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Review of the Research Strategy for Biomass-Derived Transportation Fuels
FIGURE 4-1 Schematic diagram of the conversion of biomass feedstock to ethanol fuel. Source: NREL, 1998.
A better fundamental understanding of underlying phenomena in all of these technology areas will be crucial to the development of innovative approaches to reducing costs.
An understanding of the fundamental mechanisms underlying pretreatment, cellulose and hemicellulose hydrolysis, and consolidated processing can lead to insights on the areas that have the greatest potential for improvement through R&D. As the knowledge base grows, researchers will be able to develop meaningful comparisons among technologies and investigate the effects of changes in key performance parameters on process economics. Approaches to innovation that rely largely on trial and error are inefficient, risky, and less likely to support scale-up and commercialization by industry. Investment in basic R&D will be key to identifying technical opportunities to lower the costs of manufacturing cellulosic bioethanol.
The ethanol manufacturing process that has been most thoroughly investigated by NREL is shown in Figure 4-1. Biomass is ground to an appropriate size and treated with dilute sulfuric acid to convert most of the hemicellulose to soluble pentose sugars, which are then separated from the feedstock material. The remaining plant material (mostly cellulose and lignin) is then hydrolyzed with enzymes. The resulting sugar solutions (glucose, xylose, arabinose, galactose, and mannose) are combined and fermented to produce ethanol, which is then distilled. Residual solids in the distillation mixture are burned to provide process steam and excess electricity, which is sold into the electric grid.
In the current NREL process, cellulase hydrolysis and fermentation take place simultaneously in the same vessel, a procedure referred to as SSF (simultaneous saccharification and fermentation). A portion of the biomass is also diverted to a separate fermentation step in which the enyzmes for cellulose hydrolysis are produced. Although a wide variety of types of cellulosic biomass are referred to in the literature, most laboratory and pilot-plant work to date has been focused on hardwoods (primarily poplar species). Apparently little experimental work has been done on grasses, such as switchgrass, or crop residues, such as corn stover.
The current conversion process makes use of technologies that have largely been developed in house at NREL. One technology, notably the acid hydrolyis/pretreatment, has remained essentially unchanged for almost 20 years (Lynd, 1996; NREL, 1998). Because processing downstream of the pretreatment step is greatly affected by the characteristics of the pretreated material and the hydrolyzed sugar solutions, innovation in downstream processing has also been limited.
Research on pretreatment has been underfunded relative to the high cost of this processing step and its significant effects on the costs of subsequent hydrolysis and fermentation steps (Lynd, 1996). Although large increases for research on pretreatment for fiscal year 2000 have been requested, the committee believes OFD should consider using pretreatment technologies under development elsewhere to improve bioethanol manufacturing processes.
Diverse pretreatment processes under evaluation may have the potential to unlock vast reserves of cellulosic biomass (NRC, 1999c). The most thoroughly researched pretreatment processes are dilute acid hydrolysis, steam explosion, ammonia fiber explosion, and treatment with organic solvents (Lynd, 1996). Less is known about liquid hot water pretreatment (van Walsum et al., 1996), and none of these pretreatments is currently a commercial success (NRC, 1999c). Lynd (1996) has established some criteria for determining the ideal pretreatment: produces reactive fiber; yields penroses in nondegraded form; does not significantly inhibit fermentation; requires little or no size reduction; can work in reactors of reasonable size and moderate cost; produces no solid residues; has a high degree of simplicity; and is effective at low moisture contents. This committee agrees with Lynd's assessment that the dilute acid hydrolysis process