Travertine, an American cleantech company founded in 2022, develops integrated carbon removal technology that sequesters carbon dioxide (CO₂) through mineral carbonation while producing useful by-products like sulfuric acid, green hydrogen, phosphoric acid, and lithium. The company works with fertilizer producers to turn phosphogypsum waste into phosphoric acid, hydrogen, oxygen, and carbonate minerals. The company also partners with mining companies to produce carbon-negative sulfuric acid, which is used to extract critical elements such as lithium, nickel, and cobalt.
Challenges: carbon emissions and carbon removal
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.
Carbon removal technologies
Negative emissions technologies (NET) can help companies, sectors, or countries remove more CO₂ from the atmosphere than they emit. Examples of NETs include Direct Air Capture (DAC), enhanced weathering, and Ocean Alkalinity Enhancement. According to climate models, a significant deployment of NETs will be needed to prevent catastrophic ocean acidification and global warming beyond 1.5 ºC.
Enhanced weathering of gypsum (CaSO₄·2H₂O) has been proposed for large-scale permanent mineral carbonation sequestration. Gypsum feedstocks are abundant in natural evaporite deposits and industrial phosphoric acid production byproducts. There are an estimated 700Mt of natural gypsum reserves in the United States. The global fertilizer industry produces between 100 and 280Mt phosphogypsum (PG) powder annually.
However, the lack of a sufficiently large and cost-effective source of alkalinity has prevented the mineral carbonation of gypsum on a large scale.The dissolution of gypsum releases calcium ions into solution, but the production of carbonate minerals still requires an alkalinity source. For industrial acid and base production, electrolysis methods have been developed, but they consume too much energy to be cost-competitive with conventional methods.
Travertine develops carbon removal technology that integrates an electrolyzer stack with mineral carbonization and mineral leaching to sequester CO₂ while producing valuable byproducts, such as phosphoric acid, hydrogen (H₂), and lithium.
In the cathode chamber, the water electrolyzer produces alkaline solution and hydrogen, and in the anode chamber, acid solution and oxygen. The alkaline solution acts as absorbent to capture CO₂ by forming carbonate ions (CO₃²⁻). By reacting with carbonate ions, calcium sulfate (CaSO₄) is converted to CaCO₃ precipitation, which can be disposed of underground for carbon sequestration.
The sulfate ions (SO₄²⁻) in the cathode chamber traverse the anion exchange membrane (AEM) to reach the anode chamber, where they combine with protons to form sulfuric acid (H₂SO₄). Sulfuric acid can be used to leach fluorapatite or lithium claystone to produce valuable products of phosphoric acid for phosphate fertilizer or lithium for Li-ion batteries.
Travertine carbon removal system
The diagram depicts the Travertine carbon removal system, which sequesters CO₂ while producing sulfuric acid.
The system comprises an electrolyzer stack, a catholyte tank, an anolyte tank, a gas contactor, and a precipitator for mineral carbonation. The system can be powered by solar, wind, hydroelectric, geothermal, nuclear, or fusion power plants.
The electrolyzer stack consists of multiple electrolyzer cells. Each cell has a cathode, an anode, and an anion exchange membrane (AEM) that separates the electrodes and allows the transfer of anions from the cathode to the anode.
During operation, hydroxide ions (OH⁻) and hydrogen gas are generated in the cathode chamber according to the chemical reaction:
2H₂O + 2e⁻ → 2OH⁻ + H₂↑
The green hydrogen can be used for fuel cells.
In the anode chamber, protons (H⁺) and oxygen gas are produced via the chemical reaction:
H₂O → 2e⁻ + 2H⁺ + 1/2O₂↑
Travertine’s electrolyzer can generate a higher acid concentration in the anolyte than hydroxide concentration in the catholyte, even though protons and hydroxides are produced at the same rate. This is advantageous because the flux of sulfate ions (SO₄²⁻) across the anion exchange membrane is greater than that of hydroxide ions (OH⁻), minimizing Faradaic losses and maximizing energy efficiency.
This scenario is achieved by flowing the catholyte at the rate of 500 to 1,000 liters per minute through the cathode chamber, while flowing the anolyte at the rate of 10 to 300 liters per minute through the anolyte chamber. In addition, mineral carbonation consumes a significant amount of hydroxide ions.
The alkaline catholyte solution acts as absorbent to capture CO₂ in an air contactor, forming carbonate ions (CO₃²⁻). The solution containing these ions is then fed to the precipitator for mineral carbonation.
Solid gypsum (CaSO₄·2H₂O) is also introduced into the precipitation reactor, where it is dissolved and converted to calcium carbonate (CaCO₃) precipitates according to the chemical reactions:
Ca²⁺ + CO₃²⁻ → CaCO₃↓
Calcium carbonate precipitates are separated by filtration and disposed of underground for permanent carbon sequestration. The effluent from the precipitation reactor is recirculated through the cathode chamber of the electrolyzer, where SO₄²⁻ crosses the anion exchange membrane to accumulate sulfuric acid in the recirculating anolyte solution.
How Travertine technology produces phosphoric acid
The sulfuric acid produced by the system described above can be used to produce phosphoric acid by leaching rock phosphorus, such as Ca₅(PO₄)₃F (fluorapatite), via the following reaction:
Ca₅(PO₄)₃F + 5H₂SO₄ → 3H₃PO₄ + HF + 5CaSO₄
The CaSO₄ is fed to the precipitator for mineral carbonization, as depicted in the diagram below.
How Travertine technology produces lithium
The produced sulfuric acid can also be used to leach lithium claystone for carbon removal while extracting lithium elements. The process is depicted in the following diagram.
Sulfuric acid leaching process dissolves magnesium ions and lithium ions in the lithium claystone rock. The magnesium ions react with carbonate ions (CO₃²⁻) to precipitate magnesium carbonate (MgCO₃):
Mg²⁺ + CO₃²⁻ → MgCO₃↓
MgCO₃ is removed by filtration and disposed of underground for carbon sequestration. The effluent is processed to extract lithium. The lithium-free solution is recirculated through the cathode chamber of the electrolyzer, where SO₄²⁻ crosses the anion exchange membrane to accumulate sulfuric acid in the recirculating anolyte solution.
- US20230191322A1 Systems and methods for direct air carbon dioxide capture
Travertine is currently building a 1 kg/day pilot in its existing Boulder facility in 2022 and is working towards building a pilot to demonstrate about 1 ton-per-day CO₂ sequestration on a partner mine site, with operations beginning in early 2024.
Travertine has raised a total of $3M in funding over 2 rounds:
Their latest funding was raised on Apr 17, 2023 from a Grant round.
Travertine is funded by 3 investors:
Laura Lammers is Founder.
Laura Lammers is CEO.