Limenet, an Italian cleantech company founded in 2023, has developed an innovative CO₂ removal technology. This method uses limestones to capture atmospheric CO₂ and permanently sequester it in the form of bicarbonates (HCO₃⁻) in seawater. By accelerating the natural geological carbon cycle, Limenet's technology enables effective CO₂ sequestration.
(This article contains 6 diagrams and 1617 words.)
Challenges: carbon emissions
Since the early 1900s, atmospheric carbon dioxide (CO₂) levels have surged by 50% due to human activities, primarily the combustion of fossil fuels for energy production, transportation, and industrial processes. This excess CO₂ acts as a greenhouse gas, trapping heat and causing air and ocean temperatures to rise. The resulting warming effect has elevated the global average temperature by approximately 1.1 ºC since the pre-industrial period.
This increase of the global average temperature has led to a cascade of environmental impacts, including more frequent and intense extreme weather events, melting polar ice caps and glaciers, rising sea levels, shifts in species ranges, increased risk of species extinction, challenges to agriculture and food security, and ocean acidification.
To address these pressing issues, the Paris Agreement aims to limit global warming to well below 2 ºC above pre-industrial levels.
The Intergovernmental Panel on Climate Change (IPCC) has calculated a critical "carbon budget" to address global warming. This budget, estimated at roughly 500 gigatons (GT) of CO₂, represents the maximum amount of CO₂ that can be emitted while maintaining a 66% probability of limiting global temperature rise to 1.5 ºC above pre-industrial levels.
At current emission rates, this budget would be exhausted in approximately a decade, highlighting the pressing need for swift and significant reductions in CO₂ emissions. This stark timeline emphasizes the urgency of implementing immediate and comprehensive measures to curb greenhouse gas emissions across all sectors of the global economy.
Limenet Technology
The ocean, covering over 70% of Earth's surface, stores 148,000 Gt CO₂ as dissolved ions (mainly HCO₃⁻) in seawater and solid precipitates (CaCO₃ and MgCO₃) in sediments. The ocean plays a crucial role in regulating atmospheric CO₂ levels, but this process occurs over vastly different timescales, as depicted in the diagram below.
Atmospheric CO₂ quickly equilibrates with the surface ocean, typically within a year. This initial step is relatively fast, but it's just the beginning of a much longer process. Once dissolved in the surface waters, CO₂ begins a gradual journey to the deep ocean. This mixing process takes approximately 300 years, highlighting the ocean's immense capacity as a carbon reservoir. During this descent, a series of chemical reactions occur:
CO₂ + H₂O ⇆ H₂CO₃
H₂CO₃ ⇆ H⁺ + HCO₃⁻
HCO₃⁻ ⇆ H⁺ + CO₃²⁻
As CO₂ dissolves, it increases ocean acidity. However, nature has a built-in buffering mechanism. Over about 6,000 years, carbonate minerals slowly dissolve, partially neutralizing this acidity and allowing the ocean to absorb even more CO₂ from the atmosphere. This process involves the following reactions:
CaCO₃ → CO₃²⁻ + Ca²⁺
MgCO₃ → CO₃²⁻ + Mg²⁺
CO₃²⁻ + H⁺ → HCO₃⁻
CO₂ + H₂O → H₂CO₃
H₂CO₃ → H⁺ + HCO₃⁻
On an even longer timescale—approximately 100,000 years—enhanced weathering of silicate rocks provides the necessary cations to permanently sequester anthropogenic CO₂ as carbonate sediments. This ultimate carbon sink involves the following key reactions:
HCO₃⁻ ⇆ H⁺ + CO₃²⁻
CO₃²⁻ + Ca²⁺ → CaCO₃
CO₃²⁻ + Mg²⁺ → MgCO₃
While the ocean is a powerful carbon sink, its natural processes of carbon capture and sequestration operate on geological timescales. This underscores the importance of reducing anthropogenic CO₂ emissions, as the Earth's natural systems cannot quickly compensate for rapid increases in atmospheric CO₂.
Limenet has developed an innovative carbonate-dissolution method of ocean CO₂ sequestration that uses limestone (CaCO₃) and accelerates the natural geological carbon cycle, as depicted in the diagram below.
The process begins with the calcination and hydration of limestone (CaCO₃), which produces CO₂ and calcium hydroxide (Ca(OH)₂). The generated CO₂ is then mixed with seawater to create a carbonic acid solution.
Ca(OH)₂ serves two crucial purposes in this process. A portion of it is utilized as a CO₂ adsorbent in Direct Air Capture (DAC), effectively regenerating limestone for the cyclic process. The remaining Ca(OH)₂ is used to neutralize the carbonic acid solution, resulting in the formation of soluble calcium bicarbonate (Ca(HCO₃)₂). The relatively harmless solution of Ca²⁺ and HCO₃⁻ in seawater is released back into the ocean, where it is diluted by additional seawater. Therefore, atmospheric CO₂ is effectively captured and sequestered in the oceans, primarily in the form of HCO₃⁻.
How Limenet captures and sequesters CO₂ in seawater
The diagram below depicts Limenet's carbon capture and sequestration processes, which uses limestone and seawater to capture and sequester atmospheric CO₂ in the oceans, largely as HCO₃⁻.