Page 104 - New Trends In Coal Conversion

Minimization of Hg and trace elements during coal combustion and gasification processes 67

being vaporized from the fuel, the TEs may undergo reactions both with the ash par-
ticles and the gaseous species in the flue gas. The reactions between the vaporized TEs
and the ash particles constitute an important mechanism for the retention of TEs in
large fly ash particles that are easier to be captured by the air pollution control systems
(Senior et al., 2000a). Very few elements remain in the vapor state, probably only var-
iable proportions of the total amount of Hg and the halogens, but even these may be
associated with the surfaces of very fine fly ash particles.
The final fate of TEs is determined by complex and interrelated processes such as
vaporization, reaction, nucleation, condensation, and coagulation. Fig. 3.3 shows a
general model of fine ash formation mechanisms for TEs which involves a series of
complex processes. Attalla et al. (2004) concluded the elemental form of trace species
has an influence on its final partitioning; the more volatile elements seem to vaporize
independently of the mode of occurrence; and the char particle temperature determines
the degree of volatilization. Moreover, some postcombustion zone transformations
have been identified which affect TE partitioning. For example, the interaction of
Ca with As, Se, and Cd dictates that for coals, which contain significant CaO, the par-
titioning behavior of these elements is controlled by surface reactions rather than
condensation.

Nucleation / condensation/
Metal vapor coagulation Submicron
particles
Surface reaction/
condensation
Attritive
Vaporization Porous fragmentation
fragmentation
particle
Explosive

Submicron inclusions,
residual particles,
secondary atomization
Coal
Inherently
Porous char Cenospheric
bound metal
particle
Mineral Excluded
inclusions ash
Dense
particle

Supermicron
particle
Figure 3.3 Ash formation mechanism for trace elements.
Adapted from Attalla, M.I., Morgan, S., Riley, K., Bryant, G., Nelson, P.F., 2004. Trace Element
Deportment in Combustion Processes. Research Report 50, Pullenvale, Queensland, Australia,
84 pp.