Mercury Speciation Transformation During Coal Combustion - Mercury Emission and its Control in Chinese Coal-Fired Power Plants - page 41

Transformation in Flue Gas

In Eq. (4-11), k 0 is the pre-exponential factor, is the temperature index, and E a is
the activation energy.

Table 4.4 Kinetic mechanism of mercury oxidation

k 0 (moles·cm 3 ·s) Reaction E a (cal/mol)

Hg+Cl+M=HgCl+M 9.00E+15 0.5 0

Hg+Cl 2 =HgCl+Cl 1.39E+14 0 34000

Hg+HCl=HgCl+H 4.94E+14 0 79300

Hg+HOCl=HgCl+OH 4.27E+13 0 19000

HgCl+Cl 2 =HgCl 2 +Cl 1.39E+14 0 1000

HgCl+Cl+M=HgCl 2 +M 1.16E+15 0.5 0

HgCl+HCl=HgCl 2 +H 4.64E+03 2.5 19100

HgCl+HOCl=HgCl 2 +OH 4.27E+13 0 1000

Lee [4] applied online analysis technology to study mercury reaction kinetics
with the presence of chlorine-containing substances, obtaining an order of reaction
of 1.55, an activation energy of 16.13 (kJ/mol), and a reaction rate constant of
5.07×10 2 exp(1939.68/ T ) [(μg/m 3 ) 0.55 · s 1 ]. However, the quantitative impact of
chlorine-containing substances on the reaction was not explored. Moreover, tita-
nium-based catalysts were used in the reaction, which was quite different from
actual flue gas. Dunham et al. [5] proposed a more comprehensive overview of
mercury transformation into kinetics and analyzed kinetic models of homogeneous
and heterogeneous reactions through comparisons using experiments. Sliger et al. [6]
discussed the issue from the experimental and theoretical aspects.
Niksa [7] applied the homogeneous kinetics method to theoretically predict the
importance of nitric oxide and water for mercury oxidation in coal-fired flue gas.
Liu et al. [8] applied quantum chemistry to study the reaction mechanism of mercury
and chlorine gas starting from the initiation of coal combustion. After the optimi-
zation, the reactants, transition states, intermediates and geometry of products were
determined. Zheng also calculated the activation energy and heat effect on the
reaction.
Studies on the oxidation of mercury in simulated flue gas have been conducted.
Mamani-Paco et al . [9] studied the impact of the simultaneous presence of HCl and
Cl 2 on mercury oxidation through experiments. In the experiments, the concentra-
tion of mercury was 50 g/m 3 . The experimental results showed that when the Cl 2
concentration was 50 ppm, 7% to 10% of the mercury was oxidized, when the Cl 2
concentration was 100 ppm, 36% to 45% of the mercury was oxidized, and when
the Cl 2 concentration was 300 ppm, 66% to 69% of the mercury was oxidized. On
the other hand, when HCl was added, the overall concentration of Cl 2 increased, but
the oxidation rate of mercury was not significantly different from that with only the
presence of Cl 2 at 50 ppm.
Dajnak and Lockwood [10] explored the transformation mechanism of mercury
during combustion and summarized the possible oxidation and reduction reactions
of mercury-containing substances during the combustion process (Tables 4.5 and
4.6).