Influence of an aluminium additive in aqueous and solid speciation of elements in flue gas desulphurisation (FGD) system

The acidification of the aqueous phase induced by the use of an aluminium additive (sulphate), to increase the SO2 trapping efficiency in flue gas desulphurisation (FGD) systems, results in significant differences in the speciation and partitioning of elements between the aqueous and solid phases of gypsum slurries from two Spanish coal-fired power plants (PP1 and PP2). The Al-additive increases the presence of the aqueous Al–Fx complexes in the gypsum slurry from PP2-2007. In the absence of (PP1) or using lower dosages (PP2-2008) of the Al-additive, the predominance of MgF2 and CaF2 in the FGD-gypsum slurries causes their subsequent precipitation, increasing the leachable potential of gypsum waste. This study evaluates the effect of the Al-additive on the fate of a number of elements during FGD by combining a number of experimental and modelling tools. The results suggest the precipitation of CaF2, MgF2, and CaUO4 in the PP1 FGD-gypsum, and Al2O3, AlHO2, KAl3(OH)6(SO4)2 and MgF2 in the PP2 2007 and 2008 FGD-gypsum. When Al-additive is added low amounts of heavy metals are found in the leachates of the FGD-gypsums since these remain mostly in solution of gypsum slurry. The reverse behaviour is found for Se.