INTRODUCTION

Limiting global warming to 1.5 °C above pre-industrial levels is one of the greatest challenges humanity faces in the 21^st century. Achieving this goal requires capturing CO₂ emissions not only from large point sources but also from dispersed and hard-to-abate sectors. Traditional CO₂ capture systems have been optimized for large-scale emitters, such as power and cement plants and steel industries. Therefore, a completely new approach is needed.

Direct Air Capture (DAC) emerges as a complementary technology, enabling the removal of CO₂ directly from the atmosphere by using a solvent or sorbent that binds with CO₂ in an air contactor. Then, a CO₂-concentrated gas stream is produced during the regeneration step. DAC can deliver negative emissions when combined with permanent CO₂ storage, or contribute towards carbon neutrality if the captured CO₂ is reused, for instance, in synthetic fuel production. As such, it is considered a promising option for offsetting past emissions and those generated by sectors that are difficult to decarbonize. DAC systems offer additional advantages, as they can be deployed without disrupting current systems and almost anywhere independently of emission sources, which allows them to operate in affordable locations with abundant renewable energy.

Despite its benefits, DAC faces challenges that hinder large-scale deployment stemming from the low concentration of CO₂ in ambient air—around 500 ppm. This results in high process energy requirements and capital costs for large air contactors that are needed to process vast volumes of air to reach relevant scales (i.e., capturing MtCO₂ per year), thus leading to higher costs per tCO₂ captured compared to conventional methods targeting major emitters. It is therefore essential to ensure that air contactors and functional materials are of low specific cost in order to design competitive DAC systems at large scales.

In this context, the use of solid lime-based materials such as Ca(OH)₂ is regarded as a promising solution to overcome the costly barriers associated to the capture device. Over the last few years, the CO₂ Capture Group at INCAR-CSIC (CapCO₂) has focused on the development of Ca(OH)₂-based DAC systems, leveraging its extensive experience in CO₂ capture using Ca-based sorbents at high temperatures in several Calcium Looping configurations for a range of industrial applications. Carbonation of CaO or Ca(OH)₂ sorbents with CO₂ is at the core of these systems, followed by sorbent regeneration via oxy-calcination to produce a nearly pure CO₂ stream.

This work provides a summary and update of the research conducted by CapCO₂ in the field of DAC by testing Ca-based sorbents under different configurations and operating conditions, with the aim of scaling up the proposed technology.

METHODS

Several gas–solid contact configurations were evaluated to identify optimal Ca-based materials and air contactor modes that maximize CO₂ capture rates and overall efficiency. Particle-scale experiments were conducted to determine reaction kinetics, maximum conversion and the influence of key parameters, such as air humidity and effective solid porosity, on material carbonation under ambient conditions.

Subsequent scale-up of the Ca-based air contactor was achieved at TRL3. To this end, Ca-based powders, dry mortars and other Ca-based forms were arranged to validate the gas–solid contact modes proposed.

These experimental setups enabled the evaluation of a broad range of operating conditions, including effective porosities between 0.20 and 0.55, gas velocities from 0.5 to 5.5 m/s, air humidity ranging from 0 to 90% RH and gas–solid contact lengths spanning from a few centimeters in fixed-bed configurations to up to 10 m in single-channel arrangements.

RESULTS

Tests conducted at both particle scale and TRL3 have served as a proof of concept for Ca-based air contactors designed to maximize CO₂ capture from ambient air under extended operating periods and variable conditions. More than 3,000 operating hours under CO₂ capture have been accumulated at TRL3 so far, with individual experiments lasting up to 300 hours in capture mode and demonstrating sustained average CO₂ capture efficiencies of approximately 55%, at least 20% greater than those observed in batch mode under similar conditions.

Particle-scale reaction modeling indicates that the carbonation kinetics are primarily governed by CO₂ diffusion through the carbonated product layer. The measured kinetics were subsequently incorporated into a simplified one-dimensional reactor model, which was validated against TRL3 experimental results for both batch and counter-current contact modes, thereby providing a robust basis for technology scale-up.

CONCLUSIONS

The results presented here indicate that Ca-based sorbents constitute a viable alternative for DAC under standard conditions. The CapCO₂ Group is actively advancing research in this area and will present its latest developments.

ACKNOWLEDGEMENTS

This work is part of the project PID2024-162594OB-I00 funded by MICIU/AEI/10.13039/501100011033 and by ERDF/UE.