What Are Biofuels?

Biofuels are liquid, gaseous, or solid fuels derived from biological feedstocks, including plant matter, animal fats, agricultural residues, algae, and organic waste. Unlike fossil fuels, which release carbon fixed over geological timescales, biofuels combust carbon that was recently captured from the atmosphere by growing organisms, giving them a potentially lower net carbon footprint over their life cycle. The two most commercially significant biofuels are ethanol, blended with gasoline as a transportation fuel, and biodiesel, used as a substitute or supplement for petroleum diesel. The field draws on fermentation chemistry, thermochemical processing, agricultural engineering, and life-cycle assessment.

In the United States, ethanol production totaled approximately 15.4 billion gallons in 2022, primarily from corn starch fermentation, while combined biodiesel and renewable diesel output reached about 3.1 billion gallons, according to the U.S. Energy Information Administration. The International Energy Agency tracks global biofuel deployment as part of its analysis of low-emission fuel transitions, with biofuels currently supplying roughly 4 percent of road transport fuel worldwide.

First- and Second-Generation Biofuels

First-generation biofuels are produced from food crops: sugar cane and corn for ethanol, soybeans and rapeseed for biodiesel. Their feedstocks are readily available and well-understood, but their use raises concerns about competition with food supply and the land area required for meaningful energy displacement. Second-generation biofuels address these concerns by using lignocellulosic feedstocks such as agricultural straws, forestry residues, energy grasses, and food waste. Cellulosic ethanol requires additional processing steps, including pretreatment to disrupt the lignocellulose matrix and enzymatic hydrolysis to release fermentable sugars, because cellulose and hemicellulose are far less accessible than starch. Third-generation biofuels, based on algal lipids, are still in research and demonstration phases; algae can achieve far higher yields per hectare than oilseed crops, but harvesting and dewatering costs remain a barrier to commercial viability.

Production Processes

Ethanol is produced by fermenting sugars with Saccharomyces cerevisiae or other yeasts under anaerobic conditions, followed by distillation and dehydration to fuel-grade purity. Biodiesel is made by transesterification, a reaction between vegetable oils or animal fats and a short-chain alcohol, typically methanol, in the presence of a base catalyst, yielding fatty acid methyl esters and glycerol as a co-product. Thermochemical pathways, including pyrolysis, gasification, and hydrotreating, convert lignocellulosic biomass or algal feedstocks into bio-oils and synthetic hydrocarbons. The U.S. Department of Energy Bioenergy Technologies Office focuses on these hydrocarbon drop-in fuels, which are chemically interchangeable with petroleum-derived fuels and can flow through existing pipeline and refinery infrastructure without modification.

Energy Performance and Sustainability

The energy balance and greenhouse gas performance of a biofuel depends on the feedstock, the conversion process, and the end use. The carbon intensity of corn ethanol in the United States is roughly half that of gasoline on a life-cycle basis when accounting for soil carbon dynamics and process energy, but this varies considerably with farming practices. Sugar cane ethanol produced in Brazil achieves higher displacement of fossil carbon per unit energy because the bagasse residue is combusted to power the conversion process. The IEA energy system analysis of biofuels projects that advanced biofuels from waste feedstocks, including food waste, municipal solid waste, and agricultural residues, offer the most significant emission reductions per unit volume while avoiding land-use competition with food production.

Applications

Biofuels have applications in a wide range of fields, including:

• Road transportation as ethanol blends (E10, E85) and biodiesel blends (B5, B20)
• Aviation, where sustainable aviation fuel (SAF) derived from biomass is used to reduce flight emissions
• Marine shipping as a low-sulfur alternative to heavy fuel oil
• Power generation using biogas produced by anaerobic digestion of food waste and organic effluents
• Cooking and heating in biomass-rich rural regions through solid and gaseous bioenergy