A review of life-cycle analysis studies on liquid biofuel systems for the transport sector

Given current oil prices and the heavy dependence of many countries on imported oil, the potential for producing or importing liquid transportation fuels made from biomass is attracting keen interest in many developing and industrialized countries. There is also some interest in biofuels for climate change mitigation. This article reviews the rich literature of published life-cycle analyses (LCAs) of liquid biofuels, with a focus on elucidating the impacts that production and use of such biofuels might have on emissions of greenhouse gases. Reviews of LCAs for "conventional" liquid biofuels (biodiesel and sugar/starch bioethanol) and potential "future" liquid biofuels (Fischer-Tropsch fuels, dimethyl ether, and cellulosic bioethanol) are included. Striking features of the LCAs reviewed include almost exclusive contextual focus on Europe or North America, wide ranges in net energy balance results and GHG impacts among different biofuels and even for the same biofuel, and a lack of focus on evaluating GHG impacts per unit of land area. The wide range of reported LCA GHG results is due in part to the wide range of plausible values for key input parameters, among which the four most significant parameters exhibiting the greatest variability and/or uncertainty are (1) the climate-active species included in the calculation, (2) assumptions around N₂O emissions, (3) the allocation method used for co-product credits, and (4) soil carbon dynamics. Finally, from a comparison of GHG impacts of biomass used for transportation fuels against those for stationary applications one concludes that under some conditions biofuels will provide greater GHG mitigation benefits, but under other conditions, biopower will be favored. It is difficult to make broad and unequivocal statements on this point. Case-specific analysis is required.