International shipping carries 80% of global trade and is responsible for around 3% of the world’s carbon emissions, but is not currently on track to meet climate targets.
A year ago, the International Maritime Organization, the U.N. agency that regulates shipping, tightened emissions targets for the shipping industry, bringing it in line with other industries aiming to reach net-zero emissions by 2050. But lower-emission fuels such as methanol, hydrogen and ammonia are not available quickly enough.
Jess Adkins, a chemical oceanographer at the California Institute of Technology (Caltech), thinks cargo ships can help by equipping them with reactors that can convert carbon dioxide (CO2) emitted from burning fuel into ocean salts, which he says can remain stored for 100,000 years.
The process is similar to what already happens naturally in the oceans. “This is a reaction that the planet has been doing for billions of years,” said Adkins, who founded Calcarea, a startup that designs and tests reactors.
“If we can accelerate this, we might have a shot at storing CO2 safely and permanently.”
Natural but faster
Seawater naturally absorbs about a third of the CO2 released into the atmosphere, making the water more acidic and causing it to dissolve calcium carbonate, which is abundant in the ocean. “Calcium carbonate is what coral skeletons, shells and everything else that makes up most of the sediments on the ocean floor are made of,” Adkins said.
The dissolved calcium carbonate then reacts with CO2 in the water to form bicarbonate salts, trapping the CO2. “There are 38,000 gigatons (38 trillion tons) of bicarbonate in the ocean right now,” Adkins added.
Calcarea wants to mimic this natural process by channeling the ship’s exhaust fumes into a reactor in the ship’s hull, where the fumes are mixed vigorously with seawater and limestone. Limestone is a type of rock made mostly of calcium carbonate and a common ingredient in concrete. The CO2 in the exhaust fumes reacts with the mixture, creating a brine that traps the CO2 in the form of bicarbonate salts. With a full-scale reactor, Adkins says, the goal is to capture and store about half of a ship’s CO2 emissions.
In the natural world, the reaction takes more than 10,000 years, but in Calcarea’s reactors it takes about a minute, Adkins said. It’s achieved by bringing CO2 and limestone into close contact with each other.
The brine created is simply released into the ocean, where it poses no threat to marine life or the chemical balance of seawater, Adkins said. He added that the company is also considering adding a pre-filter to the system to remove other pollutants from the exhaust that could be released into the water, such as particulates and unburned fuel, among other contaminants.
After two years of working on the project, he spun off the company in January 2023 from Caltech, where he is still a professor but on leave. He was joined by three co-founders: Caltech undergraduate Melissa Gutierrez, engineer Pierre Forin, and University of Southern California (USC) professor and geochemist Will Berelson.
They raised $3.5 million in funding and focused on the shipping industry. “The beautiful part is that the ship is a natural water pump,” Adkins said, noting that the system requires constant water movement for the reactions between the various elements to occur, which is naturally provided by the ship’s motion.
Calcarea has built two prototype reactors so far, one in a USC parking lot and one at the Port of Los Angeles. The company announced a partnership with the research and development arm of international shipping company Lomar in late May. Adkins is confident that this will lead to the first full-scale prototype of its reactor to be placed on a ship.
The reactors will be designed for different ship sizes, including the “Newcastlemax” class, which can carry 180,000 metric tons of cargo. “In one of them, we’ll be taking up about 4% to 5% of the deadweight tonnage and carrying about 4,000 metric tons of limestone. But we’re not actually going to use all of that,” Adkins said.
Carbon capture at sea
There are still some engineering challenges to solve before Calcarea is ready to build its first reactor, such as how exactly to fit the reactor onto the ship and the logistics of setting up the supply chain to load and deliver the limestone. Adkins cautions that these could be slow steps.
The system costs, according to current estimates, about $100 for every tonne of CO2 captured in exhaust, including the revenue the ship loses to make room for the reactor and the loss of commercial cargo.
Some cargo ships already have similar devices called scrubbers. These are designed to capture and discharge sulphur emissions, which are harmful to human health and the environment, but not CO2. According to the British Ports Association, they were installed on about 5% of the global trading fleet as of June 2023, but studies have found that wastewater from scrubbers can be “acutely toxic to aquatic organisms.” Calcarea’s reactors also capture sulphur as part of their CO2 removal process.
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Carbon capture technology more directly similar to Calcarea’s also exists. For example, a British company called Seabound makes a device that captures 25% to 95% of a ship’s CO2 emissions. But it produces solid carbonate gravel that must be unloaded at a port.
According to Daniel Sigman, a professor of geology and geophysical sciences at Princeton University who is not affiliated with Calcarea, the company’s approach has a number of advantages over similar strategies. First, it speeds up a natural process that is already happening. Second, because the reaction takes place in an engineered reactor on the ship and does not completely deplete the CO2 supply, it will not increase ocean acidity levels and contribute to ocean acidification, which is harmful to marine life.
He added that because Calcarea’s founders are experts on the ocean’s carbon cycle, they are well-positioned to avoid the potential pitfalls of CO2 removal: “Many other companies looking to increase ocean alkalinity do not understand the carbon cycle at all the relevant scales and are therefore prone to adopting ineffective, even counterproductive, approaches.”
Adkins believes Calcarea could help reduce carbon emissions as the industry transitions to greener fuels, and that in the more distant future reactors could occupy entire sites in special containers designed to trap CO2 captured from the atmosphere on land, rather than storing it underground.
“We think ships could actually compete with underground CO2 storage,” he said. “Specially built ships that take CO2 and limestone from a port, go out to sea and just run our reaction — these would be just machines to efficiently and safely store carbon as bicarbonate in the ocean.”
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