(639c) Evaluation of Oxidized Mercury Species Desorbed from Hg Sorbents for Coal-Fired Flue Gas Applications

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 species 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 or in cases when coal is burned and contains trace amounts of Br.

An electron ionization quadrupole mass spectrometer (EI-QMS) has been modified for the direct measurement of elemental and oxidized Hg at industrial relevant concentrations (< 15 μg m^-3  or 2 ppbv) 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. Each halogen species is investigated to determine its effectiveness at oxidizing Hg⁰. By burning Hg⁰ with the halogen allows for the effect of homogeneous oxidation to be taken into account. Although homogeneous oxidation is expected to comprise less than 10% of the total oxidation mechanism, it is important to include this component to provide an accurate representation of the mercury species postcombustion and entering the packed-bed reactor for sorbent testing. Ultimately, the extent of homogeneous and heterogeneous oxidation may be measured, with the Hg species directly 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 variety of sorbents have been evaluated, with the MS used to track any desorbed species. This information is critical for evaluating the extent of reaction on a sorbent surface and the potential for desorption of various oxidized Hg species. By investigating the desorbed species as a function of temperature provides indication of the relative strength of adsorption of the various Hg species and subsequently their relative stabilities on the various sorbents tested. These types of investigations will also increase the fundamental understanding of the surface reaction of Hg on a variety of sorbents. A commercial Hg analyzer and speciation system is used to benchmark total oxidized Hg species determined by mass spectrometry since this is the first time Hg speciation of the oxidized have been directly measured and at realistic concentrations.