Scientific uncertainties in atmospheric mercury models II: Sensitivity analysis in the CONUS domain

In this study, we present the response of model results to different scientific treatments in an effort to quantify the uncertainties caused by the incomplete understanding of mercury science and by model assumptions in atmospheric mercury models. Two sets of sensitivity simulations were performed to assess the uncertainties using modified versions of CMAQ-Hg in a 36-km Continental United States domain. From Set 1 Experiments, it is found that the simulated mercury dry deposition is most sensitive to the gaseous elemental mercury (GEM) oxidation product assignment, and to the implemented dry deposition scheme for GEM and reactive gaseous mercury (RGM). The simulated wet deposition is sensitive to the aqueous Hg(II) sorption scheme, and to the GEM oxidation product assignment. The inclusion of natural mercury emission causes a small increase in GEM concentration but has little impact on deposition. From Set 2 Experiments, it is found that both dry and wet depositions are sensitive to mercury chemistry. Change in model mercury chemistry has a greater impact on simulated wet deposition than on dry deposition. The kinetic uncertainty of GEM oxidation by O₃ and mechanistic uncertainty of Hg(II) reduction by aqueous HO₂ pose the greatest impact. Using the upper-limit kinetics of GEM–O₃ reaction or eliminating aqueous Hg(II)–HO₂ reaction results in unreasonably high deposition and depletion of gaseous mercury in the domain. Removing GEM–OH reaction is not sufficient to balance the excessive mercury removal caused by eliminating the HO₂ mechanism. Field measurements of mercury dry deposition, better quantification of mercury air-surface exchange and further investigation of mercury redox chemistry are needed for reducing model uncertainties and for improving the performance of atmospheric mercury models.     

Temporal distribution and potential sources of atmospheric mercury measured at a high-elevation background station in Taiwan

Measurements of gaseous elemental mercury (GEM), reactive gaseous mercury (RGM), and particulate mercury (PHg) have been conducted at Lulin Atmospheric Background Station (LABS) in Taiwan since April 2006. This was the first long-term free tropospheric atmospheric Hg monitoring program in the downwind region of East Asia, which is a major Hg emission source region. Between April 13, 2006 and December 31, 2007, the mean concentrations of GEM, RGM, and PHg were 1.73 ng m^?3, 12.1 pg m^?3, and 2.3 pg m^?3, respectively. A diurnal pattern was observed for GEM with afternoon peaks and nighttime lows, whereas the diurnal pattern of RGM was opposite to that of GEM. Spikes of RGM were frequently observed between midnight and early morning with concurrent decreases in GEM and relative humidity and increases in O₃, suggesting the oxidation of GEM and formation of RGM in free troposphere (FT). Upslope movement of boundary layer (BL) air in daytime and subsidence of FT air at night resulted in these diurnal patterns. Considering only the nighttime data, which were more representative of FT air, the composite monthly mean GEM concentrations ranged between 1.06 and 2.06 ng m^?3. Seasonal variation in nighttime GEM was evident, with lower concentrations usually occurring in summer when clean marine air masses prevailed. Between fall and spring, air masses passed the East Asian continent prior to reaching LABS, contributing to the elevated GEM concentrations. Analysis of GEM/CO correlation tends to support the argument. Good GEM/CO correlations were observed in fall, winter, and spring, suggesting influence of anthropogenic emission sources. Our results demonstrate the significance of East Asian Hg emissions, including both anthropogenic and biomass burning emissions, and their long-range transport in the FT. Because of the pronounced seasonal monsoon activity and the seasonal variation in regional wind field, export of the Asian Hg emissions to Taiwan occurs mainly during fall, winter, and spring.     

Studies of potential sources that contributed to atmospheric mercury in Toronto,Canada

This study identified sources of mercury (Hg) in downtown Toronto, Canada by analyzing gaseous elemental mercury (GEM), mercury associated with particles with sizes less than 2.5 microns (PHg < 2.5), and gaseous oxidized inorganic mercury (GOIM), commonly referred to as reactive gaseous mercury (RGM), and air pollutants (CO, NO_x, O₃, PM_2.5, SO₂) concentrations between Dec 2003 and Nov 2004. The data were analyzed using Positive Matrix Factorization (PMF) model, Principal Components Analysis (PCA), ratio analysis, back trajectories, and correlation analyses. The analyses suggest industrial sources (chemical production, metal production, sewage treatment), rather than coal combustion, were the major contributors to measured Hg levels. Overlap in source profiles for the Hg sources listed in the Canadian National Pollutant Release Inventory (NPRI) and lack of source profiles for urban sources were the major limitations to positively identifying sources from the PMF and PCA factors. Correlation analyses revealed direct emissions were the sources of GOIM in spring, summer, and fall, and the occurrence of GEM oxidation by ozone in the summer. Elevated Hg events are attributed to emissions from urban sources near the sampling site, regional point sources, and photochemical processes involving ozone.     

Evidence for short-range transport of atmospheric mercury to a rural,inland site

Atmospheric mercury (Hg) species, including gaseous elemental mercury (GEM), reactive gaseous mercury (RGM) and particulate-bound mercury (Hg_p), were monitored near three sites, including a cement plant (monitored in 2007 and 2008), an urban site and a rural site (both monitored in 2005 and 2008). Although the cement plant was a significant source of Hg emissions (for 2008, GEM: 2.20 ± 1.39 ng m^?3, RGM: 25.2 ± 52.8 pg m^?3, Hg_p 80.8 ± 283 pg m^?3), average GEM levels and daytime average dry depositional RGM flux were highest at the rural site, when all three sites were monitored sequentially in 2008 (rural site, GEM: 2.37 ± 1.26 ng m^?3, daytime RGM flux: 29 ± 40 ng m^?2 day^?1). Photochemical conversion of GEM was not the primary RGM source, as highest net RGM gains (75.9 pg m^?3, 99.0 pg m^?3, 149 m^?3) occurred within 3.0–5.3 h, while the theoretical time required was 14–23 h. Instead, simultaneous peaks in RGM, Hg_p, ozone (O₃), nitrogen oxides, and sulfur dioxide in the late afternoon suggested short-range transport of RGM from the urban center to the rural site. The rural site was located more inland, where the average water vapor mixing ratio was lower compared to the other two sites (in 2008, rural: 5.6 ± 1.4 g kg^?1, urban: 9.0 ± 1.1 g kg^?1, cement plant: 8.3 ± 2.2 g kg^?1). Together, these findings suggested short-range transport of O₃ from an urban area contributed to higher RGM deposition at the rural site, while drier conditions helped sustain elevated RGM levels. Results suggested less urbanized environments may be equally or perhaps more impacted by industrial atmospheric Hg emissions, compared to the urban areas from where Hg emissions originated.     

Investigation of Selective Catalytic Reduction Impact on Mercury Speciation under Simulated NOx Emission Control Conditions

Abstract

Selective catalytic reduction (SCR) technology increasingly is being applied for controlling emissions of nitrogen oxides (NO_x) from coal-fired boilers. Some recent field and pilot studies suggest that the operation of SCR could affect the chemical form of mercury (Hg) in coal combustion flue gases. The speciation of Hg is an important factor influencing the control and environmental fate of Hg emissions from coal combustion. The vanadium and titanium oxides, used commonly in the vanadia-titania SCR catalyst for catalytic NO_x reduction, promote the formation of oxidized mercury (Hg²⁺).

The work reported in this paper focuses on the impact of SCR on elemental mercury (Hg⁰) oxidation. Bench-scale experiments were conducted to investigate Hg⁰ oxidation in the presence of simulated coal combustion flue gases and under SCR reaction conditions. Flue gas mixtures with different concentrations of hydrogen chloride (HCl) and sulfur dioxide (SO₂) for simulating the combustion of bituminous coals and subbituminous coals were tested in these experiments. The effects of HCl and SO₂ in the flue gases on Hg⁰ oxidation under SCR reaction conditions were studied. It was observed that HCl is the most critical flue gas component that causes conversion of Hg⁰ to Hg²⁺ under SCR reaction conditions. The importance of HCl for Hg⁰ oxidation found in the present study provides the scientific basis for the apparent coal-type dependence observed for Hg⁰ oxidation occurring across the SCR reactors in the field.     

Investigation of the light-enhanced emission of mercury from naturally enriched substrates

Incident radiation has been reported to facilitate mercury release from soils. In this study the influence of light on mercury emissions from substrates amended with pure synthetic mercury species, and from naturally and anthropogenically mercury-enriched substrates were investigated using laboratory experiments and in situ flux measurements. Light-enhanced emissions were found to occur from substrates amended with HgS, and from elemental mercury (Hg⁰) and HgCl₂ amended iron oxide and organic containing substrates. The magnitude of light-enhanced emissions for natural substrates ranged from 1.5 to 116 times that occurring in the dark at the same substrate temperature. Substrates containing corderoite, metacinnabar and “matrix bound mercury” (that bound to organic or inorganic phases) exhibited a higher degree of light-enhanced emissions relative to that containing predominantly cinnabar. Calculated activation energies for both laboratory and field data indicate that photo-reduction is a process associated with the light-enhanced emissions. Activation energies, derived using in situ mercury fluxes and soil temperatures, indicated that photo-reduction was a dominant process facilitating release of Hg from substrates with sunrise. Activation energies, calculated using daytime data, were less than those calculated for sunrise. This is hypothesized to be due to a pool of Hg⁰ being developed with photo-reduction at first light that is released as soil temperatures and convective heat transfer increase during the day. This study demonstrated that light energy is the more dominant process controlling mercury emissions from naturally enriched substrates than soil temperature.     

Low Concentration Mercury Sorption Mechanisms and Control by Calcium-Based Sorbents: Application in Coal-Fired Processes

ABSTRACT

The capture of elemental mercury (Hg⁰) and mercuric chloride (HgCl₂) by three types of calcium (Ca)-based sor-bents was examined in this bench-scale study under conditions prevalent in coal-fired utilities. Ca-based sorbent performances were compared with that of an activated carbon. Hg⁰ capture of about 40% (nearly half that of the activated carbon) was achieved by two of the Ca-based sorbents. The presence of sulfur dioxide (SO₂) in the simulated coal combustion flue gas enhanced the Hg⁰ capture from about 10 to 40%. Increasing the temperature in the range of 65-100 °C also caused an increase in the Hg⁰ capture by the two Ca-based sorbents. Mercuric chloride (HgCl₂) capture exhibited a totally different pattern. The presence of SO₂ inhibited the HgCl₂ capture by Ca-based sorbents from about 25 to less than 10%. Increasing the temperature in the studied range also caused a decrease in HgCl₂ capture. Upon further pilot-scale confirmations, the results obtained in this bench-scale study can be used to design and manufacture more cost-effective mercury sorbents to replace conventional sorbents already in use in mercury control.     

Electricity from Fossil Fuels without CO2 Emissions: Assessing the Costs of Carbon Dioxide Capture and Sequestration in U.S. Electricity Markets

ABSTRACT

The decoupling of fossil-fueled electricity production from atmospheric CO₂ emissions via CO₂ capture and sequestration (CCS) is increasingly regarded as an important means of mitigating climate change at a reasonable cost. Engineering analyses of CO₂ mitigation typically compare the cost of electricity for a base generation technology to that for a similar plant with CO₂ capture and then compute the carbon emissions mitigated per unit of cost. It can be hard to interpret mitigation cost estimates from this plant-level approach when a consistent base technology cannot be identified. In addition, neither engineering analyses nor general equilibrium models can capture the economics of plant dispatch. A realistic assessment of the costs of carbon sequestration as an emissions abatement strategy in the electric sector therefore requires a systems-level analysis. We discuss various frameworks for computing mitigation costs and introduce a simplified model of electric sector planning. Results from a “bottom-up” engineering-economic analysis for a representative U.S. North American Electric Reliability Council (NERC) region illustrate how the penetration of CCS technologies and the dispatch of generating units vary with the price of carbon emissions and thereby determine the relationship between mitigation cost and emissions reduction.     

A New Method to Assess Mercury Emissions: A Study of Three Coal-Fired Electric-Generating Power Station Configurations

Abstract

U.S. Environmental Protection Agency (EPA) Method 7473 for the analysis of mercury (Hg) by thermal decomposition, amalgamation, and atomic absorption spectroscopy has proved successful for use in Hg assessment at coal-fired power stations. In an analysis time of ～5 min per sample, this instrumental methodology can directly analyze total Hg—with no discrete sample preparation—in the solid matrices associated with a coal-fired power plant, including coal, fly ash, bottom ash, and flue gas desulfurization (FGD) material. This analysis technique was used to investigate Hg capture by coal combustion byproducts (CCBs) in three different coal-fired power plant configurations. Hg capture and associated emissions were estimated by partial mass balance. The station equipped with an FGD system demonstrated 68% capture on FGD material and an emissions estimate of 18% (11 kg/yr) of total Hg input. The power plant equipped with low oxides of nitrogen burners and an electrostatic precipitator (ESP) retained 43% on the fly ash and emitted 57% (51 kg/yr). The station equipped with conventional burners and an ESP retained less than 1% on the fly ash, emitting an estimated 99% (88 kg/yr) of Hg. Estimated Hg emissions demonstrate good agreement with EPA data for the power stations investigated.     

Atmospheric mercury concentrations: measurements and profiles near snow and ice surfaces in the Canadian Arctic during Alert 2000

Gaseous elemental mercury (GEM) concentration measurements were made during the Alert 2000 campaign in Alert, Nunavut, Canada, between February and May 2000. GEM exhibits dramatic mercury depletion events (MDE) concurrently with ozone in the troposphere during the Arctic springtime. Using a cold regions pyrolysis unit, it was confirmed that GEM is converted to more reactive mercury species during the MDEs. It was determined that on average 48% of this converted GEM was recovered through pyrolysis suggesting that the remaining converted GEM is deposited on the snow surfaces. Samples collected during this campaign showed an approximate 20 fold increase in mercury concentrations in the snow from the dark to light periods. Vertical gradient air profiling experiments were conducted. In the non-depletion periods GEM was found to be invariant in the air column between surface and 1–2 m heights. During a depletion period, GEM was found to be invariant in the air column except at the surface where a noticeable increase in the GEM concentration was observed. Concurrent ozone concentration profiles showed a small gradient in the air column but a sharp decrease in ozone concentration at the surface. Other profile studies showed a 41% average GEM concentration difference between the interstitial air in the snow pack and ∼2 m above the surface suggesting that GEM is emitted from the snow pack. Further profile studies showed that during MDEs surface level GEM exhibits spikes of mercury concentrations that were over double the ambient GEM concentrations. It is thought that the solar radiation may reduce reactive mercury that is deposited on the snow surface during a MDE back to its elemental form which is then increasingly released from the snow pack as the temperature increases during the day. This is observed when wind speeds are very low.     

Springtime depletion of tropospheric ozone,gaseous elemental mercury and non-methane hydrocarbons in the European Arctic,and its relation to atmospheric transport

Using a trajectory climatology for the period 1992–2001 we have examined how seasonal changes in transport cause changes in the concentrations of tropospheric ozone (O₃), gaseous elemental mercury (GEM) and non-methane hydrocarbons (NMHCs) observed at the Mt. Zeppelin station, Ny-Ålesund (78.9°N, 11.9°E). During April–June O₃ depletion events were frequently observed in connection with air transport across the Arctic Basin. The O₃ loss was most pronounced in air masses advected close to the surface. This result supports the idea that the O₃ depletion reactions take place in the lowermost part of the atmosphere in the central Arctic Basin. A strong positive correlation between springtime O₃ depletion events and the oxidation of GEM to divalent mercury was found. During air mass advection from Siberia, the Barents Sea and the Norwegian Sea the strongest correlation was observed during April–May, whereas air masses originating from the Canadian Arctic and the central Arctic areas showed the highest O₃–GEM correlation in May–June. We suggest that this 1-month lag could either be due to the position of the marginal ice zone or temperature differences between the northwestern and northeastern air masses. In connection with springtime O₃ depletion events low concentrations of some NMHCs, especially ethane and ethyne, were observed, indicating that both bromine (ethyne oxidant) and chlorine radicals (ethane oxidant) are present in the Arctic atmosphere during spring. In winter, negative correlations between O₃ and NMHCs were found in connection with air transport from Europe and Siberia, which we interpret as O₃ destruction taking place in industrially contaminated plumes.     

Mercury (Hg) emissions from domestic biomass combustion for space heating

Three mercury (Hg) species (gaseous elemental mercury (GEM), gaseous oxidized mercury (GOM), and fine particulate-bound mercury (PBM_2.5)) were measured in the stack of a small scale wood combustion chamber at 400 °C, in the stack of an advanced wood boiler, and in two areas influenced by wood combustion. The low temperature process (lab-scale) emitted mostly GEM (∼99% when burning wood pellets and ∼95% when burning unprocessed wood). The high temperature wood boiler emitted a greater proportion of oxidized Hg (approximately 65%) than the low temperature system. In field measurements, mean PBM_2.5 concentrations at the rural and urban sites in winter were statistically significantly higher than in warmer seasons and were well correlated with Delta-C concentrations, a wood combustion indictor measured by an aethalometer (UV-absorbable carbon minus black carbon). Overall the results suggest that wood combustion may be an important source of oxidized mercury (mostly in the particulate phase) in northern climates in winter.     

Impact of mercury emissions from historic gold and silver mining: Global modeling

We compare a global model of mercury to sediment core records to constrain mercury emissions from the 19th century North American gold and silver mining. We use information on gold and silver production, the ratio of mercury lost to precious metal produced, and the fraction of mercury lost to the atmosphere to calculate an a priory mining inventory for the 1870s, when the historical gold rush was at its highest. The resulting global mining emissions are 1630 Mg yr^?1, consistent with previously published studies. Using this a priori estimate, we find that our 1880 simulation over-predicts the mercury deposition enhancements archived in lake sediment records. Reducing the mining emissions to 820 Mg yr^?1 improves agreement with observations, and leads to a 30% enhancement in global deposition in 1880 compared to the pre-industrial period. For North America, where 83% of the mining emissions are located, deposition increases by 60%. While our lower emissions of atmospheric mercury leads to a smaller impact of the North American gold rush on global mercury deposition than previously estimated, it also implies that a larger fraction of the mercury used in extracting precious metals could have been directly lost to local soils and watersheds.     

The influence of ozone on atmospheric emissions of gaseous elemental mercury and reactive gaseous mercury from substrates

Experiments were performed to investigate the effect of ozone (O₃) on mercury (Hg) emission from a variety of Hg-bearing substrates. Substrates with Hg(II) as the dominant Hg phase exhibited a 1.7 to 51-fold increase in elemental Hg (Hg^o) flux and a 1.3 to 8.6-fold increase in reactive gaseous mercury (RGM) flux in the presence of O₃-enriched clean (50 ppb O₃; 8 substrates) and ambient air (up to ∼70 ppb O₃; 6 substrates), relative to clean air (oxidant and Hg free air). In contrast, Hg^o fluxes from two artificially Hg^o-amended substrates decreased by more than 75% during exposure to O₃-enriched clean air relative to clean air. Reactive gaseous mercury emissions from Hg^o-amended substrates increased immediately after exposure to O₃ but then decreased rapidly. These experimental results demonstrate that O₃ is very important in controlling Hg emissions from substrates. The chemical mechanisms that produced these trends are not known but potentially involve heterogenous reactions between O₃, the substrate, and Hg. Our experiments suggest they are not homogenous gas-phase reactions. Comparison of the influence of O₃ versus light on increasing Hg^o emissions from dry Hg(II)-bearing substrates demonstrated that they have a similar amount of influence although O₃ appeared to be slightly more dominant. Experiments using water-saturated substrates showed that the presence of high-substrate moisture content minimizes reactions between atmospheric O₃ and substrate-bound Hg. Using conservative calculations developed in this paper, we conclude that because O₃ concentrations have roughly doubled in the last 100 years, this could have increased Hg^o emissions from terrestrial substrates by 65–72%.     

Pilot-Scale Study of the Effect of Selective Catalytic Reduction Catalyst on Mercury Speciation in Illinois and Powder River Basin Coal Combustion Flue Gases

Abstract

A study was conducted to investigate the effect of selective catalytic reduction (SCR) catalyst on mercury (Hg) speciation in bituminous and subbituminous coal combustion flue gases. Three different Illinois Basin bituminous coals (from high to low sulfur [S] and chlorine [Cl]) and one Powder River Basin (PRB) subbituminous coal with very low S and very low Cl were tested in a pilot-scale combustor equipped with an SCR reactor for controlling nitrogen oxides (NO_x) emissions. The SCR catalyst induced high oxidation of elemental Hg (Hg⁰), decreasing the percentage of Hg⁰ at the outlet of the SCR to values <12% for the three Illinois coal tests. The PRB coal test indicated a low oxidation of Hg⁰ by the SCR catalyst, with the percentage of Hg⁰ decreasing from ～96% at the inlet of the reactor to ～80% at the outlet. The low Cl content of the PRB coal and corresponding low level of available flue gas Cl species were believed to be responsible for low SCR Hg oxidation for this coal type. The test results indicated a strong effect of coal type on the extent of Hg oxidation.     

A diagnostic evaluation of modeled mercury wet depositions in Europe using atmospheric speciated high-resolution observations

This study is part of the Global Mercury Observation System (GMOS), a European FP7 project dedicated to the improvement and validation of mercury models to assist in establishing a global monitoring network and to support political decisions. One key question about the global mercury cycle is the efficiency of its removal out of the atmosphere into other environmental compartments. So far, the evaluation of modeled wet deposition of mercury was difficult because of a lack of long-term measurements of oxidized and elemental mercury. The oxidized mercury species gaseous oxidized mercury (GOM) and particle-bound mercury (PBM) which are found in the atmosphere in typical concentrations of a few to a few tens pg/m³ are the relevant components for the wet deposition of mercury. In this study, the first European long-term dataset of speciated mercury taken at Waldhof/Germany was used to evaluate deposition fields modeled with the chemistry transport model (CTM) Community Multiscale Air Quality (CMAQ) and to analyze the influence of the governing parameters. The influence of the parameters precipitation and atmospheric concentration was evaluated using different input datasets for a variety of CMAQ simulations for the year 2009. It was found that on the basis of daily and weekly measurement data, the bias of modeled depositions could be explained by the bias of precipitation fields and atmospheric concentrations of GOM and PBM. A correction of the modeled wet deposition using observed daily precipitation increased the correlation, on average, from 0.17 to 0.78. An additional correction based on the daily average GOM and PBM concentration lead to a 50 % decrease of the model error for all CMAQ scenarios. Monthly deposition measurements were found to have a too low temporal resolution to adequately analyze model deficiencies in wet deposition processes due to the nonlinear nature of the scavenging process. Moreover, the general overestimation of atmospheric GOM by the CTM in combination with an underestimation of low precipitation events in the meteorological models lead to a good agreement of total annual wet deposition besides the large error in weekly deposition estimates. Moreover, it was found that the current speciation profiles for GOM emissions are the main factor for the overestimation of atmospheric GOM concentrations and might need to be revised in the future. The assumption of zero emissions of GOM lead to an improvement of the mean normalized bias for three-hourly observations of atmospheric GOM from 9.7 to 0.5, Furthermore, the diurnal correlation between model and observation increased from 0.01 to 0.64. This is a strong indicator that GOM is not directly emitted from primary sources but is mainly created by oxidation of GEM.     

Ultra-high adsorption of Hg0 using impregnated activated carbon by selenium

Activated carbon was one of the main adsorptions utilized in elemental mercury (Hg⁰) removal from coal combustion flue gas. However, the high cost and low physical adsorption efficiency of activated carbon injection (ACI) limited its application. In this study, an ultra-high efficiency (nearly 100%) catalyst sorbent-Se_x/Activated carbon (Se_x/AC) was synthesized and applied to remove Hg⁰ in the simulated flue gas, which exhibited 120 times outstanding adsorption performance versus the conventional activated carbon. The Se_x/AC reached 17.98 mg/g Hg⁰ adsorption capacity at 160 °C under the pure nitrogen atmosphere. Moreover, it maintained an excellent mercury adsorption tolerance, reaching the efficiency of Hg⁰ removal above 85% at the NO and SO₂ conditions in a bench-scale fixed-bed reactor. Characterized by the multiple methods, including BET, XRD, XPS, kinetic and thermodynamic analysis, and the DFT calculation, we demonstrated that the ultrahigh mercury removal performance originated from the activated Se species in Se_x/AC. Chemical adsorption plays a dominant role in Hg⁰ removal: Selenium anchored on the surface of AC would capture Hg⁰ in the flue gas to form an extremely stable substance-HgSe, avoiding subsequent Hg⁰ released. Additionally, the oxygen-containing functional groups in AC and the higher BET areas promote the conversion of Hg⁰ to HgO. This work provided a novel and highly efficient carbon-based sorbent -Se_x/AC to capture the mercury in coal combustion flue gas.

Selenium-modified porous activated carbon and the interface functional group promotes the synergistic effect of physical adsorption and chemical adsorption to promote the adsorption capacity of Hg⁰.

     

Predicting Extents of Mercury Oxidation in Coal-Derived Flue Gases

Abstract

The proposed mercury (Hg) oxidation mechanism consists of a 168-step gas phase mechanism that accounts for interaction among all important flue gas species and a heterogeneous oxidation mechanism on unburned carbon (UBC) particles, similar to established chemistry for dioxin production under comparable conditions. The mechanism was incorporated into a gas cleaning system simulator to predict the proportions of elemental and oxidized Hg species in the flue gases, given relevant coal properties (C/H/O/N/S/Cl/Hg), flue gas composition (O₂, H₂O, HCl), emissions (NO_X, SO_X, CO), the recovery of fly ash, fly ash loss-on-ignition (LOI), and a thermal history. Predictions are validated without parameter adjustments against datasets from lab-scale and from pilot-scale coal furnaces at 1 and 29 MW_t. Collectively, the evaluations cover 16 coals representing ranks from sub-bituminous through high-volatile bituminous, including cases with Cl₂ and CaCl₂ injection. The predictions are, therefore, validated over virtually the entire domain of Cl-species concentrations and UBC levels of commercial interest. Additional predictions identify the most important operating conditions in the furnace and gas cleaning system, including stoichiometric ratio, NO_X, LOI, and residence time, as well as the most important coal properties, including coal-Cl.     

Uncertainty Analysis of Ozone Formation and Response to Emission Controls Using Higher-Order Sensitivities

Abstract

Understanding ozone response to its precursor emissions is crucial for effective air quality management practices. This nonlinear response is usually simulated using chemical transport models, and the modeling results are affected by uncertainties in emissions inputs. In this study, a high ozone episode in the southeastern United States is simulated using the Community Multiscale Air Quality (CMAQ) model. Uncertainties in ozone formation and response to emissions controls due to uncertainties in emission rates are quantified using the Monte Carlo method. Instead of propagating emissions uncertainties through the original CMAQ, a reduced form of CMAQ is formulated using directly calculated first- and second-order sensitivities that capture the nonlinear ozone concentration-emission responses. This modification greatly reduces the associated computational cost. Quantified uncertainties in modeled ozone concentrations and responses to various emissions controls are much less than the uncertainties in emissions inputs. Average uncertainties in modeled ozone concentrations for the Atlanta area are less than 10% (as measured by the inferred coefficient of variance [ICOV]) even when emissions uncertainties are assumed to vary between a factor of 1.5 and 2. Uncertainties in the ozone responses generally decrease with increased emission controls. Average uncertainties (ICOV) in emission-normalized ozone responses range from 4 to 22%, with the smaller being associated with controlling of the relatively certain point nitrogen oxide (NO_x) emissions and the larger resulting from controlling of the less certain mobile NO_x emissions. These small uncertainties provide confidence in the model applications, such as in performance evaluation, attainment demonstration, and control strategy development.     

Measurement of Particulate Bound Mercury,Gaseous Elemental Mercury,and Reactive Gaseous Mercury Concentrations by Using an Ambient Air Mercury Collection System

In this investigation, the concentrations of gaseous elemental mercury (GEM), reactive gaseous mercury (RGM) and particulate bound mercury (PBM) in ambient air were measured at the Hung Kuang (traffic) sampling site during September 27 to October 6, 2014. An ambient air mercury collection system (AAMCS) was utilized to measure simultaneously PBM, GEM, and RGM concentrations in ambient air. The results thus obtained demonstrate that the mean concentrations of PBM, GEM, and RGM were 38.57 ± 11.4 (pg/m³), 17.67 ± 5.56 (ng/m³) and 10.78 ± 2.8 (pg/m³), respectively, at this traffic-sampling site. The mean GEM/PBM and GEM/RGM concentration ratios were 458 and 1639, respectively. The results obtained herein demonstrate that AAMCS can be utilized to collect three phases of mercury simultaneously. The mean PBM, GEM, and RGM concentrations herein were compared with others found in Asia, America, Europe and Antarctica. The mean PBM, GEM, and RGM concentrations were found to be lowest in Asia and Antarctica. The mean PBM concentration in Europe was approximately eight times that in this investigation. The mean GEM and RGM concentrations in this study were 1.21 and 170 times those found in the United States.