Why Carbon Capture?
Even under optimistic scenarios for decarbonizing electricity, transport, and heating, certain sectors — cement production, steelmaking, aviation, and agriculture — are extremely difficult to fully decarbonize in the near term. Additionally, to meet the Paris Agreement target of limiting warming to 1.5°C, most scientific assessments indicate that net negative emissions (removing more CO₂ from the atmosphere than is emitted) will be necessary in the second half of this century.
Carbon capture encompasses a range of technologies that either capture CO₂ from point sources (power plants, industrial facilities) before it reaches the atmosphere (Carbon Capture and Storage, CCS), or remove CO₂ directly from ambient air (Direct Air Capture, DAC). Once captured, CO₂ must be either stored (geologically or in materials) or utilized (converted to useful products).
Post-Combustion Capture with Amine Solvents
The most commercially mature carbon capture technology is chemical absorption using amine solvents. The process occurs in two steps:
Absorption (CO₂ Capture)
Flue gas from a power plant or industrial facility (containing ~10–15% CO₂) passes through an absorber column where it contacts a liquid amine solution — typically monoethanolamine (MEA, HOCH₂CH₂NH₂) or a mixture of amines. CO₂ reacts reversibly with the amine:
2 HOCH₂CH₂NH₂ + CO₂ → HOCH₂CH₂NH₃⁺ + HOCH₂CH₂NHCOO⁻ (carbamate formation)
The CO₂-rich amine solution (CO₂ loading ~0.5 mol CO₂/mol amine) is pumped to a stripper column.
Regeneration (CO₂ Release)
In the stripper, the amine solution is heated to 100–120°C, reversing the reaction and releasing concentrated CO₂:
HOCH₂CH₂NH₃⁺ + HOCH₂CH₂NHCOO⁻ + heat → 2 HOCH₂CH₂NH₂ + CO₂↑
The lean amine is recycled back to the absorber. The concentrated CO₂ stream (>99% purity) is then compressed, transported, and stored.
Key challenges with MEA: - High energy penalty: regeneration requires approximately 3.5–4.0 GJ of heat per tonne of CO₂ — reducing plant electrical output by 20–30% - Oxidative degradation of amine in the presence of O₂, producing ammonia and heat-stable salts - Corrosion of steel equipment - Volatile amine emissions from the absorber
Research is focused on next-generation solvents: ionic liquids, piperazine (faster kinetics), amino acid salts, and mixed amine systems that reduce regeneration energy.
Solid Sorbents
Instead of liquid amines, CO₂ can be captured by solid materials with surface functional groups — primarily amine-functionalized materials (e.g., amine-grafted silica, amine-impregnated MOFs).
Temperature Swing Adsorption (TSA): CO₂ is adsorbed at low temperature, then released by heating. Avoids the large liquid volumes and corrosion issues of liquid solvents.
Pressure Swing Adsorption (PSA): CO₂ is adsorbed at high pressure and desorbed at low pressure. Used commercially for H₂ purification, increasingly relevant for CO₂ capture.
Metal-Organic Frameworks (MOFs): Crystalline porous materials with extraordinarily high surface areas (up to 7,000 m²/g) and tunable pore chemistry. MOFs can achieve very high CO₂ capture capacity and selectivity over N₂. SIFSIX-3-Ni and MOF-177 are examples studied for CO₂ capture. Commercial application remains limited by cost and water stability.
Direct Air Capture (DAC)
DAC removes CO₂ directly from the open atmosphere, where it is present at only ~420 ppm — roughly 300 times more dilute than in power plant flue gas. This extreme dilution makes DAC thermodynamically and kinetically very challenging, requiring either highly reactive sorbents or enormous air-handling systems.
Liquid Solvent DAC (Carbon Engineering / 1PointFive)
Air is contacted with a 1–3 M potassium hydroxide (KOH) solution in a large cooling tower-like structure:
CO₂ + 2 KOH → K₂CO₃ + H₂O (CO₂ absorbed as carbonate)
The potassium carbonate solution is then reacted with calcium hydroxide (Ca(OH)₂, slaked lime) to regenerate KOH and precipitate calcium carbonate (CaCO₃):
K₂CO₃ + Ca(OH)₂ → 2 KOH + CaCO₃↓
The CaCO₃ is calcined (heated to ~900°C) to release pure CO₂ and regenerate lime:
CaCO₃ → CaO + CO₂↑ (calcination, ~900°C)
CaO + H₂O → Ca(OH)₂ (slaking)
The energy penalty is enormous: ~5–9 GJ/tonne CO₂, primarily for the high-temperature calcination step. Current costs are approximately $300–600 per tonne of CO₂ — compared to roughly $50–100/tonne for amine scrubbing at point sources.
Solid Sorbent DAC (Climeworks)
Climeworks' commercial DAC units use amine-functionalized cellulose fibers that adsorb CO₂ from ambient air at room temperature. When heated to ~100°C in a vacuum, they release concentrated CO₂. The cycle: 1. Adsorption: ambient air passes through sorbent bed; CO₂ binds to amine groups 2. Desorption: bed heated under vacuum → concentrated CO₂ released 3. Sorbent cools and is ready for next cycle
Energy requirements are lower than liquid KOH routes (~2.5 GJ/tonne CO₂), but the sorbent degrades over hundreds of cycles, requiring periodic replacement. Climeworks' Orca and Mammoth plants in Iceland (powered by geothermal energy) are the world's first commercial DAC operations, currently capturing ~36,000 tonnes CO₂/year — a tiny fraction of global emissions (~37 billion tonnes/year) but an important proof of concept.
Geological Storage (Sequestration)
Captured CO₂ is compressed to a supercritical fluid above 73.8 atm and 31.1°C, where it behaves as a dense, low-viscosity fluid. Supercritical CO₂ is injected into:
- Deep saline aquifers: porous rock formations saturated with brine (saltwater) at depths > 800 m. CO₂ dissolves in brine over time (solubility trapping) and can eventually react with reservoir rock to form carbonate minerals (mineral trapping) — the most permanent storage mechanism
- Depleted oil and gas fields: well-characterized geology, existing infrastructure
- Deep unmineable coal seams: CO₂ adsorbs onto coal, displacing coalbed methane (enhanced coalbed methane recovery, ECBM)
Mineral carbonation offers permanent storage by reacting CO₂ with silicate rocks (peridotite, basalt) to form stable carbonate minerals:
CO₂ + Mg₂SiO₄ (forsterite) → MgCO₃ (magnesite) + SiO₂
Iceland's CarbFix project has demonstrated accelerated mineral carbonation in basalt: injected CO₂ dissolved in water mineralizes within 2 years in the volcanic rock — essentially turning CO₂ into stone. This approach eliminates long-term leakage risk.
Carbon Utilization
Rather than storing CO₂, it can be converted to useful products (Carbon Capture and Utilization, CCU):
- Methanol synthesis: CO₂ + 3 H₂ → CH₃OH + H₂O (over Cu/ZnO catalyst, analogous to industrial methanol synthesis)
- Urea for fertilizers: CO₂ + 2 NH₃ → CO(NH₂)₂ + H₂O (already the world's largest industrial use of CO₂)
- Polycarbonate plastics: CO₂ as a co-monomer with epoxides
- Synthetic fuels (e-fuels): Combining captured CO₂ with green hydrogen to produce jet fuel, diesel, or methane via Fischer-Tropsch synthesis
These utilization routes are most valuable when they displace fossil carbon (using CO₂ as a feedstock instead of petroleum), but the climate benefit depends critically on whether the CO₂ is permanently locked in the product or re-released during use or combustion.