Impact of additives for enhanced sulfur dioxide removal on re-emissions of mercury in wet flue gas desulfurization

The wet flue gas desulfurization process (FGD) in fossil fired power plants offers the advantage of simultaneously removing SO2 and other water soluble pollutants, such as certain oxidized mercury compounds (Hg2+). In order to maximize SO2 removal efficiency of installed FGD units, organic additives can be utilized. In the context of multi-pollutant control by wet FGD, the effect of formic and adipic acid on redox reactions of dissolved mercury compounds is investigated with a continuously operated lab-scale test-rig. For sulfite ( SO 3 2 - ) concentrations above a certain critical value, their potential as reducing agent leads to rapidly increasing formation and re-emission of elemental mercury (Hg0). Increasing chloride concentration and decreasing pH and slurry temperature have been identified as key factors for depressing Hg0 re-emissions. Both organic additives have a negative impact on Hg-retention and cause increased Hg0 re-emissions in the wet FGD process, with formic acid being the significantly stronger reducing agent. Different pathways of Hg2+ reduction were identified by qualitative interpretation of the pH-dependence and by comparison of activation enthalpies and activation entropies. While the first mechanism proposed identifies SO 3 2 - as reducing agent and is therefore relevant for any FGD process, the second mechanism involves the formate anion, thus being exclusively relevant for FGDs utilizing formic acid as additive.