Mercury Removal Based on Adsorption and Oxidation by Fly Ash: A Review
Zhuang Liu, Dunyu Liu, Bingtao Zhao, Liang Feng, Mingguo Ni, Jing Jin
Abstract
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, NOx, and SOx 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.