Cleaning flue gas with nanotubes

A collaboration between US and Chinese researchers has investigated the use of titanate nanotubes in removing CO2 from flue gas

As of April 2016, the concentration of carbon dioxide (CO₂) in Earth’s atmosphere had reached 404.08 ppm – the highest level in 400,000 years. And so, it’s not suprising that widespread efforts are underway to develop sustainable CO₂ capture technologies, suitable for use in a range of applications.  One area being investigated is the scrubbing of flue gas – the exhaust gas from power plants – through adsorption. While the gas’s composition varies depending on what’s burned, carbon dioxide (CO₂) is always present, and a Chinese-US consortium believe that amine-impregnated nanotubes may well be the key to cleaning it up.

Using aqueous amines to remove CO₂ isn’t an entirely new idea – it’s the route taken to produce the high-purity CO₂ used for freezing and preservation in the food industry. But it tends to be extremely energy-intensive, inefficient at high temperatures and costly. A number of solid sorbents have also been investigated, but most have a rather low CO₂ capacity under the conditions of flue gas (10-15 % carbon, at low pressures and at temperatures around 75 °C).

This paper, published in a recent issue of Nano Energy [DOI: 10.1016/j.nanoen.2016.04.038], combined the two approaches. The team produced a series of solid-supports from protonated titanate nanotubes, and functionalised them with a range of amines, including triethylenetetramine (TETA) and polyethyleneimine (PEI). These composite sorbents were tested in flue gas conditions, and were found to compare very favourably to other CO₂-capture compounds in the literature.

One surprising result revealed that pore volume, rather than surface area, is the dominant factor in CO₂ capacities of the composite sorbents. In addition, the larger the molecular size of the amine, the lower the sorption capacity. The researchers also showed that the amine-impregnated PTNTs performed well under cyclic tests – they remained stable even at high humidity, and could be reactivated without damaging the structure. In short, the sorbents may be recyclable, which makes them particularly interesting for use in flue gas applications. Questions remain, however, over how practical this ‘recycling’ could be at an industrial scale, and so, work is ongoing.

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L. Guo, J. Yang, G. Hu, X. Hu, H. DaCosta, M. Fan, “CO₂ removal from flue gas with amine-impregnated titanate nanotubes”, Nano Energy 25 (2016) 1–8. DOI: 10.1016/j.nanoen.2016.04.038