Variability of NOx Emissions from Precalciner Cement Kiln Systems
In 2003, there are forty-two precalciner kilns out of about 190 total portland cement kiln systems operating in the United States. As a class, precalciner kiln systems have the lowest annual mass emissions of the oxides of nitrogen (NOx) per ton of clinker when compared to wet, long-dry and preheater kiln systems, however, like all cement kilns, precalciner systems experience significant variability in hourly NOx emission rates when compared to other processes such as electric power generating plants. This inherent variability in NOx emissions frequently is not given sufficient consideration by regulators and others involved in the selection of permissible emission rates. An undesirable result can be the selection of inappropriate emission rate limits for compliance periods of less than one year. Representative annual operating data from twelve of the domestic precalciner kiln systems were obtained. These data were normalized and the variability of the NOx emissions has been presented as kiln-specific standard deviations over several of the averaging periods that commonly are used by regulators for short-term NOx emission rate limits. Normalized data for each participating plant including and excluding periods of zero NOx emissions were displayed graphically as a time series plot and as a frequency distribution. These data served to visually demonstrate the representative variability of NOx emissions from precalciner kiln systems, and to provide the basis for developing realistic NOx emission limitations from existing and new precalciner kiln systems. The results of a linear regression analysis of the data for standard deviation versus the rated daily production rate of the respective kiln systems are presented graphically and as tables of correlation coefficients. Retrospective and prospective permitting strategies are proposed.