Reemission of elemental mercury and mercury halides in wet flue gas desulfurization

The wet flue gas desulfurization process enables the removal of sulfur dioxide (SO2) and water soluble mercury compounds (e.g. HgCl2) from coal derived flue gas. The aim of this study is to improve the state of science about subsequent processes in the liquid phase of the slurry, leading to the undesired reemission of mercury. For that purpose, experiments are carried out on a continuously operated lab-scale wet flue gas desulfurization test-rig. The parameters investigated include slurry temperature, pH-value, individual halide concentration and slurry aeration flow rate. The results indicate that the reemission of elemental mercury results from the chemical reduction of dissolved oxidized mercury species with sulfite being the reducing agent. For increased slurry temperature and within individual concentration ranges of the halides bromide and iodide, simultaneous desorption of both, elemental mercury and oxidized mercury compounds into the clean gas is observed. The underlying mechanism is proposed to be the formation of the volatile uncharged oxidized mercury species HgBr2 and HgI2 respectively. Reemissions of both elemental and oxidized mercury species decrease with decreasing temperature and increasing share of charged halogenidomercurate(II)-complexes. Formation constants, inert redox character and volatility of halogenidomercurate(II)-complexes increase in the order Cl− < Br− < I−.