Page 330 - Air pollution and greenhouse gases from basic concepts to engineering applications for air emission control

10.8 Control of Trace Metals 307

Further mercury removal may be accomplished by the wet scrubber desulfur-
ization systems. A typical wet scrubber FGD shows Hg removal of 34 %, while dry
scrubber shows an efficiency of about 30 % with injected sorbent collection on a
fabric filter [6]. In a wet scrubber, water may capture some HgCl 2 due to its
solubility in water. In addition, the low temperature in a wet FGD system is
favorable for volatile Hg to condense and for the removal from the flue gas stream.
However, increased concentrations of Hg vapor were found too when the flue gas
passed through a wet FGD [17].
Total Hg removal depends on the arrangement of particulate control and FGD
devices. For a coal-fired power plant, a bag-house/FGD combination can give Hg
removal efficiencies of 88–92 %, but the efficiencies were much lower (23–54 %)
for an ESP/FGD combination [31]. The reason behind this dramatic difference in
the efficiency is that oxidized mercury is reduced to elemental Hg in ESPS. This
does not happen in filters.
A filter bag-house is preferred over an ESP for the removal of mercury from the
flue gas due to more interactions between the gaseous mercury in the flue gas, and
the dust cake on the filters help capture Hg too. This explains partially why bag-
houses are progressively being installed for new plants.

10.8.2 Mercury Adsorption by Activated Carbon

For coal-fired power plants, especially those use lignite and peat, the control of
mercury may present a serious and expensive problem in the near future. Developed
countries, including Canada, will follow the implementation of Hg regulations for
coal-fired power plants as being done in the USA.
There are several unique characters that relate Hg emission to coal combustion.
One is the low concentration of Hg in high volume of flue gas streams typically in
the order of 0.01 ppmm. Another factor is the low Cl/Hg ratio and the small fraction
of oxidized Hg, indicating the relative importance of elemental Hg in the flue gas.
For coal, over 50 % of the Hg is in the form of elemental Hg when the chlorine
content of the fuel is less than 0.1 % by weight, while less than 20 % of the Hg
elemental in the flue gas when the chlorine content of the fuel is above 0.2 % by
weight.
Hg can be removed from the coal flue gases either by injection of activated
carbons in flue gas ducts or by passing the flue gas through beds packed with
activated carbons. For a packed bed system, the C/Hg ratio by mass is in the range
of 5,000–1,00,000 due to short residence time of approximately 1 s of the flue gas
in air pollution control devices. The actual C/Hg ratio depends mainly on the
temperature of the flue gas [6]. Sorbent particles larger than 20 µm in diameter may
be too large for the typical contact times of 1 s.
It was found that the capacity or reactivity of the activated carbon is limited at a
temperature of 180 °C or higher [8]. The decrease in temperature favors the removal