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

Abstract

Abstract. 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 (Hg p ). Halogen is the primary oxidizer for Hg 0 in flue gases, and active components (e.g., TiO 2 , Fe 2 O 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 Hg p , 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, Hg p 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 smelting, cement and iron and/or steel production. The higher Hg 2+ fractions shown here than previous estimates may imply stronger local environmental impacts than previously thought, caused by mercury emissions in East Asia. Future research should focus on determining mercury speciation in flue gases from iron and steel plants, waste incineration and biomass burning, and on elucidating the mechanisms of mercury oxidation and adsorption in flue gases.

Mercury levels in fly ash and Apc residue from municipal solid waste incineration before and after electrodialytic remediation

“…The WET and SD samples came from modern incinerators in Denmark using activated carbon and limestone slurry. It has been found that activated carbon adsorbs large amounts of both Hg 0 and Hg 2+ and that limestone slurry adsorbs large amounts of Hg 2+ [15]. It follows that systems using activated carbon and slaked limes, such as our samples WET and SD, are more effective in capturing Hg, so higher levels are found in the collected residues, and only a minor fraction of mercury is released to the atmosphere.…”

“…According to Zhang et al [15]), particulate-bound mercury accounts for less than 5% of mercury in flue gas. ESP are designed to capture solid particles, not vapours, so the Nuuk incinerator is possibly capturing only a small fraction of total mercury in the flue gas, which is the fraction adsorbed onto the surface of the FA.…”

A synthesis of research needs for improving the understanding of atmospheric mercury cycling

“…To improve estimation of mercury emission from anthropogenic sources, several measurement studies are recommended (Pacyna et al, 2016;UNEP, 2013;Zhang et al, 2016a). These include (1) measurements of mercury behavior across newly employed air pollution control devices (APCDs), e.g., wet electrostatic precipitators (WESPs) for coal-fired power plants, flue gas desulfurization (FGD) systems, and novel mercury reclaiming towers (MRTs) for non-ferrous metal smelters; (2) continuous measurements of mercury transformation and speciation under different operational conditions, e.g., raw mill on/off modes and the whole cycle of fly ash circulations in cement plants; (3) measurements of mercury transformation and speciation in flue gases from sources (e.g., iron and steel plants, waste incinerators) and assessing the influence of raw materials on mercury transformation and speciation; (4) continuous measurements of mercury emission from sources with large fluctuation (e.g., waste incinerators, crematories), which can be achieved through mercury flow analysis and statistical methods for the inventory developments of these sources; (5) measurements of mercury emission factors and speciation of potential large sources, e.g., mobile oil combustion, reutilization of by-products (e.g., fly ash, waste acid), which will become potential large sources once mercury emissions from current dominant sources are controlled; and (6) mercury emission from artisanal and small-scale gold mining activities in developing countries, a source category that has been poorly characterized.…”

“…Sampler designs for passive GEM sampling were reviewed in McLagan et al (2016), and the suitability of these devices for measuring high and low GEM concentrations was identified. Mercury transformation mechanisms and speciation profiles for Hg formed in and released from flue gases of coal-fired boilers, non-ferrous metal smelters, cement plants, iron and steel plants, waste incinerators and biomass burning were documented in Zhang et al (2016a). Worldwide measurements of Hg spanning the past 4 decades were reviewed in Mao et al (2016) together with the mecha-nisms driving the observed spatiotemporal variations of speciated Hg in various environments including oceans, continents, high elevation, the free troposphere, and low to high latitudes.…”

Development of the application of speciation in chemistry

“…Table 5 Application areas for speciation trace analysis [74] Element Application area for speciation analysis Aluminum Al Aggregates [75] Antimony Sb Redox forms and organoantimony compounds in the environment [76,77] Arsenic As Redox forms and organoarsenic compounds in the environment [77] Arsenic in food products [78] Forms of arsine in air [79] Cadmium Cd Cadmium in food products [80] Chromium Cr Redox forms of chromium, Cr(VI) in the environment [81,82] Lead Pb Forms of lead compounds in the environment [83] Mercury Hg Forms of mercury compounds in the environment [84][85][86] Selenium Se Inorganic and organometallic selenium compounds in the environment [87] Thallium Tl Thallium compounds in river water [88] Tellurium Te Tellurium compounds in the environment [89] Uranium U Forms of uranium compounds in seawater [90] Zinc Zn Forms of zinc compounds in the environment and food [91] The usual analytical procedure comprises filtration (to obtain the soluble and insoluble fractions), separation from the matrix and fractionation (e.g., by high-performance liquid chromatography (HPLC), size exclusion chromatography (SEC), gel permeation chromatography, anion/cation exchange chromatography, CZE, ultrafiltration, dialysis or gas chromatography in the case of volatile species). The isolated species are then determined by atomic absorption spectrometry, flame atomic absorption spectrometry, ICP-optical emission spectrometry, ICP-MS, MS, fluorimetry, atomic fluorescence spectrometry, UV-Vis, or electrochemical methods depending on the type of analyte [74].…”