Disposal of hazardous wastes has become an increasing problem because of diminishing availability of space and a growing awareness of contamination of the environment by conventional methods of disposal, such as by dumping and incineration. Toxins present in hazardous wastes often decompose at a rate which is substantially slower than the decomposition rate of other types of wastes, such as paper and metal components found in municipal rubbish. Release of toxins to the environment contaminates water supplies, and introduction of toxins to the atmosphere, such as by incomplete incineration of hazardous wastes, can pollute the atmosphere and generally diminish the quality of life in surrounding populations.
Landfills are becoming less available as a means of disposing of wastes. In the absence of suitable landfills, hazardous wastes often must be converted to benign and, preferably, useful substances. There has been tremendous investment in development of alternate methods for suitably treating hazardous wastes. Various types of reactors which have been employed for decomposition of hazardous wastes include, for example, liquid injection, multiple hearth, multiple chamber, fluidized bed, molten salt and high-efficiency boiler reactors. However, many systems release gases which must be contained or destroyed. Often these gases are burned, which generally causes formation of molecular fragments or free radicals because of the short residence time of the gases at flame temperature.
A more recent method for disposing of hazardous wastes includes introduction of the wastes into a molten bath. The molten bath is at a temperature which is sufficient to convert at least a portion of the hazardous waste to its atomic constituents. For example, hydrocarbons introduced to the molten bath are reduced to atomic carbon and atomic hydrogen. The atomic constituents can then either remain within the molten bath or react with other components of the molten bath to form more stable compounds.
One problem commonly associated with decomposition of hazardous wastes in molten metal baths is volatilization and release of components of the hazardous wastes from the molten bath before conversion to the atomic constituents of the hazardous wastes is complete. The components can be volatilized components of the hazardous wastes or molecular fragments of such components. Both the components and fragments thereof are often toxic and generally require that off-gases generated by the molten bath be processed to remove the toxins from the off-gases before the off-gases are released to the atmosphere. Toxins which are collected typically must be further treated, such as by return to the molten bath, for completion of decomposition reactions forming atomic constituents and the subsequent formation of more stable compounds, such as carbon monoxide and water.
One attempt to diminish the amount of toxins released from a molten bath includes injection of hazardous wastes beneath the surface of the molten bath. One example of a method for introducing hazardous wastes beneath the surface of a molten bath is by directing a consumable lance, containing the hazardous wastes, into the bath. The lance is degraded by the molten bath, while releasing the waste into the bath beneath the surface. However, use of a consumable lance limits introduction of waste to staggered operation, increases environmental risk due to handling of the lance and requires addition of materials other than the waste, such as the materials in the lance itself, into the molten bath.
A method of continuously injecting waste, such as carbonaceous waste, into a molten bath, includes directly injecting the waste beneath the surface of the bath through a tuyere, which typically includes one tube concentrically disposed within at least one other tube. Generally, an oxidant, such as oxygen, is directed through a central tube of the tuyere, while the waste is conjointly and continuously directed through a tube surrounding the central tube. Similarly, a third tube can be employed to direct a coolant, or shroud gas, into the molten bath at the point of injection of oxidant and waste into the molten bath.
Continuous and conjoint introduction of the oxidant and waste into the molten bath is typically required in order to prevent capping of the tuyere tube by metal at the point of injection. Capping can be caused by endothermic conversion of the waste upon injection to the molten bath. The oxidant exothermically reacts with the atomic constituents formed by conversion of the waste, thereby maintaining a temperature at the tuyere tube which is sufficient to prevent capping. A shroud gas, such as argon or methane, is introduced through the outermost tube to prevent premature wear of the tuyere tube as a consequence of exposure to the heat of the molten bath and exothermic reaction of the oxygen upon introduction to the molten bath.
However, conjoint introduction of the waste and oxidant at a single point within the reactor, such as through a tuyere tube, can cause the waste and oxidant to blow through the molten bath to a gas layer disposed above the molten bath, thereby allowing direct release of waste and partially decomposed toxic components of the waste to the atmosphere. Further, partial reaction occurring in the tuyere envelope, caused by conjoint introduction of the waste and oxidant, can cause release of incompletely decomposed waste to the gas layer and incomplete oxidation of atomic constituents formed by conversion within the molten bath. Portions of the molten bath can thereby become saturated in atomic constituents, such as carbon, or the molecular fragments may have reduced solubility, relative to atomic species in the molten bath, consequently diminishing the rate of subsequent conversion and causing additional release of such waste from the molten bath into the atmosphere.
A need exists, therefore, for a new method and system for converting a waste feed to its atomic constituents for subsequent oxidation of the atomic constituents, which overcomes or minimizes the above-mentioned problems.