DESTRUCTION OF WASTE

Abstract: Apparatus and method for the pyrolysis of waste material which is not itself susceptible to heating by microwave radiation, which method comprises: (a) contacting the waste material, under an atmosphere wherein flame generation is substantially prevented, with a bed of pulverulent carbonaceous material which is susceptible to heating by microwave irradiation; and (b) heating the pulverulent material by means of microwave irradiation such that thermal energy is transferred from the pulverulent material to the waste material so as to cause substantial pyrolysis of the waste material, wherein the waste material is fed to an upper part of the bed of pulverulent material such that the waste material permitted to sink through the bed, and is pyrolysed within the bed. The apparatus comprises: (a) a container (9) which is inert to microwave radiation and capable of retaining the bed of pulverulent material; (b) a reaction chamber (8); (c) a conduit (19) for feeding the waste material to an upper part of the bed of pulverulent material; (d) a microwave generator (10); (e) air locks (12) and (18) for controlling the atmosphere in chamber (8) such that flame generation is prevented in chamber (8); and (f) outlet (20) for removal of gases, evolved on pyrolysis of the waste material, from chamber (8).

Description: Destruction of Waste The present invention is concerned with an apparatus and method for carrying out the pyrolysis of waste. The destruction of waste material produced in the medical field is a substantial problem, because ofthe increasing amount of "one-use" disposable materials (such as swabs, syringes, disposable gloves or the like) being used in this field. The problem clearly further extends to cover the increasing amounts of waste material produced in other fields, for example waste plastics material resulting from automobile bodies and packaging. Such waste material is generally disposed of either by incineration or by use of landfill; the former method is objectionable on environmental grounds because of the nature of some of the combustion gases produced, while the latter method is inefficient because of the large bulk of plastics material. Similarly, the destruction of materials such as polychlorinated biphenyls (PCB's) and non-biodegradable matter screened from sewage is also a substantial problem, as well as CFC's and biochemical waste. PCT application WO88/08871 discloses a method of pyrolysis of plastics waste products by contact with microwave-heated pulverulent material. We have now developed an improved method having several advantages over the method disclosed by WO88/08871 and an apparatus specifically designed to carry out the improved method; the method is suitable for pyrolysis of a wide range of waste materials not susceptible to microwave heating. The present invention comprises a method for the pyrolysis of waste material which is not itself susceptible to heating by microwave radiation, which method comprises the steps of: (a) contacting the waste material, under an atmosphere wherein flame generation is substantially prevented, with a bed of pulverulent material which comprises carbon in elemental form, or a material which is capable of being pyrolysed to elemental carbon by microwave irradiation, which pulverulent material is susceptible to heating by microwave irradiation; and (b) heating the pulverulent material by means of microwave irradiation such that thermal energy is transferred from the pulverulent material to the waste material, the time and intensity of said irradiation being controlled so as to cause substantial pyrolysis of the waste material, wherein the waste material is fed to an upper part of the bed of pulverulent material such that the waste material permitted to sink through the bed, and is pyrolysed within the bed. The microwave radiation is preferably employed at such a power and for sufficient time that the pulverulent material is heated to at least 400°C. When the method according to the invention is carried out in a substantially oxygen-free atmosphere, there would in general only be a danger of production of dioxin or other noxious products with certain feedstocks. In such cases, and also in cases where a small amount of oxygen is present (but without sufficient thereof to cause flame generation), it is preferred to ensure that the pulverulent material (and therefore the plastics or other waste) attains a temperature of at least 800°C, which is sufficiently high that the risk of evolution of dioxins or similar noxious products by the pyrolysed material is minimized or eliminated. If the waste material being pyrolysed in the method according to the invention is relatively easily degraded (e.g. polyethylene or nylon), then temperatures from 400 to 800°C may be satisfactorily employed. When more thermally resistant plastics and halogenated plastics (such as PTFE or PVC) are employed, the temperature of the carbonaceous material may be at least 800°C. The present invention further comprises apparatus suitable for carrying out the microwave pyrolysis of waste material which is not itself susceptible to heating by microwave radiation, which apparatus comprises:- (a) retaining means substantially inert to microwave radiation which is capable of retaining a bed of pulverulent material, and arranged to permit the pulverulent material to overflow from the retaining means; (b) a reaction chamber arranged to surround the retaining means; (c) means for feeding waste material to an upper part of the bed of pulverulent material retained in the retaining means such that, in use, the waste material will sink through the bed; (d) a microwave radiation source, the radiation source and retaining means being relatively positioned such that microwave radiation emitted by the source can cause heating of the pulverulent material and substantial pyrolysis of the waste material as the latter sinks through the bed of pulverulent material; (e) means for controlling the atmosphere in said chamber such that flame generation is substantially prevented in the chamber; and (f) means for removal of gases, evolved on pyrolysis of the waste material, from the chamber. Typically the retaining means comprises an open-topped receptacle which is generally of a material such as stainless steel or a ceramic material which is preferably resistant to temperatures of at least about 800°C or the order of 1000°C. The waste material is fed to an upper part of the bed of pulverulent material in order to maximise contact of the waste material with the pulverulent material as the former sinks through the bed of pulverulent material. The greater the extent of contact between the waste material and the pulverulent material, the more efficient is the energy transfer from the pulverulent material to the waste material and hence enhanced pyrolysis of the waste material can be achieved. The feeding means preferably comprises a conduit arranged to pass through a first opening in an upper section of the chamber wall and to further extend through the interior of the chamber such that a leading end portion of the conduit is located above the open end of the receptacle. Typically part of the conduit rests on an upper edge portion of the receptacle wall. Preferably the feeding means is arranged to be inclined so that the waste material can easily slide along the feeding means into the retaining means. Typically the inclined feeding means is orientated in such a way that the waste material can be fed in substantially continuous manner to the retaining means; in alternative embodiments the waste material may be supplied semi-continuously or intermittently (in a batch process, for example). The use of such an inclined feeding means minimises the need for mechanical feeding means to transfer the waste material to the retaining means. It is preferred to supply the waste material in such a manner that the pulverulent material is allowed to overflow from the receptacle as denser waste material sinks towards the bottom of the receptacle, displacing less dense carbonaceous material upwards as it does so. (The bulk specific gravity of finely divided elemental carbon is for example about 0.5 whereas waste material such as plastics normally has a bulk specific gravity of about 0.8 to 1.0). The pulverulent material overflowing from the static bed is preferably at least partly recycled thereto. The rate of feed depends largely on the power of the microwave discharge used during pyrolysis. Typically the microwave radiation source is arranged remote from the chamber, a passage being arranged to extend from the microwave radiation source through a second opening provided in the chamber wall to a position adjacent the open end of the receptacle. The passage defines a pathway for the microwave radiation from its source to the retaining means. The chamber is preferably provided with first and second outlets. The first outlet is provided through a lower section of the chamber wall and is typically located directly below the retaining means so as to be suitably positioned to receive any pulverulent material overflowing from the retaining means. Typically the first outlet leads to first cooling means for the pulverulent material, such that a substantially solid carbon product can be recovered therefrom. The means for removal of any gaseous products evolved during the pyrolysis generally comprises the second outlet which itself leads to gas cooling means before entering a liquid/gas separator which separates the evolved gas into oil products and waste gases. The second outlet is preferably arranged in an upper section of the chamber so as to be suitably located to collect any hot gases which rise from the waste material located in the retaining means during pyrolysis. The means for controlling the atmosphere such that flame generation is substantially prevented generally comprise first and second air locks, the first air lock being located at a position along the path of the feeding means and the second air lock being positioned downstream of the pulverulent material cooling means. The invention will now be further illustrated, by way of example only, with reference to the accompanying drawing which is a schematic representation of apparatus according to the present invention. Referring to the drawing, waste material and pulverulent carbonaceous material are supplied together to a refractory-lined stainless steel pyrolysis chamber 9, in a reactor 8, via an air lock 18. The material in the chamber is exposed to microwave irradiation from a microwave generator 10. The pulverulent material (including waste material which has been pyrolysed to elemental carbon) overflows from the pyrolysis chamber 9 and passes through a carbon cooler 11 and air lock 12 before being recovered from the process as a solid carbon product 17. Hydrocarbon gases evolved during pyrolysis are passed out of the reactor 8, and through a hydrocarbon cooler 13 before entering a liquid gas separator 14 which separates the waste gas into oil products 15 and (acidic) waste gases 16. The waste gases may then be "scrubbed" before being released to the atmosphere.

Claims: Qlaims: 1. A method for the pyrolysis of waste material which is not itself susceptible to heating by microwave radiation, which method comprises the steps of: (a) contacting the waste material, under an atmosphere wherein flame generation is substantially prevented, with a bed of pulverulent material which comprises carbon in elemental form, or a material which is capable of being pyrolysed to elemental carbon by microwave irradiation, which pulverulent material is susceptible to heating by microwave irradiation; and (b) heating the pulverulent material by means of microwave irradiation such that thermal energy is transferred from the pulverulent material to the waste material, the time and intensity of said irradiation being controlled so as to cause substantial pyrolysis of the waste material, wherein the waste material is fed to an upper part of pulverulent material such that the waste material is permitted to sink through the bed, and is pyrolysed within the bed. 2. A method according to claim 1, wherein said microwave radiation causes the pulverulent material to be heated to at least 400°C. 3. Apparatus suitable for carrying out the microwave pyrolysis of waste material which is not itself susceptible to heating by microwave radiation, which apparatus comprises: - (a) retaining means substantially inert to microwave radiation which is capable of retaining a bed of pulverulent material, and arranged to permit the pulverulent material to overflow from the retaining means; (b) a reaction chamber arranged to surround the retaining means; (c) means for feeding waste material to an upper part of the bed of pulverulent material retained in the retaining means such that, in use, the waste material will sink through the bed; (d) a microwave radiation source, the radiation source and retaining means being relatively positioned such that microwave radiation emitted by the source can cause heating of the pulverulent material and substantial pyrolysis of the waste material as the latter sinks through the bed of pulverulent material; (e) means for controlling the atmosphere in said chamber such that flame generation is substantially prevented in the chamber; and (f) means for removal of gases, evolved on pyrolysis of the waste material, from the chamber. 4. Apparatus according to claim 3, wherein said retaining means comprises an open- topped receptacle of a material which is resistant to temperatures of at least 800°C. 5. Apparatus according to claims 3 or 4, wherein said feeding means comprises a conduit arranged to pass through a first opening in an upper section of the chamber wall and to further extend through the interior of the chamber such that a leading end portion of the conduit is located above the open end of the retaining means. 6. Apparatus according to claim 5, wherein part of the conduit rests on an upper edge portion of a wall of the retaining means. 7. Apparatus according to any of claims 3 to 6, wherein the feeding means is arranged to be inclined so that the waste material can slide along the feeding means into the retaining means. 8. Apparatus according to any of claims 3 to 7, wherein the microwave radiation source is arranged remote from the chamber, with a passage arranged to extend from the microwave radiation source through a second opening provided in the chamber wall to a position adjacent the open end of the receptacle. 9. Apparatus according to any of claims 3 to 8, wherein the chamber is provided with a first outlet provided through a lower section of the chamber wall and located below the retaining means so as to receive pulverulent material overflowing from the retaining means; and wherein said means for removal of gases comprises a second outlet which is arranged in an upper section of the chamber for collection of any hot gases rising from the waste material during pyrolysis. 10. Apparatus according to claim 9, wherein the first outlet leads to first cooling means for the pulverulent material and said second outlet leads to gas cooling means and a liquid/gas separator. 11. Apparatus according to any of claims 3 to 10, wherein said means for controlling the atmosphere in said chamber comprise first and second air locks, the first air lock being located along the path of the feeding means and the second air lock being positioned downstream of the pulverulent material cooling means. 12. A method according to either claim 1 or 2, which is carried out in apparatus according to any of claims 3 to 11.

Inventors: HOLLAND KENNETH MICHAEL, HOLLAND, KENNETH, MICHAEL
Applicants: HOLLAND KENNETH MICHAEL, HOLLAND, KENNETH, MICHAEL
Application year: 1991
Main classification: C10B53
IPC codes: B01J19, B09B3, B29B17, C10B19, C10B51, C10B53