Forestry Biofuels - classification and production

58

Review

2007 Society of Chemical Industry and John Wiley & Sons, Ltd

The new forestry biofuels sectorYunqiao Pu, Dongcheng Zhang, Preet M. Singh and Arthur J. Ragauskas, Georgia Institute of Technology, USA

Received October 9, 2007; revised version received November 12, 2007; accepted November 13, 2007

Published online December 19, 2007 in Wiley InterScience (www.interscience.wiley.com); DOI: 10.1002/bbb.48;

Biofuels, Bioprod. Bioref. 2:5873 (2008)

Abstract: Societys increasing demand for transportation fuels has assured a viable future for the development of

renewable fuels. Although fi rst-generation biofuels are dependent on starches, sugars and vegetable oils, the need to

generate higher volumes of biofuels at lower cost has shifted the research focus to cellulosic ethanol. The utilization

of lignocellulosics for the sustainable manufacturing of biofuels is critically dependent on the chemical constituents

of the starting biomass and the desired fuel properties. This review examines the major chemical constituents of

biomass and the recent advances in their conversion to biofuels, with a special emphasis on the forest residues and

woody-energy crops to bioethanol. 2007 Society of Chemical Industry and John Wiley & Sons, Ltd

Keywords: cellulose; hemicellulose; lignin; wood; biorefi nery; biofuels; pretreatment; saccharifi cation; fermentation;

pyrolysis; gasifi cation

Introduction

The events of the last few years have brought into sharp focus the need to develop sustainable green technolo-gies for many of our most basic manufacturing and

energy needs. Since the beginning of the new millennium, we have witnessed an ever-increasing merger of technical, economical and societal demands for sustainable technolo-gies. Indeed, hardly a day goes by in which the issues of energy security, climate change, cradle-to-cradle product development are not discussed in public and professional forums.13 Accompanying these interests, science and engi-neering have made tremendous strides to begin to answer these challenges. Indeed, it is the intersection of science, business and public policy that has launched a new green, industrial revolution that promises to dramatically alter our world.4

At the cornerstone of this green industrial revolution is the integrated biorefi nery.5 Th is is a biomass processing facility that integrates our ability to tailor biomass productivity and

processability with conversion processes, with the equip-ment to produce a range of fuels, power, and chemicals from biomass.6 It fully utilizes all components of biomass to make a range of foods, fuels, chemicals, feeds, materials, heat and power in proportions that maximizes sustainable, economic development. As such, this vision seeks to develop a new carbohydrate-lignin economy that will initially supplement todays petroleum economy and, as these non-renewable resources are consumed, will become the primary resource for fuels, chemicals and materials.

Todays bioethanol and biodiesel plants represent the fi rst-generation biorefi neries that utilize readily proces-sable bioresources such as sucrose, starches and plant oils.79 As has been highlighted in several reviews, societys ability to displace substantial amounts of nonrenewable petroleum reserves with renewable resources rests on its ability to secure large amounts of low-cost biomass. For example, Perlack et al., identifi ed 1.3 billion dry tons of biomass potential/year in the USA which could be directed to biofuels production; enough to address approximately

Correspondence to: Arthur J. Ragauskas, School of Chemistry and Biochemistry, Georgia Institute of Technology,

500 10th Street NW, Atlanta, GA 30332. E-mail: [email protected]

2007 Society of Chemical Industry and John Wiley & Sons, Ltd | Biofuels, Bioprod. Bioref. 2:5873 (2008); DOI: 10.1002/bbb 59

Review: New forestry biofuels sector Y Pu et al.

one-third of current demand for transportation fuels in the United States.10 A subsequent workshop titled Breaking the biological barriers to cellulosic ethanol supported this hypothesis, and with a more aggressive research program on improving energy crops, the biomass replacement potential could be even greater.11 An analysis of the US bioresource basin suggests that approximately 30% of this biomass would originate from forest resources including: wood from forest-lands, wood-related mill residues, and terrestrial urban wood residues. Th e exact distribution of these resources is clearly sensitive to geographical locations and the next generation of biorefi ne ries will need to be engineered to utilize local bioresources. A unique web resource that summarizes the theoretical potential ethanol yield from biomass, including woody plants, is the US Department of Energy, Energy effi -ciency and renewable energy biomass program website.12 Th e theoretical ethanol yields for forest thinnings, hardwood sawdust and mixed paper were predicted at 81.5, 100.8 and 116.2 gallons/ton of dry feedstock, respectively.

Woody biomass resources

Th e conversion of these bioresources to value-added mate-rials and chemicals rests primarily on our abilities to manipulate the chemistry and biochemistry of cellulose, hemicellulose and lignin. Th ese three biopolymers are the main global plant resources and further research is needed to effi ciently convert these bio building blocks to biofuels, biochemicals, biomaterials and biopower.

Cellulose

Of the three bioresources, cellulose is chemically the simplest structure as it is a linear polymer of (14) glucopyranosyl (Fig. 1) with a degree of polymerization (DP) varying from ~10,000 in cotton to less than 500 in several industrially processed materials.13

Th e cellulose chain has a strong tendency to form intra- and inter-molecular hydrogen bonds by the hydroxyl groups on

these linear cellulose chains, which stiff ens the chains and promotes aggregation into a crystalline structure. Th ese properties give cellulose a multitude of crystalline fi ber structures and morphologies. Th e degree of crystallinity of select cellulose samples are presented in Table 1.14

Th e ultrastructure of native cellulose (cellulose I) has been shown to possess an additional complexity in the form of two crystal phases: I and I.15 Th e relative amounts of I and I have been found to vary between samples from diff erent origins. Th e I-rich specimens have been found in the cell wall of some algae and in bacterial cellulose, whereas I-rich specimens have been found in cotton, wood, and ramie fi bers.16 Th e crystal and molecular structure of cellu-lose I has been examined recently by Nishiyama et al., using atomic-resolution synchrotron and neutron diff raction data recorded from cellulose isolated from alga and tunicin.17 Most native samples of cellulose also have varying degrees of amorphous cellulose, which is more reactive to chemical and enzymatic attack.

Hemicellulose

Aft er cellulose, the next major polysaccharide resource is plant hemicelluloses. Unlike cellulose, hemicelluloses have lower DP values (i.e., typically 50300), frequently have side chain groups and are essentially amorphous. Th e main hemicelluloses of soft wood (SW) are galactoglucomannans (Fig. 2) and arabinoglucuronoxylan (Fig. 3), while in hard-wood (HW) it is glucuronoxylan (Fig. 4). Table 2 summarizes

Table 1. X-Ray crystallinity of some cellulose materials.

Sample X-ray crystallinity (%)Cotton linters 5663

Sulfi te dissolving pulp 5056

Prehydrolyzed sulfate pulp 4045

Viscose rayon 2740

Regenerated cellulose fi lm 4045

Figure 1. The structure of cellulose.

O

OO

HOOH

HOH2C

OHO

HOH2C

OHO

OO

HOOH

HOH2C

O HO

HOH2C

OH

O

60 2007 Society of Chemical Industry and John Wiley & Sons, Ltd | Biofuels, Bioprod. Bioref. 2:5873 (2008); DOI: 10.1002/bbb

Y Pu et al. Review: New forestry biofuels sector

Figure 2. Principal structure of galactoglucomannans in softwood.

O

O

HOH2CO

OH

ORO

O

O

OH

CH2OH

HO

O

O

HOH2COR

OHOH2C

RO O

O

HO RO

HO

OR OR

R: H or Ac

Figure 3. Principal structure of arabinoglucuronoxylan in softwood.

O

OO

O

OOH

O

O

OH

HOOCH3CO

HO

O

OOH

OOOH O

O

OH

HOH2COH

OHOH

HO

Figure 4. Principal structure of glucuronoxylan in hardwood.

O

OO

O

OOR

O

O

OH

HOOCH3CO

HO

O

OOR

ORORO O

OROR

RO

R: H or Ac

Table 2. The major hemicellulose components in softwood and hardwood.19,20

Wood Hemicellulose typeAmount

(% on wood)

Composition

~DPUnits21 Molar ratios Linkage

SW

Galacto-glucomannan 1015

-D-Manp -D-Glcp-D-GalpAcetyl

41

0.11

141416

100

Arabino-glucuronoxylan 710-D-Xylp4-O-Me--D-GlcpA-L-Araf

10 2 1.3

141213

100

HW

Glucuronoxylan 1530-D-Xylp4-O-Me--D-GlcpAAcetyl

10 1 7

1412

200

Glucomannan 25-D-Manp -D-Glcp

121

1414

200

the main structural features of hemicelluloses appearing in common soft wood and hardwood resources.18

In addition, most sugar components can take part in the formation of lignin-carbohydrate complexes (LCC) by

covalent linkages between lignin and carbohydrates.22,23 Th e most frequently suggested LCC-linkages in native wood are benzyl ester, benzyl ether, and glycosidic linkages.24



Lignin

Th is biopolymer is an amorphous, cross-linked, and three dimensional phenolic polymer.25 Th e biosynthesis of lignin stems from the polymerization of three types of phenylpropane units as monolignols: coniferyl, sinapyl, and p-coumaryl alcohol.26,27 Figure 5 depicts these three structures. Soft wood lignin is composed mainly of coniferyl alcohol units, while hardwood lignin is composed mainly of coniferyl and sinapyl alcohol units.

Th e polymerization process is initiated by an enzyme-catalyzed oxidation of the mono lignol phenolic hydroxyl groups to yield free radicals. A monolignol free radical can then couple with another monolignol free radical to gener ate a dilignol. Subsequent nucleophilic attack by water, alco-hols, or phenolic hydroxyl groups on the benzyl carbon of the quinone methide intermediate restores the aro maticity of the benzene ring. Th e generated dilignols then undergo further polym erization to form protolignin.

Although the exact structure of protolignin is unknown, improvements in methods for identifying lignin-degradation products and advancements in spectroscopic methods have

enabled scientists to elucidate the predominant structural features of lignin. Figure 6 depicts some of the common link ages found in soft wood lignin.28,29 Th e typical abun-dance of these types of linkages and functional groups in soft woods are shown in Tables 3 and 4.28 Lignin is much less hydrophilic than either cellulose or hemicelluloses and it has a general eff ect of inhibiting water adsorption and fi ber swelling.

Table 3. Proportions of different types of linkages connecting the phenylpropane units in softwood lignin.

Linkage typea Dimer structure Percentage

-O-4 Phenylpropane -aryl ether 50-5 Phenylcoumaran 9125-5 Biphenyl 1525

5-5/-O-4 Dibenzodioxicin 1015

4-O-5 Diaryl ether 4

-1 1,2-Diaryl propane 7- --linked structures 2

Figure 6. Common linkages between phenylpropane

units in softwood lignin.28

CCC

CO

O

C

O

CO

C

C

CC

C

O

O

CC

O

C

C

O

CC

C

O

C

O

CCCC

-O-4

CCC

O

O

-O-4

CCC

CC

C

O O

-5 5-5

- -14-O-5

C

O

CC

O

O

dibenzodioxocin

Figure 5. Three building blocks of lignin.

R1

OH

R2

HO

12

3

4

5

6

Coniferyl alcohol/guaiacyl: R1 = OMe, R2 = HSinapyl alcohol/ syringyl: R1 = R2 = OMep-Coumaryl alcohol: R1 = R2 = H



Conversion of biomass to biofuels

As highlighted by Petruss recent renewable fuels article, the utilization of these primary bioresources for biofuels production centers about deoxygenation chemistry.30 Table 5 summarizes that cellulose, hemicelluloses and lignin are all too-rich in oxygen-function groups in comparison to typi-cally hydrocarbon-based gasoline and diesel fuels.

To date, the most successful technological route for the conversion of plant biomass to biofuels is the fermentation route, with bioethanol derived from starch and sucrose now becoming a common 510% fuel supplement. Although all biorefi neries are regional and no one technology will address all needs, for many geographical locations to attain higher levels of renewable fuels this will require the utilization of lignocellulosics. Th is change in bioresource is based on the greater availability of lignocellulosic biomass, potential lower cost and the avoidance of the food or fuel arguments.31

Currently, the overall cost of converting lignocellulosic material to ethanol is higher than well-established commer-cial starch to bioethanol technologies. Th is is primarily due to the recalcitrance of lignocellulosics.32 Lignin is the most recalcitrant component of the plant cell wall. In general, the higher the proportion of lignin, the lower the bioavailability

of the substrate. Th e eff ect of virgin lignin, redeposited lignin aft er pretreatment33 and LCC on the bioavailability of other cell-wall components is thought to play a large role in the physical restriction mechanism. Other factors have been proposed, including non-specifi c association between lignin and deconstruction enzymes (i.e., cellulase, xylanase, etc.).3436 Interestingly, recent studies at the National Renew-able Energy Laboratory (NREL) have suggested that low levels of lignin may actually enhance cellulose hydrolysis.37 Th is eff ect has been attributed to a physical separation of microcellulose fi brils enhancing cellulase access/activity. Recent studies by Pu et al.,38 Hayashi et al.,39 and others have demonstrated that depolymerization of fi brous cellu-lose by cellulase exhibits selectivity toward the more reactive amorphous, paracrystalline and I forms of cellulose leaving behind a more recalcitrant crystalline form of cellulose. In contrast, other reports have suggested that the cellu-lose crystallinity index aft er hydrolysis does not change.40 Clearly, selectivity of cellulase hydrolysis and its impact on residual crystalline structure needs further investigation since it is well known that fungal cellulase hydrolysis of amorphous cellulose is 330 times faster than crystalline cellulose.41,42 Th e role of acetylated hemicelluloses for both soft woods and hardwoods has also been suggested to impact enzymatic deconstruction of polysaccharides.43 Th e effi cient, cost-eff ective depolymerization of these polysaccharides to monosaccharides remains a key challenge in the utilization of these bioresources for fermentation to ethanol.44,45 To date, eff ective utilization of these bioresources is predicated on a pretreatment that reduces biomass recalcitrance.

Current pretreatment technologies

Th e objective of pretreating lignocellulosics is to alter the structure of biomass in order to make the cellulose and hemicelluloses more accessible and amenable to hydrolytic enzymes that can generate fermentable sugars. Eff ective pretreatment technologies need to address several important criteria, including: minimization of hemicelluloses degrada-tion products, limiting the formation of by-products that inhibit ethanol fermentation, lignin alterations,46 minimal energy, capital and operating costs. Some of the most studied lignocellulosic/wood pretreatments are summarized below:

Table 4. Functional groups in spruce lignin.

Functional groupMilled wood lignin per 100 C9 units

Carbonyl 0.8

Olefi nic + substituted aromatic C

39

Aliphatic CHx-OR 23.6

Methoxyl 11.2

Aliphatic CHx 4.9

Table 5. General chemical composition of bioresources and petroleum.

Cellulose/starch [C6(H2O)5]n

Hemicellulose [C5(H2O)4]n/[C6(H2O)5]n

SW Lignin [C10H12O4]n

Gasoline ~C6H14C12H26

Diesel ~C10H22 to C15H32



Uncatalyzed steam explosion involves rapidly heating biomass with steam at elevated temperatures (~190240 oC) with residence times of ~38 min, followed by explosive decompression. Th is treatment promotes hemicellulose hydrolysis and opens up the plant cell structure, although enhanced digestability of cellulose is only weakly correlated with the physical eff ects.47,48 Th is autohydrolysis procedure has been shown to be eff ective with agricultural residues and hardwoods, but not as benefi cial for soft woods.49,50 To improve the effi ciency of this process, several additive tech-nologies have been examined including pre-impregnation with SO251 and post-alkaline hydrogen peroxide treatment.52

Hot water autocatalyzed pretreatments at 200230 oC for up to 15 minutes can result in extensive hemicellulose hydrolysis but, the high lignin content biomass reduces subsequent cellulase hydrolysis.53 Depending on the condi-tions employed, 3046% of the lignin of corn stovers could be removed. Th e production of possible inhibitors such as furfural and hydroxymethyl furfural was reported to account for less than 3% of the original carbohydrates54 as xylan release frequently results in oligomers. Subsequent enzymatic hydrolysis of cellulose has been reported to yield glucose in 2595% yields, with the latter only being accomplished with physical milling. Th e application of this pretreatment to hardwoods has been reported, resulting in 90% conversion of glucose to ethanol aft er simultaneous saccharifi cation and fermentation (SSF).55 Allen et al., published a comparison study of a hot-water treatment versus a dilute-acid pretreatment and both yielded compa-rable conversion to ethanol under optimized conditions, although the severity of the former pretreatment had to be much higher.56

Dilute acid pretreatment has been extensively studied and typically employs 0.42% H2SO4 (note: nitric, sulfur dioxide, and phosphoric acid have also been studied) at tempera-tures of 160220 oC to remove hemicelluloses and enhance cellulase digestion of cellulose.19 Th e acidic conditions used have been shown to enhance total sugar release aft er enzy-matic hydrolysis to ~93% for corn stovers and ~82% for soft wood.57 Th e pretreatment conditions impact not only the plant polysaccharides but also lignin.58 For soft woods, a two-stage acidic pretreatment has been used to tailor the reactivity of cellulose and hemicellulose. Th is tailored

approach has been reported to increase sugar yields by 10% and reduces cellulase requirements by about 50%.59 Th e use of SO2 on spruce woodchips is of exceptional interest as it yields a more reactive material with less inhibitory compounds than dilute acid and this is refl ected in higher ethanol yields aft er saccharifi cation and fermentation.60 Recently, diethyloxylate has been reported as a potential acidic pretreatment reagent for wood and other treatments are also being developed.61

Aqueous lime or NaOH pretreatment has been shown to be eff ective for wheat straw and sugar bagasse with lower temperatures than acid treatments; however, the treatment times are hours long. For example, Chang et al., used lime with wheat straw at 85 oC, for 3 h.62 Th e use of an alkaline treatment incurs additional capital cost, as the recovery of salts requires a lime kiln to regenerate the base. Th e effi ciency of alkaline treatments to convert recalcitrant biomass for subsequent cellulase treatments has focused on the application of a supplement oxidant, such as oxygen or hydrogen peroxide. It has been reported that this protocol dissolves the hemicelluloses, degrades lignin, and yields a cellulose fraction that is very accessible to enzymes for hydrolysis and fermentation to ethanol. An improved version of this pretreatment is the utilization of oxygen under alkaline conditions. An oxidative lime treatment63 and other wet-oxidations64 have been shown to improve the eff ectiveness of this pretreatment technology especially for wood-related bioresources.

Ammonia pretreatment involves pretreating biomass with an aqueous solution (515%) at temperatures of 160180 oC. Th e ammonia reacts with lignin causing depolymerization and cleavage of select lignin-carbohydrate bonds. Agricul-tural residuals and herbaceous plants treated in this manner exhibit an excellent response to cellulase.65,66 Unfortunately, hardwoods and soft woods are not effi ciently treated by this technology, with conversion yields of glucose to ethanol being reported to be less than 85%.67 In all cases, ammonia recovery is an additional cost and important consideration.

Organosolv pretreatment of biomass resides on the use of an organic solvent system (i.e., ethanol/water,68,69 acetone/water,70 methyl isobutyl ketone/ethanol/water71) with enhanced solubilizing properties, due to the organic component. Usually, the resultant cellulosic fraction is highly



susceptible to enzymatic hydrolysis, generating very high yields of glucose that can be readily converted to ethanol. Pan et al., have shown that ~88% of the cellulose could be recovered aft er the organosolv pretreatment of hybrid poplar and 85% was converted to glucose upon subsequent enzyme hydrolysis.72 Even better results were reported for infected lodgepole pine.73 Several authors have also indicated that the wood-based biofi nery of the future will garner addi-tional revenues from the extracted lignin and hemicellulose streams.7476

In all these and other pretreatment technologies, diff erences in cell-wall structure and chemistry impact how hardwoods and soft woods respond to chemical pretreatments. Several authors have indicted that the recalcitrance of soft wood resources is greater than hardwoods which is exhibited in reduced digestability by cellulase.77,78 Th e exact chemical constituents and ultrastructures that contribute to this eff ect are not well understood as recently highlighted by Mosier et al: Greater fundamental understanding of the chemical and physical mechanism that occur during pretreatment along with an improved understanding of the relationship between the chemical composition and physico-chemical structure of lignocellulosics on the enzymatic digestion of cellulose and hemicellulose is required for the generation of eff ective pretreatments.79 Future fundamental research into these issues promises to have a far-reaching benefi cial eff ect in accelerating the development of low-cost biofuels.

Plant genetics, recalcitrance and future pretreatments

Th e need to improve the eff ectiveness of pretreatment technologies for wood is driven primarily by the fact that it remains the most costly step in the overall conversion of wood to biofuels. Wyman has perhaps best summarized the state-of-the-art pretreatment capabilities: Th e only step more expensive than pretreatment is no pretreatment, because of its impact on virtually all other operations.80 In light of the well-known dependency of biomass recal-citrance on the plant resource, it is natural to consider the opportunity of reducing the recalcitrance of wood and other biomass via the genetic engineering of the biomass. Indeed, the forest products industry has extensively championed the use of plant genetics to tailor the composition, structure and

reactivity of soft wood and hardwood biopolymers, especially lignin. For example studies by Chiang et al., have inserted antisense 4CL and sense coniferaldehyde 5-hydroxylase genes into aspen to yield trees with each or both of these transgenes. Introduction of the former gene reduced lignin concentrations by 55% and the latter gave up to a three-fold increase in syringyl: guiacyl lignin.81 Huntley et al., reported that increased syringyl-lignin in transgenic poplars, by over-expressing F5H, increased chemical pulp ability by 60%.82 Likewise, Pilate et al., demonstrated that transgenic poplar with low CAD activity exhibited improved kraft pulping properties.83 Th ese results highlight the potential to alter specifi c biopolymer constituents in woody plants which confer benefi ts in subsequent chemical operations such as kraft and soda pulping.

It is reasonable to anticipate that as our knowledge of the benefi cial physical-chemical impacts of pretreatments on the plant cell wall is developed, it will be possible to genetically engineer low-recalcitrance wood. For example, reduced lignin content, modifi cations in cellulose crys-tallinity, diff ering hemicellulose structures and reduced lignin-carbohydrate complexes have all been shown to decrease plant recalcitrance and it should be possible to engineer these same properties into woody plants and other bioresources.84,85 A recent report by Davison et al.,86 has demonstrated this approach, since changes in lignin content and syringylguaiacyl ratios of a second-generation Populus signifi cantly benefi ted xylose release upon dilute sulfuric acid hydrolysis. Chen and Dixon have also reported compa-rable results for the acid hydrolysis of a series of alfalfa lines containing antisensing constructs for downregulating lignin.87 In brief, the lines with reduced lignin content released greater amounts of carbohydrates during acid pretreatment and in subsequent enzymatic hydrolysis. Th ese results indicate that genetic control of lignin content and composition infl uences the hydrolyzability of the biomass and sets the stage for further developments.

Given recent advances in plant genomics, it is anticipated that engineered changes in plant cell structure will yield low-recalcitrant, highly productive agro-energy crops in the near future. Th is will have a dramatic impact on pretreatment technologies reducing the severity, capital and operating costs of this key stage in the conversion of biomass to biofuels.



Cellulosic saccharifi cation-fermentation technologies

Current: Following pretreatment, woody biomass can be converted into simple sugars by enzymatic deconstruction via a cellulase treatment. Th is remains the second most expen-sive component in the bioconversion of wood to bioethanol, despite the fact that research studies over the past decade have decreased cellulase costs by greater than a ten-fold basis.88 Numerous publications and reviews have highlighted the use of (i) separate hydrolysis and fermentation (SHF) and (ii) simultaneous saccharifi cation and fermentation (SSF) to convert pretreated wood to ethanol.89,90 Ewanick et al., have reported that employing SSF on a SO2-steam exploded lodge-pole pine provided 6177% yield of the theoretical maximum ethanol yield depending upon the severity of the pretreat-ment.91 A two-stage acid treatment of spruce using SO2 or H2SO4 was reported to give ethanol yields of 81 and 70% respectively (i.e., 94 and 79 gallons/dry ton).60,92 Comparable SSF ethanol yields have also been reported for an organosolv pretreated mixed soft wood furnish.93

Th e conversion of hardwoods to ethanol has also been extensively studied. SSF treatment of steam-exploded poplar and eucalyptus has been reported to provide 71 and 62% of the maximum theoretical yield of ethanol from glucose.94 Higher SSF ethanol yields of 90% from cellulose have been reported for an acidic hot-water treatment of yellow poplar, provided that the solids were washed with hot water to remove solubilized lignin.95 Th e role of inhibitors formed during steam explosion of poplar on SSF has been exten-sively studied. Undetoxifi ed pretreated wood was reported to yield no ethanol even with high loadings of Saccharomyces cerevisiae, whereas water-rinsed biomass provided an 82% yield of ethanol.96 Bari et al., have also reported bioethanol yields of 85% from steam-exploded97 aspen chips and more recently demonstrated that SO2 impregnation can enhance this pretreatment technology.98 Although most authors have tailored their SSF stage to their exact bioresource, Berlin et al., have reported that cellulase treatments performed optimally on hardwood also exhibit superior performance on soft wood substrates.99

A promising approach to reducing cellulase cost is to capture and reuse the enzymes. For example, Tu et al., have shown that 51% of the applied cellulases could be recovered

by re-adsorption onto fresh lignocellulosic materials.100 In addition, it is well known that the lignin fraction in pretreated lignocellulosics is involved in unproductive binding to cellu-losic enzymes that reduces the performance of the enzymes. Th e development of additives including proteins101 and surfactants102 that disrupt this association has been shown to enhance the effi ciency of deconstruction enzymes.

Future: A process challenge in the conversion of wood to biofuels is the effi cient conversion of all wood sugars (i.e., C5 and C6) to ethanol, especially for hardwoods which have greater amounts of pentoses. Microorganisms that are able to ferment sugars to ethanol can be either yeasts103,104 or bacteria.105 Over the past decades, new methods in molecular biology, protein chemistry and genetic engineering have led to an increasing number of new strains, exhibiting improved characteristics to ferment the full spectrum of sugars avail-able in hydrolyzates.106,107 One promising strategy has been to take a natural hexose ethanologen and add the pathways to convert other sugars. Th is strategy has been eff ective in adding pentose conversion to Saccharomyces cerevisiae, and to Zymomonas mobilis.108,109 Th ese enhancements promise to further enhance the overall fermentation of mixed solutions of hexoses and pentoses to ethanol.110112

Although research studies over the past decade have decreased cellulase cost by greater than a ten-fold basis,113 they still remain a signifi cant cost for SSF and SHF. An alternative approach to minimize the cost of cellulose deconstruction and conversion to ethanol is consolidated bioprocessing (CBP). CBP involves (i) bioproduction of cellulolytic enzymes from thermophilic anaerobic microbes, (ii) hydrolysis of plant polysaccharides to simple sugars and (iii) their subsequent fermentation to ethanol all in one stage.114 Th is bioprocess is projected to reduce the cost of bioethanol by a factor of four over SSF and these reduced costs and simplicity of operation have heightened research in this fi eld. To date, the penultimate CBP system has not been developed but the basic pathways that need to be developed have been reported and research is ongoing.115,116

Non-biological pathways of converting biomass to biofuels

Although the biological route for converting biomass to biofuels is one of the most developed and promising



technologies, several other competing processes are also being explored and developed. As an alternative approach, the production of a bio-oil from biomass by pyrolysis certainly is of the most direct methods of liquidifying natures key bioresources. Typically, this process can be accomplished with a conventional slow pyrolysis reaction involving a reactor temperature of ~500 oC and a vapor residence time of 530 min, or fast pyrolysis conditions involving a temperature range of 425500 oC with a very short vapor resident time



processing of wood. Th e development of the next generation pulp-biofuel mills is being actively investigated on several fronts.18 For kraft pulp mills, two high-priority opportunities center about the next generation of chemical recovery opera-tions and the transition from the conventional Th ompson recovery furnace technologies to gasifi cation of black liquor which could yield a syngas process stream.136138 Alterna-tively, ongoing studies have highlighted the potential to extract select hemicelluloses from woodchips prior to kraft pulping. It is well known that kraft pulping conditions extract select hemicelluloses which do not contribute to fi nal pulp properties as summarized in Table 6.

Th ese extractable hemicelluloses could provide a valuable, high-volume resource of sugars for bioethanol production generating ~2040 million gallons ethanol/year/mill.140 Th orp has reported that the potential annual production of ethanol from pre-extraction of hemicellulose could approach 2 billion gallons of ethanol/year.141 Recent studies suggest that a pre-extraction of northern hardwood benefi ts kraft pulping whereas a soft wood furnish suff ers from yield losses which needs to be addressed.142,143

An alternative resource for bioethanol production is to utilize waste cellulosic streams from paper recycling

operations.144 Paper-mill sludge typically has a negative value as it needs to be properly landfi lled and hence is an attractive resource for SSF conversion to bioethanol.145,146 Furthermore, it has been well documented that this low-cost bioresource does not need a pretreatment prior to SSF but the presence of minerals, contaminants and diffi culties in mixing paper-mill sludge provide additional complications to the overall process. Nonetheless, a recent study by Fan and Lynd suggested that a viable SSF process could be devel-oped yielding a +15% internal rate of fi nancial return which provides a viable treatment option for the ~5 million tons of paper mill sludge generated annually in the USA.147

Tomorrows forest biorefi nery

Th e practical application of the science and engineering associated with converting wood to biofuels is a rapidly moving target that will require constant updating. Nonethe-less, near the end of 2007 several notable industrial develop-ments have been announced. In the USA, the Department of Energy recently announced an investment of up to $385 million for six biorefi nery projects with an industry cost share of more than $1.2 billion.148 When fully operational, these biorefi neries are expected to produce more than 130 million gallons of cellulosic ethanol/year. Th ree of these plants have announced the utilization of wood as a biore-source converting it to bioethanol via thermochemical or biological routes, including:

(1) ALICO, Inc., Florida will produce 13.9 million gallons of ethanol/year with a proposed 770 tons/day feedstock from yard, wood, vegetative wastes and eventually energy cane.

(2) BlueFire Ethanol, Inc., California will site a biorefi nery on an existing landfi ll and produce about 19 million gallons of ethanol/year. Th e proposed plant will consume 700 tons/day feedstock of sorted green waste and wood waste from landfi lls.

Figure 7. The scheme for integrated wood-based biorefi nery.

Table 6. Changes in carbohydrate distribution before and after kraft pulping loblolly pine.139

Source Glucose Galactose Mannose Arabinose XylanoseWood 67.9 3.5 17.7 2.1 8.8

Kraft Pulp 84.9 0.3 7.1 0.5 7.1



(3) Range Fuels, Georgia has begun constructing a plant that will produce about 40 million gallons of ethanol/year and 9 million gallons/year of methanol. Th e plant will use 1,200 tons/day of wood residues and wood-based energy crops.

Th e application of thermal and biological technologies to convert wood to ethanol clearly suggests that both technology platforms are viable, each with their own unique strengths and concerns. It is anticipated that the research developments described earlier in the review will favorably impact these commercial developments. Meanwhile, the Tembec Temis-caming sulfi te mill in Quebec is a modern example of how a pulp mill can grow into a biorefi nery.149 Along with pulp production, the mill ferments spent sulfi te cooking liquors with Saccharomyces cerevisiae to produce 4 million gallons/year of food-grade ethanol. It has installed an anaerobic biogas unit that displaces ~80% of the natural gas required for high-yield pulp fl ash drying and produces lignosulfonates for commercial markets. Although the same approach is much more technically challenging for a kraft pulp mill, its potential has been noted by several industry leaders.150

Another example is the Flambeau River paper mill in Wisconsin that has announced a partnership with American Process Inc., for a cellulosic ethanol biorefi nery. Th e biore-fi nery project will be designed to produce 20 million gallons of cellulosic ethanol/year from the mills spent pulping liquor. 151 Xethanol Corporation has reported that it has acquired a former medium-density fi berboard factory which it plans to re-open as a pilot plant to demonstrate the tech-nical and economic viability of using wood chips for the production of cellulosic feedstock.152 Additional announce-ments of research consortiums and pilot plant developments targeted at utilizing waste streams from virgin and recycled pulp mills along with wood residues occur virtually on a monthly basis on the international scene.153 Recent improve-ments in biorefi nery processing technology, energy costs and favorable government policy will only accelerate these busi-ness develop ments in the forest products industry.154

Transportation of bioethanol

With anticipated widespread usage of bioethanol, an effi cient and reliable transportation and distribution system from

biorefi nery to the end-user also needs attention. Pipelines are, by far, the most cost-eff ective means of transporting large quantities of fuel over long distances, whereas tankers are used to transport fuels, including ethanol, over short distances such as from small biorefi neries to storage and distribution centers. Use of existing pipeline infrastructure, presently used to transport gasoline products, as well as new, dedicated pipe-lines may be considered for ethanol transportation.

Existing gasoline pipelines are made out of carbon steel. Corrosion and stress-corrosion cracking of carbon steel structures, especially pipeline steel, are other concerns for ethanol storage and transportation. A 2003 survey of industry, reported by the American Petroleum Institute (API Technical Report 939-D),155 indicates that carbon steel may undergo stress corrosion cracking (SCC) in certain ethanol environments. Th is is not a widespread concern as the cracks have only been observed primarily in user terminals exposed to ethanol products, but not in ethanol producer tanks, in rail/tank car/shipping transportation, or in end-user systems (e.g., gas tanks). Preliminary studies have shown that certain minor constituents may aff ect SCC behavior of carbon steel. For example, the presence of oxygen and the aging of fuel-grade ethanol have been reported to increase SCC suscepti-bility of carbon steel.156,157 However, eff ects of ethanol source or aging, and resulting diff erences in the minor constituents, on corrosion and stress corrosion cracking of carbon steels are not very clear. Further work is needed to understand corrosion mechanisms and to identify fuel-grade ethanol environments that may cause SCC. Th is will help us fi nd mitigation strategies where some chemical additives may be used or alternative materials may be selected for transporta-tion and storage structures to reliably distribute ethanol.

Summary and conclusions

In closing, aft er an extended period of low energy costs and diff ering research priorities, a near global emphasis on renewable biofuels technologies has evolved in the new millennium. Although diff ering social, environmental and economic issues have elevated these needs, there is no denying this new challenge. Furthermore, advances in plant genomics, biotechnology, nanotechnology, catalysis, material science, life-cycle analysis and computational modeling suggest that advances in the fi eld of renewable



biofuels will progress at rates unattainable in past decades. As these technological advances leave the laboratory and impact commercial practices, it will bring to the forefront Morris vision of the new carbohydrate-economy in which major industrial sectors are dependent on the sustainable utilization of biomass, in harmony with global agricultural production.158

Acknowledgements

Th e authors acknowledge the support of key sponsors including NSF Performance for Innovation Program (Award # EEC0525746) and National Research Initiative of the USDA Cooperative State Research, Education and Extension Service, grant number 2003-35504-13620.

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Forestry Biofuels - classification and production

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