Page 115 - New Trends In Coal Conversion

78 New Trends in Coal Conversion

Table 3.5 Distribution of trace elements in the inlet and outlet of an
electrostatic precipitator system
Removal
Inlet Outlet efficiency (%)
Particle (size mm) 20 3.57 99.54
As 141.5 181.4 99.33
Be 13.3 15.1 99.41
Cd 1.9 5.2 98.57
Cr 361.7 406.1 99.41
Mn 311.6 411.0 99.31
Ni 140.6 622.6 97.68
Pb 167.9 277.2 99.13
Sb 9.2 12.9 99.26
Se 3.4 29.4 95.49

firing Eastern (Western Kentucky) bituminous coals have shown that mercury capture
can increase with increasing fly ash carbon content and decreasing gas temperature
(Newman-Sutherland et al., 2001). Baghouses can be more effective for mercury con-
trol than ESP, especially with bituminous coals, as the filter cake on the baghouse acts
as a fixed-bed reactor for unburnt carbon to enhance mercury capture. Native capture
for mercury in baghouse systems ranges from low to over 90%, again depending on the
coal and combustion conditions (Srivastava et al., 2006).

3.5.2 Flue gas desulfurization
TEs that are not captured in the ESP or FF device may be removed by a downstream
FGD system for SO 2 control. The most widely used FGD system is the wet scrubber
technology followed by spray dry scrubbing and to a lesser extent sorbent injection. At
this point, it is important to remember that TEs may also be introduced into the FGD
unit in the limestone. These include, in decreasing order, Ca, Pb, Mn > Cd, Mg, Fe, Si,
Sm > Ti, Al, W, As, and Sb. TEs may also be introduced by the process water (Sloss
and Smith, 2000).
The relatively low temperatures found in WFGD systems allow many of the more
volatile TEs to condense from the vapor phase and thus be removed from the flue gases
(Pacyna, 1997).
Meij (1997) has studied TE removal in the FGD system in unit 13 of the Gelderland
power plant in the Netherlands. Although the data are for 1986, and somewhat dated,
they still represent one of the most detailed studies in the literature today. In addition to
showing gaseous concentrations before and after the FGD unit, the table also demon-
strates the decrease of 90% in particulate loading across the FGD. Adding this to the