Environmental Engineering Reference
In-Depth Information
There was also a peak of Hg
2+
when SO
2
was added to the simulated flue gas at
16:26:50 (Fig. 5.29). The chemical oxidation of Hg
0
to Hg
2+
can be expressed in the
following equation:
Hg
0
(g)
+ SO
2
(g)
+ O
2 (g)
HgSO
4 (s, g)
, (5-5)
However, this peak disappeared very quickly. The Hg
2+
produced by oxidization
was adsorbed by AC, especially when NO
2
and NO were added in the simulated
flue gas. The oxidization of Hg
0
to Hg
2+
by NO and NO
2
may have occurred through
reactions Eqs. (5-6) to (5-8), which are respectively given by:
NO
(g)
+ O
2
(g)
NO
2 (g)
+ O, (5-6)
Hg
(g)
+ O
HgO
(s, g)
, (5-7)
Hg
(g)
+ NO
2 (g)
HgO
(s, g)
+NO
(g).
(5-8)
During the adsorption of Hg
0
by AC in the simulated flue gas (Step 4), the
concentration of Hg
0
continued to decrease, and there was no rebound of Hg
0
con-
centration during the remainder of the adsorption experiment. However, the outlet
concentration of Hg
2+
slowly increased. Finally, the outlet concentration of Hg
2+
reached the same level as the initial concentration of Hg
0
. The conversion of Hg
0
to
Hg
2+
occurred during adsorption in the presence of the simulated coal-fired flue gas.
There was a relatively stable curve of Hg
2+
when the SO
2
flow into the simu-
lated flue gas was stopped between 48:37:20 and 57:39:20; when SO
2
was added
again at 57:39:20, the concentration curve of Hg
2+
increased rapidly (Fig. 5.28).
There was a competitive adsorption between SO
2
and Hg
2+
on AC; thus, SO
2
hampered the adsorption of Hg
2+
by AC. The Hg
2+
concentration measured in the
outlet of the bench system was even higher than the inlet concentration (14.2
g/(N·m
3
)) at some periods. This result can be attributed to Hg
2+
, which was already
adsorbed on the surface of AC and desorbed at a temperature of 130 °C.
Here, Hg
0
was measured at the outlet of the fixed bed when AC reached satu-
ration in terms of Hg
0
adsorption in the presence of N
2
(Fig. 5.28). However, Hg
2+
was found at the outlet of the fixed bed when AC reached saturation in the presence
of the simulated flue gas.
Such observations suggested the conversion of Hg
0
to Hg
2+
occurred during
adsorption in the presence of the simulated coal-fired flue gas. Furthermore, the
conversion rate of Hg
0
to Hg
2+
was quite high and reached nearly 100%, based on
the results of Hg
0
saturation adsorption by AC in the simulated flue gas.
The results of Hg
0
adsorption by AC in N
2
gas showed that AC was not capable
of inducing high Hg
0
conversion. When only flue gas was present without AC, only
part of the Hg
0
was converted to Hg
2+
. The obtained results suggested the occur-
rence of oxidation with the help of both AC and flue gas during Hg
0
adsorption in
the presence of flue gas.
Huggins
[22]
found that mercury can be captured by bonding to I, Cl, S, or O
anionic species on the surfaces of AC and other sorbents (e.g., coal char sorbents,
zeolite-based sorbents, coal fly-ash sorbents, and so on), but only as ionic Hg
2+
.
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