A new combination of carbon-capturing solids and liquids could provide a low-cost solution to commercial CCS schemes.
CCS is a process that collects CO2 released by factories and power plants before storing it so as to reduce net global carbon missions, but the two prevailing methods are largely prohibitive because of engineering demands, cost and overall energy-efficiency.
The most common approach uses liquid amine solutions, which can absorb CO2 from the atmosphere, but CO2 is so strongly bound to the amine molecules that it is necessary to actually boil them in order to separate them making the process very energy intensive.
An alternative to liquids is to use solid materials known as "metal-organic frameworks" (MOFs) – fine powders whose particles’ surface is covered with nano-size pores that collect CO2 molecules - but despite its lower cost, as this method involves transporting solids it is very demanding in terms of engineering.
Berend Smit, director of the Energy Center at the École polytechnique fédérale de Lausanne (EPFL) in Switzerland, said: "Imagine trying to walk with a plateful of baby powder. It's going to go everywhere, and it's very difficult to control."
Instead Smit’s team, collaborating with scientists from the USA and China, have combined carbon-capturing solids and liquids to develop a "slurry" that is relatively simple to implement on a large scale due its liquid characterics, while maintaining the lower costs and energy efficiency of a solid.
The slurry, described in a paper in journal Nature Communications, is a combination of an MOF called ZIF-8, which is suspended in a 2-methylimidazole glycol liquid mixture.
“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,” said Smit.
“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."
ZIF-8 is a good material for carbon-capturing slurries, because it displays excellent solution, chemical and thermal stability, which is important for repeated regeneration cycles. Its crystals also have narrow pores that are smaller than the diameter of glycol molecules, preventing them from entering and reducing the material’s carbon-capturing abiliy.
The concept of the slurry comes from an idea from one of Smit's former PhD students who is now a professor in Beijing, and it could be the key to large-scale implementation of carbon capture.
"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 slurry also shows exceptionally good separation from CO2, meaning that it doesn't require excessive amounts of energy in order to regenerate, which increases its overall energy efficiency.
Following their successful proof-of-concept work, the research teams are now planning to test the ZIF-8/glycol slurry in the field.