Mercury Speciation Transformation During Coal Combustion - Mercury Emission and its Control in Chinese Coal-Fired Power Plants - page 40

as the O 2 concentration in flue gas ranged from 4%10%, that of SO 2 ranged from
100-2,000 ppm, and that of NO ranged from 100-1,000 ppm.
In this section, the reaction kinetic model was established and used to study the
factors that affect the mercury species distribution in typical coal-fired flue gas, to
understand the reaction mechanism of mercury in flue gas and to provide a bene-
ficial reference for mercury control methods.

4.3.1 Reaction Kinetic Model of Mercury and HCl in Coal-Fired
Flue Gas

4.3.1.1 Transformation and Formation Mechanisms of Mercury in Coal-Fired
Flue Gas

For better understanding the mercury transformation, the reaction kinetics mecha-
nism of mercury should be clarified. Most studies about mercury reaction kinetics
during coal combustion focused on the mechanism of mercury transformation in
vapor using heat balance calculations. Although this approach could primarily
estimate the main mercury speciation distribution in a coal-fired system under
equilibrium, in practice the system is rarely balanced. Therefore, to understand the
main reaction methods and illustrate accurately the concentrations of mercury re-
action compounds in the system, kinetics experiments must be conducted in a
coal-fired system. The description of the chemical kinetic mechanism is based on a
comprehensive and accurate understanding of mercury and its different components
and their elementary reaction kinetic parameters. This work is difficult.
Studies on the model establishing mercury oxidation have been conducted
widely. Carpi studied the chemical form transformation of mercury emissions
during the combustion process [2] . Widmer and West [3] proposed a two-step mecha-
nism-based kinetic mechanism of mercury oxidation as the following Eq. (4-10).
The model contained eight core elementary reactions, as shown in Table 4.4. The
parameters were solved according to the modified Arrhenius Eq. (4-11). The model
assumed that all reactions were reversible, and Arrhenius parameters were esti-
mated using different methods. Almost all of the reactions involved those between
radicals and molecular components, thus indicating that pre-exponential factors
were close to the collision limit. Widmer and West [3] estimated the second and third
activation energy E a in the estimate sheet, and directly substituted the enthalpy
value as the initial value because of the lack of relevant kinetic data, thus obtaining
the coarse kinetic parameters of elementary reaction. This estimation often led to
inconsistent collision frequency and a reaction constant.

(4-10)

Hg HgCl HgCl

2

kkT

E

/ RT

e (4-11)

0