Mercury Control and Mercury Stability in Byproducts - Mercury Emission and its Control in Chinese Coal-Fired Power Plants - page 135

The Cl element was consumed as adsorption proceeded; thus, AC lost the ca-
pability to adsorb Hg 0 through chemical oxidization. Breakthrough occurred as Hg 0
was measured at the outlet of the reactor; however, the adsorption of Hg 0 by AC did
not reach its full capacity.
When the gas flow was switched from N 2 to the simulated flue gas compounds,
AC again started to adsorb Hg 0 . However, with the help of carbon on the AC sur-
face, the oxidation factor of Hg 0 was not promoted by the Cl element on AC, but by
the components of simulated flue gas, such as NO, NO 2 , HCl, and so on. When the
adsorption capacity of AC for Hg 0 reached its full capacity, Hg 0 oxidation pro-
ceeded continuously, and the breakthrough occurred when Hg 2+ was detected at the
reactor outlet.
5.5 Mercury Stability in Pollution Control Production
The existing flue gas pollution control systems of SCR+ESP+WFGD and the in-
jection of AC upstream by a particle control device were both potential and effec-
tive mercury emission control technologies for coal-fired power plants. However,
the mercury stability in control products is very important to avoid possible mer-
cury reemission. Thus, the mercury stabilization in both FGD gypsum and simu-
lated mercury adsorption production of AC were studied in this research.
5.5.1 Mercury Stability in Desulfurization Gypsum
5.5.1.1 Mercury Speciation Analysis of Desulfurization Gypsum
Mercury stability was found to be intimately connected with mercury speciation in
desulfurization gypsum. In order to further understand the mercury speciation in
desulfurization gypsum, a chemical thermodynamic equilibrium model from
Factsage software was used. This model was based on the principle of minimum
Gibbs free energy, and was used to solve the possible direction of the chemical
reaction and speciation of mercury in desulfurization gypsum.
First, the model hypothesized that Hg (g) and gypsum slurry was uniformly
mixed, and the entire system was in equilibrium. In the calculation model, gypsum
slurry components (CaO, SiO 2 , Al 2 O 3 , Fe 2 O 3 , MgO, SO 3 , and H 2 O) and three kinds
of Hg (g) (i.e., Hg 0 , HgCl 2 , HgO) were considered. According to the calculation
results of the chemical thermodynamics model, the concentrations of Hg 0 , HgCl 2 ,
and HgO were set at 56%, 34% and 10%, respectively. The concentration of O 2 was
7%, CO 2 was 13%, N 2 was 79%, SO 2 was 1200 ppm, and HCl was 60 ppm.
The calculation results of mercury speciation in a desulfurizing tower based on
a chemical thermodynamics model are shown in Fig. 5.32. In a desulfurizing tower