Page 329 - Air pollution and greenhouse gases from basic concepts to engineering applications for air emission control

306 10 Post-combustion Air Emission Control

of the dry particulate control devices. These captured particles also contain a large
amount of metals, including copper, zinc, silver, and gold. Therefore, it is not
necessary to design a specific device for the control of Class I and Class II elements.
The majority of Class III elements in the flue gas will, however, penetrate
through the particulate control devices and wet scrubbers. Systems have to be
designed specifically for the removal of Class III trace elements, which include the
notorious Pb, As, Se, Cd, and Hg. Sorbents can be injected into the flue gas
upstream of the particulate control system, unless a separate reactor is employed.
Usually a large amount of sorbent is needed to compensate low removal efficien-
cies. Frequently, cost is the concern for large-scale applications.
Various sorbents such as activated carbon, silica, alumina, kaolinite, limestone,
emathlite, bauxite, and titania have been tested for trace elements control at tem-
peratures of 400–1,000 °C, by direct injection into the flue gas or passing the flue
gas through packed or fluidized beds. Alumina may be effective for As, Be and
fairly active for Cd and Ni, silica is effective for Cd, Pb, and Hg, while titania can
be a good sorbent for Cd and Pb. Limestone can be used to capture Pb, Cd, Sb, Hg,
Se, and As species [4].
At 350–600 °C, As and Se can be removed from flue gases by calcium-based
sorbents, forming CaSeO 3 and Ca 3 (AsO 4 ) 2. Since the temperature is favorable for
desulfurization as well, the concentration of SO 2 in the flue gas stream has to be
relatively low to prevent the competitive sulfation of the sorbent. Selenium can also
be removed from gases by physisorption using activated carbon at 125–250 °C[1].
Control of mercury emissions is a challenging task because of the complicated
transformations of mercury in combustion. All three types of Hg species must be
considered for effective mercury emission control:

(1) insoluble gaseous mercury
(2) soluble gaseous oxidized mercury, mainly HgCl 2 , and
(3) particle-bound mercury, Hg-p.
Existing particulate control and FGD devices or separated sorption units using
activated carbon, metal oxides, and fly ashes can be used to for the capture of
mercury from flue gas.

10.8.1 Mercury in Particulate Control and FGD Devices

Some particulate control devices also help to reduce Hg emissions. Typical Hg
removal efficiencies for particulate emission control equipment at US coal-fired
power plants with ESPs and fabric filters are about 32 and 44 %, respectively. The
majority of coal combustion plants employ ESPs for particulate control with more
and more bag-houses being installed, which will help in mercury reduction. For
example, the activated carbon injected upstream of an ESP yield lower Hg removal
efficiency than when it is injected upstream of a bag-house filter.