Nitrogen oxides (NOx)
The cement clinker burning process is a high-temperature
process resulting in the formation of nitrogen oxides
(NOx). Nitrogen monoxide (NO) accounts for about 95%,
and nitrogen dioxide (NO2) for about 5% of this compound
present in the exhaust gas of rotary kiln plants. As most of the NO
is converted to NO2 in the atmosphere, emissions are
given as NO2 per m³ exhaust gas.
Without reduction measures, process-related NOx
contents in the exhaust gas of rotary kiln plants would in most
cases considerably exceed the specifications of, for
example, European legislation for waste burning plants (0.50
g/m3 for new plants and 0.80 g/m3 for
existing plants). Reduction measures are aimed at smoothing and
optimising plant operation. Technically, staged combustion and
Selective Non-Catalytic NO Reduction (SNCR) are applied to cope
with the emission limit values.
High process temperatures are required to convert the raw
material mix to Portland cement clinker. Kiln charge temperatures
in the sintering zone of rotary kilns are in the range around
1,450°C. To reach these, flame temperatures of about
2,000°C are necessary. For reasons of clinker quality the
burning process takes place under oxidising conditions, under which
the partial oxidation of the molecular nitrogen in the combustion
air resulting in the formation of nitrogen monoxide dominates. This
reaction is also called thermal NO formation. At the lower
temperatures prevailing in a secondary firing unit, however,
thermal NO formation is negligible: here, the nitrogen bound in the
fuel can result in the formation of what is known as fuel-related
NO.