Fourteen mature technology biomass refi ning scenarios -involving both biological and thermochemical processing with production of fuels, power, and/or animal feed protein -are compared with respect to process effi ciency, environmental impact -including petroleum use, greenhouse gas (GHG) emissions, and water use-and economic profi tability. The emissions analysis does not account for carbon sinks (e.g., soil carbon sequestration) or sources (e.g., forest conversion) resulting from land-use considerations. Sensitivity of the scenarios to fuel and electricity price, feedstock cost, and capital structure is also evaluated. The thermochemical scenario producing only power achieves a process effi ciency of 49% (energy out as power as a percentage of feedstock energy in), 1359 kg CO 2 equivalent avoided GHG emissions per Mg feedstock (current power mix basis) and a cost of $0.0575/kWh ($16/GJ), at a scale of 4535 dry Mg feedstock/day, 12% internal rate of return, 35% debt fraction, and 7% loan rate.Thermochemical scenarios producing fuels and power realize effi ciencies between 55 and 64%, avoided GHG emissions between 1000 and 1179 kg/dry Mg, and costs between $0.36 and $0.57 per liter gasoline equivalent ($1.37 -$2.16 per gallon) at the same scale and fi nancial structure. Scenarios involving biological production of ethanol with thermochemical production of fuels and/or power result in effi ciencies ranging from 61 to 80%, avoided Modeling and Analysis: Comparative analysis of mature biomass refining scenarios $1.24/gallon). Most of the biofuel scenarios offer comparable, if not lower, costs and much reduced GHG emissions (>90%) compared to petroleum-derived fuels. Scenarios producing biofuels result in GHG displacements that are comparable to those dedicated to power production (e.g., >825 kg CO 2 equivalent/dry Mg biomass), especially when a future power mix less dependent upon fossil fuel is assumed. Scenarios integrating biological and thermochemical processing enable waste heat from the thermochemical process to power the biological process, resulting in higher overall process effi ciencies than would otherwise be realized -effi ciencies on par with petroleum-based fuels in several cases.

The question of whether the world needs biofuels is approached by examining the feasibility of doing without them. Even with aggressive reductions in travel growth, shifts to mass transport modes, strong effi ciency improvements, and deep market penetration by vehicles running on electricity and hydrogen, there remains a large demand for dense liquid fuels in 2050 (80% of transportation fuel) and even in 2075 (50%). This demand is due largely to aviation, ocean shipping, and long-haul trucking. Acknowledging the signifi cant uncertainties involved in such projections and the challenges faced by all candidate technologies and fuels, we conclude that it will likely be diffi cult to achieve a low-carbon transport sector without widespread use of biofuels, and that aggressive efforts to develop sustainable, low-carbon biofuels alongside other options are warranted.

The Role of Biomass in America's Energy Future (RBAEF) project, initiated during the fi rst half of 2003, has sought to identify and evaluate paths by which biomass can make a large contribution to energy services and determine means to accelerate biomass energy use. In addressing these issues, the study has focused on future, mature, technologies rather than today's technology. This perspective -the fi rst of eight papers that comprise this issue -introduces the project, providing an operative defi nition of and method for estimating mature technology, a rationale for choosing the model feedstock, a list of the conversion technologies considered, and as a point of reference, a brief overview of the energy fl ows through a typical petroleum refi nery. The subsequent papers are introduced as well.

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