Biofuel

Biofuels are fuels derived from biomass over relatively short timescales, in contrast to the slow geological formation of fossil fuels such as coal, oil, or natural gas. They can be produced from a wide range of organic materials, including plants, agricultural residues, household or industrial biodegradable waste, and certain types of algae. As a renewable form of energy, biofuels play a significant role in reducing dependence on finite fossil fuel reserves and supporting strategies for low-carbon energy transitions. Widely used in transportation and increasingly in heating and electricity generation, biofuels are considered central to contemporary discussions on sustainable energy systems.

Characteristics and Environmental Context

Biofuels are associated with lower greenhouse gas emissions compared with conventional fossil fuels when burned, as the carbon released during combustion was originally captured from the atmosphere by the feedstock crops. For this reason, many biofuels are often described as carbon-neutral. However, the overall environmental impact of biofuels depends heavily on factors such as land-use change, production methods, feedstock type, and processing technologies.
Life-cycle assessments (LCAs) show wide variability in emission outcomes. When production requires clearing forests or converting grasslands to cropland, the resulting emissions can offset or exceed the carbon savings delivered during fuel combustion. In favourable scenarios, advanced biofuels may even contribute to negative emissions, whereas in less favourable cases emissions may be comparable to fossil fuels. As a result, the climate mitigation potential of biofuels is highly situational.
Global demand for biofuels is rising rapidly. Forecasts indicate a 56% increase in consumption between 2022 and 2027, with biofuels projected to supply over half of the global transport fuel mix by 2027. Aviation biofuel demand—though still small in relation to road transport—is also expected to increase, though some policy structures continue to prioritise ground transportation fuels.

Terminology and Definitions

The term biofuel is used variably across publications. A broad definition includes any biobased solid, liquid, or gaseous fuel derived from organic materials such as crops, wood, or agricultural by-products. More specific definitions restrict the term to liquid or gaseous fuels suitable for transportation, such as bioethanol and biodiesel.
The IPCC defines biofuels as liquid fuels produced from biomass, explicitly excluding fossilised organic material. Under this definition, biomass refers to biological matter not embedded in geological formations and therefore renewable at human timescales. Common examples include sugarcane, maize, canola, soybeans, and similar crops used as feedstocks for bioethanol and biodiesel.

Conventional (First-Generation) Biofuels

First-generation biofuels are produced from food crops grown on arable land. Their production relies on converting sugars, starches, or plant oils into usable fuels:

• Sugar- and starch-based crops, such as maize, wheat, sugarcane, and sugar beet, are fermented to produce ethanol.
• Oil-rich crops, such as rapeseed, soybean, or palm oil, are processed via transesterification to produce biodiesel.

The reliance on edible feedstocks has led to extensive debate concerning the food-vs-fuel dilemma. Critics argue that diverting crops from food markets can increase food prices, intensify agricultural expansion, and contribute to deforestation and biodiversity loss.

Advanced (Second- and Third-Generation) Biofuels

Advanced biofuels are designed to reduce competition with food production by using non-food feedstocks or waste materials. They include fuels produced from:

• Agricultural residues such as rice straw, husks, and bagasse.
• Forestry residues including wood chips and sawdust.
• Perennial energy crops like switchgrass or jatropha grown on marginal land.
• Municipal solid waste, waste vegetable oils, and industrial by-products.
• Algae or other high-yield non-traditional feedstocks.

These biofuels can be produced through biochemical pathways such as enzymatic fermentation or through thermochemical methods including pyrolysis and gasification. By utilising residues and waste streams, advanced biofuels aim to minimise environmental impacts while maintaining significant energy potential.

Types of Biofuels

Liquid Biofuels

Ethanol is the world’s most widely used biofuel, particularly prominent in countries such as Brazil and the United States. It is produced through microbial fermentation of sugars extracted from crops like maize, wheat, sugarcane, molasses, and other starch-rich materials. Distillation and drying follow fermentation, processes that require substantial energy input. In Brazil, the energy required for distillation is commonly supplied by burning bagasse, while Europe often uses wood pellets, chips, or waste heat.
Ethanol can be used in its pure form (E100), but it is typically blended with gasoline to raise octane levels and reduce emissions. The increasing use of maize and sugar crops has stimulated research into cellulosic ethanol, derived from non-food sources such as grasses and woody biomass.
Other potential liquid biofuels include methanol and biomethanol, which may offer future renewable pathways if economically feasible. Biobutanol, produced through microbial fermentation, is also regarded as a promising substitute for gasoline due to its higher energy density and compatibility with existing fuel infrastructure. Genetically engineered strains of Clostridium and Escherichia coli have demonstrated potential for biobutanol production, though cost constraints remain a challenge.

Biodiesel

Biodiesel is the dominant biofuel in Europe and is produced through transesterification of plant oils or animal fats. Its chemical composition largely comprises fatty acid methyl or ethyl esters. Common feedstocks include soybean oil, rapeseed oil, sunflower oil, palm oil, jatropha, pongamia, hemp, and algae.
Pure biodiesel (B100) can reduce emissions by as much as 60% compared with conventional diesel. Research on second-generation biodiesel explores alternative feedstocks such as safflower oil and the use of biodiesel as a high-performance engine lubricant.

Other Biofuels

Further biofuel types include biomethane, syngas, and newer synthetic fuels derived from thermochemical conversion of biomass. These fuels are gaining attention for their potential roles in decarbonising sectors such as marine transport, heavy industry, and aviation.

Sustainability, Critiques, and Future Outlook

While biofuels contribute significantly to renewable energy portfolios, their sustainability is continually reassessed. Concerns include:

• Pressure on global food supplies due to crop diversion.
• Land-use change associated with expanding cultivation.
• Water consumption and fertiliser use linked to intensive agriculture.
• Biodiversity impacts from deforestation or habitat conversion.