Page 291 - Air pollution and greenhouse gases from basic concepts to engineering applications for air emission control

268 9 In-combustion Air Emission Control

among the three, CFBC performs the best in terms of desulfurization efficiency
while pulverized coal combustion is the worst. On the other hand, the solid residues
from BFBC find limited use due to the high lime content whereas PFBC residues
have better properties for concrete or cement, which is the main use of desulfur-
ization solid residues.

9.4 In-combustion NO x Control

Removal of nitrogen oxides during combustion process is much more favorable
than post-combustion by flue gas cleaning. With the recent advances in NO x control
during the combustion stage, conditions in the boiler furnaces are no longer the
same as those in the 1980s. The NO x emissions are dramatically reduced with the
development of advanced combustion techniques, such as FBC, where the com-
bustion temperatures are low.
As introduced in combustion basics, the conversion of nitrogen in the com-
bustion air to nitric oxide is temperature sensitive. As such, the formation of thermal
NO may be reduced by lowering the combustion temperature and by minimizing
the flue gas residence time. The formation rate of thermal NO appears to be
practically low if the combustion temperature is below 1,400 °C, where at the
temperatures above 1,600 °C, the formation of NO is strongly accelerated [22].
A variety of technologies have been developed to lower the combustion tem-
perature. A few examples include, air staging, fuel staging, exhaust gas recircula-
tion (EGR), reducing temperature of preheated combustion air, reducing the flame
temperature by a long flame, and reducing the excess air. However, the efficiencies
of the methods are case-specific, as NO x is not the only concern. NO x reduction is
often a matter of optimization against the falling overall thermal efficiency due to
the lower flame temperature and the increase of combustibles in ash and flue gas. In
general, the NO reduction efficiencies of above-mentioned methods remain lower
than 70 %.

9.4.1 Air Staging

NO x formation may be substantially reduced by rearranging the combustion air
supply, which is referred to as air staging [2, 10, 16]. As illustrated in Fig. 9.8, part
of the air (primary air) is supplied as oxidizer at the root of the flame, where fuel
rich combustion takes place and most of the HCN and NH 3 are oxidized to
molecular nitrogen. The remaining air needed for combustion is supplied to the
flame from the flame periphery, where little HCN or NH 3 is left to produce nitric
oxide. As a result of this air staging, the peak flame temperatures in burners remain
lower than the conventional burners, and the formation of thermal NO is reduced
too.