Direct Air Capture of Carbon Dioxide via Ambient Alkaline Oxide Looping

Carbon dioxide removal (CDR) - that is, the removal of billions of tons of CO2 from the atmosphere every year by mid-century - has become critical to avoid dangerous effects of climate change. Direct air capture (DAC) describes a suite of technologies that remove CO2 directly from the air and produce a concentrated stream of CO2 suitable for chemical utilization or geological storage. Existing methods for DAC rely on the use of liquid or solid sorbents, such as potassium hydroxide (KOH) solution and amine-grafted mesoporous silica. Additionally, these methods require large, engineered structures to bring massive amounts of air into contact with the capture agents, as well as multiple variable-temperature reactor units to enable the controlled handling and regeneration of the sorbent. Here, we detail a novel DAC approach that uses an alkaline oxide feedstock, such as magnesium oxide (MgO) and calcium oxide (CaO), to repeatedly capture CO2 from the atmosphere. In this process, magnesium carbonate (MgCO3) or calcium carbonate (CaCO3) is calcined at 600 ºC (MgCO3) up to 1,000 ºC (CaCO3). The calcination process produces magnesium oxide or calcium oxide and high-purity CO2. The oxide is spread over land to carbonate by reacting with atmospheric CO2. The carbonate minerals are then recollected and re-calcined. The reproduced oxide is then spread over land to carbonate again. In this presentation, we provide a preliminary techno-economic (TEA) analysis of the ambient oxide looping DAC process at scale, as well as recent experimental results quantifying the effect of ambient, outdoor conditions on CO2 uptake by oxide materials. The TEA indicates that this process is cost competitive with other proposed DAC methods, with costs ranging from $43 - $159 per metric ton of CO2 net removed from the atmosphere. Current solid sorbent DAC processes show costs ranging from $500 - $600 per tCO2 for solid sorbent DAC and are projected at $168 - $232 per tCO2 for liquid solvent DAC. Further verification of the ambient oxide looping process involves repeated cycling via carbonation-calcination experiments to quantify cyclic changes in CO2 uptake and the lifetime of the alkaline oxide feedstock.