Page 103 - New Trends In Coal Conversion

66 New Trends in Coal Conversion

volatile elements. Up to about 20% of the original mineral matter is collected in the
bottom ash and up to about 80% in the fly ash. The bottom ash remains in the combus-
tion area, whereas the fly ash is carried through the system. Nowadays power plants
have installed particle control devices with efficiencies higher than 99% such as elec-
trostatic precipitators or fabric filters (FFs). However, a small proportion, mostly fine
particles of less than about 10 mm in diameter, is emitted to the atmosphere with the
stack gases.
Coal is the primary input of TEs to the system. Most TEs reaching the boiler will be
liberated and redistributed into bottom ash, fly ash, fine fly ash, and the gaseous phase
(Fig. 3.2). The vaporized metals at high temperature near the combustion flame will
subsequently be nucleated or condensed at a lower temperature downstream
(Ratafia-Brown, 1994; Linak and Wendt, 1994: Helble, 1994; Bool and Helble,
1995). The vaporization of TEs is influenced by the fuel properties and combustion
conditions. The organically associated TEs are more easy to vaporize during combus-
tion than those associated with minerals (Helble, 1994). On the other hand, the vapor-
ization behavior of TEs is also influenced by parameters such as combustion
temperature, transport phenomena, oxidizing/reducing condition, and the presence
of gaseous species such as HCl and SO 2 (Galbreath and Zygarlicke, 1998a; b). In
most cases, the presence of gaseous chlorine may shift the distribution of TEs such
as Pb and Cd toward more volatile chlorides, thus promoting vaporization. However,
the presence of SO 2 may result in the formation of sulfates with relatively low vapor
pressure (Bool and Helble, 1995; Diaz-Somoano and Martínez-Tarazona, 2003). After

Flue gas
particle matter

SNCR
Input
Output

SCR
Limestone
Coal
Boiler
Stack
ESP WFGD

Bottom Fly Gypsum
ash ash
Figure 3.2 Trace element distribution in a coal-fired power plant. ESP, electrostatic precipi-
tator; SCR, selective catalytic reduction; SNCR, selective noncatalytic reduction; WFGD, wet
flue gas desulfurization.