The 7 ways of Getting Carbon Dioxide out of the atmosphere

This story by Ama Lorenz and Frank Odenthal is published in THE BEAM #13.


 

There are currently seven recognised negative emissions technologies (NETs). What are their global CO₂ removal potential, costs, and relevant side effects? An overview of the pros and cons of carbon capture and storage.

 

Afforestation and reforestation (AR)

Afforestation is the establishment of a forest or stand of trees in an area where there was no previous tree cover. Meanwhile, reforestation is the natural or intentional restocking of existing forests and woodlands that have been depleted, usually by deforestation. Both are available at large scale – theoretically – but they currently lack incentives for widespread adoption. They are likely to increase in costs as land gets more scarce. They could have positive side effects on biodiversity and soil and water quality if they are not applied as mono-cultures.

Example: 11 countries have set out to build the Great Green Wall, a 7,000-kilometre belt of trees stretching from Senegal in West Africa to the coastal areas of Djibouti in East Africa.

This, the project’s organisers hope, will trap the sands of the Sahara desert, halt the desert’s further expansion, restore 50 million hectares of land and absorb some 250 million tonnes of carbon. Around 15 percent of the wall of trees has already been planted, according to the Sahara and Sahel Great Green Wall Initiative.

The “Wall” promises a compelling solution to the many urgent threats facing not only the African continent but the entire global community – in particular climate change, drought, hunger, conflict, and migration. When completed, the Great Green Wall will be the largest living structure on the planet: three times the size of the Great Barrier Reef.

• Tech readiness: ready for large-scale deployment

• Positive side-effects: good for soil quality

• Negative side-effects: Albedo effect, threats to food security and potentially negative implications for biodiversity

• Permanence: reversible

© Axel Fassio/CIFOR © Axel Fassio/CIFOR

 

Biochar (BC) 

Biochar has a high carbon content of up to 90 percent and binds carbon material reliably, for long-term and without negative side effects. Obtained by pyrolysis from biomass, it will capture CO2 from the atmosphere during its growth. Carbon is stored in plant material while oxygen is released into the atmosphere. A large part of the carbon can be captured in a gas, a liquid and a solid phase. While providing climate-neutral energy using the gas phase (Syngas) and the liquid phase (Bio-Oil), the material use of the solid phase (Biochar) allows for carbon capture and storage, thus leading to a net positive climate process.

The costs of this technology are rather moderate. The broad application of biochar makes negative emissions possible at a large scale. Increased crop yields and improved soil carbon and nutrients, alongside reduced N2O emissions, are expected outcomes.

Example: As Europe’s first manufacturer of biochar, Swiss Biochar has been offering biochar of high-EBC quality since 2010. Together with the Ithaka Institute, they have developed humus-rich soil substrates with activated plant carbon. EBC-certified biochar meets the highest quality standards with a carbon content of over 80 percent. Since 2021, they have been part of the NovoCarbo Group to optimise their product range for a wide variety of applications – from viticulture to greenery and balcony plants.

• Tech readiness: limited pyrolysis capacity

• Positive side-effects: good for soil quality

• Negative side-effects: reliant on biomass availability

• Permanence: stable, depending on soil type

© iStock © iStock

 

Soil carbon sequestration (SCS)

Soil carbon sequestration comprises a series of practices that deliver negative emissions by organically storing CO2 in soils. Scientists have estimated that soils – mostly for agricultural uses – could sequester over one billion additional tonnes of carbon each year. This technology is also available on a large scale, but there are concerns about its permanence. There are hundreds of millions of farmers around the world, mostly farming small plots of land. To take full advantage of soil-based sequestration as a climate solution, would require many of them to change the way they farm, now and for hundreds of years in the future. This is a big social and economic challenge, and experts debate how much soil-based sequestration is really possible in the long term.

Example: Soil carbon sequestration has gained traction within the Biden administration as a way for farmers to reduce, or even reverse American agriculture’s greenhouse gas (GHG) emissions. To advance this technology, Congress proposed the bipartisan Growing Climate Solutions Act, which is intended to help farmers participate in voluntary markets that pay them to store carbon in the soil.

• Tech readiness: ready for large-scale deployment

• Positive side-effects: a possible concomitant reduction of N2O as another (hard to reduce) greenhouse gas

• Negative side-effects: insecure permanence

• Permanence: reversible in certain conditions

© Karan Mandre © Karan Mandre

 

Enhanced weathering on land and in oceans (EW)

Enhanced weathering delivers negative emissions by accelerating the mineral weathering process of rocks and distributing the ground-up rock over land. Enhanced weathering results in carbonation (i.e. carbonate rock formation), which may be considered a form of geological storage.

Example: Mission-driven companies, like The Project Vesta, are executing direct action measures by investing in research, and conducting field tests to develop practical solutions at scale to remove large amounts of CO2 from the atmosphere and to galvanise global deployment. The NGO captures CO2 by using an abundant, naturally occurring mineral called olivine. Ocean waves grind down the olivine, increasing its surface area. As the olivine breaks down, it captures atmospheric CO2 from within the ocean and stabilises it as limestone on the seafloor. This approach provides permanent sequestration with the potential for very high volume at a low cost. Questions remain about its safety and viability: to validate coastal enhanced weathering, more lab experiments and pilot beach projects must be performed.

• Tech readiness: limited mineral production

• Positive side-effects: a possible concomitant reduction of N2O as another (hard to reduce) greenhouse gas

• Negative side-effects: water and ground pollution, as well as supply chain risks involving mining, extraction, and the energy-intensive process of grinding rocks

• Permanence: stable

© Ishan Seefromthesky © Ishan Seefromthesky

 

Ocean fertilisation (OF)

Ocean fertilisation delivers negative emissions by enhancing the carbon uptake of oceans.

This is achieved by increasing the nutrient supply in the near-surface, by adding micro or macronutrients. This technology has only been tested in small-scale demonstration plants so far, but there is likely to be a large potential to increase scale. Its impact on marine biology and food web structures is unknown.

In addition to reducing emissions, seaweed cultivation may also reduce ocean acidification. In some places, this application is already in use for shellfish aquaculture to reduce acidification and improve shellfish growth.

• Tech readiness: only small-scale demonstrations

• Positive side-effects: unknown

• Negative side-effects: unknown

• Permanence: stable but uncertain

Seaweed farming in Nusa Lembongan, Bali © Janne Hellsten  Seaweed farming in Nusa Lembongan, Bali © Janne Hellsten

 

Bioenergy combined with carbon capture and storage (BECCS)

BECCS deliver negative emissions by capturing and storing the CO2 released from biomass during combustion. This technology has good market opportunities, but its impact on biodiversity and land degradation is likely negative.

Example: The British company Drax began to pilot the first bioenergy carbon capture and storage (BECCS) project of its kind in Europe at Drax Power Station in October 2018.

The pilot project with C-Capture technology captured its first carbon molecules at the UK’s largest renewable power station in early 2019.

A second BECCS pilot facility has been installed by Mitsubishi Heavy Industries (MHI) within the North Yorkshire power plant’s carbon capture usage and storage (CCUS) incubation area in autumn 2020.

• Tech readiness: only 1 full-scale demonstration

• Positive side-effects: low footprint

• Negative side-effects: risk of negative side-effects for biodiversity, air pollution, trace GHGs, and food security

• Permanence: stable

The dam at the Cruachan pumped storage plant © Drax Global  The dam at the Cruachan pumped storage plant © Drax Global

 

Direct air capture and storage (DACCS)

This is one of the few technologies that extracts carbon dioxide from the atmosphere and is viewed by scientists as vital to limit global warming. DACS technology extracts CO2 directly from the atmosphere through chemical processes. This is then permanently stored to achieve negative emissions. If CO2 captured with DAC is used in short-lived products, such as fuels, it is an example of CCU, and therefore is not considered a negative emission. The energy intensity of the direct air capture process may involve trade-offs with a scarce supply of climate-neutral electricity and heat.

Example: In September 2020, Swiss and Icelandic companies announced the start of operations for the world’s largest direct air carbon capture plant. The Orca plant – a reference to the Icelandic word for energy – consists of eight large containers similar in appearance to those used in the shipping industry, which employs high-tech filters and fans to extract carbon dioxide. The facility will capture and store up to 40,000 tonnes of carbon dioxide per year.

Direct air capture is still a fledgling and costly technology, but developers hope to drive down prices by scaling up production as more companies and consumers look to reduce their carbon footprint.

• Tech readiness: deployed in niche markets

• Positive side-effects: little known

• Negative side-effects: little known but significant potential opportunity costs

• Permanence: stable

Orca is the world's first and largest direct air capture and storage plant © Climeworks Orca is the world's first and largest direct air capture and storage plant © Climeworks

 


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