Mercury (Hg), in particular elemental mercury (Hg0) captured from coal-fired power plants, has attracted much attention because of its severe harm to human health and environment. Hg0 can be removed using the technology of adsorbent injection, and one of the promising adsorbents is fly ash. To evaluate the effect of complex multifactors of fly ash on Hg0 removal performance, this review summarizes, analyzes, and evaluates the quantitative effects of fly ash compositions, physical parameters, flue gas components, and modification reagents. The effects of electric field (EF), magnetic field (MF), and ultraviolet (UV) light on Hg0 removal using fly ash are also introduced. Unburned carbon (UBC) and Fe2O3 are two reactive components in fly ash for Hg0 oxidation. Physical parameters including higher specific surface area, larger total pore volume, and wider distribution of pores with well-developed micropores are beneficial for Hg0 retention, and no consistent relationship between particle size and Hg0 retention using fly ash has been obtained. While gas phase components including HCl, NO, and O2 promote the removal of Hg0 using fly ash, no agreement about the effects of SO2 and H2O vapor on Hg0 removal has been reached. The promotion of halogen-modified fly ash on Hg0 removal may be explained by the Langmuir–Hinshelwood mechanism, the Mars–Maessen mechanism, as well as the Eley–Ridel mechanism. The contribution of metal-modified fly ash to Hg0 removal is attributed to the oxidation ability of metal oxides as well as metal positive ions. EF, MF, and UV light enhance the oxidation of Hg0 using fly ash to different degrees. The promotion of EF may be attributed to the formation of more Cl, O, and OH radicals by a series of electron-induced reactions. The enhancement of MF is simultaneously attributed to the energy-level splitting of Hg0 as well as the magnetochemistry effect of magnetic materials in fly ash. The function of fly ash on the photocatalytic oxidation of Hg0 under the UV light has been verified, while relevant studies are still limited. More studies are still needed on the relationship between surface functional groups and carbon types, the control of Hg0 secondary emission from the spent fly ash, the development of fly ash-based products, the compromise between fly ash modification cost and Hg0 removal efficiency, and the effective and economical coremoval of Hg0, NO x , and SO x using fly ash. In particular, the promotional effects of EF, MF, and UV light on Hg0 removal necessitate extensive studies on their underlining mechanisms. This review resolves the existing controversies on the Hg0 removal mechanism and promotes the development of fly ash on Hg0 removal from coal combustion.

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Oxygen carriers in coal-fired chemical looping combustion can oxidize elemental mercury, but there are still no specific criteria to rank the efficiencies of Hg0 oxidation by classification of different oxygen carriers. Both an experimental approach and thermodynamic modeling were adopted to evaluate the Hg0 oxidation efficiencies in the case of eight commonly used oxygen carriers, applying a temperature range from 800 to 1000 °C. The efficiency of the Hg0 oxidation process in experiments within this temperature range was found to correspond well with the amounts of Cl/Cl2 in the calculated reaction products. Results have shown that the sequence of the amounts of Cl/Cl2 for different oxygen carriers was as follows: CaSO4 > Co3O4 (Mn2O3) > Fe2O3 > CuO (CeO2) > SiO2 (Al2O3). This sequence was in agreement with the simulations of the experiments investigating the efficiency of Hg0 oxidation. According to thermodynamic calculations, the oxidation mechanism of Hg0 can be classified into two categories: M x O y -based mechanisms (Fe2O3, CuO, Co3O4, Mn2O3, CeO2, Al2O3, and SiO2) and CaSO4-based processes. Hg0 oxidation by M x O y follows three reaction pathways. In the first instance, Hg0 can be oxidized by Cl2 to form HgCl2; in the second reaction pathway, Hg0 can be oxidized by Cl to form HgCl, which is oxidized afterward by Cl/Cl2 to HgCl2. In the third reaction pathway, Hg0 is oxidized by oxygen atoms to HgO, which is then oxidized by Cl2O to form HgCl2. In comparison, the oxidation of Hg0 by CaSO4 is different from that of other oxygen carriers with existing additional reaction pathways. Hg0 is converted first to HgS before being oxidized to HgCl2. The approach in this study may be used for choosing the optimum oxygen carriers for Hg0 oxidation in coal-fired chemical-looping combustion processes.

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Mercury emission is an important issue during chemical looping combustion (CLC) of coal. The aim of this work is to explore the effects of different flue gas components (e.g., HCl, NO, SO 2 , and CO 2 ) on mercury transformation in the flue gas cooling process. A two-stage simulation method is used to reveal the reaction mechanism of these gases affecting elemental mercury (Hg 0 ) oxidation. Furthermore, using this method, Hg 0 oxidation by eight oxygen carriers (Co 3 O 4 , CaSO 4 , CeO 2 , Fe 2 O 3 , Al 2 O 3 , Mn 2 O 3 , SiO 2 , and CuO) commonly used in CLC are investigated and their Hg 0 oxidation efficiencies were compared with the existing experimental results. The results show that HCl, NO, and CO 2 promote Hg 0 oxidation during flue gas cooling, while SO 2 inhibits Hg 0 oxidation. The stronger the oxygen release capacity of oxygen carriers, the higher the oxidation efficiency of Hg 0 becomes. The order of Hg 0 removal efficiency from high to low is Co 3 O 4 , CuO, Mn 2 O 3 , CaSO 4 , Fe 2 O 3 , CeO 2 , Al 2 O 3 , and SiO 2 , and this sequence is in good agreement with the existing experimental results. Different flue gas components directly or indirectly affect the O 2 content, thus affecting the content of gaseous oxidized mercury (Hg 2+ ). Different oxygen carriers have different oxygen release capacities and different Hg 0 oxidation efficiencies. Therefore, O 2 is the core species affecting the mercury transformation in CLC.