U.S. biomass supply would allow determination of the optimal size of conversion plants for particular locations in relation to the road network and the costs and greenhouse gas effects of feedstock transport. The assessment could be conducted by the U.S. Department of Energy and the U.S. Department of Agriculture and the information could be combined with the logistics of coal delivery to such plants to develop an optimal strategy for using U.S. biomass and coal resources for producing sustainable biofuels.

ENVIRONMENTAL IMPACTS BEYOND GREENHOUSE GAS EMISSIONS

Biomass Supply

Although greenhouse gas emissions have been the central environmental focus regarding biomass production for alternative liquid fuels, other key effects must also be considered. On the one hand, lignocellulosic biomass feedstocks offer distinct advantages over food crop feedstocks with respect to water-use efficiency, nutrient and sediment loading in waterways, enhancement of soil fertility, emissions of criteria pollutants, and safeguarding habitat for wildlife and other species, especially those that provide biocontrol services for crop production. On the other hand, many of the traits of dedicated fuel crops have been shown to contribute to their invasiveness.

Biochemical Conversion

The biochemical conversion of cellulosic biomass into ethanol or other biofuels requires process water for cooling, heating, and mixing with reagents that are associated with hydrolysis and fermentation. The amount of water required is estimated at 2–6 gallons per gallon of ethanol produced; the lower levels would be approached if the plant’s design included the recycling of process water. The processing of cellulosics into ethanol also results, in principle, in a residual water stream that needs to undergo wastewater treatment. However, an efficient process will ferment most of the feedstock’s sugars into ethanol, leaving only low amounts of organic residuals.

Air emissions resulting from bioprocessing include CO2, water vapor, and possibly sulfur or nitrogen. Fermentation processes release CO2 as a result of



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