Fuel from plants

Figure 1 Diagram of a calorimeter, used to measure the heat released when a sample is burnt, which gives a measure of the chemical energy the sample contains

All of the food we eat contains chemical energy initially derived from sunlight. We know that it is photosynthesis that transfers energy from light into chemical energy, and we are used to talking about ‘calories’ in our food. You may have estimated the amount of chemical energy in a foodstuff or a plant using a calorimeter (see Figure 1). This instrument measures the heat released when a sample is burnt. The greater the amount of heat energy released, the higher the calorific (chemical energy) content of the material. Combustion of fossil fuels to provide us with heat relies on the chemical energy stored in plants when they were alive (see Figure 2). With an ever-decreasing supply of fossil fuels, however, we are constantly on the lookout for alternative sources of energy.

Wind, wave and solar power are dependent on unpredictable natural elements, and they and nuclear power are controversial. So we are increasingly turning to biofuels — fuels derived from modern-day plants. Crops including wheat, maize and sugar cane can be used as biofuels (see Figure 3). The sugar, starch, or vegetable oil obtained from the crops is converted into biodiesel or ethanol. These are known as first generation biofuels, as their use is based on plant species we have long harvested for food. Their production therefore reduces both the availability of the foodstuffs and land suitable for cultivation of food crops. Second generation biofuels get around this disadvantage. These are derived from plant matter that is not normally used — biomass such as the parts left behind when grains have been extracted from a crop. However, second generation biofuels have their own disadvantage in that it is usually difficult and environmentally unfriendly to extract the fuel from such sources. A long series of physical and chemical treatments is required.