Mercury Control Strategy of a Wet Sodium Scrubber

摘要

STEAG's patented mercury removal process has been developed to cost effectively remove mercury from the flue gas of coal fired power plants. The technology has been proven to successfully remove greater than 95% of the oxidized mercury in a limestone forced oxidization scrubber. STEAG and an undisclosed western U.S. utility extended its mercury capture process for a unit that operates a sodium scrubber for SO_2 control. In addition to the chemical parameters, the operational differences between the two types of scrubbers also presented a challenge. The STEAG process utilizes an injection of powdered activated carbon (PAC) into a wet flue gas desulphurization (FGD) absorber module to capture the oxidized mercury in the solution. The industry users prefer technologies that utilize the synergies of existing air pollution control equipment, specifically wet flue gas desulfurization (FGD) for sulfur dioxide (SO_2) control. However, this power station cannot convert enough elemental mercury (Hg~0) to ionic/oxidized mercury (Hg~(2+)) to achieve the desired mercury removal efficiencies due to the lack of a Selective Catalyst Reduction (SCR) system for NO_x control upstream of the wet FGD. To ensure almost quantitative oxidation of mercury, the plant combined the STEAG process with the injection of a calcium bromide (CaBr_2) solution to the coal. The patented addition of CaBr_2 is a well proven, low cost method to oxidize mercury for facilitating its absorption by the wet FGD slurry. The absorbed mercury is captured by adsorption onto the suspended PAC particles within the FGD slurry and was then blown down to a storage pond. The first step of the process was the addition of CaBr_2 to the coal in order to maximize the oxidation of mercury in the flue gas. Once the mercury, which is released from the coal as elemental Hg~0 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. That increased the total capture rate of mercury in the wet FGD absorber, and prevents re-emission issues otherwise often associated with wet FGD's. The mercury laden PAC is then removed from the FGD slurry during the normal blowdown process. STEAG's patented mercury control technology includes the addition of PAC to the wet FGD absorber. Work began in 2004 and was extended in 2009 to include the addition of CaBr_2 to the coal. During the course of the test, the addition of CaBr_2 and a PAC injection to each of the absorbers provided an outlet mercury value of below 1.0 lb/TBTU. This paper will discuss the technologies, equipment used including measurement equipment, and details of the test results.