Ship-on-ship systems using limestone and seawater can reduce CO2 emissions by half

Scientists from USC and Caltech have worked with startup Calcarea to develop a promising onboard system that can remove up to half of the carbon dioxide released from the transport vessel by converting marine vessels into safe solutions to the ocean.

The breakthroughs described in Science Advances explain how systems can reduce carbon emissions from one of the world’s most cautious sectors: the shipping industry.

“What’s beautiful about this is how simple it is,” said William Bellelson, Paxon H. Offered Professor of Coastal and Marine Systems, co-author of USC Dawn Siffretars College of Letters, Art and Science, and the research. “We are speeding up the process the ocean is already using to buffer CO2, but we do that on ships and do it in a way that can significantly reduce large-scale emissions.”

This process mimics the natural chemical reactions of the ocean. As the ship moves through the seawater, CO2 is absorbed from the exhaust into the water pumped into the vessel, making it slightly acidic. The water passes through the limestone bed, where acid reacts with the rock to form bicarbonate. This is a safe and stable compound that naturally exists in seawater. Currently, treated water that has been removed from CO2 is discharged into the ocean.

“What’s most exciting for me is that this started out as a pure science question. How does the ocean buffer CO2?” Bellelson added. “From there, I realized there might be a real-world solution that can help combat climate change.”

From the laboratory to the sea

Maritime transport accounts for almost 3% of global greenhouse gas emissions. However, current solutions like low-carbon fuels and electrification remain expensive or unrealistic on long-distance voyages.

“We’re committed to providing a range of services to our customers,” said Jess Adkins, co-founder and CEO of Calcarea and SMITS family professor at Geochemistry and Global Environmental Science at Caltech.

In the lab, researchers tested key components of the process using controlled amounts of seawater, limestone and CO2. Their experiments closely matched theoretical predictions and gave them the confidence to scale the modeling up to the size required to work on a real vessel.

“We didn’t just understand chemistry, we wanted to show that we could predict how neutralised CO2 would be,” Bellelson said. “It allowed us to model what this would look like on a real ship.”

This study used sophisticated marine modeling to investigate what happens when bicarbonate-rich water is released into the ocean. The simulation tracked a virtual vessel repeatedly travelling between China and Los Angeles for ten years, and discharged treated water along the route. This model showed negligible effects on ocean pH and chemistry. This is an important verification of the environmental safety of technology.

Researchers estimate that widespread adoption of this technique can reduce shipping-related CO2 emissions by 50%.

“This is the type of scale you need to create a true dent in global emissions,” Bellerson said. “It doesn’t happen overnight, but it shows what is possible.”

Putting technology on the market

The academic work is being carried out in parallel with Calcurea, a startup working to bring technology to the market. The company is early discussion with commercial shippers and explores pilot programs that test technology on work vessels.

Calcrea previously announced a collaboration with Lomar Labs, Lomar Shipping’s Corporate Venture Lab, to commercialize and deploy its onboard carbon capture systems.

“Scalability is built into our design,” Adkins said. “We engineer systems that can be integrated with existing vessels and adopted throughout the fleet. By working directly with industry partners, we accelerate the path from the lab to the ocean.”

Berelson, co-founder and science advisor at Calcurea, continues to study the science behind the approach, including reaction rates and long-term impacts on marine chemistry.