Analyzing the Behavior and Reactions of Mercury in the Solid Phase of a Flue-Gas-Desulfurization Sludge
Toxic mercury emissions are released into the flue gas during coal combustion and are mainly co-treated in the flue gas desulfurization (FGD). In this process mercury(II)-species and SO2 are washed out of the flue gas and are transferred into the aqueous phase. The aim is to stabilize mercury in the aqueous phase and to transfer it for treatment in a waste water treatment plant, reduce reemissions of the scrubber solution and prevent a carry-over into the gypsum production over the solid phase. Mercury is known to create heavy metal complexes in the scrubber suspension after the absorption. Possible ligands are halides, chloride, bromide and iodine or sulfite. The Hgg - Hgaq equilibrium can be described by the halides concentration and distribution, the Henry coefficient for possible reemissions, the redox reaction caused by presumably sulfite to elemental mercury and a reemission out of the scrubber solution. The other equilibrium of the aqueous and solid phase of mercury is not that well researched. Different experiments were conducted to better understand the influences on the equilibrium and the resulting mercury species in the solid phase. First, a method was developed to analyze mercury species in a solid sample. This method is based on methods found in literature that thermally decompose mercury samples to identify specific evaporation behavior of the different species. Reported species in FGD gypsum are Hg-halides, HgS, HgO and HgSO4. In difference to the found approaches, the resulting mercury emissions in the developed method were differentiated between possible oxidation states (0 and II). This led to new evaporation information for mercury-halides that was not reported before. Mercury halides only evaporate as oxidized species while all the other species evaporate as elemental mercury. A laboratory scaled FGD scrubber was put into operation, to better understand the reaction path of aqueous mercury into the solid phase and to identify the resulting species. In literature identified indicators and process settings were executed to find the main influences on the different presumed resulting mercury species in gypsum. Most of the influences on the quantitative amount of mercury found in literature, such as halides concentration, sulfite concentration and metal concentration, were confirmed by the experiments. It was not possible to verify the redox potential due to technical restrictions of the experiment. None of the expected mercury species were found in the created samples. This can be explained by the improved mercury analysis due to the specification of the different oxidation states and a slow temperature ramp. The results indicate that a heavy metal precipitation based on pH level changes takes place in the scrubber solution. This indication was confirmed by indicative precipitation experiments. The suspected reaction is a precipitation of HgSO3 to a not known Hg complex.