http://www.newscientist.com/article.ns?id=dn7891

Coal-powered fuel cell aims for efficiency
17:42 23 August 2005
NewScientist.com news service
Duncan Graham-Rowe

A new coal-powered fuel cell may lead to a more efficient way of
extracting energy from the fossil fuel than simply burning it.

“The idea was to look at a way of converting the chemical energy in
coal directly into electrical energy,” says Douglas Weibel, at Harvard
University in Cambridge, Massachusetts, US.

In conventional power plants, coal is burned to produce heat, which is
then used to drive steam turbines and generate electricity. But during the
conversion of one form of energy to another, about 65% of the coal's
energy is lost, says Weibel.

Efforts have been made in the past to use coal in fuel cells but these
have required electrolytes of molten carbonate, involving temperatures
between 600°C and 900°C. Not only do such high temperatures reduce efficiency
but they also make the fuel cells prone to corrosion.

Weibel's new design allows electricity to be generated at just 100°C, a
temperature that is far easier to work with. But the efficiency of the
current prototype device is "horrid”, he admits. At 7%, it is roughly
one-fifth as efficient as conventional power stations in extracting
energy from coal, he says.

Inevitable emissions
Weibel, working with colleague George Whitesides, created the new cell
by adding iron ions to a slurry of coal powder, mixed into an electrolyte
of sulphuric acid. The ferric iron ions are reduced by the coal, allowing
a typical fuel cell reduction-oxidisation cycle to take place, generating
electricity.

The cells still produce the greenhouse gas carbon dioxide, but with
coal this is essentially impossible to avoid, says Weibel. But if the
efficiency of the fuel cell can be improved above that of power stations, then
more energy could be harvested from the world’s vast coal reserves, with no
concomitant increase in CO2 emissions. The US and other countries have
substantial "clean coal" research programmes.

Very simple and obvious improvements could make a huge difference to
the efficiency of the cell, says Weibel – using a finer form of coal
powder, for example, or enhancing the design of the electrodes and placing them
closer together.

Large-scale units
Such improvements would be crucial if the fuel cells were to have any
chance of competing with conventional coal generators, says Matthew Leach, at
the Energy Policy Management Group at Imperial College London, UK.

Leach says we should not think of this technology as being analogous to
hydrogen fuel cells, which might be used in cars, but as larger-scale
power generation units. “There is research on using relatively large fuel
cells in combination with gas turbines,” he says.

But Leach believes the portability of fuel cells is one of their real
benefits. With coal fuel cells, he says, this seems limited given the
difficulties of distributing a sulphuric acid-based coal slurry.

Journal reference: Angewandte Chemie International Edition (DOI:
10.1002/anie.200501192)

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