5 Transport biofuels: biodiesel and bioethanol

Biodiesel is an alternative fuel to diesel, and bioethanol can be used in place of petrol. Biodiesel cannot be used on its own in traditional internal combustion engines but can be blended with traditional petroleum fuel. Bioethanol can be used on its own or in a blend. Overall, biofuels make up 5% of the petroleum blend in the UK. The UK Government set a target that 5% of road transport fuel should be from renewable (non-fossil fuel) sources by 2010.

Figure 12 Due to increasing demand for transport fuel, alternatives to petrol and diesel are under scrutiny.

Biodiesel is produced from oil-seed crops, such as oilseed rape, sunflower oil, palm oil and soybean oil. Oilseed rape is the main oil crop in the UK and yields approximately 1300 litres of biodiesel per hectare planted. The fuel is synthesised from the oil seeds by mixing them with two industrially sourced chemicals, methanol and either sodium hydroxide or potassium hydroxide. This reaction causes the formation of fatty acids and glycerol, with the fatty acids reacting further to produce biodiesel. Most biodiesel production is currently in Europe, but it is not limited to the developed world. Tanzania, for example, has started producing biodiesel from the nuts of the croton tree (Croton megalocarpus), a plant native to South-East Asia. Several countries, notably India, are using biodiesel obtained from Jatropha curcas oil as a fuel; indeed the trains that run between Mumbai and Delhi now use diesel that contains 15% biodiesel obtained from Jatropha. This plant has several advantages as a source of biofuel: it can tolerate drought conditions, and it grows on poor land that is unsuitable for agriculture and so does not compete with food production.

Figure 13 (a) Sunflowers are just one oil crop that is being investigated as a potential biofuel. (b) Tropical plant species, such as Jatropha curcas, could also be a potential source.

Germany used significant quantities of bioethanol as a biofuel during the Second World War, but recently interest in this fuel source has spread. Currently, countries using bioethanol include Brazil, France, the USA, the UK, Argentina and South Africa. In Brazil use has been particularly widespread; however, this has been at the expense of large tracts of natural vegetation including rainforests. It is estimated that one-third of cars in Brazil run entirely on bioethanol and the remaining two-thirds use a mixture of bioethanol and petrol (gasoline). Bioethanol is produced by fermenting plant sugars, or it can be made by taking starch-rich plant material, treating it to convert the starch to sugars, and then fermenting them. A third, less productive, method is to take stems or leaves and convert the tough cellulose from cell walls into sugars for fermentation; but this requires additional steps, including treating the plant material with sulphuric acid and heating to convert the cellulose into sugars, or using enzymes to do this. Both sulphuric acid and the necessary enzymes have to be produced by industrial means and this incurs an extra cost.

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Figure 14 The production of biodiesel from waste cooking oil or newly produced plant oil is increasingly gaining favour in the developed world. However, the amounts produced are currently very small compared with conventional petroleum.

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Figure 15 The steps involved in the production of bioethanol.

Bioethanol is currently produced by the fermentation of sugar by microbes. Microbes use the sugar as a food source and convert it to ethanol. In order for the ethanol to be used as fuel it needs to be separated from the other components of the fermentation reaction, usually by the process of distillation. Typically the mixture is heated and, as the ethanol boils at a lower temperature than the other components, it is transformed into a gas first and this gas can be collected and then condensed (transformed back to liquid bioethanol). The purified and bioethanol can be mixed with petrol for use in car engines.