Environmental Engineering Reference
In-Depth Information
Table 4.5
Possible oxidizing reactions of mercury in the coal combustion
2Hg
0
(g)
+ 4HCl
(g)
+ O
2
(g)
2HgCl
2(g,s)
+ 2H
2
O
Oxidized Products
Hg
0
(g)
+ HCl
(g)
HgCl
2(g,s)
Hg
0
(g)
+ Cl
2(g)
HgCl
(g)
HgCl
(g)
+ Cl
2(g)
Hg
0
(g)
+ Cl
(g)
HgCl
2(g)
HgCl
2(g)
HgCl
(g)
+ HCl
(g)
HgCl
(g)
+ Cl
(g)
HgCl
2(g)
2Hg
0
(g,ads)
+ O
2(g,ads)
2HgO
(g,s)
HgO
2(g,s)
+ NO
(g)
Hg
0
(g)
+ NO
2(g)
HgO
(s,g)
Table 4.6
Possible reduction reactions of mercury in the coal combustion
HgO
(s,g)
+ SO
2
(g)
Hg
0
(s,g)
+ NO
(g)
Hg
0
(g)
+ SO
3(g)
Hg
0
(g)
+ NO
(g)
HgO
(s,g)
+ NO
(g)
Hg
0
(g)
+ CO
2(g)
HgO
(s,g)
+ CO
(g)
Hg
0
(g)
+ Cl
2(g)
HgCl
(s,g)
+ Cl
(g)
Hg
0
(g)
+ SO
2
Cl
2(g)
HgCl
2(s,g)
+ SO
2(g)
Based on previous studies, various factors were taken into consideration and
quantitative forecasting of major influencing factors was conducted. The apparent
reaction kinetic model of the combination of mercury and HCl was established
through experiments.
4.3.1.2 Establishment of the Kinetic Model
The mercury oxidization in coal-fired flue gas was main the result of the reaction
between the chlorine-containing substance and mercury. Therefore, the Hg
2+
in the
flue gas was generally believed to be mainly HgCl
2
. The direct reaction of HCl and
mercury that generates HgCl
2
could be expressed as follows:
(4-12)
0
Hg (g)
2HCl(g)
HgCl (g)
H (g)
2
2
Thermodynamic calculations showed that the above reaction had a high energy
barrier. At a temperature below 600 K, the reaction was very slow, and the direct
reaction between mercury and HCl was restricted. No clear pathway of reaction was
found, and there may be multiple elementary reactions. Theoretical and experi-
mental studies
[11]
had shown that chlorine molecules (Cl
2
) and chlorine atoms (Cl)
were very active chloride media, and the following reaction pathways were pro-
posed:
Pathway 1:
0
(4-13)
Hg (g)
Cl (g)
HgCl (g,s)
2
2
Pathway 2:
(4-14)
0
Hg (g)
Cl(g)
HgCl(s,g)
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