Field of dreams?

From the earliest days of the coal-powered railways through the rise of the internal combustion engine and into the jet age, mechanised transport has always depended on fossil fuels.

But concerns over climate change, increasing scarcity of fossil fuel resources and worries over energy security are leading to new ideas across industry, and vehicle fuel is no exception.

Attention is now turning to fuels derived not from fossilised vegetable matter, but from freshly harvested plants. Biofuels are big business, and are gathering headlines.

Most biofuels are derived from plant oils — rapeseed, palm oil, soya oil and so on. These are rich in fatty acids, which can be converted by a simple chemical process to produce compounds called fatty acid methyl esters (Fames), which have similar properties to diesel. They can be blended with petrochemical diesel, at concentrations of 5-10 per cent, without compromising the performance of the diesel engine.

And because they are derived from plants, they are effectively carbon-neutral when they burn — the CO₂ released from their combustion would have been released anyway when the plants decomposed.

Although biofuels are big business in South America and Malaysia, where sugar-cane derived ethanol has been used to fuel cars for decades, they have yet to make an impact in the UK.

This is set to change, thanks to legislation that will mandate a 2.5 per cent proportion of biodiesel (in diesel) or bioethanol (in petrol) by 2009, going up to five per cent by 2011. The European Union wants this to rise to 20 per cent by 2020.

And this is where some of the problems inherent in Fame production come to the fore. Fame is made from crops that are grown for food, which puts pressure on land use. It is not very efficient — according to Jeremy Tompkinson, the chief executive of the UK’s National Non-Food Crops Council (NNFCC), a hectare of land can produce about 1.2 tonnes of fuel. So as this first-generation biofuel is being developed, engineers are looking to develop a new suite of technologies, known as generation 2, which have a quite different basis.

While generation 1 technology converts the plant oils, which are chemically similar to the target fuel compounds, generation 2 uses the plant purely as a source of carbon. A series of processes ‘gasify’ the complex cellulose and other carbon-containing natural polymers in plant material, breaking them down into simple forms, to form a mixture of carbon monoxide and hydrogen known as synthesis gas or syngas. It then uses the Fischer-Tropsch process, a technology invented to fuel Germany’s World War I effort, to reassemble these simple molecules into long-chain hydrocarbons, suitable for burning in internal combustion engines.

This technology does not require crops to be grown specifically for biofuels. Rather than using the valuable and nutritious oils from crops, the waste material —wheat straw, stems and husks from corn, residues from sugar cane production - can be used instead. Waste wood could become a source of diesel. Even municipal waste and sewage could be pressed into service. Anything that contains carbon could be grist to the mill. At least, that’s the theory.

In practice, things are more complicated. The technologies needed for generation 2 biodiesel are available — gasifiers have been used for decades to convert coal into syngas in oil-poor regions such as South Africa, and the Fischer-Tropsch process is well-understood. But the versions on the market are not suitable for use with biomass. David Bown, technology manager for natural resources at engineering consultancy AMEC, said there is relatively little knowledge of how to adapt gasifiers for biomass-to-liquids (BTL) applications.