Cement

Production technologies

While there is a variety of kiln types, all cement kilns burn in excess of 2000°C, allowing the materials to be heated to about 1500°C. This heat causes chemical and physical changes that transform the raw limestone feedstock into clinker. The main chemical reaction is:

CaCO3 -> CaO + CO2

This reaction accounts for most of the process-related CO2 emissions from cement manufacturing. The energy-related emissions are also important, as they account for about 40% of total cement manufacturing CO2 emissions globally. As shown in the graph below, the process heat requirements vary substantially by technology. There are two basic cement production technologies: wet and dry. The wet production technology involves the use of high-moisture raw limestone feed, which has the advantage of allowing for better control of the chemistry and texture of the cement, but it also requires significantly more energy, due to the need to evaporate the water. The dry process has lower process heat requirements, but requires modern crushing and grinding equipment. The figure below shows cement energy intensity by a variety of production technologies and kiln types.

Energy Intensity by Cement Production Technologies

Credit:FLSMidth (2006), Cement Plant Pyro-technology, presentation to the IEA-WBCSD Workshop "Energy Efficiency and CO2 Emissions Reduction Potentials and Policies in the Cement Industry", IEA, Paris, France, September 2006. Retrieved from IEA (2007; see footnotes).

Vertical shaft kilns accounted for almost 50% of the cement production in China in 2005, far higher than any other region in the world. Vertical shaft kilns, while having high energy intensity, are better suited to the smaller individual factories found in China. However,  the share of vertical shaft kilns is declining as smaller cement plants are being replaced by larger facilities.

Emissions reduction potentials

CO2 emissions may be reduced from cement manufacturing by switching to less energy-intensive production technologies, fuel switching, blending, and possibly by Capture and Storage of the process-related CO2 emissions.⁶ Estimating the costs and emissions reduction potentials of fuel-switching is difficult because of the heterogeneity of regional fuel prices, and also because options may be limited in many regions. While regions with abundant natural gas and oil resources (e.g. the Middle East, Former Soviet Union, and Latin America) tend to use mostly gas and oil in their cement kilns⁷ , this is less realistic of an option for energy importers. The low-carbon alternatives to coal are generally biomass and tires, each of which is considered to be non-emitting for purposes of emissions reporting. However, the actual carbon content of tires is quite high, only 10 - 15% less than that of coal per unit of energy.⁸

Another factor in total CO2 emissions intensity which shows a good deal of heterogeneity worldwide is the clinker to cement ratio. Blending in by-products from other industrial activities, such as fly ash from coal power plants or blast furnace slag from iron manufacturing, can reduce the requirements for cement manufacturing substantially. This practice is not common in many regions (e.g. North America), as it is prohibited by building codes and/or environmental laws. This may change in the future, as modern-day blended cements actually tend to be stronger than Portland (high-clinker) cement, and no negative environmental or health effects of slag and fly ash in cement have been documented.⁹ Portland cement consists of about 95% clinker, and most regions have cement to clinker ratios between 80% and 90%. Several countries in Western Europe (France, the Netherlands) are below 75%.¹⁰

The possibility of carbon capture and storage from cement manufacturing has also been investigated. The capture costs would be a good deal higher than from an integrated gasification combined cycle (IGCC) electric power plant, as the flue gases from cement kilns are lower-volume CO2. The cost for most plants in North America has been estimated at about $50 to $80 per ton of CO2, which is well within the realm of possible future CO2 prices.¹¹ Still, it should be pointed out that carbon capture and storage would only be worth the substantial investment if carbon is priced, or if it is required. It should also be noted that this kind of carbon price would have a substantial impact on the price of cement itself, as the global average carbon intensity of cement manufacturing is about 800kg CO2 per ton of cement¹² , and the cost of cement is about $80 per ton (USD).¹³ A CO2 price of $25 per ton would therefore increase the cost of cement by 25% without any adjustments to cement production technologies or fuels.

Footnotes

1. Marland, G. B. Andres, and T. Boden (2008). Global CO2 emissions from Fossil-Fuel Burning, Cement Manufacture, and Gas Flaring: 1751-2005. Carbon Dioxide Information Analysis Center, Oak Ridge National Laboratory, Oak Ridge, TN, USA.

2. This is a bottom-up estimate calculated from clinker:cement ratios by region in Worrell et al. (2001), and cement production, manufacturing energy intensity, and fuel blends by region from IEA (2007). Assumes the following fuel carbon contents (kg C / GJ fuel): coal 27.3, biomass 23, gas 14.2, oil 19.6.

3. Houghton, R.A. (2005). The contemporary Carbon Cycle. In: William H Schlesinger (editor). Biogeochemistry. Amsterdam: Elsevier Science. pp. 473–513.

4. Worrell, E., L. Price, N. Martin, C. Hendricks, and L. Ozawa Meida (2001). Carbon dioxide emissions from the global cement industry. Annual Reviews of Energy and the Environment 26 (8).

5. IEA (2007). Non-Metallic Minerals. Chapter 6 in: Tracking Industrial Energy Efficiency and CO2 Emissions, International Energy Agency, Paris, France.

6. Worrell et al. (2001).

7. Table 6.6 in IEA (2007)./Worrell et al. (2001).

8. Energy Information Administration, Voluntary Reporting of Greenhouse Gases Program, Fuel and Energy Source Codes and Emission Coefficients

9. Table 6.6 in IEA (2007)./Worrell et al. (2001).

10. Table 6.6 in IEA (2007)./Worrell et al. (2001).

11. Mahasenan, N., R.T Dahowski, and C.L. Davidson (2005). The Role of Carbon Dioxide Capture and Sequestration in Reducing Emissions from Cement Plants in North America. In: Rubin, E.S., Keith, D.W., and C.F. Gilbor. Greenhouse Gas Control Technologies, Volume I. Elsevier Science.

12. Worrell et al. (2001).

13. Portland International, Cement Prices