The conversation about reducing carbon dioxide emissions from transportation mostly centers on electrification. But while electric cars are slowly gaining traction around the world, electrifying heavy-duty trucks is more challenging because it would need gigantic batteries that run for long distances.
In a new study published in Cell Reports Physical Science, researchers detail an on-board carbon capture and storage system for freight transport vehicles that could reduce carbon dioxide emissions by at least 50 percent.
The transport sector accounts for about a quarter of total carbon dioxide emissions. Transportation is expected to be the highest source of greenhouse gases by 2035, according to the World Bank. Ships, airplanes and trucks all burn a lot of fuel per distance because of the long distances that they have to carry heavy freight. Efforts to decarbonize these transport vehicles have focused on increasing engine efficiency, sustainable fuels, and hydrogen fuels.
Another option is to capture carbon emissions right on board, before it reaches the atmosphere. Until now, the technology to capture carbon dioxide where it is created has existed mostly at stationary sources like power plants and factories.
Capturing emissions on moving vehicles is challenging because of limited space and because of the energy needed to separate the carbon dioxide from other emissions. The dynamic ever-changing conditions in the engine as the vehicle drives and air flow also affect the system.
Giuseppe Pezzella, S. Mani Sarathy and their colleagues at King Abdullah University of Science and Technology in Saudi Arabia designed a system keeping these energy, space and process constraints in mind. They choose a recently discovered family of porous materials called metal-organic frameworks (MOFs). These microscopic compounds, which often look like tiny cages and are being studied for use in chemical separation, energy storage, and sensors.
MOFs are promising for carbon dioxide capture because they don’t react chemically with the gas, but instead store it in their cage-like structure. This makes it easy to separate the gas using little energy.
The team developed a special type of MOF that is especially good at capturing carbon dioxide from a mix of gases, and doing this even in humid conditions. Next, they picked a different MOF for storing the carbon dioxide. This MOF has a special pore structure made of a combination of cages and channels that have an ideal pore size for storage.
Using these materials, the researchers designed an onboard carbon dioxide capture and storage system for long-haul semi-trucks with an average 25,600 kg payload. The system captures otherwise wasted exhaust heat and uses it to separate the carbon dioxide and regenerate the MOF material. The volume of the entire system depends on the pressure at which carbon dioxide is captured and stored. Higher pressure means lower volume, but that in turn increases power consumption. Given this trade-off, the researchers calculated the ideal pressure and power needs for the onboard system given a semi’s typical daily driving conditions.
They were able to design a system that captures half of the otherwise emitted carbon dioxide at 96 percent purity, while needing only 7.6 percent extra engine power to operate and with a volume less than 1.5 cubic meter.
It’s a little too soon to determine the cost of the technology given that the MOFs are relatively new and cannot be made using industrial processes. But that should be next on the research agenda, write the authors.
Source: Giuseppe Pezzella et al. Onboard capture and storage system using metal-organic frameworks for reduced carbon dioxide emissions from vehicles. Cell Reports Physical Science, 2023.
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