Improvements in automobile fuel economy would unambiguously improve the chances of an easier and better-managed introduction of biomass-based fuel alcohols: doubling the mileage achieved by gasoline-fueled vehicles in the United States would, for exam­ple, reduce the demand for ethanol by 45% or more at ethanol/gasoline blends of 10% or higher (figure 5.12). Mandatory fuel economy standards and voluntary agreements with automobile manufacturers in OECD and other countries aim at varying degrees of improved mileage in passenger cars and light commercial vehicles (table 5.20).

This (relative) parsimony harmonizes well with three of the principles stated by the National Research Council (NRC) in its report on the future for biobased indus­trial products:107

• Reducing the potential for war or economic disruption due to oil supply interruptions

[55] Reducing the buildup of atmospheric carbon dioxide

[56] A much less widely quoted prediction by Hubbert concerned nuclear energy, that is, that recoverable uranium in the United States amounted to an energy potential several hundred times that of all the fos­sil fuels combined and that the world stood on the threshold of an era of far greater energy consump­tion than that made possible by fossil fuels.

[57] These have little effect on greenhouse gas emissions because of fuel substitution (oil or gas to coal).114

until 2025.114 In that context, the supporting case for bioethanol and other biofuels is, therefore, threefold:

1. They are derived from biomass feedstocks that can, with careful manage­ment (“husbandry”), be accessible when fossil fuels have become depleted — possibly, within the next four decades.

2. Their use at least partially mitigates greenhouse gas emission, far more so if nonfossil fuel energy sources contribute to their production energy inputs.

3. They can, in crude production cost terms, be competitive with conventional fuels.

Arguments in favor of lignocellulosic ethanol have been almost invariably defensive, guarding against the charge of consumer costs far in excess of what an open market

[58] The extraction, processing, and use as an automobile fuel of oil shale hydrocarbons would also increase net CO2 emissions above those of, for example, the use of natural gas.113

[59] Hawaii had two sites but Alaska had none.

[60] Lime application was a major agricultural input in the 2005 study but may have been overestimated by a factor of 5, that is, the application rate should have been spread over five years rather than every season; making this change reduces the energy required to only 77% of the biodiesel energy content.

• In the National Renewable Energy Laboratory study, the energy input to pro­duce soybeans and then extract the oil was divided 18:82 in favor of soybean meal, that is, after the weight split of oil and residual material; allocating only 18% of the input energy to soybean oil changes the energy balance dra­matically to 5.3 times more biodiesel energy than fossil energy input.

[61] Ethanol is another candidate H2 producer, but the reaction must be performed at higher temperatures and undesirable by-products are formed.95

[62] As a (possible) harbinger of the future, the most significant plant oil in the literature of science fiction was that elaborated by the triffids, a species whose true biological and predatory capabilities were not adequately understood before disaster struck (John Wyndham, Day of the Triffids, Michael Joseph, London, 1951).

[63] Heterotrophic cultivation was mentioned in descriptions of experiments in the 1998 review, but no data for production systems using this approach were included.2

[64] In July 2007, Honda unveiled its FCX Concept hydrogen fuel cell car capable of 100 mph and with a range of 350 miles.

[65] Green chemistry was coined in the mid-1990s by the U. S. Environmental Protection Agency; of the twelve principles of green chemistry, the ninth is that raw materials should be renewable; inorganic materials appear difficult to fit into this vision but could be accessed by recycling.6

[66] With this motivation, simply burning biomass to produce energy and replace coal (the highest specific CO2 generator) would be the strategic option of choice for Europe, if not elsewhere.

[67] The example quoted was for a site in northwestern Europe (the Netherlands).

[68] Economic and commercial outlook for advanced biofuels — see chapter 6 (after section 6.1)

• Developing butanol as a transportation fuel — see chapter 6, section 6.3.3

• Commercialization of second-generation biofuels — see chapters 6 and 8

• The role of the federal government in supporting cellulosic ethanol devel­opment — see chapter 5, in particular, sections 5.2.2 and 5.2.7

• California’s commitment to advanced biofuels — see chapter 1, section 1.6.2

• Design and engineering challenges for cellulosic ethanol plants — see chapter 4

By 1983, in experimental laboratory programs, selected Bacillus strains had achieved ethanol formation to 20 g/l (from 50 g/l sucrose as the carbon substrate) at 60°C, with ethanol as the major fermentation product; acetic and formic acids remained serious by-products, however, and evidence from laboratory studies suggested that ethanol accumulation followed (and depended on) the formation of those growth- inhibiting acids.46 The ability to run ethanol fermentations at 70-80°C with ther­mophilic microbes remains both a fascination and a conscious attempt to accelerate bioprocesses, despite the low ethanol tolerance and poor hexose-converting abilities of anaerobic thermophilic bacteria. In 2004, exploratory work at the Technical Uni­versity of Denmark tested isolates from novel sources (hot springs, paper pulp mills, and brewery wastewater), using three main criteria for suitable organisms:236

1. The ability to ferment D-xylose to ethanol

2. High viability and ethanol productivity with pretreated wheat straw

3. Tolerance to high sugar concentrations

Five good (but unidentified) strains were identified by this screening program, all from hot springs in Iceland,[29] the best isolate could grow in xylose solutions of up to 60 g/l.

Thermophilic and mesophilic clostridia also have their advocates, especially with reference to the direct fermentation of cellulosic polymers by the cellulosome multien­zyme complexes, as discussed in chapter 2, section 2.4.1). Bypassing the cellulosome is possible if cellulose degradation products (rather than polymeric celluloses) are used as carbon sources — this equates to using bacteria with cellulase-treated materi­als, including agricultural residues and paper recyclates. Laboratory studies with C. cellulolyticum tested cellobiose in this fashion but with chemostat culture so as to more closely control growth rates and metabolism.237 The results demonstrated that a more efficient partitioning of carbon flow to ethanol was possible than with cellulose as the substrate but that the fermentation remained complex, with acids being the major products (figure 3.11). Nevertheless, clostridia are open to metabolic engineering to reduce the waste of carbohydrates as acids and polymeric products or as vehicles for “consolidated bioprocessing” where cellulase production, cellulose hydrolysis, and fer­mentation all occur in one step — this is covered in chapter 4 (section 4.5).

During the 1970s, the U. S. Department of Energy (DOE) commissioned four detailed technical and economic reports from consultants on possible production routes for ethanol as a fuel supplement:12 [46]

• Wheat straw conversion via enzymic hydrolysis at a 25-million-gallon/year scale of production

• Another intermediate scale process for molasses fermentation to produce 14 million gallons/year

• A farm-based model (25 gallons/hr)

“Biogas,” that is, a mixture of CH4 and CO2 prepared usually from the anaerobic digestion of waste materials by methanogenic bacterial species (Methanosarcina, Methanosaeta, Methanobrevibacter, etc.), is a technology applied globally and is ideally suited for local use in rural communities in developing economies as a cheap source of nonbiomass direct fuel.20,21 As a low-technology but established approach to wastewater treatment, it is applicable on an industrial scale, its only disadvantages being the need to remove malodorous volatile sulfur compounds. Anaerobic diges­tion is also a relatively efficient means of capturing the energy present in biological materials (figure 2.2). As links to biofuels production, biogas production not only has a historical claim for practical implementation but is also an ideal means of purifying wastewater from bioethanol facilities for detoxification and recirculation, thus reducing production costs by generating locally an input for combined heat and power or steam generation.22

Much less widely known are the bacteria that can form H2 as an end product of carbohydrate metabolism. Included in the vast number of species capable of some kind of biological fermentation (figure 2.3) are a wide array of microbes from anaer­obic environments (including Escherichia coli) that were known as active research topics as far back as the 1920s (and some of which were even discovered by Louis Pasteur in the nineteenth century).23,24 The ability of microorganisms isolated from the digestive tract to produce H2 from cellulosic substrates is another scientific research subject with a surprisingly long history.25

In addition to isolating grains for processing, cereal-milling plants also generate fiber — rich fractions as a coproduct stream. In the wet milling of corn, the fiber fraction has traditionally been added into a feed product (figure 1.20). The University of Illinois developed a modified dry milling procedure to recover fiber fractions before fermenta­tion: this quick fiber contained 65% by weight of total carbohydrate and 32% by weight of glucans, and dilute acid pretreatment was used before fermentation of the substrate to ethanol by either Escherichia coli or S. cervisiae.102 Destarched, cellulose-rich, and arabinoxylan-rich fractions of the corn fiber support the growth of strains of Hypocrea jecorina and their secretion of hydrolases for plant polysaccharides; these enzymes act synergistically with commercial cellulases on corn fiber hydrolysate and represent a valuable source of on-site enzymes for corn fiber product utilization.103 Similarly, large quantities of wheat bran are produced worldwide as a coproduct of wheat milling; residual starch in the bran material can be hydrolyzed to glucose and oligoglucans by amylolytic enzymes, and acid hydrolysis pretreatment followed by cellulase treatment gives a sugar (pentose and hexose) yield of 80% of the theoretical: 135, 228, and 167 g/kg of starch-free bran for arabinose, xylose, and glucose, respectively.104

Rice husks are approximately 36% by weight cellulose and 12% hemicellulose; as such, this agricultural by-product could be a major low-cost feedstock for ethanol production; 60% of the total sugars could be released by acid hydrolysis and treatment with a mixture of enzymes ф-glucosidase, xylanase, and esterase) with no formation of furfuraldehyde sugar degradation products.105 Recombinant E. coli could ferment the released sugars to ethanol; high-pH treatment of the hydrolysate reduced the time required for maximal production of ethanol substantially, from 64 to 39 hours. In a study from India, rice straw was pretreated with and without exogenous acid, and the released hemicellulose sugars fermented by a strain of C. shehatae; ethanol production was also demonstrated by yeast cells immobilized in calcium alginate beads — an example of an advanced fermentation technology discussed in more detail in the next section.106

Fast-growing willow trees are a major focus of research interest as a bioenergy crop in Scandinavia; high sugar recoveries were achieved from lignocellulosic mate­rial by steaming sulfuric acid-impregnated material for a brief period (4-8 minutes) at 200°C, and then digesting the cellulose enzymically, liberating glucose with 92% efficiency and xylose with 86% efficiency. The pretreated substrate could also be used for SSF with a S. cerevisiae strain.107

Many “exotic” plant materials have been included in surveys of potential bio­mass and bioenergy sources; example of these are considered in chapter 5, section 5.5.2, when sustainability issues are covered at the interfaces among agronomy, the cultivation of bioenergy crops, land use, and food production. As a lignocellulosic, straw from the grass species Paja brava, a Bolivian high-plains resident species, can be considered here. Steamed, acid-impregnated material gave hemicellulose frac­tions at 190°C that could be fermented by three pentose-utilizing yeasts, P. stipitis, C. shehatae, and Pachysolen tannophilus, while a higher temperature (230°C) was necessary for cellulose hydrolysis.108 Much more widely available worldwide is the mixed solid waste of lumber, paper, tree pruning, and others; this is a highly digest­ible resource for cellulase, the sugars being readily fermented by S. cerevisiae and the residual solids potentially usable for combustion in heat and power generation.109

5.4.1 Upstream Factors: Biomass Collection and Delivery

The “billion ton vision” is a program to access a billion tons of dry biomass per year to produce bioethanol (and other biofuels) to replace 30% of U. S. gasoline consump­tion by 2030.69 One of the key parameters in a large-scale restructuring of the U. S. national fuel industry is that of supplying biomass raw material at such a high rate and at an economically acceptable cost.

The logistics and transportation costs of such large amounts of low-value, high- volume raw materials have only recently attracted serious consideration. Canadian studies comprise the most detailed considerations of these highly practical ques­tions now that cellulosic ethanol facilities are nearing industrial reality. For an agri­cultural economy (and climate) like Canada’s, wood is highly likely to be a large fraction of the biomass supply, initially from forest harvest residues and “energy plantations” on marginal farmland.70 For wood chips, larger production plants (up to 38 million dry tons of biomass/year) are more economic than smaller units (2 million tons), and truck delivery is limited to such small units by issues of traf­fic congestion and community acceptance.71 Combined road and rail shipping, that is, initial collection by truck followed by trans-shipping to rail, is only economic when the cost per unit distance of the rail sector is less than the trucking-only mode because of the incremental fixed costs: for woody material, the minimum economic rail shipping distance is 125 km (78 miles), whereas for cereal straw, the minimum distance extends to 175 km (109 miles).72 Existing rail networks impose, however, a serious restriction (that of their location), supplying only sites close to already positioned track; road transport is more versatile. Factoring in additional consid­erations, including air emissions during transport, definitely favors rail transport.73 Policy changes and new infrastructure investment appear therefore to be inevitable if the development of bioethanol production is not to be inhibited by objections of cost and pollution.

Focusing on corn stover and wheat straw as raw material inputs, a study of North Carolina concluded that more than 80% of these resources were located in the coastal area; four ethanol plants with feedstocks demands of between 146,000 and 234,000 dry tons/year required collection radiuses between 42 and 67 km (26-42 miles).74

The siting of production facilities to minimize transportation costs implies a contradiction with the economies of scale possible with larger production units. This is made more likely if relatively marginal biomass inputs such as municipal solid waste are to be considered.75 Urban fringes might be close to existing landfill sites and also within short distances of field crop residues, wastes from horticultural industries, and seasonal supplies of tree and plant residues from urban parkland. As discussed previously (section 5.3.2), the ability to design and build smaller-scale bioethanol production units, especially if they can utilize a variable and adventitious supply of feedstocks, would be highly beneficial to match the fragmented nature of the cheapest likely raw materials.

This C4 dicarboxylic acid is one of the key intermediates of glucose catabolism in aerobic organisms (including Homo sapiens) but can also be formed anaerobically in fermentative microbes (figure 8.4). In either case, CO2 is required to be “fixed” into organic chemicals; in classical microbial texts, this is described as “anaplerosis,” acting to replenish the pool of dicarboxylic and tricarboxylic acids when individual com­pounds (including the major industrial products citric, itaconic, and glutamic acids) are abstracted from the intracellular cycle of reactions and accumulated in the extracellular medium. Under anaerobic conditions, and given the correct balance of fermentation

GLUCOSE

products, a net “dark” fixation of CO2 can occur, and it is this biological option that has been most exploited in the development of modern biosynthetic routes:

• The rumen bacterium Actinobacillus succinogenes was discovered at Michigan State University and commercialized by MBI International, Lansing, Michi — gan.1617 Succinate yields as high as 110 g/l have been achieved from glucose.

• At the Argonne National Laboratory, Argonne, Illinois, a mutant of E. coli unable to ferment glucose because of inactivation of the genes encod­ing lactate dehydrogenase and pyruvate formate lyase spontaneously gave rise to a chromosomal mutation that reestablished glucose fermentative capacity but with an unusual spectrum of products: 1 mol succinate and 0.5 mol each of acetate and ethanol per mole of glucose consumed.1819 The second mutation was later mapped to a glucose uptake protein that, when inactivated or impaired, led to slow glucose transport into the cells and avoided any repression of genes involved in this novel fermentation.20 The result is a curious fermentation in which redox equivalents are balanced by a partition of carbon between the routes to succinate and that to acetate and ethanol (in equal measures), pyruvate being “oxidatively” decarboxylated rather than being split by pyruvate formate lyase activity or reduced to lac­tic acid, both routes lost from wild-type E. coli biochemistry in the paren­tal strain (figure 8.5). The maximum conversion of glucose to succinate by this route is 1 mol/mol, a carbon conversion of 67%; succinate titers have

with PTS

glucose

uptake

Oxaloacetic acid

Succinic acid oxidation value = 0 0.5 mole

oxidation value = +1
1 mole

FIGURE 8.5 Redox balance in the fermentation of glucose to succinic acid by Escherichia coli.

reached 75 g/l. Because E. coli is only facultatively anaerobic, biomass in the fermentation can be generated rapidly and to a high level under aerobic conditions, O2 entry then being restricted to transform the process to one of anaerobic metabolism.21 The same organism can successfully utilize both glucose and xylose in acid hydrolysates of corn straw and generate succi­nate as a fermentation product.22

• In wild-type E. coli, glucose fermentations produce complex mixtures of acid and nonacidic products, in which succinate may be only a minor component (chapter 2, section 2.2). Nevertheless, the succinate titer can be greatly increased by process optimization, and Indian researchers achieved more than 24 g/l within 30 hours with laboratory media and

17 g/l in 30 hours in a fermentor with an economical medium based on corn steep liquor and cane sugar molasses.23 24 The same group at the University of Delhi have enhanced succinate productivity with Bacteroides fragilis, another inhabitant of the human gut and intestine but an obli­gately anaerobic species.25 26

• In complete contrast, an aerobic system for succinate production was designed with a highly genetically modified E. coli, using the same glu­cose transport inactivation described above but also inactivating possible competing pathways and expressing a heterologous (Sorghum vulgare) gene encoding PEP carboxylase, another route for anaplerosis.2728 A succinate yield of 1 mol/mol glucose consumed was demonstrated, with a high pro­ductivity (58 g/l in 59 hours) under fed-batch aerobic reactor conditions. The biochemistry involved in this production route entails directing carbon flow via anaplerotic reactions to run “backward” through the tricarboxylic acid cycle, that is, in the sequence:

PEP ^ oxaloacetate ^ malate ^ fumarate ^ succinate

• At the same time, succinate is produced in the “forward” direction by block­ing the normal workings of the cyclic pathway (with a necessary loss of carbon as CO2) and the activation of a pathway (the “glyoxylate shunt”) nor­mally only functioning when E. coli grows on acetate as a carbon source:

citrate ^ isocitrate ^ succinate + glyoxylate

• The enzymes catalyzing the final two steps in the pathway from PEP, malate dehydratase and fumarate reductase, can be overexpressed in bacterial spe­cies and are the subjects of two recent patent applications from Japan and

Korea.29,30

• To return to anaerobic rumen bacteria, Anaerobiospirillum succiniciprodu — cens had a short but intense history as a candidate succinate producer.3133 Fermenting glucose in a medium containing corn steep liquor as a cost effective source of nitrogen and inorganic nutrients, succinate titers reached

18 g/l from 20.2 g/l of glucose, equivalent to a conversion efficiency of 1.35 mol/mol.34

• The same research group at the Korean Advanced Institute of Science and Technology, Daejeon, Republic of Korea, then isolated a novel rumen bacterial species, Mannheimia succiniciproducens, and has determined its complete genomic sequence as well as constructing a detailed metabolic network for the organism.35-37 Mutants of this microbe can produce suc­cinate with much reduced amounts of other acids and can anaerobically ferment xylose and wood hydrolysate to succinate.38 39

There are grounds to predict that overexpressing genes for anaplerotic pathway enzymes would enhance succinate production (and, in other genotypes or fermenta­tion conditions, the accumulation of other acids of the tricarboxylic cycle); experi­mental evidence amply confirms this prediction.40-45 With the capabilities to perform metabolic computer-aided pathway analysis with known gene arrays, comparison of succinate producers and nonproducers and between different species would be expected to greatly accelerate progress toward constructing the “ideal” microbial cell factory. Comparison of E. coli and M. succiniciproducens suggested five target genes for inactivation but combinatorial inactivation did not result in succinate over­production in E. coli; two of the identified genes — ptsG (the glucose transport system) and pykF (encoding pyruvate kinase, the enzyme interconverting PEP and pyruvic acid), together with the second pyruvate kinase gene (pykA) — increased succinate accumulation by more than sevenfold, although succinate was still greatly outweighed by the other fermentation products (formate, acetate, etc.).46 Eliminating the glycolytic pathway below PEP will clearly aid succinate production, but the pro­ducing cells will by then be highly dependent on organic nutrients (including many amino acids) for growth and maintenance.

The combination of several related technologies is particularly appealing for commercial production of succinic acid:

• Optimization of the Argonne National Laboratory strains for succinate pro­duction by the Oak Ridge National Laboratory, Oak Ridge, Tennessee

• Innovations for succinate recovery from the fermentation broth at Argonne National Laboratory

• An improved succinic acid purification process47

• The development of catalytic methods for converting succinic acid to 1,4- butanediol and other key derivatives at Pacific Northwest National Labora­tory, Richmond, Washington48

These advances have moved biologically derived succinic acid close to commercial­ization as a component of the first genuine biorefinery.

Bacteria are traditionally unwelcome to wine producers and merchants because they are causative spoiling agents; for fuel ethanol production, they are frequent contami­nants in nonsterile mashes where they produce lactic and acetic acids, which, in high concentration, inhibit growth and ethanol production by yeasts.3435 In a pilot plant constructed and operated to demonstrate ethanol production from corn fiber-derived sugars, for example, Lactobacilli were contaminants that could utilize arabinose, accumulating acids that impaired the performance of the ethanologenic yeast; the unwanted bacteria could be controlled with expensive antibiotics, but this experience shows the importance of constructing ethanologens to consume all the major carbon

sources in lignocellulosic substrates so as to maximize the competitive advantage of being the dominant microbial life form at the outset of the fementation.36

Bacteria are much less widely known as ethanol producers than are yeasts but Escherichia, Klebsiella, Erwinia, and Zymomonas species have all received serious and detailed consideration for industrial use and have all been the hosts for recom­binant DNA technologies within the last 25 years (table 3.3).37-43 With time, and perhaps partly as a result of the renewed interest in their fermentative capabilities, some bacteria considered to be strictly aerobic have been reassessed; for example, the common and much-studied soil bacterium Bacillus subtilis changed profoundly in its acknowledged ability to live anaerobically between the 1993 and 2002 editions of the American Society for Microbiology’s monograph on the species and its rela­tives; B. subtilis can indeed ferment glucose to ethanol, 2,3-butanediol, and lactic acid, and its sequenced genome contains two ADH genes.44 The ability of bacteria to grow at much higher temperatures than is possible with most yeast ethanologens led to proposals early in the history of the application of modern technology to fuel etha­nol production that being able to run high-yielding alcohol fermentations at 70°C or above (to accelerate the process and reduce the economic cost of ethanol recovery) could have far-reaching industrial implications.45,46

Bacteria can mostly accept pentose sugars and a variety of other carbon sub­strates as inputs for ethanol production (table 3.3). Unusually, Zymomonas mobilis can only use glucose, fructose, and sucrose but can be easily engineered to utilize pentoses by gene transfer from other organisms.47 This lack of pentose use by the

wild-type organism probably restricted its early commercialization because other­wise Z. mobilis has extremely desirable features as an ethanologen:

• It is a GRAS organism.

• It accumulates ethanol in high concentration as the major fermentation product with a 5-10% higher ethanol yield per unit of glucose used and with a 2.5-fold higher specific productivity than S. cerevisiae.48

• The major pathway for glucose is the Entner-Doudoroff pathway (figure 3.4); the inferior bioenergetics of this pathway in comparison with glycolysis means that more glucose is channeled to ethanol production than to growth, and the enzymes required comprise up to 50% of the total cellular protein.48

• No Pasteur effect on glucose consumption rate is detectable, although inter­actions between energy and growth are important.49

Escherichia coli and other bacteria are, as discussed in chapter 2 (section 2.2), prone to incompletely metabolizing glucose and accumulating large amounts of carboxylic acids, notably acetic acid; with some authors, this has been included under the heading of the “Crabtree effect.”50,51 For E. coli as a vehicle for the production of recombinant proteins, acetate accumulation is an acknowledged inhibitory factor; in ethanol production, it is simply a metabolic waste of glucose carbon. Other than this (avoidable) diversion of resources, enteric and other simple bacteria are easily genetically manipulated, grow well in both complex and defined media, can use a wide variety of nitrogen sources for growth, and have been the subjects of decades of experience and expertise for industrial-scale fermentations — Z. mobilis also was developed for ethanol production more than 20 years ago, including its pilot-scale use in a high-productivity continuous process using hollow fiber membranes for cell retention and recycling.52

The solids remaining at the end of the fermentation (distillers dried grain with solids, or DDGS: see figure 1.21) are a high-protein animal feed — a saleable by-product that has been suggested to be so commercially desirable that reduced ethanol yields could be tolerated to support its increased production, although, in practice, high — sugar residues pose severe practical difficulties to DDGS drying and processing.122 The rapid rise of ethanol production from cornstarch has, however, demanded some remarketing of this coproduct:269

• The product is less dark because sugars are more efficiently fermented and less available to react chemically and caramelize in the dried product.

• The essential amino acid contents are higher (figure 4.13).

• Although ruminant animals can certainly benefit from feeding with DDGS, pigs are geographically much closer to ethanol plants in the midwestern United States.

Reducing phosphate content would widen the use of DDGS by addressing animal waste disposal issues, and the development of more efficient methods for removing water in the preparation of the DDGS could greatly reduce processing costs.270 Adding on a second fermentation (or enzymic biotransformation), a dry-grind processing to generate plant oils and a higher-value animal feed from the DDGS, and separating more useful and saleable fine chemicals from the primary fermentation would increase the total mass of recovered bioproducts to the maximum achievable (figure 4.14).271 Pric­ing is crucial because the increased supply of DDGS is likely to significantly reduce its market price, and its alternate use as the feedstock for further ethanol production itself has been worthy of investigation: steam and acid pretreatments can convert the residual starch and fiber into a substrate for yeast-based ethanol production with a yield 73% of the theoretical maximum from the glucans in the initial solids.272

A much simpler option is to realize the potential in the fermented solids to provide nutrients and substrates for a new round of yeast (or other ethanologen) growth and eth­anol production: such spent media (“spent wash,” stillage, or vinasse) can be recycled in the process known as “backsetting,” found to be beneficial for yeast growth and a practical means of reducing water usage in a fuel alcohol facility.273 Backsetting is not without its accepted potential drawbacks, including the accumulation of toxic nonvola­tiles in the fermentor, increased mash viscosity, and dead cells causing problems with

High Value Animal
Feed

Carbon Dioxide Plant Oils Inositol Succinic Acid Glycerol Ethanol

FIGURE 4.14 Projected recovery of product and coproducts from the ethanol fermentation of corn starch. (Data from Dawson.271)

viability measurements, but as a crude means of adapting the fermentation to a semi­continuous basis, it has its advocates on both environmental and economic grounds. Furthermore, a study by Novozymes demonstrated that mixtures of fungal enzymes could decrease vinasse viscosity and liberate pentose sugars from soluble and insoluble arabinoxylans that would be suitable for fermentation by a suitable pentose-utilizing ethanologen.274-277 Portuguese work has also shown that such a pentose-rich product stream can be the starting point for the fermentative production of xylitol (a widely used noncalorific sweetener) and arabinitol by the yeast Debaryomyces hansenii.278,279 As a support for the immobilization of yeast cells, brewer’s spent grains were a very effective means of supplying “solid-phase” biocatalysts for ethanol production from molasses, and the solids from bioethanol plants could serve a parallel function.280

Brazil has, by far, the longest continuous history of devising methods for eco­nomically viable disposal for vinasse and the solid waste product (bagasse), espe­cially because neither had at times been considered to be saleable and both could even represent negative value as incurred disposal costs:281 [43]

• Important for minimizing fertilizer use, material dissolved in the digestion wastewater represents 70% of the nutrient demand of sugarcane fields.

6.2.1 The Renascence of an Old Chemistry for Biomass-Based Fuels?

The generation of a combustible gas, synthesis gas (“syngas”), from biomass was discussed briefly in chapter 2 (section 2.1). Technologies for the conversion of coal and natural gas to liquid fuels were also included in chapter 5 (section 5.6) as part of a survey of different strategies for adapting to potentially dwindling crude oil reserves. The chemistry of gas-to-liquid fuel transformations was developed in the first quarter of the twentieth century and utilized extensively in Germany during World War II; further evolution led to commercial production processes being initi­ated for peacetime purposes in the 1990s.84,85

The essential step, known as the Fischer-Tropsch (FT) reaction, can be written as

nCO + 2nH2 ^ [CH2]n + nH2O,

where [CH2]n represents a range of hydrocarbons, ranging from low-molecular-weight gases (n = 1, methane), by way of gasoline (n = 5-12), diesel fuel (n = 13-17), and as far as solid waxes (n > 17). The reaction requires catalysts for realistic rates to be achieved, usually iron or cobalt (although transition metals will function effectively) at high temperatures (180-350°C) and high pressures; the higher the temperature, the higher the proportion of gas and liquid hydrocarbon products.

To date, no process has been commercialized from plant biomass feedstocks, and the FT technology could be described as “radical” or “nth” generation for biofuels were it not that the key elements of the chemistry and production options are reason­ably well established in industrial processes with fossil inputs; in a climate of high crude oil prices, the environmental desirability of low-sulfur diesel, and the drive to commercialize otherwise unmarketable natural gas in remote locations are impor­tant synergies (table 6.6).86 FT biomass-to-liquid fuel (FT-BtL) from lignocellulosic sources is particularly attractive because of the high CO2 emission reduction poten­tial (up to 90% when substituting conventional gasoline and diesel) and the ability to use woody materials from low-grade land, thus avoiding the pressures on land use in OECD countries contemplating agriculture-based bioethanol or biodiesel production on a large scale.79 The principal barrier to large-scale biomass FT-BtL appears to be the suboptimal mixture of gases in syngas as prepared from plant materials: the lower the molar ratio of H2:CO, the more the proportion of high-molecular-weight products formed in the FT reaction, but biomass gasification results in a wide range of H2:CO ratios, often with an excess of CO, together with appreciable amounts of CO2, methane, and higher hydrocarbons as well as smaller amounts of condensable tars and ammonia.87

The methane can be transformed to CO and H2 by a number of different reac­tions, including the uncatalyzed (but again high-temperature and high-pressure)

processes:88

CH4 + O2 ^ CO2 + 2H2O and CH4 + H2O ^ CO + 3H2

Partial removal of CO (and formation of additional H^ is possible by the water — shift reaction:

CO + H2O ^ CO2 + H2

Finally, the physical removal (adsorption) of CO2 (an inert gas for FT reactions) is relatively straightforward, but a higher-yielding process can be devised (at least, in principle) by including a catalytic reduction of the CO2 to using multiple FT reactors in series with an intermediate water removal step:89

CO2 + 3H2 ^ [-CH2-] + 2H2O

Complete wood-based FT-BtL production involves, therefore, a multistage pro­cess, incorporating biomass pretreatment, syngas purification, and optional syngas recycling, plus gas turbine power generation for unused syngas and, for FT die­sel, a hydrocracking step to generate a mixture of diesel, naphtha, and kerosene (figure 6.8).87,88