Oil addiction: Fuelling the debate

There are two realistic immediate alternatives to oil. Chris Barton explains their advantages and disadvantages

Biodiesel

A combustible fuel that is physically similar to petroleum diesel but made from natural, renewable sources - mainly from vegetable oils such as soy and corn, but also from tallow (animal fat). It can be blended in different ratios with diesel. The fuel is made through a chemical process called transesterification whereby glycerin is separated from the fat or vegetable oil leaving behind two products - methyl esters (the chemical name for biodiesel) and glycerin (a by-product usually used in soaps and other products).

Pros:

* Significant reduction in pollutants that contribute to smog and global warming.

* Can be used in any diesel engine with few to no modifications.

* Reduces our dependence on imported crude oil, so we're less vulnerable to international oil price increases.

Cons:

* A 100 per cent biodiesel fuel may degrade certain types of rubber used in fuel hoses or fuel pump seals over time - not a concern with newer engines that contain parts designed for low-sulphur diesel.

* Like conventional diesel, biodiesel will cloud and gel at very cold temperatures, but blends like B20 are only slightly more sensitive than diesel.

* Slightly increased levels of harmful nitrogen oxide emissions.

Bioethanol

A renewable resource, easily obtained from sugar or starch in crops and other agricultural produce such as grain, sugarcane or even lactose. It's made by fermentation and distilling. In New Zealand bioethanol is made from whey, a by-product of the dairy industry. Bioethanol can be blended with petrol, and blends are widely used in the United States, Canada, Australia, Europe, South America and many Asian countries.

Pros:

* Renewable source of fuel which reduces overall emissions of greenhouse gases.

* Reduces our dependence on imported crude oil, so we're less vulnerable when it comes to international oil price increases.

* Low level blends (up to 10 per cent) of ethanol in petrol can be used in all vehicles without modification.

Cons:

* Can't be used in aircraft, boats or in vehicles or power tools which may be left unused for some time.

* May not be suitable for some vehicles more than 10 years old.

* Slight increase in volumetric fuel consumption due to the lower energy content of ethanol compared with petrol.