Patent Number: 060841478
Section: summary

FIELD OF THE INVENTION The present invention relates generally to decomposition of organic wastes. "Processing" refers to the breaking down of the wastes via a thermal route with the primary aim of affording an opportunity for reducing its volume to lessen handling and storage concerns. In particular, the present invention relates to pyrolysis of organic wastes. BACKGROUND OF THE INVENTION For decades, steam has been used to decompose organic chemicals, either to produce methane or to produce hydrogen and carbon monoxide and carbon dioxide as feed to other chemical processes. Because the basic process of steam reforming of organics is endothermic, much of the development in this art has focused on how best to meet the energy requirements. Typically, if external heat was not supplied, oxygen was added to the feedstock and thereby supply heat from exothermic oxidation. The apparatus for decomposing the waste also made use of the heat inherent in the effluents via heat exchange to preheat feedstock. Other developments in steam reforming focused on fluidized bed reactors and catalysts for achieving greater efficiencies, especially in the production of synthetic gas as fuel. The nuclear industry annually produces a significant amount of waste which is classified as radioactively contaminated ion exchange media, sludges and solvents. This waste is managed in various ways before being disposed of in bedrock chambers or by shallow land burial. Management of radioactive wastes is technically complex and, as a rule, leads to increased volumes that in turn increase storage costs. A process that results in reducing the volume and chemical reactivity of the waste disposed of is therefore highly desirable. Ion exchange media is an organic material. The media base is usually a styrene polymer to which are grafted sulfonic acid and amine groups. The material is therefore burnable, but, when air is supplied during combustion, sulfur and nitrogen oxides are formed that in turn must be separated in some manner. Additionally, during combustion, the temperature becomes sufficiently high for radioactive cesium to be partially vaporized. The radioactivity of the burning resins could also accompany the resulting fly ash. This effect necessitates a very high performance filtration system. Accordingly, both technical and economic problems are typically associated with combustion of ion exchange media. An alternative technique is pyrolysis. However, previously known pyrolysis methods in this field are deficient in several aspects and, in particular, no one has succeeded in devising a pyrolysis process that provides a comprehensive solution to the problem of sulfur and nitrogen-containing radioactive waste, and to do so under acceptable economic stipulations. See for example U.S. Pat. Nos. 5,424,042, 5,470,738, 5,427,738, 4,628,837, 4,636,335, and 4,654,172, and Swedish Patent SE-B 8405113-5. Ion exchange media are not the only types of organic wastes generated by the nuclear industry, nor are they the only types of radioactive wastes generated by other industries. Some industries generate mixed wastes that include both radioactive waste and chemical wastes. The chemical wastes, for example, can include organic solvents such as trichloroethylene or PCBs. Mixed wastes are especially difficult to deal with because different and sometimes conflicting regulations apply to their dual hazards. There is a need for a process that can efficiently decompose wastes containing radioactive contaminants and to do so in a way that reduces the volume and chemical reactivity of the waste residue remaining after decomposition. SUMMARY OF THE INVENTION According to its major aspects and briefly recited, the present invention is a method and apparatus for decomposing organic wastes using a two-stage steam-reformer. Wastes are fed into the first of the two stages along with a fluidizing gas composed of steam and oxygen. Both stages contain an inert media bed made of large, high-density beads, such as alumina beads up to 3000 microns in diameter. The fluidizing gases are injected at relatively high speeds, ranging up to 400 feet per second. In the first stage, the high speed gases pyrolyze much of the wastes at a temperature in the range of 450.degree. to 800.degree. C. and at a pressure of up to 45 pounds per square inch. Carbon and unpyrolyzed wastes are carried to the second stage from the first stage through a filter system. In the second stage, pyrolysis continues under essentially the same conditions but the use of various co-reactants and judicious selection of temperatures can be made to affect the precise nature of the final waste form depending on the initial waste form entering the second stage. Waste gases are captured and treated in conventional ways, leaving an inorganic, high-metals content grit for disposal. The use of two, back-to-back steam reformers is an important feature of the present invention. The bulk of the pyrolysis and steam reforming takes place in the first of the two allowing the second to be used not only to complete reformation but also to fine tune the final waste form. The use of relatively high fluid velocities in connection with large bead-sized, high-density inert media in a fluidized bed reactor is another important feature of the present invention. The velocity of the fluidizing gas can be as high as 400 FPS and the beads made of alumina up to 3000 microns in diameter. The high velocities agitate the media so that it grinds the softer, friable feedstock, thus accelerating its exposure to the steam and its reformation. The action of the fluidizing medium on the bed material accelerates the pyrolysis and helps in some cases to prevent undesired reactions of feedstocks such as liquid sodium or organic explosives. The use of co-reactants in the second stage to adjust the final waste form is another important feature of the present invention. For example, the oxidation state of metals such as chromium can be changed from the hazardous Cr+6 to the non-hazardous Cr+3 state. Reduction of hazardous sodium, calcium, magnesium and other metal salts to the corresponding cation oxide and/or carbonate is also advantageous. Addition of chloride or other co-reactants can be used to effectively partition certain metals such as zinc or cesium to the off gas. In this manner, the process can be used to remove high levels of cesium from high-level radioactive waste to produce concentrated cesium product that has a commercial value as well as low-activity radioactive waste that can be easily handled. The addition of carbon, together with sodium bearing wastes, can facilitate formation of high melting point sodium carbonates that can eliminate the formation of sodium eutectic salts that can melt and agglomerate the bed media. The addition of lime (calcium carbonate), together with phosphate bearing wastes, can facilitate the formation of stable calcium phosphate that can eliminate the corrosive phosphate ions in the system. Elimination or reduction of the amount of some waste forms that would otherwise require special handling may significantly reduce waste disposal costs. Another feature of the present invention when applied to radioactive ion exchange resins is the low temperature at which the pyrolysis takes place. At lower temperatures, radioactive cesium remains with the residue rather than volatizing and entering the offgas system. By avoiding all but nominal cesium carryover to the offgas system, the need for a special cesium trap is avoided leaving conventional scrubbers to remove the small amount that does enter the offgas. In addition, if cesium and chlorides are present, zinc may be added to preferentially bond with the chloride and partition the resultant zinc chloride to the off gas, leaving the radioactive cesium in the waste residue. Other features and their advantages will become apparent to those skilled in the art of organic waste disposal from a careful reading of the Detailed Description of Preferred Embodiments, accompanied by the following drawings.