Page 247 - Air pollution and greenhouse gases from basic concepts to engineering applications for air emission control

7.9 Fate of Trace Elements 223

There are some overlaps between these three classes of trace elements. For
example, B, Se, and I can be classified into Class II or Class III [39]. Combined
with information on toxicity and harmful effects on process equipment, this clas-
sification gives first indication on which trace elements will need special attention.

7.9.3 Mercury

Mercury (Hg), a Class III trace element, is very problematic due to its toxicity and
volatility (boiling point 357 °C). In combustion more than 90 % of the incoming Hg
penetrates through the flue gas cleaning system, and is discharged into the atmosphere
as vapor. Due to its high volatility, Hg is released from fossil fuels at about 150 °C,
mainly as elemental Hg, HgCl 2 ,Hg 2 Cl 2 and HgS. The release of Hg from the fuel will
be complete at 500–600 °C[17, 35]. Under oxidizing conditions, in the presence of
HCl and Cl 2 , elemental Hg is oxidized to HgCl 2 at 300–400 °C (Fig. 7.8).

Hg þ 4HCl þ O 2 $ 2 HgCl þ 2H 2 O ð7:94Þ
2
Hg þ Cl 2 $ HgCl 2 ð7:95Þ

Elemental mercury may react with NO 2 through the following reactions [6]:

NO 2 +Hg $ HgO þ NO ð7:96Þ
The product of HgO continues to react with HCl as follows.

HgO þ 2HCl $ HgCl þ H 2 O ð7:97Þ
2
As a result, three different mercury species must be considered in the flue gas
from coal combustion: gaseous elemental mercury (Hg), gaseous oxidized mercury
(e.g., HgCl 2 , HgS, HgO, HgSO 4 ) and particle-bound mercury (Hg-p). It is estimated
that Hg emissions from a coal-fired power plant leaving the furnace, entering the
3
flue gas duct are of the order 5–50 µg/m at STP.

7.10 Greenhouse Gases

By the end of the twentieth century it was widely accepted that CO 2 ,CH 4, nitrous
oxide (N 2 O), and water contribute to the global warming. Water vapor is the most
abundant greenhouse gas in the atmosphere, and it contributes to 2/3 of the
greenhouse effects. The rest of 1/3 effect caused by other GHGs is referred to as the
“enhanced greenhouse effect”, or the “anthropogenic greenhouse effect”.CO 2 is
contributing nearly ¾ of the enhanced greenhouse effect.