Environmental Engineering Reference
In-Depth Information
Generally, the flue gas cooling rate and residence time in the cooling section
have a significant effect on the measurement of mercury speciation in flue gas and
the mercury emission control. During the measurement, for example, sampling in a
high temperature area of flue gas, the sampling inlet temperature was high, and the
sample gas was cooled during the transportation from the sampling pipe to the
absorption bottle. Hg 0 in sampling gas could react with other flue gas components
and could increase Hg 2+ . Thus, the measurement results did not present the real Hg
speciation situation at sampling points. To reduce the impact of flue gas cooling on
the transformation of mercury speciation, the sample gas should be rapidly cooled
to ice water temperature from the sampling points by some special means.
4.2.3 Mercury Speciation in Flue Gas When Coal Combustion Is in
a Fluidized-Bed
Studies of the characteristics of mercury emissions from combustion of bituminous
coal were conducted on a circulating fluidized-bed (CFB). The mercury concen-
trations in mixed coal and in flue gas from the CFB combustion were analyzed.
The coals used in the study included Huaibei bituminous (HB-B) coal, 2 # bi-
tuminous (2 # -B) coal, Chafei Stone (CF-S) coal and Qingping Stone (QP-S) coal,
which were all crushed to a size of less than 10 mm. These samples were propor-
tionately mixed according to the research requests. The impact of limestone on
mercury emission was also studied. The sampling and analyzing of mercury con-
centration in the flue gas and ash was conducted in accordance with the OH method.
Table 4.2 lists the concentrations of Hg 2+ , Hg 0 , and total concentration of Hg (g)
in the standard flue gas volume flow unit under different combustion conditions.
The results show that the Hg (g) concentrations in the flue gas in each condition
ranged from 6.8 to 9.3 μg/(N·m 3 ). All concentration values were less than 10
μg/(N·m 3 ).
Table 4.2 Hg speciation distribution in flue gas with combustion of four mixed coals in CFB
respectively
Conditions
No. 1
No. 2
No. 3
No. 4
QP-S 60%
HB-B 40%
Limestone
CF-S 60%
2 # -B 40%
Limestone
QP-S 60%
HB-B 40%
CF-S 60%
2 # -B 40%
Mixing ratio of coal
Hg 2+ (μg/m 3 ˅
3.6
2.6
2.5
2.7
Hg 0 (μg/m 3 )
5.2
4.2
5.3
6.6
total Hg (μg/m 3 )
8.8
6.8
7.8
9.3
The proportions of Hg 2+ and Hg 0 that accounted for the total Hg (g) in the flue gas
under different combustion conditions are shown in Fig. 4.9 and Fig. 4.10, respec-
tively. The proportion of Hg 2+ to the Hg (g) in the flue gas ranged from 30% to 41%,
whereas the proportion of Hg 0 ranged from 59% to 70%. The concentration of Hg 0
was about 20% higher than that of Hg 2+ under conditions 1 and 2; however, the
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