In particular, the insulation may comprise a ceramic wool blanket covering and/or sheet insulation. The insulation may be protected by a galvanic sheet over the insulation. The ceramic wool blanket may be between approximately 25 mm and 100 mm thick. 
    
    
     
       The above and other characteristics, features and advantages of the present invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention. This description is given for the sake of example only, without limiting the scope of the invention. The reference figures quoted below refer to the attached drawings. 
         FIG. 1  is a perspective view of a retort according to a first embodiment of the present invention. 
         FIG. 2  is a cutaway perspective view of the interior of the outer vessel shown in  FIG. 1 . 
         FIG. 3   a  is a sectional view of one of the flues shown in  FIG. 1 . 
         FIG. 3   b  is a sectional view of a flue according to an alternative embodiment. 
         FIG. 4  is a perspective view of a deflector according to a first embodiment of the invention. 
     
    
    
     The present invention will be described with respect to particular embodiments and with reference to certain drawings but the invention is not limited thereto but only by the claims. The drawings described are only schematic and are non-limiting. In the drawings, the size of some of the elements may be exaggerated and not drawn to scale for illustrative purposes. The dimensions and the relative dimensions do not correspond to actual reductions to practice of the invention. 
     Furthermore, the terms first, second, third and the like in the description and in the claims, are used for distinguishing between similar elements and not necessarily for describing a sequence, either temporally, spatially, in ranking or in any other manner. It is to be understood that the terms so used are interchangeable under appropriate circumstances and that the embodiments of the invention described herein are capable of operation in other sequences than described or illustrated herein. 
     Moreover, the terms top, bottom, over, under and the like in the description and the claims are used for descriptive purposes and not necessarily for describing relative positions. It is to be understood that the terms so used are interchangeable under appropriate circumstances and that the embodiments of the invention described herein are capable of operation in other orientations than described or illustrated herein. 
     It is to be noticed that the term “comprising”, used in the claims, should not be interpreted as being restricted to the means listed thereafter; it does not exclude other elements or steps. It is thus to be interpreted as specifying the presence of the stated features, integers, steps or components as referred to, but does not preclude the presence or addition of one or more other features, integers, steps or components, or groups thereof. Thus, the scope of the expression “a device comprising means A and B” should not be limited to devices consisting only of components A and B. It means that with respect to the present invention, the only relevant components of the device are A and B. 
     Reference throughout this specification to “one embodiment” or “an embodiment” means that a particular feature, structure or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, appearances of the phrases “in one embodiment” or “in an embodiment” in various places throughout this specification are not necessarily all referring to the same embodiment, but may refer to different embodiments. Furthermore, the particular features, structures or characteristics of any embodiment or aspect of the invention may be combined in any suitable manner, as would be apparent to one of ordinary skill in the art from this disclosure, in one or more embodiments. 
     Similarly, it should be appreciated that in the description of exemplary embodiments of the invention, various features of the invention are sometimes grouped together in a single embodiment, figure, or description thereof for the purpose of streamlining the disclosure and aiding in the understanding of one or more of the various inventive aspects. This method of disclosure, however, is not to be interpreted as reflecting an intention that the claimed invention requires more features than are expressly recited in each claim. Rather, as the following claims reflect, inventive aspects lie in fewer than all features of a single foregoing disclosed embodiment. Thus, the claims following the detailed description are hereby expressly incorporated into this detailed description, with each claim standing on its own as a separate embodiment of this invention. 
     Furthermore, while some embodiments described herein include some features included in other embodiments, combinations of features of different embodiments are meant to be within the scope of the invention, and form yet further embodiments, as will be understood by those skilled in the art. For example, in the following claims, any of the claimed embodiments can be used in any combination. 
     In the description provided herein, numerous specific details are set forth. However, it is understood that embodiments of the invention may be practised without these specific details. In other instances, well-known methods, structures and techniques have not been shown in detail in order not to obscure an understanding of this description. 
     In the discussion of the invention, unless stated to the contrary, the disclosure of alternative values for the upper or lower limit of the permitted range of a parameter, coupled with an indication that one of said values is more highly preferred than the other, is to be construed as an implied statement that each intermediate value of said parameter, lying between the more preferred and the less preferred of said alternatives, is itself preferred to said less preferred value and also to each value lying between said less preferred value and said intermediate value. 
     The use of the term “at least one” may, in some embodiments, mean only one. 
     The invention will now be described by a detailed description of several embodiments of the invention. It is clear that other embodiments of the invention can be configured according to the knowledge of persons skilled in the art without departing from the true spirit or technical teaching of the invention, the invention being limited only by the terms of the appended claims. 
       FIG. 1  is a perspective view of a retort  10  according to a first embodiment of the present invention. 
     The retort  10  comprises an outer vessel  20 . The outer vessel  20  is substantially cylindrical in form, having two opposing substantially flat, substantially circular ends, each disposed within a respective substantially vertical plane, and an outer wall, substantially uniformly curved around a longitudinal axis of the outer vessel  20 . The outer vessel  20  is made from sheets of mild steel with a thickness of 4 mm. The outer wall is covered with a ceramic wool blanket of thickness between approximately 25 mm and 100 mm, which in turn is covered with a galvanic sheet. The outer vessel  20  has an axial length of approximately 2.5 m, and a diameter of approximately 1.7 m. 
     The outer vessel  20  is mounted, with its longitudinal axis horizontal, on a pair of elongate members  30  of length 2.5 m. Each elongate member  30  is orientated substantially parallel with the longitudinal axis of the outer vessel  20 . Each member  30  is formed from mild steel sheet of thickness 4 mm. 
     The outer vessel  20  is mounted on the members  30  by means of three independent supporting bulkheads  40 . A first bulkhead is located at one axial end of the outer vessel  20 , a second bulkhead is located at the other axial end of the outer vessel  20 , and a third bulkhead is located at the axial centre of the outer vessel  20 . Each bulkhead is formed from a single mild steel sheet/plate of thickness 4 mm, disposed substantially vertically and having two opposing planar surfaces, a substantially straight lower face, two substantially vertical and straight side faces and a concave upper face for receiving the outer vessel  20 . Each steel plate is bolted and/or welded to the outer vessel  20  on the concave upper face and to the members  30  at the edge joining the lower face to the side faces. Six through holes of differing sizes are present in each steel plate between opposing planar surfaces of the plate. Any number and/or size of holes may be used that do not compromise structural integrity. 
     A chimney  50  is located on top of the uniformly curved outer wall of the outer vessel  20 , approximately mid-way between the ends of the outer vessel  20 . The chimney  50  is substantially cylindrical, comprising a uniformly curving surface around a longitudinal axis and a hollow interior, and having a diameter of approximately 15 cm and an axial length of between approximately 5 cm and 1.5 m, and in particular 1 m. The chimney  50  is open at each axial end such that a path for gas to exit the interior of the outer vessel  20  is provided therethrough. The chimney  50  is made from sheets of mild steel with a thickness of 4 mm. 
     Two flues  60  extend through the top of the uniformly curved outer wall of the outer vessel  20 , approximately two-thirds of the way toward each end of the outer vessel  20  from the chimney  50 . Each flue  60  is substantially cylindrical, comprising a uniformly curving surface around a longitudinal axis and a hollow interior, and having a diameter of approximately 15 cm and an axial length of approximately 30 cm. Each flue projects above the top of the uniformly curved outer wall of the outer vessel  20  by approximately 5 cm, the remaining axial length of each flue being within the outer vessel  20 . Each flue  60  is open at each of its axial ends such that each provides a path for gas to exit the interior of an inner vessel (not shown) within the outer vessel  20 . Each flue  60  is made from sheets of mild steel with a thickness of 4 mm. 
     Each flue  60  is seated within a frustoconical member  235  comprising a sheet of mild steel with a thickness of 4 mm curved around a longitudinal axis. The axial ends of the frustoconical member  235  are open, such that a flue may be received therein. The frustoconical member is located within a hole in the top of the uniformly curved outer wall of the outer vessel  20 . A ceramic rope seal may be pressed and/or urged into a wedge-shaped region between the frustoconical member  235  and the flue  60 , such that an air-tight seal may be formed, preventing the escape of gas around the outer periphery of the flue  60 , through the hole in the top of the uniformly curved outer wall of the outer vessel  20 . 
     Each flue  60  is provided with a respective cap  70 , removably disposed on the upper axial end of the flue  60 , and for controlling flow of gas through the flue  60 . The cap  70  comprises a substantially disc shaped portion, for seating in a horizontal plane on the upper axial end of the flue  60 , and a ring-like downwardly depending rim having a diameter slightly larger than the diameter of the flue  60 . The rim acts to prevent accidental removal of the cap  70  from the flue  60 . Each cap is made from mild steel with a thickness of 4 mm. 
     At each axial end of the outer vessel  20  is provided a substantially circular access door  80 , disposed in a vertical plane. Each door  80  is hingeably mounted on one side by means of a lift off hinge  90 , for providing access to the interior of the outer vessel  20 . Each door has a diameter substantially equal to the diameter of the outer vessel  20 . Each door  80  may be reinforced; however, for clarity this has not been shown in the figures. In particular, each door  80  may be reinforced with a circumferential rib and/or three horizontal ribs, formed integrally with the door  80 . Alternatively, each door  80  may be reinforced with a circumferential rib and/or nine substantially radial ribs, formed integrally with the door  80 . Each door comprises a circumferential flange for effectively sealing the door  80  against the outer vessel  20 . The flange may have a ceramic rope seal located on one side thereof, arranged for engagement with the outer vessel  20 . Alternatively, the ceramic rope seal may be located on the outer vessel  20 , arranged for engagement with the flange. In this way, the seal between the door  80  and the outer vessel  20  may be made substantially air-tight. 
     Each access door  80  may be secured closed with a plurality of latches  100 . The latches are located around the circumference of the axial ends of the outer vessel  20 , and are configured to couple the circumferential flange to the outer wall of the outer vessel  20 . 
     Each access door  80  includes a hinged firebox door  110  therein, which provides access to a burning region within the outer vessel  20 . Each firebox door  110  is disposed in a lower half of the circular access door  80 , and comprises a vertically hinged door leaf. Each door leaf is substantially rectangular, having dimensions of 30 cm by 40 cm, and when in a closed position covers an opening of substantially the same size. 
     One of the firebox door leafs  110  additionally comprises a cut-away portion in an upper side of the firebox door leaf. An extraction pipe  120 , for removing fluid from the inner vessel, is arranged to pass through the cut-away portion. The cut-away portion and extraction pipe  120  are sized and arranged to allow the access doors  80  and the firebox door  110  to be opened and closed without causing damage to, and/or interference with, the extraction pipe  120 . 
     An access point  130  in the flue  60  allows insertion of a temperature probe into the outer vessel, for monitoring temperatures within the outer vessel. 
       FIG. 2  shows a cutaway perspective view of the interior of the outer vessel  20  shown in  FIG. 1 . 
     The inner vessel  140  is substantially cylindrical in form, having two opposing substantially flat, substantially circular ends, each disposed within a respective substantially vertical plane, and an outer wall, substantially uniformly curved around a longitudinal axis of the inner vessel  140 . The inner vessel  140  is made from sheets of mild steel with a thickness of 4 mm. The inner vessel  140  has an axial length of approximately 2.3 m, and a diameter of approximately 95 cm. The longitudinal axis of the inner vessel  140  is parallel to the longitudinal axis of the outer vessel  20 , and is displaced approximately 12 cm above the longitudinal axis of the outer vessel  20 . The flues  60  are welded and/or bolted to the top of the inner vessel  140 . 
     The inner vessel  140  is elevated above the interior surface of the outer vessel  20  by a series of internal support members  150 , such that an intermediate region is formed between the outer surface of the inner vessel  140  and the inner surface of the outer vessel  20 . In particular, a burning region is formed within the outer vessel  20 , underneath the inner vessel  140 . Any form of support means may be used to supporting the inner vessel  140  within the outer vessel  20 . The internal support members  150  each comprise a rigid elongate member having support plates at each end. One of the support plates is configured to engage with the outer surface of the inner vessel  140 . The other support plate is configured to engage with the inner surface of the outer vessel  20 . The support plates may be welded and/or bolted to the inner vessel  140  and the outer vessel  20 , respectively. 
     At each axial end of the inner vessel  140  is provided a substantially circular inner vessel door  160 , disposed in a vertical plane. Each inner vessel door  160  is hingeably mounted on one side, in the same manner as the access doors  80 , by inner vessel hinges  170 , for providing access to the interior of the outer vessel  20 . Each inner vessel door  160  has a diameter substantially equal to the diameter of the inner vessel  140 . Each inner vessel door  160  comprises a circumferential flange for effectively sealing the door  160  against the inner vessel  140 . The circumferential flange may have a ceramic rope seal located on one side thereof, arranged for engagement with the inner vessel  140 . Alternatively, the ceramic rope seal may be located on the inner vessel  140 , arranged for engagement with the flange. In this way, the seal between the inner vessel door  160  and the inner vessel  140  may be made substantially air-tight. 
     Each inner vessel door  160  may be secured closed with a plurality of inner vessel latches  180 . The latches  180  are located around the circumference of the axial ends of the inner vessel  140 , and are configured to couple the circumferential flange to the outer wall of the inner vessel  140 . 
     A first replaceable sacrificial member  190  resides inside the outer vessel  20 , on a lower surface of the outer vessel  20 , thereby defining a lower side of the burning region. The first replaceable sacrificial member  190  is made from a rectangular sheet of mild steel with a thickness of 4 mm. The first replaceable sacrificial member  190  is curved so as to fit in close engagement with the lower surface of the outer vessel  20  along its entire axial length. 
     A second replaceable sacrificial member  200  resides inside the outer vessel  20 , on a lower surface of the inner vessel  140 , thereby defining an upper side of the burning region. The second replaceable sacrificial member  200  is made from a rectangular sheet of mild steel with a thickness of 4 mm. The second replaceable sacrificial member  200  is curved so as to fit in close engagement with the lower surface of the inner vessel  140  along its entire length. The second replaceable sacrificial member  200  may be attached to the inner vessel  140  by any suitable means, such as by bolts. However, in preferable embodiments, the second replaceable sacrificial member  200  rests on a plurality of brackets. In this way, stress due to thermal expansion of the second replaceable sacrificial member is avoided. 
     Two metal feed pipes  210  wrap around the inner vessel  140 , having one end including a respective feed pipe inlet  220  disposed within a respective one of the flues  60 , and an opposing end located within the burning region. The feed pipes  210  have a square cross-section of dimension of approximately 5 cm. However, alternative embodiments in which the feed pipes  210  have a circular cross-section of diameter of approximately 5 cm are also envisaged. 
       FIG. 3   a  is a sectional view of one of the flues  60  shown in  FIG. 1 . The feed pipe  210  passes substantially horizontally through the uniformly curving surface around the longitudinal axis of the flue  60 , adjacent the lower axial end of the flue  60 . Inside the flue  60 , the feed pipe  210  is arranged to bend through approximately 90 degrees, and continues parallel to the longitudinal axis of the flue  60 , terminating at the feed pipe inlet  220 , which is arranged inside the flue  60  to face the cap  70 . Accordingly, gas from the inner vessel  140  will move up the flue  60  and be required to reverse direction within the flue  60  and move down to enter the feed pipe  210 . 
       FIG. 3   b  is a sectional view of a flue  60  according to an alternative embodiment. The arrangement of the feed pipe  210  differs from the arrangement of  FIG. 3   a  in that the feed pipe inlet  220  is substantially adjacent the lower axial end of the flue  60 , the feed pipe terminating soon after passing through the uniformly curving surface around the longitudinal axis of the flue  60 . In addition, the feed pipe inlet  220  is closed by a sliding shutter  240 , which may be moved to allow gas to flow into the feed pipe  210 . The feed pipe inlet  220  is arranged inside the flue  60  to face an opposing curved side of the flue  60 . Accordingly, gas from the inner vessel  140  will move up the flue  60  and be required to change direction within the flue  60  by approximately 90 degrees to enter the feed pipe  210 , once the feed pipe shutter  240  has been moved to allow gas to flow into the feed pipe  210 . 
       FIG. 4  is a perspective view of a deflector  250  according to a first embodiment of the invention. The deflector  250  comprises a collar  260  for receiving an outlet end of a feed pipe  210 , a securing bolt  270  for securing the collar  260  to the feed pipe  210 , and a deflecting plate  280  for directing gas flow from the feed pipe  210 . The deflector  250  is made from sheet mild steel with a thickness of 4 mm, and welded and/or bolted together. 
     The collar  260  is made from a single rectangular strip of sheet steel, bent into a ring shape having a substantially square perimeter, and four substantially flat and square faces. The collar  260  therefore defines a passage therethrough of square cross-section, with dimensions of approximately 5 cm by 5 cm. The passage is sized in order to receive an end of one of the feed pipes  210  therein. That is, the smallest internal diameter of the passage is slightly larger than the external diameter of the feed pipe  210 . 
     On one face of the collar  260 , a securing bolt  270  is received within a threaded bore, such that rotation of the bolt  270  causes axial movement of the bolt through the bore. In this way, the bolt  270  may be used to clamp the collar  260  onto the feed pipe  210 . The securing bolt  270  has a threaded shaft and a hexagonal head, for driving the bolt with a spanner or wrench. The securing bolt  270  may be replaced by any other means of securing the collar  260  to the feed pipe  210  known in the art. In particular, the collar  260  may be sized to provide a friction fit about the feed pipe  210 , such that no additional securing means is required. 
     The deflecting plate  280  is made from a single rectangular strip of sheet steel, bent a third of the way along its length at an angle of approximately 30 degrees to form a first plane and a second plane larger than the first plane. The deflecting plate  280  is welded and/or bolted via the second plane to one face of the collar  260 , such that gas exiting a pipe onto which the collar  260  has been secured will be deflected by the first plane of the deflecting plate  280 .