On-Line Detection of Elemental and Oxidized Hg Using Quadrupole Mass Spectrometry: Applications and Limits
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Coal-fired power plants provided between 45 and 50% of the United States net energy generation in the years between 1999 and 2010. While alternate sources of energy , including natural gas and renewable sources , will continue to grow in importance , coal will remain as a necessary item in the energy portfolio of the US. A drawback to coal combustion is the release of mercury , Hg , inherent in the coal matrix. The emissions of Hg was recently regulated by the US EPA in the Mercury and Air Toxics Standards (MATS) ruling in December 2011 and significant efforts are underway to bring power plants into compliance with the regulation. While there are many options for compliance , the standard options , such as activated carbon , rely on the oxidation of Hg and adsorption on a sorbent. Mercury exists in various forms in the flue gases of coal combustion , from its elemental state (Hg0) at the boiler exit , to various oxidized forms (Hg2+) as it interacts with fly ash particle surfaces as the flue gas cools. The exact form of the oxidized mercury is , at this point , an outstanding question. Thermodynamic calculations suggest that the favored form in coal combustion flue gas is HgCl2 , but HgO and HgBr2 could exist under certain conditions , such as the introduction of Br into the combustion process. An electron ionization quadrupole mass spectrometer (EI-QMS) has been modified for the direct measurement of elemental and oxidized Hg at industrial relevant concentrations (> 5 μg m-3) from a methane combustion flue gas. Hg0 and HCl , Cl2 or Br2 are introduced pre-combustion and passed through the methane combustion flame to obtain halogen radicals , representative of the real oxidizing environment of a coal-fired boiler. The extent of homogeneous oxidation is measured , and the Hg species are identified through mass spectrometry. In addition , the oxidized species created upon exposure to surfaces present in coal combustion , such as fly ash and activated carbon , are identified through the use of packed bed experiments. A commercial Hg analyzer and speciation system is used to benchmark Hg concentrations determined by mass spectrometry.