Mercury Speciation Transformation During Coal Combustion - Mercury Emission and its Control in Chinese Coal-Fired Power Plants - page 36

Table 4.3 Ratios of mercury concentration in fly ash and bottom ash
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 in coal (μg/g) 0.2977 0.2977 0.3911 0.3911

Hg in fly ash (μg/g) 0.2719 0.2569 0.3315 0.3124

Hg in bottom ash (μg/g) 0.0215 0.0320 0.0192 0.0362

CFB has received great attention since its introduction as a novel clean coal
combustion technology in the 1970s. CFB can realize high efficiency from various
flues during burning at low temperature, especially of low-quality and high-sulfur
coal. It can attain desulfurization using cheap and easy methods with the limestone
during burning and can reach 90% desulfurization efficiency, making it an efficient
low-pollution power generation technology. This section is focused on the impact of
limestone on mercury emission.
Fig. 4.13 and Fig. 4.14 show the ratios of the mercury concentrations in flue gas,
fly ash and bottom ash before and after the addition of limestone, respectively.
Adding limestone changed the proportion of mercury in the bottom ash, fly ash, and
flue gas. The addition adjusted the proportion of Hg (g) in flue gas, thus reducing the
Hg (g) . For example, burning of QP-S coal mixed with HB-B coal reduced the Hg (g)
from 66% to 43% after limestone addition. Burning of CF-S coal mixed with 2 # -B
coal reduced Hg (g) in the flue gas from 71% to 52%. However, the Hg p concentra-
tion in the fly ash increased slightly. For example, in conditions 1 and 2, the mer-
cury concentrations in fly-ash increased nearly 20% after limestone addition,
whereas in conditions 3 and 4, the mercury concentrations in fly ash increased to
16%. The mercury concentration in the bottom ash also slightly increased, but the
proportion of mercury concentration in the bottom ash was low, which ranged
within 3%8%. Nearly 90% of mercury existed in the flue gas (including Hg (g) and
Hg p in fly ash). According to historical statistics, the ratio of the mercury concen-
tration in fly ash was about 20% of that in coal, which value was higher than that in
this report. The reason was that the proportion of stone coal was larger, and the ash
in stone coal was very high (more than 70% in mass), which also caused a large
concentration of fly ash in the flue gas. As a result, the chance of contact between
the fly ash's surface and Hg (g) increased to form the Hg p . The Hg (g) in the flue gas
was partially transferred and adsorbed into the fly ash. The addition of limestone
had changed the distribution ratio of mercury in gas and moved it in the direction of
forming Hg p , which was easy to be captured by a particle collection device.
After adding limestone, the mercury concentration in fly ash increased signifi-
cantly, which indicated that limestone could help in controlling Hg (g) emission in an
easy way to transform to Hg p . Generally, limestone reduced the free Hg (g) in the
atmosphere, which was useful in the purification of the environment. However, at
the same time, mercury was transformed to solid ash, which might be potentially
dangerous on the environment depending on the stability of mercury in the ash. One
way of trying to deal with fly ash and bottom ash was to grind the ash into cement to
form a block. By checking the experiment, the released mercury proved to be very
small.