Mercury Removal Based on Adsorption and Oxidation by Fly Ash: A Review

Mercury (Hg), in particular elemental mercury (Hg-0) captured from coal-fired power plants, has attracted much attention because of its severe harm to human health and environment. Hg-0 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 Hg-0 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 Hg-0 removal using fly ash are also introduced. Unburned carbon (UBC) and Fe2O3 are two reactive components in fly ash for Hg-0 oxidation. Physical parameters including higher specific surface area, larger total pore volume, and wider distribution of pores with well-developed micropores are beneficial for Hg-0 retention, and no consistent relationship between particle size and Hg-0 retention using fly ash has been obtained. While gas phase components including HCl, NO, and O-2 promote the removal of Hg-0 using fly ash, no agreement about the effects of SO2 and H2O vapor on Hg-0 removal has been reached. The promotion of halogen-modified fly ash on Hg-0 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 Hg-0 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 Hg-0 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 Hg-0 as well as the magnetochemistry effect of magnetic materials in fly ash. The function of fly ash on the photocatalytic oxidation of Hg-0 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 Hg-0 secondary emission from the spent fly ash, the development of fly ash-based products, the compromise between fly ash modification cost and Hg-0 removal efficiency, and the effective and economical coremoval of Hg-0, NOx, and SOx using fly ash. In particular, the promotional effects of EF, MF, and UV light on Hg-0 removal necessitate extensive studies on their underlining mechanisms. This review resolves the existing controversies on the Hg-0 removal mechanism and promotes the development of fly ash on Hg-0 removal from coal combustion.