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

摘要

The wet flue gas desulfurization process (FGD) in fossil fired power plants offers the advantage of simultaneously removing SO_2 and other water soluble pollutants, such as certain oxidized mercury compounds (Hg~(2+)). In order to maximize SO_2 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 (Hg~0). Increasing chloride concentration and decreasing pH and slurry temperature have been identified as key factors for depressing Hg~0 re-emissions. Both organic additives have a negative impact on Hg-retention and cause increased Hg~0 re-emissions in the wet FGD process, with formic acid being the significantly stronger reducing agent. Different pathways of Hg~(2+) 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.