Direct capture of carbon dioxide from air via lime-based sorbents

Direct air capture (DAC) is a developing technology for removing carbon dioxide (CO₂) from the atmosphere or from low-CO₂-containing sources. In principle, it could be used to remove sufficient CO₂ from the atmosphere to compensate for hard-to-decarbonize sectors, such as aviation, or even for polishing gas streams containing relatively low CO₂ concentrations. In this paper, the performance of lime-based sorbents for CO₂ capture from air in a fixed bed was investigated. The effects of sorbent type, particle diameter, air flow rate, and relative humidity on the breakthrough time, breakthrough shape, and global reaction rate over a series of capture and regeneration cycles were examined. The greatest reaction rates and conversions were obtained when the sorbents were pre-hydrated and inlet air was humidified to 55% relative humidity. Humidifying the air alone leads to axial carbonation gradients since there is competition between CO₂ and water with the available CaO. Negligible conversion, over the duration of the experiment, is obtained in a dry system without pre-hydration and humid air. A shrinking-core gas–solid reaction model was fitted to the breakthrough curves in order to estimate the surface reaction and effective diffusion constants. Although the surface reaction constants of the two sorbents were similar, the pelletized limestone had a greater effective diffusivity due to its greater porosity. At mild calcination conditions with air at 850 °C, the pelletized particles maintained their activity over nine carbonation–calcination cycles with a conversion drop of only 9% points. However, calcination under oxy-fuel conditions (CO₂ at 920 °C) reduced the pellet carbonation conversion from 81 to 59% and pore surface area from 12.01 to 3.20 m²/g after only 4 cycles. This research clearly shows that DAC using lime-based sorbents is technically feasible, and that regeneration schemes compatible with technologies like calcium looping (CaL) are applicable for the air capture option. Finally, this study demonstrates that DAC using lime-based materials can be in the future a strategy to address emissions from transportation and distributed CO₂ sources and to mitigate climate change.