Mercury Speciation Transformation During Coal Combustion - Mercury Emission and its Control in Chinese Coal-Fired Power Plants - page 43

Transformation in Flue Gas

(4-15)

HgCl(s,g) HCl(g) HgCl (s,g) H

2

In Pathway 1, the formation of gaseous Cl 2 might be limited by reaction kinetics,
and the generated Cl 2 was very small in quantity, making the approach inappropri-
ate as a principal pathway of Hg 2+ in flue gas [12] . In addition, the mercury oxidation
reaction was accelerated with increasing temperature, which was also incompatible
with the pathway because the concentration of Cl 2 decreases as the temperature
increases. Senior and Morency [13] calculated that in the exterior of air pollution
control devices (at about 24 to 190 °C), only about 1% of Cl turns into Cl 2 . More-
over, at low temperatures and with a few particles, Cl 2 could be catalyzed by the
following process:

2HCl+½O 2 Cl 2 +H 2 O (4-16)

In Pathway 2, Cl was generated because radicals attack the HCl molecules and
could be expressed as follows:

HCl+OH Cl+H O (4-17)

2

According to thermodynamic principle, the above reaction could be conducted
at higher temperatures. Cl might be the main substance for the oxidation of Hg 0 in
flue gas. Pathway 2 might be the principal pathway of mercury oxidation.
The mercury oxidation reaction contained several elementary reactions, the
existence of which could not be clarified by quantitative studies, making kinetics
studies of the oxidation of mercury very difficult. However, kinetic parameters
could be calculated using some simplest expressions. Based on the above discus-
sion, the following formula was adopted:

3Hg+4HCl2HgCl+HgCl 2 +2H 2 (4-18)

Based on the above mentioned formula, in the presence of HCl, the total reac-
tion rate in the homogeneous gas-phase oxidation of mercury could be expressed as
follows:

d C

(4-19)

Hg

kC C

Hg HCl

d t

where C Hg is the concentration of Hg 0 in simulated flue gas; C HCl is the concentra-
tion of HCl in simulated flue gas; k is the total reaction rate constant; is the reac-
tion order relevant to the concentration of Hg 0 in simulated flue gas; and is the
reaction order relevant to the concentration of HCl in simulated flue gas.
In all experiments, C HCl was excessive relative to C Hg in the reaction Eq. (4-18).
In the reaction process, the oxidation reaction of mercury turned into a pseudo
-level reaction. In this case, Eq. (4-19) became: