Coal-fired power plants account for roughly 30% of global CO2 emissions. The world embraces other energy sources, such as solar and wind power, that do not generate greenhouse gases. However, finding a way to reduce the carbon output of these existing plants could help mitigate their effects while they remain in operation.
Researchers at the National Institute of Standards and Technology (NIST) have developed a simple, economical, and potentially reusable material that can remove carbon dioxide, a greenhouse gas, from fossil-fuel power plant exhaust before it ever reaches the atmosphere.
In their study, the researchers focused on Aluminum formate, one of a class of substances called metal-organic frameworks (MOFs). The MOFs have shown great potential for filtering and separating organic materials from one another. Some MOFs have demonstrated potential in refining natural gas or separating the octane components of gasoline, while others might contribute to reducing the cost of plastics manufacturing or cheaply converting one substance to another.
Researchers say Aluminum formate (ALF) has the ability to separate carbon dioxide (CO2) from the other gases that commonly fly out of the smokestacks of coal-fired power plants. Also, they assert that it does not have the drawbacks of other proposed carbon filtration materials.
“What makes this work exciting is that ALF performs really well relative to other high-performing CO2 adsorbents, but it rivals designer compounds in its simplicity, overall stability, and ease of preparation,” said NIST’s Hayden Evans, one of the lead authors of the team’s research paper. “It is made of two substances found easily and abundantly, so creating enough ALF to use widely should be possible at a very low cost.”
ALF is made from aluminum hydroxide and formic acid, two chemicals that are abundant and readily available on the market. Evans estimated that it would cost less than a dollar per kilogram, which is up to 100 times less expensive than other materials with similar performance.
ALF resembles a three-dimensional wire cage with innumerable small holes. These holes are large enough to allow CO2 molecules to enter and get trapped but small enough to exclude the slightly larger nitrogen molecules that make up the majority of flue gas. Neutron diffraction work at the NIST Center for Neutron Research (NCNR) showed the team how the individual cages in the material collect and fill with CO2, revealing that the gas molecules fit inside certain cages within ALF like a hand in a glove.
The separated CO2 – from the mixture of gases that flows up the smokestacks of coal-fired power plants – is then converted to formic acid. Formic acid is not only a naturally occurring organic material but also one of the two constituents of ALF. The idea here is that ALF could become part of a cyclic process where ALF removes CO2 from the exhaust streams and that captured CO2 is used to create more formic acid. This formic acid would then be used to make more ALF, further reducing the overall impact and cost of the material cycle.
Despite its potential, ALF is not ready for immediate use – engineers would need to design a procedure to create ALF at a large scale. Additionally, a coal-fired plant would need a compatible process to reduce the humidity of the flue gas before scrubbing it.
- Hayden A. Evans, Dinesh Mullangi, Zeyu Deng, Yuxiang Wang, Shing Bo Peh, Fengxia Wei, John Wang, Craig M. Brown, Dan Zhao, Pieremanuele Canepa, Anthony K. Cheetham. Aluminum formate, Al(HCOO) 3 An earth-abundant, scalable, and highly selective material for CO 2 capture. Science Advances, 2022; DOI: 10.1126/sciadv.ade1473