Breath of fresh air

A membrane that filters carbon dioxide from flue emissions in a similar way to how lungs rid the body of the waste gas has been developed by researchers in Norway.

Memfo, a membrane research group, based at the

(NTNU) in Trondheim, has used nanotechnology to construct a patented plastic polymer membrane that would be fitted at the exit of a coal-fired power plant's stack.

There is an enzyme in the lung which transforms CO

into an ion that is more easily transported through the biological membrane.

The plastic membrane mimics this function as it contains a 'carrier' that is fixed on to the backbone of the polymer.

The carrier, made up of amine groups, helps the CO

molecules to bind with existing moisture in the flue gases to form bicarbonate, which then transports the CO

quickly through the membrane. This has been designed so that only CO

is captured, while other waste gases, such as nitrogen, are allowed to pass freely.

'The main issue here is that by having this carrier in the membrane, we are speeding up the permeation, or the flux, of the CO

through the membrane and it is selectively captured over the other gases in the mixture,' said Prof May-Britt Hagg, head of Memfo.

Hagg indicated the advantages of the new membrane by comparing its efficiency with existing methods of CO

filtration, which she said are only available in installations for natural gas sweetening.

'Existing membranes used for natural gas sweetening have an optimum flux of approximately 0.15 cubic metres standard temperature and pressure (STP) per square metre/hour and per bar. They also have a selectivity level of CO

of up to 15, but because this is in natural gas, this represents a permeability of CO

over permeability of methane,' she said.

'But flue gas is largely a mixture of CO

. So in the new membrane, selectivity is around 200 for CO

over nitrogen, and it has a flux of twice as much or higher — 0.3 up to 0.7. These are very good numbers and mean that we can capture CO

in the most efficient way.'

According to Hagg, a standard power plant would require several thousand square metres of membranes, and the greater the gas flow, the bigger the membrane size. Nonetheless, being made from polymeric materials makes the membrane a cheaper option compared with carbon or ceramic ones.

'The membrane is very thin. The hollow fibre may be around 1mm thick, and the selective layer, which is the actual membrane, is a coating on the hollow fibre that is around one micrometer thick.

'They would be installed as modules, probably circular vessels three or four metres long and with a diameter of about 40cm. There will be bundles of hollow fibres within these vessels, and you can directly connect all these units in any order you want. This is how other gas separation membranes are installed so we know that it functions well,' she said.

Environmentally friendly

Hagg claimed that the new membrane would also be more environmentally friendly than other filtration technology for flue gas that is currently being piloted.

'The only method ready for piloting today is amine absorption. This is more expensive, takes up more space and has a larger footprint.

'Amine is a solvent that you really don't want to use in the future, so it would be an intermediate technology. When you use a solvent, you need large columns to wash out, or absorb, the CO

— and once you have absorbed it, you need to regenerate the solvent by heating it up,' said Hagg.

'This demands quite a lot of energy and is also not so environmentally friendly. But if you produce a membrane, you do not need any solvents at all.'

Since the carrier in the membrane is not a solution, it does not need to be regenerated, and Hagg said that the moisture present in the flue gas is sufficient for the membrane to function.

'The membrane needs to be wet, and the flue gas is saturated enough so that the membrane can use the humidity from the gas to transport the carrier.'

NTNU will carry out a small pilot of the technology in the spring, with a larger-scale pilot planned in four years' time at four as-yet unnamed power plants in Europe.