On-board CO2-capture method could make long range ships carbon negative
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Researchers in the US have offered a practical way to make ships carbon neutral with CO2-capturing solid oxide fuel cells on-board.
The method, proposed by a team at Northwestern University, suggests that after “burning” traditional carbon-based fuels, the fuel cell generates concentrated CO2 that can be stored on-board the ship. From there, the CO2 can either be sequestered or recycled into renewable hydrocarbon fuel.
In a study on the method, the researchers looked at various factors, including fuel storage volumes and mass requirements for a wide range of vehicle classes – from light-duty passenger vehicles to tanker ships – and compared onboard CO2 capture to battery-electric and hydrogen fuel cell options.
“It might be harder for people to see on-board CO2-capture as climate-friendly because it uses conventional, carbon-based fuels,” said Scott A Barnett, professor of materials science and engineering. “People assume hydrogen fuel cells and electric vehicles are more climate-friendly. In reality, they often are not. Electricity might come from burning coal, and hydrogen is often produced by natural gas, which generates a lot of CO2.”
According to Barnett, ships can consume up to 250 tonnes of fuel per day and produce about a gigaton of CO2 each year. They also added that while it might seem tempting to replace this massive amount of fuel with batteries, that’s simply not an option.
“Some tanker ships require enough fuel to circumnavigate the globe as a part of their regular multi-voyage operation,” Barnett explained. “We calculated that the battery pack for a long-range tanker would take up more room than the storage capacity of the ship. A hydrogen fuel tank also would be too large. For long-range vehicles, carbon-based fuel combined with on-board CO2 capture is arguably the best way to make these vehicles carbon neutral.”
The proposed method also has potential advantages for shorter-range vehicles, the researchers added. Battery electric and hydrogen fuel cells, however, are already being implemented for those vehicle types, so the researchers instead suggest implementing a CO2-neutral range extender.
To store the CO2 on-board, Barnett’s team has proposed a dual-chamber storage tank (see diagram below). One chamber stores a carbon-based fuel. After the fuel cycles through the fuel cell to create energy, the CO2 by-product is pressurised and introduced into the second chamber. The partition between the chambers can move – shrinking the fuel chamber as the fuel is used, making space for CO2 in the other chamber.
“The solid oxide fuel cell is critical because it burns the fuel with pure oxygen, yielding a concentrated CO2 product that is storable,” said Travis Schmauss, a PhD candidate in Barnett’s research group. “If we just burned the fuel with air, it would be heavily diluted with nitrogen, yielding too much gas to store. When the concentrated CO2 is compressed, it can be stored in a volume not much larger than that needed for the fuel, which saves space.”
“This technology really doesn’t have any major hurdles to making it work,” Barnett added. “You just have to replace the fuel tank with the double-chamber tank and add CO2 compressors. And, of course, the infrastructure eventually has to be developed to off-load the CO2 and either sequester or use it.”
With this scenario, the researchers said it is possible to make long-range vehicles CO2 negative. This is possible with biofuels, such as ethanol, because the plants used to produce the fuel have consumed CO2 from the atmosphere.
Then, after the vehicle has used the fuel, the captured CO2 is removed from the ship and sequestered underground or used in producing renewable fuel. If a vehicle uses a fossil fuel instead of a bio-fuel, then the resulting overall cycle is closer to net-zero, they concluded.
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