Hg0 Removal Efficiency Research Articles

Relationship between Hg0 capture performance and physicochemical properties of CeO2-CrOx mixed oxides

To effectively reduce the emission of Hg0, developing an adsorbent with a superior Hg0 removal performance constituted an important step. As a typical rare earth metal oxide, CeO2 was always used for Hg0 capture because of its tunable physicochemical properties. However, the weak acidity and the intermediate oxidative ability restrained its effective removal of Hg0, especially from the flue gas with a high Hg0 content. Thanks to the superior acidity and oxidability, CrOx addition could ensure the sufficient physisorption of Hg0 and accelerate its oxidation to HgO. Thus in this study, a serial CeO2-CrOx mixed oxides were prepared. The sample with Ce/Cr molar ratio of 3/1 exhibited the best performance; ca. 90 % Hg0 removal efficiency could be reached at 150 °C. Compared with virgin CeO2 and CrOx, the combination of these two metal oxides brought out a larger specific surface area, which would provide an increased number of active sites for Hg0 removal. Meanwhile, the presence of plentiful acid sites and reactive oxygen species favored the physisorption and oxidation of Hg0, hence contributing to a relatively high Hg0 removal efficiency of CeO2-CrOx mixed oxides. During the Hg0 adsorption cycles, Mars-Maessen route dominated; Cr6+ played the role of an active site to oxidize Hg0, while Ce4+ acted as a promoter-like site to boost the regeneration of Cr6+. Finally, CeO2-CrOx mixed oxide exhibited a satisfactory recyclability and a relatively weak gas component adaptability, suggesting that further modification should be conducted to improve the practicality of CeO2-CrOx mixed oxide.

Enhanced removal of elemental mercury using MnO2-modified molecular sieve under microwave irradiation

Developing efficient catalysis-adsorption method for elemental mercury (Hg0) removal with wide temperature window and good sulfur/water resistance is a hot topic. In this study, a novel method of microwave (MW) catalysis of MnO2 modified molecular sieve (Mn@MOS) composite for Hg0 removal was developed. Results indicated that Hg0 removal efficiency of MOS was poor, only 18% at 150 °C, while the efficiency was significantly increased to 97.8% when MW/Mn@MOS was used. Compared with thermo-catalysis way, MW irradiation method achieved better Hg0 removal efficiency (97.8% vs 86.4%) under a wider temperature window (150–450 °C), and sulfur/water resistance were greatly strengthened under MW irradiation. The dynamical adsorption capacity of Hg0 was calculated as 1.52 mg/g when the efficiency was below 80%. Kinetic analysis indicated that MW field significantly increased the rate constant and reduced the activation energy. The fate of Hg0 was unfolded: almost 96% of Hg0 was chemisorbed as HgO, less than 4% was converted to gaseous Hg2+. The mechanism was speculated as followed: (i) excited active site of *MnO2 was first generated under MW irradiation; (ii) then the oxidizing species including Mn(IV)/Mn(III), Oads, Olatt and O* dominated the Hg0 removal process; (iii) the reactive oxygen species of Oads, Olatt and O* were replenished by the interaction between oxygen vacancies and free O2. This study first demonstrates the superiority of MW catalysis towards Hg0 removal, and provides a new thought on the development of the novel Hg0 removal technologies.

Tuning sulfur vacancies in CoS2 via a molten salt approach for promoted mercury vapor adsorption

Metal sulfides are emerging and efficient adsorbents for mercury removal. However, the major active sites responsible for their superior Hg0 adsorption ability are still disputable. Here, we proposed that sulfur vacancies play an important role in Hg0 removal over metal sulfides. Moreover, tuning the sulfur vacancies in CoS2 appears to be a feasible route to strengthen its mercury adsorption ability. Since traditional methods for adjusting the sulfur vacancies in metal sulfides are very complicated and hard to scale up, a simple, and cost-effective molten salt approach is employed to tune the concentrations of the sulfur vacancies in CoS2. The Hg0 removal efficiency of CoS2 is positively correlation with the density of sulfur vacancies, which can be facilely tuned by altering the cobalt precursors and annealing temperatures. The CoS2 derived from cobalt chloride and cobalt sulfate with higher concentrations of sulfur vacancies distinctly outperforms the ones derived from cobalt nitrate and cobalt acetate. Higher annealing temperature can generate higher concentration of sulfur vacancies, leading to a better mercury capture performance. CoS2-500-CA attained by annealing treatment of KSCN and cobalt acetate at 500 °C displays excellent mercury capture ability with Hg0 removal efficiency of 100% at reaction temperatures of 80–100 °C plausibly due to its high density of sulfur vacancies and big pore diameter. CoS2-500-CA is insusceptible to 100–200 ppm NO or 150–300 ppm SO2.

Removal of multi-pollutants in flue gas via a new approach based on dielectric barrier discharge coupling MnCu/Ti oxidation

A dielectric barrier discharge (DBD) coupling MnCu/Ti oxidation and postpositioned wet electrostatic precipitator (WESP) system was built. The experimental results showed that WESP can significantly enhance the simultaneous conversion efficiency(ηC) of NO, SO2 and Hg0 by the DBD coupling MnCu/Ti reactor. The best simultaneous removal efficiency(ηR) of NO, SO2 and Hg0 reached 94.5%, 100% and 100%, respectively. The ratio of catalyst active component (Mn/Cu) and catalyst filling amount were optimized. The effects of catalyst active component ratio, filling amount, flue gas component and energy density (SED) on the ηC of NO, SO2 and Hg0 were determined. The effect of sodium humate (HA-Na) in the cleaning water on the ηR was also illustrated. Besides, we clarified the absorption mechanism and how WESP corona discharge oxidized NO, SO2 and Hg0, as well as the removal of NO2, SO2, and SO3 by HA-Na. Product analysis showed that NO2, SO2 and SO3 were converted into NO3- and SO42-, part of HgO was converted into Hg2+ in the cleaning water, and a small amount of Hg(NO3)2 and HgSO4 were converted into Hg2+, NO3- and SO42-.

Simultaneous removal of NO and elemental mercury from coal-fired flue gas using natural ferruginous manganese ore at low temperature

Natural ferruginous manganese ore (FMO) was developed as a cost-effective NH3-SCR catalyst for the simultaneous removal of NO and elemental mercury (Hg0) from coal-fired flue gas. The chemical composition, pore structure, crystal structure and surface chemistry of FMO were characterized by different methods. The results showed that FMO was constructed with sheet-like structures. There were massive slit-shaped pores in the FMO. Manganese and iron oxides existed in an amorphous form. After calcination, the NO removal performance of FMO was obviously decreased due to the decomposition of MnO2. A high NH3/NO ratio was conductive to NO removal but excessive NH3 was unnecessary for NO removal. NO could promote Hg0 removal whereas Hg0 had little effect on NO removal. The effects of different SCR atmosphere (inlet, middle layer, and outlet), reaction temperature (100 °C–400 °C), and flue gas composition (O2 and SO2) on NO/Hg0 removal was also studied. At the optimal temperature (200 °C), the NO removal efficiency was greater than 80% and the Hg0 removal efficiency was close to 100%. To some extent, FMO presented preferable SO2 poisoning resistance probably because of the presence of iron oxide. The mechanism of NO/Hg0 removal over FMO was further described. Both fast SCR and conventional SCR reactions happened over FMO during NO removal. Mn4+, Fe3+, and lattice oxygen were first consumed and then replenished by gaseous O2. FMO served as a catalyst for Hg0 oxidation during a long period of denitration process.

Induced adsorption and agglomeration under bipolar corona discharge to enhance the simultaneous removal of trace mercury and fine particles

Fine particulate matter (PM2.5) and gaseous mercury emitted from industrial flue gas are highly toxic to humans and the environment. Promoting the enrichment of non-condensable gaseous elemental mercury (Hg0) and enhancing the nucleation growth of fine particles can cause them to be simultaneously removed in dust removal devices. However, the sorbent itself is also a fine particle, and the fate of the mercury-carried sorbent is unclear. Herein, we propose a novel method that uses manganese oxides (MnOx) for the induced adsorption of Hg0 to coagulate the sorbent and fine particles, assisted by bipolar corona discharge. The results showed that Hg0 removal efficiencies increased from 10.2% to 32.2% with sorbent injection when the corona discharge was +21 kV and −24 kV. Simultaneously, the fine particle removal efficiencies increased from 23.6% to 46.3%. The distribution results confirmed that mercury was enriched on the particles through sorbent and fine particle agglomeration, and the mercury-carried particle proportion was concentrated in the size range of 0.5–2.5 μm. The induced adsorption performances were enhanced under bipolar corona discharge through the MnOx sorbent. In addition, the agglomeration of fine particles was enhanced as the average particle size increased by 43% when the MnOx sorbent was injected into the simulated gas. This study provides valuable guidance for clarifying the relationships between the fine particles and carried substances, and it contributes to achieving the synergistic removal of multiple contaminants.

Experimental and Mechanistic Study of Synergistic Removal of Hg by Evaporation from Desulfurization Wastewater

The flue evaporation of desulfurization wastewater can solve the problem that it is difficult to remove some heavy metal ions and chloride ions by conventional methods. A large amount of chloride ions in desulfurization wastewater can also promote the catalytic oxidation removal of Hg in the flue gas. The migration character of chloride ions in the flue evaporation process of desulfurization wastewater was studied by using the coal-fired thermal state experimental platform. The concentrations of Hg0 and Hg2+ in the flue gas at the inlet and outlet of selective catalytic reduction denitration (SCR), electrostatic precipitator (ESP), and wet desulfurization (WFGD) devices were tested, and the synergistic removal of traditional pollutant removal equipment by flue evaporation of desulfurization wastewater was analyzed. The influence of Hg and the effect of the evaporation of desulfurization wastewater at different positions on the removal of Hg in the flue gas were compared and analyzed, and the catalytic mechanism of Hg on the SCR surface was further revealed. The results show that 10% chloride ions enter the flue gas after the desulfurization wastewater evaporates. The content of chlorine elements and evaporation temperature influence the evaporation of desulfurization wastewater. The mechanism of SCR catalytic oxidation of Hg0 was explored; oxygen atoms have catalytic oxidation effects on Hg0 at different positions in the V2O5 molecule in SCR; and chloride ions can enhance the catalytic oxidation of Hg0 by V2O5. The intermediate product HgCl is generated, which is finally converted into HgCl2. The oxidation efficiency of Hg0 in electrostatic precipitation (ESP) is increased from 3% to 18%, and the removal efficiency of Hg is increased from 5% to 10%. The removal efficiency of Hg2+ in WFGD is basically maintained at approximately 85%. In addition, a small amount of Hg2+ was restored to Hg0 in WFGD. The removal efficiency of Hg0 in the flue gas of evaporative desulfurization wastewater before SCR is 65%, and the removal efficiency of gaseous Hg is 62%. When the evaporative desulfurization wastewater before ESP, the synergistic removal efficiency of Hg0 is 39%, and the gaseous Hg removal efficiency is 39%, and the removal efficiency of Hg is 40%. Evaporation of the desulfurization wastewater before SCR was more conducive to the coordinated removal of Hg by the device.

Hg0 Removal by a Palygorskite and Fly Ash Supported MnO2-CeO2 Catalyst at Low Temperature

MnO2-CeO2/PG-FA catalysts were prepared by supporting MnO2-CeO2 to PG-FA and used to remove Hg0 in simulated flue gas. The results show that MnO2-CeO2/PG-FA catalyst had excellent and stable Hg0 removal activity, which was mainly due to the combination effect of the catalytic oxidation activity by MnO2-CeO2 and the adsorption ability by PG-FA. Mn8-Ce0.5/PG-FA (with 8.0% MnO2 and 0.5% CeO2 loading) catalyst showed the highest Hg0 removal efficiency at 140 °C and Hg0 removal efficiency could be maintained above 95% with the space velocity of 6000 h−1 and Hg0 concentration of 160 μg/m3. O2 promoted Hg0 removal by MnO2-CeO2/PG-FA catalyst, while SO2 and H2O had inhibitory effects. In the presence of O2, the inhibitory effect of SO2 and H2O can be obviously weakened. MnO2-CeO2/PG-FA catalysts were characterized with scanning electron microscope (SEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and temperature-programmed desorption experiments (TPD). The results of SEM and XRD showed that the active components MnO2-CeO2 dispersed well on the surface of PG-FA support. The results of XPS and TPD show that the Hg0 removal process over MnO2-CeO2/PG-FA catalyst included adsorption and oxidation, HgO and HgSO4 were generated and adsorbed on the catalyst. MnO2-CeO2/PG-FA catalyst also showed excellent regeneration performance after Hg0 removal.

The Effects of Physical-Chemical Evolution of High-Sulfur Petroleum Coke on Hg0 Removal from Coal-Fired Flue Gas and Exploration of Its Micro-Scale Mechanism.

As the solid waste by-product from the delayed coking process, high-sulfur petroleum coke (HSPC), which is hardly used for green utilization, becomes a promising raw material for Hg0 removal from coal-fired flue gas. The effects of the physical–chemical evolution of HSPC on Hg0 removal are discussed. The improved micropores created by pyrolysis and KOH activation could lead to over 50% of Hg0 removal efficiency with the loss of inherent sulfur. Additional S-containing and Br-containing additives are usually introduced to enhance active surface functional groups for Hg0 oxidation, where the main product are HgS, HgBr, and HgBr2. The chemical–mechanical activation method can make additives well loaded on the surface for Hg0 removal. The DFT method is used to sufficiently explain the micro-scale reaction mechanism of Hg0 oxidation on the surface of revised-HSPC. ReaxFF is usually employed for the simulation of the pyrolysis of HSPC. However, the developed mesoporous structure would be a better choice for Hg0 removal in that the coupled influence of pore structure and functional groups plays a comprehensive role in both adsorption and oxidation of Hg0. Thus, the optimal porous structure should be further explored. On the other hand, both internal and surface sulfur in HSPC should be enhanced to be exposed to saving sulfur additives or obtaining higher Hg0 removal capacity. For it, controllable pyrolysis with different pyrolysis parameters and the chemical–mechanical activation method is recommended to both improve pore structure and increase functional groups for Hg0 removal. For simulation methods, ReaxFF and DFT theory are expected to explain the micro-scale mechanisms of controllable pyrolysis, the chemical–mechanical activation of HSPC, and further Hg0 removal. This review work aims to provide both experimental and simulational guidance to promote the development of industrial application of Hg0 adsorbent based on HSPC.

Oxygen-Induced Elemental Mercury Oxidation in Chemical Looping Combustion of Coal.

Mercury emission is an important issue during chemical looping combustion (CLC) of coal. The aim of this work is to explore the effects of different flue gas components (e.g., HCl, NO, SO2, and CO2) on mercury transformation in the flue gas cooling process. A two-stage simulation method is used to reveal the reaction mechanism of these gases affecting elemental mercury (Hg0) oxidation. Furthermore, using this method, Hg0 oxidation by eight oxygen carriers (Co3O4, CaSO4, CeO2, Fe2O3, Al2O3, Mn2O3, SiO2, and CuO) commonly used in CLC are investigated and their Hg0 oxidation efficiencies were compared with the existing experimental results. The results show that HCl, NO, and CO2 promote Hg0 oxidation during flue gas cooling, while SO2 inhibits Hg0 oxidation. The stronger the oxygen release capacity of oxygen carriers, the higher the oxidation efficiency of Hg0 becomes. The order of Hg0 removal efficiency from high to low is Co3O4, CuO, Mn2O3, CaSO4, Fe2O3, CeO2, Al2O3, and SiO2, and this sequence is in good agreement with the existing experimental results. Different flue gas components directly or indirectly affect the O2 content, thus affecting the content of gaseous oxidized mercury (Hg2+). Different oxygen carriers have different oxygen release capacities and different Hg0 oxidation efficiencies. Therefore, O2 is the core species affecting the mercury transformation in CLC.

Experimental and theoretical analysis of elemental mercury removal from syngas over Fe-Ti spinel

A magnetic Fe-Ti spinel sorbent was synthesized by the co-precipitation method and applied to remove gaseous Hg0 from syngas. A superior Hg0 removal performance was achieved in the temperature range of 120–180 °C, and the highest an average removal efficiency of was approximately 95% at 150 °C under simulated syngas. H2S was adsorbed on the Fe-Ti spinel surface，which generated active sulfur species, and enhanced the Hg0 removal efficiency significantly. CO, H2 and H2O variably suppressed the Hg0 removal performance. H2S-pretreatment, X-ray photoelectron spectroscopy (XPS) and Hg0-temperature programmed desorption (Hg0-TPD) results demonstrated that Hg0 reacted with H2S over Fe-Ti spinel followed the Eley-Rideal mechanism, in which gas-phase Hg0 reacted with the active surface sulfur species generated from H2S oxidation and formed surface-bonded HgS. The surface Fe3+ and surface-active oxygen species were involved in the H2S and Hg0 adsorption and transformation process. Stability and regeneration cycling experiments indicated that Fe-Ti spinel exhibited excellent stability for Hg removal and great regeneration ability. Density functional theory (DFT) calculations showed that H2S underwent strong chemisorption on the Fe-Ti spinel surface with an adsorption energy of −116.988 kJ/mol, and then H2S further dissociated on the sorbent surface to generate active sulfur species, which reacted strongly with gas-phase Hg0 to form HgS.

In situ grown Co3O4 nanosheets in the interlayer space of g-C3N4 for efficient removal of Hg0 from flue gas

Hg0 adsorption with sorbents is a widely used way to remove Hg0 from coal fire flue gas. The adsorption efficiency and capacity are limited by the activation and number of the surface adsorption sites. In the present work, a series of composite sorbents with nanosheet structure were successfully synthesized by in situ growth of Co3O4 nanosheets in the interlayer space of g-C3N4. The effects of the mass ratio of Co3O4/g-C3N4 on sorbent physicochemical properties were comprehensively studied. Characterization results show that, when the mass ratio of Co3O4/g-C3N4 is 20%, the corresponding sorbent of 20%Co3O4/NS-g-C3N4 has well dispersion of Co3O4 nanosheets in g-C3N4 interlayer (SEM and TEM) and a relatively large BET area (BET). It results in a larger amount of surface chemisorbed oxygen which can react with Hg0 forming HgO. Furthermore, the strong interaction between Co3O4 and g-C3N4 make electron transfer from g-C3N4 to Co3O4 improving oxygen activation. Therefore, 20%Co3O4/NS-g-C3N4 has a higher Hg0 removal efficiency of 97% at 250 °C and it maintains above 80% even after 70 h test. XPS, FTIR and Hg0 desorption experiment results confirm that the adsorbed Hg mainly stays in the form of HgO and a probable adsorption mechanism is deduced by these results.

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-Sex/Activated carbon (Sex/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 Sex/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 Sex/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 -Sex/AC to capture the mercury in coal combustion flue gas. Graphical abstract 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.

Self-template synthesis of Co3O4 nanotube for efficient Hg0 removal from flue gas

The adsorption of elemental mercury (Hg0) is a widely used technology to remove Hg0 from flue gas. The morphology and structure of sorbents, which have serious impacts on the number and availability of adsorption sites and the mass transfer of Hg0, play an important role in Hg0 removal process. Nanotube materials have through-pore structures leading to lower mass transfer resistance and more adsorption sites for Hg0. Given this, organic–inorganic hybrid nanowires were prepared by hydrothermal method as self-template. After calcination with a controlling heating rate and temperature, a series mesoporous Co3O4 nanotube sorbents were obtained. Characterization results show that the heating rate mainly affects the pore structure of the sorbents, while the calcination temperature mainly affects the surface morphology. When the calcination temperature is 400 °C and the calcination rate is 0.5 °C /min, the sorbent (Co3O4-T400-R0.5) has a clear 1D tubular structure which can effectively reduce the mass transfer resistance. And the unique internal surface of the tubular structure can provide more active sites, which can effectively avoid the competitive adsorption of Hg0 and other gas components. Furthermore, Co3O4-T400-R0.5 has the highest Co3+/Co2+ ratio leading to a better redox property, which is of benefit to Hg0 removal process. Therefore, the Hg0 removal efficiency of Co3O4-T400-R0.5 can reach more than 90% with a high gas hourly space velocity of 90,000 h− 1 and 0.02 g loading amount at 250 °C. Furthermore, Co3O4-T400-R0.5 has excellent Hg0 removal efficiencies in different flue gas condition showing a high stability.

Separation of Mercury(II) from Industrial Wastewater through Polymer Inclusion Membranes with Calix[4]pyrrole Derivative.

Polymer membranes with immobilized ligands are encouraging alternatives for the removal of toxic metal ions from aquatic waste streams, including industrial wastewater, in view of their high selectivity, stability, removal efficacy and low energy demands. In this study, polymer inclusion membranes (PIMs) based on cellulose triacetate, with a calix[4]pyrrole derivative as an ion carrier, were tested for their capability to dispose mercury (Hg(II)) ions from industrial wastewater. The impacts were assessed relative to carrier content, the quantity of plasticizer in the membrane, the hydrocholoric acid concentration in the source phase, and the character of the receiving phase on the performance of Hg(II) elimination. Optimally designed PIMs could be an interesting option for the industrial wastewater treatment due to the high removal efficiency of Hg(II) and great repeatability.

Graphitic Carbon Nitride for Gaseous Mercury Emission Control: A Review

Mercury emissions from combustion of coal, biomass, and solid waste threaten human life and the environment. As the principal mercury species in flue gas, elemental mercury (Hg0) can easily escape from available air pollution control instruments because of its chemical inertness, high volatility, and aqueous insolubility. Recently, graphitic carbon nitride (g-C3N4) seems to be an intriguing candidate for Hg0 emission control. This review highlights the latest advances in adsorptive and photocatalytic removal of Hg0 by g-C3N4-based composites. Metal oxide (MeOx)- and halide-modified g-C3N4 can effectively capture Hg0 from simulated flue gas. Nonetheless, acidic flue gas components can degrade the performances of MeOx/g-C3N4 adsorbents, while halide-modified g-C3N4 suffers from release of halogen and potential leaching risk of mercury. Metal sulfide (MeSx)-modified g-C3N4 appears to be the best candidate for Hg0 capture owing to its superior tolerance to sulfur dioxide, fast adsorption rate, and high mercury capacity. Combining bismuth oxyhalides (BiOX) with g-C3N4 can promote photocatalytic oxidation of elemental mercury under visible light irradiation; nonetheless, the Hg0 removal efficiency is still unsatisfactory, and the underlying photocatalysis mechanism is not yet clear.

New insights into elemental mercury removal from flue gas by a MoS2 heterogeneous system

Herein, MoS2 was used in a MoS2/H2O heterogeneous system for the heterogeneously catalyzed removal of Hg0 from flue gas. Hg0 removal efficiency reached 77.6% at 25 °C when the dosage of MoS2 was 0.200 g/L in aqueous suspension. The observed effects of MoS2 dosage and inlet Hg0 concentration on Hg0 removal efficiency revealed that the gas–liquid mass transfer process is the limiting step of Hg0 removal. Low pH and low temperature were conducive to Hg0 removal. SO2 in the flue gas was absorbed by MoS2, and the simultaneous decrease in solution pH resulted in a slight increase in Hg0 removal efficiency. The presence of NO in the flue gas enhanced Hg0 removal efficiency. Common anions, such as NO3−, SO42−, and CO32−, have no obvious effect on Hg0 removal. However, Cl− improved Hg0 removal efficiency by 11% because of the production of HgCln2−n. The kinetics and mechanism of Hg0 removal were studied, and the results indicated that Mo acted as a bridge and catalyst for Hg0 oxidation, while S immobilized Hg2+ by formation of HgS on MoS2.

Mercury removal in coal-fired power plants, possibilities how to attain very low emissions and minimization of hazardous waste stream

Emissions of toxic heavy metals (HMs), as Hg, As, Cd, Pb, etc., and some harmful compounds of F, Se, and B are related to waste streams from coal-fired power plants (CFPP). Coal/lignite combustion, due to relatively high content of ash, sulfur, and chlorine, generates in flue gas cleaning processes tremendous amount of fly ash, CaSO4 and CaCl2. Measures for minimization of Hg- and NOx-emissions (e.g. addition of bromides and NH3) change properties of fly ash, wastewater and speciation/partition of HMs. Wet flue gas desulfurization (FGD) consumes high amount of fresh water and generates harmful wastewater with water soluble salts. The planned, more stringent limits on emissions of dust, Hg, HCl, HF, SO2, etc. in CFPP will increase contents of polluting compounds in solid and liquid waste streams. We critically assess possibilities, measures and obstacles for higher efficiency of Hg and HMs removal from flue gas in CFPP, together with efficient removal of other pollutants including mutual influences and interrelations. The fates of mercury, selected harmful HMs, and some other pollutants in waste streams from wet FGD are critically analyzed and discussed. Non-toxic, stable forms of mercury (e.g. HgS) and other HMs in solid waste should be preferred. Schemes and measures for minimization of emissions and hazardous waste streams from air pollution control (APC) are compared and discussed for three selected technologies of coal combustion with different methods of gas cleaning.

Influence of SO3 on the MnOx/TiO2 SCR catalyst for elemental mercury removal and the function of Fe modification

Elemental mercury (Hg0) is a highly hazardous pollutant of coal combustion. The low-temperature SCR catalyst of MnOx/TiO2 can efficiently remove Hg0 in coal-burning flue gas. Considering its sulfur sensitivity, the effect of SO3 on the catalytic efficiency of MnOx/TiO2 and Fe modified MnOx/TiO2 for Hg0 removal was investigated comprehensively for the first time. Characterizations of Hg-TPD and XPS were conducted to explore the catalytic mechanisms of Hg0 removal processes under different conditions. Hg0 removal efficiency of MnOx/TiO2 was inhibited irreversibly from 92% to approximately 60% with the addition of 50 ppm SO3 at 150 ℃, which resulted from the transformation of Mn4+ and chemisorbed oxygen to MnSO4. The existence of H2O would intensify the inhibitory effect. The inhibition almost disappeared and even converted to promotion as the temperature increased to 250 ℃ and above. Fe modification on MnOx/TiO2 improved the Hg0 removal performance in the presence of SO3. The addition of SO3 caused only a slight inhibition of 1.9% on Hg0 removal efficiency of Fe modified MnOx/TiO2 in simulated coal-fired flue gas, and the efficiency maintained good stability during a 12 h experimental period. This work would be conducive to the future application of MnOx/TiO2 for synergistic Hg0 removal.

Surface characteristics of innovative TiO2{0 0 1} and CeO2/TiO2{0 0 1} catalysts and simultaneous removal of elemental mercury and nitric oxide at low temperatures

This research developed innovative photothermal catalysts with exposed (0 0 1) facets, which can be used in SCR for better redox effectiveness and thermal stability. By adding different quantities of HF, we prepared the photothermal catalysts with different percentages of face {0 0 1} and further obtained the optimal temperature for the catalytic oxidation of Hg0. Secondly, the synergetic effects on the removal efficiency of Hg0 and NOx and their reaction mechanisms were explored for different NO and NH3 concentrations. The newly developed novel photothermal catalysts combined with UV/TiO2 could cause the redox reactions of Hg0 and NO in SCR. CeO2 doped TiO2{0 0 1} could form active chemisorbed oxygen (Oα) and lattice oxygen (Oβ), where Ce4+ reacted with e- to form Ce3+ and O2 existed on the surface of the catalysts reacted with e- to form O2−. Hg0 further reacted with h+ and O2− to form HgO which could be effectively absorbed by wet scrubbing. Additionally, 7%CeO2/TiO2{0 0 1} oxidized NO to NO2 by O2−, then reacted with NH3 to form NH4NO2, and finally reacted with NO to form N2 and H2O.