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.
