GASIFICATION The present invention relates to a gasifier, a method of operating the gasifier to produce a combustible effluent, a novel gasification system and method incorporating the gasifier, and a novel filter and method for gas filtration using the filter, in particular to produce a tar and moisture free producer gas, and a novel polymer for use in the filter. Gasifiers are used for the gasification of biomass, fossil fuels and wastes either singly or mixed together, to produce a variety of combustible gases which may be used to generate energy, for example, using internal combustion gas engines, gas turbines, dual-fuel diesel engines and fuel cells. The interest in the use of gasifiers for treating waste products is increasing due to the rising cost of waste disposal by conventional means, such as by incineration or land filling, driven by the environmental concerns associated with these methods. Conventional gasifiers typically operate in batch mode wherein slag and clinker formation, as well as bridge formation, create intermittent gas and fuel flow, which in turn is associated with a high tar content in the product gas. Furthermore, clinker formed during the gasification process due to relatively high local temperatures in the oxidation zone cannot be effectively discharged. Gasification of high ash fuels can lead to the discharge from the gasifier of residues with a high carbon content, which can then be difficult to dispose of. As a result of these difficulties, conventional gasifiers are operated until complete gasification of the fuel has taken place and then shut down in order to manually remove the slag and clinker formed in the high temperature regions of the gasifier, together with the high carbon ash. A further problem arises from the batch operation of gasifiers in that undesirable operating conditions occur during the start-up phase, and if the gasifier operates at low temperatures (<850°C) during normal use, the tar content of the product gas can rise. Tar in the product gas will deposit in the inner parts of the combustion device used to combust the product gas, blocking the device and requiring cleaning. The current power and heat production devices that are usually attached to gasifiers are not suitable for utilisation of a tar and moisture laden product gas. The present inventors have discovered that a gasifier having two oxidation zones, wherein in a first zone the gas flow is in a downward direction (in the same direction as the fuel flow), and in a second zone beneath the first zone the gas flow is upwards (in the opposite direction to the fuel flow) addresses many of the difficulties in the known gasifiers. Such a device may be called a 'down-updraft gasifier' to reflect the combination of the down and up flow of the gas. Accordingly, a first aspect of the present invention provides a gasifier for the gasification of solid fuel to produce combustible effluent, comprising a fuel valve for loading solid fuel into a first oxidation zone, a first throat defining the lower edge of the first oxidation zone, a second throat defining the lower edge of a second oxidation zone, air intakes for both oxidation zones, a reduction zone linking the first oxidation zone to the second oxidation zone and two oppositely located (at the reduction zone) vortex discharge pipes for the combustible effluent wherein in the first oxidation zone the gas flow is in the same direction as fuel flow and in the second oxidation zone the gas flow is in the opposite direction to the fuel flow. In a particular advantage the gasifier is suitable for continuous operation and the method is a continuous operation method, capable of operation for extended periods without the need to shut down to remove slag and klinker. This is due to the high extent of gasification that takes place due in part to the uniformity of gasification, also the ability to remove ash and slag at the base of the gasifier during operation. A further advantage is the minimised hours of operation in start up mode, which tends to increase tar content in product gas. In the gasifier of the invention the first oxidation zone preferably operates at a temperature of at least 1000°C, whilst the reduction zone operates at a temperature of between 600 and 900°C, more preferably at about 850°C and the second oxidation zone operates at a temperature of between 700 and 800°C, more preferably at about 750°C. This ensures uniformity of gasification. It is preferred that the gasifier of the invention further comprises a pyrolysis zone above the first oxidation zone, and a fuel storage zone above the pyrolysis zone. In the use of such a gasifier in a preferred method of the second aspect of the invention, the fuel is dried in the fuel storage zone, and pyrolysed in the pyrolysis zone to yield charcoal which is then partially oxidised, reduced and further oxidised. The fuel storage and/or drying zone preferably operate at a temperature of between 80 and 120°C, and more preferably at about 100°C. The pyrolysis zone preferably operates at a temperature of between 500 and 700°C, more preferably at about 600°C. The heat to maintain these temperatures is derived from the first oxidation zone. Preferably the fuel storage zone comprises a hopper having a loading valve and leading to a grate above the first oxidation zone. The gasifier of the first aspect of the invention may have a perforated part in a jacket-wall filled with microporous catalysis defining the reduction zone located at about the same level in the gasifier body as the discharge pipe. This perforated jacket allows the effluent gas produced in the gasification process to be cleaned and removed efficiently, reducing its path through the second oxidation zone. The gasifier of the first aspect of the invention preferably further comprises means attached to the discharge pipe for maintaining the gasifier below atmospheric pressure, so that air is sucked into the gasifier through appropriate ah- inlets. Operating the gasifier at below atmospheric pressure provides a fail-safe mechanism, such that in the event of the means for maintaining the reduced pressure fail, the combustion processes in the gasifier will come to a halt due to a lack of oxygen, preventing a dangerous build up of product gases. Air inlets into the gasifier are maintained with the negative suction via main air inlet pipes. The continuously sucked air is withdrawn into the ring ducts which are located at the outer circle of the oxidation zones. The ring ducts provide air to be preheated before its injection into the reaction zones through air inlet jets mounted in the inner surface of the inclined throats. Therefore, pre-heated air also has a cooling effect to gasifier's metal surfaces at the throat levels. In the gasifier of the present invention, the amount of secondary oxidation in the second oxidation zone is controlled by the amount of air admitted by the secondary air intake valve. Such a gasifier may be used to gasify solid fuel incorporating biomass, fossil fuel, waste or combinations thereof to produce a combustible effluent. 'Solid fuel' can contain entrained liquid (such as moisture, oil, oil sludge) within the intra- or inter-particle pores of the solid fuel particles. Accordingly a second aspect of the present invention provides a method for the gasification of solid fuel to produce a combustible effluent using a gasifier of the first aspect of the invention, comprising the steps of partially oxidising a solid fuel in the first oxidation zone to produce char, reducing the char in the reduction zone to form ash, further oxidising any char residue in the ash in the second oxidation zone and extracting the combustible effluent produced in the above steps by the discharge pipe wherein in the first oxidation zone the gas flow is in the same direction as fuel flow and in the second oxidation zone the gas flow is in the opposite direction to the fuel flow. Preferably the combustible effluent at a temperature of around 850°C produced from the both zones passes through a perforated cone ring which is filled with microporous catalysis to crack the residue tars just before leaving the gasifier. Preferably solid fuel for the gasifier and method according to the invention incorporates biomass, such as liquid waste, waste oil or petroleum sludge, or fossil fuels or waste or combinations thereof as hereinbefore defined which is absorbed within the intra-particle and inner-particle pores of a suitable combustible carrier. Preferably the combustible carrier has high internal porosity and is more preferably in fibrous form to provide extensive inter-particle porosity. Preferably the liquid waste is mixed with the carrier and briquetted in order to densify the composite fuel. Biomass which may be gasified by the gasifier and method for gasification of the invention may include any of the conventional extremely varied sources, and include, for example, wood and lignocelluloses, sawdust, coal, nut shells, sewage sludge, leather waste, tyre and plastic waste, municipal refuse or household residual materials, olive pips, rape-seed meal, clinical waste, chicken and cattle litter and manure, slaughterhouse waste, sour chocolate waste, tallow, paper waste, food waste, sugar cane bagasse, waste oil, petroleum sludge, coal fines, bone waste, agricultural residues and blend of biomass with fossil waste such as petroleum sludge mked with bone waste, sewage sludge, sawdust or rape-seed meal. A suitable carrier may be selected from saw dust, crushed bone waste from slaughter house, bread/food waste, municipal waste, dried sewage sludge, chopped straw, rape seed meal and sugar cane bagasse. It is preferred that uniform size briquettes or pellets of fuel (such as biomass or waste carbonaceous materials) are used so as to achieve a uniform air distribution during gasification. It is also preferred that the maximum diameter of the briquettes of fuel is not larger than one eighth of the narrowest part of the gasifier, so as to prevent bridge formation. In the method for gasification the amount of secondary oxidation required will depend on the remaining char content in ash of the fuels. High ash fuels, such as sewage sludge, leather waste, petroleum residue sludge, house-hold waste (RDF), bone meal, chicken and cattle manure, usually result in low carbon conversion in conventional gasifiers due to the isolation of some carbon by ash in the main oxidation zone. In a further aspect of the invention there is provided a gasification system comprising in series: the gasifier of the invention, a water scrubber, a polymer filter unit, a fan, a further polymer filter unit and means to exit product gas for energy generation, such as an effluent stack leading to a location for clean ignition. Preferably the system comprises a filter bypass for use during start up. Preferably the filters comprise double filter units, each working in tandem, such that only one half of each double filter unit is operated in filter mode at any one time, the other operated in regeneration mode. In a further aspect of the invention there is provide a method for operating a gasification system as hereinbefore defined comprising gasifying a solid fuel as hereinbefore defined, passing the product gases through a scrubber to clean particulates and water soluble toxins as well as acids, a filter to absorb tar and moisture, a fan to accelerate gases, a second filter to absorb additional tar and moisture, to produce a gas for clean ignition. pH of the water scrubber is low in order to remove the acids from the gas. Preferably gas from the water scrubber bypasses the first filter during start up. Preferably gas is filtered in one half of a double filter unit at any one time, the other half being regenerated, more preferably with hot exhaust gases. In a particular advantage gas exiting the fan is at high pressure which is conducive to further condensation of tar and moisture in the second double polymer filter unit. In a further advantage of the invention the system of the invention and method for operation thereof produces gas which is suitable for power and heat production devices, being low in tar and moisture, preferably substantially tar and moisture free. We have now found that moisture and tar extraction can be further improved with the use of a microcellular open cell polymer filter. Accordingly in a further aspect of the invention there is provided a polymer filter comprising a microcellular open cell polyfflPE polymer comprising pores in the range 0.1 to 300 micron (primary pores) and optionally additionally in the range 300 to 10,000 micron (coalescence pores), wherein the polymer is effective in absorbing water and tar from gas, and a method for the preparation thereof. Both the acid and neutralised salt form of the sulphonated Micro-cellular polymers (they are also known as PolyHIPE Polymers) can be used as water absorbent materials. These polymers are prepared using the teaching of our previous patent application, (Microcellular polymers as cell growth media and novel polymers, EP 1 183 328; US 09/856 182). It is possible to use polymers with primary and optionally additionally coalescence pores for water absorption. These pore types are described as follows: Primary pores in the range 0.1-300 jim; for example small pore size: 0.1-0.5 urn and large pore size: 0.5 to 300 um. Coalescence pores in the range 300 - 10,000 um. Preferably coalescence pores have pore size in the range 300 to 1000 um. Polyhipe of particular pore diameter may be obtained by methods described hereinbelow. In open cell polymers, intercell communications are known as interconnects. PolyHIPE may have any desired ratio of interconnect (d) to pore (D) diameter, for example in the range 0 < d/D < 0.5, preferably in the range 0.1 < d/D < 0.5 when the pore diameter is approximately less than 200 micron. Interconnects may have diameter in a range of up to 100 micron, preferably 0.001 to 100 micron, more preferably 1-50 micron. Polyhipes are commercially available or may be prepared using methods as disclosed in US 5071747 and hi additional patent publications referred therein or as hereinbelow described. The generic polyhipe polymer which is commercially available comprises polyvinyl polyhipe and is made up of oil phase monomers styrene, divinyl benzene (DVB) and surfactant (Span 80 sorbitan monooleate), and may be in rigid or flexible form depending on the relative proportions of monomers, additionally in flexible form including monomer 2-ethylhexyl acrylate, and in the aqueous phase an amount of potassium persulphate as aqueous phase initiator. However, due to the presence of sulphur in the polymer, its use in gas cleaning may not be environmentally desirable. Therefore, we prefer to use other forms of micro-cellular polymers. When the use of sulphur groups in the polymer is not desirable, it is preferable to use oil phase initiators such as lauryl peroxide (1% of the oil phase). A first novel polyEQPE polymer comprised in a filter of the invention therefore incorporates oil phase initiator, preferably l,l-azobis(cyclohexane carbonitrile) or lauryl peroxide in 1% of the oil phase of the emulsion. A second novel polyHEPE polymer comprised in a filter of the invention incorporates monomer (such as 2-vinyl pyridine) to render the polymer adsorbent without causing emulsion breakdown, preferably present in an amount of 5 - 10%. Novel polymers may additionally incorporate monomer (such as 2-ethyl hexyl acrylate) to incorporate elasticity and improve mechanical shock absorbance and good attrition characteristics. Elasticity is also useful to enhance the water uptake capacity of the hydrophilic polymer. The preferred value of the 2-ethylhexyl acrylate concentration is up to Y=30% but more preferably Y = 15%. A third novel polyHIPE polymer comprised in a filter of the invention incorporates vinyl pyridine monomer (with or without 2 ethyl hexyl acrylate monomer) in the form of a skin-core polyhipe structure in which the core of the polymer is sulphonated but the skin is non-sulphonated but yet still water adsorbent. This is done by injecting small amount of sulphuric acid to the core of the polymer and subsequently sulphonating this polymer using known methods. This ensures that the sulphur containing part is encapsulated but yet that part of the polymer can adsorb water. We discovered that the polymers should not be dried excessively during the 'drying stage'. Excessive drying results in the reduction in water absorption capacity of the polymer as well as in reduced rate of water uptake. Typically, the water absorption capacity of these polymers is 10-12 times of their own weight. The polymer of the invention may be natural or synthetic, soluble or insoluble, optionally (bio)degradable crosslinked polymer, preferably selected from proteins and cellulose, polyacrylamide, polyvinyl in rigid or flexible form, poly(lactic acid), poly(glycolic acid), polycaprolactone, poly (lactide/glycolide) and polyacrylimide. The process for the preparation of microcellular polyhipe polymers comprises in a first stage the formation of a high internal phase emulsion (HIPE) of dispersed phase in continuous phase, wherein the dispersed phase may be void or may contain dissolved or dispersed materials, and monomers, oligomers and/or pre-polymers are present in the continuous phase, homogenisation and polymerisation thereof, by means of in the first stage introducing the dispersed phase by controlled dosing into the continuous phase with controlled mixing at controlled temperature to achieve an emulsion, and subsequently homogenising for controlled period under controlled deformation and polymerising, under controlled temperature and pressure. Type - 1 Pores (Basic pores): This is the basic pore structure the size of which is determined at the emulsification stage of the PHP formation. Therefore, the pore size is mainly determined by the deformation (flow) history of the emulsion. The integrity of these pores are kept during polymerisation and the interconnects are formed at this stage. Depending on the chemistry of the oil and aqueous phases, phase volume and the polymerisation conditions such as temperature and pressure, the interconnect size can be controlled in the range 01000°C). Once an operation temperature of above 1000°C has been established in the first oxidation zone 8, external ignition is stopped. Simultaneously, perforated catalysis jacket 3 can be activated by using suitable metal base catalyst to allow the hot product gas to pass through. This internal microporous catalysis arrangement in the gasifier 1 will enable to produce tar free product gas 13 before gas leaves the gasifier 1. The operating temperature of the first oxidation zone and hence the through put of the gasifier is subsequently controlled by increasing or decreasing air intake 9. The gasifier 1 can efficiently be operated at as low as 20% of its maximum throughput capacity by controlling the air intakes 9,15 and 25. The high temperature achieved in the first oxidation zone 8, results in a temperature of about 400 to 600°C in the pyrolysis zone 7, and about 100°C in the drying zone 6 by radiation of the heat upwards. During the pyrolysis, which may take about 20 minutes, the dense uniform size fuel 11 releases combustible gases and forms charcoal without the supply of oxygen. The following reactions are typical of those that take place in the pyrolysis zone: The charcoal is then dragged under its own by gravity to the first oxidation zone 8 followed by reduction zone 5 wherein exothermic and endothermic reactions respectively take place at high temperatures. The following reactions are typical of those that take place in the oxidation zone . An advantage of operating the oxidation zone 8 at a temperature of at least 1000°C is that any tars in the pyrolysis gases produced in the pyrolysis zone 7 can be cracked to some extent to produce lower chain length hydrocarbons. In the reduction zone 5 (also often referred as gasification zone), the biomass char is converted into product gas by the reaction with hot gases from the above zones 6, 7 and 8. The following reactions are typical of those that take place in the reduction zone The product gases from these zones are then sucked through the perforation jacket 3 which may be filled with highly active microporous catalysis in the wall surrounding the reduction zone 5 and exit the gasifier through the discharge pipe 18. The use of the perforated part 3 in the wall surrounding the reduction zone 5 allows the product gases to crack residue tars. It also reduces the amount of ash and particulates picked up by the product gases, particularly in the second oxidation zone 14, and carried out of the gasifier 1. The remaining char particles which are not completely gasified in the reduction zone 5 fall into the second oxidation zone 14 for secondary combustion and gasification of any remaining char in ash (by similar reactions that occur in the first oxidation zone 8). The product gas formed in the secondary combustion chamber 14 is directed upward to the discharge 18 due to the negative pressure applied to the discharge 18. The amount of secondary oxidation required will depend on the remaining char content in ash of the fuels. High ash fuels, such as sewage sludge, leather waste, petroleum residue sludge, house-hold waste (RDF), bone meal, chicken and cattle manure, usually result in low carbon conversion in conventional gasifiers due to the isolation of some carbon by ash in the main oxidation zone. In the gasifier of the present invention, the amount of secondary oxidation in the second oxidation zone 14 is controlled by the amount of air admitted by the secondary air intake valve 15. The secondary oxidation zones typically operate at between 650 and 850°C. Mixtures of hot combustible gases (at about 450°C) leave the gasifier 1 from the discharge pipes 18 after passing through perforated catalysis zone 3. In Fig 2, the product gas, which may contain a small amount of tar (less than 20-50 mg/m3) and particulate matters, is discharged to a cyclone for particulate and fly ash removal and then into one or more heat exchangers and gas-water scrubbers with final gas filters for further processing, such as precipitation of any tars or moisture in the product gas. During normal operation of the gasifier 1, the ash auger 19 and the rotary valve 20 are operated continuously such a rotation rate that to extract slag from the reactor so that gasification process is not disturbed. The mode of operation of ash auger 19 depends on the ash content of gasified biomass and/or waste. It is preferred that, the gasifier is operated such that the carbon content of the ash and slag removed from the gasifier is less than 3%. Figure 2 illustrates a gasification system comprising in series: the gasifier of the invention, a water scrubber, a double polymer filter unit working in tandem, a fan, a further double polymer filter unit and means to exit product gas for energy generation. The gas from the water scrubber can also bypass the first double polymer filter unit which is preferably in the form of two polymer box filters if needed (this can happen during start up when the tar content is high. During the normal operation, while the gas is going through one box filter, the polymer is regenerated in the second box filter by blowing hot exhaust gas coming from the internal combustion engine. The regeneration can be conducted at 80 C while the gas cleanup is conducted at temperature below 40 C. During regeneration using polymer in the second box filter, exhaust gases are also cleaned through absorption by the polymer and particulate matter is also retained. This filtration in tandem is repeated at the fan exit where the pressure is high which causes more tar and moisture condensation in the second double polymer filter unit which carries out water/tar absorption. There are restrictions on the gas temperature before it can be fed into the internal combustion engine. Preferred temperature is about 40 C. Therefore the gas from the fan outlet may have to be cooled. This helps the removal of the tar and moisture by the micro-porous polymer in the filter box at the fan exit. The invention is now illustrated in non limiting manner with respect to the following examples. Example 1 - method for operating gasifier according to the invention 1. Start-up The start-up stage includes all operations required until a steady state is reached when the gas quality for the engine is stable and uninterrupted. Pre-weighed batches of biomass fuel are loaded into the hopper to a predetermined level. Then, the air fan and the water scrubber circulation pump are switched on. The fuel is ignited on the grate using solid fuel igniters. 2. Gasifier operation The data measured are the fuel flow rate, gas flow rate, gas composition, temperature and pressure. Temperatures were recorded with an analogue to digital converter every 15 s for inlet air, drying zone, pyrolysis zone, throat and scrubber outlet. The pressure drops were also measured at the gasifier and water scrubber outlet. The product gas flow rate was measured by a gas flow meter located after the suction fan. The amounts of tar and condensate in the product gas were determined from gas samples taken at the gasifier and water scrubber outlets. Clean and dry wood chips and charcoal for use as filters are placed in the respective trays in the box filter. -The gas flow meter is regulated to the required flow rate. The air fan is switched on followed by the circulation pump (water scrubber pump) at the side of the water tank is turned on. Then the pilot lighter to ignite the product gas is lighted. The product gas flow rate was measured by gas flow meter located after suction fan. A circulation fan (gas booster fan) is used to provide the suction effect, which would be exerted by an engine coupled to the gasifier so as to pull the product gas out of the gasifier, through the scrubber and up the stack where it is flared after ignition by a pilot burner. Temperature was measured continuously at six different locations by thermocouples and data was recorded on a computer. 3. Gas cleaning A U-tube apparatus was used to collect tar and condensate, and to clean the gas samples for gas analysis. The U-tube apparatus, basically, consists of a stainless steel sampling probe 5-mm diameter linked to a plastic pipe. It contains two Pyrex "U" tubes in series for trapping tar and moisture. The first trap contains the spherical glass beads with a smaller diameter to provide a large surface area to the wet producer gas, while the second trap contains silica gel between two pieces of glass wool. A vacuum pump (AEI type BS 2406, 0.25 hp) was used to draw the gas through the sampling tube. A rotameter (MFG Fischer, 10 1/min) was placed between the U-tubes and the vacuum pump. Time for the flow of gas through the tube was noted at constant gas flow rate to find the tar content of the producer gas. 4. Shut down Shutdown procedure includes all the actions to safely seal the gasifier. Computer, water scrubber, gasifier air nozzles, circulation fan and exhaust fan of boiler are shut down in an orderly sequence with the off gas burner remaining on using a secondary natural gas burner until no combustible gas is produced. 5. Data processing For a full mass balance, cleanup procedure includes all procedures required to collect tar, char, ash and condensate. After the reactor cooled down, the amounts of ash and char were determined by collecting those from ash chamber. Finally, the top plate of the gasifier was opened so that all unused sugar cane bagasse were carefully removed and monitored. The average feed rate of fuel was calculated by dividing the total sugar cane bagasse consumed by the total operating time of the gasifier. 6. Fuel preparation Biomass fuel consistency is important in gasification in order to achieve continuous flow through the reactor and to provide reliable product gas composition and calorific value for the downstream energy conversion processes. Furthermore, densification of the biomass fuel reduces the reactor size, while shape and size of the densified fuel reduces fluctuations in gas and fuel flow rates as well as the subsequent product quality. For already dense biomass fuels such as wood chips, further densification is not crucial but desirable. However, for fuels with large variations in its content and low bulk density, such as municipal solid waste, it is absolutely inevitable. Therefore, municipal solid waste, after the removal of metals and glass, is either heat treated to obtain cellulose rich powder or it is shredded and subsequently briquetted to density the fuel. When the material to be gasified is liquid, such as refinery oil sludge, it can either be dried to obtain solid material or it can be mixed with a solid carrier, preferentially biomass. One such biomass used was bone-meal which contains large quantities of calcium. Therefore it is useful to remove sulphur from the oil by forming calcium sulphate. 7. Gas analysis A gas chromatograph (Shimadzu GC-8A) was used to analyse the gas samples using helium as a carrier gas. Gas Chromatography (GC) has dual columns (chromosorp 101 and molecular sieve) and a thermal conductivity detector. Parameters influencing retention time and quantity of the gases for each column are given in Table 4.1. At the same time the gas was passed through the U-tube apparatus to find the amounts of tar and condensate in the produced wet gas. Humidity of air in the near the test apparatus was measured form time to time throughout the experiment to make accurate calculations for the mass and energy balance. Example 2- GASIFICATION RESULTS WITH VARIOUS SOLD) FEEDSTOCK (Table Removed) Conclusion As shown in Tables 2A and 2B the operations gave good temperature control with all fuels giving a throat temperature in the first oxidation zone circa 1000 C. Product gas properties were excellent giving in all cases a good fuel composition for subsequent combustion. Particularly all fuels were characterised by 6 megaJoules / m3, in excess of the minimum of 4 megaJoules m3 needed for combustion in a combustion engines As shown in Tables 3A and 3B This is suitable for clean ignition to give low emissions for example: NOX 5 mg/m3 and SO2 7 mg / m3 and trace levels of NO2 and NO. Example 4 - preparation of polyhipe polymer for use in a filter according to the invention. The composition of the oil and aqueous phases are given below: OIL PHASE: Styrene Monomer: (78-X-Y-Z)% 2-Vinylpyridine monomer: X % 2-Ethylhexyl acrylate monomer: Y % Oil phase soluble initiator: Z % Divinyl benzene (cross linking agent): 8% Surbitan mono-monooleate (Span 80 non-ionic surfactant): 14% AQUEOUS PHASE: Water containing Z% potassium persulphate. The preferred phase volume of the aqueous phase is 80-95%, more preferably, 90-95%. The inclusion of the monomer 2-vinyl pyridine in high concentrations results in the breakdown of the emulsion. Therefore, the preferred value of X lies between 5-10 %. The use of 2-ethylhexyl acrylate is to make the micro-porous polymer elastic. Elasticity of the polymer results in better mechanical shock absorbance and good attrition characteristics. Elasticity is also useful to enhance the water uptake capacity of the hydrophilic polymer. The preferred value of the 2-ethylhexyl acrylate concentration is up to Y=30% but more preferably Y = 15%. Water phase and/or oil phase soluble initiator is present in amount Z= 0 -1 %. Oil phase initiator is present when sulphur containing water phase initiator (potassium persulphate) is not used. The preparation of the emulsion was carried out in a batch mixer from an oil phase and an aqueous phase, dosed at a predetennined rate while the emulsion was stirred at constant rotational speed. Dosing rate, deformation rate, mixing rate were predetermined as a function of volume of respective phases, diameter of the batch mixer and of the impellers, rotational speed of the impellers and homogenisation time. In order to eliminate the differences in performance of different mixing conditions we characterise the mixing through: Where: VA = Volume of aqueous phase added over a period of time tD V0 = Volume of the oil phase placed in the batch mixer DI - Diameter of the impellers D0 = Diameter of the batch mixer Q = Rotational speed We also define tH as the homogenisation time and tT as the total mixing time. tr = tD+tH Two flat paddle impellers (8 cm in diameter and 1.4 cm in width) were used in a mixing tank of 8.5 cm diameter. Impeller separation was 1 cm. 25 ml of oil phase is placed at the bottom of the tank and 225 ml of aqueous phase was dosed using 4 feed points. Temperature of the aqueous phase was ranged from -1.0 to 80°C. In the polymer preparation with vinyl pyridine polymerisation is carried out at 40 C for 8 hours followed by 50 C for 8 hours and finally another 8 hours at 60 C. This ensures that the emulsion is not destabilised during polymerisation. We discovered that the polymers should not be dried excessively during the 'drying stage'. Excessive drying results in the reduction in water absorption capacity of the polymer as well as in reduced rate of water uptake. Typically, the water absorption capacity of these polymers is 10-12 times of their own weight. We Claim: 1. A gasifier (1) for the gasification of biomass and waste to produce combustible effluent, comprising: a) a fuel valve (22) for loading solid fuel (11) into a first oxidation zone (8) b) a first throat (2) defining the lower edge of the first oxidation zone (8) c) a second throat (4) defining the lower edge of a second oxidation zone (14) d) a reduction zone (5) linking the first oxidation zone (8) to the second oxidation zone (14) and e) two oppositely located (at the reduction zone) vortex discharge pipes (18) for the combustible effluent wherein in the first oxidation zone the gas flow is in the same direction as fuel flow and in the second oxidation zone the gas flow is in the opposite direction to the fuel flow. 2. A gasifier (1) according to claim 1, further comprising a pyrolysis zone (7) above the first oxidation zone (8), and a fuel storage and drying zone (6) above the pyrolysis zone (7). 3. A gasifier (1) according to either claim 1 or claim 2, further comprising air distribution nozzles (12, 17) for providing air to the first and second oxidation zones, wherein said nozzles are provided in the first throat (2), the upper half of the reduction zone (5) and the second throat (4). 4. A gasifier (1) according to any of claims 1 to 3, further comprising a central air nozzle set (24) above the first oxidation zone (8) for supplying air to the first oxidation zone (8) 5. A gasifier according to any one of the preceding claims, wherein a microporous catalyst zone (3), preferably in the form of a perforated catalysis jacket (3), is located below the reduction zone (5) and links the first oxidation zone (8) to the discharge pipes (18). 6. A gasifier according to any one of the preceding claims, further comprising a grateless discharge mechanism (19) beneath the second oxidation zone (14). 7. A gasifier according to claim 6, wherein the grateless discharge mechanism comprises a screw conveyor auger (19) 8. A gasifier according to any one of the preceding claims, wherein the first and/or second throat (2,4) is inclined at an angle (10,16) of between 10 and 40° to the horizontal axis 9. A gasifier (1) according to any one of the preceding claims, wherein the first throat (2) and the second throat (4) are symmetrically located with respect to each other 10. A gasifier (1) according to any one of the preceding claims, further comprising means attached to the discharge pipes (18) so as to maintain the pressure in gasifier (1) below atmospheric pressure 11. A gasifier (1) according to any one of the preceding claims, wherein the cross sectional area of the first and second oxidation zones (8, 14) is smaller than that of the fuel storage (6) and pyrolysis zones (7) 12. A gasifier (1) according to claim 11, wherein the cross sectional area of the reduction zone (5) is smaller than that of the first and second oxidation zones (8,14) 13. A method for the gasification of solid fuel to produce a combustible effluent using a gasifier as described in any one of claims 1 to 12, comprising the steps of: 1) partially oxidising a biomass fuel in the first oxidation zone (8) to produce char; 2) reducing the char in the reduction zone (5) to form ash; 3) further oxidising any char residue in the ash in the second oxidation zone (14); 4) extracting the combustible effluent produced in the above steps, by the discharge pipe (18) wherein in the first oxidation zone the gas flow is in the same direction as fuel flow and in the second oxidation zone the gas flow is in the opposite direction to the fuel flow. 14. A method according to claim 13 wherein extraction is conducted through microporous catalyst (3), preferably in the form of perforated catalysis jacket (3) 15. A method according to any of claims 13 to 14 wherein the first oxidation zone operates at a temperature greater than 850°C, preferably at least 1000°C. 16. A method according to any of claims 13 to 15 comprising passing combustion gases from first oxidation zone through catalyst (3) to produce tar- free gas. 17. A method according to any of claims 13 to 16 wherein the temperature of the first combustion zone is controlled by modulating air intake (9) 18. A method according to any of claims 13 to 17 wherein the reduction zone operates at a temperature of between 600 and 900°C. 19. A method according to any of claims 13 to 18 wherein the second oxidation zone operates at a temperature of between 700 and 800°C. 20. A method according to any of claims 13 to 19 comprising pyrolysing fuel in a pyrolysis zone to form charcoal which is dried and proceeds by gravity to the first oxidation zone then to the reduction zone. 21. A method according to claim 20 wherein the temperature in the pyrolysis zone is 400-600°C and temperature in the drying zone is in the range of 100°C. 22. A method according to claim 20 or 21 wherein any tars remaining in pyrolysis gases are cracked in the first oxidation zone. 23. A method according to any of claims 13 to 22 for gasification of biomass, such as liquid waste for example waste oil or petroleum sludge which is absorbed within the intra-particle and inner-particle pores of a suitable combustible carrier. 24. A method according to claim 23, wherein the combustible carrier has high internal porosity and is preferably in fibrous form to provide extensive inter-particle porosity. 25. A method according to claims 23 or 24, wherein the liquid waste is mixed with the carrier and briquetted in order to density the composite fuel. 26. A method according to any of claims 23 to 25, wherein a suitable carrier is selected from saw dust, crushed bone waste from slaughter house, bread/food waste, municipal waste, dried sewage sludge, chopped straw, rape seed meal, sugar cane bagasse. 27. A gasification system comprising in series: the gasifier of the invention, a water scrubber, a double polymer filter unit working in tandem, a fan, a further double polymer filter unit and means to exit product gas for energy generation, such as an effluent stack leading to a location for clean ignition. 28. A gasification system according to claim 27 wherein box filters comprise a microcellular polyhipe polymer comprising pores in a range 0.1 to 300 micron (primary) and / or 300 to 10,000, preferably to 1,000, micron (coalescence) for absorbing water and tar. 29. A gasification system according to claims 27 or 28 wherein clean ignition is with an internal combustion engine for electricity generation. 30. A gas filter comprising a microcellular open cell polyHIPE polymer comprising pores in the range 0.1 to 300 micron (primary) and / or 300 to 10,000, preferably 1,000 micron (coalescence) which is effective in adsorbing water and tar from the gas. 31. A process for gas filtration comprising passing gas contaminated with water and tar through a filter according to Claim 30 preferably at a temperature in the range 10 - 70C, more preferably 20 - 45C. 32. A polyHIPE polymer comprising polyvinyl polyhipe made up of oil phase monomers styrene, divinyl benzene (DVB) and surfactant (Span 80 sorbitan monooleate), additionally incorporating monomer (such as 2-vinyl pyridine) to cause water adsorption of the polymer, preferably present in an amount of 5 - 10% and/or monomer (such as 2-ethyl hexyl acrylate) to incorporate elasticity and improve mechanical shock absorbance and good attrition characteristics, and water or oil phase initiator, preferably present in an amount of up to 1%, more preferably a sulphur free oil phase initiator such as lauryl peroxide, characterised in that the polymer comprises pores in the range 0.1 to 300 micron and / or 300 to 10,000, preferably 1,000 micron. 33. A polymer according to Claim 32 incorporating a sulphonated core and anon sulphonated but water absorbent skin. 34. The use of a gasifier, method for gasification, gasification system and method, gas filter and novel polymer in the gasification of solid fuel as hereinbefore defined, preferably to generate combustible gases for use in energy generation, for example using internal combustion gas engines, gas turbines, dual- fuel diesel engines or fuel cells.