Novel Mercury Control Strategy Utilizing Wet FGD in Power Plants Burning Low Chlorine Coal

摘要

Several processes have been developed to cost effectively remove mercury and other heavy metals from combustion gas of coal-fired power stations. Preferred technologies utilize the synergies of existing air pollution control equipment, specifically wet flue gas desulfurization (FGD) for sulfur dioxide (SO_2) control. However, several power stations cannot convert enough elemental mercury (Hg~0) to ionic/oxidized mercury (Hg~(2+)) to achieve the desired mercury removal efficiencies. This could be caused by the lack of a Selective Catalytic Reduction (SCR) system for NOx control upstream of the wet FGD or the type of coal burned in the unit, for example low-halogen coals such as Powder River Basin (PRB), lignite. To ensure almost quantitative oxidation of mercury, Evonik extended its patented mercury capture process to achieve the desired removal rates of greater 90% by utilizing a second patented technology, an additive to the coal. The patented addition of calcium bromide (CaBr_2) is a well known low cost method to oxidize mercury for facilitating its absorption in the wet FGD slurry. The absorbed mercury is captured by adsorption onto powdered activated carbon (PAC) suspended within the FGD slurry and separated in the FGD waste water treatment system.The first step of the process is Vosteen Consulting's patented technology~(14) of bromide addition to the coal prior to combustion in order to maximize the oxidation of mercury in the combustion gas. Once the mercury, which is released from the coal as elemental Hg° in the gas phase, is oxidized to Hg~(2+), it can be easily absorbed into the wet FGD slurry, to which submerged PAC has been added. The absorbed, oxidized mercury dissolved in the FGD slurry adsorbs onto the suspended PAC. This increases the total capture rate of Hg in the wet FGD absorber, and prevents re-emission issues otherwise often associated with wet FGD's. The mercury laden PAC is removed from the FGD slurry during the normal gypsum separation and processed with the FGD blowdown in the FGD's waste water treatment system.The PAC laden with mercury and other dissolved heavy metals, such as for example selenium halogenide, remains in the FGD waste water stream, which is treated in a two stage FGD waste water treatment system. The FGD waste water treatment system is designed such that it generates two distinctly different types of filter cakes. One filter cake, which amounts to approximately 97%wt of the combined filter cake, is the virtually mercury and heavy metal free fraction. The second filter cake accounts for the remaining balance and consists of a mercury and heavy metal rich fraction.Evonik's patented mercury control technologies include the addition of PAC to the wet FGD absorber as well as the selective mercury removal in a specially designed two stage FGD waste water treatment system. Work began in 2004 and was extended in 2009 to include the addition of CaBr2. Extensive full scale testing in Evonik's coal-fired power plants revealed a Hg oxidation of well over 95% as well as a total mercury capture rate of greater 95%. This paper discusses the technologies, equipment used including measurement equipment and details of the test results.