Annual Basis Carbon (ABC) Analysis of Biofuel Production at the Facility Level
Liquid biofuels such as ethanol and biodiesel are considered important renewable replacements for petroleum fuels such as gasoline and diesel. Biomass-based energy carriers are traditionally treated as inherently carbon neutral, so that only the fossil-based carbon dioxide (CO2) and other non-CO2 greenhouse gas (GHG) emissions associated with their production are counted when assessing their global warming impact. This accounting convention is embedded by construction in lifecycle assessment (LCA) models, which on a direct basis (i.e., excluding economically induced effects) often find that biofuels from efficient industrial agricultural production systems reduce GHG emissions relative to their fossil fuel counterparts.
LCA modeling can be subjected to an independent empirical test that bounds a biofuel's potential GHG reduction within the dynamics of the terrestrial carbon cycle. Such a test can be performed using annual basis carbon (ABC) accounting, which entails spatially and temporally explicit analysis of the direct GHG exchanges between the atmosphere and a physical vehicle-fuel system. An ABC case study of a corn ethanol biorefinery and the farmland that supplies it shows that using the ethanol it produced instead of gasoline provided no significant reduction in GHG emissions, in contrast to an LCA result that found a 40% GHG reduction for the same facility. ABC accounting reveals that the renewable ethanol so produced was not carbon neutral when substituting for gasoline, and sensitivity analysis indicates that its net GHG emissions impacts are likely to higher than those of petroleum gasoline in practice.
For a short summary, see "Bringing biofuels back to earth" on the Cars & Climate blog.