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| Fig. 1: Cement kiln in Union Bridge Maryland, USA. (Source: Wikipedia Commons) |
An integral part of concrete, cement has quite literally laid the foundation for modern life. From houses to skyscrapers, the material's use is ubiquitous. It is affordable, accessible, and structurally sound. In 2019, 4.1 Gt (4.1 × 1012 kg) of cement was produced, up from 1.45 Gt (1.45 × 1012 kg) in 1995. [1,2] This growth is predominately driven by developing economies. [1] China, for example, produced just 592 Mt (5.92 × 1011 kg) of cement in 2000 but manufactured over 2.0 Gt (2.0 × 1012 kg) in 2011. [3]
For all its benefits, cement production generates significant CO2 emissions. The process releases CO2 in two main ways - through the oxidation of calcium carbonate and by burning fossils fuels to heat the calcium carbonate feed. [2] The calcium rich material that exits this process is also known as clinker. Current estimates attribute 90% of cement emissions to clinker production. [4] Thus it will be the main focus of this analysis.
Process emissions result from the stoichiometric conversion of calcium carbonate to calcium oxide and carbon dioxide as shown in Eq. (1).
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(1) |
These emissions are independent of energy source and thus represent a significant challenge in decarbonizing cement production. Given an average clinker-to-cement ratio of 0.725, a CaO-to-clinker ratio of 0.656, and a CO2-to-CaO mass ratio of 0.786, one obtains Eq. (2) for process emissions. [2,5]
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(2) |
The production of clinker also requires significant energy to achieve thermodynamic favorability. This requirement is met by heating the clinker feed to over 1,400°C in a kiln such as the one shown in Fig. 1. The average energy used in this process was 3.5 GJ/t cement in 2014. [6] Given the following distribution of fuel sources and carbon intensities in Table 1 one can compute a weighted average for the carbon intensity of energy generation.
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| Table 1: Sources and carbon
intensities of fuels used in cement production using IPPC
default emission factors. [6,7] (†) Alternative sources may release CO2 but are deemed renewable so net impact is zero. |
Based on this information, the average carbon intensity of energy in cement is 84.7 kg CO2/GJ. From this, one can calculate the energy related CO2 emissions in Eq. (3).
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(3) | ||||||||||||||||||||||||
In addition to the process and energy emissions, quarrying, preparation, electricity and transport also contribute to the overall emissions of cement production. Current estimates peg this contribution at 10% of total emissions. [4] Given the process and energy emissions calculated above, other emissions streams constitute 0.297 Gt CO2 per year.
Adding all three emissions steams together, global CO2 emissions from cement constitute 2.97 Gt CO2 per year or 0.724 tCO2 per tonne cement. This is slightly lower than the estimate presented by Worrell et al. of 0.814 tCO2 per tonne cement; however, since Worrell's analysis in 2001, the fraction of coal used in cement production has fallen, lowering energy emissions. Given global CO2 emissions were 34.4 Gt in 2019, one can calculate the percent of anthropogenic CO2 attributable to cement production in Eq. (4). [8]
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(4) |
This percentage is staggering. For perspective, this is more than the emissions of Africa, Central, and South America combined. [8] Additionally, 49% of cement emissions are process related. Thus, decarbonizing cement is a challenge not easily addressed by renewable energy technologies.
© Jake Hoffman. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] "Mineral Commodity Summaries," U.S. Geological Survey, January 2021.
[2] E. Worrell et al., "Carbon Dioxide Emissions from the Global Cement Industry," Annu. Rev. Energy Env. 26, 303 (2001).
[3] Z. Cao et al.,"Estimating the In-Use Cement Stock in China: 1920-2013," Resour. Conserv. Recycl. 122, 21 (2017).
[4] M. S. Imbabi, C. Carrigan, and S. McKenna, "Trends and Developments in Green Cement and Concrete Technology," Int. J. Sustain. Built Environ. 1, 194 (2012).
[5] M. B. Ali, R. Saidur, and M. S. Hossain, "A Review on Emission Analysis in Cement Industries," Renew. Sustain. Energy Rev. 15, 2252 (2011).
[6] A. Chatterjee and T. Sui, "Alternative Fuels - Effects on Clinker Process and Properties," Cement Concrete Res. 123, 105777 (2019).
[7] A. Herold, "Comparison of CO2 Emission Factors for Fuels Used in Greenhouse Gas Inventories and Consequences for Monitoring and Reporting Under the EC Emissions Trading Scheme," European Topic Centre on Air and Climate Change, ETC/ACC Technical Paper 2003/10, July 2003.
[8] "BP Statistical Review of World Energy 2021," British Petroleum, June 2021.
