Mercury transformation and speciation in flue gases from anthropogenic emission sources: a critical review

摘要

Mercury transformation mechanisms and speciation profiles are reviewed for mercury formed in and released from flue gases of coal-fired boilers, 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 of Hg~0 during combustion or smelting in boilers, kilns or furnaces. Decreasing temperature from over 800 °C to below 300 °C in flue gases leaving boilers, kilns or furnaces promotes homogeneous and heterogeneous oxidation of Hg~0 to gaseous divalent mercury (Hg~(2+)/, with a portion of Hg~(2+) adsorbed onto fly ash to form particulate-bound mercury (Hgp/. Halogen is the primary oxidizer for Hg~0 in flue gases, and active components (e.g., TiO_2, Fe_2O_3, etc.) on fly ash promote heterogeneous oxidation and adsorption processes. In addition to mercury removal, mercury transformation also 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 by 34-85 %, electrostatic precipitator (ESP) and fabric filter (FF) remove over 99% of Hgp, and wet flue gas desulfurization system (WFGD) captures 60-95% of Hg~(2+). In non-ferrous metal smelters, most Hg~0 is converted to Hg~(2+) and removed in acid plants (APs). For cement clinker production, mercury cycling and operational conditions promote heterogeneous mercury oxidation and adsorption. The mercury speciation profiles in flue gases emitted to the atmosphere are determined by transformation mechanisms and mercury removal efficiencies by various APCDs. For all the sectors reviewed in this study, Hgp accounts for less than 5% in flue gases. In China, mercury emission has a higher Hg~0 fraction (66-82% of total mercury) in flue gases from coal combustion, in contrast to a greater Hg~(2+) fraction (29-90 %) from non-ferrous metal smelti