OCR Text |
Show fate of metals (e.g. the percentage which vaporizes and recondenses), but also the speciation (Seeker et. al., 1987). This model is at the heart of a larger modeling system employed by Ebasco for complete combustion analysis of industrial boilers and furnaces (see Tillman, Meagher, and Wegehaupt, 1989) The model created combustion conditions analogous to those in the Tacoma Steam Plant #2, with particular attention to the following parameters: 1) ultimate analysis of the blended fuel, 2) stoichiometric ratio, 3) inerts loading in the bed, and 4) combustion temperatures in the bed and freeboard zone. The model was sensitized to report specific compounds at mole fractions exceeding 1 x 10-9 • Test runs were also performed with model sensitivities at mole fractions of 1 x 10-29 • Base cases were run on the model utilizing the fuel compositions measured or specified for each fuel, and for the specified fuel blends. Because solid fuels are heterogeneous, however, and because the fate and speciation of certain metals (e.g. lead) can be influenced by various factors such as concentration of chlorine in the feed, additional sensitivity runs were performed varying relative concentrations of metals (e.g. chromium, nickel), and concentrations of chlorine and other reactants. All of these runs were then used to develop a pattern of speciation for metals in the flyash of the Tacoma Steam Plant #2. RESULTS The results of the analysis can be summarized in terms of the trace metal concentrations in the feed, the trace metals concentrations in the flyash, and the metal speciation profiles developed from literature search and computer modeling. These results are summarized below. Trace Metal Concentrations in the Fluidized Bed Feeds The concentrations of trace metals in the hogged wood waste and coal at the Tacoma Steam Plant #2 are shown in Table 4. Note the very low concentrations of trace metals in the hogged wood waste, the primary fuel for this power plant. These concentrations are well within the ranges associated with typical wood fuels (see Envirosphere, 1984; Greene, 1986). Typically, the hogged wood waste brings trace metals into the combustor in the form of entrained inert material rather than inherent ash. Management of the hogged wood waste to prevent acceptance of unusually dirty wood waste, therefore, becomes the significant issue. Table 5 compares the measured concentrations of trace metals in the coal to the specified maximum concentrations of trace metals in the coal. Note, again, that the measured concentrations of trace metals in coal are significantly below the guaranteed -6- |