Published online 3 September 2008 | Nature | doi:10.1038/news.2008.1077

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Gas can be stored in dry water

'Dry water' looks like a powder - but a quick squeeze is enough to release the fluid. A. Cooper et al / ACS

Methane and natural gas are usually shipped around in pressurized pipelines and canisters. But chemists have now developed a new way to transport the gases: as a powder.

Andrew Cooper and his colleagues at the University of Liverpool, UK, have found that they can trap methane in a bizarre material dubbed 'dry water', a mixture of silica and water that looks and acts like a fine white powder1. The methane reacts with the water to produce a crystalline material called methane gas hydrate, in which individual methane molecules sit inside ice-like cages of water molecules.

In principle, this could offer a way to store methane conveniently for use as a vehicle fuel. Methane-powered vehicles produce less pollution than those running off petroleum fuels.

“It looks like a powder, but if you wipe it on your skin, it smears and feels cold.” Andrew Cooper

University of Liverpool, UK

Natural gas is mostly methane, and so the new material could even be used as an alternative to pipelines to ship it from gas fields. "This is a very important issue", says E. Dendy Sloan, a specialist in natural hydrates at the Colorado School of Mines in Golden, who was not connected with the research. About 70% of the world's natural-gas stores, says Sloan, are in small, remote reservoirs from which it is not economical to run a pipeline.

The paradox of dry water

Methane gas hydrate forms naturally when water is mixed with methane at high pressure and low temperature. Huge deposits of the crystalline substance exist in the deep sea, where they could provide vast fuel reserves. But rising global temperatures increase the chances of the hydrate decomposing, releasing the greenhouse gas and accelerating further warming. This mechanism has been proposed as a cause of dramatic environmental change in the distant past.

Using methane gas hydrate as a kind of 'solid methane' for storage and transport has been mooted before. The Japanese company Mitsui Engineering and Shipbuilding has a pilot project for producing natural-gas hydrates on board ships that would then transport the gas from remote marine deposits — and using some of the stored gas to power the ships themselves.

The problem is that the hydrate forms only under cold, pressurized conditions, and then very slowly. Typically, a skin of the material forms at the surface of water and prevents further growth. The formation rate can be speeded up by vigorously mixing the gas with water, but that is costly and cumbersome.

Cooper and his colleagues have got round this problem by finding a way to break the water up into many tiny, stable droplets, massively increasing the surface area in contact with gas. They do this by converting water to 'dry water' by stirring it up with a special form of silica, called hydrophobic fumed silica.

This consists of tiny grains of silica – the same basic material as sand – coated with a chemical layer that makes them water-repellent. The silica particles cover the surface of water droplets and stop them from coalescing.

"If you've ever seen water drops in dry dust, it's the same thing", says Cooper. "They form a ball with the dust on the surface." The resulting 'dry water' is a very odd substance. "It looks like a powder", says Cooper, "but if you wipe it on your skin, it smears and feels cold" as the water is released.

The researchers found that their powder soaks up large quantities of methane at water's normal freezing point, producing crystalline methane gas hydrate within the silica-coated drops. A litre of methane gas can be stored in about 6 grammes of the material. This storage capacity, they say, is very close to the target set by the US Department of Energy for such materials, and compares well with that of other candidate storage media.

And crucially, it is made from cheap raw materials, helping to make this method economical relative to other, more exotic potential methane-capture materials such as designed molecular frameworks2.

Cool it down, whizz it up

But there remain many obstacles to making this a viable industrial process. For one thing, the hydrate remains stable only if kept cold. It must be refrigerated to about minus 70°C at atmospheric pressure, although this temperature threshold is higher if the hydrate sits within an environment of pressurized methane. The methane is released again if the material warms up.

Cooper says that, by using salts dissolved in water, his group has recently developed hydrates that remain stable at room temperature. But it's hard to make stable 'dry water' from this mixture. That's the next step, which Cooper admits is "very difficult".

Another problem is that the droplets start to coalesce and therefore react more slowly with methane after several cycles of gas uptake and release. But the original fine powder can be regenerated by whizzing the mixture again in a blender — handy, but not if it has to be done too regularly.

"In general with gas storage, it's difficult to hit all the targets at once", says Cooper, adding that "we're not anywhere close to working out the economics of this method yet".