Estimating Cement Kiln CO₂ Emissions

Cement Kiln Locations--Data was extracted from the EPA Aerometric Information Retrieval System (AIRS) databases (http://www.epa.gov/enviro/html/airs/index.html) using the National Industry Classification System (NAICS) Code for Cement Manufacturing (37231). This list was checked against the information from the U.S. and Canadian Plant Cement Industry: Plant Information Summary published by the Portland Cement Association (http://www.cement.org/). Where latitude and longitude information was absent in the EPA databases, the street address was used with Google Earth to locate the plant (http://earth.google.com/). Often the street address or the latitude and longitude are for an office removed from the kiln operations. In selected instances these locations have been corrected using the Google Earth imagery (The kilns and quarry operations are obvious). This procedure provided a list and locations for 126 kilns operating during 2004 in the US and Canada.

In cement manufacturing, CO₂ is emitted as a result of both process-related emissions (calcination) and fuel combustion. Most combustion-related CO₂ emissions result from clinker production, and specifically the fuel used for pyro-processing. Carbon dioxide emissions from combustion depend on whether a wet process or dry process for clinker making is used, as well as the carbon intensity of the fuel inputs.

Process Emissions--In the absence of detailed data on the CaO/MgO content of the clinker, the recommendations of the California Climate Cement Reporting Protocol¹ and the World Business Council for Sustainable Development² were adopted, which recommend a default emission factor of 525 kg CO₂/metric ton of clinker produced. As this emissions factor does not account for the fact that a percentage of the clinker precursor materials remain in the kiln in the form of cement kiln dust (CKD), the recommendation of the IPCC that emissions from CKD are equal to 2% of total process-related CO₂ emissions were followed. As a result the equation used for process related emissions is:

Clinker Production mt*0.525 mt CO₂/mt Clinker +
Clinker Production mt*0.02 CKD*0.525 mt CO₂/mt Clinker

Combustion Emissions--Although the actual dry process is more complex and dry kilns require more electricity to operate due to the need for fans and blowers, dry kilns consume significantly less energy in the pyro-processing. On average, the wet process has been estimated to require 6.3 Million Btu per short ton (MBtu/st) versus 5.5 MBtu/st for the dry process (Table 1)³.

Table 1--Energy required for processing

While cement kilns are energy omnivores, coal is responsible for the largest share of energy consumption at cement kilns, approximately 71% in 2001. Approximately 12% of energy consumption is derived from petroleum coke, 9% from liquid and solid waste fuels, 4% from natural gas, and the remainder from oil and coke^4,5. In the absence of detailed energy sources for individual kilns, we have generated maximum CO₂ emissions from energy consumption in kilns during the calcination process from the process type and primary fuel as reported by the Portland Cement Association. In addition we have used the EPA's carbon dioxide emission factors for individual fuels (Table 2)⁶.

Table 2--Carbon Dioxide Emission Factors

Cross Validation--All calculations for process and combustion emissions are based on reported clinker production. In 2004, The USGS estimated US clinker production at 86,658*103 metric tons⁷. For the US, the 2004 production in the NatCarb database totals 86,512*103 metric tons. This is a difference of much less than 1%.

Hanle and others⁵ estimated that in 2001 U.S. process-related emissions were 41.4 TgCO₂ and combustion related emissions were 36 TgCO₂ producing a total US cement plant CO₂ emissions of 77.4 Tg. The total 2004 CO₂ emissions for the US associated with the NatCarb database is 96,884M metric tons. In 2001, the USGS estimated US production at 78,451*103 metric tons. Adjusting NatCarb 2004 emissions and production to the lower production in 2001, generates an expected emissions in 2001 of 87 TgCO₂. This difference between the NatCarb estimated CO₂ emissions and that reported by Hanle and others may be acceptable given the differences in constructing the estimates.

¹California Climate Action Registry, Cement Report Protocol, 17 p., http://www.climateregistry.org/docs/PROTOCOLS/Cement_Certification_Protocol%20_1205.pdf

² The Cement CO₂ Protocol: CO₂ Accounting and Reporting Standard for the Cement Industry, Version 2.0, June 2005, The Cement Sustainability Initiative, World Business Council for Sustainable Development, 62 p., http://www.gispri.or.jp/calculation/ghg/pdf/cement_protocol_v2-or.pdf

³Worrell, E. and C. Galitsky, 2004, Energy Efficiency Improvement Opportunities for Cement Making: An Energy Star Guide for Energy and Plant Managers, Environmental Technologies Division, Lawrence Berkley National Laboratory, January 2004, LBNL-54036, 70p. http://www.energystar.gov/ia/business/industry/LBNL-54036.pdf

⁴USGS. 2003. Minerals Yearbook, Vol. 1, Metals and Minerals, 2002. U.S. Geological Survey. U.S. Department of the Interior. July 2003.

⁵Hanle, L. J., K. R. Jayaraman, and J. S. Smith, 2006, CO₂ Emissions Profile of the U.S. Cement Industry, 14p., 05-03-2006, http://www.epa.gov/ttn/chief/conference/ei13/ghg/hanle.pdf

⁶EPA Emission Factors, EIA Fuel and Energy Source Codes and Emission Coefficients; http://www.epa.gov/ttn/chief/ap42/ , http://www.eia.doe.gov/oiaf/1605/coefficients.html

⁷USGS, 2006, Cement Statistics and Information, 2p. http://minerals.usgs.gov/minerals/pubs/commodity/cement/