Removal of Hg0 in wet FGD by catalytic oxidation with air – A contribution to the development of a process chemical model

In the wet flue gas desulphurization (FGD) process with forced oxidation, very soluble oxidised mercury is retained in the scrubbing solution while Hg0 is not and is emitted to the atmosphere. We showed experimentally that the complex chemistry in FGD solution can be modified in so that dissolved Hg0 is oxidised by air introduced for the oxidation of S(IV) to S(VI) compounds. The most important chemical reactions were identified. Oxidation is enhanced by catalytic reactions with transition metals like Fe and Mn, which, when combined, show a synergistic positive effect. It seems that the radicals involved in the chain reactions of autoxidation of S(IV) are also responsible for the oxidation of Hg0. The reaction products of this system are sulphates and polythionates, which were also determined in the absorber solutions. The formation of Hg2+ complexes with ligands like Cl−, HSO 3 - , HSO 4 - , HCO 3 - , OH - and many others is very important. The formation of Hg 2 2 + seems to be involved as well and could play an important role in the elimination of insoluble compounds from the solution, shifting the equilibrium and enhancing the retention of mercury in wet FGD. Hg0 removal efficiency from the gaseous phase achieved in laboratory testing was from 70% to over 95%. The schematic chemical model which includes the conclusions derived from experimental results and from published chemical mechanisms is proposed.