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3 Biochemical Conversion of Biomass T his chapter focuses on the biochemical conversion of biomass to liquid transportation fuels. It addresses the questions raised in the statement of task related to the application of biochemical conversion to the production of alternative liquid transportation fuels from biomass by discussing the following: • The technology alternatives for converting biomass to liquid transporta- tion fuels. • The status of development of biochemical conversion of lignocellulosic biomass to ethanol. • The projected costs, performance, environmental impact, and barriers to deployment of biochemical conversion of lignocellulosic biomass to ethanol. • Challenges and needs in research and development (R&D), including basic-research needs for the long term. • Other technologies for converting biomass to liquid fuels that are not likely to be ready for commercial deployment before 2020. TECHNOLOGY ALTERNATIVES Liquid fuels can be derived from biomass through biochemical processing, chemi- cal catalysis, or thermochemical conversion. Biochemical conversion and chemical conversion typically transform the biomass into sugars as intermediates. In con- trast, thermochemical conversion uses heat to convert the biomass into building 

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 Liquid Transportation Fuels from Coal and Biomass Transformation Through Biochemical Intermediates Conversion (sugars) The main difference is in BIOMASS the primary catalysis system Reduction to Thermochemical Building Blocks Conversion (CO, H2) FIGURE 3.1 Comparison of biochemical and thermochemical routes for converting bio- mass to fuels. Source: Dayton, 2007. blocks, such as carbon monoxide (CO) and hydrogen (H2), which can be used TF 3-1 AL for the synthesis of fuels (Figure 3.1). Other thermochemical conversion processes include pyrolysis and liquefaction. Biochemical Conversion to Fuels Biochemical conversion uses enzymes to break down structural carbohydrates (for example, the cellulose1 and hemicellulose2 found in plant cell walls) into sugars, which are transformed into alcohols, organic acids, or hydrocarbons by microor- ganisms in fermentation. The conversions typically take place at atmospheric pres- sure and temperatures ranging from ambient to 70°C. Early ethanol production technology based on biochemical conversion of sugar and starch has been deployed commercially. In that technology, ethanol is 1A complex carbohydrate (C6H10O5)n that forms cell walls of most plants. 2A matrix of polysaccharides present in almost all plant cell walls with cellulose.

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Biochemical Conversion of Biomass  produced when wild-type yeast ferments six-carbon sugars. Sugar can be obtained directly from sugarcane (Brazil) and sugar beets (Europe) or indirectly from the hydrolysis of starch-based grains, such as corn (United States) and wheat (Canada and Europe). In the latter case, the starch feedstock needs to be ground to a meal that is hydrolyzed to glucose by enzymes. The resulting mash is fermented by nat- ural yeast and bacteria. Finally, the fermented mash is separated into ethanol and residues (for feed production) via distillation and dehydration (Figure 3.2). Corn grain is the major source of ethanol in the United States, and its poten- tial for growth is defined by production efficiencies, food-versus-fuel debates, and the question of sustainability and carbon footprint. Developments aimed at future processes are targeting cellulose conversions that could address those issues by providing a growth potential, a low carbon footprint, and sustainability. The infrastructure that was established by the corn grain ethanol industry will benefit the future cellulosic-ethanol industry because the use of ethanol as a transporta- tion fuel has been proved to be feasible, a distribution system exists, and automo- biles with internal-combustion engines that use ethanol efficiently are on the road. Recent analyses of the full life cycle of corn grain ethanol have indicated BIOCHEMICAL CONVERSION BIOMASS Plant Sugars and/or Starches Lignocellulose Liquefaction Pretreatment SACCHARIFICATION Glucoamylase Cellulolytic Enzymes FERMENTATION DISTILLATION ETHANOL CENTRIFUGATION Distiller’s Dry Grain Solids Residual Solids FIGURE 3.2 Schematic representation of bioprocessing elements. ALTF 3-2

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0 Liquid Transportation Fuels from Coal and Biomass that it provides society with small net energy gains over the fossil energy needed to produce it (Farrell et al., 2006; Hill et al., 2006) and might lead to only small net greenhouse gas advantages (Farrell et al., 2006; Hill et al., 2006; Fargione et al., 2008; Searchinger et al., 2008) or might release more greenhouse gas than do production and combustion of an energetically equivalent amount of gasoline once direct and indirect land-use changes are taken into account (Fargione et al., 2008; Searchinger et al., 2008). Issues with corn grain ethanol have led to increased interest in second-generation biofuel feedstocks—including switchgrass, Miscan- thus, hybrid poplar, and the other lignocellulosic feedstocks—and in conversion methods that potentially can make biofuels that, relative to corn ethanol, offer larger energy gains and greenhouse gas benefits and reduced competition with food crops. The development of biofuels needs to move toward conversion of lignocel- lulosic materials (so-called second-generation biofuels) that are unused agricultural or forestry residues, agricultural cover crops, dedicated perennial crops grown on marginal lands that are not suitable for commodity-crop production even with high commodity prices, or municipal solid wastes. The need to move away from corn grain ethanol is highlighted by the renewable fuel standard (RFS) as amended in the 2007 Energy Independence and Security Act. The RFS mandates that pro- duction of ethanol from corn grain level off from 2008 to 2015 and that produc- tion of cellulosic and other advanced biofuels increase from 2008 to 2020. The key differences in production between grain ethanol and cellulosic ethanol are the pretreatment of the biomass and the use of by-products (Figure 3.2). This chapter focuses on the conceptual design, conversion technologies, and economics of the biochemical conversion of cellulosic biomass to ethanol. It will also discuss other technologies to produce advanced biofuels that use nonfood renewable feedstocks. The other technologies could produce fuels more desirable than ethanol—for example, lipids, higher alcohols, hydrocarbons, and other products that can be separated by low-energy distillation. New routes of biochemical conversion of biomass to liquid fuels will probably encounter complications as they are being developed and scaled up; these issues will have to be addressed in a continuous R&D program. Chemical Conversion to Fuels In contrast with biochemical conversion, chemical conversion uses inorganic cata- lysts in a series of aqueous-phase reactions to convert sugars to hydrocarbons that

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Biochemical Conversion of Biomass  can be used as fuels. It is a developing technology that will not be ready for com- mercial deployment by 2020, but it is discussed later in this chapter. Thermochemical Conversion to Fuels In what is currently the most developed thermochemical route, biomass is ini- tially converted into CO and H2 via gasification. The gas stream can be cleaned of impurities and shifted to the needed H2:CO ratio, and CO2 can be removed to produce a gas stream that can be catalytically converted to liquid fuels by several routes, including Fischer-Tropsch (FT) and methanol synthesis followed by metha- nol-to-gasoline (MTG) conversion. Thermochemical conversion is discussed in Chapter 4. Other thermochemical conversion routes involve production of bio-oil by pyrolysis or liquefaction and refinement of the bio-oil (Huber et al., 2006); this technology is not as well developed as FT or MTG. BIOCHEMICAL CONVERSION OF CELLULOSIC BIOMASS This section discusses the biochemical processes for converting cellulosic biomass to ethanol in a biorefinery. The processes discussed here occur at the end of the supply chain, when the biomass has been delivered to the biorefinery (Figure 3.3). The process economies are those within the biorefinery. Process Overview The biochemical conversion of cellulosic biomass involves six major steps: feed- stock preparation, pretreatment to release cellulose from the lignin shield, sac- charification (breaking down of the cellulose and hemicellulose by hydrolysis to sugars, such as glucose and xylose), fermentation of sugar to ethanol, and distil- lation to separate the ethanol from the dilute aqueous solution (Figure 3.4). In the sixth step, the residues, primarily lignin, can be combusted to provide energy (Figure 3.4). The integration of those steps with each other and with the living microorganisms and enzymes that carry out the catalytic conversions in a bio- refinery is essential to the development of cost-effective processes.

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 Liquid Transportation Fuels from Coal and Biomass Sun CO2 Liquid Fuel Logistical Chain Production Harvest Store Transport Biorefinery Agriculture Coproducts Waste Fertilizer Seed FIGURE 3.3 Logistics of bioprocessing to convert cellulosic biomass to ethanol. Microorganisms ALTF 3-3 Feedstock Enzymes (Yeast, Bacteria) Ethanol 1 Feedstockn 2 Pretreatment 3 Hydrolysis 4 Fermentation 5 Distillation Preparatio CO2 CO2 6 Combustion Catalysts Energy Residue or Gasification Water Recycle FIGURE 3.4 Unit operations of a biorefinery. A biomass-based biorefinery should be energy self-sufficient or could even sell excess power to the grid. CO2 is recycled into plant matter through biomass production. Feedstock Preparation ALTF 3-4 Some feedstocks have to be washed to remove inorganic and other undesirable materials before pretreatment. Whether washing is needed depends on the source and the manner of storage before the feedstock is delivered to the conversion facil- ity. The biomass is then chopped or ground to the desirable size range to feed into

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Biochemical Conversion of Biomass  the pretreatment stage. The extent of grinding and size reduction will depend on the type of biomass and the pretreatment technology being used. Cellulosic feed- stock can be chopped or ground with existing forestry or agricultural techniques. Pretreatment Producing fuel ethanol from lignocellulosic feedstocks has been challenging because of the recalcitrant nature of the cellulose that is embedded in the plant cell-wall structure. Therefore, pretreatment is a key step in production of cellulosic ethanol. Pretreatment greatly increases the rates and extents of enzyme action in breaking down cellulose to fermentable sugars (Ladisch et al., 1978) by improving the accessibility of the structural carbohydrates in the cell wall (Figure 3.5). Yields of fermentable sugars from untreated native lignocellulosic materials are low because of the highly packed cellulose structure and the presence of hemicellulose and lignin, which shield cellulose from acid or enzymatic hydrolysis. Maximizing the use of all lignocellulosic material that is capable of yielding simple (six- and five-carbon) sugars is essential for improving ethanol yield and PRETREATMENT gives enzyme accessible substrate Lignin Cellulose Amorphous Pretreatment Region Crystalline Region Hemicellulose FIGURE 3.5 Schematic of pretreatment to disrupt the physical structure of biomass. Reprinted from Mosier et al., 2005. Copyright 2005, with permission from Elsevier. ALTF 3-5

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 Liquid Transportation Fuels from Coal and Biomass lowering the cost of ethanol production. Hence, pretreatment of lignocellulosic material is required to improve the hydrolytic efficiency of cellulose by removing and hydrolyzing hemicellulose, by separating the cellulose from the lignin, and by loosening the structure of cellulose and thereby increasing its porosity. The pretreatment of lignocellulosics is particularly important for enzymatic hydroly- sis to reduce the amount of enzyme and the time required to convert cellulose to glucose. Among the various pretreatment methods, hydrothermolysis with steam or water has been shown to be effective in removing and solubilizing hemicellulose and thus in improving hydrolytic efficiency (Mosier et al., 2005; Wyman et al., 2005a,b). Hot-water pretreatment of lignocellulosic biomass at a controlled pH effectively dissolves hemicellulose and some of the lignin and minimizes the forma- tion of monosaccharides and other coproducts that could interfere with biological processes downstream (Yang and Wyman, 2008). For example, monosaccharides inhibit cellulase in the hydrolysis of cellulose downstream. The sugars could degrade further to form such toxic substances as furfural during the pretreatment step (Ladisch et al., 1998; Kim and Ladisch, 2008; Hendriks and Zeeman, 2009). Other pretreatments are similarly effective, and they use acid, bases, ammonia, or other materials (Mosier et al., 2005; Jorgensen et al., 2007; Murnen et al., 2007; Sendich et al., 2008; Yang and Wyman, 2008; Hendriks and Zeeman, 2009). Sev- eral of the promising pretreatment methods have been demonstrated on a pilot scale, but the lowest-cost approach is yet to be determined. Saccharification In the saccharification step, the cellulose polymers (long chains of sugar) are broken down by hydrolysis into five-carbon and six-carbon sugars (xylose and glucose) for fermentation into alcohol (Figure 3.6). The enzymes used for hydro- lysis are referred to as cellulolytic enzymes, and they are classified into three main groups: cellobiohydrolases, endoglucanases, and beta-glucosidases. The cellobiohy- drolases and endoglucanases are modular proteins with two distinct independent domains; the first domain is responsible for the hydrolysis of the cellulose chain, and the second is a cellulose-binding domain (CBD) that has the dual activity of increasing adsorption of cellulolytic enzymes onto insoluble cellulose and affecting cellulose structure. A schematic of the action is shown in Figure 3.7. By intercalat- ing between fibrils and surface irregularities of the cellulose surface, CBDs help to reduce particle size and increase specific surface area. Microscopy of cellulose

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Biochemical Conversion of Biomass  After Pretreatment CO2 Glucose Cellulose Enzymes Microorganisms Ethanol Hemicellulose Xylose Lignin Lignin FIGURE 3.6 Schematic diagram of bioprocessing of sugars to ethanol through enzy- matic hydrolysis (catalytic step that frees the sugars) and microbial conversion of sugars to ethanol and CO2 , which are formed in approximately equal parts. Lignin remains unconverted. ALTF 3-6 Linker Cellulose Catalytic Region Binding Domain Domain Cellulose Microfibril FIGURE 3.7 Schematic representation of mechanisms of enzyme action. Source: Reprinted from Mosier et al., 1999. Copyright 1999, with permission from Springer. treated with isolated CBDs generated from recombinant organisms has shown the release of small particles from insoluble cellulose with no detectable hydrolytic 3-7 ALTF activity and an increase in the roughness of highly crystalline fibers. The cellobiohydrolases are the most important cellulolytic enzyme group in that cellobiohydrolase I makes up 60 percent of the protein mass of the cellulo-

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 Liquid Transportation Fuels from Coal and Biomass lytic system of Trichoderma reesei, and its removal by gene deletion reduces over- all cellulase system activity on crystalline cellulose by 70 percent. The concerted effects of pretreatment and enzymatic hydrolysis affect the plant at the cellular level as illustrated in Figure 3.8. According to the prevailing understanding, cello- biohydrolases attack the chain ends of cellulose polymers to release cellobiose, the repeat unit of cellulose. Endoglucanases decrease the degree of polymerization of cellulose by attacking amorphous regions of cellulose through random scission of 10 µm 10 µm A B 10 µm 10 µm C D FIGURE 3.8 Scanning electron microscopic images of enzymatically hydrolyzed 425–710 mm corn stover pretreated with hot water (at 500X magnification). (A) 3-h enzymatic hydrolysis, 43.3 percent glucose conversion. (B) 24-h enzymatic hydrolysis; 56.8 percent glucose conversion. (C) 72-hour enzymatic hydrolysis; 64.2 percent glucose conversion. (D) 168-h enzymatic hydrolysis; 63.1 percent glucose conversion. The images from a labora- tory experiment illustrate how enzymatic hydrolysis of corn stover pretreated with hot water is connected to pore formation (during pretreatment) and enlargement (during 3-8 ALTF hydrolysis). Reprinted from Zeng et al., 2007. Copyright 2007, with permission from Wiley-Blackwell.

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Biochemical Conversion of Biomass  the cellulose chains. Beta-glucosidase completes the process by hydrolyzing cello- biose to glucose. Cellulolytic systems, such as those in filamentaous T. reesei, have enzymes in all three groups: two cellobiohydrolases, four endoglucanases, and one beta-glucosidase (Mosier et al., 1999). The mechanism by which cellulolytic enzymes act in hydrolyzing cellulose is complex and requires a system of different enzymes to achieve deploymerization of the oligosaccharides to monosaccharides, such as glucose and xylose. Most studies have been done with cellulases, which are produced industrially from T. reesei. The development of the cellulases has resulted in effective systems that are capable of hydrolyzing cellulose to glucose almost completely. Similar studies are being done on hemicellulases, enzymes that are responsible for breaking down hemicellulose to xylose. Hemicellulases are not as well developed as cellulases. Despite the complexities, much progress has occurred in the development of enzymes for the hydrolysis of pretreated cellulose. Costs are being reduced, with the ultimate goal of combining cellulases with glucose- and xylose-fermenting microorganisms in a concept referred to as consolidated bioprocessing (Lynd et al., 2005). Hydrolysis and fermentation (combined bioprocessing) are being dem- onstrated on a pilot scale with the goal of reducing costs. Fermentation Pretreatment and enzymatic hydrolysis of plant matter—such as wood, corn sto- ver, or grasses—result in a mixture of five-carbon and six-carbon sugars. Many microorganisms, particularly yeasts, will ferment glucose to ethanol. Typically, however, 25–30 percent of the sugar derived from likely candidates as cellulosic feedstocks for bioprocessing (for example, hardwoods, agricultural residues, and some types of grasses) are pentoses—sugars that have five carbon atoms rather than six carbon atoms. Other potential sources of biomass, such as softwoods, have a lower proportion of hemicelluloses and hence fewer pentoses. Pentoses are not readily fermented to ethanol, so yeasts or bacteria that have been genetically modified to ferment both hexoses (six-carbon sugars) and pentoses are needed to maximize the yield of ethanol from cellulosic materials. Some researchers have been successful in engineering microorganisms that are able to use pentose effi- ciently but cannot do so naturally to produce ethanol. An alternative would be to supply ethanol-producing microorganisms with pentose-using pathways (Nevoigt, 2008). Development of such microorganisms presents a number of challenges. They have to be capable of fermenting the sugars to ethanol, and they have to

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 Liquid Transportation Fuels from Coal and Biomass dedicated or dual-purpose energy crops and microorganisms that can be used for both biofuel production and feedstock conversion. Genomics The sequencing of full genomes continues to become faster and less expensive, and this is enabling the sequencing of energy crops, such as trees, perennial grasses, and such nonedible oilseeds as castor and jatropha. Their sequence data are extremely important for improving overall yields, for enabling improved nutrient and water use, and for understanding and manipulating biochemical pathways to enhance the production of desired materials. Sequence data can also be used to target specific genes for downregulation by classical methods, such as antisense and RNA interference, and via complete inactivation with new and evolving pro- cedures for homologous recombination-based gene disruption. Rapid sequencing of breeding populations of energy crops will enable marker-assisted selection to accelerate breeding programs in ways previously not possible. Furthermore, rapid and inexpensive sequencing of fermentative and photosynthetic microorganisms is redefining and shortening the timelines associated with strain-development programs for converting sugars, lignocellulosic materials, and CO2 to alternative liquid fuels. Strains generated through classical mutagenesis that have improved biocatalytic properties can now be analyzed at the molecular level to determine the specific genetic changes that result in the improved phenotype, and this allows the changes to be implemented in additional strains. In addition, “metagenome” sequence data obtained by randomly sequencing DNA isolated from environmen- tal samples is providing huge numbers of new gene sequences that can be used in genetic engineering to improve crops and microorganisms. Synthetic Biology and Synthetic Genomics Improved technologies for synthesizing megabase DNA molecules are being devel- oped to allow the introduction of entire biochemical pathways into energy crops and biofuel-producing microorganisms. The technologies could have a great effect on scientists’ ability to generate plants and microorganisms with specific desir- able traits. For example, it is becoming conceivable to replace large portions of, or even complete, chromosomes of microorganisms (including photosynthetic microorganisms) in ways that will focus the vast majority of their cells’ biochemi- cal machinery toward production of next-generation biofuel molecules and thus provide cost and product advantages. Maintaining the purity of such cultures, and

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Biochemical Conversion of Biomass  finding ways to put at a disadvantage mutants that gain competitive ability by producing less of the desired secondary chemicals, could be serious hurdles. Metabolic and Bioprocess Engineering In addition to genetic manipulation, new bioengineering technologies that will lower the cost of biofuel formation and recovery are coming on line. Synthetic biology can now provide synthetic DNA for transferring heterologous genes into suitable host cells, but metabolic engineering is the enabling technology for con- structing functional and optimal pathways for microbial fuel synthesis. This field has matured in only a few years and has an impressive record of accomplishments, many already being applied in industry (for example, in the production of bio- polymers, alcohols, 1,3-propanediol, oils, and hydrocarbons). Microbial strains that secrete hydrophobic fuels that are similar to constituents of diesel fuel and gasoline into the culture medium have been developed. The fuels can be sepa- rated from the aqueous phase in a manner that simplifies distillation and thereby reduces energy inputs and facilitates continuous production. By taking a systems view of metabolism, metabolic engineering developed tools for overall biosystems optimization that are now facilitating the optimal construction of biosynthetic pathways and elicitation of novel multigenic cellular properties of critical impor- tance for biofuels production, such as tolerance of fuel toxicity. In bioprocessing, the successful development of membrane-based alcohol separation would greatly reduce energy costs relative to the typically used distillation process (Vane, 2008). Gas-stripping, liquid-liquid extractions of secreted fuel molecules or new adsor- bent materials that will allow continuous production modes for fermentation- based products are also being developed (Vane, 2008). For photosynthetic pro- duction of biofuels, the development of low-cost photobioreactors and associated recovery systems for algal biofuel production is of great interest and could have substantial beneficial effects on overall process economics. FINDINGS AND RECOMMENDATIONS Grain-based ethanol is a bridge to advanced biofuels that has important potential for greenhouse gas displacement. Advanced biofuels do not directly compete with food and feed supply, and they minimize indirect land-use change if appropri- ate feedstocks are selected and sustainable practices are used in their production.

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 Liquid Transportation Fuels from Coal and Biomass Grain ethanol has initiated public awareness of the use of ethanol in the current and future transportation fleet and of the pitfalls of feedstock supply for a new industry. Grain ethanol has helped to establish an industrial infrastructure for advanced biofuels and for distribution and use of fuel ethanol. Lignocellulosic feedstocks for production of advanced biofuels could be agri- cultural or forestry residues, agricultural cover crops, dedicated perennial crops grown on marginal lands that are not suitable for commodity-crop production even with high commodity prices, or municipal solid wastes. Biochemical conver- sion of cellulose to liquid fuels emulates commercial corn grain-to-ethanol technol- ogy but might require additional processing steps and could result in other types of alcohols and hydrocarbon-rich fuels. The technologies for biochemical conversion of cellulosic biomass to ethanol are in the early stages of demonstration and commercial development. Several demonstration plants are expected to be operational by 2012. The panel judges that cellulosic bioethanol will be commercially deployable before 2020, and other advanced biofuels are likely to emerge after 2020. Finding 3.1 Engineering and operational knowledge can be gained only from designing and building commercial-scale, integrated cellulosic-ethanol facilities and then operat- ing them for a reasonable period. The first few commercial plants will be more expensive than commercial facilities that follow because of the learning that occurs with a first-of-its-kind facility. The initial learning that occurs with first-of- a-kind plants will lead to further cost-reducing improvements in commercial facili- ties deployed thereafter. The pace of learning is expected to be similar to that in the chemical industry, in which costs have historically decreased by 30–40 percent over several cycles of deployment and concurrent process improvement. Recommendation 3.1 The federal government and industry should aggressively pursue technology dem- onstration or small-scale commercial plants, which will lead to full-scale com- mercial production of cellulosic ethanol to define its potential and to provide data on engineering and cost performance to help in preparation for full commercial deployment.

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Biochemical Conversion of Biomass  In the immediate term, pretreatment and enzymatic hydrolysis, fermenta- tion, or combined enzymatic hydrolysis and fermentation need to be substantially improved to allow efficient deconstruction of carbohydrate polymers to simple sugars and fermentation of the sugars to ethanol. Research in and improvement of pretreatment, with engineering of appropriate microorganisms for optimal use of the resulting simple sugars in an adverse fermentation environment, will have a direct impact on reducing the cost of transforming cellulosic feedstocks to etha- nol. The cost of producing sugars directly affects the cost of ethanol. In addition, the sugars have to be converted to ethanol efficiently to minimize feedstock and operational costs. Feedstock, pretreatment, and enzymes are key components of a cellulose-to- ethanol process, and they are all related to the goal of preparing lignocellulosic feedstocks (through agronomics, plant molecular genetics, and pretreatment) so that they are readily transformed to sugars and ethanol at low cost. Other tar- gets for improvement include increasing solids loading and developing engineered microorganisms and enzymes that have increased tolerance of toxic compounds in biomass hydrolysates and of the biofuel products themselves. Incremental improvements in biochemical conversion technologies and the learning and experience gained from R&D and demonstration can be expected to reduce non- feedstock processing costs by 25 percent by 2020 and 40 percent by 2035 (see Table 3.3). Finding 3.2 Process improvements in cellulosic-ethanol technology are expected to be able to reduce the plant-related costs associated with ethanol production by up to 40 percent over the next 25 years. Over the next decade, process improvements and cost reductions are expected to come from evolutionary developments in technol- ogy, from learning gained through commercial experience and increases in scale of operation, and from research and engineering in advanced chemical and biochemi- cal catalysts that will enable their deployment on a large scale. Recommendation 3.2 The federal government should continue to support research and development to advance cellulosic-ethanol technologies. R&D programs should be pursued to resolve the major technical challenges facing ethanol production from cellulosic

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 Liquid Transportation Fuels from Coal and Biomass biomass: pretreatment, enzymes, tolerance to toxic compounds and products, solids loading, engineering microorganisms, and novel separations for ethanol and other biofuels. A long-term perspective on the design of the programs and alloca- tion of limited resources is needed; high priority should be placed on programs that address current problems at a fundamental level but with visible industrial goals. Recommendation 3.3 The pilot and commercial-scale demonstrations of cellulosic-ethanol plants should be complemented by a closely coupled research and development program. R&D is necessary to resolve issues that are identified during demonstration and to reduce costs of sustainable feedstock acquisition. Industrial experience shows that such reductions typically occur as processes go through multiple phases of imple- mentation and expansion. Finding 3.3 Future improvements in cellulosic technology that entail invention of biocatalysts and biological processes could produce fuels that supplement ethanol production in the next 15 years. In addition to ethanol, advanced biofuels (such as lipids, higher alcohols, hydrocarbons, and other products that are easier to separate than ethanol) should be investigated because they could have higher energy content and would be less hygroscopic than ethanol and therefore could fit more smoothly into the current petroleum infrastructure than ethanol could. Recommendation 3.4 The federal government should ensure that there is adequate research support to focus advances in bioengineering and the expanding biotechnologies on developing advanced biofuels. The research should focus on advanced biosciences—genomics, molecular biology, and genetics—and biotechnologies that could convert biomass directly to produce lipids, higher alcohols, and hydrocarbons fuels that can be directly integrated into the existing transportation infrastructure. The translation of those technologies into large-scale commercial practice poses many challenges that need to be resolved by R&D and demonstration if major effects on produc- tion of alternative liquid fuels from renewable resources are to be realized.

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Biochemical Conversion of Biomass  Finding 3.4 Biochemical conversion processes, as configured in cellulosic-ethanol plants, pro- duce a stream of relatively pure CO2 from the fermentor that can be dried, com- pressed, and made ready for geologic storage or used in enhanced oil recovery with little additional cost. Geologic storage of the CO2 from biochemical conver- sion of plant matter (such as cellulosic biomass) further reduces greenhouse gas life-cycle emissions from advanced biofuels, so their greenhouse gas life-cycle emis- sions would become highly negative. Recommendations 3.5 Because geologic storage of CO2 from biochemical conversion of biomass to fuels could be important in reducing greenhouse gas emissions in the transportation sec- tor, it should be evaluated and demonstrated in parallel with the program of geo- logic storage of CO2 from coal-based fuels. REFERENCES Aden, A., M. Ruth, K. Ibsen, J. Jechura, K. Neeves, J. Sheehan, B. Wallace, L. Montague, A. Slayton, and J. Lukas. 2002. Lignocellulosic Biomass to Ethanol Process. Design and Economics Utilizing Co-Current Dilute Acid Prehydrolysis and Enzymatic Hydrolysis for Corn Stover. Golden, Colo.: National Renewable Energy Laboratory. Aristidou, A., and M. Penttila. 2000. Metabolic engineering applications to renewable resource utilization. Current Opinion in Biotechnology 11:187-198. Briggs, Michael. 2004. Widescale biodiesel production from algae. Available at www.unh. edu/p2/biodiesel/article_alagae.html. Accessed October 1, 2008. Chase, R. 2006. DuPont, BP join to make butanol: They say it outperforms ethanol as a fuel additive. USA Today, June 26. Available at http://www.usatoday.com/money/ industries/energy/2006-06-20-butanol_x.htm. Accessed October 11, 2008. Cornell, C.B. 2008. GM announces new cellulosic ethanol partnership with Mascoma Corp. Available at http://gas2.org/2008/05/01/gm-announcesnew-cellulosic-ethanol- partnership-with-mascoma-corp/. Accessed March 10, 2009. Dayton, D. 2007. R&D needs for integrated biorefineries: The 30 × 30 Vision. In 4th Annual California Biomass Collaborative Forum, Sacramento, Calif.

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