Environmental Engineering Reference
In-Depth Information
Transformation After Coal Combustion
4.4 Thermochemical Equilibrium Analysis of Mercury Speci-
ation Transformation After Coal Combustion
The significant amount of mercury emissions from coal-fired power plants was
found to depend on the mercury content of coal, temperature, composition, and the
unburned carbon in flue gas, as well as the air pollution control devices (APCD)
used. Also, it is strongly determined by the speciation of the mercury in flue gas [2] .
In general, in coal-fired flue gas, gaseous mercury exists predominantly in Hg 0 ,
Hg 2+ , along with the +1 oxidation state, although this state is very rare. As stated
from the real experimental results statistically, the Hg 0 was 52% 83% of the total
Hg (g) , Hg 2+ was 17% 48% emitted from coal-fired flue gas [16] . Cost-effective
methods for removing mercury from coal-fired flue gas has received increased
attention because of recent limitations placed on mercury emissions by the Protec-
tion Agency [17] . Therefore, a variety of possible options to reduce mercury emis-
sions were proposed and discussed while a more efficient utilization of already
existing air pollution control devices (APCD) for mercury removal purposes was
considered as a reliable and cost-effective alternative to the development of new
and mercury specific removal technologies.
The speciation of mercury in the flue gas of a coal-fired power plant affects the
amount of mercury retained in the air pollution control devices (and not emitted
from the stack) due to the chemistry of Hg 0 in flue gas being different from that of
Hg 2+ . Hg 0 is difficult to capture by typical APCD because it is highly volatile and
nearly insoluble in water. Whereas Hg 2+ can be removed in the wet flue gas
desulfurization (WFGD) facilities of coal combustion processes due to its high
solubility in aqueous solutions. So a promising approach for mercury removal from
coal-fired flue gas is to achieve oxidation of Hg 0 to Hg 2+ , subsequently captured by
WFGD [18] . Thus, the mercury speciation, no matter whether the mercury is Hg 0 or
Hg 2+ , has an extreme impact on mercury removal by WFGD.
Whereas the speciation was considerably correlated to the flue gas temperature
and particularly the flue gas species including HCl, SO 2 , NO x , Cl, O 2 , H 2 O etc. [19,20] .
Some literature on the impact of flue gas composition on the mercury speciation
transformation was reported. Hall et al . [11,21,26] studied the reaction characteristics
between Hg 0 (g) and the flue gas components, including CO 2 , HCl, Cl 2 , SO 2 , NO 2 ,
N 2 O, NO, NH 3 , H 2 S. It was shown that Hg 0 was reacting with NO 2 at a very low
rate, hardly with N 2 O, NH 3 , H 2 S, SO 2 , but sensitive to HCl, Cl 2 and O 2 [21] . Frand-
sen [22] focused on the thermodynamic equilibriums of trace elements such as mer-
cury, selenium, arsenic, using the MINGSYS program and DGFBASE database.
Hall et al . [21] concluded that Cl played a key role in mercury speciation transfor-
mation, instead of HCl, Cl 2 , NO x , O 2 . Senior et al . [1] conducted a simple model of
mercury transformation in flue gas, which was devised so that an importance was
attached to Cl in flue gas in mercury transformation, whereas, H 2 O, SO 2 , NO 2 were
also influenced slightly by the homogeneous reaction rate with mercury. Qiao Yu
proposed a kinetic model of the Hg/O/H/Cl reaction system, and the modeling
results agreed well with experimental data by Mamani-Paco et al . [9] .
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