Comprehensive Effect of Oxidant Addition in an FGD Slurry on the Removal and Distribution of Selenium: A Field Study

Flue gas desulfurization (FGD) scrubbers capture selenium in coal-fired power plants, leading to a high concentration of selenium in the slurry. This research proves that SO₃^2- is preferentially oxidized compared to SeO₃^2- by S₂O₈^2-. With the increase in the oxidation-reduction potential (ORP) caused by S₂O₈^2- addition, the conversion rate of SO₃^2- increased and the size of gypsum grains grew from 31.2 to 34.6 μm. SeO₃^2- migrates into gypsum grains during the growth of CaSO₄·2H₂O, leading to selenium fixation in gypsum. In a field study of a 350 MW unit, the ORP increased from 142 to 450 mV when Na₂S₂O₈ was fed into the FGD slurry. With the addition of the oxidant, 65.1% of selenium in the liquid phase migrated into gypsum. The concentration of selenium in the leachate of gypsum after oxidant addition decreased by 68.0%. A 2.34% increase in the selenium removal rate was observed in the scrubber. This study focuses on the migration and conversion of selenium in an actual FGD slurry via a field test. The results found in the 350 MW unit are consistent with laboratory results. The change in ORP has been proven to be effective in adjusting the selenium distribution in the FGD slurry.