Carbon Negative Cement

The cement industry emits around 1.6 billion tonnes of carbon dioxide per year (5% of global emissions) to manufacture 3 billion tonnes of cement. The use of cement is ubiquitous in the construction industry, which is essential to economic prosperity. As such, production of cement is likely to rise to 4.5 billion tonnes per year by mid century, emitting even more greenhouse gas. This proposal suggests that it may be possible to not only reduce the emissions of the cement industry to zero but to create an industry that removes carbon dioxide from the atmosphere. To do this, a number of technical and policy innovations are required. The net benefit could be an increase in the value of the cement industry by 20 to 40% (equivalent to US$74 – 184 billion per year by 2050, depending on a carbon price between US$ 40 – 100 per tonne of CO₂). Currently, the embodied emissions of cement are created during pre-calcination processes (e.g. mining the raw materials, transport, comminution), chemical decomposition of the limestone, and the fossil fuel energy consumed in the kiln. Usually, life-cycle analysis of cement stops here. However, there is growing evidence that a large proportion of the carbon dioxide emitted during the chemical decomposition of limestone in the kiln is chemically recaptured following the demolition of the structure. Utilising post-use re-carbonation of cement together with biomass energy and carbon capture and storage, could result in a cement industry that draws around 400kg of carbon dioxide (NET) out of the atmosphere per tonne of clinker. Considering the whole-life cycle of cement in this way is potentially game changing for the industry.

Cement production and forecast

Global cement production has increased dramatically since the mid 20^th century from <500 Mt a^-1 in 1960 to over 3 Gt in 2011 (Figure 1). Predictions for future cement production lie between 3.7 and 5.5 Gt a^-1 by 2050 [1-2]. Cement is manufactured by chemically decomposing limestone in a furnace, creating CO₂ and a number of unstable calcium silicate minerals (‘clinker’ e.g. the formation of larnite). As this reaction is powered by fossil fuel, the cement industry has a substantial carbon footprint. Between 3.5 and 7.0 GJ are required, and 0.89 t of CO₂ is produced, for every tonne of clinker [3] (95% from the kiln).

Reducing emissions from cement production

Realising stringent emission reduction targets in the coming decades is an enormous challenge for the industry. Energy efficiency improvements will contribute, although there is a theoretical limit given the endothermic reaction. Current average global energy intensity of cement clinker is 4.2 GJ t^-1, which is predicted to decrease to a minimum of 3.2 GJ t^-1 by 2050 [1] (although best available technology achieves maximum energy efficiencies of 2.7 GJ t^-1 [3]). The most prominent options for emission reduction include lower net carbon intensive fuels (e.g. biomass), and the deployment of carbon capture and storage.

Currently, biomass use in the cement industry is thought to be around 3%, but has been predicted to rise to 16% by 2050. [1] Based on confined assumptions relating to human appropriation of biomass, Powell and Lenton [5] estimate approximately 2.4 to 6.0 GtC yr^-1 of 'waste' biomass may be available for energy generation, which corresponds to a lower heating value of 77-186 EJ yr^-1 . The total energy requirement for cement production in 2050 may be between 12 and 18 EJ per year. Therefore, it is notionally possible that the energy demand for cement can be wholly met through biomass combustion without sacrificing food production. Currently, the infrastructure to collect and distribute this biomass is underdeveloped.

Wide scale deployment of flue gas CO₂ capture and storage is predicted to sequester almost 60% of global cement emissions by 2050 [2]. Nominally, this involves the separation and compression of CO₂ from the other flue gases for geological storage. Other sequestration methods may be possible (e.g. mineral carbonation [8-9], ocean alkalinity storage [6-7]). The flue gas in a kiln is typically composed of 50 to 80% by volume CO₂, which could be purified through pre-combustion air separation technologies or post-combustion gas separation.

Recarbonising cement

There is growing evidence that cement minerals readily transform back into carbonate minerals when exposed to appropriate conditions. This has been demonstrated in controlled reactors [10-11] and discovered in landfill [12-13] at elevated CO₂ partial pressure. However, it is also common, albeit unintentional, in ambient environments receiving demolition waste (urban soils [14][15])

The maximum carbon storage potential of this is equivalent to the emissions released from chemical decomposition in the kiln (0.49 tCO₂ per tonne of clinker). Concrete is one of largest waste streams globally, and is likely to grow in the coming decades as the urban environments constructed in the second half of the 20^th century renew and develop. The average life span of a building is ~50 years [16], which represents a lag time between production and re-carbonation.

Carbon Balance

For every tonne of clinker produced approximately 1.2t of limestone is required (creating 0.543 tCO₂ during chemical decomposition in the kiln). To produce 3.2 GJ of energy per tonne of clinker, approximately 0.21 tonnes of biomass are required which creates (0.352 tCO₂). It is assumed that the 0.895 tCO₂ produced in the kiln represents 95% of the emissions associated with cement production (0.045 tCO₂ are produced from raw material preparation). 80% of the CO₂ produced in the kiln is captured, the rest is assumed fugitive. The purification of the flue gas is achieved through oxy-fuel firing in the kiln, together with CO₂ compression for sequestration adds an additional 580 MJ of electrical energy onto the existing 360 MJ per tonne of clinker (creating 0.235 tCO₂). Finally, post-use recarbonation of cement is assumed to recapture 95% of the CO₂ lost during chemical decomposition (0.516 tCO₂). This represents a net removal of 0.409 tCO₂ per tonne of clinker created.

The need for CO₂ removal from the atmosphere

Once emitted carbon dioxide remains in the atmosphere, and is only removed slowly by natural processes [17]. Methods to remove CO₂ from the atmosphere are being investigated to accelerate natural removal should a dangerous concentration be surpassed. Indeed, the only emission scenario that is likely to result in less than a 2°C global mean surface temperature increase implicitly requires some form of removal towards the end of this century [18]. This scenario requires emissions to peak between 2020 and 2030 with a substantial reduction thereafter. As this is unlikely, a greater amount of CO₂ removal from the atmosphere will be required. The proposed modification to the cement life cycle reduces the emissions of the industry and offers two opportunities for CO₂ removal from the atmosphere (during biomass growth and post-use cement carbonation).

References

1.  IEA, Cement Technology Roadmap 2009. International Energy Agency.

2.  Lafarge-WWF, A blueprint for a climate friendly cement industry. 2008.

3.  van Oss, H.G. and A.C. Padovani, Cement Manufacture and the Environment: Part I: Chemistry and Technology. Journal of Industrial Ecology, 2002. 6(1): p. 89-105.

4.  USGS, USGS Minerals Yearbook. 2010, U.S. Department of the Interior and U.S. Geological Survey.

5.  Powell, T.W.R. and T.M. Lenton, Future carbon dioxide removal via biomass energy constrained by agricultural efficiency and dietary trends. Energy & Environmental Science, 2012. 5(8): p. 8116-8133.

6.  Rau, G.H., Electrochemical Splitting of Calcium Carbonate to Increase Solution Alkalinity: Implications for Mitigation of Carbon Dioxide and Ocean Acidity. Environmental Science & Technology, 2008. 42(23): p. 8935-8940.

7.  Rau, G.H., CO2 Mitigation via Capture and Chemical Conversion in Seawater. Environmental Science & Technology, 2011. 45(3): p. 1088-1092.

8.  Lackner, K.S., et al., Carbon dioxide disposal in carbonate minerals. Energy, 1995. 20(11): p. 1153-1170.

9.  O’Connor, W.K., et al., Aqueous mineral carbonation, Final Report - DOE/ARC-TR-04-002. 2005.

10.       Fernández Bertos, M., et al., A review of accelerated carbonation technology in the treatment of cement-based materials and sequestration of CO2. Journal of Hazardous Materials, 2004. 112(3): p. 193-205.

11.       Huntzinger, D.N., et al., Carbon dioxide sequestration in cement kiln dust through mineral carbonation. Environmental Science & Technology, 2009. 43(6): p. 1986-1992.

12.       Manning, D.A.C., Calcite precipitation in landfills: an essential product of waste stabilization. Mineralogical Magazine, 2001. 65(5): p. 603-610.

13.       Fleming, I.R., R.K. Rowe, and D.R. Cullimore, Field observations of clogging in a landfill leachate collection system. Canadian Geotechnical Journal, 1999. 36(4): p. 685-707.

14.       Renforth, P., D.A.C. Manning, and E. Lopez-Capel, Carbonate precipitation in artificial soils as a sink for atmospheric carbon dioxide. Applied Geochemistry, 2009. 24(9): p. 1757-1764.

15.       Washbourne, C.L., P. Renforth, and D.A.C. Manning, Investigating carbonate formation in urban soils as a method for capture and storage of atmospheric carbon. Science of The Total Environment, 2012. 431(0): p. 166-175.

16.       Adalberth, K., Energy use during the life cycle of buildings: a method. Building and Environment, 1997. 32(4): p. 317-320.

17.       Lowe, J.A., et al., How difficult is it to recover from dangerous levels of global warming? Environmental Research Letters, 2009. 4(1): p. 014012.

18.       Vuuren, D., et al., RCP2.6: exploring the possibility to keep global mean temperature increase below 2°C. Climatic Change, 2011. 109(1-2): p. 95-116.