Conversion Of Hg0 Research Articles

A new insight into catalytic role of copper sulfate on elemental mercury oxidation: DFT and experimental study

Copper sulfate (CuSO4) is a ubiquitous component in natural minerals and industrial dusts. However, CuSO4 has been believed usually that it has adverse effect on elemental mercury (Hg0) removal from flue gas. Here, we offered a new insight into the effect of crystal water in copper sulfate on Hg0 catalytic oxidation. The structure evolution of copper sulfate by density functional theory (DFT) calculation revealed anhydrous CuSO4 crystal contributing to a better HCl adsorption which provided a precondition for Hg0 oxidation. Experimental results about the catalytic activity sequence of CuSO4 > CuSO4·H2O > CuSO4·5H2O with HCl confirmed the negative effect of crystal water on Hg0 catalytic oxidation. To realize industrial application, supported catalysts CuSO4/α-Al2O3 synthesized by wet impregnation method was employed for elemental mercury (Hg0) oxidation and the results exhibited excellent performance in Hg0 oxidation under 250 °C–400 °C. The Hg0 conversion efficiency was above 97% in the presence of 2 ppm HCl + 6% O2 at 300 °C. Combining computational and X-ray photoelectron spectroscopy (XPS) analyses together, Cu atoms exposure on the surface of anhydrous CuSO4 were the critical active sites for Hg0 oxidation and the reaction obeyed the Eley–Rideal mechanism.

Research on reaction conditions and mechanism for simultaneous removal of NO and Hg0 over Cu[sbnd]Fe modified activated carbon at low temperature

In this study, modified microwave coconut shell activated carbon (CuFe/MCSAC) was used to simultaneously remove NO and Hg0 in the industrial tail gas. The influences of reaction condition were investigated, such as NH3 concentration, O2 content, gas hourly space velocity (GHSV), resistance of SO2 and H2O. Under the optimal reaction conditions (600 ppm NH3, 3% O2 and 30000 h−1 GHSV), CuFe/MCSAC showed high NO conversion rate (86%), high Hg0 conversion rate (61%) and low N2O concentration (81 ppm) at 225 °C. NH3 was conducive to the removal of NO, but it had a competitive adsorption effect for Hg0. O2 promoted oxidation of NO and Hg0, and led to the formation of N2O. Low GHSV had the external diffusion effect, which affected the reaction rate. High GHSV could not provide enough reaction time. H2O and SO2 were not conducive to the selectivity of SCR reaction and oxidation of Hg0.

Role of flue gas components in Hg0 oxidation over La0.8Ce0.2MnO3 perovskite catalyst in coal combustion flue gas

La0.8Ce0.2MnO3 perovskite catalyst was employed for elemental mercury (Hg0) oxidation in coal combustion flue gas. The effects of flue gas components, including O2, NO, SO2, HCl, H2O as well as the selective catalytic reduction (SCR) reductant (NH3), on Hg0 oxidation were investigated systematically. Above 90% Hg0 oxidation was obtained at 200 °C under simulated flue gas (SFG) and SCR atmosphere with a gas hourly space velocity of 60,000 h−1. Gaseous O2 regenerated and replenished surface oxygen, hence promoting the oxidation of Hg0. NO promoted the Hg0 oxidation because of the formation of active surface species like NO2. HCl facilitated the Hg0 oxidation, and 98.8% Hg0 oxidation was obtained under the atmosphere containing 10 ppm HCl in the co-presence of O2. SO2 inhibited Hg0 oxidation because of the irreversible damage on catalytic activity by reacting with the active species on the catalyst·H2O also inhibited Hg0 oxidation due to the change of catalyst surface chemistry as well as the consumption of active surface chlorine species (Cl∗). NH3 competed active sites with Hg0 and induced oxidized mercury reduction, hence limiting the Hg0 conversion. However, the inhibitive effect of NH3 could be partly offset by the promotional effect of NO. This work clarified the effects of flue gas compositions on Hg0 oxidation over a novel La0.8Ce0.2MnO3 perovskite catalyst, which is essential for further improving the catalyst activity.

Coexistence of enhanced Hg0 oxidation and induced Hg2+ reduction on CuO/TiO2 catalyst in the presence of NO and NH3

Simultaneous nitrogen monoxide (NO) reduction and elemental mercury (Hg0) oxidation in the presence of ammonia (NH3) was for the first time studied on a copper oxide/titanium dioxide (CuO/TiO2) catalyst at low flue gas temperatures. More than 80% NO reduction and 90% Hg0 oxidation were concurrently achieved on the CuO/TiO2 catalyst at 250°C under a simulated NH3 selective catalytic reduction (SCR) atmosphere. NO was likely oxidized on the CuO/TiO2 catalyst to form active nitrogenous species, particularly with the aid of oxygen. The active nitrogenous species then significantly promoted Hg0 oxidation. NH3 itself did not affect Hg0 conversion on the CuO/TiO2 catalyst. The co-occurrences of NO and NH3 led to a reduction in oxidized mercury (HgO in this work), which partially offset the Hg0 oxidation and therefore resulted in a relatively lower Hg0 conversion efficiency. However, compared to flue gas without NO and NH3, the combined presence of NO and NH3 still enhanced Hg0 oxidation over the CuO/TiO2 catalyst, primarily because of the overwhelming benefit from NO. This is superior to many other SCR catalysts, including commercial vanadia-based SCR catalysts, on which an SCR atmosphere generally inhibits Hg0 conversion.

Study on emission of hazardous trace elements in a 350 MW coal-fired power plant. Part 1. Mercury

Hazardous trace elements (HTEs), especially mercury, emitted from coal-fired power plants had caused widespread concern worldwide. Field test on mercury emissions at three different loads (100%, 85%, 68% output) using different types of coal was conducted in a 350 MW pulverized coal combustion power plant equipped with selective catalytic reduction (SCR), electrostatic precipitator and fabric filter (ESP + FF), and wet flue gas desulfurization (WFGD). The Ontario Hydro Method was used for simultaneous flue gas mercury sampling for mercury at the inlet and outlet of each of the air pollutant control device (APCD). Results showed that mercury mass balance rates of the system or each APCD were in the range of 70%–130%. Mercury was mainly distributed in the flue gas, followed by ESP + FF ash, WFGD wastewater, and slag. Oxidized mercury (Hg2+) was the main form of mercury form in the flue gas emitted to the atmosphere, which accounted for 57.64%–61.87% of total mercury. SCR was favorable for elemental mercury (Hg0) removal, with oxidation efficiency of 50.13%–67.68%. ESP + FF had high particle-bound mercury (Hgp) capture efficiency, at 99.95%–99.97%. Overall removal efficiency of mercury by the existing APCDs was 58.78%–73.32%. Addition of halogens or oxidants for Hg0 conversion, and inhibitors for Hg0 re-emission, plus the installation of a wet electrostatic precipitator (WESP) was a good way to improve the overall removal efficiency of mercury in the power plants. Mercury emission factor determined in this study was from 0.92 to 1.17 g/1012J. Mercury concentration in the emitted flue gas was much less than the regulatory limit of 30 μg/m3. Contamination of mercury in desulfurization wastewater should be given enough focus.

Simultaneous NO and mercury removal over MnOx/TiO2 catalyst in different atmospheres

In this study, manganese oxides supported on titania was prepared by sol-gel method and employed as SCR catalyst for simultaneous NO and mercury removal from coal combustion flue gas. The simultaneous NO and Hg0 conversion rate on MnOx/TiO2 was evaluated in different atmospheres, and the interaction between denitration and demercuration was investigated. The results indicated that MnOx/TiO2 had good activity in simultaneously removing NO and Hg0 in SCR atmosphere (NH3+NO+O2+N2). However, both NO conversion and Hg0 removal efficiency were inhibited in simulated coal combustion flue gas compared to that in SCR atmosphere because of the presence of SO2 leading to the generation of sulfite and sulfate on the catalyst. H2O in simulated coal combustion flue gas had negative effect on the catalytic performance as well. NO conversion in oxy-fuel combustion flue gas was lower than that in air combustion flue gas, while Hg0 removal efficiency in oxy-fuel combustion flue gas was higher than that in air combustion flue gas. Hg0 displayed no obvious influence on NO conversion and N2 selectivity over SCR process. And SCR atmosphere showed almost no inhibition to Hg0 removal when the temperature was below 300°C. At the optimal temperature for NO conversion (250°C), the inhibition of SCR atmosphere on Hg0 removal became stronger as the space velocity (GHSV) increased, but the inhibitive effect could be ignored when GHSV slowed down to 50,000h−1 or less.

Elemental mercury oxidation over manganese-based perovskite-type catalyst at low temperature

La1−xSrxMnO3 (LSx, x=0/0.2/0.4/0.6) perovskite-type oxides catalysts were synthesized for elemental mercury (Hg0) oxidation in simulated coal-fired flue gas at low temperature. LS0.4 exhibited superior catalytic behavior for Hg0 oxidation at 100–200°C. The superior performance was mainly due to the surface adsorbed oxygen species in the catalysts. Effects of different components in the flue gas including HCl, O2, SO2, H2O, NO and NH3 on Hg0 oxidation efficiencies were studied. The results suggested that HCl significantly enhanced Hg0 oxidation efficiency in the presence of O2 and 10ppmv HCl was sufficient for enhancing the oxidation process. As the catalyst had strong oxygen storage capacity, the Hg0 oxidation activity was less dependent on O2 concentration. As a result, the catalyst could be applied in the power plants burning low-chlorine coals. SO2, H2O and NH3 had inhibitory effect on Hg0 oxidation, while NO enhanced it. SO2 could deactivate the catalyst and the deactivation was irreversible. In terms of mechanism, Hg0 oxidation with LSx could be explained by the suprafacial process (Langmuir–Hinshelwood mechanism) whereby reactive species from adsorbed flue gas components reacted with adsorbed Hg0. The catalyst also had potential for selective catalytic reduction (SCR) of NO with NH3 at low temperature, and the simultaneous removal of NO and Hg0 over LS0.4 was investigated. The NO and Hg0 conversion were higher than 50% and 85% when the space velocity was 40,000h−1, respectively. The results indicated that the catalyst might be used to remove NO and Hg0 simultaneously after the cold-side electrostatic precipitators in flue gas with low concentration of ash.

Research on the catalytic oxidation of Hg0 by modified SCR catalysts

A series of metal oxides were employed to modify the commercial SCR catalyst, and the CeO2 modified SCR catalyst was selected and then subjected to detailed catalytic oxidation of Hg0 by using the simulated flue gas. The results indicated that the catalytic activity of the catalyst was increased remarkably after the CeO2 modification, and the highest catalytic oxidation of Hg0 was obtained from the modified SCR catalyst with 9% CeO2 loading, being 40% higher than that of the non-modified SCR catalyst. The BET and XRD analysis indicated that the surface area of the 1%–9% CeO2 modified SCR catalyst did not show significant changes as compared with the non-modified SCR catalyst, and the CeO2 was well dispersed on the catalyst surface without any aggregation. The flue gas condition had great effects on the Hg0 conversion. The catalytic oxidation of Hg0 would be significantly increased by HCl, and also increased as the increasing of the temperature in a certain range. The highest catalytic oxidation efficiency reached 95.11% at the optimal space velocity, temperature and flue gas components. In addition, the CeO2 doping did not affect the denitration efficiency of the SCR catalyst.

Effects of existing energy saving and air pollution control devices on mercury removal in coal-fired power plants

The impacts of a low temperature economizer (LTE) on mercury removal across an electrostatic precipitator and influence of load variation on mercury conversion over selective catalytic reduction (SCR) catalysts were determined at two coal-fired boilers. When the LTE was on, the total and elemental mercury removal efficiency increased by 42.87% and 18.85%, respectively, due to the improvement of adsorption and oxidation capacity of the fly ash at lower temperature. Mercury speciation at the inlet and outlet of the SCR system were analyzed, and the impacts of load variation and catalyst aging on Hg0 conversion were discussed. The variable loads resulted in simultaneous changes of the gas hourly space velocity, the ambient temperature, and the oxygen content. The results showed the load ratio was significant for Hg0 conversion by the SCR catalysts and load reduction benefitted Hg0 conversion. When the load ratios were 100%, 75% and 60%, the Hg0 conversion were 61.78%, 65.71% and 72.12%, respectively. Moreover, Hg0 conversion was more significantly affected by the catalyst aging than NOx reduction. Among the three factors, the most important one is the flue gas temperature based on the grey relational analysis.

Modeling of the SCR reactor for coal-fired power plants: Impact of NH3 inhibition on Hg0 oxidation

A 1D + 1D heterogeneous model of the SCR reactor with honeycomb monolith catalyst is developed; it solves the mass balances of the reacting species along the axial and radial direction, using a lumped approach to describe the gas–solid mass transfer and a differential approach to describe diffusion and reaction within the catalyst wall. The model describes the coexistence of the DeNOX reaction and the highly desired oxidation of Hg0. It is well known from the literature that the conversion of Hg0 is negatively affected by NH3, but the extent of such an inhibition is uncertain because of the inherent complexity of the mercury speciation measurement. We thus apply the model to examine the concentration and reaction rate profiles inside the monolith wall and comprehend the way and extent of the interference of NH3 on Hg0 oxidation. The estimate of the intrinsic kinetic parameters was obtained from lab scale and pilot scale tests over commercial V-based catalysts, which allowed to focus the analysis on realistic conditions. The DeNOX and the Hg oxidation processes are both affected by external and internal mass transfer limitations, but the kinetic control from diffusion is stronger in the case of the DeNOX reaction. Thus along the radial coordinate and along the axial coordinate, the decay of NH3 concentration is more pronounced than the decay of Hg0 concentration. In practice, the main operating parameter which determines the integral impact of NH3 inhibition is the area velocity, that is the sizing of the full scale reactor. Still, a parametric analysis provides useful elements for envisaging the desired catalyst characteristics which can promote Hg0 conversion and minimize NH3 inhibition.

Evaluation of discrepancy between measured and modelled oxidized mercury species

Abstract. L. Zhang et al. (2012), in a recent report, compared model estimates with new observations of oxidized and particulate mercury species (Hg2+ and Hgp) in the Great Lakes region and found that the sum of Hg2+ and Hgp varied between a factor of 2 to 10 between measurements and model. They suggested too high emission inputs as Hg2+ and too fast oxidative conversion of Hg0 to Hg2+ and Hgp as possible causes. This study quantitatively explores measurement uncertainties in detail. These include sampling efficiency, composition of sample, interfering species and calibration errors. Model (Global/Regional Atmospheric Heavy Metals Model – GRAHM) sensitivity experiments are used to examine the consistency between various Hg measurements and speciation of Hg near emission sources to better understand the discrepancies between modelled and measured concentrations of Hg2+ and Hgp. We find that the ratio of Hg0, Hg2+ and Hgp in the emission inventories, measurements of surface air concentrations of oxidized Hg and measurements of wet deposition are currently inconsistent with each other in the vicinity of emission sources. Current speciation of Hg emissions suggests higher concentrations of Hg2+ in air and in precipitation near emission sources; however, measured air concentrations of Hg2+ and measured concentrations of Hg in precipitation are not found to be significantly elevated near emission sources compared to the remote regions. The averaged unbiased root mean square error (RMSE) between simulated and observed concentrations of Hg2+ is found to be reduced by 42% and for Hgp reduced by 40% for 21 North American sites investigated, when a ratio for Hg0 : Hg2+ : Hgp in the emissions is changed from 50 : 40 : 10 (as specified in the original inventories) to 90 : 8 : 2. Unbiased RMSE reductions near emissions sources in the eastern United States and Canada are found to be reduced by up to 58% for Hg2+. Significant improvement in the model simulated spatial distribution of wet deposition of mercury in North America is noticed with the modified Hg emission speciation. Measurement-related uncertainties leading to lower estimation of Hg2+ concentrations are 86%. Uncertainties yielding either to higher or lower Hg2+ concentrations are found to be 36%. Finally, anthropogenic emission uncertainties are 106% for Hg2+. Thus it appears that the identified uncertainties for model estimates related to mercury speciation near sources, uncertainties in measurement methodology and uncertainties in emissions can close the gap between modelled and observed estimates of oxidized mercury found in L. Zhang et al. (2012). Model sensitivity simulations show that the measured concentrations of oxidized mercury, in general, are too low to be consistent with measured wet deposition fluxes in North America. Better emission inventories (with respect to speciation), better techniques for measurements of oxidized species and knowledge of mercury reduction reactions in different environments (including in-plume) in all phases are needed for improving the mercury models.

Conversion of elemental mercury with a novel membrane catalytic system at low temperature

A unique assembly, which integrated membrane delivery for oxidants with catalytic oxidation (MDCOs), was employed to convert elemental mercury (Hg0) to its oxidized form at low temperature (around 150°C). MnOx was used as the main catalytic component in MDCOs with Mo and/or Ru to improve the catalytic activity. The MDCOs was proved to be very effective for the conversion of Hg0 compared with the traditional catalytic oxidation mode (TCO). The analysis of speciation for Hg after catalytic oxidation showed that there was mainly mercury (II) chloride. The addition of transition metals of Mo and Ru obviously improved the conversion of Hg0 to Hg2+ and enhanced the activity of the MDCOs at low temperature, and the conversion efficiency of Hg0 reached 95% with Mo-Ru-Mn catalyst and 8ppmv HCl. The inhibition of SO2 to Hg0 conversion in the MDCOs was insignificant. The Hg0 removal exceeded 80% even if the concentration of SO2 reached 1000ppmv. The results also indicated that the Deacon reaction with the yield of Cl2 were significantly improved after modified, and MDCOs with Mo-Ru-Mn catalyst can work efficiently at low temperature.

Conversion of Elemental Mercury with a Novel Membrane Delivery Catalytic Oxidation System (MDCOs)

In order to overcome the shortcomings of the traditional catalytic oxidation (TCO) mode for the conversion of the trace level of elemental mercury (Hg(0)) in flue gas, we put forward a novel and unique assembly that integrated membrane delivery with catalytic oxidation systems (MDCOs), which combined the controlled delivery of oxidants with the catalytic oxidation of Hg(0). The results show that the demanded HCl for Hg(0) conversion in the MDCOs was less than 5% of that in the TCO mode, and over 90% of Hg(0) removal efficiency can be obtained in the MDCOs with less than 0.5 mg m(-3) of HCl escaped. Meanwhile, the inhibition of SO(2) to Hg(0) catalytic conversion in the MDCOs was also less significant than in the TCO. The MDCOs have high retainability for HCl, which is quite favorable to Hg(0) conversion and HCl utilization. The reaction mechanism on mercury conversion in the MDCOs is discussed. The MDCOs appear to be a promising method for emission control of elemental mercury.

Mercury Speciation and Distribution in a 660-Megawatt Utility Boiler in Taiwan Firing Bituminous Coals

Mercury speciation and distribution in a 660-MW tangential-fired utility boiler in Taiwan burning Australian and Chinese bituminous coal blends was investigated. Flue gases were simultaneously sampled at the selective catalytic reduction (SCR) inlet, the SCR outlet, the electro-static precipitator (ESP) outlet, and the stack. Samplings of coal, lime, bottom ash/slag, fly ash, and gypsum slurry were also conducted. Results indicated that flue gases at the inlet to SCR contained a great potion of particle-bound mercury (Hgp), 59–92% of the total mercury. Removal of mercury was not observed for the SCR system. However, repartitioning of mercury species across the SCR occurred that significantly increased the portion of elemental mercury (Hg0) to up to 29% and oxidized mercury (Hg2+) to up to 33% in the SCR outlet gas. Overreporting of Hgp at the inlet of SCR may cause the observed repartitioning; the high ammonia/nitric oxide circumstance in the SCR unit was also speculated to cause the mercury desorption from ash particles and subsequent reentrance into the gas phase. ESP can remove up to 99% of Hgp, and wet flue gas desulfurization (FGD) can remove up to 84% of Hg2+. Mercury mass balances were calculated to range between 81 and 127.4%, with an average of 95.7% wherein 56–82% was in ESP fly ash, 8.7–18.6% was retained in the FGD gypsum, and 6.2–26.1% was emitted from the stack. Data presented here suggest that mercury removal can be largely enhanced by increasing the conversion of Hg0 into Hgp and Hg2+.

Mercury Oxidization in Dielectric Barrier Discharge Plasma System

The pronounced volatility of elemental mercury (Hg0) and some of its compounds, coupled with their extreme toxicity, makes these substances extremely hazardous. Conversion of Hg0 to HgO would significantly enhance mercury removal from flue gases. This investigation is focused on studying the effect of some of the constituents such as O2, H2O, CO2, and NOx present in flue gases on elemental mercury oxidation in a dielectric barrier discharge (DBD) reactor. The results show that Hg vapors (6 ppbv) in a stream of 0.1% O2 and N2 are effectively oxidized at the energy density of up to 114 J/L. Hg conversion of over 80% is achieved when present in a gas mixture of 8% O2, 2% H2O, and 10% CO2 in N2 balance. The presence of NOx enhanced mercury oxidation in the DBD reactor. The oxidation chemistry is discussed. Studies show that Hg can be simultaneously removed along with the other two major pollutants, NOx and SO2, in one DBD reactor followed by a wet scrubber system. This avoids the need of three techniques for the removal of major gaseous pollutants from coal-fired power plants.

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

Selective catalytic reduction (SCR) technology increasingly is being applied for controlling emissions of nitrogen oxides (NOx) 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 NOx 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.