Alcohol

The four major alcohols that have been considered of interest as alternative fuels are methanol, ethanol, propanol and butanol. All four of these alcohols may be synthesised biologically (termed bioalcohols), and have characteristics which enable them to be used in vehicle engines. They can offer a range of environmental benefits: they are biodegradable, renewable, produce less local air pollutants than fossil fuels and reduce tailpipe emissions of many pollutants.

Ethanol

Bioethanol is a high octane liquid fuel made from plant material and can be used as a substitute for use in petrol engines or as a petrol additive. It is currently the world's main biofuel, and is used in countries such as Brazil, Sweden, Pakistan, India, China, Thailand and Japan. It can be produced from crops such as oilseeds, sugar beet, sugarcane and corn (maize), and in the future it may also be possible to produce it economically from straw, wood and even household waste.

When used as a blend of 5% with petrol, bioethanol may be used in cars without any need for engine modification. However, a number of manufacturers (for example, Ford and Saab) have produced flexible fuel vehicles that can run on either E85 bioethanol blend (a blend of 85% ethanol and 15% petrol) or normal petrol in the same fuel tank in any mix.

However, high concentrations of ethanol may degrade some plastic or rubber parts of fuel delivery systems, particularly in older cars, and has 37% less energy per litre than petrol.

Methanol

Whilst methanol may be obtained from natural gas or petroleum, it may also be acquired from agricultural biological waste, such as from wood, coal or eucalyptus. Methanol is almost always used as an additive to petrol engines, rather than on its own. However, the Indy Racing League used pure methanol from 1965 up until 2006, switching to an all-ethanol mix in 2007.

Methanol is even more corrosive than ethanol, and is highly toxic; extensive exposure to it could lead to permanent health damage, including blindness. It is also volatile, which could increase the risk of fires and explosions compared to ethanol. An additional problem of methanol is that its energy content is only 45% that of petrol, and 75% of ethanol. (Petrol = 30 megajoules/litre, ethanol = 22 megajoules/litre, methanol = 16 megajoules/litre).

Propanol and Butanol

Propanol and butanol have also been considered for use as alternative fuels; however, although they are considerably less toxic and less volatile than methanol, the fermentation process used to extract them from cellulose is relatively difficult to carry out. The conversion also produces an extremely unpleasant smell, a factor which must be taken into consideration when designing and locating a fermentation plant.

However, despite these drawbacks, British Petroleum, British Sugar Corporation and DuPont have joined forces to convert an ethanol fermentation plant in Wissington, Norfolk to produce biobutanol fuel from sugar beets. Production is expected to start in 2007. They are also looking at the feasibility of building larger plants in the future.