USE IN A GASIFICATION PROCESS

The present invention relates to a filter apparatus for use in the treatment of fluids in particular the treatment of hot gases produced within a pyrolysis process. The invention also relates to a gasifier for use in a gasification process. The filter assembly may be used with hot liquids or gases (particularly hot gases) for filtering, cleaning or separation of particulate, liquid or gaseous constituents. Filter assemblies, especially those capable of filtering gas at very high temperatures, and gasifiers, form key parts of an apparatus for use in carrying out a pyrolysis process.

In a known pyrolysis process, a feedstock of combustible organic material is fed into a heated pyrolysis kiln. Suitable feedstocks include municipal sourced waste (MSW) and refuse derived fuel (RDF) and also include wood and pellets, crops and other agricultural waste. Heat from a furnace that surrounds the kiln heats the feedstock material to a temperature at which pyrolysis of the material occurs. During this heating it is important that the flow of air into the kiln is prevented, as otherwise the heated pyrolysis gases and char would combust and prevent the production of syngas, and in extreme circumstances may cause an explosion.. The pyrolysis process converts the organic material into char by releasing volatile pyrolysis gases and tars and an amount of fine carbon particulates. The pyrolysis kiln may be a rotary kiln type kiln as described in GB2441721B that is incorporated herein by reference. This document describes a rotary kiln having a stationary inlet, a stationary outlet, and a rotary kiln, the inlet being upstream of the kiln which is in turn upstream of the outlet. A rotary seal connects the kiln to the inlet and a further rotary seal connects the kiln to the outlet, the seals preventing air entering the kiln. The kiln typically slopes down from the inlet to the outlet to encourage the feedstock that enters through the inlet to move towards the outlet.

The dust laden pyrolysis gases, which may also contain evaporated oils and water vapour, is removed from the outlet stage of the kiln and passed through a filter. It is known technology to clean gases at elevated temperatures above 250 C by use of

SUBSTITUTE SHEET RULE 26 ceramic filter systems. The cleaning mechanism for these filters is typically compressed air or gas forced down the inside of the filter element, causing a shock wave which disturbs the material attached to the outside wall of the filter medium. In the pyrolysis process, the process has to operate between the stringent pressure constraints to ensure that (a) none of the pyrolysis gas escapes the seals, and (b) that no ambient air is sucked into the system. During conventional cleaning of the filter elements the shock wave of the compressed gas disturbs the pressure system in the pyrolyser and pushes gases out of the seals.

The filter must be able to cope with high temperatures and may be a ceramic filter of the kind described in GB2409655B which is also incorporated herein by reference. The ceramic filter is located within a vessel and is scrubbed by a set of rings that can be moved up and down the outer wall of the cylindrical filter element. This removes the solids such as char and entrained dust from the gas.

The filtered gas is then passed through a quencher to cool the gas and remove entrained contaminants such as tars and water to leave a relatively pure synthesized gas known as syngas. This gas can then be burnt to drive a heat engine, such as a spark ignition gas engine or gas turbine, in order to generate electricity, or may be partially used as fuel for the furnace, or simply stored for later use or sale.

The solids that are separated from the gas by the filter, in the form of char, may also be used to extract further energy. They may typically be fed to a gasifier which again heats the solids but in the presence of both oxygen (as pure oxygen or in air) and steam, and this gasification extracts any remaining gases and turns the char into ash. These extracted gases, also a form of syngas, may be used as fuel to heat the pyrolyser kiln. A range of different gasifiers may be used. In the example taught in GB2409655B the gasification takes place towards the top end of the gasifier, and the ash that is left over from the process falls down to the bottom where it can be removed, cooled and bagged.

SUBSTITUTE SHEET RULE 26 The use of the pyrolysis process enables the energy contained in the feedstock to be extracted and put to use rather than the traditional process of sending the waste to a landfill site. Because the gases and other waste products can be carefully contained and not emitted into the atmosphere, the process is far cleaner than other combustion processes such as an incineration of organic feedstocks.

Preferred aspects of the present invention seek to provide an improved apparatus and method for cleaning filters for filtering fluids, especially hot gases, and which overcome one or more of the above problems. The aspects also seek to provide an improved gasifier for the gasification of char products produced in a process such as a pyrolysis process.

According to a first aspect, the present invention provides apparatus for filtering hot gases comprising :

a housing having a gas upstream zone and a gas downstream zone;

a removable filter cartridge that supports at least two filter elements, each defining a path along which gas can flow, the filter cartridge being removably secured to the housing such that gas can flow from upstream zone into the downstream zone through the filter elements;

the housing further defining an inlet portion for contaminated gas communicating with the gas upstream zone, the gas upstream zone defining a contaminated side of the filter elements; and an outlet portion for treated gas communicating with the gas downstream zone,

and in which the filter cartridge further includes at least two sets of scrapers, each set being associated with one of the two filter elements, and a drive mechanism for moving the two sets of scrapers relative to the filter elements, so as to displace material adhering to the filter element.

Providing a filter cartridge that is removable allows for simple maintenance or replacement of the filter elements and the scrapers and drive mechanism.

Preferably the cartridge includes more than two filter elements arranged in parallel so that each provides a flow path from the upstream zone to the downstream zone that is independent of the flow paths of the other filters, i.e. gas flowing along one path to the downstream zone does not need to flow through any of the other filters.

SUBSTITUTE SHEET RULE 26 Four filter elements may be provided, or as many as six or more. In one arrangement the filter cartridge may include four filter elements spaced uniformly around a central axis.

The drive mechanism may comprise a single actuator that is connected to both of the sets of scrapers such that movement of the actuator is converted into movement of both sets of scrapers at the same time . The drive mechanism may, in one embodiment, be movable in a reciprocating manner to displace the material adhering to the filter element. The reciprocating movement direction is preferably substantially the same as the longitudinal direction of the filter element. The drive mechanism may comprise a hydraulic or pneumatic ram comprising a piston displaceable relative to a cylinder, one of the piston and cylinder being fixed relative to the filters and the other fixed to the scrapers.

Preferably each of the two filter elements may therefore be elongate in form, and the drive mechanism may be arranged longitudinally with respect to the filter elements. The axes of the two filter elements may be in parallel with each other and with the axis of the movement of the ram of the drive mechanism.

The two (or more) filter elements of the cartridge may be the same length with the top and bottom of each filter element aligned with the other.

Filtration could occur from the inside of the filter element to the outside, with the displacement arrangement being located inside the filter element. However, as this puts limitations on the cleaning mechanism, it is preferred that it be located outside the filter element.

The filter need not be cylindrical in shape. Other cross-sections are possible, for example square instead of circular. This gives a greater filter surface area per unit of filter size.

SUBSTITUTE SHEET RULE 26 Each of the scrapers may be spaced from the associated filter element so as to act to remove material adhering to the filter element but permitting a predetermined thickness of material to remain adhered to the filter element. It has been found that optimum filtration/cleaning is achieved by permitting the contaminant present on the filter element to remain at an optimum thickness. A balance is struck between enough adhering material remaining on the filter surface of the filter element to provide a most effective filter and the filter becoming blocked to an extent where the pressure gradient across the filter becomes adverse . An object of the invention is, as far as practicable and possible, to minimise the variation from constant pressure drop across the apparatus.

The scrapers may be adjustable relative to the associated filter elements so as to enable alteration of the spacing between the filter element and the scraper. This allows simple alteration of the predetermined thickness of material permitted to adhere to the filter element according to the different applications for which the apparatus is used.

In preferred embodiments the thickness of material permitted to remain adhered to the filter element is between 1 mm and 12 mm.

Preferably each of the sets of scrapers may include at least one scraper collar or ring extending about at least a portion of the filter surface of the filter element. This provides for a simple arrangement suitable for use at high temperatures. The displacement arrangement is beneficially moved such that the scraper collars or rings move over the filter surface of the filter element to displace the material adhering to the filter element.

The filter cartridge may comprise a support body that supports the filter elements and drive mechanism. The support body may connect with and be sealed to a baffle that divides the inlet zone from the outlet zone.

Each filter element may be suspended from an upper point of the support body, and may be secured towards a lower point of the support body to prevent excess side-to- side movement of the filter element.

SUBSTITUTE SHEET RULE 26 The apparatus may include more than one cartridges. Each cartridge may include its own filter elements, sets of scrapers and drive elements. Each cartridge may be fully independent of the other, allowing for one cartridge to be removed with the other left in situ. This will allow one to be repaired without the need to shut down the apparatus.

The apparatus is preferably for treating gases at a temperature between 200 C and 1000 C.

The filter element beneficially comprises a substantially rigid filter element and may comprise a ceramic filter element. The fluid upstream zone preferably receives contaminated fluid for filtration or cleaning and the fluid downstream zone beneficially carries cleaned or filtered fluid.

Each of the filter elements may comprise a tube or cage having a longitudinally running outer filter surface and a closed end, an open and being provided opposed to the closed end preferably communicating with the fluid downstream zones.

The filter element beneficially includes a filter wall provided with apertures or pores for filtering or cleaning the contaminated fluid media. The apparatus beneficially includes extraction means for extracting contaminant material removed from the filter element. The apparatus may include a gravity fed outlet or hopper for this purpose.

The outlet hopper may be connected to a gasifier for the further gasification of the solid material.

A layer of active or other treatment material may be present on the surface of the filter element. For example, a layer (typically 4 mm to 12 mm) of activated carbon may be present to remove materials such as dioxins from a gas or mixture of gases being treated.

SUBSTITUTE SHEET RULE 26 According to a second aspect the invention provides a filter cartridge for use in a filter apparatus having the features set out hereinbefore. The removable filter cartridge may include at least two filter elements, each defining a path along which gas can flow, the filter cartridge in use being removably secured to a housing such that gas can flow from upstream zone into the downstream zone through the filter elements; and in which the filter cartridge further includes at least two sets of scrapers, each set being associated with one of the two filter elements, and a drive mechanism for moving the two sets of scrapers relative to the filter elements, so as to displace material adhering to the filter element.

According to a third aspect the invention provides a gasifier for use in a pyrolysis process for the treatment of organic solids by a process of gasification, the gasifier comprising:

a housing divided into a lower chamber and an upper chamber, the upper and lower chambers being connected by a neck,

A gas inlet for the introduction of gas into the upper chamber,

A gas outlet for the removal of gas from the upper chamber,

A rotary screw conveyor that extends upwards from a location below the bottom of the lower chamber and up through a sealed opening in the bottom of the lower chamber and onwards through the neck and into the upper chamber, the outer diameter of the screw conveyor being sufficiently small to leave a gap for gasified material to fall down around the conveyor through the neck into the lower chamber, ,

a collection disk that is located in the lower chamber at a point close to the bottom of the neck that collects any of the material that falls down from the neck, the disk in use supporting a column of gasified material that extends up into the upper chamber, a drive mechanism for rotating the disk periodically to cause material that has built up on the disk to be flung off the disk to the bottom of the lower chamber,

and an extraction mechanism for removing the material from the bottom of the lower chamber.

The collection disk may rotate about the same axis as the conveyor screw, and may be secured towards a central hub to a tube through which the conveyor screw passes, the

SUBSTITUTE SHEET RULE 26 tube being provided with a gear wheel that is driven by a drive chain or belt connected to an electric motor.

The provision of the disk allows material to build up on the disk before it is periodically spun off and removed by the extraction mechanism. Allowing the material to build up in this way. The disc supports the bed of gasified material, which needs a long residence time to drive off the carbon from the ash. The bed of char may in use extend up from the disc to the top of the char input screw. The level of the pile of material may be monitored by a radar sensor provided in or above the upper chamber. The base of the pile will be ash that needs to be cooled and this may be achieved by some preheating of the air and steam passing up through it. If the char bed is building up too much then the disc is spun to remove some ash causing the bed to drop.

The gasifier may include a supply mechanism for feeding material to be gasified onto the base part of the screw conveyor for conveyance into the upper chamber.

The supply mechanism may comprise a second conveyor screw that feeds material to be gasified from a hopper. This may extend generally horizontally.

The conveyor screw that feeds into the upper chamber may terminate midway up the upper chamber, so that material is deposited on top of an existing layer of material. This newly deposited material will then be dried with the layer beneath it having previously been dried.

The screw conveyor that feeds to the upper chamber may comprise a helical auger that is located within a tube, the outer wall of the tube being spaced all around or in places from the neck to allow the gasified ash to fall down onto the disk.

To speed up the drying process and to ensure that the material is gasified in the upper chamber it is important to control the amount of air in the upper chamber and the amount of steam, as well as to control the amount of solid fed into the chamber. To help this, along with the solid material passed into the upper chamber steam and air

SUBSTITUTE SHEET RULE 26 are also also conveyed via an annular space around the screw conveyor into the upper chamber.

The screw conveyer feeding material to the upper surface of the char piled on the disc also comprises of an annular passage than between the pipe containing the screw conveyed char and the outer pipe in contact with the ash moving down towards the disc. Preheated air, for instance that has passed through the upper chamber, together with steam, is in use passed into this annular space and from the base of the gasifier. An opening, or several openings, are provided in specifically designed sites within the wall of the tube surrounding the auger through which steam and air can be introduced to the pile of material built up on the disk..

The air and steam are at a pressure greater than that of the char bed and upper gasification chamber pressure and thus are ejected into the base of the char and ash bed.

This annular arrangement improved the efficiency of the apparatus by the abosrbtion of heat from the ash into the air and steam passing through the annular space . Subsequently the hot air and steam preheat the char passing long the centre of this screw conveyer arrangement.

A radar sensor may be located at the top of the upper chamber that measures the distance from the top of any layer of material in the upper chamber to a known point, the sensor being located behind a protective cover and the cover being located within a flow of gas introduced to the upper chamber that helps keep the cover clear of solid material that is being gasified. The flow of gas may be of relatively clean air, and may be blown across the face of the cover by a fan.

The cover may comprise a glass cover although any other material that will allow the radar signals to pass and yet prevent particulate material in the upper chamber contacting the radar sensor may be used.

SUBSTITUTE SHEET RULE 26 The radar sensor may comprise any known radar apparatus that is capable of measuring distances to a layer of material over a range of few tens of centimetres up to a few metres. The range will depend on the length of the upper chamber from the radar sensor to the bottom of the upper chamber.

The radar sensor in effect allows the height of any column of material to be measured, which in turn can be used to control the rate at which material is conveyed into the upper chamber to prevent it from overfilling. The use of the gas flow ensures it is not clogged with solid material that would stop it from working and that it does not get too hot.

The gas outlet from the upper chamber may be offset from the centre line of the upper chamber so that as gas is driven out under suction thereby to create a swirl in the upper chamber. This implied swirl will cause a cyclonic action to separate out the heavier particulate from the gas stream allowing further processing time.

The gasifer housing may be suspended from the top so that the gas outlet is fixed in position as this is close to or at the top . Suspending the gasifier allows the housing to move up and down as it expands with temperature without the housing being damaged.

In some arrangements, for example in a pyrolysis process, due to the inherent heterogeneous nature of the feedstock fed into the pyrolysis process there will a varying heat input requirement and a varying char output. The variation in heat and char are unlikely to be compatible. By de-coupling the char collection and the char input rate to the gasifier the amount of heat delivered to the pyrolysis process can be varied independently to the char collection rate.

The gas outlet of the upper chamber may be connected to the inlet zone of a second filter apparatus similar to that described in the first aspect of the invention.

This second filter apparatus will act to filter ash out from the gasification gas stream rather than filter char out from the pyrolysis gas stream as described previously.

SUBSTITUTE SHEET RULE 26 The lower chamber may be connected by a tube to the inlet side of a filter apparatus, such as the apparatus of the first aspect, to allow filtered debris scraped from the filter to be passed to the lower chamber from which it can be removed. The connection may be along a tube, and a screw drive may be provided within the tube for conveying material at the base of the filter apparatus to the lower chamber. A ball valve or other cut off valve may be provided in this path that stops air normally passing from filter to gasifier. The ball valve or other cut off valve needs to be strong enough to cut plug of material in the flow path that may be formed.

According to a fourth aspect the invention provides a gasifier for use in a pyrolysis process for the treatment of organic solids by a process of gasification, the gasifier comprising gasification chamber, a gas inlet for the introduction of gas into the chamber, a gas outlet for the removal of gas from the chamber, and

A radar sensor located in a position that permits a measurement of distance to be made within the chamber of the top of any layer of material in the chamber, the sensor being located behind a protective cover and the cover being located within a flow of relatively clean gas introduced to the upper chamber that helps keep the radar cool and prevents the hot gasification gas from damaging the radar.

The flow of gas may be of relatively clean air, and may be blown across the face of the cover by a fan. This cooling air absorbs the heat that may damage the radar. The efficiency of the process in improved by using this preheated air as the gasification oxidisation air that is passed through the tube around the screw auger. The air that has passed over the cover may therefore be a carried by a pipe to the tube around the auger, and may pass through a space that surrounds the base of the upper chamber where it is further pre-heated.

The fan may blow the inlet gas directly onto the cover or tangentially across the face of the cover. The flow forms a buffer that protects and isolated the cover from the swirling gas in the chamber. The cover may comprise a glass cover although any other material that will allow the

SUBSTITUTE SHEET RULE 26 radar signals to pass and yet prevent particulate material in the upper chamber contacting the radar sensor may be used.

According to a fifth aspect the invention provides an apparatus for gasification of organic solids and the subsequent filtering of gas produced comprising an apparatus of the first aspect connected to an apparatus of the second aspect.

The gasifier may be supplied with char produced in a pyrolysis chamber. The outlet zone of the filter apparatus may be connected to a burner that produce heat for the pyrolysis chamber.

There will now be described, by way of example only, two embodiments that fall within different aspects the present invention with reference to and as illustrated in the accompanying drawings of which:

Figure 1 is an overview of an embodiment of a pyrolysis system that incorporates a number of features that are in accordance with the present invention;

Figure 2 is a diagram showing in perspective an embodiment of an assembled filter assembly which includes two removable filter cartridges and which can be used in the system of Figure 1 , the filter assembly being within the scope of the first aspect of the invention;

Figure 3 is an exploded perspective view of one of the cartridges of the filter assembly of Figure 2;

Figure 4 is a perspective view of a part of the drive mechanism of the filter assembly for moving scrapers along the filter elements; Figures 5(a) and 5 (b) show how a filter cartridge can be inserted into the housing of the filter assembly, for example following a repair or replacement of a filter cartridge;

Figure 6 is a side view of the filter apparatus of Figure 2; and

SUBSTITUTE SHEET RULE 26 Figure 7 is a diagram showing an embodiment of a gasifier apparatus in accordance with a second aspect of the invention that is connected to the filter apparatus of Figure 2 as part of the pyrolysis system of Figure 1. Figure 1 is an overview of the different stages of a complete pyrolysis system that embodies a pyrolysis system in accordance with the present invention. The reader will appreciate that various modifications to the system can be made within the scope of the present invention, and that the modified system would also fall within the scope of the present invention.

The system starts with feedstock material 1 on the left side of Figure 1 , which may be municipal source waste or refuse derived waste although other solid wastes can be used. The feedstock moves through the process from the left to the right hand side of the flow diagram, being transformed into char and gas and other materials during the process to the final output gas and energy 2 on the right hand side. The feedstock material is fed into a feedstock conditioner 3 which shreds, dries and sorts the waste before it is weighed and then compacted 4 and fed into a pyrolysis kiln 4.

The compaction ensures an air tight seal is achieved at the input to the pyrolysis kiln 5. The kiln may be of the rotating drum type described in GB2441721.

The kiln is heated by combustion 5 to cause the material in the kiln to be pyrolsed, and the pyrolysed gas and carbon particulate is removed from the kiln at high temperature and fed into a filtering stage 6 which removes solids and the gas is then quenched 7 to cool the gas and remove effluents 8, heavy oils 9 and light oils 10. The cleaned gas is the fed via a diverter valve 1 1 to a heat engine 12, such as a gas turbine, which generates heat or electricity as the energy output 2. In the event that the rate of production of gas is too high for the engine 12 to consume- say if the engine is off line- the diverter valve 1 1 can send the gas to a flare 13 where it is burnt off.

The filter 6, as well as passing the solid free gas, also extracts char 14 which is collected and further gasified by a gasifier 15. The gas produced, together with any light oil 10 removed during quenching 7, is used as fuel to heat the combustion furnace 5 that heats the pyrolysis kiln. The hot air and exhaust from the heating of the

SUBSTITUTE SHEET RULE 26 pyrolysis chamber is recirculated to extract the heat energy in a process of regeneration 16, and the extract heat is used in turn to heat the air being fed to the furnace. This ensures that no energy is wasted. The ash produced from gasification is cooled and bagged for removal.

Once material has been added to the kiln, which may be added continuously or in batches, the kiln is heated to cause the material in the kiln to be pyrolsed, and the pyrolysed gas and carbon particulate is removed from the kiln at high temperature and fed into a filtering stage which removes particulates and then quenched to cool the gas and remove water and heavy and light oils. The cleaned gas is the fed to an engine which generates heat or electricity.

In the event that the rate of production of gas is too high for the engine to consume- say if the engine is off line- the diverter valve can send the gas to a flare where it is burnt off. The filter also extracts char which is collected and further gasified. This gas, together with any light oil removed during quenching, is used as fuel to heat the pyrolysis chamber. The hot air and exhaust from the heating of the pyrolysis chamber is recirculated to extract the heat energy, which is used in turn to heat the air being fed to the furnace. This ensures that no energy is wasted. The pressure in the kiln must be monitored or otherwise controlled to ensure it does not build up to such a high level that the system is damaged. An ID fan, as will be described later, is provided which normally controls the pressure . However, in the event of a fault such as a blockage of the outlet of the kiln a pressure relief mechanism is provided to allow excess pressure to be vented out of the kiln.

An exemplary filter apparatus 6 is shown in Figures 2 to 6 of the drawings. Figure 2 shows the filter apparatus in perspective view and Figure 6 shows the apparatus from the side . The apparatus 6 comprises a housing 17 (shown with the sides partially cut away to make the internal parts visible) having a sealed gas upstream zone 18 and a sealed gas downstream zone 19. The downstream zone 19 in the example is provided above the upstream zone 18 and communicates with an outlet conduit 20 to allow filtered gas to be removed. The upstream zone and downstream zone are separated by a baffle plate 21. To allow gas to be filtered to enter the upstream zone 18, the housing further

SUBSTITUTE SHEET RULE 26 includes an inlet portion 22 for contaminated gas. The base of the sealed downstream zone, at the bottom of the housing forms a material hopper into which material scraped from the filters drops. The upstream zone 19 of the housing accommodates two filter cartridges 24,25, each of which is supported at the top relative to the housing by the baffle plate 21 , each cartridge passing through an opening in the baffle plate . Both filter cartridges 24,25 are the same, so only one of the filter cartridges will be described below in detail. This is shown in perspective in Figure 3 of the drawings.

The filter cartridge 24 comprises four elongate tubular ceramic filter elements 26. Each of the filter elements 26 defines an independent and direct path along which gas can flow from the upstream zone to the downstream zone . Ceramic filter 26 has a circumferential filter surface having apertures or pores (not shown) dimensioned to permit a desired gas to pass through the filter from an upstream gas zone.

The filters are supported at the bottom by a threaded rod fixed to the base of the hoising that engages and internal thread in a nut that is bound into the filter element when it is being cast. This hold the filter element stationary whilst the filter cleaning mechanism moves

The applicant has appreciated that the filter elements 26 can become clogged in use . Therefore each filter cartridge further includes a set of scrapers 30 associated with each one of the four filter elements. To move the scraper elements 27 each filter cartridge also includes a drive mechanism 28. This can be seen best in Figure 6.

The drive mechanism for each filter assembly comprises an elongate carrier rod 28 carrying, at 15 cm spaced intervals, a set of cleaning plates 30. The rod is connected to a piston of a ram that is located at the top of the assembly above the cover plate for easy access. As shown in Figure 4 each of the filter elements has moulded within the base a secured nut to locate and support the filter elements. Around each group of filter elements the scraper rings 30 are located on the cleaning mechanism. This arrangement provides for each filter element a series of scraper rings 30 spaced over a co-extensive length of the tubular ceramic filter element 26. The scraper rings 30 are dimensioned to be spaced from the surface of filter element by a predetermined

SUBSTITUTE SHEET RULE 26 spacing (h) (typically h = 5 mm). The carrier rod 28 extends through the housing and is driven in a reciprocating motion in a direction corresponding to the longitudinal direction of the tubular ceramic filter element. In operation gas (or a mixture of gases) to be cleaned possibly containing contaminating particulate material is fed through the filter apparatus, entering the housing via the inlet zone 18. The apparatus may be vacuum driven, for example . The gas passes through the tubular filter elements 26 and the cleaned or filtered gas leaves the apparatus via outlet zone 19. Where contaminating particulate material (such as carbonaceous material carried in hot gases resultant for example from pyrolysis) is present in the gas to be treated, it collects on the outer circumferential surface of the tubular ceramic filter elements 24. It is advantageous to aid the filtering process by maintaining a covering of filtered particulate on the surface of the filter elements 24. It is however, important that the build up of particulate is maintained below a level where the filter element 26 becomes clogged. If this were to happen, the pressure drop across the apparatus becomes too large and the treatment process inefficient.

The provision of the filter elements in two filter cartridges 24 allows one or more of the two cartridges to be easily removed from the housing. Removing the filters, scrapers and drive mechanism in one piece allows for a quick and easy swap out, reducing down time for the system. Figures 5 (a) and (b) show the simple installation of the filter cartridge as it is dropped down vertically into the housing. The filter cartridge is supported at the top, so that when it is installed all that needs to be done is to locate the bottom of the filter cartridge to prevent it from swaying.

The filter apparatus 6 can be used in a range of processes and may be used directly to filter gas output from a pyrolysis kiln. It may also be used to filter gas that is fed to the filter by a gasifier 15 prior to being used in combustion. The gasifier takes char collected 14 from the pyrolysis chamber of the pyrolysis kiln 5 and gasifies it in the presence of controlled amounts of oxygen (air) and steam to produce a combustible gas.

Figure 6 shows an exemplary gasifier 15 that falls within the scope of the invention connected to the filter apparatus of Figure 2.

SUBSTITUTE SHEET RULE 26 As shown the gasifier 15 comprises a housing 30 divided into a lower chamber 3 1 and an upper chamber 32, the upper and lower chambers being connected by a narrow neck 33. The walls of the upper chamber are lined with insulation 34 to retain heat in the chamber, the insulation being shaped to define the neck.

A gas outlet 36 is provided for the removal of gas from the upper chamber, which is offset to encourage swirl of gas in the upper chamber 32. (Mark- the air blown across the radar glass plate A rotary screw conveyor 37 extends upwards from a point below the bottom of the lower chamber and up through an opening in the bottom of the lower chamber and through the neck 33 and into the upper chamber. This comprises a rotary auger within a sealed cylindrical tube driven by a motor 37a. The outer diameter of the tube of the screw conveyor is sufficiently small to leave a gap for material to fall down past the conveyor into the lower chamber. A supply mechanism 38 feeds material to be gasified onto the base part of the screw conveyor 37 for conveyance into the upper chamber, taking material from a hopper. As shown this supply mechanism comprises a horizontal screw conveyor 39 that conveys material from a hopper 40. A motor 41 controls the screw conveyor to supply material only as needed.

Below the neck 33 is a collection disk 42 within the lower chamber 33 that has an upper surface that is horizontal and is positioned a small distance from the bottom of the neck. The disk 42 has a central hole through which the tube of the screw conveyor 37 passes and is supported by a further tube 43 that is located concentrically around the screw conveyor. The tube 43 is free to rotate concentrically around the screw conveyor in turn to spin the disk 42. A drive mechanism including a motor 44 is provided for rotating the disk periodically to cause ash that has built up on the disk 42 having fallen through the neck to be flung off the disk to the bottom of the lower chamber.

To remove ash from the bottom of the lower chamber 3 1 , an extraction mechanism is provided. This comprises another screw conveyor 45 and a motor 46 to turn it when required.

SUBSTITUTE SHEET RULE 26 A radar sensor 50 is located at the top of the upper chamber that measures the distance from the top of any layer of material in the upper chamber to a known point, the sensor being located behind a protective glass cover 52 and the cover being located within a flow of gas introduced to the upper chamber thorugh an inlet pipe 35a that helps keep the cover clear of solid material that is being gasified. The flow of gas is of relatively clean air, and is blown across the face of the cover by a fan 70. This air, which is heated as it is blown across the glass plate by the heat in the upper chamber, is then exhausted along a pipe 35b which carries it to a region at the base of the upper chamber, and having been blown into this region is then further carried to the base of the screw conveyor 37 by a further pipe where it is mixed with steam.

The gas outlet 36 of the upper chamber 32 is connected to the inlet zone of the filter apparatus 6 of Figure 2. The lower chamber 3 1 is connected by a tube 60 to the hopper at the bottom of the filter apparatus 6, to allow filtered debris scraped from the filter elements 26 to be passed to the lower chamber from which it can be removed along with the ash that drops down from the upper chamber.

The connection between the base of the filter apparatus 6 and the lower chamber has to be airtight. In this example it comprises a tube 60 that contains a screw drive conveyor 61 for conveying material at the base of the filter apparatus to the lower chamber. This conveyor is driven by a motor 62. The rate of feed is chosen so that a plug of material is formed in the tube, this plug making the connection airtight. A ball valve 63 is provided in this path that stops air normally passing from filter to gasifier, in case the plug fails.

The outlet zone of the filter apparatus as shown is connected to a burner that produces heat for the pyrolysis chamber of the system of Figure 1.

Although not shown, the motors and valves of the gasifier will be controlled by control signals from a processor, to regulate the gasification process and prevent the pressure or temperature becoming too high or too low, and to control the supply of fuel- both char and air and steam-into the upper chamber 32 where gasification occurs.

SUBSTITUTE SHEET RULE 26