Study: Abundant Chemical Removes CO2 from Power Plant Smokestacks

Exhaust from coal-fired power plants, at left, contain large quantities of the greenhouse gas carbon dioxide (purple tripartite molecules). Aluminum formate, a metal-organic framework whose structure is highlighted at right, can selectively capture carbon dioxide from dried flue gas conditions, potentially at a fraction of the cost of using other carbon filtration materials. Credit: B. Hayes/NIST

Key Points:

• Aluminum formate has shown great potential to scrub carbon dioxide from power plant smokestacks.
• The chemical filter is made of two substances found easily and abundantly.
• Ultimately, the hope is aluminum formate will be part of a cyclic process that removes CO₂ from exhaust streams.

Researchers at the National Institute of Standards and Technology (NIST) say their new study shows the potential of aluminum formate to remove greenhouse gases from fossil fuel power plant exhaust—before it reaches the atmosphere.

Aluminum formate (ALF) is one of a class of substances called metal-organic frameworks (MOFs). As a group, MOFs have exhibited great potential for filtering and separating organic materials.

On a microscopic scale, ALF resembles a three-dimensional wire cage with innumerable small holes. These holes are just large enough to allow CO₂ molecules to enter and get trapped, but just small enough to exclude the slightly larger nitrogen molecules that make up the majority of flue gas. Neutron diffraction work at the 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.

ALF is made from aluminum hydroxide and formic acid, two chemicals that are abundant and readily available on the market. It would cost less than a dollar per kilogram, researchers say, which is up to 100 times less expensive than other materials with similar performance. Low cost is important because carbon capture at a single plant could require up to tens of thousands of tons of filtration material. The amount needed for the entire world would be enormous.  

Still, “creating enough ALF to use widely should be possible at very low cost,” said lead author Hayden Evans, a chemist at the NIST Center for Neutron Research.

Despite its potential, ALF is not ready for immediate use. Engineers would need to design a procedure to create ALF at large scales. A coal-fired plant would also need a compatible process to reduce the humidity of the flue gas before scrubbing it.

What to do with the CO₂ afterward is also a major question. There are research efforts underway to convert it to formic acid — which is not only a naturally occurring organic material but also one of the two constituents of ALF. The hope is that ALF could become part of a cyclic process where ALF removes CO₂ from 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.

Information provided by NIST.