Retención de mercurio en sistemas de desulfuración en fase húmeda

Resumen

[EN] Coal combustion is considered to be the largest source of anthropogenic mercury emissions. At present, there is no purpose-designed technology to reduce such emissions. However, recently several studies have been aimed at controlling mercury emissions from power plants. In most cases flue gas cleaning systems originally installed to prevent other forms of pollution are used to improve mercury retention. For example, desulphurization plants installed to control SO2 emissions are used to retain oxidized mercury species under appropriate operating conditions. The aim of this study was to investigate the mechanisms related to mercury retention in wet flue gas desulphurization systems (scrubbers) and to optimize the operating conditions so as to maximize mercury retention. This work is divided into two parts: an industrial-scale study and a laboratory-scale study. The behavior and distribution of mercury was studied in two separate power stations (PP1 and PP2) and in PP2 under different operating conditions. A laboratory-scale device was designed and built in such a way that the mercury reactions could be observed in the scrubber more easily than would be possible in an industrial plant. The industrial-scale results show that the mercury removal efficiency of the FGD system depended on the operating conditions and the concentration of mercury in the flue gas, although the proportion of Hg2+ in the input gases in the FGD systems was similar. Significant differences were detected in mercury capture between the two power plants and in PP2 operating under different conditions. Data obtained from the laboratory-scale studies show that the distribution of mercury in the FGD by-products was different in the two power plants due to variations in the pH of the gypsum slurry. A relationship between the removal efficiency of Hg2+ and the liquid-to-gas ratio was also detected. The retention of mercury is improved by decreasing the pH of the slurry. However, a lower pH causes the dissolution of the mercury species in the liquid fraction. Mercury retention mechanisms in FGD systems are dependent on the concentration of sulfite ions, which are formed in the slurry. In addition to sulfite ions, metal ions from the limestone and fly ash particles from the electrostatic precipitator also contribute to the re-emission of elemental mercury. Although the re-emission of mercury is inhibited by the formation of halide species in solutions containing sulfite ions, this does not occur with gypsum slurry. Re-emission in this case can be controlled by using forced oxidation or additives such as NaHS, Na2S2O3 and TMT, which serve to concentrate the mercury in the solid fraction of the slurry.