Carbon capture may appear at first blush to be a brute panacea for curing global climate change, entailing nothing more complex than taking the carbon dioxide molecules out of the atmosphere and sequestering them safely beneath the earth.
Existing carbon capture technologies do not provide practical large-scale solutions, however, because of their prohibitive cost and low overall energy efficiency, as well as the engineering challenges they entail.
An international team of scientists hailing from Switzerland, the US and China has developed a new technology which could finally overcome these shortcomings by serving as a high efficiency method for removing carbon from the atmosphere at comparatively low cost.
Their method involves combining the two of the primary methods for capturing carbon from the atmosphere - the first employing solid materials in powder form to adhere to CO2 molecules, and the second using liquids to absorb them.
Liquid amine solutions are currently the most popular method for capturing carbon from the atmosphere. While the liquid readily absorbs carbon dioxide from the air and is easy to deploy on a large scale, the process also consumes large amounts of energy because of the tight bond formed between the amine and CO2 molecules.
Boiling is needed to separate the amine and carbon dioxide molecules in a process referred to as "regeneration," which enables the subsequent reuse of the liquid solution.
Metal-organic frameworks (MOFs) are the solid alternative to liquid amine solutions. They consist of particles whose metallic atoms form a 3D structure connected together by organic linkers, producing nano-sized pores on their surface that can trap the carbon dioxide molecules.
While MOFs are more efficient and come at a lower cost, as solids they are difficult to transport, while the fact that they consist of a fine powder makes their deployment challenging and unwieldy.
In order to combine the advantages and overcome the shortcomings of both these approaches, the scientists from the Ecole Polytechnique Federal de Lausanne (EPFL), UC Berkeley and Beijing have developed a carbon capture "slurry" consisting of an MOF called ZIF-8 suspended in a 2 methyllimidazole glycol liquid mixture.
The developers of the slurry believe it can overcome the two chief impediments to the wide-spread deployment in the real world.
Because the slurry employs a solid MOF as its carbon capture mechanism, its cost is low and its efficiency is high. The essentially liquid form of the slurry, however, makes it far easier to deploy on a large scale than other MOFs which come in powdered form.
Bernard Smit, director of EPFL's Energy Centre, said developing the slurry required finding MOF whose pores were just the right size for incorporation into a liquid, as well as a liquid whose molecules are too large to slot into the pores.
"In the materials that are currently used for adsorption the pores are too large and the surrounding liquid would fill them, and not let them capture CO2 molecules," he said. "So here we looked at a material - ZIF-8 - whose pores are too small for the glycol's molecules to fit, but big enough for capturing the CO2 molecules from flue gas."
Others advantages of ZIF-8 include its strong solution, chemical and thermal stability, making it highly suited to repeated usage and regeneration.
The ability to use an MOF in slurry solution confers tremendous advantages compared to powdered versions of the material.
"Pumping slurry is much easier than transporting a pile of baby powder," said Smit. "And we can use the same technologies for heat integration as the liquid process."
The team of scientists now plan to trial usage of the ZIF-8/glycol slurry in the real world, and are using the success of the proof-of-concept work as the basis for other chemical combinations.