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THE ROLE OF COAL PROPERTIES AND COMBUSTION CONDITIONS IN THE CAPTURE OF MERCURY BY FLY ASH AND SORBENTS
Citation:
HUTSON, N. D. THE ROLE OF COAL PROPERTIES AND COMBUSTION CONDITIONS IN THE CAPTURE OF MERCURY BY FLY ASH AND SORBENTS. U.S. Environmental Protection Agency, Washington, DC, EPA/600/R-05/152, 2005.
Description:
The U. S. fleet of coal-fired power plants, with generating capacity of just over 300 GW, is known to be the major anthropogenic source of domestic mercury (Hg) emissions. As such, in March 2005, the U. S. Environmental Protection Agency (EPA) promulgated the Clean Air Mercury Rule (CAMR) to reduce emissions of mercury from these plants. Also in March 2005, EPA promulgated the Clean Air Interstate Rule (CAIR) which requires further reductions of nitrogen oxides (NOx) and sulfur dioxide (SO2) from these plants. As a result of CAIR, additional NOx controls and flue gas desulfurization (FGD) systems are expected to be installed. In fact, by 2020, over half of the U.S. coal-fired capacity is projected to be equipped with selective catalytic reduction (SCR) units and FGD technology. These NOx and SOx control retrofits may, in many cases, also effectively reduce Hg emissions. Vapor-phase mercury in the oxidized (+2) state is water soluble and can be collected in a wet SO2 scrubber. SCR units can enhance this capture by catalytically oxidizing elemental mercury and increasing the amount of oxidized Hg entering the wet scrubber. It is generally believed that most of the initial mercury reductions will come as a co-benefit of existing controls used to remove particulate matter (PM), SO2, and NOx.
Deeper mercury reductions in mercury emissions will likely require mercury-specific controls. Many such technologies have been (and continue to be) tested. Of these mercury-specific technologies, sorbent injection technologies are clearly the most developed. More specifically, activated carbon injection (ACI) with conventional activated carbon or chemically-treated carbons have been demonstrated over a wide range of coal types and combustion conditions.
The effectiveness of the various mercury control options (both "co-benefit control" and "mercury-specific control") is significantly impacted by site-specific characteristics such as the combustion conditions, the configuration of existing air pollution controls, and the type of coal burned. This report identifies the role of coal properties and combustion conditions in the capture of mercury by fly ash and injected sorbents.