Trace metal emissions from the Estonian oil shale fired power plant

Emission levels of selected trace metals from the Estonian oil shale fired power plant were studied. The plant is the largest single power plant in Estonia with an electricity production capacity of 1170 MWe (1995). Trace metals were sampled from the flue gases by a manual method incorporating a two-fraction particle sampling and subsequent absorption of the gaseous fraction. The analyses were principally performed with ICP-MS techniques. The trace metal contents of Estonian oil shale were found to be in the same order of magnitude as of coal on average. The high total particle concentrations in the flue gases of the studied oil shale plant contribute, however, to clearly higher total trace metal emission levels compared to modern coal fired power plants. Although the old electrostatic precipitators in the plant have been partly replaced by state-of-the-art electrostatic precipitators, the majority of the boilers are currently equipped with the old precipitators. The results of the study show remarkably high concentrations of toxic heavy metals in the flue gases (e.g., Pb, Zn, Mn and As: >200 μg/m3 each) and clear accumulation of Pb, Cd, Zn, Tl and As on the fly ash. Additionally, significant portions of some heavy metals (e.g., Hg, Cd, As and Pb) were found in the absorption liquids of the sampling line, indicating the presence of either vaporous metal species or metals condensed on very small particles in the flue gases. The experimental results were interpreted by theoretical modeling using Global Equilibrium Analysis. The modeling could reasonably well explain the experimental results, especially the enrichment of certain trace metals in the fly ash as a result of volatilization/condensation phenomenon.