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

temperatures involved in the burning or clinker formation step,
thermal NO formation provides the dominant mechanism for NO
x x
formation in cement manufacturing.
There are four different types of cement kilns used in the
industry: long wet kilns, long dry kilns, kilns with a preheater,
and kilns with a precalciner. The long wet and dry kilns and
most preheater kilns have only one fuel combustion zone, whereas
the newer precalciner kilns and preheater kilns with a riser duct
have two fuel combustion zones. Since the typical temperatures
in the two types of combustion zones are different, the factors
affecting NO formation are also somewhat different in different
x
kiln types. In a primary combustion zone at the hot end of a
kiln, the high temperatures lead to predominantly thermal NO x
formation; whereas in the secondary combustion zone lower gas-
phase temperatures suppress thermal NO formation.
x
In addition, to the specific NO formation mechanisms, the
x
energy efficiency of the cement-making process is also important
as it determines the amount of heat input needed to produce a
unit quantity of cement. A high thermal efficiency would lead to
less consumption of heat and fuel and would produce less NO x
emissions.
Since the four types of cement kilns exhibit different
combustion characteristics as well as energy efficiencies and
heat requirements, the available NO emissions data are grouped
x
by these cement kiln types. The data indicate substantial spread
in the reported NO emissions with large overlap for different
x
kiln types. The four different cement kiln types, however, do
appear to have different levels of NO emissions and different
x
characteristics influencing NO formation. The uncontrolled
x
emission factors based on the information obtained during this
study are given in Table 2-1. This table also includes the heat
input requirement for the different cement kiln types which
indicates a good correlation with the NO x
emission rates.
2-2