Development of Pollution Control Technology During Coal Combustion - Simultaneous Multi-Pollutants Removal in Flue Gas by Ozone - page 10

cost.
In a high-temperature area, the SNCR technology reduces NO x by injecting
NH 3 . The unreacted NH 3 further reacts with NO x with the aid of a catalyst
arranged at the later zone with low temperatures. This hybrid technology utilizes
SNCR by increasing the dose of the reducing agent. NH 3 leakage can be
controlled and reused during the SCR process. The SCR catalysts can also be
arranged inside the flue gas duct to save the capital cost. No additional catalyst
tower construction is needed. Therefore, the investment and operation cost of this
hybrid technology are lower than those of the SCR technology. The application of
the hybrid selective reduction technology in 320 MWe units in the Mercer
Generating Station power plant suggested that a 90% NO x reduction could be
achieved and the NH 3 leakage was less than 10 ppm [32] . Additionally,
catalyst-loaded air preheaters have been developed by some companies to utilize
the unreacted NH 3 , thereby effectively reducing the NH 3 leakage.
1.2.3 Hg Removal Technology
Coal-fired power plants are generally taken as the major anthropogenic emission
source of mercury. Normally, mercury speciation in flue gas is classified into three
forms, i.e., gaseous elemental mercury (Hg 0 ), gaseous oxidized mercury (Hg 2+ ),
and particulate mercury (Hg (p) ). Gaseous mercury, which consists of elemental and
oxidized mercury, accounts for about 86.5% - 97.8% of the total mercury
emission. Generally, Hg 2+ is water soluble, apt to absorb by wet scrubber. However,
Hg 0 is relatively difficult to be controlled, owing to higher volatility, insensitive to
water-solubility and a stable speciation, which cause global mercury pollution
with the help of atmospheric motion.
In general, the Hg-removal efficiency greatly depends on the mercury
speciation existing in flue gas. Various factors affect the mercury speciation
distribution, such as the type of coal used, gas temperature, combustion manner,
pollutant control devices, fly ash composition, and flue-gas composition like
chloride, NO, and SO 2 .
Control methods of coal-fired mercury emission can be divided into three
categories: (i) mercury control technologies before combustion, such as coal
washing and drying technologies which are not commonly used in industry; (ii)
mercury control technologies during combustion, which are focused on improving