Biofuels are a form of indirect solar energy, including any fuel that comes directly from biological sources:
- burning wood (to be avoided due to excessive pollution)
- plant oils and biodiesel
- ethanol and methanol
- wood gas
Like fossil fuels, biofuels produce carbon dioxide. But unlike fossil fuels, the carbon dioxide produced comes from plants that sucked it out of the air last year, rather than millions of years ago. This means that biofuels are a form of closed-end recycling, whereby the waste product goes directly into production of the fuel.
Pollution is any byproduct that cannot be fed back into the closed-end system. For biofuels, this includes particulates and unburnt hydrocarbons (smoke), oxides of nitrogen, carbon monoxide, and a few others. These are typically much lower level than when fossil fuel is combusted, but they remain a problem.
What is pollution for one technology may be the biofuel in another. For example, if wood is heated anaerobically (with limited oxygen), it produces carbon monoxide, which is normally considered a pollutant, but if collected, can be burnt as a biofuel.
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