Mercury Speciation Transformation During Coal Combustion - Mercury Emission and its Control in Chinese Coal-Fired Power Plants - page 67

Transformation After Coal Combustion

When the temperature is higher than 1,000 K, mercury exists mainly in the form
of gaseous Hg 0 . As the temperature reduces, Hg 0 slowly decreases, while Hg 2+
generally increases (Eq. (4-36)). A small part of Hg 2+ is produced from 600 K to
1,000 K. With the temperature decreases further, the conversion reaction intensifies
so that the reactions between SO 2 and Hg 0 produce solid mercuric sulfate (HgSO 4 )
(Eq. (4-37)). When the temperature is down to 500 K, 100% of the mercury exists in
the solid form of HgSO 4 in the flue gas.

1

(4-36)

Hg + O HgO

(g) 2(g) (g)

2

(4-37)

2SO +2HgO +O 2HgSO

2 (g) (g) 2(g) 4 (s)

When SO 2 concentration was set at 100, 500, 1,200 ppm respectively, the result
is depicted in Fig. 4.69(b). The impact of SO 2 concentration on conversion of the
mercury temperature curve is shown in Fig. 4.70. It shows that different SO 2 con-
centration had hardly any effect on T ts , but a bad effect on T te of Hg 0 transformation
into Hg 2+ . T te was 300 K in the flue gas without SO 2 . When SO 2 concentration was
100, 500, 1,200 ppm, T te was 500, 550, 600 K respectively. It was obvious that T te
increased as the SO 2 concentration was enhanced.

1400

1200

T ts
T te

1000

800

600

400

200

0 200 400 600 800 1000 1200

SO 2 concentration (ppm)

Fig. 4.70 Influence of SO 2 on mercury speciation conversion temperature

This conveyed the information that SO 2 contributed to the conversion of mer-
cury, and especially that SO 2 had stronger activity than O 2 at low temperature.
When SO 2 concentration was higher, Hg 0 could be converted into Hg 2+ at higher
temperatures.

4.4.3.6 Co-influence of Typical Gas Components

With regard to the real case, it is highly important to investigate the comprehensive
effects of the typical gas components in the flue gas. When the flue gas includes 7%