OVERVIEW AND EVALUATION OF FUEL ETHANOL FROM CELLULOSIC BIOMASS: Technology, Economics, the Environment, and Policy

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Abstract
Ethanol is a high performance fuel in internal combustion engines. It is a liquid, which is advantageous in terms of storage, delivery, and infrastructural com- patability. Ethanol burns relatively cleanly, especially as the amount of gasoline with which it is blended decreases. Evaporative and toxicity-weighted air toxics emissions are consistently lower for ethanol than for gasoline. It is likely that vehicles can be configured so that exhaust emissions of priority pollutants are very low for ethanol-burning engines, although the same can probably be said for most other fuels under consideration. Recent work suggests that ethanol may be more compatible with fuel cell—powered vehicles than has generally been assumed. Research and development—driven advances have clear potential to lower the price of cellulosic ethanol to a level competitive with bulk fuels. Pro- cess areas with particular potential for large cost reductions include biological processing (with consolidated bioprocessing particularly notable in this context), pretreatment, and incorporation of an advanced power cycle for cogeneration of electricity from process residues. The cellulosic ethanol fuel cycle has a high thermodynamic efficiency (useful energy/high heating valueD from 50% to over 65% on a first law basis, depending on the configuration), and a decidedly posi- tive net energy balance (ratio of useful energy output to energy input). Cellulosic ethanol is one of the most promising technogical options available to reduce trans- portation sector greenhouse gas emissions. It may well be possible to develop biomass-based energy on a very large scale in the United States with acceptable and in some cases positive environmental impacts. To do so will however require responsible management and increased understanding of relevant technological