Development of Pollution Control Technology During Coal Combustion - Simultaneous Multi-Pollutants Removal in Flue Gas by Ozone - page 8

1.2.2.6 SNCR
In the SNCR [20] technology, a NH x -based reducing agent (mainly ammonia and
urea) is injected into the furnace zone where the temperature range is around 800 -
1100 °C. NO x is rapidly converted into N 2 and H 2 O. The chemical reaction in this
technology is nearly the same as that occurring in the SCR technology, except that
the former has no catalyst demand. The denitrification effect mainly depends on
the temperature distribution in the spray area. The reduction reaction usually
begins to faint at temperatures below 900 °C, resulting in a lower NO removal and
NH 3 leakage. When the temperature is too high, NH 3 is naturally oxidized into NO,
resulting in high NO x emissions. Therefore, an appropriate temperature range for
maintaining high deNO x and sumptuously avoiding the NH 3 leakage, should be
considered in the SNCR NH 3 -injection technology [21] . It is found out that the NO x
removal with SNCR in an entrained-flow reactor, under the conditions of
n (N)/ n (NO x ) = 1.5, 900 - 1000 °C, and residence time of 2 s, and using urea
reductant, 70% - 80% NO removal and 45% - 50% NO 2 removal were obtained,
and the N 2 O is below 7×10 -6 [22] . In real-furnace application, for pulverized-coal
boilers with 1100 - 1200 °C at the furnace outlet, around 40% of NO x can be
reduced due to the short resident time at the selected temperature window. For a
CFB boiler with the combustion chamber running at around 850 - 950 °C, the flue
gas passage between the cyclone and combustion chamber is the suitable position
for the SNCR reaction. With a well mixing process in the cyclone, the NO x
reduction efficiency can usually reach 70%.
1.2.2.7 Advanced Reburning Technology
The advanced reburning technology integrates the traditional SNCR technology
with reburning to further reduce NO x . This technology can reduce NO x by more
than 85% and is much more economical than that of SCR. Shortly, the advanced
reburning technology is one of the most attractive denitrification technologies [23]
and comprehensive investigations have been reported [24-26] . The NH 3 -injection
position varies with the SNCR technology. NH 3 is sprayed into the oxidation
region after OFA in the traditional SNCR technology, but the NH 3 -sprayed
procedure quite differs within the advanced reburning technology. The advanced
reburning technology can be divided into three forms on the basis of the
relationship between the NH 3 injection location and the reburning zone, i.e., the