Naiqiang Yan

Catalytic Oxidation of Elemental Mercury over the Modified Catalyst Mn/α-Al2O3 at Lower Temperatures

In order to facilitate the removal of elemental mercury (Hg(0)) from coal-fired flue gas, catalytic oxidation of Hg(0) with manganese oxides supported on inert alumina (alpha-Al2O3) was investigated at lower temperatures (373-473 K). To improve the catalytic activity and the sulfur-tolerance of the catalysts at lower temperatures, several metal elements were employed as dopants to modify the catalyst of Mn/alpha-Al2O3. The best performance among the tested elements was achieved with molybdenum (Mo) as the dopant in the catalysts. It can work even better than the noble metal catalyst Pd/alpha-Al2O3. Additionally, the Mo doped catalyst displayed excellent sulfur-tolerance performance at lower temperatures, and the catalytic oxidation efficiency for Mo(0.03)-Mn/alpha-Al2O3 was over 95% in the presence of 500 ppm SO2 versus only about 48% for the unmodified catalyst. The apparent catalytic reaction rate constant increased by approximately 5.5 times at 423 K. In addition, the possible mechanisms involved in Hg(0) oxidation and the reaction with the Mo modified catalyst have been discussed.

Capture of gaseous elemental mercury from flue gas using a magnetic and sulfur poisoning resistant sorbent Mn/γ-Fe2O3 at lower temperatures

A series of Mn/γ-Fe(2)O(3) were synthesized to capture elemental mercury from the flue gas. Mn(4+) cations and cation vacancies on the surface played important roles on elemental mercury capture by Mn/γ-Fe(2)O(3). Furthermore, the reaction route of elemental mercury oxidization was dependent on the ratio of Mn(4+) cations to cation vacancies. As a result, the capacities of 15%-Mn/γ-Fe(2)O(3)-250 for elemental mercury capture were generally higher than those of 30%-Mn/γ-Fe(2)O(3)-400. SO(2) mainly reacted with ≡Fe(III)-OH and only a small amount of ≡Mn(4+) reacted with SO(2), so the presence of a high concentration of SO(2) resulted in an insignificant effect on elemental mercury capture by 15%-Mn/γ-Fe(2)O(3)-250 at lower temperatures. The capacities of 15%-Mn/γ-Fe(2)O(3)-250 for elemental mercury capture in the presence of 2.8 g N m(-3) of SO(2) were more than 2.2 mg g(-1) at <200°C. Meanwhile, 15%-Mn/γ-Fe(2)O(3)-250 can be separated from the fly ash using magnetic separation, leaving the fly ash essentially free of sorbent and adsorbed HgO. Therefore, 15% Mn/γ-Fe(2)O(3)-250 may be a promising sorbent for elemental mercury capture.

Gaseous Elemental Mercury Capture from Flue Gas Using Magnetic Nanosized (Fe3-xMnx)1-δO4

A series of nanosized (Fe3-xMnx)1-δO4 (x = 0, 0.2, 0.5, and 0.8) were synthesized for elemental mercury capture from the flue gas. Cation vacancies on (Fe3-xMnx)1-δO4 can provide the active sites for elemental mercury adsorption, and Mn(4+) cations on (Fe3-xMnx)1-δO4 may be the oxidizing agents for elemental mercury oxidization. With the increase of Mn content in the spinel structure, the percents of Mn(4+) cations and cation vacancies on the surface increased. As a result, elemental mercury capture by (Fe3-xMnx)1-δO4 was obviously promoted with the increase of Mn content. (Fe2.2Mn0.8)1-δO4 showed an excellent capacity for elemental mercury capture (>1.5 mg g(-1) at 100-300 °C) in the presence of SO2 and HCl. Furthermore, (Fe2.2Mn0.8)1-δO4 with the saturation magnetization of 45.6 emu g(-1) can be separated from the fly ash using magnetic separation, leaving the fly ash essentially free of sorbent and adsorbed Hg. Therefore, nanosized (Fe2.2Mn0.8)1-δO4 may be a promising sorbent for the control of elemental mercury emission.

Significance of RuO2 Modified SCR Catalyst for Elemental Mercury Oxidation in Coal-fired Flue Gas

Catalytic conversion of elemental mercury (Hg(0)) to its oxidized form has been considered as an effective way to enhance mercury removal from coal-fired power plants. In order to make good use of the existing selective catalytic reduction of NO(x) (SCR) catalysts as a cobenefit of Hg(0) conversion at lower level HCl in flue gas, various catalysts supported on titanium dioxide (TiO(2)) and commercial SCR catalysts were investigated at various cases. Among the tested catalysts, ruthenium oxides (RuO(2)) not only showed rather high catalytic activity on Hg(0) oxidation by itself, but also appeared to be well cooperative with the commercial SCR catalyst for Hg(0) conversion. In addition, the modified SCR catalyst with RuO(2) displayed an excellent tolerance to SO(2) and ammonia without any distinct negative effects on NO(x) reduction and SO(2) conversion. The demanded HCl concentration for Hg(0) oxidation can be reduced dramatically, and Hg(0) oxidation efficiency over RuO(2) doped SCR catalyst was over 90% even at about 5 ppm HCl in the simulated gases. Ru modified SCR catalyst shows a promising prospect for the cobenefit of mercury emission control.

MnOx/Graphene for the Catalytic Oxidation and Adsorption of Elemental Mercury.

MnOx/graphene composites were prepared and employed to enhance the performance of manganese oxide (MnOx) for the capture of elemental mercury (Hg(0)) in flue gas. The composites were characterized using FT-IR, XPS, XRD, and TEM, and the results showed that the highly dispersed MnOx particles could be readily deposited on graphene nanosheets via hydrothermal process described here. Graphene appeared to be an ideal support for MnOx particles and electron transfer channels in the catalytic oxidation of Hg(0) at a high efficiency. Thus, MnOx/graphene-30% sorbents exhibited an Hg(0) removal efficiency of greater than 90% at 150 °C under 4% O2, compared with the 50% removal efficiency of pure MnOx. The mechanism of Hg(0) capture is discussed, and the main Hg(0) capture mechanisms of MnOx/graphene were catalytic oxidation and adsorption. Mn is the main active site for Hg(0) catalytic oxidation, during which high valence Mn (Mn(4+) or Mn(3+)) is converted to low valence Mn (Mn(3+) or Mn(2+)). Graphene enhanced the electrical conductivity of MnOx, which is beneficial for catalytic oxidation. Furthermore, MnOx/graphene exhibited an excellent regenerative ability, and is a promising sorbent for capturing Hg(0).

Novel regenerable sorbent based on Zr–Mn binary metal oxides for flue gas mercury retention and recovery

To capture and recover mercury from coal-fired flue gas, a series of novel regenerable sorbents based on Zr-Mn binary metal oxides were prepared and employed at a relatively low temperature. PXRD, TEM, TPR, XPS, and N2-adsorption methods were employed to characterize the sorbents. The Hg(0) adsorption performance of the sorbents was tested, and the effects of the main operation parameters and the gas components on the adsorption were investigated. Zr significantly improved the sorbents mercury capacity, which was nearly 5mg/g for Zr0.5Mn0.5Oy. Furthermore, the spent sorbent could be regenerated by heating to 350°C, and the highly concentrated elemental mercury released could be facilely recycled. Therefore, a much greener process for mercury capture and recovery could be anticipated based on this regenerable sorbent.

Nanosized Cation-Deficient Fe-Ti Spinel: A Novel Magnetic Sorbent for Elemental Mercury Capture from Flue Gas

Nonstoichiometric Fe-Ti spinel (Fe(3-x)Ti(x))(1-δ)O(4) has a large amount of cation vacancies on the surface, which may provide active sites for pollutant adsorption. Meanwhile, its magnetic property makes it separable from the complex multiphase system for recycling, and for safe disposal of the adsorbed toxin. Therefore, (Fe(3-x)Ti(x))(1-δ)O(4) may be a promising sorbent in environmental applications. Herein, (Fe(3-x)Ti(x))(1-δ)O(4) is used as a magnetically separable sorbent for elemental mercury capture from the flue gas of coal-fired power plants. (Fe(2)Ti)(0.8)O(4) shows a moderate capacity (about 1.0 mg g(-1) at 250 °C) for elemental mercury capture in the presence of 1000 ppmv of SO(2). Meanwhile, the sorbent can be readily separated from the fly ash using magnetic separation, leaving the fly ash essentially free of sorbent and adsorbed mercury.

Substitution of WO3 in V2O5/WO3–TiO2 by Fe2O3 for selective catalytic reduction of NO with NH3

To improve N2 selectivity and lower the cost, WO3 in V2O5/WO3–TiO2 was substituted by a low cost composition Fe2O3 for selective catalytic reduction (SCR) of NO with NH3. The SCR reaction over V2O5/Fe2O3–TiO2 mainly followed the Eley–Rideal mechanism (i.e. the reaction between activated ammonia species and gaseous NO). There were two active components on V2O5/WO3–TiO2 for the activation of adsorbed NH3 (i.e. V5+ and Fe3+). The acid sites on V2O5/Fe2O3–TiO2 mainly resulted from the support Fe2O3–TiO2, so the adsorbed NH3 preferred to be activated by Fe3+ rather than by V5+. V5+ on V2O5/Fe2O3–TiO2 could accelerate the regeneration of Fe3+ on Fe2O3–TiO2 due to the rapid electron transfer between V5+ and Fe2+ on the surface, so the activation of adsorbed NH3 by Fe3+ was promoted. As some NH3 adsorbed on V2O5/Fe2O3–TiO2 was not activated by Fe3+, the inactivated NH3 could then be activated by V5+ on the surface. As a result, 2% V2O5/Fe2O3–TiO2 showed excellent SCR activity, N2 selectivity and H2O/SO2 durability at 300–450 °C. Furthermore, the emission of 2% V2O5/Fe2O3–TiO2 to the fly ash can be prevented by an external magnetic field due to its inherent magnetization. Therefore, 2% V2O5/Fe2O3–TiO2 could be a promising low-cost catalyst in NO emission control.

Competition of selective catalytic reduction and non selective catalytic reduction over MnOx/TiO2 for NO removal: the relationship between gaseous NO concentration and N2O selectivity

In this work, a novel phenomenon was discovered that N2O selectivity of NO reduction over MnOx/TiO2 was related to the concentration of gaseous NO and that lower concentration of gaseous NO would cause higher N2O selectivity. In situ DRIFTS and transient reaction studies demonstrated that both the Eley–Rideal mechanism (the reaction of over-activated NH3 with gaseous NO) and the Langmuir–Hinshelwood mechanism (the reaction of adsorbed NO3− with adsorbed NH3 on the adjacent sites) could contribute to the formation of N2O. Kinetic study demonstrated that N2O selectivity would be independent of gaseous NO concentration if NO reduction over MnOx/TiO2 mainly followed the Langmuir–Hinshelwood mechanism. If NO reduction over MnOx/TiO2 mainly followed the Eley–Rideal mechanism, there was competition between the selective catalytic reduction (SCR) reaction and non selective catalytic reduction (NSCR) reaction. As gaseous NO concentration increased, more –NH2 was used to reduce gaseous NO to form N2 and the further oxidization of –NH2 to –NH was restrained, resulting in an obvious decrease of N2O selectivity. The Eley–Rideal mechanism played an important role in NO reduction over MnOx/TiO2, especially at higher temperatures. Therefore, N2O selectivity of the low temperature SCR reaction over MnOx/TiO2 decreased especially at higher temperatures after the increase of gaseous NO concentration.

The performance of iodine on the removal of elemental mercury from the simulated coal-fired flue gas

In order to facilitate the removal of elemental mercury (Hg(0)) in flue gas, iodine was used as the oxidant to convert Hg(0) to the oxidized or particulate-bound form. The removal of Hg(0) by the homogenous gas phase reaction and the heterogeneous particle-involved reactions was investigated under various conditions, and a method to test the particle-involved reaction kinetics was developed. Iodine was found to be efficient in Hg(0) oxidation, with a 2nd-order rate constant of about 7.4(+/-0.2)x10(-17)cm(3)molecules(-1)s(-1) at 393 K. Nitric oxide showed significant inhibition in the homogenous gas reaction of Hg(0) oxidation. The oxidation of Hg(0) with iodine can be greatly accelerated in the presence of fly-ash or powder activated carbon. SO(2) slightly reduced Hg(0) removal efficiency in the particle-involved reaction. It was estimated that Hg(0) removal efficiency was as high as 70% by adding 0.3 ppmv iodine into the flue gas with 20 g/m(3) of fly-ash. In addition, the predicted removal efficiency of Hg(0) was as high as 90% if 10mg/m(3) of activated carbon and 0.3 ppmv iodine were injected into the flue gas with fly-ash. The results suggest that the combination of iodine with fly-ash and/or activated carbon can efficiently enhance the removal of Hg(0) in coal-fired flue gas.