Task 2.8 - Mercury Speciation and Capture in Scubber Solutions

Abstract

U.S. Environmental Protection Agency (EPA) investigation into health risks associated with mercury emissions from utility steam generators, municipal waste combustion units, and other sources was mandated by the Clean Air Act Amendments (CAAA) of 1990. In anticipation of mercury emission regulation, attention has been focused on quantification of mercury emissions, which require verifiable sampling and analytical techniques. Several sampling and analytical methods are currently under the final stages of development as well as a variety of emission control methods. In particular, wet scrubber systems designed for S2 control in coal-fired utilities have been targeted for mercury control. Conventional wet-scrubbers remove mercury in a variety of soluble oxidized forms. Oxidized mercury is highly water-soluble and can be removed by scrubber slurry, theoretically limited only by gas-film mass transfer. However, since some oxidized mercury forms such as HgClz are borh soluble and volatile, the final fate of mercury trapped in scrubber solutions is unclear. Elemental mercury is not water-soluble, remaining in the vapor state at temperatures through pollution control devices and exiting the stack into the environment. However, notable exceptions to this rule exist. Depending on the type of mercury-sampling method used, an increase ofs 10% in elemental mercury concentrations across wet scrubbers has been metiured but is yet unconllrmed. Also, significant amounts of elemental mercury (metallic form) have been removed during wet scrubber maintenance. In addition, questions concerning 1) the initial speciation between oxidized and elemental forms of mercury in flue gas from coal- fired boilers and 2) the effects of scrubber slurry composition and pH on the mercury species have been raised.