A Biochar Production Plant, A Combustion Chamber and a Method of Operating the Combustion Chamber

Abstract

The invention relates to a biochar production plant, a combustion chamber, and a method of operating same. The plant comprises a fuel hopper, a fuel delivery system, a combustion stage, and a post combustion stage. The post combustion stage includes a generator; a biochar removal stage and a flue gas scrubber. The combustion stage comprises a combustion chamber having an active fluid biochar bed comprising a pair of oscillating plates, one on top of the other; an air injection ring positioned above and spaced apart from the pair of oscillating plates; a biochar shelf, below and surrounding the active fluid biochar bed, positioned to receive biochar falling from the pair of oscillating plates, and means to transfer the biochar from the biochar shelf to a biochar outlet. There is provided at least one sensor operable to measure the depth of the active fluid biochar bed on the oscillating plates and a controller responsive to the sensor. The controller is operable to control the depth of the active fluid biochar bed and the temperature of combustion by controlling the oscillating plates and the air delivered into the combustion chamber. In this way, the construction is very simple, cost effective and has accurate combustion control leading to a quality biochar end product.

Claims

1. A biochar production plant comprising: a fuel hopper; a fuel delivery system; a combustion stage; a post combustion stage; the post combustion stage including: a biochar removal stage; and a flue gas scrubber; the combustion stage comprising a combustion chamber, the combustion chamber having: a fuel inlet for receipt of fuel from the fuel hopper, fed by the fuel delivery system; a fuel outlet for delivering the fuel into the combustion chamber; a biochar outlet for delivery of biochar to the biochar removal stage; an active fluid biochar bed having a pair of oscillating plates superimposed, one plate on top of the other; and an air injection ring positioned above and spaced apart from the pair of oscillating plates; a biochar shelf, surrounding and located at a level below the active fluid biochar bed, positioned to receive biochar falling from the pair of oscillating plates, and means to transfer the biochar from the biochar shelf to the biochar outlet; at least one sensor operable to measure the depth of the active fluid biochar bed; and a controller, responsive to the at least one sensor and operable to control the depth of the active fluid biochar bed.

2. A biochar production plant as claimed in claim 1 in which the controller operable to control the depth of the active fluid biochar bed comprises means to control the speed of oscillation of the oscillating plates.

3. A biochar production plant as claimed in claim 1 or 2 in which the controller operable to control the depth of the active fluid biochar bed comprises means to control the rate of delivery of fuel from the fuel delivery system to the combustion stage.

4. A biochar production plant as claimed in any preceding claim in which the post combustion stage further comprises a generator.

5. A biochar production plant as claimed in any preceding claim in which the combustion stage further comprises a pre-conditioner.

6. A biochar production plant as claimed in claim 5 in which the pre-conditioner is operable to heat the fuel to up to 350 degrees Celsius prior to entry of the fuel into the combustion chamber.

7. A biochar production plant as claimed in claim 5 or 6 in which heat is harnessed from the combustion chamber for use in the pre-conditioner.

8. A biochar production plant as claimed in claim 5 or 6 in which heat is harnessed from the generator for use in the pre-conditioner.

9. A biochar production plant as claimed in any preceding claim in which the at least one sensor operable to measure the depth of the active fluid biochar bed comprises a pair of pressure sensors, one of which is located above the air injection ring and the other of which is located below the air injection ring.

10. A biochar production plant as claimed in any preceding claim in which the pair of oscillating plates are star shaped.

11. A biochar production plant as claimed in claim 10 in which the circumferential edge of each of the oscillating plates has a wave configuration.

12. A biochar production plant as claimed in claim 10 or 11 in which the diameter of the upper oscillating plate is smaller than the diameter of the lower oscillating plate and in which the peaks of the upper oscillating plate coincide with the troughs of the lower oscillating plate.

13. A biochar production plant as claimed in any preceding claim in which the fuel outlet comprises a frusto-conical shaped fuel nozzle located centrally in the combustion chamber relative the sides of the combustion chamber.

14. A biochar production plant as claimed in claim 13 in which the frusto-conical shaped fuel nozzle further comprises a top plate having the fuel outlet located substantially centrally in the top plate, surrounded by an annular staging area platform for fuel entering the combustion chamber.

15. A biochar production plant as claimed in claim 14 in which the diameter of the fuel outlet is of the order of between 0.25 times and 0.5 times the diameter of the top plate.

16. A biochar production plant as claimed in any preceding claim in which the means to transfer the biochar from the biochar shelf to the biochar outlet comprises at least one scraper paddle configured to sweep the biochar from the biochar shelf.

17. A biochar production plant as claimed in claim 16 in which there are provided a plurality of scraper paddles, each of the plurality of scraper paddles being connected to and downwardly depending from the lower of the pair of oscillating plates.

18. A biochar production plant as claimed in claim 17 in which the scraper paddles are positioned circumferentially spaced apart from each other, and in which there is provided a paddle located at each of the outermost points of the lower oscillating plate.

19. A biochar production plant as claimed in any preceding claim in which the air injecting ring surrounds the fuel nozzle, and comprises a plurality of air nozzles through which air is delivered towards the centre of the combustion chamber.

20. A biochar production plant as claimed in claim 19 in which the plurality of air nozzles are inclined, downwardly depending towards the active fluid biochar bed.

21. A biochar production plant as claimed in claim 19 or 20 in which the plurality of air nozzles are circumferentially spaced about the air injection ring.

22. A biochar production plant as claimed in claim 21 in which the plurality of air nozzles are spaced apart by a distance between 0.03 m and 0.1 m.

23. A biochar production plant as claimed in any one of claims 19 to 22 in which each of the air nozzles has nozzle outlet diameter of between 0.003 m and 0.008 m.

24. A biochar production plant as claimed in any preceding claim in which the biochar removal stage comprises a water-filled wet biochar removal stage beneath the active fluid biochar bed and the biochar shelf.

25. A biochar production plant as claimed in claim 24 in which the water from the wet biochar removal stage is delivered to the flue scrubber for use in neutralizing flue gasses and removing heavy metals in the flue gas.

26. A biochar production plant as claimed in any preceding claim in which the flue gas scrubber comprises a condenser and a waste water filter.

27. A biochar production plant as claimed in claim 4 in which the generator comprises one of an Organic Rankine Cycle (ORC) generator and a hot air turbine generator.

28. A biochar production plant as claimed in any preceding claim in which the bottom plate of the pair of oscillating plates oscillates between 0.40 m and 0.60 m, and the top plate of the pair of oscillating plates oscillates between 0.2 m and 0.4 m respectively.

29. A biochar production plant as claimed in any preceding claim in which the distance between the air injection ring and the top plate of the pair of oscillating plates is between 0.065 m and 0.12 m.

30. A combustion chamber for a biochar production plant comprising: a fuel inlet for receipt of fuel; a fuel outlet for delivering the fuel into the combustion chamber; a biochar outlet; an active fluid biochar bed having a pair of oscillating plates superimposed, one plate on top of the other; and an air injection ring positioned above and spaced apart from the pair of oscillating plates; a biochar shelf, located at a level below the active fluid biochar bed, positioned to receive biochar falling from the pair of oscillating plates, and means to transfer the biochar from the biochar shelf to the biochar outlet; at least one sensor operable to measure the depth of the active fluid biochar bed; and a controller, responsive to the at least one sensor and operable to control the depth of the active fluid biochar bed.

31. A combustion chamber for a biochar production plant as claimed in claim 30 in which the controller operable to control the depth of the active fluid biochar bed comprises means to control the speed of oscillation of the oscillating plates.

32. A combustion chamber for a biochar production plant as claimed in claim 30 or 31 in which the controller operable to control the depth of the active fluid biochar bed comprises means to control the rate of delivery of fuel from the fuel delivery system to the combustion stage.

33. A combustion chamber for a biochar production plant as claimed in any of claims 30 to 32 in which the combustion stage further comprises a pre-conditioner.

34. A combustion chamber for a biochar production plant as claimed in claim 33 in which the pre-conditioner is operable to heat the fuel to up to 350 degrees Celsius prior to entry of the fuel into the combustion chamber.

35. A combustion chamber for a biochar production plant as claimed in claim 33 or 34 in which heat is harnessed from the combustion chamber for use in the pre-conditioner.

36. A combustion chamber for a biochar production plant as claimed in any of claims 30 to 35 in which the at least one sensor operable to measure the depth of the active fluid biochar bed comprises a pair of pressure sensors, one of which is located above the air injection ring and the other of which is located below the air injection ring.

37. A combustion chamber for a biochar production plant as claimed in any of claims 30 to 36 in which the pair of oscillating plates are star shaped.

38. A combustion chamber for a biochar production plant as claimed in claim 37 in which the circumferential edge of each of the oscillating plates has a wave configuration.

39. A combustion chamber for a biochar production plant as claimed in claim 37 or 38 in which the diameter of the upper oscillating plate is smaller than the diameter of the lower oscillating plate and in which the peaks of the upper oscillating plate coincide with the troughs of the lower oscillating plate.

40. A combustion chamber for a biochar production plant as claimed in any of claims 30 to 39 in which the fuel outlet comprises a frusto-conical shaped fuel nozzle located centrally in the combustion chamber relative the sides of the combustion chamber.

41. A combustion chamber for a biochar production plant as claimed in claim 40 in which the frusto-conical shaped fuel nozzle further comprises a top plate having the fuel outlet located substantially centrally in the top plate, surrounded by an annular staging area platform for fuel entering the combustion chamber.

42. A combustion chamber for a biochar production plant as claimed in claim 41 in which the diameter of the fuel outlet is of the order of between 0.25 times and 0.5 times the diameter of the top plate.

43. A combustion chamber for a biochar production plant as claimed in any of claims 30 to 42 in which the means to transfer the biochar from the biochar shelf to the biochar outlet comprises at least one scraper paddle configured to sweep the biochar from the biochar shelf.

44. A combustion chamber for a biochar production plant as claimed in claim 43 in which there are provided a plurality of scraper paddles, each of the plurality of scraper paddles being connected to and downwardly depending from the lower plate of the pair of oscillating plates.

45. A combustion chamber for a biochar production plant as claimed in claim 44 in which the scraper paddles are positioned circumferentially spaced apart from each other, and in which there is provided a paddle located at each of the outermost points of the lower plate of the pair of oscillating plates.

46. A combustion chamber for a biochar production plant as claimed in any of claims 30 to 45 in which the air injecting ring surrounds the fuel nozzle, and comprises a plurality of air nozzles through which air is delivered towards the centre of the combustion chamber.

47. A combustion chamber for a biochar production plant as claimed in claim 46 in which the plurality of air nozzles are inclined, downwardly depending towards the active fluid biochar bed.

48. A combustion chamber for a biochar production plant as claimed in claim 46 or 47 in which the plurality of air nozzles are circumferentially spaced about the air injection ring.

49. A combustion chamber for a biochar production plant as claimed in claim 48 in which the plurality of air nozzles are spaced apart by a distance between 0.05 m and 0.1 m.

50. A combustion chamber for a biochar production plant as claimed in any one of claims 46 to 49 in which each of the air nozzles has nozzle outlet diameter of between 0.003 m and 0.008 m.

51. A combustion chamber for a biochar production plant as claimed in any of claims 30 to 50 in which the bottom plate of the pair of oscillating plates oscillates between 0.40 m and 0.60 m, and the top plate of the pair of oscillating plates oscillates between 0.2 m and 0.4 m respectively.

52. A combustion chamber for a biochar production plant as claimed in any of claims 30 to 51 in which the distance between the air injection ring and the top plate of the pair of oscillating plates is between 0.065 m and 0.12 m.

53. A method of operating a combustion chamber for a biochar production plant comprising the steps of: providing an active fluid biochar bed with a pair of oscillating plates, the oscillating plates superimposed one on top of the other; providing a biochar shelf, surrounding and located at a level below the active fluid biochar bed, positioned to receive biochar falling from the pair of oscillating plates, and providing means to transfer the biochar from the biochar shelf to a biochar outlet; the method further comprising the steps of: oscillating the pair of oscillating plates; monitoring the depth of biochar of the fluid biochar bed on the oscillating plates; and controlling the intake of fuel into the combustion chamber to prevent the depth of the active fluid biochar bed exceeding a set fluid biochar bed depth parameter.

54. A method of operating a combustion chamber for a biochar production plant as claimed in claim 53 comprising the step of controlling the speed of oscillation of the pair of oscillating plates in order to maintain the biochar on the oscillating plates for a first predetermined period of time.

55. A method of operating a combustion chamber for a biochar production plant as claimed in claim 53 or 54 comprising the step of controlling the speed of the means to transfer the biochar from the biochar shelf to a biochar outlet in order to maintain the biochar on the biochar shelf for a second predetermined period of time.

56. A method of operating a combustion chamber for a biochar production plant as claimed in any of claims 53 to 55 in which the method comprises the step of monitoring the temperature of the combustion chamber and injecting air into the combustion chamber in order to keep the temperature at or above a desired temperature parameter.

57. A method of operating a combustion chamber for a biochar production plant as claimed in claim 56 in which the air is injected into the combustion chamber horizontally or inclined downwardly towards the fluid biochar bed.

58. A method of operating a combustion chamber for a biochar production plant as claimed in any one of claims 53 to 57 comprising the step of accurately controlling the delivery of air into the combustion chamber to provide adequate oxygen for the combustion of syngas while simultaneously preventing excess oxygen being present in the biochar bed.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0092] The invention will now be more clearly understood from the following description of some embodiments thereof given by way of example only with reference to the accompanying drawings, in which:

[0093] FIG. 1 is a perspective view of part of the biochar production plant according to the invention;

[0094] FIG. 2 is a diagrammatic representation of the components of the biochar production plant according to the invention;

[0095] FIG. 3 is a side cross-sectional view of the combustion chamber according to the invention;

[0096] FIG. 4(a) is a plan view of the pair of oscillating plates forming the moving grate along the lines III-III of FIG. 3;

[0097] FIG. 4(b) is a plan view similar to FIG. 4(a) showing an alternative embodiment of moving grate according to the invention;

[0098] FIG. 5 is a top plan view of the air injection ring;

[0099] FIG. 6 is a bottom plan view of the air diffuser plate with its cover plate removed;

[0100] FIG. 7 is a bottom plan view of the air diffuser with its cover plate attached; and

[0101] FIG. 8 is a perspective view of the biochar removal stage of the biochar production plant according to the invention.

DETAILED DESCRIPTION OF THE DRAWINGS

[0102] Referring to FIGS. 1 and 2, there is shown a biochar production plant, indicated generally by the reference numeral 100, comprising a fuel delivery system 101, a combustion stage 103 and a post combustion stage 105. The post combustion stage comprises a generator 107, a biochar removal stage 109 and a flue gas scrubber 111. Referring specifically to FIG. 2, there is further provided a fuel hopper 201, a fuel conveyor 202 and a fuel conditioner 203 of the fuel delivery system 101. The fuel conditioner 203 is not seen as essential and may be omitted in certain embodiments of the invention.

[0103] In use, fuel is delivered from a fuel hopper 201 or other repository along a fuel conveyor 202 of the fuel delivery system 101 to a fuel conditioner 203. The fuel for the biochar may be organic matter from agricultural and/or forestry wastes such as, but not solely limited to, woodchip, wood cuttings, leaves, plants, poultry litter, animal litter, dried animal digested (DAD) material, dried cattle slurry or the like. The fuel in the fuel conditioner is heated to up to of the order of 300? C. to 350? C. In this way, the fuel is brought to a temperature of the order of 75% of its gasification temperature. Gasification typically starts at approximately 400? C. for many of the above-identified fuels. The fuel travels from the fuel conditioner along a conveyor of the fuel delivery system 101 to the combustion stage 103.

[0104] The combustion stage comprises a combustion chamber in which the fuel is heated to of the order of 850? C. The combustion chamber and its operation will be described in more detail below with reference to FIGS. 3 to 7 inclusive. The heat and flue gases generated by the burning syngas are transferred from the combustion chamber to the generator 107 and the flue gas scrubber 111, respectively, for conversion into electricity and for cleaning prior to being released into the atmosphere. The biochar from the combustion chamber is delivered through a biochar outlet (not shown) to a biochar removal stage 109 where the biochar is collected for subsequent disposal.

[0105] The generator 107 is preferably an Organic Rankine Cycle (ORC) generator. The ORC uses an expander turbine coupled to a high-speed alternator to produce electricity with a high degree of efficiency. The electricity may be used in the facility or exported to the national electricity grid. Alternatively, the generator may be provided by a hot air turbine to generate electricity.

[0106] The flue gas scrubber 111 comprises a flue gas cleaning and heat exchanger system such as that produced by Woodtek Engineering Ltd of Welshpool Great Britain. The flue gas scrubber is operable to remove harmful gasses and heavy metals from the flue gas and treat the flue gasses so that they are effectively colourless as they exit the flue. The generator 107 and the flue gas scrubber 111 are shown housed together however this is not essential.

[0107] The biochar removal storage stage 109 comprises a wet conveyor that the biochar falls onto (as will be described in more detail below) and is carried by. The biochar is carried from the combustion chamber to a biochar repository.

[0108] Referring now to FIG. 3, there is shown a cross sectional view of the combustion chamber, indicated generally by the reference numeral 300. The combustion chamber is constructed using high grade steel combined with monolithic refractory lining and insulation. Fuel that has been treated in the pre-conditioner is delivered to the combustion chamber 300 along the fuel delivery system 101 to a screw auger 301. Once there, the fuel is progressed up along the screw auger as it rotates through a fuel outlet nozzle 303. The fuel nozzle 303 is substantially frusto-conical in shape with a discharge outlet 305 at is centre.

[0109] When the fuel exits the discharge outlet 305, the fuel momentarily sits on a top plate 306 surrounding the fuel discharge outlet before falling along the sides of the fuel nozzle 303 towards an active fluid biochar bed (not shown) on a pair of oscillating plates 307, 309. The pair of oscillating plates 307, 309 are actuated with the effect that the biochar moves towards the periphery of the oscillating plates towards a gap 311 between the lower oscillating plate 309 and the side wall 313 of the combustion chamber. From there, the biochar falls down onto a biochar shelf 314 located below and surrounding the oscillating plates. The biochar on the biochar shelf is agitated by a plurality of paddles 316, and pushed towards the centre of the combustion chamber by those paddles 316. The paddles 316 push the biochar into one of a pair of wet ash conveyors 315, 317 to be carried away from the combustion chamber.

[0110] Importantly, there is an air injection ring 319 having a plurality of air nozzles 321 surrounding the outlet nozzle 303, and positioned above and spaced apart from the pair of oscillating plates 307, 309. The air nozzles 321 are shown downwardly depending towards the active fluid biochar bed however this is not necessarily limiting. The temperature of combustion will be regulated in part by the introduction of air. The distance between the air injection ring and the top plate of the pair of oscillating plates is between 0.065 m and 0.12 m (65 mm to 120 mm).

[0111] In addition to the air injection ring 319, there is provided a plurality of secondary air inlets 323 circumferentially spaced around the combustion chamber, and a plurality of flue gas recirculation inlets 325 circumferentially spaced around the combustion chamber. In the embodiment shown in FIG. 3, the flue gas recirculation inlets 325 are positioned above the secondary air inlets. The secondary air inlets and the flue gas recirculation inlets are offset to the radial direction so that air entering into the combustion chamber through the secondary air inlets and the flue gas recirculation inlets will form a vortex flow around the fuel discharge outlet 305.

[0112] It is envisaged that the standard distance between the air injection ring and the top plate of the pair of oscillating plates will be 80 mm (0.08 m) as this has proven to be the optimum spacing to cover the widest variety of fuels tested. The standard diameter of the air nozzle outlets is preferably 8 mm (0.008 m). This configuration provides an even ring of airstream flow that can be controlled very accurately to the primary combustion zone. In this way, it is possible to control the combustion temperature of different fuels very accurately.

[0113] The combustion temperature is controlled by monitoring the furnace temperature immediately above the air injection ring and the differential pressure in the combustion chamber. More specifically, the temperature of the whole primary combustion zone as well as the pressure differential between a point below the air injection ring and a point above the air injection ring can be used to good effect. When the pressure decreases below the air injection ring, the fan speed will increase to increase the combustion rate. Additionally, should the furnace temperature rise above the pre-set limit, then the fuel feed rate will be reduced. By adjusting these two parameters, it is possible to maintain the correct depth of biochar in the fluid biochar bed as well as the set temperature point, normally of the order of 750? C. to 850? C. The volume of air delivered to the air injection ring is controlled by the inverter speed of a primary air fan and this adjusts automatically to match the fuel feed rate. The upper and lower limits can set for each fuel on commissioning.

[0114] The ring of secondary air inlets 323 are located between 500 mm and 1.2 m above the primary air injection ring (depending on the size of the model of combustion chamber). The secondary air inlets are configured so that the air passing through the inlets enters the combustion zone at an angle to create a circular vortex of the combustion gases to mix the combustion gases to ensure complete combustion and eliminating hot and cold spots in the combustion zone. The diameter of the inlets vary according to the combustion chamber capacity. The larger the combustion chamber, the larger the inlets.

[0115] The base level of volume of secondary combustion air delivered to the combustion chamber is set on commissioning of different fuels in the combustion chamber to achieve a desired O.sub.2 content of the combustion gas. It will be understood that different fuels and different fuel conditions will require different levels of added oxygen in order to ensure efficient gasification and combustion. Additionally, there is provided an auto trim control that automatically trims the volume of secondary air delivered into the combustion chamber to maintain the desired set O.sub.2 level. In some embodiments, an O.sub.2 level (volume) of between 6% and 10% of the total combustion gas volume has been found to be effective.

[0116] Immediately above the secondary air inlets 323 there is a ring of flue gas inlets 325 to allow cool clean flue gas to be injected into the combustion chamber to control the intensity and temperature of the combustion and to accurately maintain the optimum gasification and combustion temperature throughout the combustion chamber.

[0117] The secondary air injection primarily combusts the syngas. The syngas is the CO and H.sub.2 elements separating from the fuel and combusting above the fuel in the middle and upper region of the combustion chamber. The syngas is separated from the fuel as it passes out of the discharge outlet 305 and travels down the side of the fuel outlet nozzle 303 towards the fluid biochar bed. The heat generated in this stage drives the gasification process and the majority of the syngas is driven out of the fuel prior to it entering the fluid biochar bed zone. This allows the temperature of the biochar bed to remain below 750? C., preferably 700? C., which is important to the successful implementation of the plant. In this way, a high grade of biochar can be produced. When the biochar drops onto the biochar shelf 314, it will be at a temperature of the order of between 400? C. and 500? C., and remains at this temperature until it drops into the wet conveyor/wet quench. This delay of the biochar on the biochar shelf at the reduced temperature gives the biochar sufficient time to cure and complete the biochar production process.

[0118] The secondary air is controlled at least in part by the exhaust gas lambda sensor which measures the O.sub.2 content, relays the information back to the control panel, which then increases or decreases the secondary air fan speed to automatically adjust the amount of air injected. This ensures that correct amount of oxygen is injected for complete combustion to take place. Depending on the fuel being burnt, the range for optimum combustion may be between 6 and 10 percent O.sub.2 content in the exhaust gas, If the O.sub.2 content is too high, blue smoke is produced and the CO level will be high. If the O.sub.2 content is too low, black smoke is produced and again the CO level will be high.

[0119] The furnace temperature may be controlled in a number of different ways. First of all, the furnace temperature may be controlled by adjusting the moisture content in the fuel conditioner to achieve the optimum moisture content of the fuel to maintain the desired furnace temperature. For example, drier fuel will increase the furnace temperature whereas more moist fuel will decrease the furnace temperature. The fuel conditioner will add moisture or reduce moisture when required and the fuel conditioner is controlled by the system logic taking the furnace temperature as its data input. For example, it is envisaged that there may be a set minimum moisture content of the fuel set at 15% by weight of the fuel. If the moisture content is below 15%, water will be added to the fuel. In other cases, the fuel may be dried to reduce the moisture content.

[0120] A second way in which the furnace temperature may be controlled is by flue gas recirculation. With flue gas recirculation, cool flue gas recovered from after the wet scrubber is introduced adjacent to the secondary air injection points to reduce the secondary combustion temperatures. The rate is adjusted by the furnace temperature and data from a CEMS emissions monitor (not shown). The amount is set and balanced on fuel commissioning. Due to the fact that the flue gas is exhaust gas with low O.sub.2 content, this involves recirculation of cool gas that does not combust and so cools the combustion temperature. As the furnace temperature is the prime controller of O.sub.2 volume, the level of NOx detected by the CEMS is the secondary indicator and is only used during commissioning to establish the optimum target furnace temperature.

[0121] If the combustion temperature falls below the required temperature, the flue gas recirculation volume is reduced to balance the temperature. If the temperature remains under 850? C. after flue gas is reduced to the minimum amount, woodchip is automatically added to the fuel feed to increase the calorific value to maintain the temperature. If the temperature remains under 850? C., a diesel burner (not shown), if provided, may be ignited and the combustor moves to shutdown after a set period.

[0122] A plurality of sensors (not shown) are located inside the combustion chamber including a temperature sensor, and a pair of pressure sensors. One pressure sensor is located above the air injection ring and the second pressure sensor is located below the air injection ring adjacent the gap 311 between the lower oscillating plate 309 and the side wall 313 of the combustion chamber. The pressure sensors are operable to measure the negative pressure at those locations. The measurements from the pressure sensors may thereafter be used to determine the depth of the active fluid biochar bed, as will be described in more detail below.

[0123] The pressure sensors will be used to detect a change in the pressure (the pressure differential) between the upper pressure sensor and the lower pressure sensor. The higher the active fluid biochar bed is, the higher the pressure recorded by the lower pressure sensor will be. Therefore, if the pressure sensed by the lower pressure sensor should reach a given level, particularly relative to the pressure sensed by the upper pressure sensor, this will be indicative that the active fluid biochar bed has reached a certain height. If the fluid biochar bed is too high, there is a danger that incomplete carburization will occur, and steps will be taken to ensure that the fluid biochar bed height is at the desired level.

[0124] A primary air fan speed may be controlled by a variable speed drive to adjust the volume of air injected through the air injection ring. This is controlled in response to a lower furnace temperature probe and the speed of the primary air fan automatically increases or decreases to maintain the target set point which is established on commissioning of various fuels. The operator can take corrective action, automatically through a programmed controller, to reduce the height of the fluid biochar bed by adjusting the lower furnace target temperature set point, by increasing the speed of oscillation of the pair of oscillating plates and/or reducing the rate of fuel feed until the fluid bed level has decreased to a desired level. In addition to the foregoing, there is further provided a temperature sensor in the furnace wall above the combustion ring. This will detect if the biochar level rises above a pre-determined maximum (detected by a drop in temperature in the furnace) and will stop the fuel feed until the temperature rises again as the biochar level drops.

[0125] By operating the combustion chamber in this manner, it is possible to combust fuel with a very high ash content, for example, up to 30% ash content. This was not heretofore possible with the existing offerings. By closely managing the temperature of combustion and by managing the depth of the fluid biochar bed, thermal oxidation of the oscillating plate components is virtually eliminated, thereby obviating the need to replace furnace grates as often as would otherwise be the case and increasing the number of combustion hours (and hence efficiency of the plant).

[0126] The spacing between the primary air injection ring and the pair of oscillating plates may be varied depending on the particle size of the fuel. For example, a spacing of 100 mm is deemed suitable for green compost oversize fuel. This is to ensure any large non-combustible elements in the fuel, for example, rocks, stones, and metals, can pass between the combustion ring and the oscillating plates and not cause a blockage. For other materials, such as woodchip and the like, 80 mm is deemed to be an advantageous spacing setting. However, if the fuel is of a small size (e.g., 15 mm or less), then the minimum spacing setting of 65 mm would be used.

[0127] Referring now to FIGS. 4(a) and 4(b), and initially to FIG. 4(a), there is shown is a plan view of the pair of oscillating plates forming the moving grate 400, along the lines III-III of FIG. 3. It can be seen from FIG. 3 that the pair of oscillating plates 307, 309 are substantially star shaped and are mounted over a stationary base plate 401. The circumferential edge 403, 405 of each of the oscillating plates has a wave configuration with peaks and troughs. As can be seen, the upper oscillating plate 307 is smaller in diameter than the diameter of the lower oscillating plate 309 and in which the peaks of the upper oscillating plate coincide with the troughs of the lower oscillating plate.

[0128] Referring specifically to FIG. 4(b), the lower oscillating plate 309 has been replaced with a circular plate and there is shown the positions of the plurality of paddles 316. The paddles depend downwardly from the outer edges of the lower plate to agitate the biochar on the biochar shelf (not shown) and brush the biochar towards the biochar removal stage.

[0129] The circular plate provides more flexibility for the placement of paddles as well as the number of paddles. Alternatively, it is envisaged that a star-shaped lower oscillating plate could be provided with one or more paddles downwardly depending from the outermost end of each of the peaks of the lower oscillating plate. For example, two paddles could be provided on one or more of the peaks, the two paddles being splayed apart from each other. In this way, more paddles will be provided to remove biochar off the biochar shelf. If a circular lower oscillating plate is used, it is envisaged that the upper plate may be dimensioned so that the peaks of the upper plate are approximately 0.05 m (50 mm) from the edge of the round lower plate in order to ensure correct depth of biochar bed on the upper plate.

[0130] In use, the design of the plates is such that the main biochar mass sits on the top plate and during the first part of rotation, both plates rotate together thus moving the entire biochar volume at approximately 60% of the set rotation distance. The upper plate reaches a fixed stop which prevents further rotation of the top plate. The lower plate continues to rotate by the remaining approximately 40% of the set rotation angle/distance. As the lower plate continues to rotate, the biochar in contact with the lower plate will be moved under the weight of the biochar on the upper plate. In this way, the biochar is agitated and remains fluid, and will move radially outwardly towards the drop zone at the periphery of the plate.

[0131] Referring to FIG. 5, there is shown a top plan view of the air injection ring 309. In use, fuel travels up the screw auger and passes out of the fuel outlet 305 from where it falls between the inner lip 503 of the air diffuser plate 501 and the fuel nozzle onto the upper oscillating plate 307.

[0132] Referring to FIGS. 6 and 7, there is shown a bottom plan view of the air diffuser plate 501 with its cover plate removed, and a bottom plan view of the air diffuser plate 501 with the cover plate in position, respectively. In FIG. 6, it can be seen that there are a number of air outlets spaced evenly, circumferentially around the air diffuser plate. There are provided a plurality of air channels/air ducts feeding the air outlets. In use, air enters the air ducts from a chamber (not shown) on the outside of the combustion ring (air injection ring), passes along the air ducts and exits from the air outlets. In FIG. 7, there is shown a bottom plan view of the air diffuser plate with the cover plate in position. In this way, the entire component may be removed and replaced if required due to wear and tear without requiring the remainder of the air injection ring to be removed.

[0133] Finally, referring to FIG. 8, there is shown a perspective view of the biochar removal stage 109 of the biochar production plant according to the invention. The wet biochar removal stage 109 comprises a pair of wet conveyors 801, 803 on which the biochar is transported to a biochar repository.

[0134] It will be understood that the present invention also comprises a programmable controller with computer program instructions loaded thereon, that receives measurements from the pressure sensors and the temperature sensor and operates one or more of the screw auger, the oscillating plates, and the air injection ring to keep the temperature in the combustion chamber at a relatively stable, desired temperature. In this way, the combustion is clean, burning the toxins, harnessing the optimum amount of energy from the syngas. In addition, in certain implementations, there may be provided a gas or like burner (not shown) operable to get the combustion chamber up to temperature on start up.

[0135] One example of air injection content (primary, secondary and flue gas recirculation) and burning temperatures, fluid biochar bed depth and oscillation speeds of the plates will now be provided by way of example, merely to give an indication of some operating parameters that are suitable depending on fuel type and consistency. Other parameters would suit other fuels with different moisture and ash content levels and the person skilled in the art would appreciate from the foregoing how the parameters could be adjusted depending on the nature of the fuel used for the biochar production plant.

EXAMPLE 1

[0136] A whole tree woodchip with a moisture content of 15% and an ash content of 15% was processed using the following parameters: Fan speeds: Primary air fan operated at between 40% to 65% of maximum fan speed (the precise speed will be controlled in response to the lower furnace temperature sensor); Secondary air 20 to 60% (controlled by Oxygen level); Flue gas recirculation 25% TO 90% (the precise speed will be controlled in response to the upper furnace temperature sensor). The primary combustion zone temperature was 850? ? C. the secondary combustion zone temperature was at 900? C., the fluid biochar bed depth was 0.08 m (80 mm), and the grate oscillation time delay was set at between 15 seconds to 35 seconds. [The primary combustion zone is located in the combustion chamber from a point adjacent to the air injection ring and extends upwardly to the lower secondary air inlets. Above those secondary air inlets to the exhaust gas exit at the top of the combustion chamber is considered to be the secondary combustion zone.]

[0137] In this specification the terms comprise, comprises, comprised and comprising and the terms include, includes, included and including are all deemed interchangeable and should be afforded the widest possible interpretation.

[0138] The invention is not solely limited to the embodiment hereinbefore described but may be varied in both construction and detail within the scope of the appended claims.