The present invention relates to a system for pyrolysing material. 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 GB2441721 B which 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. 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.

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.

The present invention aims to ameliorate at least one limitation on the efficiency of operation of prior pyrolysis systems, in particular relating to improving the safety of the process and the quality of the gas that is produced.

According to a first aspect the invention provides a system for pyrolysing material comprising an inlet stage, a kiln and an outlet stage, the inlet stage being upstream of the kiln, the kiln being upstream of the outlet stage,

and further comprising a pressure relief mechanism that includes a liquid filled trap comprising a conduit having a first end and a second end, the conduit between the two ends being at least partially filled with a liquid to form at each end a respective column of liquid, the two columns being connected at a lower end by a body of liquid, whereby the pressure relief mechanism is arranged such that the surface of the liquid in the conduit at the first end is connected to the inlet stage so that the liquid is prevented from flowing into the inlet pipe and the gas pressure at the inlet stage acts down upon the column of liquid at the first end, and whereby in use the liquid in the conduit normally provides a seal to gases in the inlet stage from passing through the conduit, and in the event that the gas pressure in the inlet stage exceeds a predetermined level the gas escapes through the liquid in the conduit.

The gas pressure at the inlet stage in the region where it is connected to the pressure relief mechanism may be equal to the gas pressure within the kiln.

The pressure relief mechanism provides an effective air tight seal in normal use, which is important when the material in the kiln is being pyrolysed as it prevents the dirty syngas from bypassing the clean-up mechanism On the other than it also allows excess pressure to escape without the need for any control valves or sensors. Gas pressure may build up due to a blockage, typically in the outlet from the kiln or at some point downstream from the kiln. The passive design of the mechanism ensures the pressure can escape when there is a loss of electrical power, which would otherwise lead to a potential failure .

The conduit may comprise a trap such as a U-shaped trap with the first and second ends of the U-shaped conduit extending upwards from a base portion to define the two respective liquid columns.

The first end of the U-shaped conduit may have a smaller cross section than the second thus the displaced fluid from the first side of the U-shape is easily accommodated, and contained, within the second side .

The liquid in the trap may comprise water. There may be two or more different liquids which may be immiscible.

The second, outlet end, of the trap may be connected to a quenching stage for cleaning the gas, the pressure in which acts upon the surface of the column of water at that end of the trap.

The liquid in the trap is at ambient temperature .. The quantity of liquid in the trap is sufficient for the liquid to cool any gas bubbling through the trap to a temperature below that at which any oils entrain in the gas will be removed from the gas. The liquid may be cooled by passing chilled or cooled water through a jacket that surrounds the conduit.. The conduit may include fins on the outside to help dissipate heat from the liquid in the conduit.

The pressure relief mechanism may be connected to the inlet stage through a t-piece connector and a length of piping. This allows the mechanism to be conveniently located at a safe distance from the heated kiln. The pressure at which gas from the kiln is able to push past the seal of liquid depends on the relative pressures acting on the surfaces of the liquid at the first end and the second end of the conduit and also on the height of the column of liquid formed in the conduit and the density of the liquid that is used. The gas acts upon the surface of the first side and pushes the liquid level down until the gas can flow into the second part of the u-shape. The larger second side can accommodate , and contain, the displaced liquid.

The pressure relief mechanism may permit gas to pass through when the pressure exceeds a predefined value. The pressure relief is set so as to prevent syngas escaping from the system via the feed compaction plugs, and will thus be in the region of 0 to 50mbar of positive pressure between the kiln and ambient.

The outlet of the pressure relief mechanism, along which gas may escape if the pressure in the kiln is too high, may be connected to a quenching system which further cools the escaping gas and removes at least part of the water vapour entrained in the gas so as to produce a gas with fewer contaminants.

The quenching system may also receive pyrolysis gas that leaves the outlet of the drum following pyrolysis, such that a single quenching system can be employed to quench gas that correctly leaves the drum and that which leaves the drum when excess pressure is being relieved.

The pyrolysis gas leaves the kiln at about 500 degrees C and has a lot of water vapour and oils entrained within it which can be removed by the first stage as the cooling causes these to condense and drop out. The quenching system may comprise a first stage that in use cools the gas leaving the pyrolysis kiln to about 50degC by combining the heated gas with a closed loop water spray which in turn is cooled by a liquid-liquid heat exchanger. The cooling for the heat exchanger is provided by an external cooling loop coupled with an air blast cooler (radiator with a fan)

The cooling temperature may be selected as a function of the water moisture being carried over in the syngas and the type of oils and tars to be removed. Ideally the first quench system would cool to as low-a-temperature as possible to drop the water vapour out however, in doing so the tars and oils being dropped out may solidify and adhere to the heat exchanger surfaces, preventing further cooling.

The quench spray water is continually recirculated. The spray is collected at the base of the quench chamber and passes to a separation tank. Within the separation tank the water and any oils and tars that may have been condensed out/washed out are separated by density. The separated quench water then flows to a pump which in turn pushes the water to a liquid-liquid heat exchanger. The liquid-liquid heat exchanger is cooled by a secondary cooling circuit that includes an air blast cooler (radiator with a fan). From the liquid-liquid heat exchanger the recirculated quench water returns to be sprayed within the quench chamber.

The lower temperature is important because of the amount of moisture that goes through the process. If the temperate stays above 50 degrees C the water stays as vapour in the gas flow, but below this water drops out. However, if it is cooled to below 50 degrees heavy tars may start to solidify and may cause fouling and prevent the system being able to cool the gas. The first stage cooling will cause oils to condense from the gas along with the water vapour and the first stage may recover the oils from the water using a gas to oil separator tanks comprising dam and weir.

The quenching system may comprise in a second stage that follows the first stage. The second stage would be constructed in a similar manner to the first but the liquid- liquid heat exchanger would be cooled by chilled water circuit. The chilled water circuit containing a water chiller that would cool the chilled water to a temperature below ambient. For instance the water may be below 30 degrees, or below 20 degrees C or as low as 0 degrees C.

In a similar manner to the quench system first stage, in the second quench system the quench spray water is continually recirculated. The spray is collected at the base of the quench chamber and passes to a separation tank. Within the separation tank the water and any oils and tars that may have been condensed out/washed out are separated by density. The separated quench water then flows to a pump which in turn pushes the water to a liquid-liquid heat exchanger. The liquid-liquid heat exchanger is cooled by a secondary chilled water cooling circuit that includes a water chiller. From the liquid-liquid heat exchanger the recirculated quench water returns to be sprayed within the quench chamber.

The quencher may comprise in a third stage an additional cooling means for extracting further water vapour, comprising a means for increasing the pressure of the gas such as a fan, a heat exchanger that cools the compressed gas. The cooled compressed gas is then expanded through a throttle valve that causes the gas to further cool and dropout any further condensate. The cooled gas then passes through the other side of the heat exchanger and regaining temperature. The dry gas then passes to an induced draft ID fan which receives the cool dry gas and further comprising a throttle valve at the inlet to the ID fan that is controlled by a controller, the controller causing the throttle valve to open an close so as to control the pressure of the gas in the kiln.

The pressure in kiln will vary depending on how much gas is released from a feedstock in the furnace, the temperature and the rate at which material is added and gas drawn off. It is therefore important to control the pressure and keep it below the level at which it may blow past the pressure relief mechanism which is really only to be used as an emergency safety measure . The provision of the controller and the ID fan allows the pressure to be controlled.

A pressure sensor may be provided that measures the pressure of the gas in the kiln, the output of which is fed to the controller. The pressure sensor may be located in the kiln or at the inlet or outlet stages of the kiln. The ID fan may be controlled by a controller that causes the fan to run at constant speed yet varies the inlet throttle valve . The throttle valve may be controlled between a fully open and a fully closed position. When fully closed, no gas may pass through the valve.

By throttling the inlet side of the fan using control valves to control the pressure in the kiln rather than changing the speed of the ID fan, the inlet tips of the fan can be kept running at a high speed at all times so that no gas can flow back towards the kiln. And that the maximum pressure differential across the fan may be maintained at all times.

Furthermore, placing the valve in front of the fan rather than after the ID fan allows for reduced power consumption. With the inlet throttled the gas density in the ID fan is lower and thus the fan torque is reduced consequently reducing the power consumption. Whereas if the throttle was placed after the ID fan the density would be higher after the fan and power would absorbed in whirl slip losses. By placing the throttle valve directly in front of the fan the system pressure is maintained with only that part of the system.

The kiln may comprise a rotary kiln and there may be provided between the inlet stage and the rotary kin and/or between the rotary kiln and the outlet stage a rotary joint mechanism. The system may comprise a face seal between a rotating surface of a first seal member fixed to the kiln and a stationary surface of a second seal member fixed to the respective stage.

In a preferred embodiment the sealing surfaces of the first and second seal members are annular.

The seal members are preferably attached to respective inlet and outlet pipes of the rotary joint mechanism. It will be noted that the inlet stage is upstream of the kiln and that the kiln is upstream of the outlet stage. It will be also noted that the outlet stage is downstream of the kiln and that the kiln is downstream of the inlet stage. In preferred arrangements the upstream device comprises an outlet pipe which extends through an inlet pipe of larger diameter of the downstream device . Most preferably, said outlet pipe extends into the downstream device itself, which has the advantage of directing the conveyed material away from the respective rotary joint mechanism.

The rotary joint mechanism may incorporate a passageway for the introduction of an inert purging gas to prevent entry of air into the system and/or to prevent gases from leaving the system. The passageway preferably extends to the sealing surface of the stationary seal member from another surface of the stationary seal member, preferably from an outer cylindrical surface thereof.

To cater for solid materials to be pyrolysed which are not chemically volatile, or hydrocarbon based-, the inlet stage may be provided with a valve mechanism to constitute an inlet seal. The valve may be a rotary valve or a double flap valve or other mechanical sealing device.

Alternatively, to cater for liquids or slurry materials to be pyrolysed, the inlet seal is achieved by means of a pump connected to a feed pipe. The outlet side of the system preferably comprises a filter for dust-laden gases leaving the kiln, the filtered gases passing to a gas outlet. Solids emerging from the kiln pass from an outlet receptacle or drop out box to a conveying device.

A valve, such as a rotary valve or a double flap valve, may be provided between the container and the conveying device to serve as an outlet seal. Alternatively, the seal can be made by maintaining a column of material between the container (e.g. a drop out box) and the conveying device.

According to a second aspect the invention provides a system for pyrolysing material comprising an inlet stage, a kiln and an outlet stage, the inlet stage being upstream of the kiln, the kiln being upstream of the outlet stage,

in which the gas output from the kiln is fed to a quencher comprising a fan which compresses the gas and raises the pressure of the gas, and a heat exchanger that cools the compressed gas by allowing the gas to decompress and expand, and in which the output gas from the heat exchanger is fed into an induced draft ID fan, and further comprising a throttle valve at the inlet to the ID fan that is controlled by a controller, the controller causing the throttle valve to open an close so as to control the pressure of the gas in the kiln. The controller may control the fan to run at a constant speed at all times, controlling the draft using the throttle valve rather than varying the fan speed.

A pressure sensor may be provided that measures the pressure of the gas in the kiln, the output of which is fed to the controller. The pressure sensor may be located in the kiln or at the inlet or outlet stages of the kiln.

The ID may be controlled by a controller that causes the fan to run at constant speed. By throttling the inlet side of the fan using control valves to control the pressure in the kiln rather than changing the speed of the ID fan, the inlet tips of the fan can be kept running at a high speed at all times so that no gas can flow back towards the kiln

The quencher may include, prior to the fan, a first stage that in use cools the gas leaving the pyrolysis kiln to about 50degC by combining the heated gas with a blast of cooled air using a fan so as to condense water and oils from the gas.

The quenching system may comprise in a second stage following the first stage and before the ID fan a water chiller containing chilled water that is at a temperature below ambient. For instance the water may be below 30 degrees, or below 20 degrees C or as low as 0 degrees C. The water may be sprayed as a mist into the gas to cool the gas and to cause more water vapour in the gas to condense.

It is important at all times to maintain an airtight seal to the kiln during pyrolysis.

According to a third aspect the invention provides a system for pyrolysing material comprising a stationary inlet stage, a kiln and an outlet stage, the inlet stage being upstream of the kiln, the kiln being upstream of the outlet stage,

in which the inlet stage comprises a first screw conveyor housed within a hollow sleeve and having two flights spaced apart along the length of the shaft, and a second screw conveyer extending from a point within a hopper for feedstock to a compaction zone located at the end of the first screw flight furthest from the kiln, whereby in use the second screw conveyor pulls material from the hopper to the compaction zone where it is at least partially compacted, the first screw conveyor pushing the compacted material to the space between the flights where the material is further compacted to form an airtight plug, the second flight of the first screw conveyor pulling material away from the end of the plug nearest the kiln and conveying the material into the kiln.

The second screw and compaction zone act as a secondary safety plug. The second screw compactor pushes the feed material into a compaction zone causing a plug. The size of this plug is limited by the action of the first screw conveyer removing material from the end of this plug.

The flight of the first screw conveyor nearest the kiln may have a higher pitch than the flight furthest from the kiln to accelerate material away from the plug.

The sleeve containing the first screw conveyor may be located above the hopper and may extend substantially horizontally. The portion of pipe from the plug to the stationary seal may be airtight, and may be connected to a pressure relief mechanism of the first aspect.

The first and second screw conveyors may be controlled by a controller and may operate independently. They screws may be driven continuously or on demand when there is a requirement to add more material to the kiln. The skilled person will appreciate that the features of the third aspect can be combined with any of the features of the first two aspects.

There will now be described, by way of example only, one embodiment of 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 block diagram of the key parts of an inlet stage that constructs a plug of feedstock to seal the kiln; Figure 3 is a view of an exemplary inlet stage that includes the parts indicated in Figure 2 using two screw conveyors; Figure 4 is an overview of an embodiment of a pressure relief mechanism;

Figure 5 is an overview of a quencher that is provided downstream of the kiln; and

Figure 6 is a block diagram showing the system for removal of cooled char from the quencher.

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 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 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.

As mentioned, it is important to ensure the kiln is airtight, and this can be achieved where a solid feedstock is used by building up a plug of feedstock at the inlet stage of the pyrolyser 5. This is shown in the arrangement of Figure 2 of the drawings. The material is initially compacted at a first compaction zone 5 1 , and then further compacted at a second compaction zone 52 before being decompacted and fed to a hopper and onto the kiln. The material in the second compaction zone 52 forms the air tight plug for the kiln.

A suitable embodiment of an inlet stage 50 of the pyrolyser 5 is shown in Figure 3. The inlet stage comprises a first screw conveyor 53 housed within an elongate cylindrical hollow sleeve 54. The sleeve is sealed at the end furthest from the kiln although a cut out 55 for receiving feedstock is provided at the bottom of the sleeve at that end.

The first screw conveyor 53 rotates within the sleeve 54, driven by a motor. The screw has two flights 56. 57 spaced apart along the length of the shaft of the screw. The effect is to provide a gap in the flight when looking along the screw. A second screw conveyer 58 extends from a point within a hopper 59 for feedstock and passes through the cut out in the sleeve where it terminates. The region where the second screw conveyor terminates in the sleeve defines the first compaction zone 5 1.

In use the second screw conveyor 58 pulls material from the hopper to the first compaction zone 5 1 where it is at least partially compacted. The first screw conveyor 53, which has an end located in this first compaction zone 5 1 , pushes the compacted material along the sleeve to the gap between the flights 56,57 where the material is further compacted to form an airtight plug at the second compaction zone 52. Once the plug has reached a length equal to the length of the gap between the flights, the second flight of the first screw conveyor will start to pull material away from the end of the plug nearest the kiln and conveying the material into the kiln.

The plug provides the required airtight seal for the kiln. 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 pressure relief mechanism 70 is shown in Figure 4. The mechanism comprises a passive mechanism that requires no electrical power. It comprises a liquid filled trap 71 comprising a U-shaped conduit having a first end 72 and a second end 73, the portion of the conduit 74 between the two ends being filled with a liquid to form at each end a respective column of liquid, the two columns being connected at a lower end by a body of liquid. The pressure relief mechanism 70 is arranged such that the surface of the liquid in the conduit at the first end is connected to the inlet stage so that the liquid is prevented from flowing into the inlet pipe and the gas pressure at the inlet stage acts down upon the column of liquid at the first end. This pressure acting on the first end is counterbalanced by the pressure of air acting down on the surface of the column of liquid at the second end 73. The liquid therefore provides a seal to gases in the inlet stage from passing through the conduit. In the event that the gas pressure in the inlet stage exceeds a predetermined level the gas escapes through the liquid in the conduit as bubbles, reducing the pressure in the kiln. In the example shown, the liquid is water as this is easy to process and to clean and to dispose as required. The pressure relief mechanism 70 is connected to the inlet between the missing flight of the first conveyor screw and the kiln, i.e . downstream of the airtight plug of feedstock. The outlet of the pressure relief mechanism 70, along which gas may escape if the pressure in the kiln is too high, is connected to the quenching system 7 which further cools the escaping gas and removes at least part of the water vapour entrained in the gas so as to produce a gas with fewer contaminants. An exemplary quenching system is shown in the block diagram of Figure 5.

The quenching system 7 also receives pyrolysis gas that leaves the outlet of the drum following pyrolysis, such that a single quenching system can be employed to quench gas that correctly leaves the drum and that which leaves the drum when excess pressure is being relieved. In general use the primary function of the quench system is to quench the pyrolysis gas leaving the outlet of the kiln.

The quenching system 7 has three stages. Each cools the gas by taking energy out o of the gas and in doing so causes water, oils and heavy tars to condense and drop out of the vapour. The three stages allow different contaminants to be removed and separated, the end result being a gas that has very few contaminants. Such a clean gas can be used to drive an engine or sold for later use.

The first stage 80 cools the very hot gas that leaves the pyrolysis kiln from an initial temperature of about 500 degrees C to about 50degC by combining the heated gas with a blast of cooled air using a fan so as to condense water and oils from the gas. The removed oil and water is passed to an oil/water separator 85. The chosen temperature is important because of the amount of moisture that goes through the process. If the temperate stays above 50 degrees C the water stays as vapour in the gas flow, but below this water drops out. However, if it is cooled too far below 50 degrees heavy tars may start to solidify and don't want that to happen. Any solids that drop out at this stage may be screened and passed to a char bunker using a conveyor 81 as shown in Figure 6.

The first stage cooling will cause oils to condense from the gas along with the water vapour and the first stage may recover the oils from the water using a gas to oil separator tanks comprising dam and weir.

The second stage that follows the first stage comprises a water chiller 90 containing chilled water that is at a temperature below ambient. For instance the water may be below 30 degrees, or below 20 degrees C or as low as 0 degrees C. The gas leaving the first stage is bubbled up through the chilled water by exiting a pipe that is immersed below the surface of the chilled water. The removed water and oils is passed to a second oil/water separator 95. Where oils are condensed and drop out into the water, they will float to the surface. A separator device is provided for removing the oils from the water, such as a dam and weir.

The first and second stages cool the gas by as much as possible with the gas at ambient temperature as set by the chilled water. The third stage includes an additional cooling means for extracting still further water vapour, comprising a means for increasing the pressure of the gas such as a fan 100, a heat exchanger 1 10 that cools the compressed gas by allowing the gas to expand, and an induced draft ID fan 120 which receives the cooled gas from the heat exchanger, and further comprising a throttle valve 130 at the inlet to the ID fan 120 that is controlled by a controller 140. The controller causes the throttle valve 130 to open and close so as to control the pressure of the gas in the kiln.

The pressure in the pyrolysis kiln will vary depending on how much gas is released from a feedstock in the furnace, the temperature and the rate at which material is added and gas drawn off. It is therefore important to control the pressure and keep it below the level at which it may blow past the pressure relief mechanism which is really only to be used as an emergency safety measure. The provision of the controller and the ID fan allows the pressure to be controlled.

A pressure sensor 150 is provided that measures the pressure of the gas in the kiln, the output of which is fed to the controller 140. The pressure sensor may be located in the kiln or at the inlet or outlet stages of the kiln. The ID fan controller 140 in use causes the fan 120 to run at constant speed yet varies the throttle valve opening. The throttle valve 130 is controlled between a fully open and a fully closed position. When fully closed, no gas may pass through the valve .

By throttling the inlet side of the fan using the control valve 130 to control the pressure in the kiln rather than changing the speed of the ID fan, the inlet tips of the fan can be kept running at a high speed at all times so that no gas can flow back towards the kiln.