NOx Emissions from Cement Mfg - US Environmental Protection ...

to 50 percent of the fuel is burned at a lower (calcination)
temperature which is expected to reduce the thermal NOx formation
considerably. The preheater kilns may also use the secondary
firing to a lesser extent. Up to 15 to 20 percent of the fuel
may be fired in the riser duct in preheater designs1 although the
primary emphasis in these designs is on efficient heat recovery
from kiln exhaust gases.
The concept of staged combustion may also be used in the
precalciner designs by injecting part of the calciner fuel in the
kiln exhaust gas entering the calciner (secondary combustor).
The tertiary air is added at a later point to complete the
combustion of the calciner fuel. The injection of fuel in the
kiln exhaust gas creates a reducing zone which reduces the NOx
formed in the kiln combustor by following simplified reactions:
and
NO % C 6 N % CO
NO % CO 6 N % CO 2 .
5-11
(5-1)
(5-2)
As discussed in Section 4.2.2, nitrogen present in the fuel may
also participate in the reduction of NOx. The primary NOx
formation mechanism in the secondary firing is the fuel NOx
formation which depends upon the nitrogen content of the fuel
used. In order for the above reactions (5-1) and (5-2) to
proceed at reasonable rates the temperature in the reduction zone
should be maintained between 1000 and 1200 EC (1830 to 2190 EF).
These temperatures may lead to coating difficulties, particularly
if the fuel used is coal with high ash content.15 Very promising
results appear to have been obtained recently with the staged