Biofuels and Carbon Neutrality

A biofuel is a fuel produced from biomass (recently living material). Bioethanol is ethanol made by fermenting sugars from crops such as sugar cane, and it can be used as a fuel for cars, often blended with petrol.An activity is carbon neutral if it has no net annual carbon (greenhouse gas) emissions to the atmosphere. Bioethanol is claimed to be carbon neutral because the carbon dioxide released when it is made and burned was recently absorbed from the air by the growing plants.Photosynthesis (6 mol CO₂ absorbed):
6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
Fermentation (2 mol CO₂ released):
C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂
Combustion (4 mol CO₂ released):
2C₂H₅OH + 6O₂ → 4CO₂ + 6H₂OTotal released = 2 + 4 = 6 mol, which equals the 6 mol absorbed, so overall there is no net release of carbon dioxide.In practice bioethanol is not truly carbon neutral. Energy, usually from fossil fuels, is used to grow and harvest the crops (farm machinery, making fertilisers), to transport them and to distil the ethanol. These steps release extra carbon dioxide.Other issues: land used for fuel crops is not available to grow food, and forests may be cleared to plant crops. The advantages are that bioethanol is a renewable resource and it reduces the use of crude oil.