Ships can cut carbon emissions by half

Onboard systems using limestone and seawater can reduce offshore carbon dioxide emissions by up to 50%, according to research published in Science Advances.

The technology mimics natural marine processes, converting carbon dioxide from ship exhaust to bicarbonate, a safe compound that is naturally present in seawater and remains stable for tens of thousands of years.

Scientists in collaboration with startup Calcarea have developed the system to solve one of the world’s most challenging decarbonized sectors. Marine transport accounts for nearly 3% of global greenhouse gas emissions, but current solutions such as low-carbon fuels and electrification are still expensive or impractical for long-haul navigation.

Natural style chemistry

“What makes it beautiful is its simplicity,” explains William Berelson, a Paxson H. Offfield professor at the University of California’s coastal and marine systems. “We are speeding up the process that the oceans have already used to buffer CO2, but do so on board and in a way that makes sense to reduce large-scale emissions.”

This process works when the vessel moves through the sea water. The carbon dioxide in the exhaust gas is absorbed into the water-carrying water, making it slightly acidic. This water then passes through the limestone bed, and the acid reacts with the rock to form bicarbonate. The treated water of the now stripped carbon dioxide flows back to the ocean.

Laboratory results ratio to ship size

The researchers tested the system using controlled laboratory experiments on seawater, limestone and CO2. Their findings show strong consistency between experimental results and theoretical predictions, thus giving confidence in operations that expand the size of the vessel.

Key performance indicators include:

• Conversion efficiency under laboratory conditions is 20-35%
• The countercurrent design allows for 74% transient carbon dioxide removal, while the parallel flow rate is 40%
• Ship-scale reactors less than 1% of total capacity
• Limestone consumption per day for 12 transport containers is 10,000 containers

“We want to prove that we understand not only chemical reactions—we can also predict how much carbon dioxide is neutralized,” Beresen notes. This predictive capability allows researchers to model real-world marine applications.

Marine safety verification

Advanced ocean modeling examines what happens when bicarbonate is rich in water back to the ocean. The simulation tracked a hypothetical ship, traveling repeatedly between China and Los Angeles for 10 years, and discharged treated water along the route.

The results show that the effects on ocean pH and chemistry are negligible. After ten years of continuous operation, the surface alkalinity and dissolved inorganic carbon increased by less than 1.4%, starting from the natural variation range.

“We see our approach as a complementary strategy that can help vessels reduce environmental impact without major design overhaul,” said Jess Adkins, Calcarea co-founder and Caltech professor.

From the laboratory to the ocean

This technology addresses the key demands for offshore decarbonization. Current solutions face significant obstacles: Alternative fuels are still expensive, while electrification is only suitable for short circuits. Limestone – The water system can be integrated with existing vessels without extensive modification.

Calcarea has discussed the pilot program with commercial shippers. The company previously announced a partnership with Lomar Shippting’s Venture Lab to commercialize the technology.

“Scalability is in our design,” Adkins explains. “We are designing a system that can be integrated with existing vessels and adopted within scope. By working directly with industry partners, we are accelerating the road from the lab to the ocean.”

Real-world impact potential

Researchers estimate that widespread adoption may reduce transport-related carbon dioxide emissions by half. For a typical 10,000 containers traveling in 15 knots, the system will require four reaction units, 600 cubic meters, which is a moderate footprint compared to cargo space.

Marine modeling reveals another benefit: The ships naturally produce turbulence, allowing the processed water to quickly mix with the surrounding seawater, diluting it 710 times in a few minutes. This rapid mixing prevents carbon dioxide from escaping back to the atmosphere.

“If we were to really concave in global emissions, that’s the scale we needed,” Beresen concluded. “This won’t happen overnight, but it shows what’s possible.” The technology offers hope for an industry struggling to find practical decarbonization solutions while maintaining global trade efficiency.

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