Biofuels have the potential to be sustainable, secure, low carbon footprint transportation fuels. Primarily due to government mandates, biofuels have become increasingly adopted as transportation fuels over the last decade and are projected to steadily increase in production. Here the prospects of biofuels are summarized in terms of several important performance measures, including: lifecycle greenhouse gas (GHG) emissions, energy return on investment (EROI), land and water requirements, and tailpipe emissions. A review of the literature leads to the conclusion that most first-generation biofuels, including corn ethanol and soybean biodiesel produced in the United States, reduce tailpipe pollutant emissions and GHG emissions—provided their feedstocks do not replace large quantities of fixed carbon. However, their production is perhaps unsustainable due to low EROI and significant land-use and water requirements. Second-generation biofuels; for example ethanol produced from lignocellulosic biomass, have the potential for larger reductions in GHG emissions and can provide sustainable EROI with reasonable land area usage; however, they require water inputs several orders-of-magnitude greater than required by petroleum fuels. Advanced biofuels from algal oils and synthetic biological processes are further from commercial reality and require more assessment but potentially offer better performance due to their orders-of-magnitude greater yields per land area and lower water requirements; at present, the energy costs of such biofuels are uncertain.

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