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

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provided by coal. Natural gas contributed about 8 percent of
the energy demand, oil about 1 percent, and other fuels such as
waste solvents provided about 15 percent of the energy. Both oil
and natural gas have relatively low fuel-bound nitrogen content,
whereas coal may contain 1 to 3 percent of nitrogen by weight
depending upon the source of coal. Waste-derived fuels (WDF)
such as organic solvents are finding an increasing application in
the cement kilns. The nitrogen content in these fuels may be
significant depending on the chemicals included in the waste mix
being burned.
The maximum possible fuel NO formation may be estimated
x
from the fuel nitrogen content by assuming 100 percent nitrogen
conversion. The typical heat requirement for a wet process is
estimated to be about 6 million Btu for a ton of clinker and the
corresponding requirement for a dry process is estimated to be
about 4.5 million Btu for a ton of clinker. Assuming an average
heat requirement of 5.3 million Btu for a ton of clinker, and a
coal heating value of 12,000 Btu/lb, about 442 lb of coal will be
required per ton of clinker produced. With a nitrogen content of
1 percent by weight, approximately 9.5 lb of NO (14.5 lb
expressed as NO ) would be produced per ton of clinker with 100
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percent nitrogen conversion. Thus, even with only 10 percent
conversion of coal nitrogen to NO , 1.5 lb of fuel NO (expressed
x x
as NO ) may be formed per ton of clinker when coal is used as a
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primary fuel.
The amount of fuel NO x formed is difficult to identify
separately from thermal NO x as measurements indicate the overall
NO x formed. In general, however, thermal NO x is assumed to be
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the dominant mechanism in cement kilns. Typically, gas burners
produce more intense and hot flames compared to the less intense
"lazy" flames produced by coal burners. Thus, gas-fired kilns
may be expected to produce greater thermal NO as compared to
x
coal-fired kilns. Coal, on the other hand, contains much greater
amounts of fuel-bound nitrogen than natural gas which has almost
no fuel-bound nitrogen. The coal-fired kilns may thus be
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