Wednesday, May 18, 2022

Onboard CO2 capture promises emissions-free long-range shipping

Although battery electric and hydrogen fuel cell vehicles hold great promise for mitigating CO2 emissions, there are still unaddressed sectors for electrified transport, e.g., the heavy-duty and long-range global shipping industry. Today, around 80-90% of world trade is carried by the international shipping industry, producing nearly 3% of the world’s CO2 emissions.

Onboard nuclear reactors could be a solution, but they are way too expensive for cargo shipping. Now, a team of researchers from Northwestern University offers a practical way to make ships CO2 neutral – or even CO2 negative – using hydrocarbon or alcohol fuels, in which the CO2 product is captured onboard the vehicle.

The vehicle includes CO2-capturing solid oxide fuel cells (SOFC) for efficient, low-cost electrical generation from hydrocarbon or alcohol fuels. 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 be sequestered or recycled into renewable hydrocarbon fuel.

It might be harder for people to see onboard CO2 capture as climate-friendly because it uses conventional, carbon-based fuels,” said Northwestern’s Scott A. Barnett, senior author of the study. “People tend to 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 in the process.

Barnett further explained that batteries are just not an option for cargo ships as they would have to be as big as the ship to function. “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. When it comes to long-range vehicles, carbon-based fuel combined with onboard CO2 capture is arguably the best way to make these vehicles CO2 neutral.

Schematic illustration of a carbon capture fuel cell vehicle (CCFCV) and associated infrastructure.
Schematic illustration of a carbon capture fuel cell vehicle (CCFCV) and associated infrastructure. Credit: Northwestern University

To store and reuse the captured CO2 on board, the research team has invented a patent-pending dual-chamber storage tank with a movable partition. One chamber stores a carbon-based fuel while the other stores the captured CO2. 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, co-author of the paper. “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.

According to the researchers, the technology can also be used to make long-range vehicles CO2 negative. After all the carbon emissions on board are captured by this tank, they would then be offloaded as carbon to be sequestered underground or used in producing renewable fuel.

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 offload the CO2and either sequester or use it.”

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