Influence of Flue-Gas Components on Mercury Removal and Retention in Dual-Loop Flue-Gas Desulfurization

Due to the system, the dual-loop wet limestone flue-gas desulfurization process is assumed to be well-suited for mercury retention. As the behavior of mercury in the dual-loop process has been insufficiently dealt with in literature so far, the influence of sulfur dioxide and halides on mercury removal and retention was investigated. The process conditions in the loops are highly relevant for the removal and retention of mercury. The circulating slurries there differ in pH-value and oxidation–reduction conditions as well as in halides and sulfite concentrations and solid compositions. This work presents investigations that were conducted at a laboratory-scale test rig treating the flue gas of a pulverized coal combustion facility. The share of the removal of mercury in both loops was investigated as well as the removal of oxidized mercury compounds, the mercury re-emission, and the total mercury removal efficiency. Both loops showed comparable removal rates of oxidized mercury compounds. In the quencher, the removal and retention of mercury is assumed to be improved by the formation of stable mercury complexes with chloride. The decomposition of mercury complexes formed with sulfite and redox reactions are supposed to decrease mercury retention in the absorber. A higher sulfite concentration in the quencher induced by higher SO₂ concentration in the flue gas can increase mercury removal by the enhanced formation of mercury complexes with sulfite. At the same time it can promote re-emissions due to a higher share of sulfite on total S(IV) at the resulting higher pH-value of the quencher. Furthermore, high S(IV) concentrations might result in an increased mercury adsorption on particles. Increased chloride concentrations in the absorber can lead to higher removal rates of sulfur dioxide and improved mercury retention as well as to relatively high shares of mercury in wastewater, whereas the latter constitutes the desired mercury pathway.