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

excess air level also results in increased productivity per unit
amount of energy consumed and thus results in an indirect
reduction of NO emissions per unit amount of clinker product.
x
5.1.2 Process Modifications
By using the simple monitoring and feedback control
approach, NO emissions would be minimized to a certain level.
x
Process modifications that may be used for additional reductions
in the NO formation are discussed below.
x
5.1.2.1 Feed Mix Composition
Heat requirements for producing clinker are dependent on the
composition of the raw feed which varies among cement plants.
Experiments have demonstrated that by decreasing burnability of
the raw feed, the heat requirement of clinker can be reduced by
4
15 percent. If the raw feed composition can be formulated to
require less heat input per ton of clinker, less fuel is burned
and less NO is produced. This approach of changing the feed
x
composition may, however, be highly site specific and may not be
applicable at all locations.
The alkali content of finished cement needs to be below a
certain acceptable level. Low alkali requirements need higher
kiln temperatures and longer residence times at high temperatures
to volatilize the alkali present in the molten clinker. Raw
materials with greater alkali content need to be burned harder
(longer at higher temperatures) to meet alkali requirements and
thus may produce greater NO emissions. Increased volatilization
x
of alkali also results in increased alkali emissions in kiln
exhaust gases. To control alkali emissions, a part of the kiln
exhaust gases may be bypassed from a downstream unit, e.g., a
precalciner. The bypassed gases are quenched to remove alkali
and sent through a particulate collector. The bypass of kiln
exhaust gases typically involves a fuel penalty: about 20,000
Btu/ton of clinker for every 1 percent gas bypass. The
additional heat requirement may also contribute to increased NOx emissions. Reducing the alkali content of the raw feed mix will
thus reduce the NO x
emissions.
5-3