Mechanism of Elemental Mercury Oxidation over Copper-Based Oxide Catalysts: Kinetics and Transient Reaction Studies

Hg0 Conversion over Sulfureted HPMo/γ-Fe2O3 with HCl at Low Temperatures: Mechanism, Kinetics, and Application in Hg0 Removal from Coal-Fired Flue Gas

Recently, sulfureted metal oxides have been developed for the catalytic oxidation of Hg⁰ to HgCl₂ using HCl as an oxidant at low temperatures, and they exhibit excellent Hg⁰ removal performance. Owing to the lack of reaction mechanisms and kinetics, further improvement in their performance for Hg⁰ conversion is extremely restricted. In this study, the reaction mechanism of Hg⁰ conversion over sulfureted HPMo/γ-Fe₂O₃ with HCl at low temperatures was investigated using Hg balance analysis and transient reaction. The chemical adsorption of Hg⁰ as HgS and the catalytic oxidation of Hg⁰ to HgCl₂ both contributed to Hg⁰ conversion over sulfureted HPMo/γ-Fe₂O₃. Meanwhile, the formed HgCl₂ can adsorb onto sulfureted HPMo/γ-Fe₂O₃. Then, the kinetics of Hg⁰ conversion, Hg^t adsorption, and HgCl₂ desorption were developed, and the kinetic parameters were gained by fitting the Hg balance curves. Subsequently, the inhibition mechanism of H₂O and SO₂ on Hg⁰ conversion over sulfureted HPMo/γ-Fe₂O₃ was determined by comparing the kinetic parameters. The kinetic model suggested that both HgCl₂ resulting from Hg⁰ oxidation and unoxidized Hg⁰ can be completely adsorbed on sulfureted HPMo/γ-Fe₂O₃ with a moderate mass hourly space velocity. Therefore, sulfureted HPMo/γ-Fe₂O₃ can be developed as a reproducible sorbent for recovering Hg⁰ emitted from coal-fired power plants.

Enhanced elemental mercury removal in coal-fired flue gas by modified algal waste-derived biochar: Performance and mechanism

A novel biochar involving pyrolysis of dewatered algal waste combined with KOH and residual FeCl₃ co-activation was synthesized as an efficient sorbent specifically for Hg⁰ removal from coal-fired flue gas. It was found that the S_BET of biochar co-activated by KOH and FeCl₃ (BCFK) was 195.82 m² g^-1, much higher than that of single FeCl₃ activated biochar (BCF) of 133.38 m² g^-1 and un-activated biochar (UBC) of 20.36 m² g^-1. Furthermore, BCFK exhibited higher magnetization characteristics as well as elemental Fe and Cl contents of 2.71% and 10.33%, respectively, based on the combined characterization of XPS and VSM, etc., which is a jump of about 10-fold compared to BCF. This allows BCFK to show the best Hg⁰ removal capability of 689.66 μg g^-1 under the inlet Hg⁰ concentration of 100 μg m^-3 and 150 °C, according to pseudo-second-order kinetic model. Further analysis by XPS and Hg-TPD (Temperature Programmed Desorption) revealed that oxidation by Cl∗ radicals and C-Cl as well as weak chemisorption contributed to the removal of Hg⁰. Eventually, this efficient, simply prepared, low-cost and easily separable biochar distinguished itself in comparison to other materials. This will undoubtedly promote the valorization of algae and provide a reliable alternative material for the treatment of coal-fired flue gas.

Mercury removal using various modified V/Ti-based SCR catalysts: A review

Growing levels of mercury pollution has made countries urgently need a suitable mercury treatment technology. Among various technologies, heterogeneous oxidative mercury removal via different modified V/Ti-based SCR catalysts is considered as a promising approach due to excellent economic value and removal efficiency. Although various related modification experiments have been worked in recent years, the research on the performance, including activity and resistance, and mechanism of catalysts still needs to be improved, so it is necessary to summarize these experiments to guide further work. This article will review many modifications start from the V/Ti catalyst. Not only the performance of these catalysts, but also a lot of speculation about the mercury removal mechanism are include in our research. In addition, the characteristics of some modified catalysts have been linked with their oxidation mechanism and structural changes by comparing many studies, and finally attributed to some special properties of the corresponding modifiers. We expect this study will clarify the research progress of modified V/Ti-based SCR catalysts in mercury removal, and guide future modification so that some properties of the catalyst can be improved in a targeted manner.

Simultaneous Adsorption of Gaseous Hg0 and Hg(II) by Regenerable Monolithic FeMoSx/TiO2: Mechanism and its Application in the Centralized Control of Hg Pollution in Coal-Fired Flue Gas

FeMoS_x/TiO₂ was investigated as a regenerable sorbent to simultaneously adsorb Hg⁰ and Hg(II) from coal-fired flue gas for the centralized control of Hg pollution discharged from coal-fired power plants. The performance of FeMoS_x/TiO₂ for Hg(II) and/or Hg⁰ adsorption was evaluated on a fixed-bed reactor at 80 ^oC, and the mutual interference between Hg⁰ adsorption and Hg(II) adsorption was analyzed using individual adsorption, simultaneous adsorption, and two-stage adsorption. FeMoS_x/TiO₂ displayed an excellent capacity for individual Hg⁰ adsorption (41.8 mg g^-1) and a moderate capacity for individual Hg(II) adsorption (0.48 mg g^-1). Two types of adsorption sites were present on FeMoS_x/TiO₂ for gaseous Hg adsorption (S⁰ and FeS₂/MoS₃ sites). X-ray photoelectron spectroscope and kinetic analyses demonstrated that Hg⁰ and Hg(II) could adsorb onto S⁰ sites, whereas only Hg⁰ was adsorbed onto FeS₂/MoS₃ sites. As Hg⁰ competed with Hg(II) for the S⁰ sites, the amount of Hg(II) adsorbed slightly decreased by 16% in the presence of Hg⁰. However, Hg⁰ adsorption onto the FeS₂/MoS₃ sites predominated over the Hg⁰ adsorption onto FeMoS_x/TiO₂ and it was not inhibited in the presence of Hg(II). Therefore, the amount of Hg⁰ adsorbed on FeMoS_x/TiO₂ was only decreased by 2% in the presence of Hg(II).

Effects of Chlorine Addition on Nitrogen Oxide Reduction and Mercury Oxidation over Selective Catalytic Reduction Catalysts

The effect of chlorine on mercury oxidation and nitrogen oxides (NO _x ) reduction over selective catalytic reduction (SCR) catalysts was investigated in this study. Commercial SCR catalysts achieved a high Hg⁰ oxidation efficiency when Cl₂ was sprayed into the flue gas. Results indicated that an appropriate concentration of Cl₂ was found to promote NO _x reduction and Hg⁰ oxidation significantly. An optimal concentration of Cl₂ (25 ppm) was found to significantly promote NO _x reduction and Hg⁰ oxidation. Moreover, we studied the effects of Cl₂ on NO _x reduction and Hg⁰ oxidation over SCR catalysts under different concentrations of SO₂. The SO₂ poisoning effect was decreased by Cl₂ when the SO₂ concentration was low (below 1500 ppm). However, sulfate gradually covered the catalyst surface over time during the reaction, which limited the impact of Cl₂. Finally, different sulfur-poisoned catalysts were examined in the presence of Cl₂. The NO _x reduction and Hg⁰ oxidation performances of sulfate-poisoned catalysts improved when Cl₂ was added to the flue gas. Mechanisms for NO _x reduction and Hg⁰ oxidation over fresh catalysts and sulfate-poisoned catalysts in the presence of Cl₂ were proposed in this study. The mechanism of Cl₂-influenced NO _x reduction was similar to that for the NH₃-SCR process. With Cl₂ in the flue gas, the number of Brønsted active sites increased, which improved catalytic activity. Furthermore, Cl₂ reoxidized V⁴⁺-OH to V⁵⁺=O and caused the NH₃-SCR process to operate continuously. The Langmuir-Hinshelwood mechanism was followed for Hg⁰ oxidation by SCR catalysts when Cl₂ was in the flue gas. Cl₂ increased the number of Lewis active sites, and catalytic activity increased. Hg⁰ adsorbed on the surface of the catalysts and was then oxidized to HgCl₂. Adding Cl₂ to the flue gas increased the strength and number of acid sites on sulfate-poisoned catalysts.

Novel Counteraction Effect of H2O and SO2 toward HCl on the Chemical Adsorption of Gaseous Hg0 onto Sulfureted HPW/γ-Fe2O3 at Low Temperatures: Mechanism and Its Application in Hg0 Recovery from Coal-Fired Flue Gas

In this work, sulfureted phosphotungstic acid-grafted γ-Fe₂O₃ (HPW/γ-Fe₂O₃) was investigated as a regenerable monolithic sorbent to recover gaseous Hg⁰ upstream of wet flue gas desulfurizations (FGDs), and the effects of HCl, SO₂, and H₂O on the chemical adsorption of Hg⁰ onto sulfureted HPW/γ-Fe₂O₃ were investigated with Hg balance analysis and kinetic analysis. Hg⁰ conversion over sulfureted HPW/γ-Fe₂O₃ was remarkably promoted in the presence of HCl, and most Hg⁰ was catalytically oxidized to HgCl₂. Moreover, the chemical adsorption of Hg⁰ was notably restrained as the key species for Hg⁰ transformation to HgS (i.e., S₂^2-) was rapidly oxidized by Cl*. However, the effect of HCl on Hg⁰ conversion over sulfureted HPW/γ-Fe₂O₃ was almost counteracted by H₂O and SO₂ as they competed with physically adsorbed Hg⁰ and S₂^2- for the consumption of Cl*. Therefore, the chemical adsorption of Hg⁰ onto sulfureted HPW/γ-Fe₂O₃ in the presence of SO₂ and H₂O was slightly inhibited by HCl, and only a small amount of HgCl₂ was formed. Moreover, sulfureted HPW/γ-Fe₂O₃ exhibited a moderate ability for gaseous HgCl₂ adsorption. As a result, sulfureted HPW/γ-Fe₂O₃ showed excellent performance in recovering Hg⁰ from the flue gas upstream of the FGDs for the centralized control of Hg⁰ emitted from coal-fired plants.

Novel Promotion of Sulfuration for Hg0 Conversion over V2O5-MoO3/TiO2 with HCl at Low Temperatures: Hg0 Adsorption, Hg0 Oxidation, and Hg2+ Adsorption

In this work, the commercial selective catalytic reduction (SCR) catalyst V₂O₅-MoO₃/TiO₂ was sulfureted with H₂S to improve both its capability for elemental mercury (Hg⁰) removal at low temperatures and its resistance to SO₂ and H₂O. Hg⁰ removal over both V₂O₅-MoO₃/TiO₂ and sulfureted V₂O₅-MoO₃/TiO₂ involved the catalytic oxidation of Hg⁰ to HgCl₂ and the chemical adsorption of gaseous Hg⁰; therefore, the effect of sulfuration on Hg⁰ chemical adsorption and the catalytic oxidation of Hg⁰ to HgCl₂ over V₂O₅-MoO₃/TiO₂ and its resistance to SO₂ and H₂O were investigated using Hg balance analysis. Kinetic analysis showed that the rates of the chemical adsorption and oxidation of Hg⁰ were both in direct proportion to the concentration of physically adsorbed Hg⁰. The physical adsorption of gaseous Hg⁰ on V₂O₅-MoO₃/TiO₂ was remarkably promoted after sulfuration, and the physical adsorption of gaseous Hg⁰ over sulfureted V₂O₅-MoO₃/TiO₂ was scarcely inhibited by SO₂ and H₂O. Therefore, the performance of V₂O₅-MoO₃/TiO₂ for Hg⁰ removal and its resistance to SO₂ and H₂O were both improved after sulfuration. Even more remarkably, sulfureted V₂O₅-MoO₃/TiO₂ can adsorb gaseous HgCl₂, which resulted from Hg⁰ oxidation. Therefore, sulfureted V₂O₅-MoO₃/TiO₂ showed an excellent performance to recover Hg⁰ from coal-fired power plants, which can then be converted to liquid Hg.

Outstanding performance of CuO/Fe–Ti spinel for Hg0 oxidation as a co-benefit of NO abatement: significant promotion of Hg0 oxidation by CuO loading

Conversion of gaseous Hg⁰ to soluble Hg²⁺ using selective catalytic reduction (SCR) catalysts with gaseous HCl as an oxidant as a co-benefit of NO abatement is widely used for resolving Hg pollution from coal-burning power plants.

Simultaneous Removal of SO2 and Hg0 by Composite Oxidant NaClO/NaClO2 in a Packed Tower

Based on the implementation of the global Minamata Convention, developing an efficient and economical technology for mercury reduction in coal-fired flue gas becomes a hotspot in the field of air pollution control. The composite oxidant NaClO/NaClO₂ combined with limestone was used in the simultaneous removal of SO₂ and Hg⁰ in this study, and the three-factor and four-level orthogonal experiments were performed in a packed tower. The influential sequence of various factors on SO₂ and Hg⁰ removals was investigated through range analysis of the orthogonal experiments. Results showed that factors affecting desulfurization was C > A > B (liquid-gas ratio > oxidant concentration ratio > initial pH of absorption liquid), while factors affecting Hg⁰ removal was A > C > B (oxidant concentration ratio > liquid-gas ratio > initial pH of absorption liquid). Optimum conditions of simultaneous desulfurization and demercuration by NaClO/NaClO₂ were A4B1C4; that is, the oxidant concentration ratio was 10/4 (mmol/L:mmol/L), the initial pH was 5, and the liquid-gas ratio was 18 (L/m³). The simultaneous removal efficiencies of SO₂ and Hg⁰ reached 99.5 and 85.4% under these optimum conditions, respectively. Analysis of the characteristics of the solid products showed that the main products of the wet oxidation were CaSO₄ and CaSO₃. Analysis of the existing form of oxidized mercury showed that 23% of mercury was in the gypsum, while 77% was in the supernatant. Results of this research would provide a practical reference for promoting the simultaneous removal of SO₂ and Hg⁰ by NaClO/NaClO₂ with limestone in industrial application.