Environmental, economic, and energetic costs and benefits of biodiesel and ethanol biofuels
Results (cont'd)
Environmental effects from the complete production and combustion life cycles of corn grain ethanol and soybean biodiesel. (a and b) Use of fertilizers (a) and pesticides (b) per unit of net energy gained from biofuel production (Table 10). (c) Net GHG emissions (as CO2 equivalents) during production and combustion of biofuels and their conventional counterparts, relative to energy released during combustion (Table 11).
If CO2 from fossil fuel combustion was the only GHG considered, a biofuel with NEB > 1 should reduce GHG emissions because the CO2 released upon combustion of the fuel had been removed from the atmosphere by plants, and less CO2 than this amount had been released when producing the biofuel. However, N fertilization and incorporation of plant biomass into soil can cause microbially mediated production and release of N2O, which is a potent GHG (13). Our analyses (see Table 11, which is published as supporting information on the PNAS web site) suggest that, because of the low NEB of corn grain ethanol, production and use of corn grain ethanol releases 88% of the net GHG emissions of production and combustion of an energetically equivalent amount of gasoline (Fig. 2 c). This result is comparable with a recent study that estimated this parameter at 87% using different methods of analysis (1). In contrast, we find that life-cycle GHG emissions of soybean biodiesel are 59% those of diesel fuel. It is important to note that these estimates assume these biofuels are derived from crops harvested from land already in production; converting intact ecosystems to production would result in reduced GHG savings or even net GHG release from biofuel production.
Economic Competitiveness and Net Social Benefits.
Because fossil energy use imposes environmental costs not captured in market prices, whether a biofuel provides net benefits to society depends not only on whether it is cost competitive but also on its environmental costs and benefits vis-à-vis its fossil fuel alternatives. Subsidies for otherwise economically uncompetitive biofuels are justified if their life-cycle environmental impacts are sufficiently less than for alternatives. In 2005, neither biofuel was cost competitive with petroleum-based fuels without subsidy, given then-current prices and technology. In 2005, ethanol net production cost was $0.46 per energy equivalent liter (EEL) of gasoline (14–16), while wholesale gasoline prices averaged $0.44/liter (17). Estimated soybean biodiesel production cost was $0.55 per diesel EEL (16, 18), whereas diesel wholesale prices averaged $0.46/liter (17). Further increases in petroleum prices above 2005 average prices improve the cost competitiveness for biofuels. Even when not cost competitive, however, biofuel production may be profitable because of large subsidies. In the U.S., the federal government provides subsidies of $0.20 per EEL for ethanol and $0.29 per EEL for biodiesel (19). Demand, especially for ethanol, also comes from laws and regulations mandating blending biofuels in at least some specified proportion with petroleum. Ethanol and biodiesel producers also benefit from federal crop subsidies that lower corn prices (which are approximately half of ethanol production’s operating costs) and soybean prices.
Potential U.S. Supply.
In 2005, 14.3% of the U.S. corn harvest was processed to produce 1.48 × 1010 liters of ethanol (20, 21), energetically equivalent to 1.72% of U.S. gasoline usage (22). Soybean oil extracted from 1.5% of the U.S. soybean harvest produced 2.56 × 108 liters of biodiesel (20, 23), which was 0.09% of U.S. diesel usage (22). Devoting all 2005 U.S. corn and soybean production to ethanol and biodiesel would have offset 12% and 6.0% of U.S. gasoline and diesel demand, respectively. However, because of the fossil energy required to produce ethanol and biodiesel, this change would provide a net energy gain equivalent to just 2.4% and 2.9% of U.S. gasoline and diesel consumption, respectively. Reaching these maximal rates of biofuel supply from corn and soybeans is unlikely because these crops are major contributors to human food supplies through livestock feed and direct consumption (e.g., high-fructose corn syrup and soybean oil, both major sources of human caloric intake).
