Is solid calcium looping a scalable technology for mega-ton carbon dioxide removal?

[Geoengineering News]

https://www.sciencedirect.com/science/article/pii/S2212982025002392

Authors: M.M. Paulsen, S.G.R. Nielsen, F.J. Tilsted, T.H. Pedersen

28 October 2025

https://doi.org/10.1016/j.jcou.2025.103255

Highlights

•Solid calcium looping demonstrates potential for Mton CDR.

•Viability of solid calcium looping relies on successful process management.

•Relative humidity 70% enhances carbonation kinetics.

•Stoichiometric slaking conditions enhances carbonation kinetics.

•Stable sorbent performance observed over multiple cycles.

Abstract

Atmospheric CO2 removal at Gton scale is necessary to limit global temperature rise. This study provides essential insights into the energy, area, and water requirements of solid calcium looping for large-scale CO2 removal, combining experimental results with an upscaled system context.

Thermogravimetric analysis (TGA) characterizes the products, while scanning electron microscopy (SEM) qualitatively assesses sorbent performance. Experiments in a climate chamber focus on carbonation kinetics, varying relative humidity, temperature, and slaking ratios.

Results demonstrate that relative humidity above 70 % significantly enhances carbonation rates, favoring the placement of facilities in high-humidity locations. The optimal slaking ratio is identified as one mole of water per mole of CaO. Carbonation at 20 °C yields faster reaction kinetics than at 12 °C, with both temperatures achieving a carbonation degree of 66 mol.% in three and four days, respectively. Lower temperatures require a larger area to achieve similar annual capture rates. Cyclic testing shows stable capture capacity and carbonation kinetics over nine cycles.

For a one-million-ton-per-year capture unit, the process requires 4.9–6.2 GJ/ton CO2 for electric heating, with air contactor areas reduced to 0.08–0.12 km2 using vertically stacked sorbent trays. Water consumption reaches 0.7 tonH2O/tonCO2 at the optimal slaking ratio but can be minimized in high-humidity, water-scarce locations by omitting slaking, at the expense of larger area requirements.

While only northern Europe currently meets the conditions for implementing the technology, considering both electricity grid CO2 intensity and weather, the potential of solid calcium looping for large-scale CO2 removal remains promising, warranting further exploration of logistics and practicalities in real-world applications.

Source: ScienceDirect