Despite being responsible for 3% of all CO2 emissions, cargo and tanker ships are often overlooked when examining ways to reduce harmful CO2 emissions from vehicles.
In its recently published paper, ‘Viability of vehicles utilising on-board CO2 capture’, the research team suggested that by “burning” traditional carbon-based fuels, the resultant concentrated CO2 can be stored on-board a ship. This stored CO2 could then be recycled into a renewable hydrocarbon fuel or sequestered.
Such a method of CO2 capture can be seen as self-defeating when it comes to negating harmful emissions, as it still requires carbon-based fuels, but Scott A. Barnett, senior author of the study, countered this, saying, “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.”
Ships can consume up to 250 tonnes of fuel per day, causing around one gigatonne of CO2 to be released into the atmosphere each year.
Disregarding the suggestion that batteries could replace fuel, Barnett said, “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 would also be too large.”
He concluded that the best method to make these vehicles CO2 neutral, or even negative, is to combine carbon-based fuel with on-board CO2 capture.
CO2-neutral range extenders are also being studied for hydrogen-powered and electrified shorter-range vehicles.
To store the captured gas onboard the ship, the team proposed a patent-pending dual-chamber storage tank. A carbon-based fuel is stored in one chamber and, following the fuel cycling through the fuel cell to create energy, the CO2 by-product undergoes pressurisation and enters the second chamber.
Diagram of the dual-chamber tank
Source: Northwestern University
Space can be made for CO2 in the other chamber via the ability of the partition between the chambers to move, shrinking the fuel chamber as fuel is used.
Travis Schmauss, co-author of the study, said that the solid oxide fuel cell is critical because it burns the fuel with pure oxygen, yielding a concentrated CO2 product that is storable.
Continuing, he explained, “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.”
The team believe that there are no “major hurdles” preventing this technology from working, suggesting that the fuel tank just needs to be replaced by the double-chamber tank and CO2 compressors added. To off-load the CO2 for usage of sequestration, the infrastructure would also require development.
Using such methods of innovation, the researchers believe that bio-fuels, such as ethanol, have the potential make long-range vehicles CO2 negative.