Lawrence Berkeley National Laboratory Metal-organic frameworks boast a vast internal surface area, researchers say. If unfolded and flattened, they add, a sugar-cube-size piece could exceed the size of a football field and be adjusted to absorb huge quantities of power plants’ carbon dioxide.
As a climate change prevention strategy, carbon capture and storage is nowhere near ready for prime time. On the storage side of the equation, major questions remain on how and where to sequester the billions of tons of gas produced by power plants and industry every year. Another stumbling block, known as the parasitic energy cost, is the amount of energy needed to strip carbon out of power plant emissions. Carbon capture technologies being tested today, like amine scrubbing, exact an energy penalty as high as 30 percent, a vast and perhaps untenable expense to utilities and society.
Yet a breakthrough in chemistry may be able to radically reduce the cost of stripping carbon from power plant emissions, potentially making carbon capture and storage a far more realistic climate change solution. That is the hope, at least, of researchers studying a remarkable class of materials called metal-organic frameworks.
In their most common form, these crystalline powders resemble nothing more than ordinary table salt. But appearances can be deceiving: metal-organic frameworks are incredibly porous, with the highest internal surface area of any substance known to man. A single gram, unfolded and flattened, could cover a football field. And most promisingly, these crystals can be adjusted to absorb specific molecules like carbon dioxide.
Dr. Jeffrey Long, a chemist at the Lawrence Berkeley National Laboratory, is one researcher studying the carbon-capture potential of metal-organic frameworks.
“We think we can modify the surface so it will cause just the carbon dioxide to stick,” Dr. Long said in an interview. “It would be a sort of carbon-dioxide selective sponge.”
In a power plant setting, carbon would be captured simply by flowing the emissions through or over the absorbent crystals. Once full, the carbon could be “squeezed” out – probably in an underground storage chamber of some kind – and the crystals returned to use.
“Like a sponge, when it absorbs water, you can squeeze it and the water comes back out,” Dr. Long said.
Researchers have yet to identify the exact type of metal-organic framework that will work best in capturing carbon. But at the Berkeley lab, a program is under way to automate the synthesis of variants of the crystals using robots and to quickly screen them for their carbon-absorbing potential. The goal of the program is to identify a metal-organic framework that would remove carbon from a power plant’s emissions with an energy penalty of 10 percent or less.
If a suitable candidate can be found, it will still take years to develop practical applications for a power plant setting. But at least on the cost front, there is reason to be optimistic: The materials that go into making metal-organic frameworks – typically a metal salt, like zinc nitrate, and common organic solvents – are relatively inexpensive.