Biofuels can come from a wide variety of sources and can be roughly divided into four categories or "generations:"
- First generation biofuels are made from sugars, starches, oil, and animal fats that are converted into fuel using already-known processes or technologies. These fuels include biodiesel, bioalcohols, ethanol, and biogasses, like methane captured from landfill decomposition.
- Second generation biofuels are made from non-food crops or agricultural waste, especially ligno-cellulosic biomass like switch-grass, willow, or wood chips.
- Third generation biofuels are made from algae or other quickly growing biomass sources.
- Fourth generation biofuels are made from specially engineered plants or biomass that may have higher energy yields or lower barriers to cellulosic breakdown or are able to be grown on non-agricultural land or bodies of water.
Hard Biofuels refer to low density biomass, usually grass or wood byproducts (sawdust), that are pressed into pellets that are extremely dense and highly combustible. The pellets can be burned for electricity production or heating purposes.
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