Direct Ocean Capture, Another Frontier in Carbon Capture

The world of carbon capture, utilization, and sequestration (CCUS) continues to innovate, and the locus continues to shift away from the single, vertically integrated oil & gas projects of old. Cross-border CCUS value chains; the marine transportation of liquefied CO₂ at scale; large-scale deployments of highly popularized direct air capture (DAC), particularly as a possible remedy for residual emissions; and even the residential capture of carbon from New York high rise apartment buildings are all significant developments of late. Added to all this is the prospect of direct ocean capture (DOC). DOC can entail both biological and non-biological processes. This post focusses on the latter, which includes electrochemical ocean capture, ocean alkalinity enhancement, mineralization, air stripping, and ultraviolet light radiation processes.

CO₂ is far more concentrated in seawater than in ambient air. If CO₂ is removed directly from seawater, any deficit is naturally rectified given the constant free exchange between the ocean and the air. Overall, the world’s oceans are recognized as the greatest carbon sink available (due to the activity of phytoplankton) and are estimated to have absorbed between 25% and 40% of CO₂ emissions since the beginning of the Industrial Revolution. In September 2022, the U.S. Department of Energy announced up to $30 million in funding for R&D projects to remove carbon from both the atmosphere and the ocean.

Currently, both DAC and DOC are said to be energy-intensive endeavors. Capture costs are usually the most significant cost component for CCUS projects; given the challenges of DAC, which is still a relatively new technology, DOC systems may eventually offer a competitive alternative, because CO₂ in seawater may be considered already “captured.” A recent study centered on an electrochemical approach to DOC demonstrated a process that requires an energy input of 122 kJ/mol, without the use of membranes or chemicals, and could achieve an optimized cost of $56 per ton of CO₂ captured. However, this figure applies only to the electrochemical system, not a complete DOC system, which would involve intake, gas separation, and compression systems as well. As such, it would be premature and inaccurate to compare these initial findings directly with existing full-system DAC costs, which can exceed $300 per ton. Both DAC and DOC must also be juxtaposed with other popular sectors such as natural gas processing, where capture costs from highly concentrated CO₂ streams can be around $20 per ton.

In terms of existing pilot implementation, the startup SeaChange plans to install its first pilot plants this year, which will be capable of drawing at least 40 tons of CO₂ per year. Another company, Captura, launched its first solar-powered project in August 2022, removing just a ton of CO₂ per year. The company plans to install a 1,000-ton capture system in 2024 and says that its systems could need between a third and a quarter of the energy required by DAC systems. Scaling up systems to remove significant volumes of carbon will clearly be a challenge; DOC proponents have suggested repurposing offshore oil & gas platforms for carbon removal or creating synergies with existing facilities that process seawater such as desalination plants.

Overall, the emergence of technologies such as DOC should be encouraged as part of the toolbox for CO₂ removal, especially in the context of the long-term removal of residual emissions. In the short and medium term, however, new technologies must not detract focus from large-scale CCUS for single emitters or industrial clusters, which can be enacted in the here and now. Follow Guidehouse Insights’ upcoming global CCUS Tracker for a comprehensive view of project developments.