Mercury Speciation Transformation During Coal Combustion - Mercury Emission and its Control in Chinese Coal-Fired Power Plants - page 96

may be attributed to the shorter gas cooling process and the shorter reaction time in
the experiment. The concentration of Hg (g) , when using the fluidized-bed combus-
tion method, was slightly lower than that when using the suspension combustion
method. This may be attributed to the high proportion of burning stone coal, the
addition of limestone in some conditions, and the high concentration of fly ash.
These factors enhanced the mercury adsorption by particles, thus reducing the
concentration of Hg (g) .
Based on the previous studies and the experiments, the apparent reaction kinetic
model for the combination of mercury and HCl was established. The model was
used to predict the degree of transformation from Hg 0 to Hg 2+ in a coal-fired flue
gas. The model mainly considered the reaction between mercury and HCl which
could be applied to predict the impact of HCl concentration, residence time, reac-
tion temperature, Hg/Cl, etc., on the oxidation of Hg 0 to Hg 2+ .
With increasing HCl concentration during the same residence time, the Hg 0
concentration reduced. It was because increasing HCl was equivalent to increasing
Cl as the mercury oxidant. In addition, with a low HCl concentration, the impact on
mercury transformation from increased HCl concentration was relatively signifi-
cant. The reaction temperature increased while the Hg 0 concentration dropped,
indicating that a higher temperature in a certain temperature range promoted the
oxidation of Hg 0 . At lower HCl concentration, when the residence time was shorter,
the transformation of mercury speciation was less affected by temperature; when
the residence time was longer, the impact became more significant. When all other
conditions were unchanged, the increase in gas residence time decreased the
amount of Hg 0 . It meant that a longer retention period increased the oxidation of
Hg 0 , and that mercury oxidation in flue gas usually was far from a theoretically
balanced state because the residence time was usually less than reaction time in
theory. With the increasing concentration of Hg 0 inlet mercury in the same residence
time, the concentration of Hg 0 gradually increased, thus leading to a reduced
transformation rate. Cl/Hg increased while the concentration of Hg 0 dropped. It
meant that higher Cl/Hg increased the oxidation rate of Hg 0 under certain conditions.
Cl/Hg could be considered as a parameter in the discussion of regulating mercury
oxidation in coal-fired flue gas.
Based on the chemical thermodynamic equilibrium calculation result, mercury
speciation was dominating as Hg 0 in high temperature combustion conditions. The
thermodynamic equilibrium calculations suggested that mercury oxidation was the
most sensitive to inlet O 2 , HCl, SO 2 , and insensitive to NO. But by comparison, HCl
displayed a stronger oxidation ability than O 2 and SO 2 . With the concentration of
O 2 , HCl, SO 2 improvement, Hg 0 began to be oxidized at higher temperature and
also kept a similar tendency. If HCl gas is typically included in flue gas, mercury
oxidation could be achieved at higher temperature. As the flue gas cools along the
tail surface area, mercury oxidation would happen, altering from Hg 0 into mercuric
chloride, transferring from mercuric chloride into mercuric sulfate sometimes. In
particular, mercury was a mixture of mercuric sulfate, gaseous mercuric chloride
and gaseous mercury oxide in the flue gas entering the desulfurization tower ac-
cording to estimations from the model. In general, mercuric chloride and mercury