Mercury Speciation Transformation During Coal Combustion - Mercury Emission and its Control in Chinese Coal-Fired Power Plants - page 63

Transformation After Coal Combustion

is shown in Fig. 4.65, which indicated that Hg 0 did not take part in the oxidizing
reaction when there was no oxygen. With 0.2% of oxygen added into the flue gas,
mercury oxidized rapidly, which was in accordance with the report on rapid reaction
between Hg and oxygen proposed by Hall et al . [26] . While under a
high-temperature-burning condition, oxygen concentration almost had no effect on
mercury transformation. With the temperature decreasing, the mercury conversion
rate began to increase. At 300 1,400 K, mercury would transform into another
speciation and the reaction was extremely vigorous at 600 800 K. As the oxygen
concentration increased from 0.5% to 7%, this extended the range of the mercury
transformation temperature. When the oxygen concentration was set at 7%, Hg 0
began to convert at 1,400 K; at 300 K, nearly all the Hg 0 transformed into Hg 2+ .
Actually, due to the limitation of the chemical reaction rate, the mercury conversion
rate was not as high as expected. When the oxygen concentration in flue gas was
relatively low, the mercury transformation temperature and the mercury conversion
rate were relatively low too. It was clear that high oxygen concentration in flue gas
could promote the oxidation of mercury from the above analysis.

100

0%O 2
0.2%O 2
0.5%O 2
1%O 2
2%O 2
4%O 2
7%O 2

O 2 +CO 2 +N 2

80

60

40

20

0

200 400 600 800 1000 1200 1400 1600 1800 2000

T (K)

Fig. 4.65 Effect of O 2 content on Hg transformation

4.4.3.3 Influence of HCl

Fig. 4.66 is for observing the impact of HCl on mercury transformation by changing
the concentration of HCl (g) . After increasing HCl concentration (Case 2), we can
concluded that mercury begins to transform at higher temperatures with the exis-
tence of HCl. At the same time, there will be plenty of products within the mercuric
species, i.e. Hg 0 , Hg 2+ , HgH, Hg(CH 3 ) 2 , HgO, HgCl, HgCl 2 in a gaseous phase from
Fig. 4.66(a), while gaseous HgCl 2 is the dominant form according to the calculation
result. Because coal consists of chlorine, generally 60 ppm, when it burns HCl will
be released from coal and into the flue gas. HCl gas tends to decompose into a more
active chlorine atom at high temperature, which swiftly reacts with Hg 0 to form
HgCl 2 , sometimes oxidizing into HgCl first and then into HgCl 2 . The possible
reactions list is as follows: