Mercury Speciation Transformation During Coal Combustion - Mercury Emission and its Control in Chinese Coal-Fired Power Plants - page 83

During Coal Combustion

concentration of it near the wall is much lower, which is good for protecting
Shenhua coal from slagging. Fig. 4.76(e) shows the distribution of the concentra-
tion of H 2 S. Originally, we considered that S in the coal will all change into SO 2 . In
the reducing atmosphere, a lot of H 2 S generates. The highest concentration is about
50 ppm, which is possibly helpful for desulfurazation technology. Figs. 4.76(f), (g),
(h) show the distribution of the concentration of HCl, Cl and Cl 2 . We can find that
Cl almost exists as a form of HCl. The maximum concentration of HCl is up to 28
30 ppm, and HCl is distributed homogeneously in the furnace. A lot of Cl free
radical generates in the high temperature of the flame. The maximum concentration
of Cl is 1.8 2 ppm, while that of Cl 2 is as low as 0.5 ppm and narrowly ranged. The
location of Cl 2 is partly homogeneous with Cl and it is much closer to the center of
the furnace. Figs. 4.76(i), (j), (k) represent the features of the distribution of Hg,
HgCl 2 and HgO. In Fig. 4.76(i), Hg 0 is well-distributed in the furnace. There is a
large amount of Hg 0 in the center of the furnace and near the wall. Hg 0 is diluted by
the incoming secondary air. The amount of HgCl 2 is extremely low. The most in-
teresting phenomenon is that the character of its distribution is related to Cl 2 which
fits well with the observation by Hou Wenhui, who reported that the three dimen-
sional concentration distribution of mercury within the cylindrical stack was
simulated with different concentrations of Cl 2 and HCl by using CHEMKIN3.7
coupled with FLUENT6.2 software [33] . The result showed that even a small amount
of Cl 2 was much more effective on oxidation of Hg 0 than HCl. The temperature
range for the higher oxidizing rate of the Hg 0 focused on 950 1150 K.
HgO is mainly distributed in the axis of the air jet, which is similar to O 2 dis-
tribution. Fig. 4.76(l) shows the percentage of each Hg speciation with its integra-
tion in the section of secondary air. The percentage of Hg 0 was 95.29%, which took
up the most part. The percentage of HgO was 4.18%, and that of HgCl 2 was 0.53%,
which was quite rare. This was matched with the fact that Hg existed mainly in a
form of Hg 0 at high temperature. There was also some Hg 2+ in the furnace, and HgO
was the main form of Hg 2+ .
Fig. 4.77 shows the distribution of temperature, velocity, components and the
concentration of Hg at the updraft primary air section. The distribution is different
from that of the downdraft secondary air. Primary air is used for taking the coal to
the furnace. The temperature is below 160 centigrade and the velocity is below 30
m/s. It is mainly affected by the updraft and downdraft secondary air, which makes
the circle of contact bigger and the temperature average. The distribution of Hg was
highly correlated to Cl 2 and O 2 . The integral result in Fig. 4.77(l) shows the per-
centage of Hg 0 decreased, which was 95.29% at the downdraft secondary air section
to 90.95%. The percentages of HgO and HgCl both increased and the former
achieved 8.05% gradually. The concentration of HgO in updraft primary air was
enhanced definitely due to the high concentration of oxygen injection. Similarly,
with the Cl released from the coal, the concentration of HgCl 2 appeared to be
highlighted.