Capture6 ($6M Seed funding to develop integrated decarbonization technology for capturing CO2 and producing low carbon products)

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

Carbon emissions

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 Technology

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.

Electrodialysis

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.

Bipolar electrodialysis system uses salty water to generate acid and base solutions.
Bipolar electrodialysis system uses salty water to generate acid and base 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 working mechanism of a bipolar electrodialysis system.
The working mechanism of a bipolar electrodialysis system.

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

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