No abstract

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.

Nonstoichiometric Fe-Ti spinel

Lignite is widely used as the fuel for coal-fired power plants, and its flue gas temperature is about 50−100 °C higher than others. V 2 O 5 −WO 3 /TiO 2 is extremely restricted in the selective catalytic reduction (SCR) of NO from the coal-fired power plants burning lignite due to the drop of NO x conversion, low N 2 selectivity, and volatility of vanadium pentoxide at high temperatures. Therefore, a more environmental-friendly SCR catalyst with excellent SCR activity and better N 2 selectivity at 350−450 °C should be developed for this application. In this work, sulfated Fe−Ti spinel catalyst was developed for the SCR of NO from the coal-fired power plants burning lignite. The drop of NO x conversion at high temperatures was mainly related to the simultaneous occurrence of the catalytic oxidization of NH 3 to NO during the SCR reaction. Ti was incorporated into γ-Fe 2 O 3 to decease the oxidization ability of Fe 3+ on the surface, and the sites for −NH 2 adsorption and the active components for −NH 2 oxidization were separated after the sulfation to decrease the probability of the collision between −NH 2 adsorbed and Fe 3+ on the surface. They both inhibited the catalytic oxidization of NH 3 to NO over γ-Fe 2 O 3 . However, the SCR reaction over γ-Fe 2 O 3 was simultaneously restrained after the incorporation Ti and the sulfation. Therefore, NO x conversion over γ-Fe 2 O 3 at high temperatures depended on the ratio of NH 3 conversion through the catalytic oxidization of NH 3 to NO to that through the SCR reaction. This ratio decreased after the incorporation of Ti, and it further decreased after the sulfation, resulting in an obvious promotion of NO x conversion at high temperatures. Therefore, sulfated Fe−Ti spinel showed excellent SCR activity, N 2 selectivity, and H 2 O+SO 2 durability at 300−450 °C, which was suitable for the application in the coal-fired power plants burning lignite.

MnO x /Al 2 O 3 catalysts (i.e., impregnating manganese oxide on alumina) were employed to remove elemental mercury (Hg 0 ) from flue gas. MnO x /Al 2 O 3 was found to have significant adsorption performance on capturing Hg 0 in the absence of hydrogen chloride (HCl), and its favorable adsorption temperature was about 600 K. However, the catalytic oxidation of Hg 0 became dominant when HCl or chlorine (Cl 2 ) was present in flue gas, and the removal efficiency of Hg 0 was up to 90% with 20 ppm of HCl or 2 ppm of Cl 2 . In addition, the catalysts with adsorbed mercury could be chemically regenerated by rinsing with HCl gas to strip off the adsorbed mercury in the form of HgCl 2 . Sulfur dioxide displayed inhibition to the adsorption of Hg 0 on the catalysts, but the inhibition was less to the catalytic oxidation of Hg 0 , especially in the presence of Cl 2 . The analysis results of XPS and pyrolysis-AAS indicated that the adsorbed mercury was mainly in the forms of mercuric oxide (HgO) and the weakly bonded speciation, and the ratio of them varied with the adsorption amount and manganese content on catalysts. The multifunctional performances of MnO x /Al 2 O 3 on the removal of Hg 0 appeared to be promising in the industrial applications.