Mercury Speciation Transformation During Coal Combustion - Mercury Emission and its Control in Chinese Coal-Fired Power Plants - page 87

During Coal Combustion

the generation of Hg 0 greatly changed with the alternate injection of primary air and
secondary air. The concentration improved at the primary air surface, and decreased
at the secondary air surface. It was only after the injection of the primary air, that the
Hg in the pulverized coal released rapidly, and led to the Hg concentration increase.
Hg 0 concentration increased to nearly 100% at a height of 2325 m just above the
main burner, and then slightly decreased again.

100

90

Hg 0

80

Hg 0
HgCl 2
HgO

70

60

50

40

30

mid-sec air

20

OFA air

mid-sec air

up-sec air

HgO
HgCl 2

10

down-sec air

0

down-pri air mid-pri air up-pri air

tertiary air

Burner area

Fig. 4.81 The Hg species distribution at the burner area

The decrease in the Hg 0 in the furnace surely caused an increase in the other two
Hg species, i.e. HgO and HgCl 2 . According to the figure, the HgO concentration
changed frequently, and at the Hg 0 decreasing region the HgO concentration uni-
versally increased, especially at the burner region. At the height of 15 m, about 16%
HgO was generated. At the burner area, the main form was HgO. The generation of
HgO greatly changed with the alternate injection of primary air and secondary air.
The concentration decreased at the primary air surface, and increased at the sec-
ondary air surface. With the injection of secondary air, the rise in O 2 concentration
stimulated HgO generation. And above 25 m, HgO concentration slightly rose. So
while the furnace chamber temperature further reduced, the HgO concentration
tended to increase.
Figs. 4.82 and 4.83 are the distribution ratios of Hg and Cl after integration into
the whole three-dimensional furnace chamber. As Fig. 4.82 shows, Hg 0 in the fur-
nace was greatest and was about 94.22% of the total. Then HgO was about 4.96%.
HgCl 2 was only 0.82% and was the smallest. The Hg 2+ in major form was HgO. It
was different from the flue gas at the end of the flue where the major form was
HgCl 2 . Fig. 4.83 is the distribution ratio of element Cl. As can be seen from the
figure, HCl was 99.23%, Cl was 0.64%, and Cl 2 was only 0.13%. Although the
concentration of HCl was several orders of magnitude higher than Hg, it was still a
trace element compared with O 2 . So it was at a disadvantaged position during re-
action competition with Hg and the Hg 2+ in major form was HgO.