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| - Capture of Gas-Phase Arsenic Oxideby Lime: Kinetic and MechanisticStudies
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| - Trace metal emission from coal combustion is a majorconcern for coal-burning utilities. Toxic compounds suchas arsenic species are difficult to control because of theirhigh volatility. Mineral sorbents such as lime and hydratedlime have been shown to be effective in capturingarsenic from the gas phase over a wide temperaturerange. In this study, the mechanism of interaction betweenarsenic oxide (As2O3) and lime (CaO) is studied over therange of 300−1000 °C. The interaction between these twocomponents is found to depend on the temperature;tricalcium orthoarsenate (Ca3As2O8) is found to be theproduct of the reaction below 600 °C, whereas dicalciumpyroarsenate (Ca2As2O7) is found to be the reaction productin the range of 700−900 °C. Maximum capture of arsenicoxide is found to occur in the range of 500−600 °C. At500 °C, a high reactivity calcium carbonate is found tocapture arsenic oxide by a combination of physical andchemical adsorption. Intrinsic kinetics of the reaction betweencalcium oxide and arsenic oxide in the medium-temperaturerange of 300−500 °C is studied in a differential bed flow-through reactor. Using the shrinking core model, the orderof reaction with respect to arsenic oxide concentrationis found to be about 1, and the activation energy is calculatedto be 5.1 kcal/mol. The effect of initial surface area ofCaO sorbent is studied over a range of 2.7−45 m2/g usingthe grain model. The effect of other major acidic fluegas species (SO2 and HCl) on arsenic capture is found tobe minimal under the conditions of the experiment.
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