Biofuel refers to many different types of alternative energy sources that could supplement or even replace fossil fuels. Although they only account for a few percent of the world's transport fuel to date, they are increasingly popular due to higher oil prices and an increasing concern with global warming and investments into them are therefore growing each year. Biofuels are normally divided into three categories: solid biomass, liquid fuel and biogases. Each group does not only effectively describe the form of the fuel, but also hints at the uses for which the fuel is intended.
Solid biofuels are biomasses that may be burnt directly for energy, as has been done for thousands of years. Wood, charcoal, dried manure, non-food energy crops and domestic refuse are examples of these, as are things like sawdust and grass cuttings. Using especially the latter three is an excellent way of handling the disposal of these products, never mind the significant amount air pollution that they produce when burnt. However even if any of these were conveniently portable or burnt cleanly enough to be suited to engines, none of them pack enough of a punch to be viable as vehicle fuel; it would take unreasonable quantities thereof to provide enough motive power to travel a mere kilometre.
The most common liquid biofuel is ethanol, which is made from fermentation of any sugar or starch from which alcohol may be made. It can also be produced from cellulosic combustion of bagasse and similarly inedible waste products or non-food energy crops. Green diesel and biodiesel are some of many other forms of liquid biofuels that may be produced from vegetable or animal fats. All are mainly used for vehicle locomotion as they burn cleanly, are easily portable and contain high amounts of energy, thus making them compatible with combustion engines.
Gaseous biofuels tends to be used either for electricity generation or, in the case of those that can be stored in liquid form, for vehicle propulsion. An example of the former type is biogas, which is essentially methane gas produced from biodegradable waste or energy crops. Syngas, which is mixture of carbon monoxide and hydrogen derived from partial combustion of biomass, is better representative of the latter group. Not only can it be used directly in combustion engines or turbines, but it can also be used to produce methanol and hydrogen or even be converted into diesel substitutes or gasoline.
Biofuels are very much under development, for which the above types are only a sample of what might be available in only a few years' time. Generally the oil- and alcohol-based products listed above are referred to as the first generation of biofuels, as are other biofuels based on products that are normally considered part of the food chain. The second generation of biofuels is thus those that have taken a step away from consumable items, instead generating energy from human waste or non-edible plants. Cellulose-based ethanol fermentation is the best example of this in how it makes use of waste products like gabasse, which is made possible by refined techniques and new scientific discoveries. Currently third- and fourth generation biofuels are also researched by various companies all over the world. These promise not only to locate yet more potential energy sources, some possibly far more potent than existing ones in terms of energy content, but also to expand the uses of and improve upon existing techniques for producing the ones that are already in the market. In other words we are likely to hear a lot more about biofuels in the future.