Impact of the state-of-the-art of flue gas cleaning on mercury species emissions from coal-fired steam generators

When balancing the element mercury (Hg) two coal-fired power plant units — one with slag tap boilers (ST, 2 × 220 MW) and one with a dry bottom boiler (DB, 475 MW) were compared. Both systems are provided with electrostatic precipitators (ESP), nitrogen oxides removal (DeNO_x) and flue gas desulfurization (FGD) systems. The Hg in the flue gas is predominantly in gas phase. Only 15 % of the Hg introduced by the coal leaves the unit with the bottom or fly ash. Depending on the operating mode, 30 to 40 % of the Hg is separated in the FGD systems. The overall separation rate for the total system ranges between 45 to 55 %, the residue is emitted in the form of gaseous Hg species. At full load, the Hg concentration in the cleaned gas is less than 6 μg/m³. In the flue gas path of another dry bottom boiler (DB1, 480 MW) the concentrations of the gaseous species of bivalent mercury (Hg²⁺), elemental mercury (Hg⁰), and total mercury content (Σ Hg) were determined. The sum of the concentrations of Hg²⁺ and Hg⁰ is in agreement with the measurement of Σ Hg. Directly downstream of the boiler Hg²⁺ dominates with 77 %, while Hg⁰ amounts to 23 %. In the high-dust DeNO_x system Hg⁰ is oxidized almost completely to Hg²⁺ (96 %). Air heater and electrostatic precipitator do not influence the Hg species concentrations. The FGD system eliminates approximately 80 % of the Hg²⁺. At the same time the quantity of Hg⁰ increases by the factor 10. In the cleaned gas Hg⁰ dominates with 76 % as compared to Hg²⁺ with 24 %. At full load the concentration of Σ Hg in the cleaned gas is also below 6 μg/m³.     

An experimental study of two potential methylation agents of mercury in the atmosphere: CH3I and DMS

Experimental results from a study of the gas and aqueous phase reactions of elemental mercury (Hg⁰) with methyl iodide (CH₃I) and dimethyl sulfide (DMS) are presented. In aqueous phase experiments with CH₃I we found no observable increase in methyl mercury (MeHg). A small formation of MeHg, however, was observed in some (but not all) gas phase experiments in sunlight. A loss of Hg⁰ and a simultaneous formation of oxidized mercury (Hg(II)) was also observed in these experiments. No reaction, neither methylation or oxidation, was found between Hg⁰ and DMS under any conditions investigated. These experiments suggest that a simple homogeneous gas or aqueous phase methylation of Hg⁰ by DMS or CH₃I in the atmosphere cannot account for the significant levels of MeHg observed in precipitation.     

Development and application of a reactive plume model for mercury emissions

A reactive plume model that includes atmospheric chemical reactions of mercury was developed. The model simulates advective transport with the mean wind flow; horizontal and vertical turbulent diffusion; gas phase; aqueous-phase and particulate chemistry; cloud microphysics; wet deposition and dry deposition. The model was applied to the simulation of clear sky, non-precipitating cloud and precipitating cloud scenarios. No significant mercury chemistry occurs in the absence of droplets. In clouds, Hg(II) is reduced to Hg(0) with more reduction taking place in precipitating clouds than in non-precipitating clouds.     

Mercury speciation adsorption (MESA) method for combustion flue gas: Methodology,artifacts, intercomparison,and atmospheric implications

Chemical speciation of mercury (Hg) in a wide variety of combustion flue gas matrices has been determined using the mercury speciation adsorption (MESA) method. The MESA sampling system for gas phase Hg species employs a series of heated, solid phase adsorbent traps. Flue gas oxidized Hg species (Hg(II) and MMHg) are adsorbed by a potassium chloride (KCl) impregnated soda lime sorbent. Elemental Hg (Hg⁰) is collected by an iodated carbon sorbent after passing through the KCl/soda lime sorbent. Total Hg (Hg_t) is determined by summation of species. In the laboratory, cold vapor atomic fluorescence spectroscopy (CVAFS) is used for detection of Hg collected on the solid sorbents, after appropriate sample digestion and preparation. The MESA method has been evaluated for species stability, matrix effects, breakthrough, artifacts and precision. Based on eight duplicate samples a mean precision of 6.8% 11% and 4.5% (relative percent difference) has been calculated for Hg⁰, Hg(II) and Hgt respectively. Intercomparison of the MESA method with other methods shows very good agreement for Hgt. Mass balance calculations at 5 sites range from 75 to 140%, with a mean of 97±25%. Overall mean speciation results from 19 separate determinations suggest that Hg(II) has a 1 sigma range of 40 to 94% in coal combustion flue gas at, the inlet to pollution control devices.     

Use of a refluxing mist chamber for measurement of gas-phase mercury(II) species in the atmosphere

As part of current efforts to understand the cycling of mercury (Hg) in the atmosphere, information is needed on its atmospheric speciation. Almost no data exists on water-soluble Hg(II) species in ambient air. A new technique for measuring gas phase water soluble Hg(II) species has been developed, utilizing a high-flow refluxing mist chamber. Extensive testing has been carried out, including attempts to rule out production of artifact Hg(II). Measurements at two locations (East-Central Tennessee and the Ohio-Indiana border) found approximately 0.05–0.15 ng/m³ of reactive Hg(II), representing ca. 3 to 5% of the total gaseous Hg. Limited tests of artifact Hg(II) production in the mist chamber by ozone oxidation and co-sampled aerosol Hg(II) suggest that the majority of the collected Hg(II) exists in ambient air in the gas phase.     

Estimating Gaseous Mercury Emissions from Contaminated Floodplain Soils to the Atmosphere with Simple Field Measurement Techniques

The atmospheric emission of mercury (Hg) from a contaminatedwetlands system was studied in the floodplains along the riverElbe (Northern Germany). Results suggest that wetlands can beimportant transformation and phase transfer regions, linking theterrestrial, aquatic and atmospheric compartments of regionalbiogeochemical Hg cycles. Fluxes determined by flux chambermeasurements averaged 43 ± 5 ng m^-2 h^-1. Additionally,soil gas probe sampling was introduced to determine mercuryconcentrations in soil air. This technique shows some promise fordetecting and confining mercury contamination in soils. We alsopropose that measurements of total gaseous mercury (TGM) in soilair and the near-surface atmosphere, in combination with simplesoil physical parameters, may be suitable for calculatingsemiquantitative estimates of Hg evaporation from contaminatedsoils, based on laminar diffusion considerations. The results arecompared to other Hg flux measurements, and the advantages anddisadvantages of different approaches to quantify Hg emissionsfrom soils are discussed, especially with regard to possiblesystematic bias.     

Mercury Methylation Capacity and Removal of Hg Species from Aqueous Medium by Cyanobacteria

Most technologies used for decontamination presents good results for high concentrations, but limitations for lower ones. The desirable Hg concentration in the water is extremely low because of its toxicity. The aims of this study were to evaluate inorganic mercury (Hg²⁺) and methylmercury (CH₃Hg⁺) toxicity in Nostoc paludosum, to assess the potential of this cyanobacteria strain to remove these Hg species from aqueous medium and also to investigate Hg methylation by the cyanobacteria. CH₃Hg⁺ determination was performed by gas chromatography-pyrolysis-atomic fluorescence spectrometry in cultures exposed to a concentration of 20 μg L^?1 for 30 days. Both Hg species were removed from the supernatant, ranging from 73 to 96% of Hg²⁺ and from 73 to 95% of CH₃Hg⁺. Ultrastructural Hg²⁺ effects in the cyanobacteria cells investigated by transmission electron microscopy revealed higher production of glycogen, cyanophycin, and intrathylacoidal spaces than the control group. When Hg was added to the culture in the form of CH₃Hg⁺, a decrease corresponding to approximately 60% of the initial concentration due to Hg volatilization was observed. The production of CH₃Hg⁺ by the cyanobacteria was detected in concentrations near the limit of detection (0.0025%) of the bioaccumulated THg. This is an advantage for biotechnological decontamination applications, as CH₃Hg⁺ is a very toxic specie and can be bioaccumulated and biomagnified. The results demonstrated that cyanobacteria cells are an efficient alternative to retain and/or remove Hg at low concentrations and they constitute a potential tool for a “final cleaning” of contaminated waste water.     

Ambient levels and dry deposition fluxes of mercury to Lakes Huron,Erie and St. Clair

Ambient concentrations and dry deposition fluxes of Hg in the gas and particle phase to Lakes St. Clair, Erie and Huron were estimated with a hybrid receptor-deposition model (HRD). The ambient gas and particulate phase Hg concentrations were predicted to vary by a factor of 12 to 18 during the transport of air masses traversing the lakes. The ensemble average deposition fluxes of fine particle Hg ranged from 7 pg/m²-h to 15.3 pg/m²-h over Lake St. Clair, 0.5 to 4.2 pg/m²-h over Lake Huron and 5.1 to 20.6 pg/m²-h over Lake Erie. The deposition flux of coarse particle Hg was in the range of 50 to 84 pg/m²-h over Lake St. Clair, 4.7 to 24.2 pg/m²-h over Lake Huron and 5.1 to 20.6 pg/m²-h over Lake Erie. Gaseous Hg volatilized at a rate of 0.21 to 0.52 ng/m²-h from Lake Huron and 0.13 to 0.36 from Lake Erie. Gas phase Hg was deposited at a rate of 5.9 ng/m²-h and/or volatilized at a rate of 0.5 ng/m²-h from Lake St. Clair depending upon the location of the sampling site used in the HRD model. The effect of meteorological conditions, particle size distributions and type and location of the sampling sites played an important role in the transfer of atmospheric Hg to and/or from the lakes.     

Particulate mercury in the atmosphere: Its significance,transport, transformation and sources

The importance of participate mercury (Hg(p)) in the transport, chemistry and deposition of this toxic metal has long been underestimated and largely ignored. While it was once believed to constitute a small percentage of total atmospheric mercury, Hg(p) may contribute a significant portion of the deposition of this metal to adjacent natural waters. Recent measurements of Hg(p) in several urban/industrial areas have documented that Hg can be associated with large particles (>2.5 μm) and in concentrations similar to those of the vapor phase Hg (ng/m³). As part of ongoing effort to diagnose the sources, transport and deposition of Hg to the Great Lakes and other Great Waters, the University of Michigan Air Quality Laboratory (UMAQL) has investigated the physical and chemical properties of particulate-phase Hg in both urban and rural locations. It appears that particulate Hg may be the one of the most difficult of the Hg measurements to perform, and perhaps the one of the most important for deposition and source apportionment studies. Particulate Hg concentrations measured in rural areas of the Great Lakes Region and Vermont ranged from 1 to 86 pg/m³ whereas Hg(p) levels in urban/industrialized areas were in the range 15 pg/m³ to 1.2 ng/m³.     

The fate of mercury in coal-fired power plants and the influence of wet flue-gas desulphurization

The Hg concentrations in coal as fired in power plants in the Netherlands are low, 0.2 mg·kg^?1 on average. After combustion the Hg is released partly (between 1 and 98%, on average 42%) in a gaseous phase, which is finally emitted into the air. The other part of the Hg, which remains in the ash is separated from the flue gases by electrostatic precipitators. The variation of the vaporisation percentage of Hg is probably caused by the presence of two chemical forms: Hg^o and HgCl₂. This may be concluded from the observation that relatively high concentrations of HCl in the flue gases (≈150 mg·m^?3) give rise to low Hg concentration in the vapor phase. In cases when the concentrations of HCl are relatively low (≈25 mg·m^?3) the amount of Hg in the vapor phase is high. The average gas phase concentrations of Hg in the flue gases, based on 33 measurements with no FGD, is 4.1 μg·m_fo ^?3. In a wet FGD based on the lime/limestone-gypsum process 50 to 70% of the Hg in the flue gases is removed, leaving a residual concentration of 1–2 μg·m_fo ^?3. The emission factor is then about 0.5 mg·GJ^?1 or 5 μg·kWhr^?1. In one particular measuring serie the fate of Hg was studied in a FGD-installation with a prescrubber.     

Mercury and Methylmercury Dynamics in the Hyporheic Zone of an Oregon Stream

The role of the hyporheic zone in mercury (Hg) cycling has received limited attention despite the biogeochemically active nature of this zone and, thus, its potential to influence Hg behavior in streams. An assessment of Hg geochemistry in the hyporheic zone of a coarse-grained island in the Coast Fork Willamette River in Oregon, USA, illustrates the spatially dynamic nature of this region of the stream channel for Hg mobilization and attenuation. Hyporheic flow through the island was evident from the water-table geometry and supported by hyporheic-zone chemistry distinct from that of the bounding groundwater system. Redox-indicator species changed abruptly along a transect through the hyporheic zone, indicating a biogeochemically reactive stream/hyporheic-zone continuum. Dissolved organic carbon (DOC), total Hg, and methylmercury (MeHg) concentrations increased in the upgradient portion of the hyporheic zone and decreased in the downgradient region. Total Hg (collected in 2002 and 2003) and MeHg (collected in 2003) were correlated with DOC in hyporheic-zone samples: r ²?=?0.63 (total Hg-DOC, 2002), 0.73 (total Hg-DOC, 2003), and 0.94 (MeHg-DOC, 2003). Weaker Hg/DOC association in late summer 2002 than in early summer 2003 may reflect seasonal differences in DOC reactivity. Observed correlations between DOC and both total Hg and MeHg reflect the importance of DOC for Hg mobilization, transport, and fate in this hyporheic zone. Correlations with DOC provide a framework for conceptualizing and quantifying Hg and MeHg dynamics in this region of the stream channel, and provide a refined conceptual model of the role hyporheic zones may play in aquatic ecosystems.     

Effects of a fulvic acid on the speciation and mobility of mercury in aqueous solutions

The presence of humic substances in aqueous systems generally has a large impact on speciation as well as on mobility of metal ions at trace levels. At pH<pH_zpc, the humic substances tend to adsorb and enhance the uptake of trace metals from the solution phase. At pH>pH_zpc, the reverse effect is expected. Experimental data on the adsorption of Hg on an oxide (alumina) in the presence of a fulvic acid (FA; 0 to 25 mg L⁻¹) is reported in the present work. Generally the presence of the FA enhances the Hg adsorption in the whole pH-range studied (2.5 to 9.5). A Hg-FA complex is the dominant species already in the presence of 1 mg L⁻¹ FA in the solution phase. Chloride increases the adsorption at pH<pHZpc possibly related to the formation of the negatively charged HgCl₃ ⁻ species. The Hg adsorption is compared with Zn and Cd in the corresponding systems. The mobility of these bivalent metals in the aqueous environment is discussed.     

On-line measurement of mercury in simulated flue gas

Total and elemental mercury (Hg) in simulated flue gas was measured on-line by a commercial Semtech^® Hg analyzer. This instrument is based on Zeeman modulated Atomic Absorption Spectrometry. Both a wet chemical solution and a dry physical pyrolysis converter were applied to reduce the Hg(II) in the gas before leading it to the detector. Results show that the Semtech^® analyzer is suitable for measuring elemental Hg even in the presence of 2500 ppm SO₂ and 500 ppm HCl. For the measurement of Hg(II), the wet method is suitable only at SO₂ concentrations <50 ppm. The dry thermal converter filled with crushed quartz chips together with a small amount of soda lime converts Hg(II) quantitatively at laboratory scale. This result is promising, since the trace gases delivered to the system, such as SO₂, and HCl are similar to those produced in coal burning and waste incineration processes. The limiting factor for such a converter is its comparatively short life time of performance, about 20 hours.     

A contribution to the environmental biology of mercury accumulation in plants

Samples of six common plant species collected in the old mining areas near Prince George, British Columbia (Canada) and Mount Amiata, Tuscany (Italy) show remarkable similarities in the variation of plant/soil Hg concentration ratio with soil Hg content irrespective of species or other biological differences. In contrast, plants sampled in the geothermally active areas of New Zealand, Hawaii and around Mount St. Helens exhibit more individuality in the concentration ratio to soil Hg relationship, but the relationships are distinctly different from the mine site specimens. This distinction is particularly evident when the same species of Equisetum and Plantago taken from these two different areas are compared. These and other data support the hypothesis that specific local environmental factors strongly influence the accumulation of Hg in plants even when the immediate soil concentrations are the same. Our findings show that some plants contain concentrations of total Hg as high as 5500 to 14000 μg kg^?1 (dw).     

Chemical reactions of mercury in combustion flue gases

Atmospheric Hg is present in different physical and chemical forms, which determine its atmospheric transformation and transport capacities. The chemistry of Hg in flue gases is thus of importance for the deposition pattern around point source emissions. In order to apply Hg cleaning methods in flue gases its speciation is also of importance. To investigate this under realistic conditions, a 17 kW propane fired flue gas generator was used, while the kinetics of specific Hg reactions were investigated in a continuous flow reactor. Elemental Hg is readily oxidized by Cl2 and HCl both at room and at elevated temperatures (up to 900 °C) but not by NH₃, N₂O, SO₂ or H₂S. It reacts with O₂ if a catalyst, such as activated carbon, is present. A slow reaction between Hg and NO₂ has also been noted.     

Mercury methylation,uptake and bioaccumulation by the earthworm Lumbricus terrestris (Oligochaeta)

To date, most studies about mercury (Hg) methylation and bioaccumulation have focused on aquatic ecosystems. In contrast, information regarding the biogeochemical cycle of Hg in terrestrial ecosystems is scarce. Considering the relevance of earthworms in soils, it is very important to study their role in the bioaccumulation and transformation of Hg species (inorganic Hg, IHg, and monomethylmercury, MeHg). The aim of this experimental study was to compare the uptake and bioaccumulation of MeHg and IHg in the earthworm Lumbricus terrestris exposed to soils freshly spiked with inorganic Hg as well as historically contaminated soils. The study consisted of a 28-day uptake phase in Hg (spiked and natural) contaminated and non-contaminated soils followed by a 14-day depuration phase in non-contaminated soils. Soils were characterized by determining not only Hg concentrations (total Hg, MeHg and acid-labile Hg) but also analysed for other physicochemical parameters that can influence the fate of Hg within the earthworm–soil system. Mercury species were determined in earthworms (whole organism) exposed to Hg contaminated and non-contaminated soils. Mercury availability in soils seems to be the main factor controlling the uptake and bioaccumulation of Hg species because, according to kinetic data, the spiked IHg was more readily assimilated and methylated by earthworms. Bioaccumulation factors (BAFs) for MeHg and total Hg were also higher in spiked than in naturally Hg-contaminated soils. In addition, BAFs for MeHg (ranging from 0.8 to 17.3) were higher than those for total Hg (between 0.02 and 0.62) which suggests that MeHg was more easily bioaccumulated by this earthworm species and also that earthworms may actively contribute to MeHg production in soils.     

Use of the mercury cycling model (MCM) to predict the fate of mercury in the Great Lakes

In response to U.S. EPA's proposed Great Lakes water quality criteria for mercury (Hg), a fieldvalidated Hg cycling model (MCM) was used to predict Hg levels in the abiotic and biotic components of Lake Superior and Lake Erie. The U.S. EPA criteria are based on water column Hg concentrations and simple trophic level transfer and, thus, do not consider sediment interactions and water chemistry factors. The model, using data from published reports, was run to simulate a 25 year steady state period. For these simulations, methylmercury (MeHg) represented 5% of total Hg in Lake Erie and 8% of total Hg in Lake Superior. These proportions are roughly 3–5 times lower than U.S. EPA's estimate that MeHg contributes about 25% of total Hg in the water column of the Great Lakes. The predicted median concentrations of total Hg in top-carnivore fish were 0.13 mg/kg in Lake Superior and 0.16 mg/kg in Lake Erie. Predicted median MeHg concentrations in Lake Superior and Lake Erie (water column) were 0.019 and 0.075 ng/L, respectively. For both lakes, most (>55%) of the Hg was partitioned to sediments. Although the MCM simulation does have practical limitations (e.g., lakes are treated as fully-mixed open systems), the results demonstrate that generic assumptions of Hg behavior in all Great Lakes waterbodies are too simplistic.     

The impact of cloud chemistry on photochemical oxidant formation

Photochemical smog formation at the regional scale is a phenomenon of concern in northern America and Europe. It is important to include a treatment of cloud processes in regional photochemical oxidant models because clouds may affect the overall chemistry of regional photochemical oxidant formation. This paper focuses on the development and application of a model that describes the chemistry of oxidant formation in clouds. The model consists of a chemical kinetic mechanism for the gas phase, mass transfer and thermodynamic equilibrium between the bulk gas phase and the cloud droplets, and a detailed chemical kinetic mechanism for the aqueous phase. Model simulations were conducted for typical conditions in the northeastern U.S. using (1) gas-phase chemistry only and (2) gas-phase and cloud droplet chemistry. Comparisons of these two sets of model simulations show that O₃ formation is considerably reduced in clouds despite its low solubility. The principal causes of lower O₃ formation rates are (1) the high solubility of aldehydes, which are a main source of HO₂ radicals, (2) the scavenging of radicals by cloud droplets, and (3) the lower photolytic rates inside the cloud.     

Transformations of Pesticides in the Atmosphere: A State of the Art

The current knowledge about transformation rates and products of pesticides in the atmosphere is reviewed. Reactive species and their concentrations in the atmosphere are presented. Reactions of pesticides with these species (including photolysis) in the gas and the particulate phase are evaluated from available experimental data. The potential of estimation methods is discussed. Experimental techniques for laboratory and outdoor measurements are reviewed. Finally, an estimation is made of uncertainties in atmospheric lifetimes due to chemical or physical reactions. It is concluded that the most important transformation of pesticides in the atmosphere is due to reaction with OH radicals. Very few experimental data for pesticides are available though. The levels of uncertainty in OH radical concentrations are acceptable, however, for a proper estimation of atmospheric removal rates due to reactions with OH radicals of those pesticides for which experimental transformation rates (of homologues) are available.     

Atmospheric mercury in northern Wisconsin: Sources and species

The atmospheric chemistry, deposition and transport of mercury (Hg) in the Upper Great Lakes region is being investigated at a near-remote sampling location in northern Wisconsin. Intensive sampling over two years and various seasons has been completed. A multi-phase collection strategy (gas-, particle- and precipitation-phases) was employed to gain insight into the processes controlling concentrations and chemical/physical speciation of atmospheric Hg. Additional chemical and physical atmospheric determinations (e.g. ozone, particulate constituents, meteorology) were also made during these periods to aid in the interpretation of the Hg determinations. For example, correlations of Hg with ozone, sulfur dioxide and synopticscale meteorological features suggest a regionally discernible signal in Hg. Comparison to isosigma backward air parcel trajectories confirms this regionality and implicates the areas south, southeast and northwest of the site to be sources for Hg. Particle-phase Hg (Hg_p) was found to be approximately 40% in an oxidized form, or operationally defined as “reactive”. However, this was quite variable from year-to-year. Hg_p and other particle constituents (esp. sulfate) show significant correlation and similarity in behavior (concentration ratios in precipitation and in particles). These observations are part of the growing evidence to support the hypothesis that precipitation-phase Hg arises in large part from the scavenging of atmospheric particulates bearing Hg. Observed concentrations of rain and particle-Hg fit broadly the theoretical expectations for nucleation and below-cloud scavenging. Significant increases in the Hg/aerosol mass ratio appear to take place during transport. Enrichment of aerosols is taken as evidence of gas/particle conversion which could represent the step linking gas-phase Hg with rain. The refined budget indicates ca. 24% of total deposition is from summer particle dry deposition, and that this deposition also contributes ca. 24% of all reactive Hg deposition. Additionally, almost all (86%) deposition (wet and dry) occurs during the summer months.