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Boffins unveil self-decoking steampunk fuel cell

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Boffins in the States say they've come up with a way of making fuel cells run on coal gas - the fuel of Victorian streetlights - and steam. Their plans would, they say, offer much lower carbon emissions than normal coal power: and much easier sequestration of exhaust gases as their technology would emit nearly pure CO2.

"This could ultimately be the cleanest, most efficient and cost-effective way of converting coal into electricity," says Professor Meilin Liu, engineer at the Georgia Institute of Technology. "And by providing an exhaust stream of pure carbon dioxide, this technique could also facilitate carbon sequestration without the separation and purification steps now required for conventional coal-burning power plants."

The idea is to make use of solid-oxide fuel cells, capable of running on hydrocarbons directly without needing to reform them into hydrogen. These would be run on fuel gas produced from coal, the way that millions of gaslights did back in the 19th century.

The snag here is that normally if you run SOFCs on coal gas their anodes coke up with carbon deposits in short order and they cease operation in about half an hour. But Liu and his colleagues believe they've cracked this by using a new process to apply tiny "islands" of barium oxide onto their SOFCs' nickel and yttria-stabilized zirconia (Ni-YSZ) electrodes.

Then, water is introduced into the coal-gas supply as hot steam. When this touches the barium oxide it splits into protons and hydroxide (OH) ions. The OH-1 ions combine with the carbon depositing on the electrode before dissociating again as carbon monoxide and hydrogen: these gases then oxidise to power the fuel cell, resulting in water and CO2, and the cell thus cleans off the coking carbon as it goes thanks to the presence of the barium oxide speckling its electrodes.

In an operational plant, about half the CO2 would be recirculated back for use in gasifying the coal. The rest, once the water was removed, would be emitted as a pure stream of CO2: much easier to capture and use or sequester than the stack gases coming out of a normal coal power station.

On its own, according to the researchers, the self-cleaning SOFC tech would be a lot more efficient than a coal station, delivering around 50 per cent of the energy in the coal as opposed to 33. If hybridised with coal-gas burning turbines this could climb to 80 per cent. Either way you would burn less coal to get a given amount of electricity.

Thus far the self-cleaning SOFCs have been tested for only 100 hours at a stretch, so they need to show they can stand up to years of operation without coking up. They'll also need to be tried at lower temperatures to reduce manufacturing costs: but Liu and his colleagues think they ought to work well at this as coking is usually the barrier to low-temperature operation.

"Reducing the operating temperature significantly by eliminating the problem of carbon deposition could make these solid oxide fuel cells economically competitive," says the prof.

The prospect of pure CO2 emissions from future coal-powered fuel cells needn't dismay those who are sceptical about sequestration plans. There are various cunning schemes to manufacture synthetic "natural" gas or even petrol from CO2, using surplus renewable or nuclear power. One of the great obstacles facing these ideas has been the difficulty of actually obtaining a large supply of useable CO2 not full of troublesome contaminants.

The new self-decoking SOFC tech has also been tested using propane instead of coal gas, again with apparent success.

The new research is published today by Nature Communications. ®

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