Passive direct air capture using calcium oxide powder: The importance of water vapor

Calcium oxide (CaO; lime) looping is a proposed technology with the potential to capture gigatonnes of carbon dioxide (CO2) from the atmosphere to help mitigate climate change. The importance of water in carbonation reactions is widely understood as it is needed for mineral and CO2 dissolution and carbonate precipitation. However, the effects of water vapor on CaO carbonation pathways and rates have yet to be elucidated in a systematic manner. Here, we examine the impact of relative humidity (RH) on CO2 removal using CaO powder at 20% to 95% RH. Higher RH resulted in faster hydration rates, forming Ca(OH)2 (portlandite); however, passivation limited carbonation in all experiments, with the greatest carbonation occurring at 80% RH (65% CaCO3; calcite). Thus, CaO powder is highly prone to passivation when using RH to drive hydration and carbonation. In RH swing experiments, RH was changed at different times (hours or days) and by different amounts (e.g., 40–99%). Humidity swings can yield >85% CaCO3 when complete CaO hydration at a low RH (<40%) occurs before carbonation at a high RH (∼99%). Importantly, separating these two processes yields nearly complete carbonation. In this case, we achieved a CO2 removal rate of 1 t CO2 for every 1.95 t of Ca(OH)2 per day.