Carbon dioxide removal (CDR)
Introduction
To avoid reaching 1.5°C of temperature growth, the IPCC estimates that over 1 gigatonne (Gt) of removals per year will be needed by 2030 and 6 to 10 gigatonnes by 2050. This is alongside deep and rapid reduction in greenhouse gas emissions across all sectors1.
Carbon dioxide removal (CDR) approaches remove carbon dioxide from the atmosphere and store it in geological, terrestrial2 or ocean reservoirs, or in long-lived products.
Carbon removal is different from carbon capture. Carbon capture is the process of trapping CO₂ at its source (like a factory or power plant) before it is released into the atmosphere. It prevents new emissions from entering the atmosphere.
CDR approaches can be nature-based or tech-based. Nature-based approaches rely on natural processes to remove and store carbon dioxide from the atmosphere, for example planting trees. Tech-based approaches employ engineered systems or industrial technologies to capture and store carbon dioxide.
Nature-based approaches
a) Afforestation, Reforestation, and Revegetation (ARR)
Afforestation is the process of planting trees or establishing forests in areas where there were no previous forests or tree covers. Reforestation is the process of replanting trees in areas that have been deforested or degraded. Revegetation on the other hand is the process of replanting or restoring vegetation (trees, grasses, and shrubs) in areas where the ecosystem has been damaged or degraded, to improve soil quality, biodiversity, and carbon storage.
Trees play a crucial role in CDR because they use CO₂ for photosynthesis, which is essential for their growth and survival. They absorb CO₂ from the atmosphere through their leaves, and using sunlight and water, they convert it into glucose (sugar) and oxygen. The glucose is used for growth, while oxygen is released back into the air. Some of the carbon is stored in trunks, roots, and branches as biomass.
ARR approaches are significant CDR approaches since forests absorb vast amounts of CO₂, reducing greenhouse gas levels in the atmosphere. In addition, trees store carbon for decades or even centuries, keeping it out of the atmosphere, provided deforestation or degradation do not occur, which would release stored CO₂ back into the atmosphere.
The Hongera Reforestation Project is a large-scale reforestation project in Kenya that aims to restore previously-forested areas that have been affected by human activities such as logging, agricultural clearance, development, construction, and firewood collection.
This project is registered on the Verra registry3 under ID 3321.
A list of other global reforestation projects can be found on this blog post by the DFB Group.
Big tech firms and oil firms are among the largest buyers of ARR carbon credits. Meta, Microsoft, Google and Salesforce launched an alliance that aims to invest in nature-based carbon removal projects. The symbiosis coalition committed to purchasing credits that are equivalent to 20 million metric tons of carbon dioxide by 20304.
One of the major setbacks with ARR projects is that projects that regrow trees take roughly five to seven years before they start issuing any credits, and those initial volumes tend to be small. In addition, many ARR developers claim that it is difficult to grow a forest on cleared land5. Another setback is that deforestation or degradation e.g through forest fires can release stored CO₂ back into the atmosphere, reversing the effects.
b) Soil Carbon Sequestration
Soil acts as a significant carbon sink, holding more carbon than the atmosphere and all plant life combined.
Soil carbon sequestration involves absorbing and storing atmospheric carbon dioxide (CO₂) in soil. This occurs through natural processes like plant growth and decomposition. In the process of photosynthesizing, plants take in CO₂ from the air and transform it into organic compounds like sugars that they can use for energy and expansion. These plants leave behind organic content that is decomposed by soil bacteria when they die or shed their leaves, branches, and roots. The remaining carbon is converted into stable forms of carbon and integrated into the soil as soil organic matter (SOM) during this process, with part of it being released back into the environment as CO₂.
Soil carbon sequestration can also be enhanced through sustainable land management practices like agroforestry and grazing management. Agroforestry is the integration of trees with either crops or livestock on the same land. Trees and plants add more carbon to the soil, which stays healthy and rich with less erosion. In addition, deep roots lock carbon underground. Effective grazing management prevents overgrazing and allows plant regrowth.
c) Enhanced Rock Weathering (ERW)
Weathering is the breakdown of rocks and minerals at the Earth’s surface6.
Natural rock weathering is a geological process that removes around 1.1 Gt of CO₂ from the atmosphere per year7. As rocks are weathered by rain, they release elements like calcium and magnesium. Meanwhile, CO₂ in the air goes through other natural reactions to become carbonic acid (found in rain) or bicarbonate (found in the ocean). When these different compounds meet, they combine to form new rocks like calcium carbonate, better known as limestone. In this way, rocks help draw CO₂ out of the atmosphere8. This process happens at extremely slow rates over tens of thousands of years.
Enhanced rock weathering (ERW) is a process that fast-tracks the natural process of carbon removal. Silicate rocks, such as basalt and wollastonite are crushed and spread9 on agricultural land. When it rains, rain water absorbs CO₂ in the atmosphere. When this water hits the crushed rock, a reaction occurs to form carbonates. These carbonates are washed into rivers / sea where the CO₂ is permanently locked away for 100,000+ years10.
Apart from removing carbon from the atmosphere, ERW enhances soil fertility. Since the silicate rocks are mineral-rich, as they weather, they release nutrients such as magnesium, calcium and potassium, improving soil health and reducing the need for fertilisers11.
Flux, is a project developer working on carbon dioxide removal using the ERW method. They are headquartered in Nairobi, Kenya.
Flux sources crushed silicate rock powders that are high in calcium and magnesium from local quarries. Farmers spread the rock powder like fertiliser either by using existing equipment, or by hand. The dissolved rock increases soil pH and adds vital micro and macronutrients, as well as improving crop health and resistance to drought. CO₂ in rainwater reacts with the rock layer creating bicarbonates, a solid form of carbon which is washed into our oceans and stored for hundreds of thousands of years.
d) Peatland and coastal wetland restoration
See part b of the nature-based approaches in carbon dioxide avoidance and reduction.
Tech-based approaches
a) Direct Air Capture and Storage (DAC+S)
Direct Air Capture leverages technology to filter CO₂ directly from air, which can then be permanently stored underground or turned into climate-neutral carbon products, such as sustainable aviation fuels.12.
Scientists use two broad approaches for direct air capture: liquid solvents and solid sorbents13.
Solvent-based direct air capture systems pass air through chemicals that remove the CO₂. Existing systems use a combination of heat and vacuum to remove the captured CO₂ and return the chemicals to the direct air capture process. The system then returns the treated air, now with less CO₂, to the atmosphere.
Sorbent-based direct air capture systems use physical filters. These filters chemically bind with CO₂ molecules. When the filters are heated and/or placed under a vacuum, they release the CO₂, which is now in a concentrated stream. This concentrated CO₂ stream is either stored in geologic formations or used.
Climeworks is one of the companies employing DAC technologies.
In their DAC process, air is drawn in through a fan located inside the collector. Once sucked in from ambient air, it passes through a filter located inside the collector which traps the carbon dioxide particles. When the filter is completely full of CO₂, the collector closes, and the temperature rises to about 100°C, which is about the same temperature it takes to boil water for a cup of tea. This causes the filter to release the CO₂ so it can finally be collected. The CO₂ can then be safely and permanently stored underground14.
Octavia Carbon is another example of companies employing DAC technologies.
Through Project Hummingbird, Octavia partnered with Cella Mineral to employ DAC+S technologies in the Kenyan Rift Valley. The project is estimated to have a capacity to capture and securely store 1000 tons of CO₂ annually (1000tCO/yr). This project leverages Kenya’s geothermal energy capacity to power the DAC systems and geology to safely store air-captured CO₂ for a long time.
The captured CO₂ will be injected underground into basalt rock formations which are abundant in the rift valley region for storage. Through chemical reactions i.e mineralization, the injected CO₂ will transform into carbonate minerals, locking away the CO₂ for millions of years with no risk of leakage.
Direct air capture is expensive and requires high amounts of energy. It costs up to $1,000 per ton of CO₂ removed from the atmosphere. Octavia argue that despite being expensive, DAC delivers a premium product. An exact, directly measurable amount of CO₂ is removed from the air and locked up forever by turning CO₂ into a rock. They observe that many organizations see the value of this as per leading scientists’ recommendations and are very willing to pay a premium price for DAC to help push it down the cost curve15.
b) Biochar Carbon Removal (BCR)
Biochar is a charcoal-like substance that’s made by burning organic material from agricultural and forestry wastes (biomass) in a controlled process called pyrolysis. 70 percent of its composition is carbon. The remaining percentage consists of nitrogen, hydrogen and oxygen among other elements16.
During pyrolysis, organic materials, such as wood chips, leaf litter or dead plants, are burned in a container with very little oxygen. As the materials burn, they release little to no contaminating fumes. The unstable carbon in decaying plant material is converted into a stable form of carbon that is then stored in the biochar.
The feedstocks that were used for making biochar would release higher amounts of carbon dioxide to the atmosphere if they were left to decompose naturally. By heating the feedstocks in an oxygen-controlled environment and transforming their carbon content into a stable structure that doesn’t react to oxygen, biochar technology ultimately reduces carbon dioxide in the atmosphere17.
When biochar is applied to the soil, it stores the carbon in a secure place for potentially hundreds or thousands of years. It enriches the soils and reduces the need for chemical fertilizers. Biochar-altered soils can also hold more water, saving plants from drought longer than normal soils18.
Tera is a CDR company based in Kisumu, Western Kenya. They set up their first biochar production facility that utilizes biochar derived from sugarcane waste to capture CO₂.
Sugar Cane bagasse19 is a byproduct of cultivating sugar. Ordinarily, 75% of the total production is waste contributing to CO₂ and CH4 emissions through combustion or rotting.
The process begins by collating, shredding, drying and pelettising the bagasse. Tera then use a Takachar20 pyrolysis machine to convert the bagasse to biochar. The biochar is sold to a fertiliser manufacturer for mixing prior to agricultural application.
The biochar produced enhances soil’s ability to retain water, reduces the need for chemical fertilizers and strengthens crop resilience in drought-prone regions21.
Bio-Logical is a biochar CDR company based in Kenya. They produce biochar from macadamia shells and incorporate it into their bio-fertiliser “Asili” to enhance soil health, improve crop yields, and sequester carbon.
c) Bioenergy with Carbon Capture and Storage (BECCS)
Bioenergy with carbon capture and storage (BECCS), involves capturing and permanently storing CO₂ from processes where biomass is converted into fuels or directly burned to generate energy.
Because plants absorb CO₂ as they grow, this is a way of removing CO2 from the atmosphere. BECCS is the only CDR technique that can also provide energy. As an alternative to storage, the captured CO2 can also be utilised as a feedstock for a range of products22.
Next: Aspects to look out for when assessing the quality of a carbon offset project
Footnotes
Terrestrial carbon dioxide storage involves capturing carbon and storing it within ecosystems found on land e.g. in soil, forests, or other vegetation.↩︎
Carbon market registries track, verify, and record carbon credits, ensuring transparency and preventing double-counting. They certify that credits represent real, additional, and permanent emissions reductions, often in collaboration with third-party standards. Examples include Verra (VCS - Verified Carbon Standard) and Gold Standard. Carbon registries and standards will be explored on this post.↩︎
As Big Tech eyes the carbon market, will it work this time?↩︎
ARR Issuances Continue to Fall and Show No Immediate Signs of Bouncing Back↩︎
spreading the crushed rock on agricultural land, increases the surface area of the rock and therefore increasing its contact with CO₂↩︎
residue extracted during sugar processing.↩︎
Takachar technology is an advanced equipment that produces biochar with a new mobile, low-cost, MIT-developed technology.↩︎