Patent Publication Number: US-10760003-B2

Title: Process and apparatus for treating waste comprising mixed plastic waste

Description:
REFERENCE TO RELATED APPLICATIONS 
     This application is a Continuation of U.S. application Ser. No. 16/124,608, filed Sep. 7, 2018, which claims priority to U.S. application Ser. No. 14/768,066, filed on Aug. 14, 2015, which claims priority to PCT Applic. No. PCT/GB2013/052849, filed on Oct. 31, 2013, which claims priority to U.K. Applic. No. 1303005.1, filed on Feb. 20, 2013, the contents of which are incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention concerns processes and apparatus for the treatment of waste comprising Mixed Plastic Waste. 
     BACKGROUND OF THE INVENTION 
     In many countries, waste material is collected and taken to processing centres or recycling centres. Some categories of waste are separated out and sent to recycling processes, either at the centre or elsewhere. For example, waste glass is commonly recycled. Waste that is not recycled may be sent to landfill or may be burnt, typically to provide either heat or electrical energy. 
     Large amounts of plastic are used in modern goods and packaging and there is therefore a large quantity of plastic waste, typically termed Mixed Plastic Waste, passing through recycling centres. Typically, PET and HDPE are separated out for recycling and the remainder is sent to landfill. However, landfill may not be a popular option and there therefore exists a need to find other ways of dealing with Mixed Plastic Waste, either on its own or combined with organic material as Municipal Solid Waste. 
     Some solutions have been proposed for Mixed Plastic Waste. For example, Mixed Plastic Waste may be used as a fuel in a power station. However, the cost of electricity generated in such a way may be ten times the cost of electricity generated from a conventional fossil fuel, such as natural gas. It may also be difficult to use all the heat produced in such processes and much of it is therefore dissipated in cooling towers. The combination of low efficiency and high capital cost can make such solutions unattractive. 
     Pyrolysis of Mixed Plastic waste has been suggested as a solution. Examples include the use of pyrolysis to create fuel for vehicles, combining pyrolysis with plasma treatment to produce hydrogen and pyrolysis for disposal of plastic waste at sea. However, such processes may suffer from drawbacks, including the difficulty of producing a uniform, high-quality product from a highly variable feed such as waste and the difficulty of effectively using all the pyrolysis products. 
     The present invention seeks to mitigate the above-mentioned problems. Alternatively or additionally, the present invention seeks to provide an improved process and apparatus for the treatment of waste comprising Mixed Plastic Waste. 
     SUMMARY OF THE INVENTION 
     According to a first aspect of the invention, there is provided a process for treating waste comprising Mixed Plastic Waste, the process comprising:
         feeding the waste to a pyrolysis reactor;   pyrolysing the waste in the pyrolysis reactor to produce a fuel; and   using the fuel to run a generator to produce electricity.       

     By pyrolysing the waste, it is possible to create a fuel that powers a standard engine, for example a marine diesel engine attached to a generator. Such a process may be advantageous in that the capital cost of setting up such a process may be lower than for a bespoke combustion process. 
     Mixed Plastic Waste will be understood to be a mixture of waste plastics. That mixture of plastics could originate from separate streams of plastics or could originate from a single stream of comingled plastics. In many cases, Mixed Plastic Waste will result from domestic refuse, such as that traditionally collected in black bags in the UK. Such ‘black bag waste’, or Municipal Solid Waste, will comprise Mixed Plastic Waste. It may be that the Municipal Solid Waste is fed to the process, but advantageously some separation occurs to remove waste, such as glass and certain plastics, such as HDPE and PET, that can be recycled before feeding the waste to the process. The separations that occur may depend on what other facilities are available to recycle or otherwise use parts of the incoming waste to the facility. Thus, at some facilities, the waste fed to the process may be Municipal Solid Waste. At some facilities the waste fed to the process may be Mixed Plastic Waste. At some facilities the waste fed to the process may comprise greater than 10 wt % Mixed Plastic Waste or greater than 20 wt % Mixed Plastic Waste. Preferably the waste fed to the process comprises greater than 30 wt % Mixed Plastic Waste, more preferably greater than 40 wt % Mixed Plastic Waste, more preferably greater than 50 wt % Mixed Plastic Waste, more preferably greater than 60 wt % Mixed Plastic Waste, more preferably greater than 70 wt % Mixed Plastic Waste, more preferably greater than 80 wt % Mixed Plastic Waste and more preferably greater than 90 wt % Mixed Plastic Waste. It will be appreciated that waste comprising a high percentage of Mixed Plastic Waste may be advantageous because of a high energy density and also because such waste may be difficult to treat in other ways and may typically be sent to landfill. It may be that the waste further comprises organic waste. Advantageously the waste may contain greater than 70 wt % organic waste. Such waste may count as biomass for the purpose of government schemes such as the Renewable Obligations Certificate scheme in the UK. In such cases, the waste preferably comprises greater than 5 wt %, more preferably greater than 10 wt %, even more preferably greater than 20 wt % and still more preferably greater than 25 wt % Mixed Plastic Waste. Advantageously the waste composition is such that there is sufficient organic waste to qualify as biomass but that substantially the remainder of the waste is Mixed Plastic Waste so as to increase the energy density. 
     Preferably the process comprises passing the fuel through a condenser to form a liquid fraction and a gas fraction and using the liquid fraction of the fuel to run the generator. Using liquid fuel in the generator is preferable from a cost and simplicity point of view. Liquid fuels may be easier to store and handle and may be used in a wide variety of generators. 
     Preferably the process comprises storing the fuel in a buffer tank prior to using it to run the generator. The buffer tank is, for example, a tank in which the level of the fuel can vary. In that way, variations in the rate at which the fuel is produced and consumed can be accommodated by allowing the reserve of fuel stored in the buffer tank to increase or decrease. That may be particularly advantageous in a process such as the present invention, as it may be most efficient to run the pyrolysis reactor at a constant rate so as to achieve a steady operating state, but the demand for electricity varies through the day. Thus the quantity of fuel in the buffer tank is allowed to increase at times of low electricity demand and to decrease at times of high electricity demand. For example, it may be that the fuel is produced continuously and the generator is run intermittently. It will be appreciated that continuous production means that the process for producing the fuel is run continuously over an extended period of time such as days, weeks or months. 
     It may be that the process includes mixing the fuel while it is in the buffer tank. For example, some of the fuel may be drawn from one part of the tank and recirculated to a different part of the tank. If the buffer tank comprises multiple tanks, the mixing may be achieved by circulating the fuel between the tanks. Mixing the fuel in the buffer tank may smooth out fluctuations in the properties of the fuel that result from variability in the waste fed to the process. That may provide a more uniform heat of combustion of the fuel over time and may also prevent emissions spikes resulting from short-term rises in contamination in the waste feed. 
     Preferably, the fuel is stored as a liquid and/or as a solid. The fuel may thus be stored as a liquid, a solid, or a mixture of a liquid and a solid. It will be appreciated that heavy fuels, such as bunker fuel commonly used in shipping, may be solid, or a mixture of tarry liquids and solids, at ambient temperatures and become liquid when warmed, for example to around 50° C. Thus such fuels may be stored as a liquid, or a solid or a mixture of a liquid and solid depending on temperature. It will be appreciated that the tank volume required for liquid/solid storage may be significantly smaller than for storing an equivalent amount of fuel (in energy terms) as a gas. Liquid/solid storage may also be intrinsically safer. The process may comprise warming the fuel in the buffer tank so as to transfer, for example pump, it to the generator. The warming may be achieved using heat from the generator, when it is running, or by using an external source of heat, for example at start up. 
     The buffer tank may comprise a tank container, for example a tank container compatible with ISO standards for intermodal tank containers. Preferably the buffer tank comprises a so-called “20 ft” tank container. Thus the buffer tank may be 6.1 m long and 2.44 m wide and high and mounted in an ISO compatible intermodal container frame. The volume of the tank may be from 14,000 to 27,000 litres. In some embodiments the buffer tank may comprise a plurality of tank containers. Such an arrangement may be advantageous in that a plurality of container tanks may be simpler and cheaper to manufacture and deliver than a single large tank and in that capacity may be straightforwardly added by providing further container tanks. Multiple container tanks may also provide for flexibility in maintenance and operation and be safer than a single large tank. 
     Preferably the process comprises filtering the fuel to remove chemical contaminants. Emissions from generators may be subject to strict controls and it is possible that fuel produced from waste may contain significant levels of chemical contamination. For example, PVC in the plastic waste may mean that there are undesirable levels of chlorine in the fuel as it exits the pyrolysis reactor. Other chemical contaminants include fluorine and sulphur. By filtering the fuel, the chemical contamination can be removed before the fuel is used in the generator. The filtering may therefore remove chemicals associated with poor emissions, for example emissions that would contravene the Waste Incineration Directive. The fuel may be filtered in the gas phase. That is, the fuel may exit the pyrolysis reactor as a gas and be filtered in the gas phase before the fuel is condensed. It will be appreciated that the fuel is filtered before it is used in the generator. Thus the filtration occurs before combustion. There may be a significant advantage in filtering the fuel after the pyrolysis but before it is used in the generator. Pyrolysis breaks down the chain lengths of the hydrocarbons in the plastics, resulting in smaller molecules. Combustion, on the other hand combines the hydrocarbons with oxygen, typically from air, which results in a large volume of exhaust gas. The result is that the volume of fuel gas exiting the pyrolysis reactor may be significantly lower than the volume of gas that would be produced by combustion, either of the waste or the subsequent combustion of the fuel in the generator. By cleaning the pyrolysis product the volume of gas to be filtered and cleaned may be lower than the volume of gas that would need to be filtered and cleaned in a combustion exhaust; as a result smaller filters may be used with associated advantages in terms of capital and operating costs. 
     Preferably the process includes storing the waste in a vessel prior to feeding the waste to the pyrolysis reactor, wherein the waste is blended whilst stored in the vessel. It will be appreciated that waste collected tends to vary in composition from hour to hour and day to day. That may present a particular challenge when trying to convert the waste into a desired product, as the composition of the waste feed may affect the composition and attributes of the product. Even if sophisticated control of the process to adjust operating conditions to compensate for the variable composition of the waste is available, it can still be advantageous to try to minimise the variations in waste composition. By storing the waste in a blended storage tank, variations in composition may be to some extent averaged out. Even if such storage does not completely eliminate variations, it may smooth the rate at which the composition changes, and may therefore allow more time for the control system to apply adjustments. 
     Preferably the waste is dried in a dryer prior to being fed to the pyrolysis reactor. The dryer may reduce the water content of the waste to less than 5 wt %, preferably less than 3 wt %. The dryer may reduce the water content of the waste to between 2 and 3 wt %. When the waste is fed to the pyrolysis reactor, it is heated to the reaction temperature. Heating any water that is in the waste requires extra energy and therefore it may be advantageous to remove the water in a dryer before the waste is heated. If the fuel is being stored as a liquid, any water in the waste is heated to form steam in the reactor and then condensed again to water in the condenser, resulting in a waste of energy. 
     Preferably the temperature in the pyrolysis reactor is controlled so as to produce a fuel comprising C 5  to C 100  hydrocarbons. The mean chain length of the fuel, based on the number of molecules, is preferably in the range C 5  to C 40 , more preferably in the range C 10  to C 20 . It may be that 80 wt % of the fuel consists of hydrocarbons with a chain length in the range C 5  to C 40 , more preferably in the range C 5  to C 20 , more preferably in the range C 10  to C 20 . Preferably the hydrocarbons in the fuel have a chain length greater than C 5 . That may be achieved by passing the fuel through a condenser and separating off the part of the fuel that remains as a gas following the condensation (the gas fraction). The liquid fraction may then be used as the fuel and the gas fraction used elsewhere in the process or the host facility. 
     For example, the fuel may be bunker fuel. It will be appreciated that the composition of the fuel may be selected based on the generator used in the process. The hydrocarbon chain length of the fuel affects the heat of combustion of the fuel. Shorter chain lengths result in a higher heat of combustion. However, at very short chain lengths (for example, less than C 5 ), a small change in chain length can result in a large change in heat of combustion. That can result in control challenges, as, for example, a small decrease in the average chain length of the fuel can lead to a large increase in the energy released when the fuel is combusted and can therefore cause engine failures. Conversely, at very long chain lengths (for example, greater than C 40 ) a large change in the average chain length may be required to produce a significant change in heat of combustion. That may lead to control challenges in that large control variations may be needed to produce a noticeable effect on the fuel. The average chain length of the fuel is therefore preferably selected so as to be at a value where changes in the fuel chain length produce a sensible change in the heat of combustion in the fuel. In that way control variations can be used to alter the heat of combustion of the fuel as necessary, but short-term deviations outside the control range do not have catastrophic consequences. 
     As mentioned above, control of the process may be challenging due to the variability of the waste fed to the process. The process may be controlled by monitoring the waste input or by monitoring the generator but the process is preferably controlled by monitoring an attribute of the fuel and adjusting the temperature and/or the residence time of the pyrolysis reactor in response to the measured attribute of the fuel so as to maintain that attribute within a desired range. It may be that the temperature of the reactor is adjusted. It may be that the residence time of the reactor is adjusted, for example by adjusting the flowrate through the reactor. It may be that both the temperature and the residence time of the reactor are adjusted. Preferably the attribute is related to the heat of combustion, or the calorific value, of the fuel. For example, the attribute may be the heat of combustion of the fuel or it may be a parameter whose value is dependent on the heat of combustion of the fuel. The attribute of the fuel could be measured in the buffer tank, but is preferably measure at the inlet to the tank so as to avoid the large volume of fuel in the tank slowing the response time of the control system. By monitoring the fuel entering the tank, the control system can detect variations in the process more rapidly and apply any necessary adjustment. For example, if the monitor detects that the heat of combustion of the fuel entering the buffer tank is falling, the control system can increase the heat of the reactor or increase the residence time in the reactor. That may be achieved by feeding a greater quantity of the fuel to the burners heating the reactor or by reducing the flowrate through the reactor. Conversely, if the monitor detects that the heat of combustion of the fuel is rising too high, the temperature in the reactor can be decreased or the residence time in the reactor can be decreased. 
     The heat of combustion of the fuel is preferably controlled using a solvent monitor. Such a system may involve providing a hydrogen flame, feeding a sample of the fuel into the flame and recording the flame temperature. The heat of combustion of the flame may be related to the difference between the flame temperature with the fuel added and the temperature of the hydrogen flame but the process may be controlled more straightforwardly by controlling the process so as to achieve a flame temperature with the fuel added within a desired range. For example, a thermocouple may be provided to monitor the flame temperature and the output from the thermocouple used as an input to the control process for the reactor. Such a system may be a cheap and simple option for monitoring the fuel quality. By monitoring the fuel quality the response time of the control system may be improved. For example, if the control system monitors the generator operation, by the time a decrease in power is observed, a large quantity of sub-standard fuel may already have been produced. That may be particularly the case where the fuel is stored in a buffer tank. Furthermore, it may be more straightforward to monitor the fuel quality, since the fuel is a fluid, than it is to monitor the composition of the incoming waste. 
     Heat of combustion is a well-known attribute and the skilled person can measure the heat of combustion. For example, the heat of combustion of solids may be measured using methods such as ISO1928:2009 and the heat of combustion of liquid hydrocarbons may be measured using methods such as ASTM D4809. It may be that the heat of combustion of the waste is around 30 MJ/kg, for example in the range 25 MJ/kg to 35 MJ/kg. Preferably the process is controlled so as to produce a fuel having a heat of combustion less than 45 MJ/kg. More preferably the process is controlled so as to produce a fuel having a heat of combustion in the range 42 MJ/kg to 45 MJ/kg, even more preferably 44 MJ/kg to 45 MJ/kg. A fuel with a heat of combustion in that range may be at a point on a curve of heat of combustion against average chain length where controlled variations in the heat of combustion can be achieved by varying the temperature and/or residence time of the pyrolysis reactor so as to vary the average chain length. Fuels below that range may be in a region of the curve where undesirably large changes in reactor conditions are needed to vary the heat of combustion and fuels above that range may be in a region where unavoidable variations in the reactor conditions result in undesirably large fluctuations in the heat of combustion. Nevertheless, it may still be advantageous to operate in the higher or lower regions in some circumstances. For example, at lower heats of combustion the process may be very stable and at higher heats of combustion more power may be available. 
     The temperature in the reactor may, for example, be in the range 400° C. to 600° C. The reactor may be a fluidised bed reactor, with the fluidised bed having a mass of 2.5 to 8 tonnes, preferably 2.5 to 5 tonnes, more preferably 2.5 to 3.5 tonnes. The reactor or the fluidised bed may have an aspect ratio (height:width) of around 1:1, for example in the range 0.5:1 to 1:2, preferably in the range 0.8:1 to 1:1.2. Such a reactor size, shape and temperature may allow efficient treatment of the waste. 
     Preferably a product from the pyrolysis reactor is combusted to heat a fluid, the fluid being used to heat the pyrolysis reactor. The fluid may comprise the fuel. For example, a portion of the fuel output stream from the pyrolysis reactor may be directed through a heater, preferably an indirect heater, and back into the pyrolysis reactor. 
     The product from the pyrolysis reactor may comprise the fuel. The product from the pyrolysis reactor may comprise part of the fuel output stream from the pyrolysis reactor that is not sent to the generator. For example, if the process comprises passing the fuel through a condenser to form a liquid fraction and a gas fraction and using the liquid fraction of the fuel to run the generator, the product combusted to heat the fluid used to heat the pyrolysis reactor may comprise the gas fraction. It may be that the gas fraction is sufficient to heat the pyrolysis reactor but it may be that extra heat is needed, in which case it may be that the gas fraction is supplemented by some of the liquid fraction. The gas fraction is preferably supplemented by a stream of the fuel product from the pyrolysis reactor that is taken from upstream of the condenser. In that way energy is not wasted condensing the fuel only to combust it again immediately to heat the reactor. It may be that the gas fraction exceeds the amount of heat required to heat the pyrolysis reactor. In such cases the excess gas may be used elsewhere or flared. 
     It may be that the product from the pyrolysis reactor combusted to heat the fluid that is used to heat the pyrolysis reactor is a by-product of the pyrolysis. For example, the product may be char. For example, the pyrolysis reactor may be a fluidised bed reactor and the process may comprise removing a portion of the fluidised bed that has become at least partially coated in char from the reactor, separating the char, returning the portion of the fluidised bed to the reactor and combusting the char to heat a fluid that is used to heat the reactor. By combusting the char to provide the heat, none of the fuel product is used for heating the reactor so the process can provide more energy through the generator. 
     By combusting part of the product to provide heat, the process does not require a separate fuel source for normal operation. The skilled person will appreciate that for start-up a separate source of heat may still be required. By using combustion of the product to heat a further stream comprising the fuel, with the further stream being introduced into the pyrolysis reactor, the combustion products may be kept out of the reactor. Such an indirect heating system may allow combustion to provide heat, without the reactor becoming contaminated with combustion products. 
     Preferably the fuel is used to run a first generator and a second generator. For example, the first generator may be run continuously and the second generator run intermittently according to grid demand. It may be that the first generator is smaller than the second generator. It may be that the first generator is larger than the second generator. For example, the first generator may be used to provide continuous power to the host facility in which the process operates and the second generator could be used to produce power to sell to the grid. Preferably the first generator is sized according to the heat or electricity demand of the host facility. Preferably the second generator has a short start-up cycle, for example starting up in less than 10 minutes, so as to be able to provide reserve power to the grid at short notice. The ability to act as part of the grid&#39;s reserve may be advantageous in terms of the prices that the grid will pay. It may be that the first generator forms part of a control system, with the generator&#39;s performance being monitored and the measured performance being used to control the operating conditions, for example the temperature and residence time of the pyrolysis reactor, of the process. 
     The generators may, for example, be turbines, preferably gas turbines, but are preferably internal combustion engines attached to generators. For example, the engine may be a marine diesel engine, which may, for example, run on bunker fuel. In such a case, the control system could be calibrated so as to produce bunker fuel in the buffer tank. Combining the use of bunker fuel in a marine diesel with a step of filtering the fuel before it is used in the marine diesel can be advantageous as the cleaned fuel may mean that the emissions from the marine diesel engine are reduced. That may be particularly advantageous, as bunker fuel may be considered to be highly polluting and therefore require expensive exhaust clean-up unless the filtering is used. 
     Preferably the process treats from 5,000 to 20,000 tonnes per year of waste, more preferably 5,000 to 10,000 tonnes per year of waste and even more preferably 6,000 to 8,000 tonnes per year of waste. For example it may be that the process treats 7,000 tonnes per year of waste. Processes of those sizes can be conveniently combined with existing recycling facilities so as to treat Mixed Plastic Waste on-site, rather than having to transport the waste to another, larger facility such as a power station. Transporting waste uses energy and may therefore reduce or eliminate the environmental benefit of subsequent treatment. Preferably the process produces from 1.8 to 10 MW, more preferably 1.8 to 5 MW and even more preferably 2.1 to 4 MW. For example, it may be that the process produces from 2.5 to 3.5 MW of electricity. 
     Preferably the process includes cooling the exhaust from the generator by heat exchange with an air stream, wherein at least part of the air stream is used to provide heat elsewhere in the process or elsewhere in the host facility in which the process is carried out. For example, the air stream may be used to provide heat in the dryer, thus improving the overall efficiency of the process. 
     The generator may be cooled by heat exchange, for example indirect heat exchange with a fluid stream and at least part of that fluid stream may then be used to provide heat to another part of the process, for example the dryer, or to another process in the host facility. Preferably the generator is cooled by heat exchange with a water stream, wherein at least part of the water stream is used to provide heat elsewhere in the process or elsewhere in the host facility. 
     Preferably the pyrolysis reactor is a fluidised bed reactor. For example, the pyrolysis reactor may contain a fluidised bed of particles, such as sand, and a distributor for feeding a fluidisation medium into the reactor. The fluidisation medium may, for example, be recycled pyrolysis product, which may have been heated in order to supply energy to the pyrolysis reactor. The distributor design may be an important part of the fluidised bed design so as to achieve a uniform distribution of the fluidising medium across the bed. Moreover, the fluidised bed may accumulate contaminants over time and the particles of the bed, for example sand, may need cleaning. For example, tar, char or coke may accumulate on the particles. The cleaning could be achieved by shutting down the reactor and removing the particles, but it is advantageously performed on-line by a recirculation of the particles through a cleaning system. The distributor is preferably configured so as to allow a portion of the fluidised particles to fall through the distributor and the process preferably comprises removing a portion of the particles that have fallen through the distributor, cleaning the particles and feeding the particles back into the reactor. In that way a continuous recirculation of the particles through the cleaning apparatus may be achieved. For example, the distributor may comprise an array of ducts with orifices, for example in their surface, the ducts being configured such that the fluidising medium is fed to the ducts and exits the ducts through the orifices into the reactor, wherein the array of ducts is configured such that the particles can fall between the ducts. The ducts may for example be arranged in a row or a grid, with the spacing between the ducts being selected so as to allow the particles to pass between the ducts. It will be appreciated that steps may be taken to prevent the particles that are falling from the fluidised bed falling into, and blocking, the orifices. The orifices may, for example, be in the underside of the ducts. In another example, the orifices may comprise a nozzle comprising a cap to prevent particles blocking the orifice. 
     It will be appreciated that using the heat from the generator may provide significant advantages in terms of efficiency. As well as providing electrical power to the host facility, the generator can also supply heat to the host facility. Such a ‘combined heat and power’ approach allows the energy in the fuel to be efficiently transformed into useful energy for the host facility. Thus, in a broad aspect of the invention there is provided a process for treating waste comprising Mixed Plastic Waste at a host facility, the process comprising:
         providing an apparatus comprising a pyrolysis reactor and a generator at the host facility;   feeding the waste to a pyrolysis reactor;   pyrolysing the waste in the pyrolysis reactor to produce a fuel; and   using the fuel to run a generator to produce energy.       

     It may be that the energy is electrical energy; that is the generator is run to produce electricity. It may be that the energy is heat energy. Preferably the energy is both electrical and heat energy. Such a ‘combined heat and power’ approach may be advantageous because the generator may require cooling even if being run to produce electricity and it may therefore be advantageous to make use of the heat energy as well. The host facility may be a site having energy demands. Preferably the energy is used within the host facility. For example, the host facility may be a recycling plant and the energy may be used in other parts of the recycling process. 
     It will be appreciated that controlling the process is an inventive feature of the process in its own right and may be particularly important when there is significant variability in the waste feed. Thus, in a second aspect of the invention there is provided a process for treating waste comprising Mixed Plastic Waste, the process comprising:
         feeding the waste to a pyrolysis reactor;   pyrolysing the waste in the pyrolysis reactor to produce a fuel;   monitoring an attribute of the fuel; and   adjusting the temperature and/or residence time of the pyrolysis reactor in response to the measured attribute of the fuel so as to maintain the attribute within a desired range.       

     Preferably the attribute is related to the heat of combustion of the fuel. 
     The design of the distributor is an inventive feature of the process in its own right. Thus, in a third aspect of the invention there is provided a process for treating waste comprising Mixed Plastic Waste, the process comprising:
         feeding the waste to a pyrolysis reactor, the pyrolysis reactor being a fluidised bed reactor; and   pyrolysing the waste in the pyrolysis reactor to produce a fuel;   wherein the pyrolysis reactor contains a fluidised bed of particles and a distributor for feeding a fluidisation medium into the reactor, wherein the distributor is configured such that the particles can fall through the distributor and wherein the process comprises removing a portion of the particles that have fallen through the distributor, cleaning the particles and feeding the particles back into the reactor.       

     Heating the pyrolysis reactor is an inventive feature of the process in its own right. Thus, according to a fourth aspect of the invention there is provided a process for treating waste comprising Mixed Plastic Waste, the process comprising:
         feeding the waste to a pyrolysis reactor, for example a fluidised bed reactor; and   pyrolysing the waste in the pyrolysis reactor to produce a fuel; and   combusting a product from the pyrolysis reactor to heat a fluid, and feeding the fluid into the pyrolysis reactor so as to heat the pyrolysis reactor.       

     It may be that the product is a by-product, for example char. It may be that the product comprises the fuel. It may be that the product comprises a part of the fuel output stream from the pyrolysis reactor that is not used to run the generator. 
     The processes of the above aspects of the invention are particularly suited to use in small to moderate sized recycling facilities, where they complement the facilities already present. For example, the electricity and heat generated by the process of the above aspects of the invention can be used to power recycling operations at the centre. Thus, according to a fifth aspect of the invention there is provided a process for recycling waste material, the process comprising a process for treating waste comprising Mixed Plastic Waste according to any of the above aspects of the invention. 
     It will be appreciated that the process of the invention is carried out at a site, or host facility, for example a recycling or waste processing facility. According to a sixth aspect of the invention there is provided an apparatus for treating waste comprising Mixed Plastic Waste, the apparatus comprising:
         a pyrolysis reactor for pyrolysing the waste to produce a fuel; and   a generator configured to run on the fuel to produce electricity.       

     Preferably the apparatus comprises a condenser downstream of the pyrolysis reactor and upstream of the generator for condensing the fuel prior to using it to run the generator. 
     Preferably the apparatus comprises a buffer tank downstream of the pyrolysis reactor and upstream of the generator for storing the fuel prior to using it to run the generator. 
     Preferably the apparatus comprises a filter system downstream of the pyrolysis reactor and upstream of the generator for filtering the fuel to remove chemical contaminants. 
     Preferably the apparatus includes a storage vessel upstream of the pyrolysis reactor for storing the waste prior to feeding the waste to the pyrolysis reactor, wherein the storage vessel comprises a blending system for blending the waste stored in the vessel. 
     Preferably the apparatus comprises a dryer upstream of the pyrolysis reactor for drying the waste. 
     Preferably the apparatus comprises:
         a monitor, preferably a solvent monitor, for monitoring an attribute of the fuel; and   a controller for adjusting the temperature and/or residence time of the pyrolysis reactor in response to the measured attribute of the fuel so as to maintain the attribute within a desired range. Preferably the attribute is related to the heat of combustion of the fuel.       

     Preferably the apparatus comprises a combustor to combust a product from the pyrolysis reactor to heat a fluid that is fed into the pyrolysis reactor to heat the pyrolysis reactor. It may be that the product is a by-product, for example char. It may be that the product comprises the fuel. It may be that the product comprises a part of the fuel output stream from the pyrolysis reactor that is not used to run the generator. 
     Preferably the generator comprises an internal combustion engine, for example a marine diesel engine. 
     Preferably the apparatus is sized and configured to treat from 5,000 to 20,000 tonnes per year of waste, more preferably 5,000 to 10,000 tonnes per year of waste and even more preferably 6,000 to 8,000 tonnes per year of waste. 
     Preferably the apparatus is sized and configured to produce from 1.8 to 10 MW, more preferably 1.8 to 5 MW and even more preferably 2.1 to 4 MW. For example, it may be that the process produces from 2.5 to 3.5 MW of electricity. 
     Preferably the pyrolysis reactor is a fluidised bed reactor. 
     Preferably the pyrolysis reactor is configured to contain a fluidised bed of particles and the apparatus comprises a distributor for feeding a fluidisation medium into the reactor, wherein the distributor is configured such that the particles can fall through the distributor and wherein the pyrolysis reactor includes an outlet, such as a rotary valve, through which, in use, a portion of the particles that have fallen through the distributor can be removed, an apparatus for cleaning the particles and an inlet through which the cleaned particles can be fed back into the reactor. The inlet is preferably above the distributor in the fluidised bed reactor. 
     Preferably the distributor comprises an array of ducts with orifices in their surface, wherein the ducts in the array are spaced apart such that, in use, the particles can fall between the ducts. 
     According to a seventh aspect of the invention there is provided an apparatus for treating waste comprising Mixed Plastic Waste, the apparatus comprising:
         a pyrolysis reactor for pyrolysing the waste to produce a fuel;   a monitor for monitoring an attribute of the fuel; and   a controller for adjusting the temperature and/or residence time of the pyrolysis reactor in response to the measured attribute of the fuel so as to maintain the attribute within a desired range.       

     Preferably the attribute is related to the heat of combustion of the fuel. 
     According to an eighth aspect of the invention there is provided an apparatus for treating waste comprising Mixed Plastic Waste, the apparatus comprising:
         a pyrolysis reactor for pyrolysing the waste to produce a fuel, the pyrolysis reactor being a fluidised bed reactor;   wherein the pyrolysis reactor is configured to contain a fluidised bed of particles and wherein the apparatus comprises a distributor for feeding a fluidisation medium into the reactor, wherein the distributor is configured such that the particles can fall through the distributor and wherein the pyrolysis reactor includes an outlet through which, in use, a portion of the particles that have fallen through the distributor can be removed, an apparatus for cleaning the particles and an inlet through which the cleaned particles can be fed back into the reactor.       

     According to a ninth aspect of the invention there is provided an apparatus for treating waste comprising Mixed Plastic Waste, the apparatus comprising:
         a pyrolysis reactor for pyrolysing the waste to produce a fuel, for example a fluidised bed reactor; and   a combustor to combust a product from the pyrolysis reactor to heat a fluid that is fed into the pyrolysis reactor to heat the pyrolysis reactor.       

     It may be that the product is a by-product, for example char. It may be that the product comprises the fuel. It may be that the product comprises a part of the fuel output stream from the pyrolysis reactor that is not used to run the generator. 
     Advantageously the apparatus is portable in that it is constructed in such a way that it can be broken down into a number of modules, each of which is transportable. For example elements of the apparatus may be individually mounted in frames compatible with the ISO standards for freight containers to create modules that can be individually transported using equipment designed for the handling of freight containers. The modules may be made at a factory facility and then shipped to the location in which they are to be used, where they are connected together with other modules to form the apparatus. That may be a cost effective system of providing the apparatus at the location and may reduce the need for skilled labour at the location to install the apparatus. In some examples, the apparatus may be employed at a first location to treat a volume of waste stored at the first location, before being dismantled and transported to a second location to treat a volume of waste stored at that second location. In that way, a number of small waste collection facilities may be serviced on a rotational basis, rather than shipping the waste to a central location. In some examples the apparatus may be mounted in a portable manner, for example on a ship, so that the apparatus can be operated whilst the ship is in motion. Advantageously such an apparatus could be used to treat large islands of plastic waste that accumulate in the oceans. 
     According to a tenth aspect of the invention there is therefore provided an apparatus for treating waste comprising Mixed Plastic Waste, the apparatus comprising:
         a pyrolysis reactor for pyrolysing the waste to produce a fuel, for example a fluidised bed reactor;
 
wherein the pyrolysis reactor is mounted in a frame having fittings that are compatible with the load handling equipment used to transport freight containers.
       

     The frame may be part of a freight container; that is, the reactor may be mounted in a freight container, but preferably the frame is an open frame, in that the module has the shape of a freight container and comprises structural members along the module edges but does not include panelling to close the faces of the module. Such a frame may be lighter and allow better access to the equipment. 
     Preferably the apparatus comprises a generator configured to run on the fuel to produce electricity; wherein the generator is mounted in a frame having fittings that are compatible with the load handling equipment used to transport freight containers. 
     Preferably the apparatus comprises a buffer tank downstream of the pyrolysis reactor and upstream of the generator for storing the fuel prior to using it to run the generator; wherein the buffer tank is mounted in a frame having fittings that are compatible with the load handling equipment used to transport freight containers. 
     The apparatus may comprise further elements, for example: filters, dryers or storage tanks with blending systems, as described above in relation to aspects one to nine of the invention. Some or all of those elements may also be mounted in frames having fittings that are compatible with the load handling equipment used to transport freight containers. It may be that, in some frames, more than one element is mounted. Such an arrangement may be advantageous in reducing the number of frames that need transporting. It may be that each element is mounted in a separate frame. Such an arrangement may be advantageous in allowing exchangeability of components. 
     According to an eleventh aspect of the invention there is provided a recycling plant for recycling waste material, the plant comprising an apparatus for treating waste comprising Mixed Plastic Waste according to any of the above aspects of the invention. Preferably the recycling plant is sized to process from 5,000 to 100,000 tonnes of waste material per year. As such the recycling plant may be an existing facility serving a city or a group of towns and the apparatus may be installed at the plant to provide a process for treating Mixed Plastic Waste, which otherwise would have been sent away from the plant to be treated or landfilled elsewhere. 
     It will of course be appreciated that features described in relation to one aspect of the present invention may be incorporated into other aspects of the present invention. For example, the apparatus of the invention may incorporate any of the features described with reference to the process of the invention and vice versa. Similarly any process or apparatus aspect of the invention may incorporate any of the features described with reference to other process or apparatus aspects of the invention. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
       Embodiments of the present invention will now be described by way of example only with reference to the accompanying schematic drawings of which: 
         FIG. 1  is a schematic view of an apparatus according to a first embodiment of the invention; 
         FIG. 2  is a schematic view of a distributor in the pyrolysis reactor of  FIG. 1 ; and 
         FIG. 3  is a schematic view of a control system for the apparatus of  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION 
     In  FIG. 1  an apparatus  1  for treating waste comprising Mixed Plastic Waste has a loading conveyer  3 . The loading conveyer  3  feeds a de-water press  5 , the outlet of which is arranged above a conveyer  7  to a shredder  9 . The outlet of the shredder  9  is directed at a conveyer  11  to a filter  13 , which includes a ferrous and non-ferrous filter. The outlet of the filter  13  is connected to a dryer  15  and the outlet of the dryer  15  is connected to a storage tank  17 . The storage tank  17  is fed by conveyer  19  and comprises a blending system. The outlet of storage tank  17  is connected via line  21  to the inlet of a fluidised bed pyrolysis reactor  23 . The reactor  23  contains sand, which forms the fluidised bed. The bottom of the reactor  23  has a valve connected, via line  25 , to a cleaner  27 , which in turn is connected, via line  29  to a hopper  31 . The hopper  31  feeds into the reactor  23 . The outlet from the top of the reactor  23  is connected to hot gas filter  33 , the outlet of which is connected, via line  35 , to condenser  37 . 
     From line  35 , a branch  39  is connected, via pump  41 , to heat exchanger  43 . From heat exchanger  43  a line  45  feeds into reactor  23 . Fuel supply line  49  runs from line  35  to burner  53 . Fuel supply line  47  runs from the top of condenser  37  to burner  51 . Burners  51  and  53  are mounted on heat exchanger  43 . 
     Condenser  37  is on top of, and feeds into, buffer tank  55 . An outlet  57  from the buffer tank  55  is connected, via pump  59 , to engine  61 , which is attached to generator  63 . Engine  61  and generator  63  together form a generator to produce electricity and heat. Cooling water loop  65  runs through engine  61  and heat exchanger  67 . Water line  69  passes through heat exchanger  67  and on to another part of the plant. The exhaust  71  from the engine  61  passes through heat exchanger  73  and then filter  75  before being exhausted to the atmosphere. Air line  77  passes through heat exchanger  73  and on to another part of the plant. 
     In  FIG. 2 , the lower part of the reactor  23  has a distributor  79 . The distributor  79  is at the bottom of the fluidised bed  85 . The distributor  79  comprises an array of ducts  81   a - e  with orifices  89   a - e  in their upper surface. The orifices comprise nozzles  83   a - e , which sit on top of the orifices. The ducts  81   a - e  are spaced apart such that the gaps between the ducts are large enough for particles from the fluidised bed  85  to fall through. At the bottom of the reactor  23  there is a valve  87  leading to line  25 . 
     In  FIG. 3  a solvent monitor  91  is mounted on line  35 . The solvent monitor  91  is in communication with reactor management system  93 . Temperature monitor  99  is in reactor  23  and is also in communication with reactor management system  93 . Reactor management system  93  is in communication with gas burner control  95 , which controls burners  51  and  53 , and material feed control  97 , which controls the feed rate from line  21  into reactor  23 . 
     In use, 7000 tonnes per year of waste, in this embodiment Mixed Plastic Waste, is loaded continuously onto the loading conveyer  3 . The waste travels up the loading conveyer  3  and drops into the de-water press  5 , where the pressing action forces water out of the waste. The dried waste, which has a moisture content of about 15% by weight exits the de-water press  5  and is conveyed along conveyer  7  and into the shredder  9 , where it is shredded. The shredded waste exits shredder  9  and travels along conveyer  11  to filter  13 . Filter  13  comprises ferrous and non-ferrous filters and removes metallic contaminants from the waste. The de-watered, shredded and filtered waste then passes into dryer  15 , where the water content is reduced to about 2-3 wt %. The dryer  15  is powered by heat from hot water line  69  or hot air line  77 , or both. On exiting the dryer  15 , the dry, shredded, filtered waste is stored in storage tank  17 . Whilst in the storage tank  17 , the waste is constantly blended by withdrawing a portion of the waste from the bottom of one end of storage tank  17  and recirculating it to conveyer  19  to be redistributed across the top of storage tank  17 . With waste also being continuously added to and withdrawn from storage tank  17  to feed the process, the effect of the blending recirculation is to smooth variations in the composition of the waste over time. 
     Waste is withdrawn from the storage tank  17  along line  21  and fed to the fluidised bed pyrolysis reactor  23 . On entering the reactor  23 , the waste is heated to around 400 to 600° C. The heating is achieved by feeding a hot stream into the reactor  23  along line  45 . That hot stream comprises pyrolysis product drawn from line  35 , along line  39 , and heated indirectly by combustion of a portion of the pyrolysis product, which is also drawn from line  35 , along line  47  or  49 . The portion of the pyrolysis product combusted is normally drawn along line  47  from the top of the condenser  37  and comprises the gas fraction of the fuel output stream  35  from the pyrolysis reactor  23  that does not condense in the condenser  37 . When extra fuel is required, it is drawn directly from the fuel output stream in line  35  along line  49 . In that case, some of the fuel product that would in normal circumstances be used to run the engine  61  is being used to heat the pyrolysis reactor  23 . 
     The heated waste undergoes a pyrolysis reaction that decreases the hydrocarbon chain lengths to around C 5  to C 100 . The process is carried out in a fluidised bed  85  of sand, which results in good mixing and even temperature across the reactor  23 . The sand becomes contaminated with by-products over time. To prevent excessive build-up, a portion of the sand is continuously withdrawn from the bottom of reactor  23  along line  25  and cleaned in cleaner  27 . The cleaned sand is reheated and fed back into the reactor  23  via line  29  and hopper  31 . 
     The products of the pyrolysis reaction exit the top of the reactor  23  and pass through hot gas filter  33 . The filter  33  removes chemical contaminants such as chlorides (resulting from PVC in the Mixed Plastic Waste) and sulphates, resulting in a clean fuel gas which flows along line  35  and is condensed into buffer tank  55  by condenser  37 . 
     The quality of the fuel in line  35  is monitored continuously by solvent monitor  91 . Solvent monitor  91  measures a flame temperature resulting from burning a sample of the fuel in a hydrogen flame. The temperature of the flame can be related to the heat of combustion of the fuel. Solvent monitor  91  communicates the flame temperature to reactor management system  93  by means of an electronic signal from a thermocouple in solvent monitor  91 . Reactor management system  93  also receives a signal from temperature monitor  99  in the pyrolysis reactor  23 . Reactor management system  93  responds to changes in the flame temperature of solvent monitor  91  by adjusting the operation of burners  51  and  53  and the feed rate from line  21  into pyrolysis reactor  23  by means of gas burner control  95  and material feed control  97 . In that way, reactor management system  93  can adjust the temperature and/or the residence time of the pyrolysis reactor  23 . Thus, if solvent monitor  91  detects that the flame temperature is falling, indicating that the quality of the fuel is falling, the reactor management system  93  increases the temperature in the reactor  23 , or increases the residence time in the reactor  23 , or both. The average chain length of the fuel in the output from the reactor  23  should then decrease and the quality of the fuel increase. Conversely, if the solvent monitor  91  indicates that the heat of combustion of the fuel is rising, which could cause problems in engine  61 , the reactor management system  93  can reduce the temperature or residence time or both of the reactor  23  so as to decrease the pyrolysis of the waste and maintain the flame temperature of the solvent monitor  91  within its desired range. 
     The level of fuel in buffer tank  55  can be allowed to increase and decrease. Thus the reactor  23  can be run continuously at a constant steady state, but the fuel can be used in a discontinuous way or at a varying rate. At night, when the demand for electricity is low, the level of fuel in the buffer tank  55  rises, whilst at times of peak demand the level of fuel in the buffer tank  55  can be allowed to decrease. 
     The fuel in buffer tank  55  is continuously recirculated so as to mix the fuel and smooth temporal variations in the quality of the fuel entering the tank  55  from condenser  37 . The recirculation also helps to smooth spikes in contaminant concentrations that could otherwise lead to undesirable short term emissions levels. 
     The fuel from buffer tank  55  is used to run engine  61 , which is connected to generator  63 . Together, engine  61  and generator  63  form a generator that is run on the fuel to produce electricity. Engine  61  is a marine diesel engine designed to run on bunker fuel and the temperature in reactor  23  is controlled by monitoring the fuel entering condenser  37  so as to achieve the correct fuel specifications for engine  61 . By combining the fuel generation process with the electricity generation in a single process, the fuel specification can be relaxed. Engine  61  can be selected based in part on its ability to handle fuel of varying specification with the result that the acceptable specification for the fuel in buffer tank  55  can be broader than if the fuel was to be sold as commercial fuel. 
     The engine  61  requires cooling and generates hot exhaust gases. The heat from those streams can be captured and used elsewhere in the host facility. In this embodiment, the heat is used in dryer  15  and also in other processes in the host facility. Thus the process provides combined heat and power to the facility. Cooling water circulates through the engine in cooling water line  65 . The cooling water cools the engine  61 , and is heated in that process. The cooling water then passes to heat exchanger  67 , where it is cooled by indirect contact with cool water entering heat exchanger  67  along line  69 . The cooled cooling water exits heat exchanger  67  and returns to the engine  61  to repeat the cycle. The water heated in the heat exchanger  67  exits along line  69  and is used to provide heat to the dryer  15  and also to other processes in the host facility. The hot exhaust from the engine  61  is cooled by indirect contact with an air stream in heat exchanger  73 . The exhaust gases exit the engine  61  along exhaust line  71  and pass through the heat exchanger  73 . Air stream  77  also passes through heat exchanger  73  and heat is passed from the exhaust to the air stream. The exhaust gases then continue along exhaust line  71 , through filter  75  to remove contaminants and particulates and are vented to the atmosphere. The air stream  77  that has been heated in heat exchanger  73  is directed to the dryer  15 , where the heat in the stream is used to dry the incoming waste, and also to other processes in the host facility that use heat. 
     Whilst the present invention has been described and illustrated with reference to particular embodiments, it will be appreciated by those of ordinary skill in the art that the invention lends itself to many different variations not specifically illustrated herein. By way of example only, certain possible variations will now be described. 
     Engine  61  and generator  63  may be replaced by other generator systems. For example, in some embodiments a turbine may be used. In some embodiments two generators are provided. A small generator runs continuously to provide a base level of electrical power to run the remainder of the recycling facility in which the apparatus is installed. A large generator is turned on when the national electricity grid requires short-term supplies of electricity. The large generator is selected so as to have a quick start-up cycle so as to benefit from the higher price that national grids are willing to pay for electrical generating capacity that is available at short notice. 
     In some embodiments the de-watering press  5 , shredder  9  and filter  13  are provided in a different order. In some embodiments the waste may be shredded first and then de-watered and filtered. In other embodiments the three steps may be performed in other orders. 
     In some embodiments the de-watering press  5  is replaced by another de-watering system such as a de-watering centrifuge. 
     In some embodiments, the fuel gas that is heated to be fed back into the reactor  23  via line  45  in order to heat the incoming waste is drawn from upstream of the hot gas filter  33 . The volume of gas passing through the hot gas filter  33  in such embodiments is reduced as a result. 
     In some embodiments the waste fed to the process comprises Mixed Plastic Waste, but also comprises organic material. In some embodiments the waste is Municipal Solid Waste. 
     In some embodiments the fuel burners  51  and  53  are replaced or supplemented by burners designed to burn the char separated from the fluidised bed sand in cleaner  27 . In that way the char by-product of the pyrolysis process is used to heat the reactor  23 . 
     In some embodiments the buffer tank is a plurality of intermodal “20 ft” tank containers. In an example process, 1 tonne (1000 kg) per hour of waste may be fed to the process and 850 kg per hour of fuel produced by the reactor. A single “20 ft” tank container provides enough storage for around 24 hours of operations with the generator off, for example if the generator is undergoing maintenance. 
     In another example embodiment the fluidised bed reactor has a 1.5 m diameter and a 1:1 aspect ratio (diameter to height ratio). The reactor contains 3 tonnes of sand. 
     Where in the foregoing description, integers or elements are mentioned which have known, obvious or foreseeable equivalents, then such equivalents are herein incorporated as if individually set forth. Reference should be made to the claims for determining the true scope of the present invention, which should be construed so as to encompass any such equivalents. It will also be appreciated by the reader that integers or features of the invention that are described as preferable, advantageous, convenient or the like are optional and do not limit the scope of the independent claims. Moreover, it is to be understood that such optional integers or features, whilst of possible benefit in some embodiments of the invention, may not be desirable, and may therefore be absent, in other embodiments.