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Minimization of Hg and trace elements during coal combustion and gasification processes 81

3.5.4 Specific technologies

Because of the wide variability on TE removal efficiency of existing air pollutant con-
trol devices (APCDs) depending on coal type, combustion conditions, and power
plant configuration, specific solutions for TE removal have been evaluated. For this
purpose, the injection of sorbents such as activated carbon (AC), aluminum silicates,
or clay can be used then to be removed in the particle control system or in a separate
device. Large amounts of sorbent are needed to compensate the general low removal
efficiencies. A list of sorbents by Mojtahedi and Mroueh (1989) (VTT, Finland) for
TEs is summarized in Table 3.6. For large-scale applications, their application suffers
from excessive costs, the low operating temperature, or other side-effects (CIEMAT,
1998).
Arsenic (As) and selenium (Se) may be removed by calcium-based sorbents from

flue gases at 350e600 C, forming CaSeO 3 and Ca 3 (AsO 4 ) 2 (Gosh-Dastidar et al.,
1996; Diaz-Somoano and Martínez-Tarazona, 2004). It has been noted that SO 2 de-
creases TE removal. Moreover, experimental results have shown that Se can be

removed from gases by AC at 125e250 C. Removal efficiency increases with gas
moisture content (Se reacting with the sorbent as H 2 SeO 3 ) and internal surface of
the sorbent but decreases with temperature, indicating a physisorption mechanism
(Agnihotri et al., 1998).
Sorbents such as silica, alumina, kaolinite, limestone, emathlite, bauxite, and titania
have been widely tested for TEs control at 400e1000 C by direct injection into the

gas or in packed or fluidized beds. Based on thermodynamic calculations, alumina
may be a good sorbent for As and Be and less active for Cd and Ni. Silica may be
a good sorbent for Cd, Pb, and Hg, whereas titania may be a good sorbent for Cd
and Pb. Experiments with kaolinite injection showed a high removal efficiency for
Pb, which was much less active when excess chlorine was present. Other tests showed
that kaolinite effectively removes Ni, Cd, and Pb, again limited by the presence of
chlorine. Also, limestone can be used to capture Pb and Cd species and Sb, Hg, Se,
and As as well (Biswas and Wu, 1998).
Focusing on mercury control, the option receiving the most attention is AC injec-
tion. AC adsorb mercury depending on several factors: the surface area of the sorbent,
the pore volume, mercury oxidation state and concentration, and flue gas temperature
and composition. The effectiveness of AC for mercury removal has shown promising
results (Young et al., 1994). Butz et al. (2000) also noted that significantly higher car-
bon injection rates would be required to achieve consistent control levels as high as
80% in ESP configurations producing high operating costs (Cal et al., 2000; Dorman
et al., 2002). For that reason, huge efforts are focused on the improvement of the ef-
0
ficiency of ACs for the removal of Hg by chemically treating them with sulfur (Feng
et al., 2006a,b,c), chloride, bromine, and iodine (Lee et al., 2004), and some metal ox-
ides such as CuO, Cr 3 O 4 , MnO 2 ,Fe 2 O 3 , TiO 2 , and V 2 O 5 (Li et al., 2009; Mei et al.,
2008; Poulston et al., 2007; Straube et al., 2008).