Small concentrations, approximately 2-10 parts per million (ppm), of injected sulfur trioxide (SO 3 ) have improved particulate collection efficiencies of electrostatic precipitators burning lower-sulfur coal. However, the addition of extra SO 3 not only incurs costs but also presents negative environmental effects. This work explored a method that could be applied to existing coal-fired power plants to convert the sulfur dioxide (SO 2 ) already present in the flue gas to sufficient levels of SO 3 for fly ash conditioning as an alternative to adding SO 3 by burning elemental sulfur. During this research, a pre-mixed natural gas flame was used to promote the conversion of SO 2 to SO 3 in a drop-tube furnace with average non-flame, free stream gas temperatures of 450 and 1000 K. SO 3 concentrations measured by wet chemistry and confirmed using elemental balances of other sulfur species measured by gas chromatography revealed that as much as 7% of SO 2 was homogeneously transformed to SO 3 . The results also showed that at low temperatures, the rate at which SO 3 is converted back to SO 2 decreased, thus extending the time period during IMPLICATIONS Pulverized-coal-fired power plants that switch to lowsulfur coal often experience higher ash loading and lower fly ash resistivity, resulting in higher stack opacity. Many of these power plants add small concentrations of SO 3 from the oxidation of elemental sulfur to lower resistivity and improve ash collection in the electrostatic precipitator. While this technique may avoid some expense of retrofitting additional precipitator collection areas, the extra SO 3 can lead to the formation of a "blue plume" and release acid rain precursors. This paper shows that SO 2 existing in the flue gas could be used to produce sufficient SO 3 through a combustion-based process immediately upstream of the electrostatic precipitator. This technique also humidifies the flue gas, which may further enhance precipitator performance.
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