Impact of the state of-the-art of flue cleaning on mercury species emissions from coal-fired steam generators

When balancing the element mercury (Hg) two coal-fired power plant units - one with slag tap boilers (ST, 2 x 220 MW) and one with a dry bottom boiler (DB, 475 MW) were compared. Both systems are provided with electrostatic precipitators (ESP), nitrogen oxides removal (DeNOx) and flue gas desulfurization (FGD) systems. The Hg in the flue gas is predominantly in gas phase. Only 15% of the Hg introduced by the coal leaves the unit with the bottom or fly ash. Depending on the operating mode, 30 to 40% of the Hg is separated in the FGD systems. The overall separation rate for the total system ranges between 45 to 55%, the residue is emitted in the form of gaseous Hg species. At full load, the Hg concentration in the cleaned gas is less than 6 μg/m32. In the flue gas path of another dry bottom boiler (DB1, 480 MW) the concentrations of the gaseous species of bivalent mercury (Hg2+), elemental mercury (Hg0), and total mercury content (ΣHg) were determined. The sum of the concentration of Hg2+ and Hg0 is in agreement with the measurement of ΣHg. Directly downstream of the boiler Hg2+ dominated with 77%, while Hg0 amounts to 23%. In the high-dust DeNOx system Hg0 is oxidized almost completely to Hg2+ (96%). Air heater and electrostatic precipitator do not influence the Hg species concentrations. The FGD system eliminates approximately 80% of the Hg2+. At the same time the quantity of Hg0 increases by the factor 10. In the cleaned gas Hg0 dominated with 76% as compared to Hg2+ with 24%. At full load the concentration of σHg in the cleaned gas is also below 6μg/m3. 13 refs., 4 figs., 1 tab