Pathways to Decarbonize Cement Production

Introduction

Fig. 1: Cement kiln in Poland. (Source: Wikimedia Commons)

Cement enables modern infrastructure but is emissions-intensive: industry roadmaps attribute ~7% of global CO₂ to cement. [1] It is also a particularly hard industry to decarbonize because, beyond the emissions from burning fuel, the calcination process (see Fig. 1) releases CO₂ from limestone as part of its reaction. [2] Due to these inherent process emissions, progress depends on multiple decarbonization levers, such as lowering clinker content and capturing CO₂, coordinated with the cleanliness of the power that supplies any electrified steps.

Challenges go beyond the unavoidable process CO₂. Clinkering at 1,450°C requires steady and intense heat; retrofits must protect uptime and quality. Reducing clinker with SCMs cuts CO₂ per tonne of cement, but availability, performance specs (especially sensitive in this industry), and codes limit speed and scope. Long-lived kiln assets slow capture integration, and logistics upgrades. Recently, many projects have also been trying to make process changes by changing the heat source. [1-5]

Heat Required

The useful heat to make clinker is fixed and depends on (1) the reaction enthalpy of calcination and (2) the sensible heat to warm the solids to clinkering temperature. CO₂ comes from (a) the limestone during calcination and (b) the energy used to supply heat. The total useful heat can be calculated through these two steps:

• Reaction Enthalpy (Calcination): For CaCO₃ → CaO + CO₂, ΔH = 172 kJ mole^-1. Dividing by the molar mass of CaCO₃ (0.100 kg) gives 1.72 MJ kg^-1 of CaCO₃.

• Sensible Heating: Using C_p =0.92 kJ kg^-1°C^-1 and a temperature rise of 1,420°C gives 1.31 MJ kg^-1.

Together, these yield 3.03 MJ kg^-1 (3.03 GJ t^-1) of heat needed. This requirement applies to any route; changing the source mostly changes energy form and integration, not the minimum useful heat.

Electrification vs. Alternative Fuels

Two practical paths are advancing in terms of making changes to the heat source. (1) Electrified calcination delivers the required heat without mixing calcination gases with combustion products. (2) High alternative-fuel (AF) firing raises the share of biomass- or waste-derived fuels (RDF, tires, solvents) to displace fossil fuels in fired calciners and kilns, with road-mapping noting drying/handling implications at higher AF shares. [6]

The emissions, however, ultimately hinge on the grid. Because the useful heat is fixed at ~3.03 GJ t^-1, electrification lowers emissions only if marginal electricity is low-carbon (or procured with high-quality, hourly-matched clean supply). Where grids are fossil-heavy, total CO₂ may not improve considering the intrinsic challenges of this industry.

The main differences between the two processes come from qualitative aspects related to them. Electrification can ease the capture of the process CO₂ but it requires substantial electrical interconnection capacity and exposes plants to grid risks. High-AF firing reduces fossil fuel use and can valorize wastes using existing thermal assets; it does not eliminate CO₂ for the heat and demands robust fuel logistics and air-quality controls. [1,6]

Conclusion

About 3.03 GJ t^-1 of useful heat must be supplied to make clinker, regardless of heat source. Decarbonization therefore turns on system choices: (i) lower the clinker factor with qualified SCMs to reduce process CO₂ per tonne, (ii) electrify calcination where power is verifiably low-carbon and the purer CO₂ stream improves capture readiness, and (iii) deploy capture to address irreducible process emissions. Although cement is a very complex industry to decarbonize, an integrated approach can help reduce the carbon intensity of the process.

© João Almeida. 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.

References

[1] "Technology Roadmap: Low-Carbon Transition in the Cement Industry," International Energy Agency, March 2018, p. 31.

[2] Cement Production," U.S. Environmental Protection Agency, July 2024.

[3] "Energy Efficiency Improvement and Cost Saving Opportunities for Cement Making," U.S. Environmental Protection Agency, August 2013.

[4] C. A. Hendriks et al., "Emission Reduction of Greenhouse Gases from the Cement Industry, International Energy Agency, August 2004.

[5] "State of the Art Cement Manufacturing," European Cement Research Academy, 2022.

[6] "LEILAC Technology Roadmap to 2050," Low Emission Intensity Lime and Cement Project, June 2021.