Mercury transformation mechanisms and speciation profiles arereviewed for mercury formed in and released from flue gases of coal-firedboilers, non-ferrous metal smelters, cement plants, iron and steel plants, waste incinerators, biomass burning and so on. Mercury in coal,ores, and other raw materials is released to flue gases in the form ofHg during combustion or smelting in boilers, kilns or furnaces.Decreasing temperature from over 800 °C to below 300 °Cin flue gases leaving boilers, kilns or furnaces promotes homogeneous andheterogeneous oxidation of Hg to gaseousdivalent mercury (Hg), with a portion of Hg adsorbed onto flyash to form particulate-bound mercury (Hg). Halogen is the primaryoxidizer for Hg in flue gases, and active components (e.g., TiO,FeO, etc.) on fly ash promote heterogeneous oxidation andadsorption processes. In addition to mercury removal, mercury transformationalso occurs when passing through air pollution control devices (APCDs),affecting the mercury speciation in flue gases. In coal-fired power plants,selective catalytic reduction (SCR) system promotes mercury oxidation by34–85 %, electrostatic precipitator (ESP) and fabric filter (FF) removeover 99 % of Hg, and wet flue gas desulfurization system (WFGD)captures 60–95 % of Hg. In non-ferrous metal smelters, mostHg is converted to Hg and removed in acid plants (APs). Forcement clinker production, mercury cycling and operational conditionspromote heterogeneous mercury oxidation and adsorption. The mercuryspeciation profiles in flue gases emitted to the atmosphere are determinedby transformation mechanisms and mercury removal efficiencies by variousAPCDs. For all the sectors reviewed in this study, Hg accounts forless than 5 % in flue gases. In China, mercury emission has a higherHg fraction (66–82 % of total mercury) in flue gases from coalcombustion, in contrast to a greater Hg fraction (29–90 %) fromnon-ferrous metal smelting, cement and iron and/or steel production. The higherHg fractions shown here than previous estimates may imply strongerlocal environmental impacts than previously thought, caused by mercuryemissions in East Asia. Future research should focus on determining mercuryspeciation in flue gases from iron and steel plants, waste incineration andbiomass burning, and on elucidating the mechanisms of mercury oxidation andadsorption in flue gases.
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