Biofuel can be broadly defined as solid, liquid, or gas fuel consisting of or derived from biomass. It is material derived from organisms such as plants, animals and their by-products. Manure, garden waste and crop residues are all sources of biomass.

The definition used here is narrower, Biofuel is defined as liquid or gas transportation fuel derived from biomass. It can also be used directly for heating or power; that is called biomass fuel.

Biofuel is considered an important means of reducing greenhouse gas emissions and increasing energy security by providing a viable alternative to fossil fuels. and is used globally. Biofuel industries are expanding in Europe, Asia and the Americas. The most common use for biofuels is in automotive transport. Biofuel can be produced from any carbon source that can be replenished rapidly e.g. plants. Many different plants and plant-derived materials are used for biofuel manufacturing.

Biomass: It is a renewable energy source based on the carbon cycle, unlike other natural resources such as petroleum, coal, and nuclear fuels. Agricultural products specifically grown for biofuel production include corn and soybeans, in the United States; rapeseed, wheat and sugar beet in Europe; sugar cane in Brazil; palm oil in South-East Asia; and jatropha in India.

Biodegradable outputs from industry, agriculture, forestry and households can be used for biofuel production, either using anaerobic digestion to produce biogas or using second generation biofuel processes; examples include straw, timber, manure, rice husks, sewage, and food waste. The use of biomass fuels can therefore contribute to waste management as well as fuel security and climate change.

Carbon emissions: Biofuels and other forms of renewable energy aim to be carbon neutral. This means that the carbon released during the use of fuel, e.g. through burning to power transport or generate electricity is reabsorbed and balanced by the carbon absorbed by new plant growth. These plants are then harvested to make the next batch of fuel. Carbon neutral fuels lead to no net increases in atmospheric carbon dioxide levels, which mean that global warming will not get any worse.

In practice, biofuels are not carbon neutral. This is because energy is required to grow crops and process them into fuel. Examples of energy use during the production of biofuels include: fertiliser manufacture, fuel used in power machinery and for transporting crops and fuels to and from biofuel processing plants. The amount of fuel used during biofuel production has a large impact on the overall greenhouse gas emissions savings achieved by biofuels.

The carbon emissions produced by biofuels are calculated using a technique called Life Cycle Analysis (LCA). This uses a “cradle-to-grave” or “well-to-wheels” approach for calculating the total amount of carbon dioxide and other greenhouse gases emitted during biofuel production, from putting seed in the ground to using the fuel in cars and trucks. Many different LCAs have been done for different biofuels, with widely differing results. The majority of LCA studies show that biofuels provide significant greenhouse gas emissions savings when compared to fossil fuels such as petroleum and diesel.Using biofuels to replace a proportion of the fossil fuels that are burned for transportation can reduce overall greenhouse gas emissions.

The well-to-wheel analysis for biofuels has shown that first generation biofuels can save up to 60 per cent carbon emission and second generation biofuels can save up to 80 per cent as against using fossil fuels.

Bioenergy from waste: Using waste biomass to produce energy can reduce the use of fossil fuels, reduce greenhouse gas emissions and reduce pollution and waste management problems. A recent publication by the European Union highlighted the potential for waste-derived bio-energy for reducing the global warming. The report concluded that 19 million tons of oil equivalents will be available from biomass by 2020, 46 per cent from bio-wastes: municipal solid waste, agricultural residues, farm waste and other biodegradable waste streams.

The most common first generation biofuels are listed below.

Vegetable oil: Vegetable oil can be used for either food or fuel; the quality of the oil may be lower for fuel use. Vegetable oil can be used in many older diesel engines (equipped with indirect injection systems), but only in warm climates. In most cases, vegetable oil is used for manufacturing biodiesel, which is compatible with most diesel engines when blended with conventional diesel fuel. No engine manufacturer explicitly states that straight vegetable oil can be used in their engines. Used vegetable oil (e.g. from deep fat fryers) can be filtered and processed into biodiesel.

Biodiesel: Bio-diesel is the most common biofuel in Europe. It is produced from oils or fats using transesterification and is a liquid similar in composition to mineral diesel. Its chemical name is fatty acid methyl (or ethyl) ester. Oils are mixed with sodium hydroxide and methanol (or ethanol) and the chemical reaction produces biodiesel and glycerol. One part glycerol is produced for every 10 parts biodiesel.

Biodiesel can be used in any diesel engine when mixed with mineral diesel. In some countries manufacturers cover their diesel engines under warranty for 100 per cent biodiesel use, although Volkswagen (Germany), for example, asks drivers to make a telephone check with the VW environmental services department before switching to 100 per cent biodiesel. Many people run their vehicles on bio-diesel without problems. However, the majority of vehicle manufacturers limit their recommendations to 15 per cent biodiesel blended with mineral diesel. In many European countries, a five per cent bio-diesel blend is widely used and is available at gas stations.

In the USA, more than 80 per cent of commercial trucks and city buses run on diesel. The nascent US market of bio-diesel is growing at a staggering rate—from 25 million gallons per year in 2004 to 78 million gallons by the beginning of 2005.

Bio-ethanol: Ethanol is the most common biofuel worldwide. It is an alcohol fuel. It is produced by fermentation of sugars derived from wheat, corn, sugar beet and sugar cane. Production methods used are enzymatic digestion (to release sugars from stored starches e.g. from wheat and corn), fermentation of sugar, distillation and drying. Ethanol can be used in petrol engines as a replacement for gasoline and can be mixed with gasoline to any percentage. All petrol engines can run on blends of up to 15 per cent bio-ethanol with petroleum/gasoline. For higher percentage blends, engine modifications are needed. Many car manufacturers now produce flex-fuel vehicles which can run on any combination of bio-ethanol and petrol, up to 100 per cent bio-ethanol.

Methanol: Methanol is currently produced from natural gas, a fossil fuel. It can also be produced from biomass. The methanol economy is an interesting alternative to the hydrogen economy.

Alcohols: Biologically produced alcohols, most commonly ethanol and less commonly propanol and butanol, are produced by the action of micro-organisms and enzymes through fermentation.

Biogas: Biogas is produced by the process of anaerobic digestion of organic material by anaerobes. It can be produced either from bio-degradable waste materials or by using energy crops fed into anaerobic digesters to supplement gas yields. The solid byproduct, digestate, can be used as a bio-fuel or a fertiliser.

Biogas contains methane and can be recovered from industrial anaerobic digesters and mechanical biological treatment systems. Landfill gas is a less clean form of biogas produced in landfills through naturally occurring anaerobic digestion. If it escapes into the atmosphere it is a potent greenhouse gas.