Removal of Hg from flue gases in wet FGD by catalytic oxidation with air - An experimental study

About 46% of global mercury emissions are due to fossil fuel combustion for electrical and thermal energy production. Since more stringent emission standards are expected, important research efforts are being focused on the development of mercury removal technologies, mainly directed to two alternative approaches: (i) the enhancement of homogeneous oxidation in the flue gases of Hg⁰ to water soluble Hg²⁺ by the addition of chlorides or bromides to the boiler or; (ii) the adsorption of Hg²⁺ and Hg⁰ on impregnated activated carbon (AC). The latter may require the treatment of the entire gas volume of the thermal power plant and constantly consumes relatively large quantities of AC.A third option gaining more attention lately is based on the oxidation and retention of dissolved Hg⁰ in the wet flue gas desulphurisation (FGD) system. A series of chemical oxidants, such as halogens, hydrogen peroxide, sulphur and oxygen, are theoretically able to oxidize Hg⁰ in the wet FGD system. Most chemical oxidants when applied in the FGD, however, are non-selective and are largely consumed by SO₂ absorbed from the flue gas. The less expensive oxidant, non-selective as well, is oxygen (as air) which is already being dispersed into FGD absorbing suspension for the conversion of into .The experimental evidence of the present work showed that Hg⁰ present in the gaseous phase can be dissolved and oxidized to a high degree (70-90%) by air together with in wet FGD solutions. Transition metals such as Fe²⁺ and Mn²⁺ act as catalysts, chloride enhances the reaction, while some oxosulphur compounds, e.g. tetrathionate, inhibit the oxidation. A combination of several catalysts at a concentration of sulphite () below 100 mg L⁻¹ and an adequate redox potential of the solution can assure reasonable mercury removal even in the presence of oxidation inhibiting compounds.The main competitive reactions that govern final Hg⁰ removal in the FGD are as follows: (1) oxidation of Hg⁰ together with SO₂ with air, enhanced by catalysts; (2) removal of catalysts by precipitation in the form of Fe(OH)₃ and eventually as MnO₂ (to overcome this problem continuous addition of catalysts to the solution is required); (3) reduction of Fe³⁺ by tetrathionate to Fe²⁺ which (4) may reduce Hg²⁺ to Hg⁰ and probably (5) the complexation of Hg²⁺ by anions present which may play an important role in the mechanism by complexing the product(s) of the Hg⁰ oxidation reaction.