Criteria pollutants, hazardous air pollutants, and CO2 are released during cement manufacturing. Nitrogen oxides (NOx) emissions from cement kilns result primarily from the combustion process involving oxidation of fuel nitrogen (fuel NOx) and oxidation of nitrogen in the combustion air (thermal NOx). EPA's 2005 National Air Toxics Assessment (NATA) Inventory reports that cement kilns released 181,000 metric tons (200,000 short tons) of NOX emissions from the combustion of fuels .
Sulfur dioxide (SO2) emissions from cement kilns result from the sulfur in the fuel and the sulfur in the feed materials. Sulfur in the fuel will oxidize to SO2 during pyro-processing and a significant amount is likely to be captured in the form of sulfates as the flue gas passes through the calcination zone. Compared to long dry and wet kilns, preheater and precalciner kilns tend to be more effective at capturing fuel-generated SO2. Accordingly, oxidation of sulfur in the feed materials is likely to be the major component of total SO2 emissions. The 2005 NATA Inventory reflects that cement kilns released 133,000 metric tons (147,000 short tons) of SO2 emissions in 2005.
Quarrying operations, crushing and grinding of raw materials and clinker, and the kiln process result in particulate matter (PM) emissions. The NATA Inventory reflects that cement kilns released 10,000 metric tons (11,000 short tons) of PM10 emissions in 2005 . The cement industry also emits hazardous air pollutants (e.g., hydrochloric acid vapor, chlorine, and metals such as mercury, antimony, cadmium, and lead) . Also, the U.S. cement sector emits significant amounts of carbon monoxide. In 2005, these emissions amounted to 148,228 short tons .
In a cement kiln, the calcium carbonate in the limestone gets calcined to calcium oxide and in the process releases CO2. Additional CO2 is generated from the combustion of fuels in the kiln. Estimates of CO2 emissions from the U.S. cement industry amounted to 81.4 million metric tons in 2005. Of these, combustion-related emissions were estimated at approximately 35.5 million metric tons of CO2. Between 1990 and 2005, the process-related emissions resulting from the calcination of limestone increased 38%, from 33.3 million metric tons CO2 to 45.9 million metric tons CO2. A relatively small amount of off-site emissions (0.1 million metric tons CO2) also occur as a result of electricity use .
Multiple regulatory requirements currently apply to the U.S. cement industry. The New Source Performance Standards (NSPS) and the National Emissions Standards for Hazardous Air Pollutants (NESHAP) are two of the federal requirements that apply to cement facilities. On the other hand, state and local regulatory requirements might apply to individual cement facilities depending on their locations. In 2008, 44 cement facilities were located within ozone nonattainment areas and 20 facilities were within PM2.5 nonattainment areas. Seventeen facilities were found to be located in, or within 50 km, of Class 1 areas.
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