In industrialized countries electrical power has traditionally been produced from the combustion of fossil fuels such as coal, oil and gas and more recently has been produced using nuclear fuels. These means of producing energy are increasingly giving rise to environmental concerns with regard to global warming and safety respectively and furthermore such energy sources are not renewable and will eventually run out.

These concerns have led to an increasing interest in "power from renewables" i.e. electrical energy generated from fuels which can themselves be regenerated and/or will be constantly available.

Examples are hydro-electric power, tidal barage power, wind power, power from renewable biomass and direct solar power. With the exeption of hydro-electric power none of the methods mentioned above have been used for production of energy on a large scale e.g. 1,000 Megawatts or more of electricity, the sort of output from a power station connected into the U.K. national grid. Nevertheless, it is beginning to be recognized, because of the environmental drawbacks associated with traditional power generating systems, that there is potential for smaller energy generating plants which use a renewable fuel source. To this end a number of European governments are providing grants and subsidies for the development of power from renewables and in particular from biomass such as a

high energy crop.

The process of generating energy by combustion of biomass has been known since the last century. For example the heat generated from wood burned in a furnace has been used to raise steam which in turn is used to drive an engine or turbine. While steam-based technologies are well established, such a process shows low thermal efficiency i.e. only a small amount of electricity is generated compared to that expected from the amount of thermal energy produced.

It is also well-known to pyrolyse wood in a kiln in a limited supply of air so that there is only partial combustion of the wood. This breaks down the wood or other form of biomass into fuel gases such as carbon monoxide, hydrogen and methane. Depending on the amount of air introduced into the combustion chamber the gas will contain varying amounts of nitrogen as a diluent. The partial combustion in a controlled air supply of a solid carbonaceous material such as wood is commonly known as "gasification" and the combustion devices in which such a process is carried out are known as "gasifiers".

Although the energy content of fuel gases produced by gasification is rather low (4 to 6 MJ/m ³ at normal temperature and pressure) the gas can nevertheless be used directly as fuel in internal combustion engines or gas turbines and can be converted to electricity at a high thermal efficiency. Furthermore, fuel gas may be produced by gasification of different types of carbonaceous material such as high energy purpose-grown crops, or agricultural, industrial or domestic waste materials such as straw, paper, cardboard, wood, plastics and even sewage. The possibility of generating energy from wastes makes gasification of carbonaceous materials an even more

attractive prospect given current environmental concerns.

The present invention relates to a partial combustion device or gasifier of the fixed bed type. Fluidized bed wood gasifiers are known which are suitable for use on a relatively large scale and can produce between 20 and 50 MW of electricity. However, because the fluidized bed technology is so complex and expensive they are not yet cost effective and cannot compete commercially. Furthermore, to fuel such a gasifier a very large area of land (about 200 to 500 km ² ) would have to be planted with a suitable crop such as wood coppice and farmers would not be prepared to take this on without substantial government subsidies.

The fixed-bed gasifier on the other hand is simple to construct and cheap to run. There are two basic types, the down-draught and the up-draught gasifier. Both comprise a vertical cylindrical vessel as a combustion chamber but vary as to the position where the air supply is introduced and the fuel gas exited.

In a down-draught device the combustion chamber is loaded with a solid or semi-solid carbonaceous material from the top. This is set alight and a controlled air supply is drawn downwards from the top through the bed of fuel that sits on a grate near the bottom of the vessel. The fuel gas produced is drawn off at or near the bottom of the combustion chamber. An advantage of the down-draught device is that gas is removed from gasifier at the highest temperature region, perhaps as high as 1200°C, so that impurities in the gas such as tars and other organic chemicals of high molecular weight are "cracked" and converted into gaseous lower molecular weight materials. The fuel

gas from a down-draught gasifier can easily be further scrubbed by conventional gas cleaning technology and then used in an internal combustion engine. However, a major problem with the down-draught gasifier is that the amount of gas produced is limited because of air flow restrictions and it can only fuel at most a 200kW_ engine.

In an up-draught device the controlled air supply is introduced at or near the bottom of the combustion chamber so as to be counter-current to the carbonaceous fuel feed. Thus combustion occurs at the base of the biomass load and the fuel gas passes through the bulk of the load before being drawn off at or near the top of the chamber. By introducing air at the bottom, the hottest part of the chamber, the up-draught gasifier is not subject to the same air-flow restrictions as the down-draught gasifier and thus is capable of having an output of up to 5MW _e . While known up-draught gasifiers may have considerable commercial potential a drawback is that because the fuel gas leaves the combustion chamber at its coolest point, having passed through the biomass load, it is contaminated with tars and other impurities and therefore is not suitable for direct use in an internal combustion engine.

The present invention is concerned with an up-draught partial combustion device or gasifier which has been modified to produce fuel gas free of contamination with tars and other liquid impurities and with a gasifying apparatus capable of processing a plurality of different solid or semi-solid carbonaceous fuels.

In accordance with the first aspect of the invention there is provided a device suitable for the partial combustion of solid or semi-solid carbonaceous

materials to produce at least one fuel gas which comprises a combustion chamber having an inlet for air at or near the base thereof and an exit for fuel gas at or near the top thereof, a second chamber for receiving at least a portion of any partially combusted carbonaceous material from said combustion chamber and for isolating said material therefrom, means for bringing said fuel gas exiting said combustion chamber into contact with any partially combusted carbonaceous material isolated in said second chamber, said device being characterized in that between said combustion chamber and said second chamber there is an extracting device capable of transferring hot partially combusted carbonaceous material from said combustion chamber to the second chamber and in that said extracting device has means for preventing fuel gas introduced into the second chamber from passing back to the combustion chamber. Preferably the fuel gas exiting from the top of the combustion chamber is passed through the second chamber which receives the hot partially combusted carbonaceous material e.g. hot charcoal from the combustion chamber. In a particularly preferred embodiment the second chamber is directly below the combustion chamber and the extracting device transfers the hot charcoal from the bottom of the combustion chamber to the second chamber. Bringing the fuel gases produced by gasification e.g. carbon monoxide, hydrogen and possibly methane, from the cool part of the device into contact with the hot ash and charcoal from the combustion chamber, which may be at a temperature of up to 1200°C, results in "cracking" of tars and other organic chemicals of high molecular weight into lower molecular weight gaseous materials. Thus the fuel gas produced is cleaned and is suitable

for a wider variety of downstream uses, e.g. in an engine, than would otherwise be possible with the product of an up-draught gasifier.

It will be usual for a device in accordance with the invention to have additional means in the fuel gas stream for cleaning the gas, especially means for removing particulate material such as filters and cyclones. It is preferable if the fuel gas is mixed with air when it is brought into contact with the hot charcoal and this may be accomplished by having air injecting means in the conduit leading the fuel gas to the second chamber or the second chamber may have a separate air inlet.

It is important that gas fed into the second chamber is not allowed re-entry into the combustion chamber. Thus, there is a barrier between the combustion chamber and the second chamber which is capable of facilitating the controlled passage of hot combusted carbonaceous material from the combustion chamber into the second chamber while preventing the passage of gas back into the combustion chamber. The controlled passage into a second chamber, which is below the combustion chamber, of the hot material from the bottom of the combustion chamber is facilitated by an extracting device.

In one embodiment the extracting device may be a plurality of grates, stacked one above the other at least one of which can be moved in a horizontal plane with respect to the others to bring the holes of the grates in and out of alignment as required. For example the extracting device may comprise two fixed grates with a movable grate in-between so that movement thereof aligns the holes of the movable grate with the holes of each of the fixed grates in turn. The grates may be circular with the holes or slots in

the grate arranged radially in which case the movable grate rotates in relation to the fixed grates. Alternatively, the holes in the grate are not arranged radially and the movable grate oscillates from side to side in relation to the fixed grates. In either arrangement an air lock is always maintained between the combustion chamber and the second chamber.

In another embodiment the extracting device simply comprises upper and lower gates adapted to open and close alternately so an air lock is created in-between them. In use the upper gate is opened to allow the hot partially combusted material or charcoal to pass into the air lock. Once the upper gate is closed again the lower gate opens to allow the hot material to fall into the lower chamber. A control system ensures that both gates are never open at the same time.

In yet another embodiment of the invention the extracting device comprises a receptacle such as a cylinder or disc having holes or slots on at least one surface thereof which is arranged to rotate through 180°. In one position the holes or slots of the receptacle face uppermost towards the base of the combustion chamber so that hot material falls under gravity into the slot or slots. The receptacle rotates through 180°, either continuously or intermittently so that the holes or slots are on the lower surface and the hot carbonaceous material again falls under gravity into the second chamber. Each of the exemplified extracting devices acts to meter the amount of partially combusted hot carbonaceous material being isolated in the second chamber and this may be adjusted in accordance with the amount of fuel gas being produced. In order to maintain substantially constant levels of hot charcoal

in the second chamber the gasifier of the invention may have a third chamber below the second chamber to receive ash and spent material therefrom. Preferably an extracting device such as one of those described above is included to transfer the ash to the third chambe .

The combustion chamber may be constructed in any manner known in the art for gasifiers so long as air may be injected at or near the bottom and the fuel gas is drawn off near the top. Preferably at least one air injection nozzle or tuyere is fitted in the combustion chamber wall for introduction of a controlled amount of air. In view of the very high combustion temperatures involved it is preferable if the combustion chamber, second chamber and extracting device is made of a ceramic material.

A partial combustion device in accordance with the invention can, depending on size, produce a clean gas, free of tars and liquid contaminants. It thus combines the advantages of the known up-draught and down-draught gasifiers without any of the disadvantages. Its operation is not damaging to the environment as with conventional energy generating technology and is beneficial by reducing net emissions of carbon to the atmosphere. Other benefits to the environment accrue where the biomass used is a waste material such as paper, cardboard or sewage.

Generally the device in accordance with the invention is adapted for partial combustion of wood fuel and coppice wood of, for example, willow may be grown for the purpose. The processing of other carbonaceous material such as industrial wastes may require particular modifications to the gasifier especially with regard to the fuel feed means and to the shape and size of the combustion chamber. The

device of the invention may therefore be used in conjunction with other specially adapted gasifiers to produce an apparatus capable of processing a plurality of different carbonaceous materials to produce a clean fuel gas.

Thus, in accordance with a second aspect of the invention there is provided an apparatus suitable for the partial combustion of a plurality of different solid or semi-solid carbonaceous materials to produce at least one fuel gas which comprises a main device in accordance with the first aspect of the invention and one or more auxilliary devices, each auxilliary device comprising a combustion chamber for the partial combustion of a carbonaceous material, an air inlet to said chamber and a fuel gas exit from said chamber, each auxilliary device further being adapted for combustion of a carbonaceous material different from that to be used in said main device wherein said apparatus includes means for contacting the fuel gas exiting from said one or more auxilliary devices with hot partially combusted carbonaceous material produced in the main device.

The main device is one with a second chamber below the combustion chamber as described above with means for transferring the hot material from the combustion chamber to the second chamber and the fuel gas produced in the main device and in all the auxilliary devices can then be fed through the second chamber of the main device to crack tars and other organic chemicals with a high molecular weight to gaseous materials of smaller molecular weight.

An advantage of such an apparatus is that it can generate energy from a variety of different types of biomass all at one time and/or can adapt to whatever

types of waste happen to be available in a particular area or at a particular time.

In accordance with a third aspect the invention provides a method of producing at least one fuel gas from a plurality of carbonaceous materials which method comprises:-

(a) loading a partial combustion device in accordance with the first aspect of the invention with a solid or semi-solid carbonaceous material,

(b) loading at least one auxilliary partial combustion device comprising a combustion chamber, an inlet for air and an outlet for gas, with at least one carbonaceous material different from that used in step (a) above,

(c) setting light to the carbonaceous material loaded in both steps (a) and (b) above while passing air into the inlet of each combustion chamber thereof and

(d) bringing the fuel gas produced in step (c) from all of the partial combustion devices into contact with isolated hot partially combusted material from the partial combustion device in accordance with the first aspect of the invention.

The invention will now be described by way of example with reference to the accompanying drawings in which:-

Figure 1 is a vertical section of a device for the partial combustion of carbonaceous fuels in accordance with the invention,

Figure 2 is a plan view and a vertical sectional view of an extracting device in accordance with a first embodiment of the invention,

Figure 3 is a plan view and a vertical sectional view of an extracting device in accordance with the second embodiment of the invention; Figure 4 is a plan view and a vertical sectional view of an extracting device in accordance with a third embodiment of the invention and

Figure 5 is a plan view and two vertical sectional views of an extracting device in accordance with a fourth embodiment of the invention.

Referring first to Figure 1 the partial combustion device or gasifier 2 has a combustion chamber 4, a second reaction chamber 6 directly below the combustion chamber and a third chamber 14 directly below the second chamber. The combustion chamber 4 has a primary air inlet 8 and a control valve 10 to regulate the rate of air-flow into the device and an exit for the raw fuel gas 12 near the top of the chamber. The second chamber 6 has a secondary air inlet 16 which is regulated by control valve 18 and a clean fuel gas outlet 20. Extracting devices 22 and 22a are positioned between the combustion chamber 4 and the second chamber 6 and between the second chamber 6 and the third chamber 14, the operation of which will be hereinafter described. The third chamber 14 is provided with an outlet for ash 24.

To produce fuel gas by gasification the cover 25 of the combustion chamber is removed and a suitable carbonaceous fuel 26, e.g. renewable biomass such as wood, is introduced from the top into the combustion

chamber 4. Wood may also be introduced into the chamber 4 intermittently using a standard double valve air lock feed system (not shown) . The wood 26 is then set alight, air is blown into the combustion chamber through air inlet 8 and partial combustion of the wood to fuel gases such as carbon monoxide, hydrogen and methane proceeds. The fuel gas mixture travels to the top of the combustion chamber and exits through gas outlet 12 by which time it has substantially cooled compared with temperature at which it was generated and has also collected contamination with tars and other materials from the as yet un-burned load.

The extracting device 22 between the combustion chamber 4 and the second chamber 6 transfers a portion of the hot charcoal from the combustion chamber to the second chamber as shown at 28, intermittently. Another extracting device 22a transfers ash intermittently from the second chamber 6 to the third chamber 14. Ash is then removed from the ash outlet 24.

The fuel gas leaving exit 12 is conveyed by a conduit 30 to an inlet 32 in the second chamber 6. When the raw fuel gas is heated by being in contact with the hot charcoal 28, tars are cracked to smaller molecular weight gaseous molecules and the cleaned fuel gas exits from the second chamber 6 through gas exit 20. A pump (not shown) may be positioned at the exit 20 to assist the passage of gas through the device. As aforementioned the gas may be further cleaned by removing particulate materials with filters or cyclones.

The extracting devices 22 and 22a are shown in more detail in Figures 2, 3, 4 and 5. In the embodiment shown in Figure 2 the extracting device comprises two fixed grates 34 and 36 with a third

grate 38 between them which can be made to oscillate in a horizontal plane between the two fixed grates. The grates are so arranged that the holes 40 in the oscillating grate align with the holes 42 in the fixed grates in turn. Thus the extracting device 22 provides an air lock which prevents clean fuel gas from re-entering the combustion chamber 4.

The embodiment shown in Figure 3 operates in a similar manner to that shown in Figure 2 except that the three grates are circular and have slots 44 arranged radially therein. In operation the middle grate 46 rotates so that the slots 44 are aligned alternately with the slots of the fixed grates 48. The extracting device of Figure 4 consists of upper and lower gates 50 and 52 which open and close alternately so forming an air lock. When the gate 50 is opened charcoal from the combustion chamber 4 falls by gravity into the air lock. The gate 50 is then closed before the opening of the gate 52 which allows the hot charcoal to fall down into the second chamber 6.

Figure 5 shows yet another embodiment of the extracting device which consists of cylindrical grate 54 fitted to rotate about a spindle 56 within a cylindrical floor 58. The grate 54 has two slots 60 in its surface at 180° to each other. As the cylindrical grate 54 rotates charcoal falls into the upper slot and is carried around in the slot. When the grate 54 has rotated through 180° so that the slot is facing downwards the charcoal falls into the second chamber 6. As the edge of the slot 60 passes the edge of the floor 58 it acts as a shear crushing any charcoal pieces trapped between these edges.