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R E V I E W

Arthur J. Ragauskas1†, Máté Nagy1, Dong Ho Kim2,Charles A. Eckert3, Jason P. Hallett3, and Charles L. Liotta1

1School of Chemistry and Biochemistry2Institute of Paper Science and Technology3School of Chemical and Biomolecular EngineeringGeorgia Institute of Technology, Atlanta, Georgia 30332

†Corresponding authorPhone (404) 894-9701Fax (404) 894-4778 E-mail arthur.ragauskas@chemistry.gatech.edu

Key Words: Cellulose, Hemicellulose, Lignin, Extractives, Tall Oil,Turpentine, Bioethanol, Biodiesel, Pulp, Biorefinery

AbstractBioethanol currently contributes ~2% to the total US transportation

fuels mix, and another ~0.01% is based on biodiesel. To make a sub-stantial contribution to the United States’ energy portfolio, biofuelsproduction needs to grow substantially over the next decade by a factorof 10 or more. Although the contribution of agro-energy crops andagriculture waste for biofuels production is being intensively developed,the potential of the forest products industry to contribute to this efforthas been generally underestimated. The forest products industry isone of a few nationally based industries that have the necessary skillset and infrastructure available to process sufficient biomass for therapid short-term development and commercialization of biofuel andbiochemical technologies. On an annual basis, the US pulp and paperindustry collects and processes approximately 108 million tons ofwood for the production of pulp and paper in a sustainable manner.Wood extractives from pulping provide approximately 700 millionliters of turpentine and tall oil annually that could be employed forbiodiesel applications. Wood chip preextraction technologies couldmake available to the biofuels industry about 14 million tons of hemi-

celluloses annually while at the same time enhancing the production ofkraft pulps. This review highlights the chemical resources availablefrom wood and summarizes which biomaterials are needed for pulpproduction and which could be utilized for biofuels, with a specialemphasis on select hemicelluloses that are currently degraded duringkraft pulping that could be utilized for bioethanol production. Thereview further describes the operational considerations by which thebiofuels and pulp manufacturing industries could synergisticallyoperate together.

Introductionenry Ford and Rudolph Diesel are well known for their con-tribution to the industrial revolution and modern automotivehistory. What is frequently overlooked is that they bothenvisaged, at one time, that their engines would be powered

by biofuels such as bioethanol and peanut oil, respectively1,2. Untilrecently, the need for biofuels remained generally a low priority, aspetroleum supply and demand curves were satisfactorily addressed.

Nonetheless, global petroleum demands have increased steadilyfrom 57 x 106 barrels/day in 1973, to 82 x 106 barrels/day in 20043.By 2025, projected economic growth is anticipated to increase globaldemands for liquid fuels by another ~50%3. The impact of thisgrowth in demand, as well as limited global production capacity, hasbeen foretold by several organizations and individuals4. For example,a 1995 study by Oak Ridge National Laboratory predicted the impactof a 2005 supply shock driving prices from $20/bbl to $50/bbl5. Thegasoline lines and price increases following Hurricanes Katrina andRita in 2005 reinforce the predictions of these studies and act as aharbinger for the future unless we readdress our energy transportationpolicies and technologies.

Coupled with these concerns, the contribution of combustion CO2from fossil fuels to climate change has been noted in several recentreviews6. As described in a recent publication by Hoffert et al, futurereductions in the ecological footprint of energy generation will residein a multifaceted approach that includes the use of hydrogen, wind,

From wood to fuelsIntegrating biofuels and pulp production

H

nuclear, solar power, fossil fuels from which carbon is sequestered,and biofuels7. In the short term, the marketplace and societal concernssuggest that biofuels are one of the most attractive solutions for ourtransportation energy needs. Given that ~70% of US petroleumconsumption is transportation-related and ~25% is for materials andchemicals, it is clear that efficient production of biofuels and bio-materials will become one of this generation’s grand challenges8.

In the US, bioethanol currently contributes ~2% to the total trans-portation fuels mix, and ~0.01% is based on biodiesel. The proposedReliable Fuels Act would ban the use of methyl tertiary-butyl ether(MBTE) as a motor fuel additive for four years after enactment andrequire all motor fuels sold by a refiner, blender, or importer to containspecified amounts of a renewable fuel. The Congressional BudgetOffice expects that this renewable fuel standard would largely be metby adding ethanol to gasoline, resulting, on an annual average basis,in 6 billion gallons of renewable fuel consumed9.

As outlined in the USDA-DOE Billion Ton report, the United States’agriculture and forestry reserves have the potential to address aboutone third of this nation’s current petroleum demand10. This increasein biofuels would require approximately a 10- to 15-fold increase inbiofuel production capacity. Shifting societal dependency frompetroleum resources to renewable biomass resources has been pro-posed to have several positive ramifications, including enhancednational security, improved balance of trade, rural employment

opportunities, and environmental performance parameters, includingnet reductions in CO2 emissions. Although the contribution of agrocrops for biofuels production has been intensively examined, thepotential for the forest products industry to contribute to this efforthas not garnered as much interest11. This review article examinesnear-term opportunities for the forest products industry to contributeto the burgeoning biofuels industry12.

On an annual basis, the United States’ pulp and paper industrycollects and processes ~108 million tons of pulpwood for the produc-tion of pulp, paper, and paperboard13. As illustrated in Figure 1, amodern pulp mill employs ~50% of the technologies needed to con-tribute to biofuels biorefinery, which provides a strong incentive forthe development of innovative biorefinery technologies14. In general,a biorefinery is defined as a facility that integrates biomass conver-sion processes and equipment to produce fuels, power, and chemicalsfrom biomass. A biorefinery will seek to utilize fully all componentsof the biomass to make a range of foods, fuels, chemicals, feeds,materials, heat, and power in proportions that maximize economicreturn. This concept is analogous to today's petroleum refineries,which produce multiple fuels and products from petroleum12. Thisreview provides insight into select unit operations of pulp productionand their possible contribution to biofuels and biochemicals, alongwith the research challenges that remain, especially as apply to thepotential utilization of wood hemicelluloses.

Figure 1. Overview of proposed biorefinery (broken lines). Pulp mill operations are indicated by solid lines.

Pulp Bleaching

Papermaking

Energy

Wood Extractives

ChemicalMarkets

Combustion ofChemical Pulp

Waste

Unbleached PulpPapermaking

Wood ChipsDebarking,Wood Chipping

Wood Collection

BarkValue-AddedChemicals

WoodHemicellulose

Sugars

Extracted Woodchips

Fermentation

Bioethanol

Pulping

Biofuels

EnergyPulping

Chemicals

Proposed Hemicellulose-Extraction Stage

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Basic biomass resources from woodWood is composed primarily of cellulose, hemicellulose, lignin,

and small amounts of extractives15. Table 1 summarizes the typicaldistribution of major chemical constituents for several dominantwood resources as determined by Timell16a,b,c.

Recent studies have extended this work to other important pulpand paper wood resources including Picea abies and Acaciamangium17,18.

Of the three major biopolymers that constitute wood, lignin isdistinctly different from the other macromolecular polymers19. Ligninis an amorphous polyphenolic polymer that is synthesized by enzy-matic dehydrogenative polymerization of 4-hydroxyphenylpropanoid units (Figure 2)20. A review by Meister21 suggested themost likely lower limit for the degree of polymerization (DP) for soft-wood lignin is approximately 60 phenyl propanoid units yielding aMW of ~11 000. Recent studies by Guerra et al have measured soft-wood DP values of 90–10022.

The predominant polysaccharide in wood is cellulose, which is ahomopolymer of (1�4)-linked β-D-glucopyranosyl units with adegree of polymerization of approximately 10 000 and a relativelyhigh degree of crystallinity averaging 50–70%23,24.

The major hemicelluloses in softwoods are galactoglucomannansand arbinoglucuronoxylan, and minor amounts of arabinogalactan,xyloglucan, and other glucans (Figure 3). The predominant hemicel-luloses for hardwoods are glucomannan and glucuronoxylan, withlesser amounts of galactans and glucans.

Hardwood glucuronoxylan and softwood arabinoglucuronoxylanboth have a backbone of (1�4)-linked β-D-xylopyranosyl units butexhibit differences in branching and substitution patterns. In theformer, the C2-OH and C3-OH are partially acetylated (i.e., 3.5–7.0 acetylgroups/10 xylose) and lesser amounts of (1�2)-linked pyranoid 4-O-methyl-α-D-glucuronic acid units. For softwoods, the xylan polymer isnot acetylated and typically is branched with (1�2)-linked pyranoid4-O-methyl-α-D-glucuronic acid and (1�3)-linked α-L-arabinofura-nosyl units, with an arabinose : uronic acid : xylose ratio of ~1 : 2 : 8.

Galactoglucomannan is far more important than arabinoglucurono-xylan to the hemicellulose chemistry of softwoods, contributing 15–20%of the dry wood mass. This polysaccharide is comprised of (1�4)-linkedβ-D-glucopyranosyl and D-mannopyranosyl units that are partiallyacetylated at the C2-OH and C3-OH and partly substituted by (1�6)-linked β-D-galactopyranosyl units. Softwoods generally have twodifferent types of galactoglucomannans, one being highly branched,with a ~1 : 1 : 3 ratio of galactose : glucose :mannose, and another thatis less branched, with a ~0.1 : 1 : 3 ratio galactose : glucose :mannose. Inhardwoods, the glucomannan polymer has little or no branching, with

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Figure 2. Phenyl propane precursors for softwood (SW) and hardwood(HW) lignin

ConiferylAlcoholSW/HW

CoumarylAlcoholSW/HW

SinapylAlcohol

HW

OH OH OH

OCH3 OCH3H3CO

OH OH OH

Table 1. Major chemical constituents of wood

Wood macromolecules†

Wood Species Cellulose Lignin Hemicelluloses(%) (%) (%)

Softwoods

Picea glauca 41 27 31

Abies balsamea 42 29 27

Pinus strobus 41 29 27

Tsuga canadensis 41 33 23

Thuja occidentalis 41 31 26

Hardwoods

Acer rubrum 45 24 29

Ulmus americana 51 24 23

Populus tremuloides 48 21 27

Betula papyrifera 42 19 38

Fagus grandifolia 45 22 29

†All samples were analyzed extractives-free.

a typical glucose:mannose ratio of ~1:1.525.A recent series of publications by Willför et al have provided one

of the most definitive analyses of carbohydrates present in industriallyimportant wood resources26,27. Representative data from this study issummarized in Table 2.

Overview of pulp manufacturing and bioresources for biorefinery

In 2002, North American pulp manufacturers produced approxi-mately 62 million tons of paper and paperboard from 61 million tonsof softwood pulpwood and 44 million tons of hardwood pulpwood28.Prior to any pulping operations, wood needs to be debarked and inmost cases chipped. The bark and waste wood is burned in a furnaceto capture its energy value, which on a national basis generated 316trillion BTU in 200229. The chemical constituents of wood bark havebeen determined for many species and have been shown to includecarbohydrates, fatty acids, resin acids, sterols, stilbenes, low molecularweight phenols, and tannins. Several research groups have begun toreexamine the chemistry of inner and outer bark with advancedanalytical tools to identify value-added chemicals that could be pre-extracted prior to combustion30. Recent publications have identifiedseveral promising chemicals from the bark of trees commonly employedfor pulp production, including tannins for antioxidants, stilbenes thatcould serve as antimicrobials, and betulinol for emulgators31,32,33.

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Figure 3. Representative hemicelluloses of wood

OO

O OAcO

O

HOOH

O

HOO

OO

HO

OH

O-XylanO

HO

O

O

OH

O

H3COHO

OHHO 2C O

HO

HOOH

Softwood - arabinoglucuronoxylan

OO

HOOH

OO

HO

OHO

OH

O

AcO

OH

OO

HO

OHOH

OH

O

O

HO HO

OH

HO

Softwood - galactoglucomannan

Hardwood - glucomannan

OO

HOOH

OO

HO

OHOH

OH

O

HOOH

OO

HO

OHOH

OH

O

Table 2. Relative distribution of hemicellulose sugars in select wood resources26,27

Wood Species Ara Xyl Gal Clc Man Rha GlcA GalA 4-O-MeGlcA Total† %

Softwoods

Picea abies

sapwood 0.14 0.61 0.17 0.37 1.00 0.02 0.03 0.16 0.10 24.6

heartwood 0.17 0.69 0.28 0.35 1.00 0.03 0.04 0.20 0.12 24.9

Pinus banksiana

sapwood 0.18 0.57 0.18 0.40 1.00 0.02 0.05 0.13 0.10 27.2

heartwood 0.22 0.75 0.37 0.43 1.00 0.03 0.06 0.17 0.14 29.3

Hardwood

Betula pendula

stemwood 0.02 1.00 0.06 0.08 0.04 0.02 0.01 0.10 0.16 33.6

Populus tremuloides

sapwood 0.03 1.00 0.04 0.11 0.05 0.03 0.01 0.12 0.13 29.1

heartwood 0.03 1.00 0.04 0.12 0.09 0.03 0.01 0.12 0.13 28.8†mass sugar units/mass dry wood

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After debarking and chipping, wood samples are typically processedinto pulp by one of two competing technologies. In brief, the woodcan be defiberlated mechanically into mechanical pulp with +90%yields, thereby utilizing all three of the major wood biopolymers34. It iswell known that thermomechanical pulping (TMP) of softwoods resultsin the dissolution galactoglucomannans and some acidic arabino-galactans35, along with some wood resins, triglycerides, lignin, andlignans. NMR analysis of O-acetylated galactomannans isolated fromspruce TMP with hot water indicated that this polysaccharide consistedof (1�4)-linked β-D-mannopyranosyl and β-D-glucopyranosyl unitsin a 10.0-to-(1.9–2.6) ratio, with the former being acetylated on C2-OHand C3-OH units36. On average, every 10th mannopyranosyl wassubstituted at the C6-OH with an α-D-galactopyranosyl unit. A low DPO-acetylated glucomannan has also been isolated and characterizedfrom the process streams of aspen mechanical pulping operations37.Willför et al have reported that softwood TMP process water containsapproximately 11 kg carbohydrates/ton of processed TMP and 1 kg ofextractives; a combination of resin treatment followed ultrafiltrationwas shown to provide 5 kg polysaccharide/ton TMP processed38,39.With an average TMP mill producing ~250 tons of pulp/day, thisprovides a relatively clean source of galactoglucomannans that couldbe utilized for fuels or value-added chemicals.

In the United States, chemical pulping is the dominant pulp manu-facturing technology, utilizing NaOH and NaSH (i.e., kraft pulpingreagents) to delignify wood. A typical kraft pulp mill will manufacture35 000 tons of air-dried pulp per year, which requires approximately630 000 tons of wood. Typical kraft pulping conditions for bleachpulp grades are summarized in Table 3. Changes in wood fiber com-position during kraft pulping are summarized in Table 440.

Valuable by-products of kraft pulping resinous woods, such aspine, are the production of turpentine and tall oil. The former isrecovered primarily from the relief gases during kraft pulping.Turpentine is known to contain α-pinene, camphene, and limonene,

which are utilized for assorted chemical products, includingfragrances, polymer additives, and solvents43. A recent paper byForan highlighted that North America kraft operations generate150–170 million liters of crude sulfate turpentine annually44. Studieshave demonstrated that this bioresource could be used as a dieselfuel additive45.

Tall oil consists of saponified fatty and resin acids, which arerecovered from kraft pulping liquors. Typical yields are dependent onthe wood furnish employed, but Southern USA softwood kraft pulpmills usually yield ~34 kg of tall oil/ton wood, whereas Canadianmills have reported values of 50% less46. These extracts are used in awide range of products, including soap, lubricants, and paper sizingagents. The determination that these extracts are a rich source ofsitosterol and sitostanol, which are key intermediates for functionalfood additives47, has renewed interest in the analysis of tall oil com-ponents for other value-added compounds48. A recent publication byThorp49 has also highlighted the potential of tall oil as a viableresource for biodiesel, with a national production-rate potential of530 million liters per year. The processing of tall oil into a high qualitydiesel additive has been researched in the laboratory and pilot-scaled. The later studies included promising road tests by CanadaPost Corporation50,51. Given that many kraft pulp mills alreadycollect these extractives, their future utilization for fuels will bebased on competing economic considerations.

Pulp production benefits from hemicelluloseextraction

The loss of select wood polysaccharides during kraft pulping is anatural outcome of kraft pulping conditions. As summarized in Table 4,kraft pulping of softwood pulps leads to an extensive removal of

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Table 3. Typical softwood and hardwood kraft pulping conditions forbleaching grades41

Softwood Hardwood

Active alkali on dry wood 17–21% 14–18%

Liquor sulfidity (Na2S) 25% 25%

Max. cooking temperature/°C 170 170

Total cooking time/min 110–175 60–95

Kraft cook completed at lignin content 5% 2%

Yield 48–43 49–47

Table 4. Typical wood chemical distribution for wood before andafter kraft pulping42

Component Wood Components Kraft PulpComponents

Pine Birch Pine Birch(as a % of original wood)

Cellulose 38–40 40–41 35 34

Glucomannan 15–20 2–5 5 1

Xylan 7–10 25–30 5 16

Other carbohydrates 0–5 0–4 — —

Lignin 27–29 20–22 2–5 1.5–3

Extraneous compounds 4–6 2–4 0.25 < 0.5

glucomannans52, and, for hardwoods, a loss of xylans53. During kraftpulping, these extracted hemicelluloses are degraded into low-valueisosaccharinic acids54,55. The sugars and lignin extracted during kraftpulping56 are subsequently concentrated and incinerated in a recoveryfurnace. The extracted hemicelluloses provide ~25% of energyresources for a recovery furnace, as the bulk of the energy is derivedfrom the combustion of lignin57,58. Although the recovery furnacehas been refined through the generations, it remains the singlelargest capital investment in a kraft pulp mill and is not amenable toincremental increases in production. Hence, for several pulp mills,the capacity of the recovery furnace dictates the pulp mill’s productioncapacity. The replacement of recovery furnaces with black liquorgasifiers and Fischer Tropsch or turbines units has also been proposedas a means of extracting extra value from kraft cooking liquors andis under active investigation49.

The hemicelluloses extracted during kraft pulping are a naturalresource for bioethanol production, as they do not contribute topaper production. Based on basic kraft pulping principles, it is antic-ipated that preextraction of these “waste” hemicelluloses prior tokraft pulping could substantially improve pulp mill operations by:

• improving the overall kraft pulping process by reducing kraftcooking times,

• enhancing kraft cooking liquor impregnation, yieldingimproved pulp properties, and

• improving pulp production capacity for kraft pulp mills thatare recovery-furnace limited.

These process benefits and biofuels possibilities are strong driversfor the development of wood hemicellulose preextraction technologiesfor kraft pulp mills.

An important consideration that must be taken into account withany preextraction of wood chips prior to kraft pulping is the need todevelop a system that is readily integrated with modern pulpingoperations and will not deteriorate the quality of kraft pulps. A keyphysical parameter in the production of many grades of paper is thestrength of the final paper sheet. It has been well documented that ifthe DP of cellulose is decreased beyond its normal ~1 600 post pulpingto ~700 after bleaching59, the strength properties of the sheet aredegraded. This relationship is due to the fact that cellulose is theprimary load-bearing element in a lignocellulosic fiber and has adirect relationship to the fiber strength, which contributes to paperstrength. Hence, any hemicellulose preextraction technologyemployed prior to kraft pulping needs to minimize the hydrolysis ofcellulose. Furthermore, several researchers have noted that hemi-cellulose content is related to paper bond strength, which has beenattributed to the adhesive properties of hemicellulose. Studies by

Page and others suggest that for kraft pulps with an α-cellulosecontent higher than ~80%, a decrease in paper sheet strength propertiesoccurs60,61,62. This product specification defines a limit for hemicellu-lose preextraction technologies.

Hemicellulose preextractionThe preextraction of hemicelluloses from wood prior to kraft

pulping has been extensively studied and practiced since the early1930s for the production of dissolving grades of pulps63. In general,this grade of pulp requires a +90% cellulose content, which can beachieved by sulfite pulping or a modified kraft pulping process. Ineach case, the product is relatively pure cellulose, and this grade istypically used for products such as rayon, cellophane, and celluloseacetate64. The pioneering studies of Richter demonstrated that acidicor water prehydrolysis of wood prior to kraft pulp is a viable alter-native approach to preparing dissolving pulps65. The use of water asa prehydrolysis stage relies on the in situ hydrolysis of acetategroups on the hemicellulose chains yielding acetic acid. The liberatedacid lowers the solution pH to a range of 4–3. This results in thehydrolysis and solubilization of hemicelluloses. A detailed study bySears et al documented that at 175°C, the water extraction of pinesignificantly outperformed steam at removing hemicelluloses, by afactor of ~366. The water extraction procedure provided, on average,a higher molecular weight hemicellulose. Analysis of the southernpine water prehydrolysates indicates that it contained 9.9% ligninand 70.9% carbohydrates, with relative amounts of sugars varyingaccording to mannose > xylose > glucose > galactose > arabinose.Control of the prehydrolysis parameters is an important considera-tion, as more vigorous conditions will degrade the fiber resource. Forexample, Nguyen at al have reported that the use of 0.4% sulfuricacid and steam temperatures of 200–230°C resulted in 90–95% hemi-cellulose hydrolysis of Pseudotsuga menziesii woodchips, along with20% of the cellulose67.

Gustavsson et al examined the application of an initial pH 1.4prehydrolysis of mechanically refined Populus tremula followed by aseries of differing alkaline extractions which provided a 9% yield ofpolymer 4-O-methylglucuronoxylan after ultrafiltration68. Theextraction of wood hemicelluloses with sodium, potassium, and lithiumhydroxide has also been examined, although frequently these studiesare accomplished with holocellulose or kraft pulps. Under alkalineconditions reported by Scott, pine pulp xylans were preferentiallyextracted over glucomannans69. The addition of borate to the alkalinesolution was shown to enhance the extraction of the glucomannansat room temperature. Tyminski and Timell also utilized sodiumhydroxide-boric acid solutions at 80°C to extract glucomannans in

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80% yield from holocellulose Picea glauca, which they note was notas efficient with Pinus70.

The results of published prehydrolysis studies provide a wealth ofinformation for the removal of hemicelluloses for wood. In general,water prehydrolysis is more effective at removing hemicellulosesthan steam prehydrolysis, especially for softwoods. All prehydrolysistreatments also extract low levels of lignin and extractives. A keyconsideration for extracting hemicelluloses prior to kraft pulping fornondissolving grades of paper is the need to yield a wood furnishthat still yields excellent physical strength pulp properties. This willundoubtedly require an optimization of hemicellulose preextractiontechnologies providing optimal removal of hemicelluloses for biofuelproduction and sufficient retention of select hemicelluloses for theproduction of high quality kraft pulps.

Clearly, there is a need for innovative high-efficiency hemicelluloseextraction technologies for wood. Several research groups havebegun to address this challenge. Gabrielii et al studied the extractionof hemicelluloses from disk-refined aspen with 0.05 M HCl followedby dilute ammonium hydroxide and finally sodium hydroxide71. Theapproach removed approximately 55% of the wood hemicelluloses,which were subsequently purified and recovered after treatment withhydrogen peroxide, ultrafiltration, and spray-drying. The recoveredmaterial was reported to contain 65.2% xylan, 6.4% lignin, and 2.6%ash. Lundqvist et al observed improved hemicellulose extractionyields using microwave-induced heat-fractionation of milled Piceaabies wood72. Employing reaction temperatures of 180–200°C forseveral minutes yielded xylan- and mannan-based oligosaccharides,and polysaccharides with an 87% yield of mannans. Characterizationof the extracted material indicated a ratio of Man : Glu : Gal : Xyl : Araof 1.0 : 0.3 : 0.1 : 0.3 : 0.1. If an alkali solution was used in place ofwater in the microwave treatment, the extracted polysaccharidemolecular weight shifted from 3 800 to 14 000, but the yielddecreased to 3% when using an impregnation pH of 12.3. Palm andZacchi also reported comparable extraction efficiencies usingmicrowave treatment of spruce. Furthermore, these researchersdemonstrated that a steam extraction for the same time and temper-ature employed for the microwave treatment was ~50% less effectiveat extracting hemicelluloses73,74.

Of course, the energy requirements for microwave-assistedextraction of hemicelluloses and the difficulty of pulp millimplementation are issues of concern. However, it may be possible topreheat water-impregnated chips in a traditional manner to anelevated temperature of ~100°C and then employ dielectricmicrowave heating for a short time period in a flow-through reactor,to enhance hemicellulose extraction.

An alternative approach is the National Renewable EnergyLaboratory’s organosolv fractionation technology, which utilizes aternary mixture of methyl isobutyl ketone, ethanol, and water in thepresence of low concentrations of sulfuric acid (i.e., 2.0–0.2 wt %) toeffect a separation of cellulose, hemicellulose, and lignin. The methodtypically requires a treatment temperature of 140°C for 1 hour. Thisapproach has worked well to fractionate hardwoods, yielding high-purity cellulose and selectively dissolving lignin and hemicellulose75.However, the method proves difficult with softwoods, requiring moreacid, higher temperatures, and longer retention times, resulting inpoor cellulose pulps. For integration into a kraft biorefinery, theorganosolv extraction method would need to be studied further.

The use of supercritical carbon dioxide (scCO2) as an effectivereplacement for organic solvent extraction technologies has been welldocumented76. However, it requires high pressures and exhibits limitedsolvation of high molecular weight compounds77. Near critical water,250–350°C, offers exciting possibilities as a benign solvent for theextraction and processing of wood-derived components. As water isheated to near its critical region, the fluid expands and takes on severaluseful properties78,79. It dissolves both salts and organic chemicals,enabling homogeneous aqueous/organic reactions80,81. Further, theseparation following the reaction becomes simple: cooling the mixturecauses the organic chemicals to come out of solution82. Also, as thetemperature is increased, the dissociation constant for water, KW, goesup by orders of magnitude; the water itself is both a natural base andacid and can catalyze reactions, avoiding subsequent neutralizationand salt disposal83. Biofine Inc. has demonstrated the potential of thisapproach by utilizing a high-temperature, dilute-acid hydrolysisprocess that converts cellulosic biomass to levulinic acid and deriva-tives84. It is anticipated that near critical water could be utilized todevelop novel tunable solvents for the selective extraction and frac-tionation of lignin and hemicelluloses. The later component could alsobe depolymerized into fermentable solution of hexoses and pentoses.

Alternatively, recent developments in ionic liquids may, in thefuture, be employed to design new hemicellulose extraction protocols85.These liquids represent a new class of solvents with nonmolecular,ionic character that are liquids at room temperature. Many ionicliquids form biphasic systems, which give rise to the possibility ofmultiphase extraction procedures that could facilitate easy isolationand recovery of biopolymers. Swatloski et al recently documentedthe ability of select ionic liquids to solubilize cellulose86, and Li et alhave reported that 1-butyl-3-methylimidazolium chloride can beemployed to dissolve wood87. These studies hold the promise for thedevelopment of new hemicellulose extraction technologies based onionic liquids.

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Conversion of preextracted hemicelluloses tobioethanol

There are basically two techniques available for the conversion ofwood hemicelluloses into a fermentable sugar solution. The first is anacid hydrolysis process which would relinquish monosaccharides forthe production of ethanol via fermentation88,89,90. Alternatively,extracted wood polysaccharides could be enzyamtically hydrolyzedand fermented to ethanol91. Depending on what technologies areoptimized for the preextraction of hemicelluloses from wood chips,an acid hydrolysis of polysaccharides to hexoses and pentoses maybe preferred.

The enzymatic hydrolysis of pretreated cellulosic biomass hasbeen commercialized recently for the processing of wheat straw tobioethanol and is being actively pursued for other agricultural wasteresources92. An important consideration for hemicellulose preextractionand depolymerization treatment protocol is to minimize by-productsthat are inhibitors of the fermentation of sugars to ethanol, such asfurans, carboxylic acids, and phenolic compounds93. Some inhibitorsare present in the raw material, but others can be formed during thehydrolysis process94. The nature, composition, and concentration ofthese compounds are dependent on the hydrolysis conditions andmay have a profound influence on the fermentation production rateof biofuels from the hydrolysate95,96. There are several strategies fordealing with the inhibitors in hydrolysates. First, the hydrolysisconditions may be optimized not only with respect to maximal sugaryields but also to generating reduced amounts of inhibitorcompounds97. Detoxification prior to fermentation is another option,including alkali, sulfite, evaporation, anion exchange, or enzymatictreatments98,99,100.

The hydrolyzed hemicellulose sugar solution will finally need toundergo fermentation for the production of ethanol. The microor-ganisms that are able to ferment sugars to ethanol can be eitheryeasts101,102 or bacteria103. Recent advances in genetic engineering,forced evolution, and mutation and selection strategies haveenhanced the biological utilization of hexoses and pentoses for thebiological production of ethanol. The well-documented fermentationof wood hydrolysates to ethanol provides a strong technical basisfrom which practical fermentation technologies can be designed forthe conversion of preextracted wood hemicelluloses to ethanol. Forexample, Taherzadeh et al have reported the fermentation of dilute-acid hydrolysates from alder, aspen, birch, willow, pine, and spruceusing Saccharomyces cerevisiae. These wood hydrolysates containedvarying amounts of xylose, glucose, and mannose, and the efficiencyof fermentation varied substantially, depending upon wood speciesemployed104. The use of other yeast105 and fungi106 for the produc-

tion of ethanol from wood hydrolysates has also been reported, andtheir efficiencies and cost-performance properties continue to beenhanced107,108.

Summary The forest products industry could provide a significant contribution

to the United States’ biofuels industry. A kraft biorefinery operationwould provide for an optimal utilization of wood and its componentsfor the production of biofuels, biochemicals, biopower, and pulp. Thepreextraction of select hemicelluloses from kraft wood chips prior topulping has the potential to deliver approximately 14 million tons ofpolysaccharides annually to the biofuels industry on a national basis.Furthermore, this resource is already collected and shipped tocentralized locations, which eliminates the need to develop newcollection systems. Previous research studies have already establishedthe viability of extracting hemicelluloses from wood chips prior tokraft pulping for dissolving pulps. The challenge for thebiofuels and forest products industries is to develop optimizedpreextraction technologies that provide a hemicellulose stream forbiofuels production and a lignocellulosics stream for pulp production.This vision will, undoubtedly, require a cooperative research programwith multipartner stakeholders. These efforts have already begun andwill accelerate in the near future, given the significant benefits to allinterested parties.

AcknowledgmentsThe authors acknowledge the support of key sponsors including

NSF Performance for Innovation Program (Award # EEC0525746),National Research Initiative of the USDA Cooperative State Research,Education and Extension Service, grant number 2003-35504-13620,and the IPST@GT student fellowship program.

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