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Show 4.0. DISCUSSION The evidence regarding the oxidation state of arsenic, and the probable speciation of arsenic in the Tacoma Steam Plant #2 flyash, is clear. It would be highly unlikely that arsenic trioxide would be the dominant As compound formed in the combustion system. The implications of this fmding can only be understood, however, in terms of probable mechanisms for arsenic reaction in this combustion system. Certainly the analysis of mechanisms is critical in order to understand the leaching characteristics of the Tacoma Steam Plant #2 flyash as shown in Table 1. The general fate of arsenic in the Tacoma Steam Plant fluidized beds can be readily understood from a careful review of combustion literature concerning mechanisms associated with trace metals in combustion systems. 4.1. General Operations of the Fluidized Bed System In general, the Tacoma Steam Plant fluidized beds accept wood and coal, along with limestone, as continuous feeds to the fluidized beds. When permitted, RDF is fed to the units along with the wood and coal. Combustion occurs within the bubbling fluidized bed units. Typically combustion occurs within the bed, on the surface of the bed, and in the freeboard region depending upon the physical characteristics of the particle (e.g. specific gravity, fluidization characteristics), the stage of combustion, and the velocities within the bed. (see [22]). Simultaneously with the combustion process, the limestone introduced into the fluidized beds is calcined to lime according to the reaction: CaC03 => CaO + CO2 (9) and then the lime captures sulfur dioxide and hydrogen chloride according to the following reactions: CaO + S02 + 0.502 => CaS04 CaO + 2HCI => CaCl2 + H20 (10) ( 11) In this system there is always considerable excess limestone, and consequently unreacted lime, in order to ensure driving reactions 10 and 11. This unreacted lime is available for metals capture reactions. During fluidized bed operations, gaseous combustion products, along with coarse and fine solid particles, are exhausted from the fluidized beds and ducted to twin cyclones. Solid particles captured by the cyclones consist of unburned fuel, unreacted lime, calcium sulfate, calcium chloride, bed media, and ash. Cyclone discharge is recycled to the fluidized beds in order to maximize combustion efficiency and minimize use of fresh limestone. Fine particles not captured by the cyclones pass through the boilers and to the fabric fliter or baghouse system. These particles also include unburned fuel, calcium oxide, calcium sulfate, calcium chloride, bed media, and ash. These particles may well have been recycled several times through the fluidized beds until they are atritted to a size fme enough to escape cyclone capture. |