DAC vs DOC

Introduction and the Global Decarbonization Context

  • Climate Goals and Constraints: As outlined in the Paris Agreement and confirmed at COP28 (hosted by the United Nations in 2023), the global temperature rise must be limited to within 1.5 ∘C1.5\,^\circ\text{C} of pre-industrial levels to mitigate the worst effects of climate change.

  • Necessity of Negative Emissions: Meeting these targets requires capturing over 10 gigatons10\,\text{gigatons} of greenhouse gases—primarily carbon dioxide (CO2CO_2)—annually from the air by the year 2050. Negative emissions technologies (NETs) are required alongside point-source capture to offset "hard-to-abate" sectors.

  • Technological Spectrum: Direct Air Capture (DAC) and Direct Ocean Capture (DOC) are core NETs. They complement other strategies such as afforestation, Bioenergy with Carbon Capture and Storage (BECCS), and Ocean Alkalinity Enhancement (OAE).

  • Environmental Impact on Oceans: Oceans absorb approx. 27 %27\,\% of human-induced CO2CO_2 emissions. This natural process leads to acidification; for example, the pHpH of seawater recorded in Hawaii dropped from 8.158.15 in 1985 to 8.058.05 in 2020. This shift threatens marine calcifiers (coral and fish) by destabilizing their shells and exoskeletons.

Direct Air Capture (DAC) Scaled-Up Technologies

  • Overview and Status: DAC involves stripping CO2CO_2 molecules directly from the atmosphere using absorption or adsorption. As of 2023, 18 DAC plants are operational worldwide with a capacity of at least 1 ton/year1\,\text{ton/year}. Most are in North America or Europe. Their average individual capacity is 10,000 tCO2/year10,000\,\text{tCO}_2/\text{year}, which is less than 0.1 %0.1\,\% of the 2050 required target capacity.

  • Climeworks (Switzerland):

    • Facilities: Includes "Capricorn" (Switzerland, > 100\,\text{tons/year}), "Arctic Fox" (Iceland, 50 tons/year50\,\text{tons/year}), and "Orca" (Iceland, currently the largest at 4,000 tons/year4,000\,\text{tons/year}). They are building the "Mammoth" plant in Iceland, designed to capture 36,000 tons/year36,000\,\text{tons/year} by mid-2025.

    • Mechanism: Uses Temperature-Vacuum Swing Adsorption (TVSA). Capture occurs at ambient temperature; regeneration occurs at 100 ∘C100\,^\circ\text{C} under vacuum. A typical cycle takes 4–6 h4\text{--}6\,\text{h}.

    • Adsorbents: Uses amine-functionalized materials like "SI-AEATPMS" (grafted silica) and "APDES-NFC-FD" (nanofibrillated cellulose). APDES-NFC-FD achieves negative emissions of −116 kg CO2/ton capture-116\,\text{kg CO}_2/\text{ton capture}.

    • Energy Demand: Requires 200–300 kWh/ton200\text{--}300\,\text{kWh/ton} (electrical) and 1,500–2,000 kWh/ton1,500\text{--}2,000\,\text{kWh/ton} (thermal).

  • Carbon Engineering (Canada):

    • Mechanism: Employs a continuous aqueous solvent system (KOH solution) and a calcium caustic recovery loop. Air passes through a contactor with KOH; CO2CO_2 reacts to form K2CO3K_2CO_3. This is then reacted with Ca(OH)2Ca(OH)_2 to form CaCO3CaCO_3, which is calcined at ≈900 ∘C\approx 900\,^\circ\text{C} to release CO2CO_2.

    • Facility: Building "STRATOS" in Texas, USA, with a capacity of 0.5–1.0 μton/year0.5\text{--}1.0\,\mu\text{ton/year}.

    • Energy Demand: Heat demand is > 2,000\,\text{kWh/ton}. If powered by natural gas, it emits 0.5 ton CO20.5\,\text{ton CO}_2 per ton captured.

  • Airthena (CSIRO, Australia):

    • Mechanism: Uses Metal-Organic Framework (MOF) polymer nanocomposites coated on resistive heating sheets.

    • Demonstration: A single module captures 140 g/day140\,\text{g/day}; purity is between 70–80 %70\text{--}80\,\%. Regeneration occurs at the relatively low temperature of 80 ∘C80\,^\circ\text{C}.

  • Global Thermostat (USA): Uses amino-polymer adsorbents at 85–95 ∘C85\text{--}95\,^\circ\text{C}. Their Colorado plant captures > 1,000\,\text{tons/year}. Target costs are intended to drop below US$ 150/ton\text{US\$ } 150/\text{ton}.

Comparative Analysis: Low-Temperature (LT) vs. High-Temperature (HT) DAC

  • LT Systems (Solid Adsorbents): These operate at 80–120 ∘C80\text{--}120\,^\circ\text{C}. They are compatible with waste heat and geothermal energy.

    • Energy Cons: Thermal energy is 4–7.5 GJ/ton4\text{--}7.5\,\text{GJ/ton}; Electricity is 200–700 kWh/ton200\text{--}700\,\text{kWh/ton}.

  • HT Systems (Liquid Absorbents): These require 800–900 ∘C800\text{--}900\,^\circ\text{C} for calcination.

    • Energy Cons: Thermal energy is 5–9 GJ/ton5\text{--}9\,\text{GJ/ton}; Electricity is 350–770 kWh/ton350\text{--}770\,\text{kWh/ton}.

  • Efficiency and Emission Metrics:

    • Carbon Removal Efficiency: LT-DAC is typically higher (45%–70 %45\%–70\,\%—reaching up to 97 %97\,\% with low-carbon energy) compared to HT-DAC.

    • Purity: Climeworks (LT) reaches 99.9 %99.9\,\%, while Carbon Engineering (HT) reaches 97.1 %97.1\,\%.

    • Water Impact: HT-DAC evaporates approx. 0–50 tons of water/ton CO20\text{--}50\,\text{tons of water/ton CO}_2. LT-DAC can actually be a net water producer (0.8–2 tons water/ton CO20.8\text{--}2\,\text{tons water/ton CO}_2 captured).

    • Land Use: To capture 1 gigaton/year1\,\text{gigaton/year}, DAC units occupy approx. 400 km2400\,\text{km}^2, though the renewable energy supply (PV) would require an additional 11,500 km211,500\,\text{km}^2.

Direct Ocean Capture (DOC) Mechanisms and Pilots

  • The Oceanic Carbon Reservoir: Oceans hold approx. 38,000 gigatons38,000\,\text{gigatons} of carbon. DOC focuses on extracting Dissolved Inorganic Carbon (DIC) from carbonates and bicarbonates.

  • Electrochemical pH Swing Processes:

    • Bipolar Membrane Electrodialysis (BMPED): Uses voltage to create H+H^+ and OH−OH^- gradients. Seawater is acidified to pH < 4 to release CO2CO_2. Energy cost is approx. 155 kJ/mol155\,\text{kJ/mol} (900 kWh/ton900\,\text{kWh/ton}).

    • Electrochemical Cation Exchange (ECE): Extracts CO2CO_2 and H2H_2 simultaneously from oceanwater.

    • Electrochemical Hydrogen Looping (EHL): High efficiency with low operating voltage (0.48 V0.48\,V). Energy demand is approx. 104 kJ/mol104\,\text{kJ/mol} (660 kWh/ton660\,\text{kWh/ton}).

    • Cl-Mediated EC: Membrane-free system using Bismuth and Silver electrodes. Estimated energy consumption is 122 kJ/mol122\,\text{kJ/mol}.

  • Carbonate Mineralization:

    • Steel Slag Utilization: Adding 1 %1\,\% steel slag to oceanwater increases carbonate precipitation by 115 %115\,\%. It absorbs approx. 128 kg CO2/ton steel slag128\,\text{kg CO}_2/\text{ton steel slag}.

    • Cement Kiln Dust (CKD): High calcium content allows for 185 kg CO2/ton CKD185\,\text{kg CO}_2/\text{ton CKD} fixation.

Industrial DOC Projects and Emerging Concepts

  • Captura (California/Norway): Has developed a pilot plant capturing 100 tons/year100\,\text{tons/year}. Partnering with Equinor to build a 1,000 tons/year1,000\,\text{tons/year} pilot in Norway. It uses solar power and no added chemicals.

  • MIT Electro-Swing: A battery-like architecture using quinone electrodes that bypasses the need for high-cost membranes or chemicals.

  • EMPA (Swiss Federal Labs): Proposed "Ocean Floating Islands" featuring 7070 solar clusters covering 550,000 m2550,000\,m^2. Theoretically extracts 2.4 ton CO2/h2.4\,\text{ton } CO_2/h alongside H2H_2 production.

  • University of California, Los Angeles (UCLA): Single-step electrolytic precipitation that produces solid minerals directly from DIC, significantly faster than natural rock weathering.

Techno-Economic Benchmarking of DAC vs. DOC

  • Cost Realities (2023): DAC costs range from US$ 600–1,000/ton\text{US\$ } 600\text{--}1,000/\text{ton}. DOC costs are highly speculative with lab prototypes; Cl-mediated electrodialysis is estimated at US$ 56–100/ton\text{US\$ } 56–100/\text{ton}, while stand-alone ED-based DOC can reach US$ 2,000/ton\text{US\$ } 2,000/\text{ton} when including pumping and degassing.

  • Energy Demand Comparison: DOC is theoretically more favorable, utilizing approx. 1.3 kWh/kg1.3\,\text{kWh/kg} of CO2CO_2 compared to DAC’s 1.54–2.45 kWh/kg1.54\text{--}2.45\,\text{kWh/kg}.

  • Land vs. Location: DAC is versatile but land-intensive. DOC can be installed offshore, potentially above sub-seafloor storage reservoirs (like the Sn%C3%B8hvit fields in Norway), saving terrestrial land.

  • Atmospheric and Fluid Concentrations: DAC faces the challenge of low atmospheric CO2CO_2 (≈420 ppm\approx 420\,\text{ppm}). DOC handles oceanwater with low relative CO2CO_2 flux, meaning it must process vast volumes of water, leading to potential biofouling of heat exchangers and membranes.

Future Challenges and Considerations

  • Technological Readiness: DAC is scaling to megaton levels (TRL 7–9TRL\,7\text{--}9 range for some players). DOC is largely at lab or early pilot stage (TRL 1–3TRL\,1\text{--}3).

  • Environment and Ethics: Both require ethical consideration. DOC deployment needs intensive study on its impact on marine biodiversity and biodiversity loss.

  • Governmental Support: Reducing costs to the target of < \text{US\$ } 100/\text{ton} by 2050 requires massive public investment and supportive policy frameworks (like new tax credits).

  • Storage Infrastructure: Only 2 of the 18 active DAC plants (Orca and Arctic Fox) currently utilize dedicated geological storage. The rest capture CO2CO_2 for immediate industrial use (beverage carbonation, greenhouses, fuels).