Water covers about three-quarters of our planet’s surface, mostly in the form of saline oceans and seas. And these large bodies of water act as the world’s biggest carbon sink, soaking up about a third of all carbon dioxide emissions released into the atmosphere.
Researchers at the University of Pittsburgh are devising ways to capture this carbon dioxide using membranes. In two new studies, they present details of experiments and computational studies that show how two types of membrane-based capture devices could remove carbon dioxide from the ocean.
“This is a novel way of capturing carbon dioxide from the ocean that could potentially be simpler and cheaper than other methods,” says Katherine Hornbostel, professor of mechanical engineering and materials science at the University of Pittsburgh.
With the world unable to curb carbon emissions, many governments and organizations are pursuing the drastic option of capturing carbon emissions. Capturing carbon dioxide from power plant and factory smokestacks is an established technology, although still not widely used. Some startup companies are also trying to pull carbon dioxide directly out of air. This typically involves large arrays of fans that blow air over carbon-absorbing materials.
Removing carbon dioxide from the oceans could be more efficient than air capture, because the concentration of the greenhouse gas is 150 times higher than that in the atmosphere. Plus, says Hornbostel, ocean capture needs less land space, and it could be coupled with offshore wind and offshore storage of carbon dioxide.
A few research groups and startups are now starting to explore direct ocean capture. But Hornbostel and her colleagues are the first to demonstrate using a technology called membrane contactors, which have been used before for traditional capture of carbon dioxide from the exhaust gas of a power plant.
Membrane contactors bring two fluids into contact with each other on the two sides of a membrane. For ocean capture, this would be ocean water on one side and a solvent on the other.
The Pitt team built and tested two types of membrane contactors, detailed separately in the Chemical Engineering Journal. In one paper, they demonstrate hollow fiber membrane contactors, which look like straws. In the other, they report bead-like encapsulated solvent membrane contactors.
“Thankfully all of the materials in our proposed system are relatively cheap and scalable,” she says. The team chose affordable, abundant solvents such as sodium carbonate and sodium hydroxide. Hollow fiber membrane contactors are already commercially available.
The team tested the performance of these two designs using laboratory-scale experiments and computational models. They also did an economic analysis to estimate the cost of a system that captures one million metric tons of carbon dioxide a year. While the bead-like material had an exorbitant cost of over $8,000 per metric ton of carbon dioxide, the hollow fiber contactor could be competitive with direct air capture technologies at about $500/metric ton, they found.
Increasing the carbon dioxide removal rate will be critical to bring down costs and scale this technology up, Hornbostel says. She is now looking into special chemical coatings for the membranes that will make carbon dioxide bubble out for easier capture.
The important thing to remember, she adds, is that “direct ocean capture isn’t meant to replace direct air capture but rather to complement it as another approach to reduce carbon dioxide levels and mitigate global warming.”
- Joanna Rivero et al. Demonstration of direct ocean carbon capture using hollow fiber membrane contactors. Chemical Engineering Journal, 2023.
- Austin Lieber et al. Demonstration of direct ocean carbon capture using encapsulated solvents. Chemical Engineering Journal, 2023.