Process for the destruction of organic waste material

Abstract: A system for the destruction of organic waste material such as halogenated organic compounds, e.g., PCB's and related waste organic matter, comprises subjecting the waste material to reduction with a gaseous reducing agent, preferably hydrogen, at a temperature above about 600°C and thereafter subjecting the hot reaction mixture thus obtained to oxidation with a gaseous oxidizing agent at a temperature above about 1000°C. The combined reduction followed by immediate oxidation is effective in achieving substantially complete breakdown of the halogenated compounds to substantially non-noxious gases. The system includes a reduction vessel wherein hydrogen is used directly in intimate mixing relation with the pulverized waste material, and after completion of a residence time sufficient to reduce the organic compounds, the gaseous by-products pass to an adjacent combustor for oxidation therein.

Description: This invention relates to a process for the destruction of organic waste material and more particularly it relates to a process for the destruction of organic waste material which may or may not contain polyhalogenated waste material, and to apparatus for carrying out the process. It is known to destroy halogenated organic waste material by reduction procedures using, for example, sodium metal or sodium napthalide. It is also known to destroy halogenated organic waste material by oxidation, using, for example, high temperature incineration. These known processes have certain limitations or disadvantages in that they can only be used for particular kinds of waste material. Moreover, the chemical reagents used for the destruction are sometimes hazardous to handle and the destruction may lead to the formation of highly toxic by-products. The dimensions of the problem relating to the disposal of polyhalogenated liquids, particularly polychlorinated biphenyls (known generally, and to the public as PCB's) and associated wastes has been reviewed in the Environment Canada Economic and Technical Review Report EPS 3-EC-83-1. Bearing in mind the suspected carcinogenic nature of PCB's and the apparent absence of degradation in nature, over extremely long time periods, the identified quantities of PCB's presently in use in electrical transformers and capacitors (Table 7 of the Report), comprising a mass value of 14.8 million kilograms, conveys some idea of the scale of the problem. Furthermore, the accumulation of these materials in numerous localities, mostly remote from the existing large-scale incinerators available in the United States and Canada for disposal purposes further emphasizes the extreme nature of the problem. Public awareness to the potential danger of PCB's to public health also further complicates the situation in precluding transportation thereof to existing combustion facilities. The occurrence of certain, well-publicized, PCB spills has further exacerbated this aspect of the problem. The existing method of disposing of these types of substance has been by incineration, in large installations. While claims have been made to the achievement of very high percentile effectiveness of disposal by this method, there is concern that the reports may prove less than valid, on a long-term, continuing basis. In addition incineration or oxidation will form highly toxic by-products such as chlorinated dioxins if operated outside of exact optimal temperatures and residence time requirements. One identified problem, referred to at Page 28 of the above-noted Report, which contributed to the demise of the related PCB incinerator, was the formation of a ring of "agglomerated material" during incineration. This type of "glop" formation may well be a characteristic by-product of incineration systems, and possibly is indicative of partial recombinations of molecules into ring compounds of a suspect type. This constitutes a further reason for providing a changed process. Pyrolysis or starved air thermal destruction or degradation of solid and liquid organic waste products of hydrocarbons has previously been hampered by the formation of tars and polynuclear aromatic hydrocarbons containing from 1 to 5 aromatic rings. WO-A-8202001 discloses a process for the destruction of vaporised heavy halogenated wastes such as polychlorinated biphenyls, which comprises mixing the waste with hydrogen, and oxidising the mixture with oxygen at a temperature of at least 2000°C. An article entitled "Thermal Hydro-Dechlorination of (Poly)chlorinated Organic Compounds", published in Chemistry and Industry , vol 102, no. 19, 3rd October 1983, pages 259 to 760, Letchworth, Herts, Great Britain, by R. Louw et al., describes the substitution of H for Cl in a number of chlorinated benzenes, alkenes and alkanes by reaction with hydrogen. Under the conditions used, the major organic products derived from starting chlorobenzenes were the corresponding dechlorinated benzenes and benzene. Biaryls arose in 1 to 2 mol per cent proportions on HCl only, together with trace amounts of naphthalene and biphenylene. GB-A-2159137 describes a process for the destruction of chlorinated organic compounds by pyrolysis at least 825°C in a reducing atmosphere, e.g. comprising hydrogen. Apparatus is disclosed which comprises a reducing vessel comprising heating means, a gas inlet, means for raising the temperature within the vessel, waste material feed means, and a nozzle via which a reducing gas can be directed into a zone for mixing with the waste material. The obtained reduced materials may be used as fuel. They are said to contain small amounts of carbon and higher aromatics. DE-A1551838 describes a two-stage process for incinerating industrial waste. The waste is collected in batches, preheated and homogenised in a reducing atmosphere. Apparatus for this purpose comprises a mixing drum having a gas inlet and heating means, for the maintenance of a reducing atmosphere. The mixing chamber is connected to a combustor vessel via an outlet for the mixed waste. EP-A-0 175 406 discloses a process for the destruction of waste material in which said material is heated with an excess of hydrogen/hydrogen donor to a temperature in the range 700-1220°C, followed by quenching the gaseous effluent of the reaction and separation into a hydrocarbon - and hydrogen-containing phase and a phase containing hydrogen halides and/or nitrogen-, sulphur- and/or oxygen-containing compounds. Soot is formed in the process, necessitating interruption of the process for its removal. Fearing of the hydrocarbon and hydrogen containing phase is generally mentioned. SUMMARY OF THE INVENTION We have now found that organic waste material can be destroyed efficiently and completely by use of a process which combines a gas-phase chemical reduction in a reducing atmosphere at a high temperature, followed by oxidation of the hot reaction mixture from the chemical reduction in a high-temperature incinerator. According to the present invention, a process for the destruction of organic waste material, comprises intimately mixing the waste material with a gaseous reducing agent; subjecting the waste in the mixture to reduction at a temperature of at least 600°C in an isolated oxygen-free first zone using sufficient of the reducing agent to saturate or reduce the molecules produced such that aromatic and polyaromatic structures are eliminated as by-products; and, in a second zone, subjecting the hot reaction mixture thus obtained to oxidation with a gaseous oxidizing agent at a temperature of at least 1000°C. Further, according to the present invention apparatus suitable for carrying out the above process is defined in claim 14. DESCRIPTION OF THE INVENTION In a preferred aspect of the invention, the waste material is subjected to reduction at a temperature of 700 to 900°C with a residence time of 5 to 40 seconds, and the hot reaction mixture is subjected to oxidation at a temperature of 1000 to 1400°C over a residence time of 1 to 4 seconds. The process of this invention may, for example, be conveniently carried out by the direct injection of organic waste material, for example, halogenated organic waste material in the form of a liquid, a pumpable sludge, a fine particulate slurry such as contaminated sediment/water mixtures, or a pulverised solid, upwardly into a pressurised reaction vessel, in the absence of oxygen. The reaction vessel is heated and maintained at temperatures above about 600°C, preferably at a temperature of from about 700°C to about 900°C. The waste may be co-injected with a gaseous reducing agent such as hydrogen, gaseous ammonia, natural gas, methane, propane or water vapour, or a mixture of such reducing agents. The reduction may be carried out with or without the addition of a metal catalyst such as iron, zinc, tin or nickel in the form of iron filings or powdered zinc, tin or nickel which may be co-injected into the vessel to promote the reduction reaction. However, the preferred process embodiment comprises the steps of co-injecting (on a continuous basis) the waste, with hydrogen, into a preheated reduction vessel, maintaining the vessel at an internal pressure above atmospheric, up to one atmosphere above ambient, without the addition of catalyst materials. The reaction vessel can be arranged and the rate of injection may be adjusted, such that the residence time of gaseous material in the reduction vessel is greater than about 5 seconds and preferably from about 15 seconds to about 45 seconds. A particularly effective residence time in which to effectively complete the reduction is about 30 seconds. The reduction may be carried out at a temperature of from about 600°C to about 1100°C, preferably within the range of from about 700°C to about 900°C and particularly at a temperature of from about 800°C to about 900°C. The reduction may also be carried out in the presence of a metal, such as an iron, nickel, zinc or tin catalyst. The catalyst may be in the form of iron filings, powdered nickel, powdered zinc or powdered tin. The organic waste material may or may not contain organic compounds such as halogenated biphenyls, halogenated benzenes, halogenated phenols, halogenated cycloalkanes, halogenated alkanes, halogenated dioxins and halogenated dibenzofurans. For example, the organic waste material may contain commonly used chlorinated organic compounds such as chlorinated biphenyls, also known as polychlorinated biphenyls (PCB's), chlorinated benzenes, chlorinated phenols, chlorinated cycloalkanes, chlorinated alkanes, chlorinated dioxins and chlorinated dibenzofurans. The organic waste material may be in the form of a liquid, a pumpable sludge, a fine particulate slurry such as contaminated sediment/water mixtures, or a pulverized or shredded solid such as contaminated wood waste or soils. Such waste material may include, for example, oils containing polychlorinated biphenyls (PCB's) as waste products from capacitor and transformer manufacturing processes, or from obsolete electrical or non-electrical equipment, and products used in various industries as plasticizers, hydraulics fluids and lubricants. The organic waste material may also be such that it contains non-halogenated organic compounds. It may, for example, be in the form of shredded or particulate organic solid material, such as shredded pathogenic waste material. The reaction vessel for the reduction stage of the process is lined with suitable chemical and thermal resistant materials to withstand gaseous by-products generated, such as hydrogen halides, for example, hydrogen chloride. It also is fitted with a clean-out auger to remove solid debris or by-products, such as metals, metal salts, silicates or other solid matter, which accumulates in the vessel. While gaseous reducing agents such as gaseous ammonia are less costly and also potentially less explosive than hydrogen, methane or propane, certain advantages are obtained with these latter reducing agents. However, while the use of gaseous hydrogen is preferred for a number of reasons, it is contemplated that propane may be used if the BTU content of the waste is potentially too low for proper oxidation to take place. However, the use of a reducing vessel and selection of gaseous hydrogen as the reducing agent has many beneficial and optimizing effects. Thus, in the case of one embodiment, a road-mobile system for the on-site destruction of accumulated PCB's, the utilization of hydrogen minimizes the necessary size of the reducing vessel for carrying out a continuous process and the potential for production of carbon. Reduction of PCBs or hazardous waste to gaseous fuel further reduces the necessity of additional fuel and the additional required combustion air for the fuel, therefore greatly reducing the size of the secondary incinerator and overall destruction apparatus. The safety requirements for the reduction vessel involve the use of a multiple purge by inert gas, such as nitrogen, to ensure the absence of oxygen (by way of air) within the vessel, so as to preclude the likelihood of explosion. As the process is generally proposed to be a continuous process, the purging requirement becomes proportionately less onerous. The reduction zone is maintained at a pressure sufficiently higher than that within the combustor as to warrant no flow-back of oxidizing agents, including air, from the combustor, into the reduction vessel. Furthermore, the arrangement of the reduction zone has the mixing nozzles, wherein the pressurized reducing gas mixes intimately with and atomizes the incoming waste by high velocity impact therewith, located at a low level within the zone, and a gaseous residence zone containing reduced gaseous products located thereabove, so as to isolate the reduction zone from the outlet to the oxidation zone. This interface zone between reduction and oxidation may include a ceramic firestop to prevent flash backs. The hot reduced gas is introduced into the oxidation zone through a combustion mantle, combustion nozzles or suitable apparatus to allow adequate mixing with the combustion air introduced into the oxidation zone and to allow optimal positioning of the flame front within the oxidation chamber. A further advantage of utilizing hydrogen, generally in gaseous form, is the capability of utilizing a jet or jets of hydrogen in impacting relation with the substances being reduced, so as to achieve a highly active mixing zone wherein the as-supplied state of the hydrogen is utilized in order to optimize the mixing efficiency, and the reduction process. In the case of fluid waste, a jet of the waste can be impacted by a transversely directed jet of hydrogen, to effect atomization with intimate mixing, to promote the chemical effectiveness of the reduction process. One embodiment incorporates the use of a radially inward gas cross-flow nozzle of the CALDYN (TM) type. The subject system is capable of handling fluid waste incorporating particles up to 6 mm (1/4 inch) mesh size therein, and droplets sized down to as small as 40 µm can be economically obtained. It is to be understood that the reduction may also be carried out in the presence of water vapour which does not inhibit the reduction reaction. Thus, it is possible to destroy organic waste material, sludges or sediments, such as contaminated harbour sludges or sediments, containing substantial quantities of water. The hot reaction mixture from this reduction procedure will generally be dehalogenated, hydrogenated or reduced hydrocarbons or substantially dehalogenated hydrocarbons together with hydrogen halide, such as hydrogen chloride, water and excess hydrogen. The reaction vessel to be used for the reduction in a preferred embodiment is vertically interfaced with a second vessel to be used for the second, oxidation phase. The hot reaction mixture from the reduction stage, at a temperature of from about 600°C to about 1000°C, and more particularly from about 800°C to about 900°C, may be forced through a short insulated ceramic or refractory lined tube by convection and the pressure created as a result of evaporation and volatilization of the injected liquid or partially liquid waste together with continuous expansion of the gases as the reduction breakdown occurs. Excess air or oxygen can be introduced into the second vessel in such a way as to create a turbulent flow of hot gases of the hot reaction mixture together with oxygen which will promote the complete combustion of those gases at a temperature of above about 1000°C. Particularly useful temperatures are those within the range of about 1000°C to about 1500°C and especially a temperature of from about 1200°C to about 1400°C. The size of the second vessel for the oxidation procedure may be such that the retention or residence time of the hot reaction mixture in the combustion chamber will be from about 1 second to about 4 seconds, preferably for a residence time of about 2 seconds, or more. The combustion chamber of the second vessel may also be lined with suitable material to withstand the hot acidic gases, such as hydrogen chloride, which will pass through it. The hot emission from this second vessel is then rapidly cooled and scrubbed with water and aqueous alkali such as sodium hydroxide mist or sodium carbonate in order to remove and neutralize the acidic gases. In carrying out the present invention at an effective production level, it is necessary to provide a reduction chamber suitably preheated to a reaction sustaining temperature. In view of the advantages that obtain by use of hydrogen, as previously set forth, and the imperative need to provide an effective gaseous purge, the preheating of the subject vessel is preferred by way of passive heating means, such as electrical heating elements, as opposed to active heating methods, such as gas combustion. Owing to the highly active chemicals generally evolved from the subject process, the use of protective, chemically resistant vessel linings is important. This requirement and the use of radiant heat are not incompatible. The use of steam or superheated steam as both a purge gas and as the pre-heating agent, is contemplated. In one embodiment, the combustion chamber is superposed over the reduction vessel, to receive the gases therefrom in upward flowing relation, through a central passage, equipped with a ceramic fire stop to prevent flash back. The hot reduced gas is introduced into the oxidation zone through a combustion mantle, combustion nozzles or other suitable apparatus to promote mixing with the combustion air being admitted to the oxidation zone, and provide optimal positioning of the flame front within the oxidation chamber. The system may provide flue gas treatment means to handle the highly acidic flue gas by way of an alkaline scrub down, prior to release to the atmosphere. A control system, not forming a part of the present invention, provides an automated control, in order to monitor and maintain appropriate feed rates of the process constituents, and safe and suitable environmental conditions for the respective stages of the process. BRIEF DESCRIPTION OF THE DRAWINGS Certain embodiments of the invention are described, reference being made to the accompanying drawings, wherein; Figure 1 is a schematic elevation, in diametrical section, of a reduction chamber; combined with a combustor, shown partly sectioned; Figure 2 is a scrap view, in diametrical section, of an atomizing nozzle portion of the reduction apparatus; Figure 3 is a diagramatic arrangement of a plant for carrying out the process; and Figure 4 is an impression of a mobile plant embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to Figure 1, the reduction vessel 10 for carrying out the subject process has a metal shell 12 and is substantially free-standing, having a combustion chamber 14 mounted thereon. The reduction vessel 10 has one or more inlet nozzles 20 (see Fig. 2) for injecting waste, including pulverized solids, for atomization of the liquid portion thereof by jets of hydrogen gas through an annulus of nozzles 27. The vessel 10 has a bank of radiant electrical heaters 22 of known commercial type such as Carborundum silicon carbide glow bar heaters lining the walls thereof. Chemically resistant FIBERFRAX (TM) ceramic thermal insulation 24 protects the shell 12, while also ensuring a safe working thermal environment in the locality of the vessel 10. A bottom portion 26 of shell 12 includes an auger 28, and a sealed outlet 30 whereby cleaning out of solid inorganic residues can be effected. A passage 32 connects vessel 10 with vessel 14 for upward passage therethrough of the reduced gaseous products. The passage 32 is positioned and sized to allow the pressure within vessel 10 to be controlled, in order to ensure a safe, positive pressure differential between reduction chamber 10 and combustion chamber 14. A blow-out panel (not shown) safeguards the vessel 10 against explosive over pressure. An air supply nozzle and combustion grid combination 31 ensures safe and stable combustion within chamber 14. Referring to Figure 2, the nozzles 20, positioned as shown in Figure 1, each have a liquid inlet 21 and a gaseous inlet 23. The gaseous inlet 23 connects, by way of control valves 201, 203 with respective pressurized nitrogen and pressurized hydrogen supplies (not shown), for use in initially purging the combined vessel 10/14, and for subsequently operating the chemical reduction process, respectively. The exhaust opening 36 of chamber 14 connects with an acidic gas scrubber and centrifugal fan or particulate removal apparatus 61, particularly in view of the hydrogen chloride content of the exhaust gases. Referring to Figure 3, the system 40 is shown schematically, with reduction chamber 10 supporting combustion chamber 14. Feed of waste-with-liquid or liquids such as PCB or PCB-containing sludge is fed by way of a controllable feed valve 50 to the inlets 21 of the nozzles 20 in chamber 10. The reduced gases pass by way of passage 32 to combustion chamber 14. Variable air inlet controls 54 permit regulation of combustion chamber 14, operating substantially at atmospheric pressure. Exhaust gases leave by way of passage 56, passing through a scrubber system 60 centrifugal fan particulate removal apparatus 61, and cooling showers, shown schematically at 62, to leave by stack 65, to atmosphere. The subject system includes neutralizing water tank 67, cooling water tank 69, and associated pumps and controls. The overall system does not preclude other types of scrubbers. Figure 4 is an artist's impression of the system as set up as a low-bed trailer 90, having a hydraulic actuator 92 for positioning the composite reduction/combustion vessel 10/14. It is contemplated that steam from a steam generator may be utilized, both as the purging agent for the combined vessel 10/14, and as the pre-heating agent for the reduction and combustor chambers 10/14. Process control instrumentation located in ancilliary trailer 95 provides automated control and back-up by way of instrumentation and controls (not shown). In order to improve the economics of the operation it is contemplated to pass the high temperature exhaust gases through a suitable gas turbine power section (i.e. without combustors), connected in driving relation with an alternator, to generate useful electrical power, for sale to a local utility, and also to supply the power needs of the process, once start-up has been effected.

Claims: A process for the destruction of organic waste material, which comprises intimately mixing the waste material with a gaseous reducing agent; subjecting the waste in the mixture to reduction at a temperature of at least 600°C in an isolated oxygen-free first zone, using sufficient of the reducing agent to saturate or reduce the molecules produced such that aromatic and polyaromatic structures are eliminated as by-products; and, in a second zone, subjecting the hot gaseous reaction mixture thus obtained to oxidation with a gaseous oxidizing agent at a temperature of at least 1000°C. A process of claim 1, wherein the reducing agent is one or more of hydrogen, ammonia, natural gas, methane, propane and water vapour. A process of claim 1 or claim 2, wherein the reduction is conducted in a zone which has been purged of oxygen-containing gases. A process of any preceding claim, wherein the reduction is carried out at a temperature of 600 to 1100°C. A process of any preceding claim, wherein the waste material is subjected to reduction over a residence time greater than 5 seconds. A process of any preceding claim, wherein the reduction is carried out in the presence of a medal catalyst, selected from iron, nickel, zinc and tin. A process of any preceding claim, wherein the hot reaction mixture is at a temperature of 600 to 1000°C, prior to oxidation. A process of any preceding claim, wherein the oxidizing agent is oxygen. A process of any preceding claim, wherein the oxidation is carried out at a temperature of 1000 to 1500°C. A process of any preceding claim, wherein the hot reaction mixture is subjected to oxidation over a residence time of 1 to 4 seconds. A process of any preceding claim, wherein the waste material contains organic compounds selected from halogenated biphenyls, halogenated benzenes, halogenated phenols, halogenated cycloalkanes, halogenated alkanes, halogenated dioxins and halogenated dibenzofurans. A process of claim 11, wherein the organic compounds are selected from chlorinated biphenyls, chlorinated benzenes, chlorinated phenols, chlorinated cycloalkanes, chlorinated alkanes, chlorinated dioxins and chlorinated dibenzofurans. A process of any preceding claim, wherein the waste material is subjected to reduction at a temperature of 700 to 900°C over a residence time of 5 to 40 seconds, and the hot reaction mixture is subjected to oxidation at a temperature of 1000 to 1400°C over a residence time of 1 to 4 seconds. Apparatus for the destruction of organic waste material, which comprises a reducing vessel (10); a purge gas inlet (20); means (22) for raising the temperature within the vessel; an outlet (32) for combustible gaseous components; means (21) for feeding the waste material in liquid and/or particulate form into a localised mixing zone within the reducing vessel, and a nozzle (27), via which a reducing gas is directed into the mixing zone; a combustor vessel (14) connected to the outlet of the reducing vessel; an air or oxygen inlet into the combustion vessel; and a conduit along which hot reduced gaseous products can be transferred unidirectionally from the reducing vessel to the combustor vessel, for exothermic combustion therein, wherein the reducing vessel is directly adjacent to and in direct connection with the combustor vessel, optionally there being a ceramic fire stop at an interface between the reducing vessel and the combustor vessel. Apparatus according to claim 14, which additionally comprises scrubber means (60) for receiving fluid combustion products from the combustor vessel. Apparatus according to claim 14 or claim 15, which additionally comprises electrical energy-generating means. Apparatus according to claim 14, 15 or 16, wherein the feed means is adapted to discharge the waste material as a jet within a body portion of the gas admission nozzle, and the nozzle is adapted to discharge the reducing gas in impacting relation with the jetof the waste material.

Inventors: HALLETT DOUGLAS J, HALLETT, DOUGLAS J.
Applicants: ONTARIO INC 655901, 655901 ONTARIO INC.
Application year: 1987
Main classification: A62D3
IPC codes: F23G5, B09B3, F23G7, A62D101, A62D3