The potential of biomass fuels in the context of global climate change: Focus on transportation fuels

An ultimate limit on the extent that biomass fuels can be used to displace fossil transportation fuels, and their associated emissions of CO2 will be the land area available to produce the fuels and the efficiencies by which solar radiation can be converted to useable fuels. Currently, the Brazil ca ne-ethanol system captures 33% of the primary energy content in harvested c ane in the form of ethanol. The US corn-ethanol system captures 54% of the primary energy of harvested corn kernels in the form of ethanol. If ethanol is used to substitute for gasoline, avoided fossil fuel CO2 emissions woul d equal those of the substituted amount minus fossil emissions incurred in producing the cane- or corn-ethanol. In this case, avoided emissions are es timated to be 29% of harvested cane and 14% of harvested corn primary energ y. Unless these efficiencies are substantially improved, the displacement o f CO2 emissions from transportation fuels in the United States is unlikely to reach 10% using domestic biofuels. Candidate technologies for improving these efficiencies include fermentation of cellulosic biomass and conversio n of biomass into electricity, hydrogen, or alcohols for use in electric dr ive-train vehicles.