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 work is to improve the state of science about the absorption of these compounds and their subsequent reactions in the liquid phase of the slurry. For that purpose, experiments are carried out on a continuously operated lab-scale wet flue gas desulfurization test-rig.
The removal rate of SO2 is strongly dependent on the concentration gradient in the liquid boundary layer. Thus, measures leading to a decrease in concentration of physically dissolved SO2 promote its absorption rate. In this context, the addition of dicarboxylic acids to the slurry enhances the SO2 removal rate significantly. For operating conditions of the test-rig which enable high SO2 removal rates, gaseous HgCl2 is absorbed almost completely.
As a consequence of redox reactions in the liquid phase of the slurry, the reemission of elemental mercury is observed. Within this work, the interaction of HgCl2 with gypsum particles, ligands and reducing agents is investigated in detail. Formation constants, inert redox character and volatility of halogenidomercurate(II)-complexes increase in the order Cl- < Br- < I-. The interaction of HgCl2 with dissolved sulfite is diverse. For low sulfite-concentrations, the share of reactive HgCl2 and thus the reemission of elemental mercury are decreased due to the formation of sulfitomercurate(II)-complexes and particle bound mercury compounds. In contrary, for elevated concentrations and pH-value, sulfite acts as an effective reducing agent for HgCl2, promoting reemissions of elemental mercury. For pH-values higher than pH 8, the chemical reducing of HgCl2 by the hydroxide ion is gaining increasing importance. The addition of organic acids to the slurry leads to decreasing redox-potentials and subsequent increasing reemissions of mercury. The reducing strength of organic acids depends on their functional groups and the chain length.