New Way to Develop Long-Range Vehicles Capable of Capturing CO2 Onboard

The conversation about eliminating carbon dioxide (CO₂) emissions from vehicles often revolves around electrifying cars, buses and trucks. However, cargo and tanker ships that contribute 3% of all CO₂ emissions are hardly discussed.

Researchers from Northwestern University now provide a practical method to make ships CO₂ neutral or CO₂ negative by using CO₂-capturing solid oxide fuel cells. Once the conventional carbon-based fuels are “burnt,” the fuel cell produces concentrated CO₂ that can be stored on-board the ship. It is then possible to sequester and recycle the CO₂ into renewable hydrocarbon fuel.

The researchers have explained their findings in the study “Viability of vehicles utilizing on-board CO₂ capture,” published in the journal ACS Energy Letters on August 18^th, 2021.

As part of the study, the team focuses on various factors such as fuel storage volumes and mass needs for a wide range of vehicle classes, ranging from light-duty passenger vehicles to tanker ships. It also compares onboard CO₂ capture to battery-electric and hydrogen fuel cell options.

It might be harder for people to see onboard CO₂ capture as climate friendly because it uses conventional, carbon-based fuels. 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.

Scott A. Barnett, Study Senior Author, Northwestern University

Barnett is an expert on solid oxide fuel cells and a professor of materials science and engineering at Northwestern’s McCormick School of Engineering. He co-authored the paper along with Travis Schmauss, a PhD candidate in his research group.

Why Batteries aren’t a Viable Solution

Ships are responsible for producing almost a gigaton of CO₂ every year and use up to 250 tons of fuel per day. Although it is tempting to replace a huge amount of fuel with batteries, it is not considered an option.

Some tanker ships require enough fuel to circumnavigate the globe as a part of their regular multivoyage operation. 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 on-board CO₂ capture is arguably the best way to make these vehicles CO₂ neutral.

Scott A. Barnett, Study Senior Author, Northwestern University

The suggested technique holds potential benefits for shorter-range vehicles. Battery electric and hydrogen fuel cells are already used in those types of vehicles, but the researchers recommend implementing a CO₂-neutral range extender.

Storage Solution

For storing the CO₂ on board, the research team suggested a patent-pending dual-chamber storage tank. Carbon-based fuels are stored in one chamber. Energy is generated when the fuel cycles through the fuel cells and the CO₂ byproducts are pressurized and moved into the second chamber. The partition between the chambers is designed to move, where the fuel chamber shrinkage during the use of fuel makes space for CO₂ in another chamber.

The solid oxide fuel cell is critical because it burns the fuel with pure oxygen, yielding a concentrated CO₂ product that is storable. 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.

Travis Schmauss, PhD Candidate, Northwestern University

“This technology really doesn’t have any major hurdles to making it work. 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 CO₂and either sequester or use it,” Barnett stated.

Moving Toward Net-Zero

The researchers, referring to this scenario, state that it is viable to produce long-range vehicles CO₂-negative. This can be realized with biofuels like ethanol because the plants utilized to make the fuel have fed on CO₂ from the atmosphere.

Once the fuel is consumed by the vehicle, the captured CO₂ is drawn out from the ship and sequestered underground or employed to produce renewable energy. When a vehicle consumes fossil fuel instead of a bio-fuel, then the resulting overall cycle is near to net-zero.

Journal Reference:

Schmauss, T A & Barnett, S A (2021) Viability of Vehicles Utilizing On-Board CO₂ Capture. ACS Energy Letters. doi.org/10.1021/acsenergylett.1c01426.