Enrichment and Speciation of Elements in Re-circulated Waters from Desulphurisation at Two Spanish Power Plants

摘要

The enrichment of Al, F, B, Se, Hg, and U and other elements in water streams is the most important consequence of the flue gas desulphurisation (FGD) systems with water re-circulation at two Spanish power plants. The knowledge of the speciation of these elements in FGD waters and in gypsum is of paramount importance should the re-circulation system be interrupted since the waters would require a decontamination treatment. The PHREEQC code 2.0 was used for calculation of the aqueous speciation of these elements and for the saturation index (SI) in the re-circulated waters. The preliminary calculations of PHREEQC reveal that, at pH 7.6 of the re-circulated water at PP1, Al(OH)4~- is the main specie of Al; MgF~+ of F; Hg~ (0) of Hg; SeO3~(-2) of Se, and (UO2)3(OH)5~+ of U. At PP2, at pH～5.0 of the re-circulated water, AlF3 is the main specie of Al and F, Hg ~(0) of Hg, SeO3~(-2) Se, H3BO3 of B, and UO2(SO4)2~(-2) and UO2(SO4) of U. A relationship between water enrichment and gaseous Hg retention may explain the differential Hg speciation OUT-FGD between power plants observed in previous studies. The different speciation of Hg OUT-FGD may be attributable to: a) the high soluble salt concentrations of the FGD water streams at PP2 which reduce the gaseous solubility and probably, the gas retention efficiencies; b) the entraining of HgCl2 droplets by the gas OUT-FGD; and c) the operational such as, limestone purity, use of additives, fluoride and/or sulphate complexes, and the S/F and S/Cl ratios in the scrubber. The potential parameters controlling Hg speciation and partitioning are currently investigated.