Carbon removal or carbon capture

What is carbon removal?

Carbon removal, also known as carbon dioxide removal (CDR) or carbon drawdown, is the process of capturing carbon dioxide (CO2) from the atmosphere and locking it away for decades or centuries in plants, soils, oceans, rocks, saline aquifers, depleted oil wells and other geological reservoirs, or long-lived products like cement. Scientists have proposed many different methods of carbon removal. Some of these are already in use at relatively small scales, whereas others remain in the early stages of research and development. Technologies and practices for implementing carbon removal are often called negative emissions technologies (NETs).

Carbon removal is not the same as carbon capture

Although they are often conflated, carbon removal is importantly different than fossil carbon capture and use or storage (fossil CCUS). Carbon capture and storage (CCS) captures CO2 from a smokestack or flue, such as in a gas-fired power plant or a cement factory, and then sequesters that CO2 underground. Processes that capture CO2 and use it to produce commercial products, such as methanol or cement, are known as carbon capture and use. CCUS includes either process. We use the term “fossil CCUS” to identify processes where the carbon in the captured CO2 comes from fossil fuels or carbonate minerals.

To say this more simply: carbon removal removes CO2 from the atmosphere, while fossil CCUS can only reduce the amount of CO2 entering the atmosphere. Proponents argue that fossil CCUS could play a valuable role in climate policy, but it is crucial to recognize that fossil CCUS and carbon removal would play very different roles in long-term climate strategies.

Carbon removal technologies and techniques

There are many ways to remove CO2 from the atmosphere and many ways to sequester or store it. Some of the most discussed approaches include.

1.       Biochar.

2.       Bioenergy with carbon capture and storage (BECCS).

3.       Direct air capture with carbon storage (DACCS).

4.       Enhanced mineralization.

5.       Forestation.

6.       Ocean-based approaches.

7.       Soil carbon sequestration.

Biochar:

Biochar is a kind of charcoal produced by heating biomass in a low-oxygen environment. When buried or ploughed into soils, it locks carbon away for decades or centuries while enhancing soil quality. Biochar can also be used in building materials. The amount of carbon ultimately removed with biochar depends on what kind of biomass is used, how it is sourced and heated, whether the soils are eventually disturbed, and other details of the process.

BECCS:

(BECCS) involves growing or collecting biomass, processing it, converting it to biofuels or energy, capturing the resulting CO2, and storing it underground or in long-lasting products. There are many different ways to implement BECCS, depending on whether the biomass is purpose-grown or collected from agricultural wastes, forest residues, or other sources, whether it is converted to liquid or gaseous fuels or pelletized and burned to generate heat or electricity; whether it is sequestered in depleted oil fields, saline aquifers, basalt formations, or long-lasting products; and so on, all with major implications for BECCS’ climate impact and sustainability.

DACCS:

Direct air capture with carbon storage (DACCS) refers to processes that capture CO2 with purpose-built machines and store the CO2 in the same kinds of geological reservoirs or long-lasting products used for BECCS. These machines capture CO2 from ambient air using various chemical processes and then separate the CO2 for sequestration. Whereas other forms of carbon removal take various kinds of natural materials, such as biomass or rocks, as their primary inputs, the primary input in DACCS is energy. 

The most mature direct air capture (DAC) technologies require both heat and electricity, but several companies are developing DAC technologies that only require electricity.

Enhanced mineralization:

Enhanced mineralization involves accelerating the natural processes by which various minerals absorb CO2 from the atmosphere. The process begins by mining specific kinds of rock, such as olivine or basalt. One prominent proposal for implementation would involve grinding those rocks into powder and spreading the powder over soils, where it would react with the air to form carbonate minerals.

 Other options include exposing powdered rock to CO2-rich fluids or spreading it over the ocean. Enhanced mineralization remains at the very early stages of research and development, but the long-term potential may be quite large.

Forestation

Forestation involves planting trees over large areas or allowing forests to regrow naturally. Growing trees on land that was recently covered in forest is called reforestation; growing trees on land that has not been recently covered in forest is called afforestation. Forest restoration refers to helping degraded forests recover their natural forest structure and rebuild ecological processes and biodiversity.

Agroforestry, in which farmers integrate trees into agricultural practices, is sometimes counted under the heading of carbon removal, too. These new or restored forests would absorb carbon in both the trees and the soil as they grow, with the rates and side effects depending on the mix of trees being planted and whether the forest regains its natural ecological functions. Forests would sequester the captured carbon for as long as they remain standing, which means that, as with other biological methods of carbon removal, the climate benefits of forestation are reversible.

Ocean-based approaches

Scientists are exploring a wide variety of ocean-based approaches to carbon removal. These include the restoration of ocean and coastal ecosystems, such as mangroves, oyster reefs, kelp forests, and open-ocean ecosystems; fertilizing the ocean with micro- or macronutrients; artificial upwelling and down welling; electrochemical approaches, such as processes that react seawater with limestone to produce hydrogen and bicarbonate; adding alkaline materials, such as lime, to the ocean; and cultivating seaweed for bioenergy or to sink into the deep ocean. Aside from restoration of coastal habitats, most of these approaches remain in the early stages of research.

Soil carbon sequestration

Soil carbon sequestration refers to a number of different practices for increasing the amount of carbon stored in soils, especially agricultural soils. Prominent examples include no-till agriculture, manuring, and cover crop rotation. Because they improve soil quality, these practices can contribute to improved crop yields and help protect fields against both floods and droughts. Soil carbon sequestration methods are already in use and ready to scale up, but major challenges remain, including encouraging widespread adoption and ensuring long- term maintenance of the practices to keep the carbon in the ground.