Abstract:
A method and apparatus for the volume reduction and processing of solid organic waste, in particular ion exchange media from nuclear facilities containing sulfur or chlorine. A method and apparatus for the volume reduction of nuclear waste is disclosed comprising: subjecting the waste to pyrolysis in a pyrolysis vessel, and gasifying the solid pyrolysis residue in a steam reformer to remove residual carbon. A further method and apparatus for the processing of nuclear waste is disclosed comprising: grinding the nuclear waste, addition of iron powder, pyrolysis, gasification of the pyrolysis residue, and combustion in a submerged bed heater. An evaporator cools gases from the submerged bed heater. Off gases from the evaporator are treated by a fiber bed scrubber to remove acids.

Description:
RELATED APPLICATIONS 
     This application is a continuation-in-part of U.S. patent application Ser. No. 08/403,758, filed on Mar. 17, 1995, U.S. Pat. No. 5,536,896, entitled &#34;Waste Processing&#34; by Rolf Hesbol and Lars E. Holst and assigned to the assignee of the present invention. 
    
    
     FIELD OF INVENTION 
     The present invention relates to the field of processing organic waste, &#34;processing&#34; in the present case referring to the breaking down of said waste via the thermal route with the primary aim of affording opportunities for reducing its volume to thereby lessen handling and storage problems. More particularly, it concerns a new method and new apparatus for processing solid organic sulphur or chloride containing waste in which the carbon content of solid residue resulting from an initial pyrolysis of the waste is gasified. 
     BACKGROUND TO THE INVENTION 
     The new method of the present invention not only achieves the aim of volume reduction, but also provides, for example, such benefits as the elimination of sulphur or chloride content from exhaust gases, and similarly any radioactive content, in an effective and straight forward manner. The invention is therefore especially useful for the processing of ionic exchange media from nuclear facilities, which media display a certain degree of radioactivity and therefore would otherwise require conventional measures in relation to ultimate waste disposal and deposition. The nuclear industry annually produces a significant amount of waste which is classified as radioactively contaminated ion exchange media. Such waste is managed in various fashions prior to ultimate disposal in bedrock chambers or shallow land burial. This management is technically complex and as a rule leads to increased volumes which influences storage costs. A process resulting in diminished volume at reasonable cost is therefore highly commercially desirable. 
     Ion exchange medium is an organic material. The base is usually a styrene polymer with grafted sulfonic acid and amine groups. The material is therefore burnable, but air is supplied during combustion and sulphur and nitrogen oxides are formed which in turn must be separated in some manner. Additionally, during combustion the temperature becomes sufficiently high for radioactive cesium to be partially vaporized. The residual radioactivity will also accompany the resulting fly ash to some extent. This necessitates a very high performance filter system. Accordingly, both technical and economic problems are typically associated with standard combustion techniques. 
     An alternative to straight combustion is pyrolysis. However, previously known pyrolysis methods in this technical field are deficient in several aspects and in particular no one has earlier succeeded in devising a pyrolysis process which provides a comprehensive solution to the problem of sulphur and nitrogen-containing radioactive waste, and to do so under acceptable economic stipulations. The following can be mentioned as examples of the known technology in this respect: 
     U.S. Pat. No. 5,424,042 to J. Bradley Mason et al. describes a system for vitrification of nuclear waste (incorporating of a portion of the nuclear waste into a stable glass matrix) including several subsystems: a feed conditioning system for conditioning &#34;dry waste,&#34; &#34;wet waste&#34; or ion exchange resins and &#34;liquid waste&#34;; a feed preparation system for blending the waste types; a feed melter chamber with an upper zone and a lower zone for oxidizing the waste into ash and off-gas; a glass handling system for packing and storing the glass product; and an off-gas cleaning and control system. 
     U.S. Pat. No. 5,470,544 describes a system for the detoxification of hazardous waste utilizing a moving bed evaporator and a steam-reforming detoxification reactor. U.S. Pat. No. 5,427,738 also describes a system for the detoxification of solid waste which first mechanically particularized the waste in a spinning knife cutter, size reduction grinder or like device, and then subjects the particularized waste to a gas flow of a hot gas in the range of 250-750° C. The particularized waste is agitated to enhance exposure to the hot gaseous flow. 
     SE-B 8405113-5 which describes single stage pyrolysis in a fluidised bed followed by conversion of tars in the resulting gas to non-condensable gas using limestone as catalyst. 
     U.S. Pat. Nos. 4,628,837, 4,636,335, and 4,654,172 describe pyrolysis of ion exchange resins where the pyrolysis is carried out in two stages. Both of these stages, however, are directed towards pyrolysis of the ion exchange media itself, i.e. the solid product. Speaking generally, both stages moreover are carried out at relatively low temperatures. Furthermore, none of these specifications recites any comprehensive solution to the problem of solid organic sulphur or chloride containing waste such as is the case with the method of the present invention. 
     SUMMARY OF THE INVENTION 
     The principal objective of the present invention is to provide a method and apparatus for processing solid wastes of the above-mentioned type, which method results in a &#34;dead&#34; (to use a biological term) pyrolysis residue and thereby an effective reduction in the volume of the waste. 
     Another objective of the invention is to provide a method and apparatus which, in addition to the above-mentioned volume reduction, affords effective processing of the resulting exhaust gases. 
     Another objective of the present invention is to provide a method and apparatus for the further reduction of the pyrolysis residue volume through the gasification of at least a portion of the carbon content of the pyrolysis residue. 
     A further objective of the invention is to provide a method and apparatus which also affords an extremely high retention of the radioactivity present in the pyrolysis residue. 
     A still further objective of the invention is to provide a method and apparatus which is straight forward in technical respects and which is therefore also cost effective taking everything into account as regards volume reduction of the solid waste and management of the resulting exhaust gases. 
     The above-mentioned objectives are attained by pyrolyzing the solid waste at a relatively low temperature. The pyrolysis residue is passed through a steam reformer to gasify the carbon content of the solid residue. The resulting gases from the pyrolysis vessel and steam reformer vessel are mixed and oxidized (combusted) in a submerged bed heater. Residual acid gases and particulates are removed from the offgas stream by a fiber bed scrubber. The method and apparatus of the present invention can process sulfur and chloride containing waste having high organic content, including salt depleted resins, steam generator cleaning solutions, antifreeze and sludges. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a more complete understanding of the present invention, reference is made to the following Detailed Description taken in conjunction with the accompanying drawings in which: 
     FIGS. 1A and 1B are diagrams of a system embodying the present invention. 
    
    
     DETAILED DESCRIPTION 
     Preferred embodiments of the present invention will now be described with continued reference to the drawings. 
     The sulphur or chloride containing waste is subjected to pyrolysis at a temperature of no more than 700° C., and preferably no more than 600° C., to form a gas which contains organic sulphur or chloride compounds and a solid pyrolysis residue which contains radioactive material from the waste. 
     The pyrolysis residue (ash) 6 is fed to a steam reformer 61 to gasify the residual carbon. Significant volume reduction and overall cost savings to the waste generator can be realized by gasifying the carbon content of the solid pyrolysis residue (ash) 6 to CO 2  or CO. The final waste residue 7 can be reduced from the volume of the pyrolysis residue 6 by a factor of 2 to 5 times. The steam reformer 61 is preferably a fluid bed or rotary contact design. An optional compressing device may be added for further reducing the residue volume. 
     The final waste residue 7 from the steam reformer 61 is pelletized and packaged 62. The residue pellet package 62 is decontaminated 63 and gamma scanned 64 through standard procedures known in the art. The residue pellet package 62 is stored 65 until prepared for final shipment 66 for burial. 
     Gases from both the pyrolysis reactor 20 and the steam reformer 61 are preferably mixed and oxidized (combusted) in a submerged bed heater (afterburner) 30 operating at a preferred temperature of 1800-2200° F. The present invention overcomes many of the problems of sulfur and nitrogen oxide formation during standard combustion of ion exchange media. The pyrolysis step utilizes substoichiometric quantities of air which forms a reducing atmosphere. In such reducing conditions sulfur forms H 2  S, or in the presence of Fe it readily forms FeS and/or FeS 2 . The nitrogen components remain as N 2 , and minimal NO x . 
     Although described herein as utilizing a two stage process of pyrolysis-combustion, the steam reformer process of the present invention may also be used with other treatment process, such as the two stage pyrolysis-pyrolysis process described in copending U.S. application Ser. No. 08/403,758. 
     Heat recovered from the submerged bed heater 30 may be used to heat the process facility and/or to provide heat to dry the resin as described below or to evaporate water from dilute solutions to provide concentration. 
     The gases from the submerged bed heater are quickly reduced in temperature by a submerged bed evaporator (quencher). The use of a submerged bed evaporator allows the water temperature to be closely controlled to provide zero liquid discharge from the facility by vaporizing all of the process water. The vaporized process water is then passed out the exhaust stack as gaseous discharge. The submerged bed evaporator also serves as a scrubber to remove acid gases from the offgas stream. The acid gases are converted to salts which are concentrated to 5-15% wt. solids by the removal of water to offgas. This avoids the need for a separate concentrator unit. 
     The offgas stream is fed to a standard, commercially-available, fiber bed scrubber 51 to remove residual acid gases and particulates. The fiber bed scrubber 51 preferably provides a low pressure drop of 5 to 10 inches of water and preferably removes more than 99% of particulates and entrained mists. 
     The pyrolysis step involves subjecting the solid waste to pyrolysis at a temperature of no more than 700° C., and preferably not more than 600° C. The term &#34;pyrolysis&#34; is used herein in its conventional sense, i.e. chemical decomposition or breakdown of a substance by the action of heat and without any real supply of oxygen or at least so little oxygen supply that no real combustion is effected. The pyrolysis thereby leads to breaking down of the carbonaceous waste to a relatively fluffy pyrolysis residue which can be drawn off from the bottom of the pyrolysis reactor employed and can thereafter be imparted a significantly smaller volume by gasifying the carbon content of the residue, and, optionally compression. 
     Additionally, by keeping the temperatures no higher than those recited above, practically speaking all of the radioactive materials, in particular  134  Cs and  137  Cs, are retained in the pyrolysis residue and therefore measures and consequent costs to remove additional radioactivity can be minimized. Any fly ash formed can, however, be removed from the resulting gas in a per se known manner, preferably in a ceramic or metal filter in the pyrolysis reactor. In this way, the radioactive material in the fly ash caught in the filter can be returned to the pyrolysis residue. 
     In the practice of the invention, it has proven possible in this fashion to attain very high retention of the radioactivity in the pyrolysis residue. In this regard, trials carried out on ion exchange media from a nuclear power station show a retention of almost 10 6  :1, i.e. the decontamination factor (&#34;DF&#34;) is of the order 10 6 . Aside from said radioactive material, the pyrolysis residue contains carbon and possibly iron compounds such as iron oxides and iron sulphides. Trials in this connection, show the retention of sulphur, in the pyrolysis residue to be &gt;90%. 
     No immediately critical lower limit is apparent for the pyrolysis in the pyrolysis reactor 20 but rather this limit is dictated, if anything, by effectiveness and/or cost. However, for practical purposes, a lower limit can generally be set at 400° C. and therefore a preferred embodiment of the method of the invention involves the pyrolysis unit 20 being operated at a temperature in the range 400-700° C., preferably 400-600° C., especially 450-600° C., and most preferably 450-550° C. 
     Additionally, as the method of the invention as a whole has proven to be extremely effective both as regards the solids content and the evolved cases, the pyrolysis stage is preferably carried out without any catalyst for the breakdown of the carbon compounds in the waste which, of course, means that the method of the invention is very cost effective as the catalyst costs in comparable contexts often represent a large part of the total costs. 
     The pyrolysis stage can be carried out in per se known fashion as regards the type of pyrolysis reactor 20, e.g. in a fluidised bed, but in the overall set-up of the method in the context of the invention, &#34;flash pyrolysis&#34; has proven to give exceptionally good results. The expression flash pyrolysis is used herein in its conventional sense, i.e. with a relatively rapid, flow-through of material. In other words, it is a matter of a short residence time, normally less than 30 seconds and even more usually a significantly shorter time, e.g. less than 15 seconds. An especially preferred &#34;flash pyrolysis&#34; is carried out in a gravity or flash reactor for which a suitable residence time can be 3-15 seconds, even better 4-10 seconds and most preferred 5-8 seconds such as around 6 seconds. Suitable residence times are, however, easily determined by the man skilled in the art in each individual case. 
     An important additional consideration found is that the flash pyrolysis process of resins for about 5-8 seconds can be used under high throughput production operations. The flash pyrolysis of resins will convert the external surface of small resin particles. Subsequent residence times in the bottom of the pyractor vessel 20 for about 0.1-1 hours will complete the pyrolysis conversion. The particles have been found not to stick or adhere to each other because any incomplete flash pyrolysis that occurs in the drop zone is sufficient to convert the outer surface, thus preventing adhesion between the hot particles which accumulate in the bottom of the pyractor vessel 20. 
     The pyractor 20 is a gravity drop vessel and may utilize either multiple drop tubes or a single large diameter drop. 
     In the present case, it will be understood that &#34;solid waste&#34; does not concern a solution of the material in question. It need not however necessarily concern a dry material but also material with a certain degree of moisture content, e.g. up to 50%, usually 10-30% such as is often the case when using ion exchange media. However, for flash pyrolysis, for example, it can be convenient to condition the material prior to pyrolysis, which means a certain degree of drying and optionally, comminution. In this regard, a material in powder form has proven to give very good results in the initial pyrolysis step. 
     The ion exchange medium is fed from a cask/liner 10 to a resin storage container 11. The ion exchange medium 5 is preferably ground in a grinder 12, centrifuged 13, and dried in a dryer/feeder 14, prior to being fed into the pyrolysis vessel 20. Excess gas/vapor from the dryer/feeder 14 is fed to condenser 15. Water from condenser 15 is pumped 44 from container 41 to ion exchange unit 42. The ionized water is stored in container 43 and pumped 45 with tap water to the submerged bed evaporator 31. 
     Fine iron powder, preferably Fe 2  O 3 , may be added with the resin feed to the pyrolysis vessel 20. The Cl from chloride containing waste combines with the iron in the pyrolysis vessel 20 to form FeCl 2 , which is an inert, high melting point inorganic solid which will remain in the carbon rich residue. Sulfur containing wastes effectively combine with the iron powder to form FeS and FeS 2  compounds. 
     The fine iron oxide powder will also collect on the ceramic or metal filters exhausting the offgas from the pyrolysis vessel 20 to the submerged bed heater 30. The iron powder effectively converts H 2  S to FeS and SO 2  to H 2  S and to FeS. 
     The gas, which is formed during the pyrolysis step, contains decomposition products from the organic waste referred to as &#34;tars&#34;. These tars principally contain pure hydrocarbons and water vapor, and organic sulphur or chloride compounds and amines when the waste is of the nitrogen and sulphur or chloride containing ion exchange media type. The gas is separated from the pyrolysis residue and subjected to oxidation (combustion) in a second stage 30. 
     When the gases from the pyrolysis vessel 20 and the steam reformer 61 contain tar products and water, a preferred embodiment of the method of the invention will subject the gas, prior to being fed to the submerged bed heater, to condensation conditions such that tar products therein condense out and are separated before the gas is conducted to said submerged bed heater 30. In this context, &#34;tar products&#34; will be understood to include carbonaceous compounds which are, of course, in gaseous form after pyrolysis in the first stage but which drop out in the form of a more or less viscous tar mixed with water. The condensate can be separated by fractionated condensation (not shown) into a low viscosity tar of high calorific value, water and a viscous sulphur-rich tar. 
     If sulphur oxides, especially SO 2 , are present in the cases emanating from the pyrolysis vessel 20, they must be attended to in an appropriate manner bearing in mind the strict requirements which now apply to the release of sulphur oxides and other sulphur compounds. 
     As noted above, this may be directly attained by feeding offgases to a demister/fiber bed scrubber 51. The demisted gases are pumped 52 to a HEPA filter 53 and then vented through a standard ventilation stack 54 with radiation monitors. 
     One alternate method for attending to any sulfur oxides present in the offgas is to expose the offgas from the submerged bed evaporator 31 to a bed (not shown) of a solid reductant under reducing conditions so that the sulphur oxides are reduced, principally to hydrogen sulfide and carbon disulfide. Carbon, in particular, has proven to work extremely well as a reductant in relation to the method of the invention. Additionally, carbon results in the sort of end products, especially carbon dioxide, which are harmless and in principle can be released direct to the atmosphere. 
     The temperature for the reduction is selected by one skilled in the art in this field in such a fashion that the sought after reactions are attained. The reduction is carried out at a temperature in the range 700° C. to 900° C., the approximately 800° C. temperature level probably lying near the optimum. Alternately, one may add air to the reformer, allowing operation to as low as 375° C. The reduction bed additionally leads to a reduction in nitrogen oxides in the event that these are present in the gas after the pyrolysis steps. In the event that a high temperature filter of the carbonaceous filter type or similar is utilized for filtering out the soot in the reformer offgas, this filter can be regarded as a reduction means for use in this optional reduction step of the invention. 
     Finally, the gas may optionally be exposed to a bed (not shown) of a sulphide-forming metal under conditions in which the remaining sulphur compounds form metal sulphides with said metal. In this context, it is the gas from the prior optional reduction step, if present, or the gas from the submerged fuel evaporator 31. In each case it is primarily a matter of transforming hydrogen sulfide to metal sulfide. Preferably, iron is used as sulfide-forming metal as iron is a cheap material and results in a harmless product, principally in the form of the iron disulfide, pyrite. Other metals, however, are also conceivable of which nickel can be mentioned as an example. The temperature for this optional step is also selected by the man skilled in the art in this field so that the sought after reactions are attained. An especially preferred temperature range, however, is 400-600° C. the approximately 500° C. level being especially suitable in many cases. 
     As has been touched upon earlier, both the solid end-product and the gaseous end-products of the method of the invention are amenable to handling. The resulting ash, for example, is thus particularly suitable for post-treatment in the form of simple compression, where the practice of the invention has proven that the volume can be reduced by as much as up to 75%. Furthermore, the resulting gases are rich in light organic compounds which implies a gas with a high heat content which can be burnt. Additionally, the sort of gases being referred to are non-injurious to the surroundings, e.g. carbon dioxide, gaseous nitrogen, gaseous hydrogen and water vapor, and therefore the method of the invention, as a whole, represents unparalleled advantages in relation to the known technique. 
     In order that the method should proceed in an effective fashion and especially in order that the release of radioactive or unpleasant or dangerous gases through system leakage should be avoided, with consequent risks to working personnel, a further preferred embodiment involves carrying out the method under a certain degree of vacuum or negative pressure, conveniently by arranging a suction pump 52 or gas evacuation pump downstream of the demister/fiber bed scrubber 51. 
     The following especially preferred embodiments of the apparatus can be mentioned. 
     Preferably, the pyrolysis reactor 20 is a gravity reactor. 
     An optional condenser (not shown) for the condensation of tar products in the gas may be located prior the submerged bed heater 30. 
     A metal or ceramic filter (not shown) for the separation of any fly ash from the gas is preferably located in the pyrolysis reactor 20 and the steam reforming vessel 61. 
     Optionally, a compactor (not shown) may be included for compression of the carbon reduced pyrolysis residue 7 resulting from the steam reformer 61. 
     It is understood that various other modifications will be apparent to, and can be readily made by, those skilled in the art without departing from the scope and spirit of the present invention. Accordingly, it is not intended that the scope of the claims be limited to the description or illustrations set forth herein, but rather that the claims be construed as encompassing all features of patentable novelty that reside in the present invention, including all features that would be treated as equivalents by those skilled in the art.