LP/LIF study of the formation and consumption of mercury (I) chloride: kinetics of mercury chlorination

The laser photolysis/laser induced fluorescence (LP/LIF) technique has been applied to studies of gas-phase mercury (Hg) chlorination. Mercury (I) chloride (HgCl) was been detected via LIF at 272 nm from reactions of elemental Hg with Cl atoms generated from the 193 nm photolysis of carbon tetrachloride. While the formation of HgCl was too fast to be observed on millisecond time scales, the kinetics of the consumption of HgCl have been determined at temperatures characteristic of post-combustion conditions. Rate coefficients and Arrhenius parameters for the reaction of HgCl with Cl2, HCl and Cl atoms were determined. The reaction of HgCl with Cl2 was the fastest reaction studied, while the reaction of HgCl with HCl was the only reaction to show any measurable temperature dependence. Estimates of the rate coefficient for the reaction Hg + Cl --> HgCl were determined using a modeling approach. Comparisons of these new measurements with model predictions are discussed.     

Hg reactions in the presence of chlorine species: homogeneous gas phase and heterogeneous gas-solid phase

The kinetics of Hg chlorination (with HCl) was studied using a flow reactor system with an online Hg analyzer, and speciation sampling using a set of impingers. Kinetic parameters, such as reaction order (alpha), overall rate constant (k'), and activation energy (Ea), were estimated based on the simple overall reaction pathway. The reaction order with respect to C(Hg), k', and Ea were found to be 1.55, 5.07 x 10(-2) exp(-1939.68/T) [(microg/m3)(-055)(s)(-1)]. and 16.13 [kJ/ mol], respectively. The effect of chlorine species (HCl, CH2Cl2) on the in situ Hg capture method previously developed (28) was also investigated. The efficiency of capture of Hg by this in situ method was higher than 98% in the presence of chlorine species. Furthermore, under certain conditions, the presence of chlorine enhanced the removal of elemental Hg by additional gas-phase oxidation.     

Influence of HCl on oxidation of gaseous elemental mercury by dielectric barrier discharge process

The influence of HCl on the oxidation of gaseous elemental mercury (Hg0) has been investigated using a dielectric barrier discharge (DBD) plasma process, where the temperature of the plasma reactor and the composition of gas mixtures of HCl, H2O, NO, and O2 in N2 balance have been varied. We observe that Cl atoms and Cl2 molecules, created by the DBD process, play important roles in the oxidation of Hg0 to HgCl2. The addition of H2O to the gas mixture of HCl in N2 accelerates the oxidation of Hg0, although no appreciable effect of H2O alone on the oxidation of Hg0 has been observed. The increase of the reaction temperature in the presence of HCl results in the reduction of Hg0 oxidation efficiency probably due to the deterioration of the heterogeneous chemical reaction of Hg0 with chlorinated species on the reactor wall. The presence of NO shows an inhibitory effect on the oxidation of Hg0 under DBD of 16% O2 in N2, indicating that NO acts as an O and O3 scavenger. At the composition of Hg0 (280 microg m(-3)), HCl (25 ppm), NO (204 ppm), O2 (16%) and N2 (balance) and temperature 90 degrees C, we obtain the nearly complete oxidation of Hg0 at a specific energy density of 8 J l(-1). These results lead us to suggest that the DBD process can be viable for the treatment of mercury released from coal-fired power plants.     

Issues related to solution chemistry in mercury sampling impingers

Analysis of Hg speciation in combustion flue gases is often accomplished in standardized sampling trains in which the sample is passed sequentially through a series of aqueous solutions to capture and separate oxidized Hg (Hg2+) and elemental Hg (Hg0). Such methods include the Ontario Hydro (OH) and the Alkaline Mercury Speciation (AMS) methods, which were investigated in the laboratory to determine whether the presence of Cl2 and other common flue gas species can bias the partitioning of Hg0 to front impingers intended to isolate Hg2+ species. Using only a single impinger to represent the front three impingers for each method, it was found that as little as 1-ppm Cl2 in a simulated flue gas mixture led to a bias of approximately 10-20% of Hg0 misreported as Hg2+ for both the OH and the AMS methods. Experiments using 100-ppm Cl2 led to a similar bias in the OH method, but to a 30-60% bias in the AMS method. These false readings are shown to be due to liquid-phase chemistry in the impinger solutions, and not necessarily to the gas-phase reactions between Cl2 and Hg as previously proposed. The pertinent solution chemistry causing the interference involves the hypochlorite ion (OCl-), which oxidizes Hg0 to soluble Hg2+. Addition of sodium thiosulfate (Na2S2O3) to the front impinger solutions eliminates this false positive measurement of Hg2+ by selectively reacting with the OCl- ion. In general, the presence of SO2 also mitigates this interference in the same way, and so this bias is not likely to be a factor for Hg speciation measurements from actual coal combustion flue gases. It might, however, be a problem for those few combustor flue gas measurements and research studies where Cl2 is present without appreciable amounts of SO2.     

Pilot-scale study of the effect of selective catalytic reduction catalyst on mercury speciation in Illinois and Powder River Basin coal combustion flue gases

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 (NOx) 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 approximately 96% at the inlet of the reactor to approximately 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.     

Kinetics of OH radical reactions with dibenzo-p-dioxin and selected chlorinated dibenzo-p-dioxins

The pulsed laser photolysis/pulsed laser-induced fluorescence (PLP/PLIF) technique has been applied to obtain rate coefficients for OH + dioxin (DD) (k1), OH + 2-chlorodibenzo-p-dioxin (2-CDD) (k2), OH + 2,3-dichlorodibenzo-p-dioxin (2,3-DCDD) (k3), OH + 2,7-dichlorodibenzo-p-dioxin (2,7-DCDD) (k4), OH + 2,8-dichlorodibenzo-p-dioxin (2,8-DCDD) (k5), OH + 1,2,3,4-tetrachlorodibenzo-p-dioxin (1,2,3,4-TCDD) (k6), and OH + octachlorodibenzo-p-dioxin (OCDD) (k7) over an extended range of temperature. The atmospheric pressure (740 +/- 10 Torr) rate measurements are characterized by the following Arrhenius parameters (in units of cm3 molecule(-1) s(-1), error limits are 1 omega): k1(326-907 K) = (1.70+/-0.22) x 10(-12)exp(979+/-55)/T, k2(346-905 K) = (2.79+/-0.27) x 10(-12)exp(784+/-54)/T, k3(400-927 K) = 10(-12)exp(742+/-67)/T, k4(390-769 K) = (1.10+/-0.10) x 10(-12)exp(569+/-53)/T, k5(379-931 K) = (1.02+/-0.10) x 10(-12)exp(580+/-68)/T, k6(409-936 K) = (1.66+/-0.38) x 10(-12)exp(713+/-114)/T, k7(514-928 K) = (3.18+/-0.54) x 10(-12)exp(-667+/-115)/T. The overall uncertainty in the measurements, taking into account systematic errors dominated by uncertainty in the substrate reactor concentration, range from a factor of 2 for DD, 2-CDD, 2,3-DCDD, 2,7-DCDD, and 2,8-DCDD to +/- a factor of 4 for 1,2,3,4-TCDD and OCDD. Negative activation energies characteristic of an OH addition mechanism were observed for k1-k6. k7 exhibited a positive activation energy. Cl substitution was found to reduce OH reactivity, as observed in prior studies at lower temperatures. At elevated temperatures (500 K < T < 500 K), there was no experimental evidence for a change in reaction mechanism from OH addition to H abstraction. Theoretical calculations suggest that H abstraction will dominate OH reactivity for most if not all dioxins (excluding OCDD) at combustion temperatures (>1000 K). For OCDD, the dominant reaction mechanism at all temperatures is OH addition followed by Cl elimination.     

Hg Reactions in the Presence of Chlorine Species: Homogeneous Gas Phase and Heterogeneous Gas-Solid Phase

Abstract

The kinetics of Hg chlorination (with HCl) was studied using a flow reactor system with an online Hg analyzer, and speciation sampling using a set of impingers. Kinetic parameters, such as reaction order (α), overall rate constant (k′ ), and activation energy (E _a), were estimated based on the simple overall reaction pathway. The reaction order with respect to C _Hg, k′, and E _a were found to be 1.55, 5.07 x 10^-2exp(-1939.68/T) [(μg/m³)^-0.55(s)^-1], and 16.13 [kJ/mol], respectively. The effect of chlorine species (HCl, CH₂Cl₂) on the in situ Hg capture method previously de-veloped²⁸ was also investigated. The efficiency of capture of Hg by this in situ method was higher than 98% in the presence of chlorine species. Furthermore, under certain conditions, the presence of chlorine enhanced the removal of elemental Hg by additional gas-phase oxidation.     

Effects of sulfur dioxide and nitric oxide on mercury oxidation and reduction under homogeneous conditions

This paper is particularly related to elemental mercury (Hg0) oxidation and divalent mercury (Hg2+) reduction under simulated flue gas conditions in the presence of nitric oxide (NO) and sulfur dioxide (SO2). As a powerful oxidant and chlorinating reagent, Cl2 has the potential for Hg oxidation. However, the detailed mechanism for the interactions, especially among chlorine (Cl)-containing species, SO2, NO, as well as H2O, remains ambiguous. Research described in this paper therefore focused on the impacts of SO2 and NO on Hg0 oxidation and Hg2+ reduction with the intent of unraveling unrecognized interactions among Cl species, SO2, and NO most importantly in the presence of H2O. The experimental results demonstrated that SO2 and NO had pronounced inhibitory effects on Hg0 oxidation at high temperatures when H2O was also present in the gas blend. Such a demonstration was further confirmed by the reduction of Hg2+ back into its elemental form. Data revealed that SO2 and NO were capable of promoting homogeneous reduction of Hg2+ to Hg0 with H2O being present. However, the above inhibition or promotion disappeared under homogeneous conditions when H2O was removed from the gas blend.     

The impact of wet flue gas desulfurization scrubbing on mercury emissions from coal-fired power stations

This article introduces a predictive capability for Hg retention in any Ca-based wet flue gas desulfurization (FGD) scrubber, given mercury (Hg) speciation at the FGD inlet, the flue gas composition, and the sulphur dioxide (SO2) capture efficiency. A preliminary statistical analysis of data from 17 full-scale wet FGDs connects flue gas compositions, the extents of Hg oxidation at FGD inlets, and Hg retention efficiencies. These connections clearly signal that solution chemistry within the FGD determines Hg retention. A more thorough analysis based on thermochemical equilibrium yields highly accurate predictions for total Hg retention with no parameter adjustments. For the most reliable data, the predictions were within measurement uncertainties for both limestone and Mg/lime systems operating in both forced and natural oxidation mode. With the U.S. Environmental Protection Agency's (EPA) Information Collection Request (ICR) database, the quantitative performance was almost as good for the most modern FGDs, which probably conform to the very high SO2 absorption efficiencies assumed in the calculations. The large discrepancies for older FGDs are tentatively attributed to the unspecified SO2 capture efficiencies and operating temperatures and to the possible elimination of HCl in prescrubbers. The equilibrium calculations suggest that Hg retention is most sensitive to inlet HCl and O2 levels and the FGD temperature; weakly dependent on SO2 capture efficiency; and insensitive to HgCl2, NO, CA:S ratio, slurry dilution level in limestone FGDs, and MgSO3 levels in Mg/lime systems. Consequently, systems with prescrubbers to eliminate HCl probably retain less Hg than fully integrated FGDs. The analysis also predicts re-emission of Hg(O) but only for inlet O2 levels that are much lower than those in full-scale FGDs.     

Treatment of phenylmercury salts by heterogeneous photocatalysis over TiO(2)

UV/TiO(2) photocatalysis of phenylmercury salts in aqueous solutions has been performed starting from both acetate (C(6)H(5)HgCH(3)CO(2), PMA) and chloride (C(6)H(5)HgCl, PMC) salts, in the presence or the absence of oxygen at acidic pH. Removal of Hg(II) in solution took place with the simultaneous deposit of dark or pale gray solids on the photocatalyst, identified as metallic Hg (when starting from PMA) or mixtures of Hg(0) and Hg(2)Cl(2) (when starting from PMC). Partial mineralization of the organic part of both compounds has also been achieved. Hg(II) removal and mineralization were enhanced in the absence of oxygen. PMA photocatalysis followed a saturation kinetics, going from first order at low concentration to zero order at higher concentrations (>0.5mM). For PMA, reaction was faster at high pH (11) with formation of mixtures of Hg and HgO. Phenol was detected as a product of the reaction in both cases, PMA and PMC, and no formation of dangerous methyl- or ethylmercury species was observed in the first case. A mechanism for the photocatalytic reaction has been proposed. The fact that calomel was found as a deposit when starting from PMC under nitrogen suggests that the mechanism of Hg(II) transformation proceeds through successive one-electron transfer reactions passing by mercurous forms.     

The adsorptive capacity of vapor-phase mercury chloride onto powdered activated carbon derived from waste tires

Injection of powdered activated carbon (PAC) upstream of particulate removal devices (such as electrostatic precipitator and baghouses) has been used effectively to remove hazardous air pollutants, particularly mercury-containing pollutants, emitted from combustors and incinerators. Compared with commercial PACs (CPACs), an alternative PAC derived from waste tires (WPAC) was prepared for this study. The equilibrium adsorptive capacity of mercury chloride (HgCl2) vapor onto the WPAC was further evaluated with a self-designed bench-scale adsorption column system. The adsorption temperatures investigated in the adsorption column were controlled at 25 and 150 degrees C. The superficial velocity and residence time of the flow were 0.01 m/sec and 4 sec, respectively. The adsorption column tests were run under nitrogen gas flow. Experimental results showed that WPAC with higher Brunauer-Emmett-Teller (BET) surface area could adsorb more HgCl2 at room temperature. The equilibrium adsorptive capacity of HgCl2 for WPAC measured in this study was 1.49 x 10(-1) mg HgCl2/g PAC at 25 degrees C with an initial HgCI2 concentration of 25 microg/m3. With the increase of adsorption temperature < or = 150 degrees C, the equilibrium adsorptive capacity of HgCl2 for WPAC was decreased to 1.34 x 10(-1) mg HgCl2/g PAC. Furthermore, WPAC with higher sulfur contents could adsorb even more HgCl2 because of the reactions between sulfur and Hg2+ at 150 degrees C. It was demonstrated that the mechanisms for adsorbing HgCl2 onto WPAC were physical adsorption and chemisorption at 25 and 150 degrees C, respectively. Experimental results also indicated that the apparent overall driving force model appeared to have the good correlation with correlation coefficients (r) > 0.998 for HgCl2 adsorption at 25 and 150 degrees C. Moreover, the equilibrium adsorptive capacity of HgCl2 for virgin WPAC was similar to that for CPAC at 25 degrees C, whereas it was slightly higher for sulfurized WPAC than for CPAC at 150 degrees C.     

Chlorination of dibenzofuran and dibenzo-p-dioxin vapor by copper (II) chloride

Dibenzofuran (DF) is formed from phenol and benzene in combustion gas exhaust streams prior to particle collection equipment. Subsequent chlorination at lower temperatures on particle surfaces is a potential source of chlorinated dibenzofuran (CDF). Gas streams containing 8% O₂ and approximately 0.1% DF vapor were passed through particle beds containing copper (II) chloride (0.5% Cu, mass) at temperatures ranging from 200 to 400 °C to investigate the potential for CDF formation during particle collection. Experiment duration was sufficient to provide an excess amount of DF (DF/Cu=3). The efficiency of DF chlorination by CuCl₂ and the distribution of CDF products were measured, with effects of temperature, gas velocity, and experiment duration assessed. Results of a more limited investigation of dibenzo-p-dioxin (DD) chlorination by CuCl₂ to form chlorinated DD (CDD) products are also presented.

The efficiency of DF/DD chlorination by CuCl₂ was high, both in terms of CuCl₂ utilization and DF/DD conversion. Total yields of Cl on CDF/CDD products of up to 0.5 mole Cl per mole CuCl₂ were observed between 200 and 300 °C; this suggests that nearly 100% CuCl₂ was utilized, assuming a conversion of two moles of CuCl₂ to CuCl per mole Cl added to DD/DF. In a short duration experiment (DF/Cu=0.3), nearly 100% DF adsorption and conversion to CDF was achieved. The degree of CDF chlorination was strongly dependent on gas velocity. At high gas velocity, corresponding to a gas–particle contact time of 0.3 s, mono-CDF (MCDF) yield was largest, with yields decreasing with increasing CDF chlorination. At low gas velocity, corresponding to a gas–particle contact time of 5 s, octa-CDF yield was largest. DF/DD chlorination was strongly favored at lateral sites, with the predominant CDF/CDD isomers within each homologue group those containing Cl substituents at only the 2,3,7,8 positions. At the higher temperatures and lower gas velocities studied, however, broader isomer distributions, particularly of the less CDD/CDF products, were observed, likely due to preferential destruction of the 2,3,7,8 congeners.     

The fate and behavior of mercury in coal-fired power plants

For the past 22 years in the Netherlands, the behavior of Hg in coal-fired power plants has been studied extensively. Coal from all over the world is fired in Dutch power stations. First, the Hg concentrations in these coals were measured. Second, the fate of the Hg during combustion was established by performing mass balance studies. On average, 43 +/- 30% of the Hg was present in the flue gases downstream of the electrostatic precipitator (ESP; dust collector). In individual cases, this figure can vary between 1 and 100%. Important parameters are the Cl content of the fuel and the flue gas temperature in the ESP. On average, 54 +/- 24% of the gaseous Hg was removed in the wet flue-gas desulfurization (FGD) systems, which are present at all Dutch coal-power stations. In individual cases, this removal can vary between 8% (outlier) and 72%. On average, the fate of Hg entering the power station in the coal was as follows: <1% in the bottom ash, 49% in the pulverized fuel ash (ash collected in the ESP), 16.6% in the FGD gypsum, 9% in the sludge of the wastewater treatment plant, 0.04% in the effluent of the wastewater treatment plant, 0.07% in fly dust (leaving the stack), and 25% as gaseous Hg in the flue gases and emitted into the air. The distribution of Hg over the streams leaving the FGD depends strongly on the installation. On average, 75% of the Hg was removed, and the final concentration of Hg in the emitted flue gases of the Dutch power stations was only -3 microg/m3(STP) at 6% O2. During co-combustion with biomass, the removal of Hg was similar to that during 100% coal firing. Speciation of Hg is a very important factor. An oxidized form (HgCl2) favors a high degree of removal. The conversion from Hg0 to HgCl2 is positively correlated with the Cl content of the fuel. A catalytic DENOX (SCR) favors the formation of oxidized Hg, and, in combination with a wet FGD, the total removal can be as high as 90%.     

Chlorination of dibenzofuran and dibenzo-p-dioxin vapor by copper (II) chloride

Dibenzofuran (DF) is formed from phenol and benzene in combustion gas exhaust streams prior to particle collection equipment. Subsequent chlorination at lower temperatures on particle surfaces is a potential source of chlorinated dibenzofuran (CDF). Gas streams containing 8% O₂ and approximately 0.1% DF vapor were passed through particle beds containing copper (II) chloride (0.5% Cu, mass) at temperatures ranging from 200 to 400 °C to investigate the potential for CDF formation during particle collection. Experiment duration was sufficient to provide an excess amount of DF (DF/Cu=3). The efficiency of DF chlorination by CuCl₂ and the distribution of CDF products were measured, with effects of temperature, gas velocity, and experiment duration assessed. Results of a more limited investigation of dibenzo-p-dioxin (DD) chlorination by CuCl₂ to form chlorinated DD (CDD) products are also presented.The efficiency of DF/DD chlorination by CuCl₂ was high, both in terms of CuCl₂ utilization and DF/DD conversion. Total yields of Cl on CDF/CDD products of up to 0.5 mole Cl per mole CuCl₂ were observed between 200 and 300 °C; this suggests that nearly 100% CuCl₂ was utilized, assuming a conversion of two moles of CuCl₂ to CuCl per mole Cl added to DD/DF. In a short duration experiment (DF/Cu=0.3), nearly 100% DF adsorption and conversion to CDF was achieved. The degree of CDF chlorination was strongly dependent on gas velocity. At high gas velocity, corresponding to a gas–particle contact time of 0.3 s, mono-CDF (MCDF) yield was largest, with yields decreasing with increasing CDF chlorination. At low gas velocity, corresponding to a gas–particle contact time of 5 s, octa-CDF yield was largest. DF/DD chlorination was strongly favored at lateral sites, with the predominant CDF/CDD isomers within each homologue group those containing Cl substituents at only the 2,3,7,8 positions. At the higher temperatures and lower gas velocities studied, however, broader isomer distributions, particularly of the less CDD/CDF products, were observed, likely due to preferential destruction of the 2,3,7,8 congeners.     

Modeling interactions of Hg(II) and bauxitic soils

The adsorptive interactions of Hg(II) with gibbsite-rich soils (hereafter SOIL-g) were modeled by 1-pK surface complexation theory using charge distribution multi-site ion competition model (CD MUSIC) incorporating basic Stern layer model (BSM) to account for electrostatic effects. The model calibrations were performed for the experimental data of synthetic gibbsite-Hg(II) adsorption. When [NaNO(3)] > or = 0.01M, the Hg(II) adsorption density values, of gibbsite, Gamma(Hg(II)), showed a negligible variation with ionic strength. However, Gamma(Hg(II)) values show a marked variation with the [Cl(-)]. When [Cl(-)] > or = 0.01M, the Gamma(Hg(II)) values showed a significant reduction with the pH. The Hg(II) adsorption behavior in NaNO(3) was modeled assuming homogeneous solid surface. The introduction of high affinity sites, i.e., >Al(s)OH at a low concentration (typically about 0.045 sites nm(-2)) is required to model Hg(II) adsorption in NaCl. According to IR spectroscopic data, the bauxitic soil (SOIL-g) is characterized by gibbsite and bayerite. These mineral phases were not treated discretely in modeling of Hg(II) and soil interactions. The CD MUSIC/BSM model combination can be used to model Hg(II) adsorption on bauxitic soil. The role of organic matter seems to play a role on Hg(II) binding when pH>8. The Hg(II) adsorption in the presence of excess Cl(-) ions required the selection of high affinity sites in modeling.     

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.     

Mercury oxidation promoted by a selective catalytic reduction catalyst under simulated Powder River Basin coal combustion conditions

A bench-scale reactor consisting of a natural gas burner and an electrically heated reactor housing a selective catalytic reduction (SCR) catalyst was constructed for studying elemental mercury (Hg(o)) oxidation under SCR conditions. A low sulfur Powder River Basin (PRB) subbituminous coal combustion fly ash was injected into the entrained-flow reactor along with sulfur dioxide (SO2), nitrogen oxides (NOx), hydrogen chloride (HCl), and trace Hg(o). Concentrations of Hg(o) and total mercury (Hg) upstream and downstream of the SCR catalyst were measured using a Hg monitor. The effects of HCl concentration, SCR operating temperature, catalyst space velocity, and feed rate of PRB fly ash on Hg(o) oxidation were evaluated. It was observed that HCl provides the source of chlorine for Hg(o) oxidation under simulated PRB coal-fired SCR conditions. The decrease in Hg mass balance closure across the catalyst with decreasing HCl concentration suggests that transient Hg capture on the SCR catalyst occurred during the short test exposure periods and that the outlet speciation observed may not be representative of steady-state operation at longer exposure times. Increasing the space velocity and operating temperature of the SCR led to less Hg(o) oxidized. Introduction of PRB coal fly ash resulted in slightly decreased outlet oxidized mercury (Hg2+) as a percentage of total inlet Hg and correspondingly resulted in an incremental increase in Hg capture. The injection of ammonia (NH3) for NOx reduction by SCR was found to have a strong effect to decrease Hg oxidation. The observations suggest that Hg(o) oxidation may occur near the exit region of commercial SCR reactors. Passage of flue gas through SCR systems without NH3 injection, such as during the low-ozone season, may also impact Hg speciation and capture in the flue gas.     

Theoretical and experimental investigations of mercury adsorption on hematite surfaces

One of the biggest environmental concerns caused by coal-fired power plants is the emission of mercury (Hg), which is toxic metal. To control the emission of Hg from coal-derived flue gas, it is important to understand the behavior and speciation of Hg as well as the interaction between Hg and solid materials in the flue gas stream. In this study, atomic-scale theoretical investigations using density functional theory (DFT) were carried out in conjunction with laboratory-scale experimental studies to investigate the adsorption behavior of Hg on hematite (α-Fe₂O₃). According to the DFT simulation, the adsorption energy calculation proposes that Hg physisorbs to the α-Fe₂O₃(0001) surface with an adsorption energy of ?0.278 eV, and the subsequent Bader charge analysis confirms that Hg is slightly oxidized. In addition, Cl introduced to the Hg-adsorbed surface strengthens the Hg stability on the α-Fe₂O₃(0001) surface, as evidenced by a shortened Hg-surface equilibrium distance. The projected density of states (PDOS) analysis also suggests that Cl enhances the chemical bonding between the surface and the adsorbate, thereby increasing the adsorption strength. In summary, α-Fe₂O₃ has the ability to adsorb and oxidize Hg, and this reactivity is enhanced in the presence of Cl. For the laboratory-scale experiments, three types of α-Fe₂O₃ nanoparticles were prepared using the precursors Fe(NO₃)₃, Fe(ClO₄)₃, and FeCl₃, respectively. The particle shapes varied from diamond to irregular stepped and subrounded, and particle size ranged from 20 to 500 nm depending on the precursor used. The nanoparticles had the highest surface area (84.5 m²/g) due to their highly stepped surface morphology. Packed-bed reactor Hg exposure experiments resulted in this nanoparticles adsorbing more than 300 μg Hg/g. The Hg L_III-edge extended X-ray absorption fine structure spectroscopy also indicated that HgCl₂ physisorbed onto the α-Fe₂O₃ nanoparticles.

Implications: Atomic-scale theoretical simulations proposes that Hg physisorbs to the α-Fe₂O₃(0001) surface with an adsorption energy of ?0.278 eV, and the subsequent Bader charge analysis confirms that Hg is slightly oxidized. In addition, Cl introduced to the Hg-adsorbed surface strengthens the Hg stability on the α-Fe₂O₃(0001) surface, as evidenced by a shortened Hg-surface equilibrium distance. The PDOS analysis also suggests that Cl enhances the chemical bonding between the surface and the adsorbate, thereby increasing the adsorption strength. Following laboratory-scale experiment of Hg sorption also shows that HgCl₂ physisorbs onto α-Fe₂O₃ nanoparticles which have highly stepped structure.     

Mercury toxicity induces oxidative stress in growing cucumber seedlings

In this study, the effects of exogenous mercury (HgCl(2)) on time-dependent changes in the activities of antioxidant enzymes (catalase and ascorbate peroxidase), lipid peroxidation, chlorophyll content and protein oxidation in cucumber seedlings (Cucumis sativus L.) were investigated. Cucumber seedlings were exposed to from 0 to 500microM of HgCl(2) during 10 and 15 days. Hg was readily absorbed by growing seedlings, and its content was greater in the roots than the in shoot. Time and concentration-dependent reduction in root and shoot length was observed at all concentrations tested, equally in the roots and shoot, at both 10 and 15 days. At 50microM HgCl(2), root fresh weight of 15-day-old seedlings increased, and at other concentrations, it reduced. For 10-day-old seedlings, reduction in root and shoot fresh biomass was observed. At 15 days, only at 50microM HgCl(2) was there no observed reduction in shoot fresh biomass. Dry weight of roots increased at 500microM both at 10 and 15 days, though at 250microM HgCl(2) there was only an increase at 15 days. There was a significant effect on shoot dry weight at all concentrations tested. Hg-treated seedlings showed elevated levels of lipid peroxides with a concomitant increase in protein oxidation levels, and decreased chlorophyll content when exposed to between 250 and 500microM of HgCl(2). At 10 days, catalase activity increased in seedlings at a moderately toxic level of Hg, whereas at the higher concentration (500microM), there was a marked inhibition. Taken together, our results suggest that Hg induces oxidative stress in cucumber, resulting in plant injury.     

The role of copper(II) chloride in the formation of organic chlorine in fly ash

In the de-novo synthesis and formation of PCDD/PCDF, the transfer of inorganic chlorine to the carbonaceous material of fly ash plays an important role. Here, copper acts as a catalyst in the chlorination reaction. In experiments in the range of 250-350 degrees C under helium, we determined the stoichiometry of the chlorination reaction with model systems. Therefore, it was necessary to develop a method to quantify the copper(II) and copper(I) ions. In a combination of solid electron paramagnetic (spin) resonance spectroscopy (EPR) for Cu(I), and X-ray fluorescence spectroscopy (XRFA) analysis for Cu (total), we found a way for the quantification of copper(I) and (II). With these experiments, we can show that the chlorination reaction is relatively fast and comes to a stop under helium, after the copper(II) is reduced. The ratio between the organic chlorine formed and copper(II) reduced is, at the end of the reaction, 0.5, which is in agreement with the following reaction: 2CuCl2 + R-H-->2CuCl + R-Cl + HCl.