Hill Jason, Nelson Erik, Tilman David, Polasky Stephen, Tiffany Douglas
Department of Ecology, Evolution, and Behavior, University of Minnesota, St. Paul, MN 55108, USA.
Proc Natl Acad Sci U S A. 2006 Jul 25;103(30):11206-10. doi: 10.1073/pnas.0604600103. Epub 2006 Jul 12.
Negative environmental consequences of fossil fuels and concerns about petroleum supplies have spurred the search for renewable transportation biofuels. To be a viable alternative, a biofuel should provide a net energy gain, have environmental benefits, be economically competitive, and be producible in large quantities without reducing food supplies. We use these criteria to evaluate, through life-cycle accounting, ethanol from corn grain and biodiesel from soybeans. Ethanol yields 25% more energy than the energy invested in its production, whereas biodiesel yields 93% more. Compared with ethanol, biodiesel releases just 1.0%, 8.3%, and 13% of the agricultural nitrogen, phosphorus, and pesticide pollutants, respectively, per net energy gain. Relative to the fossil fuels they displace, greenhouse gas emissions are reduced 12% by the production and combustion of ethanol and 41% by biodiesel. Biodiesel also releases less air pollutants per net energy gain than ethanol. These advantages of biodiesel over ethanol come from lower agricultural inputs and more efficient conversion of feedstocks to fuel. Neither biofuel can replace much petroleum without impacting food supplies. Even dedicating all U.S. corn and soybean production to biofuels would meet only 12% of gasoline demand and 6% of diesel demand. Until recent increases in petroleum prices, high production costs made biofuels unprofitable without subsidies. Biodiesel provides sufficient environmental advantages to merit subsidy. Transportation biofuels such as synfuel hydrocarbons or cellulosic ethanol, if produced from low-input biomass grown on agriculturally marginal land or from waste biomass, could provide much greater supplies and environmental benefits than food-based biofuels.
化石燃料对环境造成的负面影响以及对石油供应的担忧，促使人们寻找可再生的交通生物燃料。要成为一种可行的替代能源，生物燃料应实现净能量增益，具有环境效益，具备经济竞争力，并且能够大量生产而不减少粮食供应。我们运用这些标准，通过生命周期核算，对玉米谷物乙醇和大豆生物柴油进行评估。乙醇产出的能量比生产过程中投入的能量多25%，而生物柴油产出的能量则多93%。与乙醇相比，每获得单位净能量，生物柴油释放的农业氮污染物、磷污染物和农药污染物分别仅为乙醇的1.0%、8.3%和13%。相对于它们所替代的化石燃料，乙醇的生产和燃烧使温室气体排放量减少了12%，生物柴油则减少了41%。每获得单位净能量，生物柴油排放的空气污染物也比乙醇少。生物柴油相对于乙醇的这些优势源于较低的农业投入以及原料到燃料的更高效转化。如果不影响粮食供应，这两种生物燃料都无法大量替代石油。即便将美国所有的玉米和大豆产量都用于生产生物燃料，也仅能满足12%的汽油需求和6%的柴油需求。在近期石油价格上涨之前，高昂的生产成本使得生物燃料在没有补贴的情况下无利可图。生物柴油具有足够的环境优势，值得给予补贴。诸如合成燃料碳氢化合物或纤维素乙醇等交通生物燃料，如果由种植在农业边际土地上的低投入生物质或废弃生物质生产，可能会比基于粮食的生物燃料提供更多的供应并带来更大的环境效益。
