Biofuel is any fuel made from recently living biological material. Wood, crop residues, algae, and animal waste all qualify. The category includes ethanol, biodiesel, biogas, and solid biomass pellets. The common thread is that the carbon in the fuel came from the atmosphere recently, not from underground deposits formed millions of years ago.

Biofuels fall into generations. First-generation fuels come from food crops like corn and sugarcane. Second-generation fuels use non-food sources: straw, wood chips, and energy grasses. Third-generation fuels come from algae. Each generation tries to avoid the problems of the one before it.

The promise is carbon neutrality. Plants absorb CO2 as they grow. Burning the fuel releases it back. In theory, the net addition is zero. In practice, farming, processing, and transport all use energy, often from fossil fuels. The real carbon balance depends on how the fuel is made. Some biofuels cut emissions sharply. Others barely help.

Common biofuels

• Ethanol from corn or sugarcane
• Biodiesel from oils and fats
• Biogas from waste
• Wood pellets
• Algal fuels