Mercury Control and Mercury Stability in Byproducts - Mercury Emission and its Control in Chinese Coal-Fired Power Plants - page 133

None of the observations made by the reported XAFS spectroscopy study are con-
sistent with the capture of mercury in the elemental state, i.e., physisorption. Phy-
sisorption seemed to be very unlikely in the adsorption Hg 0 by AC in simulated flue
gas.
Based on previous experience with saturated adsorption, the adsorption of Hg 0
by AC in N 2 may be physical adsorption. However, further research suggests that
the adsorption of Hg 0 by AC in N 2 gas may also occur through a chemical adsorp-
tion process. The AC used in the current study was produced by a chemical activa-
tion method using ZnCl 2 , which may reside on the surface of AC. The Cl element
has been suggested to be strongly supportive for AC adsorption of Hg 0 in previous
studies.
For the purpose of proving the key chemical element (e.g., Cl) in the Hg 0 ad-
sorption by AC, a high temperature desorption system was utilized to remove the
element on the surface of AC. After heat treatment at a temperature of 1200 °C, AC
had almost the same physical characteristics, but the chemical elements were all
removed by decomposition.
The Hg 0 adsorption curves of the treated AC in N 2 gas are shown in Fig. 5.30.
When the gas flow was switched to the adsorption bed at 5 min, the decrease in Hg 0
concentrations was about 30% of the initial Hg 0 concentration. The adsorption
curve rose quickly, reaching the initial level of Hg 0 concentration about 5 min later.
This finding was completely different from the adsorption phenomena in N 2 (Fig.
5.28).
Fig. 5.30 The Hg 0 adsorption curves of treated AC in the N 2 gas
The treated AC only had limited adsorption capacity for Hg 0 in N 2 gas (Fig.
5.30). After losing the supply of surface chemical elements (e.g., Cl, among others),
AC was unable to develop the capability to adsorb Hg 0 in N 2 gas. The Hg 0 adsorp-
tion capacities of ACs produced by the physical activation method were also
compared. The three ACs produced by the physical activation method showed
almost no capacity for Hg 0 adsorption in N 2 gas. However, those ACs had high Hg 0
adsorption capacities in simulated flue gas.