Performance evaluation of non-thermal plasma injection for elemental mercury oxidation in a simulated flue gas

Research progress on elemental mercury (Hg0) removal in flue gas using non-thermal plasma technology

Elemental mercury (Hg0) removal is a crucial target for mercury pollution control in flue gas. This article focuses on Hg0 removal in flue gas using corona discharge (CD) and dielectric barrier discharge (DBD) technologies, and provides a mechanistic perspective on the development and influencing factors of non-thermal plasma (NTP) technology for Hg0 removal. The influence factors include reactor configurations, power supplies, energy density, residence time, oxidation methods, gas composition, and the synergy between NTP and catalysis/adsorption, etc. This study reveals that the use of a pulsating electrical power supply significantly increases electron densities in both CD and DBD systems, thereby ensuring high energy efficiency and economic viability. Cl2 proves to be more effective than HCl as a chlorine source for Hg0 removal. NO significantly reduces Hg0 oxidation efficiency, while the effects of SO2 and H2O remain unclear. Energy density distribution is closely related to plasma devices, power supplies, and overall reactor configurations. Direct oxidation proves to be more effective than indirect oxidation for Hg0 removal. The combination of NTP with adsorption/catalysis technologies shows significantly better Hg0 removal efficiency compared to using NTP alone. This study can provide theoretical support for enhancing Hg0 removal mechanisms and optimizing process control parameters in industrial applications of NTP technology.

Calibration Approach for Gaseous Oxidized Mercury Based on Nonthermal Plasma Oxidation of Elemental Mercury

Atmospheric mercury measurements carried out in the recent decades have been a subject of bias largely due to insufficient consideration of metrological traceability and associated measurement uncertainty, which are ultimately needed for the demonstration of comparability of the measurement results. This is particularly challenging for gaseous Hg^II species, which are reactive and their ambient concentrations are very low, causing difficulties in proper sampling and calibration. Calibration for atmospheric Hg^II exists, but barriers to reliable calibration are most evident at ambient Hg^II concentration levels. We present a calibration of Hg^II species based on nonthermal plasma oxidation of Hg⁰ to Hg^II. Hg⁰ was produced by quantitative reduction of Hg^II in aqueous solution by SnCl₂ and aeration. The generated Hg⁰ in a stream of He and traces of reaction gas (O₂, Cl₂, or Br₂) was then oxidized to different Hg^II species by nonthermal plasma. A highly sensitive ¹⁹⁷Hg radiotracer was used to evaluate the oxidation efficiency. Nonthermal plasma oxidation efficiencies with corresponding expanded standard uncertainty values were 100.5 ± 4.7% (k = 2) for 100 pg of HgO, 96.8 ± 7.3% (k = 2) for 250 pg of HgCl₂, and 77.3 ± 9.4% (k = 2) for 250 pg of HgBr₂. The presence of HgO, HgCl₂, and HgBr₂ was confirmed by temperature-programmed desorption quadrupole mass spectrometry (TPD-QMS). This work demonstrates the potential for nonthermal plasma oxidation to generate reliable and repeatable amounts of Hg^II compounds for routine calibration of ambient air measurement instrumentation.

Abatement of NO/SO2/Hg0 from flue gas by advanced oxidation processes (AOPs): Tech-category, status quo and prospects

This review article provides a state-of-art insight into the removal of NO, SO₂ and elemental mercury (Hg⁰) from flue gas by using advanced oxidation processes (AOPs) method. Firstly, the main flue gas purification strategies based on AOPs would be classified as gas-gas, gas-liquid and gas-solid systems preliminarily, and the primary chemistry/mechanism of the above homogeneous/heterogeneous reaction systems were presented as the oxidation of NO, SO₂ and Hg⁰ by the oxidative free radicals (OH, O₂ and SO₄^-etc.). Secondly, the research progress and reaction pathways for separately or simultaneously removing NO, SO₂ and Hg⁰ from flue gas by AOPs has been reviewed elaborated and analyzed in more details. Notably, the wet/dry oxidation coupled with efficient absorption process would be a promising method of efficient removal of above gaseous pollutants. Subsequently, four types of assumed layout modes were described graphically. The application prospects of AOPs for the purification of flue gas from coal-fired boiler or industrial furnace were evaluated and found that the operation cost and utilization of oxidants must be reduced and improved respectively. Finally, the limitations in the current removal technologies based on AOPs are highlighted, meanwhile the future research directions are suggested, such as cut down the cost of oxidants and catalysts, improve the yield and valid utilization of highly reactive radicals and enhance the reactivity, resistance and stability of catalysts. Significantly, it is also envisaged that the review could enrich the knowledge repository to function as a scientific reference for the sustainable development of economical, effective and environment-friendly technologies for the abatement of a wide variety of emissions from flue gas, and further improve the feasibility and reliability of the strategies for moving from laboratory studies to large-scale development and industrial application.

Highly efficient simultaneous removal of HCHO and elemental mercury over Mn-Co oxides promoted Zr-AC samples

Mn_xCo_y/Zr_z-AC prepared by impregnation method was investigated on the simultaneous removal of HCHO and Hg⁰. The samples were characterized by BET, SEM, XRD, H₂ pulse chemisorption, H₂-TPR, XPS, Hg-TPD and in-situ DRIFTS. Thereinto, the optimal Mn_2/3Co₈/Zr₁₀-AC achieved 99.87% HCHO removal efficiency and 82.41% Hg⁰ removal efficiency at 240 °C, respectively. With increased surface area and pore volume, Zr-AC support facilitated higher dispersion of MnO_x-CoO_x. Moreover, the co-doping of MnO_x-CoO_x endowed the sample with more active oxygen species and higher reducibility, which further facilitated the removal of HCHO and Hg⁰. Chemisorption was proved to predominate in Hg⁰ removal, and oxidation also worked as Hg²⁺ was detected in outlet gas. Besides, HCHO predominated in the competition of active oxygen species, especially for lattice oxygen, thus suppressed the Hg⁰ removal. According to in-situ DRIFTS, HCHO removal proceeded as HCHO_ads → DOM → formate species → CO₂ + H₂O, and was boosted by active oxygen species. Furthermore, Mn_2/3Co₈/Zr₁₀-AC was proved with excellent regeneration performance, indicating its potential in practical application.

Removal of Hg0, NO, and SO2 by the surface dielectric barrier discharge coupled with Mn/Ce/Ti-based catalyst

In this study, power parameters (power, frequency, and voltage), initial Hg⁰ concentration, and residence time are investigated for the removal of the increased Hg⁰ concentration via surface dielectric barrier discharge (SDBD). The synergistic effect of a Mn/Ce/Ti catalyst with SDBD is verified with a mixture of flue gas (Hg⁰, NO, and SO₂). Results show that Hg⁰ oxidation efficiency has an optimal frequency, which declines as the input voltage increases. The amplification of the Hg⁰ removal efficiency decreases as voltage increases. The effect of the initial Hg⁰ concentration gradually decreases as the peak voltage increases. The residence time slightly affects the Hg⁰ removal efficiency at a high peak voltage. The cooling water temperature behaves differently on Hg⁰ oxidation under high and low voltages. X-ray photoelectron spectroscopy (XPS) reveals the relative atomic concentrations of Mn²⁺ and Mn³⁺ in the Mn-TiO₂ and Mn-Ce-TiO₂ catalysts are 66.84% and 65.80%, respectively, which indicate that Ce addition will not affect surface Mn. Mn has a limited catalytic action on the removal of flue gas with and without SDBD. Nevertheless, SDBD can stimulate the oxygen storage capacity of Mn to increase the NO₂ conversion rate. Mn-Ce-TiO₂ greatly improves the removal efficiencies of NO and SO₂ because of the existence of the redox pairs of Mn⁴⁺/Mn³⁺, Ce⁴⁺/Ce³⁺, and Ti⁴⁺/Ti³⁺. However, the three catalysts slightly differ on Hg⁰ removal when combined with SDBD, indicating that the effect of the catalyst was weakened after SDBD was added.

Surface modification of fly ash by non-thermal air plasma for elemental mercury removal from coal-fired flue gas

The fly ash from a coal-fired power plants was modified with non-thermal plasma in air to improve the elemental mercury (Hg⁰) removal performance. The Hg⁰ adsorption experiments were implemented via a bench-scale fixed-bed reactor system. Brunauer-Emmett-Teller (BET), X-ray photoelectron spectroscopy (XPS), ultimate, X-ray diffraction (XRD) and XRF analysis were employed to characterize the fly ash. The effect of non-thermal plasma voltage and time on Hg⁰ removal efficiency was investigated. The results showed that fly ash had better Hg⁰ removal performance when treatment voltage was 3.0 kV and treatment time was 7 min. Furthermore, the results showed that non-thermal plasma treatment had little influence on the specific surface area. However, non-thermal plasma treatment increased the relative content of oxygen. XPS and temperature programmed desorption results indicated that Hg⁰ removal process included adsorption and oxidation of Hg⁰. Moreover, ester and carbonyl groups played extremely vital roles in the improvement of Hg⁰ removal performance, and their temperature programmed desorption peak occurred at around 250°C and 320°C, respectively.

Radical-induced oxidation removal of multi-air-pollutant: A critical review

Sulfur dioxide (SO₂), nitric oxide (NO) and elemental mercury (Hg⁰) are three common air pollutants in flue gas. SO₂ and NO are the main precursors for chemical smog and Hg⁰ is a bio-toxicant for human. Cooperative removal of multi-air-pollutant in flue gas using radical-induced oxidation reaction is considered as one of the most promising methods due to the high removal efficiency, low cost and less secondary environmental impact. The common radicals used in air pollution control can be classified into four types: (1) hydroxyl radical (OH), (2) sulfate radical (SO₄^-), (3) chlorine-containing radicals (Cl, ClO₂, ClO, HOCl^-, etc.) and (4) ozone. This review summarizes the generation methods and mechanism of the four kinds of radicals, as well as their applications in the removal of multi-air-pollutant in flue gas. The reactivity, selectivity and reaction mechanism of the four kinds of radicals in multi-air-pollutant removal were comprehensively described. Finally, some future research suggestions on the development of new technique for cooperative removal of multi-air-pollutant in flue gas were provided.

Immobilization of mercury using high-phosphate culture-modified microalgae

This study developed a novel Hg(II) immobilization strategy by firstly incubating algal cells in high-phosphate cultures for surface modification, followed by obtaining the P-rich biomass as adsorbents for enhanced Hg(II) removal and then charring the Hg-loaded biomass to prevent leaching of phosphate and to immobilize Hg(II). For algal surface modification, Scenedesmus obtusus XJ-15 were cultivated under different P concentrations and obtained the highest sites concentration of surface phosphoryl functional groups in 80 mg L^-1 P cultures. For Hg(II) adsorption, biomass from 80 mg L^-1 P cultures (B-80) achieved the highest saturated sorption capacity of 95 mg g^-1 fitting to Langmuir isotherm model under the optimum pH of 5.0. For charring stabilization, the Hg-loaded B-80 was calcinated under different temperatures, and the product obtained from 300 °C charring showed the lowest Hg(II) leaching rate without P release. Moreover, FT-IR and XPS analysis indicate that the surge of surface phosphoryl functional groups dominated the enhancement of Hg(II) sorption and also Hg(II) charring immobilization. The above results suggested that the developed strategy is promising for both phosphate and mercury removal from water and for co-immobilization of P and Hg(II) to prevent leaching.

Simultaneous removal of SO₂, NO and Hg⁰ through an integrative process utilizing a cost-effective complex oxidant

A novel process of pre-oxidation combining with post-absorption to simultaneously remove SO2, NO and Hg(0) from flue gas was proposed. A vaporized complex oxidant (CO) consisted of cost-effective H2O2 and NaClO2 was prepared to oxidize Hg(0) and NO, then the oxidation products were absorbed by the Ca(OH)2 solution that was followed. For the establishment of the optimal reaction conditions, the influences of various reaction factors on the simultaneous removal of SO2, NO and Hg(0) were investigated, i.e., the molar ratio of H2O2 to NaClO2 in CO, the adding rate of CO, the pH of CO, the reaction temperature, the flue gas residence time and the coexistence gases. The experimental results indicated that the desulfurization was constant in all tests, whereas the removal of NO and Hg(0) was primarily affected by the NaClO2 addition, the adding rate of CO, the pH of CO, and the reaction temperature. Meanwhile, NO and SO2 were characterized as the promoters for the Hg(0) removal. Under the optimal reaction conditions, the best simultaneous removal efficiencies were 100% for SO2, 87% for NO and 92% for Hg(0). According to the characterizations of removal products by UV-vis, EDX, XRD, AFS and XPS, the reaction mechanism was speculated.