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'Dammit, Number One, this fuel's gone mouldy'

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A study by marine engineering experts has set out the likely effects of using biodiesel fuel in Royal Navy warships. As British warships are mostly powered by gas turbines derived from aircraft jet engines, the results are also interesting in the context of future biofuelled aviation and power generation.

The current Jane's Defence Weekly reports on the study by John Buckingham, chief mechanical engineer at naval architecture and technology firm BMT Defence Systems. The study also draws on postgraduate research by naval engineering officer Lieutenant Roy Casson RN.

Biodiesel isn't the same as the headliner, mainstream biofuel ethanol. Ethanol must currently be made from food crops such as corn, and has as a result enjoyed strong political support from farming lobbyists in Europe and America. But ethanol is nowadays seen by many as causing starvation by driving up food prices. There is also serious doubt about its carbon-emissions benefit and indeed how much of it could ever be produced, given finite amounts of farmland.

Biodiesel is different. Chemically it is a fatty acid methyl ester, and it can be made from various feedstocks such as rapeseed (canola) oil or palm oil. These are subject to most of the same criticisms levelled at corn ethanol; in particular, palm-oil farming is said to have caused significant rainforest destruction. However, useable biodiesel can also be made from the jatropha plant, which can grow in deserts where nothing much else will. There are also hopes that waterborne algae farms might one day produce economically viable, relatively green fuel.

Many renewable fuel fanciers reckon that biodiesel could be the way ahead for transport fuel. As its name implies, it can be used in diesel engines, so it's OK for cars, trucks and merchant shipping. But can you use it in a warship?

The answer seems to be a qualified yes. Buckingham and Casson say that one of Her Majesty's ships could expect to lose around five per cent of its top speed and 13 per cent of its endurance running entirely on suitable types of biodiesel, which wouldn't usually be a deal-breaker. Palm oil fuel won't do, though, as it solidifies too easily at low temperatures. For a warship, the best current feedstocks are rapeseed/canola, sunflower and linseed oils.

There would be a few other issues. In some circumstances biofuel will go off, due to pesky microbes multiplying in it or simple "reversion to fatty acids". This is especially likely if water gets in the tanks, always an issue for ships and harbour bunkers. Buckingham says that warships of the future would do well to keep a stock of microbe-killing biocide preservatives and other handy chemicals on board, but it might still be necessary to scoop out build-ups of microbial sludge to prevent it damaging the engines. The best thing would be to use the biodiesel up as quickly as possible, before it started rotting or going all fatty-acid.

All in all, though, running gas turbine warships on biodiesel would seem to be relatively practical. Similar recent studies indicate that combined-cycle gas powerplants, of the sort which generate much of the UK's electricity (and give the grid its ability to deal with surges in demand) can do it too. And there has already been a successful airliner test using similar juice.

As ever with biofuel though, it's the sustainable fuel production that's the real challenge. Jatropha, algae and the rest of the non-cropland, starvation- and deforestation-free solutions remain unproven. Even if they can be got to work, they may still require energy and carbon intensive fertilisers and processing, robbing them of any serious greenness.

The green warship would seem to be a long way off, then. But at least the Royal Navy might enjoy a bit of freedom from oil imports out of this; the UK currently grows large amounts of rapeseed.

The Jane's article requires a subscription, but the full Buckingham paper can be read here (pdf). ®

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