Development of Pollution Control Technology During Coal Combustion - Simultaneous Multi-Pollutants Removal in Flue Gas by Ozone - page 7

NH x , CH x , and CO, as well as longer residence time of flue gas in the reduction
zone, thereby reducing NO x .
The technology is based on the utilization of hydrocarbon radicals, i.e., CH,
HCN, and NH 3 , and other radicals decomposed from reburning fuel, to reduce
NO x . Fuel is fed into the furnace in stage, and the reburning fuel is injected at the
top of the flame to establish a fuel-rich zone and to deoxygenize the generated
NO x . The OFA is then injected to ensure complete combustion. Generally, 80% -
85% of fuel (known as the primary fuel) is introduced into the main combustion
zone and burned out with the excess air. The remaining 15% - 20% of fuel
(known as the reburning fuel) is sent into the upper part of the main burner
(reburning zone) to form a reductive atmosphere for converting NO into N 2 . The
pulverized coal and natural gas are usually considered as reburning fuels. However,
the latter is superior to the former in terms of NO x reduction. Generally, the
optimum NO x reduction efficiency with acceptable burnout levels can reach up to
50%.
1.2.2.5 SCR
The SCR method [18] is developed based on the fact that a number of reagents can
directly convert NO x into N 2 and H 2 O through metal catalyst at a relatively low
temperature. NH 3 is a typical reducing agent that can be sprayed into the flue gas
between economizer and air pre-heater. A mixture of NH 3 and flue gas goes
through the catalyst bed and undergoes the following reactions:
4NH 3 +4NO+O 2 = 4N 2 +6H 2 O (1.1)
4NH 3 +2NO 2 +O 2 = 3N 2 +6H 2 O (1.2)
Reaction time, flue-gas temperature, flow rate, and NH 3 /NO mole ratio are the
main parameters affecting NO x removal efficiency. When NH 3 /NO=l, the deNO x
of SCR is approximately 80% - 90%. Metal-based and C-based catalysts can also
be used [19] . The most common metal-based catalysts are V 2 O 5 and TiO 2 , and their
corresponding operating temperature is approximately 300 - 400 °C. C-based
catalysts aim to remove NO x and SO 2 at the same time, in which C is used to
absorb SO 2 . The operation temperature of C-based catalysts is about 200 - 250 °C,
which is lower than that of metal-based catalysts. The operation temperature of
C-based catalysts has a passive effect on deNO x and enhances the SO 2 adsorption.