Cellulosic Energy Cropping Systems (Karlen/Cellulosic) || Introduction to Cellulosic Energy Crops

1

Introduction to CellulosicEnergy Crops

Mark Laser and Lee LyndThayer School of Engineering, Dartmouth College, U.S.A.

1.1 Cellulosic Biomass: Definition, Photosynthesis, and Composition

Plants, through photosynthesis, convert solar energy, carbon dioxide, and water into sugarsand other derived organic materials, referred to as biomass, and release oxygen as aby-product. Humans have long used plant biomass for a variety of applications, such as fuelfor warmth and cooking, lumber and other building materials, textiles, and papermaking.More recently, plant biomass has been considered as a feedstock for biofuels production –the focus of this book – with first-generation fuels being made from edible portions ofplants, including starch, sucrose, and seed oils. Next-generation biofuels will be producedfrom non-edible cell wall components (described below) that comprise the majority ofplant biomass.Photosynthesis consists of two stages: a series of light-dependent reactions that are

independent of temperature (light reactions) and a series of temperature-dependent reactionsthat are independent of light (dark reactions). The light reactions convert light energy intochemical energy in the form of adenosine triphosphate (ATP) and nicotinamide adeninedinucleotide phosphate (NADPH). The dark reactions, in turn, use the chemical energystored in ATP and NADPH to convert carbon dioxide and water into carbohydrate.About half of the light energy falls outside the photosynthetically active spectrum; some

of the available energy is reflected away and not captured. Further energy is lost during thelight absorption process, and during carbohydrate synthesis and respiration. As a result,photosynthesis typically converts less than 1% of the available solar energy into chemicalenergy stored in the chemical bonds of the structural components of biomass [1].

Cellulosic Energy Cropping Systems, First Edition. Edited by Douglas L. Karlen.© 2014 John Wiley & Sons, Ltd. Published 2014 by John Wiley & Sons, Ltd.

2 Cellulosic Energy Cropping Systems

Plants have evolved three photosynthetic pathways, each in response to distinct envi-ronmental conditions. One is called the C3 pathway because the initial product of carbonfixation is a three-carbon compound (phosphoglyceric acid, or PGA). When carbon dioxidelevels inside a leaf become low, especially on hot dry days, a plant is forced to close its stom-ata (microscopic pores on the surface of land plants) to prevent excess water loss. If the plantcontinues to fix carbon when its stomata are closed, carbon dioxide is depleted and oxygenaccumulates in the leaf. To alleviate this situation, the plant uses a process called photores-piration in which a molecule ordinarily used in carbon fixation (ribulose-1,5-bisphosphate,or RuBP) combines instead with oxygen, catalyzed by the enzyme RuBisCO, which alsofigures prominently in carbon fixation. This reduces photosynthetic efficiency in two ways:firstly, it creates competition between oxygen and carbon dioxide for the active sites ofRuBisCO – sites that take up oxygen are not available for carbon dioxide; secondly, theprocess re-releases carbon dioxide that had been fixed. Photorespiration reduces photosyn-thetic efficiency by 35–50%, depending upon environmental conditions, with warm, aridhabitats promoting greater photorespiration [1].In response, many plant species in warm, dry climates have evolved two alternative

photosynthetic pathways – the C4 pathway and crassulacean acid metabolism (CAM) pho-tosynthesis, both of which significantly reduce photorespiration and enhance efficiency.Both convert carbon dioxide into a four-carbon intermediate using the enzyme phospho-enolpyruvate (PEP) carboxylase –which does not react with oxygen – rather than RuBisCO.C4 plants fix carbon dioxide during the day; CAM plants, to keep stomata closed duringthe day, fix carbon dioxide at night [2].The highest reported solar energy conversion efficiency is about 2.4% for C3 plants and

3.7% for C4 species [3]. CAM plants are estimated to be 15%more efficient than C3 plants,but 10% less efficient than C4 plants [4]. Zhu et al. [3] estimate the theoretical maximumefficiency to be 4.6 and 6% for C3 and C4 crops, respectively. The C3 pathway is the oldest –originating around 2800 million years ago – and most widespread, both taxonomically andenvironmentally, accounting for about 95% of total plant species [5]. C4 photosynthesis isfound in about 1% of plant species [5] and is most prevalent in grasses, with about 50% ofthe species using the pathway [6]. CAM occurs in about 4% of total plant species [5].The energy crops considered in this volume all have either a C3 or C4 photosynthetic

pathway. They include:

• C3 pathway: wheat straw, eucalyptus, poplar, willow, pine• C4 pathway: miscanthus, switchgrass, sugarcane, energy cane, sorghum, corn stover.

Though not considered here, examples of potential energy crops having the CAM path-way include agave and opuntia. More detailed treatments of photosynthesis are availableelsewhere [2, 7].Each of the above plant species contains cellulosic biomass, that is, the fibrous, generally

inedible portions of plants, rich in the polysaccharide cellulose, which make up the majorityof all plant material. Cellulosic biomass can generally be grouped into four categories:herbaceous plants, woody plants, aquatic plants, and residual material such corn stover,sugarcane bagasse, paper sludge, and animal manure. Terrestrial cellulosic energy cropsand agricultural crop residues are the primary focus of this book.Cellulosic biomass contains varying amounts of cellulose, hemicellulose, lignin, pro-

tein, ash, and extractives. Cellulose, a structural component of the primary cell wall

Introduction to Cellulosic Energy Crops 3

in plants, generally comprises the largest fraction, with 40–50% on a dry weight basisbeing typical. The material is a polymer of glucose, a six-carbon sugar, joined by1–4 beta-linkages. Linear cellulose chains, which have an average molecular weight ofabout 100 000, are generally arrayed in parallel and held together with extensive hydro-gen bonding forming macromolecular fibers 3–6 nm in diameter called microfibrils. Thematerial is well ordered, largely crystalline, and highly recalcitrant to rapid reaction undermany conditions.Hemicellulose, another polysaccharide – one that binds tightly, but non-covalently, to

the surface of each cellulose microfibril – usually comprises 20–35% of the dry massof biomass. In contrast to cellulose, hemicellulose is composed of multiple sugars – theidentity and proportion of which depend on the type of plant – and has a heterogeneous,non-crystalline branched structure. As a result, hemicellulose is generally more reactivethan cellulose and is readily hydrolyzed by dilute acid or base as well as hemicellulaseenzymes. Xylose, a five-carbon sugar, is the dominant constituent of hemicellulose in plantsother than softwoods; for softwoods, mannose is often the most abundant sugar.Lignin is an amorphous polymer of phenyl–propane subunits (six-carbon rings linked to

three-carbon chains) joined together by ether and carbon–carbon linkages, and covalentlybound to hemicellulose. The subunits may have zero, one, or two methoxyl groups attachedto the rings, giving rise to three structures – denoted I, II, and III, respectively. Theproportions of each structure depend on the plant type. Structure I is commonly found ingrasses, structure II in softwoods, and structure III in hardwoods. Lignin both creates anet around carbohydrate-rich microfibrils in plant cell walls and penetrates the interstitialspace in the cell wall, driving out water and strengthening the wall. The dry mass fractionof lignin in plants typically ranges from 7–30%. Leafy herbaceous plants are generally atthe low end of this range, woody plants at the high end, with softwoods having more ligninthan hardwoods.Smaller amounts of protein and minerals are also present in plant tissues. As plants

mature, wall composition shifts from moderate levels of protein and almost no lignin tovery low concentrations of protein and substantial amounts of lignin. Protein content canbe significant (e.g. 10% dry mass) in early-season herbaceous crops, but is relatively lowin late-season harvests and minimal in most woody crops.Plants require a variety of inorganic minerals for proper growth, including both macronu-

trients (N, P, K, Ca, S, Mg) and micronutrients, or trace elements (B, Cl, Mn, Fe, Zn, Cu,Mo, Ni, Se, Na, Si). Plant roots, mediated by transport proteins, absorb mineral nutrientsas ions in soil water. Each mineral participates in distinct biological functions within theplant. Nitrogen, for example, is involved in all aspects of plant metabolism, with its fore-most function being to provide amino groups in amino acids, the building blocks of everyprotein. Potassium, meanwhile, is essential for activating a multitude of enzymes, includingpyruvate kinases involved in glycolysis, and is one of the most important contributors tocell turgidity in plants. Another vital macronutrient, calcium, is essential for providingstructure and rigidity to cell walls, and is used as a signaling compound in response tomechanical stimuli, pathogen attack, temperature shock, drought, and changes in nutrientstatus. When plant biomass is converted to fuels, chemicals, electricity, and/or heat, inor-ganic minerals remain as ash, with the amount residual ash being dependent upon plantspecies. Herbaceous plant species typically have higher levels of ash (e.g. 5–10% dry mass)than do woody species (<2% dry mass).


The term “extractives” is also commonly used when characterizing the compositionof plant biomass. Extractives are materials in the biomass that can be dissolved in asolvent (typically water and/or ethanol), including resins, fats and fatty acids, phenolics,phytosterols, salts, minerals, soluble sugars, and other compounds.More detailed consideration of the composition of cellulosic biomass can be found

elsewhere [8,9]. Representative compositions for many of the biomass crops considered insubsequent chapters are listed in Table 1.1.

1.2 Cellulosic Biomass Properties and Their Relevanceto Downstream Processing

The choice of biomass feedstock is a critical driver in determining key performance metricsof bioenergy – including economic viability, scale of production (both at individual facilitiesand in aggregate), and environmental impact. For commodities such as fuels or electricity,feedstock cost typically represents two-thirds of the product cost, or more [26]; therefore,selecting a cost-effective feedstock is essential. As is discussed in Part IV of this book,the logistics of growing, harvesting, storing, and transporting biomass – unique for a givenfeedstock type – affects the feasible size of the processing facility, which, in turn, impactsthe overall sector scale. Each feedstock also has a particular set of environmental attributes –for example, water use, wildlife habitat, soil quality, and so on – that significantly affectsthe environmental performance of the bioenergy system.In assessing the suitability of a biomass feedstock for a given conversion process,

several material properties are important to consider, including: (1) moisture content;(2) energy density; (3) fixed carbon/volatile matter ratio; (4) ash content; (5) alkali metalcontent; and (6) carbohydrate/lignin ratio. The first five properties are especially importantin thermochemical processing. For biological conversion, the first and last properties are ofprimary concern.

1.2.1 Moisture Content

Biomass moisture content is defined as the amount of water in the biomass expressed asa percentage of the material’s weight; reporting on a wet basis is most common. Moisturecontent at harvest for woody feedstocks is usually 40–60% (wet basis); for herbaceouscrops, it typically ranges from 10 to 70% (wet basis) depending upon the species, climate,geographic location, and stage of maturation. Biomass net energy density per unit massdecreases with increasing moisture content. Transport efficiency of biomass feedstock,therefore, decreases as moisture content increases. Storage of high-moisture biomass isalso less efficient, both because of reduced energy density and increased probability ofbiological degradation, fire risk, and mold formation. Moisture content also affects down-stream processing, especially for thermochemical conversion. High-moisture feedstocksmust be dried to levels of less than 50% for conventional combustion and less than 20%for gasification and pyrolysis. In biological processing for which some form of thermalpretreatment is used, moisture content can also significantly affect the energy efficiency ofthe process.

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1.2.2 Energy Density

Energy density, often termed “heating value”, refers to the amount of energy released perunit fuel combusted, usually measured in terms of energy content per unit mass for solids(e.g. MJ/kg) and per unit volume for liquids (e.g. MJ/l). Energy density can be expressed intwo forms, higher heating value (HHV) or lower heating value (LHV). HHV represents thetotal energy released when the fuel is combusted in air, including the latent heat containedin the resulting water vapor product – the maximum potentially recoverable energy froma given feedstock. The latent heat contained in the water vapor, however, typically cannotbe used effectively. LHV, therefore, is the appropriate value to use when quantifying theenergy available for subsequent use. As noted above, moisture content significantly affectsbiomass feedstock energy density. Freshly cut wood, for example, might have as much as60% moisture and a relatively low energy content (e.g., 6 MJ/kg). In contrast, oven-driedwood with little moisture might have up to 18 MJ/kg. Representative LHV values for manyof the biomass crops considered in subsequent chapters are listed in Table 1.1.

1.2.3 Fixed Carbon/Volatile Matter Ratio

Fuel analysis that quantifies the amount of chemical energy stored as volatile matter (VM)and fixed carbon (FC) has been developed for solid fuels such as coal. The VM of a solidfuel is the portion released as gas (including moisture) by heating to 950◦C in the absenceof air for seven minutes; the FC is the mass remaining after the volatiles have been drivenoff, excluding the ash and moisture contents. Fuel analysis based upon VM content, ash,and moisture, with the FC determined by difference, is termed the proximate analysis ofa fuel. Elemental analysis of a fuel, presented as C, N, H, O and S, together with theash content, is termed the ultimate analysis of a fuel. The ratio of FC to VM provides anindication of the ease with which the solid fuel can be ignited and subsequently gasified,or oxidized, depending on how the fuel is to be converted. Representative proximate andultimate analyses for many of the biomass crops considered in subsequent chapters arelisted in Table 1.1.

1.2.4 Ash Content

Conversion of biomass feedstock, either thermochemically or biochemically, results in asolid residue. In themochemical processing via combustion in air, the residue consists solelyof ash. For biochemical processing, it contains both ash and other unconverted material,especially lignin. The bioprocess residue can be further processed thermochemically toyield ash as the final solid residue. The ash content negatively affects the energy density ofthe feedstock. Ash can also pose operational problems in thermochemical processing, suchas slagging in which the ash melts and fuses together. Relatively low-cost control measures,such as leaching the raw feedstock with water and using different mineral additives (e.g.kaolinite, clinochlore, ankerite), can be used to reduce negative effects [27]. Potential enduses of ash include mineral agricultural fertilizer [28] and construction material additive[29]. Representative ash content values for many of the biomass crops considered insubsequent chapters are listed in Table 1.1. As can be seen from the table, herbaceousfeedstocks tend to have higher ash contents (e.g.≥5%) than woody feedstocks (e.g.<2%).


1.2.5 Alkali Metal Content

During thermochemical conversion, alkali metals (Na, K, Mg, P, Ca) present in the ashreact with silica – originating both from the biomass itself and from soil introduced dur-ing harvesting – to produce a sticky, mobile liquid phase that can contribute to slag-ging, deposition, and corrosion of process equipment. As noted above, water leaching andfuel additives can be used to reduce the damaging effects of ash components, includingalkali metals.

1.2.6 Carbohydrate/Lignin Ratio

In biological processing, carbohydrate present in cellulose (and potentially hemicellulose)is converted to fuels and/or chemicals, while the lignin fraction remains unaffected. Fur-thermore, the recalcitrance of cellulosic biomass to bioconversion typically increases withincreasing lignin content, requiring more severe pretreatment, which decreases processefficiency. Bioconversion processes, therefore, favor feedstocks with high carbohydrate tolignin ratios. Representative cellulose, hemicellulose, and lignin values for many of thebiomass crops considered in subsequent chapters are listed in Table 1.1.

1.3 Desirable Traits and Potential Supply of Cellulosic Energy Crops

Given the world’s finite land resource, the most important trait for cellulosic energy cropsis productivity – the annual dry matter produced per unit land area. As listed in Table 1.1,productivity of the crops considered in this book ranges from 0.1 to 1.75 Mg/ha/yr (drybasis) for wheat straw, to as high as 44 Mg/ha/yr (dry basis) for miscanthus. The bestenergy crops will also have few inputs and low production costs. Easily established, robustperennial crops having long life spans (e.g.≥10 years) are favored over annual crops, as arethose having low fertilizer, pesticide, and insecticide requirements. Native, non-invasivespecies that provide good habitats for wildlife are preferred.Feedstocks used in thermochemical processing should be harvested when moisture con-

tent is relatively low to minimize preliminary energy intensive drying. Low moisture is notas critical in bioconversion feedstocks, for which wet storage can sometimes be a viableoption. Ideally, ash content should be low (e.g. <1%), ash melting temperatures should behigh (e.g. >1500◦C), with low levels of particularly damaging elements, including alkalimetals, alkaline earth metals, silicon, chlorine, and sulfur.Conventional plant breeding – which involves manipulating the genes of a species via

selection and hybridization so that desired genes are packaged together in the same plantand as many detrimental genes as possible are excluded – has traditionally been usedto enhance desired agronomic traits such as productivity, water use efficiency, and croplifespan. Breeding systems have been developed, and continue to be developed, that canbe used to improve virtually all plant species. The productivity of corn, for example,has more than quadrupled since the 1930s largely through conventional breeding [30].Biomass productivity can potentially be increased even further using more sophisticatedbiotechnology techniques. Recent molecular and genetic studies have identified a numberof regulators of plant biomass production – for example, vegetative meristem activities,


cell elongation, photosynthetic efficiency, and secondary wall biosynthesis – that might bemanipulated to enhance energy crop yields [31].The potential to produce viable energy feedstocks is vast. A detailed study led by the Oak

RidgeNational Laboratory estimates that theUnited States could produce 602–1009milliondry tons annually by 2022, and 767–1305 million annual dry tons by 2030, at a price of$60 per dry ton [32]. (The low value in the range assumes a 1% annual increase in yield; thehigh value, a 4% annual increase.) This excludes resources that are currently being used,such as corn grain and forest products industry residues. When currently used resourcesare included, the total biomass estimate jumps to over one billion dry tons per year for thelower productivity case – enough to displace about half of the country’s current gasolineconsumption (134 billion gallons/year) if converted to ethanol at a yield of 100 gallons/dryton. Estimates for the global annual supply of biomass feedstocks range from 100 to400 EJ/year – equivalent to 6 to 24 billion dry tons. If converted to ethanol, this represents120–460% of current global gasoline consumption (338 billion gallons/year).

1.4 The Case for Cellulosic Energy Crops

With ever-increasing indications that resource use is exceeding the planet’s biocapacity[33] – largely driven by non-renewable fossil fuel consumption – it is clear that humankindmust shift to sustainable practices in order for a peaceful, equitable, and thriving futureto be possible. Furthermore, given mounting evidence of climate change – to the pointthat some say we are now living in a new geologic epoch, the Anthropocene [34] – thistransformation must begin now and be completed within decades, not centuries. Indeed,it is fair to characterize this transition, moving from finite resource capital to renewableresource income, as the defining challenge of our time.Most sustainable paths from primary resources to human needs pass through either

plant biomass or renewable electricity, with biomass being the only foreseeable source oforganic fuels, chemicals, and materials, as well as food. In comparison, other large-scalesustainable energy sources are most readily converted to electricity and heat. Becauseliquid organic fuels have a greater energy density than batteries, both today and withanticipated improvements in battery technology, it is reasonable to expect that organic fuelswill meet a significant fraction of transportation energy demand for the indefinite future.This is particularly true for long-distance travel via personal vehicles and for heavy-dutyapplications, such as aviation and long-haul trucking, which account for more than halfof global transportation energy [35]. Biofuels would, therefore, appear to be an essentialcomponent of tomorrow’s sustainable world rather than a discretionary option.Cellulosic biomass energy potentially offers many environmental benefits that contribute

to its sustainability, some of which are:

• Fossil fuel displacement.• Lower emissions of greenhouse gases and other air pollutants.• Enhanced soil quality.• Reduced soil erosion.• Reduced nutrient run-off.• Enhanced biodiversity.


Demirbas [36], Rowe et al. [37], Arjum [38], and Skinner et al. [39] provide moredetailed reviews and discussion of these and other potential benefits.In addition to the environment, cellulosic biomass energy also has the potential to enhance

energy security and rural economic development. Nations dependent upon petroleum faceincreasing security costs to ensure the steady supply of oil. The United States, for example,according to the RAND Corporation [40], spends about $75.5–93 billion per year – repre-senting between 12 and 15% of its current defense budget – to secure the supply and transitof oil. Furthermore, major oil supplying countries hold leverage over nations relying uponimports, as the oil producers control price stability. This directly affects foreign policy,forcing import nations to prioritize stability over values such as democracy, transparency,and human rights. Even if a country could produce 100% of the oil it uses, its consumerswould still be vulnerable to global price fluctuations based on supply disruptions in unstableregions. Beyond consumerism, modern militaries invest for the long term – new airplanes,ships, and vehicles are expected to last decades. This requires alternative energy sources tobe able to accommodate infrastructure that is likely to be in place for years.In recognition of this, the United States Department of Defense has developed an alter-

native fuels policy to “ensure operational military readiness, improve battle space effec-tiveness,” and increase “the ability to use multiple, reliable fuel sources [41].” Consistentwith this, the US Navy has plans to deploy a “Great Green Fleet” strike group of ships andaircraft running entirely on alternative fuel blends – including cellulosic fuels – by 2016[42]. It also has a goal of meeting 50% of its total energy consumption from alternativesources by 2020. To help enable these goals, the Navy – together with the Departments ofEnergy and Agriculture – signed a Memorandum of Understanding (MOU) to “assist thedevelopment and support of a sustainable commercial biofuels industry [43].” The MOUcalls for $510 million in funding over three years to develop advanced biofuels that meetmilitary specifications, are price competitivelywith petroleum, are at geographically diverselocations with ready market access, and have no significant impact on the food supply.A cellulosic biofuels industry, by generating demand for agricultural products, has the

potential to significantly increase employment in rural areas in sectors ranging from farmingto feedstock transportation to plant construction and operation. Workers would be requiredin a variety of occupations, including: scientists and engineers conducting research anddevelopment; construction workers building plants and maintaining infrastructure; agricul-tural workers growing and harvesting energy crops; plant workers processing feedstocksinto fuel; and sales workers selling the biofuels. Brazil’s sugar/ethanol industry directlyemploys about 489 000 workers, with an additional 511 000 workers engaged in supportingagricultural activities [44]; the United States corn ethanol industry directly employs about400 000 [45]. A study forecasting the impact of advanced biofuels on the US economyestimates that the industry could create over 800 000 jobs by 2022 [46].Cellulosic biofuels also have the potential to promote rural economic activity within

developing nations and improve the lives of the world’s poor. Farmers would have increaseddemand for their products, including crop residues from existing crops, and employ addi-tional workers to produce the energy feedstocks. They would also be able to make useof degraded or marginal land not suitable for food production. Care must be taken, how-ever, to include small landholders in the sector’s development and to adequately invest inlocal workforce training for feedstock production, production facilities construction, andprocess operation. In addition, to the extent possible, the sector should be developed around


existing industries, such as sugarcane processing, to lower investment barriers [47]. Also,selection of feedstock supply chains that do not compromise food security is critical. Signif-icant potential exists to actually enhance food security through bioenergy production – byusing inedible crops grown onmarginal land, for example, or integrating production of food,animal feed, and bioenergy. One can envision many benefits that might be realized: employ-ment and development of marketable skills; introduction of agricultural infrastructure andknowledge; energy democratization, self-sufficiency, and availability for agricultural pro-cessing; and an economically rewarding way to restore degraded land. Bioenergy couldpotentially improve both food security and economic security for the rural poor [48].Such benefits, however, are by no means guaranteed. The environmental impact of

biomass energy very much depends upon how the given system is designed and imple-mented. Detractors of bioenergy have called into question its sustainability, citing a numberof concerns, including:

• Food versus fuel.• Land use change (direct and indirect).• Water use.• Invasive species.• Biodiversity.

This productive debate has prompted an expanding literature analyzing and discussingthe keys to “getting biofuels right,” so that the promise of sustainable bioenergy can berealized [49–51]. To minimize both competition with food production and land use changeeffects, multiple classes of feedstocks are available, including energy crops grown onabandoned agricultural lands; food crop residues such as corn stover and wheat straw;sustainably harvested forest residues; double crops grown between the summer growingseasons of conventional row crops; mixed cropping systems in which food and energy cropsare grown simultaneously; municipal and industrial wastes; and harvesting invasive speciesfor bioenergy [49, 50, 52–54]. Water use can be minimized by selecting crops having lowirrigation requirements, by using non-potable sources such as wastewater or high-salinewater for any necessary irrigation [55,56], and using subsurface drip irrigation to minimizeevaporative losses [57]. The potential for non-native energy crops becoming invasive can belimited by proper preliminary risk assessment, including test plots [58], regular monitoringand stewardship programs [59], and by using sterile plant varieties [60]. The impact of agiven energy crop upon biodiversity depends strongly on specific regional circumstances,the type of land and land use shifts involved, and the associated management practices[61]. Herbaceous perennial crops, in particular, appear to be capable of providing suitablehabitats for a variety of species, especially with careful attention to crop placement andwhen mixed cultures are used [62–65]. By incorporating many of the above strategies, Daleet al. [51] calculated that, using the 114 million hectares of cropland currently allocated inthe United States for animal feed, corn ethanol, and exports, 400 billion liters of cellulosicethanol (80% of current gasoline demand) could be made – all while producing the sameamount of food. In summarizing their findings, the authors write:

Our analysis shows that the U.S. can produce very large amounts of biofuels, maintain domesticfood supplies, continue our contribution to international food supplies, increase soil fertility,and significantly reduce GHGs. If so, then integrating biofuel production with animal feed


production may also be a pathway available to many other countries. Resolving the apparent“food versus fuel” conflict seems to be more a matter of making the right choices ratherthan hard resource and technical constraints. If we so choose, we can quite readily adapt ouragricultural system to produce food, animal feed, and sustainable biofuels.

Any human activity involving new technology can potentially be harmful if not thoughtfullyplanned and appropriately conducted. The early-generation Altamont Pass wind farm inCalifornia, for example, unwittingly located on a major bird migratory route, results inthousands of bird deaths every year [66]. To remedy the problem, the farm’s owners areinstalling new, less destructive turbines and shutting down a significant fraction of theturbines during the migration season [67]. In the case of cellulosic biomass, if care istaken to address the key concerns noted above, the resource could very likely contributesubstantially – indeed, uniquely and essentially, by accommodating energy services noteasily met by other means – towards achieving a sustainable global energy future. Klineet al. [50] succinctly capture the promise of this vision:

When biofuel crops are grown in appropriate places and under sustainable conditions, theyoffer a host of benefits: reduced fossil fuel use; diversified fuel supplies; increased employment;decreased greenhouse gas emissions; enhanced habitat for wildlife; improved soil and waterquality; and more stable global land use, thereby reducing pressure to clear new land.

This book – through detailed consideration of cellulosic energy crop production; the logis-tics of feedstock harvest, storage, and transport; and commercial deployment that is mindfulof economic, environmental, and social concerns – seeks to disseminate knowledge thatcan help make large-scale, sustainable bioenergy a reality.

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