Mercury and dioxins in a MercOx-scrubber

Abstract: The removal of mercury and the absorption of dioxins in an acid wet scrubber have been studied in industrial scale. Hydrogen peroxide is added as an oxidizing agent to the scrubber liquid with the purpose to remove SO2 and enhance the removal of mercury. The scrubber is operating as the final flue gas cleaning stage at the SAKAB plant, a hazardous waste incinerator in Sweden.Mercury speciation was measured in flue gases upstream and downstream of the scrubber with three different techniques: Impingers, solid phase absorbents and a continuous emission monitor. During the flue gas sampling periods also the scrubber liquid was sampled. It was found that mercury entering the scrubber is efficiently oxidized at normal operating conditions. An increased fraction of elemental gaseous mercury was found in flue gases after the scrubber at redox potentials below 700 mV and pH below 1. The mercury emissions were more dependent on the scrubber liquid composition than on the inlet concentrations.Equilibrium calculations of the mercury speciation in the scrubber liquid and flue gases after the scrubber showed that mercury is normally emitted as HgBr2 or HgCl2. Elemental bromine and chlorine are formed in the scrubber. Both species probably take part in the mercury oxidation process.Results from 18 months of dioxin sampling, using a continuous long term sampling device (the AMESA system) were evaluated by multivariate data analyses. It was shown that the elevated emissions observed were caused by a memory effect in the scrubber due to high start-up concentrations. The memory effect was modelled in a diffusion model where the dioxins were considered to be physically absorbed in and desorbed from the polypropylene fillings of the scrubber. From the diffusion model, the diffusivities for tetra to hexa chlorinated dioxins in polypropylene were calculated. The diffusivity of tetra chlorinated dioxins was calculated to be 10-14 m2/s. The diffusivity decreases with increasing chlorination. For hexa chlorinated dioxins it was calculated to be 10-16 m2/s.Brominated dioxins were measured before and after a fabric filter. The fraction of brominated dioxins increased after the filter. Di brominated furans were the most abundant homologue.