WO2003050450A1

WO2003050450A1 - Feeder for injecting fibrous biomass fuels into a reactor at atmospheric pressure or into a pressurized reactor

Info

Publication number: WO2003050450A1
Application number: PCT/AU2002/001655
Authority: WO
Inventors: Terence Francis Dixon; Christopher Mark Downing; Neil James Mckenzie
Original assignee: Sugar Research Limited; University Of Queensland; Stanwell Corporation Limited
Priority date: 2001-12-11
Filing date: 2002-12-06
Publication date: 2003-06-19

Abstract

A feed apparatus to feed a fibrous fuel into a pressurised vessel, the feed apparatus comprising an inlet into which the fuel can pass, an outlet to pass the fuel into the reactor, a barrel between the inlet and the outlet and through which the solid fuel passes, and at least one feeding roller to feed the fuel into and through the barrel, the construction and arrangement being such that the fuel forms a sealing plug in the barrel and said sealing plug is pushed through the barrel by virtue of the at least one feeding roller.

Description

FEEDER FOR INJECTING FIBROUS BIOMASS FUELS INTO A REACTOR AT ATMOSPHERIC PRESSURE OR INTO A PRESSURIZED REACTOR

FIELD OF THE INVENTION

This invention relates to a novel feeder for appropriately or fittingly injecting fibrous biomass fuels (such as bagasse) into a reactor at atmospheric pressure or into a pressurized reactor. The invention results in a rise in the performance of the reactor to a very high level which in turn results in a very substantial commercial advantage.

BACKGROUND ART The technology available for feeding into a pressurised system bagasse and similar fibrous biomass (coconut husk, straw, reed, alfalfa, etc) has been reviewed. The main problem with the handling of bagasse and similar fibrous biomass resides in the flow properties of said bagasse or biomass. In the sugar industry, bagasse, which is the cane fibre residue which remains after the sugar juice is extracted, is burnt in spreader stoker industrial boilers to produce steam and power for the sugar mill. Electricity may also be produced for the grid.

Conceptually, discrete particle materials such as woodchips or wheat will flow relatively easily due to the low inter-particle forces. However, the long (high aspect ratio) and flexible bagasse and similar fibrous biomass particles will interact cohesively in a manner similar to hemp fibres that can be wound to form a rope.

Prior art relating to the feeding of bagasse and similar fibrous biomass into a pressure vessel includes, but is not limited to, the application of devices such as lock hoppers, plug screw feeders, plug piston feeders, coaxial screw- piston feeders, non-purging rotary valves, self-purging rotary valves and wet feed systems.

These special devices are required in order to push the biomass into the pressure vessel and against the positive pressure in the pressure vessel. The devices require the existence of a seal to prevent against gas leakage. The pressure effects in chemical and/or physical reactors are well-known. In particular, the effect of pressure is significant in a gas-phase reaction, such as gasification. A change in pressure may also influence the activation energy of some reactions. Operating pressure also affects the equilibrium conversion of the reaction and the product composition. The optimisation of the pressure within a pressurised vessel, particularly a reactor, is therefore important to the amount and quantity of products formed. Said devices are generally affected by one or more of the following problems:

• Device is not scaleable up to the large size necessary for commercial scale pressurized gasification of bagasse or similar fibrous biomass.

• Device is too energy intensive. • Device degrades the bagasse and similar fibrous biomass particle size too severely.

• Device does not feed due to the bridging of the fibrous bagasse or biomass.

• Device does jam due to the fibrous bagasse or biomass particles blocking the close tolerances necessary between components.

• Device is a high maintenance item due to wear between the close tolerance components.

• Device risks leakage of combustible gases at the high pressure involved and/or does not allow a smooth or continuous operation or process. OBJECT OF THE INVENTION

It is an object of the invention to provide an apparatus which may be an energy efficient, compact, continuously feeding and self-sealing feeder for bagasse or similar fibrous biomass and blended fuels (bagasse/coal, bagasse/woodchip, straw/woodchip, green municipal waste, etc). The invention has a particular application to, but is not limited to, biomass fibrous fuels with long soft fibre biomass such as bagasse, straw, grass, switch grass, alfalfa etc. Said long biomass fuels with long soft fibre are most difficult to handle as fibres grab each other and bind up.

The invention has a particular application to, but is not limited to, reactors for combustion and gasification. Boilers to produce steam and power are typical combustion reactors. In gasification reactors, a reaction with a shortage of oxygen (or steam plus oxygen) is performed in order to produce gaseous fuels. The gasification reactor contains a mixture of steam and air (or air plus oxygen) and the reaction with the fuel feed produces inert gases (such as nitrogen and carbon dioxide), reactive gases (such as carbon monoxide, hydrogen, methane and lower hydrocarbons) and ashes. Said ashes are removed from the gaseous fuel produced by filtration or any other suitable means.

In one form, the invention resides in a feed apparatus to feed a fibrous fuel into a pressurised vessel, the feed apparatus comprising an inlet into which the fuel can pass, an outlet to pass the fuel into the vessel, a barrel between the inlet and the outlet and through which the solid fuel passes, and at least one feeding roller to feed the fuel through the barrel, the construction and arrangement being such that the fuel forms a sealing plug in the barrel and said sealing plug is pushed through the barrel by virtue of the at least one feeding roller.

In this manner, the sealing plug reduces leakage of combustible gasses back through the feed apparatus, this being especially pertinent if the reactor is under a fairly high pressure.

Although the feed apparatus is designed particularly for fibrous biomass fuels, it should be appreciated that the apparatus may also find use with other types of fuels where an apparatus having a similar construction may be considered appropriate. For example, the fuel may be a mixture of feed products such as bagasse and/or wood chips.

Suitably a pair of feeding rollers is provided to feed and push the fuel through the barrel. The feeding rollers may comprise counter-rotating rollers.

The rollers may be provided with grip enhancing means on the peripheral surface. The grip enhancing means of the peripheral surface may be circumferential grooves or teeth in the form of grooves, profiles, a textured surface, hard facing particles (i.e. tungsten chip) and the like. The rollers can function to prevent the fuel from being pushed back through the barrel towards the inlet by virtue of the backpressure within the reactor. Ideally, then may be two feeding rollers, positioned on opposing sides of the barrel.

The inlet may be associated with a feed chute. The feed chute may be a pressure feed chute and may include one or more separate rollers to pressure feed the fuel into and through the feed chute. The inlet may also preferably be associated with one or more pre-feeder devices such as dewatering belts or compacting conveyors. The pre-feeder device may also accomplish mixing of more than one type of fuel. As such, the pre-feeder may be a mixing bin. As the moisture content of the fuel may be important in the production and maintenance of plug seal quality, there may also preferably be a means to precondition the feed stock of fuel to achieve the required moisture content.

The at least one roller is suitably positioned adjacent to one end of the barrel. Preferably, the at least one roller is positioned at the end furthest from the pressurised vessel so as to push or otherwise drive the fuel through the barrel. The rollers may comprise a driven roller and an idler roller, or a pair of driven rollers.

The barrel may comprise an elongate substantially straight member. One end of the barrel may be associated with the at least one roller. The other end of the barrel may be associated with an expansion area or zone. The barrel may also preferably converge or diverge along its length in order to properly control the compaction, friction forces and sealing characteristics of the plug. The expansion area or zone may comprise a chamber that is typically positioned between the barrel and the reactor, although it may also form part of the reactor. The chamber may function to allow the compressed plug of fuel to expand and loosen thereby facilitating combustion of the fuel in the reactor. The chamber may function to preheat the fuel before entry into the pressurised vessel.

Means may be provided to assist in loosening the compressed plug of fuel. The means may comprise a kicker, a fluffer, or any other type of suitable device. The means may be positioned in the chamber and adjacent the outlet end of the barrel. Ideally, the means to assist in the loosening of the compressed plug of fuel may be provided in the expansion area or zone. Guide means may be provided adjacent the outlet end of the barrel to assist in the maintenance of the plug integrity during normal operations and also provide a constriction for plug formation at start-up as the plug passes through the outlet end of the barrel. The guide means may comprise a pair of pinch plates that allow maintenance of plug integrity or formation of the plug of fuel. The guide means may be positioned between the outlet end of the barrel and the kicker, fluffer and the like. A suitable arrangement of rollers or any other suitable low friction device may comprise the guide means to control or assist plug compaction.

The guide means may also comprise a plug control flap which is movable between a first position and a second position, wherein the plug control flap does not impinge the plug in the first position but in the second position the plug control flap at least partially occludes the barrel thereby assisting plug formation. The plug control flap may be movable using a ram. Movement of the plug control flap between the first position and the second position is preferably controlled by control means which may be automatic or manual. A transfer means such as a conveyor may be positioned in the chamber to convey the expanded fuel into the reactor.

In a particularly preferred form, the invention resides in a feed apparatus in which the barrel diverges. In the particularly preferred form, the invention possesses two feeding rollers, positioned on opposing sides of the barrel, one above the barrel and one below the barrel. The preferred form of the invention also has a plug control flap located in the expansion zone, adjacent the end of the barrel. According to this particularly preferred form, the barrel is oriented substantially horizontally.

In a second particularly preferred form, the barrel of the feed apparatus may be oriented substantially vertically. In this preferred form, the invention may use gravity to enhance the effectiveness of the invention. For example, gravity will allow fuel to fall to the feeding rollers, after which the back pressure from the pressurised vessel will allow or enhance the formation of a plug by pushing against the compressed fuel plug as it enters the barrel. Accordingly, gravity may also assist in the break-up of the sealing plug upon exit from the barrel. Preferably, the vertical form of the invention may be more conducive to treating a mix of fuels such as bagasse and woodchips. There may preferably also be more than one set of feeding rollers. A first set of feeding rollers may preferably compress the fuel and pass the fuel into a pre-feed barrel. At the end of the pre-feed barrel closest to the pressurised vessel, the fuel may preferably be further compressed by a second set of feeding rollers prior to entry into the barrel leading to the expansion zone. In this embodiment, the second set of feeding rollers may be associated with a fuel and or gas leakage collection apparatus. The fuel and/or gas leakage collection apparatus may comprise a shroud substantially surrounding the second set of feeding rollers. The shroud may preferably be associated with a cyclone or like device for collection and/or separation of the fuel particles entrained in the gas.

The parameters controlled to assist in plug formation may suitably include fuel moisture content, bulk density of the fuel or feedstock and the particle size and size distribution of the feed.

BRIEF DESCRIPTION OF THE DRAWINGS Embodiments of the invention will be described with reference to the following drawings in which:

Figure 1 illustrates a schematic view of a feed apparatus. Figure 2 illustrates a perspective view of a feed apparatus according to a preferred embodiment of the present invention. Figure 3 illustrates a side elevation view of the feed apparatus shown in Figure 2.

Figure 4 illustrates a sectional top view along the line AA. Figure 5 illustrates a sectional side view along the line BB. Figure 6 illustrates an end elevation view of the feed apparatus shown in Figure 2.

Figure 7 illustrates a sectional end elevation view along the line CC. Figure 8 illustrates a perspective view of a second preferred embodiment of the present invention.

Figure 9 illustrates a section a sectional side elevation view of the feed apparatus as shown in Figure 8.

Figure 10 illustrates an end elevation view of the feed apparatus as shown in Figure 8. BEST MODE

Referring to Figure 1 , there is illustrated a schematic of a feed apparatus 10 which functions to push a formed sealing plug of biomass fuel into a reactor 11 , and functions efficiently even if reactor 11 has a positive pressure.

Apparatus 10 has an inlet 12, an outlet 13, and a barrel 14 which is an extruder barrel and which is positioned between inlet 12 and outlet 13. In the embodiment shown in Figure 1 , one end of the barrel is associated with inlet 12 while the other end of the barrel is associated with outlet 13.

At least one feeding roller is provided, and in the embodiment, a pair of oppositely rotating feeding rollers 15, 16 are provided. Rollers 15, 16 can comprise a driven roller and an idler roller, or a pair of driven rollers. The roller periphery may be formed with grip enhancing means and this may be in the form of grooves, profiles, a textured surface, hard facing particles (i.e. tungsten chip) and the like. The rollers are spaced apart sufficiently to allow the solid fuel (for instance bagasse) to pass between the oppositely rotating rollers. The bagasse may be compressed during this process to form or to begin to form the sealing plug. The pair of rollers is positioned adjacent one end of the extruder barrel

14 and therefore pushes the bagasse into the extruder barrel and also compresses the bagasse. As the bagasse is pushed through the extruder barrel a sealing plug of bagasse is formed, and the sealing plug is continuously pushed through the barrel and through outlet 13. The sealing plug can therefore be seen as a "dynamic biomass seal". The barrel functions to maintain the integrity of the plug. The rollers 15, 16, positioned at one end of the barrel 14 and the guide means 17 at the other end of the barrel 14 also assist in maintaining plug density and integrity.

The length and cross-sectional dimension of the barrel can vary but should be sufficient to allow a dynamic biomass seal to form and therefore reduce escape of pressurised gas from reactor 11 back through feed apparatus 10.

Adjacent outlet 13 is an expansion area or zone that is in the form of a pair of movable pinch plates 17. In this area, the compressed bagasse can begin to naturally expand in a controlled manner.

As the bagasse passes through the expansion area, it contacts a means to assist in loosening the compressed bagasse, the means being in the form of a break up device such as a kicker or fluffer 18. This device functions to loosen and break up the bagasse plug.

The loosened bagasse then passes onto a conveyor 19 that conveys the loosened bagasse into reactor 11.

Outlet 13 of barrel 14 typically passes into a side chamber or expansion zone 20 that contains the pinch plates 17, the break up device 18 and conveyor 19.

Inlet 12 is associated with a feed chute 21. The feed chute 21 can gravity feed the bagasse (or other solid fibrous biomass fuel) into inlet 12. If desired, feed chute 21 can comprise a pressure feed chute and may be provided with one or more feed rollers.

The feed apparatus can be used to feed various types of solid fuels which would be otherwise difficult to efficiently feed into a reactor. The solid fuels include fibrous fuels such as bagasse, bagasse/coal, bagasse/woodchip, straw, straw/woodchip or other straw mixtures, green municipal waste, grasses, switch grass, alfalfa and other types of long biomass fuels with a long soft fibre.

The feed apparatus can feed this material into reactors for combustion and/or gasification.

As is illustrated in Figures 2 to 7, the feed apparatus may be used in association with a non-combusting or non-gasifying pressure vessel. As illustrated in Figure 2, the expansion zone 20 may be fitted with an end cap 22 when not in use to protect the innards of the expansion zone 20 and the feed apparatus 10 from damage. The end cap 22 is removable to allow the expansion zone 20 to be positioned adjacent an inlet to a furnace. The embodiment illustrated in Figure 2 to 7 possesses a pair of feeding rollers 15, 16 and these rollers are positioned substantially vertically, one above the other on either side of the entry to the barrel 14. Each feeding roller 15, 16 is a cylindrical roller with a grip enhancing profile on the circumferential surface. The pair of feeding rollers 15, 16 is maintained in position adjacent to the barrel 14 using a pair of frame members 30. Each frame member is equipped with a pair of spaced apart openings 31 through which the axle 32 for each of the feeding rollers 15, 16 extends. The pair of feeding rollers 15, 16 is spaced apart a small distance at the nearest point, which is adjacent the entry to the barrel 14, in order to apply pressure and correctly feed the fuel into the barrel 14. Ideally, both of the feeding rollers 15, 16 are driven rollers, and a motor or other drive apparatus (not shown) is associated with each axle 32. As is illustrated in Figure 5, the barrel 14 of this embodiment diverges at an inner end, being the end closest to the pressurised vessel or reactor 11. The barrel comprises two substantially planar plates, the first plate 23 positioned substantially horizontally, and the second plate 24 is inclined at a small angle to the horizontal to form the divergent barrel 14. At the inner end of the barrel 14, a bagasse plug control flap 25 is located. The bagasse plug control flap 25 is a substantially planar member having an attachment to allow movement between a first position wherein the bagasse plug control flap 25 at least partially obstructs the inner end of the barrel 14 and a second position where in the bagasse plug control flap 25 does not obstruct the inner end of the barrel 14. Movement of the bagasse plug control flap 25 into the first position allows the formation of a plug within the barrel 14, to at least partially seal the barrel 14 and prevent any back flow of the fuel due to the pressurised environment of the pressurised vessel or reactor 11. The bagasse plug control flap 25 is moveable between the first position and the second position by a hydraulic ram 26. The hydraulic ram 26 is connected to the bagasse plug control flap 25 and is also linked to the expansion zone wall.

According to the embodiment shown in Figure 5, there are effectively two expansion zones, a first expansion zone 20 and a second expansion zone 27. Associated with each expansion zone is an access hatch 28, to allow access into each expansion zone. As the expansion zones 20, 28 are also pressurised, the access hatches are sealed with hatch covers 29 securely enough to withstand the pressure.

As the feed apparatus 10 of this embodiment is used to feed fuel into a pressurised vessel, the feed apparatus 10 itself must be capable of withstanding an equal pressure. Therefore, the feed apparatus 10 as a whole, and particularly the expansion zones 20, 27 should be designed according to the appropriate Standard for Pressure Vessel Design.

The feed apparatus as seen in Figures 2 to 7 is mounted on a support frame 33. The support frame 33 comprises a number of H-section metal girders arranged as a mounting frame to provide the requisite support to the feed apparatus 10. The feed apparatus is secured to the support frame by mounting straps 50.

As can be seen in Figures 8 to 10, another embodiment of the feed apparatus 10 may also be associated with a gasifier drying column 34. The essence of gasification process is the conversion of solid carbon fuels into carbon monoxide by thermochemical process. The gasification of solid fuel is accomplished in an air sealed, closed chamber, usually under pressure relative to ambient pressure. As the embodiment of the invention shown in Figures 8 to 10 provides a feed apparatus to a gasifier drying column 34 or gasifier reactor vessel, it is useful to understand the operating parameters of a typical gasifier, in order to fully appreciate the operation of the feed apparatus 10.

Gasification is quite a complex thermochemical process. Splitting of the gasifier into strictly separate zones is not realistic, but nevertheless it is conceptually essential. Gasification occurs at the same time in different parts of gasifier. There is generally a drying section, a pyrolysis section and a reaction section.

Biomass fuels generally consisting of moisture ranging from 5 to 35% are fed into the dryer 34. At a temperature above 100 ° C, the water is removed and converted into steam. In the drying stage, fuels usually do not experience any kind of decomposition.

Air introduced in the oxidation zone contains, besides oxygen and water vapours, inert gases such as nitrogen and argon. These inert gases are considered to be non-reactive with fuel constituents. The oxidation takes place at the temperature of 700-2000° C. As can be appreciated, at temperatures such as these, the pressure in the gasifier could be considerable. A heterogenous reaction takes place between oxygen in the air and solid carbonized fuel, producing carbon monoxide. In the reduction zone, a number of high temperature chemical reactions take place in the absence of oxygen.

Producer gas is the mixture of combustible and non-combustible gases which results from the gasification operation. The quantity of gases constituents of producer gas depends upon the type of fuel and operating conditions of the gasifier.

A wide range of biomass fuels such as wood, charcoal, wood waste (branches, roots, bark, saw dust) as well agricultural residues- maize cobs, coconut shells, cereal straws, rice husks, can be used as fuel for biomass gasification. Theoretically, almost all kinds of biomass with a moisture content of 5-35% can be gasified. However, not every biomass fuel leads to successful gasification. One key to successful design of a gasifier is to understand properties and thermal behaviour of fuel as fed to the gasifier. The properties of fuel which influence the gasification and which can be controlled with the feed apparatus according to the present invention are described below. Fuel moisture content

The moisture content of the most biomass fuel depends on the type of fuel, its origin and treatment before it is used for gasification. Moisture content of the fuel is usually referred to as inherent moisture plus surface moisture. A moisture content below 15% by weight is desirable for trouble free and economical operation of a gasifier. Higher moisture contents can reduce the thermal efficiency of a gasifier and results in low gas heating values. Fuel with higher moisture content becomes increasingly difficult to ignite, and the gas quality and yield are also poor. Particle size and distribution The fuel size affects the pressure drop across the gasifier and the power that must be supplied to draw the air and gas through gasifier.

Excessively large sizes of particles give rise to reduced reactivity of fuel, causing start-up problems, such as difficulty with plug formation and poor gas quality. Therefore, in order to optimise the gasification process the compacted fuel exiting the feed apparatus should ideally be fully expanded prior to being fed to a gasifier drying column 34.

Bulk density of fuel

Bulk density is defined as the weight per unit volume of fuel. Bulk density varies significantly with moisture content and particle size of fuel, as fuel with a higher moisture content and a smaller particle size will tend to aggregate or clump together. It is also recognised that bulk density has considerable impact on gas quality, as it influences the fuel residence time in the gasifier, fuel velocity and gas flow rate. Ideally, the compacted fuel exiting the feed apparatus should therefore be expanded prior to entry to the gasifier drying column 34.

Fuel form

The form in which fuel is fed to gasifier has an economical impact on gasification. Compaction of biomass such as bagasse has been practiced in the USA and other countries for the past 40 years. Cubers and pelletisers compact all kinds of biomass and municipal waste into "energy cubes".

Generally these have been produced in batch quantities and gravity fed to a gasifier to overcome the pressure effects. These cubes are available in cylindrical or cubic form and have a high density of 600-1000 kg/m ³. The specific volumetric content of cubes is much higher than the raw material from which they are made.

Reactivity of fuel

Reactivity determines the rate of reduction of carbon dioxide to carbon monoxide in the gasifier. Reactivity depends upon the type of fuel. There is a relationship between reactivity and the number of active places on the fuel surfaces, which is in turn related to the bulk density of the fuel.

Each of the above properties may also assist in the formation or maintenance of the sealing plug. The features and operation of the feed apparatus according to the present invention provide the fuel with the requisite properties for gasification and a solution to the problem of continuously feeding a biomass material into a pressurised gasifier environment. Specifically, the embodiment illustrated in Figures 8 to 10, possesses a two-stage feed apparatus. There is a first feeder stage 35 and a second feeder stage 36. The first feeder stage 35 is a pre-feed compaction stage, having a pair of feeding rollers 15, 16 spaced apart a wider distance than the pair of feeding rollers 15, 16 in the second feeder stage 36. The first feeder stage compacts the feed slightly, while also achieving a partial dewatering effect. It also contributes to the formation of a plug.

The fuel is fed by a slat supply conveyor 37 into an entry chute 38. The fuel falls through the entry chute 38 into a first barrel 39. At the lower end of the first barrel 39, the first feeder stage 35 is located. In the first feeder stage, the fuel undergoes a pre-compaction and exits into an intermediate barrel 40. At the lower end of the intermediate barrel 40, is the second feeder stage 36. The fuel enters a pair of feeding rollers 15, 16 which are closely spaced. Upon exiting the pair of feeding rollers 15, 16 the fuel is forced into the barrel 14. Each pair of feeding rollers 15, 16 is contra-rotating. Upon exiting the lower end of the barrel 14, the fuel undergoes expansion in the expansion zone 20. It is important to note that the bagasse plug control flap shown in Figure 9 is in the first position, obstructing the lower end of the barrel 14. This will enhance plug formation. Once a plug has been formed, the plug control flap may be moved to the second position and plug formation is maintained due to the back pressure from the gasifier drying column 34. Each pair of feeding rollers 15, 16 is associated with a drive 46 and gears 47 so that a single drive can drive both feeding rollers.

As can be seen in Figure 9, the bagasse plug control flap 25 is movable between the first position and the second position by a ram 26. The ram 26 has an access hatch 28 associated with it for servicing and replacement. At the lower end of the expansion zone 20 there is an isolation valve 41.

The second feeder stage 36 is associated with a leakage collection system. The leakage collection system comprises a shroud 42 substantially surrounding the pair of rollers 15, 16 of the second feeder stage 36. The shroud 42 is associated with a cyclone 43. The cyclone 43 has a rotary valve 44 at its base to remove any build-up of fuel which has leaked from the second feeder stage 36. The cyclone 43 also has an gas outlet pipe 45 which is connected to a suction device, for example a fan (not shown).

Also shown in Figure 8 is the mechanism for mixing the fuel's where more than one fuel is used. The mixing mechanism comprises a mixing box 48 positioned at the end of two conveyors 49. For example, one of the conveyors 49 may convey the bagasse to the mixing box 48 and the other conveyor 49 may convey wood waste. The speeds of the respective conveyors 49 can be optimised to provide particular proportions of each fuel component in the mixing box 48. The mixing box 48 is associated with the slat supply conveyor 37. It should be appreciated that various other changes and modifications can be made to the embodiment described without departing from the spirit and scope of the invention.

Claims

CLAIMS:

1. A feed apparatus to feed a fibrous fuel into a pressurised vessel, the feed apparatus comprising an inlet into which the fuel can pass, an outlet to pass the fuel into the reactor, a barrel between the inlet and the outlet and through which the solid fuel passes, and at least one feeding roller to feed the fuel into and through the barrel, the construction and arrangement being such that the fuel forms a sealing plug in the barrel and said sealing plug is pushed through the barrel by virtue of the at least one feeding roller.

2. The feed apparatus as claimed in claim 1 wherein a pair of feeding rollers is provided, positioned on opposing sides of the barrel to feed and push the fuel through the barrel.

3. The feed apparatus as claimed in claim 2 wherein the feeding rollers comprise counter-rotating rollers.

4. The feed apparatus as claimed in any one of the preceding claims wherein at least one roller is provided with grip enhancing means on the peripheral surface.

5. The feed apparatus as claimed in claim 4 wherein the grip enhancing means of the peripheral surface comprises circumferential grooves or teeth in the form of grooves, profiles, a textured surface, or hard facing particles for example tungsten chip.

6. The feed apparatus as claimed in any one of the preceding claims wherein the inlet is associated with a feed chute.

7. The feed apparatus as claimed in any one of the preceding claims wherein, the inlet is associated with one or more pre-feeder devices such as dewatering belts, mixing devices or compacting conveyors.

8. The feed apparatus as claimed in any one of the preceding claims wherein, the feed apparatus is associated with a means to precondition the feed stock of fuel to achieve the required moisture content.

9. The feed apparatus as claimed in any one of the preceding claims wherein, the feed apparatus is associated with a means to precondition the feed stock of fuel to achieve the required bulk density of the fuel.

10. The feed apparatus as claimed in any one of the preceding claims wherein, the feed apparatus is associated with a means to precondition the feed stock of fuel to achieve the required particle size and size distribution.

11. The feed apparatus as claimed in any one of claims 8 to 10 wherein, the parameters of the feed are controlled to assist plug formation.

12. The feed apparatus as claimed in any one of the preceding claims wherein, the barrel diverges along its length in order to properly control the compaction, friction forces and sealing characteristics of the plug

13. The feed apparatus as claimed in any one of the preceding claims wherein, the expansion zone allows the compressed plug of fuel to expand and loosen thereby providing an enlarged surface area.

14. The feed apparatus as claimed in any one of the preceding claims wherein, loosening means are provided to assist in loosening the compressed plug of fuel, said means comprising for example a kicker, a fluffer, or any other type of suitable device.

15. The feed apparatus as claimed in claim 14 wherein, the loosening means is positioned in the expansion zone and adjacent an outlet end of the barrel.

16. The feed apparatus as claimed in any one of the preceding claims wherein, guide means are provided adjacent the outlet end of the barrel to assist in the maintenance of the plug integrity during normal operations and also provide a constriction for plug formation at start-up as the plug passes through an outlet end of the barrel.

17. The feed apparatus as claimed in claim 16 wherein, the guide means comprises a pair of pinch plates that allow maintenance of plug integrity or formation of the plug of fuel.

18. The feed apparatus as claimed in claim 16 wherein, the guide means is a suitable arrangement of rollers or any other suitable low friction device to control or assist plug compaction.

19. The feed apparatus as claimed in claim 16 wherein, the guide means comprises a plug control flap which is movable between a first position and a second position, wherein the plug control flap does not occlude the barrel in the first position but in the second position the plug control flap at least partially occludes the barrel assisting plug formation.

20. The feed apparatus as claimed in claim 19 wherein, the plug control flap is movable using a ram controlled by control means.

21. The feed apparatus as claimed in any one of the preceding claims wherein, a transfer means such as a conveyor is positioned in the expansion zone to convey the expanded fuel into the pressurised vessel.

22. The feed apparatus as claimed in any one of the preceding claims wherein, more than one set of feeding rollers is provided.

23. The feed apparatus as claimed in claim 22 wherein, a first set of feeding rollers compresses the fuel and passes the fuel into a pre-feed barrel, a second set of rollers at an outlet of the pre-feed barrel, the fuel being further compressed by the second set of feeding rollers prior to entry into the barrel leading to the expansion zone.

24. The feed apparatus as claimed in claim 22 wherein, the second set of feeding rollers is associated with a fuel leakage collection apparatus.

25. The feed apparatus as claimed in claim 22 wherein, fuel leakage collection apparatus comprises a shroud substantially surrounding the second set of feeding rollers, the shroud associated with a cyclone or like device for collection and/or separation of the fuel.

26. A feed apparatus substantially as described herein with reference to the accompanying drawings.