Trace element behavior in coal-fired power plants

Abstract

The element concentrations and distributions in coal, bottom ash, pulverized-fuel ash (PFA, ash collected in the ESPs) and fly ash (as present in the flue gases downstream of the ESP) of coal-fired power plants in the Netherlands were studied. Also, special attention was given to minor and trace elements present in the vapor phase in the flue gases downstream of the ESP (As, B, Br, Cl, F, Hg, I, Se). An important goal of this research programme is to establish the relationship between the elements in the different process streams. For this purpose mass balances were determined to obtain a good understanding of the accuracy of the measurements. A total of 16 test series were performed. The research established the collection efficiency of the new generation of high-efficiency cold-side electrostatic precipitators (ESPs) in relation to their collection both of total particulate matter and of each individual element. In an ESP hardly any gaseous inorganic trace elements are removed. All large coal-fired power plants in the Netherlands are equipped with wet flue gas desulfurization (FGD) plants. In one test series the fate of (trace) elements in such an installation was studied in detail. It appeared that 90% of the total particulate matter in the FGD plant was removed, but that the fly dust emitted consisted of about 40% fly ash and 10% gypsum particles; 50% of the fly dust emitted originates in evaporated droplets (saturated with gypsum). Hence, the demisters are of vital importance to minimize emissions. In a wet FGD plant the gaseous inorganic trace elements are also removed. The removal of these elements was measured at all FGD plants in the Netherlands. The heavy metals are introduced in the FGD plant partly by the flue gases, but primarily by the limestone. The gaseous elements are mainly introduced by the flue gases. The (heavy) metals leave the FGD through the following media, in order of decreasing importance: gypsum, sludge, wastewater effluent (the majority of the gaseous elements) and flue gases (only a few gaseous elements).