Mercury and its compounds are toxic pollutants whose emissions are a global problem due to their distribution and subsequent bioaccumulation. Dual-loop flue-gas desulphurization (FGD) systems can absorb water-soluble components and hereby reduce the mercury emissions of thermal power plants. Although the technology is well-known and developed in full scale application, the behaviour of mercury in dual-loop systems has to date been insufficiently dealt with in literature. To improve the removal and retention of flue-gas mercury compounds, experiments were conducted at a laboratory and technical scale dual-loop FGD.
The dual-loop FGD, comprising both quencher and absorber loop, are compared with single-loop FGD subsystems. Both loops show comparable removal efficiencies of oxidized mercury compounds and are therefore relevant for mercury removal in the system. In the dual-loop process, sulphite and halides are key parameters, influencing the formation of mercury complexes and redox reactions, leading to mercury reemissions. As the quencher is fed by the raw gas, the largest share of the flue-gas load of mercury compounds is removed. The quencher is a more favourable loop than the absorber, as the formation of mercury complexes with chloride is enhanced. At high pH-values, higher shares of sulphite on sulphur(IV) promote the formation of [Hg(SO3)2]2- in the absorber and accelerate the mass transfer from gaseous to liquid phase. The reemission ratio of the absorber is slightly higher, as complexes with sulphite are assumed to be less stable under the prevailing conditions. The oxidation air is a key parameter, as it both influences the oxidation-reduction conditions and promotes the oxidation of sulphite to sulphate. Regarding formation of gypsum in the dual-loop process, the adsorption of mercury on particles is reduced at a lower operational pH-value, which respectively goes along with a higher oxidation-reduction potential. The dual-loop process constitutes a system in which interactions between the loops can lead to mercury reemissions. Furthermore, the flue-gas sulphur and the hydrogen halide loads influence the process.
Improvements in total mercury removal are focused on the removal and the retention of mercury in both quencher and absorber loops and on the adaption of the operation to the actual flue-gas composition with regard to the sulphur dioxide load as well as on the increase in halide concentrations in the absorber. The use of small shares of burnt lime in the dosed limestone has been shown to improve operation conditions for mercury removal and retention.
The aim of this thesis is to widen the understanding of the mercury behaviour in the dual-loop FGD. As the dual-loop system is influenced by interactions between the two loops and the flue-gas, it is believed that the simple transfer of single-loop mercury studies on the dual-loop process is not possible. It is hoped that increased knowledge of the processes in the dual-loop FGD will help to improve the environmental impact of the system.