Carbon Dioxide Removal (CDR) involves actively taking carbon dioxide (\(CO_2\)) out of the ambient air and storing it durably. This removal is necessary for meeting global climate objectives, as deep cuts in emissions alone will not be sufficient to limit global warming to target levels. CDR technologies counterbalance emissions from sectors that are difficult to decarbonize, such as industrial processes and long-distance transport. The goal of CDR is to achieve net-zero and eventually net-negative emissions by reducing the overall stock of greenhouse gases accumulated in the atmosphere.
Enhancing Natural Carbon Sinks
Methods that enhance natural carbon sinks leverage the biosphere’s existing capacity to absorb and store \(CO_2\). Primary examples include afforestation, establishing forests on previously un-forested land, and reforestation, replanting forests in deforested areas. Trees remove atmospheric \(CO_2\) through photosynthesis, storing the carbon in their biomass and the soil. This process makes forests effective carbon reservoirs, mitigating climate change while providing co-benefits like soil conservation and biodiversity enhancement.
Soil carbon sequestration focuses on maximizing the amount of carbon stored in agricultural and grazing lands, often through regenerative agriculture practices. Techniques like no-till farming, cover cropping, and diverse crop rotations enrich soil organic matter. This organic matter, consisting of decomposed plant material and microbial biomass, is where the carbon is held.
Coastal blue carbon refers to the \(CO_2\) captured and stored by coastal ecosystems such as mangroves, salt marshes, and seagrass meadows. These ecosystems are highly efficient carbon sinks because their plants grow rapidly and they thrive in anaerobic, oxygen-deprived soils. The lack of oxygen in these waterlogged soils drastically slows the decomposition of organic matter. This allows the carbon to remain sequestered for hundreds or even thousands of years, making protection and restoration a durable removal strategy.
Direct Air Capture Technologies
Direct Air Capture (DAC) is an engineered process that chemically scrubs \(CO_2\) directly from the atmosphere, unlike methods that capture emissions from point sources like factory smokestacks. The concentration of \(CO_2\) in ambient air is low, around 420 parts per million, making the chemical separation challenging and energy-intensive. DAC systems draw in large volumes of air using fans and pass it over a specialized chemical medium that selectively binds with the \(CO_2\) molecules.
Sorbent-based capture uses solid materials, often in a filter or structured bed, that chemically or physically adsorb \(CO_2\). Once the sorbent is saturated, the system is isolated from the air flow. The captured \(CO_2\) is released in a concentrated stream by applying heat, typically between 80°C and 120°C, and sometimes a vacuum. This process regenerates the sorbent for reuse and is often favored for its modularity and lower temperature requirements.
The liquid solvent system uses an aqueous alkaline solution to chemically absorb the \(CO_2\). Air passes through a contactor unit where the \(CO_2\) dissolves into the liquid, forming a carbonate ion. The \(CO_2\)-rich solvent is pumped to a regeneration section where high-temperature heat, often around 900°C, is applied to release the concentrated \(CO_2\) stream and recycle the solvent. Liquid solvent systems have a high capacity for absorption and utilize existing liquid handling infrastructure. Both sorbent and solvent systems require significant energy, and the source of this energy must be low-carbon to ensure the overall process achieves net-negative emissions.
Permanent Storage and Utilization
Once \(CO_2\) is captured, the removal process requires the gas to be stored permanently or converted into a stable product. Geological sequestration involves injecting the captured \(CO_2\), compressed into a supercritical fluid state, deep underground into porous rock formations. Ideal sites include deep saline aquifers or depleted oil and gas reservoirs, typically at depths greater than 800 meters.
The \(CO_2\) is trapped through several mechanisms. These include structural trapping beneath an impermeable caprock, and solubility trapping where the \(CO_2\) dissolves into the formation water. Over longer timescales, mineral trapping occurs as the \(CO_2\) reacts with reservoir rocks to form stable, solid carbonate minerals. This mineral form represents the most secure and irreversible sequestration mechanism, though it proceeds slowly over millennia.
Mineral carbonation, also known as carbon mineralization, mimics natural rock weathering. It reacts captured \(CO_2\) with metal-oxide bearing materials like magnesium and calcium silicates. This reaction forms stable, non-toxic carbonate minerals, such as calcite, which permanently lock the \(CO_2\) away. The resulting solids are extremely stable, offering a massive storage capacity given the abundance of suitable silicate minerals in the Earth’s crust.
Carbon Capture and Utilization (CCU) involves using the captured \(CO_2\) as a feedstock for industrial processes, such as curing concrete or manufacturing synthetic fuels. For CCU to count as true carbon removal, the carbon must be embedded in a product that provides long-term sequestration. Utilization in materials like concrete, where the \(CO_2\) is permanently mineralized, satisfies this requirement. However, using captured \(CO_2\) to create synthetic fuels is temporary, as the carbon is released back into the atmosphere when the fuel is combusted.