| OCR Text |
Show extensively examined (see [6], [7] , [14] , [16] , [17] . The work by Chakraborti and Lynch [7] is particularly useful as it deals with the probable reaction sequences which volatilize arsenic in high temperature oxidizing environments. Under oxidizing conditions, these authors demonstrate that the arsenic volatilized is, indeed, initially oxidized to arsenic trioxide (A~03)' Subsequently, the arsenic oxidizes to arsenic pentoxide (A~Os) and then precipitates, because that compound does not exist in the vapor state. This work concludes by stating: "The thennodynamic calculations reveal that at extreme reducing conditions A~S3 and AS4S4 will be the primary arsenic-bearing vapor species. At higher partial pressures of O2, the data reveal that arsenic sulfides and AS40 6 constitute the main arsenic-bearing species in the vapor phase. The data also reveal that for conditions typically found in the freely flowing gas of an industrial roaster, A~Os( s) precipitation is favored ." The work of Lynch and Chakraborti [16] suggests that in highly oxidizing fluidized bed environments, at temperatures above 1,400oP (1,035°K) arsenic pentoxide does decompose. Such decomposition could potentially produce As4, AS20 3 and O2, However, in cooler regions of the system such as the boiler, economizer, and baghouse; thennodynamics favor oxidation of any arsenic trioxide to arsenic pentoxide with the consequent precipitation of the A~Os. Ho et al. [11] also conclude that volatile arsenic may chemisorb or react with fine CaO particles to fonn highly stabilized solid products as well. 4.3. Conclusions Regarding the Fate of Arsenic in Fluidized Bed Combustion The data generated by numerous researchers are sufficiently compelling to support the arguement that a significant majority of the arsenic in a fluidized bed setting where an excess of CaO exists will never become volatilized and, due to polymerization, may never become accessable to oxygen. This arsenic will remain in the solid phase. Its final product slate, although unknown at this time, will be highly stable due to the physical and chemical mechanisms operating in the fluidized bed and also because of the expected As-CaO reactions. Of the volatile arsenic, some initial oxidation to A~03' or AS40 6 will occur, followed by some completion of the oxidation process, to A~Os, in cooler regions of the system (e.g. the boiler and baghouse). The literature available, then, suggest that the fate of arsenic in the fluidized bed is complex, and follows the following conceptual pathways: As => As(m) +As(g) => AsX(m) + AsX(g) A~03(S) => A~O/g) => AsX(g) AsX(m) + CaO +A120 3 + Si02 => As-CaO-Al20 3-Si02 complexes AsX(g) + CaO +A1203 + Si02 => As-CaO-A1203-Si02 complexes AsX(m) + CaO + other reactants => non-volatile polymers AsX(g) + CaO + other reactants => non-volatile polymers 2As(v) + 302 => AS20 3 A~03 + O2 => As20 s (12) (13) (14) (15) (16) (17) ( 18) ( 19) |
