Environmental Engineering Reference
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4.4.3 Thermochemical Equilibrium Calculation Results
4.4.3.1 Influence of Temperature
To explore the elements affecting mercury speciation transformation from
coal-fired flue gas, the typical flue gas temperature was firstly considered. The
simulated flue gas temperature was set at 3002,000 K as in a real power plant,
along with the flue gas components consisting of 7% O 2 , 13% CO 2 , 80% N 2 , Hg
(real working case 1). Table 4.13 shows mole fractions of the input flue gas com-
ponents and Table 4.12 provides the various mercury compounds concerned. From
Fig. 4.64(b), as the temperature went down, the mercury conversion rate increased
definitely, which was in line with the general law shown in Fig. 4.63. At 1,200 K,
only a small amount of mercuric oxide gas was detected and a large amount of
mercuric oxide (HgO) gas-solid mixture at 473 K, while at 300 K mercuric oxide
existed in the form of a solid phase in Fig. 4.64(a). The conclusion could be drawn
that Hg 0 tended to react with oxygen molecules to form mercuric oxide, which was
observed by Hall and his colleagues as well [21] .
100
10
Hg 2+
80
8
Hg 0 (g)
Hg 2+ (g)
HgO (g)
HgO (s)
case 1
case 1
60
6
40
4
2
20
0
0
0
400
800
1200
1600
2000
400
800
1200
1600
2000
(a) (b)
Fig. 4.64 Effect of temperature on mercury speciation transformation
T (K)
T (K)
From the chemical and thermodynamics equilibrium discussed above, it is in-
ferred that most of the mercury evaporates and exists dominantly in the form of Hg 0 .
As the flue gas entered the convection pass and temperature decreased gradually,
Hg 0 reacted with the oxygen molecules to form a mercuric oxide gas-solid mixture
in the oxidizing atmosphere.
4.4.3.2 Influence of Oxygen Concentration
From Case 1 above, it was concluded that oxygen was the key factor in promoting
mercuric oxide generation. Upon consideration of the variability of oxygen con-
centration in the flue gas in an actual power plant, the influence of oxygen con-
centration changes on mercury speciation transformation was simulated. The result
 
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