Magnetic pyrrhotite, derived from the thermal treatment of natural pyrite, was developed as a recyclable sorbent to recover elemental mercury (Hg) from the flue gas as a cobenefit of wet electrostatic precipitators (WESP). The performance of naturally derived pyrrhotite for Hg capture from the flue gas was much better than those of other reported magnetic sorbents, for example Mn-Fe spinel and Mn-Fe-Ti spinel. The rate of pyrrhotite for gaseous Hg capture at 60 °C was 0.28 μg g min and its capacity was 0.22 mg g with the breakthrough threshold of 4%. After the magnetic separation from the mixture collected by the WESP, the spent pyrrhotite can be thermally regenerated for recycle. The experiment of 5 cycles of Hg capture and regeneration demonstrated that both the adsorption efficiency and the magnetization were not notably degraded. Meanwhile, the ultralow concentration of gaseous Hg in the flue gas was concentrated to high concentrations of gaseous Hg and Hg during the regeneration process, which facilitated the centralized control of mercury pollution. Therefore, the control of Hg emission from coal-fired plants by the recyclable pyrrhotite was cost-effective and did not have secondary pollution.
In this work, the novel relationships of N 2 selectivity of NO reduction over MnO x −CeO 2 with the gas hourly space velocity (i.e., GHSV) and the reactants' concentrations were discovered. Meanwhile, the mechanism of N 2 O formation during the low temperature selective catalytic reduction reaction (SCR) over MnO x −CeO 2 was studied using in situ DRIFTS study and the transient reaction study. N 2 O formation over MnO x −CeO 2 mainly resulted from the Eley−Rideal mechanism (i.e., the reaction between overactivated NH 3 and gaseous NO), and the Langmuir−Hinshelwood mechanism (i.e., the reaction between adsorbed NH 3 species and adsorbed NO x ) did not contribute to N 2 O formation. There was an excellent linear relationship of NO reduction and N 2 formation with gaseous NO concentration. Meanwhile, the reaction order of N 2 O formation with respect to gaseous NO concentration was nearly 1. However, the reaction orders of NO reduction, N 2 O formation, and N 2 formation over MnO x −CeO 2 with respect to gaseous NH 3 concentration were all higher than 0 due to the adsorption competition between NH 3 and NO+O 2 . Therefore, N 2 selectivity of NO reduction over MnO x −CeO 2 remarkably increased with the increase of gaseous NO concentration, and it slightly decreased with the increase of gaseous NH 3 concentration.
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