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

NOxemissions will form. The heater exhaust gas could be combined
with the kiln exhaust gas prior to the SCR unit.
The SNCR processes may also increase CO concentrations in
the flue gas, although this increase for urea-based systems is
apparently much less than that due to combustion modifications
such as overfire air and sub-stoichiometric combustion used in
utility boilers.7 On the other hand, one reference claims that
ammonia injection has no effect on CO emissions.8 Interestingly,
the intentional addition of CO in the reaction zone of the
process broadens the operating temperature window for urea-based
systems, even at CO concentrations as low as 500 ppm, although it
increases N2O emissions.9 At a demonstration of the Nalco urea
based SNCR system, CO and SO2 emissions were unaffected when the
NSR was less than 0.7 or when the preheater exit oxygen was
greater than 2.3%. At lower O2 and at NSR's greater than 0.7,
both SO2 and CO increased.6
The SCR employs catalysts that are composed of various
active materials, such as titanium dioxide, vanadium pentoxide,
and tungsten trioxide, as well as inert ingredients to impart
structural strength and ease of forming.5 Most concerns over
hazards have centered on vanadium pentoxide, which is a toxic
compound. After deactivation, these catalysts must be disposed
of properly. Some catalyst manufacturers may take back any spent
catalyst for reactivating and recycling. Where the spent
catalyst cannot be recycled, it must be disposed in an approved
landfill.7
7.2 ENERGY IMPACTS OF NOx CONTROLS
7.2.1 In-combustion NOx Control Systems
Combustion control techniques for NOx reduction rely on
reducing excess air and excess burning (or high flame
temperatures) for optimum conditions. Both of these factors will
likely increase the energy efficiency of the cement-making
process. Thus by intuition, NOx reduction measures based on
combustion process controls are also likely to reduce the energy
requirement per ton of clinker produced. Techniques such as
secondary firing allow combustion of low grade fuels such as
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