Among the technologies studied for DAC, the use of aqueous hydroxide sorbents is one of the most interesting approaches.
Example 1: Calcium oxide (CaO): Calcium oxide (quicklime) will absorb CO2 from atmospheric air mixed with steam at 400 °C (forming calcium carbonate) and release it at 1,000 °C. This process, proposed by A. Steinfeld, can be performed using renewable energy from thermal concentrated solar power. Quicklime is made by heating limestone to release the CO2 within it. Quicklime is mixed with sand for brick building as mortar, where it hardens by absorption of CO2.
Example 2: Sodium hydroxide (NaOH): Zeman and Lackner outlined a specific method of air capture using sodium hydroxide. Carbon Engineering, a Calgary, Alberta firm founded in 2009 and partly funded by Bill Gates, is developing a process to capture carbon dioxide using a solution of potassium hydroxide mixed with water at their pilot plant . They hope to create and sell synthetic fuels at a cost of $100 a ton.
Direct air capture (DAC) process example using NaOH
The chemistry of this process is explained in the “Sodium Hydroxide” section in the carbon dioxide scrubber page. In short, CO2 from the air is chemically dissolved into NaOH(aq) solution as Na2CO3. The Na2CO3 is then reacted with solid Ca(OH)2, which regenerates the solvent and produces CaCO3 crystals; lastly, heat is applied to the CaCO3 crystals to produce pure CO2 gas.
Air is pumped through the CO2 absorber as the first step of this process. CO2 absorbers for DAC are designed either as counter-current spray towers or as thin-falling-film contactors to maximize the contact area between the air and the solvent. The solvent is regenerated in the causticization unit by reacting the Na2CO3 with Ca(OH)2, which also converts the captured CO2 to CaCO3 in solid crystal form. A mechanical filter is then used to separate the CaCO3 crystals from the water. Since the crystals come out wet from the filter, they are dried in a steam dryer. Then the dry crystals are heated in a furnace to produce CaO and pure CO2 gas. The CaO is then hydrated to regenerate the Ca(OH)2 used for the causticization reaction. The pure CO2 stream is then compressed and ready to be transported for geologic sequestration, EOR, or other commercial applications.
1 M NaOH (aq) is a typical solvent concentration because this concentration is limited by the causticization reaction that regenerates the solvent. It is not too far from the practical maximum of 2 M NaOH. The furnace/kiln can be powered renewably or by burning fuel on-site with pure oxygen produced in an on-site air separation unit.
NaOH is economically competitive with other absorbents (e.g. amines) used for DAC processes. DAC processes are energy intensive. Calcination (at the furnace) is the most energy intensive step of this process.
A crucial issue for DAC methods is their cost, which varies substantially among the different technologies: some of these are not sufficiently developed to perform cost assessments. The American Physical Society estimates the costs for direct air capture to be $600/tonne with optimistic assumptions. The IEA Greenhouse Gas R&D Programme and Ecofys estimates that 3.5 billion tonnes could be removed annually from the atmosphere with BECCS (Bio-Energy with Carbon Capture and Storage) at carbon prices as low as €50 per tonne, while a report from Biorecro and the Global Carbon Capture and Storage Institute estimates costs “below €100” per tonne for large scale BECCS deployment.
Risks, problems and criticisms
Carbon Dioxide Removal (CDR) is slow to act, and requires a long-term political and engineering program to be effective. CDR is even slower to take effect on acidified oceans. In a Business as usual concentration pathway, the deep ocean will remain acidified for centuries, and as a consequence many marine species are in danger of extinction.