The Fate of Arsenic at the Tacoma Steam Plant #2

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Title The Fate of Arsenic at the Tacoma Steam Plant #2
Publisher Digitized by J. Willard Marriott Library, University of Utah
Date 1992
Spatial Coverage presented at Cambridge, Massachusetts
Abstract On three occasions, Tacoma Steam Plant #2 experienced relatively high concentrations of arsenic (e.g. > 100 ppmw) in the flyash produced by fluidized bed combustion of refuse derived fuel (RDF), coal, and hogged wood waste. These concentrations produced apparent exceedences of state regulations for As in the flyash, because Washington regulations assume that all arsenic reports as arsenic trioxide (As2O3) , and because those regulations state that flyash containing> 75 ppmw As must be handled as a dangerous (e.g. hazardous) waste. Ebasco Environmental undertook research to identify the sources of such arsenic in the fuel feed, the probable arsenic product slate in the flyash, and potential mechanisms capable of generating such products. The sources of the arsenic were probably a few loads of wood waste containing some residual slag from the abandoned Tacoma copper smelter. The sources also could have included random loads of construction wood containing measurable quantities of copperchromium-arsenate (CCA) wood preservative. Coal and RDF did not prove significant sources of As in the fuel feed. Ebasco Environmental then evaluated the oxidation state and binding energy of the arsenic in the flyash, recognizing that the bulk of recent experience and research demonstrates that arsenic does not behave in strict accordance with thermodynamic equilibrium calculations. Using the University of Utah as a subcontractor, Electron Scanning for Chemical Analysis (ESCA) techniques were used to identify arsenic species. This research led to the following conclusions: (1) the dominant species had an oxidation state of +3, and (2) the dominant species had binding energies sufficiently low to preclude their existence as As2O3. Through a detailed review of the literature, Ebasco Environmental evaluated potential mechanisms, reaction pathways, and product slates associated with the arsenic in the flyash at Tacoma Steam Plant #2. The most probable mechanisms and reaction pathways involve mobilizing or volatilizing the arsenic. The mobile or volatile As compounds then react with CaO and potentially alumina and silica, or other compounds, to form large polymers and complexes with vapor pressures which preclude evolution in the vapor phase and oxidation to arsenic trioxide. Some arsenic trioxide may evolve or form. The ASzO3 which does evolve or form will oxidize, at least partilly, to As2O5. On this basis it has been concluded that significantly less than 50 percent of the As in the Tacoma Steam Plant #2 flyash exists as As2O3.
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Language eng
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Scanning Technician Cliodhna Davis
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Setname uu_afrc
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