E

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Global Agriculture: Industrial Feedstocks for Energy and MaterialsBM Jenkins, University of California, Davis, CA, USA

r 2014 Elsevier Inc. All rights reserved.

GlossaryAnaerobic digestion The biochemical conversion orfermentation of organic materials, such as biomass, bymixed bacterial communities under anaerobic (without freeoxygen) conditions. The process is typically used to producea methane-containing biogas. Anaerobic digestion alsooccurs naturally in solid waste landfills.Biochar Charcoal or black carbon produced by thepyrolysis or other thermochemical processing of biomass.Biochemical conversion Conversion by biological andchemical processing, typically through microbial systems butalso involving the use of enzymes or chemical pretreatment.Biofuel A fuel product made from a biomass feedstock.Biomass Living material. In the context of energy andmaterials, biomass is nonfossil material of biogenic origin.Biomaterial A material synthesized or produced frombiological resources or biomass.Biorefinery A processing facility, similar in function to apetroleum refinery, used to refine or upgrade biomassfeedstocks to higher value fuels, materials, and other products.

cyclopedia of Agriculture and Food Systems, Volume 3 doi:10.1016/B978-0-444

Gasification The thermochemical conversion of a biomassfeedstock or other organic material by heating and reactionat elevated temperatures through partial oxidation using acontrolled amount of oxygen, steam, or other oxidant.Gasification is used principally to produce fuel or synthesisgases containing carbon monoxide and hydrogen alongwith other species.Lifecycle analysis An accounting technique used forevaluating and comparing the environmental and otherimpacts of a product, process, or system from the start toend of its useful life.Pyrolysis The thermochemical heating of a biomassfeedstock or other organic material in the absence of freeoxygen. The process typically produces liquid, gaseous, andsolid (char) products.Thermochemical conversion Conversion by thermal andchemical processing such as by combustion, gasification, orpyrolysis.

Introduction

Technological innovations and the complex composition ofbiomass offer immense opportunities for new materials andproducts from agriculture. Many of these opportunities alsocome with immense challenges. Biomass can be used to pro-duce a wide variety of biofuels to replace petroleum and naturalgas, for example, but large-scale production is at present avigorously contested global issue with many uncertainties as tonet environmental, social, and economic benefits despite thepromise of increased renewable energy supplies and enhancednational and global energy security. Less heavily debated is theproduction of new pharmaceuticals, nutritional products, spe-cialty chemicals, and biomaterials, but these also are subject toconcerns over the lifecycle implications of feedstock productionand product manufacturing. Together with new tools andtechniques for crop improvement, genetic modification, andbiomass conversion, agriculture has the capacity to add sig-nificant new economic and development value. Agriculture, as aprimary land use sector, also has the option to transform intonew ways that have little to do with food or biomass pro-duction, including land conversion to support wind and solarenergy deployment that may be only partially compatible withmore conventional agricultural production but which may offergreater profit to the land owner. With the overall sustainabilityof the current agricultural system still an open question, theexpansion, intensification, or redirection of agriculture towardnew markets will require careful analysis and management.

Although agriculture has long provided many products inaddition to food, it has the productive capacity to supply muchlarger quantities of industrial feedstock for other markets, in

many cases in direct competition with food production. Euro-pean, US, and other national or regional policies to encouragebiofuel production have been heavily criticized for this reasonand for their potential to increase greenhouse gas emissionsrather than reducing them as intended. Increased demand forbiomass, whether food crop or otherwise but especially forfood crops like corn (maize) and soybeans for industrialproducts, is also criticized for increasing food and feed priceswith particular impact on the world’s poor. This food versusfuel debate is characterized by large uncertainties that lead todifficulties in the design of effective policies for the broader useof products from agriculture. As the development of agriculturehas had significant global impact due to land conversion,transitions in agriculture to take advantage of new technologiesand new markets for energy, chemicals, materials, and othernontraditional products require an improved understanding ofglobal consequences and a larger systems perspective beyondwhat has been applied to date. The success of agricultural andindustrial research in producing new conversion methods andnew products from biomass has created enormous economicand environmental potential, as well as a greater imperative forbetter information and strategies in managing land resourcesand the global agricultural enterprise.

Resources and Feedstocks

Photosynthetic Pathways, Efficiencies, and Global BiomassProduction

Biomass is living material, and in the context of energy andmaterials from agriculture and other sources, biomass is

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462 Global Agriculture: Industrial Feedstocks for Energy and Materials

interpreted to mean nonfossil material of biogenic origin.Biomass includes purpose-grown organisms and crops as in-dustrial feedstocks, crop, and processing residues from agri-cultural, industrial, and commercial operations, and biogenicfractions of municipal solid wastes and wastewaters amongother sources. Agriculture is increasing its production of pur-pose-grown crops for energy and materials including trees andshrubs, grasses and other herbaceous materials, algae, andother aquatic and terrestrial species. Primary agricultural resi-dues include cereal straws and stovers, animal manures,orchard and vineyard prunings, forest slash from timber op-erations, forest stand improvement thinnings (such as smalltrees and brush removed to reduce wildfire intensity), andgreen waste from yard- and landscape maintenance. Secondaryresidues arise from food processing, lumber production, andother industrial operations. Black-liquor is a lignin-containingsecondary residue from pulp and paper production, but ismost commonly burned in recovery boilers at the mill in orderto regenerate pulping chemicals and generate steam andpower. Tertiary residues are associated with end-of-use ma-terials such as wastepaper, food scraps and other biogenicfractions of municipal solid wastes, biosolids from waste watertreatment, and waste fats, oils, and greases (FOG) althoughadvances in reuse, recycling, and product recovery are ex-panding the perception of these as resources rather than wastesand altering their overall economic value and utility in society.Large amounts of biomass reside in waste landfills that arenow being considered for materials mining to reclaim energyand product value. Distinct in arising from different economicobjectives, residues and purpose-grown crops nonethelessshare many similarities in composition, use, and lifecycleimpacts.

The total global resource potential in biomass from agri-culture and other activities has been variously estimated. Theprincipal photosynthetic pathway produces carbohydratebiomass from carbon dioxide and water (Calvin, 1976). Theprocess is endothermic and requires energy in the form of light(photons, ν) as well as water as a source of electrons for theoverall reduction of CO2 to carbohydrate:

nCO2 þ nH2Oþ ν¼ ðCH2OÞn þ nO2 ½1�

where n indicates the mean polymer chain length of thecarbohydrate. The photosynthetic efficiency dictates the num-ber of photons needed per mole of carbohydrate. Anoxygenicphotosynthesis also occurs that does not result in oxygen as aproduct and does not use water as an electron donor (seereaction [2] below) (Sato-Takabe et al., 2012; Buchanan, 1992;Cohen et al., 1975).

An estimated 0.02% of the 175 PW (PW¼Petawatt¼1015 W) of incoming solar radiation to the Earth (Hubbert,1971) is used to produce approximately 70� 1012 kg (70 Gt(Gt¼Gigaton¼109 metric tons¼1012 kg)) of biomass eachyear through oxygenic photosynthesis (based on an averagedry matter heating value of approximately 16 MJ kg�1) (Jen-kins et al., 1998). More detailed analyses of biomass pro-duction by type of ecosystem yield global estimates in therange of 170–220 Gt year�1 (Klass, 1998; Hall et al., 1993).Total plant biomass currently accumulated in all global eco-systems is estimated at approximately 1015 kg (1000 Gt) dry

matter, or approximately 10 times annual production (Hallet al., 1993; Salisbury and Ross, 1992).

Biomass is also produced through processes other thanoxygenic photosynthesis. Hyperthermophilic bacteria, such asthose associated with hydrothermal vents at the deep oceanfloor, use chemosynthetic pathways for energy and biomassproduction and are utilized in symbiotic relationships by otherorganisms (e.g., tube worms) for survival in these extremeenvironments (Kato et al., 2010). Overall, sulfide chemo-synthesis produces elemental sulfur and water in convertingcarbon dioxide and hydrogen sulfide:

nCO2 þ 2nH2S¼ ðCH2OÞn þ nH2Oþ 2nS ½2�

Oxygen gas is not released by this mechanism. Sulfur metab-olizing bacteria that utilize chemosynthetic pathways are nowbeing investigated for their use in hydrogen sulfide removalfrom biogas produced during anaerobic digestion of manureand other feedstocks (Ho et al., 2013; Camarillo et al., 2013).Sulfur removal is required for the successful application ofmost postcombustion catalysts for NOx emission reductionsfrom engines burning biogas for power generation (Camarilloet al., 2013; Liu and Gao, 2011).

Biomass provides approximately 15% of world energyneeds, but in developing countries constitutes a much higherfraction of energy supply: 35% overall, and in excess of 80% inmany rural areas (Hall et al., 1993; Bain et al., 1998). Globally,agriculture produces approximately 5 Gt year�1 of crop resi-dues of which perhaps less than half may be sustainablyavailable for energy and industrial purposes although largerfractions may be used in developing countries (Jenkins, 1995).Resource-focused studies of global bioenergy potentials rangewidely, differing by at least an order of magnitude dependingon the assumptions made regarding land availability andcrop yields (Berndes et al., 2003). Considerations of lifecycleenvironmental effects, particularly greenhouse gas emissions,also influence global potentials (Searchinger et al., 2008;Fargione et al., 2008).

Plants utilize three principal pathways in assimilating at-mospheric carbon dioxide and synthesizing carbohydratestructures and other compounds through photosynthesis. Thebiological or dry matter yield is dependent in part on thepathway used. Light energy is absorbed in two pigment sys-tems called photosystem I (PS-I) and photosystem II (PS-II).In both the systems, absorption of light by chlorophyll andaccessory pigments leads to the emission and transport ofelectrons against an adverse voltage gradient. As noted above(refer to reaction [1]), in oxygenic photosynthesis the electronsare derived from the photolysis of water mediated by a man-ganese-containing enzyme in PS-II (Salisbury and Ross, 1992;Marschner, 1986). Electrons are transferred from PS-II throughwhat is called the Z-scheme to PS-I, storing energy in the car-riers adenosine triphosphate (ATP) and the reduced form ofnicotinamide adenine dinucleotide phosphate (NADPH) forlater use in CO2 reduction and carbohydrate synthesis. Mineralnutrients are directly involved in electron transport, and inother processes of the plant. The mineral or ash concentrationand composition are often quite important in the subsequentuse of the biomass, and can influence the design of the pro-duction and utilization system.

Global Agriculture: Industrial Feedstocks for Energy and Materials 463

The light reactions store energy in NADPH and ATP. In theso-called C3 plants, CO2 and water react with ribulose-1,5-diphosphate to produce 3-phosphoglyceric acid as part of theCalvin–Benson cycle. The glyceric acid is subsequently con-verted using NADPH and ATP into 3-phosphoglyceraldehydeand then into hexose phosphate and ribulose-5-phosphate.The latter reacts with ATP to regenerate ribulose-1,5-di-phosphate, whereas hexose is used in the synthesis of theprimary storage products, sucrose and starch. The C3 pathwaytakes its name from the 3-carbon intermediates producedduring the cycle. C3 plants include the cereals barley, oats,rice, and wheat, alfalfa (lucerne), cotton, Eucalyptus, sun-flower, soybeans, sugar beets, potatoes, tobacco, Chlorella, andothers. Gymnosperms (with a few possible exceptions),bryophytes, and algae are C3 organisms, as are most trees andshrubs.

Along the C4 pathway, CO2 combines with phosphoe-nolpyruvate (PEP) via a PEP-carboxylase catalyzed reaction toform oxaloacetate, which is reduced to malic-acid (malate) oraspartic-acid (aspartate), 4-carbon intermediates giving thepathway its name. These are translocated from the mesophyllcells where the primary CO2 fixation occurs to the bundlesheath cells from which CO2 is released for subsequent fix-ation through reactions of the Calvin–Benson cycle as in C3plants. Decarboxylation of the acids regenerates PEP. C4 plantsare usually of tropical origin and have higher photosynthesisrates and higher biological yields (dry matter production)compared with C3 plants. C4 plants include sugarcane, sor-ghum, maize, and Bermuda grass. Euphorbia species consideredfor direct hydrocarbon production are mostly C3, but a fewhave evolved to use the C4 pathway.

The third primary pathway is that of crassulacean acidmetabolism (CAM) used by many succulents. The CAMpathway also fixes CO2 via PEP carboxylase, but in the CAMspecies the stomata are open at night rather than during theday in order to reduce water loss. Malate is stored in thevacuoles during the night and then released during the daywhen the stomata are closed. CAM plants have lower growthrates than the C4 species but have high water use efficiency dueto their adaptation to low-water environments, includingsemiarid and saltmarsh regions, and epiphytic sites, such asthose used by orchids.

For photosynthetic organisms, biological yield (total bio-mass) is a function of the net production by photosynthesisand consumption by respiration, the latter including photo-respiration in C3 plants. Respiration provides energy throughthe oxidation of organic compounds in generating substratesfor the synthesis of other plant products in essentially the re-verse of reaction [1] above. Maximum theoretical photo-synthetic efficiencies can be derived based on a minimumrequirement of eight photons of photosynthetically active ra-diation (PAR, light of 400–700 nm wavelength) per moleculeof CO2 used to produce glucose, the minimum ν in reaction[1] (Klass, 1998; Hall et al., 1993; Loomis and Williams,1963). Approximately 43% of the energy in sunlight at theground level is contained within the PAR band, and of this amaximum of approximately 80% is actively absorbed duringphotosynthesis. Only approximately 28% of this energy isstored in glucose. In C4 plants respiration consumes some-where between 25% and 40% of the energy in glucose. The

maximum net efficiency of photosynthesis based on incidentsunlight is therefore 6–7%. Photorespiration in C3 plantsgenerally leads to efficiencies of approximately 3%, lower thanthe C4 plants. Photosynthetic efficiencies can be translated tobiomass yields using site-specific insolation data and the en-ergy content of the biomass (heating value). At maximumefficiency, theoretical yields can exceed 400 metric tons of drymatter per hectare per year. Agricultural yields are generally farbelow this due to nonoptimal crop conditions, limited inputs,and losses to diseases and pests. In practice, yields are alsolower because not all biomass is or can be harvested. Geneticmodifications and other crop improvements are widely in-vestigated to increase productivity and yields.

Efficiencies for agricultural crops typically are of the orderof 1%, although tropical crops, such as sugarcane and high-yielding grasses, can produce at 2–3% efficiency with drymatter yields of 50–100 Mg ha�1 year�1. Intensive productionof green algae can approach 5% efficiency, similar to the bestefficiencies with C4 crops under research conditions. A maize(corn) crop grown in Davis, California achieved 5.6% photo-synthetic efficiency under optimized conditions where onlylight was limiting (Loomis and Williams, 1963). Seasonal ef-ficiency for many C3 crops when given sufficient water andnutrients is closer to 2% (Monteith, 1977). Natural forest ef-ficiencies trend lower. The production of purpose-grown in-dustrial and energy crops seeks to produce biomass atrelatively high photosynthetic efficiency. Owing to the costs ofinputs, an optimal production system based on maximumprofit may not operate at maximum yield; however, yields arenevertheless of high relative importance to the overall eco-nomic feasibility.

Biomass can be used for remediation of environmentalcontamination (phytoremediation), and the use of biomass asfuel in the substitution of fossil resources can help mitigategreenhouse gas and global climate change impacts, but itsproduction and use requires careful lifecycle assessment toensure net environmental benefits. As noted earlier in theSection Introduction, many policies and incentives for theproduction of biofuels are highly controversial.

Biomass production has significant utility in serving tostore solar energy. Hybrid renewable energy systems usingbiomass can take advantage of this attribute in helping tostabilize electricity grids supplied with high levels of inter-mittent solar and wind power generation.

Biomass is a distributed resource, and its use to supplylarge quantities of energy and materials requires relatively largeamounts of land. The overall conversion efficiency from solarenergy to final energy product is low due to the inherently lowefficiency of photosynthesis. The overall electrical efficiency,for example, using biomass produced at 2% photosyntheticefficiency as feedstock for power plants operating at 25%average thermal efficiency is 0.5%. Solar photovoltaic systems,including inversion of DC to AC power for grid inter-connection, currently generate at efficiencies of 6–12% (withpeak research efficiencies well above this), but suffer from theintermittency of sunlight. The higher efficiency of direct solarenergy conversion coupled with declining manufacturing costsgenerates competition between the agricultural and energysectors for land resources, a subject of additional controversyand developing policy.

464 Global Agriculture: Industrial Feedstocks for Energy and Materials

Improvements in biomass-fueled power systems will in-crease conversion efficiencies, but the overall solar conversionefficiencies for biomass are likely to remain below 1% forpower. To meet the current (2013–14) world primary energydemands of approximately 600 EJ (EIA, 2013a) would requireapproximately 600 million hectares of land (1 TJ of energy inbiomass per hectare per year) for crops continuously pro-ducing at 2% photosynthetic efficiency (approximately 6 kgper square meter or 60 Mg ha�1 year�1 dry matter). This isapproximately 40% of the world’s cultivated land area, and15% of forest lands. It is only approximately 30% of the areaof degraded tropical lands, and half the area of these landsconsidered suitable for reforestation (Hall et al., 1993).Abandoned agricultural land is estimated to range globallyfrom 385 to 470 million hectares (Campbell et al., 2008). Theactual land requirement to meet the world energy demandwould be much greater than this because of the differences inconversion efficiencies between the current energy resourcesand biomass to satisfy the same end uses, and because not allland would be kept in continuous production and the overallphotosynthetic efficiency would not likely reach 2%. This es-timate does not include aquatic or marine species such as algaethat might also contribute. More considered estimates of thecontributions from bioenergy range from 50 to 240 EJ year�1

and closer to 10% of the global energy demand (Berndes et al.,2003). Meeting the world energy demand from biomass isneither necessary nor desirable, but the potential scale for in-dustrial feedstock production is large, even for relatively smallshares of the energy market alone and highly significant interms of land use impacts, both direct and indirect.

Types of Biomass

Agricultural residuesAgricultural residues are coproducts of the principal com-modity production system. These include primary vegetationas well as animal manures composed of digested feeds. Cornstover, for example, which is a residue of grain production, isnow receiving considerable attention as an energy resource inaddition to its potential use in new materials. Increasing eco-nomic value of residue biomass can lead to changes in theoverall crop production system, such as attention to improvingyields of residues as well as the primary crop. Increased fer-tilization may be needed to replace nutrients exported with theresidue biomass when harvested. Alternatively, the farmingsystem may adapt to the application of recycled ash or othernutrients returned from the biomass utilization system.Changes in residue management can also lead to addition orloss of soil carbon, adjustment of fertilizer composition toreduce the uptake of chlorides and other constituents that canbe detrimental to downstream conversion, modified tillagestrategies to protect against soil erosion when residue cover isreduced or to take advantage of decreased amounts of residueneeding to be incorporated into the soil, changes in irrigationpractices to manage soil and crop moisture for biomass har-vesting, modified chemical applications due to changes inweed, pest, and disease pressure from residue removal, andchanges in the harvesting system design and operation to in-tegrate primary crop and residue collection.

Controversy over indirect land-use change effects and foodversus fuel impacts associated with purpose-grown crops forbiofuels shifted focus onto agricultural residues as seeminglybenign sources of feedstock. However, residue use also needsto be carefully evaluated for more global sustainability effects.Harvesting of corn stover in the midwestern US for biofuels,for example, may result in losses of soil carbon that increasethe net greenhouse gas emissions above the levels for pet-roleum-based fuels (Murphy, 2013). Purpose-grown cropssuch as mixtures of native grassland perennials may provesuperior in overall environmental performance (Tilman et al.,2006; Murphy, 2013). Broad generalizations relating to therelative impacts of agricultural residues and industrial cropsshould be carefully inspected.

Other changes to the production system may occur whenmodifications are desired in the properties of the biomass.One example is the delayed harvesting of cereal straws to takeadvantage of natural precipitation in the leaching of alkalimetals and chlorine to improve the combustion or gasificationproperties for biomass power generation and to reduce theexport of nutrients from the field when harvesting residues.Residue harvesting reduces air pollution when substituted fortraditional open burning disposal practices with some crops,and also removes nonvolatile nutrients such as phosphorous(P) and potassium (K) (Jenkins et al., 1992). Leaching byprecipitation readily removes soluble potassium and a numberof other constituents and returns them to the field beforeharvesting, although in-field decomposition of the residue anddry matter loss may reduce the overall economic value (Bakkerand Jenkins, 2003).

Residue yields from selected crops are listed in Table 1. Theestimated production rates for animal manures are alsoshown. When not measured directly, residue yields are fre-quently estimated from primary crop yields using the harvestindex, the ratio of crop yield to total above ground biomass,typically determined from test plots (Huehn, 1993). Harvestedyields are typically lower than listed due to losses in collectionand handling, adding to the uncertainty associated with re-source supply.

Forest residues and stand improvement biomassForest residues include tree tops and branches from timberharvesting operations referred to as forest slash (due to thepractice of removing them from the commercially valuablebole of the tree). Biomass can also be collected in the form offorest thinnings of two forms: (1) lower quality stock that isoften unsuitable for traditional markets and which contributesto poor forest health or (2) as increased production from moreintensively managed regrowth forests. In many temperateforests, the mean annual growth far exceeds the mean annualharvest, and the overall stand quality can be diminished.Understory brush and dense stands of trees can contribute tohigh fuel loadings in forests with increased danger of cata-strophic wildfire, higher incidence of crown fires and greaterdamage and mortality among mature trees, and high costs offire suppression if practiced. Many forests of the westernUnited States are now particularly at risk of catastrophic firedue to high fuel loads resulting from more than a century offire suppression (Jenkins, 2005).

Table 1 Agricultural residue yields and manure production rates

Crop residue Yield (Mg ha� 1 year� 1)

Almonds 2.2Apples 3.7Apricots 3.4Artichokes 3.8Asparagus 4.9Avocados 2.5Barley 2.5Beans 1.9Cherries 0.7Citrus 1.7Corn (maize) 9.0Cotton 2.9Cucumbers 3.8Dates 1.7Figs 3.7Grapes 3.4Lettuce 2.2Melon/squash 2.7Oats 2.3Peaches 3.4Pears 3.8Plums 2.5Potatoes 2.7Prunes 1.7Rice 6.7Safflower 1.9Sorghum (grain and milo) 5.0Sugar beets 4.6Tomatoes 2.9Walnuts 1.7Wheat 3.7

Livestock manures Production (kg dry matter per animal per day)

Beef cattle 4.1Dairy cattle 5.9Chickens (layers) 0.04Chickens (broilers) 0.02Turkeys 0.1Swine 0.5

Source: Adapted from Knutson, J., Miller, G.E., 1982. Agricultural residues inCalifornia, factors affecting utilization. UC Cooperative Extension Leaflet No. 21303.Berkeley, CA: University of California.

Global Agriculture: Industrial Feedstocks for Energy and Materials 465

Industrial and energy cropsTerrestrial (land-based) purpose-grown industrial and energycrops are typically classified as woody or herbaceous althoughnot all industrial crop types are readily classified in this way.Jatropha, for example, includes approximately 170 species ofsucculent plants, shrubs, and trees. The more common speciesof this plant considered for biofuel purposes is the drought-resistant shrub Jatropha curcas that produces an oil-bearingseed useful for biodiesel production among other uses. Theplant also contains toxic compounds including phorbol estersand other terpenoid compounds, but even these also havesome beneficial uses (Devappa et al., 2011; Wang et al.,2013a). The phorbol esters demonstrate insect deterrent andcytotoxic antitumor and antimicrobial properties (Devappaet al., 2011). Woody crops are predominantly plantation trees,

frequently grown in short-rotation intervals of from 1 to 20years between harvests. Cultural practices for these crops havebeen well established for roundwood and papermaking, andhave been more recently extended to the production of fuelwood. Herbaceous crops include annual and perennial grassesand other nonwood plants. Production practices for thesecrops are in most cases similar to other agricultural crops, al-though in both woody and herbaceous crop production, theend use for the biomass can influence the management andcultural inputs and practices employed to optimize the pro-duction system. A number of more commonly consideredindustrial and energy crops are listed in Table 2 but manymore have been investigated for specific material recovery orenergy purposes.

Just as they are for current agriculture, water availabilityand water costs are key constraints in industrial biomassproduction. Water requirements vary considerably but aretypically in the range of 300–1000 kg per kg of dry matterproduced. Arid or semiarid regions of the world are not an-ticipated to produce substantial quantities of biomass for en-ergy markets even where water is commonly imported foragriculture. The exception may be in the production of algaedue to the high availability of solar radiation and the potentialfor the use of brackishwater, waste water, or seawater (Renukaet al., 2013). Lands in these areas might be used in the pro-duction of feedstocks for higher value industrial and consumerproducts.

Biomass production can play an integral role in managingsalts and remediating other undesirable impacts of irrigatedagriculture in arid or semiarid regions and in reclaiming landsdegraded by unsustainable agricultural practices. Integratedfarm drainage management (IFDM) systems have beenevaluated in California and other irrigated agricultural regionsto reclaim salt-affected soils and to reduce the environmentalimpacts of agriculture in these areas (Lin et al., 2002). IFDMsystems employ sequential reuse of water through a cascadeof increasingly salt-tolerant crops with the objective of con-tinuously reducing water volume and increasing salt concen-tration for ultimate recovery or removal. Fresh irrigation waterapplied to high-value vegetable crops, for example, results indrainage water that can be applied to more salt-tolerant agri-cultural crops such as cotton or barley. Biomass crops, suchas Eucalyptus or Jose tall wheat grass (Agropyron elongatum)are suitable for irrigation with the secondary drainage waterfrom these, with tertiary drainage from the biomass cropsapplied to halophytes, and solar evaporators or other saltseparation and purification systems used for final treatmentand salt removal. The biomass crops transpire large amountsof water and act as biopumps to lower groundwater tablesand reduce salt accumulation in the root zone, andalso help in removing toxic elements, such as selenium, thatis taken up and accumulated in the plant biomass. Cropharvesting removes undesirable materials from the field fordownstream recovery, utilization, or disposal. Biomass yieldsunder these conditions are substantially less than the bestyields obtained for the same species under optimal con-ditions. Where other agronomic or environmental purposesare involved, yield is not necessarily the primary consi-deration for these systems although still an important con-sideration for economic performance. Biomass crops can in

Table 2 Selected industrial and energy feedstock crops

Woody species – biomass/fiber/pulp Wetland species – biomass/fiberAlder (Alnus spp.) Cattail (Typha sp.)Australian pine (Casuarina) Cordgrass (Spartina spp.)Birch (Onopordum nervosum) Giant reed (Arundo spp.)Black locust (Robinia pseudoacacia) Giant reed (Phragmites spp.)Eucalyptus (Eucalyptus spp.) Reed canary grass (Phalaris arundinacea)Lucaena (Lucaena leucocephala)Poplar (Populus spp.)

Seed/oilseed/terpenesWillow (Salix spp.)Amaranth (Amaranthus spp.)

Herbaceous species – biomass/fiber/energy grain Castor (Ricinus communis)Alfalfa (Medicago sativa) Crambe (Crambe abyssinica)Corn/Maize (Zea mays) Euphorbia (Euphorbia spp.)Flax (Linum usitatissimum) Jojoba (Simmondsia chinensis)Hemp (Cannabis sativa) Linseed (Linum usitatissimum)Jose tall wheatgrass (Agropyrum elongata) Oilseed rape (Brassica spp.)Kenaf (Hibiscus cannabinus) Safflower (Carthamus tinctorius)Miscanthus (Miscanthus spp.) Soybean (Glycine max)Napier grass/Bana grass (Pennisetum purpureum) Sunflower (Helianthus annuus L.)Spanish thistle or Cardoon (Cynara cardunculus) Jatropha (Jatropha curcas)Spring barley (Hordeum vulgare)Switchgrass (Panicum virgatum L.) Aquatic species – biomass/lipidsTriticale (Triticosecale) Brown algae (Sargassum spp.)Winter rye (Secale cereale) Giant kelp (Macrocystis pyrifera)Winter wheat (Triticum aestivum) Microalgae (Botryoccus braunii)

Red algae (Gracilaria tikvahiae)Herbaceous species – sugar/starch/biomass Unicellular algae (Chlorella and Scenedesmus)Buffalo gourd (Curcurbita foetidissima) Water hyacinth (Eichhornia crassipes)Cassava (Manihot esculenta)Jerusalem artichoke (Helianthus tuberosus)Sugar/energy cane (Saccharum spp.)Sugar/fodder beet (Beta vulgaris)Sweet sorghum (Sorghum bicolor)

466 Global Agriculture: Industrial Feedstocks for Energy and Materials

general be used for the restoration of abandoned or degradedagricultural lands (Campbell et al., 2008), and economic andenvironmental reasons are frequently proposed for plantingin marginal areas.

Short-rotation woody crops grown for energy or otherpurposes are typically grown in plantations with stand dens-ities ranging up to 10 000 trees per hectare depending on thesize of the tree desired at harvest, harvesting technique em-ployed, and end use intended for the biomass (Smith et al.,1997). More than 100 million hectares are in industrial treeplantations (Hall et al., 1993), most in longer rotations forroundwood and pulp production. Production site selectiondepends on soil properties, water availability, slope, climate,and distance from market. As with agricultural crops, highyields are associated with better soil types, although trees cangenerally take advantage of higher groundwater tables (Sirenet al., 1987). Soil preparation commonly involves land clear-ing to remove the existing vegetation and eliminate weeds, soilamelioration to adjust pH and improve tilth, drainage, andnutrient concentrations, and leveling and in some casesmulching. The plant bed is prepared in a manner similar toagricultural crops, although tillage is typically deeper than forcereals. Following soil preparation, cuttings, slips, or seedlingsare planted either manually or by machine or by machine-assist to manual planting (Golob, 1987). Soil moisture man-agement following planting is critical as deeper tillage

increases drying rates, potentially leading to inadequate wateravailability without rain or irrigation. Cultivation, chemicalapplication, or mulching (including plastic sheet mulching) tocontrol weeds, and in some cases pruning to improve shootvigor, are typically required following planting for properstand establishment. Some crop species, such as Salix (willow),are highly susceptible to common herbicides. Biological weedcontrol is sometimes practiced, usually by planting weed-competitive crops such as Trifolium, although these can alsocompete for nutrients and water with the primary crop untilgood canopy cover is achieved. Fencing to control grazing byanimals may also be needed. Irrigation and fertilization can beaccomplished in the same manner as for agricultural crops.Drip irrigation systems can be combined with chemigation(injection of nutrients and other chemicals into the irrigationsystem for distribution to the plants). Frost protection is notcommonly practiced, but frost damage is a concern in manylocales. Where frosts are frequent, more tolerant species orvarieties are selected. Fire suppression may be needed in dryareas, and varietal or species selection is important in reducingdamage from insects and diseases. Plantation design can in-clude set-aside areas harboring native predators of pests, anddivision of the plantation into blocks of different clones orspecies to make the overall plantation less susceptible. Owingto the lower frequency of planting and other operations in treeplantations, soil erosion rates are generally lower compared

Global Agriculture: Industrial Feedstocks for Energy and Materials 467

with agricultural crops. Seedling survival rates in well-managed plantations are typically approximately 85%. Cop-pice (resprouting) crops are harvested in 3–10 year cycles, withup to six cycles before replanting. Mean annual dry matterincrements in practice are 10–20 Mg ha�1 year�1, generallydeclining at higher latitudes for reasons of climate and inso-lation. Crop improvements may extend these yields to15–30 Mg ha�1 year�1 or higher.

Production of herbaceous crops in many respects bearsgreater resemblance to conventional forage and other agri-cultural crops, although many of the same considerationsapply to the production system design as do for short-rotationwoody crops (Smith et al., 1997; Gosse, 1996). Regional mixedcultivation of perennial and annual species, as opposed tolarge-scale monoculture, is seen in many cases to be of benefitboth in terms of environmental and economic performance(Tilman et al., 2006). Depending on the location, the regionalmix might include legumes and warm and cool season grasses.Total dry matter yields for herbaceous species under goodmanagement are typically in the range of 10–30 Mg ha�1

year�1. Sugarcane, a C4 species, is one of the most productivecrops, with world average yields of approximately 35 Mgha�1 year�1 (Hall et al., 1993) with fresh wet-weight yieldsincluding sugar above 250 Mg ha�1 year�1 although fewcountries exceed 100 Mg ha�1 year�1 fresh-weight (Duke,1983). Where proposed as energy or industrial crops, par-ticular attention must be given to the composition of theplant. Herbaceous species, especially grasses, generally containmore ash than woody species, and the ash is higher in alkalimetals and silica. The latter, for example, combine at hightemperatures in thermochemical systems to form slags anddeposits that increase maintenance costs. Leaching with watercan remove most of the alkali, and delayed harvest to takeadvantage of rain-washing can improve the combustionproperties in the same manner as noted earlier for residues(Jenkins et al., 1998; Huisman, 1999). Chlorine, primarilysupplied in potassium fertilizer (e.g., as muriate of potash orKCl) or present as chloride in irrigation water, facilitates alkalivolatilization and fouling at high temperatures, acceleratescorrosion of metals, and contributes to acid gas (principallyHCl) and toxic emissions such as dioxins and furans fromcombustion systems (Jenkins et al., 2011). Sulfur, althoughtypically present in plant biomass at low concentrations, alsocontributes to fouling and air pollutant emissions. Restrictingchlorine from fertilizers in energy crop production is generallyadvantageous, as is proper management of sulfur. These con-cerns are reduced for crops grown as feedstock for directchemical extraction, fermentation, or manufactured fiberproducts, where oil, sugar, starch, or structural carbohydratesare of more importance, unless ligneous residues of the fer-mentation are to be burned as fuel as is commonly consideredfor cellulosic biorefineries. However, in all cases the com-position of the feedstock needs to be considered in terms ofthe requirements of the manufacturing or conversion processand any accompanying waste disposal. Consideration alsoneeds to be given to the introduction of exotic species that maygrow well initially but lack biological disease and pest controlsin new environments and are, therefore, more susceptible tolater damage or that may prove adaptive and invasive to thedetriment of native species or other crops.

Except for phytoplankton, aquatic species tend to demon-strate higher yields than terrestrial crops (Klass, 1998). Aquaticspecies include both unicellular (e.g., Chlorella, Scenedesmus)and macroscopic multicellular (e.g., Macrocystis, Gracilaria,Sargassum) algae (seaweeds), and a number of water and saltmarsh plants such as cordgrass (Spartina spp.), reed (Arundo,Phragmites), bulrush (Scirpus), and water hyacinth (Eichhornia).Dry matter yields range between 5 and 75 Mg ha�1 year�1.Water hyacinth is considered a nuisance plant in many inlandwaterways and is difficult to control, but is hardy and diseaseresistant. Many of the aquatic species are currently producedunder much more intensive conditions, and generally for morevaluable products than energy and fuels, for example, Spirulinafor food protein or for antihistamine peptides used in theprevention of atherosclerosis (Spolaore et al., 2006; Vo andKim, 2013), although benefits of its use as a supplement foranimal feeds are still in question (Holman and Malau-Aduli,2013).

Algae and cyanobacteria represent a significant fraction(20–30%) of global photosynthetic productivity and arewidely investigated for their energy and industrial productpotential (Waterbury et al., 1979; Spolaore et al., 2006;Pisciotta et al., 2010). High lipid concentrations in somestrains of algae (up to 50% in Nannochloropsis and 60% inBotryococcus braunii (Scott et al., 2010)) have encouragedresearch and commercial development into the production ofbiodiesel fuel from algal extracts and fermentation of algae foralcohol, biogas, and other products using algal production inopen natural systems, ponds and raceways, as well as in highlyengineered bioreactor systems (Sheehan et al., 1998; Chenget al., 2013). Petroleum resources that supply current motorfuels and other energy demands predominantly originate fromalgal and zooplankton biomass deposited during periods ofglobal warming approximately 90 and 150 Ma ago (Campbell,1988). Interest in algae is also driven by perceived highproductivities and the potential for carbon uptake and se-questration in mitigating carbon dioxide emissions frompower plants. Single-day maximum productivities obtained inresearch trials are up to 50 g of dry matter per square meter perday, which if sustained throughout the year would equate to183 Mg ha�1 year�1, well above yields from the best pro-ducing C4 species. Commercial yields are unlikely to exceedapproximately 80 metric tons dry matter per hectare per year(Scott et al., 2010) although more optimistic estimates arefrequently made. Sea-based floating membrane bioreactorshave also been proposed to enable large-scale off-shorefarming of algal biomass using municipal wastewaters as nu-trient sources (Howell, 2009; Harris et al., 2013).

Structure, Composition, and Properties of Biomass

Biomass is a chemically rich feedstock that can be used in itsnative form, converted into intermediate chemicals and ma-terials for further downstream processing to final products, orconverted directly to the final product. Composition varies bythe type of feedstock, location of production, and by a numberof other cultural attributes, but can be generally classified intoorganic and inorganic constituents all of which can be im-portant in the conversion to energy, materials, and fuels. These

468 Global Agriculture: Industrial Feedstocks for Energy and Materials

constituents can be further divided into a number of com-ponents and properties subject to international standards ofanalysis (Figure 1).

Fresh or crude biomass contains water as moisture and drymatter, the latter comprises organic constituents that arevolatile under heating (volatile solids) and an inorganic frac-tion represented as ash. Moisture content influences the entireproduction chain due to its importance in determining theharvesting conditions, transportation modes and costs, con-version technology type and performance. For the processesinvolving thermal transformations (e.g., combustion), the or-ganic fraction is typically classified into volatile matter (not tobe confused with volatile solids) and fixed carbon, but com-bined these two fractions comprise most of the structuralcomponents of biomass (principally hemicellulose, cellulose,and lignin but also pectin and other cell wall polymers) andextractives (e.g., lipids in algae or oilseeds, sugars, starches, andterpenes and also many other compounds including some ofthe inorganic constituents). The ash consists typically of saltsand other minerals, metals, and adventitious material, that is,materials not inherent in the biomass but added insteadthrough handling and processing, such as soil accumulatedduring harvesting. Organic materials may also be adventitious,and alterations in the organic composition and structure canoccur through degradation or decomposition of the feedstockduring handling, storage, and processing. More funda-mentally, the biomass can be characterized by its elementalcomposition that will contain essentially every natural elementbut more practically is limited to a smaller number of majorelements with a large number of minor and trace elements atconcentrations of significance, some of which will be im-portant to industrial processing or energy conversion (e.g.,

N

Moi

stur

e

Wet

bio

mas

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Dry

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ter

(tot

al s

olid

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ids

Ash

(met

als,

min

eral

s,ad

vent

itiou

sm

ater

ials

)E

lem

enta

l com

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tion

Figure 1 Biomass composition (dashed lines indicate variable concentratio

lead, mercury, and other heavy metals). The design of anyindustrial system requires a careful analysis of the chemicaland physical properties of the intended feedstock to evaluateinfluences on process operations and product quality.

The structure of biomass varies depending on the speciesand the tissue type. Microstructures of some higher plants(woods and grasses) are shown in Figures 2–4. The term‘wood’ more specifically refers to the secondary xylem tissuearising from cell division in the vascular cambium (Hon andShiraishi, 2001), but bark and other tissues are also generallyincluded when referring to feedstock materials even thoughcompositions can be widely different. Ash content, for ex-ample, is frequently higher in bark than in wood. Its distri-bution also varies in herbaceous species such as rice (Summerset al., 2003). Woods are classified as softwoods and hard-woods. Tracheid and parenchyma cells are the principal typesin softwoods, with tracheids in some comprising 90% of thewood volume. Hardwoods are more complex. Principal celltypes in hardwoods are fibers, vessels, and parenchyma.Herbaceous species such as the grasses have tubular or solidstems (culm) divided into node and internode regions, thelatter typically surrounded by leaf sheaths originating from thenodes. Algae constitute a highly diverse group of eukaryoticorganisms that can be unicellular or multicellular and differ infine structure compared with terrestrial green plants (Bouck,1965). They range in size from microalgae under 5 mm (e.g.,Scenedesmus) to the macroscopic giant kelp (Macrocystispyrifera) exceeding 50 m in length. They also vary substantiallyin lipid, carbohydrate, nucleic acid, and protein composition(Sheehan et al., 1998). Fresh-weight moisture is more than65% and more commonly more than 85%; dry weight ashcontents can exceed 30%; protein concentrations can approach

C

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Pectin,...

Lignin

Hemi-cellulose

Cellulose

, S, Cl,...

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Extractives(sugars, starch,lipids, proteins,inorganics,...)

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Figure 3 Scanning electron micrograph of birch hardwood showinga more complex structure compared with softwoods. Large holes arevessel elements. Reproduced from Society of Wood Science andTechnology, 2013. Structure of Wood. Available at: http://www.swst.org/edu/teach/teach1/structure1.pdf (accessed 11.11.13).

Global Agriculture: Industrial Feedstocks for Energy and Materials 469

half the dry weight; and lipid concentrations range up to 60%dry weight (Butler, 1931; Renaud et al., 1999; Sheehan et al.,1998). Cell wall components include cellulose, hemicellu-loses, lignin, pectins, arabinogalactan proteins, extensin, car-rageenan, agar, and in diatoms sculpted silica composites(Blumreisinger et al., 1983; Abo-shady et al., 1993; Domozych,2011; Domozych et al., 2012). The so-called blue-green algae(prokaryotic cyanobacteria) are now classified with the bac-teria. Chitin (polymeric N-acetylglucosamine, also found inthe exoskeletons of arthropods) rather than cellulose is theprimary structural component of the cell wall among the truefungi (Blumenthal and Roseman, 1957; Sikkema and Lovett,1984).

The dominant structural compounds making up the plantbiomass are cellulose (six-carbon or C6 polymers) and hemi-cellulose (predominantly C5 polymers but including C6 spe-cies) produced via condensation polymerization of themonosaccharides (Chum and Baizer, 1985; Schultz andTaylor, 1989; Sudo et al., 1989; Lynd, 1990; Wyman andHinman, 1990; Hon and Shiraishi, 2001). The other primarystructural components are lignins, aromatic polymers ofvariable structure derived in one proposed pathway fromconiferyl, sinapyl, and p-coumaryl alcohols. The alcohols arisethrough the shikimic acid pathway, and are polymerized intolignin via free-radical reactions (Salisbury and Ross, 1992).Organic compounds in biomass also include proteins, trigly-cerides (fats and oils), terpenes (including isoprenes), waxes,cutin, suberin, phenolics, phytoalexins (antimicrobial com-pounds produced by the plant), flavonoids, betalains, alkal-oids, and other secondary compounds as well as sugars andstarch. Plants accumulate inorganic materials (ash), sometimesin concentrations exceeding those of the hemicellulose orlignin. The composition of wood can depend on the appliedforces during growth (‘reaction’ wood). Compression woodtypically has highly lignified tracheid walls, in contrast totension wood with lower lignin concentrations.

Figure 2 Scanning electron micrographs of softwood showing longitudinal(rc, a resin canal; bp, a bordered pit). Right: pine wood. Reproduced from SAvailable at: http://www.swst.org/edu/teach/teach1/structure1.pdf (accessed 1

Cellulose is a linear crystalline polysaccharide of β-D-glucopyranose units linked with (1–4) glycosidic bonds. Cel-lulose serves as a framework substance, making up 40–50% ofwood. In the cell wall, cellulose exists in the form of thinthreads or microfibrils, each 2–5 nm wide in wood, but up to30 nm wide in algae (Valonia). The polymer is formed fromrepeating units of cellobiose, a disaccharide of β-linked glucosemoieties (Figure 5). The structure of cellulose renders it highlyinsoluble and resistant to biochemical degradation. The α-linked disaccharide of glucose forms starch (Figure 6). Starchis an amorphous material, and as such is more readily de-graded by biochemical means than is cellulose and hence therelative ease of fermenting starch to ethanol whereas celluloserequires substantial pretreatment to deconstruct it beforefermentation.

Hemicelluloses are matrix substances laid down betweencellulose microfibrils. They are polysaccharides of variable

tracheids, horizontal rays, and other structures. Left: spruce woodociety of Wood Science and Technology, 2013. Structure of Wood.1.11.13).

Sugarcane

70×Figure 4 Scanning electron micrographs of a wheat straw stem (left) showing hollow tubular structure of vascular bundles in culm, andsugarcane (right) showing solid stem with distributed vascular bundles. Left: Reproduced from Malhotra, V., 2013. Bio-Composites fromAgricultural Raw Materials. Available at: http://www.physics.siu.edu/malhotra/vivek/biocomposites.htm (accessed 11.11.13). Right: Reproduced fromNTNU, 2013. Monocotyledons. Norwegian University of Science and Technology – Trondheim. Available at: http://www.chemeng.ntnu.no/research/paper/Online-articles/Nonwoods/Nonwood.html (accessed 11.11.13).

OH

OH n

HOH

O

H

CH2OH

CH2OH

HO

H H

H

HO

O OH

Figure 5 The β-linked glucose units forming cellobiose, therepeating unit of cellulose. Reproduced from Salisbury, F.B., Ross, C.W., 1992. Plant Physiology. Belmont, CA: Wadsworth Publishing Co.

OH

OH

OH n

H

OH O

HCH2OH CH2OH

H

O

HH

H

H

H

HO

O

O

H

Figure 6 The α-linked glucose units making up starch. Reproducedfrom Salisbury, F.B., Ross, C.W., 1992. Plant Physiology. Belmont,CA: Wadsworth Publishing Co.

470 Global Agriculture: Industrial Feedstocks for Energy and Materials

composition containing both five- (including xylose and ara-binose) and six-carbon (including galactose, glucose, andmannose) monosaccharide units. Like starch, hemicellulosesare mostly amorphous, making them more readily hydro-lysable than cellulose. Hemicelluloses constitute 20–30% ofwood, generally with higher concentrations in hardwoodsthan softwoods. Hemicelluloses also constitute 20–30% of drymatter in most other biomass. Partial structures for the pri-mary forms of hemicellulose in hardwood and softwood areshown in Figure 7.

Lignin is the encrusting substance (‘glue’) solidifying the cellwall. Lignin is an irregular polymer of phenylpropane units andthe structure varies among different plants. Lignin is thought to

occur through the enzymatic dehydrogenation of phenylpro-panes followed by radical coupling (Hon and Shiraishi, 2001).Softwood lignin is composed principally of guaiacyl unitsstemming from the precursor trans-coniferyl alcohol (Figure 8).Hardwood lignin is composed mostly of guaiacyl and syringylunits derived from trans-coniferyl and trans-sinapyl alcohols.Grass lignin contains p-hydroxyphenyl units deriving fromtrans-p-coumaryl alcohol. Almost all plants contain all threeguaiacyl, syringyl, and p-hydroxyphenyl units in lignin. A partialstructure of softwood lignin is shown in Figure 9.

Partial compositions of selected biomass materials are lis-ted in Table 3. The four materials – Monterey pine (Pinusradiata, a softwood), eastern cottonwood (Populus deltoides, ahardwood), sugarcane bagasse (from Saccharum spp.), andwheat straw (from Triticum aestivum var. Thunderbird) – wereanalyzed as reference materials for the purposes of providingreference characterizations of industrial and energy feedstocks(NIST, 2003).

Plants producing large amounts of free sugars, such assugarcane and sweet sorghum, are attractive as feedstocks forethanol fermentation, as are starch crops such as maize (corn)and other grains. Current attention is focused on the fermen-tation of the cellulosic components because of the perceivedeconomic and energetic advantages for large-scale liquid fuelproduction. Cellulose and hemicellulose are not fermentableby conventional means, as more aggressive pretreatment andhydrolysis must precede the fermentation step. Lignins are notyet generally considered fermentable, and thermochemicalmeans are usually proposed for their conversion to fuels.Typically, 60–80% of the biomass feedstock mass is ultimatelyfermentable based on cellulose and hemicellulose (holo-cellulose) concentrations.

Oil crops such as rape and canola, soybean, sunflower,safflower, algae, and others have been widely investigated for

OH

OH

O

O

O

COOH

OAc

OH

OHO

O

O

OH

OOH

OH

OAcO

OOH

OAc

O

O

O

CH3O

CH2OAc

OH

OH

O

O

O

CH2OH

CH2OH CH2OH

OH OH OHO

O

HO

O

OOH OH

CH2OH CH2OH

OAc OHO

OOH

OH

O

O

O

(a)

(b)

Figure 7 Partial structures of the principal hemicelluloses in wood (Hon and Shiraishi, 2001): (a) O-acetyl-4-O-methylglucuronoxylan fromhardwood; (b) O-acetyl-galactoglucomannan from softwood. Ac¼acetyl group.

Coniferyl alcohol Sinapyl alcohol

OCH3

OCH3

HC OH

CHH

HOC

H

OCH3

HC OH

CHH

HOC

H

p-Coumaryl alcohol

HC OH

CHH

HOC

H

Figure 8 Phenolic subunits of lignin. Reproduced from Salisbury, F.B., Ross, C.W., 1992. Plant Physiology. Belmont, CA: Wadsworth Publishing Co.

Global Agriculture: Industrial Feedstocks for Energy and Materials 471

their potential in producing diesel fuel substitutes (commonlyreferred to as ‘biodiesel’). Although raw plant oils are notgenerally satisfactory as fuels for diesel-type (compression-ig-nited) engines, methyl and ethyl esters formed by reacting theoil with an alcohol (methanol or ethanol) have lower viscosityand improved injection and combustion properties makingthem suitable for engine use (Peterson et al., 1992). Othercrops, such as Euphorbia, have been investigated for directproduction of hydrocarbons (Nishimura et al., 1977; Nemethyet al., 1981).

Roughly half of the plant organic matter is carbon, the restbeing made up of 5–7% hydrogen, 30–50% oxygen, alongwith nitrogen, sulfur, chlorine, and other elements (Jenkinsand Ebeling, 1985; Jenkins et al., 1998). The ash or inorganicfraction of biomass may account for more than 30% of the drymass in some cases (e.g., animal manures and other wastematerials, some algae). Temperate region woods typically haveash contents below 1%, whereas some tropical woods containup to 5% ash (Chum and Baizer, 1985). Leaves and youngtissues contain more ash than mature wood. Cereal grain

straws, hulls (husks), and other agricultural biomass have ashcontents ranging up to approximately 25% of the dry mass.The amount of ash in a biomass material, and the com-position of the ash are important for the selection of theconversion technology. As noted earlier in the Section Types ofBiomass, alkali metals (especially the macronutrient potas-sium) in the ash of many agricultural residues can causeslagging and boiler fouling and corrosion in the case of directfiring as a result of high temperature reactions with silica andother minerals. Chlorine is important in corrosion and as afacilitator in the alkali reactions leading to deposition inboilers. Biochemical methods avoid this problem because ofthe lower temperatures involved although corrosion can stillbe of concern. Other methods, such as supercritical wateroxidation, in which the oxidation is carried out at a relativelylow temperature but high pressure, may suffer less from ashreactions although salts can be an issue, but such processes arenot yet commercial. In certain circumstances, fouling is re-duced by the extraction of the offending elements beforeintroducing the fuel to the boiler. Such is the case of sugarcane

CH2OH

HC

HCH

HC

HC

CH

HCOH

CH

CH

HC

HC

OH

C O

O

OCH

HC

HC

HOCH2

OCH3

OCH3

OCH3

O

O

CH3O

CH3O

CH3O

CH3O

CH2OH

HOCH2

HOH2C

H2COH

CH2OH

CH2OH

OCH2OH

CH2OH

H2C

CH2OH

CH2OH

HOH2C

CH3OCH3O

CH3O

OCH3

OCH3

CH3O

CH3O

OCH3

OCH3

OCH3

OCH3

CH2

O

OH

OO

O

O

HC

HC

OH

H

HCC

O

O

O

O

O

O

O

O

CH

CH

CH

CH

CHHC

HCHC

HC

HCOH

HCOH

HCOH

HCOH

HCOH

HCOH

Figure 9 Partial lignin structure of softwood. Reproduced from Salisbury, F.B., Ross, C.W., 1992. Plant Physiology. Belmont, CA: WadsworthPublishing Co.

Table 3 Partial compositions (%) of biomass materials (NIST, 2003)

Biomass constituent Populus deltoides (rm #8492) Pinus radiata (rm #8493) Sugarcane bagasse (rm #8491) Wheat straw (rm #8494)

Ash 1.0 0.3 4.0 10.395% ethanol extractives 2.4 2.7 4.4 13.0Acid soluble lignin 2.4 0.7 2.0 2.3Acid insoluble lignin 23.9 25.9 22.3 15.9Total lignin 26.2 26.6 24.2 18.0Glucuronic acid 3.7 2.6 1.3 2.1Arabinan 0.7 1.5 1.8 2.5Xylan 13.9 6.2 21.5 21.7Mannan 2.0 10.9 0.4 0.3Galactan 0.6 2.4 0.6 0.8Glucan 43.2 42.9 40.2 37.6

Each value is mean from round-robin testing of approximately 20 laboratories.

472 Global Agriculture: Industrial Feedstocks for Energy and Materials

Global Agriculture: Industrial Feedstocks for Energy and Materials 473

bagasse, commonly used as a fuel in the sugar industry, butwhich has passed through the sugar extraction step and hadmost of the alkali leached in the process. Direct firing of canetrash, currently under investigation for increasing the powergeneration from the cane industry, does not extract theseelements, and may suffer from deposition common to strawsand other high ash, high alkali biomass fuels.

The energy content or heating value (heat of combustion)of biomass can be partially correlated with ash concentration.Woods with less than 1% ash typically have heating valuesnear 20 MJ kg�1. Each 1% increase in ash translates roughlyinto a decrease of 0.2 MJ kg�1 (Jenkins, 1989) because ashdoes not contribute substantially to the overall heat releasedby combustion, although elements in the ash may be catalyticto thermal decomposition. Heating values can also be correl-ated to carbon concentration, with each 1% increase in carbonelevating the heating value by approximately 0.39 MJ kg�1, aresult identical to that found by Shafizadeh (1981) for woodsand wood pyrolysis products. The heating value relates to theamount of oxygen required for complete combustion,with 14 022 J released for each gram of oxygen consumed(Shafizadeh, 1981). Cellulose has a smaller heating value(17.53 MJ kg�1) than lignin (26.7 MJ kg�1) because of itshigher degree of oxidation. Other compounds, such ashydrocarbons, with lower degrees of oxidation or containingno oxygen tend to raise the heating value of the biomass.

Proximate and elemental compositions and heating valuesfor selected biomass are listed in Table 4. The heating valueshown in the table is the constant volume higher heating valuethat is one of four that can be defined based on whether thetest is carried out at constant pressure or constant volume andwhether water produced from the hydrogen in the biomass isin the vapor or liquid phase at the end of the test (Jenkins,1989). Wheat straw and sugarcane bagasse listed in Table 4 arenot the same samples listed in Table 3 and hence have dif-ferent compositions. The proximate analysis yields ash contentand the fraction referred to as volatile matter (Figure 1)evolved during a short heating at 950 1C in a nonoxidizingatmosphere representing pyrolysis. The difference between thetotal dry matter and the sum of ash and volatile matter is fixedcarbon (essentially charcoal). The ultimate analysis yieldselemental concentrations of carbon, hydrogen, nitrogen, andsulfur, with oxygen usually determined by difference. Al-though not part of the standard for ultimate analysis, chlorineis also sometimes analyzed due to its importance in manyconversion systems. The Jose tall wheat grass listed in thetable was grown as a phytoremediation crop under salineconditions, and has a high Cl content.

Logistical Supply Chains

The use of biomass for industrial products and energy typicallyrequires a supply chain of multiple operations to deliver bio-mass feedstock or biomass-derived intermediates from the siteof production to the conversion facility and then the finishedproduct into final demand (Figure 10). For food processingfacilities, lumber mills, wastewater treatment plants, dairies,and other animal operations, the supply chain may be rela-tively simple due to the generation of the feedstock on-site.

For crop and forest residues and energy crops, supply op-erations upstream of the conversion facility will typically in-clude collection or harvesting, packaging and processing suchas baling, pelleting or grinding, loading and unloading ontotransport vehicles, transportation on roads, rail, river or mar-ine routes, and in most cases storage, possibly at one or moreintermediate sites (depots) in addition to the primary site ofconversion. Densification of the bulk biomass may be pre-ferred for the purposes of achieving high transportation pay-loads on trucks and other vehicles but contributes to the costof processing. Whether the cost can be justified depends in parton transport distance and other factors associated with thehandling and use of the feedstock and the cost of transportotherwise. Conversion into pyrolysis oils, sugars, and otherintermediate products at depots or other satellite facilities mayalso be considered for logistical purposes to reduce the totalcosts of feedstock transportation to larger central facilities forfinal processing and production.

Drying to reduce moisture content may be needed, espe-cially for feedstocks intended for many types of thermo-chemical conversion facilities. For some facilities, for example,biochemical conversion facilities using fermentation systems,the moisture in feedstock may be an important contributiontoward meeting the overall process water demands and mayenhance conversion but adds to transportation costs and in-creases the risk of spoilage during handling.

The costs associated with feedstock supply, conversion, andthe distribution of finished products influence the optimal sizeof a conversion facility (Jenkins, 1997). Economies of scaletypically exist for capital and operating costs so that as theproduction capacity or size increases, these costs typically donot increase in the same proportion and the cost per unit ofproduct output decreases. However, as the size of the facilityincreases, the feedstock demand increases as well, and fordistributed feedstocks such as crop and forest residues andpurpose-grown crops, this means the supply region will gen-erally increase unless crop productivity and yields can be in-creased to compensate for the added supply needs.Transportation costs, therefore, increase, but not generally indirect proportion to the facility size. These competing effectslead to a system size at which either total cost of production isminimized (Figure 11) or profit is maximized depending onthe objective function. For example, economies of scale in thecapital and operating costs of facilities to generate electricity(power plants) from biomass coupled with increasing costs offeedstock acquisition yield a minimum in the levelized(amortized) cost of energy from the plant (Figure 11). Thescaling in capital and operating cost may take a functionalform such as

CCo

¼ MMo

� �s

½3�

where C is the cost; M is the facility size or capacity; the sub-script denotes a reference facility of known cost and size; and sis the scaling factor that specifies how cost varies with size. Theoptimum size is sensitive to the scaling relationship and thevalue of s. The two cases of Figure 11 were derived for s isconstant for all sizes and s is variable and increasing with sizeto account for increasing risk as the facility size becomes very

Table 4 Proximate and elemental compositions and heating values of selected biomass materials (Jenkins et al., 1998), Composition of selected biomass materials (Jenkins et al., 1998)

Type Alfalfastraw

Ricestraw

Wheatstraw

Miscanthus Switchgrass

Jose tall wheatgrass

Hybridpoplar

Willow Waterhyacinth

Nonrecyclable wastepaper

Sugarcanebagasse

Municipal digester sludge (class Bbiosolids)

Typical harvest moisture (% wet basis)14 14 10 14 14 10 45 45 85 6 50 75

Proximate composition (% dry matter)Ash 4.88 18.67 14.48 4.90 6.53 12.34 1.6 0.95 22.40 8.21 3.61 37.91Organic fraction 95.12 81.33 85.52 95.10 93.47 87.66 98.40 99.05 77.60 91.79 96.39 62.09Volatiles 76.48 65.47 69.94 78.20 77.03 72.18 86.14 85.23 82.50 84.51 53.68Fixed carbon 18.64 15.86 15.58 16.90 16.44 15.48 12.26 13.82 9.29 11.88 8.41

Higher heating value (MJ kg� 1)Moisture free(dry)

18.16 15.09 16.33 18.05 18.90 17.86 18.93 19.38 16.02 21.52 18.50 15.38

Moisture and ashfree

19.09 18.55 19.09 18.98 20.22 20.38 19.24 19.56 20.64 23.44 19.19 24.77

Wet 15.62 12.97 14.69 15.52 16.26 16.08 10.41 10.66 2.40 20.22 9.25 3.85

Structural composition (% dry matter)a

Cellulose 29 34 40 45 41 49 16 36Hemicellulose 12 28 29 30 33 28 56 30Lignin 9 14 21 26 22 6 19

Ultimate elemental composition (% moisture and ash free)Carbon 48.40 47.02 48.08 53.31 50.69 52.86 51.65 49.56 52.96 53.66 49.99 58.29Hydrogen 6.15 6.39 6.08 4.63 6.08 5.05 5.99 5.95 6.82 7.60 5.86 7.18Oxygen (bydifference)

43.05 44.77 43.99 41.59 42.57 39.59 41.75 44.11 37.16 38.18 43.94 24.17

Nitrogen 1.19 1.07 1.19 0.21 0.60 2.00 0.60 0.35 2.53 0.38 0.15 9.08Sulfur 0.19 0.22 0.28 0.11 0.06 0.37 0.02 0.03 0.53 0.22 0.08 1.72Chlorine 1.17 0.71 0.71 0.21 0.10 2.21 0.01 0.04 0.16

Ash analysis (% ash)SiO2 7.04 74.67 54.64 70.60 66.53 46.71 1.17 8.08 19.44 41.87 47.11Al2O3 1.12 1.04 5.73 1.10 6.98 3.09 0.41 1.39 63.97 22.25 17.9TiO2 0.05 0.09 0.23 0.06 0.34 0.09 0.21 0.06 3.81 3.87 1.22Fe2O3 0.41 0.85 6.16 1.00 3.56 1.00 0.76 0.84 0.42 20.90 5.64CaO 21.37 3.01 5.02 7.50 7.14 4.59 59.16 45.62 8.37 3.50 8.65MgO 5.83 1.75 2.45 2.50 3.17 2.03 5.76 1.16 1.68 1.45 2.98Na2O 11.2 0.96 2.16 0.17 1.03 8.60 0.31 2.47 0.83 0.26 1.33K2O 22.9 12.3 14.09 12.80 7.00 15.68 26.76 13.2 0.23 2.59 1.32P2O5 6.32 1.41 2.43 2.00 2.80 2.70 0.20 10.04 0.10 1.13 14.65SO3 4.27 1.24 3.03 1.70 2.00 2.05 5.26 1.15 1.14 0.90 1.38Cl 13.43 0.01CO2 0.05 0.21

aStructural data representative only and not necessarily from same sample used for elemental analysis and heating value.

474GlobalAgriculture:IndustrialFeedstocks

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Production

• Residues

crops

Harvesting and handling

• Collection• Processing• Storage• Transportation

(May be performedmultiple times and inany order depending onoptimal system design)

Conversion

• PretreatmentThermalBiologicalChemicalMechanical

• ThermochemicalCombustionGasificationPyrolysisHydroprocessing

• BiochemicalAnaerobic (fermentation)

Anaerobic digestionAlcohol fermentation

AerobicCompostingActivated (oxygenated)waste treatment

Direct hydrogen• Physicochemical

ExtractionEsterification

• Manufacturing (may alsoemploy physical and chemicalprocessing)

Final demand

• Bioenergy (heat and power)Process and space heatingPower generation

• Biofuels (fuels)Biomass solidsCharcoal/BiocharSynthesis gases (CO + H2)HydrogenBiogas (methane + CO2)including digester gas andlandfill gasMethanolEthanolPyrolysis liquidsBiodieselOthers

• Bioproducts (chemicals andmaterials)

FibersStructural materialsComposite materialsCitric and other acidsPesticidesLubricantsSurfactantsOthers

• Purpose-grown

Figure 10 Supply-chain processes for biomass to energy and products.

0.20

Mopt=1252 MWconstant s

Total cost

Fuel cost Fuel cost

Capital + O&M costCapital + O&M cost

Mopt=305 MWvariable s

0.15

0.10

(US

$ kW

h−1)

0.05

0.001 10 10100

M (MW) M (MW)

1001000 1000 10 0001

Total cost

Figure 11 Economy of scale and capacity optimization in biomass conversion (Jenkins, 1997).

Global Agriculture: Industrial Feedstocks for Energy and Materials 475

large. The cost–size dependence for regionally distributedfeedstock typically follows a different relationship, contrib-uting an increasing cost per unit of production as the facilityscale increases (Figure 11) (Jenkins, 1997). The estimated costof power generation is plotted in Figure 11 against the facilitysize on a logarithmic scale, which masks to some degree thebroad range around the optimum in which cost does not varysubstantially, a characteristic of larger size facilities. The opti-mal sizes in both examples are quite large for biomass power

generators and few facilities approach these sizes in practice.To some extent, this is due to regulations that also affect fa-cility siting and permitting. In California and the US, for ex-ample, new facilities must undergo a siting review when thesize exceeds 50 or 80 MW electrical capacity and this adds tothe cost of facility development (Jenkins, 1997).

Consideration of the feedstock properties and availability,the regional product markets, and the costs and prices asso-ciated with production and sales are important to the size,

Dry mil Forest

Grease

HEC

MSWOVW

Wood pulp

Seed oils

AG Residue

Animal fat

Corn/grain

FAHCFAMEFT DieselLCEWal mill

Figure 12 Optimized potential biorefinery locations in the US identified through geospatial modeling of biomass resources, supply infrastructure,and fuel demand (Parker, 2011).

476 Global Agriculture: Industrial Feedstocks for Energy and Materials

type, and location of conversion or production facilities.Geospatial modeling using resource location data to give thespatial distribution of feedstock coupled with technoeconomicmodels of conversion facilities and distribution models offinished product have been used, for example, to project policyoutcomes for biorefinery development in the US and tospeculate on the optimized structure of the future industry(Figure 12) (Parker et al., 2010; Tittmann et al., 2010; Parker,2011).

Chemicals and Materials from Biomass

Products from biomass originate both from direct use or ex-traction of materials from the parent tissues and via conversionof the biomass to alternate forms. In related bioprocessing, thebiomass can serve as a substrate by which the cellular mech-anisms are used by another organism to express the desiredproduct, for example, the transient expression of a proteinthrough the process of agroinfiltration (Joh and Vander-Gheynst, 2006).

The potential to produce a much larger range of chemicals,materials, and other products from biomass has been widelyrecognized, including direct replacement of products nowsynthesized from petroleum, natural gas, and other non-renewable resources (Johnson, 2007). Biobased plastics, lu-bricants, solvents, surfactants, composites, and other productsare increasing in market shares as technology improves, newmarkets are developed, and demand increases for more

sustainable or ‘green’ products (BIOCHEM, 2010). Globalproduction of bioplastics is projected to increase five-fold by2016 to close to 6 million metric tons compared with theproduction in 2011, although in this case demand for bio-degradable plastics does not appear to be the major driver ofgrowth as only 13% of the total is anticipated to be in poly-mers of this type (European Bioplastics, 2013). Biobasedpolyethylene terephthalate (PET) is likely to make up thelargest fraction (greater than 40%) of the total. Other drivers,such as displacement of petroleum and reduced greenhousegas emissions, plus economic and market factors account forthe increasing demand of biologically derived materials. Forexample, in the making of riboflavin (vitamin B2), single-stepinstead of six-step processing, increased productivity, and re-duction in cost are quoted as the main forces in the adoptionof a biobased process (Johnson, 2007). Similar rationale existsfor the production of antibiotics and acrylamide monomers(Johnson, 2007; Scott et al., 2007). Improved processes andefficiencies in the manufacturing of biobased intermediatesand final products are the subject of substantial research, suchas the use of biomass-derived furans in the catalytic pro-duction of p-xylene that is, in turn, used to make terephthalicacid, a monomer for PET (polyester) plastics (Williams et al.,2012), or the conversion of biomass-derived carbohydrates tochemical feedstocks, such as alkenes and aromatics, that arenow principally produced from petroleum (Arceo et al., 2009;Arceo et al., 2010).

Agriculture and forestry have long supplied materials otherthan food and feed for construction, pulp and paper, textiles,

Global Agriculture: Industrial Feedstocks for Energy and Materials 477

medicine, cosmetics, soaps, and other uses, and the pro-duction of materials such as plastics from biomass is not new,although many new and novel chemicals are now becomingavailable. Crude plastics were historically made from naturalproteins, such as those from blood, egg, and milk. Goodyear’svulcanization of natural rubber beginning in 1838 provided apathway to thermoset materials (Richardson, 2010). Shellacwas produced by dissolving the natural resin secreted by thelac bug in ethanol (Patel et al., 2013). The conversion of cel-lulose to nitrocellulose by the addition of nitric acid was dis-covered in 1846 and used for the preparation of guncotton,and nitrocellulose modified by camphor as a plasticizer wasbeing used by 1889 for photographic film by Eastman Kodak(Michel, 2006; PHS, 2013a). Nitrocellulose is now applied insuch diverse uses as membranes for protein immobilizationand analysis and solid-rocket motors. The compound (as cel-luloid) was also famously used in the attempted replacementof natural ivory for billiard balls but the balls had a tendencyto explode on impact. Cellulose acetate, the acetate ester ofcellulose, was first prepared in 1865 and is still widely used forplastic films and other applications (PHS, 2013b). Viscose, apolymer of wood pulp, was patented in 1892 and the processfor making cellophane, a viscose plastic film produced bydissolving cellulose in alkali and carbon disulfide was estab-lished by 1913 (PHS, 2013c). Despite the pollution issuesassociated with disulfide in the manufacturing process, cello-phane is biodegradable and has remained in continuousproduction since the 1930s especially for food packaging(Rojo et al., 2010; PHS, 2013c). Rayon fiber used in textiles ismade through a similar process.

Comprehensive studies have examined candidate materialsfrom sugars, synthesis gases (syngas), and lignin, all derivedfrom biomass (Werpy and Petersen, 2004; Holladay et al.,2007). The study by Werpy and Petersen (2004) identified aninitial set of 300 possible building-block chemicals fromsugars and syngas in integrated biorefineries. Such bior-efineries conceptually involve various processing operations

Feedstock

Processing technologies

ProductsMaterials and energy

Figure 13 Basic integrated biorefinery concept. Adapted from Johnson, F.Xand Challenges for the Developing World. Vienna: Stockholm Environment In

within the same production facility (Figure 13) for themanufacturing of multiple value-added products, similar tobut potentially more complex than modern petroleum re-fineries that produce a variety of commodity chemicals andfuels (Johnson, 2007). Screening criteria for the chemical se-lection included feedstock cost, estimated processing costs,market information, and compatibility with current and pro-jected future refining operations. From this initial set, afoundational suite of chemicals was selected based on morestrategic criteria (direct product replacement, novel products,and building-block intermediates) and the value chains asso-ciated with processing to products for final demand(Figure 14). The resulting network of product interactions hasbeen widely referenced in relation to potential biorefinerydesign and opportunities for products from biomass. The finalsuite includes compounds with carbon numbers (number ofcarbon atoms in the molecule) ranging from C1 to C6(Table 5). These compounds have multiple functionality forfurther downstream conversion and could be produced fromboth lignocellulose and starch in making sugars or syngas asintermediate chemical platforms, some of the principal plat-forms under development for energy and products from bio-mass. The study by Holladay et al. (2007) later identified otherproducts from lignin using the criteria based on technicaldifficulty in production, market potential, market risk, build-ing-block utility, and whether the product could be made as asingle compound or would be present in a mixture with othercompounds (Table 5).

As illustrated in Figure 14, a wide spectrum of products canbe produced from biomass to serve multiple markets in whatis loosely classified a biobased economy. These products canrange from higher value, generally lower production and salesvolume chemicals such as pharmaceuticals, food additives,and cosmetics, to lower value, higher volume chemicals andmaterials such as biobased plastics and composites, chemicalfeedstocks, fuels, and environmental products such as biocharor black carbon now being investigated for its agronomic

• Food and feed grains• Lignocellulosic biomass• Forest biomass• Municipal solid waste• Algae and others

• Biological processes• Chemical processes• Thermochemical processes• Thermal processes• Physical processes

• Fuels• Chemicals• Materials (e.g., polymers)• Specialty chemicals and materials• Commodities and goods

., 2007. Industrial Biotechnology and Biomass Utilization, Prospectsstitute and United Nations Industrial Development Organization.

Ar

SG

C6

C5

C2

BiobasedSyn Gas

TextilesCarpets, Fibers, fabrics, fabric coatings, foam cushions, upholstery, drapes, lycra, spandex

Safe food supplyFood packaging, preservatives, fertilizers, pesticides, beverage bottles, appliances, beverage can coatings, vitamins

TransportationFuels, oxygenates, anti-freeze, wiper fluids molded plastics, car seats, belts hoses, bumpers, corrosion inhibitors

HousingPaints, resins, siding, insulation, cements, coatings, varnishes, flame retardents, adhesives, carpeting

RecreationFootgear, protective equipment, camera and film, bicycle parts & tires, wet suits, tapes-CD’s-DVD’s, golf equipment, camping gear, boats

Health and hygienePlastic eyeglasses, cosmetics, detergents, pharmaceuticals, suntan lotion, medical-dental products, disinfectants, aspirin

C3

C4EnvironmentWater chemicals, flocculants, chelators, cleaners and detergents

CommunicationMolded plastics, computer casings, optical fiber coatings, liquid crystal displays, pens, pencils, inks, dyes, paper products

IndustrialCorrosion inhibitors, dust control, boiler water treatment, gas purification, emission abatement, specialty lubricants, hoses, seals

Intermediateplatforms

Buildingblocks

Secondarychemicals

Products/usesIntermediatesBiomassfeedstocks

SugarsGlucoseFructoseXylose

ArabinoseLactoseSucroseStarch

Starch

Cellulose

Lignin

Oil

Protein

Hemicellulose

Figure 14 Chemical platforms for biobased products from biomass feedstocks (Werpy and Petersen, 2004).

478GlobalAgriculture:IndustrialFeedstocks

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Table 5 Building-block chemicals and products from sugars, syngas, and lignin (Werpy and Petersen, 2004; Holladay et al., 2007)

Source Chemical

Sugars/syngas

C1: Carbon monoxide (CO) and hydrogen (H2) (syngas)C3: Glycerol, 3 hydroxypropionic acid, lactic acid, malonic acid, propionic acid, and serineC4: Acetoin, aspartic-acid, fumaric acid, 3-hydroxbutyrolactone, malic-acid, succinic acid, and threonineC5: Arbinitol, furfural, glutamic acid, itaconic acid, levulinic acid, proline, xylitol, and xylonic acidC6: Aconitic acid, citric acid, 2,5 furan dicarboxylic acid, glucaric acid, lysine, levoglucosan, and sorbitol

Lignin Process heat and power, syngas, methanol, dimethyl ether (DME), ethanol, mixed alcohols, hydrocarbons (Fischer–Tropsch liquids), C1 –C7 hydrocarbon or oxygenates, alkylates (benzene, toluene, xylene, and higher), cyclohexane, styrenes, biphenyls, phenol, substitutedphenols, catchols, cresols resorcinols, eugenol, syringols, coniferols, guaiacols, vanillin, vanilic acid, DMSO, aromatic acids, aliphaticacids, syringaldhyde and aldehydes, quinones, cyclohexanol/al, beta-keto-adipate, carbon fiber, polyelectrolites, polymer alloys, fillers,polymer extenders, substituted lignins (carbonylated, ethoxylated, carboxylated, epoxidized, and esterified), thermoset resins,composites, formaldehyde-free adhesives and binders, wood preservatives, nutraceuticals and drugs, and mixed aromatic polyols

Global Agriculture: Industrial Feedstocks for Energy and Materials 479

utility in soils and as a means to sequester carbon (BIOCHEM,2010; Keiluweit et al., 2010; Woolf et al., 2010).

Biobased plastics (bioplastics) constitute the class ofmanufactured plastics that are either entirely or partiallycomposed of biologically derived materials, that is, biomass.Plastics certified as compostable frequently contain a highproportion of biobased materials but not all biobased poly-mers are biodegradable (BIOCHEM, 2010). Current bioplas-tics include the bulk materials polylactic acid (PLA),polyamides (nylon), polyhydroxyalkanoates, polybutylene-terephthalate and PET and polyethylene from among a largernumber of thermoplastics, and polyurethane from among thethermosetting materials. These are typically prepared fromsugars and starches or from processing residues (e.g., lignin).PLA is principally made from plant sugars polymerizedthrough a process of fermentation. Corn is a major source.Biobased succinic acid, a precursor to various bioplastics, canbe produced from glucose and sucrose sugars and is a re-placement for petroleum-based adipic acid (Chimirri et al.,2010; Liang et al., 2013).

Composite materials are standard products of the woodindustry (e.g., particleboard) but a wider variety of biomassfeedstocks are now being investigated for their application inthis market, including woody and herbaceous biomass such asEucalyptus, Athel (Tamarix aphylla), and Jose tall wheatgrass (A.elongatum) used also in phytoremediation of salt-affectedsoils (Pan et al., 2007; Zheng et al., 2006, 2007a). Particle-boards made from saline wood generally had equal or bettermechanical qualities compared with boards made from moreconventional nonsaline wood materials (Zheng et al., 2006,2007a).

Nanomaterials from biomass are also being investigated fora wide variety of applications. Low-cost carbon fiber pro-duction has been the subject of much interest especially in thewake of recent legislation to increase vehicle fuel economystandards in the US that add motivation to finding new light-weight materials with good strength and from sustainablesources. Toward this purpose, lignin has been investigated forits use in producing carbon fiber but is required to be of highpurity with a narrow molecular weight distribution, andgreater research is needed in the refining of this potentialfeedstock (Baker and Rials, 2013). Carbon nanofibers, nano-tubes, graphenes, and other materials have also been investi-gated for use in energy storage devices, particularly ultra- and

supercapacitors. Carbon nanosheets for use in high energydensity supercapacitors have been made from hemp bastfibers, yielding a maximum energy density of 12 Wh kg�1 and8–10 Wh kg�1 after charge times of less than 6 s, comparableto or greater than the conventional activated carbons (Wanget al., 2013b). The multilayered structure of the feedstockbiomass in this case provides a unique starting morphologyfor producing the carbon sheets.

Biobased lubricants and solvents are similarly classified asbeing comprised entirely or in part from biomass. Biobasedlubricants predate petroleum-based compounds and are eithervegetable oil-based or manufactured from modified oils in-cluding synthetic esters made from mineral oil-based products(BIOCHEM, 2010). Finished products can be biodegradablewith improved environmental properties and lower toxicityallowing use in more sensitive environments. Canola and anumber of other vegetable oils have been investigated asfeedstocks in the production of more environmentally benignmotor oils for engines (Johnson et al., 2002). Solvents are usedin many different applications, such as additives in paints,degreasing agents, pharmaceutical extractions, strippingagents, and cleaners. A key property of some biobased solventsis their low emission of volatile organic compounds that areboth direct health hazards and contribute to reduced airquality, for example, in reactions with oxides of nitrogen orNOx in the formation of tropospheric ozone, a breathinghazard and regulated air pollutant (U.S. EPA, 2013). The fattyacid methyl ester of soy oil made from soybeans, in additionto its use as biodiesel, is also a biosolvent, as are lactate esters,limonene, and other citrus terpenes. Although limonene (asD-, L-, and DL-limonene), for example, is listed as a generallyrecognized as safe synthetic flavoring substance by the USFood and Drug Administration (CFR, 2006), and high-purityproducts may generally prove of low allergenic properties,oxidized products (such as obtained by exposing to air forprolonged periods) have been shown to produce sensitivity inanimals (Karlberg et al., 1991) and safety properties need to becarefully considered in application.

Plant oils are also used in the production of surfactants –chemicals that lower the surface tension of liquids to allowbetter mixing. By definition, biosurfactants have at leastone of the hydrophobic or hydrophilic groups associated withthe surfactant molecule from a biobased source and so canalso be either entirely or partially derived from biomass.

480 Global Agriculture: Industrial Feedstocks for Energy and Materials

Feedstocks include palm oil, sorbitol, sucrose, glucose, andanimal fats such as tallow. Surfactants are common ingredi-ents in detergents but are used in a wide range of otherproducts including agricultural chemicals, textiles, paints, andlubricants (BIOCHEM, 2010), and nonionic surfactants havebeen employed in the enzymatic hydrolysis of biomass aspretreatment for fermentation and biofuel production (Zhenget al., 2007b).

Biochar or black carbon from biomass has received muchattention in recent years for its perceived utility in maintainingsoil carbon stocks and soil fertility. Although carbon producedfrom biomass in various stages of activation has long servedfor water treatment and environmental remediation, the dis-covery of the terra preta do índio soils in the Amazon stimulatednew research related to understanding the properties of car-bons for soil applications including chars produced in con-junction with energy conversion, especially biomass pyrolysisbut also thermal gasification. The potential new market inbiocarbons is fortuitous also for thermochemical energy con-version systems where char coproducts have in the past poseda disposal problem given the limited capacity of the activatedcarbon markets. The terra preta soils, which originate from pre-Columbian times and accumulated charcoal, bone, and ma-nure as part of kitchen middens from food preparation andother activities, have been shown to have lower greenhouse gasemissions (as CO2) per unit of carbon in the soil comparedwith similar soils with low or no black carbon (Liang et al.,2010). In a study of black carbon from 12 different biomassfeedstocks including hardwoods, softwoods, algal digestate,walnut shell, and turkey litter, char surface area, an importantmeasure of sorptive properties, increased with charring tem-perature whereas chemical functionality could be separatedinto wood and nonwood feedstock groups (Mukome et al.,2013). The presence of carbonate and chloride salts increasedthe basicity of chars from nonwood feedstock. Agronomicproperties and impacts can be related to feedstock com-position (Mukome et al., 2013). Biochar applications to soilsmay also reduce the overall lifecycle greenhouse gas emissionsfor bioenergy systems (Gaunt and Lehmann, 2008).

Energy and materials from biomass frequently compete forcommon resources, hence in addition to lifecycle environ-mental concerns, debate also arises over the highest and thebest use of biomass. Policies favoring one use over another canintroduce economic distortions, as in the food-versus-fueldebate where both sectors may receive government incentivesor subsidies but without substantial coordination aimed atimproving the overall sustainability. Given the limited sup-plies of biomass, the potential to extract multiple productsthrough integrated biorefining (cascading the biomass use)has been suggested as a way to maximize the value of theresource (Keegan et al., 2013). Such practice already exists inboth the forestry and agriculture sectors, for example, in pulpand paper manufacturing and sugar milling in which high-value materials are extracted before processing residues (e.g.,lignin from wood or bagasse from sugarcane) are utilized forenergy conversion. As energy values increase, however, eco-nomic distinctions tend to disappear in many cases, shiftingfocus onto other performance metrics as indicators of socialgood. Greenhouse gas emissions may be reduced through theproduction of biomaterials, many of which can sequester

renewable carbon over long periods of time in addition todisplacing fossil emissions from petroleum or other non-renewable resources. Significant opportunities exist to improvematerial flows through the society to meet both material andenergy needs. Just as integrated technologies may improve theeconomic benefits for energy and materials from biomass,policies that address system-level resource management effectsmay realize a greater social benefit. Uncertainties remain in theassessment of net benefits for both materials and energyconversion from agricultural feedstocks and distinct needsexist for improved information.

Energy from Biomass

The potential for biomass to supply much larger amounts ofuseful energy has stimulated substantial research and devel-opment into systems to convert biomass into the principalforms of heat, electricity, and solid, liquid, and gaseous fuels(Bungay, 1981; Hall and Overend, 1987; Kitani and Hall,1989; Klass, 1998; Brown, 2003). Although substantial debatecontinues over the sustainability of large-scale bioenergy de-velopment, so do advances continue across a wide range ofconversion technologies and systems to improve the overallefficiencies and fuel yields, fuel properties, lifecycle costs, andenvironmental performance.

Three principal routes exist for converting biomass to en-ergy: (1) thermochemical, (2) biochemical, and (3) physico-chemical. In practice, combinations of two or more of theseroutes may be used in the generation of the final product orproducts. Chemical or biological catalysts are employed inmany cases. Thermochemical conversion includes com-bustion, thermal gasification, and pyrolysis along with anumber of variants involving microwave, plasma arc, super-critical fluid, hydrothermal, and other processing techniques(Brown, 2011). Products include heat, fuel gases, liquids, andsolids. Thermochemical techniques tend to be of high rate andrelatively nonselective for individual biomass components inthat the chemically complex biomass is substantially degradedinto simple compounds (e.g., CO, CO2, H2, and H2O), whichmay later be reassembled into more complex compoundsthrough either chemical or biological processing (Chum andBaizer, 1985). Biochemical conversion includes fermentationto produce alcohols (ethanol being perhaps the best known),fuel gases (such as methane by anaerobic digestion), acids, andother chemicals (Klass, 1998). Among the physicochemicalmethods are alkaline and acid processes, esterification, mech-anical milling, steam and ammonia freeze explosion and otherexplosive decompression processes, and pressing and extru-sion, many times in combination with a biochemical orthermochemical reaction process. One major process underthis category is vegetable oil extraction and esterification tomanufacture biodiesel as a substitute for engine fuel. Bio-chemical and physicochemical processes in general are in-tended to be more selective for biomass components andhigher value products, although thermochemical routes canalso be used, as in the indirect production of methanol viathermal gasification (Hos and Groeneveld, 1987). There areother means of producing useful energy from biomass, such asbiophotolysis for the production of hydrogen by plants

Global Agriculture: Industrial Feedstocks for Energy and Materials 481

(Bungay, 1981), and other direct hydrogen routes (Cortrightet al., 2002), but these are not yet demonstrated on a com-mercial scale. Ethanol is already produced on a large scale as abiofuel, mostly for light-duty automotive use including as anoxygenate blendstock for motor gasoline as well as in higherconcentrations for flexible fuel (FlexFuel) and neat (100%)fuel vehicles. Hydrocarbon liquids from biomass are of par-ticular interest due to their similarity to current gasoline, die-sel, and aviation fuels and their fungibility in existing fueltransportation and distribution systems such as pipelines.These and other so-called drop-in biofuels avoid the need fornew infrastructure or the use of more expensive truck transportfor large-scale application.

Thermochemical routes generally involve the heating andthermal decomposition of feedstock biomass to compoundsthat can either be directly used or reacted to produce otherand potentially more valuable chemicals and fuels. Gasifi-cation and pyrolysis are thermochemical approaches that in-volve thermolysis of the feedstock to intermediate gases (e.g.,CO and H2 as synthesis gas or syngas) and liquids (e.g.,biocrude) that can be burned directly, chemically synthesizedinto drop-in liquid fuels along with a wide variety of otherfuels including methane, hydrogen, methanol, and dimethy-lether, or fermented biochemically to alcohols. These thermalprocesses can also produce biochar (see the discussion onmaterials from biomass above). Torrefaction is a light pyr-olysis that typically is carried out as a pretreatment at tem-peratures between approximately 200 and 300 1C to producea carbonized feedstock or biochar with improved grindability,densification, and other handling characteristics. Biocharproduction can add coproduct value to gasification, pyrolysis,and other thermochemical conversion processes althoughthere remain many uncertainties regarding its value for thesepurposes. Intermediates such as biocrude produced throughpyrolysis and lipids extracted from algae or oil seeds(vegetable oils) can be hydrotreated through the addition ofhydrogen for upgrading to renewable diesels and otherhydrocarbons. Combustion of the feedstock generates heatwhich can then be used in the generation of electricity orprocess steam and hot water, and is historically the largestenergy use of biomass.

Biochemical routes, predominantly fermentation, relyprincipally on microbes (yeasts, bacteria, or fungi) for theconversion to useful fuels and chemicals. In most instances,hydrolysis of structural polymers (starch, hemicellulose, andcellulose, see above) through enzymatic or chemical means,possibly in combination with thermal or mechanical pre-treatment, is necessary to generate the sugars or other simplercompounds upon which the microbes feed. Starch and sugarfermentation by yeast is well known and has long been used inmaking beverage alcohol (ethanol) and is currently used inproducing most of the world’s fuel alcohol from corn (maize)and sugarcane. The use of lignocellulosic biomass is less welldeveloped and the subject of much research directed at im-proving hydrolysis, particularly enzymatic hydrolysis, of boththe hemicellulose and cellulose fractions and simultaneouscofermentation of the resulting five and six-carbon sugars forwhich recombinant organisms have also been developed towork in association with or in place of yeast. Lignin is mostlyrecalcitrant to biological conversion and is often considered

for use as boiler fuel in making process heat or steam (such asthat used in distilling the alcohol) and electricity for thebiorefinery but higher economic value may reside in fuelproducts if effective conversion techniques can be developed,for example, gasification to syngas for subsequent catalyticconversion to substitute natural gas or liquid fuels. Anaerobicdigestion is another type of fermentation that typically usesmixed populations of bacteria to digest biomass to makebiogas, a multicomponent gas consisting mostly of methane(CH4, the major component of natural gas) and CO2 withother more minor species including hydrogen sulfide (H2S).Sulfide removal is typically needed before the use of biogas inengines especially those employing catalytic aftertreatment ofcombustion products for control of oxides of nitrogen (NOx)and other air pollutants. Anaerobic digestion is commonlyused in waste water treatment facilities and is becoming morecommon for managing animal wastes in concentrated animalfeeding operations such as dairies and hog and poultry farms.Biogas is used as engine and boiler fuel, sometimes in com-bined heat and power (CHP) operations, such as for cheesemaking at the dairy. Biogas can be upgraded to pipelinequality methane to complement natural gas or for use as ve-hicle fuel as compressed natural gas (CNG) or liquefied nat-ural gas (LNG), and can serve as chemical feedstock. Biogashas also been used in microturbines and for fuel cells fordistributed power applications. Landfill gas is a biogas that isproduced by the anaerobic decomposition of organics in solidwaste landfills and is recovered for electricity generation andother energy purposes and to reduce hazardous migration ofthe gas into nearby structures.

Physicochemical processes are used in the making of bio-diesel, a product of the catalyzed reaction between an oil, suchas the vegetable oil, and an alcohol, such as methanol orethanol, all of which can be produced from biomass. In theconventional biodiesel process, triacylglycerides that make upvegetable oils and other FOG are converted by transester-ification into long-chain esters and coproduct glycerol. Theesters (biodiesel) have lower viscosity than the originalvegetable oil and have better injection and spray properties foruse in diesel engines. Biodiesel can also be produced usingenzymatic techniques. Coproduct markets for glycerol exist insoaps, cosmetics, and many other products but the potentiallylarge production of fuel could saturate these markets so thatadditional uses are being investigated, including production ofother fuels.

Industrial facilities to produce fuels and chemicals frombiomass are typically classified as biorefineries. Integratedbiorefineries can include multiple processing operations inmaking a suite of products from the same production facility,similar to modern petroleum refineries (Figure 15). Biofuelsare often classified as first generation, second generation, andbeyond, the terms intended to convey the development statusand to some extent the perceived sustainability of the pro-duction systems although there are not always clear dis-tinctions. First-generation biofuels include ethanol from starchand sugar as well as biodiesel, both mostly from food crops,but may also include biogas, syngas, and pyrolytic biooil.Second-generation biofuels include ethanol, hydrocarbons,and other fuels produced from lignocellulosic and algal feed-stocks. Later generations are associated with more advanced

Bioethanol

Electricityand heat

Bio-H2FertilizerLink among biorefinery pathways

Materialproducts

Energy products

Platform

Feedstock

Legend

Biogas

H2

Chemicalprocess

Mechanical/physical process

Biochemicalprocesses

Thermochemicalprocess

Biomethane Biomaterials Chemicals andbuilding blocks

Synthetic biofuels(FT, DME...)

Polymers andresins

Organic residuesand others

Grasses

Separation

LigninFiber

separation

Anaerobicdigestion

Organicjuice

Syngas

C6 sugars C5 sugars

Starchcrops

Sugarcrops

Lignocellulosiccrops

Lignocellulosicresidues

Oil cropsMarine

biomassOil-basedresidues

Oil

Pyrolyticliquid

Food Animalfeed

BiodieselElectricityand heat

Glycerin

Figure 15 Partial classification system for biorefining options (Cherubini et al., 2009).

482 Global Agriculture: Industrial Feedstocks for Energy and Materials

production and processing and improved lifecycle perform-ance. Third-generation biofuels are based on feedstocksmodified to improve processing, such as reduced lignin orincreased oil content. Fourth-generation technologies involveboth improved feedstocks and improved processing and con-version techniques, including genetically modified micro-organisms, that result in net negative carbon impacts as anoutcome of the biofuel production system (Lu et al., 2011).

The conversion strategies are integrally coupled to theproperties of the biomass. Moisture has often been used as adiscriminating parameter in the selection between thermo-chemical and biochemical processes, although by itself this isnot usually sufficient. In many cases, the properties of thebiomass necessary for engineering design have not beenproperly characterized before commercial implementation of atechnology. Operation of combustion-type power-generatingstations intended to burn certain types of agricultural residues(in particular cereal straws and other herbaceous biomass) hasbeen hindered by fouling of boiler heat transfer surfaces fromthe inorganic compounds in the biomass (Turnbull, 1991;Wiltsee, 2000; Jenkins et al., 1998, 2011), a phenomenon thatwas largely overlooked in the design phase of many facilities,but is well known for coal (Baxter, 1992). Effective biomassenergy development demands a continuing program in re-source and technology development.

Thermochemical Conversion

CombustionHistorically, and still so today, the most widely applied energyconversion method for biomass is combustion. The chemicalenergy of the fuel is converted into heat energy. Heat can beused directly and also transformed by heat engines of all sortsinto mechanical and electrical energy. Theoretically, biomasscould be directly converted into electricity by magnetohydro-dynamic power generation in the same way coal might be, butthe technology is still mostly speculative (Rosa, 1961; Mikheevet al., 2006). Burning of wood and agricultural materials inopen fires and simple stoves for cooking and space heating iscommon around the world and a vital source of heat, al-though less desirable than advanced conversion techniquesfrom the perspective of atmospheric pollution and unduehealth impacts from incomplete combustion (Jenkins et al.,2011). Electricity generation using biomass fuels has receivedconsiderable attention in recent years, although sugarcanebagasse has long been used as fuel for electricity generation atsugar mills and is now an integral component of biofuelproduction from sugarcane with surplus power exported fordistribution. Wood also has been widely used for cogenerationof heat and electricity in sawmills, with heat and steam gen-eration being the primary function for kiln drying of lumber.

Global Agriculture: Industrial Feedstocks for Energy and Materials 483

A large independent power-generation industry in the USdeveloped in response to incentives provided directly andindirectly by federal legislation, with approximately 10 GWe ofelectrical-generating capacity from biomass currently operatingin the United States alone (EIA, 2013b). CHP facilities usingwood, straw, and municipal solid wastes are operating inEurope and elsewhere for electricity generation and districtheating (COGEN Europe, 2011). Additional incentives exist inthe form of renewable portfolio standards (RPS) such as thatin California that now calls for a third of retail electricity salesto come from renewable resources by the year 2020. Totalinstalled capacity in biomass power generation around theworld is approaching 50 000 MWe including large-scale solidfuel combustion as well as small-scale digester and landfill gasapplications (IEA, 2007). In many regions of the world, Asiabeing an exception, biomass utilization is below the sustain-able resource capacity and potential exists to increase uses forfuels, heat, and power (Parikka, 2004).

Viewed simply, the complete combustion of biomass in airtransforms the organic fraction into carbon dioxide and water:

CxHyOz þ n1H2Oþ n2 1þ eð ÞðO2 þ 3:76N2Þ ¼ n3CO2

þ n4H2Oþ n5N2 þ n6O2 ½4�

where the stoichiometric coefficients n depend on the con-centrations of carbon, hydrogen, and oxygen in the originalfeedstock (here expressed on a moisture-free, ash-free basis),and on the amount of excess oxygen, e, added to the reaction.Feedstock moisture is accounted for in the reaction by thecoefficient n1, and air is approximated in the normal engin-eering fashion as consisting of 21% by volume oxygen and79% equivalent nitrogen.

A more sophisticated global combustion reaction for bio-mass that better accounts for the range of species present isrepresented by the mass balance of eqn [5] in which the masscoefficients mr,i designate the masses of the reactant species andmp,j designate the product species. In this case, the biomass isdivided into three fractions: an organic phase, a moisture phase,and an ash phase. The feedstock elemental mass fractions de-scribed by the coefficients yi arise from the analysis of the dryfeedstock for C, H, O, N, S, and Cl. Moisture in biomass isrepresented by a separate water fraction (free and boundmoisture). The oxidation medium, air, for example, is con-sidered to consist of O2, CO2, H2O, and N2, all in the gas phase.The reaction is represented as resulting in eleven main gas phaseproducts and a residual mass containing ash and unreactedportions of the other element masses from the biomass. Theresidual can consist of solids such as particulate matter, carbonin ash as charcoal or carbonates, and chlorides and sulfates infurnace deposits. Equation [5] is reasonably general in that itcan equally be used to describe the combustion of other fuelssuch as (bio)methane, biodiesel, Fischer–Tropsch liquids, andother hydrocarbons, biooils from pyrolysis, and many others:

mr,1 ∑iyi

�����C,H,O,N,S,Cl,ash

!þmr,2H2OðlÞ þ :::

:::þmr,3O2 þmr,4CO2 þmr,5H2OðgÞ þmr,6N2 ¼ ::::::¼mp,1CH4 þmp,2COþmp,3CO2 þmp,4H2 þmp,5H2Oþmp,6HClþ ::: :::þmp,7N2 þmp,8NOþmp,9NO2

þmp,10O2 þmp,11SO2 þmresidual ½5�

In addition to the product composition, eqn [5] allows forthe estimation of other properties of the reaction, such as theflame temperature under different burning conditions, animportant parameter for estimating heat transfer, efficiency,and thermal performance.

Equation [5] also specifies the oxidant-to-fuel ratio, or inthe case of air as the source of oxygen, the air–fuel ratio, alongwith the equivalence ratios often used to specify combustionconditions. The air–fuel ratio, AF, defines the mass of airadded relative to the mass of feedstock, expressed on either awet or dry basis. The stoichiometric value, AFs, defines thespecial case in which only the amount of air theoreticallyneeded to completely burn the feedstock is added to the re-action. The air–fuel ratio, the fuel–air equivalence ratio, ϕ, theair–fuel equivalence ratio or air-factor, λ, and the excess air, e,are all related as follows:

ϕ¼ AFsAF

λ¼ 1ϕ

e¼ λ� 1 ½6�

The combustion regimes are defined by the value of ϕ withϕ¼1 (e¼0) being the stoichiometric combustion, ϕ41 thefuel-rich regime (insufficient air), and ϕo1 the fuel-lean re-gime (excess air). The equivalence ratio is particularly im-portant in characterizing the potential air pollutant emissionsfrom a combustion system. For fuel-rich conditions, concen-trations increase for products of incomplete combustion suchas hydrocarbons, CO, and particulate matter. NOx emissionsmay peak under slightly fuel-lean conditions due to high flametemperatures and reaction of nitrogen and oxygen in air (theso-called thermal NOx). NOx is also produced from nitrogenin the feedstock (fuel NOx). Other pollutant species are alsoproduced in varying amounts under all three combustion re-gimes but are not specifically included in eqn [5].

Fuel moisture is a limiting factor in biomass combustion.The autothermal limit for most biomass fuels is approximately65% moisture content wet basis (mass of water per mass ofmoist fuel). Above this point, insufficient heat is liberated bycombustion to satisfy the energy needs for evaporation andproduct heating. Practically, most combustors require a sup-plemental fuel, such as natural gas, when burning biomass inexcess of 55% moisture wet basis. In the US, Federal EnergyRegulatory Commission regulations developed after the en-actment of the Public Utilities Regulatory Policies Act in 1978permit up to 25% of the power plant energy input to comefrom natural gas or other fossil fuel (depending on air per-mits) for qualifying biomass cogenerators. Many independentpower producers and biomass facilities employ cofiring forstartup and flame stabilization.

The most common type of biomass-fueled power planttoday utilizes the conventional Rankine or steam cycle, as il-lustrated in Figure 16. The fuel is burned in a boiler, whichconsists of a furnace and one or more heat exchangers to makesuperheated steam. Typical medium efficiency units utilizesteam temperatures and pressures of approximately 500 1Cand 6 MPa. The steam is expanded through one or more tur-bines that drive an electrical generator. The steam from theturbine exhaust is condensed, and the water recirculated to theboiler through the feedwater pumps. Combustion productsexit the combustor, are cleaned, and vented to the atmosphere.

Turbine

Stack

Emission control

Flue gas

Flyash Bottom ash Coolant

Boiler

Generator Electricity

Air

Superheated steam

Fuel

Water PumpCondenser

Figure 16 Schematic of a Rankine or steam cycle power plant.

484 Global Agriculture: Industrial Feedstocks for Energy and Materials

Typical cleaning devices include wet or dry scrubbers for sulfurand chloride control, cyclones or other inertial separationdevices, baghouses (filters) or electrostatic precipitators forparticulate matter removal, and selective catalytic or selectivenoncatalytic reduction of NOx. Low CO and hydrocarbonemissions are maintained primarily by proper control of air–fuel ratio in the furnace and boiler.

Ash fouling on the superheaters, the heat exchangers usedto increase steam temperature above its saturation temperatureand that are commonly located in the hottest parts of thefurnace, has been a severe problem in many biomass-fueledboilers. Pretreatment of the feedstock to remove much of thealkali and chloride before firing to the boiler has provedbeneficial in reducing fouling (Jenkins et al., 1998, 2011).Coproduct bottom and fly ash can be used for industrialmaterials or land applied as fertilizers, potentially from thesame fields that produced the feedstock. Ash from municipalsolid waste incinerators, which commonly employ additionalcontrol of chlorides, mercury, and other hazardous products,may require hazardous waste disposal.

Individual power plants using biomass fuels in the mannerdescribed here typically range up to approximately 50 MWelectrical capacity, which in the US is sufficient to supply theneeds of from 25 000 to 50 000 people. Larger sizes are pos-sible, and size selection is accomplished through an analysis offuel resource availability, plant economy, and local regulationsas described earlier under feedstock logistics. The distributednature of biomass fuels and the limited economy of scale as-sociated with plants of this type have kept the size of indi-vidual facilities relatively small in comparison with coal,petroleum, or nuclear-fueled power-generating stations. Thelargest biomass-fueled power station is a 240 MWe unit inFinland that operates at 545 1C and 16.5 MPa and producesprocess steam and hot water for district heating, but this unitcan also accept peat and coal as fuel (Alholmens Kraft, 2013).

A larger project in the UK (750 MWe) based on replacing coalwith biomass was recently abandoned for financial and feed-stock supply reasons, the latter based on uncertainties aboutlocal supplies from the region (Ross, 2013). Conversion ofcoal-fired facilities to biomass continues to be considered,however, in order to help meet the standards for reducedgreenhouse gas emissions.

The efficiencies of biomass power plants are generallylower than comparable fossil-fueled units because of higherfuel moisture content, lower steam temperatures and pressures(due to the need to control fouling at higher combustion gastemperatures), and to some extent the smaller sizes, with aproportionately higher parasitic power demand for pumps,fans, and other electrical devices associated with the powerplant itself. Large fossil-fueled units normally incur approxi-mately 3% parasitic demand but with biomass plants the de-mand is more commonly approximately 10%. Currently,biomass power plant efficiencies are in the range of 17–28%,compared with good single-cycle fossil-fueled units ranging upto 40% overall efficiency. Gas and distillate (diesel) firedcombined cycle power plants have efficiencies ranging morethan 50% overall. Biomass integrated combined cycles areprojected to exceed 35% electrical efficiency. Cofiring of bio-mass at 10–15% of energy input in higher capacity, higherefficiency fossil (e.g., coal) stations can also lead to a higherefficiency in biomass conversion. Increasing the power gener-ation efficiency is a major goal for advanced biomass designs(Jenkins et al., 2011).

Gasification and pyrolysisOn heating, biomass fuels will decompose into a number ofgaseous and condensable species, leaving behind a solid car-bonaceous residue known as char. This is an early stage ofcombustion, and the luminous flame seen when burningwood and other biomass is a result of the oxidation of volatile

Global Agriculture: Industrial Feedstocks for Energy and Materials 485

compounds emitted during pyrolysis and gasification of thefeedstock and thermal radiation from soot particles from theflame giving a characteristic yellow color.

When the fuel–air equivalence ratio, ϕ, of eqn [6] is sub-stantially greater than unity (fuel-rich), the fuel will be onlypartially oxidized due to the insufficient oxygen, and the re-action products will consist not only of carbon dioxide andwater, but of large amounts of carbon monoxide and hydro-gen in addition to variable amounts of gaseous hydrocarbonsand condensable compounds (tars and oils), along with charand ash. Other oxidants, including steam, can also be usedinstead of air, in which case the reaction product suite willdiffer. Reaction conditions can be varied to maximize theproduction of fuel gases, fuel liquids, or char (as for charcoal),depending on the intended energy market or markets. Theterm gasification is applied to processes that are optimized forfuel gas production (principally CO, H2, and light hydro-carbons). Under heating alone without the addition of anoxidant the feedstock will pyrolyze. Pyrolysis reactors aretypically designed to maximize the production of liquidsthrough fast rather than slow heating although increasinginterest in biochar or black carbon is now shifting the preferredproduct mix. Catalysts are sometimes employed to promotevarious reactions, especially the cracking of high molecularweight hydrocarbons produced during gasification and also inchemical catalytic synthesis of liquid hydrocarbons and otherproducts in making transportation biofuels.

Gasification technology was developed more than 200years ago (Kaupp and Goss, 1984), and lately has been im-proved primarily for the purposes of providing solid fuels(biomass, coal, and coke) access to some of the same com-mercial markets as natural gas and petroleum. Gasifiers havelong been used to convert solid fuels into fuel gases for op-erating internal combustion engines, both spark ignited (gas-oline) and compression ignited (diesels). They can also beused for external combustion devices, such as boilers andStirling engines. The most common types are the direct gasi-fiers, where the partial oxidation of the feedstock in the fuelbed provides the heat for pyrolysis and gasification reactions,which are mostly endothermic. Indirect gasifiers and pyrolysisreactors use external heat transfer to provide the heat necessaryto pyrolyze the fuel. The heat may be produced by the com-bustion of some of the original biomass fuel, or by the com-bustion of output fuel gases, liquids, or char. Allothermalreactors have been developed to supply heat by internal butseparate burning of the char phase following gasification of thefeedstock, mostly in dual reactor systems (Wilk and Hofbauer,2013). Gasifiers may have less difficulty with ash slaggingbecause of the lower operating temperatures compared withcombustors, although slagging, fouling, and bed agglomer-ation remain problems with some fuels (e.g., straw).

When direct gasifiers are supplied with air to react thefeedstock, the fuel gases will contain large amounts of nitrogenand the heating value or energy content of the gas will be low(3–6 MJ m�3) in comparison to natural gas (compare me-thane at 36.1 MJ m�3) and other more conventional fuels.Nonsupercharged engines operating on such gas will be de-rated in power output relative to their operation on gasoline ordiesel (Jenkins and Goss, 1988). In the case of diesel engines,the gas cannot be used alone, and pilot amounts of diesel fuel

are injected to provide proper ignition and timing. For spark-ignited engines, the engine power output is about half that ofthe same engine on gasoline, because the air capacity of theengine (the amount of air drawn into the engine cylinderduring the intake stroke) is reduced by the large volume oc-cupied by the fuel gas, and so not as much fuel can be burnedduring each cycle (Jenkins and Goss, 1988). Supercharging theengine can overcome this in part. For dual-fuel diesel engines,the gas can generally supply up to 70% of the total fuel energywithout encountering severe knock, which results from thelong ignition delay associated with the producer gas, thesame property which gives the gas excellent octane rating(Chancellor, 1980; Ogunlowo et al., 1981). The same prop-erties of producer gas which lead to late ignition and knock ina diesel engine make it quite knock resistant in a spark-ignitedengine, so compression ratios well above 10 can be used. Withproper design of the cylinder head at increased compressionratio, the engine efficiency can be improved over gasoline,offsetting some of the derating due to reduced air capacity.

If the gasification reactor uses enriched or pure oxygen, thefuel gas, or syngas, produced is of higher quality. The cost ofproducing the oxygen is high, however, and such systems aregenerally proposed for larger scales or for producing highervalue commodities, such as chemicals and liquid fuels.Methanol, a liquid alcohol fuel, CH3OH, is produced by thecatalytic reaction

COþ 2H2 ¼CH3OH ½7�This reaction is favored by low temperature (400 1C) but byhigh pressure (30–38 MPa). Zinc oxide and chromic oxide arethe common catalysts. With copper as the catalyst, the reactiontemperature and pressure can be reduced (260 1C, 5 MPa), butcopper is sensitive to sulfur poisoning and requires good gasscrubbing (Probstein and Hicks, 1982). Fischer–Tropsch re-actions can be utilized to produce a spectrum of chemicalsincluding alcohols and aliphatic hydrocarbons. Temperatureand pressure requirements are reduced, and greater selectivitycan be obtained by the choice of catalyst.

Liquids, such as gasolines, can be produced via indirectroutes involving the gasification or pyrolysis of the solid bio-mass to produce reactive intermediates that can be catalyticallyupgraded (Kuester et al., 1985; Prasad and Kuester, 1988;Kuester, 1991; Brown, 2011). Liquids produced directly bypyrolysis are usually corrosive, suffer from oxidative instabil-ity, and cannot be directly used as engine fuels. Many productsare also carcinogenic. Refining of some sort is generally ne-cessary to produce marketable compounds. Despite this, fastpyrolysis reactors employing biomass and other fuels are incommercial startup to produce biooils (Ensyn Corp, 2014).Liquid fuels can also be produced by direct thermochemicalroutes, such as by hydrogenation in a solvent with a catalystpresent (Elliott et al., 1991; Bridgwater and Bridge, 1991).

One of the principal technical hurdles, especially at thesmall scale, in applying gasifiers for applications other thandirect burning of the raw gas is gas cleaning and purifica-tion. Removal of particulate matter and tars from the gas iscritical in downstream power generation and fuel synthesis.Tars comprise a class of heavy organic materials that are par-ticularly difficult to remove or treat. Systems are available toproduce acceptable gas quality, but generally rely on some

486 Global Agriculture: Industrial Feedstocks for Energy and Materials

combination of wet and dry scrubbing and filtering and addexpense to the conversion system. Small-scale gasifiers used forremote power generation have often been deployed withoutadequate handling procedures for tar separated from the gas.Gas cleaning and tar handling remain critical engineeringchallenges for a wider adoption of the technology at all scales.

Advanced power generation options from biomass includethe use of a biomass gasifier to produce fuel gas for a gasturbine in an integrated gasification combined cycle system(Figure 17; Meerman et al., 2013). The efficiency of thesesystems could be considerably higher than the conventionalRankine cycle power generation systems. Major engineeringchallenges include hot gas cleaning to provide a gas of

Compressor

Gaspurification Producer

gas orsyngas

Crude gas

Gasifier

Fuel

Tar/impurities

Ash/char

Air

Stac

Emission control

Flue gas

Heat

Bottomingcycle

Recovery/disposal

Recovery

GeneratorSteam

Burner

Pressurizedair

Superste

Exhga

Water

Figure 17 Integrated gasification combined cycle advanced power generatioturbine option is also shown (IG/STIG).

adequate quality to avoid fouling the turbine, and the devel-opment of reliable high pressure reactors or compressors andfuel feeding systems. The use of a gasifier is thought to be anadvantage over a direct combustor because heat loss in the gascleaning system is of less concern, most of the fuel energybeing in the form of chemical energy in the product gas. Otheradvantages for gasifiers over combustors include the ability tooperate at lower temperatures, and lower gas volumes per unitof feedstock converted, which assists in the removal of sulfurand nitrogen compounds to reduce pollutant emissions. Sys-tems of this type are currently under development and severallarge-scale demonstration projects have been completed, butthe technology has not yet been deployed commercially for

k

Coolant

Gasturbine

Generator Electricity

Toppingcycle

Steaminjection

Generator ElectricitySteamturbine

heatedam

austs

PumpCondenser

n concept. Pressurized air gasification shown. A steam-injected gas

Global Agriculture: Industrial Feedstocks for Energy and Materials 487

biomass although it has for coal at larger scales (Stahl andNeergaard, 1998). Figure 17 also illustrates the possible use ofsteam injection for reducing thermal NOx emissions and en-hancing the power output of the gas turbine. The high heatcapacity of the steam in comparison to the combustionproducts leads to a power increase, and the addition of steamreduces the flame temperature, which is beneficial for reducingthermal NOx formation (Weston, 1992). Many other ther-mochemical conversion options are under development(Brown, 2011).

Biochemical Conversion

Biochemical conversion relies primarily on the abilities ofmicroorganisms to convert biomass components into usefulliquids and gases. Yeast fermentation is the principal means ofproducing ethanol. Biogas is a product of bacterial fermen-tation in anaerobic digestion, although bacteria are also usedin alcohol fermentation. Fermentation is a widely used in-dustrial process and an active area of biotechnology researchand development.

FermentationEthanol (C2H6O) is widely produced by fermentation and isthe predominant liquid fuel derived by biochemical meansfrom biomass. In the United States, ethanol is currently usedas a oxygenate blending agent in gasoline to help reduce airpollutant emissions from vehicles as well as a major fuelproduct in the form of E85, an ethanol–gasoline blend con-taining nominally 85% ethanol and 15% gasoline althoughactual blends may range as low as 70% ethanol. In Brazil,ethanol is used both as a pure or ‘neat’ (100% ethanol) fuel,and as a blending agent with gasoline at 20% concentration(E20). The primary feedstock for ethanol production in theUnited States is corn (maize) grain as a source of readilyfermentable starch but sorghum and other agricultural feed-stocks are used as well. In Brazil it is sugarcane as a source ofsugar. The yeast Saccharomyces cerevisiae has been the mostwidely used organism for ethanol fermentation (Sen, 1989).The fermentation process involves the necessary pretreatmentof the feedstock to produce a fermentable substrate (sugar),fermentation, and product separation.

Corn biorefineries producing ethanol are of two types: wetmills and dry mills. Wet mills extract multiple products fromthe grain including oil, fiber, gluten, and starch, with the starchfraction used for fermentation. Dry mills (Figure 18) use theentire corn kernel for fermentation. The grain is not directlyfermentable, but requires milling, hydration and gelatin-ization, and enzymatic or acid hydrolysis of the starch tofermentable sugars before the actual fermentation. Theoreticalyields from glucose (C6H12O6) are 51% ethanol and 49%carbon dioxide (mass basis). The overall reaction for the fer-mentation of glucose to ethanol is

C6H12O6 ¼ 2C2H6Oþ 2CO2 ½8�

The theoretical yields from starch and sucrose are somewhathigher. Actual yields from fermentation are lower due toproduction of the microbial cell biomass and incompleteconversion of the substrate.

Ethanol concentrations reach approximately 10% in thefermentation beer before separation; higher concentrationsinhibit the microbial activity. The ethanol is typically separ-ated by steam distillation to 95% concentration, the azeo-tropic point at which water and ethanol evaporate to yield thesame concentration in the vapor as in the liquid and no furtherseparation by distillation is possible. The distilled product isdehydrated using solvents or other desiccants to produce an-hydrous ethanol. A high protein stillage is produced and isused as animal feed (distillers grains). Beverage grade alcoholincludes removal of fusel oils during distillation. For fuel gradealcohol, this step is usually eliminated.

Cellulosic feedstocks are more difficult to hydrolyze intomonosaccharides for fermentation, thereby incurring morecostly pretreatment. Current methods under development forcellulosic biomass hydrolysis and fermentation essentiallyfall into four categories: (1) concentrated acid hydrolysis(Figure 19), (2) dilute acid hydrolysis, (3) enzymatic hy-drolysis (Figure 20), and (4) thermochemical conversion(gasification and pyrolysis) followed by fermentation of syn-thesis gases, a process that attempts to circumvent problemsassociated with hydrolysis (Skidmore et al., 2013). Concen-trated acid processes have long been used to produce ethanolfrom cellulosic materials, but for economic and environmentalreasons in modern applications require substantial recoveryand recycling of the acid, adding expense. Enzymatic hy-drolysis is widely investigated in an effort to improve yieldsand reduce costs. Enzymatic processes developed followingresearch during World War II to control deterioration ofmilitary clothing and equipment. This research identified thefungal organism Trichoderma viride, later renamed Trichodermareesei, important to the development of cellulose enzymes(cellulase) for decrystallization and hydrolysis of cellulose.Genetically modified and recombinant organisms have beendeveloped to enhance both the hydrolysis and fermentation ofcellulosic materials (Luli et al., 2008; Singh, 2010). The cost ofenzyme production remains high, however, and major re-search investigations continue to explore means of reducingthe cost. Alternative techniques are under development toproduce sugar aldonates as the reactive intermediates ratherthan sugars to replace the pretreatment, cellulase production,and enzymatic hydrolysis processes with a single biologicalstep (Fan et al., 2012).

Cellulose hydrolysis yields hexoses, primarily glucose,fructose, mannose, and galactose. Hemicellulose hydrolysisyields mostly pentoses, principally xylose and arabinose. Lig-nin is currently unfermentable in any commercial application.The sugars are fermented to ethanol whereas lignin is mostlyconsidered as boiler fuel (e.g., in steam production for distil-lation and power generation) or for conversion to otherproducts, such as aryl ethers (Salanitro, 1995; Sergeev andHartwig, 2011; Parthasarathi et al., 2011). Sequestering oflignin has also been proposed to reduce net greenhouse gasemissions from biochemical conversion (Murphy, 2013).

Energy balances associated with fermentation of lig-nocellulosic materials are thought to be improved relative tothose for corn grain, although commercial data are not yetsubstantially available. For lignocellulosic feedstocks, overallethanol yields are likely to be in the range of 300 l Mg�1 of dryfeedstock compared with 400 l Mg�1 for corn grain. The ideal

Corn grain

Grind

Cook(starch solubilization)

Water

Cool

Cool

Fermentation

Beer still

Beer

Yeast

Steam

Steam

Water

Solventstripper

Anhydrous ethanol

Anhydroustower

Steam

FeedStillage

Drying

CO2

Enzyme (glucoamylase)pH adjust (acid)

Rectifying column

Make-up solvent

Hydrolysis

Water

pH adjust (lime)Enzyme (�-amylase)

Figure 18 Process for the production of anhydrous ethanol from corn.

488 Global Agriculture: Industrial Feedstocks for Energy and Materials

energy recovery assuming complete conversion into ethanolfor cellulose fermentation is approximately 97% of the feed-stock energy. The overall reaction of cellulose to glucose is

ðC6H10O5Þn þ nH2O¼ nðC6H12O6Þ ½9�

with glucose fermented to ethanol as in reaction [8]. Thefractional energy yield for cellulose to ethanol is shown inTable 6. Actual energy yields are substantially lower. At300 l Mg�1 the energy yield from feedstock is 40%. This doesnot include the supplemental energy involved in feedstockproduction, handling, and conversion.

As an improvement over the separate hydrolysis andfermentation (SHF) of the cellulosic biomass, simultaneoussaccharification and fermentation (SSF) processes are per-ceived to have certain advantages in avoiding end-pointinhibition in the fermentation stage and the possibility ofreducing the fermentation time. The cost of ethanol from SSFprocesses may, therefore, be reduced relative to SHF. In SSF, amixed culture of organisms is used, typically Brettanomycesclaussenii and S. cerevisiae, with B. clausenii being active earlyand the more robust S. cerevisiae continuing to completion.More recent developments employ simultaneous sacchar-ification and cofermentation of multiple sugars, or com-bined enzyme production, saccharification, and fermentation

PressSolids 5−10% C5

5−10% C6

9.5% H2SO4

10% glucose

Cellulosepretreatment

20−30% H2SO41−2 h

Cellulosereactor

Press

Solids (Lignin +unreacted cellulose/acidand sugar recovery)

Recycle acid

Acid

Dryer

Yeast

Steam

Feedstock

Hemicellulosereactor

100 °C/1−6 h

Water

Steam

Stream

Water

Steam

Lime

Neutralization

Filter

Ethanol

Fermentation

Beer

Distillation Stillage

CO2

Solids/gypsumrecovery

Mill

+ Sugars

Figure 19 Process for fermenting cellulosic feedstocks using concentrated acid hydrolysis with acid recycle. Adapted from Barrier, J.W., Moore,M.R., Broder, J.D., 1986. Integrated Production of Ethanol and Coproducts from Agricultural Biomass. Muscle Shoals, AL: Tennessee ValleyAuthority.

Global Agriculture: Industrial Feedstocks for Energy and Materials 489

processing in consolidated bioprocessing (Olson et al.,2012).

Gasification systems coupled with a bioconversion stageare proposed to take advantage of the capability of anaerobicorganisms, such as Clostridium ljungdahlii to ferment syngas toethanol. These systems potentially would operate at a higherrate than the hydrolytic fermentation systems (Skidmore et al.,2013).

Thirty-five percent of the weight of ethanol is due to oxy-gen. In the United States, ethanol is used as an oxygen sourcein gasolines where it assists in controlling CO emissions fromengines. At present, ethanol can be blended up to 10% byvolume. Fuel oxygen promotes CO oxidation to CO2, and theethanol in the gasoline tends to create a leaning effect in mostspark-ignited engines, causing them to operate at highereffective air–fuel ratios with lower CO emission rates (OTA,1990). Ethanol has a high octane rating, as does methanol,and engines using neat or blended fuels can be designed forhigher compression ratios and higher efficiencies. Therefore,even though ethanol has a heating value which is 60% that ofgasoline (Table 7), the effective fuel consumption on aproperly configured engine should only be increased by ap-proximately 25% relative to gasoline. Stable blends of ethanolwith gasoline require anhydrous ethanol to avoid phase sep-aration, an issue in ethanol transportation and distribution.

The use of ethanol as blending stock for gasoline inCalifornia has also been controversial due to the higher vapor

pressures when blended compared with gasoline alone, al-though the vapor pressure of neat ethanol is less than gasoline.The higher blend vapor pressure can result in higher evap-orative losses of reactive hydrocarbons, although improvedautomotive fuel systems have reduced this concern in latemodel vehicles and blending ratios have shifted from 5.7% to10%. The hydrocarbons react with NOx in the atmosphere toproduce ozone, a lung irritant and undesirable troposphericpollutant. The level of ethanol blending is also a point ofcontention between oil refiners and ethanol producers due torules under the US federal renewable fuel standard (RFS), apart of the Energy Independence and Security Act of 2007(U.S. Congress, 2007; Podkul, 2013). A 10% blend ratio limitsthe total amount of ethanol that can be used in motor fuel(the so-called blend wall), and changes in gasoline demandcould result in refiner demands that are lower than called forunder the RFS, making financing difficult for new biorefiningcapacity additions. Vehicle warranties, other than for E85-capable vehicles, mostly cover only up to 10% ethanol, addingto the blend-wall constraints. Further constraints to the growthof a corn ethanol industry in the United States and elsewherearise in California from estimates of lifecycle carbon intensities(net greenhouse gas emissions) under the low-carbon fuelstandard (LCFS) that requires fuel suppliers to reduce intensityby 10% (CARB, 2012). Where coal is used for process energy inbiorefineries producing fuel ethanol, estimated carbon inten-sities exceed those of the reference gasoline from petroleum.

Liquid

PressSolids

Saccharification/fermentor

Ethanolrecovery

Press

Wastewatertreatment

Cellulaseenzymes

Ethanol

Solids (lignin utilization)

Feedstock

Sizereduction

Dilute acidpretreatment

WaterAcid

Lime

Neutralization/conditioning

FilterSolids/gypsumrecovery

Liquid

Figure 20 Process for fermenting cellulosic feedstocks using enzymatic hydrolysis. Adapted from McMillan, J.D., 2004. Biotechnology routes tobiomass conversion. DOE/NASULGC Biomass and Solar Energy Workshops, August 3–4. Golden, CO: National Renewable Energy Laboratory.

Table 6 Ideal fractional energy yields in the conversion of cellulose into ethanol (actual yields lower)

Compound Molecular weight (kg kmol� 1) Higher heating value (MJ kg� 1) Molar energy (MJ) Fractional energy yield

Cellulose 162 17.53 2840 1.000Glucose 180 15.67 2820 0.993Ethanola 46 29.78 2740 0.965

a2 mol of ethanol produced per mole cellulose reacted.

Table 7 Properties of ethanol and gasoline

Property Ethanol Unleaded regulargasoline

490 Global Agriculture: Industrial Feedstocks for Energy and Materials

Although concerns such as these will likely be resolved over thelonger term, the role of policy is evident in influencing systemdesign and industrial capacity.

Specific gravity (15 1C) 0.79 0.78Lower heating value(MJ kg� 1)

26.9 44.0

Lower heating value (MJ L� 1) 21.2 34.3Octane number ((RþM)/2)a 98 88Stoichiometric air–fuel ratio 9.0 14.7Lower heating value ofstoichiometric air–fuelmixture (MJ kg� 1)

2.7 2.8

Enthalpy of vaporization(kJ kg� 1 at 15 1C)

840 335

Reid vapor pressure (kPa)b 16 55–103

aAverage of research (R) and motor (M) octane test methods (pump value).bReid vapor pressure of 10% ethanol in gasoline is 3−7 kPa higher than gasolinealone.

Anaerobic digestionFermentation by anaerobic bacteria is used to produce biogas,a gaseous fuel consisting of 50–80% by volume methane, 15–45% carbon dioxide, and 5% water, with small concentrationsof H2S and other species (Krich et al., 2005). The technology isextensively employed in municipal waste water treatment.Often the biogas is burned in engines to generate power, withengine waste heat used to heat the digester for improvedperformance. The same biological processes are active in wastelandfills, where gas recovery has become an integral part oflandfill design, both as a means to control gas migration andas a means of energy recovery. Other products can also beproduced through anaerobic digestion including carboxylicacids, ketones, and alcohols (Thanakoses et al., 2003).

Global Agriculture: Industrial Feedstocks for Energy and Materials 491

The overall reaction for anaerobic digestion of the organicportion of the feedstock (represented as CxHyOz) to biogas is

CxHyOz þ x� y4� z2

� �H2O¼ x

2� y8þ z4

� �CO2

þ x2þ y8� z4

� �CH4 ð10Þ

The actual concentration of CO2 is typically reduced due toits solubility in water.

The digestion of the feedstock is commonly described asoccurring in three stages although the cooperative mechanismsamong the organisms involved has been emphasized(McInerney and Bryant, 1981; Hills and Roberts, 1982; Parkinand Owen, 1986). After loading feedstock to the digestervessel, much of the organic material is solubilized by bacterialmetabolism or chemical hydrolysis. Acid-forming bacteriautilize the soluble compounds and produce low molecularweight organic acids, principally acetic acid, that are used bymethanogenic (methane-forming) bacteria for the final con-version to biogas. In a properly operating digester, these pro-cesses take place simultaneously within the digester contents,although in some recent designs the hydrolysis and metha-nogenic processes have been largely separated (Zhang andZhang, 1999). Acid-forming bacteria are generally more robustthan the methanogens, and can overwhelm the system withacids, causing a decrease in pH and unfavorable conditions forthe methanogens. A properly functioning digester will have apH in the range of 7–8. The health of the digester is controlledby proper management of nutrient loading, retention time,temperature, and, in dilute slurry digesters, mixing.

Digesters most often operate with dilute slurries. Bioreactordesigns include covered lagoons commonly used in dairy andother agricultural operations, batch digesters, plug-flow di-gesters, completely stirred tank reactors, upflow anaerobicsludge blanket systems, anaerobic sequencing batch reactorsystems, anaerobic-phased solids systems, and in landfillsenhanced bioreactors employing leachate recycle. Plug-flowreactors utilizing higher solids concentrations are also used.High solids or dry-fermentation digesters have been developedto reduce some of the problems associated with handling di-lute slurries in tank reactors (Jewell, 1982). These high solidsdigesters are similar in nature to landfills, but implemented atsmaller scales and with greater automation.

Digesters can operate on almost all biomass feedstocks,although with differing conversion rates and efficiencies. Theyare well suited for wet or moist feedstocks that would requiredrying for thermal conversion systems. Nitrogen availability islimiting in most cases, particularly when woods or herbaceousmaterials are used alone. Animal manures and mixtures ofmanures with agricultural residues, such as straw, give betterperformance due to improved carbon-to-nitrogen (C/N)ratios, which are recommended to be in the range of 25–30overall (Hills and Roberts, 1981). The biogas produced typi-cally has a heating value of 22–24 MJ m�3 with yields of 50–400 l per kg dry solids depending on feedstock and digesterconditions, including temperature, inoculation levels, andhydraulic retention time. Design operating conditions includethe psychrophilic (ambient temperature or below), mesophilic(30–40 1C), and thermophilic temperature regimes (50–60 1C) with conversion rates generally increasing with

temperature although heating requirements also increase.Hyperthermophiles that can withstand even higher tempera-tures have not yet been employed in commercial systems.Energy conversion efficiencies for anaerobic digesters arerelatively low if based on the yield of the gas alone. Typicalenergy efficiencies of biogas are 20–50%, and up to approxi-mately 20% for electricity. The stabilized sludge remainingafter digestion can be used as fertilizer, unless contaminatedwith heavy metals or other toxic materials from the input feed.Nutrient management is an important issue in many agri-cultural systems and digesters can be used to advantage, es-pecially in mineralizing nitrogen before land application ordischarge of effluent.

Biogas is a reasonably versatile fuel and can be burneddirectly for heating the digester (to improve performance), toheat water, or for cooking and lighting purposes. The gas canalso be used as fuel for engines, turbines, and boilers, in-cluding cogeneration, and more recently in fuel cells (Krichet al., 2005; Lo Faro et al., 2013). As noted earlier, NOx

emissions from engines are currently a key issue for small di-gester-power systems. H2S can be scrubbed from the gas toreduce corrosion and SO2 emissions, although in small-scaleapplications such as individual dairy operations, the cost ofmore effective removal systems is a concern. Siloxanes typi-cally present in landfill gas may need scrubbing (usually oncharcoal) for some applications, turbines in particular and alsoin fuel cells and reciprocating engines. Biogas has been used astransportation fuel, including low pressure applications withlarge flexible bags on the roof of the vehicle serving as the fueltank (Stout, 1990), but is now more widely considered forCNG or LNG applications. CO2 stripping followed by gascompression is mostly required for these purposes. CO2

stripping has also been used to produce pipeline quality gasfor blending into natural gas distribution systems and toproduce transportation fuels. Small-scale digesters have beenpromoted in many areas to improve sanitation as well as tosupply fuel gas. The process is also considered in the design ofintegrated biorefineries (Thomsen et al., 2013).

Physicochemical Conversion

Biodiesel has recently emerged as a commercial renewablealternative to petroleum-derived diesel. Biodiesel can be pro-duced from virgin plant and algal oils and also from waste oilssuch as cooking oils and greases, the latter incurring lowerprocurement costs and hence lower cost of production. In theUnited States, the RFS mandates increasing the amounts ofbiodiesel although the amount required is a fairly small shareof the total biofuel under the standard (U.S. Congress, 2007).Recent reversals in European Union (EU) policies due to sus-tainability concerns have reduced mandated biofuel volumeswith impacts on the existing biodiesel producers and suppliers.Although potentially resource and policy constrained, thecurrent production of biodiesel represents one of the pre-dominant physicochemical conversion techniques.

Unlike petroleum-based diesel fuels, which are primarilystraight or branched chain hydrocarbons, vegetable oils areprimarily mixtures of triacylglycerols composed of fatty acidsesterified to the three –OH positions on glycerol (Goering

492 Global Agriculture: Industrial Feedstocks for Energy and Materials

et al., 1982; Robbelen et al., 1989). Although crude or refinedvegetable oils can be used as fuel for compression-ignited(diesel) engines, such use is not recommended except on anemergency basis for short periods of time (Peterson, 1989).Engine damage arising from fuel polymerization and ringsticking, along with lubricating oil contamination, is likely tooccur after prolonged operation. Vegetable oils have heatingvalues similar to diesel fuel (approximately 90% of ASTM No.2 diesel), but the viscosity is one to two orders of magnitudehigher. The high viscosity causes irregular spray patterns fromfuel injectors inside the engine cylinders, which in turn leadsto injector coking (carbonization of the fuel on the injectornozzles) and deposits on cylinder walls and pistons.Vegetable oils also have higher cloud point and pour pointtemperatures, which makes them inferior to diesel fuels forcold weather operation. However, they have higher flash pointtemperatures, and are therefore somewhat safer, and are lesstoxic than diesel fuels and mostly biodegradable. Stable blendsof diesel fuel with vegetable oils can be made to improve thefuel properties, but modification of the oil is preferred forgeneral engine application.

The reaction of a triglyceride, such as a vegetable oil, withan alcohol in the presence of a catalyst such as sodium hy-droxide, produces upon separation a fatty acid ester and gly-cerol. The ester is an improved diesel fuel substitute over theoriginal oil and the basis for current biodiesel manufacturing.When methanol is used, the result is a fatty acid methyl ester.Fatty acid ethyl esters, produced from oil and ethanol, havealso emerged as diesel fuel replacements, with some possibleadvantages over the methyl esters in reduced smoke opacityand lower injector coking. The esters have viscosities that areonly 2–3 times that of diesel fuel, which leads to reducedinjector coking and decreased deposit formation comparedwith the original vegetable oil. The esters have improved ce-tane numbers relative to the original oils, and in some casesrelative to diesel fuel. This makes them useful as blendingstocks for diesel. Although esters reduce smoke emissionscompared with diesel fuel, NOx emissions are about the sameor higher, as are acrolein emissions. Aromatic aldehyde emis-sions depend on the aromaticity of the fuel such that blends ofbiodiesel with regular diesel tend to have higher emissionsthan pure biodiesel fuel (B100) (Qi et al., 2013; Cahill andOkamoto, 2012).

Fuel specification standards for biodiesel have been de-veloped in Europe (Mittelbach et al., 1992) and the US in-cluding standards (ASTM D6751, 2012) for 100% biodieselfuel (B100) as blending stock for distillate fuels. Owing to itstypically low sulfur content, biodiesel can be blended withpetroleum diesel that otherwise does not meet newer low-sulfur fuel standards. B20, a 20% biodiesel blend, can be usedin some unmodified diesel engines. The use of B100 may re-quire modifications to the fuel supply system if the engine hasnot been warranted for it.

The Challenge of Sustainability

The production of industrial feedstocks by agriculture involvesland and other resources that provide many different agro-nomic and ecosystem services. As the scale of production

capacity for fuels and other commodity chemicals has in-creased, so has the debate around the wisdom of convertingforest lands and prime or marginal agricultural lands to pur-poses other than traditional food, feed, and fiber production,especially food for a growing global population. Policies in-tended to improve sustainability in one sector, such as energysecurity, have often been contested for their narrow focus andlack of analysis and consideration of larger system issues, suchas net global greenhouse gas emissions or net energy benefits(Searchinger, 2008; Pimentel, 1991). The reversal in EU pol-icies regarding biofuels was motivated largely by reconsider-ation of sustainability impacts associated with resourcing largesupplies of biodiesel and other fuels from palm oil and othercrops where global environmental, social, and economic im-pacts had not been systematically evaluated (Turner et al.,2008).

Challenges to US federal policies relating to biofuels de-veloped following the first oil shock of 1973–74 when the USinitiated policy supporting alternative fuels. Debate initiallycentered on net energy yields associated with producingethanol from corn with suggestions that the amount of energyembodied in fossil fuels and other inputs to grow corn andmanufacture ethanol exceeded the energy in the ethanol, thusnegating any intended energy benefits (Pimentel, 1991).Criticisms were largely addressed through improvements inconversion technologies (Shapouri et al., 2002; Farrell et al.,2006), but other questions of sustainability continue to arise.In addition to a more recent debate over the net greenhousegas emissions from industrial feedstock production, the effectson food prices have been called into question (the food-ver-sus-fuel debate) with concerns over food security, especiallyfor the poor. The Brazilian program to produce ethanol fromsugarcane, widely viewed as a successful national response toimprove domestic energy security, also generates concerns overenvironmental sustainability although the net greenhouse gasimpacts are mostly regarded as lower than for corn (Smeetset al., 2008).

The global energy markets are large and can easily absorblarge quantities of feedstock biomass. Estimates for the UnitedStates suggest that only approximately a third or less oftransportation fuel could be supplied from biomass (Perlackand Stokes, 2011; Parker et al., 2011). Increasing biofuel de-mand potentially leads to direct land-use changes (dLUCs) aswell as indirect land-use changes (iLUCs) through market-mediated effects (Searchinger et al., 2008) unless yield in-tensification on the same land base is able to compensate, agoal of crop research but not so far achieved. iLUC can causesubstantial emissions of carbon to the atmosphere fromclearing of standing biomass and depletion of carbon stocks inthe soil in response to crop shifting elsewhere in the world(Fargione et al., 2008; Searchinger et al., 2008). The amount ofcarbon released, like many other issues in agricultural sus-tainability, is highly uncertain and estimates vary widely. In-clusion of iLUC effects in environmental lifecycle assessmentscan result in higher attributed greenhouse gas emissions forsome biofuels in comparison with petroleum-derived gasolineor diesel fuels. Policies to encourage biofuels based solely ondirect substitution effects therefore may fail to achieve theintended outcomes for total greenhouse gas emission re-ductions. In California, the development of a LCFS to limit the

Global Agriculture: Industrial Feedstocks for Energy and Materials 493

carbon intensity of transport fuels and electricity so as to re-duce carbon emissions has been particularly contentious(CARB, 2012). Other standards, such as the US renewable fuelstandard (U.S. Congress, 2007), also require set levels of life-cycle greenhouse gas emission reductions to qualify for a re-newable identification number for the fuel, a key objective formarket access and financial success in meeting fuel blendingobligations (U.S. Congress, 2005; 2007). Sustainability con-siderations for biofuels also apply to issues of environmentaljustice and other social implications in addition to environ-mental and economic effects (RSB, 2010).

Bioenergy production has been viewed as a way to provideadditional economic opportunities for farms and rural areas,improve the environment, enhance energy security, and helpto stabilize fuel prices. Substitution of biofuels for gasolineand other petroleum products can result in direct reductions ingreenhouse gas emissions and other benefits, but can also leadto changes in the type of air pollutants that are emitted, affectwater demand and quality, alter land use, and increase ordestabilize other commodity prices. Demand for wood andother biomass feedstocks for traditional uses in cooking,heating, and charcoal making has contributed to deforestationin many regions of the world in addition to causing adversehealth effects from exposure to smoke (Jenkins et al., 2011).Concerns have long been expressed over harvesting of agri-cultural residues for energy and other industrial uses due topossible agronomic impacts associated with nutrient de-pletion, changes to soil organic matter and carbon, and soilerosion (Jenkins et al., 1981). Similar criticisms arise in thelarge-scale production of chemicals and materials from agri-culture. In adopting new uses for crops and resources, carefulconsideration must be given to the lifecycle impacts and sus-tainability of the entire production system and feedstocksupply chain.

Economic sustainability is a primary factor in motivatinginvestment financing for industrial feedstock production,where environmental and social sustainability factors are sat-isfied. The costs of producing energy and other products frombiomass are heavily influenced by feedstock acquisition costsas well as by the costs of conversion. For comparison, costs togenerate electricity in the existing biomass-fueled power plantsare approximately US$0.06–0.08 kWh�1 (US$17–22 per GJ),sometimes higher, at an average fuel cost of US$30 per metricton, principally as wood residue (Jenkins, 2005). Cost pro-jections for purpose-grown crops range substantially higher,typically beyond $50 per metric ton dry weight and often morethan $100 per metric ton (Parker et al., 2011).

Exclusive of harvesting and downstream processing andconversion operations, production costs for agricultural andother biomass residues are typically allocated to the primarycrop production system and not always separately accounted.Byproduct or waste biomass may be available at no cost, or insome cases disposal fees (tipping fees) can be applied to coverthe costs of handling, an advantage to systems receivingmunicipal solid wastes. In contrast, industrial and energy cropsassume full allocation of production costs. These costs arequite variable depending on the species, production site, levelof management, and resulting yield. Total average deliveredcosts, including harvesting and transportation (85 km),for Eucalyptus plantations in northeast Brazil producing at

12.5 Mg ha�1 year�1 have been estimated at US$2.50–3.40 perGJ, or approximately US$52–67 per metric ton dry matter (CPIinflation adjusted 2013 US dollars) (Hall et al., 1993). Of thetotal, 40% is associated with stand establishment includingnursery production, land, planting, and administration, andanother 10% is associated with plantation maintenance in-cluding management, cultivation, and research. Half the de-livered cost is in the production of the biomass. UnderCanadian conditions, the total delivered cost including har-vesting, chipping, and transportation for a typical 5 yearrotation, 4 rotation cycle forestry crop with a yield of 12 Mgha�1 year�1, was similarly estimated at US$58 per metric ton(CPI adjusted 2013 US dollars), of which 40% was allocatedto the production system including land, nursery, planting,and tending (Golob, 1987).

Other energy and chemical feedstock costs influence de-cisions to produce industrial feedstocks from agriculture.Natural gas prices in the US in the period 1999–2001, forexample, fluctuated between US$2 and US$15 per GJ. Owingto the development of hydraulic fracturing (fracking) techni-ques and expansion of natural gas reserves, prices are now inthe vicinity of US$3 per GJ and anticipated to remain belowUS$4 per GJ through 2020, increasing to US$8 per GJ by2040 (EIA, 2013c). Although policies such as California’sRPS require increasing supplies of electricity from renewablesources, resource neutrality within the policy provides nospecific advantage for biomass over other renewable resourcesand principal capacity additions have been in wind and solar(EIA, 2013d). Policy stability is also important in motivatingfinancing for biorefineries and other industrial applications.Reversals such as that of the EU over biofuels (colloquiallyreferred to as the ‘biofuels disaster’) and possible adjustmentsin the renewable fuel obligations under the RFS in the UScreate significant uncertainties associated with security of in-vestments in facilities with decades-long economic lifetimes.Research along with genetic and cultural improvements areprojected to reduce biomass production costs, but biomassproduction levels will also be influenced by direct and in-direct environmental and socioeconomic consequences ex-ternal to the direct costs of production (Hanegraaf et al.,1998).

Sustainable large-scale production of industrial feedstockswill require a broad systems view and well-designed standardsand best practices. International standards for certifying sus-tainable production of biofuels are currently in developmentwith early versions in use (RSB, 2010). The principles onwhich these are based – attention to law, stakeholder partici-pation, climate change mitigation, human, labor and landrights, rural and social development, food security, wastemanagement, resource conservation and environmental pro-tection – apply equally well to agricultural production systemsfor biobased products as they do for bioenergy, a point re-inforced by the recent name change of one of the principlecooperative initiatives in international standardization tobroaden from biofuels to biomaterials and biomass pro-duction, more generally (RSB, 2013). Local standards may insome cases exceed international standards but will need closecoordination to avoid conflicts with international agreementsand rules such as those of the World Trade Organization. Thecomplex issues now being researched and addressed in

494 Global Agriculture: Industrial Feedstocks for Energy and Materials

sustainable industrial feedstock production should providenew perspectives on the improved sustainability of agricultureoverall.

See also: Agroforestry: Practices and Systems. Air: GreenhouseGases from Agriculture. Climate Change: Agricultural Mitigation.Computer Modeling: Applications to Environment and FoodSecurity. Economics of Natural Resources and Environment inAgriculture. Global Food Supply Chains. International and RegionalInstitutions and Instruments for Agricultural Policy, Research, andDevelopment. International Trade. Land Use, Land Cover, and Food-Energy-Environment Trade-Off: Key Issues and Insights forMillennium Development Goals. Markets and Prices. NaturalCapital, Ecological Infrastructure, and Ecosystem Services inAgroecosystems. Soil: Carbon Sequestration in Agricultural Systems

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Zheng, Y., Pan, Z., Zhang, R.H., Jenkins, B.M., Blunk, S., 2006. Properties ofmedium-density particleboard from saline Athel wood. Industrial Crops andProducts 23 (3), 318–326.

Relevant Websites

http://www.astm.org/American Society for Testing and Materials International.

http://www.bioenergykdf.net/Bioenergy Knowledge Discovery Framework.

http://www.arb.ca.gov/homepage.htmCalifornia Air Resources Board.

http://www.energy.ca.gov/California Energy Commission.

http://www.caiso.com/Pages/default.aspxCalifornia Independent System Operator.

http://www.cpuc.ca.gov/puc/California Public Utilities Commission.

http://en.european-bioplastics.org/European Bioplastics Organization.

http://www.iea.org/International Energy Agency.

http://www.iso.org/iso/home.htmlInternational Standards Organization.

http://www.wikipedia.orgOn-Line Encyclopedia.

http://www.plastiquarian.com/index.phpPlastics Historical Society.

http://rsb.org/Roundtable on Sustainable Biomaterials.

498 Global Agriculture: Industrial Feedstocks for Energy and Materials

http://www.usda.gov/wps/portal/usda/usdahomeU.S. Department of Agriculture.

http://energy.gov/U.S. Department of Energy.

http://www.eia.gov/U.S. Energy Information Administration.

http://www.epa.gov/U.S. Environmental Protection Agency.

http://www.nist.gov/index.htmlU.S. National Institute for Standards and Technology.

http://www.nrel.gov/U.S. National Renewable Energy Laboratory.

http://www.nsf.gov/U.S. National Science Foundation.