Outside primary scientific journals, data from a range of sources (including reports prepared for governments and conference proceedings) were compiled on the basis of 2003 costings as a baseline for future cost modeling.32 Ethanol produced from sugar, starch (grain), and lignocellulosic sources covered production cost estimates from less than $1/gallon to more than $4/gallon (table 5.15). Even with the lower production costs for lignocellulosic ethanol in the United States, taking into account financial outlays and risks ($260 million for a 50-million-gallon annual production plant), an ethanol price of $2.75/gallon would be more realistic.33

The International Energy Agency’s most recent assessment of sugar — and starch- derived ethanol (2005 reference basis) is that Brazil enjoys the lowest unit costs ($0.20/l, or $0.76/gallon), starch-based ethanol in the United States costs (after pro­duction subsidies) an average of around $0.30/l (or $1.14/gallon), and a European cost (including all subsidies) is $0.55/l (or $2.08/gallon).34 Brazilian production costs for fuel alcohol, close to $100/barrel in 1980, decreased rapidly in the 1980s, and then more slowly, but only a severe shortage of sugarcane or a marked rise in sugar prices would interrupt the downward trend in production costs.35,36

With due allowance of the lower fuel value of ethanol, therefore, the historical trend of fuel ethanol production costs versus refinery gate price[50] of gasoline is show­ing some degree of convergence (figure 5.5). In particular, the real production costs of both sugar — and corn-derived ethanol have fallen so that the production costs (with all tax incentives in place, where appropriate) now is probably competitive with the production cost of gasoline, as predicted for biomass ethanol in 1999.37 Critics of the corn ethanol program have, however, argued that the price of fuel ethanol is arti­ficially low because total subsidies amount to $0.79/gallon for production costs of $1.21/gallon, that is, some $3 billion are expended in subsidizing the substitution of only 1% of the total oil use in the United States.38 Although incentives for domestic
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ethanol use in Brazil were discontinued by 1999 (except as part of development poli­cies in the northeast region), a cross-subsidy was created during the 1990s to subsi­dize ethanol production through taxation on gasoline and diesel; this was operated via a tight government control of the sales prices of gasoline, diesel, and ethanol, and the monopoly represented in the country by PETROBRAS.36

Brazilian sugarcane ethanol has reached the stage of being an importable com­modity to the United States, promoting the development of shipping, port handling, and distribution network infrastructures. In Europe, grain alcohol might be cost — competitive — with the much higher tax rates prevailing in Europe, the scope for regulating the end user price is much higher. In contrast, the economics of lignocel — lulosic ethanol remain problematic, although it is possible that in the United States, at least, production costs may become competitive with gasoline within the next five to ten years unless, that is, crude oil prices decrease significantly again.

8.1 THE BIOREFINERY CONCEPT

As a neologism, “biorefinery” was probably coined in the early 1990s by Charles A. Abbas of the Archers Daniel Midland Company, Decatur, Illinois, extrapolating the practices implicit in the fractionation of corn and soybean — the wet milling process was an excellent example of a protobiorefinery (figure 1.20). Certainly, by the late 1990s, the word (or, in an occasional variant usage “biomass refinery”) was becoming increasingly popular.1 The concept has carried different meanings according to the user, but the central proposition has been that of a comparison with the petrochemical refin­ery that produces not only gasoline and other conventional fuels but also petrochemical feedstock compounds for the chemical industry: from a biorefinery, on this formal anal­ogy, the fuels would include ethanol, biodiesel, biohydrogen, and/or syngas products, whereas the range of fine chemicals is potentially enormous, reflecting the spectrum of materials that bacterial metabolism can fashion from carbohydrates and other mono­mers present in plant polysaccharides, proteins, and other macromolecules (figure 8.1).

The capacity to process biomass material through to a mixture of products (includ­ing biofuels) for resale distinguishes a biorefinery from, for example, a “traditional” fermentation facility manufacturing acids, amino acids, enzymes, or antibiotics, indus­trial sites that may use plant-derived inputs (corn steep liquor, soybean oil, soy protein, etc.) or from either of the two modern polymer processes producing any one output from biomass resources that are often discussed in the context of biorefineries:

• Cargill Dow’s patented process for polylactic acid (“Natureworks PLA”), pioneered at a site in Blair, Nebraska; this was the first commodity plastic to incorporate the principles of reduced energy consumption, waste genera­tion, and emission of greenhouse gases and was awarded the 2002 Presi­dential Green Chemistry award.2

• 1,3-Propanediol (1,3-PD) produced from glucose by highly genetically engineered Escherichia coli carrying genes from baker’s yeast and Kleb­siella pneumoniae in a process developed by a DuPont/Tate & Lye joint venture; 1,3-PD is a building block for the polymethylene terphthalate poly­mers used in textile manufacture.3

However good are these example of the use of modern biotechnology to support the bulk chemistry industry, they center on single-product fermentations (for lactic acid and 1,3-PD, respectively) that are not significantly different from many earlier bacte­rial bioprocesses — in particular, lactic acid has a very long history as a microbial ingredient of yogurts and is used in the food industry to control pH, add flavor, and

control microbial growth in products as diverse as alcoholic beverages, frozen desserts, and processed meat; the lactic acid production sector has major manufacturers in China, the United States, and Europe that utilize lactobaccili, bacilli, or Rhizopus molds in large-scale fermentations.

The following three definitions for biorefineries focus on the multiproduct (usu­ally) biofuel-associated nature of the envisaged successors to fossil-based units:

1. The U. S. Department of Energy: “A biorefinery is an overall concept of a processing plant where biomass feedstocks are converted and extracted into a spectrum of valuable products.”4

2. The National Renewable Energy Laboratory: “A biorefinery is a facility that integrates biomass conversion processes and equipment to produce fuels, power, and chemicals from biomass. The biomass concept is analogous to today’s petroleum refineries, which produce multiple fuels and products from petroleum.”5

3. “Third generation (generation-III) and more advanced biorefineries … will use agricultural or forest biomass to produce multiple product streams, for example ethanol for fuels, chemicals, and plastics.”1

In early 2008, no such biorefineries exist, but the concept provides a fascinating insight into how biofuel-production facilities could develop as stepping stones toward the global production of chemical intermediates from biomass resources if lignocel — lulosic ethanol fails to meet commercial targets or if other developments (e. g., the successful emergence of a global hydrogen economy) render liquid biofuels such as bioethanol and biodiesel short-lived experiments in industrial innovation.[64]

The sheer scale of chemical endeavor possible from biomass resources is moreover extremely persuasive:6

• By 2040, a world population of 10 billion could be supported by 2 billion hectares of land for food production leaving 800 million hectares for nonfoods.

• With a “modest” increase in agricultural productivity to 40 tonnes/ hectare/year, this land surplus to food production could yield 32 billion tonnes/year.

• Adding in 12 billion tonnes annually from forests and other agricultural waste streams yields 50 billion tonnes.

Of this total, only 1 billion tonnes would be required to generate all the organics[65] required as chemical feedstocks — leaving the rest for biofuels, including traditional biomass as a direct source of power and heat.

Calculations prepared from and for German industry show that agricultural waste only, that is, cereal straw, could match the total demand for E10:gasoline blends as well as all the ethylene manufactured for the national chemical and plas­tics industries plus surplus ethanol for use in E85 blends and other chemical uses (figure 8.2).7

Included in the range of roles proposed for biorefineries, as codified by biorefin — ery. nl, the umbrella organization in the Netherlands tasked with developing strategic aspects of biorefineries (www. biorefinery. nl), are the following:

1. Primary processing units for waste streams from existing agricultural endeavors

2. Essential technologies for ensuring that biomass-derived ethanol and other biofuels can be produced at costs competitive with conventional fuels

3. New additions to be integrated with the infrastructure of agricultural processing — these might include (in Europe) beet sugar refineries

As corollaries and (probably) axiomatic truths, biorefineries will only become “interest­ing” (as players in the industrial economy) when they reach large scales of operation and contribute significant amounts of materials to widely used and/or specialist chemistry platforms while being driven not essentially or solely as means to reduce greenhouse gas emissions[66] but by considerations of the future depletion of fossil fuel reserves and the desire to broaden the substrate base, with governments being instrumental in cata­lyzing these developments by favorable taxation regimes and economic subsidies.

The International Energy Agency’s 2006 analysis concludes that biodiesels are not price competitive with conventional diesels if all subsidies to crops and production are excluded; if the biodiesel source is animal fat, however, the derived biodiesel would be competitive at crude oil prices below $50-55/barrel.2 By 2030, assum­ing various process improvements and economies of scale, biodiesel from vegetable oils were also predicted to be competitive at crude oil prices below $50-55/barrel; European biodiesel would continue to be more expensive than U. S. biodiesel, with feedstock costs being the largest contributor (figure 6.4). This continues a tradition of cost assessments that commenced in the 1990s. Rapeseed oil-derived biodiesel was estimated to require a total subsidy of between 10% and 186% of the price of conventional diesel in 1992, the variation reflecting the price of the seeds sown to grow the crop; this was equivalent to the cost of biodiesel being between 11% and 286% of the refinery gate cost of conventional diesel — the contemporary price for seeds would have resulted in biodiesel being 243% of the conventional diesel

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cost.4 These calculations required both coproducts (glycerol and rapeseed meal) to be income generators; in addition, the spring-sown crop (although with lower yields per unit of land) had lower production costs.

An evaluation of costs from U. S. soybean and sunflower in 2005 concluded that soybean biodiesel would have production costs 2.8-fold that of conventional diesel (using 2003 price data), whereas sunflower-derived biodiesel was more than five­fold more expensive to produce than petroleum diesel fuel.49 Sunflower seeds were accepted to have a higher oil content (25.5%) than soybeans (18%) but to also have a lower crop productivity (1500 versus 2700 kg/hectare); in both cases, oil extraction was calculated to be highly energy intensive.

The production costs for biodiesels also depend on the production route. With waste cooking oil as the feedstock, although an alkali-catalyzed process using virgin vegetable oil had the lowest fixed capital cost, an acid-catalyzed process using the waste oil was more economically feasible overall, providing a lower total manufac­turing cost, a more attractive after-tax rate of return, and a lower biodiesel breakeven price; in addition, plant capacity was found to be a significant factor affecting the economic viability of biodiesel manufacture.50 51 The U. S. Department of Agricul­ture’s Agricultural Research Service has developed a computer model to estimate the capital and operating costs of a moderately sized industrial biodiesel production facility (annual production capacity, 10 million gallons):52

• Facility construction costs were calculated to be $11.3 million.

• The largest contributors to the equipment cost (accounting for nearly one — third of expenditures) were storage tanks to contain a 25-day capacity of feedstock and product.

• At a value of $0.52/kg for feedstock soybean oil, a biodiesel production cost of $0.53/l ($2.00/gallon) was predicted.

• The single greatest contributor to this value was the cost of the oil feed­stock, which accounted for 88% of total estimated production costs. An analysis of the dependence of production costs on the cost of the feedstock indicated a direct linear relationship between the two.

• Process economics included the recovery of coproduct glycerol generated during biodiesel production, and its sale into the commercial glycerol mar­ket, which reduced production costs by approximately 6%.

Waste cooking oils, restaurant grease, and animal fats are inexpensive feedstocks; they represent 30% of total U. S. fats and oil production but are currently devoted mostly to industrial uses and animal feed, and because the free fatty acids may represent more than 40% of the material, the production process may be complex.53 Nevertheless, such unconventional feedstocks may become increasingly important because soybean oil prices reached a peak not seen since 1984: an article posted online in Biodiesel Magazine traced the rapid inflation in soybean oil from early 2006.54 This surge in the price of biodiesel feedstock has occurred despite the stocks of soybean oil being at near-record levels — and in only three individual months between January 2006 and March 2007 did soybean oil use exceed production (figure 6.5). With its main feedstock being increasingly expensive, U. S. biodiesel is

not competitive on price with diesel fuels or heating oil; tax incentives may be neces­sary to overcome these production price issues.54

Confounding these feedstock problems, the great expansion of biodiesel produc­tion both in Europe and the United States has caused such a glut of glycerol-containing waste (or coproduct) that, in the absence of glycerol valorization mechanisms in place and on site, disposing of this glycerol is proving an increasingly expensive disposal cost outlay. Because of the enormous potential of this renewable source of a potentially valuable chemical intermediate, however, biodiesel waste glycerol is best considered an example (rather premature) of “biocommodity engineering” and is discussed at length in chapter 8 (section 8.3.3) when the broader topic of replacing petrochemicals by biobased products is considered.

Although a few pioneering studies attempted cost estimates of wood-derived ethanol from the 1980s onward, they focused on aspects of the technological processes required rather than making firm conclusions about market prices.24,25 Swedish studies appear to have been the first to present detailed cost breakdowns for ethanol production from accessible large-scale woody biomass sources.26,27 The first of the two to be published was highly unusual in that it used recent advances in pentose utilization by recombinant bacteria to model a pentose stream process, that is, using the solubilized sugars from the pretreatment of wood (willow) feedstock — the procedure involved the impregna­tion of the material with SO2 and subsequent steaming — and included a detoxification procedure to reduce the levels of inhibitors from the hydrolysate.26 The fermentation with Escherichia coli KO11 (chapter 3, section 3.3.2.1) was assumed to consume 96%

[49] The authors did not discriminate between xylose and xylooligosaccharides or between xylose and arabinose, and the implicit assumption was that more than 95% of the available pentoses were in a readily metabolizable form, that is, monomeric xylose.

A very similar analysis of three different approaches to utilizing the full carbo­hydrate potential of pine wood, digesting the cellulose component with concentrated acid, dilute acid, and enzymatic methods, calculated a full manufacturing costs for ethanol of between 500 and 530/l ($1.89-2.01/gallon).27 The bulk of the production cost (up to 57.5%) was accounted for by the financial costs of installing the hardware for generating fermentable carbohydrates (hexoses as well as pentoses) in more com­plex total processes with longer cycle times.

It is vital at this point to differentiate commercial realities from strategic (or geopo­litical) and all other considerations. Although (as discussed in chapter 1) historical, environmental, political, and macroeconomic arguments have all been adduced in support of bioenergy programs, fiscal considerations now play an important role in both encouraging (prompted by political agendas) the take-up of novel alternative fuels and in partitioning the market for first and subsequent generations of rival but not equally readily commercialized biofuels. Indeed, taxation issues were quickly recognized and seized on by proponents of bioethanol, particularly because they were useful to counter the gasoline versus gasohol price differential: for example, in the United States, the indirect costs of regulating air pollution and of military protection for oil supplies from the Middle East are calculable and greatly inflate the nominal price of crude oil but are not (explicitly) passed on to the consumer.5 This distortion of the transportation fuel market by “hidden” subsidies has also led to economics models in which other indirect factors are included in the cost-benefit analysis:

• Technological developments that improve the national scientific base for employment, patents and overseas licensing, and engineering advances that “spillover” into related fields

• Reduced foreign currency payments and associated “banking” costs—highly important for a developing economy such as Brazil’s

• Higher income and sales tax returns from greater rural employment

• Reduced longer-term economic impacts of climate change and air pollution

All these arguments are, to varying degrees, contentious, and skeptics can be found from opposite ends of the economic spectrum, from oil industry analysts to academ­ics who foresee only accelerated land degradation from the industrial agronomy of energy crop cultivation.67

Taxation as an instrument of social and economic policy has, moreover, obvi­ous limitations if wasteful subsidies or punitive levels of taxation on standard gasoline and diesel products are to be avoided. Consider the following three scenarios:

• Bioethanol production can generate a commercial fuel with pump prices no greater than those of standard gasoline grades at equivalent tax rates, the comparison being valid when average prices during a period of one to five years are calculated, thus avoiding false comparisons at peaks and troughs caused by fluctuations in both agricultural feedstock prices (as an important cost input to biofuel production) and oil price movements if they continue to move inside the wide limits evident since the early 1980s (figure 5.1).

• Bioethanol can be produced commercially at a total (production, distribu­tion, and resale) cost that averaged, during a 5- to 10-year period, 10-50% higher than that of gasoline.

• Bioethanol production can only generate an unsubsidized product with a total cost more than twice that of the refinery gate price of standard gasoline (a price differential quoted for the United States in the late 1990s8) — or per­haps, even up to 10 times higher than conventional fuels where, for example, local conditions of climate and biomass availability are consistently much less favorable than for sugarcane production in Brazil or corn in the United States or where only refractory lignocellulosic feedstocks can be accessed with poorly developed bioprocessing technology.

The first (optimistic) case approximates that of Brazilian consumers with flexibly fueled cars after 2000.9 The second case is the conclusion most often reached in technoeconomic studies, whereas the third scenario is parallel to the emergency or “wartime” case discussed when the energy yields of conventional and alternative fuels were considered in chapter 1 (section 1.6.1): even if biofuels are prohibitively expensive now, technical developments may erode that differential or be obviated if (or when) fossil fuel shortages become acute in the present century (see later, section 5.6). In all three cases, taxation policy can (and will) influence consumer choice and purchasing patterns, whether for short — (tactical) or longer-term (strategic) reasons and when legislation enforces alternative or reconstituted fuels to achieve environ­mental targets.

A snapshot of data from October 2002 in Brazil, however, reveals the complexity of the interaction between production/distribution costs and imposed taxation on the final at-pump selling price.10 Although gasohol mixtures, hydrous ethanol, and diesel all had very similar production costs, equivalent to approximately 150/l (570/gal — lon) at that time, the final cost to the user was determined by the much higher taxes applied to gasohol (figure 5.2). Brazil exemplifies the extensive use of taxation to determine and direct the perceived prices of gasoline and alternative fuels as a delib­erate instrument of national policy. Such management of the fuel economy is likely to be instigated in societies where not only economics but social and environmental considerations are taken into account, but runs the risk of experiencing budgetary shortfalls if the total tax raised is severely reduced when the policy is too successful in achieving its aims — this becomes even worse if private transportation is perceived as being subsidized by other taxation sources (e. g., sales tax, income tax). For all the various interest groups in biofuels development, therefore, the priority is to establish viable production processes with the minimum requirement for tax incentives.

5-Hydroxymethylfurfural (HMF) was discussed in chapter 3 as a toxic product of acidic pretreatment techniques for biomass. The boiling point of HMF is too high (291°C) to be considered as a liquid fuel, but if HMF is subject to chemical hydroge — nolysis of two of its C-O bonds, a more volatile product, 2,5-dimethylfuran (DMF) is formed (figure 7.3).12 DMF has a boiling point of 93°C, 20°C higher than ethanol, and has a Research Octane Number of 119 — the by-product 2-methylfuran has an even higher RON (131) but is more water-soluble than DMF.

HMF is most readily formed by the dehydration of fructose, a naturally occur­ring sugar and a straightforward isomerization product of glucose; mineral acids such as hydrochloric acid (HCl) can be used to catalyze the reaction, 88% conversion being achieved at 180°C.12 A solvent such as n-butanol (chapter 6, section 6.3.3) can then be employed to extract the HMF before hydrogenolysis over a mixed Cu-Ru catalyst at 220°C.

Such production routes, beginning with enzymic conversion of glucose to fruc­tose and proceeding via entirely thermochemical processes, have been described as “hybrid.”13 They have the advantage of avoiding reliance on large fermentation vessels for the production step(s), therefore being potentially much more rapid. Their economics could be similar to, or an improvement on, those for Fischer-Tropsch liquid fuels (chapter 6, section 6.2). The conversion of glucose to fructose, catalyzed by the enzyme glucose isomerase, has been a major industrial application of enzy- mology since the 1960s, the product (high-fructose corn syrup) being introduced as a substitute for Cuban sugar in the U. S. reduced-calorie sweetener market.14 The enzyme technology has been continuously improved, evolving to immobilized forms of the enzyme; the potential of enzymes from hyperthermophilic microbes has now been explored, with a stability at 80oC rivaling that of conventional enzyme pro­cesses operated at 55-650C.15 Rapid and efficient processing of glucose solutions to high concentrations of fructose is feasible if the desirable biocatalytic and thermo­stability properties of suitable enzymes can be realized.

5.3.1 Near-Future Projections for Bioethanol Production Costs

The persistently high production costs of lignocellulosic ethanol (particularly in Europe) have catalyzed several attempts to predict trends in 5-year, 10-year, and lon­ger scenarios, with the implicit or explicit rationale that only lignocellulosic biomass is sufficiently abundant to offer a means of substituting a sizeable proportion of the gasoline presently used for transportation.

In 1999, the National Renewable Energy Laboratory published a projection of the economic production costs for lignocellulosic ethanol that, starting from a baseline of $1.44/gallon (in 1997 dollars), computed a decrease to $1.16/gallon with a 12% increase in yield (to 76 gallons/ton of feedstock), with a 12% increase in plant produc­tion capacity together with a 12% reduction in new capital costs.31 A price trajectory envisaged this price deceasing to below $0.80/gallon by 2015 based on developments in cellulase catalytic efficiency and production and in ethanologenic production organisms:

• Improved cellulose-binding domain, active site, and reduced nonspe­cific binding

• Improved cellulase producers genetically engineered for higher enzyme production

• Genetically engineered crops as feedstocks with high levels of cellulases

• Ethanologens capable of producing ethanol at temperatures higher than 50°C

• Ethanologens capable of direct microbial conversion of cellulose to ethanol

These topics were covered in chapters 2 to 4; to a large extent, they remain research topics, although cellulase production and costs have certainly been improved greatly at the industrial scale of production.

A European study suggested that by 2010 lignocellulosic ethanol production costs could decrease to $0.53/l ($2.00/gallon), $0.31/l ($1.18/gallon) by 2020, and $0.21/l ($0.79/gallon) after 2025.32 These improvements in process economics were considered to result from the combined effects of higher hydrolysis and fermenta­tion efficiencies, lower specific capital investment, increases of scale, and cheaper biomass feedstock costs, but the prospect of ethanol ever becoming cost-competitive to gasoline was nevertheless considered to be “unlikely” — much of the analysis was, however, undertaken during the early stages of the great surge in oil prices and gasoline production costs after 2002 (figure 5.5). A more recent publication by the same research group (Utrecht, the Netherlands) computed a 2006 production cost for lignocellulosic ethanol of €22/GJ ($2.00/gallon, assuming exchange parity between the currencies) that was anticipated to fall to €11/GJ ($1.00/gallon) by 2030 — the single largest contributor to the production cost in 2006 was capital-related (46%) but this was expected to decrease in both absolute and relative terms so that biomass costs predominated by 2030.39

The International Energy Agency’s most recent prognosis is for lignocellulosic ethanol (from willow, poplar, and Miscanthus biomass sources) to reach production costs below €0.05/l (€0.18/gallon) by 2030 with achieved biomass yield increases per unit land area of between 44% and 100% (table 5.16); biomass-derived etha­nol would, on this basis, enjoy lower production costs than other sources in Europe (cereal grain, sugarbeet, etc.).34 These three biomass options each target different areas: [51]

decrease in the starch content (from 59.5% to 55%, w/w) can similarly negatively impact the productivity of an ethanol production unit.40

Projecting forward from a 1995 baseline, feedstock costs for a mature biomass ethanol technology were anticipated as being within the price range $34-38.6/dry ton.22 Switchgrass farming has been estimated to cost $30-36/dry tonne; in compari­son with straw or corn stover, the collection of switchgrass is probably less expensive because of the high yield of a denser biomass — nevertheless, delivered costs of the switchgrass to a production plant could be as low as $37/dry tonne of compacted material to $47/tonne of bales.41 Similarly, corn stover is difficult to handle on account of its low bulk density; chopped corn stover can be compacted into briquettes that can reach a density of 950 g/l, and these more easily transported briquettes are more durable if produced from stover with low water content (5-10%).42 A more radical option is the pipeline transport of corn stover (e. g., at 20% wet solids concentration); this is cheaper than trucking at more than 1.4 million dry tonnes/year and allows the possibility of conducting a partial saccharification during transport if enzymes are added, thus reducing the need for investment in the fermentation plant and lowering production costs by 7-80/gallon.43

In general, as bioethanol plant capacity is increased to cut unit production costs, the land area required for collection of sufficient biomass feedstock increases; as biomass supplies are sought from larger and larger distances, the costs of moving the raw material increases, introducing possibly diseconomies into production mod­els. One solution is to introduce more flexibility into the feedstock “diet,” taking advantage of whatever surpluses of other biomass material may seasonally occur; for example, a Californian study investigated what biomass supplies could be considered for a 40-million-gallon facility in the San Joaquin Valley: locally grown corn was significantly more expensive than midwestern corn ($1.21/gallon of ethanol versus $0.92/gallon), but surplus raisins and tree fruit and (although much more expensive) grapes and citrus fruit might all be included in a biomass harvest.44 The proximate example of feedstock diversity is, however, that of cane sugar bagasse: the Dedini Hidrolise Rapida (Rapid Hydrolysis) process uses organic solvent extraction of sug­arcane bagasse as a pretreatment method and aims to double the alcohol production per hectare of sugarcane harvested (www. dedini. com. br).

Using paper sludge as a feedstock for ethanol production has been claimed to be profitable as it provides a near ideal substrate for cellulase digestion after a com­paratively easy and low-cost pretreatment; even without xylose conversion to ethanol, such a technology may be financially viable at small scales, perhaps as low as 15 tonnes of feedstock processed/day.45 This is a genuinely low-cost feedstock because, in the absence of any productive use, paper sludge goes into landfills — at a cost to its producer. This has led to several attempts to find viable means of converting such waste into fuel, and in late 2006, New York state contributed $14.8 million to fund the development of a demonstration facility in Rochester (New York) to use paper sludge as well as wood chips, switchgrass, and corn stover as feedstocks, the facility being operated by Mascoma (www. mascoma. com). The pulp and paper industry in Canada is estimated to produce at least 1.3 million tonnes of sludge every year, and at up to 70% cellulose, this raw material is economically competitive with cereal grain as a substrate for ethanol production.46 Hungarian researchers have also identified paper sludge and other industrial cellulosic wastes as being cost-effective routes to bioethanol.47

This illustrates the proposition that biomass ethanol facilities might be designed and constructed to adventitiously utilize a range of biomass substrates as and when they become available. As in the Californian example noted above, investigation of wastes from fresh and processed vegetables defined a sizeable resources of plant material (450,000 tonnes/year) in Spain; easily pretreated with dilute acid, such inputs could be merged with those for starch or lignocellulosic production lines at minimal (or even negative) cost.48

Substantial cost savings in cereal-based ethanol production can be achieved by a more integrated agronomic approach: although fermentor stillage could be used as a substitute for mineral fertilizer, total ethanol production was 45% lower if cereals were grown after a previous nitrogen-fixing legume crop; intensifying cereal yields certainly increased crops per unit land area, but ethanol production costs per liter dropped as the ethanol yield per unit land area outweighed the other costs; field trials also suggest that barley may (under German conditions) be economically favorable in comparison with wheat and rye).49 All these conclusions may be directly applicable to cereal straw as well as cereal grain. Moreover, since one of the strongest candidates for lignocellulosic ethanol production (wheat straw) also has one of the lowest ethanol yields per unit dry mass, the ability to flexibly mix substrates has capacity advantages if feedstock handling and processing regimes can be harmonized (figure 5.6).50

Once in the ethanol facility, biomass pretreatment and hydrolysis costs are important contributors to the total production cost burden; for example, with hard­woods and softwoods, enzymes represent 18-23% of the total ethanol production costs; combining enzyme recycling and doubling the enzyme treatment time might

improve the economic cost by 11%.51 Bench-scale experiments strongly indicated that production costs could be reduced if advanced engineering designs could be adopted, in particular improving the efficiency of biomass pretreatment with dilute acid in sequential cocurrent and countercurrent stages.52 Different pretreatment techniques (dilute acid, hot water, ammonia fiber explosion, ammonia recycle per­colation, and lime) are all capital-intensive, low-cost reactors being counterbalanced by higher costs associated with catalyst recovery or ethanol recovery; as a result, the five rival pretreatment options exhibited very similar production cost factors.53 Microbial pretreatments could greatly rescue the costs and energy inputs required by biomass hydrolysis techniques because enzymic digestibility is increased and hydrodynamic properties improved in stirred bioreactors; a full economic analysis remains to be undertaken in industrial ethanol facilities.54 Continued optimization of chemical pretreatments has resulted in a combined phosphoric acid and organic solvent option that has the great advantage of requiring a relatively low temperature (50°C) and only atmospheric pressure.55

Being able to generate higher ethanol concentrations in the fermentation step would improve overall economic performance by reducing the costs of ethanol recov­ery; one solution is to run fermentations at higher biomass substrate loadings, that is, accomplish biomass liquefaction and saccharification at high solids concentrations; wheat straw could be processed to a paste/liquid in a reactor system designed for high solids content, the material then being successfully fermented by Saccharomy- ces cerevisiae at up to 40% (w/v) dry matter in the biological step.56

What is the optimum processing of the various chemical streams (soluble sugars and oligosaccharides, pentoses and hexoses, cellulose, and lignin) resulting from the pretreatment and hydrolysis of lignocellulosic biomass? The utilization of pentose sugars for ethanol production is certainly beneficial for process economics, a conclu­sion reached as early as 1989 in a joint U. S.-New Zealand study of pine as a source of woody biomass, where ethanol production costs of $0.75/l ($2.83/gallon) were cal­culated, decreasing by 5% if the pentose stream was used for fermentable sugars.57 The stillage after distillation is also a source of carbohydrates as well as nutrients for yeast growth; replacement of up to 60% of the fresh water in the fermentation medium was found to be possible in a softwood process, with consequential reduc­tions in production costs of as much as 17%.58

Because larger production fermentors are part of the drive toward economies of scale savings in production costs, reformulating media with cheaper ingredients becomes more important. In the fermentation industry at large, devising media to minimize this operating cost parameter has had a long history; as recombinant eth — anologens are increasingly engineered (chapter 3), suitable media for large scales of production are mandatory. With E. coli KO11, for example, laboratory studies showed that expensive media could be substituted by a soya hydrolysate-containing medium, although the fermentation would proceed at a slightly slower rate; both this and a corn steep liquor-based medium could contribute as little as 60/gallon of ethanol produced from biomass hydrolysates.5960

Operating a membrane bioreactor (in a demonstration pilot plant of 7,000-l capacity) showed that the yearly capital costs could be reduced to $0.18-0.13/gallon, with total operating costs for the unit of $0.017-0.034/gallon.61 Another advanced engineering design included continuous removal of the ethanol product in a gas stream; compared with a conventional batch process, ethanol stripping gave a cost saving of $0.03/gallon with a more concentrated substrate being used, thus resulting in less water to remove downstream.62 Simply concentrating the fermented broth if the ethanol concentration is low in a conventional process is feasible if reverse osmo­sis is employed but not if the water is removed by evaporation with its high energy requirement.63

Incremental savings in bioethanol production costs are, therefore, entirely pos­sible as processes are evolved; many of the steps involved obviously require higher initial investment when compared with basic batch fermentation hardware, and it is unlikely that radical innovations will be introduced until a firm set of benchmark costings are achieved in semiindustrial and fully production-scale units. Efficient utilization and realization of the sales potential of coproducts remains, on the other hand, an immediate possibility. Coproduct credits have long been an essential fea­ture of estimates of ethanol production (section 5.2); among these, electricity genera­tion has been frequently regarded as readily engineered into both existing and new ethanol production facilities, especially with sugarcane as the feedstock for ethanol production. For example, in Brazil steam turbines powered by combustion of sugar­cane bagasse can generate 1 MWh/m3 (1,000 l) of ethanol, and economic analysis shows that this is viable if the selling price for electricity is more than $30/MWh, the sales price in Brazil in 2005.64 When electricity credits cannot be realized or where coproducts can be used as substrates for other chemical or biological processes, dif­ferent criteria come into play — this is discussed in chapter 8 (section 8.2). Pyrolysis of sugarcane wastes to produce “bio-oil” could yield 1.5 tons of saleable products per ton of raw sugar used, but the selling cost of the product will be crucial for establish­ing a viable coproduction process.65

The potential of biorefineries to generate a wider collection of chemical feedstocks presently derived from petroleum is best visualized with biomethanol as the starting point (figure 8.3). Methanol obtained from the gasification of biomass (chapter 6, section 6.3.1) can be transformed by well-known purely chemical reactions to form, among many other chemicals:8

• Formaldehyde (CH2O) is used in the production of resins, textiles, cosmet­ics, fungicides, and others; formaldehyde accounted for 35% of the total worldwide production of methanol in the mid-1990s and is prepared by the oxidation of methanol with atmospheric O2 using a variety of catalysts.

• Acetic acid (CH3COOH) is a major acid in the food industry and as feed­stock for manufacturing syntheses, in the production of some plastics, fibers, and others; acetic acid is manufactured by the carbonylation of methanol with CO.

• Formic acid (HCOOH) is a preservative.

• Methyl esters of organic and inorganic acids are used as solvents and meth — ylation reagents and in the production of explosives and insecticides.

• Methylamines are precursors for pharmaceuticals.

• Trimethylphosphine is used in the preparation of pharmaceuticals, vita­mins, fine chemicals, and fragrances.

• Sodium methoxide is an organic intermediate and catalyst.

• Methyl halides are solvents, organic intermediates, and propellants.

• Ethylene is used for plastics and as an organic intermediate.

3.5.1 Ethanol Recovery from Fermented Broths

The distillation of ethanol from fermented broths remains the dominant practice in ethanol recovery in large and small ethanol production facilities.250 Other physical techniques have been designated as having lower energy requirements than simple distillation, and some (vacuum dehydration [distillation], liquid extraction, super­critical fluid extraction) can yield anhydrous ethanol for fuel purposes from a dilute aqueous alcohol feed (figure 4.12).251 Only water removal by molecular sieving* has, however, been successful on an industrial scale, and all new ethanol plants are built with molecular sieve dehydrators in place.252

Nevertheless, the economic costs of dehydration are high, especially when anhydrous ethanol is to be the commercial product (figure 4.12). In the early 1980s, the energy requirements were so high that the practical basis for fuel ethanol pro­duction was questioned because the energy required for distillation approximated the total combustion energy of the alcohol product.253,254 The investment costs of rivals to distillation were, however, so high (up to 8.5 times that of conventional distillation) that little headway was made and attention was focused on improving

Synthetic zeolite resins are crystalline lattices with pore sizes of 0.3 nm, sufficiently small to allow the penetration of water molecules (0.28 nm in diameter) but exclude ethanol (molecular diameter 0.44 nm).

process efficiencies and energy cycling with the development of low-energy hydrous ethanol distillation plants with 50% lower steam-generating requirements.252

The economics of downstream processing are markedly affected by the concen­tration of ethanol in the fermented broth; for example, the steam required to produce an ethanol from a 10% v/v solution of ethanol is only 58% of that required for a more dilute (5% v/v) starting point, and pushing the ethanol concentration in the fermenta­tion to 15% v/v reduces the required steam to approximately half that required for low conversion broth feeds.253

Inevitably, an essential facet of the public discussions on costs and subsidies of biodiesel production has been that of its potential amelioration of greenhouse gas emissions. If significant, this would augment the case for production and consump­tion incentives to offset higher production costs than for conventional diesel. At the scientific level, this debate has mirrored that for bioethanol (chapter 1, section 1.6) and has proved equally contentious and acrimonious.

In the early 1990s, net energy balance (NEB) values of up to 3.8:1 were calculated for rapeseed-derived biodiesel, depending on how the coproducts and crop straw were assessed in the calculations (figure 6.6).4 Unpublished reports and communica­tions quoted in that report were from 1.3 to 2.1 without coproduct credits and from 2 to 3 if thermal credits for the meal and glycerol coproducts were included. Radi­cally different conclusions were reached in a 2005 publication: biodiesel production

2.0

from soybean oil required 27% more fossil energy than the biodiesel energy content, whereas sunflower oil was even less viable (requiring 118% more fossil energy than in the product).49

Midway (in time) between these conflicting estimates was a report from the National Renewable Energy Laboratory whose main conclusion was that biodiesel (from soybean oil) yielded 3.2 units of fuel product energy for every unit of fossil energy consumed in its life cycle, whereas conventional diesel yielded only 0.83 unit per unit of fossil fuel consumed, that is, that biodiesel was eminently “renew­able.”55 Direct comparison of these conflicting results shows that the disagreements are major both for the stages of soybean cultivation and biodiesel production (fig­ure 6.6). As so often in biofuel energy calculations, part of the discrepancy resides in how the energy content of coproducts is allocated and handled in the equations (chapter 1, section 1.6.1). As the authors of the 2005 study pointed out, if the energy credit of soybean meal is subtracted, then the excess energy required for biodiesel production falls to 2% of the biodiesel energy content.49 A paper posted on the Uni­versity of Idaho bioenergy site suggests other factors:56 [60]

• Even adding in energy requirements for oil transport and transesterification as well as biodiesel transport produces a favorable energy balance of 2.9:1.

The energy balance, in any of these scenarios, is highly dependent on viewing the process as a biorefinery producing coproducts as well as biodiesel. If the energetic (and economic) value of the soybean meal cannot be realized, then the balance will be negative — even using the soybean meal as a “green manure” spread on the soybean fields would only partially offset the major loss of replaced fossil energy in the total process. If biodiesel production from oil seed crops is viewed as an “opportunity” over their alternative uses as, for example, foodstuffs, then the NEB can be recalculated to be favorable.57 As discussed previously (chapter 1, section 1.6.1), this is a contentious argument, and much media comment in Europe dur­ing 2006-7 has pointed to increased areas of arable land being devoted to oilseed rape as demand for the crop as a source of biodiesel increases, thus fulfilling the prediction of a subsidized cash crop.4 A focus of future attention may be that of realizing an economic return on the greatly increased amounts of seed meal and of finding a viable use for glycerol — refining the glycerol coproduct to a chemically pure form is expensive, and alternative uses of glycerol for small — and medium — scale biodiesel facilities are being explored, for example, its use as an animal feed supplement.58

As the number of industrial units producing biodiesel increases, assessments of energy balances should be possible from collected data rather than from calculations and computer simulations. A report on activities in six Brazilian and Colombian biodiesel facilities using palm oil as the agricultural input attempted precisely this.59 NEBs were in the range of 6.7-10.3, with differences arising because of

• Different rates of fertilizer application

• Different uses of plant residues as fertilizers or as boiler fuel for electric­ity production

• On-site electricity generation at some sites, whereas others were entirely dependent on purchased electricity

• Differing efficiencies in the generation of coproducts and the recovery of unused palm oil

Taken together as a group, these palm oil biodiesel producers were assessed as being more energy efficient than reference manufacturers in Europe or the United States — the most recent (2006) detailed estimate of biodiesel from soybean oil in the United States arrived at a NEB of 1.93:1, but this was critically dependent on full credits being taken for soybean meal and glycerol coproducts (without them, the balance decreases to only 1.14:1).60

The energy balance is an important parameter that defines the extent of the biodiesel’s capacity to reduce greenhouse gas emissions, because, in the extreme case, if biodiesel requires more fossil energy in its production than can be usefully recovered in the product, no savings could possibly accrue.49 With a favorable energy balance for soybean biodiesel, its use could displace 41% of the greenhouse gas emissions relative to conventional diesel.60 As headline statements, the National

Renewable Energy Laboratory study on biodiesel use for public transport concluded the following:55

1. Substituting 100% biodiesel (B100) for petroleum diesel reduced the life cycle consumption of petroleum by 95%, whereas a 20% blend (B20) reduced consumption by 19%.

2. B100 reduced CO2 emissions by 74.5%, B20 by 15.7%.

3. B100 completely eliminated tailpipe emissions of sulfur oxides and reduced life cycle emissions of CO, sulfur oxides, and total particulate matter by 32%, 35%, and 8%, respectively.

4. Life cycle emissions of NO* and hydrocarbons were higher (13.4% and 35%, respectively) with B100, but there were small reductions in methane emissions.

Earlier assessments indicated that only 55% of the CO2 emitted from fossil diesel could be saved if biodiesel were to be used because of the CO2 emissions inher­ent in the production of biodiesel and that, other than a marked reduction in sulfur oxides, effects on CO, hydrocarbons, NO*, and polyaromatic hydrocarbons were inconsistent.4 As the use of biodiesel has widened globally, the number of publi­cations exploring individual pollutants or groups of greenhouse gas emissions has expanded, especially after 2000 (table 6.5).61-68 The report of increased mutagenicity in particulate emissions with a biodiesel is unusual as two earlier reports from the same research group in Germany found reduced mutagenicity with rapeseed oil — and soybean-derived biodiesels.66 6970 A high sulfur content of the fuel and high engine speeds (rated power) and loads were associated with an increase in mutagenicity of diesel exhaust particles. This is in accord with the desirability of biodiesels because of their very low sulfur contents, zero or barely detectable, as compared with up to 0.6% (by weight) in conventional diesels.4 There are suggestions that exhaust emis­sions from biodiesels are less likely to present any risk to human health relative to petroleum diesel emissions, but it has been recommended that the speculative nature of a reduction in health effects based on chemical composition of biodiesel exhaust needs to be followed up with thorough investigations in biological test systems.71