Biofuels are fuels derived from biomass. Biomass is organic matter taken from or produced by plants and animals. It comprises mainly wood, agricultural crops and products, aquatic plants, forestry products, wastes and residues, and animal wastes. In its most general meaning, biofuels are all types of solid, gaseous and liquid fuels that can be derived from biomass. Examples of solid biofuels include wood, charcoal and bagasse. Wood and charcoal are widely used as fuel for domestic purposes such as cooking in the rural areas of most developing countries. Waste bagasse, the fibrous material produced from sugar cane processing, is extensively used for steam and electrical power generation in raw sugar mills. Examples of gaseous biofuels include methane gas and producer gas. Methane gas is produced from the anaerobic fermentation of animal wastes, wastewater treatment sludge and municipal wastes in landfills. On the other hand, producer gas can be made from the pyrolysis or gasification of wood and agricultural wastes. Examples of liquid biofuels include methanol, ethanol, plant oils and the methyl esters produced from these oils commonly referred to as biodiesel.
The focus of this book is on liquid biofuels that are used primarily as transport fuel. In particular, it covers biofuels from plant oils. However, this chapter will briefly discuss the production of ethanol from sugar crops, starchy crops, and cellulosic biomass in order to provide a more complete overview of the two major types of biofuels that are commercially available, namely, ethanol and biodiesel.