Biofuels are plant material and animal waste, and specifically grown crops, which can be burnt to produce energy. It is sometimes known as "biomass burning". Biomass material may include tree and grass crops, and forestry, agricultural and urban waste. It is the oldest source of renewable energy known to humans. Biofuels are considered to be renewable sources of energy source because the energy they contain comes from the Sun.
Unlike other renewables, biofuel energy does release carbon dioxide into the atmosphere, but it is only returning to the atmosphere as much as was removed through photosynthesis during the plants lifetime. Burning fossil fuels, by contrast, returns carbon dioxide to the atmosphere that has been locked away in the Earths crust for millions of years.
Crops can be grown with the purpose of being burnt to produce energy, for example willow and oil seed. Energy from waste can also be generated by burning the methane given off from waste landfill sites. Methane is a powerful greenhouse gas, and burning it would help to reduce the amount of methane in the atmosphere.