Mercury Speciation Transformation During Coal Combustion - Mercury Emission and its Control in Chinese Coal-Fired Power Plants - page 32

Mercury in the flue gas included gaseous mercury and Hg p . The latter was partly
removed by dust collection devices. Carbon in fly ash had the ability to adsorb
mercury, which mostly depended on the flue gas temperature, carbon concentration
of fly ash, and surface properties, and so on. The finer fly-ash particles adsorbed
more mercury on the surface. There was more than 90% of mercury that existed in
the fly-ash particles whose size was less than 0.125 mm. The proportion of mercury
entering the fly ash was about 27% for layer combustion and about 23% for pul-
verized coal suspension combustion. The ratio of mercury in the bottom ash was
estimated to be about 20%. Fig. 4.8 shows the distribution of gaseous mercury and
Hg p in the flue gas of burning pulverized coal in suspension. The Hg p and Hg (g) were
about 20% and 80% of the total mercury, respectively, indicating that most mercury
was emitted into the atmosphere in gaseous form.

90

80

70

CG gaseous mercury
YZ gaseous mercury
YZ particul a te mercury
CG particul a te mercury

60

50

40

30

20

10

1100 1150 1200 1250 1300

Temperature (°C)

Fig. 4.8 Distribution of Hg (g) and Hg p in flue gas when coal combustion is in a state of suspension

From the perspective of chemical reaction dynamics, the cooling rate of flue gas
can affect the reaction between Hg 0 and other flue gas components. The combustion
experiment was conducted at a temperature of 1,300 °C. Under the same conditions,
the effect of the sampling cooling rate between a sampling point temperature of 415
°C and the solution adsorption point temperature of 125 °C in the Hg speciation
distribution was also studied. The results are shown in Table 4.1.

Table 4.1 Effect of sampling cooling rate on the Hg speciation distribution
Concentration (g/(N·m 3 )) Ratio (%)

Cooling rate
(K/s)

Experiments

Hg 2+ Hg 0 Hg 2+ Hg 0

No. 1 490 5.76 8.49 40 60

No. 2 360 6.77 6.68 50 60

During the two sampling processes, the sampling cooling rate changed by al-
tering the distance between the sampling point and the solution adsorption point.
Residence time was lengthy at the low sampling cooling rate, thus enhancing the
opportunities for reactions between Hg 0 and other flue gas components and im-
proving the oxidization from Hg 0 to Hg 2+ .