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

temperature and excess air may be used to ensure optimum
operating conditions and as such would not involve any additional
environmental or energy impacts. Although the concept of
secondary combustion of a part of the fuel in the calcining zone
may reduce NOx emissions, as discussed in Section 6.2.1.3,
confirming data regarding the extent of NOx reduction are not
available.
7.1.2 Emission Tradeoffs
In-combustion control approaches attempt to change
combustion conditions to reduce NOx formation. Since the
combustion conditions also strongly affect formation of CO, SO2,
hydrocarbons, and possibly hazardous air pollutants (HAPs), the
in-situ control approaches are likely to affect these emissions.
To reduce NOx emissions, these approaches generally attempt to
reduce excess air and combustion temperature which may lead to
incomplete combustion and may lead to increases in CO and
hydrocarbon emissions. However, with optimum operating
conditions, the oxygen content in the combustion air may be
reduced to an optimum level without increasing CO and hydrocarbon
emissions. In a study to determine the effect of excess air on
CO and NOx emissions in a long dry kiln, an excess air level of 1
percent was found to reduce NOx emissions without increasing CO
emissions.1 Such behavior of CO emissions was also observed in
two other studies, where the CO concentration in the exhaust
gases did not appear to increase until the excess air level was
reduced below 1.5 percent.2,3 Thus, it appears to be possible to
operate a cement kiln at a certain optimum level of excess air
without impacting CO and hydrocarbon emissions.
The low NOx burner designs targeted for cement kilns induce
internal flue gas recirculation and staging of combustion.
According to one supplier of low NOx burners, these burners would
not have any effect on CO emissions, and their impact on
hydrocarbon emissions is not known.4
The sulfur present in the fuel and in the raw kiln feed is
converted to SO2. The calcined lime present in the solids in the
kiln is an excellent absorber for SO2, forming calcium sulfites
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