| The production of ethanol from lignocellulosic materials, e.g. wood, agricultural residues, and municipal solid wastes, is considered. The conversion of these materials to ethanol in the U.S. could annually yield approximately 430 million tons ethanol, or about 9.8 quads, within the next 20 years. Thermophilic bacteria have advantages over yeasts for ethanol production because various species produce an active cellulase enzyme and utilize pentose sugars. However thermophiles have lower ethanol tolerance and usually lower ethanol yields. The potential of thermophilic ethanol production from hardwood chips is examined in detail. It is concluded that if high ethanol yield can be achieved this process could have economics competitive with either ethanol production from corn via yeast or synthetic production from ethylene. Low ethanol tolerance is not a major problem provided concentrations {dollar}geq{dollar}1.5% are produced, ethanol is continuously removed from the fermentor, and IHOSR/extractive distillation is employed.; Research was undertaken aimed at closing the gap between the attractive potential of thermophiles for ethanol production, and that which is possible based on present knowledge, which is not practical. Major topics were the activity of Clostridium thermocellum cellulase on pretreated mixed hardwood and Avicel in vivo, continuous culture of C. thermocellum on pretreated mixed hardwood and Avicel, and the continuous culture of Clostridium thermosaccharolyticum at high xylose concentrations in the presence and absence of ethanol removal. Major conclusions were that: (1) there is overlap between the concentrations of ethanol which can be produced by thermophilic bacteria in continuous culture and those which can be economically recovered; (2) the presence of insoluble lignin appears to have no effect on the rates of hydrolysis catalyzed by the cellulase of C. thermocellum; (3) rates and yields of hydrolysis of preteated wood catalyzed by C. thermocellum cellulase are comparable to the most effective known cellulase at similar pretreatment conditions and enzyme loadings; (4) continuous cultures of C. thermocellum achieve high extents of utilization ({dollar}>{dollar}85%) of pretreated hardwood in reasonably short residence times (12-16 hours); (5) continuous ethanol removal via sidestream stripping appears to offer an attractive way to increase the productivity of thermophilic fermentation, and possibly also the ethanol yield. The significance and limitations of these results are discussed. |
