Capture6, an American cleantech company founded in 2021, has developed integrated Direct Air Capture (DAC) systems that uses electrochemical method to convert seawater or brine into base and acid solution to capture atmospheric CO₂ while producing valuable byproducts such as clean water, calcium carbonate (CaCO₃) for concrete, and lithium salts for lithium-ion batteries. The company has announced plans to construct a pilot facility in California with Palmdale Water District to test its DAC technology.
Challenges: carbon emissions and direct air capture
Since the early 1900s, carbon dioxide (CO₂) levels in the atmosphere have increased by 50% due to human activities. When fossil fuels (such as coal, oil, and natural gas) are burned for energy production, transportation, and industrial processes, CO₂ is released into the atmosphere. This excess CO₂ acts as a greenhouse gas, trapping heat and causing the air and ocean temperatures to rise. CO₂ emissions play a crucial role in driving climate change.
This warming effect has caused the global average temperature to rise by about 1.1 ºC since the pre-industrial period. This has led to rising in the frequency and intensity of extreme weather events, melting of polar ice caps and glaciers and rising sea levels, shifts in species ranges and increased risk of species extinction, agriculture and food security, and ocean acidification.
To mitigate these impacts, the Paris Agreement aims to limit global warming to well below 2 ºC above pre-industrial levels. The Intergovernmental Panel on Climate Change (IPCC) estimates that a “carbon budget” of about 500 GtCO₂, which corresponds to about ten years at current emission rates, provides a 66% chance of limiting global warming to 1.5 ºC.
Direct air capture
Direct Air Capture (DAC) is a process that extracts diluted carbon dioxide (CO₂) directly from the atmosphere, as opposed to industrial emissions with a high CO₂ content. The captured CO₂ can then be either utilized in various industrial applications or buried to prevent its release back into the atmosphere.
The basic principle of DAC involves using large-scale machines or facilities equipped with specialized filters or sorbents that selectively bind with CO₂ from the air. After the CO₂ is captured, it is separated from the sorbent through a regeneration process, resulting in the release of CO₂.
DAC technology employs a variety of methods, but chemical sorbents or solvents are typically used to capture CO₂. These sorbents can chemically react with CO₂ to form solid compounds or dissolve the CO₂ in a solvent. The captured CO₂ is then released from the sorbent or solvent via heating or other processes, allowing for its storage or utilization.
As carbon dioxide removal from ambient air is an energy-intensive process, DAC technology is more expensive per ton of CO₂ removed than many mitigation strategies and natural climate solutions. Today, the price range for DAC ranges between $250 and $600/mtCO₂e (million tons of CO₂ equivalent). If further industrialization is accomplished within the ecosystem of this emerging industry, prices may fall to between $100 and $200/mtCO₂e.
Capture6 has developed integrated Direct Air Capture (DAC) systems for carbon removal, with the valuable byproducts of clean water, calcium carbonate (CaCO₃) for concrete, and lithium salts for lithium-ion batteries.
The systems use well-established technology of electrodialysis to convert seawater or brine into sodium hydroxide (NaOH) and hydrochloride (HCl) solutions. The NaOH base solution is used as an absorbent to capture atmospheric CO₂ and generate CaCO₃ and/or as precipitant for lithium hydroxide (LiOH). The acid solution of HCl can be used as a leaching solvent for olivine processing to dissolve mineral metal ions, which are precipitated as mineral carbonates by reacting with atmospheric CO₂. The mineral carbonates are disposed of underground to permanently remove carbon.
Capture6’s carbon removal systems use the established water purification technology of bipolar electrodialysis, which can be powered by renewable electricity from solar panels and wind turbines.
The diagram below depicts the structure of a bipolar electrodialysis system which uses saline water input to produce acidic and basic solutions.
The electrodialysis stack comprises a series of salt, acid and base chambers that are respectively separated by ion-permeable membranes.
Each salt chamber is separated from each adjacent acid chamber and base chamber by an anion-exchange membrane and a cation-exchange membrane, respectively. The adjacent acid and base chambers are separated by a bipolar membrane. The electrodialysis stack also includes electrodes that apply an electric field across the salt, acid and base chambers and cause ions to pass through the ion-exchange membranes.
During operation, the saline water stream flows through the salt chambers, while the acid and base solution flow through the acid chambers and a base stream from the base buffering tank through the base chamber.
The diagram below illustrates how the electrodialysis generates acid and base solutions.
The applied electric field causes chloride ions (Cl⁻) to pass from the salt chambers through the anion-exchange membranes into the acid chambers, and also causes sodium ions (Na⁺) to pass from the salt chambers through the cation-exchange membranes into the base chambers. In the bipolar membrane that separates adjacent acid and base chamber, water is dissociated into protons (H⁺) and hydroxide ions (OH⁻). H⁺ transfers into the acid chamber, where it combines with Cl⁻ to form acid (HCl), while OH⁻ transfers to the base chamber, where it combines with Na⁺ to form base (NaOH).
Consequently, the acid and base streams leaving the electrodialysis stack have a higher concentration of acid and base substance, respectively, than the acid and base streams entering the stack. Therefore, the properties of concentration and pH change as each acid/base fluid stream passes through the electrodialysis stack.
Cacture6 carbon removal systems
In the following video, Capture6 has presented three carbon removal systems.
The carbon removal system produces CaCO₃
The picture below shows the carbon removal system that uses seawater input to capture atmospheric CO₂ and disposes of mineral carbonates such as CaCO₃ underground for permanent carbon sequestration.
The following diagram depicts how this system works.
Before entering the electrodialysis system, seawater is pretreated in a nano-filtration unit and ion-exchange unit. Divalent ions such as calcium (Ca²⁺) and magnesium (Mg²⁺) are removed by the pretreatment. The divalent-ion free salt water flows into the electrodialysis system, which produces streams of NaOH and HCl solutions as described previously.
The NaOH stream is then directed through an air contactor, such as a cooling tower, where it absorbs CO₂ from the air to produce sodium carbonates (Na₂CO₃). The Na₂CO₃ stream is then pumped to a precipitator, where it is combined with the divalent ion stream from the nano-filtration and ion exchange units to produce CaCO₃. The CaCO₃ solution is then separated into a slurry of CaCO₃ solids, which are dried and disposed of underground for permanent carbon removal.
The HCl stream is stored and can be sold. The dilute sodium chloride solution is returned to the sea.
The carbon removal system produces clean water, CaCO₃, and mineral carbonates
The picture below presents another carbon removal system that uses seawater input to capture atmospheric CO₂ while producing valuable by-products such as clean water and CaCO₃. The acid solution of HCl produced by electrodialysis is used to leach olivine to dissolve mineral metal ions, which can be used as an absorbent to capture atmospheric CO₂ by precipitating mineral metal carbonates and disposing of them underground for carbon removal.
The following diagram depicts how this system works.
Seawater is pumped to the desalination facility, where it is processed by a reverse osmosis unit to produce two streams: the low TDS water and ion-concentrated water. Low TDS water is returned back for residential and industrial use because it contains fewer dissolved salts. The ion-concentrated water is then fed to a nano-filtration unit and an ion exchange unit to remove divalent ions. The divalent ion-free salt water is sent to an electrodialysis system, which produces streams of NaOH and HCl solutions.
The NaOH stream is then passed through an air contactor to absorb atmospheric CO₂ and produce Na₂CO₃ stream. The Na₂CO₃ stream is then pumped to a precipitator where it is mixed with the divalent ion stream from the nano-filtration and the ion exchange units to form CaCO₃ as described above. The CaCO₃ solution is then separated into a slurry of CaCO₃ solids which are dried and can be used as concrete raw material.
The HCl stream is used to leach olivine to dissolve mineral metal ions. The leaching solution serves as an absorbent to capture atmospheric CO₂, resulting in the precipitation of mineral metal carbonates. These mineral metal carbonates are dried and disposed of underground for permanent carbon removal.
Dilute NaCl stream is sent back to the desalination facility, where it is processed with a secondary reverse osmosis unit to produce low TDS water for residential and industrial use.
The carbon removal system produces clean water, CaCO₃, lithium salt, and mineral carbonates
The picture below presents the third carbon removal system that uses brine input to capture atmospheric CO₂ while producing valuable by-products such as clean water, CaCO₃, and LiOH. The acid solution of HCl is used to leach olivine. The leaching solution is used as absorbent to capture atmospheric CO₂ as described above.
The following diagram depicts how this system works.
The brine is passed through filtration and concentration systems for pretreatment. The treated liquid is fed to the electrodialysis system to produce base and acid streams. In this system, the electrodialysis system produces a strong base solution for CO₂ capture and lithium extraction.
The strong base stream is divided into two streams. One is sent to a precipitator for lithium hydroxide extraction. Another stream is used to absorb CO₂ from the air. The electrodialysis can increase the alkalinity of base solution, which facilitates the conversion of bicarbonates (HCO₃⁻), carbonic acid, and trapped CO₂ into carbonate ions (CO₃²⁻). Therefore, as pH increases, CaCO₃ precipitates more and more.
The acid stream can be used as a leaching solvent to leach olivine for capturing atmospheric CO₂ and precipitating mineral metal carbonates which are disposed of underground for carbon removal as described above.
- US20230191322A1 Systems and methods for direct air carbon dioxide capture
Capture6 Monarch project
Capture6’s Project Monarch is a joint pilot facility in California between Palmdale Water District (PWD) that aims to produce freshwater resources and simultaneously capture atmospheric CO₂. The project integrates Capture6’s Direct Air Capture (DAC) process with PWD’s water management technology. Project Monarch aims to achieve zero brine discharge and save PWD 20% to 40% on the lifetime costs of their new water management facility.
Capture6 has raised a total of $8M in funding over 2 rounds:
Their latest funding was raised on Jun 21, 2023 from a Non-equity Assistance round.
Capture6 is funded by Carbon to Value Initiative (C2V Initiative).
Ethan Cohen-Cole is CEO.