Carbon Capture with Nanotubes

Startup Porifera is developing membranes to separate greenhouse gases from smokestacks.

Membranes made with carbon nanotubes could reduce the amount of energy needed to capture carbon-dioxide emissions from smokestacks, and therefore cut costs, according to a company that will receive $1 million from the new advanced-research projects agency for energy, Arpa-e, to develop the technology.

The company, Hayward, CA-based Porifera, claims that its carbon-nanotube membranes could capture one billion to three billion tons of carbon dioxide a year and save $10 billion a year compared to existing CO₂ capture technology. At this point, however, the work is at an early stage, says Olgica Bakajin, Porifera's chief technology officer. She expects that it will be another year before the first prototype is ready.

The company hopes to make use of some peculiar properties of nanotubes to capture carbon dioxide. Membranes for capturing CO₂ from smokestacks need to have two features. They need to be selective, allowing carbon dioxide to pass through and not the other exhaust gases. This produces a concentrated stream of carbon dioxide that can then be compressed and stored. The membranes also need to be highly permeable--allowing CO₂ to pass through freely to minimize the energy needed to pump it.

Carbon nanotube membranes are particularly good for this second property. Gases can move through the interior of nanotubes extremely quickly--at rates 100 times as fast as through conventional membrane materials, according to experiments Bakajin led at Lawrence Livermore National Laboratory. Those results were published in the journal Science in 2006. As a result, membranes based on nanotubes would require far less energy than conventional membranes.

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The challenge with carbon nanotube membranes is selectively transporting carbon dioxide and not the other gases in a smokestack. This is particularly difficult because the main component of flue gas, nitrogen has many properties that are very similar to CO₂, says Karl Johnson, professor of chemical and petroleum engineering at the University of Pittsburgh. One approach to selecting the carbon dioxide is to bind compounds to the ends of the carbon nanotubes that chemically attract carbon dioxide but not other gases. Attracting the CO₂ would create high concentrations of it near the membrane, increasing the amount of carbon dioxide that gets transported through relative to the nitrogen and other flue gases. Attaching these compounds is particularly easy because the ends of nanotubes have open locations for binding with such molecules, Bakajin says.