Biofuel is a term used to describe all fuels of organic origin. Coal, natural gas, and oil are biofuels found underground that were formed by the decomposition of plants millions of years ago. Plant material trapped between layers of sediment was subjected to temperature and pressure changes by movements of Earth’s crust, which converted it to carbon and hydrocarbon compounds. Because of their age, such materials are known as fossil fuels and are nonrenewable. More recently, the term biofuel has been used to describe renewable sources of energy derived from wood, waste materials, and crop plants.
All plants contain stored chemical energy in their cells that can be converted into useful forms of energy, such as heat. The process of converting organic material, or biomass, into energy is called bioconversion. Energy can be released from biomass by a number of processes—burning, fermentation, or chemical or bacterial breakdown. Burning provides heat that can be used directly or to drive turbines to produce electricity. Fermentation converts plant sugars into carbon dioxide and ethanol, which can then be used as a liquid fuel. Chemical breakdown can provide synthetic gases and fuel oils, while bacterial processes can be tailored to produce methane, alcohols, and a range of other chemicals.
Wood and charcoal
Wood is the world’s oldest fuel and is still the main source of energy for cooking and heating in developing countries. Its use in industrialized nations has been increasing since the 1980s, when massive rises in the price of oil and improvements in wood-burning stoves made it an attractive proposition to rural homeowners in countries including the United States. Industry, too, is reverting to wood at a growing rate. Furnaces burning sawdust, wood chips, or special wood pellets are becoming very popular since the introduction of a new type of burner, the fluidized bed combustor, which is extremely efficient and can even burn wood that is wet.
Electric power plants also burn wood, and in many areas where there is no coal and nuclear power plants are too expensive to build, it is still the best fuel. Small power plants, of about 50 MW output, can supply rural communities with electricity economically by burning wood chips. The Vermont Gasifier in Burlington, Vermont, uses a low-pressure process to turn wood chips into gas that is then fed to high-efficiency turbines that convert it to electricity. Burning wood can cause pollution problems—the smoke from some wood fires contains many substances that may cause illness, even cancer. However, wood contains little sulfur, so the acid rain that is caused by the release of sulfur oxides from coal and oil does not occur with wood combustion. Where wood is the only cheaply available fuel, as in less-developed countries, this advantage may be counterbalanced by the destructive environmental effects of deforestation.
More promising is the conversion of wood to wood alcohol, better known as methanol or methyl alcohol. Being liquid, it is suitable for powering aircraft and motor vehicles that currently run on oil-based products. Motor cars will run satisfactorily on methanol, and diesel engines will work well on a mixture of 95 percent methanol and 5 percent oil. Blends of methanol and gasoline are already in use in Europe and many other countries are considering methanol-from-wood pilot plants. When engines are fueled with methanol, no lead is added and much less pollution is produced than when they are fueled by gasoline or diesel.
Charcoal is another fuel made by burning wood in a much reduced air supply. This process converts the complex chemicals in the structure of wood into simpler ones that burn more easily.
