The fate of mercury in coal-fired power plants and the influence of wet flue-gas desulphurization

Abstract

The Hg concentrations in coal as fired in power plants in the Netherlands are low, 0.2 mg·kg−1 on average. After combustion the Hg is released partly (between 1 and 98%, on average 42%) in a gaseous phase, which is finally emitted into the air. The other part of the Hg, which remains in the ash is separated from the flue gases by electrostatic precipitators. The variation of the vaporisation percentage of Hg is probably caused by the presence of two chemical forms: Hgo and HgCl2. This may be concluded from the observation that relatively high concentrations of HCl in the flue gases (≈150 mg·m−3) give rise to low Hg concentration in the vapor phase. In cases when the concentrations of HCl are relatively low (≈25 mg·m−3) the amount of Hg in the vapor phase is high. The average gas phase concentrations of Hg in the flue gases, based on 33 measurements with no FGD, is 4.1 μg·mfo−3. In a wet FGD based on the lime/limestone-gypsum process 50 to 70% of the Hg in the flue gases is removed, leaving a residual concentration of 1–2 μg·mfo−3. The emission factor is then about 0.5 mg·GJ−1 or 5 μg·kWhr−1. In one particular measuring serie the fate of Hg was studied in a FGD-installation with a prescrubber.