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Show Disposal of Solid Residue Although incineration reduces the volume and weight of scrap tires by a factor of approximately 20, some solid residues will remain. The steel beads and belts and the glass belts will oxidize or melt and remain in the ash. Zinc oxide, ZnO, a white pigment used in white wall tires will also be present in the ash. These residual materials must be collected and disposed of in an environmentally acceptable manner, which is discussed in the following subsection. When burning high-sulfur coal, limestone and dolomite are added to the fluidized-bed combustion boiler to absorb the sulfur in the form of CaS04. Combined with the silica from coal ash, and/or glass belting from tires, the ash makes an excellent raw material for cement manufacture. Discussion with a leading cement manufacturer on the feasibility of making cement from the solid residues from a fluidized-bed combustion boiler and results of an in-house study indicated that these solid residues can be used to make cement. Pollutant Controls All energy conversion and generation processes should be conducted in an environmentally acceptable manner, and tire incineration is no exception. Pollution control in scrap tire incineration can be subdivided into the following discussion categories: o Sulfur Dioxide, S0?. The sulfur content in scrap tires is relatively moderate, roughly 1 percent by weight, making it a feasible blending stock for higher-sulfur coals to reduce the sulfur content of the total fuel. The attractiveness of this option depends on future actions of the EPA on S02 emission standards. Conventional boilers are not equipped to handle high-sulfur coals. Add-on units such as wet scrubbers are normally contemplated to implement the sulfur dioxide control program; however, problems have been demonstrated with these cleanup units in terms of costs, efficiency, and operability. Fluidized-bed combustion boilers do not need a scrubber system to control sulfur dioxide emissions, since an appropriate feed rate of limestone can capture sulfur in the FBC process itself, thus maintaining S02 emissions within the standard limits. o Nitrogen Oxides, NO A number of nitrogen compounds are used in tires as catalysts and additives; they vary from company to company. Our experience in fluidized-bed combustion of coal indicates that most of the nitrogen oxides formed are from fuel nitrogen. The combustion temperature in a fluidized-bed combustor is low enough (1550°F to 1650°F) that, based on thermodynamic equilibrium calculations, it is not feasible to expect a significant level of nitrogen oxides to form from air nitrogen. Consequently, this is not a potential problem. 11-17 |