Carbon sequestration

Carbon sequestration is the capture, sequestration, and removal of carbon dioxide from the atmosphere to mitigate atmospheric CO2 pollution. The process is heavily studied as a way to slow, stop, or reverse global warming. Atmospheric carbon dioxide is naturally collected and converted through biological and chemical processes in plants, but is also capable of occurring through artificial processes. Artificially or naturally improving carbon sequestration is done in many ways, including storing CO2 in carbon sinks such as subsurface saline aquifers, ocean water, or aging oil fields.

Carbon sequestration methods fall into three categories: oxy-combustion, pre-combustion capture, and post-combustion capture. The majority of methods involve capturing carbon emissions from polluters and transforming them into a solid state or moving them to another medium which prevents them from leaking into the environment.

Schematic showing both terrestrial and geological sequestration of carbon dioxide emissions from a coal-fired plant.

Continuous research aimed at the discovery of catalysts and cost-cutting processes to make carbon sequestration more viable is being carried out. Caltech and USC scientists have discovered that the addition of a common enzyme to the process accelerates a delaying part of the chemical reaction and allows for carbon dioxide to be safely sequestered into the ocean 500 times over the normal rate.

There are three categories of carbon sequestration: biosequestration, geosequestration, and technological sequestration. The illustration below shows “open” (e.g. forests), “closed” (e.g. building materials), and “cycling” (e.g. CO2-based fuels) utilization pathways, signified by purple, red and yellow arrows respectively.

The utilization of CO2 does not guarantee climate benefits and may not always contribute to mitigation. Potential inhibiting factors include other greenhouse gas emissions, land-use alteration, leakage, and only temporary displacement.

Biosequestration is a subset of carbon sequestration, focused on the capture of atmospheric greenhouse carbon dioxide gasses by enhanced continual biological processes. This form usually occurs through increased rates of photosynthesis, often created through land-use practices such as reforestation, sustainable forest management and genetic engineering.

Also known as terrestrial sequestration, this process entails the removal of CO2 from Earth’s atmosphere by vegetation (such as grasslands, forests, crop lands, pastures, or meadows) and microorganisms and its subsequent storage in plants, soil, and oceans.

The EPA (Environmental Protection Agency) estimated that in 2009, 15.3% of the total US greenhouse gas emissions were partly offset by carbon sequestration in forests, food waste, landfilled grass clippings, agricultural soils, and trees in urban areas.

Geosequestration (or geological sequestration) is the process of separating and capturing carbon dioxide (CO2) and injecting it into suitable geological formations where it can be stored safely for hundreds and even thousands of years. This process is also referred to as carbon capture and storage (CCS), or carbon capture and geological storage (CCGS). There is no universal agreement regarding nomenclature.

In geosequestration, carbon dioxide must first be separated from methane before being captured and transported to an appropriate storage location. Some industrial operations, such as natural-gas processing and fertilizer manufacturing, emit relatively pure CO2 and cost less to capture and separate. Geosequestration is normally a costlier and slower process compared to biosequestration.

Although it can be very effective, geosequestration faces obstacles such as legal barriers, scientific uncertainty, possible adverse impacts on the environment, and high costs of implementation, limiting its utility.

Aside from innovative ways of removing and storing carbon, scientists are seeking new ways of utilizing carbon in industrial processes. Carbon dioxide can be utilized in the production of graphene, which can be applied in the transport, energy, defense, medical, electronics, and other industries.

Carbon can also be applied in molecular engineering and direct air capture (DAC) systems: new compounds capable of capturing CO2 are being engineered by scientists. These custom molecules are designed to filter every non-carbon element. DAC is an energy and cost intensive (approximately $500-$800 per ton of carbon) carbon capture method which enables the capture of carbon directly from the air by advanced technology plants.

The following methods of carbon utilization are estimated by Ella Adlen and Cameron Hepburn at the University of Oxford to become cost-effective and potentially widespread in the future:

• CO2-based chemicals: reduction to basic elements through catalyst-accelerated chemical reactions for use in the production of urea, methanol, or polymers.
• Hydrocarbon fuels: CO2 can potentially be combined with hydrogen to produce fuels; synfuel (synthetic fuel), syngas (synthetic gas) ,and methanol among them.
• Microalgae: fuel and chemical production can be enabled by post-carbon sequestration microalgae biomass processing.
• Concrete building materials: long-term displacement potential in carbon dioxide-cured cement or aggregate production.
• Enhanced oil recovery (EOR): by injecting CO2 into the earth through oil wells, the output of extracted oil can be increased and carbon dioxide can be safely stored.
• Accelerated weathering: through the dissolution of silicate minerals on the land surface, CO2 may be captured in large quantities, theoretically up to 430 billion tonnes of CO2.
• Forestry: timber used in construction can displace a fraction of CO2-heavy cement production and store CO2 in buildings.
• CO2 sequestration in soil: carbon dioxide can be stored in soil, also potentially improving agricultural yields.